Carlin-type Deposits Research Articles

The giant Carlin gold province: a protracted interplay of orogenic, basinal, and hydrothermal processes above a lithospheric boundary

Northern Nevada hosts the only province that contains multiple world-class Carlin-type gold deposits. The first-order control on the uniqueness of this province is its anomalous far back-arc tectonic setting over the rifted North American paleocontinental margin that separates Precambrian from Phanerozoic subcontinental lithospheric mantle. Globally, most other significant gold provinces form in volcanic arcs and accreted terranes proximal to convergent margins. In northern Nevada, periodic reactivation of basement faults along this margin focused and amplified subsequent geological events. Early basement faults localized Devonian synsedimentary extension and normal faulting. These controlled the geometry of the Devonian sedimentary basin architecture and focused the discharge of basinal brines that deposited syngenetic gold along the basin margins. Inversion of these basins and faults during subsequent contraction produced the complex elongate structural culminations that characterize the anomalous mineral deposit “trends.” Subsequently, these features localized repeated episodes of shallow magmatic and hydrothermal activity that also deposited some gold. During a pulse of Eocene extension, these faults focused advection of Carlin-type fluids, which had the opportunity to leach gold from gold-enriched sequences and deposit it in reactive miogeoclinal host rocks below the hydrologic seal at the Roberts Mountain thrust contact. Hence, the vast endowment of the Carlin province resulted from the conjunction of spatially superposed events localized by long-lived basement structures in a highly anomalous tectonic setting, rather than by the sole operation of special magmatic or fluid-related processes. An important indicator of the longevity of this basement control is the superposition of different gold deposit types (e.g., Sedex, porphyry, Carlin-type, epithermal, and hot spring deposits) that formed repeatedly between the Devonian and Miocene time along the trends. Interestingly, the large Cretaceous Alaska–Yukon intrusion-related gold deposits (e.g., Fort Knox) are associated with the northern extension of the same lithospheric margin in the Selwyn basin, which experienced an analogous series of geologic events.

Nature, diversity of deposit types and metallogenic relations of South China

The South China Region is rich in mineral resources and has a wide diversity of deposit types. The region has undergone multiple tectonic and magmatic events and related metallogenic processes throughout the earth history. These tectonic and metallogenic processes were responsible for the formation of the diverse styles of base and precious metal deposits in South China making it one of the resource-rich regions in the world. During the Proterozoic, the South China Craton was characterised by rifting of continental margin before eruption of submarine volcanics and development of platform carbonate rocks, and the formation of VHMS, stratabound copper and MVT deposits. The Phanerozoic metallogeny of South China was related to opening and closing of the Tethyan Ocean involving multiple orogenies by subduction, back-arc rifting, arc–continent collision and post-collisional extension during the Indosinian (Triassic), Yanshanian (Jurassic to Cretaceous) and Himalayan (Tertiary) Orogenies. The Late Palaeozoic was a productive metallogenic period for South China resulting from break-up and rifting of Gondwana. Significant stratabound base and precious metal deposits were formed during the Devonian and Carboniferous (e.g., Fankou and Dabaoshan deposits). These Late Palaeozoic SEDEX-style deposits have been often overprinted by skarn systems associated with Yanshanian magmatism (e.g., Chengmenshan, Dongguashan and Qixiashan). A number of Late Palaeozoic to Early Mesozoic VHMS deposits also developed in the Sanjiang fold belt in the western part of South China (e.g., Laochang and Gacun). South China has significant sedimentary rock-hosted Carlin-like deposits, which occur in the Devonian- to Triassic-aged accretionary wedge or rift basins at the margin of the South China Craton. They are present in a region at the junction of Yunnan, Guizhou, and Guangxi Provinces called the ‘Southern Golden Triangle’, and are also present in NW Sichuan, Gansu and Shaanxi, in an area known as the ‘Northern Golden Triangle’ of China. These deposits are mostly epigenetic hydrothermal micron-disseminated gold deposits with associated As, Hg, Sb + Tl mineralisation similar to Carlin-type deposits in USA. The important deposits in the Southern Golden Triangle are Jinfeng (Lannigou), Zimudang, Getang, Yata and Banqi in Guizhou Province, and the Jinya and Gaolong deposits in Guangxi District. The most important deposits in the Northern Golden Triangle are the Dongbeizhai and Qiaoqiaoshang deposits. Many porphyry-related polymetallic copper–lead–zinc and gold skarn deposits occur in South China. These deposits are related to Indosinian (Triassic) and Yanshanian (Jurassic to Cretaceous) magmatism associated with collision of the South China and North China Cratons and westward subduction of the Palaeo-Pacific Plate. Most of these deposits are distributed along the Lower to Middle Yangtze River metallogenic belt. The most significant deposits are Tonglushan, Jilongshan, Fengshandong, Shitouzui and Jiguanzui. Au–(Ag–Mo)-rich porphyry-related Cu–Fe skarn deposits are also present (Chengmenshan and Wushan in Jiangxi Province and Xinqiao, Mashan-Tianmashan, Shizishan and Huangshilaoshan in Anhui Province). The South China fold belt extending from Fujian to Zhejiang Provinces is characterised by well-developed Yanshanian intrusive to subvolcanic rocks associated with porphyry to epithermal type mineralisation and mesothermal vein deposits. The largest porphyry copper deposit in China, Dexing, occurs in Jiangxi Province and is hosted by Yanshanian granodiorite. The high-sulphidation epithermal system occurs at the Zijinshan district in Fujian Province and epithermal to mesothermal vein-type deposits are also found in the Zhejiang Province (e.g., Zhilingtou). Part of Shandong Province is located at the northern margin of the South China Craton and the province has unique world class granite-hosted orogenic gold deposits. Occurrences of Pt–Pd–Ni–Cu–Co are found in Permian-aged Emeishan continental flood basalt (ECFB) in South China (Jinbaoshan and Baimazhai in Yunnan Province and Yangliuping in Sichuan Province). South China also has major vein-type tungsten–tin–bismuth–beryllium–sulphide and REE deposits associated with Yanshanian magmatism (e.g., Shizhuyuan and Xihuashan), important world class stratabound base metal–tin deposits (Dachang deposit), and the large antimony deposits (Xikuangshan and Woxi). During the Himalayan Orogeny, many giant deposits were formed in South China including the recently emerging Yulong and Gangdese porphyry copper belts in Tibet and the Ailaoshan orogenic gold deposits in Yunnan.

