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SEM and FIB-TEM analyses on nanoparticulate arsenian pyrite: Implications for Au enrichment in the Carlin-type giant Lannigou gold deposit, SW China
ORE CLASSIFICATION OF PSEUDOBRECCIA ORE IN THE 144 ZONE GOLD DEPOSIT: A CHEMICAL REPLACEMENT MODEL, BARE MOUNTAIN RANGE, NEVADA
A Nanoscale Investigation of Carlin-Type Gold Deposits: An Atom-Scale Elemental and Isotopic Perspective
Magmatic Origin for Sediment-Hosted Au Deposits, Guizhou Province, China: In Situ Chemistry and Sulfur Isotope Composition of Pyrites, Shuiyindong and Jinfeng Deposits
Nevada’s Carlin-Type Gold Deposits: What We’ve Learned During the Past 10 to 15 Years
Abstract This contribution provides brief introductions to research on Carlin-type gold deposits completed since publication of the 2005 review paper on the deposits in the Economic Geology 100th Anniversary Volume ( Cline et al., 2005 ). Major advances in our understanding of the deposits have resulted from these studies that cover a broad range of topics, from the geology of deposits to recent discoveries and current geologic models. Studies of host rocks include expanded application of sequence stratigraphy that is refining our understanding of favorable host rocks, now known to have formed on shallow carbonate platforms during lowstands as well as in deep-water slope to basin environments. Sparse igneous dikes at the surface that were emplaced coincident with formation of deposits of the Carlin trend indicate that a batholith of about 1,000 km 2 underlies the trend. Reactivated and inverted normal Neoproterozoic faults formed anticlines and fed ore fluids into structurally prepared reactive rock types. Collaborative district studies determined that structural preparation of host rocks along the Carlin trend occurred during three discrete contractional events followed by Eocene extension and coincident mineralization. Ore and alteration studies identified systematic trace element and sulfur isotope zoning in ore-stage pyrite rims that formed from temporally discrete ore fluids fed by separate structures. Deposit-scale studies determined that ore minerals in shallowly formed deposits are similar to late ore-stage minerals of typical, more deeply formed Carlin-type gold deposits. Breccias containing high-grade ore formed both by replacement and by calcite dissolution and collapse processes. Halos useful in vectoring toward mineralization include rock quality designation values, trace elements above mineralization in premineral rock and in postmineral clay, oxygen isotope ratios, and soil, soil gas, vegetation, and groundwater chemistry. Isotopic studies have indicated relative timing of ore fluid movement through discrete structures. Deposit ages coincide with spatially related intrusion ages, from about 42 to 35 Ma, and both young from northeast to southwest. Magmatism and deposit formation are interpreted as related to Eocene delamination of subcontinental lithospheric mantle. Apatite fission track data indicate that the Betze-Post deposit, which contained >1, 240 tonnes (40 Moz) of gold, formed in <15,000 to 45,000 years. New geologic maps illustrate structural and stratigraphic relationships that will contribute to exploration efforts and potential new discoveries. Recent Nevada discoveries include South Arturo on the northern Carlin trend, the Long Canyon deposit in Cambrian-Ordovician rocks in the newly recognized Pequop district in northeastern Nevada, the giant Goldrush deposit on the Battle Mountain-Eureka trend, and the North Bullion deposit at the southern end of the Carlin trend. Two potential new districts of deposits are being actively explored in the Yukon Territory, Canada, and the Golden Triangle, southern China. Deposits in the Golden Triangle and prospects in the Yukon are currently much smaller than deposits in Nevada, and the presence of proximal coeval magmatism, now recognized in Nevada, is unclear. Studies of some of the Chinese deposits indicate that they formed at conditions intermediate to Carlin-type and orogenic deposits. Recently published geologic models propose that either shallow, basin-related processes or deep magmatic processes provided gold for the Nevada deposits. Studies evaluating the Harrison Pass pluton and the Emigrant Pass volcanic rocks, both the same age as the Carlin deposits, addressed the magmatic model and provide information about potential magmatic ore fluids and systems that may have formed the deposits.
