Low-sulfidation Epithermal Deposit Research Articles

A cold-water trap as an essential process for the generation of low-sulfidation epithermal deposits: Geological and numerical studies of the Hosokura deposit, northern Japan

Although low-sulfidation epithermal deposits, including the Hosokura deposit (Ag–Pb–Zn) in the Miyagi Prefecture, northern Japan, are globally significant metal sources, their ore deposition mechanisms are not thoroughly understood . This study aimed to construct a genetic ore deposition model for this deposit by focusing on the richest vein, the Fuji, and test the model using a numerical simulation of the formation and transitions of this fossil geothermal system. It was proposed that the mixing of neutral hydrothermal fluids (~260 °C) ascending through the vein with oxygenated cold groundwater caused early stage mineralization and the transport of hydrothermal fluids into the surrounding host rocks, forming a hydrothermal alteration halo around the vein that results in silicification, adularia alteration, and argillization. The progress of this alteration decreased the permeability of the halo and generated a mushroom-shaped low-permeability alteration halo (LPAH) in permeable zones surrounding the vein. The low-permeability and insulating properties of the LPAH increased the fluid temperature, the vein fluid boiled, and consequently the middle stage mineralization occurred. After a further decrease in LPAH permeability, hydrothermal brecciation resulting from increasing fluid pressure occurred at the top of the vein, resulting in late-stage mineralization. A TOUGH2 numerical simulation tested these proposed processes: early stage cooling of hydrothermal fluids by mixing with groundwater, the development of vapor in the vein by the formation of an LPAH during the middle stage, and an increase in fluid pressure leading to hydrothermal brecciation during the late-stage. The results of the simulation confirmed that a cold-water trap is an essential process for the generation of low-sulfidation epithermal deposits. Mushroom-shaped LPAHs, silicification zones, and brecciated veins caused by cold-water traps have been found in representative low-sulfidation epithermal deposits in Japan, such as the richest Au deposit, the Hishikari. Therefore, identification of cold-water-trap footprints can contribute to the exploration of this type of deposit.

Geology, geochronology, and geochemistry of the Sinongduo Ag–Pb–Zn deposit in the Gangdese metallogenic belt: Implications of intermediate sulfidation mineralization in the Linzizong volcanic succession

The Sinongduo Ag–Pb–Zn deposit was first defined as a typical low sulfidation (LS) epithermal deposit in the widespread Linzizong volcanic succession (LVS), Gangdese belt, Tibet. This paper we describe the lithology, mineralization, and hydrothermal alteration based on current advances in exploration, and use multiple chemical and geochronology methods, including whole-rock geochemistry (major and trace elements) of ores and the lithocap, the mineral chemistry of sphalerite, and the LA–MC–ICP-MS zircon U–Pb dating of host volcanic rocks to better constrain the genesis of the Sinongduo deposit and give some exploration advice.In the Sinongduo deposit, the host crystal tuff yields a zircon U–Pb age of 61.9 ± 0.4 Ma, indicating nearly simultaneous diagenesis and mineralization. The metallic minerals are mainly FeS-poor sphalerite, galena, pyrite, and minor chalcopyrite, along with silver and silver-based precious minerals. These minerals, particularly FeS–poor sphalerite, pyrite, and chalcopyrite, represent IS states. Mineral chemistry shows that the content of FeS in sphalerite is lower than 4.94 mol% and the Ag concentration is quite high, ranging from 16 to 1798 ppm with a mean concentration of 245 ppm. Low Ga/in ratios (<1) and enrichment of In and Sn in sphalerite imply that the ore-forming fluid is dominated by magmatic water with intermediate temperature. Gangue minerals include the silicic group (quartz and chalcedony), carbonates group (calcite, siderite, ankerite, etc.), and hematite. Alterations, including illite, montmorillonite, and sericite, are widely distributed. The results suggest that the Sinongduo deposit is not a LS epithermal deposit but a typical IS epithermal deposit and we believe that there is potential for Au and Ag exploration.

Low-Sulfidation Epithermal Carbonate-Basemetal-Gold Mineralization Hosted by Tertiary Sedimentary Rocks in Bastem Prospect, Luwu District, Sulawesi Island, Indonesia: A Preliminary Study

