Pyrite Inclusions Research Articles

Occurrence, source and enrichment mechanism of silver in black shale-hosted Baiguoyuan Ag-V ore deposit, Hubei Province, China

The Baiguoyuan Ag-V ore deposit in Xingshan County, Hubei Province, is hosted in Neoproterozoic black shale, containing approximately 1860 tons Ag at average grade of 116g/t and 21.8 Mt. V at average grade of 1.5%. The occurrence, source and enrichment mechanism of silver in this deposit are still a topic of debate. In this study, we applied high-resolution electron microprobe analysis (EMPA) and LA-ICP-MS technique to determine the occurrence of silver and to infer its source and enrichment mechanism. Mineralography observation revealed that pyrite is the main metallic mineral in the ores, which shows three-stage formation histories: (1) sedimentation, (2) diagenesis, and (3) deformation. BSE imaging detected tiny naumannite inclusions in pyrite. EMPA showed high contents of Ag in sedimentary pyrite. LA-ICP-MS analysis displayed both solid solution and tiny naumannite inclusion of Ag in pyrite. A mass balance calculation suggested that Ag occurred mainly as solid solution and minor as tiny naumannite inclusions in pyrite. The high contents of Co and Se, and high ratios of Co/Ni and Se/S in sedimentary pyrite indicated that Ag was probably supplied by submarine hydrothermal venting. The correlation between Ag and Fe or Se in sedimentary pyrite indicated that Ag was enriched through Ag and Se replacing Fe and S in pyrite, respectively.

Magma mixing and gold mineralization in the Maevatanana gold deposit, Madagascar

The Maevatanana gold deposit in Madagascar is hosted by Archean metamorphic rocks in quartz–sulfide veins that are structurally controlled by NNW–SSE trending shear zones. Fluid inclusion data show that the trapping conditions in quartz range from 0.87 to 2.58kbar at temperatures of 269–362°C. Laser Raman spectroscopy confirms that these inclusions consist of CO2, SO2, and H2O. The δ34S values of the pyrites range from 1.7‰ to 3.6‰, with an average of 2.25‰, supporting a magmatic origin. Noble gases (He, Ne, Ar, Ke, Xe) are chemically inert, thus will not be involved in chemical reactions during geological processes. Also due to the low concentration of He in the atmosphere and the low solubility of He in aqueous fluids, the atmosphere-derived He is unlikely to significantly affect He abundances and isotopic ratios of crustal fluids, ensures that He production should have the typical crust 3He/4He ratios. The 3He/4He ratios of fluid inclusions in pyrite from the deposit range from 0.06 to 0.12Ra, while the 40Ar/36Ar ratios range from 6631 to 11441. We infer that the ore-forming fluids could have been exsolved from a granitic magma. The oxygen and hydrogen isotope compositions of the ore-forming fluids (1.5‰≤δ18OH2O≤7.8‰; –72‰≤δD≤–117‰) indicate they were derived from a granitic magma. Four pyrite samples from the gold deposit yield a precise Re–Os isochron age of 534±13Ma. Given that the post-collisional granites in northern and central Madagascar were derived by melting of sub-continental lithospheric mantle and formed between 537 and 522Ma, we can state that the gold metallogenesis was coeval with the crystallization age of these parental magmas. These data could be accounted for the formation of the Maevatanana gold deposit. First, the shear zones hosting the deposit formed around 2.5Ga, when the Madagascan micro-continental blocks collided with other continental blocks, triggering large-scale tectono-magmatic activity and forming NNW–SSE trending shear zones. The gold mineralization at Maevatanana is coeval with the crystallization age of the Cambrian post-collisional A-type granitoid plutons in northern and central Madagascar, implying that this deposit is associated with extensional collapse of the East African Orogen. This extension in turn induced asthenospheric upwelling, melting of sub-continental lithospheric mantle. These magmas underplated the lower crust, generating voluminous granitic magmas by partial melting of the lower crust. The mixing magma during tectono-thermal reactivation of the East African Orogen produced large volumes of volatiles that extracted gold from the granitic magma and produced Au–S complexes (e.g., Au(HSO3)2−). The shear zones, which were then placed under extensional collapse of the East African Orogen in the Cambrian, formed favorable pathways for the magmatic ore-forming fluids. These fluids then precipitated gold-sulfides that form the Maevatanana gold deposit.

Alteration paragenesis and the timing of mineralised quartz veins at the world-class Geita Hill gold deposit, Geita Greenstone Belt, Tanzania

