Pyrite Inclusions Research Articles

Phenomenon of the Evolution of Hydrothermal Fluids of Mineral Formation at the Archean-Proterozoic Boundary

Based on isotope-geochemical study of mineral-forming fluid of the gold and uranium deposits of two megablocks of the Ukrainian shield, a significant difference between the hydrothermal systems of the Archean and Proterozoic (3042—1750 million years) has been established. The study included the experimental determination of the hydrogen, oxygen and carbon isotopic composition, and the ratio of water and carbon dioxide of fluids of gas—liquid inclusions in quartz, pyrite and feldspars.

Hydrothermal evolution and ore genesis of the Hongshi copper deposit in the East Tianshan Orogenic Belt, Xinjiang, NW China: Constraints from ore geology, fluid inclusion geochemistry and H–O–S–He–Ar isotopes

The Hongshi lode-type copper deposit is located in the Kalatag region in the East Tianshan, Xinjiang, NW China. The lode ore bodies are hosted in the Late Ordovician to Early Silurian Daliugou Formation, and the Cu mineralization is closely associated with andesite. Minor amounts of intrusive rocks, such as the quartz porphyry, are also exposed. The country rocks have undergone extensive silicification, chlorite alteration and sericite alteration. The metallic minerals are pyrite and chalcopyrite, with minor magnetite, sphalerite and galena. The gangue minerals are quartz, sericite, epidote, calcite and chlorite. The mineralized veins in the Hongshi deposit can be divided into: an early quartz–pyrite vein stage, the main quartz–chalcopyrite–pyrite ± sphalerite vein stage, and a late quartz–calcite–chlorite vein stage. Fluid inclusion petrography shows that two-phase liquid-rich (type I), two-phase vapor-rich (type II), and daughter-bearing multiphase (type III) fluid inclusions are present in the hydrothermal quartz-sulfide veins in the Hongshi deposit. The fluid inclusions in the early-stage minerals are mainly type I and type II with minor type III. Their homogenization temperatures and salinities range from 198 °C to 315 °C (with a peak value of 253 °C) and from 1.40 to 8.81 wt% NaCl equiv., respectively. The coexistence of type II and type III inclusions and their similar homogenization temperatures but contrasting salinities suggest that fluid boiling processes occurred in the early stage. The main-stage minerals feature dominantly type I and type II inclusions, with homogenization temperatures and salinities of 168 °C–280 °C (with a peak value of 209 °C) and 1.40–6.16 wt% NaCl equiv., respectively. The late-stage minerals feature dominantly type I and type II inclusions, with homogenization temperatures and salinities of 109 °C–179 °C (with a peak value of 157 °C) and 0.53–3.23 wt% NaCl equiv., respectively. The initial ore-forming fluids of the Hongshi deposit were therefore associated with an H2O–NaCl system with a near-neutral pH, low–intermediate temperatures and low salinities. The oxygen and hydrogen stable isotopic analyses of FIs in quartz from the different ore-forming stages show that the ore-forming fluids were of volcanic origin, with gradually increasing proportions of meteoric water during/after mineralization. The δ34S values of chalcopyrite and pyrite from the main ore-forming stage vary between 1.9% and 11.9%, suggesting a volcanic source. The 3He/4He ratios of fluid inclusions in the main-stage pyrite mainly range from 0.12 to 1.40 Ra, and the 40Ar/36Ar values vary from 310 to 337. All these isotopic characteristics suggest that the ore fluids were likely derived from a combination of crustal and mantle components. Combining the integrated analysis of ore deposit geology, the fluid evolutionary process, and H–O–S–He–Ar isotopes, we suggest that the Hongshi deposit should be classified as a volcanic–subvolcanic hydrothermal copper deposit. Processes including local fluid unmixing, decreasing temperature and fluid mixing may have been responsible for the hydrothermal alteration and Cu precipitation.

