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 H 2 S-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 δ D values by magma degassing and higher δ 18 O and δ 13 C 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 H 2 O, CO 2 , and H 2 S. 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.

Abstract At least 14 porphyry copper-gold deposits and 19 epithermal gold deposits are known within 60 km of Cajamarca. The partly explored porphyry deposits vary in grade, Cu-Au-Mo proportions, and depth of erosion. Associated epithermal mineralization occurs at Perol, Peña de las Águilas, Kupfertal, Yanacocha Norte, Maqui Maqui, and Pampa Verde but not at Michiquillay, El Galeno, Chailhuagón, Cerro Corona, La Sorpresa, Colpayoc, and Chamis. These deposits are associated with Miocene magmatic activity, northwest-trending folds and thrusts, and northeast-trending faults. In the porphyry deposits, granular A quartz veins, associated with K-feldspar-biotite alteration and disseminated chalcopyrite-magnetite with bornite or pyrite, are typically present within and about multiple coeval porphyry intrusions. Banded quartz veins occur near the tops of some shallowly eroded systems, and late sericite-pyrite ± chalcopyrite is superimposed on most. Epithermal mineralization is mostly of high-sulfidation character, with pyrite-enargite-covellite typically underlying oxide Au zones leached of Cu. Epithermal Au-Cu is associated with multiple stages of brecciation and intense silicification, zoned outward and downward with decreasing SiO 2 and Au through quartz-pyrophyllite-diaspore-alunite-dickite to quartz-alunite and kaolinite. Structurally controlled, high-grade Au is apparently late and associated locally with intermediate-sulfidation assemblages, barite, and chalcedony. The transition between porphyry and epithermal environments is exposed at Perol and Huaylamachay, La Zanja, and especially Tantahuatay and Yanacocha. At Perol and Huaylamachay, porphyry gold-copper deposits are adjacent to generally contemporaneous volcanic vents altered to quartz-alunite with minor structures containing quartz-pyrophyllite-alunite-Au. At Perol, the dacitic vent is intruded by a later mineralized porphyry, whereas at Huaylamachay the vent breccia contains clasts with quartz-molybdenite veins and is cut by banded quartz veins, which we interpret as indicating a second, deeper porphyry Au system. At Tantahuatay, an andesitic dome complex is pervasively brecciated and altered to quartz-alunite-pyrophyllite-diaspore ± dickite, with extensive pyrite-enargite-covellite-(bornite) veins and disseminations beneath Aurich oxide mineralization. A gusano texture of soft, round patches of pyrophyllite-diaspore and/or alunite in a silicified matrix is widespread and associated with anomalous concentrations of Mo. Only one of several drill holes to 600-m depth encountered A quartz veins and minor porphyry intrusions. This hole provides evidence for prograde advance of quartz veining associated with one or more porphyry intrusions into the epithermal environment and subsequent retrograde collapse. At Yanacocha, the most abundant evidence of direct, albeit complex, spatial and temporal relationships between multiple centers of epithermal mineralization and porphyry intrusion and mineralization has been partially deciphered. At Kupfertal, the matrix of gusano alteration above the top of the porphyry becomes increasingly silicified and patchy downward, developing very contorted wormy quartz veins that overlap the top of A quartz veins. Intense pyritic quartz-pyrophyllite-diaspore-alunite and underlying sericite alteration is superimposed on K-feldspar-biotite alteration of the early stage. Fluid inclusions in quartz are vapor dominant, with downward-increasing proportions of high-salinity inclusions and amounts of minute relict chalcopyrite ± bornite grains “locked” in A vein quartz. A-veined and advanced argillic-altered xenoliths in pyroclastic rocks intruded by porphyries and hosting gold mineralization demonstrate multiple generations of porphyry and epithermal mineralization. Early Cu and Au of the porphyry event appear to have been remobilized and incorporated into the overlying epithermal system.