The world-class Geita Hill deposit is one of the largest gold deposits located within the Geita Greenstone Belt in NW Tanzania. The deposit is hosted within a complexly deformed sedimentary package dominated by ironstone and intruded by diorite dykes. The gold mineralisation is spatially associated with the Geita Hill Shear Zone which, is a NE-trending, moderately NW dipping deformation zone consisting of a network of discontinuous shear fractures that record early thrusting overprinted by later strike-slip and normal events. The regional metamorphic assemblage in the meta-sediments is characterised by biotite+chlorite+actinolite+K-feldspar+magnetite±pyrrhotite±pyrite indicating upper greenschist facies conditions. The gold-related alteration overprints the regional metamorphic assemblage, and is characterised by silicification and sulfidation fronts that generally extend out from the mineralised zone by no more than one meter. The alteration assemblage includes sub-vertical, mineralised quartz veins that trend approximately E-W. The mineralised quartz veins are accompanied by alteration halos of quartz+biotite+K-feldspar+pyrite which overprints the peak metamorphic assemblage. Gold is closely associated with secondary pyrite and occurs as free gold and gold tellurides (sylvanite, calaverite and nagyagite). It occurs mainly as inclusions in pyrite and as invisible gold in pyrite but also as gold inclusions in biotite and along quartz grain boundaries. Two distinct textural styles of auriferous pyrite can be distinguished: inclusion rich subhedral pyrite, hosting invisible gold, and inclusion free euhedral pyrite, hosting visible gold grains. It is common for the inclusions rich pyrite to have thick rims of inclusion free pyrite. The mineralising alteration is overprinted by barren, multiphase quartz-carbonate, and carbonate-chlorite veins. This alteration is characterised by the assemblage calcite+siderite+chlorite±quartz±pyrite±barite. Palaeostress analysis of mineralised shear fractures along the Geita Hill Shear Zone are indicative of sigma 1 being vertical and sigma 3 trending N-S, indicating N-S extension, which is consistent with the orientation of the mineralised quartz veins.

Genesis of the Dianfang breccia-hosted gold deposit, western Henan Province, China: Constraints from geology, geochronology and geochemistry

The Dianfang gold deposit, located in Songxian, western Henan province, China, is one of two breccia-hosted gold deposits in the Xiong’ershan–Waifangshan region. Previous studies suggest that the breccia and host volcanic rocks of the Xiong’er Group formed coevally. The deposit consists of seven vein-type orebodies, occurring within the southern and hanging wall margins of the breccia pipe, as well as in fractures in the rhyolites of the Jidanping Formation, and are structurally controlled by ENE-trending faults. Their mineral assemblage includes K-feldspar, quartz, sericite, chlorite, epidote, carbonate, adularia, and sulfides formed from hydrothermal alteration related to mineralization. Pyrite is the dominant ore mineral, and is associated with minor galena, sphalerite, and chalcopyrite. The trace minerals include magnetite, rutile, cervelleite, matildite, and bismuthinite. Gold occurs as native gold, and electrum inclusions in pyrite, or along microfractures in sulfides and quartz. Four stages of hydrothermal mineralization have been identified: quartz–K-feldspar (I), quartz–pyrite (II), polymetallic sulfide (III), and carbonate–adularia (IV). Zircon U–Pb geochronology of the granitic matrix of the breccia constrains the timing of breccia pipe formation to 145.8Ma, indicating it formed at the end of the Late Jurassic, rather than the Paleoproterozoic, as suggested by previous studies. Hydrothermal sulfides from the main orebody yield a Rb–Sr isochron age of 121.5±1.7Ma, placing gold mineralization at the end of the Early Cretaceous. The sulfur and lead isotope data suggest that the ore-forming materials have a close connection with magmatic fluid. The initial 87Sr/86Sr value of the sulfides is 0.71056±0.00012, suggesting an ancient crustal source, likely the Archean Taihua Group, mixed with a juvenile mantle component. The trace element characteristics of pyrite indicate the mineralizing fluid was magmatic in origin, sourced from a magma derived from the partial melting of the Taihua Complex, and may have been affected by water–rock interaction at high temperatures under reducing conditions. Geological, geochronological, and geochemical data suggest the Dianfang ore deposit is genetically related to a hidden late Mesozoic granite, and forms part of an intrusion-related auriferous system. Gold deposits of similar ages are widespread at the southern margin of the North China Craton, and are coeval with the emplacement of felsic and mafic intrusions, exhumation of a metamorphic core complex, and formation of extensively faulted basins. This suggests that the Dianfang gold deposit formed under conditions of lithospheric extension caused by crustal thinning, and the destruction of the lithosphere mantle beneath the craton during the Late Jurassic to Early Cretaceous.

Dispersive and directional electrical conductivity and dielectric permittivity of conductive-mineral-bearing samples derived from multifrequency tensor electromagnetic induction measurements

Organic-rich mudrocks, hydrocarbon-bearing conventional formations, and source rocks generally contain pyrite, rutile, graphite, graphitic precursors, and other electrically conductive minerals in the form of veins, laminations, flakes, and grains. Under redox-inactive subsurface conditions, when an external electromagnetic (EM) field is applied to geomaterials containing conductive mineral inclusions, ions in pore-filling brine and charge carriers (electrons and holes) in electrically conductive mineral inclusions migrate, accumulate/deplete, and diffuse around impermeable host-inclusion interfaces. These EM-field-induced phenomena are referred to as perfectly polarized interfacial polarization (PPIP) phenomena, and they alter the effective electrical conductivity [Formula: see text] and effective relative dielectric permittivity [Formula: see text] of geomaterials. In addition, the relaxation process associated with such polarization phenomena and the time required to fully develop and dissipate the EM-field-induced polarization gives rise to frequency dispersion of [Formula: see text] and [Formula: see text] of geomaterials containing conductive mineral inclusions. A laboratory-based EM apparatus, referred to as a whole-core EM induction tool, was used to measure the directional, multifrequency EM response of brine-saturated 4 in diameter (10.16 cm diameter), 2 ft long (0.61 m long), glass-bead packs containing uniformly distributed pyrite and graphite inclusions. We then implemented a semianalytic (SA) EM forward model, referred to as the SA model, to compute the [Formula: see text] and [Formula: see text] of these conductive-mineral-bearing glass-bead packs. The estimated [Formula: see text] and [Formula: see text] of conductive-mineral-bearing packs exhibit directional and frequency dispersive characteristics, which can be explained using the theory of PPIP phenomena. Relative variations in [Formula: see text] and [Formula: see text] due to frequency dispersion were as large as [Formula: see text] and [Formula: see text], respectively, between the values estimated at 20 and 260 kHz. Computed values of [Formula: see text] of conductive-mineral-bearing packs were unusually large in the range of 103–106, whereas the corresponding values of [Formula: see text] exhibited strong dependence on volume content, size, and metallic nature of conductive mineral inclusions, brine salinity, and frequency. Furthermore, packs containing uniformly distributed pyrite and graphite inclusions exhibited conductivity and permittivity anisotropy in the range of one to two.

