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Corresponding author: e-mail, tpercival@uisreno.com

Abstract

The Bau mining district, on the island of Borneo in the southwestern Pacific, has produced gold (45.5 tonnes [t] or 1.46 Moz), antimony (83,000 tons), and mercury (1,100 t or 32,000 flasks) from calcic skarn, calcite-quartz veins, and sedimentary rock-hosted replacement deposits that are concentrically arranged around microgranodiorite intrusions with Cu-(Mo) quartz stockwork mineralization. Ores are exceptionally enriched in arsenic. Oxidized disseminated replacement ores, which are chemically, texturally, and isotopically similar to Carlin-type gold deposits in northern Nevada, have contributed the majority of the gold production of the district. The Tai Parit mine, the largest in the district, has produced about 22 t (0.7 Moz) Au at an average grade of about 7.5 g/t (0.22 oz/t) Au. Ores were mainly treated by cyanidation. The concentric zonation pattern led previous workers to propose that these and other Carlin-style gold deposits are distal manifestations of magmatic-hydrothermal systems. This investigation presents new fluid-inclusion, isotopic, and mineralogical data in the context of previously obtained geological, chemical, and other information that advance our understanding of this district, enabling comparisons with Carlin-type deposits in Nevada and distal disseminated deposits elsewhere in the world.

Bau is situated on the western end of the Eocene to Miocene Central Kalimantan magmatic arc. A new K-Ar date on hydrothermal sericite of 10.4 ± 0.3 Ma from a stock with gold-bearing calcic skarns is within the age range of nearby intrusions dated at 11.6 to 9.3 Ma that form part of a NNE-trending, adakitic, magmatic belt. The subvolcanic intrusions and gold deposits are localized by NNE-striking normal faults that transect marine calcareous rocks of the Upper Jurassic Bau Formation and siliciclastic rocks of the Lower Cretaceous Pedawan Formation that are exposed along the axis of the NE-trending Bau anticline. Gold ore is best developed at the intersection of the Krian fault and the contact between these two formations.

The outward zonation from wollastonite-bearing skarn, through calcite-quartz veins, to decalcified and silicified limestone and clastic rock is indicative of decarbonation and Si metasomatism of limestone as hydrothermal fluids cooled. The predominance of sericite over kaolinite shows that fluids were near neutral to moderately acidic and contained a significant amount of potassium. The spatial distribution and paragenetic sequence of native antimony, aurostibite, stibnite, sphalerite (1–7 wt % Fe), pyrrhotite, pyrite, arsenopyrite, native arsenic, and realgar is evidence for cooling and desulfidation of ore fluids.

In sedimentary rock-hosted replacement deposits, mass loss due to carbonate dissolution is shown by enrichment of Ti and Al and depletion of Ca, Mg, and Sr. The strong introduction of Si, Fe, Mn, Zn, Pb, and Ag together with Au, As, and Sb is suggestive of cooling and mixing of saline and H2S-bearing fluids. Laser ablation-inductively coupled plasma-mass spectrometry analyses show that most of the Au resides in arsenopyrite and that Cu and Te are present in Sb and As minerals.

Cooling and decompression are shown by hypersaline fluid inclusions (25–38 wt % NaCl equiv) in Cu (Mo) stockworks and calcic skarn, which were trapped between 500° and 240°C and 400 and 30 bar, while low-salinity fluid inclusions (0–6 wt % NaCl equiv) in vein and Carlin-style deposits were trapped between 350° to 100°C and 200 and 1 bar. Fluid inclusions of intermediate salinity are indicative of fluid mixing. The maximum pressure corresponds to depths of 1.6 (lithostatic) to 4 km (hydrostatic).

The H, O, and C isotope compositions of sericite, wollastonite, quartz, calcite, and inclusion fluids strongly suggest that each deposit type formed from magmatic fluids that were shifted to lower δD values by magma degassing and higher δ18O and δ13C values by exchange with marine limestone. Only fluid inclusion water extracted from late drusy quartz is shifted toward, and late calcite plots on, the meteoric water line. The isotopic composition of S in pyrrhotite and pyrite is magmatic, whereas S in Te, Sb, and As minerals was derived from country rock. The data show how readily hydrothermal fluids of magmatic origin can be modified by reaction with wall rock, mixing with other fluids, and selective loss of lighter components.

In comparison to Nevada’s Carlin-type gold deposits, the Carlin-style gold deposits in the Bau district are smaller and more structurally controlled, have zonation in mineralogy and geochemistry indicative of steep thermal and chemical gradients around exposed porphyry intrusions, and formed from less acidic fluids by cooling and fluid mixing. In addition, Au resides in arsenopyrite, ore has more introduced Fe, Mn, Zn, Pb, Ag, Sb, and As and less Tl and Hg, and there is clear isotopic evidence for magmatic H2O, CO2, and H2S. The genetic links between magmatism and distal disseminated gold mineralization at Bau are a significant contribution to a growing body of evidence that Au and related trace elements in many Carlin-style gold deposits may be derived from magmas.

Introduction

The Bau mining district, Sarawak, Malaysia (Fig. 1A) has long been known for its gold, antimony, and mercury production (Emmons, 1937). Gold mineralization at Bau was first reported by Geikie (1906). Recorded production between 1820 and 1981 from arsenic-rich ores includes 45.4 tonnes (t) (1.46 Moz) of gold (Wolfenden, 1965), 75,300 t (83,000 tons) of metallic antimony (Hon, 1981), and 1,100 t (32,000 flasks) of mercury (Roe, 1950). Current proven and probable gold reserves are 10.66 Mt (0.34 Moz) at 1.7 g/t (0.05 oz/t) (Bau Gold Project, Sarawak, Malaysia, www.mining-technology.comprojects/bau-gold-project-sarawak, accessed December 2015), which corresponds to an additional 18 t (0.58 Moz). Wilford (1955), Wolfenden (1965), Pimm (1967), and Schuh (1993) described the geology and general features of the Au-, Sb-, and As-bearing calcic skarns and vein deposits. Percival et al. (1989, 1990) and Sillitoe and Bonham (1990) pointed out that the major gold production came from Carlin-style gold deposits with physical and chemical characteristics similar to Carlin-type gold deposits in the northern Great Basin of western North America.

Fig. 1.

A. Location map (inset) and geologic map of the Bau mining district, Sarawak, Malaysia. Geology modified from Wilford (1955). B. Schematic cross section (modified after Percival et al., 1990) showing the general spatial geometry of the various styles of alteration and mineral deposits in the Bau district relative to porphyritic igneous intrusions.

Fig. 1.

A. Location map (inset) and geologic map of the Bau mining district, Sarawak, Malaysia. Geology modified from Wilford (1955). B. Schematic cross section (modified after Percival et al., 1990) showing the general spatial geometry of the various styles of alteration and mineral deposits in the Bau district relative to porphyritic igneous intrusions.

Geologic relationships and zonation patterns led previous investigators to conclude that the diverse types of Au-As-Sb mineralization at Bau are genetically related to nearby porphyritic intrusions (Percival et al., 1989, 1990; Sillitoe and Bonham, 1990; Schuh, 1993; Percival and Hofstra, 2002). This paper describes each type of gold mineralization in the district and presents new geochronologic, mineralogical, geochemical, fluid inclusion, and stable isotopic data that substantiate a magmatic-hydrothermal origin. The evidence for magmatic-hydrothermal ore fluids at Bau is significant, because it bears on the origin of similar deposit types in the western United States and other parts of the world, which has been the subject of considerable long-standing debate.

Geology

Regional geology

Borneo was produced by the Mesozoic accretion of ophiolites, an island arc, and microcontinental fragments onto the Paleozoic core of Sundaland (Schuh, 1993; Hall, 2002). Southward subduction of the proto-South China Sea below Borneo produced an accretionary complex and the 1,200-km-long, Eocene to Miocene Central Kalimantan magmatic arc, until closure and collision with Palawan in early Miocene time (Schuh, 1993; Hall, 2002; Garwin et al., 2005). Pre- and post-collisional igneous rocks have different characteristics from early Miocene (23.7–21.9 Ma) high-K, calc-alkaline diorites to late Miocene (14.6–6.4 Ma) adakitic microtonalites and dacites (Prouteau et al., 2001). In addition, a petrologically similar tonalite stock dated at 16 Ma is present about 64 km south-southeast of Bau (Wilford, 1955). The adakite suite may have been produced by partial melting of subducted slivers of young oceanic crust in the upper mantle (Prouteau et al., 2001) or by fractionation of normal hydrous arc magmas originating in the metasomatized wedge of asthenospheric mantle (Richards and Kerrich, 2007). Bau is spatially and temporally associated with the postcollisional episode of adakitic magmatism at the western end of the Central Kalimantan magmatic arc.

Hamilton (1979) identified two subduction mélange complexes in westernmost Sarawak in which oceanic rocks were emplaced onto continental rocks during Lower Cretaceous to Eocene time. The older allochthonous sequence is the Serabang Formation (Wilford, 1955). It consists of clastic sedimentary rocks with subordinate greenstone, ultramafic rocks, and limestone that crop out as a NW-trending belt extending from Sarawak southward into Kalimantan (Fig. 1A). In the Bau region, this Jurassic to Lower Cretaceous polymictic subduction mélange comprises deformed shale, sandstone, felsic to intermediate volcanic rock, and subordinate limestone that was obducted onto continental rocks of Lower Permian to Upper Cretaceous age during the Cretaceous (Hamilton, 1979). During Eocene time, an imbricated tectonic wedge of oceanic rocks comprised of a northeast-southwest series of crescent-shaped mélange belts was emplaced upon a continental platform throughout northern Borneo (Hamilton, 1979).

District geology

A brief summary of district geology is presented here; descriptions that are more detailed are given by Wolfenden (1965), Pimm (1967), Percival et al. (1990), and Schuh (1993). Gold, antimony, and mercury deposits at Bau are hosted by rocks of the Mesozoic allochthonous sequence, composed of the Late Jurassic to Early Cretaceous Bau Limestone Formation and the Late Jurassic to Late Cretaceous Pedawan Formation (Figs. 1A, 2). Beneath the Bau Limestone Member is the Krian Member, which is a near-shore, transgressing, shallow marine, siliciclastic facies (Schuh, 1993). The Krian Member unconformably overlies the Upper Late Triassic Serian Volcanics, which are exposed directly north and east of the Bau district. The Serian Volcanics are up to 1,500 m thick (Pimm, 1967) and are composed of a bimodal sequence of tuff, lava, and breccia consisting of basal oceanic island-arc-type tholeiitic basalt and andesite overlain by rhyolite and trachyte.

Fig. 2.

Geologic map of the Bau district showing the spatial distribution of deposit types discussed in this paper. The deposits define a district-wide zoning pattern from porphyry-hosted base metal mineralization at the core to an outer zone of sedimentary rock-hosted gold deposits. The geologic formations are the same as in Figure 1.

Fig. 2.

Geologic map of the Bau district showing the spatial distribution of deposit types discussed in this paper. The deposits define a district-wide zoning pattern from porphyry-hosted base metal mineralization at the core to an outer zone of sedimentary rock-hosted gold deposits. The geologic formations are the same as in Figure 1.

The Bau Limestone is a thick-bedded sequence of light-gray, massive, nearly pure micritic limestone approximately 550 m thick, containing subordinate amounts of interbedded calcarenite, argillaceous limestone, and polymictic breccia. Siliciclastic material in the less pure sections contain dark-gray to black carbonaceous material. The bulk of the Bau Limestone appears to consist of rudist reef complexes (Lau, 1973). The basal Krian Member includes about 100 m of sandstone, shale, and argillaceous limestone. Finegrained clastic rocks are dark gray, contain carbonaceous material, and are pyritic. An apparent erosional gap occurs above the Serian and below the Krian Member; therefore, a complete stratigraphic section is not exposed in the Bau district. The Pedawan Formation consists of about 1,200 m of dark-gray and brown impure siltstone and sandstone and pyritic carbonaceous shales, with minor to rare radiolarian cherts, mudstone, calcareous sedimentary rock, sandstone, and volcanic rocks. The Pedawan clastic sedimentary section exhibits the classic features of a Bouma turbidite facies sequence deposited on an unstable marine slope. These clastic rocks typically underlie areas of low topographic relief, in marked contrast to the cliff-forming limestones of the Bau Formation.

The Bau district lies along the axial zone of a prominent, ENE-trending regional anticline (Fig. 1A, B). The anticline, which is symmetrical with a broad axial crest exposing the Bau and Pedawan Formations at the core, is flanked by tightly folded synclinal basins. Several sets of near-vertical and highangle faults transect the crest of the Bau anticline (Fig. 1A, B). The anticline is cut by a set of NNE-striking normal faults and fractures with significant offsets that produced a graben into which Tertiary porphyritic stocks were emplaced. These structures were dilatant and controlled the emplacement of dikes and the subsequent circulation of hydrothermal fluids (Hon, 1981; Percival et al., 1990). Fault dilation may have occurred in response to counterclockwise rotation of Borneo during mid-Miocene postsubduction regional extension (Schuh, 1993; Garwin et al., 2005). An orthogonal set of NW-striking faults and fractures as well as other weakly developed fracture sets with other orientations typically lack significant offset and mineralization.

Microgranodiorite and dacite porphyry stocks and dikes of middle to late Miocene age (13–10 Ma, Metal Mining Agency of Japan, 1985; 14.6–6.4 Ma, Prouteau et al., 2001) intrude the Mesozoic sedimentary rocks along a NNE-trending belt that extends from north of Bau southwestward into Indonesia (Fig. 1A; Kirk, 1968). Subordinate amounts of andesite porphyry, tonalite, and gabbro also occur along this magmatic belt (Pimm, 1967). Microgranodiorite porphyry, the most abundant rock type, contains abundant phenocrysts of quartz, orthoclase, plagioclase (oligoclase-andesine), hornblende, and biotite set in an equigranular groundmass of quartz, orthoclase, and plagioclase with accessory apatite, magnetite, zircon, and sphene. Dacite porphyry contains phenocrysts of zoned andesine, hypersthene, clinopyroxene, and hornblende in a microcrystalline groundmass of sodic plagioclase, quartz, and subordinate biotite (Pimm, 1967). The equilibrium textures of ilmenite and magnetite are indicative of temperatures <900°C and oxidizing conditions (NNO + 1) in underlying magma chambers (Prouteau et al., 2001). The porphyritic textures suggest that the intrusions were emplaced in a shallow (epizonal) environment.

The intrusions contain xenoliths of recrystallized sedimentary rock and are surrounded by contact metamorphic aureoles comprised of marble, calc-silicate minerals, and hornfels. Thermal aureoles range in size from narrow (<20 m) to wide (>350 m) and grade outward from marble and hornfels to weak distal recrystallization. The outcrop pattern and relatively small diameters (100–1,000 m) of the stocks, the abundance, distribution, and orientations of the dikes, and the large aerial extent of the associated contact aureoles suggest these intrusions merge into a single large pluton at depth (cf. Norton, 1982).

Hydrothermal Alteration

Hydrothermal alteration is widespread in the Bau district and affects all rock units. Types of alteration include silicification, decalcification, sericitization, argillization, propylitization, and the formation of calcic skarn. Supergene alteration and tropical weathering are extensive and well developed. Because most of the hydrothermally altered and mineralized rocks were originally strongly sulfidic, supergene oxidation has transformed them into red-, yellow-, brown- purple-, and black-stained (Fe and Mn oxides) masses of fine-grained unidentified clay minerals containing less altered blocks of densely silicified rocks and massive limestone. Such weathered material is variably auriferous (as much as 1 oz/t Au) depending upon original gold content (Wolfenden, 1965; Pimm, 1967).

Sedimentary rocks

Silicification is widespread in the Bau district. Intensity varies from weak to complete replacement by microcrystalline quartz to form dark-gray jasperoid and is directly proportional to the degree of fracturing and brecciation. Rocks that originally contained carbonate minerals are generally more strongly silicified than clastic rocks. Silicification of the Bau limestone typically results in the formation of a dark-gray to black, vuggy jasperoid in which original textural features are completely destroyed, whereas bluish-gray to gray silicified Pedawan shale retains some of its textural detail. Pervasively silicified rocks are commonly crosscut by later coarsely crystalline quartz and quartz + calcite veins and veinlets (millimeter to centimeter scale) and contain quartz-lined vugs. Brecciated zones, common in clastic rocks, are infilled and cemented by coarsely crystalline quartz exhibiting comb, cockade, and drusy textures. Breccia clasts are replaced by microcrystalline quartz. Quartz vein stockworks occur where vein-filled fractures and zones of brecciation are intensely developed. Pyrite and other metallic minerals are common throughout the silicified and quartz-veined rocks. Silicified rocks are bordered by narrow (centimeter to millimeter scale) bleached zones comprised of argillically and sericitically altered and decalcified rock. Massive limestone wall rock typically contains white calcite veins beyond the limit of megascopic hydrothermal alteration.

Sericitic and argillic alteration are commonly found adjacent to and intermixed with zones of replacement silicification and gold mineralization. These rocks contain variable amounts of kaolinite, dickite, illite, sericite, calcite, quartz, pyrite, marcasite, and secondary iron oxide/hydroxide phases (Wolfenden, 1965; Percival et. al., 1990). The Bau Limestone exhibits light-colored zones of mixed sericitic and argillic alteration between zones of replacement silicification and unaltered limestone. The calcareous clastic rocks of the Krian clastic member are extensively decalcified and variably argillized (sericite, smectite, kaolinite) and contain fine-grained quartz and pyrite. Incomplete silicification of limestone typically results in the formation of mixtures of microcrystalline quartz and sericite. Sericite, pyrite, and fine-grained clay minerals replace partially silicified shale within and adjacent to brecciated fault zones. Zones of strong argillic alteration are characterized by kaolinite and rarely dickite. Quartz veins that crosscut unsilicified shale are surrounded by selvages of sericite or sericite plus other fine-grained unidentified alteration minerals. Dissolution of calcareous rocks by hypogene hydrothermal solutions resulted in an increase in secondary porosity and permeability of the altered rock. The massive, impermeable nature of the Bau Limestone severely restricted the decalcification process to fractured or brecciated areas.

Calcic skarn forms discontinuous and erratically distributed pods and irregular masses at and near the contacts between the microgranodiorite porphyry stocks and the Bau Limestone (Wolfenden, 1965; Pimm, 1967; Percival et al., 1990). The skarns are composed principally of Fe-poor calc-silicate minerals such as wollastonite, garnet (grossular to andradite), diopside, epidote, vesuvianite, and chlorite in varying proportions accompanied by calcic plagioclase (Wolfenden, 1965; Percival et al., 1990). In the massive calcic skarns, later-formed quartz and calcite enclose, crosscut, and infill open spaces in the calc-silicate mineral assemblages. Distal to the massive skarns, calc-silicate minerals occur as idioblastic crystals and xenoblastic grains in complex and compact intergrowths that form crosscutting veins and irregular replacements in the thermally recrystallized carbonate host rocks.

Intrusive rocks

Several of the porphyritic intrusions are cut by stockworks of porphyry-style quartz veins that contain subeconomic concentrations of Cu + Mo ± Au (Fig. 2) associated with potassic and sericitic alteration characteristic of weak porphyry-style mineralization (Sillitoe and Bonham, 1990). Intense sericitically altered zones within these stocks and locally within the metamorphosed wall rocks host gold-bearing veins, pervasive disseminations, and breccias containing pyrite, arsenopyrite, sphalerite, galena, stibnite, and chalcopyrite (Pimm, 1967; Sillitoe and Bonham, 1990). Grains of native gold as much as 0.1 mm in diameter and native silver are reported by Wolfenden (1965) and Pimm (1967) in the mineralized quartz veins. Although the intrusion-hosted mineralization was not examined in detail as part of this study, it is significant because it documents the metalliferous nature of fluids emanating from the central intrusions. Metasomatism occurring immediately adjacent to and within the intrusions and along nearby fault and fracture zones resulted in the formation of variably mineralized calcic skarn. Proximity of the gold-bearing skarns to the Miocene intrusions implies a genetic relationship (Percival et al., 1990; Sillitoe and Bonham, 1990).

The intrusions are partially to pervasively hydrothermally altered, although weathering has commonly masked or obliterated the hydrothermal minerals. Sericitization is the most common and intensely developed alteration type in the intrusive rocks. Feldspar and ferromagnesian phenocrysts and groundmass minerals are replaced by sericite in association with calcite, quartz, and pyrite that occur as disseminations and as veinlets and vug linings with quartz and calcite. Primary textures are obliterated in the most strongly altered igneous rocks while less intensely sericitized intrusive rocks contain abundant calcite, clay minerals, and chloritized ferromagnesian minerals.

Argillically altered rocks are less widely distributed and are less pervasively developed than sericitically altered rocks. Primary rock-forming silicate minerals are altered to kaolinite, montmorillonite, sepiolite, and sericite. Argillic alteration is associated with introduced quartz and disseminated finegrained to coarsely crystallized (>2 mm) pyrite (Wolfenden, 1965; Pimm, 1967). Porphyritic dikes in or near zones of gold mineralization are commonly argillically altered (e.g., Tai Parit, Bukit Young; Fig. 1A, B). Black unidentified Mn oxide minerals and fine-grained Mn oxide stained clay minerals are very common in weathered sericitic and argillically altered igneous rocks. Therefore, some of the argillic alteration is due to supergene weathering and oxidation of sulfides.

Propylitically altered intrusive rocks are widespread throughout the district. Phenocrysts and groundmass of the affected rocks are replaced by chlorite, epidote, zoisite, albite, pyrite, prehnite, unidentified radiating zeolite minerals and, rarely, actinolite, which impart a greenish-gray color. Some intrusions are strongly altered to zeolites (stilbite and unidentified zeolites) and/or calcite and contain variable amounts of other propylitic minerals. Ferromagnesian minerals are replaced by chlorite, magnetite, and calcite, whereas feldspars are typically replaced by epidote. Late veinlets containing epidote, prehnite, and calcite commonly crosscut the altered rocks.

Geochronology of hydrothermal alteration

We obtained a potassium-argon date of 10.4 ± 0.3 Ma (1σ) on a mixture of finely intergrown quartz and sericite separated from a specimen of pervasively altered intrusive rock from Gunung Bau (Fig. 2). Analytical data are K2O = 1.735 wt %, 40Ar = 2.600 × 10–11 mol/g, and 40Ar = 35.5%. This sample is representative of alteration and mineralization in the Gunung Bau and Lucky Hill deposits (Figs. 1A, 2) and is near the center of the 14.6 to 6.4 Ma age range for the belt of adakitic intrusions and, specifically, the 11.6 to 9.3 Ma age range of intrusions on the north side of the district (Prouteau et al., 2001).

Gold Deposits

In the Bau district, gold is found in early calcic skarn, base metal-bearing veins, and late Carlin-style replacement deposits (Table 1; Percival et al., 1990). The spatial distribution of each type of mineralization relative to igneous intrusions is shown on the cross section of the district (Fig. 2) modified after Percival et al. (1990). Gold-bearing calcic skarn occurs closest to the exposed magmatic centers, whereas the sedimentary rock-hosted replacement types are typically distally located, and the vein deposits occupy an intermediate position (Fig. 2; Percival et al., 1990; Schuh, 1993). Most of the gold produced from the district has come from the Carlin-style deposits. Although calcic skarn deposits have been the least productive, they contain some of the highest gold grades. Tai Parit (Fig. 1A) is the largest Carlin-style deposit in the district with historic (1898–1921) and more recent (1991–1997) combined production of about 22 t (0.7 Moz) Au at an average grade of about 7.5 g/t (0.22 opt) Au from oxidized siliceous replacement and disseminated ores that were mainly treated by cyanidation (Hon, 1981). Recorded gold production from the next largest mine, Bukit Young, is about 0.31 t (10,000 oz) from 77,000 t (85,000 tons) of ore. Tai Ton, Bidi, and Jambusan (Fig. 1A) are other large Carlin-style deposits. In addition, approximately 50 mines in the district were mined by small companies and artisanal miners from 1820 to recent times. New and ongoing exploration work in the district has resulted in a new technical report, feasibility study, and a large district-wide resource (www.besra.com/malaysia).

Table 1.

