Epithermal Ore Research Articles

Understanding the links between volcanic systems and epithermal ore formation: A case study from Conical Seamount, Papua New Guinea

The Tabar-Lihir-Tanga-Feni (TLTF) island chain in northeastern Papua New Guinea formed by tectonic and alkaline to shoshonitic magmatic activity since the Pliocene. Several volcanic centers are CuAu mineralized including the world-class Ladolam Au deposit and Conical Seamount south of Lihir. The latter has been recognized as a juvenile analogue to the Ladolam deposit located on-shore. Whereas the mineralization at Conical Seamount is reasonably well studied, the specific magmatic processes that promote epithermal mineralization at this seamount but not at others are poorly understood. Here, we present new petrological and geochemical data from Conical Seamount, and compare them with those from the barren (unmineralized) Edison, Tubaf and New World seamounts nearby. We focus on whole rock compositions and major and trace element analysis of melt inclusions and minerals including clinopyroxene, sulfide and magnetite. We combine our observations with modelled constraints on mantle source composition and partial melting as well as magma evolution. A first-stage melting leaves a residual mantle source enriched in Au. Second-stage melting of a previously subduction-metasomatized mantle generally promotes the transfer and concentration of metals and volatiles in the ascending melts. These magmas are unlikely to control ore formation as all seamounts show evidence for similar mantle sources and parental melt composition. However, the presence of a shallow crustal magma chamber is unique to Conical Seamount. It is characterized by frequent melt replenishments and extensive magma fractionation leading to sulfide and magmatic volatile saturation. These specific magma chamber processes lead to the pre-enrichment of the magma in chalcophile elements including Au, while sulfide saturation coeval with magmatic volatile exsolution provide the way for an effective Au transfer from the magmatic to the epithermal system.

Petrography and Geochemistry of Hydrothermal Alteration in the Low-Sulfidation Epithermal Kestanelik Au Deposit, Biga Peninsula, NW Turkey

In this study, the facies and degrees of hydrothermal alteration related to the low-sulfidation epithermal Kestanelik Au deposit in the Biga Peninsula metallogenic province are identified through petrographic studies and analysis of geochemical characteristics, such as mass changes, molar element ratios, and alteration indices. The gold mineralization is located in silicified zones containing veins and stockwork veinlets of silica. In the Kestanelik Au deposit, common hydrothermal alteration is mainly found in the Permian-Upper Cretaceous Çamlıca basement metamorphics and the Eocene granodiorite, and less often in the Eocene Şahinli volcanic rocks of the Karabiga Massif on the Peninsula. Based on mineralogical and geochemical studies conducted on altered samples, four different alteration facies are defined as silicic, sericitic, argillic, and propylitic, which show remarkable differences in the behavior of REEs, Si, K, Al, Na, and Ca elements. The hydrothermal fluids that caused alteration in the Kestanelik Au mineralization and host rocks had low REE contents because of REE mobilization. In addition, the kaolinization of feldspars and micas, and the chloritization of biotite and feldspars, may have caused negative Eu anomalies. The characterization of rocks subjected to hydrothermal alteration that are most influenced by diverse K-metasomatism with the largest K gains and losses in Na–Ca is illustrated by molar element ratio plots. Depending on the intensity of K-metasomatism, gold mineralization increases with increasing K trends toward gold ore veins. In the Kestanelik Au field, the argillic, sericitic, and propylitic alteration types from the zones enclosing the Au ore veins are revealed using the Ishikawa alteration index and chlorite–carbonate–pyrite index. Mass changes in the altered rocks indicate that there are gains in Si, K, and Al, and losses in Na and Ca with the increasing intensity of alteration toward the ore veins. The results confirm the presence of silicic and K–metasomatic (sericite and argillic) and propylitic (Fe-rich chloride) alteration zoning extending from the inner regions to the outer regions, which characterize the epithermal ore systems.

