Epithermal-style Mineralization Research Articles

Lithological Control to the Gold Mineralization in the Main Ridge Complex of the Bakan Gold District, Northern Sulawesi, Indonesia

Abstract Main Ridge complex is characterized by prominent high sulfidation epithermal-style mineralization with windows of porphyry style mineralization in the deeper zone. Several lithologic features play important controlling role in the occurrences of those mineralization. The oldest rock observed in the Main Ridge complex is unmineralized hornblende diorite (IDO-1) which encompasses the whole Bakan district. Several late intrusive stocks were concealed below the current topographic as mentioned as the microdiorite (IDO-2) and quartz diorite (IDO-3) in which the latter exhibits windows of porphyry-style mineralization. Those intrusive units were then covered by two unconformable volcanic tuff sequences which are andesitic tuff and dacitic tuff in compositions. Dacitic tuff is more favourable unit for gold mineralization compared to andesitic tuff which shown by near-surface ‘mushroom-like’ deposit form. In more peripheral area, the mineralization is also controlled by the bedding features of dacitic tuff which underlain by the less-mineralized andesitic tuff. In the latest stage, diatreme breccia cut all the lithologic units and becomes the most favourable host rock in sub-vertical deposit form. The comprehensive understanding of the lithological features is important to understand the mineralization control for further exploration target.

Hydrothermal evolution from porphyry- to epithermal-style mineralization in the Naozhi deposit, NE China

The Naozhi deposit in the Yanbian district of northeastern China contains polymetallic epithermal veins and subeconomic porphyry-style Cu mineralization within 2 km of each other. These two mineralization styles are genetically associated with 130–126 Ma andesites and dioritic–granitic intrusions. Isotopic ages determined on sericite (128.2 ± 2.9 Ma; 40Ar–39Ar plateau age) and sulfide (124.6 ± 2.7 Ma; Rb–Sr isochron) associated with the epithermal mineralization constrain the hydrothermal activity to at approximately 128 Ma. Three types of fluid inclusions, including halite-bearing, vapor-rich, and liquid-rich inclusions were trapped in quartz from porphyry-style veinlets associated with an assemblage of K-feldspar–biotite–magnetite. Based on microthermometry and the homogenization behavior of halite-bearing inclusions, they were trapped at lithostatic pressures of > 500 bars and at temperatures of ≥ 350 °C, whereas the vapor- and liquid-rich inclusions were trapped at pressures and temperatures of 110 ± 35 bars and ≤ 350 °C.Epithermal mineralization comprises four vein stages, which, from oldest to youngest, are quartz–pyrite, quartz–base metal sulfide, quartz–precious metal, and quartz–carbonate. The ore assemblage precipitated from low-salinity fluids (<10 wt% NaCl equiv.) at temperatures and pressures of ≤ 350 °C and ≤ 100 bars. The δ18OH2O (2.5 to −2.4 ‰) and δDH2O (−85 to −114 ‰) compositions of fluids related to porphyry- and epithermal-style mineralization indicate a mixture of magmatic and meteoric waters. The δ34SH2S (−3.1 to 4.5 ‰) values of fluids become isotopically heavier from the quartz–pyrite stage to the quartz–base metal sulfide stage, reflecting continuous reduction during epithermal mineralization. The mineral assemblage and chemical composition of sulfide minerals indicate a transition from oxidizing, high sulfidation porphyry-style fluids to reducing, intermediate–low sulfidation epithermal fluids, with log ƒO2 values ranging from −31 to > −33, and log ƒS2 values ranging from −7.0 to ≤ −13.4 during the epithermal veining stage. Metal deposition in the Naozhi magmatic–hydrothermal system resulted from cooling, sulfidation, neutralization, and boiling of predominantly magmatically sourced hydrothermal fluids that mixed with meteoric water.

Mineralization styles and ore-forming conditions of the quartz-fragment-rich breccia (QBX) at the Didipio alkalic porphyry Cu-Au deposit, Nueva Vizcaya, Philippines

