Carlin-type Deposits Research Articles

Large-scale basement mobilization endows the giant Carlin-type gold mineralization in the Youjiang Basin, South China: Insights from mercury isotopes

The metal source and genesis of hydrothermal mercury-rich gold metallogenic systems occurring far away from active continental margins remain puzzling. The Youjiang Basin of South China, where exists numerous Carlin-type gold deposits and some synmineralization hidden intrusions, is a natural laboratory to address this issue due to it was up to 1000 km inward from the active continental margins of South China when mineralization. Here, we use mass-independent fractionation of mercury isotope ratios (reported as ∆199Hg), which is predominantly generated during Hg photochemical reactions on Earth’s surface and has superiority of isotopic inheritance during hydrothermal processes, to address the metal source of the Youjiang Carlin-type gold deposits. Ore-associated sulfides from seven representative deposits display negative to near-zero ∆199Hg values (−0.29‰ to 0.04‰), which fall in between that of the regional Precambrian basement rocks (−0.21‰ to 0.06‰) and deep magmatic-hydrothermal systems (∼0‰), suggesting a binary mixing of Hg from these two sources. An isotope mixing model and mass balance calculations demonstrate that ∼1000 km3 of the basement rocks, which contributed to 86% of Hg budget, were leached and remobilized by magmatic-hydrothermal fluids and deep-circulating crustal fluids to endow the gold reserves of these deposits. Given that traditional S, Pb, C, and O isotopic data yielded indirect and ambiguous constraints on metal source due to their complex evolution processes and isotope fractionation during the fluids ascended. Our results, therefore, highlight the great advantage of using Hg isotope as a new tracer to understand metal sources of hydrothermal deposits.

Thermal history of Carlin-type gold deposits in Yukon (Canada) as revealed by organic matter geothermometry, clumped isotope data, fluid inclusion microthermometry, and apatite fission-track analyses

Sediment-hosted gold deposits in central Yukon have most of the diagnostic characteristics of Carlin-type gold deposits in Nevada. This study combines organic matter geothermometry with fluid inclusion microthermometry, clumped isotope data (Δ47) for late ore-stage hydrothermal calcite, and apatite fission-track analyses to constrain the thermal evolution of Carlin-type gold zones in central Yukon. The Tmax parameter derived from pyrolysis analyses indicates that organic matter is overmature and records regional temperatures of > 150 °C. Calcite and fluorite associated with the waning stage of mineralization at ca. 74 Ma have mean fluid inclusion homogenization temperatures of 123–173 °C, with an average salinity of 4.8 wt.% NaCl equiv. These temperatures overlap values of 91–162 °C determined from calcite clumped isotope measurements and are similar to data from Carlin-type deposits in Nevada. Fluid mixing is suggested by a variation of the isotopic composition of the fluid in equilibrium with calcite, with the higher temperature end-member having higher δ18OH2O values. In thermal models based on Tmax, fluid inclusion microthermometry, clumped isotope measurements, and apatite fission-track data, a higher temperature event at approximately 220 °C is consistent with pyrobitumen reflectance measurements. This event is either related to maximum tectonic burial prior to mineralization or to the flow of acidic hydrothermal fluids early in the main ore-stage. Each analytical technique used in this study is independent of the others and records part of the thermal and chemical evolution of the Yukon Carlin-type gold deposits and their host rocks.

Application of an approximate geostatistical simulation algorithm to delineate the gold mineralized zones characterized by fractal methodology

The Zarshuran Carlin-type gold deposit is located in the NW part of the Sanandaj-Sirjan metamorphic zone which hosts Neoproterozoic carbonate and black shale rock units. Concentration-Volume (C–V) fractal model and Turning Band Simulation method have been applied to effectively separate gold mineralized stages in the Zarshuran deposit. Three mineralized stages determined by the means of C–V fractal model and Turning Bands Simulation method were correlated with the mineralized stages considering geological studies. The results revealed the presence of enriched (>37 ppm) and Au high grade (5.5–37 ppm) mineralized zones within the deposit. A log ratio matrix was used to correlate the results obtained by the C–V fractal model and turning band simulation for Au and lithological units which showed that the main mineralized zones occur in the black gouges and Jasperoids.

