Epithermal Au-Ag Mineralization Research Articles

Physico-chemical constraints associated with hypogene hydrothermal alteration and supergene oxidation at the Farallón Negro intermediate-sulfidation epithermal Au-Ag deposit, NW Argentina

The Farallón Negro deposit is located in the Farallón Negro Volcanic Complex (FNVC; ∼9.40 to 4.88 Ma) in NW Argentina and includes intermediate-sulfidation, epithermal Au-Ag mineralization in the Alto de la Blenda deposit. Detailed mineralogical and geochemical investigations of the Esperanza-Esperanza Sureste, Cecilia veins, both hosted in the Alto de la Blenda monzonite, and the Laboreo vein, hosted in the volcaniclastic Morada Central andesitic breccia, reveal four hypogene vein stages. Each of these four stages are comprised of a texturally early quartz, followed by a sulfide-sulfosalt and a late Mn-carbonate sub-stage. Stage 1 is characterized by a quartz, base metal and Mn-carbonate sub-stage showing high δ18OVSMOW (δ18OQuartz = 13.7 ± 0.4‰ and δ18OMn-Calcite = 11.6 ± 0.2‰) and low δ13CVPDB values (δ13CMn-Calcite = –4.7 ± 0.2‰). Stage 2 includes most gold and silver minerals in a tennantite-tetrahedrite-sphalerite-pyrite-galena±electrum sub-stage. Fluid inclusions trapped in vein-anhydrite, only associated with stage 2, record the highest temperatures (TEntrapment = 327–335°C) and highest salinity (3.8NaCleq.wt.%) when compared to all other stages. Stage 2 quartz (δ18OQuartz = 9.9 to 11.9‰) and carbonate display lower δ18OVSMOW (δ18OMn-Calcite = 7.9 to 10.3‰), as well as lower δ13CVPDB values (δ13CMn-Calcite = -5.2 to 4.7‰) than all other stages. Stages 3 and 4 consist of barren quartz and carbonate sub-stages that show lower temperatures (TEntrapment = 187–262°C) and higher δ18OVSMOW (δ18OQuartz = 13.4 to 13.9‰ and δ18OMn-Calcite = 10.7 to 11.8‰) and δ13CVPDB values (δ13CMn-Calcite = –3.5 to −3.9‰) when compared to earlier stages.Petrographic, sulfide mineral chemistry, microthermometric, and stable isotope data combined with distinct alteration mineral assemblages allow the reconstruction of a model for the Alto de la Blenda deposit. During the hypogene hydrothermal fluid flow stage, around 7.16 Ma, at ∼2.2 km depth (PHYDROSTATIC = 220 bars), the high temperature alteration mineralogy, low fluid salinities and 18O and 13C isotope compositions indicate cooling of ascending magmatic-hydrothermal fluids. These fluids were derived from a contracted magmatic vapor phase that exchanged with unexposed sedimentary rocks of the Sierras Pampeanas. Hypogene quartz-R3 illite/smectite-chlorite-calcite-pyrite alteration in the andesitic-breccia and quartz-illite-R3 illite/smectite-chlorite-calcite-pyrite alteration in the monzonite are temporally and spatially to the hydrothermal vein minerals deposited between 187 and 335°C. During the post-hydrothermal supergene weathering event, around 2.70 Ma, sulfuric acids generated by meteoric fluids and supergene oxidation of hypogene sulfides, particularly pyrite, descended along normal fault zones and related vein and fracture networks to at least 450 m depth below the surface level, forming the supergene alteration assemblage of dioctahedral smectite-kaolinite-Mn-/Fe-oxides/-hydroxides-gypsum±interstratified chlorite/smectite±interstratified R1 illite/smectite in both wall rock types at temperatures <50°C.The dominant hypogene alteration assemblage, present in the monzonite and volcaniclastic andesitic breccia, may be indicative of deeper-seated, transitional magmatic-hydrothermal systems (>1 km depth), generally controlled by fluid cooling, as is the case at Farallón Negro. Pervasive supergene alteration can conceal these systems but the identified supergene alteration assemblage of dioctahedral smectite-kaolinite-interstratified chlorite/smectite-interstratified R1 illite/smectite that replaces the hypogene alteration assemblage along the permeable fault, vein and fracture zone network may present a useful vectoring tool towards concealed, deeper-seated, epithermal Au-Ag mineralization in arid climates.

Late Mesozoic tectonic evolution and thermal history in the northeast Xing'an Block, NE China: Implications for gold mineralization and preservation