In Indonesia, gold is typically mined out from epithermal, porphyry and skarn deposit types occurred within volcanic belts along magmatic arc or active continental margin setting. Numerous gold prospects, however, are recently discovered in association with metamorphic and sedimentary rocks. This paper is aimed to discuss a preliminary study on the occurrences and characteristics of the sedimentary rock-hosted low-sulfidation (LS) epithermal gold mineralization in Bastem (Bessengan Tempe) prospect, Luwu district, Sulawesi Island, Indonesia. Detailed geological and hydrothermal alteration mapping was performed to understand the distribution and characteristics of ore mineralization. Representative ore samples taken were analysed for ore chemistry by means of Fire Assay – Atomic Absorption Spectrometry (FA-AAS) and Acid Geochemical Digest - Atomic Absorption Spectrometry (GA-AAS). The results from this work that the Bastem prospect is stratigrapically occupied by Tertiary sedimentary rocks of Toraja formation, which is adjacent to volcanic rocks of Lamasi formation. Gold-bearing quartz±carbonate veins are hosted by mudstone and siltstone of Toraja formation. The quartz-carbonate veins show a typical LS epithermal open space filling texture containing erratic gold grade of up to 7.16 g/t with relatively high besemetals (Pb and Zn) grades of up to &gt;0.4 and &gt;1%, respectively. Based on those various features, the LS epithermal deposit is categorized as “carbonate-basemetal-gold mineralization type”, which might be originated in back arc rift/basin setting. This ‘unconventional’ sedimentary rock hosted-gold mineralization type would be the new target of gold exploration in Indonesia.

Geochemical, fluid inclusion and O–H–S isotope studies of the Milajerd Au-polymetallic prospect, Central Iran: Implications for ore genesis

The Milajerd Pb-Zn ± Au polymetallic prospect in Naein-Qom Magmatic Zone is a favorable target for exploration. Geological and geochemical characteristics such as setting, alteration assemblages, fluid inclusion microthermometry and stable isotopes are consistent with the Milajerd Pb-Zn ± Au polymetallic prospect being a typical low-sulfidation (LS) epithermal deposits. The Au-polymetallic mineralization occurs mainly as structurally controlled veins, and brecciated dikes, hosted by calc-alkaline volcanic rocks. Mineralization at the Milajerd prospect is controlled by an annular fault system. The sulfide mineralogy of the deposit includes sphalerite, galena, pyrite, arsenopyrite, and marcasite. The principal gangue minerals are quartz, calcite, sericite, and adularia. The alteration assemblages include quartz ± pyrite, quartz-sericite-pyrite and chlorite-calcite-epidote. Fluid inclusions in quartz-sulfide veins have homogenization temperatures (Th) of 158–255 °C and salinities of 4.7–7.7 wt% NaCl equiv., whereas those in barren quartz ± carbonate veins yield Th values of 135–176 °C and salinities of 1.1–4.3 wt% NaCl equiv. The H and O isotopic composition (δDwater = − 120 to − 92‰; inferred δ18Owater = − 6.2 to 3.5‰) of fluid inclusions in quartz veins imply that cooling a mixed magmatic and meteoric waters was the main mechanisms of ore deposition at Milajerd deposit. The δ34S values of ore sulfides are mostly close to zero (avg. − 0.1‰), indicating a magmatic source of sulfur, which could have emanated directly from magmatic activity or have been leached from the surrounding volcanic rocks.

3D Mineral Prospectivity Modeling for the Low-Sulfidation Epithermal Gold Deposit: A Case Study of the Axi Gold Deposit, Western Tianshan, NW China

The Axi low-sulfidation (LS) epithermal deposit in northwestern China is the result of geological controls on hydrothermal fluid flow through strike-slip faults. Such controls occur commonly in LS epithermal deposits worldwide, but unfortunately, these have not been quantitatively analyzed to determine their spatial relationships with gold distribution and further guide mineral prospecting. In this study, we conduct a 3D mineral prospectivity modeling approach for the Axi deposit involving 3D geological modeling, 3D spatial analysis, and prospectivity modeling. The spatial analysis of geometric features revealed the gold mineralization trends in convex segments (0–20 m) with a specific distance from fault 2, the lower interface of late volcanic phase, and the upper interface of phyllic alteration with steep slopes (&gt;65°), implying that gold deposition was significantly controlled by the morphological characteristics and distance fields of geologic features. The present alteration–mineralization zone at Axi has a larger width in bending sites (sections No. 35–15 and No. 40–56) than elsewhere, indicating the location of two fluid conduits extending to depth. The prediction-area plots and receiver operating characteristic curves demonstrated that (genetic algorithm optimized support vector regression (GA-SVR)) outperformed multiple nonlinear regression and fuzzy weights-of-evidence, which was proposed as a robust method to solve complicated nonlinear and high-dimensional issues in prospectivity modeling. Our study manifests spatial controls of structure, host rock, and alteration on LS epithermal gold deposition, and highlights the capability of GA-SVR for identifying deposit-scale potential epithermal gold mineralization.