Comparison of Mineralogical and Chemical Features of the Three Main Styles of Gold Mineralization in the Bau District, Malaysia

 Replacement depositsVein depositsCalcic skarn and type 2 vein deposits
Mineralization
ReplacementCarbonate host rocks replaced by microcrystalline (jasperoidal) quartz with subordinate crystallized quartz filling open spaces and as breccia matrix cementCarbonate and clastic rocks replaced by microcrystalline (jasperoidal) quartz crosscut by veins and veinlet stockworks adjacent to mineralized areas.Calc-silicate (cs) minerals (wo, gr, an, vs, ep, and pl) occurring as masses and vein-like zones of calcic skarn associated with subordinate microcrystalline quartz
VeiningMinor quartz ± calcite veinlets and narrow veins (<1 cm) crosscutting silicified rocksAbundant quartz, quartz + calcite, and calcite as major veins (>1 m), vein zones, narrow (±1 cm) veins, veinlets, and stockworksQuartz, quartz + calcite, and calcite veins with and rarely without calc-silicate minerals
Ore mineralogyAs, St, Asp, Py, Rl, Orp (?), Sp, and AuSt, Asp, As, Py, Au ± Rl, and Orp (?)St, Asp, Sb, Aur, and Sar
 Trace Cu-Pb-Zn sulfides and possibly sulfosalt mineralsCu-Pb-Zn sulfides and sulfosalt mineralsCu-Pb-Zn sulfides and sulfosalt minerals
GeochemistryAs, Sb, Au, Zn; minor Cu and PbSb, As, Au, Cu, Pb, and ZnSb, As, Au, Cu, Pb, and Zn
As/Sb ratiosAs/Sb ratios increase distal to the porphyritic intrusionsVariable As/Sb ratiosAs/Sb ratios decrease proximal to the porphyritic intrusions
Primary gold associationAs + Sb + AuAs + Sb + AuAs + Sb + Au; Sb + Au
Fluid inclusions
Average Th220°C (qz)265°C (qz + cc + Sp)290°C (qz + cc + cs)
Salinity range2–4 wt % NaCl equiv1–10 wt % NaCl equiv18–38 wt % NaCl equiv
Stable isotopes (ranges)δ18O = 16.6–22.6‰ (qz + cc)δ18O = 18.2–22.6‰ (qz + cc)δ18O = 7.5–8.3‰ (cs)
δ13C = –5.1 to –6.1‰ (cc)δ13C = –2.4 to –7.2‰ (cc)δ18O = 19.0–20.2‰ (late qz)
δD H2o = –62 to –101‰ (qz + cc + St + As)δD H2O = –58‰ 
δ18OH2O = –5.4 to 4.3δ18OH2O = 9.5–13.9 
δ13CCO2 = –5.9 to –12.4δ13CCO2 = –2.4 to –7.1 
Deposit examplesTai ParitBukit YoungLucky Hill
Tai TonRumohGunung Bau
JambusanSaburanGunung A. Bukit
BoringKojok
Bidi
 Replacement depositsVein depositsCalcic skarn and type 2 vein deposits
Mineralization
ReplacementCarbonate host rocks replaced by microcrystalline (jasperoidal) quartz with subordinate crystallized quartz filling open spaces and as breccia matrix cementCarbonate and clastic rocks replaced by microcrystalline (jasperoidal) quartz crosscut by veins and veinlet stockworks adjacent to mineralized areas.Calc-silicate (cs) minerals (wo, gr, an, vs, ep, and pl) occurring as masses and vein-like zones of calcic skarn associated with subordinate microcrystalline quartz
VeiningMinor quartz ± calcite veinlets and narrow veins (<1 cm) crosscutting silicified rocksAbundant quartz, quartz + calcite, and calcite as major veins (>1 m), vein zones, narrow (±1 cm) veins, veinlets, and stockworksQuartz, quartz + calcite, and calcite veins with and rarely without calc-silicate minerals
Ore mineralogyAs, St, Asp, Py, Rl, Orp (?), Sp, and AuSt, Asp, As, Py, Au ± Rl, and Orp (?)St, Asp, Sb, Aur, and Sar
 Trace Cu-Pb-Zn sulfides and possibly sulfosalt mineralsCu-Pb-Zn sulfides and sulfosalt mineralsCu-Pb-Zn sulfides and sulfosalt minerals
GeochemistryAs, Sb, Au, Zn; minor Cu and PbSb, As, Au, Cu, Pb, and ZnSb, As, Au, Cu, Pb, and Zn
As/Sb ratiosAs/Sb ratios increase distal to the porphyritic intrusionsVariable As/Sb ratiosAs/Sb ratios decrease proximal to the porphyritic intrusions
Primary gold associationAs + Sb + AuAs + Sb + AuAs + Sb + Au; Sb + Au
Fluid inclusions
Average Th220°C (qz)265°C (qz + cc + Sp)290°C (qz + cc + cs)
Salinity range2–4 wt % NaCl equiv1–10 wt % NaCl equiv18–38 wt % NaCl equiv
Stable isotopes (ranges)δ18O = 16.6–22.6‰ (qz + cc)δ18O = 18.2–22.6‰ (qz + cc)δ18O = 7.5–8.3‰ (cs)
δ13C = –5.1 to –6.1‰ (cc)δ13C = –2.4 to –7.2‰ (cc)δ18O = 19.0–20.2‰ (late qz)
δD H2o = –62 to –101‰ (qz + cc + St + As)δD H2O = –58‰ 
δ18OH2O = –5.4 to 4.3δ18OH2O = 9.5–13.9 
δ13CCO2 = –5.9 to –12.4δ13CCO2 = –2.4 to –7.1 
Deposit examplesTai ParitBukit YoungLucky Hill
Tai TonRumohGunung Bau
JambusanSaburanGunung A. Bukit
BoringKojok
Bidi

Abbreviations: an = andradite garnet, As = native arsenic, Asp = arsenopyrite, Au = native gold, Aur = aurostibite, cc = calcite, cs = calc-silicate minerals, ep = epidote family minerals, gr = garnet, Orp = orpiment, pl = plagioclase feldspar, Py = pyrite, qz = quartz, Rl = realgar, Sar = sarabauite, Sb = native antimony, Sp = sphalerite, St = stibnite, vs = vesuvianite, wo = wollastonite

Calcic skarn deposits

Gold-bearing calcic skarns are most abundant in the central portion of the district within and near the Gunung Bau (Lucky Hill) and Gunung A. Bukit deposits (Fig. 2; Wolfenden, 1965; Percival et al., 1990). The calcic skarns formed in marbleized limestone along intrusion-carbonate contacts and typically occur as discontinuous pods, lenses, and irregular masses ranging from <5 to 50 m along strike and up to 5 m wide. Although the gold-bearing calcic skarn deposits are volumetrically minor and high grade (e.g., 55,000 tons at 6.3 g/t Au at Gunong Arong Bakit; Metal Mining Agency of Japan, 1985), they provide a key genetic link between the porphyry and neighboring deposit types (Percival et al., 1989, 1990; Sillitoe and Bonham, 1990; Percival and Hofstra, 2002). Gold grades in many massive skarns are typically <5 g/t. However, calcic skarn veins developed in marbleized Bau limestone typically contain higher concentrations (10–30 ppm) of gold; Wolfenden (1965) reported contents of 30 to 130 ppm Au and up to 1,100 ppm Au (Metal Mining Agency of Japan, 1985). Fine-grained (≤0.5 mm) gold and electrum (800–850 fine) can be observed in high-grade zones. The paragenetic assemblage of the metal-bearing minerals in the skarns provides a link to the intrusions and includes electrum, chalcopyrite, tennantite, and minor amounts of stibnite, hessite, galena, Pb-Sb sulfosalts, jalpaite, and rare uytenbogaardite (Schuh, 1993).

Veins containing calc-silicate minerals were classified as type 2 vein deposits by Percival et al. (1990); however, because of their similar mineralogy and close spatial and probable genetic relationship to massive calcic skarn, both are discussed here as a single deposit type (Fig. 2). As with other veins in the district, calcic skarn veins fill high-angle faults, fractures, and joints within metamorphosed, competent, and massive Bau Limestone. They crosscut the massive skarns and extend several meters outboard of the Miocene intrusions.

The skarns are composed of varying mixtures of calc-silicate minerals with infillings of quartz + calcite containing intergrown and disseminated sulfides (Percival et al., 1990) as shown in Figure 3. The calc-silicate minerals consist primarily of wollastonite, grossular-andradite garnet and diopside, with lesser vesuvianite and some calcic plagioclase. Retrograde alteration of the skarns is spotty and characterized by epidote, chlorite family minerals, prehnite, quartz, and calcite (Figs. 3, 4A-C). The skarns are associated with anhedral masses of quartz and calcite in open spaces. Wollastonite is most abundant, occurring as fibrous masses replacing marble and radiating aggregates in calcite and quartz. Small sulfide grains, including stibnite, pyrite, arsenopyrite, sphalerite, and pyrrhotite, are intimately intergrown with calc-silicate minerals and finely disseminated within later microcrystalline quartz. Coarser-grained (0.2–>1 cm) sulfides occur as open space fillings within the calc-silicates and are in turn encased in late, coarse white calcite and crystalline quartz. Calc-silicate + quartz with later quartz + calcite veins crosscut the fine-grained massive skarn and contain more coarse-grained sulfides (e.g., acicular stibnite). The calcic skarn veins that emanate from massive skarns contain more sulfides, native gold, and late microcrystalline quartz and calcite.

Fig. 3.

Mineral paragenesis of deposits in the Bau district.

Fig. 3.

Mineral paragenesis of deposits in the Bau district.

Fig. 4.

Photomicrographs and scanning electron microscopy (SEM) images of calcic skarn, vein, and siliceous replacement ores. A. Backscattered electron (BSE) image of calcic skarn gold ore showing native antimony (SB, N) grain intergrown with wollastonite (WO), andradite, quartz (QZ), and calcite (CC). B. BSE image of calcic skarn ore showing aurostibite (ast) in a goethite (go) surrounded by quartz (qz); two grains of silver-bearing copper-lead antimony sulfosalt (tetrahedrite? [th]) also occur in the vug. C. Transmitted light image of calcic skarn ore from the Lucky Hill deposit showing acicular wollastonite (wo) in contact with prismatic sarabauite (black; sab). D. BSE image of euhedral arsenopyrite in a starburst-type pattern; arsenopyrite is enveloped by microcrystalline quartz (qz) associated with anhedral and massive stibnite (white). E. BSE image showing a close-up view of compositional zoning developed in arsenopyrite (asp) of Figure 4D; antimony-bearing core (sb), quartz (qz), stibnite (stib), and the compositional zone boundary (zb) are shown in detail in this electron micrograph. F. BSE image of siliceous replacement ore showing botryoidal, concentrically banded, disseminated grains of native arsenic (as) in microcrystalline quartz (QZ).

Fig. 4.

Photomicrographs and scanning electron microscopy (SEM) images of calcic skarn, vein, and siliceous replacement ores. A. Backscattered electron (BSE) image of calcic skarn gold ore showing native antimony (SB, N) grain intergrown with wollastonite (WO), andradite, quartz (QZ), and calcite (CC). B. BSE image of calcic skarn ore showing aurostibite (ast) in a goethite (go) surrounded by quartz (qz); two grains of silver-bearing copper-lead antimony sulfosalt (tetrahedrite? [th]) also occur in the vug. C. Transmitted light image of calcic skarn ore from the Lucky Hill deposit showing acicular wollastonite (wo) in contact with prismatic sarabauite (black; sab). D. BSE image of euhedral arsenopyrite in a starburst-type pattern; arsenopyrite is enveloped by microcrystalline quartz (qz) associated with anhedral and massive stibnite (white). E. BSE image showing a close-up view of compositional zoning developed in arsenopyrite (asp) of Figure 4D; antimony-bearing core (sb), quartz (qz), stibnite (stib), and the compositional zone boundary (zb) are shown in detail in this electron micrograph. F. BSE image of siliceous replacement ore showing botryoidal, concentrically banded, disseminated grains of native arsenic (as) in microcrystalline quartz (QZ).

In calcic skarns, evidence for antimony introduction (in addition to stibnite) directly from magmatic fluids is provided by antimony minerals with relatively high depositional temperatures such as sarabauite, vesuvianite, and aurostibite. Sarabauite (CaSb10O10S6) and its alteration product kermesite (Sb2S2O) were first described by Nakai et al. (1978) from the Gunung Bau (Lucky Hill) deposit (Figs. 2A, 5D). Antimonian vesuvianite (Ca-Sb10Mg2Al4(Si2O7)2(SiO4)5(OH)4) with up to 15 wt % Sb2O3 in the lattice structure was documented by Bradshaw (1972) from Bau in the calcic skarn mineralization. Sarabauite and stibnite, which are each locally up to several vol %, occur with minor amounts of arsenopyrite, pyrrhotite, pyrite, and sphalerite that are intergrown with wollastonite, calcite, and microcrystalline quartz (Fig. 4C). Rare native antimony and aurostibite (AuSb2) (Fig. 4A, B) are spatially associated with garnet, wollastonite, quartz, and calcite in some of the calcic skarn mineralization (Percival et al., 1990). Later-stage coarse white calcite and coarsely crystalline quartz fill remaining open spaces.

Fig. 5.

Photographs of outcrops and rock samples from the Bau district. A. Massive Bau Limestone outcrop with large quartz + calcite veins (type 1) exposed along the cliff face with abundant iron and manganese oxides. Note several mine adits exposed in the center and lower half of the photo. B. Shallowly dipping altered microgranodiorite intrusion crosscutting altered and variably mineralized sediments of the Krian Member of the Bau Formation at Bukit Young mine area. C. Hand sample of massive Bau Limestone with white calcite veinlets. D. Wollastonite skarn with red sarabauite and minor reddish-brown supergene kermesite from the Lucky Hill mine. E. Partially oxidized botryoidal native arsenic from the Bidi mine. F. Cut slab of botryoidal native arsenic on stibnite-bearing drusy quartz and jasperoid from the Bidi mine. G. Arsenical ore with botryoidal native arsenic, stibnite, drusy quartz, and realgar from the Bidi mine. H. Type 1 coarse calcite vein with large stibnite crystals.

Fig. 5.

Photographs of outcrops and rock samples from the Bau district. A. Massive Bau Limestone outcrop with large quartz + calcite veins (type 1) exposed along the cliff face with abundant iron and manganese oxides. Note several mine adits exposed in the center and lower half of the photo. B. Shallowly dipping altered microgranodiorite intrusion crosscutting altered and variably mineralized sediments of the Krian Member of the Bau Formation at Bukit Young mine area. C. Hand sample of massive Bau Limestone with white calcite veinlets. D. Wollastonite skarn with red sarabauite and minor reddish-brown supergene kermesite from the Lucky Hill mine. E. Partially oxidized botryoidal native arsenic from the Bidi mine. F. Cut slab of botryoidal native arsenic on stibnite-bearing drusy quartz and jasperoid from the Bidi mine. G. Arsenical ore with botryoidal native arsenic, stibnite, drusy quartz, and realgar from the Bidi mine. H. Type 1 coarse calcite vein with large stibnite crystals.

Based on phase equilibria, Einaudi et al. (1981) constrained typical calcic skarn formation in the epizonal environment to temperatures between ~400° and ~500°C. Aurostibite has a melting point of 460°C (Hansen and Anderko, 1958) and sarabauite undergoes a phase transformation at 420°C (Nakai et al., 1978). Because wollastonite and sarabauite are intimately intergrown in some of the calcic skarns and fluid inclusion homogenization temperatures of primary inclusions in early quartz associated with the calc-silicate mineral assemblages range from approximately 468° to 230°C (Schuh, 1993; this study), a temperature range of approximately 450° to 500°C is believed to be reasonable for skarn formation at Bau.

Vein deposits

Gold-bearing vein deposits are abundant and widely distributed throughout the Bau district (Figs. 1, 2) but account for only a relatively minor portion of the total gold production. Two distinct types of gold-bearing veins can be recognized: quartz + calcite and base metal-sulfide. Both occupy high-angle faults, major joints, fractures, and bedding planes within massive Bau Limestone (Figs. 2B, 5A; Wolfenden, 1965; Pimm, 1967). Vein mineralization occurs in a zone that extends approximately 500 m from the exposed granodiorite stocks and postdates the calc-silicate veins, although very small amounts of calc-silicate minerals do occur in some of the later type 1 and type 2 veins. Calcic-skarn and type 1 veins are crosscut by type 2 veins. Although the veins typically contain high concentrations of gold (5–30 ppm), they are generally narrow (1–10 m; mainly <3 m) and discontinuous along strike.

Type 1 veins, the most abundant in the district, are composed of quartz and calcite, the proportions of which vary from >85% quartz to >90% calcite within and among individual veins (Fig. 5H). Most veins contain more than about 50% calcite. Although one or more generations of quartz and calcite may be present, field and petrographic relationships suggest that calcite was deposited before quartz. Finely to coarsely crystalline (≤5 cm) calcite that ranges in color from white to gray to dark-brown and black (manganiferous) is crosscut by microcrystalline to coarsely crystalline quartz veins and irregular pods of quartz. Calcite vein breccias are generally cemented by quartz. A late stage of coarse calcite locally lines open spaces and cavities within the quartz.

Type 1 veins contain highly variable amounts of sulfide minerals intimately intergrown with calcite and quartz (Fig. 3). Stibnite is most abundant and occurs as large crystal aggregates embedded in calcite gangue. It is widely distributed and occurs with or without native arsenic, pyrite, arsenopyrite, realgar, and sphalerite as well as supergene Mn oxides and trace malachite. These minerals occur as disseminations, as finely to coarsely crystalline aggregates, and as isolated crystals within calcite and quartz gangue. Mixtures of quartz and stibnite commonly form the cement to vein breccias. A large proportion of the gold and As-bearing minerals is associated with quartz. Manganiferous calcite contains minor amounts of gold and higher than normal silver values.

Most type 1 veins exhibit a clear paragenetic sequence that suggests there has been a significant change in fluid chemistry and/or temperature with time. Early vein filling was dominated by calcite + stibnite and was followed by overgrowths of quartz and As-bearing minerals containing abundant gold. In some veins, this change in chemistry is evidenced by Sb-bearing arsenopyrite cores that are overgrown by Sb-absent (below detection) rims (Fig. 4D, E).

Type 2 base metal-sulfide veins are uncommon in the district and typically occur adjacent to or within hydrothermally altered intrusive rocks (e.g., Say Seng). Less commonly, they are found with Carlin-style deposits in massive limestone. The genetic and temporal relationships between type 2 veins and replacement deposits remain unclear because of poor exposures. The type 2 veins contain considerable amounts of Au (2–20 ppm) and Ag (50 to several hundreds of ppm) and are typically characterized by sphalerite, chalcopyrite, and galena in varying proportions associated with less abundant pyrite and arsenopyrite. Tennantite-tetrahedrite, bornite, pyrrhotite, and marcasite are locally present. Quartz, the dominant gangue mineral, contains fine-grained inclusions of pyrite, arsenopyrite, and subhedral to anhedral grains of base metal sulfides followed by open-space deposition of more massive aggregates of sulfide and sulfosalt minerals (Fig. 3).

Sedimentary rock-hosted replacement deposits

Three types of replacement ore are recognized at Bau: siliceous (jasperoid) ore, arsenical ore, and disseminated ore. This deposit type typically occurs within 0.5 to 1.5 km of the intrusive center and accounts for most past gold production and known reserves. Type 1 and type 2 veins occur in close proximity to some of the Carlin-style deposits, and at Bukit Young and Say Seng, the veins and Carlin-style ores overlap locally in the same structural zones. The relative timing between the formation of Carlin-style and vein deposits is variable; however, vein material is typically overprinted by microcrystalline quartz masses associated with Carlin-style deposits. This suggests that there is a transitional zone between the more proximal veins and the more distal Carlin-style deposits that is accompanied by a shift from lower to higher As/Sb ratios.

Field relationships between each Carlin-style ore type suggest that the siliceous ores formed slightly (?) before and grade into the disseminated ores. The arsenical ores contain a low-temperature mineral assemblage that overprints the siliceous and disseminated ores. Characteristic petrologic and mineralogical features of each type are described below.

Siliceous ore: Siliceous ores are characterized by decalcification and pervasive silica replacement of massive limestone whose intensity ranges from moderate to dense jasperoid (e.g., Tai Parit, Tai Ton, etc.; Figs. 1A, 2). Silicification is most intense and best focused in and along structural zones consisting of faults, fractures, and brecciation. The intensely silicified zones commonly grade into areas of moderately intense silicification that contain a higher proportion of vuggy quartz, sericite, and argillic clays. These ores are typically dark gray to black and are composed mainly of fine-grained, massive, microcrystalline quartz with xenomorphic to granular textures (Fig. 6A), which are characteristic of quartz that precipitated directly from hydrothermal fluids (Dong et al., 1995) by cooling at temperatures greater than 180° to 200°C (Rimstidt, 1997). Jigsaw mosaic and feathery chalcedonic textures, indicative of an amorphous silica precursor, are absent or are late features in these rocks (Fig. 6F). Later quartz veinlets and veins (<1 mm to >2 cm in width) exhibiting comb and cockade textures locally form dense stockworks or infill open spaces between silicified breccia fragments. Vugs are lined with crystalline quartz and sulfide minerals.

Fig. 6.

Thin section images of ore specimens from Bau. A. Cross-polarized transmitted light image of siliceous ore (jasperoid) comprised of interlocking quartz crystals containing inclusions of carbonate derived from the Bau limestone. Sample BA-36. Field of view is 1.2 mm. B. Scanning electron microscopy (SEM) backscatter image of jasperoidal quartz (dark gray) containing small arsenopyrite (Aspy) crystals and one sphalerite (Sph) crystal that is mantled by drusy quartz (Qtz) and native arsenic (As) with minor pyrite (Py) and later stibnite (Stib) and realgar (Real). Remaining void space is black. Sample BA-36. Field of view is 0.6 mm. C. Transmitted light image of jasperoidal quartz with disseminated arsenopyrite crystals (black), large translucent sphalerite crystals, and later drusy quartz. Sample BA-42a. Field of view is 1.2 cm. D. SEM backscatter image of jasperoidal quartz (black; qz) containing small white rhomb-shaped crystals of gold-bearing arsenopyrite (asp) and large sphalerite crystals (sphl) with inclusions of anhydrite (anh) and arsenopyrite. Sample BA-42a. Field of view is 0.6 mm. E. Reflected light image of siliceous ore (jasperoid and drusy quartz) with native arsenic (white) showing hexagonal growth zones. Sample BA-42a. Field of view is 0.6 mm. F. Cross-polarized transmitted light image of drusy quartz with a chalcedonic silica rim and dark realgar in siliceous ore. Sample BA-42b. Field of view is 0.6 mm.

Fig. 6.

Thin section images of ore specimens from Bau. A. Cross-polarized transmitted light image of siliceous ore (jasperoid) comprised of interlocking quartz crystals containing inclusions of carbonate derived from the Bau limestone. Sample BA-36. Field of view is 1.2 mm. B. Scanning electron microscopy (SEM) backscatter image of jasperoidal quartz (dark gray) containing small arsenopyrite (Aspy) crystals and one sphalerite (Sph) crystal that is mantled by drusy quartz (Qtz) and native arsenic (As) with minor pyrite (Py) and later stibnite (Stib) and realgar (Real). Remaining void space is black. Sample BA-36. Field of view is 0.6 mm. C. Transmitted light image of jasperoidal quartz with disseminated arsenopyrite crystals (black), large translucent sphalerite crystals, and later drusy quartz. Sample BA-42a. Field of view is 1.2 cm. D. SEM backscatter image of jasperoidal quartz (black; qz) containing small white rhomb-shaped crystals of gold-bearing arsenopyrite (asp) and large sphalerite crystals (sphl) with inclusions of anhydrite (anh) and arsenopyrite. Sample BA-42a. Field of view is 0.6 mm. E. Reflected light image of siliceous ore (jasperoid and drusy quartz) with native arsenic (white) showing hexagonal growth zones. Sample BA-42a. Field of view is 0.6 mm. F. Cross-polarized transmitted light image of drusy quartz with a chalcedonic silica rim and dark realgar in siliceous ore. Sample BA-42b. Field of view is 0.6 mm.

Orebodies hosted in silicified shale (e.g., Bukit Young; Fig. 1A) are intensely brecciated and contain angular silicified shale fragments cemented by finely crystalline quartz. They are overprinted by late veins containing drusy quartz and calcite. Sericite and sericite + clay occur within incompletely silicified shale fragments and as selvages along quartz veins that cut variably silicified shale. Stockworks of Au-bearing quartz veinlets are found in both limestone and shale, typically on the periphery of siliceous replacement ores. The intensity of brecciation, groundmass silicification of matrix, and late mineral veining decreases with increasing distance from primary mineralized zones.

Siliceous ores contain arsenian pyrite, native arsenic, stibnite, arsenopyrite, realgar, and rare native gold (Figs. 3, 5, 6). Base metal sulfides-sulfosalts, altaite, hessite, and other Ag-Pb tellurides were reported by Schuh (1993) in mineralized samples from the dewatered Tai Parit pit. Mustard (1997) reported that gold occurs in solid solution in arsenian pyrite. The abundance of the metallic minerals is typically proportional to the amount of introduced silica. Sulfide content is also dependent upon host-rock composition, with mineralized limestone containing significantly greater concentrations of sulfide than adjacent shales. Pyrite, the most abundant sulfide, occurs as disseminated and crystalline aggregates in the quartz gangue, particularly in the Tai Parit and Jugan deposits (Schuh, 1993; Mustard, 1997). However, pyrite is often a minor to rare component in numerous mineral occurrences where native arsenic, arsenopyrite, and stibnite are the dominant metallic minerals. Pyrite was not a significant component in the samples we studied petrographically, although it occurs as individual anhedral grains from a few micrometers to aggregates of subhedral and euhedral crystals up to about 0.2 mm. Native arsenic occurs in quartz as botryoidal crystalline aggregates (Fig. 4F), exhibits well-developed internal growth structures (Fig. 7A), and serves as nuclei for micro-crystalline and drusy quartz grains (Fig. 7B). Rims of native arsenic are commonly oxidized to arsenolite (As2O3) and other unidentified supergene arsenic minerals. Arsenopyrite occurs as euhedral crystals within a quartz matrix and is spatially associated with native arsenic, acicular stibnite, native antimony, and late realgar. Stibnite overgrowths occur on both native arsenic and arsenopyrite. Rare particles of gold (80–85 wt % Au) have been petrographically identified in native arsenic in unoxidized ore from Tai Ton by Wolfenden (1965) and in this study, although it only accounts for a small portion of the assayed gold content. The unoxidized ores are refractory, and historic mining usually ceased when unoxidized gold-bearing minerals, such as arsenopyrite, were encountered. Supergene oxidation of primary native elements and sulfide minerals has resulted in the formation of arsenic, antimony, and iron oxides and liberated gold that was redeposited as rare particles of native gold (Fig. 7C). The oxidized ores are readily amenable to the cyanide extraction of gold, accounting for much of the historic mining production (Wolfenden, 1965; Pimm, 1967). Laboratory studies indicate that 75 to 93% gold recovery from Tai Ton and Bukit Young are oxidized ores (Dawson Metallurgical Laboratories, Inc., unpub. report, 1983).

Fig. 7.

Photomicrographs and scanning electron microscopy (SEM) images of siliceous replacement and arsenical ores from the Bau district. A. Reflected light photomicrograph of siliceous replacement ore showing native arsenic grain exhibiting well-defined growth lines in a matrix to crystalline quartz (qz). B. Reflected light image of dark-gray to black native arsenic nuclei overgrown by drusy microcrystalline quartz in siliceous replacement ore. C. SEM image of oxidized brecciated siliceous replacement ore showing a native gold grain within a mixture of goethite (go) and quartz (qz). D. Backscattered electron image of arsenical ore showing sphalerite (SPHL), native arsenic (AS), and stibnite (STIB) grains enveloped by microcrystalline quartz (QZ). E. SEM image of arsenical ore showing native arsenic within microcrystalline quartz (QZ) matrix. Late-formed realgar (rel) fills open fractures. Arsenolite crystals (ars) are supergene oxidation products of arsenic minerals.

Fig. 7.

Photomicrographs and scanning electron microscopy (SEM) images of siliceous replacement and arsenical ores from the Bau district. A. Reflected light photomicrograph of siliceous replacement ore showing native arsenic grain exhibiting well-defined growth lines in a matrix to crystalline quartz (qz). B. Reflected light image of dark-gray to black native arsenic nuclei overgrown by drusy microcrystalline quartz in siliceous replacement ore. C. SEM image of oxidized brecciated siliceous replacement ore showing a native gold grain within a mixture of goethite (go) and quartz (qz). D. Backscattered electron image of arsenical ore showing sphalerite (SPHL), native arsenic (AS), and stibnite (STIB) grains enveloped by microcrystalline quartz (QZ). E. SEM image of arsenical ore showing native arsenic within microcrystalline quartz (QZ) matrix. Late-formed realgar (rel) fills open fractures. Arsenolite crystals (ars) are supergene oxidation products of arsenic minerals.

Arsenical ore: Arsenical ore at Bau (Fig. 5E-G) contains more than 15 wt % As, as dark-gray to black, irregularly shaped bodies within siliceous deposits (Tai Parit, Tai Ton, etc.) and as fault-controlled, vein-like bodies within massive Bau Limestone (e.g., Bidi; Fig. 1). The arsenical ores were studied by Lau (1970), who subdivided them based upon mineralogy and mineral abundance into microcrystalline quartz + native arsenic (type 1 ore) and dominantly native arsenic with lesser amounts of gangue and sulfide minerals (type 2 ore). These two ore types commonly occur in the same deposit and are typically compositionally gradational, although the type 1 ore is usually paragenetically later and volumetrically minor.

Quartz-rich arsenical ores (type 1) are principally composed of microcrystalline quartz containing finely disseminated, small (<2 mm) botryoidal masses of native arsenic as the most abundant ore mineral. Less abundant arsenopyrite, acicular stibnite, and rare sphalerite and pyrite are disseminated within microcrystalline quartz in association with native arsenic (Fig. 7D). Coarser-grained (>1 cm) acicular stibnite and botryoidal native arsenic occur in vugs and open spaces with late-stage overgrowths of quartz and calcite. Realgar is paragenetically very late and occurs in quartz- and calcite-lined vugs and open fractures with gray-black arsenolite (As2O3) (Fig. 7E).

Type 2 arsenical ores consist of one or more generations of native arsenic in large cavities locally developed within the type 1 ores. The type 2 ores consist of dense masses of coarse (1–>10 cm) botryoidal aggregates with well-defined concentric growth bands. Dark crusts of arsenolite coat the massive native arsenic. Stibnite, the second most abundant metallic mineral, occurs within native arsenic, as crystals encased within quartz or quartz + calcite filling open spaces, or as acicular crystals within open vugs. Late realgar fills fractures and cavities. Pyrite, arsenopyrite, and sphalerite also occur as sparsely disseminated crystals within native arsenic and microcrystalline quartz.

Rare particles of native gold were detected using petrographic techniques within aggregates of native arsenic (Lau, 1970; this study). Electron microprobe studies by Lau (1970) determined that most particles are 4 to 15 μm in diameter and contain approximately 80 wt % Au; these particles account for less than 1% of the total assayed gold content in arsenical ores. Chemical analyses show that a large proportion of the gold in arsenical ores resides in native arsenic, although arsenopyrite and rare pyrite likely account for some of the assayed gold.