Mineral composition and gemological characteristics of jewelry-ornamental ammonites of the Republic of Adygea

For the first time, data on the mineral composition and gemological characteristics of the jewelry-ornamental ammonites of the Republic of Adygea have been presented. Ammonites consist mainly of the calcite with inclusions of quartz, glauconite, kaolinite, aragonite, pyrite, apatite and hematite. The walls and partitions of the shells mainly lost the original aragonite composition and consist of calcite with inclusions of pyrite and apatite. Aragonite is preserved fragmentary in the walls and partitions of the shells. Calcite contains some elements-admixture, which average volumes, (wt. %), are: Mg0,62, Mn0,31 and Fe 1,26. The color and transparency of calcite is determined by the structural features of the crystals and mineral inclusions. Ammonites of the Republic of Adygea have a wide range of sizes and are associated with sediments of the lower Cretaceous. According to the decorative and technological characteristics ammonites can be used as a jewelry-ornamental material for the production of a wide range of ware.

Fluid inclusion and stable isotope (O, H, C) constraints on the genesis of the Pedra Branca gold deposit, Troia Massif, Borborema Province, NE Brazil: An example of hypozonal orogenic gold mineralization

At the Archean-Paleoproterozoic Troia Massif, NE Brazil, two major Paleoproterozoic greenstone belts are documented (Algodões and Serra das Pipocas). They share similar lithostratigraphic characteristics with other 2.2–2.1 Ga greenstone belts of the surrounding cratonic domains (e.g. Guiana shield and West Africa craton). Gold mineralization in the Serra das Pipocas greenstone belt is associated with a regional NE-trending shear zone. In the mineralized area (the Pedra Branca gold deposit) the main gold mineralization is found in association with quartz veins, calc–silicate alteration (e.g. diopside, K-feldspar, amphibole, titanite, biotite, pyrite, albite, magnetite ± carbonates) and albitization. Pyrrhotite is commonly found as inclusions in pyrite, and late magnetite replaces pyrite, suggesting progressive oxidation of the ore–forming fluid. Free-milling gold commonly precipitates at the late stages of alteration, in close association with magnetite and tellurides. The main tellurides found are hessite, altaite, sylvanite, calaverite and tellurobismuthite. Two fluid inclusion assemblages were identified from mineralized quartz veins. Assemblage 1 is characterized by pseudo-secondary trails that show the coexistence of CO2–rich and low salinity (0–8 wt% NaCl equiv.) CO2–H2O–NaCl and H2O–NaCl inclusions, suggesting formation during phase separation (fluid immiscibility). Assemblage 2 is represented by late secondary low–temperature (Th < 200 °C) H2O–NaCl inclusions, probably unrelated to gold mineralization. The δ18O, δD and δ13C values of hydrothermal minerals (quartz, calcite, biotite, hornblende and magnetite) and fluid inclusions are compatible with a magmatic–hydrothermal ore-forming fluid that underwent fluid–rock interaction with the greenstone sequence. The calculated fluid δ18O values range from +8.3 to +11.0‰ (n = 59), fluid δD from −98 to –32‰ (n = 24) and δ13C values of calcite from −6.35 to −9.40‰ (n = 3). Oxygen isotope thermometry for quartz–magnetite pairs yields temperatures from 467 to 526 °C (n = 7, average 503 °C). This probably represents the temperature of gold deposition, because gold is generally associated with magnetite in the ore mineral assemblage. The estimated PT conditions of the gold mineralization range from 467 °C and 1.93 kbar (7 km) to 526 °C and 3.48 kbar (13 km), repectively, and they were obtained from the intersection of isochores with the temperature range derived from the quartz–magnetite isotope geothermometer. This PT range is akin to hypozonal orogenic gold deposits. The association of gold with magnetite and tellurides implies an ore–forming fluid derived from oxidized magmas, similar to those interpreted as ‘orogenic oxidized intrusion-related gold deposits’ in other Precambrian greenstone belts (e.g. Abitibi and Eastern Goldfields).