Positive Δ199Hg anomalies in Mesozoic porphyry Mo deposits of Northeastern China and their implications to the metallogeny of arc-related hydrothermal systems at convergent margins

Mercury (Hg) isotopes display unique mass-independent isotope fractionation (MIF, reported as Δ199Hg), which is primarily generated via Hg photochemical processes in the land-ocean-atmosphere system. Magmatic and hydrothermal processes do not trigger MIF of Hg isotopes, therefore, Hg-MIF signals detected in deep reservoirs (mantle and crust) are theoretically sourced from recycled surface materials. Here, we observed positive Δ199Hg signals (0.10 ± 0.07‰, SD) in Mesozoic porphyry Mo deposits of NE China. These values correspond to the recently reported Δ199Hg signature of Mesozoic epithermal AuAg systems in NE China (Δ199Hg: 0.11 ± 0.07‰, SD), suggesting that both mineral systems receive large amounts of Hg from subducted marine materials, given that marine Hg has positive Δ199Hg composition. Our study thus provides novel geochemical evidence on the linkage between oceanic plate subduction and metallogeny of various hydrothermal systems at convergent margins. Subduction contributes large amounts of volatiles to arc magmas, which then release metal-rich magmatic-hydrothermal fluids of the porphyry to epithermal ore deposit spectrum.

Structural control on the alteration and fluid flow in the lithocap of the Allumiere-Tolfa epithermal system

Zones of advanced argillic alteration with general low permeability (i.e., lithocaps) are common place in the shallow parts of porphyry and epithermal ore deposits and active geothermal systems. The study of structural control on alteration distributions is of paramount importance for exploitation purposes as it really influences the caprock efficiency. We present the results of a structural-mineralogical study carried out in the lithocap of the Allumiere-Tolfa epithermal system (Northern Apennines). We characterized the composition, textures and physical properties (i.e., in-situ permeability; relative rock strength) of alteration facies. We then integrated field structural analysis with analysis of a virtual outcrop model to reconstruct the geometry of principal fluid-corridors. It resulted that advanced argillic alteration was promoted by circulation of highly reactive fluid(s) along a complex network of NE- and NW-striking faults and fractures dissecting the acidic volcanic dome. Such structures likely developed in response to a local disturbance of the regional stress field due to the extrusion of the Tolfa dome, which controlled the syn-extensional mineralization of the Allumiere-Tolfa area.

Geochemistry and P-T Conditions of Hydrothermal Fluids Associated with Porphyry, Metasomatic and Epithermal Ore Deposits at Oued Belif-Ain El Araar Magmatic Structure (North-African Alpine Orogeny, Tunisia)

Geochemistry and P-T Conditions of Hydrothermal Fluids Associated with Porphyry, Metasomatic and Epithermal Ore Deposits at Oued Belif-Ain El Araar Magmatic Structure (North-African Alpine Orogeny, Tunisia)

Porphyry and Epithermal Au-Cu Systems of the Southern Caucasus and Northern Iran

This article presents tangible geological evidence for coexistence of porphyry copper and epithermal gold systems within single polygenic deposits and provides a paleothermophysical model for their origins. Brief metallogenic analysis of the Southern Caucasus and Northern Iran has shown that such deposits are confined to long-living calc-alkaline island arcs and were formed during their orogenesis. Examples of complex Sonajil (Iran), Gharta, and Merisi (Georgia) deposits are considered. Investigation has shown that for combined porphyry and epithermal ore formation some preconditions are suggested to exist: (i) Source of anomalous energy, which exceeds thermodynamics of the enclosing environment; (ii) Existence of temperature gradient, which determines conventional flows of fluids composed of endogenous and meteoric constituents (proven by rhythmical zoning of ore lodes); (iii) Stability of such conditions for a period of sulfide ore formation. However, such a process of sulfide ore formation cannot explain formation of high sulfidation gold deposits. Mass precipitation of free gold requires phreatic collapse in the ore conduit channel already after formation of hydrothermally altered rocks, and this event results in creation of either hydrothermal breccias, often with jigsaw-fit texture or brecciated vuggy silica where host rocks and hydrothermally altered rocks are cemented by a gold-bearing quartz matrix.