Are There Carlin-Type Gold Deposits in China? A Comparison of the Guizhou, China, Deposits with Nevada, USA, Deposits
Abstract Carlin-type Au deposits in Guizhou Province, China, have similarities to and differences from the Carlin-type Au deposits in Nevada, USA. The Shuiyindong and Jinfeng deposits, located in the Guizhou Province of southern China, are compared with the Getchell and Cortez Hills Carlin-type Au deposits of Nevada in terms of ore paragenesis and pyrite chemistry. The Guizhou deposits formed in a tectonic setting similar to Nevada with the deposition of passive-margin sequences in a rifted cratonic margin context with subsequent deformation. In both districts, orebodies are preferentially hosted in limestone and calcareous siltstone and are related to faults, gold is invisible and ionically bound in arsenian pyrite, and ore-stage minerals include quartz and illite with late ore-stage minerals, including calcite, realgar, orpiment, and stibnite. Despite major similarities, however, the Guizhou deposits have characteristics that contrast with those of Carlin-type deposits of Nevada. Significant differences include the following: Guizhou ore-stage pyrite is commonly subhedral to euhedral, and typical Nevada fuzzy ore pyrite is absent. Guizhou ore pyrite contains significantly less Au, As, Hg, Tl, Cu, and Sb than the Nevada ore pyrite. Decarbonatization in Nevada deposits is expressed by extensive removal of calcite, dolomite, and Fe dolomite. In contrast, decarbonatization in the Guizhou deposits results in loss of most primary calcite, but Fe dolomite was instead sulfidized, forming ore pyrite and dolomite. This alteration is a key process in the formation of ore pyrite in the Guizhou deposits. Silicification in Nevada deposits is characterized by jasperoid replacement of calcite, dolomite, and Fe dolomite, whereas in the Guizhou deposits jasperoid replaced mainly calcite but not Fe dolomite or dolomite. Minor vein quartz, which formed during the early ore stage in Guizhou deposits, has not been identified in Nevada deposits. Clay alteration in the Nevada deposits is characterized by formation of significant illite and variable kaolinite/dickite; however, in the Guizhou deposits, trace to minor illite is present and kaolinite is uncommon. Late ore-stage arsenopyrite and vein quartz are common in Guizhou deposit but are rare in Nevada deposits. Guizhou ore fluids contained significantly more CO 2 and were higher in temperature and pressure compared with the ore fluids in Nevada deposits. To date, magmatism spatially or temporally associated with the Guizhou deposits has not been recognized. Conversely, the Nevada deposits coincide in time and space with the southward sweep of Eocene magmatism and related extension. Dolomite-stable alteration in Guizhou formed from less acidic, CO 2 -rich ore fluids at higher temperature and pressure compared with Nevada deposits, reflecting similarities between Guizhou deposits and orogenic systems. Study results are consistent with Guizhou deposits having formed in a transitional setting between typical orogenic gold and shallow Carlin-type deposits, as indicated by estimated pressure-temperature conditions at the time of gold deposition and ore-forming fluid chemistry.
