Hydrothermal Gold Deposits Research Articles

Archean atmospheric sulfur stocked in the Cretaceous gold deposits

The giant Cretaceous Jiaodong gold province is the world's third largest gold mining district, while the source of the ore-forming fluids and AuS remains controversial. The elemental sulfur is a key carrier for gold and the isotopic composition is an ideal tracer of the sulfur source, which makes it crucial for understanding gold deposit genesis and guiding ore exploration. Here, we show the Au-bearing pyrite in the gold deposits from Jiaodong bears a non-zero Δ33S signature from −0.07 ‰ to 0.31 ‰ showing the mass-independent fractionation (S-MIF). This clearly indicates involvement of Archean atmospheric sulfur during the Cretaceous gold mineralization. The MIF-bearing sulfur was likely remobilized from the Archean supracrustal rocks in the basement of the Jiaodong region via fluid-rock interaction while the hydrothermal ore-forming fluids passed through the crustal-scale fault zones. The Phanerozoic gold deposits from Jiaodong have relatively minor S-MIF signals compared with the Archean orogenic gold deposits due to more material (S) cycling and dilution. This new discovery of S-MIF signal in the Phanerozoic ore deposits implies that the recycling of crustal sulfur may be a more common process and play an important role in forming hydrothermal gold deposits.

Metal remobilization at the Nibao gold deposit in Youjiang basin, SW China: Implications from mineral texture, trace element and sulfur isotope compositions of sulfides and TiO2 polymorphs

Multistage mineralization overprinting is important for the formation of giant hydrothermal gold deposits. The Youjiang basin in SW China has undergone at least three major deformation phases in the Mesozoic, but their relationship with the large-scale gold accumulation in the region remains enigmatic. Here, we study the large Nibao gold deposit (reserve > 70 t Au) in the northern part of the basin, which has both fault-controlled and stratabound orebodies. We identified a pre-ore stage and syn-ore stage including four tectono-hydrothermal (T-H) substages (i.e., T-H stage 1, 2, 3 and 4) for the local mineralization at Nibao. The pre-ore stage is dominated by coarse-grained pyrite coexisting with TiO2 polymorphs in the Middle-Late Permian basaltic rocks. The T-H substage 1 is characterized in that the sedimentary pyrites are brecciated in bedding-parallel faults. Fluorination and silicification occur in the T-H substage 2, which is re-brecciated at the T-H substage 3 forming the highest gold grade ore in high-angle thrust fault. The T-H substage 4 presents as fault reactivation within minor calcite and pyrite deposition. Mineralogy and in-situ trace element analyses reveal that mineral dissolution and reprecipitation (incl. dolomite, auriferous pyrite and TiO2 polymorphs) is ubiquitous during the multistage tectono-hydrothermal process, and the continuous alteration overprinting may have promoted gold accumulation. This replacement process of different generations of auriferous pyrite is also recorded by their inherited sulfur isotopes, giving a narrow δ34S (−3.1 to + 2.1 ‰) range. Anatase is the main accessory phase of TiO2 polymorphs, as identified with LA-ICP-MS trace element and laser Raman spectroscopic analyses. Moreover, the anatase can been divided into two types (type-I and II) based on their texture, alteration and chemistry. The type-I anatase commonly occurring in the stratabound ore is characterized by higher Zr (mean 1,730 ppm), V (mean 1,412 ppm), and Cr (mean 110 ppm) contents than type-II that coexists with zoned pyrite in the fault-controlled ore (mean 26 ppm, 187 ppm, and 38 ppm, respectively). Additionally, W (mean 636 ppm) and Mg (10,021 ppm) in type-II are much higher than type-I ones (mean 135 ppm W and 476 Mg). These features show that the sedimentary-stage Zr-V-Cr-rich type-I anatase was dissolved via fluid metasomatism, and reprecipitated as the hydrothermal-stage W-Mg-rich and Zr-poor anatase, with gold gradually enriched. Besides, the anatase phase of TiO2 polymorphs also indicates that the Nibao gold deposit was formed under low pressure–temperature condition. Overall, we propose that the Nibao is a Carlin-type gold deposit, dominated by multistage tectono-hydrothermal events.