Recognition of the Xiayu intermediate-sulfidation epithermal Ag-Pb-Zn-Au(-Cu) mineralization in the East Qinling polymetallic ore belt, China: Constraints from geology and geochronology

Porphyry Cu deposits are often temporal-spatial-genetically associated with epithermal polymetallic (Ag-Pb-Zn-Au-Cu) veins in some important metallogenic belts, forming a porphyry–epithermal ore systems. However, the genetic type of the large-scale vein-type Ag, Pb, Zn and Au mineralization in the world-class East Qinling porphyry Mo ore belt (Central China) has not been delineated. Here, we provide a case study of the Xiayu orefield as an example for a typical intermediate-sulfidation epithermal Ag-Pb-Zn-Au(-Cu) deposit. The Xiayu Ag-Pb-Zn-Au(-Cu) orefield, the largest Ag producer in the East Qinling ore belt, contains the Haopinggou Ag-Pb-Zn-Au, Shagou Ag-Pb-Zn, Tieluping Ag-Pb(-Cu), and Houzhanggou Ag(-Cu) deposits, which share similar geological characteristics. The Ag-Pb-Zn-Au(-Cu) mineralization occurs as open-space fillings, veins and local hydrothermal breccias and stockworks, infilled with carbonates (siderite, ankerite, and calcite), quartz, pyrite, sphalerite, galena, chalcopyrite, tetrahedrite, electrum, silver minerals (argentiferous tetrahedrite, stromeyerite, pyrargyrite, polybasite, argentite and native silver). The vein crosscutting and overprinting relationships indicate four hydrothermal stages: (I) siderite-pyrite-quartz-electrum ± magnetite, (II) dark-brown sphalerite-siderite-quartz ± galena, (III) galena-tetrahedrite-chalcopyrite-silver minerals-ankerite-quartz ± light-brown sphalerite, and (IV) ankerite-calcite-chalcedony-fluorite. Mineral zoning at the orefield-scale is recognized from north to south: pyrite, sphalerite-galena, and tetrahedrite-native silver, which corresponds to ore metal zoning from Au ± Zn to Zn-Pb-Ag and Cu-Ag.The Haopinggou granite porphyry consists of the megacrystic granite porphyry (P1) and granite porphyry (P2), which were emplaced at 134.6±1.2 Ma and 125.9±0.7 Ma, respectively. Anhedral monazite (Mon1) encapsulated in pyrite and siderite from stage I of the Haopinggou and Shagou deposits constrains the early hydrothermal activity at 133.3±1.0 Ma. U-Pb dating of syn-ore monazite (Mon2) and xenotime (intergrown with electrum, galena, sphalerite and quartz) from the Shagou, Haopinggou, and Tieluping deposits constrains the Ag-Pb-Zn-Au(-Cu) mineralization age to be 125.7±1.8 Ma to 123.3±1.7 Ma. The consistency of the magmatic and mineralization ages indicates that the magmatic-hydrothermal process may have played an important role in the Ag-Pb-Zn-Au(-Cu) mineralization. Gold distribution, mineral association, and geochronological data, all suggest that the Au and Zn-Pb-Ag ores were formed from the same mineralization event rather than two discrete mineralization events as previously suggested. Considering the geological characteristics, metal zoning, mineralization ages, mineral chemical composition and published data of fluid inclusion, we conclude that the Xiayu Ag-Pb-Zn-Au(-Cu) veins belong to typical intermediate-sulfidation epithermal mineralization and further propose a porphyry–breccia–intermediate-sulfidation epithermal Ag-Pb-Zn-Au(-Cu) mineralization system. It is the first reported intermediate-sulfidation epithermal mineralization in the East Qinling ore belt, which implies that the epithermal mineralization may have been developed in the world-class East Qinling porphyry Mo ore belt.

Organic‐Inorganic Interaction Recorded by Polycyclic Aromatic Steranes in Continental Epithermal Ore Deposits