Late Triassic–Jurassic orogenic and Early Cretaceous epithermal Au deposits are significant producers of Au, Ag, and potentially critical elements (e.g., Te, Se and Sb) in the North Heilongjiang Belt in the Xing'an Block, NE China. However, the preservation mechanism of these Au deposits associated with tectonic evolution is unclear. Here, we conducted zircon U–Pb, sericite Ar–Ar, zircon and apatite (U–Th)/He dating as well as whole‐rock geochemistry in the Xing'an Block. Zircon U–Pb dating shows the ages of 174.1 ± 2.4 Ma for the granodiorite, 169.9 ± 1.6 Ma for the tonalite, 154.0 ± 2.0 Ma for the monzogranite and ca.122 Ma for the diorite. Sericite Ar–Ar dating presents plateau and isochron ages of 163.2 ± 1.8 Ma and 163.4 ± 2.8 Ma. The average apatite (U–Th)/He (AHe) and zircon (U–Th)/He (ZHe) ages from the Jurassic rocks have the ranges of ca. 146–89 Ma and ca. 150–116 Ma and from the Cretaceous are ca. 109–76 Ma and ca. 117–111 Ma, respectively. The Early‐Middle Jurassic intrusions are featured by the tonalite‐trondhjemite‐granodiorite (TTG) rock type. Late Jurassic monzogranite has the geochemical signature of adakitic rocks. The Early Cretaceous granitoids are classified as volcanic arc granites with a part of the Early Cretaceous granodiorite featured by adakitic. These new results reveal five group ages associated with tectonic evolution, Au mineralization and the preservation of the Au deposits. (i) Magmatic events were related to the subduction of the Mongolia‐Okhotsk Ocean during the Early Jurassic (ca. 177–170 Ma) and then the compressional settings with regional deformation (ca. 167–163 Ma). (ii) Thickening crust and orogenic Au mineralization were related to the closure of the Mongolia‐Okhotsk Ocean (ca. 154–145 Ma). (iii) The transformation was from compressional to extensional settings followed by exhumation during the Early Cretaceous (ca. 145–120 Ma), which is the factor of the exhumation of the orogenic Au deposits formed from the Late Triassic to Late Jurassic. (iv) Epithermal Au–Ag mineralization was associated with an extension related to the subduction of the Palaeo‐Pacific Plate (ca. 120–100 Ma). (v) Thermal history shows one thermal event during the Late Cretaceous (ca. 89–76 Ma). The Au deposits have been preserved since the Late Cretaceous.

Precious-Metal Mineralization and Formation Conditions of the Biche-Kadyr-Oos Epithermal Au-Ag Ore Occurrence (Eastern Sayan, Russia)

The Biche-Kadyr-Oos epithermal Au-Ag ore occurrence is a prospective object in the Ak-Sug porphyry copper ore cluster (Eastern Sayan) in the northern part of the Central Asian orogenic belt (CAOB). The mineralization consists of gold-sulfide-quartz and gold-polysulfide-carbonate-quartz veins with argillic zones in the Lower Cambrian volcanic-sedimentary rocks. The origin of the Au-Ag ore occurrence is still debatable. To determine the origin, we examined the mineralogical and geochemical features, conditions of formation, and fluid sources of the Biche-Kadyr-Oos ore. A mineralogical and geochemical investigation outlines three stages of mineral formation: early argillic stage; gold-sulfide-quartz stage with pyrite, marcasite, pyrrhotite, arsenopyrite, chalcopyrite, less frequently sphalerite, hessite, gold, and electrum; and late gold-polysulfide-carbonate-quartz stage with gold, electrum, Hg-electrum, Se-acanthite, Se-galena, bornite, tennantite, tetrahedrite, hessite, tellurobismuthite, bismuthinite, matildite, jamesonite, ourayite, native Bi, and barite. Fluid inclusion study (thermometry, Raman spectroscopy) in quartz and mineral thermometry (electrum and sphalerite paragenesis) determined that ore veins were formed at P~0.5 kbar from CO2-water Na-K-chloride fluid (4.9–9.6 wt % NaCl eqv) and temperatures from 300 to 200 °C (early gold-sulfide-quartz veins at 300–230 °C, and late gold-polysulfide-carbonate-quartz veins at 290–200 °C) and variations in fO2, fS2, fSe2 and fTe2. The S isotopic composition in sulfides and δ34SH2S values of the fluid are +1.3‰ and +4.7‰, respectively, (T = 300–275 °C) indicating magmatic S in ore formation. The oxygen isotope data indicate that during the formation of veins, the magmatic fluid mixed with meteoric water (δ18Ofluid is from +3.4 to +6.4‰). The isotopic data that were obtained combined with mineralogical and geochemical features and conditions of ore formation indicate the similarity of Biche-Kadyr-Oos ore occurrence with epithermal Au-Ag deposits of intermediate sulfidation (IS) type. The presence of epithermal Au-Ag mineralization of the Biche-Kadyr-Oos IS type in ore cluster of the Ak-Sug Cu-Au-Mo porphyry deposit indicates the existence of a single porphyry-epithermal ore-magmatic system.

МОРФОЛОГИЧЕСКИЕ ХАРАКТЕРИСТИКИ, ХИМИЧЕСКИЙ СОСТАВ РОССЫПНОГО ЗОЛОТА БАССЕЙНА РЕКИ КЕНГЕВЕЕМ И ПЕРВЫЕ СВЕДЕНИЯ О СОПУТСТВУЮЩЕЙ ПЛАТИНОВОМЕТАЛЛЬНОЙ МИНЕРАЛИЗАЦИИ (МАГАДАНСКАЯ ОБЛАСТЬ)

The paper presents the results of the study of typomorphic characteristics of placer gold of the tributaries of the Kengeveem River, Magadan region. Placer gold was studied by optical and scanning electron microscopy at the Institute of Volcanology and Seismology of the Far East Branch of the Russian Academy of Sciences. As a result, the first information on the morphology, internal structure and chemical composition of placer gold from the southeastern part of the Taigonos Peninsula was obtained. Three types of the gold were determined. The first type is dominant. The grains are chacterized by sub-ore morphology, with weak and medium degrees of roundness. Only Au-Ag is present in the composition of native gold, other macrocomponents were not found. The gold grade is 750–950 ‰. Some gold grains bear signs of hypergene transformation, expressed in the development of high-grade gold rims and intergrain veins. The second type of gold is represented by well-rounded lamellar extractions with a grade of 980–990 ‰. The third type is loose aggregates of secondary high-grade mustard gold. The obtained data allowed us to determine the different drift distances of native gold and to consider the low-sulfide epithermal Au-Ag mineralization and presumably far-distant gabbroids as the primary sources of gold.

