Epithermal Au Research Articles

Raman Spectroscopy Coupled with Reflectance Spectroscopy as a Tool for the Characterization of Key Hydrothermal Alteration Minerals in Epithermal Au-Ag Systems: Utility and Implications for Mineral Exploration.

Raman spectroscopy of fine-grained hydrothermal alteration minerals, and phyllosilicates in particular, presents certain challenges. However, given the increasingly widespread recognition of field portable visible–near infrared–shortwave infrared (Vis-NIR-SWIR) spectroscopy as a valuable tool in the mineral exploration industry, Raman microspectroscopy has promise as an approach for developing detailed complementary information on hydrothermal alteration phases in ore-forming systems. Here we present exemplar high-quality Raman and Vis-NIR-SWIR spectra of four key hydrothermal alteration minerals (pyrophyllite, white mica, chlorite, and alunite) that are common in precious metal epithermal systems, from deposits on the island of Newfoundland, Canada. The results reported here demonstrate that Raman microspectroscopy can accurately characterize pyrophyllite, white mica, chlorite, and alunite and provide details on their compositional variation at the microscale. In particular, spectral differences in the 1000–1150 cm−1 white mica Raman band allows the distinction between low-Tschermak phases (muscovite, paragonite) and phases with higher degrees of Tschermak substitution (phengitic white mica composition). The peak position of the main chlorite Raman band shifts between 683 cm−1 for Mg-rich chlorite and 665 cm−1 for Fe-rich chlorite and can be therefore used for semiquantitative estimation of the Fe2+ content in chlorite. Furthermore, while Vis-NIR-SWIR macrospectroscopy allows the rapid identification of the overall composition of the most abundant hydrothermal alteration mineral in a given sample, Raman microspectroscopy provides an in-depth spectral and chemical characterization of individual mineral grains, preserving the spatial and paragenetic context of each mineral and allowing for the distinction of chemical variation between (and within) different mineral grains. This is particularly useful in the case of alunite, white mica, and chlorite, minerals with extensive solid solution, where microscale characterization can provide information on the alteration zonation useful for mineral exploration and provide insight into mineral deposit genesis.

Genetic Environments of the Eunjeok Au–Ag Deposit in the Yeongam District: Implications for Cretaceous Epithermal Au–Ag Mineralization in South Korea

Eunjeok Au–Ag deposits are situated in the Yeongam district, Cheollanamdo-province, South Korea. They are genetically related to the Bulgugsa magmatic event (ca. 110–60 Ma), caused by the transition in the subduction direction and style of the Izanagi Plate. Three gold- and silver-bearing hydrothermal veins filled the fractures of the Cretaceous rhyolitic tuff. The major ore minerals were arsenopyrite (31.47–32.20 at.% As), pyrite, chalcopyrite, sphalerite (8.58–10.71 FeS mole%) and galena with minor amounts of electrum (62.77–78.15 at.% Au), native silver, and argentite. Sericitization was dominant in the alteration zone. The various textures of quartz veins (i.e., breccia, crustiform, comb, and vuggy) may indicate the formation of an epithermal environment. The auriferous fluids with the H2O–NaCl system have homogenization temperatures and salinities of 204 °C to 314 °C, less than 10 wt.% equiv. NaCl, and experienced mixing (dilution and cooling) events during mineralization. Considering the characteristics of the geologic setting, major fault system, and host rock, the Eunjeok Au–Ag deposit within the Yeongam district tends to share the general geologic characteristics of Haenam–Jindo epithermal mineralization episodes. However, the age of gold–silver mineralization (86.0 Ma) is older than that of Haenam–Jindo epithermal mineralization episodes (<70.3 Ma), implying some differences exist in the genetic sequence of extensional characteristics caused by transcurrent Gwangju–Yeongdong faults.

LA-ICP-MS trace element analysis of pyrite from the Dafang gold deposit, South China: Implications for ore genesis