Supergene arsenical ores consist of light-colored masses of honeycomb-textured microcrystalline quartz coated with gray-black arsenolite. Secondary antimony oxides and traces of iron oxide are also common.

Disseminated ore: Disseminated ore contains lesser amounts of hydrothermal silica and metallic minerals (typically <1%) and a higher proportion of clay minerals than the siliceous and arsenical types. This ore type was mined at the large Tai Parit deposit (Fig. 1), which consists of decalcified, clay-rich, variably silicified, brecciated carbonate and clastic rocks of the relatively permeable Krian Member. Disseminated ore was not as well developed in the massive carbonate of the Bau Limestone. The disseminated ores are associated with lesser amounts of siliceous replacement and arsenical ores. Mustard (1997) reports that clay matrix and clay breccia are prominent constituents of the ores and are commonly manganiferous. Because the Tai Parit open pit was filled with water at the time of our study, and outcrops of disseminated ore in the main ore zone were inaccessible, gold-bearing material was collected from small exposures and pits adjacent to and as much as 500 m from the Tai Parit pit. More recent (1991–1997) mining activity at Tai Parit allowed Mustard (1997) and Schuh (1993) to describe the main ore zone in situ.

The unoxidized ore is medium gray and is composed of fine-grained quartz, unidentified clay minerals, sericite, and remnant calcite. Pyrite and arsenian pyrite, gold, arsenopyrite, stibnite, and native arsenic are disseminated throughout the hydrothermally altered host rocks. Minor amounts of sphalerite, galena, chalcopyrite, bournonite, famatinite, altaite, hessite, and other Ag-Pb tellurides also occur (Schuh, 1993). Late realgar, pyrite, arsenopyrite, native arsenic, orpiment, and calcite veins cut mineralization (Schuh, 1993). Disseminated gold mineralization is also present at Jugan, approximately 7 km northeast of the town of Bau. Gold mineralization with grades ranging from 1 to 4 ppm occurs in decalcified, argillized, weakly silicified carbonaceous shale and weakly calcareous siltstone and sandstone of the Pedawan Formation. Mineralization is associated with intensely altered porphyritic dikes and contains disseminated pyrite, arsenopyrite, and arsenian pyrite (Sillitoe and Bonham, 1990; Schuh, 1993; Mustard, 1997). Microprobe work by Schuh (1993) indicates gold is in solid solution in the sulfides and occurs as overgrowth rims on arsenian pyrite. Supergene oxidation of disseminated ore produces mixtures of light-gray to tan and red-brown iron oxides and hydroxides, clay, and secondary arsenic and antimony oxides.

The Jugan resource hosted in the Pedawan Formation contains much less introduced silica, subtle alteration, pervasive disseminations of gold in arsenopyrite and pyrite, and low quartz depositional temperatures of less than 165°C (reported by Schuh, 1993). Recent work by Besra (Fulton, 2013) indicates that the ores contain about 2 to 2.5% arsenopyrite and 4.5 to 5% pyrite; however, approximately 70% of the gold occurs in arsenopyrite, 25% in pyrite, and 5% in silica.

Ore Controls

Structure and lithology control the location and geometry of the orebodies in the Bau district. Structural controls are imparted by regional and local features such as faults and folds, while important lithologic controls to ore formation include differences in permeability, porosity, and chemical reactivity of the host rocks.

Orebody geometry

The orebodies at Bau are crudely tabular or ovoid in plan view, and in cross section they commonly are lens shaped (Fig. 2B). The long axis of the larger orebodies (e.g., Tai Parit) coincides with the trace of the primary localizing fault zone (Tai Parit fault zone), resulting in a crudely tabular geometry. In cross section, the thickness of mineralization thins along feeder faults with increasing depth (Fig. 2B).

Gold is also found at the contact between the Bau Limestone and porphyry dikes at Tai Parit, Bukit Young, and Tai Ton; pod-shaped ore zones occur at shale-limestone contacts. The porphyry dikes are intensely sericitically altered, variably silicified, and pyritized and locally contain gold mineralization along their margins. At Tai Parit, late-stage pebble dikes containing sulfide clasts are a distinct link with the proximal copper-porphyry intrusions (Schuh, 1993). Most of the gold is concentrated within competent sedimentary wall rock, where faulting, fracturing, and brecciation created open spaces for mineralization.

Structural controls

Deposits and prospects in the Bau district occur within a NE-trending regional metallogenic zone. The gold-bearing mercury deposits at Tegora and Gading, Sarawak (Fig. 2A) lie to the southwest, and farther south, areas of known gold mineralization are in neighboring Kalimantan, Indonesia (Bowles, 1984; Carlile and Mitchell, 1994). This alignment is generally believed to reflect a deeply seated linear break in the crust that localized magmatic activity and the movement of gold-bearing solutions. Tertiary stocks were emplaced along this zone at approximately 1.5 km centers in the Bau district, indicating this structural zone existed prior to the late Tertiary magmatism and mineralization. Mineralization in the Bau district is most intense in areas of abundant intrusions.

Several prominent normal faults, traceable for 3 to 4 km, are present within the Bau district. These faults occur within and parallel to the northeast trend of the regional gold belt and are believed to be the high-level expressions of deep-seated structural features that were produced by an episode of mid-Miocene postsubduction extension (Percival et al., 1990; Schuh, 1993). The preferred occurrence of gold deposits along or near one of these faults, the Tai Parit fault, suggests that it was the primary conduit for gold-bearing hydrothermal fluids. The major focus of gold mineralization in the district occurs where these NE-trending structures intersect the axial zone of the ENE-trending Bau anticline (Fig. 1). Intrusive rocks and mineralization are both concentrated along major faults and at fault intersections. Less prominent NW- and N-trending fault-fracture sets are also abundant and mineralized (Hon, 1981), especially where the Tai Parit fault transects the Bau anticline. The sedimentary host rocks are often brecciated at the intersections between these structures.

Lithological and chemical controls

The two principal sedimentary rock types at Bau—the massive, pure Bau limestone and the siliceous shales of the Pedawan Formation—are relatively impermeable and were not particularly favorable host rocks for large-tonnage disseminated-type gold deposits. The limestone was chemically reactive to the ore fluids, but its low permeability limited silicification to fractures. Tectonically brecciated limestone along faults facilitated the movement of ore fluids away from the feeder faults and into the limestone breccias. Relatively complete dissolution and replacement of limestone by silica formed jasperoid breccias and silica replacement orebodies. Gold-bearing quartz + calcite veins occur along fracture zones within the limestone wall rocks adjacent to the replacement ores.

The low permeability and massive nature of the Pedawan shale and its resistance to shattering along fault zones facilitated the formation of many of the larger orebodies near the base of this unit. At shale-limestone contacts, the weakly fractured shale is incipiently to moderately silicified above larger volumes of jasperoidal gold ore occurring in the silicified and brecciated limestone. The relative lack of calcite in the shale precluded the development of increased permeability by decalcification.

The paucity of permeable and reactive sedimentary rocks at Bau, such as calcareous siltstone and silty, thin-bedded carbonate rocks, hindered the development of large-tonnage, stratiform, disseminated replacement-type orebodies. The exception is the clastic Krian Member of the Bau Formation. Increased permeabilities and porosities in the clastic rocks of the Krian Member allowed fluid penetration into a much larger volume of rock and, in conjunction with structural preparation, account for the significantly larger tonnage of disseminated ore developed at the Tai Parit deposit.

Multielement Geochemistry

Multielement geochemical analyses were performed on hydrothermally altered and mineralized rocks collected from most of the deposits across the entire Bau district (Table 2). Analyses for Au, Ag, As, Sb, Cu, Pb, Zn, Hg, and Tl were performed by Hunter Analytical Laboratory, Sparks, Nevada, on 51 selected samples of replacement and calc-silicate ores and vein materials using fire assay and atomic absorption methods. Twenty specimens were analyzed in laboratories of the U.S. Geological Survey for a wide range of elements using a variety of instrumental procedures. More recently, eight specimens were analyzed by ALS-Chemex for major elements and a broad suite of minor elements. The analyses were done during the early 1980s through 1990s using methods commonly employed at the time, and although detection limits, precision, and analytical methods have since improved, these analyses document important chemical changes and relative elemental abundances between ore types and for the district as a whole. Selected analyses are presented in Table 2, and the entire body of data is presented in Figure 8.

Table 2.

Major and Minor Element Data for Selected Specimens from the Bau District, Malaysia

No.TypeField no.Si %Al %Fe %Mg %Ca %Na %K %Ti %Mn %P %F %
Type of analysis
 USGS/Hunter Laboratory--GICPGICPGICPGICPGICPGICPGICPGICPGICPGSP
 ALS ChemexXRFXRF/ICPXRF/ICPXRF/ICPXRF/ICPXRF/ICPXRF/ICPIMS/XRFXRF/ICPXRF/ICP--
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.220.140.010.0942.13<0.010.02<0.010.00150.001--
2SBA-32--0.60.80.010.00.020.10.020.008<0.01<0.01
3SBA-36--0.50.30.012.00.010.1<0.010.090<0.010.01
 SBA-3636.60.400.300.012.140.010.080.010.0160.006--
4SC-1--2.21.80.142.40.090.70.060.240.030.01
5SC-4--3.61.00.090.70.040.30.160.0100.020.01
6SF-3--3.85.30.114.80.060.50.130.610.050.02
7ASG-4--0.50.40.010.10.010.10.020.0050.01<0.01
8ASBA-4119.60.330.290.042.68<0.010.10<0.010.170.003--
9CSBA-5216.20.280.100.1726.620.010.02<0.010.500.012--
10CSLH-1014.90.050.380.1423.680.030.01<0.010.940.006--
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-34--3.16.80.103.70.450.40.110.850.050.02
12SC-3--5.04.60.132.30.080.70.170.390.050.02
13SD-1--5.24.80.312.30.091.60.160.430.090.03
14NBA-3--5.71.70.26110.142.30.100.0240.030.02
15NBA-22--9.313.00.130.20.161.50.650.0920.020.02
16NBA-27--5.914.00.130.10.300.60.372.10.040.01
17NBA-33--3.12.50.120.90.030.50.072.50.010.02
 NBA-3325.92.511.450.11110.040.760.120.970.007--
18NBA-35--9.63.70.080.20.030.60.160.0140.010.02
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-4722.57.796.473.902.691.945.831.100.290.087--
20SBA-3944.20.430.320.010.630.010.07<0.010.0120.001--
Vein ores
21VBA-7--0.030.040.06 <0.010.01<0.01>1<0.001--
22VBA-13--1.00.90.12340.010.10.050.680.00.01
23VBA-14--2.21.60.12350.040.20.070.330.10.02
24VBA-15--2.01.50.13310.020.10.081.40.00.01
25VBA-19--9.52.90.100.30.242.10.380.0060.0<0.01
 VBA-19--0.230.07<0.010.790.01<0.01<0.010.0170.003--
Intrusive bodies
26ZBA-4--9.91.40.120.10.851.60.130.0070.010.01
27ZBA-10--9.11.30.120.10.831.20.140.0020.010.01
No.TypeField no.Si %Al %Fe %Mg %Ca %Na %K %Ti %Mn %P %F %
Type of analysis
 USGS/Hunter Laboratory--GICPGICPGICPGICPGICPGICPGICPGICPGICPGSP
 ALS ChemexXRFXRF/ICPXRF/ICPXRF/ICPXRF/ICPXRF/ICPXRF/ICPIMS/XRFXRF/ICPXRF/ICP--
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.220.140.010.0942.13<0.010.02<0.010.00150.001--
2SBA-32--0.60.80.010.00.020.10.020.008<0.01<0.01
3SBA-36--0.50.30.012.00.010.1<0.010.090<0.010.01
 SBA-3636.60.400.300.012.140.010.080.010.0160.006--
4SC-1--2.21.80.142.40.090.70.060.240.030.01
5SC-4--3.61.00.090.70.040.30.160.0100.020.01
6SF-3--3.85.30.114.80.060.50.130.610.050.02
7ASG-4--0.50.40.010.10.010.10.020.0050.01<0.01
8ASBA-4119.60.330.290.042.68<0.010.10<0.010.170.003--
9CSBA-5216.20.280.100.1726.620.010.02<0.010.500.012--
10CSLH-1014.90.050.380.1423.680.030.01<0.010.940.006--
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-34--3.16.80.103.70.450.40.110.850.050.02
12SC-3--5.04.60.132.30.080.70.170.390.050.02
13SD-1--5.24.80.312.30.091.60.160.430.090.03
14NBA-3--5.71.70.26110.142.30.100.0240.030.02
15NBA-22--9.313.00.130.20.161.50.650.0920.020.02
16NBA-27--5.914.00.130.10.300.60.372.10.040.01
17NBA-33--3.12.50.120.90.030.50.072.50.010.02
 NBA-3325.92.511.450.11110.040.760.120.970.007--
18NBA-35--9.63.70.080.20.030.60.160.0140.010.02
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-4722.57.796.473.902.691.945.831.100.290.087--
20SBA-3944.20.430.320.010.630.010.07<0.010.0120.001--
Vein ores
21VBA-7--0.030.040.06 <0.010.01<0.01>1<0.001--
22VBA-13--1.00.90.12340.010.10.050.680.00.01
23VBA-14--2.21.60.12350.040.20.070.330.10.02
24VBA-15--2.01.50.13310.020.10.081.40.00.01
25VBA-19--9.52.90.100.30.242.10.380.0060.0<0.01
 VBA-19--0.230.07<0.010.790.01<0.01<0.010.0170.003--
Intrusive bodies
26ZBA-4--9.91.40.120.10.851.60.130.0070.010.01
27ZBA-10--9.11.30.120.10.831.20.140.0020.010.01
No.TypeField no.Li (ppm)Ba (ppm)Cs (ppm)Sr (ppm)Ga (ppm)Be (ppm)La (ppm)Ce (ppm)Nd (ppm)Y (ppm)Th (ppm)
Type of analysis            
 USGS/Hunter LaboratoryGICPGICP--GICPGICPGICPGICPGSGICPGICPGICP
 ALS ChemexIMSICPIMSICP/IMSIMSICP/IMSIMSIMS--IMSIMS
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.8<100.0512200.050.50.56--1.30.2
2SBA-321654--11<8<25<8<84<8
3SBA-361648--14<8<2<4<8<84<8
 SBA-36--30--11--<0.5----------
4SC-125170--43<8<2109<816<8
5SC-44377--28<8<217221120<8
6SF-336300--7110<221201429<8
7ASG-41545--8<8<2<4<8<84<8
8ASBA-41--<10--14--<0.5----------
9CSBA-52--<10--121--<0.5----------
10CSLH-10--<10--104--<0.5----------
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-3434400--689<220191227<8
12SC-345200--6010<223281729<8
13SD-130230--7611<225262042<8
14NBA-315200--11015<223231212<8
15NBA-2218170--8720<21028156<8
16NBA-2752570--23013<2327132559
17NBA-3341160--53<8<21415102811
 NBA-33--210--78--<0.5----------
18NBA-354365--2218<2713<84--
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-47542,1907.224222<3.91227.8 31.31.2
20SBA-3914201.5551.2<0.50.50.48 0.30.2
Vein ores
21VBA-7--10--92--<0.5----------
22VBA-131248--130<8<211<8<810<8
23VBA-1424110--46<8<21591115<8
24VBA-1520110--1309<215131217<8
25VBA-19170730--15019<226532814<8
 VBA-19--<10--3--<0.5----------
Intrusive bodies
26ZBA-448540--22020<237834839<8
27ZBA-1046520--22019<237854535<8
No.TypeField no.Li (ppm)Ba (ppm)Cs (ppm)Sr (ppm)Ga (ppm)Be (ppm)La (ppm)Ce (ppm)Nd (ppm)Y (ppm)Th (ppm)
Type of analysis            
 USGS/Hunter LaboratoryGICPGICP--GICPGICPGICPGICPGSGICPGICPGICP
 ALS ChemexIMSICPIMSICP/IMSIMSICP/IMSIMSIMS--IMSIMS
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.8<100.0512200.050.50.56--1.30.2
2SBA-321654--11<8<25<8<84<8
3SBA-361648--14<8<2<4<8<84<8
 SBA-36--30--11--<0.5----------
4SC-125170--43<8<2109<816<8
5SC-44377--28<8<217221120<8
6SF-336300--7110<221201429<8
7ASG-41545--8<8<2<4<8<84<8
8ASBA-41--<10--14--<0.5----------
9CSBA-52--<10--121--<0.5----------
10CSLH-10--<10--104--<0.5----------
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-3434400--689<220191227<8
12SC-345200--6010<223281729<8
13SD-130230--7611<225262042<8
14NBA-315200--11015<223231212<8
15NBA-2218170--8720<21028156<8
16NBA-2752570--23013<2327132559
17NBA-3341160--53<8<21415102811
 NBA-33--210--78--<0.5----------
18NBA-354365--2218<2713<84--
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-47542,1907.224222<3.91227.8 31.31.2
20SBA-3914201.5551.2<0.50.50.48 0.30.2
Vein ores
21VBA-7--10--92--<0.5----------
22VBA-131248--130<8<211<8<810<8
23VBA-1424110--46<8<21591115<8
24VBA-1520110--1309<215131217<8
25VBA-19170730--15019<226532814<8
 VBA-19--<10--3--<0.5----------
Intrusive bodies
26ZBA-448540--22020<237834839<8
27ZBA-1046520--22019<237854535<8
No.TypeField no.U (ppm)Co (ppm)Cr (ppm)Ni (ppm)Sc (ppm)V (ppm)Ag (ppm)Ag (ppm)Au (ppm)Au (ppm)Bi (ppm)
Type of analysis            
 USGS/Hunter Laboratory--GICPGICPGICPGICPGICPGICPHFAGCSAAHFAGICP
 ALS ChemexIMSICP/IMSICPICP/IMS--ICPICP/IMSFAFA/AASFAICP/IMS
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.80.610<0.2--20.05--0.01--0.01
2SBA-32--312<4<41383.42035.7<20
3SBA-36--<216<4<41164.57.018.5<20
 SBA-36--12276--94.4-->1017.2<2
4SC-1--5221864318--14--<20
5SC-4--5191564717--7.5--<20
6SF-3--1737357646--9.1--<20
7ASG-4--<212<4<4217--3.0--<20
8ASBA-41--31383--8>100185.1>1020.3<2
9CSBA-52--1171--19.6--7.35--<2
10CSLH-10--1131--<18.6--6.86--<2
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-34--16332866085.12533.6<20
12SC-3--2043459695--9.2--<20
13SD-1--154861168519--10--<20
14NBA-3--420225384834.68.414.7<20
15NBA-22--242504843240<4<0.340.550.55<20
16NBA-27--3101205823110<40.340.250.21<20
17NBA-33--52830668<41.711312.7<20
 NBA-33--56011--545.6--9.98--2
18NBA-35--<218<4748<4<0.340.650.55<20
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-471.23525169--2240.9--0.03--0.04
20SBA-391.80.63192.2--610.2--9.56--0.01
Vein ores
21VBA-7--219<1--389.2--0.37--<2
22VBA-13--4912<4222428.56.97.2<20
23VBA-14--61313528<4<0.340.100.41<20
24VBA-15--515175428161.09.28.3<20
25VBA-19--13402219140<42.70.050.07<20
 VBA-19--<11271--<11.8--0.18--<2
Intrusive bodies
26ZBA-4--7187852<4--0.20.03<20
27ZBA-10--6246752<4--0.70.07<20
No.TypeField no.U (ppm)Co (ppm)Cr (ppm)Ni (ppm)Sc (ppm)V (ppm)Ag (ppm)Ag (ppm)Au (ppm)Au (ppm)Bi (ppm)
Type of analysis            
 USGS/Hunter Laboratory--GICPGICPGICPGICPGICPGICPHFAGCSAAHFAGICP
 ALS ChemexIMSICP/IMSICPICP/IMS--ICPICP/IMSFAFA/AASFAICP/IMS
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.80.610<0.2--20.05--0.01--0.01
2SBA-32--312<4<41383.42035.7<20
3SBA-36--<216<4<41164.57.018.5<20
 SBA-36--12276--94.4-->1017.2<2
4SC-1--5221864318--14--<20
5SC-4--5191564717--7.5--<20
6SF-3--1737357646--9.1--<20
7ASG-4--<212<4<4217--3.0--<20
8ASBA-41--31383--8>100185.1>1020.3<2
9CSBA-52--1171--19.6--7.35--<2
10CSLH-10--1131--<18.6--6.86--<2
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-34--16332866085.12533.6<20
12SC-3--2043459695--9.2--<20
13SD-1--154861168519--10--<20
14NBA-3--420225384834.68.414.7<20
15NBA-22--242504843240<4<0.340.550.55<20
16NBA-27--3101205823110<40.340.250.21<20
17NBA-33--52830668<41.711312.7<20
 NBA-33--56011--545.6--9.98--2
18NBA-35--<218<4748<4<0.340.650.55<20
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-471.23525169--2240.9--0.03--0.04
20SBA-391.80.63192.2--610.2--9.56--0.01
Vein ores
21VBA-7--219<1--389.2--0.37--<2
22VBA-13--4912<4222428.56.97.2<20
23VBA-14--61313528<4<0.340.100.41<20
24VBA-15--515175428161.09.28.3<20
25VBA-19--13402219140<42.70.050.07<20
 VBA-19--<11271--<11.8--0.18--<2
Intrusive bodies
26ZBA-4--7187852<4--0.20.03<20
27ZBA-10--6246752<4--0.70.07<20
No.TypeField no.Cd (ppm)Cu (ppm)Hg (ppm)Mo (ppm)Pb (ppm)Tl (ppm)W (ppm)Zn (ppm)As (ppm)Sb (ppm)Te (ppm)
Type of analysis
 USGS/Hunter LaboratoryGICPGICPGCVAAGICPGICPGCSAAGTGICPGICPHASSGCSAA
 ALS ChemexICP/IMSICPFASSICPICP/IMSIMSICP/IMSICPHASSIMSIMS
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.1<1.00.01<0.21<0.020.4<2.085.10.05
2SBA-32<412--6<82.9<180240,0005,4000.25
3SBA-36<417--<4<80.5<16077,00011,0000.35
 SBA-362.5210.39<16--<1058>10,000>10,000--
4SC-1<4280.34<41800.733604,100--2.7
5SC-4<4341.40<40412.31807,700--1.3
6SF-3<4320.40<4691.3<12603,400--1.1
7ASG-4<412--<4<81.6<1100270,000--0.25
8ASBA-4124.01442.36<1<2--<103,520>10,000>10,000--
9CSBA-521.50120.13<142--<106611>10,000--
10CSLH-1026.5160.66<118--<103,20041>10,000--
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-34<436--<4550.9101,00023,0008501.3
12SC-3<4300.40<4931.1102702,800--1.0
13SD-1<4940.40<45501.1126804,300--3.6
14NBA-3<41600.20<41,8000.75<14904,600470.30
15NBA-22<41100.20<4560.5514904,100400.20
16NBA-27<4780.10<42200.4<1290650101.2
17NBA-33<4490.20<43301.163404,10016025.0
 NBA-330.5190.10<140--<10384,570670--
18NBA-35<4150.20<4120.4<11,50088097<0.05
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-478.262,6100.32<0.21,5702.32.83,51050240.05
20SBA-391.42180.60<0.6121.30.4206>10,000>1,0003.2
Vein ores
21VBA-72.0460.12<1440--<10260271180--
22VBA-13<4950.22<46900.525001,400621.7
23VBA-14<4200.20<4151.21054330330.20
24VBA-15<46600.60<45,5000.731,0003,4001604.6
25VBA-196240.20<4230.15<14631035,0000.05
 VBA-190.591.09<1<2--<10212>10,000--
Intrusive bodies
26ZBA-4<460.06<480.352531080.25
27ZBA-10<490.24<490.42102302000.20
No.TypeField no.Cd (ppm)Cu (ppm)Hg (ppm)Mo (ppm)Pb (ppm)Tl (ppm)W (ppm)Zn (ppm)As (ppm)Sb (ppm)Te (ppm)
Type of analysis
 USGS/Hunter LaboratoryGICPGICPGCVAAGICPGICPGCSAAGTGICPGICPHASSGCSAA
 ALS ChemexICP/IMSICPFASSICPICP/IMSIMSICP/IMSICPHASSIMSIMS
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.1<1.00.01<0.21<0.020.4<2.085.10.05
2SBA-32<412--6<82.9<180240,0005,4000.25
3SBA-36<417--<4<80.5<16077,00011,0000.35
 SBA-362.5210.39<16--<1058>10,000>10,000--
4SC-1<4280.34<41800.733604,100--2.7
5SC-4<4341.40<40412.31807,700--1.3
6SF-3<4320.40<4691.3<12603,400--1.1
7ASG-4<412--<4<81.6<1100270,000--0.25
8ASBA-4124.01442.36<1<2--<103,520>10,000>10,000--
9CSBA-521.50120.13<142--<106611>10,000--
10CSLH-1026.5160.66<118--<103,20041>10,000--
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-34<436--<4550.9101,00023,0008501.3
12SC-3<4300.40<4931.1102702,800--1.0
13SD-1<4940.40<45501.1126804,300--3.6
14NBA-3<41600.20<41,8000.75<14904,600470.30
15NBA-22<41100.20<4560.5514904,100400.20
16NBA-27<4780.10<42200.4<1290650101.2
17NBA-33<4490.20<43301.163404,10016025.0
 NBA-330.5190.10<140--<10384,570670--
18NBA-35<4150.20<4120.4<11,50088097<0.05
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-478.262,6100.32<0.21,5702.32.83,51050240.05
20SBA-391.42180.60<0.6121.30.4206>10,000>1,0003.2
Vein ores
21VBA-72.0460.12<1440--<10260271180--
22VBA-13<4950.22<46900.525001,400621.7
23VBA-14<4200.20<4151.21054330330.20
24VBA-15<46600.60<45,5000.731,0003,4001604.6
25VBA-196240.20<4230.15<14631035,0000.05
 VBA-190.591.09<1<2--<10212>10,000--
Intrusive bodies
26ZBA-4<460.06<480.352531080.25
27ZBA-10<490.24<490.42102302000.20

Analytical procedures: Branch of Geochemistry, U.S. Geological Survey (USGS): GCSAA = chemical separation atomic absorption spectrometry, GCVAA = cold-vapor atomic absorption spectrometry, GICP = semiquantitative induction-coupled plasma spectrometry, GSP = specific analytical procedure for F Hunter Analytical Laboratory: HAAS = atomic absorption analysis, HFA = classic fire assay; ALS Chemex: FA = classic fire assay analysis, FA-ASS = fire assay collection with atomic absorption finish, FASS = flameless atomic absorption spectrometry, HASS = hydride generator-atomic absorption spectrometry, ICP = induction coupled plasma optical emission analysis, IMS = induction coupled plasma mass spectrometric analysis, XRF = X-ray fluorescence analysis; italicized numbers were determined by the italitized analytical method

Sample types: AS = arsenical ore, BM = base-metal mineralized shale, CS = calcic skarn, N = clay-rich ore derived from limy clastic rock, S = silicified (jasperoid) ore, UL = unaltered limestone, V = vein ore, Z = intrusive rocks

-- = no data available

Fig. 8.

Scatter plots of rock sample data for selected elemental pairs. Concentrations reported at below detection limits are plotted at one-half the limit of detection.

Fig. 8.

Scatter plots of rock sample data for selected elemental pairs. Concentrations reported at below detection limits are plotted at one-half the limit of detection.