MINERAL COMPOSITION AND STRUCTURAL FEATURES OF JEWELER AMMONITES FROM KOSTROMA REGION

Detailed mineralogical examination of ammonites of jewelry quality has been carried out by means of radiographical, X-ray microanalyses and electronic microscopy. The ammonites consist mainly of calcite, apatite and some cells are constituted of phosphorite. The walls and shell’s septums consist of apatite and calcite, which have replaced original aragonite and maintain inclusions of pyrite of globular shape, aluminosilicate and organic matter. Dispersed apatite determines the black color of walls and shell’s septums. The color and transparency of the calcite, filling up the shell’s chambers, are determined by the quantity of micropores and microinclusions, mainly of apaTite. Ammonites from Kostroma region by its decorative and technology characteristics will be able to use widely in jewelry and interior articles.

Mineralogical and isotopic constraints on the genesis of the Jingchong Co–Cu polymetallic ore deposit in northeastern Hunan Province, South China

The Jingchong Co–Cu polymetallic deposit and a few other Co (–polymetallic) ore deposits are located in northeastern Hunan Province, in the central segment of the Jiangnan Orogen, South China. These deposits occur along the NE-trending Changping (Changsha–Pingjiang) deep fault zone and show a spatial association with the Late Jurassic–Early Cretaceous Lianyunshan granitoids, which intruded the Archean to Paleoproterozoic Lianyunshan Group comprising metaclastic-sedimentary rocks interlayered with mafic-intermediate igneous rocks. The Jingchong deposit is hosted within an altered fracture zone at the footwall of the Changping fault zone, and comprises an upper Pb–Zn ore zone and a lower Co–Cu ore zone. Wall rock alteration is dominated by silicification and chloritic alteration. Three types of ores were identified, i.e., quartz–sulfide veins, altered structural breccias and altered fractured rocks. Integrated field work and microscopy observations reveal a three-stage hydrothermal process related to mineralization: 1) quartz+chlorite (ChlI)+pyrite, 2) quartz+chlorite (ChlII)+pyrite+chalcopyrite+cobaltite, and 3) quartz+carbonate+galena+sphalerite±pyrite±chalcopyrite. LA-ICPMS analyses reveal that the Co contents are 1315–6473ppm, 76–3866ppm, and 5–435ppm for pyrite from the first-, the second-, and the third stages, respectively. EPMA analyses of chlorite show that ChlI and ChlII have Fe/(Fe+Mg) ratios of 0.65–0.80 and 0.76–0.92, and AlIV contents of 1.30–1.65 apfu (atom per formula unit) and 1.18–1.48 apfu, respectively. Empirical geothermometry of chlorite yielded temperatures of 256–318°C (av. 281°C) and 239–290°C (av. 256°C) for first- and second-stage chlorite, respectively. The δ34SVCDT values of sulfides (−4.9‰ to 0.2‰, av. −1.6‰) suggest a magmatic source for the sulfur, and the 3He/4He (0.017–0.30Ra) and 40Ar/36Ar (285–306) ratios of fluid inclusions in pyrite suggest that the ore-forming fluids were mainly crust-derived magmatic hydrothermal fluids with possible meteoric water inputs. The 206Pb/204Pb (18.042–18.746), 207Pb/204Pb (15.592–15.741) and 208Pb/204Pb (38.237–39.150) ratios of ore sulfides are similar to those of the Lianyunshan Group, suggesting that the metals may have been derived from the latter. Integrating the geological, mineralogical and geochemical data documented in this study, we propose that the Jingchong Co–Cu polymetallic deposit formed from a hydrothermal system related to the Lianyunshan pluton, which was derived from crustal anataxis of the Lianyunshan Group in a back-arc extensional setting in the Late Mesozoic.

Geology, mineralogy, geochemistry and δ34S of sedimentary rock-hosted Au deposits in Song Hien structure, NE Vietnam

The Song Hien rift basin is considered to be one of the most important regions of gold mineralisation in North East Vietnam. A number of gold deposits in the Song Hien rift basin are hosted in Triassic and Devonian sedimentary formations of the basin. The largest among them are the Bo Va, Tham Riem and Khung Khoang deposits. The Bo Va deposit is hosted in carbonaceous sedimentary rocks of Triassic age, whereas the Tham Riem and Khung Khoang deposits are hosted in carbonaceous sedimentary rocks of Devonian ages. Based on the mineral composition of the ores, the deposits can be divided into to two types: (i) pyrite dominated and (ii) pyrite-arsenopyrite dominated. The Khung Khoang is of the first type and the Bo Va and Tham Riem deposits belong to the second type. The isotopic composition of pyrite and arsenopyrite in the Tham Riem deposit however, is close to that for the ores of the Bo Va deposit. The δ34S value for pyrite ranging from −3.7‰ to −7.4‰ and for arsenopyrite ranging from −3.2‰ to 7.4‰. The δ34S of pyrite in the ore from the Khung Khoang deposit however, has a much heavier isotopic composition of +18.9 to +20.2‰. A narrow range of the variation of sulfur isotopic composition of pyrite and arsenopyrite, the presence of visible gold as inclusions, the presence of chalcopyrite, sphalerite and other inclusions in arsenopyrite and pyrite, the large size of the grains of major ore minerals allow us to assume that the primary gold ores of the Bo Va and Tham Riem deposits underwent metamorphic transformations. The absence of arsenic, antimony, mercury and other characteristic elements in the ores of the Khung Khoang deposit, and substantially heavier isotopic composition of sulfur similar to the sulfur isotopic composition of marine sulfates in the Devonian, allow us to assume another source of the ore components, not connected with the Triassic sedimentary rocks of the Song Hien rift.