The quartz-fragment-rich breccia (QBX) in the Didipio alkalic porphyry deposit of Nueva Vizcaya, Philippines is a high-grade Cu-Au orebody dominantly composed of quartz fragments. The QBX can be subdivided into different facies, each characterized by a particular spatial distribution and mineral composition. This study utilized petrographic, electron-probe, scanning electron microscope, and fluid inclusion analyses on the samples from different QBX facies to provide new insights about mineralization styles, gold occurrence, and ore-forming conditions. Based on their respective mineral assemblages, two styles of mineralization were determined for the QBX: (1) porphyry and (2) epithermal.The porphyry-style mineralization is characterized by early-stage disseminated and vein-hosted sulfides and alteration consisting of chalcopyrite, bornite, chlorite, biotite, magnetite, quartz, K-feldspar, and actinolite. The assemblage formed from high temperature (420–440 °C), high salinity brines (22–49 wt% NaCl) possibly derived from a magmatic source. The porphyry-style mineralization occurs mainly as disseminations in quartz-sulfide veins within diorite wall rocks, Quan porphyry, and Bufu syenite. These altered intrusive bodies were eventually fragmented to form a low-grade (<5 ppm Au) distal facies (QBX-Monz, QBX-Dio) and a lower contact facies (QBX-Sy), above and below the breccia core, respectively. Gold occurs as Au-Ag-Cu-Te blebs (gold telluride) at phase boundaries in bornite-chalcopyrite symplectites.The epithermal-style mineralization is embodied by a late-stage gold mineralization event that precipitated chalcopyrite, pyrite, sphalerite, galena, tennantite, calcite, and Te- and Se-minerals. The later epithermal fluids were of relatively low temperature (300–320 °C) and low salinity (3–4 wt% NaCl), indicative of diluted hydrothermal fluids. The vein-hosted epithermal mineral assemblage is observed in clasts of the upper contact facies (QBX-FP) and in the matrix and cement of the moderate- to high-grade (∼10–83 ppm Au) central facies (QBX-Un, QBX-Act). In this paragenetic stage, gold occurs as native gold hosted in chalcopyrite, pyrite, and tellurides – within fractures or as inclusions – and as invisible gold in pyrite.The close relationship between the gold and chalcogenides supports the Ag-Bi-Te scavenger model, which suggests that the presence of Ag, Bi, Se, and Te in magmatic hydrothermal ore-forming fluids aids in concentrating Au at high temperatures and exsolving it at lower temperatures.

Multi-Stage Introduction of Precious and Critical Metals in Pyrite: A Case Study from the Konos Hill and Pagoni Rachi Porphyry/Epithermal Prospects, NE Greece

The Konos Hill and Pagoni Rachi porphyry-epithermal prospects in northeastern Greece are characterized by abundant pyrite that displays important textural and geochemical variations between the various ore stages. It is commonly fine-grained and anhedral in the porphyry-related mineralization (M- and D-type veins), while it forms idiomorphic, medium- to coarse-grained crystals in the late, epithermal style veins (E-type). Porphyry-style pyrite from both prospects is characterized by an enrichment in Co, Se, Cu, and minor Zn, and a depletion in other trace elements, like Bi, Mo, Ag, etc. Pyrite in epithermal-style mineralization is mostly characterized by the presence of As, Bi, Pb, Ni, and Se. Gold in pyrite from all mineralization stages occurs as a non-stoichiometric substituting element, and its abundance correlates with As content. Arsenic in pyrite from Konos Hill records an increase from the porphyry stage to the epithermal stage (along with gold); however, at Pagoni Rachi, the highest Au and As contents are recorded in D-type pyrite and in the epithermal stage. The composition of the studied pyrite marks changes in the physico-chemical conditions of the ore-forming fluids and generally follows the geochemical trends from other porphyry-epithermal systems elsewhere. However, a notable enrichment of Se in the porphyry-style pyrite here is a prominent feature compared to other deposits and can be considered as an exploration tool towards Au-enriched mineralized areas.

Tectonic Triggers for Postsubduction Magmatic-Hydrothermal Gold Metallogeny in the Late Cenozoic Anatolian Metallogenic Trend, Turkey

Abstract The newly defined, 1,500-km-long, late Cenozoic Anatolian metallogenic trend of Turkey is the central segment of the western Tethyan metallogenic belt and formed after the closure of the southern Neotethys Ocean. Mineral deposit discoveries along this trend show that the Oligocene to Miocene igneous units are highly prospective for gold-rich porphyry- and epithermal-style mineralization (~27 Moz) but that copper endowment is poor. However, the temporal and spatial constraints on late Cenozoic gold districts and isolated prospects and their tectonic affinity are poorly known, despite recent efforts. We herein provide new U-Pb and Re-Os age data and field observations from Miocene gold prospects and deposits throughout the Anatolian trend, which we interpret together with previously published age data in the region. We define nine new porphyry and epithermal districts: Simav, İzmir, Uşak, Bodrum, Konya, Aksaray, Kayseri, Tunceli, and Ağri. Gold-rich porphyry and epithermal systems peaked at (1) 25 to 17 Ma in eastern Anatolia, (2) 21 to 9 Ma in western Anatolia, and (3) 10 to 3 Ma in central Anatolia. The westward migration of porphyry and epithermal mineralization from eastern to central Anatolia is interpreted to reflect slab break-off propagation and gap opening after the onset of the Arabian continental collision. Conversely, the southwestward migration of the magmatic front and associated mineralization in western Anatolia resulted from the acceleration of the Aegean slab rollback and subsequent lateral tearing (15–8 Ma). Thus, the bulk of gold mineralization formed in response to the slab segmentation and thermal events at 25 and 15 Ma.