The Alshar epithermal Au-As-Sb-Tl deposit, southern Macedonia

The results of tectono-metallogenic, geological-structural, mineralogical, isotopic, and thermobarogeochemical studies at the Alshar deposit and in the adjacent area are discussed. The data obtained show that the gold mineralization at the deposit is similar to that observed at the Carlin-type deposits in the western United States. Similar characteristics include the Au-As-Sb-Tl-Hg geochemical assemblage; low Pb, Zn, Cu, and Ag contents in ore; widespread jasperoid and argillic metasomatic alterations of host siliciclastic-carbonate sedimentary rocks; and the spatial relations to fault zones. At the same time, the Alshar deposit differs from the Carlin-type deposits by the following features: (1) Pliocene age of mineralization, (2) expansion of mineralization over younger volcanics, (3) a high Tl grade in ore, and (4) localization of the ore field in a long-lived central-type magmatic structure. The results obtained can be used as an exploration model.

Thermal and isotopic profiling of the Pipeline hydrothermal system: Application to exploration for Carlin-type gold deposits

New exploration techniques are vital to the search for new orebodies in mature terranes, as well as for extensions of existing orebodies. This research focused on application of low-temperature dating techniques (primarily apatite fission-tracks) and stable isotope measurements (carbon and oxygen in carbonate rocks) in and around the Pipeline deposit, a Carlin-type gold system. The primary purpose of the project was to assess whether these techniques could provide exploration vectors that might be used in conjunction with other geologic, geochemical, and geophysical techniques to determine the locus of fossil hydrothermal fluid flow, and the attendant possibility of finding economic mineral deposits. At Pipeline, measurements of apatite fission-tracks and (U − Th) / He geochronometry yield a clear indication of the elevated temperatures associated with the fossil hydrothermal system. The pattern is one of a central target (Pipeline deposit) with decreasing thermal effects as far as several kilometers laterally from the known ore zone. Because of the irregular nature of fluid flow through fractures, a significant number of samples are required to discern this pattern, but the pattern is quite clear from the 32 samples in and around the Pipeline pit. Stable isotope measurements of carbonate rocks yield patterns centered on the Pipeline pit area. Oxygen isotopes in particular are shifted toward lower values as the result of interaction between the hydrothermal fluids and carbonate rocks. Carbon isotopes show a pattern, but it is somewhat more difficult to interpret than the oxygen isotope pattern. As with the geochronometric patterns, isotopic indications of fluid flow are present several kilometers from the ore zone at Pipeline. Also as with the geochronometric data, a relatively large sample set is required to see the pattern. At Pipeline, the patterns are evident in approximately 45 surface samples and very clearly in the cross-sections containing approximately 100 samples. From these data, it is clear that thermal and stable isotopic measurements on rocks at a significant distance from the known Pipeline hydrothermal system record the passage of hot fluids through the rock. Both techniques provide a footprint of the Pipeline system that is several diameters larger than the ore zone (as presently known). Therefore, thermochronologic and stable isotopic measurements can be utilized in conjunction with other techniques as part of an overall exploration strategy for Carlin-type deposits. Although these techniques do not provide a direct indication of the metal content of the fossil hydrothermal fluids, they do provide an indication of the robustness of fluid flow and the potential size of a hydrothermal system.