High-Grade Gold Deposition and Collapse Breccia Formation, Cortez Hills Carlin-Type Gold Deposit, Nevada, USA
Ore Fluid Properties and Sources from Quartz-Associated Gold at the Betze-Post Carlin-Type Gold Deposit, Nevada, United States
UNCLOAKING INVISIBLE GOLD: USE OF NANOSIMS TO EVALUATE GOLD, TRACE ELEMENTS, AND SULFUR ISOTOPES IN PYRITE FROM CARLIN-TYPE GOLD DEPOSITS
Carlin-Type Gold Deposits in Nevada: Critical Geologic Characteristics and Viable Models
Abstract Abstract:Carlin-type Au deposits in Nevada have huge Au endowments that have made the state, and the United States, one of the leading Au producers in the world. Forty years of mining and numerous studies have provided a detailed geologic picture of the deposits, yeta comprehensive and widely accepted genetic model remains elusive. The genesis of the deposits has been difficult to determine owing to difficulties in identifying and analyzing the fine-grained, volumetrically minor, and common ore and gangue minerals, and because of postore weathering and oxidation. In addition, other approximately contemporaneous precious metal deposits have overprinted, or are overprinted by, Carlin-type mineralization. Recent geochronological studies have led to a consensus that the Nevada deposits formed ~42 to 36 m..y ago, and the deposits can now be evaluated in the context of their tectonic setting. Continental rifting and deposition of a passive margin sequence followed by compressional orogenies established a premineral architecture of steeply dipping fluid conduits, shallow, low dipping “traps” and reactive calcareous host rocks. Sedimentary rock sequences that formed following continental margin rifting or in a foreland basin ahead of an advancing thrust front contain reactive pyritic and carbonaceous silty limestones, the primary host rocks in almost every deposit. The largest deposits now lie in the lower plate to the Devonian to Mississippian Roberts Mountain thrust, which placed nonreactive, fine-grained siliciclastic rocks with less inherent rock permeability, above more permeable carbonate stratigraphy, forming a regional aquitard. North-northwest- and west-northwest-striking basement and Paleozoic normal faults were inverted during postrifting compressional events and formed structural culminations (anticlines and domes) that served as depositional sites for auriferous fluids in the Eocene. These culminations are now exposed as erosional windows through the siliciclastic rocks of the Antler allochthon. During the Eocene, northwesterly to westerly extension reopened favorably oriented older structures as strike-slip, oblique-slip, and normal-slips faults. Fluid flow and mineral deposition appear to have been fairly passive as there is minimal evidence for overpressured hydrothermal fluids, complicated multistage vein dila-tancy, or significant synmineralization slip. Geologic reconstructions and fluid inclusions indicate that deposits formed within a few kilometers of the surface. Ore fluids were moderate temperature (~180°-240°C), low salinity (~2-3 wt % NaCl equiv), CO 2 bearing (<4 mol %), and CH 4 poor (<0.4 mol %), with sufficient H2S (10 –1 –10– 2 m) to transport Au. Ore fluids decarbonatized, argillized, and locally silicified wall rocks, and deposited disseminated pyrite containing submicron Au as Fe liberated from wall rock reacted with reduced S in the ore fluid. Isotopic studies indicate multiple sources for ore fluids and components and require either different models for different districts or call upon meteoric waters to overwhelm a deep ore-fluid signal in most districts. Oxygen and H isotope ratios of minerals and fluid inclusions indicate a deep magmatic or metamorphic fluid source at the Getchell deposit; however, most similar studies in other districts have identified meteoric water. A large range in S isotopes in ore pyrite from all districts suggests derivation from a sedimentary source; yet studies at Getchell and a few studies in the northern Carlin trend are consistent with a magmatic S source. As a result of these inconsistencies, current models relate deposits to (1) metal leaching and transport by convecting meteoric water, (2) epizonal intrusions, and (3) deep metamorphic and/or magmatic fluids. With the exception of the isotopic studies, compiled data from all Nevada trends and districts indicate compelling similarities, suggesting that all Nevada Carlin-type deposits formed in response to similar geologic processes. We propose a model in which removal of the Farallon slab promoted deep crustal melting that led to prograde metamorphism and devolatilization, thus generating deep, primitive fluids. Such fluids were likely incorporated in deep crustal melts that rose buoyantly and ultimately exsolved hydrothermal fluids, possibly containing Au. Metamorphism at midcrustal levels may have contributed fluids, all of which were collected into basement-penetrating rift faults, where they continued to rise and scavenge various components, evolving in composition to become ore fluids. North-northwest–trending paleo-normal faults and northeast-trending paleo-transform faults, preferentially dilated during Eocene extension, controlled the regional position, orientation, and alignment of the deposits. Eventually the ore fluids accumulated in areas of reduced mean effective stress, particularly boundaries of older Jurassic and Cretaceous stocks and structural culminations. The ore fluids were diluted by exchanged meteoric water as extension increased fault permeability in the upper crust. Within a few kilometers of the surface, fluids were diverted by structural and stratigraphic aquitards into reactive host rocks, where they sulfidized host rock iron and deposited Au. Sedimentary rock-hosted disseminated Au deposits in other parts of the world exhibit many similarities to Nevada Carlin-type Au deposits, yet no district has been discovered anywhere else that approaches Nevada’s Au productivity. The deposits found in other parts of the world are products of diverse, well-recognized, hydrothermal systems (e.g., low-sulfidation epithermal, porphyry Cu-Mo-Au, reduced intrusion-related epizonal orogenic, and sedimentary exhalative or sedex). Of these, the deposits in southern China are remarkably similar to Nevada Carlin-type deposits and are interpreted to have formed where metamorphic fluids reacted with wall rocks and local meteoric water.