Gold and silver

Gold and silver have a wide range of concentrations (Fig. 8; Table 2). The Au/Ag ratio varies by more than three orders of magnitude from about 0.01 to 10. Samples with more than about 10 ppm Au have a median Au/Ag ratio of about 0.5 to 1. The Au/Ag ratio of some samples may have been affected by supergene oxidation (Table 2), which increases the Au/Ag ratio as Ag is preferentially removed. However, analyses of unoxidized Bau ores by Schuh (1993) also show Au/Ag ratios exhibit a wide range of values (0.04–87) for skarn, vein, and replacement mineralization, with the values for Carlin-style ores generally ranging from 0.5 to 22.9 (avg 6.5). Silver shows at best a modest correlation with the base metals and Au (Fig. 8). The wide range in the Au/Ag ratio, the comparable variations in the ratios of Ag to the base metals, and the marked range in absolute and relative concentrations of other elements suggest that the chemical and thermal gradients across the district were rather steep.

Pathfinder elements

Ores from the Bau district contain elevated concentrations of pathfinder elements, As, Sb, Hg, and Tl (Table 2; Fig. 8). Calcic skarn ores are enriched in Sb (1–10 wt %) and have low As/Sb ratios (0.001–0.01). The other ore types are enriched in As (commonly 1,000 ppm to 20 wt %) and have high As/Sb ratios (1–10) (Fig. 8). Arsenic values show a general positive correlation with gold, with many gold-bearing specimens having As/Au ratios of between 100 and 1,000 (Fig. 8).

Our data show that Hg concentrations are low in calcic skarn ores (<0.1 ppm) and high in the replacement ores (0.2–3 ppm; Fig. 8). However, Schuh (1993) reported similar Hg values for calcic skarn, vein, and Carlin-style ores and somewhat higher values for arsenical ores (4 and 11 ppm) and much higher values for the most distal mercury deposits (>1%). Thallium generally ranges from about 0.5 to 2 ppm. A single sample of arsenic-rich shale-hosted Carlin-style ore analyzed by Schuh (1993) contained 9.7 ppm Tl, and distal mercury ore from Tegora contained 20 ppm Tl. Although Te ranges from <0.1 to more than 10 ppm, it is present in many ores in the range of 1 to 4 ppm (Fig. 8; Table 2). Analyses of calcic skarn and base metal-bearing veins (presented by Schuh, 1993; Table 2) show concentrations of 10 to >1,000 ppm Te proximal to the porphyritic intrusions. Tellurium exhibits no correlation with Tl and shows only a modest association with Au (Fig. 8).

Base metals

The concentrations of Zn, Pb, and Cu in mineralized samples range from less than 10 to more than 1,000 ppm, with a weak correlation among these elements (Fig. 8; Table 2). Higher concentrations generally occur in base metal-bearing veins near porphyry intrusions. Molybdenum contents are less than 4 ppm, and tungsten ranges from <1 to 10 ppm. The concentration is <10 ppm for Sn and ≤2 ppm for Bi.

Al, Fe, Ti, and the rare earth elements and transition metals

Several samples of Bau ores contain elevated concentrations of Fe, Al, Ti, V, and rare earth elements (REEs) (Table 2; Fig. 8). The positive correlations between Al, Fe, Ti, the REEs, and transition metals, such as V, reflect original concentrations in the calcareous sedimentary rocks that were concentrated by the dissolution of calcite, as shown below. The significant amount of arsenopyrite, arsenian pyrite, and pyrite in many ores reflects the localized introduction of high concentrations of Fe and S by hydrothermal solutions.

Alkali and alkaline earth elements

Potassium varies between <0.1 and 2.3 wt %, with a median of 0.5 wt %. The K/Al ratios are much less than K-feldspar (1.45) and approach that of muscovite (0.48), which suggests that potassium is present in a K-deficient white mica or illite. The lowest ratios (<0.1; Fig. 8) reflect the presence of clay minerals such as kaolinite or smectite, which may in part be supergene. The wide range of K/Al ratios (0.4–0.05) reflects the proportion of illite and clay minerals in the samples. Sodium is less abundant than K, with a median of about 0.06 wt %, and Li ranges between 15 and 52 ppm, with a median of 32 ppm. Although Sr is more abundant than Ba in calcic skarn and in some veins, Ba is more abundant than Sr in Carlin-style ore (Fig. 8).

Manganese

The content of manganese in mineralized Bau rocks is higher than in fresh host rocks (Table 2). Twelve of the 18 samples of mineralized rocks and veins for which data are available contain at least 900 ppm Mn, and three samples contain more than 1.0 wt % Mn. Manganese content shows very little or no correlation to Fe (Fig. 8) or any other element. Although some of the Mn in oxidized rocks may have been enriched by supergene weathering processes, a number of unoxidized specimens contain 2,000 to 6,500 ppm Mn, which suggests that Mn was introduced by hydrothermal solutions. This is supported by the abundance of Mn (~1,000–>10,000 ppm) in samples of unoxidized skarn, vein, and replacement ores reported by Schuh (1993).

Base metal mineralized shale

A shale sample from the Pedawan Formation (Table 2, specimen 19) provides evidence for a previously unrecognized type of disseminated base metal mineralization at Bau. The sample does not show megascopic evidence of alteration or mineralization and was collected to provide a baseline for evaluating the addition and loss of elements. However, the rock contains 2,610 ppm Cu, 3,510 ppm Zn, 1,570 ppm Pb, and 6.47 wt % iron in very fine grained sulfide minerals. Potassium is very high (5.83 wt %) and Ba and Sr (2,190 and 242 ppm, respectively) are higher than in the gold-bearing rocks. The transition elements Ni (69 ppm), Co (35 ppm), and V (224 ppm) are also elevated. Values of Au (0.03 ppm), Ag (0.9 ppm), Sb (24 ppm), and As (50 ppm) are low, both absolutely and in relation to the base metals. Although syngenetic/diagenetic base metal mineralization has not previously been recognized in the Pedawan Formation, the geochemical features and location of the sample, well away from intrusive centers, allow that possibility.

Gain and loss of elements

To document mass transfer associated with alteration and mineralization (Fig. 9A), the concentration of elements in representative samples of silicified, arsenical, and calcic skarn ore are plotted against the concentration of elements in unaltered Bau limestone (Table 2, specimen 1). The depletion of Ca, Mg, and Sr and the enrichment of Si, Al, K, Na, and Li relative to the line of constant mass are indicative of carbonate dissolution, silicification, and potassium metasomatism. The strong enrichments of Fe, Mn, and Zn together with As, Sb, Ag, and Au suggest that they precipitated together in an assemblage of Fe, Zn, As, and Sb minerals. Lesser amounts of P, Ba, Pb, Cu, Ni, Cd, Te, Tl, and Hg were also introduced by the ore fluids.

Fig. 9.

A. Logarithmic isocon plot (i.e., Hofstra and Cline, 2000) comparing the abundance of elements in unaltered Bau limestone to those in calcic skarn, arsenical, and silicified ores. The isocon for silicified ore is based on the immobility of Ti and Al. Elements that plot above the isocon are introduced, along the isocon are immobile, and below the isocon are depleted. The enrichment of immobile elements (Ti and Al) relative to the line of constant mass is due to the loss of Ca, Sr, and Mg. Values reported as less than detection are plotted at one-half the limit of detection. B, C. Element enrichment factors (relative to average crust) in gold ore from Bau (colored diamonds) compared to gold ore in 27 Carlin-type deposits in Nevada (black bars).

Fig. 9.

A. Logarithmic isocon plot (i.e., Hofstra and Cline, 2000) comparing the abundance of elements in unaltered Bau limestone to those in calcic skarn, arsenical, and silicified ores. The isocon for silicified ore is based on the immobility of Ti and Al. Elements that plot above the isocon are introduced, along the isocon are immobile, and below the isocon are depleted. The enrichment of immobile elements (Ti and Al) relative to the line of constant mass is due to the loss of Ca, Sr, and Mg. Values reported as less than detection are plotted at one-half the limit of detection. B, C. Element enrichment factors (relative to average crust) in gold ore from Bau (colored diamonds) compared to gold ore in 27 Carlin-type deposits in Nevada (black bars).

To document similarities and differences between gold ore at Bau and gold ore in Carlin-type deposits (Fig. 9B, C), enrichment factors for Tl, Hg, Sb, As, Au, Ag, Pb, Zn, Cu, Mo, and W in ore (>1 ppm Au) from Bau are compared to those in 27 Carlin-type gold deposits from Nevada. The results show that in comparison to Carlin-type gold deposits, Bau is less enriched in Hg, Tl, and W and more enriched in As, Sb, Ag, Pb, and Zn.

Trace elements in minerals

Electron microprobe analyses of pyrite were conducted at the U.S. Geological Survey (USGS) in Denver, Colorado, using instrumentation and methods similar to those described in Lowers et al. (2007). Multielement analyses of ore-related minerals by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were conducted at the USGS in Denver using instrumentation and methods similar to that described by Ridley (2000). A laser spot size of 25 μm and a linear travel rate of 10 μm/s were used to generate multielement profiles across sulfide minerals in polished thin sections.

Electron microprobe analysis of pyrites (five spots) from microgranodiorite porphyry for arsenic and gold contained 0.77 to 1.14 mol % As but did not detect any gold. Vein pyrites (11 spots) contained 0.12 to 0.25 mol % As, and no gold was detected. Therefore, to identify the residence of Au and other elements in ore-related minerals from Carlin-style ores, Fe minerals were analyzed by LA-ICP-MS (Fig. 10A-E).

Fig. 10.

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) multielement profiles across the following minerals: A. sphalerite, arsenopyrite, and realgar (sample BA-42); B. native arsenic (alloy) (sample Bau-3); and C. stibnite (BA-36). D. Proportional box plot showing the abundance of Fe in sphalerite (sample BA-42). E. Au vs. As plot of LA-ICP-MS analyses of arsenopyrite (gray diamonds; samples BA-36, BA-42, G-4) and whole-rock analyses of mineralized rocks (yellow squares; Table 2). Except for calcic skarn, the array of whole-rock data appears to reflect the abundance of arsenopyrite in the rocks. Fields adapted from Large et al. (2011). Abbreviations: asp = arsenopyrite, real = realgar, sph = sphalerite.

Fig. 10.

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) multielement profiles across the following minerals: A. sphalerite, arsenopyrite, and realgar (sample BA-42); B. native arsenic (alloy) (sample Bau-3); and C. stibnite (BA-36). D. Proportional box plot showing the abundance of Fe in sphalerite (sample BA-42). E. Au vs. As plot of LA-ICP-MS analyses of arsenopyrite (gray diamonds; samples BA-36, BA-42, G-4) and whole-rock analyses of mineralized rocks (yellow squares; Table 2). Except for calcic skarn, the array of whole-rock data appears to reflect the abundance of arsenopyrite in the rocks. Fields adapted from Large et al. (2011). Abbreviations: asp = arsenopyrite, real = realgar, sph = sphalerite.

Sphalerite (sample BA-42a; Fig. 10A) was found to contain, in decreasing order of abundance, Fe, Cd, Mn, Sb, Cu, Hg, Sn, and Te. Arsenopyrite from three samples (BA-42a [e.g., Fig. 10A], BA-36, and G-4) was characterized, and it contains Sb, Cu, Au, Sn, Te, Ag, Hg, Tl, and Pb. Native arsenic (sample Bau-3; Fig. 10B) contains Sb, Ag, Pb, Au, In, Hg, and Te. Realgar (sample BA-42a; Fig. 10A) contains Sb, Sn, Ag, Cu, Te, Hg, Au, Pb, and Tl. Stibnite (BA-36; Fig. 10C) contains As, Se, Pb, Cu, and Te. Pyrite was not analyzed, because it occurs in only minor quantities in our samples relative to arsenopyrite, native arsenic, and stibnite.

The abundance of Fe in sphalerite associated with Au-bearing arsenopyrite (sample BA-42a; Fig. 10D) decreases from core (7.4 wt %) to rim (1.4 wt %). The decrease in Fe suggests either that log aS2 remained constant (e.g., –10) as temperatures dropped from about 300° to 250°C or that log aS2 increased by two orders of magnitude at constant temperature (Scott, 1983). The former interpretation is favored, because it is consistent with the cooling required to precipitate quartz that encloses the sphalerite and arsenopyrite.

The abundance of Au in arsenopyrite, of 100 to 10,000 ppm (Fig. 10E), indicates that arsenopyrite harbors a significant proportion of the Au in the ore. This interpretation is supported by whole-rock data from the vein and Carlin-style deposits that have Au/As ratios that are similar to those based on microanalyses of arsenopyrite (Fig. 10E). The Au/As ratios of arsenopyrite and whole-rock data are generally lower than Au/As ratios within Carlin pyrite and Carlin ore (Fig. 10E; Large et al., 2011). Calcic skarn has higher Au/As ratios, which is supported by the common occurrence of native gold in skarn.

Fluid Inclusions

Analytical procedures

Doubly polished plates were prepared from 22 representative specimens from calcic skarns, each vein-type deposit, and the Carlin-style deposits. Thermometric data were collected on fluid inclusions in 11 of these samples using a modified USGS-type gas flow heating-freezing stage constructed by Fluid Inc., Denver, Colorado. Synthetic fluid inclusions (–56.6° and 0°C) and a chemical standard (398°C) were used for calibration.

The petrographic characteristics and origin of fluid inclusions in the host minerals were determined prior to analysis by standard microthermometric techniques. Freezing was carried out before heating on some samples. The extremely small sizes of inclusions in microcrystalline quartz permitted only heating studies. Homogenization temperatures were generally bracketed in increments of 5°C or less by slowly raising and lowering temperature and monitoring the vapor bubble. Microthermometric measurements are repeatable to about ±3°C for heating and ±0.2°C for freezing runs. Freezing experiments were accomplished by slowly increasing the temperature in 0.2°C increments near the ice-melting point. Fluid inclusion results are summarized in Table 3 and displayed in Figure 11A and B.

Table 3.

Summary of Fluid Inclusion Data from the Bau District, Malaysia

DepositSampleOre typeMineralTh (°C)Tm (°C)TmH (°C)
 nRangeMode °CnRangenRange
Calcic skarn deposits
Gunug BauCS-1Calc-silicateQuartz, zone 114230–339230–3107–2.5 to –0.5  
   Quartz, zone 2, s9223–368- -12–1.9 to –0.7  
   Quartz, zone 3, s17247–335- -  14186–223
Lucky HillLH-2Calc-silicateQuartz9102–271- -2–3.1 to –1.5  
   Calcite5255–288250–2903–16.2 to –14.4  
   Sarabauite1172- -    
Vein deposits
Quartz-calcite veins (type 1)
RumohBA-7Quartz-calciteQuartz56160–298220–28034–2.1 to –0.7  
   Calcite3151–172- -2–0.1 to –0.1  
KojakBA-19Quartz-calciteQuartz12198–218190–22012–1.2 to –1.1  
Tai TonC-5Quartz-calciteQuartz, zone 116240–349290–35013–2.7 to –1.1  
   Quartz, zone 232285–358290–34012–2.1 to –1.3  
Base metal veins (type 2)
NegaraBA-40Base metalSphalerite6302–308300–3104–6.4 to –4.6  
Replacement deposits
Bukit YoungBA-47NormalQuartz, zone 17105–118100–120    
   Quartz, zone 216148–279150–2209–2.2 to –2.0  
Bukit YoungBY-1NormalQuartz, s13241–373280–330  9197–268
 60–380         
 C-1SilicifiedQuartz13295–341290–3209–2.3 to –1.6  
 BA-43SilicifiedQuartz7155–303210–240    
BidiBAU-1ArsenicalQuartz26201–265200–2509–2.5 to –1.1  
BidiBA-41ArsenicalQuartz22182–263200–25017–2.2 to –1.3  
DepositSampleOre typeMineralTh (°C)Tm (°C)TmH (°C)
 nRangeMode °CnRangenRange
Calcic skarn deposits
Gunug BauCS-1Calc-silicateQuartz, zone 114230–339230–3107–2.5 to –0.5  
   Quartz, zone 2, s9223–368- -12–1.9 to –0.7  
   Quartz, zone 3, s17247–335- -  14186–223
Lucky HillLH-2Calc-silicateQuartz9102–271- -2–3.1 to –1.5  
   Calcite5255–288250–2903–16.2 to –14.4  
   Sarabauite1172- -    
Vein deposits
Quartz-calcite veins (type 1)
RumohBA-7Quartz-calciteQuartz56160–298220–28034–2.1 to –0.7  
   Calcite3151–172- -2–0.1 to –0.1  
KojakBA-19Quartz-calciteQuartz12198–218190–22012–1.2 to –1.1  
Tai TonC-5Quartz-calciteQuartz, zone 116240–349290–35013–2.7 to –1.1  
   Quartz, zone 232285–358290–34012–2.1 to –1.3  
Base metal veins (type 2)
NegaraBA-40Base metalSphalerite6302–308300–3104–6.4 to –4.6  
Replacement deposits
Bukit YoungBA-47NormalQuartz, zone 17105–118100–120    
   Quartz, zone 216148–279150–2209–2.2 to –2.0  
Bukit YoungBY-1NormalQuartz, s13241–373280–330  9197–268
 60–380         
 C-1SilicifiedQuartz13295–341290–3209–2.3 to –1.6  
 BA-43SilicifiedQuartz7155–303210–240    
BidiBAU-1ArsenicalQuartz26201–265200–2509–2.5 to –1.1  
BidiBA-41ArsenicalQuartz22182–263200–25017–2.2 to –1.3  

Notes: Primary and pseudosecondary inclusions unless noted otherwise by “s” for secondary inclusions; zones in minerals refer to different growth zones, from earliest (1) to latest (3); Th is homogenization temperature of complete fluid inclusion to liquid; Tm is the temperature of ice melting; TmH is the temperature of halite dissolution; - = no data available

Fig. 11.

A. Histograms showing the homogenization temperatures of fluid inclusions in quartz and other minerals in samples from the various types of mineralization studied in the Bau district. Colorless area within histograms represents data from quartz. B. Salinity vs. homogenization temperature plot for fluid inclusions in the various types of mineralization at Bau. Data are shown relative to phase boundaries and isobars in the H2O-NaCl system (Atkinson, 2002). Abbreviations: H = halite, L = liquid, SRH = sedimentary rock-hosted, V = vapor.

Fig. 11.

A. Histograms showing the homogenization temperatures of fluid inclusions in quartz and other minerals in samples from the various types of mineralization studied in the Bau district. Colorless area within histograms represents data from quartz. B. Salinity vs. homogenization temperature plot for fluid inclusions in the various types of mineralization at Bau. Data are shown relative to phase boundaries and isobars in the H2O-NaCl system (Atkinson, 2002). Abbreviations: H = halite, L = liquid, SRH = sedimentary rock-hosted, V = vapor.

Petrography

Most fluid inclusions are less than 20 μm in largest dimension and contain two phases—a liquid and a vapor bubble—at room temperature. Liquid-rich inclusions containing about 10 to 30 vol % vapor (all volume percentages are visual estimates) are by far the most common. Some planes are composed of liquid-rich inclusions with uniform phase ratios, while other planes have variable phase ratios that are due to heterogeneous trapping of liquid and vapor or necking down. Vapor-rich inclusions containing about 50 to more than 90 vol % vapor locally occur with liquid-rich inclusions. Planes composed entirely of rounded, mature secondary vapor-rich inclusions are common in some vein samples, indicating that a low-density fluid phase was trapped. Liquid CO2 was not observed in any inclusions. Two of the samples studied contain secondary hypersaline inclusions with halite, and some of these inclusions contained another unidentified daughter mineral. Most hypersaline inclusions are liquid rich and homogenize by vapor disappearance. Rare vapor-rich inclusions that contain a halite daughter mineral may reflect heterogeneous trapping of vapor and halite or necking down. All of the microthermometric data reported in this paper were collected from the liquid-rich and hypersaline inclusions, because no phase changes were perceived in the vapor-rich inclusions.

Quartz typically contains abundant fluid inclusions. Several forms of quartz are present in the Bau ores. For example, microcrystalline quartz (jasperoid) occurs in replacement deposits, calcic skarns, and vein deposits, whereas coarsely crystalline quartz occurs in vein deposits and as veins and breccia matrix in replacement deposits. Turbid equigranular microcrystalline quartz with interlocking mosaic grains is commonly intergrown with fine-grained sphalerite, native arsenic, stibnite and/or arsenopyrite. This type of quartz contains very small (±1–4 μm) primary fluid inclusions that are irregular in shape, are necked, and are commonly distributed along growth zones near the center of the grains. In Sb-rich jasperoid, this quartz contains rare primary inclusions along growth zones with rounded or negative crystal shapes that are up to 15 μm in their largest dimension. Coarsely crystalline open-space-filling quartz contains abundant primary inclusions on growth zones and secondary and/or pseudosecondary inclusions along healed fractures.

Calcite from vein and calcic skarn deposits contains abundant liquid-rich inclusions, many with negative crystal shapes, occurring in distinct planes parallel to cleavage directions that are probably secondary in origin. Primary liquid-rich inclusions as large as 50 μm in greatest dimension were observed in one sample of early vein calcite that contained abundant manganese oxide (type 1 vein; specimen BA-7). Massive sphalerite from a type 2 vein (specimen BA-40) contains sparse primary liquid-rich inclusions as large as 25 μm in largest dimension in the form of negative crystals. Sarabauite (CaSb10O10S10) intergrown with wollastonite in a calc-silicate vein (specimen LH-2) contains abundant elongated tubular inclusions; although most of the inclusions are dark and appear to be one phase, a few inclusions with small vapor bubbles were observed. Inclusions in wollastonite and other calc-silicate minerals were too small to perform heating and freezing experiments.

Results

Calcic skarn: Fluid inclusions in two veins composed of calc-silicate minerals were studied. Fine-grained quartz that cuts skarn at Gunung Bau (specimen CS-1) contains two populations of primary fluid inclusions. The earlier population yielded homogenization temperatures of 230° to 339°C and ice-melting temperatures of –2.5° to –0.5°C that correspond to salinities of 0.9 to about 4.2 wt % NaCl equiv (Fig. 11). A later population has variable liquid to vapor ratios, homogenization temperatures from 223° to 368°C, and ice-melting temperatures of –1.9° to –0.7°C, indicating salinities of 1.2 to 3.2 wt % NaCl equiv (Fig. 11; Table 3).

Small primary fluid inclusions in transparent microcrystalline quartz with a mosaic texture of uncertain paragenetic position at Lucky Hill (specimen LH-2) yielded homogenization temperatures ranging from 102° to 271°C (Fig. 11; Table 3). The inclusions were too small to obtain reliable ice-melting temperatures. Primary inclusions in calcite, believed to have formed after the sarabauite and minor wollastonite at Lucky Hill, yielded homogenization temperatures of 255° to 288°C and ice-melting temperatures from –16.2° to –14.4°C, indicating salinities of about 18 to 20 wt % NaCl equiv (Table 3). A single homogenization temperature of 172°C obtained from an inclusion in sarabauite intergrown with wollastonite is substantially lower than temperatures measured for the crosscutting quartz and calcite and therefore is considered unreliable.

Secondary hypersaline fluid inclusions were observed in quartz phenocrysts in microgranodiorite porphyry from Gunung Bau and Lucky Hill and in hydrothermal quartz from a calcic skarn vein at Gunung Bau and a replacement deposit at Bukit Young (Table 3). In the hypersaline inclusions with uniform phase ratios, halite dissolved at temperatures lower than liquid-vapor homogenization. At Gunung Bau (specimen CS-1), halite dissolution temperatures ranged from 186° to 223°C, and the temperature of total homogenization to liquid ranged from 247° to 335°C, with most values between 290° and 320°C (Fig. 11). Hypersaline inclusions at Bukit Young (specimen BY-1) are very irregular and in some cases are obviously necked. Halite dissolution occurred between about 197° and 268°C and final homogenization between 241° and 373°C. The halite dissolution temperatures of the hypersaline inclusions indicate salinities of about 31 to 37 wt % NaCl equiv (Fig. 11; Table 3).

Veins: Fluid-inclusion data were collected from three types of veins: calcic-skarn, quartz + calcite (type 1), and base metal (type 2) veins (Table 3).

Data were obtained on three quartz and calcite (type 1) veins from the Kojak, Rumoh, and Tai Ton deposits. Homogenization temperatures for primary and pseudosecondary inclusions in quartz from the individual veins have ranges of 198° to 218°C at Kojak, 160° to 298°C at Rumoh, and 240° to 349°C at Tai Ton. Ice-melting temperatures (Tm ice) for all quartz vein samples range from –2.7° to –0.7°C, yielding salinities ranging from about 1.2 to 4.2 wt % NaCl equiv (Hall et al., 1988). Primary fluid inclusions in atypical coarse vein quartz that cements breccia fragments of turbid microcrystalline quartz (Tai Ton, specimen C-5; siliceous ore) yielded relatively high homogenization temperatures of 285° to 358°C, with most between 290° and 310°C (Fig. 11) and exhibits temperature and salinity ranges similar to those of the individual quartz vein discussed above. The Tm ice ranged from –2.1° to –1.3°C, indicating salinities of 3 to 4 wt % NaCl equiv (Fig. 11B).

Primary inclusions in paragenetically early, dark, manganese oxide-bearing calcite from the Rumoh vein (specimen BA-7) yielded lower homogenization temperatures between 151° and 172°C. The Tmice of several inclusions is very close to zero, but others have positive Tm ice of as much as 6.2°C, indicating metastable ice melting or clathrate melting. The calcite in this sample contains numerous planes of secondary inclusions along healed cleavage traces. Inclusions from one such plane homogenized to liquid between 288° and 290°C—temperatures that are similar to those for primary inclusions in later-formed quartz of the same sample. The secondary inclusions along the healed cleavage trace in calcite record a later-stage, higher-temperature fluid that was trapped after the formation of the calcite.

Primary fluid inclusions in a sphalerite vein at Negara (specimen BA-40) homogenized to a liquid between 302° and 308°C, temperatures that are similar to those found for some quartz veins (Fig. 11). The measured Tm ice values, however, are lower than for quartz veins, ranging from –6.4° to –4.6°C, and indicate higher salinities of 7.3 to 9.7 wt % NaCl equiv (Table 3).

Carlin-style mineralization: Fluid inclusions in quartz from two replacement deposits (Bidi and Bukit Young) were studied. Two samples of arsenical ore consisting of turbid microcrystalline quartz intergrown with stibnite, native arsenic, arsenopyrite, and fine sphalerite from Bidi (specimens BA-41 and BAU-1) contained fluid inclusions that yielded a broad normal distribution centered on a homogenization temperature of 225°C (Fig. 11). Ice-melting temperatures of –2.5° to –1.1°C correspond to fluid salinities of about 2 to 4 wt % NaCl equiv. Primary inclusions along a growth zone in slightly coarser quartz infilling breccia fragments in the same sample appeared to possess a relatively constant liquid to vapor ratio but gave a wide range of homogenization temperatures between 148° and 279°C with most between 150° and 220°C (Fig. 11); ice-melting temperatures were constant at –2.2° to –2.0°C (Table 3). The variable Th and constant salinity of these inclusions are indicative of necking down. Samples of silicified and disseminated ore from Bukit Young (C-1 and BA-43) ranged from 155° to 341°C and exhibit two distinct populations, 290° to 320°C and 210° to 240°C (Table 3).

Although liquid CO2 was not seen in any of the inclusions, clathrate formation during freezing was observed in some inclusions in calcite, and crushing experiments indicate the presence of a compressed gas phase, most likely CO2, in samples from the vein and replacement deposits. This inference is supported by the fact that CO2 is produced during the replacement of limestone by skarn minerals (Einaudi et al., 1981).