Integrated Approach to Reserve Estimation and Optimal Open Pit Design for Hydrothermal Gold Deposit

Integrated Approach to Reserve Estimation and Optimal Open Pit Design for Hydrothermal Gold Deposit

Mineralogy and mineral chemistry of Bi-Te minerals: Constraints on mineralization process of the Dulanggou gold deposit, Dadu River Metallogenic Belt, China

There is a widely recognized association of bismuth (Bi)-tellurium (Te) minerals with gold in many hydrothermal gold deposits. However, the specific process for gold enrichment related to Bi-Te minerals remains enigmatic. The Dulanggou gold deposit, located on the southeastern margin of the Songpan-Garzê orogenic belt, China, provides an opportunity to research this process. Gold in the deposit is mainly associated with coarse-grained Bi-Te minerals and bismuth-rich polyphase metal droplets. Three mineralization stages are identified in the deposit. In Stage 1, a small amount of gold was deposited within dominate pyrrhotite. In Stage 2, a large amount of visible gold grains were precipitated with coarse-grained Bi-Te minerals. The various Bi-Te phases are mainly tsumoite, pilsenite, unnamed minerals (Bi3Te2, Bi2Te), joséite-B and trace hedleyite, which have a common characteristics of 2 ≥ Bi/Te > 1. In stage 3, around the Stage 2 large Bi-Te phase patches, bismuth-rich polyphase metal droplets pervasively occurred with Bi/Te ≥ 2. Other Bi-Te minerals in Stage 3 include hedleyite, unnamed minerals (Bi3Te, Bi8Te3, Bi5Te), joséite-B, and joséite-A, all intergrowth with gold and maldonite in the metal droplets. Mineral chemistry and phase diagrams of the Bi-Te mineral assemblages indicate that the mineralization temperatures at Stages 2 and 3 are > 325 ℃ and around 235 ℃ respectively, and the fTe2 and fS2 gradually decrease from Stage 2 to Stage 3. Importantly, a large amount of native gold was deposited with native Bi during Stage 3 and Bi contents in Bi-Te minerals increase with gold fineness from 799 ∼ 876 at Stage 2 to 833 ∼ 939 at Stage 3. These indicate that the metal droplets could be Bi-rich melts which were separated from Bi-Te phase patches and scavenged gold from earlier mineral assemblages to enhance gold enrichment.

Occurrence of State of Gold in Crude Oil and Its Economic Significance

Gold and petroleum are also strategic resources of great importance to national security. With the increasing demand for energy, multi-energy cooperative exploration has become an inevitable trend of resource development and utilization. Petroleum and hydrothermal gold deposits may form together, with similar evolutionary trends in their formation, migration, and enrichment. Petroleum reservoirs and gold deposits are closely coupled under certain geological conditions. The solubility of gold in crude oil and its forms of occurrence are important in determining the mechanisms of interaction between gold and petroleum and in facilitating the recovery of gold from gold-bearing petroleum. In this study, the occurrence of gold in crude oil from the Linnan Depression in the Bohai Bay Basin, China, was studied using inductively coupled plasma–mass spectrometry and X-ray photoelectron spectroscopy. Concentrations of gold in crude oil from the Linpan and Shanghe oilfields averaged 44.5 ppb, which is well above the minimum concentration required for hydrothermal gold mineralization. Gold has an affinity with carbon, oxygen, and sulfur, and its concentration in crude oil is positively correlated with total acid and sulfur contents. We speculate that gold may exist in crude oil as complexes with organic acids or thiols, with crude oil thus being a transport medium for gold.