Activation of Dolomite Flotation by Ferrous Hydroxide and Carbonate

The major problem with Carlin-type gold deposit flotation is that the high dolomite content in the concentrate decreases the quality of gold. Further, the activation mechanisms involved in dolomite flotation are still not fully understood. Herein, the correlation of Fe2+ conversion with xanthate adsorption and dolomite flotation was investigated to reveal the effect of dolomite embedded with pyrite. Flotation tests suggested that Fe2+ rather than Fe3+ improved the floatability of dolomite from 20% to 45%. Contact angles and thermodynamic tests indicated that the hydrophobicity of Fe2+-modified dolomite corresponds to the adsorption of xanthate. Importantly, time-of-flight secondary ion mass spectroscopy (Tof-SIMS) and x-ray photoelectron spectroscopy (XPS) attributed the activation of dolomite flotation to the formation of Fe(OH)2 and FeCO3. The coordination model of flotation successfully elucidated the selective adsorption of xanthate between Fe(OH)2, FeCO3 and FeOOH surfaces. The density function theory (DFT) simulation calculation was performed to identify the reaction rate at the atomic level, and the density of states (DOS) was also conducted to verify the conclusions at the electronic level. This study presents important surface chemistry evidence for understanding and regulating the poor selectivity in the flotation of Carlin-type gold deposits.

Carbonate U[sbnd]Pb and illite Rb[sbnd]Sr geochronology of sediment-hosted gold: A case study of Yata gold deposit

In situ U–Pb dating of carbonate minerals and Rb–Sr dating of illite was carried out on the Yata sediment-hosted disseminated-type gold deposit in the Youjiang basin, South China. The new study determines the timing of gold mineralization and clarifies the geological conditions required for the successful application of in situ U–Pb dating of alteration-related carbonate minerals. The results suggest that the Yata gold deposit likely formed during two metallogenic episodes. Illite Rb–Sr ages of 178.6 ± 5.8 Ma (MSWD = 1.2) for the 2–0.5 μm fraction and 171.3 ± 3.4 Ma (MSWD = 0.62) for the 0.5–0.2 μm fraction constrain an earlier episode of gold mineralization to >178.6 Ma. Hydrothermal carbonate minerals within the Yata gold deposit are associated with organic matter. In situ U–Pb dating of paragenetically early ankerite and later calcite veins yields ages of 149.0 ± 10.4 Ma (MSWD = 2.0) and 147.3 ± 32.6 Ma (MSWD = 1.8), respectively. These mineral ages and their petrographic relationships provide unequivocal evidence that another episode of gold mineralization occurred in the Early Cretaceous. Importantly, this study demonstrates in situ U–Pb dating is applicable to low-temperature deposits consisting of organic-rich hydrothermal carbonates characteristic of relatively high common Pb. Such deposits potentially include hydrothermal antimony, Carlin-type gold deposits, gold‐uranium, and carbonate-hosted uranium deposits.

Geodynamic factors in the formation of large gold-bearing provinces with Carlin-type deposits on continental margins in the North Pacific

<abstract> <p>Several similar indicators in Nevada (USA) and South Yakutia (Russia) gold-bearing provinces have been identified based on modern tectonic, geophysical and seismic tomography observations, followed by the analysis of the main geodynamic factors of the formation and distribution of large gold-bearing provinces in the North Pacific. One of the significant metallogenic peculiarities is a wide variety of formational and mineral deposits concentrated in the areas. Both provinces are situated at active margins surrounded by fold-thrust belts. In South Yakutia, a combination of sublatitudinal Baikal-Elkon-Ulkan and submeridional Seligdar-Verkhnetimpon gravity field gradient zones is recorded. In contrast, significant positive gravity anomalies of the Northern Nevada Rift and higher-order gradient zones are presented in Nevada. Large pluton and transform fault zones in both provinces support a conclusion about the fundamental role of geodynamic factors in developing ore-magmatic systems in the regions. Significant differences in the scale of the gold mineralization in the considered provinces are explained by the existence under the North American continent not only of the Mendocino transform fault zone but also of the Juan de Fuca paleo-spreading center. In contrast, the Inagli-Konder-Feklistov magmatic-metallogenic belt alone controls mineralization under the Asian continent.</p> </abstract>

The Relationship between Fluid Evolution and Hydrocarbon Accumulation and Metallization in the Nanpanjiang-Youjiang Basin: Evidence from Calcite Petrography and Fluid Inclusions