Geometry and scale property of a gold-silver-bearing vein system associated with an oblique-slip fault zone at Gasado Island, Korea

Gasado Island, on the southwestern Korean Peninsula, comprises c. 50 Ma volcanic rocks that are cut by faults and fractures hosting low-sulfidation epithermal Au-Ag mineralization. In this study, we examined the relationship between the fault-fracture networks and auriferous vein systems to better understand the role of brittle structures in controlling hydrothermal fluid flow. Geometrical and fractal analyses were performed to express and quantitatively compare the vein geometries and thickness population. The results were as follows: (1) The geometrical analysis revealed a strong relationship between Au-Ag-bearing quartz veins and NE-SW-striking, oblique sinistral-normal faults, indicating that these structures are a key control on mineralization. (2) Box-counting fractal analysis applied to veins hosted by fault zone demonstrated that the D-values (fractal dimensions) are directly proportional to vein density – indicating that damage zones control vein density. (3) The linear traverse line analysis yielded cumulative frequency plots of vein thicknesses that showed obvious changes in the slope, which are reflected in the variable D-values. The cause of this slope variability is interpreted to be the presence of hybrid veins that fill secondary fault planes, which disturbs the original linear relationship shown by cumulative thickness distribution data. This study shows that vein density and physical properties are strongly controlled by the types, or existence, of fault damage zones. Understanding fault zone architecture may assist with the prediction of locations that concentrate mineralizing fluids and may benefit the initial stage of mineral exploration.

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

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

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

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

The Intrusive Phase in the Evolution of a Volcanic Structure and of Au–Ag Low Sulfidation Mineralization: The Nevenrekan Deposit, Magadan Region, Russia

This paper provides information on the structural setting and ore composition of the Nevenrekan deposit. We provide an account of the regional and local factors of mineralization control. The ore bodies are confined to the junction between regional faults and annular ruptures of a circular subsidence. The geochemical zoning of the ore field is disturbed. We show how geochemical associations are distributed in the ore zones of this deposit. The structural geological and geochemical data, as well as results from a comparison among the Au‒Ag low sulfidation (LS) deposits at the Okhotsk–Chukchi, Uda–Murgali, and Central Kamchatka volcanogenic belts, shows that the mineralization occurred in two phases. The first phase involved the generation of epithermal Au–Ag mineralization, while the second, plutogenic, phase produced intrusion-related gold system that is seen as epidotization and silicification of rocks in the ore field, as well as in the metamorphism of volcanogenic mineralization. The presence of the later mineralization phase associated with granitoids is typical of the Nevenrekan deposit as distinguished from the other Au–Ag (LS) deposits in the Evensk ore region, and makes it similar in this respect to the Karamken, Nyavlenga, and Cirotan deposits.

ПРОГНОЗНО-ПОИСКОВАЯ МОДЕЛЬ ЭПИТЕРМАЛЬНЫХ Au-Ag МЕСТОРОЖДЕНИЙ КИСЛОТНО7СУЛЬФАТНОГО ТИПА ПО ДАННЫМ ДЕШИФРИРОВАНИЯ СОВРЕМЕННЫХ КОСМИЧЕСКИХ СНИМКОВ (НА ПРИМЕРЕ РУДНОГО ПОЛЯ СВЕТЛОЕ, ХАБАРОВСКИЙ КРАЙ)

Актуальность. Расширение минерально-сырьевой базы действующих горнодобывающих предприятий требует современных подходов в изучении сопредельных площадей. Одним из таких современных методов в изучении площадей является использование современных спектрозональных космических снимков в целях выделения потенциально рудоносных площадей на основе структурно-вещественного дешифрирования. Основная цель исследования заключалась в разработке критериев эпитермального золото-серебряного оруденения в пределах Охотско-Чукотского вулканического пояса на основе материалов дистанционных съемок. Объектами исследования послужили известные рудные таксоны юго-восточного фланга Ульинской минерагенической зоны Охотско-Чукотского вулканического пояса. Методы: обработка, спектральный анализ и дешифрирование современных спектрозональных космических снимков Landsat, Aster и снимков высокого пространственного разрешения Ikonos, WorldView, QuickBird. Результаты. Разработаны региональные, локальные и детальные критерии Au-Ag оруденения на примере одного из участков Ульинской минерагенической зоны. Структурными критериями регионального уровня являются участки пересечения разнонаправленных разрывных нарушений во вложенных соподчиненных кольцевых структурах диаметром от 20 до 90 км. Структурно-вещественные критерии локального уровня – области сопряжения систем вложенных соподчиненных кольцевых структур размером от 1 до 10 км с зонами развития Fe-окисных индексов. К структурно-вещественным критериям детального уровня отнесены малые кольцевые структуры, выполненные телами вторичных кварцитов различных минеральных ассоциаций, сопряженные с зонами развития Fe-окисных индексов, внутри участков пропилитового типа изменений. На основе выявленных особенностей строения рудного поля Светлое, дан прогноз о перспективности южного и юго-восточного фланга на выявление золото-серебряного эпитермального оруденения. Предложена иерархическая дистанционная прогнозно-поисковая модель эпитермальных Au-Ag месторождений.