Noble and Rare Metals in Some Coal Deposits of Kazakhstan

The concentrations of the impurities in brown coals some of Kazakhstan deposits — «Kulan», «Shoptikol’», «Oi Karogai», «Sarykum» — were determined. Surface inclusions in coals were studied using optical USB microscopes. The composition of inclusions and total composition of coals were analyzed by both X-ray spectrometry with energy dispersion and atomic emission spectrometry using «Grand-Potok» complex with sample introduction by the spill-injection method into an arc discharge and recording spectra with an analyzer MAÉS. Gold was determined on a «Grand-Potok» complex using scintillation method. The coals of «Oi Karogai», «Sarykum», and «Kulan» deposits exhibited inclusions of pyrite in crystalline form. It was shown, that both gold and silver are present only in the coals of the «Shoptikol’» deposit in the form of flat round inclusions. Inclusions formed particles of gold-silver alloy upon melting according to data of scintillation analysis. The average concentrations of gold and silver in the coal samples of the «Shoptikol’» deposit are 0.007 and 0.03 ppm, respectively. It is shown that scintillation determination of precious metals in brown coals of Kazakhstan on a «Grand-Potok» complex with MAÉS analyzer is more rapid compared to the methods of inductively-coupled plasma-mass spectrometry (ICP-MS), atomic emission spectrometry (AES) and atomic absorption spectrometry (AAS) at comparable detection limits.

Cassiterite and Rutile as Indicator Minerals for Exploring the VMS System

Cassiterite and rutile strongly resist physical and chemical weathering and can be used as indicator minerals for mineralisation. These minerals are residually concentrated in a near-surface silcrete duricrust capping a deeply weathered profile developed over the Scuddles Cu-Zn-Pb VMS deposit, Golden Grove, Western Australia. These minerals were extracted by panning of rock chips from Reverse Circulation drill holes and studied by optical, scanning electron microscopy and mapped and analysed by electron probe. The formation of silcrete involved destruction of all labile minerals, removal of clays and leaching of alkali, alkaline earth and transition (e.g. Fe, Mn, V, Ni, Co, Cu, Zn) elements. This is followed by residual concentration of high field strength elements in resistate minerals and silica cementation. The main driving force in the formation of silcrete is the oxidation of massive sulphides generating strongly acidic and highly saline fluids. Under these conditions, silica was probably transported as either silicic acid from destabilising aluminosilicates or as an aqueous sol under neutral to low pH conditions. In addition, Au and Ag were remobilised from the massive sulphide mineralisation as a halide complex. Cassiterite, rutile and quartz in silcrete contain inclusions of pyrite, chalcopyrite, sphalerite, galena and argentite identical to those in the underlying VMS. Thus, they are considered as indicator minerals for VMS exploration. Bioturbation and organic acids produced by decay of plants from the upper lateritic profile remobilised clays downward as colloid, recycled Au from Ag halides and attacked resistant heavy minerals. Oxidation of sulphide inclusions in cassiterite, rutile and quartz intensely corroded these minerals and liberated trace elements (i.e., Sn, Ti, Pb, Sb, Bi, Hg, Se, Ge, W, Mo and Te) to form cements of complex chemical compositions. These cements and Au-Ag halides form a multi-element anomaly in silcrete delineating the underlying VMS deposits at Scuddles.

Reactive fronts in chemically heterogeneous porous media: Experimental and modeling investigation of pyrite oxidation

The spatial distribution of reactive minerals in subsurface porous media is an important control for groundwater quality. In this study we investigate pyrite oxidation reactive fronts in chemically heterogeneous porous media by combining laboratory experiments and reactive transport modeling. We performed experiments in different setups including batch, 1-D column, and 2-D flow-through systems. The flow-through experiments were performed in physically homogenous but chemically heterogeneous domains with embedded reactive pyrite inclusions at different spatial locations and with different concentrations. The setups were initially maintained under anoxic conditions and subsequently flushed with an inflowing oxic solution. A non-invasive optode technique was used for high-resolution monitoring of oxygen. This allowed us to capture the dynamics of the reactive oxygen fronts in the 1-D columns and in the 2-D flow-through chamber. Water quality analyses of the products of pyrite oxidation, iron (Fe) and sulfur (S), were also carried out in the different setups. The concentration of these species released in the 1-D columns (up to 6.2 × 10−5 mol/L Fe and 13.7 × 10−5 mol/L S) and in the 2-D setup (up to 3 × 10−6 mol/L Fe and 9 × 10−6 mol/L S) could be quantitatively related to the consumption of oxygen (up to 1.9 × 10−4 mol/L consumed in the 1-D and 2-D setups). The reaction rates were found to be different between the setups and dependent on the spatial location and concentration of the pyrite inclusions.A modeling approach coupling 1-D and 2-D transport codes with the geochemical simulator PHREEQC is proposed to simulate the spatial and temporal dynamics of oxygen transport, the kinetics of pyrite oxidative dissolution, and the changes in water quality in the chemically heterogeneous flow-through setups. The model allowed the quantitative interpretation of the experimental results and represents a valuable tool to capture the coupling between multidimensional transport and geochemical reactions both in laboratory and in field scale applications.