Igneous Geology of the Carlin Trend, Nevada: Development of the Eocene Plutonic Complex and Significance for Carlin-Type Gold Deposits

The Carlin trend contains the largest concentration of Carlin-type gold deposits in the world. Two major controversies about these giant gold deposits have been their age, which is now firmly established as Eocene, and the source of heat, fluids, and metals, which remains debated. We present data that demonstrate an intense period of Eocene magmatism coincided in time and space with deposit formation and was arguably the primary heat source. Geologic studies over the last 40 years have emphasized the stratigraphy and structure of Paleozoic sedimentary rocks, which are the major ore hosts. However, four igneous episodes affected the Carlin trend, in the Jurassic, Cretaceous, Eocene, and Miocene. A Jurassic diorite-granodiorite laccolith and related dikes were emplaced at about 158 Ma in the northern Carlin trend. A Cretaceous granite intruded the north-central part of the trend at 112 Ma. Abundant Eocene dikes intruded along most of the trend and were accompanied by lavas in a large volcanic field along the southwest edge of the trend between ~40 and 36 Ma. Miocene rhyolite lavas erupted just west of and across the southern part of the trend at ~15 Ma. Exposed Eocene rocks consist predominantly of silicic to intermediate dikes, lavas, and epizonal intrusions, which we interpret to be sourced from a large Eocene plutonic complex underlying the Carlin trend. Eocene dikes are present in most deposits, are generally altered, but, with a few exceptions, were poor ore hosts. Distinct Eocene igneous suites, which are restricted to specific areas of the trend, are from north to south: (1) 40.3 to 39.0 Ma rhyolite to dacite dikes in the northern Carlin trend, centered approximately on the Betze-Post deposit and the richest part of the trend; (2) 37.6 Ma porphyritic rhyolite dikes including those that host ore at the Beast deposit; (3) 38.6 Ma intermediate to silicic intrusions of Welches Canyon; (4) 38.1 to 37.4 Ma andesite to dacite lavas and shallow intrusions of the Emigrant Pass volcanic field; (5) 36.2 Ma rhyolite dikes in the Emigrant Pass field that are indistinguishable from the 37.6 Ma suite except for age and location; and (6) ~37.5 Ma rhyolite to dacite intrusions and lavas of the southern Carlin trend (Rain subdistrict). Additionally, a few basaltic andesite dikes were emplaced at 37.8 Ma near Dee in the northernmost part of the trend and at 38.2 Ma near Rain. The petrography, distribution, and age of the Eocene igneous suites and aeromagnetic data indicate that each suite is underlain by a major, silicic pluton. The longer lived suites require either multiple plutons or long-lived magma chambers. All Eocene dikes cannot have come from any single magma chamber, for example, from a chamber beneath the Welches Canyon intrusions as proposed by some. In this case, some igneous suites would have been emplaced only 12 to 15 km north of the northern edge of the source pluton, would not have been emplaced above or symmetrically around the source pluton, and would be distinct in age from the proposed source pluton. These requirements are not consistent with the distribution and age of the igneous suites. The combined data for the Eocene igneous rocks require a plutonic complex about 50 km long (north-south), essentially coincident with the northern and central Carlin trend, and between 12 and 23 km across, underlying an area of ~1,000 km2. This complex was emplaced over ~4 million years that coincided with the formation of the Carlin-type deposits of the Carlin trend. Although many factors contributed to the formation of the deposits and the Carlin trend, magmatic heat was abundant in the right place and at the right time to generate the deposits. The Carlin trend may be the largest concentration of Carlin-type deposits because the Eocene igneous episode there was the largest and longest lived of the Great Basin.

Ore fluid geochemistry of the Jinlongshan Carlin-type gold ore belt in Shaanxi Province, China

The Jinlongshan gold ore belt in southern Shaanxi Province contains a number of Carlin-type gold deposits in the Qinling collisional orogenic belt. Their fluid inclusions are of the Na+ − Cl− type. From the main metallogenic stage to later stages, the total quantity of anions and cations, temperature and deoxidation parameter (R) for fluid inclusions all gradually decreased, suggesting the gradual intensification of fluid oxidation, the reduction of metallogenic depth and the input of meteoric water and organic components. The deposits were formed during crustal uplifting and hence had similar tectonic settings to orogenic gold deposits. The CO2 contents and CO2/H2O values of the ore fluid increased from early to late stages, and the wall-rock alteration is represented by decarbonation, which is inconsistent with the characteristics of orogenic gold deposits. It is also discovered that Na+, K+, SO 4 2− , Cl− and the total amounts of anions and cations in the inclusions in quartz are higher than those in the coexisting calcite. The H, O and C isotope ratios indicate that the ore fluid was sourced from meteoric water and metamorphic devolatilisation of the sedimentary rocks that host the ores. The high background δ18O and δ13C values of wall rocks resulted in high δ18O and δ13C values of ore fluid and also high δ18O and δ13C values of hydrothermal minerals such as quartz and carbonate. The carbon in ore fluid stemmed largely from the hosting strata. The δ18O and δ13C values of Fe-calcite and the δD values of fluid inclusions are lower than those of calcite and quartz. In terms of the theory of coordination chemistry, all these differences can be ascribed to water-rock interaction in the same fluid system, instead, to the multisource of ore fluid.

