Abstract The Eocene Goldstrike system on the Carlin Trend in Nevada is the largest known Carlin-type gold system, with an endowment of 58 million ounces (Moz) distributed among several coalesced deposits in a structural window of gently dipping carbonate rocks below the regional Roberts Mountains thrust. The 3.5- × 2.5-km Goldstrike system is bounded to the east by the Post normal fault system and to the south by the Jurassic Goldstrike diorite stock and is partly hosted in the favorable slope-facies apron of the Bootstrap reef margin that passes through the system. The carbonate and clastic sedimentary sequence is openly folded, cut by sets of reverse and normal faults, and intruded by the Jurassic Goldstrike stock and swarms of Jurassic and Eocene dikes, establishing the structural architecture that controlled fluid flow and distribution of Eocene mineralization. A proximal zone of permeability-enhancing decarbonatization with anomalous gold (>0.1 ppm) extends a few hundreds of meters beyond the ore footprint and lies within a carbonate δ 18 O depletion anomaly extending ~1.4 km farther outboard. The full extent of the larger hydrothermal system hosting Goldstrike and adjacent deposits on the northern Carlin Trend is outlined by a 20- × 40-km thermal anomaly defined by apatite fission-track analyses. The bulk of the mineralization is hosted in decarbonatized sedimentary units with elevated iron contents and abundant diagenetic pyrite relative to background. Gold is associated with elevated concentrations of As, Tl, Hg, and Sb, and occurs in micron-sized arsenian pyrite grains or in arsenian pyrite overgrowths on older, principally diagenetic pyrite, with sulfidation of available iron as the main gold precipitation mechanism. The intersection of a swarm of Jurassic lamprophyre dikes with the edge of the limestone reef provided a favorable deeply penetrating structural conduit within which a Jurassic stock acted as a structural buttress, whereas the reef’s slope-facies apron of carbonate units, with high available iron content, provided a fertile setting for Carlin-type mineralization. The onset of Eocene extension coupled with a southwestward-sweeping Cenozoic magmatic front acted as the trigger for main-stage gold mineralization at 40 to 39 Ma. All these factors contributed to the exceptional size and grade of Goldstrike.

Abstract Carlin-type gold deposits in Nevada account for ~5% of worldwide annual gold production, typically about ~135 metric tons (t) (~4.5 Moz) per year. They are hydrothermal epigenetic replacement bodies hosted predominantly in carbonate-bearing sedimentary rocks. They are known for their “invisible” gold that occurs in the crystal structure of pyrite. Over 95% of the production from these deposits is from four clusters of deposits, which include the Carlin trend and the Cortez, Getchell, and Jerritt Canyon camps. Despite differences in the local geologic settings, the characteristics of the deposits are very similar in the four clusters. Shared characteristics include: (1) alteration characterized by carbonate dissolution, silicate argillization, and silicification; (2) ore formation characterized by auriferous arsensian pyrite, typically as rims on preore pyrite, followed by late open-space deposition of orpiment, realgar, stibnite, and other minerals; (3) Ag/Au ratios of <1 in ore; (4) an As-Hg-Sb-Tl geochemical signature; (5) low temperatures (~160°–240°C) and salinities of ore fluids (~1–6 wt % NaCl equiv) and fairly shallow depths of formation (<~2–3 km); and (6) lack of mineral and elemental zoning around ore. The four clusters share regional geologic controls related to formation as follows: (1) along the rifted margin of a craton, (2) within the slope facies of a passive margin sequence dominated by carbonates, (3) in the lower plate of a regional thrust fault, and (4) during a narrow time interval in the late Eocene (~42–34 Ma). The geometries and ore controls of the deposits in the four clusters are also very similar. At the deposit scale, ore and hydrothermal alteration are commonly associated with high-angle faults and preore low-angle contractional structures, including thrust faults and folds. The high-angle faults acted as fluid pathways for upwelling ore fluids, which were then diverted into lower angle favorable strata and contractional structures, where fluid-rock interaction led to replacement of carbonate and formation of ore. Rheologic contrasts between lithologies were also critical in diverting fluids into wall rocks. Common rheologic contrasts include contacts between thin- and thick-bedded lithologic units and the margins of contact metamorphic aureoles associated with Mesozoic intrusions. The similarities suggest common processes. Four critical processes are apparent: (1) development of source(s) for gold and other critical components of the ore fluids, (2) formation of fluid pathways, (3) water-rock interaction and gold deposition, and (4) a tectonic trigger, which was renewal of magmatism and a change from contraction to extension in the late Eocene. Consensus exists on these processes, except for the source of gold and other components of the ore fluid, with most models calling upon either a magmatic-hydrothermal source or a crustal source, where metals were scavenged by either meteoric or metamorphic fluids. Future research should focus on Carlin-style deposits in Nevada that exhibit epithermal characteristics and deposits that appear to have a clear genetic association with magmatic-hydrothermal systems associated with upper crustal intrusions. Rather than discrete types of ore deposits, there may be continua between Carlin-type gold deposits, epithermal deposits, and distal disseminated deposits, with the four large camps representing an end member.