Genetic association between carbonates and gold precipitation mechanisms in the Jinshan deposit, eastern Jiangnan orogen

Carbonates have been traditionally regarded as post-ore minerals in hydrothermal deposits, but they can also occur in pre- and syn-ore stages. However, the genetic relationships between multistage carbonates and gold precipitation mechanisms remain unclear. The Jinshan gold deposit, the largest gold deposit in the eastern Jiangnan orogen, is characterized by the occurrence of abundant hydrothermal carbonates. The nature of the carbonates, especially their association with gold precipitation, was examined in this study through petrographic and geochemical analyses as well as geochemical modeling. Field and petrographic work recognized four paragenesis stages in the Jinshan deposit: (1) quartz (Q1)-carbonate, (2) pyrite-quartz (Q2), (3) gold-polysulfide-quartz (Q3)-carbonate, and (4) chlorite-quartz (Q4)-carbonate. Carbonates associated with gold mineralization are mainly present in the first and third stages. Among them, the first-stage carbonates are pre-ore ankerites (Ank1), which are generally crosscut by syn-ore sulfides. Such carbonates mostly occur in altered host rocks that are characterized by bleaching. Mineralogical and geochemical analyses revealed that chlorite was consumed while ankerite was produced, and Fe contents of the host rocks remained generally unchanged during alteration. Consequently, ankerite (Ank1) is interpreted to have been generated by the interaction of CO2-bearing fluids and chlorite in the host rocks. Geochemical modeling revealed that both Fe-bearing chlorite and ankerite (Ank1) can lead to gold precipitation by triggering sulfidation, but ankerite has higher chemical reactivity and therefore is kinetically favorable for more efficient gold deposition. Syn-ore carbonates were mainly formed in the third stage, primarily consisting of ankerite (Ank2) with minor calcite, and they were coprecipitated with native gold, galena, sphalerite, and chalcopyrite. Two stages of carbonates shared similar δ13CVPDB values and Pb, Zn, and Cu contents but different Ca, Mg, Mn, and Sr contents, indicating they formed at different stages of the same hydrothermal event. Geochemical modeling demonstrated that pH increases, as indicated by the formation of syn-ore carbonates, together with the consumption of S due to the precipitation of pyrite and arsenopyrite in the second stage, resulted in the decrease of Au, Pb, Zn, and Cu solubility in aqueous fluids. This would have led to the coprecipitation of native gold with galena, sphalerite, and chalcopyrite, forming the general small-scale but commonly occurring native gold−polysulfide veins in the hydrothermal gold deposits. Consequently, both pre- and syn-ore carbonates in the Jinshan deposit were genetically associated with efficient Au-polymetallic deposition through distinct mechanisms. Pre-ore carbonates caused the large-scale bleaching of the host rocks, while syn-ore carbonates were closely related to native gold−polysulfides, and they can both be used as exploration indicators for the Jinshan deposit and other similar hydrothermal gold deposits worldwide.

Geology and Geochemistry of K‐feldspar Veins in Lamprophyre at the Zhenyuan Gold Deposit, Yunnan, Southwest China: Implications for Gold Mineralization

AbstractLamprophyres typically appear in hydrothermal gold deposits. The relationship between lamprophyres and gold deposits is investigated widely. Some researchers suggest that the emplacement of lamprophyres triggers gold mineralization, whereas others hypothesize that the formation of lamprophyres increases the fertility of mantle sources and ore‐forming fluids. K‐feldspar veins, with ages between those of lamprophyres and gold deposits, appear in lamprophyres in Zhenyuan. Therefore, K‐feldspar veins are ideal for investigating the relationship between lamprophyres and gold deposits. Phlogopite in K‐feldspar veins has lower Mg#, Ni, and Cr contents and higher TiO2, Li, Ba, Sr, Sc, Zr, Nb, and Cs contents than phlogopite in lamprophyres. The in‐situ Sr isotopic values of apatites (0.7063–0.7066) in K‐feldspar veins are within the range for apatites (0.7064–0.7078) from lamprophyres. High large‐ion lithophile element concentrations and low Nb and Ta concentrations in phlogopite from lamprophyres, in addition to high (87Sr/86Sr)i values of apatite (0.7064–0.7078), indicate that the magma parental to these phlogopite and apatite crystals is derived from an enriched mantle. K‐feldspar veins are genetically correlated with lamprophyres, whereas sulfide mineral assemblage and trace element compositions of pyrite in K‐feldspar veins suggest that K‐feldspar veins in lamprophyres are not directly related to gold mineralization of the Zhenyuan deposit.