Stable Isotopic Data

Analytical procedure

Stable isotopic data for oxygen and carbon were obtained at the USGS laboratory in Menlo Park, California, and are reported in standard per mil (δ‰) notation; oxygen values are reported relative to Vienna-standard mean ocean water (V-SMOW), and carbon is reported relative to the Pee Dee belemnite (PDB) standard. Oxygen from silicate minerals was liberated for isotopic analysis by reaction with ClF3 at 550°C in nickel bombs (Borthwick and Harmon, 1982). Oxygen and carbon from carbonate minerals were released as CO2 by reaction with 100% H3PO4 (McCrea, 1950). All isotopic measurements were performed in duplicate with a reproducibility of approximately ±0.1‰. The isotopic compositions of H2O and CO2 in fluid inclusion extracts were determined in R.O. Rye’s lab at the USGS in Denver using methods described in Richardson et al. (1988) and are reported relative to the V-SMOW and PDB standards. The isotopic compositions of sulfides in Schuh (1993) were determined by laser fluorination and are reported relative to the Canyon Diablo Troilite (CDT) standard. The temperature-dependent fractionation factors used to calculate δDH2O, δ18OH2O, δ13CCO2, and δ34SH2S compositions are as follows: quartz-H2O (Clayton et al., 1972), calc-silicate-H2O (Zheng, 1993), sericite-H2O (Sheppard and Gilg, 1996), calcite-H2O (Friedman and O’Neil, 1977), and calcite-CO2 and sulfides-H2S (Ohmoto and Rye, 1979). Stable isotopic data for minerals and fluid inclusion extracts are provided in Table 4 and displayed in Figure 9A-D.

Results

Hydrogen and oxygen: The range of measured δ18O values for Bau Limestone (Kakisaki et al., 2013), quartz phenocrysts and sericite in altered granodiorite porphyry, calc-silicates in skarn, and quartz in veins and jasperoid are shown in the inset of the δD vs. δ18O diagram (Fig. 12B). Corresponding water δ18O values were calculated for appropriate temperature ranges. The calculated quartz and sericite compositions are similar to magmatic water. The calc-silicate fluid extends from the magmatic water field to higher δ18O values that are indicative of isotopic exchange between magmatic fluid and limestone. Similarly, fluid inclusion extracts from base metal veins (galena and sphalerite) have δDH2O and δ18OH2O values that plot to the right of the magmatic water box. The high δDH2O value for water extracted from native arsenic (–60‰) and its similarity to values for sphalerite and galena suggest that As was introduced by a similar fluid. Fluid inclusion extracts from vein quartz, calcite, stibnite, and native arsenic have measured δDH2O values between –60 and –82‰. Calculated δ18OH2O values for vein calcite and quartz and distal jasperoid at temperatures of 320° to 170°C are between 6 and 14‰. Together, the δDH2O and δ18OH2O data define a field that extends below and to the right of the magmatic water box (Fig. 9B). The highest δ18OH2O values reflect isotopic exchange between magmatic water and host limestones. The lowest δDH2O values may be due to reservoir effects during magma degassing, brine-vapor immiscibility, and preferential trapping of brine inclusions (Hedenquist and Richards, 1998). In contrast, fluid inclusion extracts from late drusy quartz have lower δDH2O and δ18OH2O values that reflect mixing with local meteoric water (Stephens and Rose, 2005), and fluid extracted from late calcite plots on the meteoric water line (Fig. 9B). The δDH2O and δ18OH2O data indicate that ore fluids consisted of magmatic water that generally evolved by reactions with carbonate rocks and in the latest stages by mixing with meteoric groundwater.

Table 4.

Stable Isotopic Data for Mineralized Rocks, Intrusive Rock, and Unaltered Limestone from the Bau District, Malaysia

SampleMineralδ18Oδ13CT1δ18OH2O2δH2Oδ18OH2Oδ13CCO2Comments
Intrusion
BI-1qtz9.2      Phenocrysts
 ser12.5 350310.3   Alteration
Calcic skarn         
CS-1qtz20.2 29013    
 cs7.5       
 cs + sulf9.6       
LH-1wo7.8       
LH-2qtz20.1      Very fine grained
 qtz19      Later coarse-grained
Lucky H 5wo8.3       
Replacement
BA-32stibnite    –82   
BA-39cal18.3–6.1     Late stage
 drusy qtz18.8 2007.2–71–3.1–5.9Stockwork veining
BA-41qtz19.1 2259   Late stage
 qtz19.6 2259.5   Early qtz + native As
BA-42Adrusy qtz    –68   
BA-42Bdrusy qtz    –67–4.8–7.6 
BA-47qtz20.1 1756.8   Late stage
 qtz19.7 1756.4   Early stage
BAU-1arsenic    –62   
BAU-2stib + qtz    –86   
BAU-3qtz18.3 2258.2    
 drusy qtz20.4 22510.3    
 cal17.1–5.1     Late stage
 late cal    –824.3–12.4 
 drusy qtz    –101–5.4–8.2 
BAU-4cal18.6–6.1     Late stage in vug
 qtz18.3 2258.2    
BY-1qtz18 2509.1   Late stage
 qtz16.6 2256.5   Early stage
 qtz17.3 2508.4   Middle stage
BY-2qtz19.5 2259.4    
 qtz22.1 22512    
Vein
C-1qtz20 25011.1   Early fine grained
 qtz20.5 30013.6   Late coarse-grained
Kusa 3qtz19.1 2259   Stockwork veining
 cal18.5–6.1     Late stage
BA-7cal20.1–7.21708.8    
BA-19qtz21.9 20012.4   Early stage
 qtz22.6 20011   Late stage
C-5qtz + sulf18.2 32012    
Base metal vein
Negara 10cal21.4–3.6      
BA-51gal    –5813.9–2.4 
BA-56sph     9.5–7.1 
Bau Limestone
BA-46ls23.60.1 23.6   Fresh limestone
SampleMineralδ18Oδ13CT1δ18OH2O2δH2Oδ18OH2Oδ13CCO2Comments
Intrusion
BI-1qtz9.2      Phenocrysts
 ser12.5 350310.3   Alteration
Calcic skarn         
CS-1qtz20.2 29013    
 cs7.5       
 cs + sulf9.6       
LH-1wo7.8       
LH-2qtz20.1      Very fine grained
 qtz19      Later coarse-grained
Lucky H 5wo8.3       
Replacement
BA-32stibnite    –82   
BA-39cal18.3–6.1     Late stage
 drusy qtz18.8 2007.2–71–3.1–5.9Stockwork veining
BA-41qtz19.1 2259   Late stage
 qtz19.6 2259.5   Early qtz + native As
BA-42Adrusy qtz    –68   
BA-42Bdrusy qtz    –67–4.8–7.6 
BA-47qtz20.1 1756.8   Late stage
 qtz19.7 1756.4   Early stage
BAU-1arsenic    –62   
BAU-2stib + qtz    –86   
BAU-3qtz18.3 2258.2    
 drusy qtz20.4 22510.3    
 cal17.1–5.1     Late stage
 late cal    –824.3–12.4 
 drusy qtz    –101–5.4–8.2 
BAU-4cal18.6–6.1     Late stage in vug
 qtz18.3 2258.2    
BY-1qtz18 2509.1   Late stage
 qtz16.6 2256.5   Early stage
 qtz17.3 2508.4   Middle stage
BY-2qtz19.5 2259.4    
 qtz22.1 22512    
Vein
C-1qtz20 25011.1   Early fine grained
 qtz20.5 30013.6   Late coarse-grained
Kusa 3qtz19.1 2259   Stockwork veining
 cal18.5–6.1     Late stage
BA-7cal20.1–7.21708.8    
BA-19qtz21.9 20012.4   Early stage
 qtz22.6 20011   Late stage
C-5qtz + sulf18.2 32012    
Base metal vein
Negara 10cal21.4–3.6      
BA-51gal    –5813.9–2.4 
BA-56sph     9.5–7.1 
Bau Limestone
BA-46ls23.60.1 23.6   Fresh limestone

Abbreviations: cal = calcite, sulf = sulfide minerals, cs = calc-silicate minerals, gal = galena, ls = limestone, qtz = quartz, ser = sericite, sph = sphalerite, wo = wollastonite

1

Approximate value from fluid inclusion homogenization temperatures

2

Calculated from the equation of Clayton et al. (1972)

3

Estimated

Fig. 12.

A. Histogram showing the distribution of δ18O values among the various mineralization types in the intrusive and carbonate host rocks at Bau. B. Inset, colored bars show the range of δ18O values for the Bau limestone (Kakisaki et al., 2013), hydrothermal minerals, and fluids in equilibrium with minerals at their respective temperatures of formation. δD vs. δ18O plot showing the compositions of calculated fluids and water extracted from fluid inclusions (FI) in minerals. The hydrogen isotope composition of water extracted from various minerals is indicated by the labeled ticks on the y-axis. The light-blue box shows the composition of fluids that produced the vein and Carlin-style deposits. Black arrows show how fluid compositions may have shifted in response to water/rock exchange (W/R) and fluid mixing. Data are shown relative to seawater (SW), the meteoric water line (MWL), local meteoric water (small gray boxes; Stephens and Rose, 2005), the primary magmatic water (PMW) box, and metamorphic water box. C. δ13C vs. δ18O plot showing the composition of Bau Limestone (large blue square, this study; small blue squares, Kakisaki et al., 2013), calcite in base metal veins (green squares), late calcite (yellow squares), fluid inclusions in galena (gray square), sphalerite (brown square), quartz (blue square), and local meteoric water (small gray squares). The δ13C of CO2 extracted from fluid inclusions in quartz is indicated by the white bar on the y-axis. The calculated δ18O of water in equilibrium with quartz is indicated by the white bar on the x-axis. The light-blue box shows the composition of fluids that produced the vein and Carlin-style deposits. Fluid in equilibrium with calcite in the base metal veins at the temperature of formation (green polygon) extends from the carbonatite box toward the Bau limestone. Fluid in equilibrium with late calcite (yellow polygon) at temperatures of 200° to 50°C extends from the carbonatite box toward local meteoric water (small gray squares; Stephens and Rose, 2005). Data are shown relative to the carbonatite box and marine field of typical marine limestones. D. Histogram showing the distribution of δ34S values among the various mineralization types in the intrusive and carbonate host rocks at Bau (based on data in Schuh, 1993). The blue bars show the isotopic composition of H2S in equilibrium with the minerals shown at their respective temperatures of formation. The lower blue bar, labeled magmatic, is based on pyrite and pyrrhotite from the stock. The upper blue bars are based on minerals in the calcic skarn, vein, and Carlin-style deposits. Pyrite and base metal sulfides contain magmatic sulfur, whereas As, Sb, and Te minerals contain more country rock sulfur. Data are shown relative to sulfides in the Pedawan Formation and Serian Volcanics and marine sulfate in Triassic and Tertiary time. Abbreviations: BM = base metal, Cal = calcite, Calc = calc-silicate rock, CDT = Canyon Diablo Troilite, gal = galena, Jasp = jasperoid, Ls = limestone, Qtz/qtz = quartz, PDB = Pee Dee belemnite, PMW = primary magmatic water, Po = pyrrhotite, Py = pyrite, Ser = sericite, SMOW = standard mean ocean water, Sph/sph = sphalerite, Stib/stib = stibnite.

Fig. 12.

A. Histogram showing the distribution of δ18O values among the various mineralization types in the intrusive and carbonate host rocks at Bau. B. Inset, colored bars show the range of δ18O values for the Bau limestone (Kakisaki et al., 2013), hydrothermal minerals, and fluids in equilibrium with minerals at their respective temperatures of formation. δD vs. δ18O plot showing the compositions of calculated fluids and water extracted from fluid inclusions (FI) in minerals. The hydrogen isotope composition of water extracted from various minerals is indicated by the labeled ticks on the y-axis. The light-blue box shows the composition of fluids that produced the vein and Carlin-style deposits. Black arrows show how fluid compositions may have shifted in response to water/rock exchange (W/R) and fluid mixing. Data are shown relative to seawater (SW), the meteoric water line (MWL), local meteoric water (small gray boxes; Stephens and Rose, 2005), the primary magmatic water (PMW) box, and metamorphic water box. C. δ13C vs. δ18O plot showing the composition of Bau Limestone (large blue square, this study; small blue squares, Kakisaki et al., 2013), calcite in base metal veins (green squares), late calcite (yellow squares), fluid inclusions in galena (gray square), sphalerite (brown square), quartz (blue square), and local meteoric water (small gray squares). The δ13C of CO2 extracted from fluid inclusions in quartz is indicated by the white bar on the y-axis. The calculated δ18O of water in equilibrium with quartz is indicated by the white bar on the x-axis. The light-blue box shows the composition of fluids that produced the vein and Carlin-style deposits. Fluid in equilibrium with calcite in the base metal veins at the temperature of formation (green polygon) extends from the carbonatite box toward the Bau limestone. Fluid in equilibrium with late calcite (yellow polygon) at temperatures of 200° to 50°C extends from the carbonatite box toward local meteoric water (small gray squares; Stephens and Rose, 2005). Data are shown relative to the carbonatite box and marine field of typical marine limestones. D. Histogram showing the distribution of δ34S values among the various mineralization types in the intrusive and carbonate host rocks at Bau (based on data in Schuh, 1993). The blue bars show the isotopic composition of H2S in equilibrium with the minerals shown at their respective temperatures of formation. The lower blue bar, labeled magmatic, is based on pyrite and pyrrhotite from the stock. The upper blue bars are based on minerals in the calcic skarn, vein, and Carlin-style deposits. Pyrite and base metal sulfides contain magmatic sulfur, whereas As, Sb, and Te minerals contain more country rock sulfur. Data are shown relative to sulfides in the Pedawan Formation and Serian Volcanics and marine sulfate in Triassic and Tertiary time. Abbreviations: BM = base metal, Cal = calcite, Calc = calc-silicate rock, CDT = Canyon Diablo Troilite, gal = galena, Jasp = jasperoid, Ls = limestone, Qtz/qtz = quartz, PDB = Pee Dee belemnite, PMW = primary magmatic water, Po = pyrrhotite, Py = pyrite, Ser = sericite, SMOW = standard mean ocean water, Sph/sph = sphalerite, Stib/stib = stibnite.

Carbon and oxygen: Our sample of fresh Bau Limestone (BA-46; Table 4) yielded a δ18O value of 23.6‰ and a δ13C value of 0.1‰ that are within the range of values for Bau Limestone reported by Kakisaki et al. (2013); these data are characteristic of normal marine limestone (Fig. 12C; Veizer and Hoefs, 1976). The δ13C and δ18O values of fluid inclusion CO2 and H2O extracted from sphalerite plot in the carbonatite box, and fluid extracted from galena plots between the carbonatite box and Bau Limestone. The calculated composition of fluid in equilibrium with calcite (from a base metal vein) at temperatures of 175° to 250°C plots between sphalerite and galena. The δ13C values for CO2 extracted from quartz and the calculated δ18O composition of water in equilibrium with quartz define a field that overlaps the carbonatite box and fluid extracted from sphalerite. Fluid inclusions in drusy quartz plot between the carbonatite box (Hoefs, 1987) and local meteoric water. Calculated fluid compositions for late calcite overlap the carbonatite box at 200°C or local meteoric water at 50°C (Fig. 12C; Stephens and Rose, 2005). Given that fluid inclusion water extracted from late calcite has δD and δ18O values that plot on the meteoric water line, we surmise that it formed at low temperature (e.g., 50°C). The lowest δ13C values of local meteoric water (Fig. 12C) indicate that some of the CO2 was produced by the oxidation of organic matter, which is common in the tropics. Overall, the δ13C and δ18O data indicate that ore fluids consisted of magmatic fluids that evolved by reactions with carbonate rocks. Incursion of local meteoric ground water is only evident in late calcite and fluid inclusion extracts from drusy quartz.

Sulfur: Diagenetic pyrite laminae in one sample of Cretaceous Pedawan Formation siltstone yielded a δ34S value of 7.0‰. The Bau Limestone was deposited in a shallow marine environment and generally lacks diagenetic pyrite. The isotopic composition of a mixture of chalcopyrite, sphalerite, and galena in underlying Serian volcanic rocks is 18.7‰ (Schuh, 1993). The δ34S values of pyrite and pyrrhotite in granodiorite porphyry range from –1.4 to 5.2‰ with a mean of 3.0‰. The isotopic composition of H2S in equilibrium with pyrite, sphalerite, and galena from porphyry, skarn, and polymetallic veins at 250° to 400°C ranges from 0.7 to 5.0‰ with a mean of 2.7‰. These data suggest that sulfur in the proximal base metal-rich deposits was largely derived from a magmatic source. The isotopic composition of H2S in equilibrium with stibnite and realgar from the vein and Carlin-style deposits at 150° to 250°C is 5 to 16‰. Tennantite, tetrahedrite, and tellurides from a variety of stages and proximal to distal positions in the system also have high sulfur values of 6.3 to 10.4‰. These data suggest that sulfur in certain base metal stages and distal positions in the system was derived from the overlying Pedawan Formation or underlying Serian volcanic rocks or was produced by reduction of marine sulfate (Fig. 12). In contrast, H2S in equilibrium with disseminated pyrite and arsenopyrite from Carlin-style mineralization at 150° to 250°C has lower δ34S values of –2.3 to –1.5‰ (Fig. 12D), which may be due to oxidation of magmatic H2S (Ohmoto and Rye, 1979). The preferential occurrence of country rock S in As, Sb, and Te minerals may indicate that the concentration of H2S in magmatic ore fluids was less than the concentration of base metals (Fe, Zn, Cu, Zn, and Pb) and semimetals (As, Sb, and Te). If magmatic H2S was consumed by precipitation of base metal sulfides, then As, Sb, and Te minerals may have precipitated where H2S-depleted fluids reacted with reduced sulfur derived from the country rocks.

Discussion

Hydrothermal processes deduced from mineralogy, geochemistry, and fluid inclusions

The change in mineralogy of the ores from the proximal skarns to the distal Carlin-style deposits has important implications for the process of ore formation. The occurrence of pyrrhotite, pyrite, arsenopyrite, native antimony, and stibnite in skarn; stibnite and arsenopyrite in veins; and stibnite, arsenopyrite, pyrite, arsenian pyrite, native arsenic, and realgar in the Carlin-style deposits is indicative of an overall decrease in temperature and sulfidation state of the hydrothermal fluids over time in each deposit type (Fig. 13). Since Au transport in porphyry-epithermal systems is largely as sulfide complexes, the consumption of H2S associated with precipitation of the aforementioned sulfide minerals would have been an effective gold precipitation mechanism.

Fig. 13.

Temperature vs. log fS2 diagram (adapted from Hofstra and Cline, 2000) showing the inferred cooling and desulfidation path (green arrow) based upon the minerals present in calcic skarn, vein, and Carlin-style deposits. See text for further explanation.

Fig. 13.

Temperature vs. log fS2 diagram (adapted from Hofstra and Cline, 2000) showing the inferred cooling and desulfidation path (green arrow) based upon the minerals present in calcic skarn, vein, and Carlin-style deposits. See text for further explanation.

The strong introduction of Fe, Mn, Zn, Pb, and Ag together with the Carlin suite (Au, As, Sb, Hg, and Tl) of elements requires explanation. Chemical modeling suggests that precipitation of gold with pyrite, sphalerite, and galena can occur where a saline fluid containing Fe, Zn, and Pb mixes with an H2S-bearing fluid containing Au (Hofstra, 1994). In the context of a porphyry Cu system, such mixing could occur where condensed magmatic vapor containing Au, Ag, As, Sb, Hg, and Tl mixes with brine containing Fe, Mn, Zn, and Pb (Heinrich, 2005). This interpretation is supported by the detection of Cu, Sn, and Te in sphalerite, arsenopyrite, realgar, and stibnite as well as In and Te in native arsenic, which suggests that these minerals formed from magmatic fluids (Halley et al., 2015). Alternatively, this assemblage of elements could also be introduced by cooling and neutralization of a single, hot, saline, S-bearing magmatic fluid containing all of these elements (Reed et al., 2013).

Considering that the salinity of aqueous fluids exsolved from porphyry intrusions is typically between 13 to 2 wt % (Richards, 2011), the high- and low-salinity data arrays likely reflect brine-vapor immiscibility and condensation of vapor. This interpretation is supported by the phase ratios and microthermometry of fluid inclusions described above. Both the high- and low-salinity data arrays record a decrease in temperature from about 350° to 200°C as pressure decreased from about 150 to 20 bar (Fig. 11). The population of brine inclusions was trapped on the liquid side of the solvus in the H2O-NaCl system. The low-salinity population may represent condensed vapor or contraction of a low-salinity fluid exsolved from magma (i.e., Heinrich, 2005). Homogenization temperatures of fluid inclusions in quartz veins that crosscut the Bau calcic skarns range from 220° to 370°C (Fig. 11), suggesting that overprinting by quartz veins began at or just below the minimum temperature of calc-silicate formation (~400°C) and continued within the lower range of temperatures found for the vein and replacement deposits. The average homogenization temperature for inclusions in quartz in type 1 veins is approximately 265°C (Fig. 11). Therefore, the type 1 veins formed from lower-temperature and probably less saline fluids than the calcic skarns but at temperatures slightly higher than those for quartz in replacement deposits (~225°C; Fig. 11; Table 3). Most of the fluid inclusions within the microcrystalline jasperoidal quartz were not measurable because of their small size. Many of those that could be measured, however, homogenized below 200°C (Fig. 11). In general, the results show that texturally similar types of quartz from the different deposits within the district have fluid inclusions that yield similar ranges of homogenization temperatures. Although there is evidence for temperature and salinity oscillations in some deposits, the data document an overall decrease in homogenization temperature and salinity with increasing distance from igneous intrusions.

Reconnaissance fluid inclusion studies reported by Schuh (1993) on various ore types in the district yielded a similar pattern: porphyry Cu-Mo-(Au) stockwork (422°–508°C; 25–38 wt % NaCl equiv); calcic skarn bodies and veins (331°–425°C; 14–31 wt % NaCl equiv); quartz-calcite vein deposits (240°–341°C; 2–10 wt % NaCl equiv); base metal veins (272°–310°C; 9–14 wt % NaCl equiv); siliceous and disseminated Carlin-style ores (174°–292°C; 0.8–6.4 wt % NaCl equiv); arsenical ores (122°–240°C; 0–5 wt % NaCl equiv); and the distal shale-hosted disseminated ore at the Jugan deposit (85°–162°C; 0–3 wt % NaCl equiv). These data ranges are shown relative to our results in Figure 11. The highest temperature and salinity data from the porphyry stockwork mineralization was trapped on the brine side of the solvus at a pressure of about 400 bar, which corresponds to a minimum depth of about 1.6 km under lithostatic conditions or a maximum depth of 4 km under hydrostatic conditions. Data from skarn and vein deposits reflect mixing between brine and low-salinity liquid. Together with our results, the pattern of fluid inclusion data at Bau is remarkably similar to the mixing arrays between brine, low-salinity liquid, and meteoric ground water documented in other porphyry-epithermal systems (Heinrich, 2005).

To ascertain the source of salt in hydrothermal fluids, the Na/Cl and Cl/Br ratios of fluid inclusion extracts from sphalerite (BA-56), stibnite (BA-32, Bau-2, Bau-Cc + Stb), and quartz (BA-42) in vein and Carlin-style deposits were determined by Hofstra and Emsbo (2005). Fluid inclusion extracts from sphalerite and stibnite have Na/Cl (0.72–0.99) and Cl/Br (950–1,400) ratios that plot within the field of diverse magmatic hydrothermal deposits (Hofstra and Emsbo, 2005), whereas an extract from late drusy quartz has Na/Cl (0.97) and Cl/Br (220) ratios that are typical of sodium bicarbonate fluids in geothermal systems (Ellis and Mahon, 1977; Hedenquist, 1990). The extracts from sulfides confirm that the salt in ore fluids was derived from magmas, whereas those from quartz are consistent with condensation of CO2-rich vapor into groundwater.

Interpreted evolution of the Bau hydrothermal system

The evidence presented in this report substantiates claims made by previous workers that Carlin-style deposits in the Bau district are distal manifestations of a Cu-Mo porphyry system. The hot, saline fluids responsible for calcic skarn formation contained magmatic H2O and CO2 that were shifted to higher δ18O and δ13C values by exchange with the Bau limestone. The vein and Carlin-style deposits formed from low-salinity fluids with magmatic δD, δ18O, and δ13C values under progressively cooler conditions. Influx of meteoric water into the system is only recorded by fluid inclusions in paragenetically late quartz and calcite. Although proximal iron sulfides contain magmatic sulfur, influx of country rock sulfur is indicated by the δ34S values of As, Sb, and Te minerals in the vein and replacement deposits. Nevertheless, the abundance of Cu and Te in As and Sb minerals suggests that As and Sb were derived from a magmatic source. The cause of the low abundance of Cu, Mo, Zn, Pb, and Ag and the high abundance of Au, As, and Sb in this system is poorly understood and merits further study.

Postcollisional, hydrous, oxidized, adakite porphyry intrusions (Fig. 1) at the western end of the Central Kalimantan magmatic arc were emplaced in response to repeated dilation of a deep-penetrating, NE-striking fracture zone between 15 and 6 Ma. Heat from these intrusions produced contact metamorphic halos in the Pedawan Formation and Bau Limestone consisting of calc-silicate hornfels and marble. If pyrite was converted to pyrrhotite in these haloes, it may have been a source for sedimentary H2S with high δ34S values. Intrusions emplaced at about 10 Ma exsolved hydrothermal fluids that moved upward and outward along dilatant high-angle faults (e.g., Krian fault) and laterally along the axis of the SW-trending Bau anticline through permeable strata and especially along shale-limestone contacts. As these fluids unmixed into brine and vapor at depths of 4 to 1.6 km and reacted with porphyry, limestone, and overlying carbonaceous pyritic silica-clastic rocks, they produced subeconomic porphyry Cu-Mo mineralization and an array of Au-, As-, and Sb-enriched calcic skarn, base metal vein, distal vein, and Carlin-style deposits and remote mercury deposits. From central to distal deposits, fluids cooled from about 500° to less than 100°C, salinities decreased from 38 to 0 wt % NaCl equiv, and magmatic brine mixed with condensed magmatic vapor and local meteoric water as the system waned. The transition from calcic skarn to quartz and calcite occurred at intermediate temperatures and can be attributed to proximal decarbonation reactions and distal cooling. Cooling promoted dissociation of acid volatiles, which resulted in formation of sericite and dissolution of carbonate. Magmatic fluids that cooled in contact with carbonaceous pyritic rocks in the Pedawan Formation were buffered to reduced H2S-rich compositions. Cooling and mixing between oxidized magmatic brine and reduced fluids may have led to precipitation of base metal sulfides. Gold-bearing arsenopyrite and pyrite in vein and replacement deposits also likely precipitated by fluid mixing and, to a lesser extent, sulfidation of Fe-bearing minerals in wall rocks. Cooling of fluids in the Bau limestone led to the formation of quartz veins and jasperoid containing native arsenic, stibnite, and realgar. Incursion of reduced S and meteoric water from adjacent country rocks took place during later stages as Te, As, and Sb minerals, drusy quartz, and calcite precipitated. Subsequent uplift and erosion of the epithermal part of this system resulted in deep weathering of sulfide ores, argillic alteration, and formation of oxide ores that are amenable to gold recovery by cyanide heap leach methods.