Zircon Petrochronology of the Meghri-Ordubad Pluton, Lesser Caucasus: Fingerprinting Igneous Processes and Implications for the Exploration of Porphyry Cu-Mo Deposits

AbstractThe trace element composition of zircon, especially in tandem with U-Pb geochronology, has become a powerful tool for tracing magmatic processes associated with the formation of porphyry copper deposits. However, the use of the redox-sensitive Eu and Ce anomalies as a potential mineral exploration proxy is controversial. This study presents a comprehensive, temporally constrained data set of zircon trace element compositions (n = 645) for three compositionally distinct magmatic series identified in the Meghri-Ordubad pluton, southernmost Lesser Caucasus. The 30 million years of Cenozoic magmatism in the Meghri-Ordubad pluton are associated with several ore-forming pulses leading to the formation of porphyry copper deposits and epithermal-style mineralization. Our zircon geochemical data constrain the thermal and chemical evolution of this complex intrusive suite and allow an evaluation of the usefulness of zircon as a mineral exploration proxy for porphyry copper deposits. Our results combined with Rayleigh fractionation modeling indicate that the trace element composition of zircon (Th/U, Hf, Ti, YbN/DyN, Eu anomalies) is influenced by the composition and the water concentration of the parental magma, as well as by co-crystallizing titanite and apatite. In contrast, the variations of Ce anomalies remain difficult to explain by magmatic processes and could rather be ascribed to relative fluctuations of the redox conditions. In the Meghri-Ordubad pluton, we do not observe any systematic patterns between the trace element composition in zircons and the different ore-forming pulses. This questions the reliability of using the trace element composition in zircon as an exploration mineral proxy, and it rather emphasizes that a good knowledge of the entire magmatic evolution of a metallogenic province is required.

Petrological and Mineralogical Aspects of Epithermal Low-Sulfidation Au- and Porphyry Cu-Style Mineralization, Navilawa Caldera, Fiji

The Navilawa caldera is the remnant of a shoshonitic volcano on Viti Levu, Fiji, and sits adjacent to the low-sulfidation Tuvatu epithermal Au–Te deposit. The caldera occurs along the Viti Levu lineament, approximately 50 km SW of the Tavua caldera, which hosts the giant low-sulfidation Emperor epithermal Au–Te deposit. Both calderas host alkaline rocks of nearly identical age (~5.4–4.6 Ma) and mineralization that occurred in multiple stages. The gold mineralization in these locations is spatially and genetically related to monzonite intrusions and low-grade porphyry Cu-style mineralization. Potassic, propylitic, phyllic, and argillic alteration extends from the Tuvatu Au–Te deposit towards the central, northern, and eastern parts of the Navilawa caldera where it is spatially associated with low-grade porphyry Cu–Au mineralization at the Kingston prospect and various epithermal Au–(Te) vein systems, including the Banana Creek and Tuvatu North prospects. Chalcopyrite, and minor bornite, occurs in quartz–calcite–(adularia) veins in the Kingston deposit associated with weak propylitic and phyllic alteration, whereas NE-trending epithermal gold veins at the Banana Creek and Tuvatu North prospects are associated with weak potassic alteration that is overprinted by propylitic and phyllic alteration. Gold is accompanied by chalcopyrite, galena, and sphalerite in quartz–pyrite veins that also have a Ag–As–Hg–Te signature. The temperature range for phyllosilicates in the phyllic alteration (chlorite ± smectite ± corrensite ± illite) is in good agreement with temperatures recorded from previous fluid inclusion studies of quartz at the Banana Creek Au prospect (~260 °C) and the nearby Tuvatu Au–Te deposit (205 to 382 °C). Sulfur isotope compositions of pyrite (−6.2 to +0.4‰) from the Banana Creek prospect indicate a likely magmatic source of sulfur. Oxidation of the ore fluids or a direct addition of volatiles to the hydrothermal fluids may account for the lighter isotopic values. The similarities of the igneous rock types and compositions, transition from porphyry- to epithermal-style mineralization, alteration assemblages, paragenetic relationships, and stable isotope data suggest a common origin for the porphyry- and epithermal-style mineralization within the Navilawa and between the Navilawa and Tavua calderas.