A review on the mineralogy of stibnite in hydrothermal gold deposits

A review on the mineralogy of stibnite in hydrothermal gold deposits

A lightweight convolutional neural network with end-to-end learning for three-dimensional mineral prospectivity modeling: A case study of the Sanhetun Area, Heilongjiang Province, Northeastern China

With the continuous exploitation of surface and shallow mineral resources, the global demand for concealed ore deposit exploration is increasing. However, concealed mineral prospectivity modeling (MPM) requires significant efforts, particularly in terms of methods and technologies for three-dimensional modeling. In this study, we propose a lightweight three-dimensional convolutional neural network (3D CNN) for MPM, which adopts the inception structure of GoogleNet and combines the idea of end-to-end learning. We replace the fully connected layer in the conventional CNN with deconvolution, which can greatly reduce the required parameters during the training process and accelerate the convergence. The proposed method overcomes the disadvantage that other shallow machine learning methods cannot extract spatial neighborhood information, while it can extract cross-correlations among geological factors and generates less parameters by a lightweight network when facing massive data. Additionally, compared with the patch-wise method commonly used in previous studies, we use the pixel-wise method for end-to-end learning, which not only overcomes the drawbacks of random sampling but also considers the influence of each voxel when calculating the loss function. The three-dimensional multi-source geoscientific characteristics obtained from the geophysical inversion and 3D geological models are not discretized in order to promote effective CNN training while facilitating the ore-controlling representation. Comparing the predicted results between the 3D weight of evidence (WofE) and our proposed 3D CNN method for MPM, our proposed method and WofE delineated 100% of the known mineralization in high-favorability areas with voxel numbers of 70% and 95%, respectively. A case study of a structure-controlled hydrothermal gold deposit in the Sanhetun area of Heilongjiang Province demonstrates that the proposed 3D CNN method performs better than WofE in terms of prediction effectiveness and efficiency and effectively reveals the correlation between mineralization and adjacent ore-controlling characteristics. Moreover, the proposed 3D CNN method can simulate non-linear metallogenic processes and mine hidden relationships to reveal complex ore-controlling characteristics. In conclusion, the proposed 3D CNN method can reduce the exploration effort in 3D MPM, thus greatly improving the efficiency of discovering concealed ore deposits.

A Deep-Penetrating Geochemical Prospecting Experiment of Mahuagou Gold Deposit in the Core of the Huangling Anticline, Western Hubei, China

The Mahuagou gold deposit is among the most important gold deposits in the core of the Huangling Anticline. However, the geochemical exploration on the surface of the mining area presents challenges due to the thin overburden. This paper focuses on the overburden soil of the Fengxiangshugou (FXS)-Mahuanggou (MHG) section as the research object. It utilizes chemical form analysis of gold, soil halogen survey, and heat-released mercury survey to determine the key deep-penetrating geochemical methods for the mining area. The results indicated that Si and Al components of samples exhibit minimal variation, suggesting that drift loads did not influence the overburden soil. Based on the systematic clustering, As, Sb, Mo, Bi, W, and Hg emerge as ore-body or ore-belt front elements of hydrothermal gold deposits. In the study area, the predominant chemical form of gold in soil is the strong organic bond. Compared to the total amount, strong organic bound gold and heat-released mercury show higher anomaly contrasts, making them crucial indicators of faults, intrusions, and hidden ore bodies. Consequently, chemical form analysis of gold and heat-released mercury surveys can enhance the anomaly contrast, proving beneficial for geochemical prospecting for weak anomalies in this area.