Carlin-type Gold Prospects in Surigao del Norte, Mindanao Island, Philippines: Their Geology and Mineralization Potential

. Three calcareous sedimentary rock-hosted Carlin type-like gold prospects were mapped in a mineral production sharing agreement area of Philex Gold Philippines Inc. in Taganaan municipality, Surigao del Norte province in Mindanao island in the Philippines. They occur along a 20–25 km long trend of known epigenetic gold and porphyry copper deposits that lie close to several splays of the Philippine Fault Zone. The gold district forms part of the Late Cretaceous Eastern Mindanao Range that hosts early Paleogene and late Pliocene to Quaternary intrusive rocks. Gold is invisible in the jasperoid outcrops in Lascogon, Napo, and Danao prospects. The jasperoids occur in lenses of marls belonging to the Taganaan Marl Member that is associated to a turbiditic member of the Middle Miocene Mabuhay Formation. The marl lenses include gently dipping interbedded silty limestones and calcareous shales. The “invisible gold” mineralization in silicified calcareous rocks resembles Carlin-type deposits. Based on the mapped igneous and sedimentary rocks, a possible heat source for the gold mineralization is either or both of the two main phases of intrusion, Mabuhay An-desite or Alipao Andesite Porphyry. Forty-eight rock samples, fifteen stream sediment samples, and one soil sample were critical in delineating the general features of the potential Carlin-type prospects. The gold grades of jasperoids in the three prospects range from trace amounts to 20 g/t Au. Regional studies of gold and porphyry copper mineralization in the Surigao del Norte mineral district are important in delineating ore targets for drilling in the three prospects.

Solubility of gold in arsenian pyrite

Although Au and As can be enriched up to the weight percent level in arsenian pyrite, there is little knowledge of their limiting concentrations and nature of incorporation. This study reports SIMS and EMPA analyses showing that As and Au contents of arsenian pyrites plot in a wedge-shaped zone with an upper compositional limit defined by the line C Au = 0.02 ⋅ C As + 4 × 10 − 5 indicating a maximum Au/As molar ratio of ∼0.02. Arsenian pyrites with Au/As ratios plotting above this limit contain nanoparticles of native Au, as observed by HRTEM imaging/EDS analysis and SIMS depth profiling. In this case, a significant amount of the total Au is present in its elemental form. In arsenian pyrites with Au/As < 0.02, native Au nanoparticles were not observed by HRTEM, and all of the Au measured is inferred to be structurally bound in solid solution. The microanalytical results, coupled with previously published XANES-EXAFS spectroscopic measurements confirm that arsenian pyrite compositions plotting above this limit contains Au 0, whereas arsenian pyrite compositions plotting below the limit contain Au +1. On the basis of these observations, the upper bound is interpreted to represent a solubility limit for solid solution of Au as a function of As in arsenian pyrite between ∼150°C and ∼250°C, the approximate conditions under which samples used in the study were deposited. The Au-As composition of arsenian pyrite relative to this limit can be used to predict the chemical state of Au as well as the saturation state of Au in the hydrothermal solution that deposited it. These observations confirm that the parent hydrothermal solutions for the giant Carlin-type deposits, where solid solution of Au is dominant in arsenian pyrite, were largely unsaturated with respect to Au 0.

FORMATION OF A PALEOTHERMAL ANOMALY AND DISSEMINATED GOLD DEPOSITS ASSOCIATEDWITH THE BINGHAM CANYON PORPHYRY Cu-Au-Mo SYSTEM, UTAH--A DISCUSSION

Sir: The recent paper by Cunningham et al. (2004) presents reconnaissance apatite and zircon fission track, conodont alteration index, vitrinite and solid bitumen reflectance, and stable isotope data documenting the extent of the paleothermal anomaly associated with the 37 Ma Bingham Canyon porphyry Cu-Au-Mo deposit. Based on these data, the paleothermal anomaly encompasses the Barneys Canyon and Melco sediment-hosted gold deposits. The authors conclude that the gold deposits are genetically related to the Bingham porphyry system and that the maximum sustained temperature was 100° to 140°C at Melco and near 100°C at Barneys Canyon. Cunningham et al. (2004) contend that Barneys Canyon and Melco are, perhaps, distal disseminated Ag-Au deposits rather than Carlin-type deposits. Fluid inclusion data from ore-grade samples at Barneys Canyon (Presnell, 1992; Presnell and Parry, 1996) and Melco (R.D. Presnell, 1991, Constraints on a genetic model for the Melco gold deposit based on mineralogy, alteration, and fluid inclusions, unpub. report for Kennecott Exploration, 12 p.) yield homogenization temperatures in excess of 200°C. We contend that the paleothermal data support our previous conclusion that the temperature at Barneys Canyon during formation of the Bingham system was near ambient (i.e., 70°C, assuming 2 km of cover and a geothermal gradient of 35°C/km), and that the formation of Barneys Canyon was related to a hydrothermal system separate from Bingham. The data presented in Presnell (1992) and Presnell and Parry (1996) are from drill core through the center and periphery of the orebodies at Barneys Canyon, and the data from Melco (R.D. Presnell, 1991, unpub. report for Kennecott Exploration) are also from drill core through the orebody. All of the samples from Barneys Canyon used in the study by Cunningham et al. (2004) were collected from the pit wall after the orebody was mined, and only two samples from their …