Sulphur and lead isotope geochemistry of sulphide minerals from the Zn-Pb-Cu-Ag-Au Lemarchant volcanogenic massive sulphide (VMS) deposit, Newfoundland, Canada

The Lemarchant Zn-Pb-Cu-Ag-Au volcanogenic massive sulphide (VMS) deposit, in the Newfoundland Appalachians, Canada, is an important example of a precious metal-bearing VMS deposit. The deposit contains VMS- and sulphosalt-rich mineral assemblages, which are paragenetically distinct. Initial Zn-Pb-sulphosalt mineralization was deposited from low temperature (<250 °C), weakly oxidized (SO4 ≥ H2S), near-neutral fluids with high sulphur activity. Later Cu-rich, polymetallic mineralization was deposited from higher temperature (>300 °C), less oxidized hydrothermal fluids with lower sulphur activity. In situ microanalyses of pyrite, chalcopyrite and galena yield a wide range of δ34S values (−6.4 to +15.1‰). Sulphides from paragenetically early, lower temperature, Zn-Pb-sulphosalt assemblages have the lowest average δ34S values (+4.6‰), whereas the highest δ34S values occur in the late-stage, high-temperature, Cu-rich sulphide assemblages. Variability in δ34S occurs in all styles of mineralization, and is consistent with S derived from thermochemical reduction of seawater sulphate, igneous leaching and, potentially, disproportionated magmatic SO2. Although it is difficult to distinguish explicitly between leached igneous and magmatic sources of sulphur, mineralogical and geochemical data from Lemarchant suggest a possible contribution of magmatic SO2 during the deposition of epithermal-style mineralization at the earlier stages of deposit formation.Lead isotope compositions of galena show little variability with style of mineralization, suggesting similar Pb sources throughout ore deposition. Most Pb at Lemarchant is derived from underlying Neoproterozoic Sandy Brook group volcanic rocks, with potential input of juvenile Pb from the Lake Ambrose basaltic rocks of the Tally Pond group and/or from more juvenile Neoproterozoic basement rocks.

Genesis and tectonic setting of the Late Devonian Tawuerbieke gold deposit in the Tulasu ore cluster, western Tianshan, Xinjiang, China

ABSTRACTThe Tawuerbieke deposit is one of the important mining gold deposits in the Tulasu basin, which is known as an epithermal gold ore cluster in the Boluokenu Polymetallic Belt (BPB), western Tianshan, China. The orebodies of this deposit are hosted by late Palaeozoic volcanic rocks and granitic porphyries. LA-ICP-MS U–Pb dating on zircons from a hosting andesite gave a weighted average age of 367.1 ± 3.2 Ma. Geochemical characteristics of the Late Devonian volcanic rocks are similar to those of arc-type magma, which could be formed in a continental arc due to the North Tianshan oceanic plate southward subduction under the Yili-Central Tianshan plate. Diorite aplite dike crosscutting orebody has a late Carboniferous crystallization age (315.2 ± 3.5 Ma), constraining the mineralization age within the range of 367 ~ 315 Ma. Considering that all the other deposits with various types of mineralization in the BPB are products of the Late Devonian–early Carboniferous magmatism, we further suggest that the Tawuerbieke deposit formed during Late Devonian to early Carboniferous. Isotopic geochemistry data of S, Pb, C, and O suggest that the ore-forming materials were mainly derived from the host volcanic-subvolcanic rocks. The ore-hosting rock types, hydrothermal alteration, and ore mineral assemblage and textures of the Tawuerbieke distinctly differ from those of the low- and high-sulphidation epithermal Au deposits in the Tulasu basin, indicating that it could experience the telescoping/juxtaposing between porphyry- and epithermal-style mineralization. The epithermal- and porphyry-type Au mineralization in the Tawuerbieke deposit, together with the low- and high-sulphidation epithermal Au, porphyry Cu, and skarn-type Fe–Cu mineralization in the Tulasu basin, constitutes an epithermal–porphyry–skarn polymetallic mineralization system. The development of such mineralization system and magmatic-arc setting in the Tulasu basin suggests that porphyry-type Cu–Au mineralization should have a high potential at the deep sites of the Tawuerbieke district.

Mineralogy of tin-sulfides in the Zijinshan porphyry–epithermal system, Fujian Province, China