Abstract A 40 Ar/ 39 Ar age of 16.94 ± 0.34 Ma on hypogene alunite from the La Virgen sedimentary and volcanic rockhosted Au deposit and new ages of 16.06 ± 0.11 to 15.58 ± 0.12 Ma on porphyry Cu-Au deposits at Minas Conga confirm the existence of early Miocene and earliest middle Miocene mineralization in the eastern part of the Miocene metallogenic belt of northern Peru. A new K-Ar age of 15.3 ± 0.3 Ma for the Magistral porphyry-skarn Cu-Mo deposit extends the belt of known early and middle Miocene deposits southeast of La Virgen. A 40 Ar/ 39 Ar age of 20.02 ± 0.15 Ma is reported for a late intrusive phase at the Michiquillay porphyry Cu deposit. A 40 Ar/ 39 Ar plateau age of 15.61 ± 0.12 Ma on alunite from the San Pedro Sur zone at the La Zanja epithermal Au district is consistent with the location of the district within the middle Miocene Quiruvilca-Pierina subbelt of the Miocene metallogenic belt. 40 Ar/ 39 Ar mineralization ages of 20.79 ± 0.10 and 21.78 ± 0.11 Ma have been obtained on mineral deposits of the Malvas and Huinac districts in the Cordillera Negra to the south. Together with a published age of 18.7 ± 0.6 Ma on the Churropampa Au prospect in the eastern part of the Cordillera Negra and a new Re-Os age of 18.15 ± 0.06 Ma on the Pachagón porphyry Cu-Ag prospect northeast of Trujillo, an early Miocene mineral activity in the western part of the metallogenic belt is indicated. The new ages show that clearly magmatically related mineralization was formed at the same latitudes in both the eastern and western parts of the Miocene metallogenic belt during early Miocene and earliest middle Miocene times. The deposits of the giant Yanacocha Au district, which were formed at about 11 Ma, are younger than deposits to the west and to the east. Deposits of the Hualgayoc district to the north, which have yielded mineralization ages from 12.4 ± 0.4 to 14.4 ± 0.2 Ma, may overlap in time with those of Yanacocha. These districts, which are located within large coeval volcanic fields, could reflect a younger episode of magmatic activity of late middle Miocene age.

Abstract The Miocene metallogenic belt of central and northern Perú, extending for at least 900 km along the Western Cordillera and the adjacent high plateaus province, is defined by a large number of hydrothermal mineral deposits of different types that formed between about 6 and 20 Ma. The belt, centered east of the Mesozoic and early Paleogene Coastal batholith, is on mature continental crust that has undergone multiple episodes of compressive deformation from at least middle Paleozoic to latest Neogene time. Mineralization began before the early Miocene Quechua I compressive event and spanned later Quechua II tectonism. Mineral deposits are mostly hosted by shelf carbonates and other sedimentary rocks of Late Triassic,Jurassic, and Cretaceous age and by volcanic and intrusive rocks mainly of Neogene age. Base metal and precious metal mineralization was intimately associated in time and space with the eruption of calc-alkalic volcanic rocks of intermediate composition and the emplacement of mineralogically and chemically similar dikes and stocks. These igneous rocks are moderately potassic and the few available data suggest relatively nonradiogenic Sr, Nd, and Pb isotope compositions. Mineral deposits range from porphyry and associated proximal skarn deposits to polymetallic, precious metal, and mercury deposits formed at relatively lower temperatures. Porphyry deposits include the La Granja Cu porphyry, the Au-bearing Michiquillay Cu porphyry, the Mo-bearing Cu porphyritic rocks of Toro Mocho, Pashpap, and Páraq, the Mundo Nuevo-Tamboras-Compaccha Mo-W porphyry system, and the Cerro Corona, Minas Conga, Collpayoc, Laguna Chamis, Carhuacayán, and Puy-Puy Au-Cu porphyry deposits. Many of the classic base and precious metal deposits of central and northern Perú are within zoned polymetallic districts, some with one or more porphyry centers. Many districts have veins or replacement bodies containing enargite in their central parts, and a number are characterized by deposits of both vein and limestone replacement type. At a number of polymetallic districts, for example, Julcani, Yauricocha, Morococha, Casapalca, Huarón, Raura, Antamina, Pasto Bueno, Quiruvilca, Algamarca, and Hualgayoc, stocks containing high-salinity fluid inclusions are exposed, known from drill-hole data, or can be confidently inferred from fluid-inclusion or isotope data. Vein and limestone-replacement Pb-Zn ± Ag ± Cu deposits are common, and range from vertically persistent, high-temperature deposits, such as the veins of Casapalca, to largely stratabound deposits such as Cercapuquio and Azulcocha, that were formed at temperatures below 200°C. Although certain writers have interpreted some manto deposits to be diagenetic or syndiagenetic, field relations and lead isotope compositions argue strongly for an epigenetic origin. Vein systems or epithermal paragenetic stages in which silver is the economically most important metal, such as those of Milluachaqui, Millotingo, and Colqui, typically contain appreciable amounts of base metals and can best be considered a variant of the polymetallic vein group. The Huancavelica mercury deposit represents an extremely large geochemical anomaly, perhaps developed at the top of a