Epithermal Au Deposits Research Articles

Nature and genesis of the Zaglic Au deposit, NW Iran: Constraints from geochemical studies

The Zaglic Au deposit is located in a Late Oligocene magmatic arc within the Arasbaran Magmatic Belt of NW Iran, which formed during the subduction of Mesotethys oceanic crust. Country rocks comprise intermediate to mafic volcanic and volcanoclastics of andesitic to trachy–andesitic composition, and exhibit late granite plutonism. Host assemblages display medium– to high–K calc–alkaline affinities with fractionated REE profiles, including Nb and Ti depletion and Rb, Ba, Th, U enrichment, consistent with subducted terranes. Wall–rock alteration comprises multi–stage silicic, sericitic, argillic and/or propylitic domains, associated with (i) pre–, (ii) main–, and (iii) post–ore emplacement. Pyrite is the dominant sulfide with spatially associated chalcopyrite and bornite in quartz vein and veinlets, with free gold occurring as microscopic grains in quartz, or as inclusions in pyrite. Fluid inclusion analysis of pre– (quartz I), main– (quartz II) and post–ore stages (quartz III) identifies liquid–rich, multi–stage, and vapor–rich varieties with decreasing temperatures and salinities, with the following Th and NaCl wt% equivalent ranges: pre–mineralization: 230–382 °C and 6.1–8.2; main–ore: 179–311 °C and 0.35–3.22%; and post–ore: 150–195 °C and 2.2–2.9%. Stable isotope analysis identifies variable δ18Ofluid and δDfluid signatures, including: quartz I: 2–3.4‰; −86 to −78‰; quartz II: −0.6–0.2‰; −89 to −85‰; sericite: −0.2 to 3.4‰; −107 to −91‰; and chlorite: −0.6 to 1.4‰; −115 to −104‰. Such ranges imply a mixture of magmatic and meteoric fluids, characteristic of a wide range of epithermal Au deposits. Sulfur isotope composition (δ34S) of this deposit's two principal sulfide minerals is consistent with compositionally reduced mineralizing fluids including: pyrite: −1.4 to 2.7‰ and chalcopyrite: −2.6 to 1.8‰. Geochronological analysis records 40Ar/39Ar dating of biotite from andesite porphyry with a plateau Eocene age of 48.20 ± 0.73 Ma, together with ~8 Ma or Late Miocene ages from fine–grained sericite.

Epithermal Au–Ag–Se–Te Deposits of the Chukchi Peninsula (Arctic Zone of Russia): Metallogeny, Mineral Assemblages, and Fluid Regime

Abstract Numerous epithermal Au–Ag deposits and ore occurrences of the Chukchi Peninsula are localized in the Cretaceous Okhotsk–Chukotka (OCVB) continent-marginal and Late Jurassic–Early Cretaceous Oloi (OVB) island arc volcanic belts and in Early Cretaceous postcollisional volcanic troughs. Volcanotectonic depressions, calderas, and volcanic domes control the location of the deposits. The orebodies of the deposits are quartz–adularia veins, sometimes en-echelon ones forming extending vein zones, as well as isometric and linear stockworks. The auriferous veins of most deposits display complex breccia–crustification structures. The vein ores have rhythmically and colloform–banded structures, with a predominantly fine distribution of ore mineral grains, often with banded clusters of ore minerals (ginguro). Native gold is of low fineness; the dispersion of this index varies from low to high. Acanthite is widespread in the ores. Its highest contents are specific to deposits with the repeated redistribution of substance (Kupol, Corrida, and Valunistoe). Based on the results of mineralogical studies, most of the epithermal Au–Ag deposits of the Chukchi Peninsula can be assigned to the Se type. The ores of some deposits (Valunistoe, Dvoinoe, etc.) contain both Se and Te minerals. The telluride-richest sites of the Sentyabr’skoe and Televeem deposits are far from the main orebodies. Most of the Chukchi epithermal Au–Ag deposits have many common characteristics (low and moderate temperatures of fluids, low fluid salinity, domination of carbon dioxide over methane, etc.) typical of low-sulfidation deposits. The maximum temperatures and salinity are specific to fluids in the Central Chukchi sector of the OCVB and in the Baimka zone of the OVB, and the minimum ones are typical of fluids in the East Chukchi flank zone and inner zone of the OCVB. The average salinity of mineral-forming fluids in the inner zone of the OCVB is half as high as the salinity of fluids in the East Chukchi flank zone of this belt, although the sulfate content is higher. At the same time, the fluids in the inner zone of the OCVB are richer in carbon dioxide and bicarbonate ion than the fluids in the East Chukchi flank zone of this belt. The fluid inclusion data permit the Vesennee deposit (Baimka zone) to be regarded as an intermediate-sulfidation one and suggest the presence of epithermal high-sulfidation deposits in the inner zone of the OCVB.