The Zijinshan orefield is a Cu–Au–Mo–Ag porphyry–epithermal mineralization system of Cretaceous age in southeastern China, comprising the Zijinshan high-sulfidation (HS) Au–Cu deposit, Luoboling porphyry Cu–Mo deposits, Yueyang low-sulfidation Au–Ag–Cu deposit, and Wuziqilong and Longjiangting intermediate-sulfidation (IS) Cu deposit. A number of Sn-(W)-sulfides occur in the Zijinshan orefield, including kiddcreekite, hemusite, colusite, vinciennite, stannoidite, mawsonite and kësterite. The occurrence represents the first report of hemusite, colusite and vinciennite in China. The relative abundance and diversity of Sn-(W)-sulfide-bearing assemblages make the Zijinshan orefield possibly unique in the world, and carries implications for the evolution of the mineralizing environment. Detailed electron probe studies of the Sn-minerals reveal that kiddcreekite, hemusite, mawsonite, stannoidite and kësterite have compositions close to ideal formulae. In contrast, colusite displays grain-scale zoning expressed by W, Sn and As. Vinciennite displays compositional variation depending on mineralization style: an average composition of Cu10.74Fe3.88Sn(As0.85,Sb0.08)S15.4 in the Zijinshan HS deposit, Cu10.71Fe3.9SnAs0.85S15.34 in the Wuziqilong Cu deposit and Cu10.76Fe3.7Sn1.02 (As0.5,Sb0.45)S15.54 with higher Sb contents (3.32%) in the adjacent Longjiangting Cu deposit.In the Zijinshan HS Au–Cu deposit, the main Cu-mineral assemblage varies from bornite-rich to digenite- to covellite-rich with increasing depth. Correspondingly, tin mineralogy varies from kësterite at upper levels to stannoidite-dominant at mid-to-upper levels of the copper orebody, to vinciennite- and mawsonite-dominant at depth, implying an increase in sulfidation and oxidation state downwards. Moreover, kiddcreekite is always partially replaced by wolframite and (hypogene) covellite at depth, also indicating overprinting by high-fO2 and -fS2 fluids in the covellite stage of mineralization. Kiddcreekite, along with a primary chalcopyrite–pyrite mineral association, occurs widely in the Zijinshan orefield (Zijinshan, Yueyang, Luoboling, Wuziqilong, Longjiangting and Dayanli), and is likely indicative of the early IS stage of epithermal system. Analogous with published data, kiddcreekite most commonly occurs in porphyry-style mineralization, implying the presence of the mineral has potential as an indicator of porphyry systems. Vinciennite-bearing Cu–Sn–As mineral associations occur in a specific position in the Zijinshan HS Au–Cu, Wuziqilong and Longjiangting Cu deposits, indicative of a transitional zone between porphyry and HS epithermal-style mineralization. The occurrence of the identified Sn-(W)-sulfide-bearing assemblages has considerable potential implications for understanding the architecture of a complex porphyry–epithermal environment and for exploration for underlying porphyry copper deposits. There is potential for discovery of a subjacent porphyry Cu deposit within the Zijinshan orefield.

Petrochemistry of igneous rocks of the California-Vetas mining district, Santander, Colombia: Implications for northern Andean tectonics and porphyry Cu (–Mo, Au) metallogeny

Porphyry Mo and Cu mineralization in the California-Vetas mining district is contemporaneous with 10.9 to 8.4 Ma granodiorite porphyry stocks and overprinted by Au–Ag mineralization of epithermal affinity. Mineralization is hosted by Grenvillian aged paragneisses (Bucaramanga Gneiss of the Santander Massif) and late Triassic to early Jurassic granitic rocks. All intrusive rocks are high-K calc-alkaline. Late Triassic to early Jurassic rocks include peraluminous granites with more than 70 wt.% SiO2 as well as metaluminous diorites, tonalites and granodiorites with SiO2 between 54.9 and 60.4 wt.%. Late Miocene rocks are weakly peraluminous granodiorite porphyries with SiO2 between 61 and 67 wt.% SiO2. Late Miocene rocks share some characteristics with adakite-like rocks which are widely associated with porphyry and epithermal style mineralization elsewhere in the Andes. They have high Ba (930 to 1500 ppm) and high Ba/La (28 to 50), high Sr (850 to 1100 ppm) and Sr/Y (48–78) and depleted middle rare earth elements (MREE) compared to the Mesozoic granites, which have 400 to 700 ppm Ba (Ba/La 14 to 25) and 80 to 150 ppm Sr (Sr/Y 2.5 to 14), and Mesozoic diorites and tonalites, which have ~ 900 to 1200 ppm Ba (Ba/La 20 to 32) and ~ 610 to 750 ppm Sr (Sr/Y 22 to 25). Miocene granodiorite porphyries, in contrast to Mesozoic intrusive rocks have only weak negative Eu anomalies. The Miocene rocks have 87Sr/86Sr ratios of 0.7052 to 0.7067 and εNd of − 1.9 to − 5.4 and are significantly more isotopically primitive than all other rocks in the study area including the Mesozoic diorites to tonalites (87Sr/86Sr = 0.7082 and 0.7092; εNd = − 6.7 and − 7.2), granites (87Sr/86Sr = 0.730 (n = 2); εNd = − 8.2 and − 8.3) and Bucaramanga Gneiss (0.718 to 0.743; εNd = − 10.8 to − 14.1). Lead isotope data are broadly consistent with the Sr and Nd isotope data and the Miocene porphyries have the lowest 207Pb/204Pb ratios but overlap with the Mesozoic diorites to tonalites in their 206Pb/204Pb and 208Pb/204Pb ratios. The geochemical signature suggests that late Miocene granodiorite porphyries represent oxidized and water-rich melts. However, in contrast to many other porphyry districts in the Andes, they do not have low Y (< 15 ppm) or depleted HREE, which together with the high Sr, Al and Na content indicates that neither plagioclase nor garnet were important residual phases in the MASH zone at the base of the crust. The trough shaped MREE suggest that amphibole was present in the source. The 87Sr/86Sr values of the Miocene rocks are consistent with a mantle derived melt which interacted with the radiogenic continental crust. The magmatism occurred above the shallow subducting Caribbean slab attributed to subduction of part of the Caribbean plateau (a.k.a. proto-Caribbean oceanic complex). It may be related to slab break-off and incursion of hot asthenospheric mantle, but dehydration of the Caribbean slab probably provided additional volatiles which made amphibole a major residual phase and provided water for the hydrous porphyry related magmas.