polymetallic system. High-sulfidation-type Au-Ag deposits, such as Pierina and those of the Yanacocha district, are economically important. At Tantahuatay and Colquijirca, oxidized Au-bearing, vuggy silica rock occurs at higher elevations than surrounding, zoned, enargite-cored Cu-Pb-Zn-Ag veins and strata-bound replacement deposits. In contrast to the association of precious metals with enargite, tetrahedrite, and barite at Julcani and other reduced-type deposits, in moderate- to high-grade ores at Pierina and probably certain deposits in the Tantahuatay and Yanacocha districts, most of the gold is very late, following initial quartz-alunite-pyrite alteration, the destruction of alunite to form vuggy silica rock, and the subsequent deposition of pyrite and enargite accompanied by small amounts of gold. Gold and silver in economic quantities were then introduced by compositionally distinct, late fluids that oxidized pyrite and enargite, leached Cu, Zn, Se, Te, Tl, and other elements, and introduced Hg, Pb, Bi, Sb, and large amounts of barite. An analogous case for a distinct, compositionally different Au-Ag mineralizing pulse perhaps can be made for the sedimentary rock-hosted gold deposits of Purísima Concepción in Yauricocha. The ubiquitous presence of enargite, and the spatial and temporal association in several districts of pyrite + enargite, with modest gold content, and oxidized Au-rich ores, support the interpretation that bulk-mineable, volcanic-hosted gold deposits of a high-sulfidation type represent one of the many types of deposits related to the general class of porphyry-related, zoned polymetallic systems. The sandstone-hosted gold deposits of northern Perú also appear to be related to subjacent magmatic systems, although there are certain geological, mineralogical, and chemical differences from both volcanic-hosted, high-sulfidation and Purísima-type gold deposits. High W and Sn content of many of the sandstone-hosted ores of the Angasmarca district suggest that they are high-level manifestations of subjacent W-Mo ±Au systems such as are exposed at the nearby, more deeply seated Mundo Nuevo-Tamboras-Compaccha and Pasto Bueno districts. Several subsidiary belts are recognized within the Miocene metallogenic belt. A group of deposits in northern Perú, including the polymetallic deposits of the Quiruvilca district, the several Cu-Mo porphyry systems at Pashpap, and the Pierina high-sulfidation Au deposit, defines the 13 to 15 Ma or older Quiruvilca-Pierina subbelt in the western part of the metallogenic belt. The provisional Michiquillay-El Toro subbelt, including the Michiquillay Cu porphyry, the El Toro Au prospect, and probably the Au-Cu porphyry systems of the Minas Conga district, appears to have formed in northern Perú along the eastern margin of the metallogenic belt between about 18 and 20 Ma. A narrow, late Miocene subbelt that comprises a number of deposits dated at less than about 10 Ma, including Huachocolpa, Yauricocha, San Cristóbal, Morococha, Puy-Puy, Carhuacayán, Huarón, Raura, Huanzalá, Antamina, Pasto Bueno, and Angasmarca, extends from the Huachocolpa district at the southern end of the belt to the latitude of Santiago de Chuco in northern Perú. Deposits of the late Miocene subbelt postdate the 9 to 10 Ma Quechua II compressive pulse, and the initiation, location, and narrowness of the subbelt may have been related in some manner to this tectonic event. Intersections of successive, magmatic mineral axes with northeast-trending and other fault systems of probable crustal scale may have combined to influence the location of individual mineral deposits or clusters of deposits. Mineralization had ceased, and possibly was terminated, by the 5 to 7 Ma Quechua III compressive event. The emplacement of the 5.2 Ma late phase of the Cordillera Blanca batholith and the eruption of approximately coeval units of silicic ash-flow tuff and lava in northern and central Perú may reflect the subsequent relaxation of compressive stress, leading to the switching of axes of least and greatest principal stress indicated by 4 Ma north-south-trending dike systems in central Perú. Four important older districts within the Miocene metallogenic belt (Quicay, ca. 37.5 Ma; Uchucchacua, ca. 24.5 Ma), or bordering it on the east (Atacocha and Milpo, ca. 29-30 Ma), are related to older, and perhaps in part less intense, periods of magmatic activity. Although gold deposits may prove to be more important in northern than in central Perú, there is little indication that the concentrations of other metals vary markedly along or normal to the Miocene metallogenic belt. For example, porphyry molybdenum deposits are found in both the eastern and western parts of the belt. Moreover, particular types of deposits do not appear to be preferentially restricted to a given time period: several sandstone-hosted gold deposits in northern Perú have yielded ages ranging from less than 9 to greater than 18 Ma, and Au-bearing porphyry systems include examples of early, middle, and late Miocene age. Local geology and depth of erosion may be more important controls of deposit type. If future work shows that individual subbelts are as narrow and continuous as the present data suggest, areas within the narrow subbelts may prove to be the most prospective for mineral exploration.