Chemical Forms of Mercury in Pyrite: Implications for Predicting Mercury Releases in Acid Mine Drainage Settings.

Pyrite (cubic FeS2) is the most abundant metal sulfide in nature and also the main host mineral of toxic mercury (Hg). Release of mercury in acid mine drainage resulting from the oxidative dissolution of pyrite in coal and ore and rock resulting from mining, processing, waste management, reclamation, and large construction activities is an ongoing environmental challenge. The fate of mercury depends on its chemical forms at the point source, which in turn depends on how it occurs in pyrite. Here, we show that pyrite in coal, sedimentary rocks, and hydrothermal ore deposits can host varying structural forms of Hg which can be identified with high energy-resolution XANES (HR-XANES) spectroscopy. Nominally divalent Hg is incorporated at the Fe site in pyrite from coal and at a marcasite-type Fe site in pyrite from sedimentary rocks. Distinction of the two Hg bonding environments offers a mean to detect microscopic marcasite inclusions (orthorhombic FeS2) in bulk pyrite. In epigenetic pyrite from Carlin-type Au deposit, up to 55 ± 6 at. % of the total Hg occurs as metacinnabar nanoparticles (β-HgSNP), with the remainder being substitutional at the Fe site. Pyritic mercury from Idrija-type Hg deposit (α-HgS ore) is partly divalent and substitutional and partly reduced into elemental form (liquid). Divalent mercury ions, mercury sulfide nanoparticles, and elemental mercury released by the oxidation of pyrite in acid mine drainage settings would have different environmental pathways. Our results could find important applications for designing control strategies of mercury released to land and water in mine-impacted watersheds.

Ore‐forming physicochemical conditions of tellurium–gold deposits: A case study from the Guilaizhuang deposit, eastern North China

Tellurium–gold deposits differ from normal gold mineralization in their significant content of tellurium‐bearing minerals with a close relation between Au and Te. The Guilaizhuang gold deposit is a major tellurium–gold deposit in the Shandong Province of China, and is located in the eastern margin of the North China Craton. The main sulfide minerals in this deposit are pyrite, As‐bearing pyrite, arsenopyrite, chalcopyrite, galena, sphalerite, chalcocite, and argentite. Tellurides contain native tellurium, altaite, coloradoite, petzite, and calaverite. Here, we investigate the fluid characteristics based on fluid inclusion studies and evaluate the intensive variables through thermodynamic computations to evaluate the ore genesis. We identify four stages for the mineralization: quartz‐pyrite stage, gold‐polymetallic sulfide stage, gold‐telluride stage, and carbonate stage. The gold‐telluride stage represents the main stage of gold precipitation. In gold‐polymetallic sulfide stage, Au formed within pyrite and As‐bearing pyrite in the form of invisible gold, whereas in the gold‐telluride stage, Au occurs as visible gold, such as native gold, electrum, calaverite, and petzite. Our data show that the mineralizing fluids were partly acidic and reduced during the gold‐polymetallic sulfide stage. Gold was transported as AuCl2− and Au(HS)2− complexes which was reduced by Fe2+. Gold might also occur as submicroscopic Au0 inclusions in pyrite and As‐bearing pyrite. The acidic state of the fluids favoured Au leaching from the surrounding rocks. During the gold‐telluride stage, gold might have migrated mostly as Au(HTe)2− and the Te combined at first with Ag to form hessite and subsequently with Au and Ag to form petzite and calaverite. With the decrease in Te content and increase in Au saturation, tellurium and native gold were precipitated. Our study provides insights into the physicochemical conditions of the formation of Te–Au deposits.