Petrogenesis of the Rio Blanco epithermal Au-Ag mineralization in the Cordillera Occidental of southwestern Ecuador: Assessment from host rocks petrochemistry and ore constituents isotopic (O, S, H, and Pb) compositions

The petrogenesis of the Rio Blanco Au-Ag mineralization in the Cordillera Occidental (Western Cordillera), southwest Ecuador, was assessed using magmatic host rocks and ore constituents geochemical and isotopic (S, O, H, Pb and Sr) composition. The magmatic rocks spatially associated with the Rio Blanco Au-Ag mineralization are of arc affinity and mostly include lava flows of andesitic composition, as well as plutons and pyroclastic falls that range from dioritic (andesitic) to granodioritic (dacitic) compositions. These arc rocks as evidenced by their initial Sr isotope compositions (87Sr/86Sri: 0.70443–070694), which are symptomatic of crustal contamination, are of two contrasting magmatic suites (transitional and calc-alkaline), and typically display enrichment in LREE with respect to HREE, with resulting moderately elevated La/Yb ratios of 5.75–8.79.Unlike parental melts of most Miocene intrusions associated with porphyry-related mineralization from the Cordillera Occidental of Ecuador, which differentiated at a deep crustal level, the Rio Blanco parental magma, which was generated in a relatively thin crustal setting (as suggested by Sr/Y ratios) differentiated in the upper crust (as supported by the relatively low Sr/Y) at <0.4 GPa, where significant plagioclase fractionation took place as evidenced by their consistent negative Eu anomaly (Eu/Eu* ranges from 0.065 to 0.102), as well as the positive correlation between Sr/Y and Eu/Eu*. Important plagioclase fractionation was accompanied by minor amphibole and pyroxene fractionation as evidenced by their sporadic occurrence as phenocrysts in Rio Blanco magmatic rocks under investigation.Magmatic rocks spatially associated with the Rio Blanco Au-Ag mineralization also show a significant radiogenic signature, which was acquired through assimilation of various basement rocks within the upper crust, thus leading to heterogeneous and wide Pb isotopic composition ranges (206Pb/204Pbi = 18.605–19.174; 207Pb/204Pbi = 15.621–15701; 208Pb/204Pbi = 38.442–39.043). Sulfides and electrum are radiogenic as well (206Pb/204Pb ratios are between 19.002 and 19.143, whereas 207Pb/204Pb and 208Pb/204Pb ratios span from 15.644 to 15.697 and from 38.827 to 38.998, respectively) and overlap the compositions of the highly radiogenic magmatic rocks, indicating Pb and by inference, the other metals such as Au and Ag are of magmatic origin like S, which δ34S (V-CDT) values range between −3 and 4.9‰. Unlike S and metals of magmatic origin, the degased (as indicted by δD (V-SMOW) values range between −92.54 and −78.89‰) mineralizing H2O is a mixture between magmatic and meteoric fluids as supported by their δ18O (V-SMOW) values ranging from −0.959–4.21‰.

Features of Ore Mineralization of Alyarmaut Rise (Western Chukotka)

The Au mineralization of Alyarmaut Rise was formed from the ore-forming fluids of two types strongly differing in pressure. The first type of fluid is similar to fluids of the orogenic Au–quartz deposits in terms of the physicochemical parameters and composition. These fluids were formed at a high pressure from heterogeneous, carbon dioxide–aqueous fluids with a low salinity at a depth of 5–7 km. The second type of fluid, which is also heterogeneous, has a shallower origin with a wide range of salinity variation and a small amount of carbon dioxide. This type of fluid is similar to the fluids of epithermal deposits. The wide range of pressure variation characterizing the mineralization of the rise indicates the vertical displacement of the block of the host rocks followed by erosion. Judging by the presence the secondary halos of Au, Ag, As, Sb, Pb, Zn, and Cu in the Upper Paleozoic carbonate–terrigenous rocks of the Alyarmaut Rise, there are prospects to reveal sites with new epithermal Au–Ag mineralization, which are essential for the area and unusual for the Anyui-Chukotka Fold System.

Insights into the genesis of the epithermal Au-Ag mineralization at Rio Blanco in the Cordillera Occidental of southwestern Ecuador: Constraints from U-Pb and Ar/Ar geochronology

The genesis of the Au-Ag mineralization at Rio Blanco in the Cordillera Occidental (Western Cordillera) of southwest of Ecuador is here constrained. This was done by investigating the temporal and by inference the genetic relationship between the Au-Ag mineralization and the spatially associated magmatic host rocks using zircon U-Pb [chemical abrasion (CA) IDTIMS and laser ablation (LA) ICPMS] and adularia 40Ar/39Ar geochronology.Whereas volcanics hosting the Au-Au mineralization range in age from 37.35 ± 0.30 to 33.09 ± 0.20 Ma (Late Eocene-Early Oligocene), the spatially associated intrusions are of at least two discontinuous phases of magmatism and these include: (i) Late Eocene intrusions that range in age from 35.77 ± 0.19 to 36.03 ± 0.19 Ma, and; (ii) Miocene intrusions of 15.58 ± 0.04 Ma. The 40Ar/39Ar age of adularia from a Bonanza Au (334 g/t)-Ag (2060 g/t)-bearing epithermal vein is bracketed between 14.3 and 14.9 Ma.The temporal relationship between adularia and by inference mineralization and the spatially associated rocks therefore rules out any temporal link between the Late Eocene-Early Oligocene Rio Blanco Formation, but rather favors a possible genetic relationship between the Rio Blanco Au-Ag mineralization and the Miocene intrusions. The determined Rio Blanco Au-Ag mineralization age is consistent with the established Oligocene-Miocene period of ore deposits in Ecuador and coincides with the extensional tectonic event, which developed intramontane basins in southern Ecuador.