Metallogeny of the Sarsuk polymetallic Au deposit in the Ashele Basin, Altay Orogenic Belt, Xinjiang, NW China: Constraints from mineralogy, fluid inclusions, and He–Ar isotopes

The Sarsuk polymetallic Au deposit is located in the Devonian volcano-sedimentary Ashele Basin of the Altay Orogenic Belt (AOB), Xinjiang. Mineralization in the deposit is confined to an area of 400m long and 50–120m wide, and occurs as dense massive, banded, disseminated, stockwork, and veinlet ore structures. Exhalative sedimentary, subvolcanic hydrothermal, and supergene periods have been distinguished. Although syn-depositional ores can be found, the main orebodies from this period have been eroded. Thus, the orebodies being mined are hosted mainly in a rhyolite porphyry subvolcanic intrusion within the volcanic conduit of a hydrothermal system. Electron microprobe analysis of the main ore minerals indicates that pyrite is the main Au-bearing mineral. The Au occurs mainly as either inclusions or in fissures in the form of tellurides, and is closely associated with Ag. Zn/Fe ratios of sphalerite vary from 11.37 to 251.36, with an average of 96.65, and the formation temperatures of chlorites are 155–206°C, with an average of 175°C. The compositions of sphalerite and chlorite indicate that the main mineralization occurred in a low- to moderate-temperature environment. Homogenization temperatures of fluid inclusions from ores of the main mineralization stage vary generally from 130°C to 390°C, with peaks at 140°C, 190°C, and 310°C. Their corresponding salinities range from 3.0 to 10.0wt.% NaCl equivalent, with densities from 0.56 to 1.03g/cm3. The main mineralizing fluid of the deposit is characterized by high–low temperatures, moderate–low salinities, and moderate–low densities. The 3He/4He ratios of fluid inclusions in pyrite are 0.02 to 0.44 Ra, and the percentage of mantle-derived He ranges from 0.06% to 7.45%. 40Ar/36Ar ratios range from 319.6 to 458.5, and the percentage of 40Ar* varies from 7.53% to 35.55%. Both fluid characteristics and He and Ar isotope compositions suggest that the ore-forming fluids were a mixture of magmatic fluid and seawater. The CO2-rich fluids in the ores suggest that boiling occurred. The mixing of a high-temperature and high-pressure magmatic fluid with deeply circulating seawater caused a decrease in temperature, and phase separation in the ore-forming fluids. Thus, the precipitation of the most economically important ore materials was the result of this mixing process.

ORE MINERALS FROM KUROKO TYPE DEPOSIT OF TOYA-TAKARADA MINE, HOKKAIDO, JAPAN

Toya-Takarada mine is Au- and Ag-rich Kuroko-type deposit located in Takarada, Toya-mura, southwest Hokkaido, Japan. The deposits were hosted in rhyolitic tuff and mudstone of Middle Miocene age. Ore samples of fine-grained black ore, vuggy black-yellow ore, granular vuggy black ore, quartz-sulfide ore and massive quartz-barite ore were studied to identify the ore minerals association in the Toya-Takarada mine. The ore minerals are dominated by sphalerite, galena, chalcopyrite and pyrite with fewer amounts of electrum, tetrahedrite-tennantite, and other sulfosalt minerals with secondary mineral of covellite.The quantitative chemical analysis of ore minerals by EPMA indicated that FeS contents in sphalerite is low (0.3-1.2 mol.%) in all kinds of ore samples. Small grains of electrum as inclusions in pyrite are identified in vuggy black-yellow ore with Ag content around 32-33 atm %.In general, the silver minerals in Kuroko-type deposits occurred mainly in the black and yellow ores zone dominantly composed of sphalerite, galena, pyrite, chalcopyrite and barite as a form of electrum and/or argentian tetrahedrite-tennantite series. Thus, the massive quartz-barite ore sample of Toya-Takarada mine are also contain some rare silver sulfosalt minerals such as proustite, Cu-rich pearceite, geocronite-jordanite and fizelyite. Those minerals were found together in association with sphalerite. It seems that sphalerite was crystallized first followed by proustite and Cu-rich pearceite, then geocronite-jordanite and fizelyite are crystallized later.Sphalerites from quartz-sulfide ore of Toya-Takarada contain some fluid inclusions and measured homogenization temperatures are in the range of 164-247°C (av. 208°C) with salinity ranging from 1.9 to 4.7 wt.% NaClequiv. (av. 3.9 wt.% NaClequiv.). The mineral assemblage, iron content in sphalerite and silver content in electrum were indicated that sulfur fugacity was slightly higher during ore mineralization in Toya-Takarada mine.