In the process of diagenesis and burial of sedimentary basins, basin fluid activities participate in the process of hydrocarbon accumulation and metal mineralization. Understanding the evolution of basin fluid is of great significance in revealing the related hydrocarbon accumulation and mineralization. Paleo-reservoirs are closely associated with Carlin-type gold deposits in the Nanpanjiang-Youjiang Basin, South China. Calcite, the fluid activity product, is closely related to bitumen and gold-bearing pyrite. By integrating petrographic, cathode luminescence, and fluid inclusion analysis, as well as the relevant chronological results of predecessors, this paper attempts to establish the relationship between fluid evolution, hydrocarbon accumulation, and gold mineralization. Two types of calcite (black/gray and white) developed in the Banqi-Yata-Laizishan area, the Nanpanjiang-Youjiang Basin. Black/gray calcite is symbiotic with bitumen and features dark red colors in cathode luminescence. Many hydrocarbon inclusions developed along with fluid inclusion analysis at low homogenization temperatures (65.7~173.1 °C). Combining the previously reported U-Pb ages (~250–230 Ma) of this kind of calcite with some geochemistry data on the associated reservoir and gold deposit, this calcite records the consecutive hydrocarbon accumulation and Carlin-type gold mineralization from the Late Permian to the Late Triassic periods controlled by Indosinian tectonic movement. The white calcite featuring bright red in cathodoluminescence is symbiotic with gold-bearing pyrite and realgar, and the associated fluid inclusions have high homogenization temperatures (128.2~299.9 °C). Combined with regional tectonic background and isotopic chronology (~140–106 Ma), it seems to record the early Cretaceous Carlin-type gold mineralization controlled by the subduction of the paleo-Pacific plate in the late Yanshanian period.

Combined leaching of Carlin-type gold deposit in Guizhou by potassium chlorate and bleaching powder

Cyanidation has been considered an effective process for extracting gold from gold-bearing ores for over 100 years. However, this process also has several disadvantages including it being toxic, inefficient, and unsuitable for leaching gold from carbonaceous gold ores containing organic and inorganic carbons. Moreover, it is not feasible for the extraction of gold from refractory gold ores , which are increasingly being used.Therefore, there is an urgent need to find an environmentally friendly and efficient leaching method to improve the extraction rate of gold from refractory gold ores. This study discusses using potassium chlorate and bleaching powder for combined leaching of refractory gold ore. Compared with other leaching methods, this method does not require the pre-oxidizing of the refractory gold ore. Moreover, the optimal reaction temperature required for the reaction is lower, thus achieving efficient and environmentally friendly leaching of refractory gold ore. To improve on the method, we used response surface methodology (RSM). RSM can quickly determine the quadratic function connection between the response value and each component and optimize the experimental settings to precisely forecast the experimental findings, saving a significant amount of time and money.This study presents a response surface approach based on a thermodynamic analysis of gold-bearing mineral oxidation to analyze the primary composite design experiment. The response surface model uses four components and three levels of potassium chlorate concentration, bleaching powder addition, reaction temperature, and reaction pH. The model gives 0.16 mol l−1 potassium chlorate, 37.5 g bleaching powder, 25 °C reaction temperature, and reaction pH of 13 as the optimal conditions for leaching gold using potassium chlorate and bleaching powder. Under these conditions, the gold leaching rate can reach 90.84%. In addition, the following parameters influence combined gold leaching in decreasing order: bleaching powder dose, reaction temperature, potassium chlorate concentration, and reaction pH.The results show that potassium chlorate and bleaching powder combined is advantageous for leaching of Carlin-type gold ore in Guizhou. It provides a high leaching rate. There is no need for pre-oxidation of gold ore. It is a simple inexpensive process that can be operated at a low optimum reaction temperature. Thus, it is a feasible method in industrial applications and provides a new way for gold leaching.

Sulfidation And Gold Precipitation In The Jugan Gold Deposit In Bau, Sarawak, East Malaysia: Insights From Correlation Plots And Factor Analysis

The Jugan Gold Deposit (JGD) is part of the Bau Mineral District (BMD) in Bau, Sarawak, East Malaysia. Although the mineral district is well studied, limited studies were conducted on the JGD. This paper presents the results of statistical studies using the multi-element geochemical data emphasizing the trace elements association with gold, alteration-mineralization, and the precipitation mechanism of the gold-bearing sulfide minerals. The correlation matrix displays positive correlation associations, particularly those associated with gold mineralization, i.e., arsenic, sulfur, antimony, and bismuth. Factor analysis grouped the trace elements into eight factors that reflect lithologies, mineralization, alteration, and geological processes in the JGD. Elements comprising the gold mineralization assemblage have the most significant factor (Factor 1) with the highest variance. The mineral assemblage was enriched during the alteration-mineralization process, as confirmed by the isocon plot. The barren samples (<0.01g/t) and gold-bearing samples (>0.2 g/t) plotted in a Fe vs. S diagram indicate that sulfidation is the precipitation mechanism of gold-bearing sulfide minerals. The immobility of iron and the vertical trajectory trend in the Fe vs. S diagram suggest that the possible source of Fe for gold-bearing sulfide minerals is the sedimentary host rock. Collectively, characteristics such as (1) the association of gold-bearing sulfide minerals with carbonate mineral assemblage and (2) high bismuth loadings on the main mineralization stage suggest a distinct geochemical characteristic of JGD relative to both Carlin-type deposits (CTD) and Carlin-like deposits (CLD); hence it is inferred to be a sedimentary-hosted gold deposit (SHGD). Establishing the JGD characteristics will contribute to a better understanding of the deposit and the BMD. Exploration-wise, it will assist future exploration work in delineating Au mineralization zones.