AbstractContinental epithermal ore deposits are commonly associated with sedimentary organic matter, oils or solid bitumen. These organics embedded in mineral deposits can convey valuable information of the ore genesis. However, the extent to which the formation of ore minerals was recorded by organic compounds remains largely unknown, as also is how metal‐rich ores interfere with the molecular proxies in the temperature regime envisaged for hydrothermal activity. The molecular compositional changes of various polycyclic aromatic steranes and polycyclic aromatic hydrocarbons and compounds derived from the Jinding Pb/Zn deposit, SW China provide new data. Aliphatic regular steranes are present as traces. The transformation from polycyclic aromatic steranes to unsubstituted polycyclic aromatic hydrocarbons is observed to show an increased trend with increasing hydrothermal alteration levels; this is consistent with the transformation from unsubstituted polycyclic aromatic hydrocarbons to heterocyclic compounds. Dehydrocyclization (aromatization) of polycyclic biological compounds and hydrodecyclization (dearomatization) of polycyclic aromatic compounds are two important reaction pathways in hydrothermal systems with moderate temperature. This detailed investigation of organic‐inorganic interactions of two groups of polycyclic compounds with metal‐rich ores provides insights into the questions on how and to what extent the formation of Pb/Zn deposits can be recorded by organics. This work will improve our understanding of carbon reduction, oxidation or condensation in the deep Earth and the carbon exchange between the Earth's crust and mantle, and may shed light on the processes for ultra‐deep hydrocarbon exploration.

Fluid inclusion, zircon U-Pb geochronology, and O-S isotopic constraints on the origin and evolution of ore-forming fluids of the tashvir and varmazyar epithermal base metal deposits, NW Iran

Tashvir and Varmazyar deposits are part of the epithermal ore system in the Tarom–Hashtjin Metallogenic Belt (THMB), NW Iran. In both deposits, epithermal veins are hosted by Eocene volcanic-volcaniclastic rocks of the Karaj Formation and are spatially associated with late Eocene granitoid intrusions. The ore assemblages consist of pyrite, chalcopyrite, chalcocite, galena, and sphalerite (Fe-poor), with lesser amounts of bornite and minor psilomelane and pyrolusite. Fluid inclusion measurements from the Tashvir and Varmazyar revealed 182–287 and 194–285°C formation temperatures and 2.7–7.9 and 2.6–6.4 wt.% NaCl equivalent salinities, respectively. The oxygen isotope data suggested that the mineralizing fluids originated dominantly from a magmatic fluid that mixed with meteoric waters. The sulfur isotope data indicated that the metal and sulfur sources were largely a mixture of magma and surrounding sedimentary rocks. LA-ICP–MS zircon U–Pb dating of the granitoid intrusion at Tashvir and Varmazyar, yielded a weighted mean age of 38.34–38.31 and 40.85 Ma, respectively, indicating that epithermal mineralization developed between 40.85 and 38.31 Ma. Our data indicated that fluid mixing along with some fluid boiling were the main drives for hydrothermal alteration and mineralization at Tashvir and Varmazyar. All these characteristics suggested an intermediate-sulfidation epithermal style of mineralization. The THMB is proposed to be prospective for precious and base metal epithermal mineralization. Considering the extensional tectonic setting, and lack of advanced argillic lithocaps and hypersaline fluid inclusions, the THMB possibly has less potential for economically important porphyry mineralization.

Mineral mapping in the Duolong porphyry and epithermal ore district, Tibet, using the Gaofen-5 satellite hyperspectral remote sensing data

Remote sensing technology is an exploration method that can directly map distribution of alteration minerals or zones associated with different types of mineral systems. Hyperspectral remote sensing can identify key pathfinder minerals and compositional variations within minerals. The Duolong porphyry-epithermal ore district, which has proven reserves of approximately 20 million tons of Cu and 420 tons of Au, is part of an emerging world-class metallogenic area in the western part of the Bangong Co-Nujiang metallogenic belt, Tibet, China. The Gaofen-5 hyperspectral (GF-HS) reflectance data were used to map the distribution patterns of hydrothermal alteration and to locate areas potentially containing mineral deposits in the Duolong ore district (DOD). Based on the mixture tuned matched filtering (MTMF) technique and the absorption feature wavelength positions of the minerals, an alteration map was generated using GF-HS data for mineral mapping in the DOD with sparse vegetation. The widely distributed advanced argillic alteration (alunite + pyrophyllite + kaolinite ± dickite), phyllic alteration (high content of Al-rich white mica), propylitic alteration (Mg-rich chlorite), and limited argillic alteration (kaolinite ± dickite ± white mica) were mapped in detail. The distribution patterns of alteration in known deposits were fairly consistent with the mineralogical information obtained in previous studies. In addition, we identified potential mineralization areas with characteristics similar to the Duobuza, Bolong and Naruo porphyry deposits.