Phosphorite deposits on the Namibian shelf

Phosphorite deposits provide a source of fertilizer to feed an increasingly populated world and they provide a record of paleoceanographic changes in upwelling systems linked to climate. The Benguela Upwelling System (BUS) is among the most productive today and is associated with major phosphorite deposits exposed over an area of 24,700km2 on the Namibian shelf. Analyses of cores associated with recent offshore mineral exploration provide new insights into the age and origin of these phosphorite deposits. The deposit consists of coarsening upward muddy to gravelly pelletal phosphorite sand, up to several meters thick, on the middle to outer shelf (190 and 350m water depth) offshore of Lüderitz and Walvis Bay. Less extensive and less continuous deposits occur offshore of Walvis Bay as far north as the Kunene River mouth on the inner to middle shelf (50 and 250m water depth). Pelletal phosphorite sand (some concentrically banded) and concretionary phosphorite pebbles are the dominant grain types consisting of up to 90wt.% carbonate fluorapatite (francolite) cement and inclusions of organic matter, pyrite and terrigenous mud. Strontium isotope stratigraphy and foraminiferal biostratigraphy indicate that phosphogenesis was initiated in the latest Miocene but that most phosphorite formed in the Plio/Pleistocene during early burial diagenesis of organic-rich mud as it does today in the Holocene diatomaceous mud belt. The highly-condensed, coarsening-upward succession reflects increasingly high-amplitude Pleistocene sea-level fluctuations. Phosphorite formation correlates to terrestrial aridification as well as to marine proxies of intensified coastal upwelling in the 600m thick equivalent successions on the upper slope. Repeated phosphorite formation and reworking over Pleistocene glacial to interglacial cycles resulted in the economic concentration of phosphorite, with an estimated total resource of 7800milliontons of phosphate rock at an average grade of 19wt.% P2O5 on the Namibian shelf.

Origin and evolution of hydrothermal fluids in epithermal Pb-Zn-Cu ± Au ± Ag deposits at Koru and Tesbihdere mining districts, Çanakkale, Biga Peninsula, NW Turkey

The Koru and Tesbihdere mining districts in Biga Peninsula, Northwestern Turkey, consist of twelve deposits covering approximately 12km2. The epithermal Au-Ag enriched base metal veins and associated low-grade breccia and stockwork at Koru and Tesbihdere are hosted by Oligocene subaerial and calc-alkaline volcanic rocks including basaltic andesite lavas, dacitic lava-tuffs, rhyolitic lava-domes and tuffs. NW- to N-trending strike-slip faults and E- and NE-trending faults constitute the most important ore-controlling structures in the Koru and Tesbihdere districts respectively. In the Koru mining district, galena is the dominant ore mineral in barite-quartz veins containing sphalerite, chalcopyrite, pyrite, bornite, enargite and tennantite. According to base metal content, the Tesbihdere mining district can be subdivided into sphalerite-galena dominated Tesbihdere mineralization and chalcopyrite-pyrite dominated Bakır and Kuyu Zones mineralization. Gold is present in small quantities with maximum 3.14g/t Au values either as free grains in quartz or as micro inclusions in pyrite and galena. The most widespread silver minerals are polybasite, pearceite, argentite and native silver which commonly occur as replacements of galena, sphalerite and pyrite, and other sulfides, or as fillings of microfractures in sulfides and quartz.Microthermometric measurements of primary liquid-rich fluid inclusions in sphalerite, barite and quartz in Koru indicate that the veins were formed at temperatures between 407 and 146°C from fluids with salinities between 0.7 and 12.5wt.% equiv. NaCl. Barite from the Tahtalıkuyu, Kuyutaşı and 5th Viraj mineralization show the highest homogenization temperatures. Fluid inclusion data for ore-stage quartz and sphalerite from the Tesbihdere mining district, indicate that these minerals were deposited at temperatures between 387 and 232°C from more diluted fluids with moderate salinities between 0.2 and 10.6wt.% NaCl equiv. Tahtalıkuyu and 5th Viraj mineralization show only boiling trends while Kuyutaşı, Tesbihdere, Bakır and Kuyu Zones mineralization show both boiling and isothermal mixing trends. The O and H isotope compositions of ore fluids from the Tahtalıkuyu (δ18O=−1.40 to 0.25‰; δD=−72.49 to −52.68‰) and Kuyutaşı (δ18O=−2.29 to 3.59‰; δD=−90.70 to −70.93‰) mineralization indicate that there was a major contribution from a magmatic component to ore genesis. Based on 9 quartz samples associated with orebodies at the Tesbihdere mining district, the relatively higher δ18O and lower δD isotope compositions from hydrothermal fluids could be attributed to a relatively dilute fluid derived by the mixing with meteoric water. The Pb isotope compositions also reveal that most of the lead in both mining districts is derived from the Oligocene-Miocene magmatic rocks, possibly with smaller contributions from the Eocene magmatic rocks.