The Dafang is a hydrothermal gold deposit (6 t Au with an average grade of 1.8 g/t) in periphery of the Sn-Cu metallogenic system in Nanling Range in south China. This deposit locates in the contact zone between late Jurassic granodiorite porphyry and late Paleozoic limestone as well as the fractures of them. The mineralization comprises three stages, pyrite + arsenopyrite ± chalcopyrite + quartz (I), pyrite + arsenopyrite + gold + calaverite + sulfosalt ± sphalerite ± galena + quartz (II), and pyrite + arsenopyrite + quartz + calcite (III). Pyrite is the dominant gold-bearing mineral, which can be classified into three generations (Py1 to Py3), corresponding to three ore stages (Stage I to Stage III), respectively. Different stage pyrites show textural homogeneity in both reflected light and backscattered electron images. In addition, the LA-ICP-MS results show that all pyrites have relatively low Au concentrations (0.17 ppm for Py1, 0.48 ppm for Py2 and 0.58 ppm for Py3), and trace element mapping for Py1 demonstrate oscillatory zoning patterns for Au, As, Sb, Te, Co, and Ni. Compared to Py2 and Py3, Py1 has higher Co, Ni, Cu, Pb, Zn, Ag, and Te contents, but lower Au and Tl contents. Gold contents for all stage pyrite plot well below the solubility limit for gold in arsenian pyrite, indicating that invisible gold occurs as Au1+ in the pyrite lattice. Silver occurs mainly as Ag-bearing sulfosalt inclusion or galena micro-inclusion in pyrite, while Pb and Zn as galena and sphalerite micro-inclusions in pyrite. Trace element mapping yields that Au-rich zone coincides well with As rich zones, demonstrating that the occurrence of As in pyrite promotes the enrichment of lattice gold. From Stage I to Stage III, the decrease of Co and Ni contents indicates the decrease of the ore-fluid temperature, and the change of Se and Te contents suggests the fO2 increased from Stage I to Stage II and then decreased to Stage III. The Co and Ni contents suggest that all studied pyrite is dominantly of hydrothermal origin. Combined geological and mineralogical evidences indicate that the Dafang gold deposit is an epithermal Au deposit.

Dispersed Gold and Associated Disseminated Mineralization of Leucogranites of the Dukat Ore Field as Indicators of the Generation Conditions of Magmatogene Gold-Bearing Fluids

Abstract —We present data on dispersed gold and associated disseminated mineralization of leucogranites controlling the location of the Dukat epithermal Au–Ag deposit. The data suggest the formation and burial of small portions of oversaturated gold-bearing fluids in hypabyssal magmatic bodies. The lower (relative to silver) contents of gold in the deposit ores and the high Ag/Au ratios (350–550) are due to the limited occurrence of oxidized gold-bearing fractions of magmatogene fluids. This limitation is explained by the active interaction of late magmatic portions of fluids and the material of the host carbonaceous volcanosedimentary unit and by the formation of an environment favorable for the concentration, migration, and deposition of manganese, silver, and nonferrous-metal compounds.

The Dvoinoye and September Northeast high-grade epithermal Au–Ag veins, Vodorazdelnaya district, Chukotka region, Russia

The Dvoinoye and September Northeast high-grade epithermal Au–Ag veins, Vodorazdelnaya district, Chukotka region, Russia

Fluid inclusions, isotopes and mineralogy of the Kuruer epithermal Cu-Au deposit, Chinese Western Tianshan: Implications for ore genesis and exploration

The Kuruer Cu-Au deposit is located in the Wusun Range of Yili Block, Chinese Western Tianshan, where numerous epithermal Au, skarn Fe-Cu and porphyry Mo-Cu deposits have been recognized. Petrography, fluid inclusions, S-Pb-H-O isotopes, field emission-scanning electron microscopy (FE-SEM) plus energy dispersive spectrometer (EDS) and electron microprobe analysis (EMPA) have been carried out to constrain the genesis of the Kuruer deposit and its implication for further exploration. The minerlization is hosted by volcanic rocks and occur as veins and veinlets structurally controlled by NW-trending faults. Three ore-forming stages were recognized, including quartz-sulfides-bismuth-gold stage (I), quartz-chalcopyrite stage (II) and calcite-chlorite-chalcopyrite stage (III).Six types of fluid inclusions were identified in stage I quartz, including monophase vapor inclusions (type 1), liquid-rich biphase inclusions (type 2), vapor-rich biphase inclusions (type 3), H2O-rich CO2-H2O triphase inclusions (type 4), CO2-rich CO2-H2O triphase inclusions (type 5) and solid halite-bearing triphase inclusions (type 6), whereas only type 2 inclusions were recognized in stage II quartz and stage III calcite. Types 2, 3, 4 and 6 inclusions in stage I have homogenization temperature (Th) ranges of 184–359 °C, 209–252 °C, 250–319 °C and 229–263 °C, with salinity ranges of 4.1–8.5 wt% NaCleq, 5.9–6.9 wt% NaCleq, 1.0–6.0 wt% NaCleq and 32.7–33.9 wt% NaCleq, respectively. Type 2 inclusions in stages II and III have Th varying from 144 to 212 °C and 114 to 163 °C, with salinities of 0.2–6.7 wt% NaCleq and 0.1–3.1 wt% NaCleq, respectively.The in-situ δ34S values of sulfides in stage I range from −4.66 to 0.34‰. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of stage I chalcopyrite vary from 18.295 to 18.954, 15.568 to 15.673 and 38.109 to 38.817, respectively, which are consistent with those of Early Carboniferous Dahalajunshan Formation volcanic rocks and granitoids from the Wusun Range. The values of δ18OH2O-VSMOW calculated from δ18Oquartz and δDH2O-VSMOW of stage I quartz range from 0.0 to 2.4‰ and −118.6 to −109.3‰, respectively. These isotopic compositions indicate that both metals and sulfur were sourced from the hosting volcanic rocks and hidden causative pluton, while the initial ore-forming fluids were magmatic in origin, with increasing involvement of meteoric water with time.Evidence from mineralogy, mineral geochemistry and fluid inclusions indicates that the initial fluid phase separation triggered the precipitation of “invisible” gold within pyrite and chalcopyrite during stage I. Subsequently, the decrease of fS2 due to the escape of H2S(g) and sulfide precipitation and cooling caused the formation of liquid bismuth from solutions. The bismuth melts have scavenged Au in situ from the residual solutions, resulting in the formation of Bi-Au inclusions within the fractures and intergranular spaces of earlier pyrite. For stages II and III, dilution and cooling due to input of meteoric water facilitated the chalcopyrite precipitation. The Kuruer Cu-Au deposit is classified as a subepithermal mineralization associated with porphyries. Porphyry Cu-Au mineralization could have developed beneath the Kuruer and its vicinity.