Rare-earth-element abundances have been determined for 26 rhyolite obsidians and glass separates of both peralkaline and subalkaline chemical character for which a history of marked differentiation had been inferred from very low Sr, Ba, Mg, and minor transition-element contents. All but five of the specimens possess very strong negative Eu anomalies, with values of Eu/Eu* of 0.06 or less. Eight have values of Eu/Eu* of 0.02 or less, and subalkaline rhyolites from the Fish Creek Mountains Tuff, Nevada, have Eu/Eu* of <0.003. Calculations using coefficients appropriate for the distribution of Eu between Ca-poor feldspar and silicic melt suggest that, if the Eu anomalies are the result of crystal fractionation, many of the glasses represent 10% to 20% or less of hypothetical unfractionated parent liquids. Distribution coefficients for Eu between Ca-poor feldspar and peralkaline silicic melt are significantly lower than for subalkaline systems; this implies high degrees of differentiation for peralkaline rocks having even modest negative Eu anomalies. A number of the specimens studied represent relatively large rock units, and some are from the lower parts of large, compositionally zoned ash-flow sheets. The mechanism—whether crystal fractionation or thermogravitational diffusion—called upon to produce large-scale vertical compositional variations within bodies of highly silicic magmas must be able to explain the profound depletion of large volumes of magma in such elements as Sr, Eu, Ba, Mg, Co, and Ni. The moderate negative Eu anomaly of glass from the Summit Lake Tuff, Nevada, can be explained by crystallization of the anorthoclase phenocrysts present in the rock. This suggests that this compositionally unzoned ash-flow sheet was produced from rhyolitic magma that possessed little or no Eu anomaly before it rose to a high-level chamber where it subsequently underwent appreciable intratelluric crystallization, but little or no crystal segregation, prior to eruption. In contrast, the strong Eu depletion (Eu/Eu* = 0.05 to 0.15) of crystal-rich specimens of the Fish Creek Mountains Tuff indicates that considerable differentiation had taken place before growth of the present generation of phenocrysts. Peralkaline specimens from the northern Great Basin and Snake River Plain have lower La/Lu ratios (2.4 to 6.2) than do specimens from the southern Great Basin (La/Lu = 6.4 to 15.7), which suggests that they may have been derived from source regions that have significantly different rare-earth-element abundance patterns.

A late Miocene (13 to 15 m.y.) volcanic center is located on eastern Pahute Mesa near lat 37° 18′ N., long 116°20′ E., Nevada. Rocks of the center are predominantly comendites and subordinately trachytic soda rhyolite and trachyte. The distinctive and unusual mineralogy and trace- and major-element chemistry of these peralkaline rocks indicate that they are closely related genetically and allows them to be distinguished readily from rocks of other centers. Activity began with emplacement of numerous lava flows, domes, and dikes over an area of at least 450 sq miles. Most were erupted in the general area of later caldera collapse, but some vents were 15 to 20 miles from the caldera’s midpoint. Two compound ash-flow sheets — the Tub Spring and overlying Grouse Canyon members of the Belted Range Tuff — were then erupted. The more voluminous Grouse Canyon covered 3000 sq miles and had an original volume of about 50 cu miles. The Silent Canyon caldera formed during and/or just after eruption of the Belted Range Tuff. Only the eastern third of the caldera is exposed or reflected as steep initial dips in overlying tuffs. That the caldera is the source of the Belted Range Tuff is shown by: (1) facies changes in the Grouse Canyon Member, (2) thickness variations in underlying bedded tuffs, (3) presence of one Grouse Canyon unit and of thick post-Grouse Canyon tuffs and lavas only near the caldera, (4) occurrence of an arc of small welded-tuff vents outside and concentric with the caldera, and (5) other field evidence. The location is consistent with the regional distribution of the Belted Range Tuff.

The Kane Wash Tuff was erupted from a vent area in the southern Delamar Range, Nevada, at lat 37°14′ N., long 114°47′ E. The source location is shown by distribution, thickness, and facies variations of individual cooling units, field evidence for caldera collapse, and petrographic and chemical similarity of pre- and post-collapse rocks. The Kane Wash Tuff, with a K/Ar age of 14 m.y., is the youngest ash-flow sequence in south-central Lincoln County. It covered an area of 3000 to 4000 sq miles. Original volume was > 200 cu miles. Correlations of tuffs in Nye County, Nevada, Washington County, Utah, and the north Pahranagat Range with the type formation are not considered valid. Activity of the Kane Springs Wash center began with eruption of lavas. Six or more Kane Wash ash-flow cooling units, at least three of which are multiple-flow, were then emplaced, followed by collapse forming an E.-W. elongated, scalloped caldera 8 by 12 miles in diameter. The depression was filled to overflowing by a complex sequence of welded and nonwelded tuff, lava flows and intrusives, and talus. No resurgence is apparent. Rocks erupted from the center are mainly comendites, but include trachytic soda rhyolite, trachyte, basalt, and silicic rocks without obvious peralkaline affinities. The peralkaline rocks contain phenocrysts of alkali feldspar, clinopyroxene, fayalite, ± quartz and have the high, B, Be, Ga, Nb, Zr, rare-earth, and halogen contents typical of the comendite-pantellerite-trachyte suit. Basin-range faulting has only slightly disturbed and tilted the caldera area. Subsequent erosion has magnificently exposed geologic relations.