Orogenic gold: is a genetic association with magmatism realistic?

Many workers accept a metamorphic model for orogenic gold ore formation, where a gold-bearing aqueous-carbonic fluid is an inherent product of devolatilization across the greenschist-amphibolite boundary with the majority of deposits formed within the seismogenic zone at depths of 6–12 km. Fertile oceanic rocks that source fluid and metal may be heated through varied tectonic scenarios affecting the deforming upper crust (≤ 20–25 km depth). Less commonly, oceanic cover and crust on a downgoing slab may release an aqueous-carbonic metamorphic fluid at depths of 25–50 km that travels up-dip along a sealed plate boundary until intersecting near-vertical structures that facilitate fluid migration and gold deposition in an upper crustal environment. Nevertheless, numerous world-class orogenic gold deposits are alternatively argued to be products of magmatic-hydrothermal processes based upon equivocal geochemical and mineralogical data or simply a spatial association with an exposed or hypothesized intrusion. Oxidized intrusions may form gold-bearing porphyry and epithermal ores in the upper 3–4 km of the crust, but their ability to form economic gold resources at mesozonal (≈ 6–12 km) and hypozonal (≈ > 12 km) depths is limited. Although volatile saturation may be reached in magmatic systems at depths as deep as 10–15 km, such saturation doesn’t indicate magmatic-hydrothermal fluid release. Volatiles typically will be channeled upward in magma and mush to brittle apical roof zones at epizonal levels (≈ < 6 km) before large pressure gradients are reached to rapidly release a focused fluid. Furthermore, gold and sulfur solubility relationships favor relatively shallow formation of magmatic-hydrothermal gold systems; although aqueous-carbonic fluid release from a magmatic system below 6 km would generally be diffuse, even if in cases where it was somehow better focused, it is unlikely to contain substantial gold. Where reduced intrusions form through assimilation of carbonaceous crustal material, subsequent high fluid pressures and hydrofracturing have been shown to lead to development of sheeted veins and greisens at depths of 3–6 km. These products of reduced magmatic-hydrothermal systems, however, typically form Sn and or W ores, with economic low grade gold occurrences (< 1 g/t Au) being formed in rare cases. Thus, whereas most moderate- to high-T orogens host orogenic gold and intrusions, there is no genetic association.

Mobilisation of deep crustal sulfide melts as a first order control on upper lithospheric metallogeny

Magmatic arcs are terrestrial environments where lithospheric cycling and recycling of metals and volatiles is enhanced. However, the first-order mechanism permitting the episodic fluxing of these elements from the mantle through to the outer Earth’s spheres has been elusive. To address this knowledge gap, we focus on the textural and minero-chemical characteristics of metal-rich magmatic sulfides hosted in amphibole-olivine-pyroxene cumulates in the lowermost crust. We show that in cumulates that were subject to increasing temperature due to prolonged mafic magmatism, which only occurs episodically during the complex evolution of any magmatic arc, Cu-Au-rich sulfide can exist as liquid while Ni-Fe rich sulfide occurs as a solid phase. This scenario occurs within a ‘Goldilocks’ temperature zone at ~1100–1200 °C, typical of the base of the crust in arcs, which permits episodic fractionation and mobilisation of Cu-Au-rich sulfide liquid into permeable melt networks that may ascend through the lithosphere providing metals for porphyry and epithermal ore deposits.

Mineralogical and petrographical characterization of the Vizcaya, Octubrina and Gabi veins from the Zaruma-Portovelo gold epithermal ore deposit (Ecuador)