Evidence for a magmatic origin for Carlin-type gold deposits: isotopic composition of sulfur in the Betze-Post-Screamer Deposit, Nevada, USA

We report here new sulfur isotope analyses from the Betze-Post-Screamer deposit, the largest Carlin-type gold deposit in the world. Carlin-type deposits contain high concentrations of arsenic, antimony, mercury, tellurium and other elements of environmental interest, and are surrounded by large volumes of crust in which these elements are also enriched. Uncertainty about the source of sulfur and metals in and around Carlin-type deposits has hampered formulation of models for their origin, which are needed for improved mineral exploration and environmental assessment. Previous studies have concluded that most Carlin-type deposits formed from sulfide sulfur that is largely of sedimentary origin. Most of these studies are based on analyses of mineral separates consisting of pre-ore diagenetic pyrite with thin overgrowths of ore-related arsenian pyrite rather than pure, ore-related pyrite. Our SIMS spot analyses of ore-related pyrite overgrowths in the Screamer zone of the Betze-Post-Screamer deposit yield δ34S values of about −1 to 4‰ with one value of about 7‰. Conventional analyses of realgar and orpiment separates from throughout the deposit yield δ34S values of about 5–7‰ with one value of 10‰ in the Screamer zone. These results, along with results from an earlier SIMS study in the Post zone of the deposit and phase equilibrium constraints, indicate that early arsenian pyrite were formed from fluids of magmatic origin with variable contamination from sulfur in Paleozoic sedimentary rocks. Later arsenic sulfides were formed from solutions to which sulfur of sedimentary origin had been added. The presence of Paleozoic sedimentary sulfur in Carlin-type deposits does not require direct involvement of hydrothermal solutions of sedimentary origin. Instead, it could have been added by magmatic assimilation of Paleozoic sedimentary rocks or by hydrothermal leaching of sulfur from wall rocks to the deposit. Thus, the dominant process delivering sulfur, arsenic, gold and mineralizing fluids to Carlin-type systems and their surrounding country rocks was probably separation of fluids from a magmatic source.

The physical and chemical evolution of low-salinity magmatic fluids at the porphyry to epithermal transition: a thermodynamic study

Fluid-phase relationships and thermodynamic reaction modelling based on published mineral solubility data are used to re-assess the Cu–Au-mineralising fluid processes related to calc-alkaline magmatism. Fluid inclusion microanalyses of porphyry ore samples have shown that vapour-like fluids of low to intermediate salinity and density (~2–10 wt% NaCl eq.; ~0.1–0.3 g cm−3) can carry percentage-level concentrations of copper and several ppm gold at high temperature and pressure. In epithermal deposits, aqueous fluids of similar low to intermediate salinity but liquid-like density are ubiquitous and commonly show a magmatic isotope signature. This paper explores the physical evolution of low-salinity to medium-salinity magmatic fluids of variable density, en route from their magmatic source through the porphyry regime to the near-surface epithermal environment, and investigates the chemical conditions required for effective transport of gold and other components from the magmatic to the epithermal domain. Multicomponent reaction modelling guided by observations of alteration zonation and vein overprinting relationships predicts that epithermal gold deposits are formed most efficiently by a specific succession of processes during the evolution of a gradually cooling magmatic–hydrothermal system. (1) The low-salinity to medium-salinity fluid, after separating from the magma and possibly condensing out some hypersaline liquid in the high-temperature porphyry environment, must physically separate from the denser and more viscous liquid, and then cool within the single-phase fluid stability field. By cooling under adequate confining pressure, such a vapour will evolve above the critical curve and contract, without any heterogeneous phase change, to an aqueous liquid of the same salinity. (2) High concentrations of gold, transported as stable Au bisulphide complexes supporting >1 ppm Au even at 200°C, can be maintained throughout cooling, provided that the fluid initially carries an excess of H2S over Cu+Fe on a molal scale. This condition is favoured by an initially high sulphide content in a particularly low-salinity magmatic fluid, or by preferential partitioning of sulphur into a low-salinity vapour and partial removal of Fe into a hypersaline liquid at high temperature. (3) Acid neutralisation further optimises gold transport by maximising the concentration of the HS− ligand. This may occur by feldspar destructive alteration along pyrite±chalcopyrite±sulphate veins, in the transition zone between the porphyry and epithermal environments. An alternative acid/base control is the dissolution of calcite in sediments, which may enable long-distance gold transport to Carlin-type deposits, because of the positive feedback between acid neutralisation and permeability generation. The three physical and chemical transport requirements for high-grade epithermal gold mineralisation are suggested to be the common link of epithermal gold deposits to underlying magmatic–hydrothermal systems, including porphyry-Cu–Au deposits. Both mineralisation types are the result of gradual retraction of isotherms around cooling hydrous plutons in similar tectonic and hydrologic environments. As magmatic fluid is generated at increasing depths below the surface the importance of vapour contraction increases, leading to the typical overprinting of potassic, phyllic and advanced argillic alteration and their related ore styles.