Fluid Inclusion Study of Epithermal Gold-Base Metal Mineralization System in the Shwebontha Prospect, Monywa Mining District, Central Myanmar Monywa

The Shwebontha Prospect area is one of prominent epithermal Au-Ag prospects in Monywa mining district, central Myanmar, characterized by the appearance of gold-bearing and base metal quartz veins with gold grade is around 3g/t -10.4g/t. The geology of the area consists of the volcanic and volcaniclastic rocks of Upper Oligocene-Middle Miocene Magyigon Formation that served as the host rock of the ore mineralization. This research focused on fluid inclusion study is aimed to know the characteristics of hydrothermal fluids during ore mineralization as well as the possible paleo- depth and temperature of formation of gold-bearing and base metal quartz veins. The mineralization styles are gold-bearing brecciated quartz veins and chalcedonic quartz veins where sulfides are clustered as well as disseminated both in quartz gangue and volcanic host rocks. Those quartz veins include pyrite, sphalerite, galena, chalcopyrite and gold (electrum). Fluid inclusion microthermometry indicates that the ore mineralization is characterized by the values of homogenization temperature range from 158°C to 310°C and salinities range from 0.35 to 2.41wt.% NaCl equiv. This temperature is consistent with the formation temperature of 250°C to 270 °C and also their estimate paleo-depth of formation is between 440m and 640m respectively. Microthermometric data indicates that fluid mixing and dilution were significant processes during ore mineralization and evolution of hydrothermal fluids. Based on the petrography of fluid inclusion, microthermometric measurements and ore minerals assemblage as well as estimation of paleo-depth from the Shwebontha Prospect imply that forming in under shallow level epithermal environment

Gold in Paleoproterozoic (2.1 to 1.77 Ga) Continental Magmatic Arcs at the Tapajós and Juruena Mineral Provinces (Amazonian Craton, Brazil): A New Frontier for the Exploration of Epithermal–Porphyry and Related Deposits