GEMMOLOGICAL CHARACTERISTICS OF SENGILIT — A NEW KIND OF JEWELRY-ORNAMENTAL RAW MATERIALS OF THE ULYANOVSK REGION

Sengilite is a trade mark of florogenic jewelry and ornamental material being mined around the village Sengilei of the Ulyanovsk region. The sangilite is a lithofected marine plant remains cemented in marl. The sangilite mainly consists of calcite (from 86 to 94 wt. %). The following minerals have been established (wt. %): pyrite 2,5—6, quartz 2, montmorillonite 3, dolomite 2, in minor quantities illite, kaolinite, jarosite, gypsum, chalcopyrite can be presented. Among the elements-admixture, Sr is presented up to 0,053 wt. % and Ba up to 0,023 wt. %. The content of radioactive and carcinogenic elements is at the background level. The original structure of plant remains is inherited in sengilite. The differently colored zones of sengilite are determined by the size and structural features of the calcite crystals, as well as the amount of pyrite inclusions, and the presence of marl. In the calcite, the following elements-admixture, (wt. %) are noted: Mg from 0,68 to 1,29, Fe from 0,14 to 0,52, insignificant amounts of Mn. According to its technological and decorative characteristics, sengilite is a new high-quality jewelry and ornamental material that allows to produce a wide range of jewelry and souvenirs. Collection and processing of sengilit in the Ulyanovsk region along with ammonites and other types of jewelry and ornamental raw materials will increase the profitability of the deposit and the range of products.

Three-D Mineralogical Mapping of the Kovdor Phoscorite-Carbonatite Complex, NW Russia: II. Sulfides

The world largest phoscorite-carbonatite complexes of the Kovdor (Russia) and Palabora (South Africa) alkaline-ultrabasic massifs have comparable composition, structure and metallogenic specialization, and can be considered close relatives. Distribution of rock-forming sulfides within the Kovdor phoscorite-carbonatite complex reflects gradual concentric zonation of the pipe: pyrrhotite with exsolution inclusions of pentlandite in marginal (apatite)-forsterite phoscorite, pyrrhotite with exsolution inclusions of cobaltpentlandite in intermediate low-carbonate magnetite-rich phoscorite and chalcopyrite (±pyrrhotite with exsolution inclusions of cobaltpentlandite) in axial carbonate-rich phoscorite and phoscorite-related carbonatite. Chalcopyrite (with relicts of earlier bornite and exsolution inclusions of cubanite and mackinawite) predominates in the axial carbonate-bearing phoscorite and carbonatite, where it crystallizes around grains of pyrrhotite (with inclusions of pentlandite-cobaltpentlandite and pyrite), and both of these minerals contain exsolution inclusions of sphalerite. In natural sequence of the Kovdor rocks, iron content in pyrrhotite gradually increases from Fe7S8 (pyrrhotite-4C, Imm2) to Fe9S10 (pyrrhotite-5C, C2 and P21) and Fe11S12 (pyrrhotite-6C) due to gradual decrease of crystallization temperature and oxygen fugacity. Low-temperature pyrrhotite 2C (troilite) occurs as lens-like exsolition inclusions in grains of pyrrhotite-4C (in marginal phoscorite) and pyrrhotite-5C (in axial phoscorite-related carbonatite). Within the phoscorite-carbonatite complex, Co content in pyrrhotite gradually increases from host silicate rocks and marginal forsterite-dominant phoscorite to axial carbonate-rich phoscorite and carbonatite at the expense of Ni and Fe. Probably, this dependence reflects a gradually decreasing temperature of the primary monosulfide solid solutions crystallization from the pipe margin toward its axis. The Kovdor and Loolekop phoscorite-carbonatite pipes in the Palabora massif have similar sequences of sulfide formation, and the copper specialization of the Palabora massif can be caused by higher water content in its initial melt allowing it to dissolve much larger amounts of sulfur and, correspondingly, chalcophile metals.