Characteristics of Cu–Mo Mineralization in the Chatree Mining Area, Central Thailand

AbstractCu–Mo mineralization occurs in southern part of the Chatree Au–Ag deposit, central Thailand. Quartz veins of Cu–Mo mineralization are divided into five types: Types A, B, C, D and E. Quartz veins of Types A, B and C are hosted in altered granodiorite porphyry, and quartz veins of Types D and E occur in altered andesite lava. Mineral assemblages of Types A, B and C quartz veins are composed of qz–chl–ilt–mol–py–ccp, qz–chl–ilt–ccp–py and qz–chl–ilt–ccp–py–sp–po, respectively. Types D and E quartz veins consist of qz–chl–py–ccp–sp–po and qz–ep, respectively. Fluid inclusions of quartz veins are divided into liquid‐rich two‐phases fluid inclusion, vapor‐rich two‐phases fluid inclusion and multiphase solid‐bearing fluid inclusion. Coexistence of a halite‐bearing fluid inclusion having salinity of 37 equiv. wt.% NaCl and a vapor‐rich two‐phases fluid inclusion having salinity of 1 equiv. wt.% NaCl suggests that the Cu–Mo‐bearing quartz veins were formed at temperature of 450°C and pressure of 250 bars (depth of approximately 1.5 km from the paleosurface). Based on the formation temperature of 450°C of quartz veins and the δ18O values of quartz of the quartz veins, the δ18O value of fluid responsible for the Cu–Mo‐bearing quartz vein is estimated to be +9.9‰. The origin of fluid forming the Cu–Mo‐bearing quartz veins in the N prospect of the Chatree mining area would be magmatic water. Based on the characteristics of geology, age, mineral assemblage and the formation environment, Cu–Mo mineralization would be different from the epithermal Au–Ag mineralization of the Chatree mining area.

Geochemical features of Paleozoic Au–Ag epithermal deposits (Northeastern Russia)

The geochemical features and conditions of formation of the Paleozoic epithermal Au–Ag mineralization in the pre-accretion Kedon (D2–3) volcanoplutonic belt located within the Omolon craton terrain are described. The new data on the composition and contents of trace and rare-earth elements (REEs) in igneous ores of epithermal deposits is provided. The elevated grades of a wide range of trace elements as compared to the average values of the upper crust have been identified.

The Engineer Mine, British Columbia: An example of epithermal Au-Ag mineralization with mixed alkaline and subalkaline characteristics

The Engineer Mine epithermal precious metal deposit in British Columbia, Canada, is related to Eocene Sloko-Skukum-Group (SSG) volcanism and, according to previous studies, contains roscoelite in intimate association with electrum. Roscoelite, a vanadian mica, is considered characteristic of low-sulfidation epithermal deposits related to alkaline magmatism. This contradicts the fact that the SSG volcanics are subalkaline. In order to address this ambiguity and to accurately classify the style of epithermal mineralization at the Engineer Mine we conducted detailed petrographic, mineralogical, geochronological, fluid inclusion, and stable isotope studies.The principal ore assemblage of the Engineer Mine epithermal veins precipitated in response to boiling during a hydrothermal event at 49.90Ma±0.25Ma. During this event electrum, arsenopyrite, pyrite±chalcopyrite±sphalerite±löllingite±tetrahedrite-group phases±allargentum±acanthite±hessite±dyscrasite±stibarsen±galena and an unidentified Ag-rich phase were deposited in conjunction with amorphous silica, platy and rhombic calcite, K-feldspar, and vanadian illite. Fluid inclusion and stable isotope data suggest that the ore-forming fluid was boiling at ∼220°C during vein mineralization and had an isotopic composition derived from local meteoric water. Based on these results the Engineer Mine is classified as an epithermal low-sulfidation deposit, which shares similarities with alkaline and subalkaline epithermal low-sulfidation deposits. This is attributed to the fact that the SSG volcanic rocks are borderline subalkaline to alkaline in character and that the sedimentary host rocks are vanadium-bearing. These sedimentary rocks contributed the bulk of the vanadium to the Engineer Mine epithermal system. The presence of roscoelite at the Engineer Mine could not be confirmed during this study. The mica referred to as roscoelite in previous publications instead is vanadian illite. To our knowledge the only alkaline low-sulfidation epithermal precious metal deposit that contains V-mica which exclusively qualifies as true roscoelite is the Porgera deposit, Papua New Guinea.