This review paper aims to integrate geological, tectonic and metallogenetic data, including new data, and propose a regional model for the gold (and base metal) mineralization in the south Amazonian Craton to support the mineral exploration concerning magmatic–hydrothermal deposits. The Proterozoic evolution of the Amazonian Craton comprises the accretion of terrains to the Archean Carajás Mineral Province. In the Tapajós and Juruena mineral provinces, located at the south part of the Amazonian craton, a long-lived ocean–continent subduction event produced ca. 2.0 to 1.77 Ga continental magmatic arcs. Extensive lava flows, volcaniclastic, sedimentary, and plutonic rocks were originated during at least four major orogenic magmatic events (ca. 2.1, 1.9, 1.88, and 1.80 Ga) and two post- to anorogenic events (ca. 1.87 and 1.77 Ga). Gold mineralization occurs in: (i) alluvial/colluvial occurrences, (ii) orogenic carbonate–sulfide-rich quartz veins in shear zones, (iii) stockworks, veins, and dissemination in granites, (iv) contact of basic dikes, (v) well-preserved high-, intermediate- and low-sulfidation epithermal mineralization, and (vi) porphyry-like and intrusion-related gold systems associated with late- to post-orogenic epizonal granites. The estimated historical gold production, mainly in secondary deposits, is over 27 Moz at the Tapajós and 6 Moz at the Juruena provinces. A total resource of over 5 Moz Au is currently defined in several small to large primary gold deposits. Andesite to rhyolite, volcaniclastic, and clastic sedimentary rocks (1.96–1.88 Ga) host epithermal (high-, intermediate-, and low-sulfidation) Au–(Ag–Pb–Zn) mineralization, whereas Au–Cu and Cu–Mo–Au mineralization is hosted in sub-volcanic tonalitic to granitic plutons. Advanced argillic alteration (alunite, pyrophyllite, enargite) associated with high-sulfidation mineralization occurs in ring volcanoes around nested volcanic calderas. This zone grades outward to propylitic or chlorite alteration, often covered by silica caps with vuggy silica. Lava flows and volcaniclastic rocks within faults or associated with volcanic edifices and rhyolitic domes host low- and intermediate-sulfidation mineralization. Low-sulfidation alteration zones typically have adularia and illite or sericite. Chalcopyrite, sphalerite, galena, pyrite, digenite, and manganiferous calcite are related to intermediate-sulfidation gold mineralization. Late- to post-orogenic evolved oxidized I-type granitoids host alkalic-type epithermal and porphyry-like gold mineralization. Porphyry-style hydrothermal alteration is analogous to those of modern systems, with inner sodic and potassic (potassic feldspar ± biotite or biotite) alterations grading to propylitic, muscovite-sericite, chlorite–sericite, and chlorite alterations. Potassic alteration zones are the locus of Cu–Mo mineralization, and gold-rich zones occur in muscovite/sericite–quartz–pyrite alteration. The Paleoproterozoic epithermal and porphyry-like mineralization in these large provinces defines a new frontier for the exploration of world-class gold deposits in the worldwide Proterozoic arc-related magmatic terrains.

Особенности структур распада твердого раствора Cu-Ag cульфидов на месторождении Теплое (Северо-Восток России)

The authors consider features of the triple structure of Ag-Cu-S solid solution decomposition resulted from redistribution of matter in the ores of the epithermal Au-Ag Teploye deposit due to the spatial combination of volcanogenic Au-Ag mineralization, with relatively later copper porphyry. The discovery of the mckinstryite, copper-silver sulfide, permits to consider the occurrence of copper in acanthite a non-structural impurity element, while the term "cupreous acanthite" appears insufficiently substantiated. It has been suggested that the mckinstryite-jalpaite (+ acanthite) structure was formed not by the decomposition of a solid solution or the crystallization of a normal hydrothermal solution, but but in the course of solidification of the sluggish, viscous, colloid-like substance in a metastable environment under low-temperature conditions, and was accompanied by structural rearrangement of its mineral constituents.

Study of the Influence of Non-Deposit Locations in Data-Driven Mineral Prospectivity Mapping: A Case Study on the Iskut Project in Northwestern British Columbia, Canada

The accuracy of data-driven predictive mineral prospectivity models relies heavily on the training datasets used. These models are usually trained using data for “known” deposit locations as well as “non-deposit” locations that are based on randomly generated point patterns. In this study, data related to the Seabridge Gold Inc Iskut project, an epithermal Au deposit in northwestern British Columbia (BC), Canada, are used to test the utility of data-driven mineral prospectivity modeling. The input spatial dataset is comprised mostly of publicly available data. Data for 18 vein and epithermal Au known mineral occurrences (KMO) are obtained from the BC Geological Survey’s MINFILE repository and selected as training deposit locations. A total of eleven sets of non-deposit locations (NDL) were also created, including one set of selected non-prospective KMO for Au deposits from the MINFILE and ten sets of random point patterns. Given the scale of this study, most of the KMO recorded on the property are of the epithermal deposit type. Hence, they could not be used as a selection criterion. Data-driven mineral potential models are generated using the random forest (RF) algorithm and trained on multiple data sets. The comparison of RF models demonstrated that using non-prospective KMO generates more accurate predictions than the random point pattern. The produced mineral prospectivity maps delineated multiple areas with higher discovery potential, which matched viable targets for the Au-Cu epithermal-porphyry system identified through previous Seabridge Gold Inc. (Toronto, ON, Canada) field reconnaissance and drilling programs.

Effects of arsenic on the distribution and mode of occurrence of gold during fluid-pyrite interaction: A case study of pyrite from the Qiucun gold deposit, China

Abstract This paper presents the results of an investigation of the incorporation of Au within pyritic ore from the Qiucun epithermal Au deposit, China. The new data provide insights into the mode of occurrence of Au during fluid-rock interactions within epithermal systems. The distribution and mode of occurrence of Au within arsenian pyrite were investigated using a chemical and structural characterization-based approach combining laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element analysis, megapixel synchrotron X-ray fluorescence analysis (MSXRF), and atom probe tomography (APT). The resulting data indicate that invisible Au is present at elevated concentrations in the form of a homogeneous solid solution within As-rich pyrite domains, which yields Au concentrations that positively correlate with As. Arsenic-induced lattice defects, such as stacking faults and the expansion of the pyrite unit cell, provide evidence of the effect of As on the incorporation of Au into pyrite. The nucleation and crystallization of electrum preferentially occurred at the fluid-pyrite reaction interface or along fractures and grain boundaries within the pre-existing pyrite. This study indicates that changes in physicochemical conditions (e.g., temperature, pH, and sulfur fugacity) during fluid-pyrite interactions are key controls on the development of nanometer- or micrometer-scale clusters of gold. The systematic compositional and textural observations documented in this study provide new insights into the mechanisms responsible for the different modes of occurrence of Au (ionic vs. particulate) and enable us to further understand the processes involved in the formation of Au mineralization.