3D inversion modelling of gravity data to identify gold mineralization zones in region “X”, Pongkor

Gold originates from hydrothermal liquids and is deposited in the form of epithermal deposits. Structures such as faults play an important role in the process of gold deposition because they form hydrothermal flow paths to the surface. One method that can be used for gold exploration is the gravity method. This method can identify gold mineralization zones in the faults of a research area. Three-dimensional (3D) inversion modeling can be used to obtain further information on a target. Such modeling allows the process to be more realistic because the geometry model can be fitted to the actual conditions in the field, which allows the measurements to be more accurate. The gravity data acquisition for this study was performed by PT. Antam Tbk gold mining company in Pongkor. The hydrothermal deposit area is categorized as containing low sulfidation epithermal deposits. Analysis and interpretation of the gravity data were performed using derivative analyses, a spectrum analysis, and 3D gravity inversion modeling. The results show gold mineralization zones in the middle of the study area that have high gravity anomalies at the faults. There are eight faults detected in the potential gold mineralization zone. From the inversion result, the density of the rocks in the gold mineralization zone was determined to be in the range of 2.80–3.34 g/cm3.

Pyrite chemistry: A new window into Au-Te ore-forming processes in alkaline epithermal districts, Cripple Creek, Colorado

Tellurium has a wide variety of applications, most importantly in the solar energy industry and is eco-toxicologically significant; however, the magmatic-hydrothermal processes causing the pronounced Te enrichment together with Au in some epithermal districts are still poorly constrained. Hydrothermal and alkaline magmatic activity in post-subduction environments are suggested to be a critical component in the evolution of this Te-rich sub-class of low-sulfidation epithermal deposits. Cripple Creek represents an example for a world-class low-sulfidation epithermal Au-Te anomaly in the continental crust. This area represents a natural laboratory to investigate the processes of ore-formation and Au-Te enrichment in alkaline igneous rock-hosted epithermal systems.Here, we present the first micro-analytical approach that combines petrographic observations with in situ LA-ICP-MS analyses and trace element mapping to define the key ore-forming processes of Au and Te in the Cripple Creek epithermal complex. Two main styles of mineralization can be distinguished: (1) low-grade Au disseminated pyrite-rich ores in the permeable brecciated host rocks and (2) high-grade quartz-fluorite veins rich in calaverite (AuTe2), coloradoite (HgTe), petzite (Ag3AuTe2), altaite (PbTe) and native Au.Pyrite trace element mapping revealed distinct variations and decoupling of elements (Au-Te) and element pairs (Au-As vs. Co-Ni) within and between different sites in the epithermal district. This is reflected by the concentric/growth zoning of these elements in pyrite that were interpreted to be caused by fluid boiling associated with the deposition of the low-grade disseminated host rock ores at temperatures between 220 and 350 °C leading to the precipitation of Au, As, Co and Ni from the liquid phase and the preferential partitioning of Te into the vapor phase. The subsequent condensation of the Te-rich vapors in metal-bearing meteoric waters led to the Te precipitation being decoupled from Au in the low-grade ores.The high-grade Au-Te mineralization was likely initiated by the influx of magmatically derived oxidized fluids. Subsequently, fluid temperatures dropped due to mixing with meteoric waters resulting in fluid boiling under low pressure conditions at shallower crustal levels (<1000 m) compared to the low-grade ores. Elevated fO2 minimized the loss of Te to the vapor phase and the strong boiling conditions at lower fluid temperatures (105–200 °C) caused the contemporaneous precipitation of Au and Te in the high-grade veins.

Vein Mineralogy and Hydrothermal Gold Mineralization at the Kyaw Soe Thu Prospect in the Wabo Deposit, Mandalay Region, Central Myanmar

The Kyaw Soe Thu Prospect is a part of the Wabo deposit which is situated along Mogok Metamorphic Belt (MMB) in Central Myanmar. Gold bearing veins are hosted in the banded garnet-biotite gneiss, calc-silicate, marble and granitoid of the Wabo area. The granitic rocks intruded into banded garnet-biotite gneiss. The MMB was intensely deformed and it is a highly fractured metamorphic terrain in which metamorphic facies ranges from amphibolite to granulite facies. Gold mineralization at the Kyaw Soe Thu Prospect in the Wabo deposit occurred in the forms of gold-quartz veins. The quartz veins are a few centimeters in width. The color of the quartz vein at the Kyaw Soe Thu Prospect is commonly smoky and milky. The quartz vein exhibits chalcedonic, saccharoidal and granular textures. Three mineralizations were indicated. Stage I is the thin layered crustiform chalcedonic quartz. Stage II is chalcedonic quartz vein and stage III is Ankerite-quartz-sericite vein. Stage II and stage III are gold rich hydrothermal stages, and Ankerite-quartz-sericite veins were formed with a small amount of sericites in the Kyaw Soe Thu Prospect. The Ankerite-quartz-sericite veinlets range from a centimeter up to three centimeter in width within the stage II chalcedonic quartz vein. Textures of mineralized quartz vein samples were petrographically examined. The textures include a feathery texture that is most closely associated with electrum in both stage II chalcedonic quartz veins and stage III Ankerite-quartz-sericite veins. Ankerite varies in color from pink, white to buff. Among them, pink color is more common in the Kyaw Soe Thu Prospect. Ankerite in the Kyaw Soe Thu Prospect exhibits rhombohedral idiomorphic, which exhibits growth patterns. Ore minerals of the veins are electrum, pyrite, galena, and trace amount of very fine-grained chalcopyrite. On the basis of textures of the chalcedonic quartz veins, the Kyaw Soe Thu Prospect is ascribed to be a low-sulfidation epithermal deposit.