Cu–Ag sulfides as indicators of pre-porphyritic epithermal Au–Ag deposits in Northeastern Russia

Au–Ag mineralization of the Olcha and Teploe epithermal deposits underwent thermal metamorphism due to porphyritic intrusions. The presence of Bi-bearing galena and matildite in the ores (Teploe), Cu–Te-bearing naumannite (Olcha), the occurrence of middle- and high-temperature facies of metasomatic rocks (epidote and actinolite), and temperature formation conditions are related, firstly, to the influence of granitoids on the ore process, which supplied not only Cu and Mo, but also Bi, Te, and, secondly, to the heating of host rocks containing pre-porphyritic epithermal Au–Ag mineralization. The abundance of Cu–Ag sulfides and Cu-acanthite resulted from the enrichment of later mineral phases in Cu and Ag under the substance redistribution with the formation of Ag-acanthite ores. The data considered in the paper are of practical importance for regional forecasting of metallogenic constructions, exploration, and evaluation of the epithermal Au–Ag deposits.

Low-sulfidation epithermal Au-Ag mineralization in the Sındırgı District, Balıkesir Province, Turkey

The Sındırgı District (Balıkesir, western Turkey) lies within the Western Anatolian volcanic and extensional province, adjacent to the WNW-trending Simav graben, approximately 130 km NE of İzmir. The Sındırgı mining district is underlain mainly by Miocene volcanic rocks and hosts several low-sulfidation epithermal Au-Ag deposits and prospects located near the towns of Sındırgı and Bigadiç. The Kızıltepe low-sulfidation epithermal gold-silver deposit is located southeast of Yusufçam village (Sındırgı, Balıkesir), and other prospects, including the Kepez, Kavaklıdüz, and Karadüz prospects, are located northeast of Kızıltepe. Potentially economic grades occur at Kızıltepe, which contains a measured and indicated resource of 1.754.790 Mt @3.0 g/t Au, 44 g/t Ag, hosted by quartz veins showing colloform/crustiform banding, quartz pseudomorphs after bladed calcite, and multiphase brecciations, all typical textures noted in low-sulfidation epithermal deposits. Alteration minerals include mixed-layer illite/smectite, high-crystallinity illite, and kandite group minerals (dickite and nacrite). Precious metal minerals include traces of electrum, acanthite, Au-rich acanthite, and Ag-Hg-Au-Tl-Pb series, occurring mainly within quartz. Pyrite is the most common opaque mineral at Kızıltepe. ^{40}Ar/^{39}Ar dating of adularia from the quartz veins indicates an age of mineralization of 18.3 ± 0.2 Ma. The ore mineralization is divided into three main phases. These comprise the deposition of: coarse-grained quartz, illite, pyrite, and minor precious metals (Phase I); major gold–silver-bearing medium-grained quartz, which commonly exhibits crustiform banding, carbonate replacement, and hydrothermal breccia textures (Phase II); and fine-grained chalcedonic quartz with colloform/crustiform banding (Phase III). Phase II is economically the most important in terms of precious metal content. Phase II quartz contains fluid inclusions, which range from predominantly vapor-rich to predominantly liquid-rich with homogenization temperatures (Th) varying from 157 to 330 °C, showing a cluster between 190 and 300 °C, and ice-melting temperatures (Tm) ranging from -0.2 to -2.9 °C (salinity from 0.5-4.8 wt.% NaCl equiv.). Moderate to strong positive correlations occur between Au-Ag (R = 0.8) and Au-Cu (R = 0.5), whereas there is no correlation between As and Au or Ag.