Mineralogical, stable isotope, and fluid inclusion studies of spatially related porphyry Cu and epithermal Au-Te mineralization, Fakos Peninsula, Limnos Island, Greece

The Fakos porphyry Cu and epithermal Au-Te deposit, Limnos Island, Greece, is hosted in a ~20 Ma quartz monzonite and shoshonitic subvolcanic rocks that intruded middle Eocene to lower Miocene sedimentary basement rocks. Metallic mineralization formed in three stages in quartz and quartz-calcite veins. Early porphyry-style (Stage 1) metallic minerals consist of pyrite, chalcopyrite, galena, bornite, sphalerite, molybdenite, and iron oxides, which are surrounded by halos of potassic and propylitic alteration. Stage 2 mineralization is composed mostly of quartz-tourmaline veins associated with sericitic alteration and disseminated pyrite and molybdenite, whereas Stage 3, epithermal-style mineralization is characterized by polymetallic veins containing pyrite, chalcopyrite, sphalerite, galena, enargite, bournonite, tetrahedrite-tennantite, hessite, petzite, altaite, an unknown cervelleite-like Ag-telluride, native Au, and Au-Ag alloy. Stage 3 veins are spatially associated with sericitic and argillic alteration. Fluid inclusions in quartz from Stage 1 (porphyry-style) mineralization contain five types of inclusions. Type I, liquid–vapor inclusions, which homogenize at temperatures ranging from 189.5°C to 403.3°C have salinities of 14.8 to 19.9 wt. % NaCl equiv. Type II, liquid–vapor-NaCl, Type III liquid–vapor-NaCl-XCl2 (where XCl is an unknown chloride phase, likely CaCl2), and Type IV, liquid–vapor-hematite ± NaCl homogenize to the liquid phase by liquid–vapor homogenization or by daughter crystal dissolution at temperatures of 209.3 to 740.5 °C, 267.6 to 780.8 °C, and 357.9 to 684.2 °C, respectively, and, Type V, vapor-rich inclusions. Stage 2 veins are devoid of interpretable fluid inclusions. Quartz from Stage 3 (epithermal-style) veins contains two types of fluid inclusions, Type I, liquid–vapor inclusions that homogenize to the liquid phase (191.6 to 310.0 °C) with salinities of 1.40 to 9.73 wt. % NaCl equiv., and Type II, vapor-rich inclusions. Mixing of magmatic fluids with meteoric water in the epithermal environment is responsible for the dilution of the ore fluids that formed Stage 3 veins. Eutectic melting temperatures of −35.4 to −24.3 °C for Type I inclusions hosted in both porphyry- and epithermal-style veins suggest the presence of CaCl2, MgCl2, and/or FeCl2 in the magmatic-hydrothermal fluids. Sulfur isotope values of pyrite, galena, sphalerite, and molybdenite range from δ34S = −6.82 to −0.82 per mil and overlap for porphyry and epithermal sulfides, which suggests a common sulfur source for the two styles of mineralization. The source of sulfur in the system was likely the Fakos quartz monzonite for which the isotopically light sulfur isotope values are the result of changes in oxidation state during sulfide deposition (i.e., boiling) and/or disproportionation of sulfur-rich magmatic volatiles upon cooling. It is less likely that sulfur in the sulfides was derived from the reduction of seawater sulfate or leaching of sulfides from sedimentary rocks given the absence of primary sulfides in sedimentary rocks in the vicinity of the deposit. Late-stage barite (δ34S = 10.5 per mil) is inferred to have formed during mixing of seawater with magmatic ore fluids. Petrological, mineralogical, fluid inclusion, and sulfur isotope data indicate that the metallic mineralization at Fakos Peninsula represents an early porphyry system that is transitional to a later high- to intermediate-sulfidation epithermal gold system. This style of mineralization is similar to porphyry-epithermal metallic mineralization found elsewhere in northeastern Greece (e.g., Pagoni Rachi, St. Demetrios, St. Barbara, Perama Hill, Mavrokoryfi, and Pefka).