Ore petrography through optical microscopy and electron microprobe analysis (EMPA) can be applied to the characterization of ore deposits, allowing accurate and representative information on the evolutionary sequences of mineral phases (paragenesis). In this studywork mineralogical and petrographical characteristics of three veins (Vizcaya, Octubrina and Gabi) of the Zaruma-Portovelo hydrothermal deposit (Ecuador) have been studied. The main objective of this research was to identify and quantify the ore minerals present in the polished section studied by optical microscopy and EPM, and to correlate the data obtained with the previously defined paragenetic evolution. In addition, the systematics we describe is revealed as an appropriate method for the characterization and basic knowledge of the mineralized areas. The mineralogical characterization was completed by scanning electron microscopy, optical image analysis and X-ray diffraction. The results have provided us with allowed: i) the identification and quantification of gold, sphalerite, chalcopyrite, galena, pyrite, hematite, chalcocite, covellite and tetrahedrite in the polished sections of the veins studied; ii) the characterization of the mineral associations as a complement to the previous studies of the mineral paragenesis of the deposit and indicative in the geometallurgy of the deposit. The mineralogical and ore petrographical characterization of the ores represent the basis of knowledge to understand the characterization of the mineral deposit and, in addition, improve the processing of the mineral of interest.

Lithogeochemistry of various hydrothermal alteration types associated with precious and base metal epithermal deposits in the Tarom-Hashtjin metallogenic province, NW Iran: Implications for regional exploration

In this study, the styles, types, and degrees of hydrothermal alteration associated with Pb-Zn-Cu-Ag-Au epithermal deposit systems in the Tarom-Hashtjin metallogenic province are evaluated using petrographic studies, as well as mass changes and molar elements ratios (MER), in order to examine regional exploration implications. The MER plots show that the hydrothermally altered rocks were most affected by various types of K-metasomatism, mainly characterized by K gains and Na and Ca losses. The intensity of K-metasomatism is related to precious and base-metal mineralization, showing increasing K trends toward the ore veins. The alteration index (AI) and chlorite-carbonate-pyrite index (CCPI) reveal that argillic, sericitic, and propylitic alteration types are the main zones of alteration surrounding ore veins indicating that AI increases and CCPI decreases toward more proximal to the structures linked to these ore systems. Determination of mass changes in altered rocks surrounding selected epithermal deposits suggest Si, K, and Al gains and Na and Ca losses increasing in intensity toward ore veins, consistent with alteration zoning from the silicic and K-metasomatic (sericite and argillic) inner zones to the propylitic (Fe-rich chlorite alteration) outer zone typical of epithermal ore systems. Data from this study show that the intensity of K-metasomatism in the acidic rocks is greater than in the intermediate and mafic rocks, where it is related to high to intermediate sulfidation epithermal systems. We suggest that exploration can be focused on highly altered volcanic rocks (especially andesite-dacite) with both high intensity of K-metasomatism and higher Pb, Zn, Ag, Au, Ba and Rb and lower Sr.

Sphalerite Composition in Low- and Intermediate-Sulfidation Epithermal Ore Bodies from the Roșia Montană Au-Ag Ore Deposit, Apuseni Mountains, Romania

We evaluated the significance of the iron and manganese content in sphalerite as a tool for distinguishing between low-sulfidation and intermediate-sulfidation epithermal deposits on the basis of new and previously published electron probe microanalyses data on the Roșia Montană epithermal ore deposit and available microchemical data from the Neogene epithermal ore deposits located in the Apuseni Mountains and Baia Mare region, Romania. Two compositional trends of the Fe vs. Mn content in sphalerite were delineated, a Fe-dominant and a Mn-dominant, which are poor in Mn and Fe, respectively. The overlapping compositional range of Fe and Mn in sphalerite in low-sulfidation and intermediate-sulfidation ores suggests that these microchemical parameters are not a reliable tool for distinguishing these epithermal mineralization styles.

Burial and exhumation of late paleozoic arc-related rocks in the Tulasu basin, Western Chinese Tianshan: Implication for the preservation of epithermal deposits in old orogenic belts