Toward a mineral potential map of central Aceh, North Sumatra: contributions from spectrum-area and local singularity fractal analysis of stream sediment geochemistry and its relationships to magmatic-hydrothermal Cu and Au mineralisation

ABSTRACT A reassessment of the geochemistry of stream sediments collected by the British-led North Sumatra Project from 1975 to 1980 in central Aceh with spectrum–area and local singularity fractal techniques refines previously determined areas with anomalous Cu and delineates As anomalies (proxy for Au) that correspond to porphyry Cu and epithermal Au–Ag deposits and prospects discovered in the 1980s and 1990s. Additionally, the application of these methods to stream sediment samples collected by an Australian company in the mid-1990s from part of central Aceh identifies Au, As, and Cu anomalies that are aligned with regional geochemical trends. Integrating singularity metal-enriched areas and the distribution of mineral occurrences and structures reveals northerly and easterly tectono-magmatic corridors controlling epithermal Au–Ag and porphyry Cu mineralisation. The above results attest to the usefulness of legacy data for first-pass mineral assessments and for providing evidence layers for a future mineral prospectivity map.

Mineral Composition and Physicochemical Conditions of Formation of the Pepenveem Epithermal Au–Ag Deposit (Chukchi Peninsula)

Abstract —We have studied the mineral composition of ores from the Pepenveem epithermal Au–Ag deposit, which is a promising new object of the Chukchi Peninsula. It has been found that the ore formation process was developed in the following sequence: Pyrite, arsenopyrite, and marcasite were deposited at the early stage, next were Pb, Zn, and Cu sulfides; at the late stage, native gold, pyrargyrite, stephanite, proustite, minerals of the pearceite–polybasite series, acanthite, and other Ag minerals were deposited. The results of fluid inclusion studies indicate that the Au–Ag mineralization formed from low-temperature (236–137 °C) low-concentration chloride hydrotherms (0.18–1.57 wt.% NaCl eq.). The results of calculation of thermodynamic equilibria have shown that in the temperature range from 200 to 100 °C, there were a decrease in the fugacity of sulfur (lg fS2 from –10 to –21) and oxygen (lg fO2 from <–36 to <–48) and a change from near-neutral to acidic solutions. Compared to other Au–Ag deposits on the Chukchi Peninsula (Corrida and Valunistoe), which are characterized by wide distribution of Se- and Te-bearing Au–Ag chalcogenides (naumannite, cervelleite, and hessite), ore formation with gold–silver–sulfosalt mineralization at the Pepenveem deposit took place at lower temperatures and lower selenium, tellurium, and oxygen fugacity. The data obtained permit us to refer the Pepenveem deposit to the group of epithermal low-sulfidation (LS) deposits.