3D Plotting of Gold Solubility and Gold Fineness: Quantitative Analysis of Ore-Forming Conditions in Hydrothermal Gold Deposits

Abstract The 3D plotting of gold solubility and gold fineness aims to illustrate how to quantify their correlations with ore-forming conditions in hydrothermal gold deposits. The thermodynamic calculation of the Au-Ag solid solutions in Mathematica and the 3D plotting in MATLAB are used to build isopleths of gold solubility and gold fineness at different temperatures (200℃, 400℃), pressures (0.1, 5 kbar), salinities (1, 40 wt% NaCl eq.), and sulfur concentrations (0.01, 0.5 mol/kg). The plot indicates that the ore-forming conditions have different correlations with gold solubility and gold fineness. Average rates of change for the correlations are quantified, showing distinct values in the four pH-logfO2 fields of (I) HSO4−, (II) SO42−, (III) H2S, and (IV) HS−, where dominant gold and silver complexes have different dependencies on the conditions. The quantification of the plots illustrates that a decrease in gold solubility by one order of magnitude is possibly caused by a decrease in temperature of ≥40℃, the salinity of ≥9.6 wt% NaCl eq. or sulfur concentration of ≥0.14 mol/kg, or an increase in pressure of ≥3 kbar, while a decrease in gold fineness by 100 units is possibly caused by a decrease in temperature of ≥14 ℃, pressure of ≥1.4 kbar, or salinity of ≥4 wt% NaCl eq., or an increase in sulfur concentration of ≥0.07 mol/kg. Quantification results suggest that a sharp decrease in temperature may result in large-scale gold mineralization and a great variation in gold fineness. In addition, the quantification reveals that the correlation between gold solubility and gold fineness can be expressed by a function, providing a rapid method for 3D plotting.

A track record of Au–Ag nanomelt generation during fluid-mineral interactions

Recent studies have reported the significant role of Au-bearing nanoparticles in the formation of hydrothermal gold deposits. Despite the ever-increasing understanding of the genesis and stability of Au-bearing nanoparticles, it is still unknown how they behave when exposed to hydrothermal fluids. Here, we study the nanostructural evolution of Au–Ag nanoparticles hosted within Co-rich diarsenides and sulfarsenides of a natural hydrothermal deposit. We use high-resolution transmission electron microscopy to provide a singular glimpse of the complete melting sequence of Au–Ag nanoparticles exposed to the hydrothermal fluid during coupled dissolution–precipitation reactions of their host minerals. The interaction of Au–Ag nanoparticles with hydrothermal fluids at temperatures (400–500 ºC) common to most hydrothermal gold deposits may promote melting and generation of Au–Ag nanomelts. This process has important implications in noble metal remobilization and accumulation during the formation of these deposits.

Apatite records metamorphic and hydrothermal fluid evolution at the large Shuangqishan orogenic gold deposit, SE China