Epithermal deposits are susceptible to erosion owing to their shallow emplacement depth of 1 to 2 km, resulting in the observation that the majority of epithermal deposits worldwide are generally Cenozoic in age. Several large and super-large epithermal deposits likely associated with Late-Devonian to Carboniferous arc magmatism were recognized in the Tulasu volcanic-clastic sedimentary basin, western Chinese Tianshan. In order to further clarify the preservation processes of these old epithermal orebodies, apatite fission track (AFT) analysis has been undertaken in this study. Our AFT results gave a range of ages from 54.7 ± 6.3 Ma to 272.3 ± 8.7 Ma, and the mean confined fission track lengths are between 12.1 ± 0.2 μm and 13.5 ± 0.1 μm. Our new data, in combination with the 1:50,000 geological mapping, suggest that the ore-hosting Tulasu basin may have undergone multiple burial and exhumation episodes postdating the mineralization. The main burial scenario, probably occurring in the late Early Carboniferous (350–330 Ma) and Permian (300–250 Ma), may be related to subsidence on syn-volcanic faults concurrent with retreat of the Junggar Ocean and strong uplift of the neighboring mountain range induced by final closure of the Paleo-Asian Ocean, respectively. The calculated thickness of ca. 4.83 km of sedimentary rocks deposited during the burial events form a critical shield for the epithermal ore bodies that have been protected from substantial erosion by subsequent geological processes. Exhumation occurred in two episodes: Late Triassic-Cretaceous (ca. 240–100 Ma) and Oligocene (ca. 35 Ma). The Late Triassic-Cretaceous erosion is characterized by a slow exhumation rate (ca. 0.012 mm/y), corresponding to a total unroofing thickness of ca. 1.67 km. In contrast, the Oligocene event exhibits as a higher exhumation rate (up to ca. 0.057 mm/y), with removal of ca. 3.83 km of sedimentary cover. Therefore, the epithermal deposits have undergone a complex geological path from formation to final exposure. Initial burial and the following sluggish exhumation are two fundamental factors for the preservation of epithermal deposits in the old orogenic belts.

Mineralogy of the Svetloye epithermal district, Okhotsk-Chukotka volcanic belt, and its insights for exploration

The Svetloye epithermal district (SED) is located within the Okhotsk-Chukotka volcanic belt (OCVB), 220 km southwest of Okhotsk. The OCVB, which formed during the Cretaceous with ore potential, is a large marginal volcanic rock belt associated with continental subduction. Svetloye occupies two volcanic edifices of different ages – the northwestern volcanic center (including the Emmy deposit) consists of basaltic-andesite of the Khetanian suite (Coniacian-Santonian time (K2), and the southeastern volcanic center (hosts the Elena, Tamara, Lyudmila, and Larisa deposits) comprises the dacite-rhyolite-leucogranite formation of the Urak suite (Campanian-Maastrichtian time (K2). Distinct host rocks and different levels of erosion in the two edifices of Svetloye led to various altered rocks, mineral assemblages, and diverse types of gold mineralization. The alteration halo of the Emmy deposit contains vuggy residual quartz, with Au-Ag-telluride and Au-Ag mineralization plus other tellurium-bearing minerals (goldfieldite, kawazulite, coloradoite, melonite, altaite, tellurobismuthite, tellurantimony, and native tellurium). Altered rocks of the other deposits are characterized by variable halos, including vuggy residual quartz, alunite, dickite, quartz-alunite-dickite, illite-chlorite, illite, illite–smectite, smectite, quartz-carbonate, and quartz-white mica alteration, hypogene and supergene processes. The hydrothermal (hypogene) and supergene ore processes within the SED occurred in 3 stages: leaching and barren quartz-rutile-pyrite zone with halos of quartz-alunite-dickite-pyrite (or quartz-illite-chlorite-calcite-pyrite), then quartz-pyrite with polysulfides and gold, gold-silver telluride, and finally supergene with 'mustard' gold. The alteration halo, epithermal ore bodies as hydrothermal veins and breccias with vug infill, colloform, and crustiform textures, hypogene sulfide assemblage represented by galena, sphalerite, tennantite-tetraedrite, and chalcopyrite of SED are typical of epithermal styles with an high-intermediate sulfidation signature, possibly indicates a porphyry deposit at depth. The lack of high-sulfidation-state sulfides such as hypogene covellite, enargite, famatinite, luzonite in the residual quartz may be due to their erosion at shallower paleodepth, and/or to their supergene oxidation below the present day surface.