“Invisible” gold revealed: Direct imaging of gold nanoparticles in a Carlin-type deposit

Although As-rich, hydrothermal overgrowths on pyrite have been recognized as the primary host phase for Au in Carlin-type deposits in Nevada, the chemical and structural state of the Au has remained unresolved. Spectroscopic and electron imaging techniques have suggested that Au is either structurally bound (e.g., Au 1+ ) or occurs as particles of native Au (Au 0 ), but the latter has never been observed directly. We have determined that Au is present in signiÞ cant quantities as discrete nanoparticles of native Au (~5–10 nm) in As-rich overgrowths on pyrite from the Screamer deposit in the Carlin trend, Nevada, using analytical and high-resolution TEM and high-angle annular dark-Þ eld (HAADF) imaging in STEM mode. Electron microprobe and secondary ion mass spectrometry (SIMS) analyses of the As-rich rims containing the Au-particles reveal that these rims (1–20 ∝m) contain up to 0.8 wt% Au, among the highest Au-contents ever reported for arsenian pyrite. These observations suggest two possible mechanisms for nanoparticle formation: that Au exceeded its solubility limit in arsenian pyrite causing it to be deposited as nanoparticles of native metal; or that exsolution of native metal from metastable arsenian pyrite was caused by a later event in the history of the deposit.

Controversies on the Origin of World-Class Gold Deposits, Part I: Carlin-type Gold Deposits in Nevada

Editor’s note: This is the first of a two-part preview of an SEG-sponsored forum to be held in Reno, Nevada, on May 14, 2005. Meeting details are on p. 46.

Arsenopyrite and As-bearing pyrite from the Roudný deposit, Bohemian Massif

AbstractThe major- and trace-element chemistry of pyrite and arsenopyrite from the mesothermal Roudný gold deposits was studied by electron microprobe and laser ablation ICP-MS techniques. In total, four generations of pyrite and two of arsenopyrite were distinguished. The pyrite is enriched in As through an Fe (AsxS1–x)2substitution mechanism. The As-rich zones of pyrite-2 (up to 4.5 wt.% As) are also enriched in gold (up to 20 ppm), lead (commonly up to 220 ppm, exceptionally up to 1500 ppm) and antimony (commonly &lt;600 ppm, rarely up to 1350 ppm). Positive correlation of As and Au in the studied pyrites is not coupled with an Fe deficiency, in contrast to Au-rich As-bearing pyrites in Carlintype gold deposits. The As-rich pyrite-2 coprecipitated with the Sb-rich (1 –4.2 wt.%) and Au-rich (40 –150 ppm) arsenopyrite-1. The younger arsenopyrite-2 is significantly less enriched in these elements (0 –70 ppm of Au).The chemical zonality of pyrites in the Roudný gold deposits reflects the chemical evolution of orebearing fluids that are not observed in any other mineral phases. The data available suggest relatively high activity of sulphur and low activities of arsenic and gold during crystallization of the older pyrite generation (pyrite-1). Later, after particular dissolution of pyrite-1, Au-rich As-bearing pyrite-2 and arsenopyrite precipitated. These facts suggest a marked increase in the arsenic and gold activities in ore-bearing fluids. The As-content of pyrite-2 decreases in an oscillatory manner from the core to the rim, reflecting changes in the As activity or/and in theP-Tconditions. The As-bearing pyrites were formed at temperatures of at least 320–330°C, based on arsenopyrite thermometers and fluid inclusion data.

Alligator Ridge District, East-Central Nevada: Carlin-Type Gold Mineralization at Shallow Depths

Carlin-type deposits in the Alligator Ridge mining district are present sporadically for 40 km along the north-striking Mooney Basin fault system but are restricted to a 250-m interval of Devonian to Mississippian strata. Their age is bracketed between silicified ca. 45 Ma sedimentary rocks and unaltered 36.5 to 34 Ma volcanic rocks. The silicification is linked to the deposits by its continuity with ore-grade silicification in Devonian-Mississippian strata and by its similar δ 18 O values (~17‰) and trace element signature (As, Sb, Tl, Hg). Eocene reconstruction indicates that the deposits formed at depths of <300 to 800 m. In comparison to most Carlin-type gold deposits, they have lower Au/Ag, Au grades, and contained Au, more abundant jasperoid, and textural evidence for deposition of an amorphous silica precursor in jasperoid. These differences most likely result from their shallow depth of formation. The peak fluid temperature (~230°C) and large δ 18 O H2O value shift from the meteroric water line (~20‰) suggest that ore fluids were derived from depths of 8 km or more. A magnetotelluric survey indicates that the Mooney Basin fault system penetrates to mid-crustal depths. Deep circulation of meteoric water along the Mooney Basin fault system may have been in response to initial uplift of the East Humboldt-Ruby Mountains metamorphic core complex; convection also may have been promoted by increased heat flow associated with large magnitude extension in the core complex and regional magmatism. Ore fluids ascended along the fault system until they encountered impermeable Devonian and Mississippian shales, at which point they moved laterally through permeable strata in the Devonian Guilmette Formation, Devonian-Mississippian Pilot Shale, Mississippian Joana Limestone, and Mississippian Chainman Shale toward erosional windows where they ascended into Eocene fluvial conglomerates and lake sediments. Most gold precipitated by sulfidation of host-rock Fe and mixing with local ground water in zones of lateral fluid flow in reactive strata, such as the Lower Devonian-Mississippian Pilot Shale.