Hydrologic, Magmatic, and Tectonic Controls on Hydrothermal Flow, Taupo Volcanic Zone, New Zealand: Implications for the Formation of Epithermal Vein Deposits

Geologic controls on development of high-flux hydrothermal conduits that promote epithermal ore formation are evaluated at large and small scales for geothermal systems of the Taupo Volcanic Zone, New Zealand. Most geothermal systems occur within a rifted volcanic arc (~150 km long) dominated by silicic volcanism, and they occur in association with major faults near caldera structures or within accommodation zones that transfer ex tension between rift segments . The geothermal systems are hosted in a thick sequence (1:>3 km) of young vol canic deposits that rest unconformably on weakly metamorphosed Mesozoic argillite and graywacke. Flow regimes and permeability controls in one extinct (Ohakuri) and six active (Broadlands-Ohaaki, Waiotapu, Roto kawa, Waimangu, Te Kopia, and Orakeikorako) geothermal systems show that in general, hydrothermal fluid flow is controlled by (1) heat from magmatic intrusions which drives convective circulation; (2) intergranular host-rock porosity and permeability; (3) fault-fracture network permeability produced by tectonism, volcanism, and/or diking; (4) pipelike vertical conduits produced by volcanic and hydrothermal eruptions; and (5) hydro thermal alteration and mineral deposition that may cause heterogeneity in the porosity and permeability of a fluid reservoir. Such controls influence fluid flow within three distinctive depth zones: (1) a feed zone (>2,000 m depth), (2) an epithermal mineralization zone ( 100 km 3 that encloses geothermal reservoirs and high-flux fluid conduits. Fracture-dominated flow becomes important with decreasing porosity induced by hydrothermal alteration. In the discharge zone, the re duction in confining pressure, combined with mineral deposition and alteration, hydrothermal eruptions, and interplay of hot and cold waters create complex, but strongly localized flow paths that feed hot springs. The permeability structure conducive to epithermal vein formation is analogous to a geothermal well: short in horizontal dimension (10s:100s m) but long in vertical dimension (>1,500 m) and possibly pipelike in shape. Episodic high-flux occurs over time scales of tens to thousands of years to accumulate sufficient amounts of gold and silver to form orebodies. During these episodes when faults and fractures are dilated, development of an upward-expanding column of boiling fluid promotes rapid ascent and high mass flow but also promotes silica and calcite precipitation, which can quickly reduce hydrothermal flow. Seismic activity and/or dike intru sion create and reactivate these high-flux pathways through extension and extensional shearing, caused by low differential stresses. The Taupo Volcanic Zone is highly prospective for epithermal-style mineralization, but the predominance of weak porous host rocks at shallow depths is prone to disseminated-style mineralization (e.g., Ohakuri). Structurally controlled mineralization forms in volcanic rocks where they have been embrittled by silicification through seismicity and fault displacement, caldera-forming eruptions, and dike intrusion.

Gold-bismuth-telluride-sulphide assemblages at the Viceroy Mine, Harare-Bindura-Shamva greenstone belt, Zimbabwe

AbstractGold mineralization at the Viceroy Mine is hosted in extensional veins in steep shear zones that transect metabasalts of the Archaean Arcturus Formation. The gold mineralization is generally made up of banded or massive quartz carrying abundant coarse arsenopyrite. However, most striking is a distinct suite of Au-Bi-Te-S minerals, namely joseite-A (Bi4TeS2), joseite-B (Bi4Te2S), hedleyite (Bi7Te3), ikunolite (Bi4S3), ‘protojoseite’ (Bi3TeS), an unnamed mineral (Bi6Te2S), bismuthinite (Bi2S3), native Bi, native gold, maldonite (Au2Bi), and jonassonite (AuBi5S4). The majority of the Bi-Te-S phases is characterized by Bi/(Se+Te) ratios of &gt;1. Accordingly, this assemblage formed at reduced conditions at relatively low fS2 and fTe2. Fluid-inclusion thermometry indicates depositional temperatures of the main stage of mineralization of up to 342°C, in the normal range of mesothermal, orogenic gold deposits worldwide. However, melting temperatures of Au-Bi-Te phases down to at least 235°C (assemblage (Au2Bi + Bi + Bi7Te3)) imply that the Au-Bi-Te phases have been present as liquids or melt droplets. Furthermore, the close association of native gold, native bismuth and other Bi-Te-S phases suggests that gold was scavenged from the hydrothermal fluids by Bi-Te-S liquids or melts. It is concluded that a liquid/melt-collecting mechanism was probably active at Viceroy Mine, where the distinct Au-Bi-Te-S assemblage either formed late as part of the main, arsenopyrite-dominated mineralization, or it represents a different mineralization event, related to rejuvenation of the shear system. In either case, some of the gold may have been extracted from pre-existing, gold-bearing arsenopyrite by Bi-Te-S melts, thus leading to an upgrade of the gold ores at Viceroy. The Au-Bi-Te-S assemblage represents an epithermal-style mineralization overprinted on an otherwise mesothermal (orogenic) gold mineralization.