Ore genesis of Axi post-collisional epithermal gold deposit, western Tianshan, NW China: Constraints from U–Pb dating, Hf isotopes, and pyrite in situ sulfur isotopes

The Axi gold deposit has long been regarded as a typical Paleozoic low sulfidation epithermal deposit, located in the western Tianshan, in the Central Asian Orogenic Belt (CAOB). SIMS zircon U–Pb dating of the ore-bearing andesite and dacite yielded concordia ages of 350.8 ± 2.7 Ma and 351.3 ± 3.1 Ma, respectively. In situ SIMS hydrothermal rutile U-Pb dating yielded two 207Pb corrected ages as 306.1 ± 16.9 Ma and 303.8 ± 14.6 Ma, respectively. These ages are considerably younger than the zircon SIMS U-Pb ages of ore-bearing volcanic rocks, which precludes a genetic link between the epithermal gold mineralization and the subduction-related Late Devonian-Early Carboniferous magmatism in the western Tianshan. Instead, the gold mineralization is interpreted to be genetically related to a buried ~300 Ma pluton that formed in a post-collisional environment. Combined with previously published zircon Hf isotopic compositions of ~300 Ma felsic intrusive rocks from western Tianshan, the results show that ~300 Ma felsic rocks have higher zircon εHf(t) values than those from ~351 Ma felsic rocks in the Axi deposit. These elevated εHf(t) values suggest that mantle material inputs of ~300 Ma Axi epithermal deposit in a post-collisional environment. Pyrite, the most common sulfide in the Axi ores, displays heterogeneous textures and has a large variation of As contents and sulfur isotopes, suggesting complex ore-forming processes in the Axi deposit. This study highlights the importance of in situ isotopes of hydrothermal minerals to decode the ore-forming histories of epithermal systems.

Metallogenesis and ore-forming time of the Changtuxili Mn–Ag–Pb–Zn deposit in Inner Mongolia: Evidence from C–O–S isotopes and U–Pb geochronology

This paper reports new geochronological (U–Pb) and isotope (C, O, and S) data to investigate the timing of mineralization and mode of ore genesis for the recently discovered Changtuxili Mn–Ag–Pb–Zn deposit, located on the western slopes of the southern Great Hinggan Range in NE China. The mineralization is hosted by intermediate–acidic lavas and pyroclastic rocks of the Baiyingaolao Formation. Three stages of mineralization are identified: quartz–pyrite (Stage I), galena–sphalerite–tetrahedrite–rhodochrosite (Stage II), and quartz–pyrite (Stage III). δ13C and δ18O values for carbonate from the ore vary from −8.51‰ to −4.96‰ and 3.97‰ to 15.90‰, respectively, which are indicative of a low-temperature alteration environment. δ34SV-CDT values of sulfides range from −1.77‰ to 4.16‰ and show a trend of equilibrium fractionation (δ34SPy ​> ​δ34SSp ​> ​δ34SGn). These features indicate that pyrite, sphalerite, and galena precipitated during the period of mineralization. The alteration mineral assemblage and isotope data indicate that the weakly acidic to weakly alkaline ore-forming fluid was derived largely from meteoric water and the ore-forming elements C and S originated from magma. During the mineralization, a geochemical barrier was formed by changes in the pH of the ore-forming fluid, leading to the precipitation of rhodochrosite. On the basis of the mineralization characteristics, new isotope data, and comparison with adjacent deposits, we propose that the Changtuxili Mn–Ag–Pb–Zn deposit is an intermediate-to low-sulfidation epithermal deposit whose formation was controlled by fractures and variability in the pH of the ore-forming fluid. The surrounding volcanic rocks yield zircon U–Pb ages of 160−146 ​Ma (Late Jurassic), indicating that the mineralization is younger than 146 ​Ma.