Evaluation of the Role of Sulfidation in Deposition of Gold, Screamer Section of the Betze-Post Carlin-Type Deposit, Nevada

This study was undertaken to evaluate the relation between and gold deposition in the Screamer section of the Betze-Post Carlin-type deposit. We also attempted to determine the source of sulfur in the deposit and the possibility that more than one gold-depositing event contributed metal to the Screamer system. Gold ore at Screamer forms a generally stratiform body hosted largely by the Wispy member of the Devonian Popovich Formation and lacks obvious alteration of wall-rock gangue minerals. Gold at Screamer is hosted by arsenian pyrite that forms disseminated grains and overgrowths on diagenetic pyrite. A strong correlation is observed between the gold content of rock samples and their proportion of ore-related (arsenian) pyrite, as determined by point counts. Isocon plots show that mineralization at Screamer involved addition of both sulfur and iron, along with gold, arsenic, antimony, tungsten, and local silica, barium, and phosphorus. Fe/Al vs. S/Al plots show that most ore samples at Screamer do not contain enough sulfur to account for all of their iron as pyrite; petrographic examination shows that the iron occurs in pyrite, ferroan dolomite, and iron-bearing micas in order of decreasing abundance. The Fe/Al vs. S/Al plots also show that Screamer samples with high gold contents contain more pyrite than samples with low gold contents and that samples inside the ore zone have more pyrite than those in the surrounding area. These relations are interpreted to indicate that Screamer has undergone at least two pyrite-forming events. The first event, which probably took place during diagenesis, involved incomplete that left some iron in carbonate and silicate minerals. The second event, which probably took place during gold mineralization, sulfidized most remaining iron and added pyrite in the Screamer ore zone. The δ 34 S values of chemically extracted sulfur from disseminated pyrite at Screamer range from –13.8 to 16.5 per mil, and δ 34 S values for hand-picked separates of pyrite and other sulfides in veins cutting these rocks range from –21.2 to 11.7 per mil. A significant fraction of samples with high gold contents and of samples in the ore zone regardless of gold content have δ 34 S values between –1 and 5 per mil. These data suggest that early diagenetic sulfur with a wide range of δ 34 S values was overprinted by sulfur with isotopic compositions in the –1 to 5 per mil range that was associated with gold mineralization. The δ 34 S values for gold-related sulfur at Screamer are lower than those reported for bulk mineral separates from most other Carlin-type deposits and from SIMS analyses of sulfides from the proximal Post part of the Betze-Post system and could be magmatic. Limited evidence can be found for multiple gold-forming events in the Screamer zone. Tungsten, which might have been introduced by the Jurassic-age Goldstrike stock, is widespread in the deposit and correlates closely with gold. A few samples with high tungsten/gold ratios found along fault zones might be part of an earlier phase of mineralization related to the Goldstrike stock. A δ 34 S value for pyrite in one of these samples is similar to the high values reported for sulfides in auriferous, polymetallic mineralization in the Post section of the deposit, but this mineralization is not reported to contain tungsten. Other veins containing sphalerite have lower δ 34 S values and lack consistent gold values. Whereas there is no correlation between gold values and the degree to which the host rocks have undergone sulfidation, there is a strong and highly significant correlation between gold values and the abundance of ore-related pyrite. This shows that simple of immediately adjacent wall rock cannot account alone for gold deposition at Screamer and it might not be the only ore-depositing process in some other Carlin deposits. A more general, extended sulfidation process could be important, however. One likely process involves mixing of an invading, mineralizing fluid containing sulfur and gold with a wall-rock fluid containing iron derived from adjacent or distal wall rocks. Recognition of the source(s) for this iron and fluid flow pathways responsible for introducing it to the ore zone could provide useful guidance in exploration for the next Carlin trend.