해남 모이산 천열수 금-은 광호대의 지질구조와 광화작용 당시의 지구조환경

옥천변성대 남서부에 위치하는 전남 해남군 황산면 모이산 지역에는 백악기 말 유천층군 해남층의 황산 화산쇄설 암을 모암으로 하는 천열수 금-은 광화대가 발달한다. 본 연구는 광화대의 형성과 관련된 지질구조를 파악하기 위해 모이산 지역의 층리배열, 습곡, 단층, 단열계, 석영맥의 특성, 단열계의 상대적인 발달순서 등을 조사하였고, 이를 바탕으로 광화작용이 발생할 당시의 지구조환경을 고찰해 보았다. 층리는 동북동 주향 방향이 우세하며 북북서 내지 남남동 방향의 저각 경사를 보이고 분산된 층리 배열은 경사-이동 단층 이외에 (동)북동 방향의 준수평-직립 개방습곡에 의해 규제되어 있다. 단열계는 적어도 6~7회의 변형단계를 걸쳐 형성되었다. D1 단계; 연장성이 우세한 동서 (D1-1)와 남북(D1-2) 방향의 주 단열조 형성기, D2 단계; 북서 방향의 인장단열 형성기, 동서(우수향)와 남북(좌수향) 방향의 공액 전단단열 운동기, D3 단계; 북동 방향의 인장단열 형성기, 동서(좌수향)와 남북(우수향) 방향의 공액 전단단열 재활동기, D4 단계; 연장성이 미약한 남북 방향의 인장단열 형성기, D5 단계; 북서 방향의 인장단열 재형성기. 동서(우수향)와 남북(좌수향)방향의 공액 전단단열 재활동기, D6 단계; 연장성이 미약한 동서 방향의 인장단열 형성기. 변형단계별 단열조의 출현빈도는 D1-1(19.73 %)> D1-2(16.44 %)> D3=D5(14.79 %)> D2(13.70 %)> D4(12.33 %)> D6(8.22 %) 순으로 우세하게 나타나고 평균개수/1 m는 D6(5.00개)> D5 = D4(4.67개)> D2(4.60개)> D3(4.13개)> D1-1(3.33개)> D1-2(2.83개) 순으로 높게 나타난다. 전체 단열조의 평균 조밀도는 420개로서 모이산 지역의 단열조는 평균 23.8 cm 간격 이상으로 발달한다. 석영맥의 방향성별 출현빈도는 동서 방향(52 %)> 북서 방향(28 %)> 남북 방향(12 %)> 북동 방향(8 %) 순으로 우세하게 나타난다. 전체 석영맥의 평균 조밀도는 4.14개로 나타나고 모이산 지역의 석영맥은 평균 24.2 cm 간격 이상으로 발달한다. 석영맥에 대한 미구조 연구결과, 천열수 금-은 광화작용(약 77.9~73.1 Ma)은 지구조적 응력이 아닌 천열수 파괴 단열작용과 직후의 정적인 지구조환경 하에서 기존에 형성된 연장성이 우세한 D1 단계의 동서 방향과 남북 방향의 단열조를 따라 발생한 것으로 보인다. 모이산 지역의 D1 단열작용은 유천층군과 불국사 화성압류의 화성활동과 변형을 초래하는 후기 백악기 이자나기판의 북향-사교 섭입작용과 관련하여 남북 방향의 압축력과 인정력이 교호하는 불안정한 지구조환경 하에서 발생하였던 것으로 고찰된다. An Epithemal Au-Ag mineralized zone is developed in the Moisan area of Hwangsan-myeon, Haenam-gun, Jeol-lanam-do, Korea, which is located in the southwestern part of the Ogcheon metamorphic zone. It is hosted in the Hwangsan volcaniclastics of the Haenam Formation of the Late Cretaceous Yucheon Group. This research investigated the characteristics of bedding arrangement, fold, fault, fracture system, quartz vein and the time-relationship of the fracture system to understand the geological structure related to the formation of the mineralized zone. On the basis of this result, the tectonic environment at the time of the mineralization was considered. Beds mainly trend east-northeast and gently dip into north-northwest or south-southeast. Their poles have been rearranged by subhorizontal-upright open fold of (east)-northeast trend as well as dip-slip fault. Fracture system was formed through at least 6~7 different deformation events. D1 event; formation phase of the main fracture set of EW (D1-1) and NS (D1-2) trends with a good extensity, D2 event; that of the extension fracture of NW trend, and conjugate shear fracturing of the EW (dextral) and NS (sinistral) trends, D3 event; that of the extension fracture of NE trend, and conjugate shear refracturing of the EW (sinistral) and NS (dextral) trends, D4 event; that of the extension fracture of NS trend showing a poor extensity, D5 event; that of the extension fracture of NW trend, and conjugate shear refracturing of the EW (dextral) and NS (sinistral) trends, D6 event; that of the extension fracture of EW trend showing a poor extensity. Frequency distribution of fracture sets of each deformation event is D1-1 (19.73 %)> D1-2 (16.44 %)> D3=D5 (14.79 %)> D2 (13.70 %)> D4 (12.33 %)> D6 (8.22 %) in descending order. The average number of fracture sets within 1 meter at each deformation event is D6 (5.00)> D5 = D4 (4.67)> D2 (4.60)> D3 (4.13)> D1-1 (3.33)> D1-2 (2.83) in descending order. The average density of all fractures shows 4.20 fractures/1 m, that is, the average spacing of all fractures is more than 23.8 cm. The frequency distribution of quartz veins at each orientation is as follows: EW (52 %)> NW (28 %)> NS (12 %)> NE (8 %) trends in descending order. The average density of all quartz veins shows 4.14 veins/1 m, that is, the average spacing of all quartz veins is more than 24.2 cm. Microstructural data on the quartz veins indicate that the epithermal Au-Ag mineralization (ca. 77.9~73.1 Ma) in the Moisan area seems to occur mainly along the existing D1 fracture sets of EW and NS trends with a good extensity not under tectonic stress but non-deformational environment directly after epithermal rupture fracturing. The D1 fracturing is considered to occur under the unstable tectonic environment which alternates compression and tension of NS trend due to the oblique northward subduction of the Izanagi plate resulting in the igneous activity and deformation of the Yucheon Group and the Bulguksa igneous rocks during Late Cretaceous time.

Whole-Rock Geochemical Techniques for Evaluating Hydrothermal Alteration, Mass Changes, and Compositional Gradients Associated with Epithermal Au-Ag Mineralization