Abstract Epithermal, Carlin, and orogenic Au deposits form in diverse geologic settings and over a wide range of depths, where Au precipitates from hydrothermal fluids in response to various physical and chemical processes. The compositions of Au-bearing sulfidic hydrothermal solutions across all three deposit types, however, are broadly similar. In most cases, they comprise low-salinity waters, which are reduced, have a near-neutral pH, and CO 2 concentrations that range from <4 to >10 wt %. Experimental studies show that the main factor controlling the concentration of Au in hydrothermal solutions is the concentration of reduced S, and in the absence of Fe-bearing minerals, Au solubility is insensitive to temperature. In a solution containing ~300 ppm H 2 S, the maximum concentration of Au is ~1 ppm, representing a reasonable upper limit for many ore-forming solutions. Where Fe-bearing minerals are being converted to pyrite, Au solubility decreases as temperature cools due to the decreasing concentration of reduced S. High Au concentrations (~500 ppb) can also be achieved in strongly oxidizing and strongly acidic chloride solutions, reflecting chemical conditions that only develop during intense hydrolytic leaching in magmatic-hydrothermal high-sulfidation epithermal environments. Gold is also soluble at low to moderate levels (10–100 ppb) over a relatively wide range of pH values and redox states. The chemical mechanisms which induce Au deposition are divided into two broad groups. One involves achieving states of Au supersaturation through perturbations in solution equilibria caused by physical and chemical processes, involving phase separation (boiling), fluid mixing, and pyrite deposition via sulfidation of Fe-bearing minerals. The second involves the sorption of ionic Au on to the surfaces of growing sulfide crystals, mainly arsenian pyrite. Both groups of mechanisms have capability to produce ore, with distinct mineralogical and geochemical characteristics. Gold transport and deposition processes in the Taupo Volcanic Zone, New Zealand, show how ore-grade concentrations of Au can accumulate by two different mechanisms of precipitation, phase separation and sorption, in three separate hydrothermal environments. Phase separation caused by flashing, induced by depressurization and associated with energetic fluid flow in geothermal wells, produces sulfide precipitates containing up to 6 wt.% Au from a hydrothermal solution containing a few ppb Au. Sorption on to As-Sb-S colloids produces precipitates containing tens to hundreds of ppm Au in the Champagne Pool hot spring. Sorption on to As-rich pyrite also leads to anomalous endowments of Au of up to 1 ppm in hydrothermally altered volcanic rocks occurring in the subsurface. In all of these environments, Au-undersaturated solutions produce anomalous concentrations of Au that match and surpass typical ore-grade concentrations, indicating that near-saturated concentrations of dissolved metal are not a prerequisite for generating economic deposits of Au. The causes of Au deposition in epithermal deposits are related to sharp temperature-pressure gradients that induce phase separation (boiling) and mixing. In Carlin deposits, Au deposition is controlled by surface chemistry and sorption processes on to rims of As-rich pyrite. In orogenic deposits, at least two Au-depositing mechanisms appear to produce ore; one involves phase separation and the other involves sulfidation reactions during water-rock interaction that produces pyrite; a third mechanism involving codeposition of Au-As in sulfides might also be important. Differences in the regimes of hydrothermal fluid flow combined with mechanisms of Au precipitation play an important role in shaping the dimensions and geometries of ore zones. There is also a strong link between Au-depositing mechanisms and metallurgical characteristics of ores.

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.