Orogenic gold deposits are currently the world’s major source of Au, but uncertainty exists in the timing of mineralization relative to the ages of host rocks, metamorphism, and magmatism. Consequently, the origin and detailed evolution of ore-forming fluids have long been elusive. Apatite occurs widespread in various types of mineral deposits and can provide valuable information on ore genesis. Here, we present textural, high-precision in situ U-Pb geochronological, trace elements, and Sr isotope data for metamorphic and hydrothermal apatite in the Shuangqishan gold deposit in southeastern China. These data, together with U-Pb dates of metavolcanic host rocks, granitic and mafic intrusive rocks, allow us to precisely constrain the timing of mineralization and reconstruct the history of the ore-forming fluids there. The apatite crystals within metamorphic and auriferous quartz veins can be grouped into three generations according to cathodoluminescence imaging and trace element concentrations: Ap1 precipitated in the pre-mineralization metamorphic stage, whereas Ap2 and Ap3 formed in the auriferous hydrothermal stage. Metamorphic Ap1 displays a negative Eu anomaly, indicating a predominance of Eu2+ and crystallization under relatively reduced conditions. On the other hand, syn-gold hydrothermal Ap2 and Ap3 have marked positive Eu anomalies. Radiometric ages for metamorphic Ap1 are at ca. 461 Ma, whereas hydrothermal Ap2 and xenotime from auriferous quartz gave U-Pb ages of 425 ± 15 Ma and 416 ± 15 Ma, interpreted as the time of gold mineralization. Emplacement ages determined by U-Pb zircon data are 439 ± 2 Ma for granitic intrusion and 427 ± 2 Ma for a mafic dike in the mining district. Mineralization, therefore, postdates the metamorphism of local host rocks and granitic magmatism by ∼35 ± 24 m.y. and ∼14 ± 17 m.y., respectively. The synchronism of gold genesis and mafic magmatism provides additional evidence for a mantle derivation of the ore-forming fluids. The highly radiogenic 87Sr/86Sr ratios obtained for Ap1 (0.7178−0.7302) are consistent with its precipitation from a metamorphic fluid. Higher Sr concentrations (Ap2 = 1228−2884 ppm, Ap3 = 2325−3169 ppm) and positive Eu anomalies (Eu/Eu*: Ap2 = 1.03−2.84, Ap3 = 2.00−4.30) but lower and variable 87Sr/86Sr ratios in hydrothermal Ap2 (0.7100−0.7165) and Ap3 (0.7086−0.7116) are mainly ascribed to extensive fluid-wall rock exchange of Sr during mineralization. It is therefore suggested that fluid-rock reaction played a vital role in the observed positive Eu anomalies of hydrothermal apatite. Our study highlights the usefulness of apatite as a novel and robust geochronological, geochemical, and isotopic indicator of complex mineralization processes in hydrothermal gold deposits.

Application of the k-Prototype Clustering Approach for the Definition of Geostatistical Estimation Domains

The definition of geostatistical domains is a stage in the estimation of mineral resources, in which a sample resulting from a mining exploration process is divided into zones that show homogeneity or minimal variation in the main element of interest or mineral grade, having geological and spatial meaning. Its importance lies in the fact that the quality of the estimation techniques, and therefore, the correct quantification of the mineral resource, will improve in geostatistically stationary areas. The present study seeks to define geostatistical domains of estimation for a mineral grade, using a non-traditional approach based on the k-prototype clustering algorithm. This algorithm is based on the k-means paradigm of unsupervised machine learning, but it is exempt from the one-time restriction on numeric data. The latter is especially convenient, as it allows the incorporation of categorical variables such as geological attributes in the grouping. The case study corresponds to a hydrothermal gold deposit of high sulfidation, located in the southern zone of Peru, where estimation domains are defined from a historical record of data recovered from 131 diamond drill holes and 37 trenches. The characteristics directly involved were the gold grade (Au), silver grade (Ag), type of hydrothermal alteration, and type of mineralization. The results obtained showed that clustering with k-prototypes is an efficient approach and can be used as an alternative or complement to the traditional methodology.

Quantitative resource assessment of hydrothermal gold deposits based on 3D geological modeling and improved volume method: Application in the Jiaodong gold Province, Eastern China

Quantitative assessment of deep mineral resources is important for the mineral exploration decisions of governments and companies. Complex structural controls make the volume method difficult to apply to hydrothermal gold deposits. Quantitative assessment of Au resources in hydrothermal goldfields remains a challenging task. Here we proposed a quantitative assessment approach for hydrothermal gold deposits via an improved volume method combined with 3D geology modeling. A highly reliable 3D model of ore-controlling structure was first constructed to obtain the spatial association between Au mineralization and structure. On this basis, we made an optimal quantitative estimation of the ore-bearing ratio and similarity index, which are the most critical parameters in the volume method, before evaluating gold resources. Specifically, (1) an ore-bearing ratio prediction model was established by support vector machine based on measured data and 3D models, which was further applied in predicting the ore-bearing ratio of the evaluation area at depth and peripherals; (2) the calculation of the similarity index was improved by novelly using the Mahalanobis distance of the regional geochemical data with regard to the similar geochemical anomalies of gold mineralization in adjacent space. This proposed method was applied to quantitatively assess the gold resources in the Jiaojia and Dayingezhuang goldfields in the Jiaodong gold province, China, where gold deposits show significant structure controls on mineralization. The results of gold grade prediction are statistically significantly correlated to measured values by the Wilcoxon signed rank test, showing the model for gold grade prediction can be extensible in the deep parts. Our approach produced similarity indices for the model and evaluation areas at Jiaojia and Dayingezhuang that accurately reflected the geology complex. The above lines suggest that the proposed approach can be used to estimate the gold resources of Jiaodong. Potential gold resources in the Jiaojia and Dayingezhuang districts were estimated to be 1221.82 t and 254.95 t.