Svetlanaite, SnSe, a new mineral from the Ozernovskoe deposit, Kamchatka peninsula, Russia

AbstractSvetlanaite, SnSe is a new mineral discovered from the high-sulfidation epithermal Au-deposit Ozernovskoe, Kamchatka peninsula, Russia. It forms tiny euhedral spindles (0.5–2 μm × 10–15 μm) in quartz, in close association with cassiterite, rutile, mohite, mawsonite, kiddcreekite, hemusite, tellurium, kostovite and Se-bearing ‘fahlores’ (Se-goldfieldite–Se(Bi)-tetrahedrite–Se-tennantite). In plane-polarised light, svetlanaite is light-grey, pleochroic from white to cream and strongly anisotropic in shades of light blue, dark blue, khaki and orange–brown; it exhibits no internal reflections. Reflectance values of synthetic analogue of svetlanaite in air (R1,R2 in %) are: 50.9, 56.5 at 470 nm, 50.2, 56.7 at 546 nm, 49.5, 55.3 at 589 nm and 48.7, 53.4 at 650 nm. Twelve electron-microprobe analyses of svetlanaite give an average composition: Sn 61.30, Se 37.22 and S 1.25 total 99.79 wt.%, corresponding to the empirical formula Sn1.01(Se0.92S0.07)Σ0.99 based on 2 atoms; the average of seven analyses on its synthetic analogue is: Sn 59.98 and Se 39.71, total 99.59 wt.%, corresponding to Sn1.00Se1.00. The density, calculated on the basis of the empirical formula, is 6.08 g/cm3. The mineral is orthorhombic, space group Pnma, with a = 11.500(2), b = 4.154(2), c = 4.445(2) Å, V = 212.34(14) Å3 and Z = 4. The crystal structure was solved and refined from the powder X-ray-diffraction data of synthetic SnSe. It crystallises in the GeS-structure type. It is isostructural with the mineral herzenbergite (SnS). The mineral name is in honour of Svetlana K. Smirnova, a Russian mineralogist, for her contributions to geology in the epithermal Au–Ag deposits of the Tien–Shan region.

Late Jurassic epithermal mineralization of Shitouban Au deposit, Dehua-Youxi area, southeast China: Quartz Rb[sbnd]Sr dating, fluid inclusions, and H-O-S-Pb-Sr isotope analyses

Shitouban Au deposit is a vein-type deposit, which located in the Dehua-Youxi area (Fujian Province, southeast China) and hosted by late Jurassic volcanic-sedimentary rocks. In this study, a detailed investigation on fluid inclusions, multiple isotopic compositions (H-O-S-Pb-Sr) and RbSr isotope dating of quartz have been carried out in an attempt to constrain the ore-forming processes and ore genesis. The deposit shares a characteristic of low-sulfidation epithermal Au deposit, such as low-temperature (peak Th 160–180 °C, average 168 °C) and low-salinity (2-3 wt% NaCl equiv., average 2.47). The hydrogen and calculated oxygen isotope values for fluids are −64 to −54‰ (average − 60‰) and - 0.95 to 0.76‰ (average 0.15‰) respectively, indicating that the ore-forming fluid was a mixture of dominant meteroric water and subsidiary magmatic fluid. Pyrite from the deposit shows a δ34S range from - 4.5 to - 1.2‰ (average −2.4‰), indicating a magmatic sulfur. The lead isotope ratios of pyrite, 206Pb/204Pb (18.399–18.494), 207Pb/204Pb (15.695–15.841), 208Pb/204Pb (38.842–39.297), are more radiogenic than the Jurassic magmatic rocks in the area, indicating a mixing metal source of magmatic and basement rocks. Based on the RbSr isotopic isochron age of Au-bearing quartz, the metallogenic epoch of Shitouban Au deposit is 157 ± 1 Ma, and consistent with the Jurassic volcanism in the Dehua-Youxi area. It is proposed that the primary hydrothermal fluids may have exsolved from the late Jurassic magma chamber and subsequently ascent into the shallower crust level near paleosurface to mix with meteoric water, which cause precipitation of Au and sulfides in the epithermal environment due to a decrease in metal solubility triggered by fluid boiling, cooling, and mixing.