Timing of Rhyolite Intrusion and Carlin-Type Gold Mineralization at the Cortez Hills Carlin-Type Deposit, Nevada, USA

AbstractCarlin-type gold deposits (CTDs) of Nevada are the largest producers of gold in the United States, a leader in world gold production. Although much has been resolved about the characteristics and origin of CTDs in Nevada, major questions remain, especially about (1) the role of magmatism, whether only a source of heat or also metals, (2) whether CTDs only formed in the Eocene, and (3) whether pre-Eocene metal concentrations contributed to Eocene deposits. These issues are exemplified by the CTDs of the Cortez region, the second largest concentration of these deposits after the Carlin trend.Carlin-type deposits are notoriously difficult to date because they rarely generate dateable minerals. An age can be inferred from crosscutting relationships with dated dikes and other intrusions, which we have done for the giant Cortez Hills CTD. What we term “Cortez rhyolites” consist of two petrographic-geochemical groups of siliceous dikes: (1) quartz-sanidine-plagioclase-biotite-phyric, high-SiO2 rhyolites emplaced at 35.7 Ma based on numerous 40Ar/39Ar dates and (2) plagioclase-biotite-quartz ± hornblende-phyric, low-SiO2 rhyolites, which probably were emplaced at the same time but possibly as early as ~36.2 Ma. The dikes form a NNW-trending belt that is ~6 to 10 km wide × 40 km long and centered on the Cortez Hills deposit, and they require an underlying felsic pluton that fed the dikes. Whether these dikes pre- or postdated mineralization has been long debated. We show that dike emplacement spanned the time of mineralization. Many of both high- and low-SiO2 dikes are altered and mineralized, although none constitute ore. In altered-mineralized dikes, plagioclase has been replaced by kaolinite and calcite, and biotite by smectite, calcite, and marcasite. Sanidine is unaltered except in a few samples that are completely altered to quartz and kaolinite. Sulfides present in mineralized dikes are marcasite, pyrite, arsenopyrite, and As-Sb–bearing pyrite. Mineralized dikes are moderately enriched in characteristic Carlin-type elements (Au, Hg, Sb, Tl, As, and S), as well as elements found in some CTDs (Ag, Bi, Cu, Mo), and variably depleted in MgO, CaO, Na2O, K2O, MnO, Rb, Sr, and Ba. In contrast, some high-SiO2 rhyolites are unaltered and cut high-grade ore, which shows that they are post-ore. Both mineralized and post-ore dikes have indistinguishable sanidine 40Ar/39Ar dates. These characteristics, along with published interpretations that other giant CTDs formed in a few tens of thousands of years, indicate the Cortez Hills CTD formed at 35.7 Ma. All Cortez-area CTDs are in or adjacent to the Cortez rhyolite dike swarm, which suggests that the felsic pluton that fed the dikes was the hydrothermal heat source. Minor differences in alteration and geochemistry between dikes and typical Paleozoic sedimentary rock-hosted ore probably reflect low permeability and low reactivity of the predominantly quartzofeldspathic dikes.Despite widespread pre-35.7 Ma mineralization in the Cortez region, including deposits near several CTDs, we find no evidence that older deposits or Paleozoic basinal rocks contributed metals to Cortez-area CTDs. Combining our new information about the age of Cortez Hills with published and our dates on other CTDs demonstrates that CTD formation coincided with the southwestern migration of magmatism across Nevada, supporting a genetic relationship to Eocene magmatism. CTDs are best developed where deep-seated (~6–8 km), probably granitic plutons, expressed in deposits only as dikes, established large, convective hydrothermal systems.

Origin and re-Os geochronology of vein bitumen in the Nanpanjiang basin, SW China: Implication for the ore-forming age of Carlin-type gold deposits