Complex fluid source of the multistage pyrite-bearing Huilvshan gold deposit (west Junggar, NW China): Insight from pyrite texture, sulfur isotope and trace element compositions

Hydrothermal gold deposits commonly form during protracted multistage ore-forming processes, however, discriminating among different ore-forming fluids and tracing their origin to unique or different repositories is a challenging task. The Huilvshan gold deposit in West Junggar (Xinjiang province, NW China), was affected by three hydrothermal stages and thus provides an opportunity to examine complex ore-forming processes. The deposit consists of gold-bearing quartz-sulfide veins and disseminated sulfides, hosted within Early Carboniferous basalts and tuffs. Three stages of hydrothermal pyrite (Py1, Py2, and Py3) were identified. Gold is only present in Py2 where it occurs as native gold inclusion and in invisible form. Anhedral Py1, disseminated in the altered basalt, is characterized by higher Ni (13.7 to 635 ppm) concentrations than Py2 and Py3. Euhedral-subhedral Py2 is the richest in Au (1.46 to 25.7 ppm) among three stages of pyrite. Py3 is subdivided into grains with zonal texture (Py3a) and irregular grains (Py3b). Both sub-types have high Sb (56.8 to 1599 ppm) and Tl (0.02 to 68.7 ppm) concentrations. In situ δ34S values of Py1 (-6.0 to 2.7 ‰) are similar to those of Py2 (-5.9 to 3.9 ‰), whereas Py3 has extremely negative δ34S values (-44.3 to-18.0 ‰).Ar-Ar ages of hydrothermal muscovite coexisting with Py2 indicate that the Huilvshan gold deposit formed at ∼300 Ma. The sulfur isotopic systematics of Py1 and Py2, close spatial and temporal relationship between the Huilvshan gold deposit and adjacent felsic intrusions, and paucity of regional metamorphic rocks in the region all suggest that hydrothermal fluids in stage I (Py1) and II (Py2) were derived from magmatism. On the other hand, stage III pyrite and associated sulfides more likely precipitated from a hydrothermal fluid that circulated in tuffs, based on the extremely negative δ34S values of Py3, similar to those measured from framboidal pyrite in the host tuffs.Therefore, textural, trace elemental and sulfur isotope data from pyrite suggest different sources for the hydrothermal fluids in the Huilvshan gold deposit, and that magmatic activity may have contributed the most to the main ore-forming stage. The genesis of this Huilvshan gold deposit could provide significant insights into the origin of other hydrothermal gold deposits that show multiple sulfide generations.

Ore Genesis of the Toudaochuan Gold Deposit in Central Jilin Province, NE China: Constraints from Fluid Inclusions and C–H–O–S–Pb Isotopes

The Toudaochuan gold deposit is a recently discovered lode gold deposit in Central Jilin Province. Gold ore bodies are dominantly controlled by NE-trending fault. The major hydrothermal period can be further divided into the quartz–pyrite stage (stage I), quartz–gold–polymetallic sulfides stage (stage II, major gold mineralization stage), and quartz–carbonate stage (stage III). Primary fluid inclusions (FIs) identified in quartz at different hydrothermal stages include liquid-rich aqueous FIs (L-type), CO2 FIs (C-type, including CO2-bearing C1-type FIs and CO2-rich C2-type FIs), and minor vapor-rich aqueous FIs (V-type). Microthermometry studies on different fluid inclusions indicate that the original ore-forming fluids belonged to the CO2–H2O–NaCl system characterized by a moderate–low temperature and low salinity in stages I and II, and they finally evolved into a H2O–NaCl system characterized by low temperature and low salinity in stage III. Fluid immiscibility is considered to be the key ore-forming mechanism. The initial ore-forming fluid was originated from magmatic water and was mixed with meteoric water in the later stage. The S and Pb isotope data suggest that the ore metal materials were derived from the mixed source of mantle and crust. Based on all the above data, therefore, it can be proposed that the Toudaochuan gold deposit is a mesothermal magmatic–hydrothermal 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 altered slate type ores in the Huangjindong gold deposit, Jiangnan Orogenic Belt, South China