The Tuscarora Au-Ag District: Eocene Volcanic-Hosted Epithermal Deposits in the Carlin Gold Region, Nevada
Alteration Associated with Gold Deposition at the Getchell Carlin-Type Gold Deposit, North-Central Nevada
Timing of Gold and Arsenic Sulfide Mineral Deposition at the Getchell Carlin-Type Gold Deposit, North-Central Nevada
Ore-fluid evolution at the Getchell Carlin-type gold deposit, Nevada, USA
Abstract Carlin-type gold deposits are restricted to a small part of the North American Cordillera, in northern Nevada and northwest Utah, and formed over a short interval of time (42–30 Ma) in the mid-Tertiary when the Yellowstone mantle plume is inferred to have been located below the subduction zone. They formed after a change in plate motions (43 Ma) at, or soon after, the onset of extension in an east-west-trending, subduction-related magmatic belt. The deposits do not show consistent spatial relationships to mid-Tertiary magmatic centers, rather, most are located along long-lived, deep crustal structures inherited from Late Proterozoic rifting and formation of a passive margin. These structures influenced subsequent patterns of sedimentation and deformation and localized multiple episodes of igneous and hydrothermal activity, many of which contain anomalous concentrations of gold. The mid-Tertiary surface topography was relatively flat and many systems were located below large shallow lakes. Most deposits are hosted in a Paleozoic miogeoclinal carbonate sequence that is either structurally overlain by a eugeoclinal siliciclastic sequence, the Roberts Mountains allochthon emplaced in Early Mississippian time, or stratigraphically overlain by a miogeoclinal siliciclastic sequence deposited in the resulting foredeep. These siliciclastic sequences are less permeable than underlying carbonate rocks and apparently caused fluids ascending along major structures to flow laterally into permeable and reactive rocks below them. In these areas, gold ore is localized at intersections of a complex array of structures with permeable and reactive strata. The common alteration, mineralogy, and geochemical signature of these deposits is a direct expression of the P, T, and composition of ore fluids. The deposits generally formed at depths of >2 km at temperatures of 250° to 150°C, from moderately acidic (pH ≈5), reduced fluids containing <6 wt percent NaCl equiv, <4 mole percent CO 2 , <0.4 mole percent CH 4 , and >0.01 mole percent H 2 S. The H 2 S concentration was critical because it suppressed the solubility of Fe, base metals, and Ag as chloride complexes and enhanced the solubility of Au and associated trace elements (e.g., As, Sb, Tl, and Hg) as sulfide complexes. Gold was transported as AuHS° and/or Au(HS) 2 −1 complexes. The main ore stage formed during cooling and neutralization of ore fluids by reactions with the host rocks. It is characterized by carbonate dissolution, argillization of silicates, sulfidation of ferroan minerals, and silicification of limestone. Gold occurs as submicron inclusions or solid solution in arsenian pyrite and precipitated as H 2 S was consumed by sulfidation of Fe released from ferroan minerals. The other common trace elements (e.g., Sb, Tl, Hg) also reside in arsenian pyrite. The ideal host rock consists of permeable ferroan carbonate that is completely dissolved and its contained iron completely sulfidized such that all that remains is gold-bearing arsenian pyrite. Accordingly, large tonnage, low-grade gold deposits (e.g., Gold Quarry) are in siliceous rocks with low carbonate and reactive iron contents, and small tonnage, high-grade gold deposits (e.g., Meikle) are in carbonate rocks with high concentrations of reactive iron. Late ore-stage quartz, calcite, orpiment, realgar, stibnite, and barite occur in open fractures and pores and their abundance varies tremendously from deposit to deposit. These minerals precipitated as the systems cooled and ore fluids mixed with local ground water. Boiling was generally not important. Isotopic data from different