Oxygen isotope evidence for input of magmatic fluids and precipitation of Au-Ag-tellurides in an otherwise ordinary adularia-sericite epithermal system in NE China

Abstract Tellurium-rich (Te) adularia-sericite epithermal Au-Ag deposits are an important current and future source of precious and critical metals. However, the source and evolution of ore-forming fluids in these deposits are masked by traditional bulk analysis of quartz oxygen isotope ratios that homogenize fine-scale textures and growth zones. To advance understanding of the source of Te and precious metals, herein, we use petrographic and cathodoluminescence (CL) images of such textures and growth zones to guide high spatial resolution secondary ion mass spectroscopy (SIMS) oxygen isotope analyses (10 µm spot) and spatially correlated fluid inclusion microthermometric measurements on successive quartz bands in contemporary Te-rich and Te-poor adularia-sericite (-quartz) epithermal Au-Ag vein deposits in northeastern China. The results show that large positive oxygen isotope shifts from –7.1 to +7.7‰ in quartz rims are followed by precipitation of Au-Ag telluride minerals in the Te-rich deposit, whereas small oxygen isotope shifts of only 4‰ (–2.2 to +1.6‰) were detected in quartz associated with Au-Ag minerals in the Te-poor deposits. Moreover, fluid-inclusion homogenization temperatures are higher in comb quartz rims (avg. 266.4 to 277.5 °C) followed by Au-Ag telluride minerals than in previous stages (~250 °C) in the Te-rich deposit. The Te-poor deposit has a consistent temperature (~245 °C) in quartz that pre- and postdates Au-Ag minerals. Together, the coupled increase in oxygen isotope ratios and homogenization temperatures followed by precipitation of Au-Ag tellurides strongly supports that inputs of magmatic fluid containing Au, Ag, and Te into barren meteoric water-dominated flow systems are critical to the formation of Te-rich adularia-sericite epithermal Au-Ag deposits. In contrast, Te-poor adularia-sericite epithermal Au-Ag deposits show little or no oxygen isotope or fluid-inclusion evidence for inputs of magmatic fluid.

The Gold Mineralization of the Baranyevskoe Au-Ag Epithermal Deposit in Central Kamchatka

The Baranyevskoe Au-Ag epithermal deposit of low-sulfidation (LS) type is located on the Kamchatka Peninsula in the Neogene-Quaternary Central Kamchatka Volcanic Belt, where Au-bearing quartz veins are usually accompanied by veinlet stockworks. Two economic associations are typical of the Baranyevskoe deposit. The first corresponds to gold-pyrite-quartz association with low-grade native gold (521–738‱) intergrown with pyrite. Some accessory Au-Ag minerals within the early association were also identified: acanthite AgS2, hessite AgTe2, lenaite Ag(Fe,Cu)S2, petzite Ag3AuTe2, utenbogardite Ag3AuS2 and unnamed Ag-Sb-As sulfosalts. The former Au-Ag minerals were most likely formed in the temperature range of 320–330 °C based on the study of arsenopyrite thermometers and fluid inclusions. The second, a gold-sulfosalt-quartz association, includes high-grade native gold (883-941‱) in intergrowth with chalcopyrite. Cuprous phases (bornite, chalcocite, heerite, native copper, Cu-Zn solid solutions), Bi-rich sulfosalts (aikinite PbCuBiS3, emplectite CuBiS2, witticenite Cu3BiS3), stannoidite Cu8Fe3Sn2S12, mawsonite Cu6Fe2SnS8), Au-bearing galena, Te-free and Bi-rich tetrahedrite-tennantite represent this association. Fluid inclusions in gold-sulfosalt-quartz association are characterized by homogenization temperature ranging from 226 to 298 °C, and salinity from 0.4 to 1.2 wt. % NaCl eq.

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.

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.

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.

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.

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.