A Special Issue Devoted to Gold Deposits in Northern Nevada: Part 2. Carlin-Type Deposits

A Special Issue Devoted to Gold Deposits in Northern Nevada: Part 2. Carlin-Type Deposits

Geology and Geochemistry of the Deep Star Gold Deposit, Carlin Trend, Nevada

Deep Star is a high-grade Carlin-type gold deposit located in the northern part of the Carlin trend. The deposit averages 34.0 g/t Au and by year end 2000 had produced 37.8 t (1,217,000 oz) gold with a remaining reserve of 16.0 t (513,698 oz) gold. The deposit is primarily hosted in brecciated calc-silicate rocks of the Devonian Popovich Formation, with a minor amount of gold in the Jurassic Goldstrike diorite. Intrusion of the syn- and postore Deep Star rhyolite constrains the age of the mineralization. The postore rhyolite is compositionally and mineralogically similar to the synore dike and yielded an average 40Ar/39Ar isochron age of 38.3 Ma. Eocene rhyolite dikes intruded active, dilatant north- to northeast-striking faults and/or fractures, providing an important age constraint on the local stress regime at Deep Star during mineralization. Essentially horizontal, west-northwest-directed Eocene extension (291°) is consistent with dextral-normal oblique slip observed on north-south-striking, east-dipping portions of the Gen-Post fault system and dilation and sinistral shear on dike-filled, northeast-striking structures. A right-stepping, releasing bend in the Deep Star fault at its intersection with northwest- and north-northwest-striking subsidiary structures created a deep-tapping dilatant conduit for gold-bearing hydrothermal fluids. Five stages of hydrothermal alteration are present at Deep Star. Stages 1 and 2 are related to thermal metamorphism, metasomatism, and possibly retrograde alteration associated with intrusion of the Goldstrike stock. Stage 3 consists of preore quartz-carbonate alteration. The carbonate minerals are zoned both laterally and vertically, with dolomite as the dominant mineral closest to ore. Stage 3 overlaps with stage 4 quartz-kaolinite alteration associated with the introduction of gold and is therefore interpreted to be part of the Eocene mineralizing event. Gold is hosted in As-rich rims on marcasite and pyrite within a kaolinite and quartz matrix. Postore stage 5 is dominated by calcite with lesser siderite and barite. A three-dimensional geochemical model of the Deep Star deposit and its environs reveals that Carlin-type systems may have a geochemical expression involving a much broader suite of elements than previously recognized. Elements with distribution patterns considered to be related to the mineralizing event include Ag, As, Au, Ba, Bi, Ca, Cd, Co, Cu, Fe, Hg, Mg, Mo, Mn, Ni, P, Pb, S, Sb, Se, Te, Tl, U, V, W, and Zn. The multielement anomaly associated with the Deep Star deposit is best described as a vertical plume that is (1) focused between the Deep Star and Post faults below the deposit, (2) present in the footwall of the Deep Star fault and the Deep Star-Post fault corridor at the level of the deposit, and (3) broadly dispersed above, and to the west of, the deposit under the influence of northeast- and northwest-trending structural fabrics. Most closely associated with gold deposition are enrichments in As, Sb, Hg, Tl, Ag, and Zn within a halo of Ca, Mg, Ba, and Sr depletion. The elements Fe, Mn, Co, Ni, and P are most elevated in the immediate hanging wall of the Deep Star fault and above the deposit in a region where secondary carbonate veins (ankerite, kutnahorite, and Mn-rich dolomite); open-space-filling carbonate minerals (siderite, calcite) have been observed. Some lateral zonation is evident at levels above the deposit, with increases of Pb and Bi on its eastern margin and more Mo, U, V, and W west of the deposit. Isotopic analyses of kaolinite are interpreted to indicate that gold-mineralizing fluids originated from either a magmatic or possibly a deep metamorphic source. Fluids range from near the magmatic water field for those near the center of the Deep Star orebody along a mixing path toward exchanged mid-Tertiary meteoric water as represented by the fluids near the eastern orebody margin.

Lead in the Getchell-Turquoise Ridge Carlin-Type Gold Deposits from the Perspective of Potential Igneous and Sedimentary Rock Sources in Northern Nevada: Implications for Fluid and Metal Sources

Lead in the Getchell-Turquoise Ridge Carlin-Type Gold Deposits from the Perspective of Potential Igneous and Sedimentary Rock Sources in Northern Nevada: Implications for Fluid and Metal Sources

Source of Iron for Sulfidation and Gold Deposition, Twin Creeks Carlin-Type Deposit, Nevada

This study was undertaken to determine the source of iron in Comus Formation sedimentary rocks that were sulfidized during deposition of gold in the Megapit area of the Twin Creeks Carlin-type deposit. Sedimentary rocks in and near the Megapit contain ferroan dolomite, largely as overgrowths on iron-poor dolomite. Iron to form these overgrowths appears to have been released from mafic volcanic rocks that are interlayered with the sedimentary rocks. These igneous rocks have undergone two stages of hydrothermal alteration. The first stage involved formation of albite and iron-rich chlorite, possibly caused by interaction with seawater. The second stage involved destruction of the iron-rich chlorite by illite or sericite, which released iron to form ferroan dolomite in the sedimentary rocks. Comparisons show that transfer of iron from the igneous rocks to the sedimentary rocks can account for the present distributions of iron in these rocks. Relative to basalts, Comus Formation igneous rocks are enriched in iron and potassium. These results suggest that ferroan dolomite in sedimentary rocks is not solely a product of diagenetic processes and can form when iron is released from adjacent iron-bearing igneous rocks. Recognition of this additional mechanism for formation of ferroan dolomite expands the range of geologic settings that can be favorable for formation of gold deposits formed by sulfidation.