Petrogenesis of the Late Jurassic Cu-Mo mineralization-related Xianyang granite porphyry in the Dehua district, Southeast China: Response to the subduction of paleo-Pacific slab and implication for regional exploration

The Dehua Au-polymetallic ore cluster, as one of the most important area in the Wuyi Au-Cu-Mo metallogenic belt, consists of many large to middle sized epithermal Au deposits and recently discovered porphyry Cu-Mo deposits. We first study the geochronology and geochemistry characteristics of the Cu-Mo mineralization-related Xianyang granite porphyry to constrain its petrogenesis, tectonic setting and metal endowment. LA-ICP-MS zircon U-Pb dating yield a precise age of 157.2 ± 0.8 Ma for the Xianyang granite porphyry, indicating that the granite porphyry was emplaced during the Late Jurassic. The Xianyang granite porphyry was suggested to be generated under continental arc setting, associated with subduction of the paleo-Pacific slab beneath the South China Block. The Xianyang granite porphyry belongs to the high-K calc-alkaline series, with SiO2 of 71.16–72.02 wt% and K2O of 4.28–4.69 wt%. It shows light rare earth element-enriched distribution patterns with (La/Yb)N of = 9.74–13.93, enrichment in large ion lithophile elements, but depletion of high field strength elements, with negative Nb, Ta and Ti anomalies, suggesting an arc magma geochemical affinity. It has lower initial 87Sr/86Sr ratios of 0.7079–0.7086 and relatively higher εNd(t) and εHf(t) values of −8.03 to −7.69 and − 8.86 to −5.48, respectively, than those of basement rocks of the Cathaysia block, suggesting that it was most likely generated by partial melting of the regional ancient crustal materials, with the involvement of asthenosphere mantle-derived materials. The Xianyang granite porphyry has remarkably high zircon Ce4+/Ce3+ values of 119–404 (mean = 245) and EuN/EuN* values of 0.37–0.60 (mean = 0.51), indicating a high oxygen fugacity, which is favor of the formation of the Xianyang porphyry Cu-Mo mineralization. Combining with published research results for regional subvolcanic rocks-related epithermal Au deposits, the porphyry Cu-Mo deposits and epithermal Au deposits in the Dehua ore cluster share similar petrogenetic-metallogenic ages and magma sources in the continental arc setting during the Late Jurassic period. The identification of the coeval porphyry-epithermal Cu-Mo-Au metallogenic system in the Dehua ore cluster highlights the possibility of finding the hidden porphyry Cu-Mo orebodies beneath the discovered epithermal Au deposits.

Constraints on the nature of metamorphism in the Kalgoorlie gold camp (Yilgarn Craton, Western Australia) and implications for genetic models of gold mineralisation

The timing and nature of metamorphism in the Kalgoorlie gold camp (Yilgarn Craton, Western Australia) are key aspects to genetic models for gold mineralisation. Despite this, existing constraints on metamorphism are equivocal as to whether the Fimiston and Oroya Au–Te lodes formed pre- or post-peak regional metamorphism. Evaluation of an actinolite (S1)–chlorite (S2) crenulation cleavage in the Hannan’s Lake Serpentinite on the western margin of the Golden Mile Super Pit supports a syn-D1 timing for peak metamorphism and a post-peak metamorphic timing for the Fimiston and Oroya Au–Te lodes. These results support a key aspect of metamorphic models for gold mineralisation. If intrusion-related models are correct, they should not be premised on the argument that the Fimiston and Oroya Au–Te lodes are equivalent to other examples of metamorphosed, epithermal-type mineralisation in orogenic belts. KEY POINTS The definition of an S1 cleavage in the Hannan’s Lake Serpentinite by actinolite supports a syn-D1 timing (ca 2685–2680 Ma) for peak regional metamorphism (sub- to lower-greenschist facies) in the Kalgoorlie gold camp. A syn-D1 timing for peak regional metamorphism suggests that the D2 (ca 2675–2665 Ma) Fimiston and Oroya Au–Te lodes and D3 (ca 2650–2640 Ma) Mt Charlotte stockwork Au orebodies developed in post-peak metamorphic settings. A post-peak metamorphic timing for the Fimiston and Oroya Au–Te lodes suggests that these mineralisation types do not represent metamorphosed equivalents of epithermal Au–Te mineralisation.