Geological, fluid inclusion, and H–O–S–Pb isotopic studies of the Xiaban epithermal gold deposit, Fujian Province, southeast China: Implications for ore genesis and mineral exploration

The Xiaban gold deposit is located in the Dehua goldfield of central-eastern Fujian Province, southeast China. Detailed field observations combined with mineral exploration in the study area indicate that the majority of mineralization within the Xiaban gold deposit is hosted by porphyritic rhyolite units in areas proximal to E–W trending faults with lesser amounts located within alteration zones along lithological contacts. The deposit contains silicified banded, disseminated, brecciated, fine-grained stockwork, and quartz–sulfide vein-hosted mineralization, all of which is associated with a quartz–sericite–illite–chalcedony alteration assemblage. The main ore minerals are electrum and pyrite with lesser amounts of galena, Fe-rich sphalerite, and marcasite, all of which are associated with sericitic and argillic alteration as well as silicification. Fluid inclusion microthermometric analysis of quartz-hosted fluid inclusions associated with the main stage of mineralization yielded homogenization temperatures (Th) of 157–207 °C (mean of 180 °C) and salinities of 0.7–2.9 wt% NaCl equivalent, which are indicative of an epithermal-type system. The precipitation of gold was most likely caused by fluid mixing between dominant meteoric and minor magmatic fluid components. Fluids within quartz-hosted fluid inclusions have H and O isotopic compositions (δDH2O = −106.3‰ to −81.8‰; δ18OH2O = −5.8‰ to −3.0‰) that are indicative of ore-forming fluids derived from a meteoric-water-dominated source. Pyrite in the study area has S isotopic compositions that vary between −2.8‰ and −1.6‰, indicative of S derived from a homogeneous magmatic source. Lead isotopic compositions of the pyrite within the deposit (206Pb/204Pb = 18.2953–18.3203, 207Pb/204Pb = 15.6458–15.6502, and 208Pb/204Pb = 38.6974–38.7271) plot between the orogenic-belt and upper-crustal evolution lines on tectonic discrimination diagrams as well as in the upper-crustal zone of a Δγ–Δβ genetic classification diagram, indicating that Pb within the Xiaban deposit was derived from a homogeneous upper-crustal source. Taken together, these mineralization and alteration, gangue and ore mineral assemblage, fluid inclusion, and H–O–S–Pb isotopic characteristics suggests that the Xiaban gold deposit is a low-sulfidation epithermal deposit. This result, together with the inferred minor amount of denudation that has affected the study area, means that the surrounding region should be considered prospective for this style of mineralization, with further exploration for epithermal mineralization within the Dehua goldfield ideally focusing on areas that show similar alteration characteristics to those found in the Xiaban deposit.

The Distribution of Precious Metals in High-Grade Banded Quartz Veins from Low-Sulfidation Epithermal Deposits: Constraints from µXRF Mapping

High-grade ore zones in low-sulfidation epithermal deposits are commonly associated with the occurrence of banded quartz veins. The ore minerals in these veins are heterogeneously distributed and are mostly confined to ginguro bands, which can be identified in hand specimen based on their distinct dark gray to black color. Micro-X-ray fluorescence element maps obtained on representative samples of banded quartz veins show that Au occurs together with Ag minerals in some of the ginguro bands, but Au can also be present in quartz bands that are light gray to white and cannot be macroscopically distinguished from barren bands. The occurrence of compositionally distinct ginguro and gankin bands, the latter being a new term coined here for colloform quartz bands containing primarily electrum or native gold, can be explained by temporal changes in the composition of the ore-forming thermal waters or variations in the conditions of ore deposition. Textural relationships, including the dendritic shape of ore minerals that appear to have grown in a matrix of silica microspheres, suggest that the ginguro and gankin bands have formed as a result of rapid deposition associated with vigorous boiling or flashing of the thermal waters.

Characterization of Illite Clays associated with the Sinongduo low sulfidation epithermal deposit, Central Tibet using field SWIR spectrometry

Field measurements using shortwave infrared (SWIR) reflectance from drill hole samples provided an effective method to map variations in illite crystallinity and the illite assemblage minerals associated with hydrothermal alteration zonation of the Sinongduo low sulfidation epithermal Ag-Pb-Zn deposit, Tibetan Plateau, China. The SWIR mineral spectral information was combined with electron probe micro-analysis (EPMA) measurements, and microscope observations to provide geological insights into: (a) the likely metallogenic fluid nature of the mixture of magmatic fluids and a cooler water source; (b) the illite physico-chemical properties of the associated hydrothermal fluids; (c) the illite, montmorillonite and sericite evolutionary interactions among these fluids, country rocks, and alteration minerals; and (d) the alteration minerals relationships and illite crystallinity to the economic mineralization. The results suggest that illite characterizations can be used as an indicator for epithermal deposit prospecting.