Granulite-Hosted Gold: Tectonic Setting and Lithogeochemistry of the Tropicana Deposit, Western Australia

The Tropicana gold deposit (7.9 Moz) is hosted by the Neoarchean, amphibolite- to granulite-grade Tropicana Gneiss. These rocks form part of the Plumridge terrane, a tectonic unit that separates the in situ eastern margin of the Yilgarn craton from Proterozoic elements of the Albany-Fraser orogen, Western Australia, The gneissic host package to the orebody originally constituted a suite of medium-to high-K calc-alkaline and adakitic basaltic to rhyolitic lavas emplaced in a relatively thick crustal continental margin arc. Amphibolitized mafic intrusions have back-arc basin basalt-like compositions consistent with their having formed during extension and incipient rifting of the arc. Gold mineralization occurs mainly within a sheet-like, syenitic gneiss that ranges compositionally from basaltic andesite to rhyolite, and derived through metasomatism of compositionally equivalent lavas within an interpreted shear zone. Coarse perthite textures in microcline, ruffle exsolution in brown biotite, and occasional pyrite inclusions away from fractures in microcline porphyroblasts indicate that this shear zone predated peak metamorphism. Relative to the unmineralized host gneisses, the mineralized syenitic gneisses show significant enrichments in S and the ore elements (Mo, Te, Tl, Ag, Au, W and, occasionally, Bi and Sb, but not As, Cu, or Zn), in the K-group elements, and also in P, light rare earth elements (LREE), Th, U, Zr, and Hf. Depletions in most major elements (Si, Ti, Fe, Mg, and Ca), as well as V, Cr, Ni, and Co, are considered to reflect dissolution of relatively low volume mafic phases (hornblende, augite, Fe-Ti oxides) and plagioclase in the protoliths. Some of the distinctive enrichments (P, LREE, Zr, Hf) are linked to volume loss (in excess of 50%) associated with dissolution of silica and concentration of insoluble apatite and zircon in the mineralized unit, but the ore elements and strongly enriched (similar to X10) Th and U were added from the hydrothermal fluids involved. Exhumation of the Tropicana Gneiss into the upper crust was accompanied by fluid ingress and retrogression. This produced mm-to cm-scale, wavy biotite-pyrite shears, solution seams, and disseminations throughout the syenitic gneiss which were further retrogressed to sericite-chlorite assemblages in places, both of which host most of the gold in the deposit. Thus, although the present deportment of gold is unambiguously associated with the postpeak metamorphic, greenschist-grade retrogression at ca. 2520 Ma, there is compelling evidence that the paucity of quartz and carbonate veining, atypical element enrichments, and sheet-like nature of the Tropicana deposit in part reflect metasomatism associated with a prepeak metamorphic, relatively deep shear zone. At present, we have no direct evidence for the timing of introduction of gold, and a protracted history of gold precipitation, redistribution, and enrichment is implied.

Origin and evolution of ore fluids in the late Mesozoic Naozhi epithermal Au–Cu deposit, Yanbian area, Northeast China: evidence from fluid inclusion and isotopic geochemistry

The Naozhi Au–Cu deposit is located on the continental margin of Northeast China, forming part of the West Pacific porphyry–epithermal gold–copper metallogenic belt. In this paper, we systematically analyzed the compositions, homogenization temperatures, and salinity of fluid inclusions as well as their noble gas isotopic and Pb isotopic compositions from the deposit. These new data show that (1) five types of fluid inclusions were identified as pure gas inclusions (V-type), pure liquid inclusions (L-type), gas–liquid two-phase inclusions (W-type, as the main fluid inclusions (FIs)), CO2-bearing inclusions (C-type), and daughter-mineral-bearing polyphase inclusions (S-type); (2) W-type FIs in quartz crystals of early, main, and late stage are homogenized at temperatures of 324.7–406.7, 230–338.8, and 154.6–308 °C, with salinities of 2.40–7.01 wt% NaCleq, 1.73–9.47 wt% NaCleq, and 6.29 wt% NaCleq, respectively. S-type FIs in quartz crystals of early stage are homogenized at temperatures of 328.6–400 °C, with salinities of 39.96–46.00 wt% NaCleq; (3) Raman analysis results reveal that the vapor compositions of early ore-forming fluids consisted of CO2 and H2O, with H2O gradually increasing and CO2 being absent at the late mineralization stage; (4) fluid inclusions in pyrite and chalcopyrite have 3He/4He ratios of 0.03–0.104 Ra, 20Ne/22Ne ratios of 9.817–9.960, and 40Ar/36Ar ratios of 324–349. These results indicate that the percentage of radiogenic 40Ar* in fluid inclusions varies from 8.8 to 15.5 %, containing 84.5–91.2 % atmospheric 40Ar; (5) the 206Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb ratios of sulfides are 18.1822–18.3979, 15.5215–15.5998, and 38.1313–38.3786, respectively. These data combined with stable isotope data and the chronology of diagenesis and metallogenesis enable us suppose that the ore-forming fluids originated from the melting of the lower crust, caused by the subduction of an oceanic slab, whereas the mineralized fluids were exsolved from the late crystallization stage and subsequently contaminated by crustal materials/fluids during ascent, including meteoric water, and the mineral precipitation occurred at a shallow crustal level.