Comparison with Nevada Carlin-type gold deposits

To facilitate comparisons with Carlin-type and Carlin-style gold districts around the world, the geologic, geochemical, fluid inclusion, and stable isotopic attributes of ore deposits in the Bau district are summarized in Table 5. The Bau Carlin-style deposits differ from Nevada Carlin-type deposits (Hofstra and Cline, 2000; Cline et al., 2005), because they fit into a concentric pattern of alteration and mineralization produced by steep chemical and thermal gradients around exposed porphyry intrusions. Sericitic alteration is more abundant than hypogene argillic alteration, which suggests that the fluids were higher in temperature and less acidic. Sphalerite is present in macroscopic amounts, arsenopyrite is more abundant than pyrite, and there is more native arsenic and less orpiment. Gold resides primarily in arsenopyrite instead of in arsenian pyrite. Together, these minerals suggest the deposit formed at a lower sulfidation state. Geo-chemically, there is clear evidence for the introduction of Fe and S (fluid mixing, pH increase, cooling) and less evidence for immobile Fe and introduced S (wall-rock sulfidation), which is characteristic of Carlin-type deposits. The ores have a wider range of Au/Ag ratios and contain more Ag, Pb, Zn, Mn, Sb, As, and Te and less Tl and Hg. Stibnite and realgar contain detectable concentrations of Cu, Sn, and Te that suggest a magmatic source. Isotopic data and fluid inclusions clearly show that H2O and CO2 were derived from magmas, whereas isotopic evidence for magmatic-hydrothermal fluids in Carlin-type deposits is equivocal (Cline and Hofstra, 2000). Although the ore grades are similar, the gold endowment of the Bau district is at the small end of the spectrum for Carlin-type deposits. The field and geochemical evidence for decalcification and silicification is similar, and the S isotope compositions of As and Sb sulfides indicate that reduced sulfur was derived from country rocks during the late-ore stage. Most importantly, if only the Carlin-style deposits in the Bau district had been exposed and studied, a magmatic connection would have been inferred.

Comparison with other distal disseminated gold deposits

The geology and characteristics of the Carlin-style deposits in the Bau district are most similar to those in other districts that are spatially and temporally related to epizonal intrusive centers (Table 5). We therefore conclude that the Carlin-style deposits in the Bau district are best classified as distal disseminated gold deposits (Hofstra and Cline, 2000). Such deposits include McCoy-Cove (Johnston et al., 2008), Lone Tree (Bloomstein et al., 1993; Theodore, 1998, 2000), Bald Mountain (Hitchborn et al., 1996; Nutt and Hofstra, 2007), Sterling, Mother Lode, and related deposits in southwestern Nevada (Noble et al., 1989; Castor and Weiss, 1992; Weiss, 1996), deposits in the Drum Mountains in western Utah (Nutt et al., 1991), Alšar in Macedonia (Percival and Radtke, 1994), Purísima Concepción in the Yaricocha district (Alvarez and Noble, 1988), and Santa Bárbara in the Huancavelica district of central Perú (Alvarez and Noble, 1988). A key difference between the distal disseminated gold deposits in these districts and Carlin-type gold deposits in Nevada is that they are interpreted to occur in the distal parts of porphyry systems (e.g., Sillitoe, 2010). Below we briefly describe aspects of the McCoy-Cove and Sterling-Mother Lode deposits, because they are similar to Bau and are thought to be related, respectively, to porphyry Cu and porphyry Mo systems.

Table 5.

Characteristics of the Bau Gold District in Borneo

SettingKalimantan subduction-related magmatic arc, postaccretion magmatism
District-scale structuresSouth side of an WNW-striking regional fault zone (Lupar line), at intersection of the ENE-striking Bau anticline and a NE-striking belt of Miocene intrusions
Level of exposureSubvolcanic, Cu-Mo stockwork fluid inclusion homogenization pressure of 400 bar corresponds to a depth of 1.6 km (lithostatic) or 4 km (hydrostatic)
Character of intrusionsPostcollisional, adakitic microtonalite to dacite porphyries
Age of intrusionsMagmatic belt 14.6 to 6.4 Ma, intrusions north of
 the ore deposits 11.6 to 9.3 Ma
Age of alterationSericite 10.4 Ma
Structural ore controlsNortheast faults, axis of east-northeast anticline, northwest faults, dike margins
Lithologic ore controlsPermeable clastic Krian member of the Bau Formation is preferentially mineralized, contact between brittle fractured Bau limestone and overlying Pedawan shale
Zoning of auriferous alteration and mineralizationProximal porphyry Cu-Mo stockwork with potassic alteration; to calcic skarn bodies and veins with aurostibnite, sarabauite, and kermesite; to carbonate and quartz veins with phyllic halos that are zoned laterally from base metal sulfides, to stibnite, arsenopyrite, native arsenic, and realgar; to distal Carlin-style disseminated replacement deposits with arsenopyrite, pyrite, jasperoid, calcite, barite, and cinnabar
Mineral constraintsAurostibite and sarabauite in calcic skarn ≤460° to 420°C, decreasing Fe in sphalerite from core to rim records cooling or increasing fs2, sequence of arsenic minerals records cooling and desulfidation of fluids, jasperoid textures >180° to 200°C
Au/Ag ratio0.01 to 100
Mass transfer in Carlin-style depositsDepletion of Ca, Mg, and Sr; immobile Ti, Al, and K; strong introduction of Si, Fe, S, As, Sb, Mn, Zn, Ag, and Au; lesser introduction of Cu, Pb, Te, Tl, and Hg
Mineral compositionsElevated Cu, Sn, Te, and In in stibnite, arsenopyrite, arsenic, and realgar suggests that Sb and As are derived from a magmatic source
Residence of goldNative grains ± solid solution in arsenopyrite and pyrite
Fluid inclusionsHypersaline, vapor-rich, and low-salinity liquid-rich, immiscible brine and vapor that condensed to liquid ± contracted to low-salinity liquid, cooling and decompression from 500°C and 400 bar in Cu-Mo stockwork to 150°C and 20 bar in distal disseminated gold mineralization
Source of saltSphalerite, stibnite are magmatic; drusy quartz is Na bicarbonate
Source of waterProximal magmatic water, exchange with carbonate country rocks, distal/late mixing with local meteoric water
Source of CO2Proximal magmatic CO2, exchange with lime-stone, distal mixing with local meteoric water
Source of H2SProximal magmatic, distal/late country rocks
ProcessesCooling, decompression, phase separation, fluid mixing, decarbonation and dissolution of lime-stone, iron sulfide precipitation by cooling, pH increase, fluid mixing, and sulfidation of country rocks
System typeCu-Mo-Au porphyry-epithermal unusually enriched in As and Sb
Carlin-style gold deposit typeDistal disseminated
SettingKalimantan subduction-related magmatic arc, postaccretion magmatism
District-scale structuresSouth side of an WNW-striking regional fault zone (Lupar line), at intersection of the ENE-striking Bau anticline and a NE-striking belt of Miocene intrusions
Level of exposureSubvolcanic, Cu-Mo stockwork fluid inclusion homogenization pressure of 400 bar corresponds to a depth of 1.6 km (lithostatic) or 4 km (hydrostatic)
Character of intrusionsPostcollisional, adakitic microtonalite to dacite porphyries
Age of intrusionsMagmatic belt 14.6 to 6.4 Ma, intrusions north of
 the ore deposits 11.6 to 9.3 Ma
Age of alterationSericite 10.4 Ma
Structural ore controlsNortheast faults, axis of east-northeast anticline, northwest faults, dike margins
Lithologic ore controlsPermeable clastic Krian member of the Bau Formation is preferentially mineralized, contact between brittle fractured Bau limestone and overlying Pedawan shale
Zoning of auriferous alteration and mineralizationProximal porphyry Cu-Mo stockwork with potassic alteration; to calcic skarn bodies and veins with aurostibnite, sarabauite, and kermesite; to carbonate and quartz veins with phyllic halos that are zoned laterally from base metal sulfides, to stibnite, arsenopyrite, native arsenic, and realgar; to distal Carlin-style disseminated replacement deposits with arsenopyrite, pyrite, jasperoid, calcite, barite, and cinnabar
Mineral constraintsAurostibite and sarabauite in calcic skarn ≤460° to 420°C, decreasing Fe in sphalerite from core to rim records cooling or increasing fs2, sequence of arsenic minerals records cooling and desulfidation of fluids, jasperoid textures >180° to 200°C
Au/Ag ratio0.01 to 100
Mass transfer in Carlin-style depositsDepletion of Ca, Mg, and Sr; immobile Ti, Al, and K; strong introduction of Si, Fe, S, As, Sb, Mn, Zn, Ag, and Au; lesser introduction of Cu, Pb, Te, Tl, and Hg
Mineral compositionsElevated Cu, Sn, Te, and In in stibnite, arsenopyrite, arsenic, and realgar suggests that Sb and As are derived from a magmatic source
Residence of goldNative grains ± solid solution in arsenopyrite and pyrite
Fluid inclusionsHypersaline, vapor-rich, and low-salinity liquid-rich, immiscible brine and vapor that condensed to liquid ± contracted to low-salinity liquid, cooling and decompression from 500°C and 400 bar in Cu-Mo stockwork to 150°C and 20 bar in distal disseminated gold mineralization
Source of saltSphalerite, stibnite are magmatic; drusy quartz is Na bicarbonate
Source of waterProximal magmatic water, exchange with carbonate country rocks, distal/late mixing with local meteoric water
Source of CO2Proximal magmatic CO2, exchange with lime-stone, distal mixing with local meteoric water
Source of H2SProximal magmatic, distal/late country rocks
ProcessesCooling, decompression, phase separation, fluid mixing, decarbonation and dissolution of lime-stone, iron sulfide precipitation by cooling, pH increase, fluid mixing, and sulfidation of country rocks
System typeCu-Mo-Au porphyry-epithermal unusually enriched in As and Sb
Carlin-style gold deposit typeDistal disseminated

McCoy and Cove: The McCoy and Cove deposits, located approximately 25 km south of the Copper Canyon gold-copper porphyry-skarn complex, are spatially associated and genetically related to the Eocene (41.5–39 Ma) Brown stock of porphyritic granodiorite to tonalite composition (Brooks et al., 1991; Kuyper et al., 1991; Johnston et al., 2008; Thompson et al., 2015). A younger (39 Ma) intrusive phase is an ilmenite-series (reduced) type that is interpreted to be temporally related to the concentrically zoned types of mineralization. These types include proximal calcic skarns (McCoy), an intermediate zone of Au-Ag–bearing, polymetallic (base metal-rich) vein deposits, and distal disseminated Carlin-style gold (Cove) mineralization (Johnston et al., 2008). Abundant dikes and sills cut folded Mesozoic carbonate and clastic rocks, fill structures, and occur as a NE-trending dike swarm along a structural corridor that extends from the Brown stock to the Cove deposit. At Cove, Carlin-style mineralization is focused within the broad, gently plunging, faulted, and brecciated Cove anticline. This zonal arrangement of deposits is analogous to that of the Bau district in both character and scale.

The Carlin-style mineralization at Cove is significantly larger in volume than either the skarn or the polymetallic vein ores (Johnson et al., 2008). Hydrothermal alteration is similar to that at Bau, including decarbonatization, silicification and jasperoid formation, and argillization. Mineralized rock contains disseminated pyrite, arsenian pyrite, and lesser amounts of arsenopyrite and includes veinlets and veins of pyrite and/or marcasite. Erratically distributed veinlets, veins, and pods of realgar ± stibnite ± orpiment and realgar ± orpiment are present in the gold ores. Fluid inclusion homogenization temperatures of the polymetallic ores range from 208° to 371°C (avg 304°C). Hydrogen and oxygen isotope values indicate a magmatic source of the mineralizing fluids (Johnston et al., 2008). Sulfur isotope values establish a link between the 39 Ma intrusions and the sulfur-bearing ore minerals of the skarn, polymetallic vein, and Cove Carlin-style mineralization.

Sterling and Mother Lode: The Sterling, Mother Lode, and other small deposits surrounding Bare Mountain near Beatty, Nevada, are Carlin-style deposits temporally and spatially related to a Miocene (ca. 14.5–14.0 Ma) silicic porphyry dike swarm (Noble et al., 1989; Castor and Weiss, 1992; Weiss, 1996). Finely disseminated gold mineralization has high Au/Ag ratios (>1) in fine-grained pyrite, arsenian pyrite, and marcasite in both sedimentary rocks and igneous dikes. Gold is associated with anomalous levels of As, Sb, Hg, and Tl with elevated Mo, Bi, Te, and F and low concentrations of base metals. Mineralization is associated with decarbonatization, argillic, and illitic alteration and locally with adularia. Secondary hypersaline inclusions are present in quartz phenocrysts of altered dikes. The hydrothermal alteration and disseminated gold and fluorite mineralization throughout the district are slightly younger than the dike swarm based on K-Ar dates on illite-smectite (12.7 Ma) and adularia (12.9 Ma) (Weiss, 1996). Spatial, temporal, and geochemical relationships suggest the Carlin-style gold mineralization is the shallow expression of a concealed porphyry Mo system.

Conclusions

The evidence presented in this report shows that the Carlin-style gold deposits in the Bau district are spatially centered on and genetically related to porphyry intrusions and therefore are best classified as distal disseminated gold deposits. As in many other intrusion-related districts that contain distal disseminated gold deposits, the styles of alteration and mineralization at Bau vary from deposit to deposit with increasing distance from central intrusions and from early to late stages in the paragenesis. Establishing the genetic links between magmatism and distal disseminated gold mineralization at Bau is a significant contribution to the growing body of evidence that indicates that magmatism is fundamentally involved in the formation of many Carlin-style gold deposits. The detailed attributes of the distal disseminated gold deposits at Bau may lead to improved classifications of Carlin-style gold deposits in other districts around the world, including some for which other formative processes have been proposed.

Acknowledgments

Acknowledgments

Fieldwork and initial laboratory research in the Bau district from 1982 through 1984 was funded and supported by Blakeney Stafford, managing partner of Nassau Limited. This work was partially supported by the Development of Assessment Techniques Program at the U.S. Geological Survey. We are also thankful to Robert Oscarson, U.S. Geological Survey, for helping with scanning electron microscopy studies. Reviews by James Rytuba, Harold F. Bonham, Jr., A.C. Pimm, Jean Cline, Steve Garwin, David A. John, Jacob Margolis, and Mark Hannington of earlier versions of this manuscript were very helpful in clarifying several sections and are much appreciated. Finally, we would like to thank two reviewers for Economic Geology, Greg Corbett and Wolfram Schuh, and editor of this volume, John Muntean, for many thoughtful comments and suggestions that significantly improved the quality of this paper. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Fieldwork and initial laboratory research in the Bau district from 1982 through 1984 was funded and supported by Blakeney Stafford, managing partner of Nassau Limited. This work was partially supported by the Development of Assessment Techniques Program at the U.S. Geological Survey. We are also thankful to Robert Oscarson, U.S. Geological Survey, for helping with scanning electron microscopy studies. Reviews by James Rytuba, Harold F. Bonham, Jr., A.C. Pimm, Jean Cline, Steve Garwin, David A. John, Jacob Margolis, and Mark Hannington of earlier versions of this manuscript were very helpful in clarifying several sections and are much appreciated. Finally, we would like to thank two reviewers for Economic Geology, Greg Corbett and Wolfram Schuh, and editor of this volume, John Muntean, for many thoughtful comments and suggestions that significantly improved the quality of this paper. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

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APPENDIX

Descriptions of geochemically analyzed samples from the Bau district in Table 2.

  1. Near the Kusa deposit. Bluish-gray to gray, massive, finegrained, unaltered Bau Limestone. Very minor white calcite veinlets. Field no. BA-46C.

  2. Tai Ton deposit. Jasperoidal ore composed of black micro-crystalline quartz containing finely disseminated native arsenic and possibly arsenopyrite and containing abundant vugs and voids lined with quartz and stibnite crystals. Field no. BA-32.

  3. Tai Ton deposit. Jasperoidal ore composed of gray-black, vuggy, microcrystalline quartz with finely disseminated native arsenic and minor arsenopyrite. Some vugs contain stibnite crystals and realgar as overgrowths on drusy quartz. Field no. BA-36.

  4. Bukit Young deposit. Jasperoidal ore consisting of silicified and brecciated Bau Limestone with abundant crosscutting quartz veins and veinlets. Rock is composed of gray to dark-gray, fine-grained microcrystalline quartz with minor pyrite, native arsenic, and a trace of stibnite. Cockade and crustiform quartz veins cut silicified groundmass and occur as open-space infillings in brecciated rock. Field no. C-1.

  5. Tai Ton deposit. Jasperoidal ore composed of gray to dark-gray microcrystalline quartz crosscut by abundant quartz microveinlets. Contains disseminated native arsenic, arsenopyrite, and pyrite. Vugs are lined with drusy quartz, minor stibnite, and limonite. Field no. C-4.

  6. Tai Ton deposit. Jasperoidal ore similar to sample 5 (C-4) except that it appears to contain less abundant ore minerals. Field no. F-3.

  7. Tai Ton deposit. Arsenical ore consisting of gray to black microcrystalline quartz replacing limestone. Contains abundant disseminated and coarse granular native arsenic. Vugs and voids are lined with drusy quartz and stibnite. Uncommon quartz veinlets contain traces of native arsenic and abundant stibnite. Brecciated areas are infilled with coarsely crystalline quartz. Field no. G-4.

  8. Kusa deposit. Arsenical ore consisting of dark-gray mixtures of native arsenic, microcrystalline quartz, and stibnite replacing limestone. Early siliceous replacement mineralization is overgrown by massive native arsenic and stibnite in association with drusy quartz and lesser realgar. Field no. BA-41.

  9. Lucky Hill deposit. Calcic skarn mineralization consisting of wollastonite, diopside, garnet, microcrystalline quartz, stibnite, and sarabauite crosscutting bleached and weakly altered Bau Limestone. Field no. BA-52.

  10. Lucky Hill deposit. Calcic skarn veining and replacement of weakly quartz and calcite veined Bau Limestone. The calc-silicate minerals consist primarily of wollastonite, vesuvianite, and garnet associated with stibnite and sarabauite and containing crosscutting microcrystalline quartz veins and drusy quartz-lined vugs. Late calcite fills open spaces. Field no. LH-10.

  11. Tai Ton deposit. Jasperoidal ore composed of gray to black microcrystalline quartz containing finely disseminated native arsenic, arsenopyrite, and minor pyrite. Vuggy with drusy quartz, stibnite, and minor realgar lining open spaces. Field no. BA-34.

  12. Bukit Young deposit. Siliceous ore less intensely silicified than jasperoidal ores; oxidized to tan and yellow-brown, relatively soft. Consists of a mixture of replacement silica, argillic clay, and secondary iron oxides. Concentrations of disseminated and localized iron oxide (limonite > hematite) are pervasive, and quartz microveinlets are common. Field no. C-3.

  13. Bukit Young deposit. Siliceous ore, very similar to sample 12 (C-3); however, containing more abundant replacement silicification and quartz veining. Field no. D-1.

  14. Bukit Young deposit. Tan to light-brown, soft, porous clay. Typically extremely vuggy and porous with ubiquitous limonite lining and filling open spaces. Variably developed silicification. Field no. BA-3.

  15. Gunong Kolong Bau deposit. Reddish-brown to yellow-brown, soft, somewhat friable argillized, and weakly silicified clastic sedimentary rock. Original rock was calcareous sandy siltstone of the Krian Member of the Bau Formation. Ore is thoroughly oxidized to secondary iron and black manganese (?) oxides and contains fine networks of quartz veinlets and weak stockworks and patches of silica of replacement origin. Field no. BA-22.

  16. Gunong Kolong Bau deposit. Similar to sample 15 (BA-22). Field no. BA-27.

  17. Tai Ton deposit. Dark-brown to black soft, friable clay. Secondary clay is intermixed with significant amounts of manganese and limonitic iron oxides. Field no. BA-33.

  18. Tai Ton deposit. Argillic clay altered and variably silicified Bau Formation. Intensely developed porous and friable iron oxides (locally a gossan) mixed with clays and lesser amounts of microcrystalline quartz containing vugs lined with iron oxides. Field no. BA-35.

  19. Jambusan Road. Dark-gray to black, thinly bedded and laminated carbonaceous shale and mudstone of the Pedawan Formation, containing subordinate thin carbonaceous siltstone interbeds. Rare, fine-grained pyrite occurs in the carbonaceous shale. Field no. BA-47.

  20. Bukit Young deposit. Gray to dark-gray microcrystalline quartz replacement of Bau Formation. Fine-grained disseminated native arsenic, with subordinate arsenopyrite and stibnite. Fractured and brecciated with quartz linings showing cockade textures with stibnite, arsenopyrite, and rare realgar and containing late calcite infillings. Field no. BA-39.

  21. Rumoh deposit. Calcite vein comprised of coarse, dark-gray to black calcite crosscut by later colorless to white calcite in fractures and open spaces in association with minor amounts of crystallized quartz and yellow-brown iron and black manganese oxides. Field no. BA-7.

  22. Rumoh deposit. Quartz-calcite iron oxide vein composed of tan to gray, microcrystalline quartz, minor calcite, and abundant soft, friable, dark-brown to black iron and manganese oxides. Clay is abundant. Field no. BA-13.

  23. Rumoh deposit. Quartz-calcite vein, similar to BA-13, containing dark-brown to black soft, friable clay intermixed with significant amounts of manganese and limonitic iron oxides. Field no. BA-14.

  24. Rumoh deposit. Quartz-calcite vein composed of gray to tan microcrystalline quartz associated with minor white calcite. Clay with secondary limonite and manganese oxides is abundant. Field no. BA-15.

  25. Kojak deposit. Quartz-calcite vein with abundant stibnite. Oxidized areas consist of secondary clays, antimony, and iron oxides. Field no. BA-19.

  26. Bukit Young deposit. Argillized dike consisting of tan to grayish-white kaolinite and montmorillonite with dispersed pyrite. Rock is moderately silicified and contains abundant vugs lined with drusy quartz. Field no. BA-4.

  27. Gunong Kapor, Bidi area. Intensely argillized dike (?) consisting of dark-brown to black iron and manganese oxides mixed with secondary clays. Calcite veins and irregular massive replacements are common. Field no. BA-10.

Figures & Tables

Fig. 1.

A. Location map (inset) and geologic map of the Bau mining district, Sarawak, Malaysia. Geology modified from Wilford (1955). B. Schematic cross section (modified after Percival et al., 1990) showing the general spatial geometry of the various styles of alteration and mineral deposits in the Bau district relative to porphyritic igneous intrusions.

Fig. 1.

A. Location map (inset) and geologic map of the Bau mining district, Sarawak, Malaysia. Geology modified from Wilford (1955). B. Schematic cross section (modified after Percival et al., 1990) showing the general spatial geometry of the various styles of alteration and mineral deposits in the Bau district relative to porphyritic igneous intrusions.

Fig. 2.

Geologic map of the Bau district showing the spatial distribution of deposit types discussed in this paper. The deposits define a district-wide zoning pattern from porphyry-hosted base metal mineralization at the core to an outer zone of sedimentary rock-hosted gold deposits. The geologic formations are the same as in Figure 1.

Fig. 2.

Geologic map of the Bau district showing the spatial distribution of deposit types discussed in this paper. The deposits define a district-wide zoning pattern from porphyry-hosted base metal mineralization at the core to an outer zone of sedimentary rock-hosted gold deposits. The geologic formations are the same as in Figure 1.

Fig. 3.

Mineral paragenesis of deposits in the Bau district.

Fig. 3.

Mineral paragenesis of deposits in the Bau district.

Fig. 4.

Photomicrographs and scanning electron microscopy (SEM) images of calcic skarn, vein, and siliceous replacement ores. A. Backscattered electron (BSE) image of calcic skarn gold ore showing native antimony (SB, N) grain intergrown with wollastonite (WO), andradite, quartz (QZ), and calcite (CC). B. BSE image of calcic skarn ore showing aurostibite (ast) in a goethite (go) surrounded by quartz (qz); two grains of silver-bearing copper-lead antimony sulfosalt (tetrahedrite? [th]) also occur in the vug. C. Transmitted light image of calcic skarn ore from the Lucky Hill deposit showing acicular wollastonite (wo) in contact with prismatic sarabauite (black; sab). D. BSE image of euhedral arsenopyrite in a starburst-type pattern; arsenopyrite is enveloped by microcrystalline quartz (qz) associated with anhedral and massive stibnite (white). E. BSE image showing a close-up view of compositional zoning developed in arsenopyrite (asp) of Figure 4D; antimony-bearing core (sb), quartz (qz), stibnite (stib), and the compositional zone boundary (zb) are shown in detail in this electron micrograph. F. BSE image of siliceous replacement ore showing botryoidal, concentrically banded, disseminated grains of native arsenic (as) in microcrystalline quartz (QZ).

Fig. 4.

Photomicrographs and scanning electron microscopy (SEM) images of calcic skarn, vein, and siliceous replacement ores. A. Backscattered electron (BSE) image of calcic skarn gold ore showing native antimony (SB, N) grain intergrown with wollastonite (WO), andradite, quartz (QZ), and calcite (CC). B. BSE image of calcic skarn ore showing aurostibite (ast) in a goethite (go) surrounded by quartz (qz); two grains of silver-bearing copper-lead antimony sulfosalt (tetrahedrite? [th]) also occur in the vug. C. Transmitted light image of calcic skarn ore from the Lucky Hill deposit showing acicular wollastonite (wo) in contact with prismatic sarabauite (black; sab). D. BSE image of euhedral arsenopyrite in a starburst-type pattern; arsenopyrite is enveloped by microcrystalline quartz (qz) associated with anhedral and massive stibnite (white). E. BSE image showing a close-up view of compositional zoning developed in arsenopyrite (asp) of Figure 4D; antimony-bearing core (sb), quartz (qz), stibnite (stib), and the compositional zone boundary (zb) are shown in detail in this electron micrograph. F. BSE image of siliceous replacement ore showing botryoidal, concentrically banded, disseminated grains of native arsenic (as) in microcrystalline quartz (QZ).

Fig. 5.

Photographs of outcrops and rock samples from the Bau district. A. Massive Bau Limestone outcrop with large quartz + calcite veins (type 1) exposed along the cliff face with abundant iron and manganese oxides. Note several mine adits exposed in the center and lower half of the photo. B. Shallowly dipping altered microgranodiorite intrusion crosscutting altered and variably mineralized sediments of the Krian Member of the Bau Formation at Bukit Young mine area. C. Hand sample of massive Bau Limestone with white calcite veinlets. D. Wollastonite skarn with red sarabauite and minor reddish-brown supergene kermesite from the Lucky Hill mine. E. Partially oxidized botryoidal native arsenic from the Bidi mine. F. Cut slab of botryoidal native arsenic on stibnite-bearing drusy quartz and jasperoid from the Bidi mine. G. Arsenical ore with botryoidal native arsenic, stibnite, drusy quartz, and realgar from the Bidi mine. H. Type 1 coarse calcite vein with large stibnite crystals.

Fig. 5.

Photographs of outcrops and rock samples from the Bau district. A. Massive Bau Limestone outcrop with large quartz + calcite veins (type 1) exposed along the cliff face with abundant iron and manganese oxides. Note several mine adits exposed in the center and lower half of the photo. B. Shallowly dipping altered microgranodiorite intrusion crosscutting altered and variably mineralized sediments of the Krian Member of the Bau Formation at Bukit Young mine area. C. Hand sample of massive Bau Limestone with white calcite veinlets. D. Wollastonite skarn with red sarabauite and minor reddish-brown supergene kermesite from the Lucky Hill mine. E. Partially oxidized botryoidal native arsenic from the Bidi mine. F. Cut slab of botryoidal native arsenic on stibnite-bearing drusy quartz and jasperoid from the Bidi mine. G. Arsenical ore with botryoidal native arsenic, stibnite, drusy quartz, and realgar from the Bidi mine. H. Type 1 coarse calcite vein with large stibnite crystals.

Fig. 6.