Fluid inclusion and stable isotope study of the Lubin-Zardeh epithermal Cu-Au deposit in Zanjan Province, NW Iran: Implications for ore genesis

The Lubin-Zardeh deposit is located in the Western Alborz Magmatic belt, 45 km NE of Zanjan, Iran. This deposit contains polymetallic (Cu, Au, Pb, Zn) mineralization which is hosted by the Cenozoic volcanic-intrusive rocks of mainly intermediate and acidic composition. Pyrite, chalcopyrite, galena, sphalerite, bornite and tennantite-tetrahedrite, with native gold and silver are present in the ores. Quartz, adularia and clay minerals (illite-montmorillonite) with minor calcite are the most abundant gangue minerals and generally occur as fine-grained assemblages; some are, however, medium- and coarse-grained assemblages. The ore minerals show disseminated, vein-veinlet, replacement, and brecciated textures while the gangue minerals represent replacement, cockade, crustiform, colloform, comb, vein-veinlet and plumose textures. Alteration haloes along the main vein are silicic, argillic (illite-montmorillonite), sericitic and pyritic. Gold contents of the quartz-chalcopyrite-gold veins range from 0.002 to 10 ppm with an average of 0.52 ppm. The average values for total reserve of 0.5 Mt of Lubin-Zardeh deposit has been determined 1.3%, 0.7% and 0.4% for Cu, Pb and Zn, respectively. Most of fluid inclusions in the quartz-gold-sulfide veins consist of two vapor and liquid phases at room temperature. The primary fluid inclusions have a low to intermediate homogenization temperature between 127 and 327 °C and salinity between 4.07 and 13.98 wt% NaCl equivalent. Stable isotope and fluid inclusion studies indicate that mineralization occurred in two stages at Lubin Zardeh deposit i.e. stage I; along which base metal deposited and stage II which is associated with copper and gold mineralization. The δ18Oquartz values from stage I and stage II range from 9.56 to 12.77‰ and from 7.23 to 9.24‰, respectively while their δ34S values of sulfide minerals range from −0.9‰ to +6.2‰ and form −6.6‰ to −1.9‰. The isotopic and microthermometric data indicate that the hydrothermal solution evolved through mixing of magmatic fluids with meteoric waters. Boiling of magmatic fluid and mixing with meteoric waters were the main mechanisms of ore deposition in the Lubin-Zardeh low sulfidation epithermal deposit.

Fluid inclusions, isotopic characteristics and geochronology of the Sinongduo epithermal Ag-Pb-Zn deposit, Tibet, China

The Sinongduo Ag-Pb-Zn deposit is the first documented low-sulfidation epithermal deposit in Tibet and located on the southern side of the Longge’er-Gongbujiangda arc fault belt of the Lhasa terrane. The Ag-rich base metal veins and relatively low-grade breccias and stockworks in the Sinongduo deposit occur in the Paleocene high-potassium calc-alkaline to shoshonitic series volcanic rocks, including rhyolitic welded tuff breccia and volcanic breccia, rhyolitic ignimbrite, rhyolitic crystal fragment tufflava, rhyolite, and rhyolitic tuff. A nearly N-S-trending fault is the most important ore-controlling structure in the deposit. Galena and sphalerite are the dominant ore minerals in quartz-sericite-illite ± adularia veins, with subordinate argentite, pyrargyrite, native silver, chalcopyrite, pyrite and tennantite. The most widespread silver minerals are argentite, pyrargyrite, pearceite and native silver, which occur in fissures in Fe-Mn carbonates, jasper and early sulfides. The mineralization process can be divided into a pre-ore stage, main ore stage, and post-ore stage, and the main metallogenic stage is divided into the galena substage, sphalerite substage and silver minerals substage.Microthermometric analyses of the primary liquid-rich and vapour-rich fluid inclusions (FIs) in the sphalerite (SpII), main ore stage quartz (QzII) and post-ore stage quartz (QzIII) of the Sinongduo deposit indicate that the FIs in SpII, QzII and QzIII are mainly homogenized at temperatures of 197.2–280.8 °C, 185.6–290.4 °C and 148.2–218.7 °C with salinities of 1.40–5.71 wt% NaCl equiv., 1.59–5.71 wt% NaCl equiv. and 2.07–8.94 wt% NaCl equiv., respectively. These data indicate that the deposit was formed in a low-temperature and low-salinity environment, corroborating an epithermal system of mineralization. The O and H isotope compositions of the ore-forming fluids (δ18O = −0.6 to −6.3‰; δD = −177 to −151‰) indicate there was a major contribution from a meteoric water component to metallogenesis. All of the Pb isotope data from the metal sulfides and country rocks in the Sinongduo deposit in this study are similar and imply a close genetic relationship. The δ34S values of galena and sphalerite exhibit narrow distributions, ranging from +4.1‰ to +5.6‰ and from +7.6‰ to +8.4‰ respectively, with average values of +5.0‰ and +8.1‰, respectively, suggesting a uniform source. The Ar-Ar ages determined from sericite in the quartz-sericite-illite-sulfide veins range from 60.9 ± 0.7 to 63.1 ± 0.7 Ma, showing that the timing of epithermal Ag-Pb-Zn mineralization in the study area is consistent with the eruption of the Dianzhong Formation volcanic rocks (DVR) (U-Pb dating, 65–62 Ma), which indicates a genetic relationship. The characteristics of FIs, the H-O-S-Pb isotope data and the mineralization ages suggest that the ore-forming fluids were primarily sourced from circulating meteoric water, while S and Pb were derived from the DVR. Our results, combined with previous data on the regional geology, ore geology, FIs, and tectonic setting, indicate that the Sinongduo Ag-Pb-Zn deposit was produced in an epithermal system that formed on the southern side of the Longge’er-Gongbujiangda fault-block uplift belt, following the northward subduction of the Neo-Tethyan oceanic plate beneath the Asian continent.