Fluid evolution and ore genesis of the giant Sanshandao gold deposit, Jiaodong gold province, China: Constrains from geology, fluid inclusions and H–O–S–He–Ar isotopic compositions

The Sanshandao gold deposit is located at the northwestern edge of the Jiaodong peninsula, North China Craton. It is mainly hosted in the Mesozoic granitoids. Gold mineralization occurs as disseminated- and stockwork-style ores, typically enveloped by broad alteration selvages. Mineralization and alterations are structurally controlled by the Sanshandao–Cangshang fault. Three paragenetic stages were identified with the early stage represented by quartz±pyrite, the middle stage by gold+quartz+pyrite or gold+quartz+base metal sulfides, and the late stage by quartz+carbonate±pyrite, respectively. Four types of fluid inclusions were distinguished, including (1) pure CO2 fluid inclusions (type I), (2) H2O–CO2–NaCl fluid inclusions (type II), (3) H2O–NaCl fluid inclusions (type III), and (4) daughter mineral-bearing or multiphase fluid inclusions (type IV). The early- and middle-stage quartz mainly contain primary type II fluid inclusions that completely homogenized at temperatures of 241–390°C with salinities of 2.96–18.39wt.% NaCl equivalent, and temperatures of 207–336°C with salinities of 2.06–17.57wt.% NaCl equivalent, respectively. A few primary type I fluid inclusions can be observed in the early-stage quartz, while the middle-stage quartz contains a few primary type I, type III and type IV fluid inclusions. In contrast, the late-stage quartz only contains type III fluid inclusions showing homogenization temperatures of 101–268°C, and salinities of 0.17–15.47wt.% NaCl equivalent. CH4, derived from extensive water–rock interaction between the ore-forming fluids and wall rocks in the main fault, is the key factor controlling the gold mineralization. CH4 changed the physico-chemical conditions of the ore fluids, and led to gold precipitation.Oxygen and hydrogen stable isotopic analyses for quartz from different ore-forming stages show that the ore-forming fluids are of magmatic origin, with gradual incorporation of meteoric water during/after mineralization. δ34S values of pyrite from different ore-forming stages vary between 1.9‰ and 11.9‰. 3He/4He ratios of fluid inclusions in the middle-stage pyrite mainly range from 0.14 to 2.94Ra, while it is from 675.10 to 5926.44 for 40Ar/36Ar. All these isotopic signatures suggest that the ore fluids could be derived from a combination of crustal and mantle components.

ASTER spectral analysis for alteration minerals associated with gold mineralization

This study presents classifications of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery based on spectral analysis of alteration minerals associated with gold mineralization in Abo Marawat area which is located in the Central Eastern Desert of Egypt. Spectral analysis of continuum removed reference spectra of alteration minerals alunite, kaolinite, sericite, and calcite from USGS and JPL spectral libraries show shifts in position, shape, and strength most probably related to changes in sample purity and particle size of analyzed samples. Spectral Information Divergence (SID) classification method proved successful for mapping sericite, calcite, and clay minerals. Spectral Angle Mapper (SAM) classification identified only sericite and calcite alteration minerals. The identified alteration zones are coincidence with the field sampling and geological map of the study area. The microscopic examination of samples collected from the quartz veins and hydrothermally altered wall-rocks from near surface and subsurface at Abu Marawat gold mine shows sericitization, argillaceous, and carbonatization alteration zones. Gold occurs as very fine inclusions in pyrite, chalcopyrite, and sphalerite and also found filling the fractures between chalcopyrite grains. This study concludes that ASTER image classifications using reference spectra are a stable and reproducible technique for mapping gold related hydrothermal alteration zones in areas with no dominant vegetation cover.

Local and Regional Mass Transfer During Thrusting, Veining, and Boudinage in the Genesis of the Giant Shale-Hosted Paracatu Gold Deposit, Minas Gerais, Brazil

Multiscale structural and geochemical studies have been applied to understand the genesis of the Paracatu deposit in Brazil, a shallow-dipping, bulk-tonnage, vein-style low-grade Au(-Ag-Pb-Zn) orebody hosted in ~1000 Ma black phyllites of the Paracatu Formation, associated with intense shearing accompanying thrusting during the ~680 Ma Brasiliano orogeny. Massive to laminated quartz-sulfide-carbonate veins and associated alteration and Au mineralization formed early in the deformation history, and these veins were boudinaged during subsequent progressive increase in shear strain. Geochemical profiles across the ore allow recognition of a relatively homogeneous protolith at 100-m scales with respect to Ti, Al, Zr, V, and rare earth elements, including a footwall with protolith attributes similar to those of the ore. Oxygen, hydrogen, and sulfur isotopes do not reveal a distinctive external fluid source; rather, they reflect fluid-rock equilibration with the host phyllites during greenschist facies regional metamorphism. Detailed geochemical sampling around a prominent population of smaller boudinaged veins shows that Si, Ca, and Sr were removed and Al, K, and Ti were residually concentrated during formation of synboudinage dark selvages. This process of mass transfer occurred at scales more local than the processes responsible for ore genesis. Despite the local processes of chemical equilibration and redistribution, many of the main ore components appear externally derived at scales broader than the orebody. Gold, As, Ag, Sb, Bi and, to some extent, Pb and Zn show a strong spatial and statistical correlation with boudinaged quartz-sulfide ± carbonate veins, and these are surrounded by a broader (10–50-m vertical scale) halo of enriched K, Ba, and volatiles (loss on ignition), with depleted Na and Sr. The majority of gold is present as inclusions in pyrite and arsenopyrite within and immediately surrounding the veins, and gold-bearing pyrite shows mineral chemistry consistent with a hydrothermal origin. The main population of large veins locally preserve preboudinage, complex internal vein textures, including sulfide-rich wall-rock laminae similar to laminated fault-fill veins in many orogenic-style vein deposits. These larger veins contain most of the gold in the Paracatu resource. Metals were most likely sourced distal to the Paracatu Formation and were precipitated in sheeted shear or tensile veins during the early stages of the Brasiliano thrusting. The oblate strain recorded by the boudinaged, mineralized veins is anomalous and distinctive to Paracatu at km scales, even though the thrusting is regional. Although the orebody may have been physically dislocated by thrusting from the place where the early veins formed, the anomalous strains recorded suggest a perturbation in the footwall to the regional thrust or, perhaps, a spatially restricted competency contrast within the stratigraphy, which contributed to the localization of the present mineralization during the thrusting.