Mineralogical, Petrological, Stable Isotope, and Fluid Inclusion Characteristics of the Tuvatu Gold-Silver Telluride Deposit, Fiji: Comparisons with the Emperor Deposit

The Tuvatu gold-silver telluride deposit with reserves of 13 t Au is the second largest gold deposit in Fiji after the large Emperor gold telluride deposit (production, and proven and probable reserves of 280 t Au). The deposits are 50 km apart and occur along the >250-km east-northeast–trending Viti Levu lineament. They are spatially associated with alkaline rocks of almost identical age (~5.4–4.6 Ma) and having a shoshonitic affinity. The gold mineralization in both deposits is spatially and genetically related to monzonite intrusions and to a low-grade porphyry copper-style system. The Emperor deposit occurs along the margins of the Tavua volcano whereas the Tuvatu deposit may occur adjacent to an eroded shoshonite volcano. At both locations, low-sulfidation, epithermal gold telluride mineralization occurs in flat-lying veins, steep faults, shatter zones, stock-works, and hydrothermal breccias. Mineralization in both deposits formed in multiple stages and is characterized by the presence of quartz-roscoelite telluride veins in which gold-rich tellurides were deposited prior to silver-rich tellurides. Gold tellurides and vanadium minerals were deposited at approximately 250°C from moderately saline fluids. Oxygen and hydrogen isotope compositions of ore fluids at Emperor and Tuvatu are similar to the composition of waters exsolved from arc magmas. Previously published values of δ34S of sulfides (–20.3 to +3.9‰) from Emperor are like those obtained from the Tuvatu deposit (−15.3 to −3.2‰) and indicate, along with mineral assemblages, that the ore fluids were oxidizing and near the hematite-pyrite buffer. The similar igneous lithological units of almost identical age, transition from porphyry- to epithermal-style mineralization, paragenetic relationships, and comparable fluid inclusion and stable isotope data suggest a common origin for sulfide and gold telluride mineralization at the Tuvatu and Emperor deposits. Potential exists for additional epithermal gold telluride mineralization near volcanic centers in shoshonitic rocks (Ba and Koroimavua Volcanic groups) spatially related to the Viti Levu lineament in northern Viti Levu.

鉱物資源探査における空中物理探査の現状と将来

Airborne geophysical survey methods, such as magnetics, electromagntics, and radiometrics, are well known, have numerous applications, and are regularly employed in regional surveys around the world by both governments and private organizations. Technology regarding each of these methods has gradually improved and along with it, the introduction and necessary accuracy in Global Positioning Systems (GPS), the introduction of image processing techniques allowing a greater range in data display compared with conventional contouring, and the availability of multi-client data sets resulting in significant cost reduction. Airborne geophysical methods have long been recognized as a valuable geological survey tool since the first airborne magnetic survey was flown in 1936 as part of a mineral exploration program. The Metal Mining Agency of Japan (MMAJ) has for more than three decades employed airborne geophysical methods in both its domestic and overseas mineral exploration programs. In general, the surveys provide a wide range of data concerning lithological variation, structure, and features peculiar to mineralization. Domestically, the MMAJ has employed airborne magnetic surveys since 1969 as part of their regional exploration program, and airborne electomagnetic and magnetic surveys since 1975, initially as part of their research into the application of these methods in the search for Kuroko and epithermal style mineralization. A direct result of this work was the discovery of the Hishikari deposit, Japan's premier gold mine. In 1996, the MMAJ embarked on a new program of high-resolution airborne magnetic and radiometric surveys, beginning in the Hokusatsu region of southwest Kyushu. The results confirmed the importance and application of these sensors in the search for low sulphidation epithermal style gold mineralization. High-resolution geophysical surveys remain an integral part of the domestic survey departments current activities. Overseas, the MMAJ began its history of airborne surveys in Zaire in 1968 with a fixed-wing magnetic survey. Since that time a range of sensors have been employed around the world for various styles of mineralization, the most recent being a helicopter survey this year in the Bicol region of the Philippines targeting porphyry and epithermal copper-gold deposits. Although the above mentioned methods have been successfully employed in the search for mineral deposits, they are equally important in other fields. The identification and surveying of faults, slope stability, dam site identification, transport and land-use planning, electric power line construction, underground surveying, and the monitoring of radioactive elements, are just a few of the applications currently employed by both government and private organizations. Airborne geophysical surveys provide an important and essential data source which should be undertaken as part of Japan National Mapping Program with both current and future multipurpose users in mind. Additionally, it is critical that the government continue to develop the necessary technology concerning data acquisition, processing, analysis, and interpretation, and build upon the high standards, already established within the MMAJ. Such a program is believed to be a matter of national importance.