Tracking fluid mixing in epithermal deposits – Insights from in-situ δ18O and trace element composition of hydrothermal quartz from the giant Cerro de Pasco polymetallic deposit, Peru

Ore precipitation in mineral deposits formed in the upper parts of a porphyry system, at shallow crustal level (< 1.5 km), such as epithermal Au-Ag-(Cu)-(As) and polymetallic deposits is often triggered by fluid cooling and/or mixing between fluids from different sources. Commonly, in such deposits, two main fluid sources are identified a deeply sourced magmatic fluid and a shallow meteoric water stored in a surficial aquifer. Oxygen and hydrogen isotope compositions of gangue and alteration minerals using conventional bulk isotopic methods support the existence of mixing between these two fluid types. However, bulk isotope analysis provides only limited information on the exact mixing mechanisms and on the changing proportions of the involved fluids. Due to their high spatial resolution, SIMS in-situ oxygen isotope and LA-ICP-MS trace element analyses, in transects across growth zones of single crystals are adequate tools to trace the dynamics of this fluid mixing. In this study, in-situ SIMS oxygen isotope and LA-ICP-MS trace element analyses were performed on 10 selected quartz crystals from the giant Cerro de Pasco porphyry-related epithermal polymetallic deposit in central Peru. The results, combined with previous microthermometric and LA-ICP-MS fluid inclusion studies on the same or equivalent crystals, allow quantifying and documenting the mixing between different types of fluids that formed the large Cerro de Pasco epithermal polymetallic deposit. The δ18Oquartz values range between 4‰ and 20‰ and display variations up to 11.5‰ inside single crystals that cannot be only, nor mainly ascribed to fluid temperature changes. Rather, these variations record variable mixing proportions of a rising moderate-salinity magmatic fluid with a δ18OH2O around 10‰ and a low-salinity fluid with a δ18OH2O between 0 and 4‰, the latter stored below the paleo-water table. Each analyzed quartz crystals also display important variation of their trace element content, with Al (43 to 2098 ppm), Li (0.7 to 18 ppm), Ge (1.1 to 24ppm) and Ti (0.8 to 10 ppm). These variations do not systematically correlate with oxygen isotope compositions. This suggests that quartz trace element content is controlled by a complex interplay of fluid composition, temperature, pressure, and growth rate. Application of published Ti-in-quartz geothermometers on quartz grains from which the precipitation temperature is well constrained by fluid inclusion microthermometry, shows that it can lead to overestimation or underestimation of precipitation temperatures by more than 50 °C.The obtained δ18Oquartz patterns measured along profiles in the studied quartz crystals, and less clearly the in-situ trace element compositions, reveal abrupt changes and suggest that mixing between magmatic and surface-derived low-salinity fluids was not a continuous process. It rather took place through the influx of multiple short-lived pulses of magmatic fluid into the surface-derived low-salinity fluid surface aquifer.

Extreme sulfur isotope fractionation of hydrothermal auriferous pyrites from the SW fringe of the Taupo Volcanic Zone, New Zealand: Implications for epithermal gold exploration

The Taupo Volcanic Zone and its environs, New Zealand, represent an ideal place for epithermal gold exploration. We have documented structurally-bound gold in low temperature hydrothermal pyrite veins from the Erua Basin area on the outside fringe of the Taupo Volcanic Zone. Extreme 34S/32S fractionation of 122.2‰ from auriferous pyrite is documented in hydrothermal veins hosted in alluvial deposits from the Erua Basin. We document three generations of pyrite based on petrographic and Nano-SIMS microstructural analysis. First generation pyrite (Py assemblage -1) has extremely high δ34S values of +70.3 to +100.0‰ probably resulting from sequential-cyclic bacterial reduction and Rayleigh fractionation of limited sulfate. Second generation pyrite (Py assemblage -2) has relatively lower δ34S values of -39.8 to -3.3‰, interpreted to reflect the influx of magmatic-related oxidized fluids mixed with remobilized, reduced sedimentary sulfides that then formed the host calcite-pyrite veins. Third generation pyrite (Py assemblage -3) incorporated much heavier 34S resulting in positive δ34S values ranging from +6.8 to +72.2‰, indicating greater reduction of sulfate to H2S in an anoxic (reducing) environment. One single composite ~ 10 um pyrite grain encompassing all three types of subgrains has a large δ34S range from -22.2‰ along its rim and +100.0‰ from its core (122.2‰ for a single grain, and 139.8‰ for the system). This is the largest known variation in δ34S ever reported in a single pyrite grain from any system on Earth, but is comparable to δ34S ranges in pyrite systems of up to 186‰ reported from microbial sulfate reduction in deep bedrock fracture systems in Sweden. The growth stages from Py-2 to Py-3 indicate a transformation of the environment from an open to a closed (highly reducing) system, and it is these late-stage vein pyrites that are slightly auriferous. This process is interpreted to be induced by the precipitation of sulfides perhaps near the margin of the hydrothermal system or closed growth of pyrite in hydrothermal veins, which both restrict circulation and convection of sulfate within the veins. Our results suggest that the Erua Basin may sit on the fringe of a low-temperature epithermal gold system; other “low-temperature” areas around the TVZ may also preserve similar, potentially enriched zones on the edge of an epithermal Au belt.