New insights into the geology and tectonics of the San Dimas mining district, Sierra Madre Occidental, Mexico

The San Dimas district is a world-class silver-gold low-sulfidation epithermal deposit located in the central part of the Sierra Madre Occidental of Mexico, within the eastern part of the Gulf of California extensional province. Previous works assumed a single period of mineralization between ∼38.5 and 31.9 Ma, which is at odds with the existence of vein systems with two different orientations and Ag/Au ratios. We present a re-evaluation of this district based on new zircon U/Pb and apatite fission-track ages as well as petrographic and field observations of mineralization styles. Our study also includes two new prospective areas of Causita and Mala Noche located to the south of the main district.Within the Lower Volcanic Complex, we identify a Late Cretaceous volcanic succession (∼77 to 69 Ma) correlative with the Tarahumara formation from southern Sonora and coeval with the San Ignacio batholith exposed to the west. This volcanic succession hosts Au-rich mineralized sub-volcanic felsic bodies yielding slightly younger ages and is covered by Paleocene intermediate lavas with hypabyssal intrusions with ages around 48 Ma. A voluminous intrusive suite (Piaxtla batholith and associated dike swarms) was emplaced in the region between ∼49 and 44 Ma. Early extensional basins were filled by a continental sedimentary sequence (Palmas formation), which yielded U/Pb age peaks at 66 and 56 Ma from detrital zircons and a maximum depositional age of ∼43 Ma. The last magmatic activity, as in the rest of the Sierra Madre Occidental, consists of silicic ignimbrites and less basaltic lava flows clustered in two pulses of ∼31.5–29 Ma and 24–20 Ma.NNW–SSE extensional fault systems expose the mineralization and tilted all the succession prior to the emplacement of a ∼24 Ma ignimbrite package. This late Oligocene extension, associated to the early stage of the Gulf of California rift, is confirmed by apatite fission-track dating of samples from the Piaxtla batholith, which consistently indicate an episode of cooling at 25–23 Ma followed by a second episode at ∼12.5 Ma. Our absolute ages and geologic mapping allow to infer that an older, WSW–ENE trending normal fault system with up to 1 km of displacement must exist between the San Dimas district and the Causita area to the south. This fault system, currently buried beneath Oligocene–Miocene ignimbrites, may have controlled the intrusion of the Piaxtla batholith and played a crucial role in the preservation of large vein systems of San Dimas in a tectonic depression setting.The main epithermal mineralization is associated with two kinds of structures: Ag/Au veins with WSW–ENE to E–W orientation and Au/Ag veins with NNW–SSE to N–S orientation. The dominant Ag/Au veins slightly post-date the Piaxtla intrusive suite and partly recycled older felsic intrusion with porphyry mineralization. The NNW–SSE to N–S Au/Ag veins are most probably associated with the Oligocene silicic volcanism.

Mineral composition and paragenesis of altered and mineralized zones in the Gadir low sulfidation epithermal deposit (Lesser Caucasus, Azerbaijan)

Mineralogy, gold mineralization and metal contents of the Gadir deposit have been investigated during current research in order to determine the geological conditions, temporal and spatial relationship with certain mineral assemblages and associations. The mineralogy of orebodies is mainly composed of pyrite, chalcopyrite, sphalerite, galena, petsite, native gold, electrum and subordinate molybdenite. Gold is hosted by pyrite and chalcopyrite minerals in fracture-filling textures and forms a thin dispersion condition. The native gold was observed in chalcopyrite, which is probably related to the second stage of ore deposition. The Gadir deposit can be classified to Au-Ag-Cu-Zn±Pb stockwork-type mineralization which is characteristic of low sulfidation epithermal deposit.