Intermediate sulfidation epithermal Pb - Zn (± Ag ± Cu ± In) and low sulfidation Au (± Pb ± Ag ± Zn) mineralization styles in the Gonzalito polymetallic mining district, North Patagonian Massif

The Gonzalito polymetallic mining district is located within the northeast tip of the 600 km-long NE-trending polymetallic belt of the North Patagonian Massif, in Patagonia, Argentina. It comprises Paleozoic igneous-metamorphic rocks of the Mina Gonzalito complex, which are intruded by Middle Triassic trachytic-andesitic dykes of the Monasa Fm. through a complex system of high-angle fractures. Rhyolitic dykes crosscut the pre-Jurassic rocks. Numerous semi-parallel vein-like mineral deposits, whose classification and age have been subject of debate, sharply crosscut the igneous-metamorphic, trachytic-andesitic and rhyolitic units. Mineralization shows a strong structural control through a complex system of semi-parallel NW-N-NE oriented fracture-hosted mineral deposits. Extensional jogs and open dilational fractures allow multiband vein infills, vein-like hydrothermal and fault-tectonic breccias, stockwork veins and veinlets with primary growth open-space and boiling-derived textures, indicating a brittle geological setting that is proper of the epithermal environment. Based on the structural setting, sulfide-gangue-alteration mineral assemblages and textures, sulfide mineral chemistry and whole-rock geochemical analyses, two different styles of hydrothermal deposits can be distinguished: massive-sulfide intermediate sulfidation epithermal Pb – Zn ± Ag ± Cu ± In and disseminated-sulfide low sulfidation Au ± Pb ± Ag ± Zn. The first has a main-stage pyrite – In-rich sphalerite – chalcopyrite – galena ore assemblage in scarce quartz - carbonates gangue. The second shows disseminated pyrite ± sphalerite ± chalcopyrite ± galena with economic grades of Au in abundant quartz – adularia ± fluorite gangue. SHRIMP U–Pb zircon data of rhyolites reveal Early Jurassic ages of 193 ± 2 and 191 ± 2 Ma and, along with their bulk-rock geochemical features, they can be included within the Chon Aike magmatic province, locally known as Marifil volcanic complex. Crosscutting relationships between the Lower Jurassic rhyolites and the epithermal veins in the Gonzalito district, and the presence of Oligocene basalts partially covering the epithermal deposits towards the western area, constrain the mineralization event between the Lower Jurassic and the Oligocene. However, the crosscutting relationships of this district along with the already proven genetic, temporal and spatial links between epithermal ore and fluorite-rich deposits and the Jurassic magmatism-rifting of Patagonia suggest that the studied epithermal deposits might be yielded closer or during the Lower Jurassic.

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.

Porphyry-type mineralization associated with epithermal deposits in the Tarom metallogenic belt of NW Iran: Constraints from fluid inclusions

Research is conducted to investigate whether or not, apart from the Chodarchay deposit, there are other examples of porphyry-type mineralization associated with epithermal deposits in the Tarom metallogenic belt of the Alborz zone, NW Iran. Homogenization temperature, final ice-melting, and salinity values are determined for fluid inclusions in mostly quartz and partly calcite, fluorite, sphalerite, and barite samples from eight epithermal ore deposits to unravel the enigma on porphyry-type mineralization in Tarom. The homogenization temperatures and salinities of the fluid inclusions range from 82 to 520 °C and 0.2 to 37 wt% NaCl equiv., respectively. The fluid inclusions show varying densities from 0.4 to ~1.2 g/cm3 with most values between 0.7 and 0.95 g/cm3. Tarom epithermal deposits in order of maximum mineralization depth are Chargar (520 m), Zajkan (640 m), Goloujeh (770 m), Lohneh (1075 m), Nikoueieh (1150 m), Khalifelou (1500 m), Aghkand (1690 m), and Chodarchay (1900 m). It seems that magmatic water played an important role in the Tarom epithermal deposits except for Chargar where mixing magmatic and meteoric waters is considered as the main mechanism for ore deposition. The physicochemical variations suggest differences among epithermal deposits in the Tarom district. Chodarchay, Lohneh, Khalifelou, and Goloujeh have some fluid inclusion features of porphyry mineralization associated with an epithermal part. Minerals from Tarom contain fluid inclusions indicative of porphyry mineralization beneath some intermediate- and high-sulfidation epithermal deposits. The findings of this study are instrumental in the exploration of deeper porphyry parts of the Tarom epithermal deposits.