Altered rock ores, closely associated with fluid-rock interactions, represent a typical mineralization style in hydrothermal gold deposits. In the Huangjindong gold deposit of the Jiangnan Orogenic Belt, South China, altered slate ores are generally characterized with bleaching, but the genesis remains not understood. Based on field work, petrographic observations and Tescan Integrated Mineral Analyzer (TIMA) analysis, the alteration of the slate can be defined as carbonatization-sericitization, characterized with the abundant siderite spots and sericite, as well as locally occurred cryptocrystalline quartz. Abundant gold-bearing sulfides are locally present in altered rocks, which constitute the altered slate ores. In altered slates, pyrite and arsenopyrite aggregate near carbonate spots, sharing similar geochemical compositions with those in quartz veins. These sulfides locally crosscut siderite grains, demonstrating that the carbonatization-sericitization alteration took place before gold mineralization. μ-XRF analysis and mass balance calculation show that Fe is not prominently gained or lost but aggregated to form siderite spots in the alteration zone. In addition, K contents increased during alteration, consistent with the presence of sericitization. Thermodynamic modeling of the dissolution of siderite manifests that the chemical reactions between siderite and S-Au-bearing fluids leads to the co-precipitation of gold, pyrite, arsenopyrite and quartz. Moreover, taking account of the siderite contents in altered slates and Au contents in sulfides, the water-rock interaction could generate altered slate ores with economic significance. These results show that large amounts of siderite spots are generated during the pre-mineralization fluid-rock reaction, and thus provide favorable chemical traps for gold mineralization. Abundant siderites contribute to gold mineralization by triggering sulfidation, which is the major genesis for the altered slate ores in the Huangjindong gold deposit.

Intracontinental and arc-related hydrothermal systems display distinct δ202Hg and Δ199Hg features: Implication for large-scale mercury recycling and isotopic fractionation in different tectonic settings

Mercury isotopes display both mass-dependent and mass-independent fractionation and allow the tracing of pathways and storage of surface-derived Hg in the lithosphere. While the subduction-related orogenic recycling of Hg from marine reservoirs into hydrothermal systems in continental arc settings has been documented recently, the source of Hg in intracontinental hydrothermal systems remains unclear. We measured Hg isotopes in two intracontinental anorogenic/postorogenic Late Mesozoic hydrothermal gold deposits in the South China craton and the Central Asian orogenic belt of northern China, respectively. The ore and sulfide samples from the studied systems have positive δ202Hg (0.70 ± 0.39‰, 1SD, n = 49) and negative Δ199Hg values (−0.12 ± 0.05‰, 1SD, n = 49). These values are different from their country rocks and regional geological environment (volcanic arc granites, marine sedimentary rocks) which have positive Δ199Hg values, but similar to that of their Precambrian supracrustal basement rocks of largely non-marine continental materials. We conclude that Hg in the intracontinental hydrothermal systems was leached from basement rocks by upper crustal basinal fluid circulation driven by regional heat flow, likely due to lithospheric thinning and upwelling of the asthenosphere in the Late Mesozoic. The intracontinental hydrothermal systems and their continental sources with positive δ202Hg and negative Δ199Hg values are complementary to volcanic-arc and marine sedimentary rocks with opposite δ202Hg - Δ199Hg compositions. The distinct Hg isotopic features of hydrothermal systems in different tectonic settings, in particular the indelible Δ199Hg signature, allow the tracing of large-scale material cycling in the Earth.