Auriferous pyrite formed by episodic fluid inputs in the Akeshi and Kasuga high-sulfidation deposits, Southern Kyushu, Japan

Pyrite geochemistry has proven useful for tracking changes in the composition and physico-chemical conditions of hydrothermal fluids in ore-forming environments. Here, we investigated the microtextural features and chemical composition of pyrite, a main Au-bearing phase in the Akeshi and Kasuga deposits (Southern Kyushu, Japan), to better constrain the ore-forming processes in these high-sulfidation epithermal Au deposits. Despite the widespread distribution of Au-bearing pyrite in both deposits, no visible Au minerals coexist with pyrite. However, in situ laser ablation inductively coupled plasma mass spectrometry results show that Au concentrations in pyrite vary from below the detection limit to 41 ppm and are positively correlated with Cu (r = 0.4; up to 7400 ppm) and Bi concentrations (r = 0.44; up to 640 ppm). In both deposits, high Cu and Au concentrations occur in small (< 25 μm) anhedral grains of pyrite, which are interpreted to have rapidly crystallized from the ore-forming hydrothermal fluid. In addition, dissolution–reprecipitation textures and thin, concentric, Cu-rich overgrowths were identified in a number of larger (> 25 μm) pyrite grains and aggregates. These abrupt changes in the trace element compositions of pyrite grains likely record episodic metal-rich fluid inputs. We also propose that gold adsorption onto growing pyrite surfaces played a key role in the mineralization of these deposits.

Different metal sources in the evolution of an epithermal ore system: Evidence from mercury isotopes associated with the Erdaokan epithermal Ag-Pb-Zn deposit, NE China

Epithermal metal deposits along convergent plate margins are of large economic value, however, the origin of the typical metal spectrum (Au, Ag, Sb, As, Hg) in these deposits remains debated. Here, we study the Hg isotope geochemistry of the vein-style Erdaokan Ag-Pb-Zn deposit of epithermal type in the eastern part of the Central Asian Orogenic Belt in NE China, to provide direct constraints on its metal source. Bulk ore samples and early-stage hydrothermal minerals (magnetite, pyrite, and quartz) show negative δ202Hg (−1.52 to −0.18‰) and positive Δ199Hg (−0.03 to 0.17‰) values similar to those of previously reported epithermal Au deposits in NE China, and also of marine systems in general. However, late-stage calcite shows weak negative to positive δ202Hg (-0.61 to 0.90‰) and significant negative Δ199Hg (−0.18 to −0.02‰), similar to the Late Paleozoic sandstone country rocks (δ202Hg, −1.09 to 0.76‰; Δ199Hg, −0.09 to 0.04‰). Hydrothermally altered gabbro and diorite show large variations in δ202Hg (−1.83 to 0.50‰) and Δ199Hg (−0.14 to 0.15‰) which overlap with those of the ore and country rocks, indicating a mixed source. The Δ199Hg signature in the bulk ore and early-stage hydrothermal minerals likely derives from dehydration of the subducted oceanic slab, whereas Hg in late-stage calcite is likely derived from the sedimentary country rocks. Hydrothermal deposits in different tectonic settings have distinct Δ199Hg signals, and our study suggests that Hg isotopes are a robust tool to distinguish metal sources of hydrothermal ore deposits.