Abstract Mining of Carlin-type gold deposits in Nevada has made the United States one of the leading gold producers in the world for almost four decades. These deposits constitute an endowment of ~255 Moz (7,931 tonnes) of gold, of which 89% occurs in four main clusters of deposits: the Carlin trend, Getchell, Cortez, and Jerritt Canyon. These four clusters share many characteristics, including (1) formation during a narrow time interval (42–34 Ma), (2) lithologic and structural controls to fluid flow and ore deposition, (3) geochemical signature of the ores, (4) hydrothermal alteration and ore paragenesis, (5) relatively low temperatures and salinities of ore fluids, (6) fairly shallow depths of formation, and (7) lack of mineral and elemental zoning. A mineral systems approach to exploring for Carlin-type gold deposits in Nevada and elsewhere is presented, in which critical processes are laid out: (1) development of source(s) for gold and other critical components of the ore fluid, (2) formation of fluid pathways, (3) water-rock interaction and gold deposition, and (4) a tectonic trigger. The critical processes are then converted into a practical targeting system for Carlin-type gold deposits within and outside of Nevada, ranging from regional to district to drill target (<~20 km 2 ) scales. The critical processes of the Carlin mineral system are translated into targeting elements and mappable targeting criteria. At the regional scale, targeting elements for magmatic sources of gold and ore fluid components include (1) intrusive centers with a mantle component to the magmas, (2) processes that could result in metasomatized subcontinental lithospheric mantle, (3) high-K, H 2 O-rich calc-alkaline magmas, and (4) evidence for fluid release. For crustal sources of gold, targeting elements include (1) carbonaceous sedimentary rocks with diagenetic/syngenetic sulfides enriched in Au-As-Hg-Tl-Sb-(Te) and sulfates and (2) a heat source to drive convection of meteoric and/or formation of metamorphic fluids. Targeting elements for fluid pathways at the regional scale include (1) basement suture zones and rifted continental margins, (2) long-lived upper crustal faults that may be linked to basement faults, and (3) a reduced crustal section to ensure long transport of gold by sulfide-rich fluids. Targeting elements at the regional scale for water-rock interaction and gold deposition include (1) passive margin dominated by carbonate rocks, (2) contractional deformation and formation of regional thrust faults and fold belts, and (3) a regional Au-As-Hg-Tl-Sb-(Te) geochemical signature. Targeting elements for tectonic triggers include (1) changes from contraction to extension, (2) periods of intense magmatism, especially related to slab rollback, and (3) plate reorganization. At the district scale, targeting elements for fluid pathways include (1) old reactivated high-angle fault zones, (2) zones of abundant low-displacement, high-angle extensional faults, (3) fault intersections, and (4) lithologic rheology contrasts, such as preore intrusions and contact aureoles. For water-rock interaction and gold deposition, targeting elements include (1) carbonate-bearing stratigraphy, (2) low-angle features that could divert upwelling fluids out of high-angle faults and into reactive wall rocks, (3) hydrothermal system of targeted age, (4) alteration consistent with wall-rock reaction with acidic, sulfide-rich hydrothermal fluids, and (5) Fe-rich rocks in the stratigraphic section, which will drive sulfidation. At the drill target scale, the targeting elements for fluid pathways are zones of increased fault/fracture permeability. The targeting elements for water-rock interaction and gold deposition include (1) zones of increased low-angle permeability in carbonate rocks proximal to high-angle faults, (2) favorable alteration, especially hydrothermal carbonate dissolution and silicification, (3) Fe-rich rocks including ferroan carbonates and mafic volcanic rocks and intrusions, (4) favorable Au-As-Hg-Tl-Sb-(Te) geochemical signature with low base metals and Ag/Au ratios, and (5) favorable mineralization, especially arsenian pyrite with textures and chemistry consistent with Carlin-type deposits.

Abstract For the last several decades, gold exploration in Nevada has been strongly focused on sedimentary rock-hosted gold deposits in the Carlin, Cortez, Independence, and Getchell trends in north-central Nevada. Accordingly, less exploration activity has been directed toward the search for similar gold deposits in the eastern Great Basin, south and east of the major trends. Deposits in the central and northern Carlin and Cortez trends are hosted primarily in Upper Devonian middle slope soft-sediment slumps and slides and base-of-slope carbonate debris flows, turbidites, and enclosing in situ fractured lime mudstones. This is in marked contrast to gold deposits in the eastern Great Basin that are hosted primarily in three chronostratigraphic horizons: (1) shallow-water, Cambrian and Ordovician carbonate platform interior, supratidal karsted horizons and shelf lagoon strata, associated with eustatic sea-level lowstands and superjacent, transgressive calcareous shale and siltstone horizons that are deposited as sea level begins to rise, (2) Early Mississippian foreland basin turbidites and debris flows overlying karsted Late Devonian platform strata, and (3) Pennsylvanian and Permian shallow marine basin strata. Stratigraphic architecture in these three horizons was influenced in part by Mesozoic (Elko and Sevier) contractional deformation, including low-angle thrust and attenuation faults, boudinage, and large-scale folds, which in turn affected the orientation and localization of synmineral brittle normal faults. A compilation of past production, reserves, and resources (including historic and inferred) suggests an overall endowment of over 41 Moz of gold (1,275 tonnes) discovered to date in the eastern Great Basin, some in relatively large deposits. Significant clusters of deposits include the Rain-Emigrant-Railroad and Bald Mountain-Alligator Ridge areas on the southern extension of the Carlin trend, the Ruby Hill-Windfall-South Lookout-Pan on the southern extension of the Cortez trend, and the Long Canyon-West Pequop-Kinsley Mountain area near Wells, Nevada. Sedimentary rock-hosted gold deposits extend to the eastern edge of the Great Basin in Utah and Idaho and include the past-producing Black Pine, Barney’s Canyon, Mercur, and Goldstrike mines. The recognition of widespread, favorable host rocks and depositional environments on the Paleozoic platform-interior shelf in the eastern Great Basin opens up vast areas that have been relatively underexplored in the past. A basic premise throughout this paper is that the better we understand the origin of rocks and the depositional and postdepositional processes under which they formed, the more accurately we can make well-founded stratigraphic, sedimentological, structural, geochemical, and diagenetic interpretations. Without this understanding, as well as the rigorous application of multiple working hypotheses to explain our observations, the advance of science and the discovery of gold deposits is problematic.

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.