Two fluid sources and genetic implications for the Hatu gold deposit, Xinjiang, China

The Hatu gold deposit is located in the West Junggar domain, Xinjiang, China. It contains 56tonnes of gold. The deposit is hosted in the volcanic and sedimentary rocks of the Early Carboniferous Tailegula Group, and an ~E–W trending ductile shear zone controls the mineralization. The gold mineralization is hosted in quartz veins and hydrothermally altered country rocks with <3vol.% sulfides developed along the faults. The fluid inclusions are characterized by the presence of dominant CO2 and minor CH4 fluid. The δ13CVPDB values of CH4 (−18.7‰ to −35.4‰) and CO2 (−9.7‰ to −13.9‰) in fluid inclusions from quartz and their ∆13CCO2–CH4 values (5.5‰ to 24.6‰) equilibrated at a temperature range from 299°C to 1013°C; these data indicate that the carbon had a dominantly sedimentary source with a minor contribution from the mantle. The oxygen and hydrogen isotope compositions (δ18O=8.5‰ to 12.2‰, δD=−87‰ to −105‰) of water from fluid inclusions in quartz indicate that ore fluids comprised largely metamorphic water with a minor contribution of magmatic water. The δ34S values of sulfide from the mineral zones are between −2.3‰ and +2.5‰, with the exception of two values (−9.7‰ and +7.4‰), showing a heterogeneous sulfur source. Fluid inclusions in pyrite and arsenopyrite that are intergrown with auriferous quartz have 3He/4He ratios of 0.2 to 0.3 Ra, and their 40Ar/36Ar ratios range from 310 to 650, indicating a crustal source for much of the ore fluids. We propose that the Hatu deposit is a structurally controlled orogenic gold system which is characterized by two different fluids one dominantly derived from a crustal source with a minor component of mantle derived fluids.

PGE distribution in the Chromite bearing mafic-ultramafic Kondapalli Layered Complex, Krishna district, Andhra Pradesh, India

Abstract The Kondapalli Layered Complex (KLC) is a dismembered mafic-ultramafic layered intrusion, mainly composed of gabbroic and anorthositic rocks with subordinate ultramafics and chromitite. Chromitite occurs as lenses, pods, bands and disseminations. Platinum group of minerals (PGMs) occur as inclusions within chromite and silicates. The study indicates an inhomogeneous distribution of PGMs and distinct dominance of IPGEs over the PPGEs. The average ΣPGE content of chromite of KLC varies from 64 ppb to 576 ppb with Pt ranging from 5 to 495 ppb, Pd 5 to 191 ppb, Ir 3 to 106 ppb, Ru 3 to 376 ppb and Rh 3 to 135 ppb. The PGMs identified in the KLC indicate primary deposition of the IPGE, preceding chromite, indicating its orthomagmatic nature. Most of the PGM grains are usually below 10 μm. The identified PGMs are Laurite (RuS2), irarsite (Ir, As, S), iridosmine (Os, Ir), undetermined Os-Ir sulphide and Ru-Os-Ir-Zn alloys. Chromite also contains inclusions of pentlandite, millerite, chalcopyrite and pyrite. Study indicating that the KLC have orthomagmatic origin for PGE which are dominated by IPGE group and formed under surpa-subduction zone peridotite setting.

Copper–zinc albite porphyry in the Hersai porphyry copper deposit, East Junggar, China: A transition between late magmatic and hydrothermal porphyry copper deposit

A 418Ma albite porphyry dike emplaced in a 429Ma granite pluton in the Hersai porphyry copper deposit (Nom, East Junggar, China) consists of euhedral and pale-orange luminescing albite phenocrysts and groundmass, zircons, blue-violet luminescing apatites and chalcopyrite–sphalerite-rich miarolitic cavities. This dike is characterized by two-domain textures presented by albites and disseminated intergranular miarolitic cavities, and by geochemical features of continental island arc and adakitic intrusions.The miarolitic cavities consist of various minerals that crystallized in the following sequence: epidote, rutile and sphalerite–chalcopyrite solid solution, titanite and K-feldspar; chalcopyrite with pyrite inclusions; and chlorite, zoisite and calcite. The miarolitic cavities exhibit magmatic to hydrothermal internal textures, such as aplitic textures presented by intergrowths of albite, epidote, rutile and sphalerite–chalcopyrite solid solution, zoning and intersertal texture, poikilitic textures, and layered chlorites. The sphalerite contains a high concentration of exsolved chalcopyrite (up to 10 vol.%). The rutiles intergrown with epidotes contain 137 to 528ppm Zr corresponding to Zr-in-rutile temperatures of 507 to 625°C, and the rutiles intergrown with sphalerite–chalcopyrite solid solutions have 381 to 420ppm Zr and Zr-in-rutile temperatures of 585 to 605°C. Mineralization temperatures of sphalerite–chalcopyrite solid solutions are possible approximately 600°C. The formation of the chalcopyrite–sphalerite-rich miarolitic cavities is related to interstitial residual sulfide-rich fluids. The copper–zinc albite porphyry may represent a copper–zinc (Cu–Zn) subtype of porphyry copper deposit that formed as a transition between late magmatic and classical hydrothermal porphyry copper deposits.