The Nanpanjiang basin has the second-largest concentration of Carlin-type gold deposits in the world. Although significant progress has been made during the past two decades, the age of gold mineralization has not been well constrained. Field observation shows that these gold deposits are spatially associated with hydrocarbon reservoirs. At the Lannigou gold deposit, solid bitumens exist as films within structural fractures and cut through the orebodies, indicating that these bitumens formed after gold mineralization. In this work, we systematically studied bitumen reflectance (Rb), trace element, carbon isotope, and Re-Os isotopic compositions. These results show that these bitumens have high Rb values ranging from 2.40 to 2.71, indicating that they are high-maturity pyrobitumen. Trace element analysis reveals that bitumens in the deposit have high V/Ni, V/(V + Ni), and Zr/Sc ratios, but low Ni/Co, La/Sc ratios, and exhibit relatively flat rare-earth element (REE) patterns with no obvious Ce or Eu anomalies, comparable to the corresponding elemental ratios and REE patterns of Permian source rocks in the region. These bitumens also have similar δ13C values (-24.4 ‰ to –23.8 ‰, mean = -24.1 ‰) to the Permian source rocks (–22.4 ‰ to 24.7 ‰, mean = –23.6 ‰), indicating that they originate from the Permian source rocks. Five bitumen samples cut through the orebody yield a well-defined isochron age of 173 ± 6 Ma, indicating that the Lannigou gold deposit formed before this time. Our new data, combined with previously published geochronological data, suggest that the majority of Carlin-type gold deposits in the Nanpanjiang basin were mineralized at late Triassic to early Jurassic. The Re-Os data of this study also imply that the bitumen Re-Os chronometer has a large potential to constrain the timing of metal mineralization in low-temperature hydrothermal deposits.

The roles of hydrocarbons on the mineralization of Carlin-type gold deposits, Nanpanjiang Basin, South China

The spatial association of hydrocarbons with metalliferous ore deposits, like MVT lead–zinc deposits, sandstone uranium deposits and various type gold deposits (epithermal Au-Ag (-Hg) deposits, orogenic gold deposits and Carlin-type gold deposits) is found worldwide. However, the genetic relationship between hydrocarbon evolution and mineralization is still under controversy. Integrating isotope dates related to timing of petroleum evolution and gold mineralization from published data, as well as high-resolution transmission electron microscopy (TEM) analysis of pyrobitumen, electron probe microanalysis (EPMA) on both pyrobitumen and pyrite, the In-situ S isotope analyses of the gold-bearing pyrite in this study, we consider different type hydrocarbons (liquid oil and gas) played different roles on the mineralization of Carlin type gold deposits. As the residue of liquid oil cracking, the widespread high maturity pyrobitumen within the reservoir indicate both oil and gas has ever accumulated in the Nanpanjiang Basin. The high gold abundance (EPMA: ∼120 ppm; TEM: ∼1500 ppm) within the pyrobitumen in agreement with the metal solubility experiment suggest liquid oils could have acted as gold carrier before metal precipitation. The late Triassic timing overlapping between gas generation and gold mineralization and the δ34S features with gold bearing pyrite ∼ 10 ‰ lighter than the coeval seawater sulfate indicate gas dominated TSR process may play an important role on the supply of gold bisulfide complex (Au(HS)2−) in the ore-forming fluid and directly involved the precipitation of the Carlin-type gold deposit.

Progressive crushing 40Ar/39Ar dating of a gold-bearing quartz vein from the Liaotun Carlin-type gold deposit, Guangxi, southern China

Carlin-type gold deposits are among the largest hydrothermal gold deposits in the world. However, direct dating the metallogenic age of these deposits is difficult, because not all deposits provide material suitable for conventional radiometric methods. Syn-mineralization stage quartz veins from these deposits usually contain abundant fluid inclusions, which allow fluid inclusion 40Ar/39Ar dating. In this study, progressive crushing 40Ar/39Ar dating has been performed on a gold-bearing quartz vein from the Liaotun Carlin-type gold deposit in northwestern Guangxi, China. Argon isotopes liberated from the later steps yielded an isochron age of 200.7 ± 2.1 Ma. We infer that Ar-bearing gas was extracted from the primary fluid inclusions, and that the age of ca. 200.7 Ma reflects the timing of gold mineralization. The initial 40Ar/36Ar ratio corresponding to the isochron is 298.0 ± 4.3, which is statistically indistinguishable from the value for air, indicating that the ore-forming fluids probably mainly derived from gravitational pressure flow in the basin of air-saturated water. Our preliminary study shows the feasibility and great potential of 40Ar/39Ar dating of gases from fluid inclusions by progressive crushing of quartz veins to date the mineralization age and decipher the fluid origins of Carlin-type gold deposits.

Genesis of the Tangshang Au Deposit in Southeast Yunnan Province, China: Constraints from In Situ Chemical and S-Sr Isotope Analyses