Au‐Ag‐S‐Se‐Cl‐Br Mineralization at the Corrida Deposit (Russia) and Physicochemical Conditions of Ore Formation

The mineral and chemical compositions of ores from the Corrida epithermal Au-Ag deposit (Chukchi Peninsula, Russia) were studied using the optical and scanning electron microscopy with X-ray energy-dispersion microanalysis. The deposit was formed at the time close to the period when the basic volume of acid magmas had been emplaced within the Okhotsk–Chukotka belt (84 to 80 Ma). The Au–Ag mineralization is distinguished with Au-Ag sulphides and selenides (uytenbogaardtite-fischesserite solid solution, Se-acanthite, S-naumannite) and Ag halides of the chlorargyrite-embolite-bromargyrite series. The ores were formed in two stages. Using microthermometric methods, it has been established that the ore-bearing quartz was formed in the medium-temperature environment (340–160 °C) with the participation of low-salt (3.55 to 0.18 wt.% NaCl eq.) hydrotherms, mostly of the NaCl composition with magnesium, iron and low-density СО2. According to our results of thermodynamic modeling at temperatures from 300 to 25 °C and data on mineral metasomatic alterations of the host rocks, the Au-Ag-S-Se-Cl-Br mineralization was formed at decreasing temperature and fugacity of sulphur (logƒS2 from −6 to −27), selenium (logƒSe2 from −14 to −35), and oxygen (logƒО2 from −36 to −62), with near-neutral solutions replaced by acid solutions. Analysis of the obtained data shows that the Corrida refers to the group of the LS-type epithermal deposits. This deposit is a new example of epithermal deposits with significant quantities of Au–Ag chalcogenides (acanthite, uytenbogaardtite, fischesserite, naumannite and others).

Characteristics and genesis of phyllosilicate hydrothermal assemblages from Neoproterozoic epithermal Au-Ag mineralization of the Avalon Zone of Newfoundland, Canada

The Avalon Zone hosts well-preserved examples of high- and low-sulfidation epithermal Au-Ag mineralization and associated phyllosilicate alteration. Using examples from the Hope Brook and Hickey's Pond high-sulfidation systems (HSS) and the Heritage low-sulfidation system (LSS), this study presents new data on the mineralogical and chemical characterization of phyllosilicates associated with epithermal Au-Ag deposits. Electron probe microanalysis and X-ray diffraction studies revealed that the HSS examples contain Fe muscovite ((K0.62–0.79Na0.10–0.37Ca0.00–0.01)Al1.31–1.72(Mg0.01–0.08Fe0.05–0.19)(Al1.00–1.24Si2.76–3.00O10)(OH)2), paragonite ((K0.05–0.30Na0.78–1.02Ca0.00–0.01)Al1.66–1.84(Mg0.00–0.01Fe0.02–0.04)(Al1.01–1.11Si2.89–2.99O10)(OH)2), Al clinochlore ((Mg2.42–2.65Fe1.62–1.78)(Si2.57–2.69Al1.31–1.43O10)(OH)8), pyrophyllite, kaolin minerals, illite-smectite (I-Sme) and chlorite-smectite (C-Sme). Conversely, the LSS examples contain Fe-Mg muscovite ((K0.44–0.76Na0.00–0.02Ca0.00–0.01)Al1.55–1.82(Mg0.17–0.31Fe0.05–0.12)(Al0.55–0.71Si3.29–3.45O10)(OH)2), Al clinochlore ((Mg2.17–2.56Fe1.59–2.01)(Si2.71–2.92Al1.08–1.29O10)(OH)8), and I-Sme and C-Sme intermediates. Chlorite geothermometry indicated that HSS and LSS assemblages crystallized at average temperatures of 261 °C and 145 °C, respectively. Mica and chlorite from HSS and LSS are defined by Tschermak substitution and high-temperature polytypism (2M1 and IIb, respectively). In the HSS examples, one or more pulses of hot, acidic fluid reacted with host rocks generating an envelope of advanced argillic alteration (Fe-Ms, Kln, Pg, Prl) around a vuggy silica core. This is bounded by a zone of argillic alteration (Fe-Ms, Kln), with sericitic/chloritic alterations (Fe-Ms, Al-Clc) developed in distal parts of the systems. In the LSS, near-neutral hydrothermal fluids led to the formation of broad phyllic/chloritic zones (Fe-Mg Ms and Al-Clc). A low temperature overprint with 1M mica, Ib clinochlore ((Mg2.48–2.74Fe1.58–1.73)(Si3.05–3.15Al0.85–0.95O10)(OH)8), kaolinite and Sme-poor I-Sme and C-Sme is attributed to late waning stage hydrothermal activity and/or weathering. A close paragenetic link was established between Fe-Mg muscovite and Ag mineralization at Heritage, as well as between Fe-Mg muscovite and bladed calcite, suggesting boiling as the precipitation mechanism. The present study provides guidance on how phyllosilicate assemblages have the potential to discern HSS versus LSS mineralization during early prospection in terranes with limited exposure, and to contribute to the reconstruction of their hydrothermal record.