Volcanic rocks hosting quartz ± calcite ± adularia ± illite epithermal deposits undergo elemental mass changes associated with K metasomatism, K-H metasomatism, and H metasomatism that are developed progressively upward and outward from the site of mineralization as hydrothermal fluid ascends, boils, and cools. Resultant hydrothermal mineral zonation shows increasingly K-rich mineral assemblages with proximity to deposits, thus patterns of K enrichment provide a vector toward those deposits. We describe whole-rock geochemical techniques for identifying these patterns and for evaluating attendant hydrothermal mineral zonation. Whole-rock geochemical anomalies are evaluated by calculating mass changes associated with hydrothermal alteration, using a modified version of Gresens’ (1967) equation to compare the composition of altered rocks to fresh-rock equivalents. Hydrothermally altered rocks most affected by K metasomatism will be characterized by the largest K gains and, generally, Na and Ca losses. Mass changes associated with K metasomatism are also evaluated graphically using plots of molar (2Ca + Na + K)/Al versus molar K/Al. Since molar values are used to construct the plot, compositions of altered rocks can be compared to the compositions of primary and secondary K-, Na-, Ca-, and Al-bearing minerals that are located in the same compositional space, allowing identification of important hydrothermal minerals (e.g., adularia, illite, smectite) and alteration processes that are reflected in trends from fresh-rock compositions toward the compositions of hydrothermal minerals. The intensity of K metasomatism, encompassing both K gains and Na and Ca losses, can be represented by the slope of the line between an altered rock composition and the origin (i.e., the molar K/(2Ca + Na + K) value). Determinations of mass changes in altered rocks surrounding selected epithermal deposits demonstrate the predominance of K metasomatism proximal to, and commonly increasing in intensity toward, mineralized veins. Comparisons of K mass changes to trace element concentrations indicate that the area affected by K metasomatism is more extensive (100s to 1,000s of meters) than that containing anomalous concentrations of precious metals, base metals, and pathfinder elements (10s to 100s of meters); therefore, whole-rock geochemical techniques potentially extend the area over which geochemical targeting may be effective. Data from this study show that intensity of K metasomatism (molar K/(2Ca + Na + K) values) and concentrations of precious metals and pathfinder elements increase toward ore and are greatest proximal to ore, so that pathfinder element and whole-rock geochemical anomalies are complementary.

Magmatic Fluids in the Breccia-Hosted Epithermal Au-Ag Deposit of Rosia Montana, Romania

The breccia-hosted epithermal Au-Ag deposit of Rosia Montana is located 7 km northeast of Abrud, in the northern part of the South Apuseni Mountains, Romania. Estimated total reserves of 214.91 million metric tons (Mt) of ore at 1.46 g/t Au and 6.9 g/t Ag (10.1 Moz of Au and 47.6 Moz of Ag) make Rosia Montana one of the largest gold deposits in Europe. At this location, Miocene calc-alkaline magmatic and hydrothermal activity was associated with local extensional tectonics within a strike-slip regime related to the indentation of the Adriatic microplate into the European plate during the Carpathian orogenesis. The host rocks of the magmatic complex consist of pre-Mesozoic metamorphosed continental crust covered by Cretaceous turbiditic sediment (flysch). Magmatic activity at Rosia Montana and its surroundings occurred in several pulses and lasted about 7 m.y. Rosia Montana is a breccia-hosted epithermal system related to strong phreatomagmatic activity due to the shallow emplacement of the Montana dacite. The Montana dacite intruded Miocene volcaniclastic material (volcaniclastic breccias) and crops out at Cetate and Cârnic Hills. Current mining is focused primarily on the Cetate open pit, which was mapped in detail, leading to the recognition of three distinct breccia bodies: the dacite breccia with a dominantly hydrothermal matrix, the gray polymict breccia with a greater proportion of sand-sized matrix support, and the black polymict breccia, which reached to the surface, contains carbonized tree trunks and has a dominantly barren clastic matrix. The hydrothermal alteration is pervasive. Adularia alteration with a phyllic overprint is ubiquitous; silicification and argillic alteration occur locally. Mineralization consists of quartz, adularia, carbonates (commonly Mn-rich), pyrite, Fe-poor sphalerite, galena, chalcopyrite, tetrahedrite, and native gold and occurs as disseminations, as well as in veins and filling vugs within the Montana dacite and the different breccias. The age of mineralization (12.85 ± 0.07 Ma) was determined by 40 Ar- 39 Ar dating on hydrothermal adularia crystals from vugs in the dacite breccia in the Cetate open pit. Microthermometric measurements of fluid inclusions in quartz phenocrysts from the Montana dacite revealed two fluid types that are absent from the hydrothermal breccia and must have been trapped at depth prior to dacite dome emplacement: brine inclusions (32–55 wt % NaCl equiv, homogenizing at T h > 460°C) and intermediate density fluids (4.9–15.6 wt % NaCl equiv, T h between 345°–430°C). Secondary aqueous fluid inclusion assemblages in the phenocrysts have salinities of 0.2 to 2.2 wt percent NaCl equiv and T h of 200° to 280°C. Fluid inclusion assemblages in hydrothermal quartz from breccias and veins have salinities of 0.2 to 3.4 wt percent NaCl equiv and T h from 200° to 270°C. The oxygen isotope composition of several zones of an ore-related epithermal quartz crystal indicate a very constant δ 18 O of 4.5 to 5.0 per mil for the mineralizing fluid, despite significant salinity and temperature variation over time. Following microthermometry, selected fluid inclusion assemblages were analyzed by laser ablation-inductively coupled-plasma mass spectrometry (LA-ICPMS). Despite systematic differences in salinity between phenocryst-hosted fluids trapped at depth and fluids from quartz in the epithermal breccias, all fluids have overlapping major and trace cation ratios, including identical Na/K/Rb/Sr/Cs/Ba. Consistent with the constant near-magmatic oxygen isotope composition of the hydrothermal fluids, these data strongly indicate a common magmatic component of these chemically conservative solutes in all fluids. Cu, Pb, Zn, and Mn show variations in concentration relative to the relatively non-reactive alkalis, reflecting the precipitation of sulfide minerals together with Au in the epithermal breccia, and possibly of Cu in an inferred subjacent porphyry environment. The magmatic-hydrothermal processes responsible for epithermal Au-Ag mineralization at Rosia Montana are, however, not directly related to the formation of the spatially associated porphyry Cu-Au deposit of Rosia Poieni, which occurred about 3 m.y. later.