The Yunnan–Guizhou–Guangxi district (also known as the Dian–Qian–Gui “Golden Triangle”) in southwestern China contains numerous Carlin-type Au deposits (CTGDs). However, the sources of Au and Au-bearing fluids in these deposits remain controversial. The Tangshang Au deposit is a middle-sized CTGD in southeastern Yunnan Province. This study involved in situ chemical and S isotope analyses of sulfides and in situ trace elemental and Sr isotope analyses of ore-related calcite; these data were used to trace the sources of fluids and Au, as well as the genesis of this deposit. Four pyrite types (Py1, Py2, Py3, and Py4) and two arsenopyrite types (Apy1 and Apy2) were identified based on their textural characteristics. It was found that Py1 contains relatively lower Au, Sb, Cu, and Tl contents than those of Py2, Py3, and Py4. Py1 is wrapped by rim-Py2 and Py3, which indicates an early-ore-stage genesis. The Carlin-type mineralization elements are elevated in the pyrites (Au = 3.04–38.1 ppm; As = 40,932–65,833 ppm; Tl = 0 to 3.3 ppm; Sb = 1.2 to 343 ppm; and Cu = 10 to 102 ppm), and the average Co/Ni ratio is 0.54. Additionally, Au has a positive correlation with Tl and Cu. The high concentrations of As and Au in all types of pyrite indicate that the ore-forming fluids are rich in both elements. The sulfides in the ores were shown to produce similar S isotope ratios, which are obviously higher than the S isotope value of sulfide (~0‰) in Emeishan basalt; therefore, the integration of these and elemental composition data indicated that all pyrites (Py1, Py2, Py3, and Py4) form during the ore stage. These results also demonstrate that the δ34S values of the Au-bearing fluids are higher than those of basalt wall rocks. The flat chondrite-normalized REEs pattern and positive Eu anomaly of the calcite were similar to those obtained from Emeishan basalt, which suggests a reducing characteristic of hydrothermal fluids. The 87Sr/86Sr ratios (0.70557–0.70622) of calcite were also comparable to the range obtained from Emeishan basalt. Some slightly higher 87Sr/86Sr ratios, which ranged between those obtained from Emeishan basalt and limestone from the Maokou Formation, indicated that the Sr isotope ratios of the Au-bearing fluids are higher than those of Emeishan basalt. Based on data generated in the present study and the regional geology of this area, a genetic model involving a metamorphic fluid system was proposed for the Tangshang gold deposit, and a gold mineralization event related to metamorphic fluid in the south of the Dian–Qian–Gui “Golden Triangle” was indicated.

Monitoring the evolution of sulfur isotope and metal concentrations across gold-bearing pyrite of Carlin-type gold deposits in the Youjiang Basin, SW China

The Youjiang Basin in Southwest China is the world’s second largest Carlin-type gold (Au)-producing region. However, the source of reduced sulfur that accounts for Au transport in ore-forming fluids remains controversial. Finely characterizing the sulfur isotopic compositions (δ34S values) in micron-scale zonation of Au-bearing pyrite is the key to clearly identify sulfur source. Here, we used high-resolution nanoscale secondary ion mass spectrometry (Nano-SIMS) to characterize the temporal variation in δ34S values and its relationship with metal contents across Au-bearing pyrite from the Linwang and Badu deposits in the Youjiang Basin, with the aim of monitoring the source and evolution of reduced sulfur in auriferous fluids. The Au-bearing pyrite rims in the Linwang deposit contain three growth stages that record episodic injections of Au- and As-rich fluids. Within these rims, the δ34S values vary inversely with Au concentrations. The inner rims with the high Au contents have δ34S values of −1.7‰ to +3.3‰ that are comparable to those of magmatic sulfur. The outer rims with decreasing Au contents have δ34S values of +1.3‰ to +15.7‰ that gradually approach those of pre-ore pyrite in the host rock. Such a variation indicates that the reduced sulfur in the initial Au-bearing ore-forming fluids was primarily originated from deep magmatic-hydrothermal systems while the host rock-derived 34S-enriched sulfur increasingly dominated through fluid-rock interactions during mineralization. In contrast, Au-bearing pyrite from the Badu deposit has positive δ34S values ranging from +9.0‰ to +25.8‰, which overlap those of diagenetic pyrite in the Devonian sedimentary rocks. Combining the intimate spatial association between Au mineralization and the Devonian strata, we propose that the initial ore-forming fluids have leached substantial sulfur from the Devonian strata. Significant contaminations of sedimentary sulfur erased the primary sulfur isotopic signals of the initial auriferous fluids. Our interpretations of these two deposits may also apply to other Carlin-type Au deposits in the Youjiang Basin, where δ34S values of Au-bearing pyrite show host rock-dependent variations. This study demonstrates that high-resolution Nano-SIMS sulfur isotope and elemental analysis of Au-bearing pyrite is a potent tool for tracing the source and evolution of reduced sulfur in ore-forming fluids for sedimentary-host Au deposits worldwide.