districts yield conflicting indications as to the source of ore fluids. Abundant stable isotope data (δD, δ 18 O, δ 13 C, δ 34 S) and limited radiogenic isotope data (Pb, Sr, Os) from the major trends and districts are consistent with models involving the circulation of meteoric water through sedimentary rocks. In contrast, δD, δ 18 O, and δ 13 C data from the Getchell trend suggest that gold was introduced by a deep-sourced fluid that was of metamorphic or magmatic origin. The apparent lack of mid-Tertiary intrusions in this district argues for a metamorphic fluid, although the characteristics of certain portions and stages of the deposits suggest there was a magmatic fluid component characterized by higher Cl, Fl, K, Fe, and Cs contents. N 2 /Ar/He ratios of fluid inclusions suggest there were inputs of mantle He. Carlin-type deposits do not fit neatly into any one of the models proposed for them. Although variably evolved meteoric water is present in all of them, they are deeper than low-sulfidation epithermal veins and there is little or no evidence of boiling. They are shallower than orogenic veins and metamorphic fluids have only been detected in one district. Magmatic models call upon concealed intrusions that are so far removed from the deposits that no coeval contact metamorphic rocks, breccia pipes, or zoned geochemical halos are recognized at current levels of exposure or drilling. If the numerous similarities among Carlin-type deposits reflect the presence of a common ore fluid, then only one of the fluids detected by isotopic methods can be the ore fluid and the others must be due to contamination. In this case, we find the metamorphic fluid model most attractive, because both Carlin-type and orogenic gold deposits form in broad thermal anomalies, are distributed along major crustal structures, form during a change in stress regime, have similar ages over wide areas, have monotonous geochemical signatures, and contain similar endowments of gold. If we rely on the best data available from each district, a variety of models is needed and the only common factor is the geologic setting. These considerations suggest that Carlin-type deposits are unique, or too complex, to neatly fit into any one of these models.
Abstract The Getchell gold deposit is one of several Carlin-type gold systems located along the Getchell Trend in north-central Nevada. Deposits located along this trend exhibit all of the characteristics typical of Carlin-type systems including the presence of submicron-sized gold particles in arsenic-rich pyrite. Although Carlin-type deposits have been mined for thirty years and are currently responsible for Nevada being one of the leading gold pr~ducers in the world, there is little consensus concerning the geological processes that concentrated gold in these deposits. The lack of understanding of formation conditions stems from the submicron size of the gold particles, a paucity of ore stage fluid inclusions in most systems, and a lack of readily datable minerals that are unequivocally related to gold mineralization. The lack of visible gold, even under the microscope, has made it difficult to identify minerals and textures associated with gold precipitation, complicating determination of gold precipitation mechanisms. Fluid inclusions are sparse or absent in the fine-grained replacement ore present in most systems, making it difficult to determine system pressure and temperature, and ore fluid chemistry. The lack of datable minerals has prevented identification of key geological events responsible for gold concentration. The Getchell deposit is somewhat unusual in that a significant amount of open-space-filling mineralization is hosted within the Getchell fault zone. This mineralization exhibits cross-cutting and textural relationships that provide constraints for the ore paragenesis. Additionally, these slightly more coarse-grained minerals trapped several populations of fluid inclusions, including primary inclusions contained within growth zones. Examination of