Hydrothermal evolution and ore genesis of the Jiamante Au deposit in Western Tianshan, Xinjiang, China: Evidence from the geology, fluid inclusions, and C–H–O–S–Pb isotopes

The Jiamante Au deposit is one of the important deposits in the Axi ore cluster of the Western Tianshan, Xinjiang. The orebodies occurring mainly as quartz‐sulphide veins are hosted in Late Devonian volcanoclastic rocks and granitic porphyries, and structurally controlled by a group of NNW‐ and NNE‐ trending extensional faults and volcanic breccias. Primary metallic minerals are chalcopyrite, galena, pyrite, sphalerite, and native gold, with minor amounts of bismuthinite and bismuth. Nonmetallic minerals are dominated by quartz, sericite, chlorite, and calcite. Medium to low temperature hydrothermal alterations are well developed and characterized by silicification, beresitization, phyllic alteration, and chloritization. Two hydrothermal stages were identified: (a) the main‐ore quartz‐sulphide stage; and (b) the post‐ore quartz‐calcite stage. Microthermometric measurements indicate that the primary biphase liquid‐rich aqueous fluid inclusions within quartz of the main‐ore stage homogenize at temperatures (Th) of 176 to 251°C and have salinities of 2.1 to 7.4 wt% NaCleqv. A lower range of homogenization temperatures (169 to 212°C) and salinities (1.6 to 5.2 wt% NaCleqv) were obtained in calcite of the post‐ore stage. The fluid characteristics of moderate to low temperatures and salinities imply that the ore‐forming process occurred in an epithermal environment. In addition, there is a positive relationship between Th and salinities consistent with the fluid mixing trend. Together with H–O isotopic data (δDH2O = −118 to −97‰ and δ18OH2O = −5.5 to −1.6‰) and fluid characteristics, it is suggested that the ore‐forming fluids were a mixture of magmatic and meteoric waters. The δ13C values for CO2 in the hydrothermal fluids range from −3.2 to −2.7‰, and the sulphides have δ34S values of −5.2 to 2.9‰ with relatively uniform Pb isotopic compositions (206Pb/204Pb = 18.228 to 18.287, 207Pb/204Pb = 15.574 to 15.617, and 208Pb/204Pb = 38.151 to 38.251), indicating that the ore‐forming materials were magmatic sources that were derived directly from magmatic fluids and/or leached from volcanic‐subvolcanic rocks by hydrothermal fluids. Combined with the evidence from the mineralization and alteration, fluid inclusions, and C–H–O–S–Pb isotopes suggest that the Jiamante deposit is an intermediate‐sulphidation epithermal Au deposit. A combination of fluid mixing and fluid–rock interactions is inferred as the important factors causing ore deposition. According to the close spatial and temporal relationship between intermediate‐epithermal deposits and porphyry deposits, it is further suggested that porphyry‐type mineralization should have a good potential at the deep sites of the Jiamante deposit and its vicinity.