The Paleo-Tethys subduction and related mineralization in the SW Yangtze Block: Evidence from the Zhesang Carlin-type gold deposit

The Zhesang gold deposit is a Carlin-type gold deposit located in the “Golden Triangle” structural belt of Yunnan-Guizhou-Guangxi. Pyrite and arsenopyrite are important gold-bearing minerals of the deposit. In order to understand the typomorphic characteristics of pyrite and arsenopyrite and the enrichment mechanism of gold, morphological and structural features of these sulphides and the occurrence state of gold were investigated by using reflected light microscope and electron probes (EPMA). The results show that pyrites are characterized mainly by medium-fine broken pentagonal-dodecahedron crystal forms. Most of the arsenopyrites display needle and columnar crystal forms. Compositionally, pyrite is depleted in both Fe and S, whereas arsenopyrite is As-depleted but S-enriched. The w(Co)/w(Ni) of pyrite is between 1.731 and 3.215 (i.e., within the range of ratio of the hydrothermal pyrite, which is between 1 and 5). The crystal forms and compositional features of pyrite and arsenopyrite indicate that the Zhesang gold deposit is typical of medium-low temperature hydrothermal origin. The positive correlation between As and Au and the Au/As (mol%) ratio are lower than the solubility limit of Au in arsenic pyrite. Combined with previous studies, our data suggest that gold is likely to exist in the form of Au+ complex. Arsenic pyrite and arsenopyrite condensed Au in unsaturated ore-forming hydrothermal fluids relative to Au0 through surface voids and/or defects and the As content, forming a Zhesang Carlin-type gold deposit. Based on the age data of the Mesozoic polymetallic deposits in South China, it can be concluded that the age of the Zhesang gold deposit (215.3 ± 1.9 Ma), is consistent with the age of the main body of the Mesozoic polymetallic deposits (230–200 Ma) in South China Plate and the age of the granites (230–200 Ma) related to tungsten-tin mineralization in the Cathaysian Block. Therefore, we believe that the Zhesang gold deposit is the product of the subduction regime of the Paleo-Tethys Ocean in the Middle-Late Triassic.

Sediment-hosted disseminated gold deposits in orogenic belts: An example from the giant Jiagantan gold deposit in the West Qinling orogen, China

Sediment-hosted disseminated gold deposits in orogenic belts share many similarities to both epizonal orogenic and Carlin-type gold deposits, leading to a lot of difficulties in their genetic classification. The giant Jiagantan gold deposit was newly discovered in the West Qinling orogen, central China, and was selected to provide a better understanding of this issue. Gold mineralization at Jiagantan is hosted within sandstone and silty slate, and the gold is intimately associated to disseminated pyrite and arsenopyrite. Three generations of pyrite (Py0, Py1, and Py2) and two generations of arsenopyrite (Apy1 and Apy2) are recognized and used to perform in situ textural, chemical, and sulfur isotopic analyses.The diagenetic Py0 is framboidal and porous, and contains elevated concentrations of As, Sb, Co, Ni, Cu, Ti, and V. The early mineralization stage Py1 and Apy1 are disseminated within sandstone and silty slate, and have the highest average Au concentrations of 7.7 ppm and 12.6 ppm, respectively. The intermediate mineralization stage Py2 and Apy2 are hosted by quartz veins and veinlets, coexisting with stibnite, chalcopyrite, and minor amounts of other sulfides. Gold mineralization mainly took place during the early mineralization stage. Microscopic observations and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) output graphs confirm the majority of gold may reside in the crystallographic structures of Py1 and Apy1.The diagenetic Py0 grains show negative δ34S values of −25.2 to −20.1‰, implying the derivation of sulfur by bacterial reduction of marine sulfate. The hydrothermal Py1, Py2, Apy1, and Apy2 have higher but still negative δ34S values ranging from −16.4 to −5.0‰, suggesting an origin of sulfur from metamorphism of the underlying Devonian sedimentary rocks.The Jiagantan gold deposit can best be classified as an epizonal orogenic gold deposit owing to the possible metamorphic origin of Au, As, S, and other associated elements. The source of ore-forming materials is the key to discriminating orogenic gold deposits from Carlin-type gold deposits, while the type of host rocks, the style of mineralization, and the occurrence state of gold may not be the effective indicators.

Trace elements in pyrite from five different gold ore deposit classes: a review and meta-analysis