Thin section images of ore specimens from Bau. A. Cross-polarized transmitted light image of siliceous ore (jasperoid) comprised of interlocking quartz crystals containing inclusions of carbonate derived from the Bau limestone. Sample BA-36. Field of view is 1.2 mm. B. Scanning electron microscopy (SEM) backscatter image of jasperoidal quartz (dark gray) containing small arsenopyrite (Aspy) crystals and one sphalerite (Sph) crystal that is mantled by drusy quartz (Qtz) and native arsenic (As) with minor pyrite (Py) and later stibnite (Stib) and realgar (Real). Remaining void space is black. Sample BA-36. Field of view is 0.6 mm. C. Transmitted light image of jasperoidal quartz with disseminated arsenopyrite crystals (black), large translucent sphalerite crystals, and later drusy quartz. Sample BA-42a. Field of view is 1.2 cm. D. SEM backscatter image of jasperoidal quartz (black; qz) containing small white rhomb-shaped crystals of gold-bearing arsenopyrite (asp) and large sphalerite crystals (sphl) with inclusions of anhydrite (anh) and arsenopyrite. Sample BA-42a. Field of view is 0.6 mm. E. Reflected light image of siliceous ore (jasperoid and drusy quartz) with native arsenic (white) showing hexagonal growth zones. Sample BA-42a. Field of view is 0.6 mm. F. Cross-polarized transmitted light image of drusy quartz with a chalcedonic silica rim and dark realgar in siliceous ore. Sample BA-42b. Field of view is 0.6 mm.

Fig. 6.

Thin section images of ore specimens from Bau. A. Cross-polarized transmitted light image of siliceous ore (jasperoid) comprised of interlocking quartz crystals containing inclusions of carbonate derived from the Bau limestone. Sample BA-36. Field of view is 1.2 mm. B. Scanning electron microscopy (SEM) backscatter image of jasperoidal quartz (dark gray) containing small arsenopyrite (Aspy) crystals and one sphalerite (Sph) crystal that is mantled by drusy quartz (Qtz) and native arsenic (As) with minor pyrite (Py) and later stibnite (Stib) and realgar (Real). Remaining void space is black. Sample BA-36. Field of view is 0.6 mm. C. Transmitted light image of jasperoidal quartz with disseminated arsenopyrite crystals (black), large translucent sphalerite crystals, and later drusy quartz. Sample BA-42a. Field of view is 1.2 cm. D. SEM backscatter image of jasperoidal quartz (black; qz) containing small white rhomb-shaped crystals of gold-bearing arsenopyrite (asp) and large sphalerite crystals (sphl) with inclusions of anhydrite (anh) and arsenopyrite. Sample BA-42a. Field of view is 0.6 mm. E. Reflected light image of siliceous ore (jasperoid and drusy quartz) with native arsenic (white) showing hexagonal growth zones. Sample BA-42a. Field of view is 0.6 mm. F. Cross-polarized transmitted light image of drusy quartz with a chalcedonic silica rim and dark realgar in siliceous ore. Sample BA-42b. Field of view is 0.6 mm.

Fig. 7.

Photomicrographs and scanning electron microscopy (SEM) images of siliceous replacement and arsenical ores from the Bau district. A. Reflected light photomicrograph of siliceous replacement ore showing native arsenic grain exhibiting well-defined growth lines in a matrix to crystalline quartz (qz). B. Reflected light image of dark-gray to black native arsenic nuclei overgrown by drusy microcrystalline quartz in siliceous replacement ore. C. SEM image of oxidized brecciated siliceous replacement ore showing a native gold grain within a mixture of goethite (go) and quartz (qz). D. Backscattered electron image of arsenical ore showing sphalerite (SPHL), native arsenic (AS), and stibnite (STIB) grains enveloped by microcrystalline quartz (QZ). E. SEM image of arsenical ore showing native arsenic within microcrystalline quartz (QZ) matrix. Late-formed realgar (rel) fills open fractures. Arsenolite crystals (ars) are supergene oxidation products of arsenic minerals.

Fig. 7.

Photomicrographs and scanning electron microscopy (SEM) images of siliceous replacement and arsenical ores from the Bau district. A. Reflected light photomicrograph of siliceous replacement ore showing native arsenic grain exhibiting well-defined growth lines in a matrix to crystalline quartz (qz). B. Reflected light image of dark-gray to black native arsenic nuclei overgrown by drusy microcrystalline quartz in siliceous replacement ore. C. SEM image of oxidized brecciated siliceous replacement ore showing a native gold grain within a mixture of goethite (go) and quartz (qz). D. Backscattered electron image of arsenical ore showing sphalerite (SPHL), native arsenic (AS), and stibnite (STIB) grains enveloped by microcrystalline quartz (QZ). E. SEM image of arsenical ore showing native arsenic within microcrystalline quartz (QZ) matrix. Late-formed realgar (rel) fills open fractures. Arsenolite crystals (ars) are supergene oxidation products of arsenic minerals.

Fig. 8.

Scatter plots of rock sample data for selected elemental pairs. Concentrations reported at below detection limits are plotted at one-half the limit of detection.

Fig. 8.

Scatter plots of rock sample data for selected elemental pairs. Concentrations reported at below detection limits are plotted at one-half the limit of detection.

Fig. 9.

A. Logarithmic isocon plot (i.e., Hofstra and Cline, 2000) comparing the abundance of elements in unaltered Bau limestone to those in calcic skarn, arsenical, and silicified ores. The isocon for silicified ore is based on the immobility of Ti and Al. Elements that plot above the isocon are introduced, along the isocon are immobile, and below the isocon are depleted. The enrichment of immobile elements (Ti and Al) relative to the line of constant mass is due to the loss of Ca, Sr, and Mg. Values reported as less than detection are plotted at one-half the limit of detection. B, C. Element enrichment factors (relative to average crust) in gold ore from Bau (colored diamonds) compared to gold ore in 27 Carlin-type deposits in Nevada (black bars).

Fig. 9.

A. Logarithmic isocon plot (i.e., Hofstra and Cline, 2000) comparing the abundance of elements in unaltered Bau limestone to those in calcic skarn, arsenical, and silicified ores. The isocon for silicified ore is based on the immobility of Ti and Al. Elements that plot above the isocon are introduced, along the isocon are immobile, and below the isocon are depleted. The enrichment of immobile elements (Ti and Al) relative to the line of constant mass is due to the loss of Ca, Sr, and Mg. Values reported as less than detection are plotted at one-half the limit of detection. B, C. Element enrichment factors (relative to average crust) in gold ore from Bau (colored diamonds) compared to gold ore in 27 Carlin-type deposits in Nevada (black bars).

Fig. 10.

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) multielement profiles across the following minerals: A. sphalerite, arsenopyrite, and realgar (sample BA-42); B. native arsenic (alloy) (sample Bau-3); and C. stibnite (BA-36). D. Proportional box plot showing the abundance of Fe in sphalerite (sample BA-42). E. Au vs. As plot of LA-ICP-MS analyses of arsenopyrite (gray diamonds; samples BA-36, BA-42, G-4) and whole-rock analyses of mineralized rocks (yellow squares; Table 2). Except for calcic skarn, the array of whole-rock data appears to reflect the abundance of arsenopyrite in the rocks. Fields adapted from Large et al. (2011). Abbreviations: asp = arsenopyrite, real = realgar, sph = sphalerite.

Fig. 10.

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) multielement profiles across the following minerals: A. sphalerite, arsenopyrite, and realgar (sample BA-42); B. native arsenic (alloy) (sample Bau-3); and C. stibnite (BA-36). D. Proportional box plot showing the abundance of Fe in sphalerite (sample BA-42). E. Au vs. As plot of LA-ICP-MS analyses of arsenopyrite (gray diamonds; samples BA-36, BA-42, G-4) and whole-rock analyses of mineralized rocks (yellow squares; Table 2). Except for calcic skarn, the array of whole-rock data appears to reflect the abundance of arsenopyrite in the rocks. Fields adapted from Large et al. (2011). Abbreviations: asp = arsenopyrite, real = realgar, sph = sphalerite.

Fig. 11.

A. Histograms showing the homogenization temperatures of fluid inclusions in quartz and other minerals in samples from the various types of mineralization studied in the Bau district. Colorless area within histograms represents data from quartz. B. Salinity vs. homogenization temperature plot for fluid inclusions in the various types of mineralization at Bau. Data are shown relative to phase boundaries and isobars in the H2O-NaCl system (Atkinson, 2002). Abbreviations: H = halite, L = liquid, SRH = sedimentary rock-hosted, V = vapor.

Fig. 11.

A. Histograms showing the homogenization temperatures of fluid inclusions in quartz and other minerals in samples from the various types of mineralization studied in the Bau district. Colorless area within histograms represents data from quartz. B. Salinity vs. homogenization temperature plot for fluid inclusions in the various types of mineralization at Bau. Data are shown relative to phase boundaries and isobars in the H2O-NaCl system (Atkinson, 2002). Abbreviations: H = halite, L = liquid, SRH = sedimentary rock-hosted, V = vapor.

Fig. 12.

A. Histogram showing the distribution of δ18O values among the various mineralization types in the intrusive and carbonate host rocks at Bau. B. Inset, colored bars show the range of δ18O values for the Bau limestone (Kakisaki et al., 2013), hydrothermal minerals, and fluids in equilibrium with minerals at their respective temperatures of formation. δD vs. δ18O plot showing the compositions of calculated fluids and water extracted from fluid inclusions (FI) in minerals. The hydrogen isotope composition of water extracted from various minerals is indicated by the labeled ticks on the y-axis. The light-blue box shows the composition of fluids that produced the vein and Carlin-style deposits. Black arrows show how fluid compositions may have shifted in response to water/rock exchange (W/R) and fluid mixing. Data are shown relative to seawater (SW), the meteoric water line (MWL), local meteoric water (small gray boxes; Stephens and Rose, 2005), the primary magmatic water (PMW) box, and metamorphic water box. C. δ13C vs. δ18O plot showing the composition of Bau Limestone (large blue square, this study; small blue squares, Kakisaki et al., 2013), calcite in base metal veins (green squares), late calcite (yellow squares), fluid inclusions in galena (gray square), sphalerite (brown square), quartz (blue square), and local meteoric water (small gray squares). The δ13C of CO2 extracted from fluid inclusions in quartz is indicated by the white bar on the y-axis. The calculated δ18O of water in equilibrium with quartz is indicated by the white bar on the x-axis. The light-blue box shows the composition of fluids that produced the vein and Carlin-style deposits. Fluid in equilibrium with calcite in the base metal veins at the temperature of formation (green polygon) extends from the carbonatite box toward the Bau limestone. Fluid in equilibrium with late calcite (yellow polygon) at temperatures of 200° to 50°C extends from the carbonatite box toward local meteoric water (small gray squares; Stephens and Rose, 2005). Data are shown relative to the carbonatite box and marine field of typical marine limestones. D. Histogram showing the distribution of δ34S values among the various mineralization types in the intrusive and carbonate host rocks at Bau (based on data in Schuh, 1993). The blue bars show the isotopic composition of H2S in equilibrium with the minerals shown at their respective temperatures of formation. The lower blue bar, labeled magmatic, is based on pyrite and pyrrhotite from the stock. The upper blue bars are based on minerals in the calcic skarn, vein, and Carlin-style deposits. Pyrite and base metal sulfides contain magmatic sulfur, whereas As, Sb, and Te minerals contain more country rock sulfur. Data are shown relative to sulfides in the Pedawan Formation and Serian Volcanics and marine sulfate in Triassic and Tertiary time. Abbreviations: BM = base metal, Cal = calcite, Calc = calc-silicate rock, CDT = Canyon Diablo Troilite, gal = galena, Jasp = jasperoid, Ls = limestone, Qtz/qtz = quartz, PDB = Pee Dee belemnite, PMW = primary magmatic water, Po = pyrrhotite, Py = pyrite, Ser = sericite, SMOW = standard mean ocean water, Sph/sph = sphalerite, Stib/stib = stibnite.

Fig. 12.

A. Histogram showing the distribution of δ18O values among the various mineralization types in the intrusive and carbonate host rocks at Bau. B. Inset, colored bars show the range of δ18O values for the Bau limestone (Kakisaki et al., 2013), hydrothermal minerals, and fluids in equilibrium with minerals at their respective temperatures of formation. δD vs. δ18O plot showing the compositions of calculated fluids and water extracted from fluid inclusions (FI) in minerals. The hydrogen isotope composition of water extracted from various minerals is indicated by the labeled ticks on the y-axis. The light-blue box shows the composition of fluids that produced the vein and Carlin-style deposits. Black arrows show how fluid compositions may have shifted in response to water/rock exchange (W/R) and fluid mixing. Data are shown relative to seawater (SW), the meteoric water line (MWL), local meteoric water (small gray boxes; Stephens and Rose, 2005), the primary magmatic water (PMW) box, and metamorphic water box. C. δ13C vs. δ18O plot showing the composition of Bau Limestone (large blue square, this study; small blue squares, Kakisaki et al., 2013), calcite in base metal veins (green squares), late calcite (yellow squares), fluid inclusions in galena (gray square), sphalerite (brown square), quartz (blue square), and local meteoric water (small gray squares). The δ13C of CO2 extracted from fluid inclusions in quartz is indicated by the white bar on the y-axis. The calculated δ18O of water in equilibrium with quartz is indicated by the white bar on the x-axis. The light-blue box shows the composition of fluids that produced the vein and Carlin-style deposits. Fluid in equilibrium with calcite in the base metal veins at the temperature of formation (green polygon) extends from the carbonatite box toward the Bau limestone. Fluid in equilibrium with late calcite (yellow polygon) at temperatures of 200° to 50°C extends from the carbonatite box toward local meteoric water (small gray squares; Stephens and Rose, 2005). Data are shown relative to the carbonatite box and marine field of typical marine limestones. D. Histogram showing the distribution of δ34S values among the various mineralization types in the intrusive and carbonate host rocks at Bau (based on data in Schuh, 1993). The blue bars show the isotopic composition of H2S in equilibrium with the minerals shown at their respective temperatures of formation. The lower blue bar, labeled magmatic, is based on pyrite and pyrrhotite from the stock. The upper blue bars are based on minerals in the calcic skarn, vein, and Carlin-style deposits. Pyrite and base metal sulfides contain magmatic sulfur, whereas As, Sb, and Te minerals contain more country rock sulfur. Data are shown relative to sulfides in the Pedawan Formation and Serian Volcanics and marine sulfate in Triassic and Tertiary time. Abbreviations: BM = base metal, Cal = calcite, Calc = calc-silicate rock, CDT = Canyon Diablo Troilite, gal = galena, Jasp = jasperoid, Ls = limestone, Qtz/qtz = quartz, PDB = Pee Dee belemnite, PMW = primary magmatic water, Po = pyrrhotite, Py = pyrite, Ser = sericite, SMOW = standard mean ocean water, Sph/sph = sphalerite, Stib/stib = stibnite.

Fig. 13.

Temperature vs. log fS2 diagram (adapted from Hofstra and Cline, 2000) showing the inferred cooling and desulfidation path (green arrow) based upon the minerals present in calcic skarn, vein, and Carlin-style deposits. See text for further explanation.

Fig. 13.

Temperature vs. log fS2 diagram (adapted from Hofstra and Cline, 2000) showing the inferred cooling and desulfidation path (green arrow) based upon the minerals present in calcic skarn, vein, and Carlin-style deposits. See text for further explanation.

Table 1.

Comparison of Mineralogical and Chemical Features of the Three Main Styles of Gold Mineralization in the Bau District, Malaysia

 Replacement depositsVein depositsCalcic skarn and type 2 vein deposits
Mineralization
ReplacementCarbonate host rocks replaced by microcrystalline (jasperoidal) quartz with subordinate crystallized quartz filling open spaces and as breccia matrix cementCarbonate and clastic rocks replaced by microcrystalline (jasperoidal) quartz crosscut by veins and veinlet stockworks adjacent to mineralized areas.Calc-silicate (cs) minerals (wo, gr, an, vs, ep, and pl) occurring as masses and vein-like zones of calcic skarn associated with subordinate microcrystalline quartz
VeiningMinor quartz ± calcite veinlets and narrow veins (<1 cm) crosscutting silicified rocksAbundant quartz, quartz + calcite, and calcite as major veins (>1 m), vein zones, narrow (±1 cm) veins, veinlets, and stockworksQuartz, quartz + calcite, and calcite veins with and rarely without calc-silicate minerals
Ore mineralogyAs, St, Asp, Py, Rl, Orp (?), Sp, and AuSt, Asp, As, Py, Au ± Rl, and Orp (?)St, Asp, Sb, Aur, and Sar
 Trace Cu-Pb-Zn sulfides and possibly sulfosalt mineralsCu-Pb-Zn sulfides and sulfosalt mineralsCu-Pb-Zn sulfides and sulfosalt minerals
GeochemistryAs, Sb, Au, Zn; minor Cu and PbSb, As, Au, Cu, Pb, and ZnSb, As, Au, Cu, Pb, and Zn
As/Sb ratiosAs/Sb ratios increase distal to the porphyritic intrusionsVariable As/Sb ratiosAs/Sb ratios decrease proximal to the porphyritic intrusions
Primary gold associationAs + Sb + AuAs + Sb + AuAs + Sb + Au; Sb + Au
Fluid inclusions
Average Th220°C (qz)265°C (qz + cc + Sp)290°C (qz + cc + cs)
Salinity range2–4 wt % NaCl equiv1–10 wt % NaCl equiv18–38 wt % NaCl equiv
Stable isotopes (ranges)δ18O = 16.6–22.6‰ (qz + cc)δ18O = 18.2–22.6‰ (qz + cc)δ18O = 7.5–8.3‰ (cs)
δ13C = –5.1 to –6.1‰ (cc)δ13C = –2.4 to –7.2‰ (cc)δ18O = 19.0–20.2‰ (late qz)
δD H2o = –62 to –101‰ (qz + cc + St + As)δD H2O = –58‰ 
δ18OH2O = –5.4 to 4.3δ18OH2O = 9.5–13.9 
δ13CCO2 = –5.9 to –12.4δ13CCO2 = –2.4 to –7.1 
Deposit examplesTai ParitBukit YoungLucky Hill
Tai TonRumohGunung Bau
JambusanSaburanGunung A. Bukit
BoringKojok
Bidi
 Replacement depositsVein depositsCalcic skarn and type 2 vein deposits
Mineralization
ReplacementCarbonate host rocks replaced by microcrystalline (jasperoidal) quartz with subordinate crystallized quartz filling open spaces and as breccia matrix cementCarbonate and clastic rocks replaced by microcrystalline (jasperoidal) quartz crosscut by veins and veinlet stockworks adjacent to mineralized areas.Calc-silicate (cs) minerals (wo, gr, an, vs, ep, and pl) occurring as masses and vein-like zones of calcic skarn associated with subordinate microcrystalline quartz
VeiningMinor quartz ± calcite veinlets and narrow veins (<1 cm) crosscutting silicified rocksAbundant quartz, quartz + calcite, and calcite as major veins (>1 m), vein zones, narrow (±1 cm) veins, veinlets, and stockworksQuartz, quartz + calcite, and calcite veins with and rarely without calc-silicate minerals
Ore mineralogyAs, St, Asp, Py, Rl, Orp (?), Sp, and AuSt, Asp, As, Py, Au ± Rl, and Orp (?)St, Asp, Sb, Aur, and Sar
 Trace Cu-Pb-Zn sulfides and possibly sulfosalt mineralsCu-Pb-Zn sulfides and sulfosalt mineralsCu-Pb-Zn sulfides and sulfosalt minerals
GeochemistryAs, Sb, Au, Zn; minor Cu and PbSb, As, Au, Cu, Pb, and ZnSb, As, Au, Cu, Pb, and Zn
As/Sb ratiosAs/Sb ratios increase distal to the porphyritic intrusionsVariable As/Sb ratiosAs/Sb ratios decrease proximal to the porphyritic intrusions
Primary gold associationAs + Sb + AuAs + Sb + AuAs + Sb + Au; Sb + Au
Fluid inclusions
Average Th220°C (qz)265°C (qz + cc + Sp)290°C (qz + cc + cs)
Salinity range2–4 wt % NaCl equiv1–10 wt % NaCl equiv18–38 wt % NaCl equiv
Stable isotopes (ranges)δ18O = 16.6–22.6‰ (qz + cc)δ18O = 18.2–22.6‰ (qz + cc)δ18O = 7.5–8.3‰ (cs)
δ13C = –5.1 to –6.1‰ (cc)δ13C = –2.4 to –7.2‰ (cc)δ18O = 19.0–20.2‰ (late qz)
δD H2o = –62 to –101‰ (qz + cc + St + As)δD H2O = –58‰ 
δ18OH2O = –5.4 to 4.3δ18OH2O = 9.5–13.9 
δ13CCO2 = –5.9 to –12.4δ13CCO2 = –2.4 to –7.1 
Deposit examplesTai ParitBukit YoungLucky Hill
Tai TonRumohGunung Bau
JambusanSaburanGunung A. Bukit
BoringKojok
Bidi

Abbreviations: an = andradite garnet, As = native arsenic, Asp = arsenopyrite, Au = native gold, Aur = aurostibite, cc = calcite, cs = calc-silicate minerals, ep = epidote family minerals, gr = garnet, Orp = orpiment, pl = plagioclase feldspar, Py = pyrite, qz = quartz, Rl = realgar, Sar = sarabauite, Sb = native antimony, Sp = sphalerite, St = stibnite, vs = vesuvianite, wo = wollastonite

Table 2.

Major and Minor Element Data for Selected Specimens from the Bau District, Malaysia