The Jbel Saghro Au(–Ag, Cu) and Ag–Hg Metallogenetic Province: Product of a Long-Lived Ediacaran Tectono-Magmatic Evolution in the Moroccan Anti-Atlas

The Jbel Saghro is interpreted as part of a long-lived silicic large igneous province. The area comprises two lithostructural complexes. The Lower Complex consists of folded metagreywackes and N070–090°E dextral shear zones, which roughly results from a NW–SE to NNW–SSE shortening direction related to a D1 transpressive tectonic stage. D1 is also combined with syntectonic plutons emplaced between ca. 615 and 575 Ma. The Upper Complex is defined by ash-flow caldera emplacements, thick and widespread ignimbrites, lavas and volcaniclastic sedimentary rocks with related intrusives that were emplaced in three main magmatic flare ups at ca. 575, 565 and 555 Ma. It lies unconformably on the Lower Complex units and was affected by a D2 trantensive tectonic stage. Between 550 and 540 Ma, the magmatic activity became slightly alkaline and of lower extent. Ore deposits show specific features, but remain controlled by the same structural setting: a NNW–SSE shortening direction related to both D1 and D2 stages. Porphyry Au(–Cu–Mo) and intrusion-related gold deposits were emplaced in an earlier stage between 580 and 565 Ma. Intermediate sulfidation epithermal deposits may have been emplaced during lull periods after the second and (or) the third flare-ups (560–550 Ma). Low sulfidation epithermal deposits were emplaced late during the felsic alkaline magmatic stage (550–520 Ma). The D2 stage, therefore, provided extensional structures that enabled fluid circulations and magmatic-hydrothermal ore forming processes.

Hydrothermal processes at the Axi epithermal Au deposit, western Tianshan: Insights from geochemical effects of alteration, mineralization and trace elements in pyrite

As the largest low-sulfidation (LS) epithermal deposit in the Tulasu basin, western Tianshan, NW China, the Axi gold deposit has a poorly understood geologic history. It is not clear whether hydrothermal mineralization occurred during a single event or multiple events. In this paper, new data consist of whole rock geochemistry and mass-change calculations for hydrothermal alteration. LA-ICP-MS trace element analysis of pyrite was conducted in order to develop a comprehensive metallogenic model. The pyrite-sericite-quartz alteration at the Axi deposit is characterized by replacement of plagioclase and mafic phenocrysts by sericite, quartz, pyrite (py1), smectite, and illite, indicating slightly acidic and reducing conditions during the early ore stage. Medium subhedral pyrite crystals (py1) with porous texture are typically sited adjacent to iron-rich minerals (e.g., hornblende) and are characterized by Co-Ni-Mn enrichment with high ratios of Co/Ni (avg. 1.114) and Cu/As (avg. 0.655), implying preferential sulfidation reactions in magmatic hydrothermal systems. The quartz ± calcite ± pyrite ± adularia assemblages in silicified rocks and the strong mobility of LREE both reflect a slightly acidic chloride-rich solution phase during the later ore stage. Coarse anhedral pyrites with cataclastic texture (py2) and medium euhedral pyrites (py3) have a mixed hydrothermal origin and were deposited with quartz during Au-pyrite-quartz and Au-polysulfide-quartz stages, respectively. Py2 and py3 both have significantly lower Cu/As ratios (avg. 0.103 and avg. 0.088, respectively) than py1, indicating the input of meteoric fluid during the later ore stage. The three generations of pyrite, especially py3, have significant lattice-bound gold contents (py1: 0.01–38.86 ppm, median 1.51 ppm; py2: 0.03–234.37 ppm, median 1.21 ppm; py3: 3.31–231.56 ppm, median 42.72 ppm). Gold precipitation is associated with the destabilization of Au-bearing chlorine (Au(Cl)2−) and bisulfide complexes (Au(HS)2−) via sulfidation and fluid boiling. All of these characteristics indicate the Axi epithermal deposit is the result of two stages of mineralization.

Textural Characteristics of Noncrystalline Silica in Sinters and Quartz Veins: Implications for the Formation of Bonanza Veins in Low-Sulfidation Epithermal Deposits

Silica sinters forming at the Wairakei geothermal power plant in New Zealand are composed of noncrystalline opal-A that deposited rapidly from cooling geothermal liquids flashed to atmosphere. The sinter is laminated with alternating layers of variably compacted silicified filamentous microbes encased by chains of fused silica microspheres. Microscopic inspection of bonanza quartz vein samples from the Buckskin National low-sulfidation epithermal precious metal deposit in Nevada showed that colloform bands in these veins exhibit relic microsphere textures similar to those observed in the silica sinters from the Wairakei power plant. The textural similarity suggests that the colloform bands were originally composed of noncrystalline opal-A that subsequently recrystallized to quartz. The colloform bands contain dendrites of electrum and naumannite that must have grown in a yielding matrix of silica microspheres deposited at the same time as the ore minerals, implying that the noncrystalline silica exhibited a gel-like behavior. Quartz bands having other textural characteristics in the crustiform veins lack ore minerals. This suggests that ore deposition and the formation of the colloform bands originally composed of compacted microspheres of noncrystalline silica are genetically linked and that ore deposition within the bonanza veins was only episodic. Supersaturation of silica and precious metals leading to the formation of the colloform bands may have occurred in response to transient flashing of the hydrothermal liquids. Flashing of geothermal liquids may thus represent a key mechanism in the formation of bonanza precious metal grades in low-sulfidation epithermal deposits.