Abstract Scientific studies that use pyrite trace element signatures to facilitate interpretations of the geological history of hydrothermal gold deposits are becoming more common. Given this rise in popularity in pyrite trace element chemistry studies, a review of trace element incorporation in pyrite from various gold deposit classes is highly timely. We provide a global compilation of over 20 000 chemical data points resulting from over 5520 spot measurements of pyrites from five separate ore deposit classes, each of which is known to have high gold concentrations. The ore deposit classes considered here include orogenic gold deposits, Carlin-type gold deposits, high- and low-sulfidation gold deposits, and Au-bearing porphyry systems, whereas the pyrite minor- and trace elements that we consider include Au, As, Bi, Co, Cu, Ni, Pb, Sb, Se, Te, Tl and Zn. Meta-analyses of these data suggest that, on a global scale, both existing pyrite trace element chemistry discrimination tools and the newly-established principal component analysis (PCA) approach have a relatively low efficacy in correctly predicting the metallogenic setting in which the pyrite formed. At a deposit scale, however, pyrite trace element chemistry continues to be a useful tool for understanding Au mineralizing processes, especially when coupled with additional analytical techniques or lines of geological evidence. Further meta-analysis of the data tentatively suggests the solubility limits for the various trace elements in pyrite mineral structure and reveals that the correlations between temperature and trace element incorporation (i.e. pyrite geothermometry) are complex. The study highlights that pyrite, by virtue of its mineralogy and propensity for element substitutions, remains an important repository of chemical information related to the evolution of hydrothermal gold mineralizing systems. In as much as this repository is now readily accessed utilizing modern analytical techniques (e.g. routine LA-ICP-MS mapping of trace element distributions at ppb concentrations; mapping element distributions at the nanoscale using atom probe tomography), its full value will only be realized through continued study into the kinetics and mechanisms of trace element incorporation into pyrite structure at conditions relevant to ore-forming processes.

Nanoscale isotopic evidence resolves origins of giant Carlin-type ore deposits

Abstract The western North American Great Basin's Carlin-type deposits represent the largest accumulation of gold in the Northern Hemisphere. The controversy over their origins echoes the debate between Neptunists and Plutonists at the birth of modern geology: were the causative processes meteoric or magmatic? Sulfur isotopes have long been considered key to decoding metal cycling in the Earth's crust, but previous studies of Carlin-type pyrite lacked the spatial resolution to quantify differences among the numerous generations of sulfide mineralization. We developed a new dual-method, nanoscale approach to examine the fine-grained ore pyrite. The δ34S of the ore pyrite varies systematically with Au concentration at the nanoscale, indicating that both magmatic and meteoric fluids contributed during mineralization, but the magmas brought the gold. Repeated oscillations in fluid ratios upgraded the metal content, resulting in high gold endowment. Our results demonstrate that high-spatial-resolution studies are key to elucidate the spatiotemporal evolution of complex hydrothermal systems.

“Roll-Front” Mass Transfer of Carbonate Cations in Carlin-Type Gold Deposits: Insights from UV-Fluorescent Calcite Veins

Abstract Hydrothermal fluids dissolve and precipitate vast quantities of carbonate during the formation of Carlin-type gold deposits. This mass transfer results in extensive volumes of decarbonatized rock and hydrothermal calcite with distinct δ18O and δ13C signatures, occurring as both veins and wall-rock pseudomorphs. However, the processes which cause the fluids to switch between carbonate-dissolving and carbonate-precipitating are not well understood. Here, we present a model for the formation of ore-stage calcite through the pseudomorphic replacement of pre-existing calcite based on a detailed study of UV-fluorescent (UVF) calcite veins from the Nadaleen trend Carlin-type gold deposits in Yukon. The UV fluorescence in the veins is a visual indicator of their high-Mn, low-Fe chemistry, which we use to develop our model. The proximity of UVF calcite veins to ore-stage alteration and the realgar within the veins indicate that they formed during Au mineralization. Vein orientation and crosscutting relationships with faults and dikes suggest they formed by replacing pre-existing calcite veins that developed during folding. Cathodoluminescence responses, showing the selective replacement of pre-ore calcite by Mn-rich calcite, support this hypothesis. We propose that differences in Mn, Sr, and Mg content between pre-existing calcite and UVF calcite drive this replacement reaction as the fluids approach calcite saturation. UVF calcite veins have fluid-dominated δ13C and δ18O compositions, suggesting that the fluids dissolved fluid-buffered hydrothermal carbonate upstream along the flow path. In our model, unreactive and high-permeability decarbonatized rock allows upstream fluids to reach reactive calcite at the edge of the dissolution zone, where they replace the calcite with ore-stage calcite pseudomorphs. Over several iterations of this process, the fluids repeatedly precipitate and dissolve ore-stage calcite, moving it downstream as a roll front, which concentrates Mn at the edge of the dissolution zone. We suggest that this mass transfer also occurs at the deposit scale to form zoned carbonate alteration along the flow path. Additionally, we present carbonate clumped isotope thermometry results that indicate UVF calcite veins formed at temperatures of ~140°C, although higher temperatures of up to 300°C are possible if the veins exhumed slowly after their formation. Calculated fluid δ13C and δ18O values at these temperatures suggest a meteoric fluid source, although the isotopic data are not inconsistent with a magmatic source if fluid temperatures were ≥200°C because less isotopic fractionation occurs at higher temperatures.