Subduction: The recycling engine room for global metallogeny

The formation of ore deposits, within mineral systems that represent highly anomalous concentrations of economic elements in localized parts of the Earth’s crust, first requires an enriched and available source of those elements. In early Earth history, initial enrichment in the upper mantle was arguably through mantle plumes that even recycled metals from the core-mantle boundary. From the Mesoarchean onwards, the development of plate tectonics allowed subduction processes to be increasingly important in both recycling of elements from the upper mantle into the lower crust and from there into the upper crust where mineral deposits could form and be exploited. The most obvious expression of this recycling process is the direct association of porphyry-high-sulfidation epithermal-skarn Cu-Au-Ag +/- Mo systems and inboard fractionated granite-associated Sn-W systems with magmatic-hydrothermal fluids derived from subduction-related magmas in convergent margins, where epithermal Au-Ag systems also developed due to high thermal gradients. Syn-subduction VMS Cu-Zn-Pb(Au) systems were accreted at convergent margins. Most orogenic gold systems relate to fluids released through devolatilization of down-going subduction slabs and overlying metal-rich sediment wedges. More indirect associations relate to the ability of subduction zones to cause structural damage and metasomatize and fertilize sub-continental mantle lithosphere along the margins of bordering craton margins. Orogen collapse in the final stages of the subduction-related orogenic event led to asthenosphere upwelling and formation of proximal intrusion-related and more distal Carlin-type gold systems in continental basins on the craton margins. During subsequent orogenic events, previously metasomatized and fertilized mantle lithosphere was reactivated to release hydrothermal fluids to form many of the orogenic gold systems in eastern China. Reactivation of the metasomatized lithosphere also resulted in development of post-subduction hybrid or alkaline magmas that generated giant magmatic Kiruna-type Fe-P, carbonatite-related REE-Nb or Cu-P, and lamproite-related diamond systems, together with magmatic-hydrothermal IOCG mineral systems near craton margins. Subduction-damaged craton margins and their rift-related marginal basins became the sites of formation of Zambian Copper Belt-type deposits and the craton margins were reactivated during subsequent rifting allowing intrusion of giant Ni-Cu mafic–ultramafic bodies along lithosphere-tapping fault zones at those margins. Thus, subduction acts as the recycling engine room or factory responsible for many of the global giant mineral systems that are currently exploited.

Epithermal Zeolite Alteration Associated with Siliceous Sinters, Hydrothermal Eruption Breccias, and Gold-Silver Mineralization, Central Taupo Volcanic Zone, New Zealand

Abstract In the central Taupo Volcanic Zone, extensive zeolite (mordenite ± clinoptilolite) alteration occurs in late Quaternary rhyolitic vitric tuffs that were deposited in a lake formed by caldera collapse following the ~290 ka Ohakuri ignimbrite eruptions. Glass shards in lacustrine vitric tuffs of the Ngakuru Formation and in the underlying Ohakuri Formation ignimbrite are replaced by mordenite ± clinoptilolite, along with hydrothermal adularia, opal-A, opal-CT, and cristobalite. This mineral assemblage is also found in the outer alteration zones of the nearby Ohakuri and Tahunaatara epithermal gold prospects. Evaluation of whole-rock chemical analyses indicates that the zeolitized vitric tuffs show a slight gain in K, and Na, Ca loss relative to unaltered Ohakuri Formation pumice, which is reflected in the presence of hydrothermal adularia in the alteration assemblage. The mordenite ± clinoptilolite alteration is associated with siliceous sinters and hydrothermal eruption breccias that were formed in recently active (39–1.5 ka) geothermal systems. By analogy with geothermal systems elsewhere in the Taupo Volcanic Zone at Wairakei and Ohaaki, the mordenite ± clinoptilolite alteration was formed from dilute alkali-chloride aqueous liquid at 60° to 150°C. Based on the close association of the mordenite ± clinoptilolite alteration with siliceous sinters and hydrothermal eruption breccias in the central Taupo Volcanic Zone, it is classified as shallow, low-temperature, epithermal alteration. Mordenite ± clinoptilolite alteration has also been identified in Quaternary rhyolitic caldera settings in Japan and the United States, where it is termed “caldera-type zeolitization.” In exploration for epithermal Au-Ag deposits in rifted arc settings, such alteration may be overlooked, given its subtle appearance and distal location relative to veins that mark upflow areas.

Geochemical signatures in native gold derived from Au-bearing ore deposits

Native gold is a significant indicator mineral for Au-bearing deposits. The chemical composition of gold in diverse geological settings is a function of fluid composition under various physicochemical conditions. Native gold grains from various Au-bearing deposits and districts, including orogenic, volcanogenic massive sulfide (VMS), porphyry Cu-Au deposits, epithermal Au-Ag, and reduced intrusion-related gold (RIRG) deposits, were investigated by electron probe microanalyzer (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to establish discriminant chemical signatures to apply indicator mineral surveys for exploration for gold-bearing deposits. Detailed textural observation of gold identifies compositional heterogeneities in gold grains from epithermal and VMS deposits. Partial least square-discriminant analysis (PLS-DA) identifies that Pd, Ag, Sb, Pb, Cu, Hg, and Te contents in gold grains have a high potential to discriminate gold from different deposit types. Comparison of gold compositional data from different deposit types using Random Forests classification models can be used as a predictive modeling tool to provide an accurate indication of gold-bearing deposit types.