No.TypeField no.Si %Al %Fe %Mg %Ca %Na %K %Ti %Mn %P %F %
Type of analysis
 USGS/Hunter Laboratory--GICPGICPGICPGICPGICPGICPGICPGICPGICPGSP
 ALS ChemexXRFXRF/ICPXRF/ICPXRF/ICPXRF/ICPXRF/ICPXRF/ICPIMS/XRFXRF/ICPXRF/ICP--
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.220.140.010.0942.13<0.010.02<0.010.00150.001--
2SBA-32--0.60.80.010.00.020.10.020.008<0.01<0.01
3SBA-36--0.50.30.012.00.010.1<0.010.090<0.010.01
 SBA-3636.60.400.300.012.140.010.080.010.0160.006--
4SC-1--2.21.80.142.40.090.70.060.240.030.01
5SC-4--3.61.00.090.70.040.30.160.0100.020.01
6SF-3--3.85.30.114.80.060.50.130.610.050.02
7ASG-4--0.50.40.010.10.010.10.020.0050.01<0.01
8ASBA-4119.60.330.290.042.68<0.010.10<0.010.170.003--
9CSBA-5216.20.280.100.1726.620.010.02<0.010.500.012--
10CSLH-1014.90.050.380.1423.680.030.01<0.010.940.006--
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-34--3.16.80.103.70.450.40.110.850.050.02
12SC-3--5.04.60.132.30.080.70.170.390.050.02
13SD-1--5.24.80.312.30.091.60.160.430.090.03
14NBA-3--5.71.70.26110.142.30.100.0240.030.02
15NBA-22--9.313.00.130.20.161.50.650.0920.020.02
16NBA-27--5.914.00.130.10.300.60.372.10.040.01
17NBA-33--3.12.50.120.90.030.50.072.50.010.02
 NBA-3325.92.511.450.11110.040.760.120.970.007--
18NBA-35--9.63.70.080.20.030.60.160.0140.010.02
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-4722.57.796.473.902.691.945.831.100.290.087--
20SBA-3944.20.430.320.010.630.010.07<0.010.0120.001--
Vein ores
21VBA-7--0.030.040.06 <0.010.01<0.01>1<0.001--
22VBA-13--1.00.90.12340.010.10.050.680.00.01
23VBA-14--2.21.60.12350.040.20.070.330.10.02
24VBA-15--2.01.50.13310.020.10.081.40.00.01
25VBA-19--9.52.90.100.30.242.10.380.0060.0<0.01
 VBA-19--0.230.07<0.010.790.01<0.01<0.010.0170.003--
Intrusive bodies
26ZBA-4--9.91.40.120.10.851.60.130.0070.010.01
27ZBA-10--9.11.30.120.10.831.20.140.0020.010.01
No.TypeField no.Si %Al %Fe %Mg %Ca %Na %K %Ti %Mn %P %F %
Type of analysis
 USGS/Hunter Laboratory--GICPGICPGICPGICPGICPGICPGICPGICPGICPGSP
 ALS ChemexXRFXRF/ICPXRF/ICPXRF/ICPXRF/ICPXRF/ICPXRF/ICPIMS/XRFXRF/ICPXRF/ICP--
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.220.140.010.0942.13<0.010.02<0.010.00150.001--
2SBA-32--0.60.80.010.00.020.10.020.008<0.01<0.01
3SBA-36--0.50.30.012.00.010.1<0.010.090<0.010.01
 SBA-3636.60.400.300.012.140.010.080.010.0160.006--
4SC-1--2.21.80.142.40.090.70.060.240.030.01
5SC-4--3.61.00.090.70.040.30.160.0100.020.01
6SF-3--3.85.30.114.80.060.50.130.610.050.02
7ASG-4--0.50.40.010.10.010.10.020.0050.01<0.01
8ASBA-4119.60.330.290.042.68<0.010.10<0.010.170.003--
9CSBA-5216.20.280.100.1726.620.010.02<0.010.500.012--
10CSLH-1014.90.050.380.1423.680.030.01<0.010.940.006--
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-34--3.16.80.103.70.450.40.110.850.050.02
12SC-3--5.04.60.132.30.080.70.170.390.050.02
13SD-1--5.24.80.312.30.091.60.160.430.090.03
14NBA-3--5.71.70.26110.142.30.100.0240.030.02
15NBA-22--9.313.00.130.20.161.50.650.0920.020.02
16NBA-27--5.914.00.130.10.300.60.372.10.040.01
17NBA-33--3.12.50.120.90.030.50.072.50.010.02
 NBA-3325.92.511.450.11110.040.760.120.970.007--
18NBA-35--9.63.70.080.20.030.60.160.0140.010.02
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-4722.57.796.473.902.691.945.831.100.290.087--
20SBA-3944.20.430.320.010.630.010.07<0.010.0120.001--
Vein ores
21VBA-7--0.030.040.06 <0.010.01<0.01>1<0.001--
22VBA-13--1.00.90.12340.010.10.050.680.00.01
23VBA-14--2.21.60.12350.040.20.070.330.10.02
24VBA-15--2.01.50.13310.020.10.081.40.00.01
25VBA-19--9.52.90.100.30.242.10.380.0060.0<0.01
 VBA-19--0.230.07<0.010.790.01<0.01<0.010.0170.003--
Intrusive bodies
26ZBA-4--9.91.40.120.10.851.60.130.0070.010.01
27ZBA-10--9.11.30.120.10.831.20.140.0020.010.01
No.TypeField no.Li (ppm)Ba (ppm)Cs (ppm)Sr (ppm)Ga (ppm)Be (ppm)La (ppm)Ce (ppm)Nd (ppm)Y (ppm)Th (ppm)
Type of analysis            
 USGS/Hunter LaboratoryGICPGICP--GICPGICPGICPGICPGSGICPGICPGICP
 ALS ChemexIMSICPIMSICP/IMSIMSICP/IMSIMSIMS--IMSIMS
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.8<100.0512200.050.50.56--1.30.2
2SBA-321654--11<8<25<8<84<8
3SBA-361648--14<8<2<4<8<84<8
 SBA-36--30--11--<0.5----------
4SC-125170--43<8<2109<816<8
5SC-44377--28<8<217221120<8
6SF-336300--7110<221201429<8
7ASG-41545--8<8<2<4<8<84<8
8ASBA-41--<10--14--<0.5----------
9CSBA-52--<10--121--<0.5----------
10CSLH-10--<10--104--<0.5----------
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-3434400--689<220191227<8
12SC-345200--6010<223281729<8
13SD-130230--7611<225262042<8
14NBA-315200--11015<223231212<8
15NBA-2218170--8720<21028156<8
16NBA-2752570--23013<2327132559
17NBA-3341160--53<8<21415102811
 NBA-33--210--78--<0.5----------
18NBA-354365--2218<2713<84--
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-47542,1907.224222<3.91227.8 31.31.2
20SBA-3914201.5551.2<0.50.50.48 0.30.2
Vein ores
21VBA-7--10--92--<0.5----------
22VBA-131248--130<8<211<8<810<8
23VBA-1424110--46<8<21591115<8
24VBA-1520110--1309<215131217<8
25VBA-19170730--15019<226532814<8
 VBA-19--<10--3--<0.5----------
Intrusive bodies
26ZBA-448540--22020<237834839<8
27ZBA-1046520--22019<237854535<8
No.TypeField no.Li (ppm)Ba (ppm)Cs (ppm)Sr (ppm)Ga (ppm)Be (ppm)La (ppm)Ce (ppm)Nd (ppm)Y (ppm)Th (ppm)
Type of analysis            
 USGS/Hunter LaboratoryGICPGICP--GICPGICPGICPGICPGSGICPGICPGICP
 ALS ChemexIMSICPIMSICP/IMSIMSICP/IMSIMSIMS--IMSIMS
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.8<100.0512200.050.50.56--1.30.2
2SBA-321654--11<8<25<8<84<8
3SBA-361648--14<8<2<4<8<84<8
 SBA-36--30--11--<0.5----------
4SC-125170--43<8<2109<816<8
5SC-44377--28<8<217221120<8
6SF-336300--7110<221201429<8
7ASG-41545--8<8<2<4<8<84<8
8ASBA-41--<10--14--<0.5----------
9CSBA-52--<10--121--<0.5----------
10CSLH-10--<10--104--<0.5----------
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-3434400--689<220191227<8
12SC-345200--6010<223281729<8
13SD-130230--7611<225262042<8
14NBA-315200--11015<223231212<8
15NBA-2218170--8720<21028156<8
16NBA-2752570--23013<2327132559
17NBA-3341160--53<8<21415102811
 NBA-33--210--78--<0.5----------
18NBA-354365--2218<2713<84--
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-47542,1907.224222<3.91227.8 31.31.2
20SBA-3914201.5551.2<0.50.50.48 0.30.2
Vein ores
21VBA-7--10--92--<0.5----------
22VBA-131248--130<8<211<8<810<8
23VBA-1424110--46<8<21591115<8
24VBA-1520110--1309<215131217<8
25VBA-19170730--15019<226532814<8
 VBA-19--<10--3--<0.5----------
Intrusive bodies
26ZBA-448540--22020<237834839<8
27ZBA-1046520--22019<237854535<8
No.TypeField no.U (ppm)Co (ppm)Cr (ppm)Ni (ppm)Sc (ppm)V (ppm)Ag (ppm)Ag (ppm)Au (ppm)Au (ppm)Bi (ppm)
Type of analysis            
 USGS/Hunter Laboratory--GICPGICPGICPGICPGICPGICPHFAGCSAAHFAGICP
 ALS ChemexIMSICP/IMSICPICP/IMS--ICPICP/IMSFAFA/AASFAICP/IMS
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.80.610<0.2--20.05--0.01--0.01
2SBA-32--312<4<41383.42035.7<20
3SBA-36--<216<4<41164.57.018.5<20
 SBA-36--12276--94.4-->1017.2<2
4SC-1--5221864318--14--<20
5SC-4--5191564717--7.5--<20
6SF-3--1737357646--9.1--<20
7ASG-4--<212<4<4217--3.0--<20
8ASBA-41--31383--8>100185.1>1020.3<2
9CSBA-52--1171--19.6--7.35--<2
10CSLH-10--1131--<18.6--6.86--<2
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-34--16332866085.12533.6<20
12SC-3--2043459695--9.2--<20
13SD-1--154861168519--10--<20
14NBA-3--420225384834.68.414.7<20
15NBA-22--242504843240<4<0.340.550.55<20
16NBA-27--3101205823110<40.340.250.21<20
17NBA-33--52830668<41.711312.7<20
 NBA-33--56011--545.6--9.98--2
18NBA-35--<218<4748<4<0.340.650.55<20
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-471.23525169--2240.9--0.03--0.04
20SBA-391.80.63192.2--610.2--9.56--0.01
Vein ores
21VBA-7--219<1--389.2--0.37--<2
22VBA-13--4912<4222428.56.97.2<20
23VBA-14--61313528<4<0.340.100.41<20
24VBA-15--515175428161.09.28.3<20
25VBA-19--13402219140<42.70.050.07<20
 VBA-19--<11271--<11.8--0.18--<2
Intrusive bodies
26ZBA-4--7187852<4--0.20.03<20
27ZBA-10--6246752<4--0.70.07<20
No.TypeField no.U (ppm)Co (ppm)Cr (ppm)Ni (ppm)Sc (ppm)V (ppm)Ag (ppm)Ag (ppm)Au (ppm)Au (ppm)Bi (ppm)
Type of analysis            
 USGS/Hunter Laboratory--GICPGICPGICPGICPGICPGICPHFAGCSAAHFAGICP
 ALS ChemexIMSICP/IMSICPICP/IMS--ICPICP/IMSFAFA/AASFAICP/IMS
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.80.610<0.2--20.05--0.01--0.01
2SBA-32--312<4<41383.42035.7<20
3SBA-36--<216<4<41164.57.018.5<20
 SBA-36--12276--94.4-->1017.2<2
4SC-1--5221864318--14--<20
5SC-4--5191564717--7.5--<20
6SF-3--1737357646--9.1--<20
7ASG-4--<212<4<4217--3.0--<20
8ASBA-41--31383--8>100185.1>1020.3<2
9CSBA-52--1171--19.6--7.35--<2
10CSLH-10--1131--<18.6--6.86--<2
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-34--16332866085.12533.6<20
12SC-3--2043459695--9.2--<20
13SD-1--154861168519--10--<20
14NBA-3--420225384834.68.414.7<20
15NBA-22--242504843240<4<0.340.550.55<20
16NBA-27--3101205823110<40.340.250.21<20
17NBA-33--52830668<41.711312.7<20
 NBA-33--56011--545.6--9.98--2
18NBA-35--<218<4748<4<0.340.650.55<20
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-471.23525169--2240.9--0.03--0.04
20SBA-391.80.63192.2--610.2--9.56--0.01
Vein ores
21VBA-7--219<1--389.2--0.37--<2
22VBA-13--4912<4222428.56.97.2<20
23VBA-14--61313528<4<0.340.100.41<20
24VBA-15--515175428161.09.28.3<20
25VBA-19--13402219140<42.70.050.07<20
 VBA-19--<11271--<11.8--0.18--<2
Intrusive bodies
26ZBA-4--7187852<4--0.20.03<20
27ZBA-10--6246752<4--0.70.07<20
No.TypeField no.Cd (ppm)Cu (ppm)Hg (ppm)Mo (ppm)Pb (ppm)Tl (ppm)W (ppm)Zn (ppm)As (ppm)Sb (ppm)Te (ppm)
Type of analysis
 USGS/Hunter LaboratoryGICPGICPGCVAAGICPGICPGCSAAGTGICPGICPHASSGCSAA
 ALS ChemexICP/IMSICPFASSICPICP/IMSIMSICP/IMSICPHASSIMSIMS
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.1<1.00.01<0.21<0.020.4<2.085.10.05
2SBA-32<412--6<82.9<180240,0005,4000.25
3SBA-36<417--<4<80.5<16077,00011,0000.35
 SBA-362.5210.39<16--<1058>10,000>10,000--
4SC-1<4280.34<41800.733604,100--2.7
5SC-4<4341.40<40412.31807,700--1.3
6SF-3<4320.40<4691.3<12603,400--1.1
7ASG-4<412--<4<81.6<1100270,000--0.25
8ASBA-4124.01442.36<1<2--<103,520>10,000>10,000--
9CSBA-521.50120.13<142--<106611>10,000--
10CSLH-1026.5160.66<118--<103,20041>10,000--
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-34<436--<4550.9101,00023,0008501.3
12SC-3<4300.40<4931.1102702,800--1.0
13SD-1<4940.40<45501.1126804,300--3.6
14NBA-3<41600.20<41,8000.75<14904,600470.30
15NBA-22<41100.20<4560.5514904,100400.20
16NBA-27<4780.10<42200.4<1290650101.2
17NBA-33<4490.20<43301.163404,10016025.0
 NBA-330.5190.10<140--<10384,570670--
18NBA-35<4150.20<4120.4<11,50088097<0.05
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-478.262,6100.32<0.21,5702.32.83,51050240.05
20SBA-391.42180.60<0.6121.30.4206>10,000>1,0003.2
Vein ores
21VBA-72.0460.12<1440--<10260271180--
22VBA-13<4950.22<46900.525001,400621.7
23VBA-14<4200.20<4151.21054330330.20
24VBA-15<46600.60<45,5000.731,0003,4001604.6
25VBA-196240.20<4230.15<14631035,0000.05
 VBA-190.591.09<1<2--<10212>10,000--
Intrusive bodies
26ZBA-4<460.06<480.352531080.25
27ZBA-10<490.24<490.42102302000.20
No.TypeField no.Cd (ppm)Cu (ppm)Hg (ppm)Mo (ppm)Pb (ppm)Tl (ppm)W (ppm)Zn (ppm)As (ppm)Sb (ppm)Te (ppm)
Type of analysis
 USGS/Hunter LaboratoryGICPGICPGCVAAGICPGICPGCSAAGTGICPGICPHASSGCSAA
 ALS ChemexICP/IMSICPFASSICPICP/IMSIMSICP/IMSICPHASSIMSIMS
Protolith is massive pure carbonate of the Limestone member of the Bau Formation
1ULBA-46-C0.1<1.00.01<0.21<0.020.4<2.085.10.05
2SBA-32<412--6<82.9<180240,0005,4000.25
3SBA-36<417--<4<80.5<16077,00011,0000.35
 SBA-362.5210.39<16--<1058>10,000>10,000--
4SC-1<4280.34<41800.733604,100--2.7
5SC-4<4341.40<40412.31807,700--1.3
6SF-3<4320.40<4691.3<12603,400--1.1
7ASG-4<412--<4<81.6<1100270,000--0.25
8ASBA-4124.01442.36<1<2--<103,520>10,000>10,000--
9CSBA-521.50120.13<142--<106611>10,000--
10CSLH-1026.5160.66<118--<103,20041>10,000--
Protolith is sandy shale and argillaceous limestone of the Kirian member of the Bau Formation
11SBA-34<436--<4550.9101,00023,0008501.3
12SC-3<4300.40<4931.1102702,800--1.0
13SD-1<4940.40<45501.1126804,300--3.6
14NBA-3<41600.20<41,8000.75<14904,600470.30
15NBA-22<41100.20<4560.5514904,100400.20
16NBA-27<4780.10<42200.4<1290650101.2
17NBA-33<4490.20<43301.163404,10016025.0
 NBA-330.5190.10<140--<10384,570670--
18NBA-35<4150.20<4120.4<11,50088097<0.05
Protolith is calcareous and carbonaceous shale of the Pedawan Formation
19BMBA-478.262,6100.32<0.21,5702.32.83,51050240.05
20SBA-391.42180.60<0.6121.30.4206>10,000>1,0003.2
Vein ores
21VBA-72.0460.12<1440--<10260271180--
22VBA-13<4950.22<46900.525001,400621.7
23VBA-14<4200.20<4151.21054330330.20
24VBA-15<46600.60<45,5000.731,0003,4001604.6
25VBA-196240.20<4230.15<14631035,0000.05
 VBA-190.591.09<1<2--<10212>10,000--
Intrusive bodies
26ZBA-4<460.06<480.352531080.25
27ZBA-10<490.24<490.42102302000.20

Analytical procedures: Branch of Geochemistry, U.S. Geological Survey (USGS): GCSAA = chemical separation atomic absorption spectrometry, GCVAA = cold-vapor atomic absorption spectrometry, GICP = semiquantitative induction-coupled plasma spectrometry, GSP = specific analytical procedure for F Hunter Analytical Laboratory: HAAS = atomic absorption analysis, HFA = classic fire assay; ALS Chemex: FA = classic fire assay analysis, FA-ASS = fire assay collection with atomic absorption finish, FASS = flameless atomic absorption spectrometry, HASS = hydride generator-atomic absorption spectrometry, ICP = induction coupled plasma optical emission analysis, IMS = induction coupled plasma mass spectrometric analysis, XRF = X-ray fluorescence analysis; italicized numbers were determined by the italitized analytical method

Sample types: AS = arsenical ore, BM = base-metal mineralized shale, CS = calcic skarn, N = clay-rich ore derived from limy clastic rock, S = silicified (jasperoid) ore, UL = unaltered limestone, V = vein ore, Z = intrusive rocks

-- = no data available

Table 3.

Summary of Fluid Inclusion Data from the Bau District, Malaysia

DepositSampleOre typeMineralTh (°C)Tm (°C)TmH (°C)
 nRangeMode °CnRangenRange
Calcic skarn deposits
Gunug BauCS-1Calc-silicateQuartz, zone 114230–339230–3107–2.5 to –0.5  
   Quartz, zone 2, s9223–368- -12–1.9 to –0.7  
   Quartz, zone 3, s17247–335- -  14186–223
Lucky HillLH-2Calc-silicateQuartz9102–271- -2–3.1 to –1.5  
   Calcite5255–288250–2903–16.2 to –14.4  
   Sarabauite1172- -    
Vein deposits
Quartz-calcite veins (type 1)
RumohBA-7Quartz-calciteQuartz56160–298220–28034–2.1 to –0.7  
   Calcite3151–172- -2–0.1 to –0.1  
KojakBA-19Quartz-calciteQuartz12198–218190–22012–1.2 to –1.1  
Tai TonC-5Quartz-calciteQuartz, zone 116240–349290–35013–2.7 to –1.1  
   Quartz, zone 232285–358290–34012–2.1 to –1.3  
Base metal veins (type 2)
NegaraBA-40Base metalSphalerite6302–308300–3104–6.4 to –4.6  
Replacement deposits
Bukit YoungBA-47NormalQuartz, zone 17105–118100–120    
   Quartz, zone 216148–279150–2209–2.2 to –2.0  
Bukit YoungBY-1NormalQuartz, s13241–373280–330  9197–268
 60–380         
 C-1SilicifiedQuartz13295–341290–3209–2.3 to –1.6  
 BA-43SilicifiedQuartz7155–303210–240    
BidiBAU-1ArsenicalQuartz26201–265200–2509–2.5 to –1.1  
BidiBA-41ArsenicalQuartz22182–263200–25017–2.2 to –1.3  
DepositSampleOre typeMineralTh (°C)Tm (°C)TmH (°C)
 nRangeMode °CnRangenRange
Calcic skarn deposits
Gunug BauCS-1Calc-silicateQuartz, zone 114230–339230–3107–2.5 to –0.5  
   Quartz, zone 2, s9223–368- -12–1.9 to –0.7  
   Quartz, zone 3, s17247–335- -  14186–223
Lucky HillLH-2Calc-silicateQuartz9102–271- -2–3.1 to –1.5  
   Calcite5255–288250–2903–16.2 to –14.4  
   Sarabauite1172- -    
Vein deposits
Quartz-calcite veins (type 1)
RumohBA-7Quartz-calciteQuartz56160–298220–28034–2.1 to –0.7  
   Calcite3151–172- -2–0.1 to –0.1  
KojakBA-19Quartz-calciteQuartz12198–218190–22012–1.2 to –1.1  
Tai TonC-5Quartz-calciteQuartz, zone 116240–349290–35013–2.7 to –1.1  
   Quartz, zone 232285–358290–34012–2.1 to –1.3  
Base metal veins (type 2)
NegaraBA-40Base metalSphalerite6302–308300–3104–6.4 to –4.6  
Replacement deposits
Bukit YoungBA-47NormalQuartz, zone 17105–118100–120    
   Quartz, zone 216148–279150–2209–2.2 to –2.0  
Bukit YoungBY-1NormalQuartz, s13241–373280–330  9197–268
 60–380         
 C-1SilicifiedQuartz13295–341290–3209–2.3 to –1.6  
 BA-43SilicifiedQuartz7155–303210–240    
BidiBAU-1ArsenicalQuartz26201–265200–2509–2.5 to –1.1  
BidiBA-41ArsenicalQuartz22182–263200–25017–2.2 to –1.3  

Notes: Primary and pseudosecondary inclusions unless noted otherwise by “s” for secondary inclusions; zones in minerals refer to different growth zones, from earliest (1) to latest (3); Th is homogenization temperature of complete fluid inclusion to liquid; Tm is the temperature of ice melting; TmH is the temperature of halite dissolution; - = no data available

Table 4.

Stable Isotopic Data for Mineralized Rocks, Intrusive Rock, and Unaltered Limestone from the Bau District, Malaysia

SampleMineralδ18Oδ13CT1δ18OH2O2δH2Oδ18OH2Oδ13CCO2Comments
Intrusion
BI-1qtz9.2      Phenocrysts
 ser12.5 350310.3   Alteration
Calcic skarn         
CS-1qtz20.2 29013    
 cs7.5       
 cs + sulf9.6       
LH-1wo7.8       
LH-2qtz20.1      Very fine grained
 qtz19      Later coarse-grained
Lucky H 5wo8.3       
Replacement
BA-32stibnite    –82   
BA-39cal18.3–6.1     Late stage
 drusy qtz18.8 2007.2–71–3.1–5.9Stockwork veining
BA-41qtz19.1 2259   Late stage
 qtz19.6 2259.5   Early qtz + native As
BA-42Adrusy qtz    –68   
BA-42Bdrusy qtz    –67–4.8–7.6 
BA-47qtz20.1 1756.8   Late stage
 qtz19.7 1756.4   Early stage
BAU-1arsenic    –62   
BAU-2stib + qtz    –86   
BAU-3qtz18.3 2258.2    
 drusy qtz20.4 22510.3    
 cal17.1–5.1     Late stage
 late cal    –824.3–12.4 
 drusy qtz    –101–5.4–8.2 
BAU-4cal18.6–6.1     Late stage in vug
 qtz18.3 2258.2    
BY-1qtz18 2509.1   Late stage
 qtz16.6 2256.5   Early stage
 qtz17.3 2508.4   Middle stage
BY-2qtz19.5 2259.4    
 qtz22.1 22512    
Vein
C-1qtz20 25011.1   Early fine grained
 qtz20.5 30013.6   Late coarse-grained
Kusa 3qtz19.1 2259   Stockwork veining
 cal18.5–6.1     Late stage
BA-7cal20.1–7.21708.8    
BA-19qtz21.9 20012.4   Early stage
 qtz22.6 20011   Late stage
C-5qtz + sulf18.2 32012    
Base metal vein
Negara 10cal21.4–3.6      
BA-51gal    –5813.9–2.4 
BA-56sph     9.5–7.1 
Bau Limestone
BA-46ls23.60.1 23.6   Fresh limestone
SampleMineralδ18Oδ13CT1δ18OH2O2δH2Oδ18OH2Oδ13CCO2Comments
Intrusion
BI-1qtz9.2      Phenocrysts
 ser12.5 350310.3   Alteration
Calcic skarn         
CS-1qtz20.2 29013    
 cs7.5       
 cs + sulf9.6       
LH-1wo7.8       
LH-2qtz20.1      Very fine grained
 qtz19      Later coarse-grained
Lucky H 5wo8.3       
Replacement
BA-32stibnite    –82   
BA-39cal18.3–6.1     Late stage
 drusy qtz18.8 2007.2–71–3.1–5.9Stockwork veining
BA-41qtz19.1 2259   Late stage
 qtz19.6 2259.5   Early qtz + native As
BA-42Adrusy qtz    –68   
BA-42Bdrusy qtz    –67–4.8–7.6 
BA-47qtz20.1 1756.8   Late stage
 qtz19.7 1756.4   Early stage
BAU-1arsenic    –62   
BAU-2stib + qtz    –86   
BAU-3qtz18.3 2258.2    
 drusy qtz20.4 22510.3    
 cal17.1–5.1     Late stage
 late cal    –824.3–12.4 
 drusy qtz    –101–5.4–8.2 
BAU-4cal18.6–6.1     Late stage in vug
 qtz18.3 2258.2    
BY-1qtz18 2509.1   Late stage
 qtz16.6 2256.5   Early stage
 qtz17.3 2508.4   Middle stage
BY-2qtz19.5 2259.4    
 qtz22.1 22512    
Vein
C-1qtz20 25011.1   Early fine grained
 qtz20.5 30013.6   Late coarse-grained
Kusa 3qtz19.1 2259   Stockwork veining
 cal18.5–6.1     Late stage
BA-7cal20.1–7.21708.8    
BA-19qtz21.9 20012.4   Early stage
 qtz22.6 20011   Late stage
C-5qtz + sulf18.2 32012    
Base metal vein
Negara 10cal21.4–3.6      
BA-51gal    –5813.9–2.4 
BA-56sph     9.5–7.1 
Bau Limestone
BA-46ls23.60.1 23.6   Fresh limestone

Abbreviations: cal = calcite, sulf = sulfide minerals, cs = calc-silicate minerals, gal = galena, ls = limestone, qtz = quartz, ser = sericite, sph = sphalerite, wo = wollastonite

1

Approximate value from fluid inclusion homogenization temperatures

2

Calculated from the equation of Clayton et al. (1972)

3

Estimated

Table 5.

Characteristics of the Bau Gold District in Borneo

SettingKalimantan subduction-related magmatic arc, postaccretion magmatism
District-scale structuresSouth side of an WNW-striking regional fault zone (Lupar line), at intersection of the ENE-striking Bau anticline and a NE-striking belt of Miocene intrusions
Level of exposureSubvolcanic, Cu-Mo stockwork fluid inclusion homogenization pressure of 400 bar corresponds to a depth of 1.6 km (lithostatic) or 4 km (hydrostatic)
Character of intrusionsPostcollisional, adakitic microtonalite to dacite porphyries
Age of intrusionsMagmatic belt 14.6 to 6.4 Ma, intrusions north of
 the ore deposits 11.6 to 9.3 Ma
Age of alterationSericite 10.4 Ma
Structural ore controlsNortheast faults, axis of east-northeast anticline, northwest faults, dike margins
Lithologic ore controlsPermeable clastic Krian member of the Bau Formation is preferentially mineralized, contact between brittle fractured Bau limestone and overlying Pedawan shale
Zoning of auriferous alteration and mineralizationProximal porphyry Cu-Mo stockwork with potassic alteration; to calcic skarn bodies and veins with aurostibnite, sarabauite, and kermesite; to carbonate and quartz veins with phyllic halos that are zoned laterally from base metal sulfides, to stibnite, arsenopyrite, native arsenic, and realgar; to distal Carlin-style disseminated replacement deposits with arsenopyrite, pyrite, jasperoid, calcite, barite, and cinnabar
Mineral constraintsAurostibite and sarabauite in calcic skarn ≤460° to 420°C, decreasing Fe in sphalerite from core to rim records cooling or increasing fs2, sequence of arsenic minerals records cooling and desulfidation of fluids, jasperoid textures >180° to 200°C
Au/Ag ratio0.01 to 100
Mass transfer in Carlin-style depositsDepletion of Ca, Mg, and Sr; immobile Ti, Al, and K; strong introduction of Si, Fe, S, As, Sb, Mn, Zn, Ag, and Au; lesser introduction of Cu, Pb, Te, Tl, and Hg
Mineral compositionsElevated Cu, Sn, Te, and In in stibnite, arsenopyrite, arsenic, and realgar suggests that Sb and As are derived from a magmatic source
Residence of goldNative grains ± solid solution in arsenopyrite and pyrite
Fluid inclusionsHypersaline, vapor-rich, and low-salinity liquid-rich, immiscible brine and vapor that condensed to liquid ± contracted to low-salinity liquid, cooling and decompression from 500°C and 400 bar in Cu-Mo stockwork to 150°C and 20 bar in distal disseminated gold mineralization
Source of saltSphalerite, stibnite are magmatic; drusy quartz is Na bicarbonate
Source of waterProximal magmatic water, exchange with carbonate country rocks, distal/late mixing with local meteoric water
Source of CO2Proximal magmatic CO2, exchange with lime-stone, distal mixing with local meteoric water
Source of H2SProximal magmatic, distal/late country rocks
ProcessesCooling, decompression, phase separation, fluid mixing, decarbonation and dissolution of lime-stone, iron sulfide precipitation by cooling, pH increase, fluid mixing, and sulfidation of country rocks
System typeCu-Mo-Au porphyry-epithermal unusually enriched in As and Sb
Carlin-style gold deposit typeDistal disseminated
SettingKalimantan subduction-related magmatic arc, postaccretion magmatism
District-scale structuresSouth side of an WNW-striking regional fault zone (Lupar line), at intersection of the ENE-striking Bau anticline and a NE-striking belt of Miocene intrusions
Level of exposureSubvolcanic, Cu-Mo stockwork fluid inclusion homogenization pressure of 400 bar corresponds to a depth of 1.6 km (lithostatic) or 4 km (hydrostatic)
Character of intrusionsPostcollisional, adakitic microtonalite to dacite porphyries
Age of intrusionsMagmatic belt 14.6 to 6.4 Ma, intrusions north of
 the ore deposits 11.6 to 9.3 Ma
Age of alterationSericite 10.4 Ma
Structural ore controlsNortheast faults, axis of east-northeast anticline, northwest faults, dike margins
Lithologic ore controlsPermeable clastic Krian member of the Bau Formation is preferentially mineralized, contact between brittle fractured Bau limestone and overlying Pedawan shale
Zoning of auriferous alteration and mineralizationProximal porphyry Cu-Mo stockwork with potassic alteration; to calcic skarn bodies and veins with aurostibnite, sarabauite, and kermesite; to carbonate and quartz veins with phyllic halos that are zoned laterally from base metal sulfides, to stibnite, arsenopyrite, native arsenic, and realgar; to distal Carlin-style disseminated replacement deposits with arsenopyrite, pyrite, jasperoid, calcite, barite, and cinnabar
Mineral constraintsAurostibite and sarabauite in calcic skarn ≤460° to 420°C, decreasing Fe in sphalerite from core to rim records cooling or increasing fs2, sequence of arsenic minerals records cooling and desulfidation of fluids, jasperoid textures >180° to 200°C
Au/Ag ratio0.01 to 100
Mass transfer in Carlin-style depositsDepletion of Ca, Mg, and Sr; immobile Ti, Al, and K; strong introduction of Si, Fe, S, As, Sb, Mn, Zn, Ag, and Au; lesser introduction of Cu, Pb, Te, Tl, and Hg
Mineral compositionsElevated Cu, Sn, Te, and In in stibnite, arsenopyrite, arsenic, and realgar suggests that Sb and As are derived from a magmatic source
Residence of goldNative grains ± solid solution in arsenopyrite and pyrite
Fluid inclusionsHypersaline, vapor-rich, and low-salinity liquid-rich, immiscible brine and vapor that condensed to liquid ± contracted to low-salinity liquid, cooling and decompression from 500°C and 400 bar in Cu-Mo stockwork to 150°C and 20 bar in distal disseminated gold mineralization
Source of saltSphalerite, stibnite are magmatic; drusy quartz is Na bicarbonate
Source of waterProximal magmatic water, exchange with carbonate country rocks, distal/late mixing with local meteoric water
Source of CO2Proximal magmatic CO2, exchange with lime-stone, distal mixing with local meteoric water
Source of H2SProximal magmatic, distal/late country rocks
ProcessesCooling, decompression, phase separation, fluid mixing, decarbonation and dissolution of lime-stone, iron sulfide precipitation by cooling, pH increase, fluid mixing, and sulfidation of country rocks
System typeCu-Mo-Au porphyry-epithermal unusually enriched in As and Sb
Carlin-style gold deposit type