Paragonite-chloritoid alteration in the Trafalgar Fault and Fimiston- and Oroya-style gold lodes in the Paringa South mine, Golden Mile, Kalgoorlie: 2. Muscovite-pyrite and silica-chlorite-telluride ore deposited by two superimposed hydrothermal systems

The Golden Mile deposit (1751 t Au to 2017) in the Archean Eastern Goldfields Province, Yilgarn Craton, Western Australia, is located in a km-scale envelope of chlorite-calcite-albite alteration overprinting metamorphosed tholeiitic rocks. The propylitic zone (310–340 °C) is centered on D2 strike-slip faults segmenting D1 folds. At the southeast margin of the deposit, 200 m wide hydrolytic paragonite-ankerite-chloritoid alteration ( 1.5 km northwest to the Paringa South mine. In the mine, D2 faults control Fimiston ore locally overprinted by Oroya ore, whereas D3 reverse faults offsetting the D2-system control only Oroya ore. Representative ore bodies (lodes) were studied by petrographic, geochemical, and thermometric methods. Fimiston ore (4–20 g/t Au) is refractory, characterized by 10% pyrite ± chalcopyrite ± tennantite, and varies in oxidation state (magnetite stable to hematite stable). Gold and tellurides form inclusions in pyrite. In propylitic GMD, lode alteration is zoned from albite stable in veins and selvages (400 °C) to outer hydrolytic phengite-quartz-ankerite replacement (370 °C). The younger Oroya ore in D3 faults contains free gold and is oxidized (5 g/t Au; pyrite-hematite) or reduced (50–120 g/t Au; pyrite-pyrrhotite) close to carbonaceous black schist. Oroya Stage 1 silica-chlorite-siderite replacement (400–425 °C) and As-rich pyrite-arsenopyrite-sphalerite mineralization is crosscut by Stage 2 silica-ankerite fill (385 °C) and Sb-rich pyrite-tetrahedrite-telluride mineralization associated with vanadian muscovite. Mass-balance calculations indicate that the Oroya fluid carried higher concentrations of silica, Fe, V, Ni, and Te than the older Fimiston fluid.

Минералогия аллювиальных отложений Авзянского золоторудного района (Южный Урал)

The Avzyan gold region is located within the Bashkirian anticlinorium and includes lode gold deposits and placers. The Gorny Priisk, Bogryashka and Ulyuk-Bar gold deposits are hosted in the Riphean metamorphosed carbonaceous sequence. The article describes the mineralogy of the heavy concentrates from alluvial sediments of the streams of Bolshoy Avzyan basin which drains the Gorny Priisk, Bogryashka and Ulyuk-Bar gold ore deposits. The comparison of mineralogical and chemical feature of the studied heavy concentrates is done. Samples and Methods. Samples from the streams were collected every 50-100 m. Hand specimens of ore and host rocks from the lode gold deposits were collected from outcrops and dumps. The content of metals in the heavy concentrates estimated using X-ray fluorescence analyzer Innov-X alfa. Chemical composition of the accessory minerals was studied using electron microscope Vega-3 Tescan with EDA X-Act Oxford. Discussion and Results. The source of the alluvial sediments was the lode gold deposits located in the immediate vicinity of placers. Heavy concentrates of the Kamenny stream are characterized by a high content of As and Cu while ones of the Bogryashka and Bolshoy Klyuch streams show a high content of Cr and Ba. Goethite is major ore mineral for all studied samples. Ilmenite, rutile, epidote and barite are also widespread in the samples from the Bogryashka and Bolshoy Klyuch streams. Native gold is present in the sediments of all studied stream. The greatest number of gold grains was found in the samples from the Bolshoy Klyuch stream. The weak roundness of the golds and the presence of unoxidized sulfides (pyrite, chalcopyrite and pyrrhotite) indicate a relatively small age of placers. Monazite and xenotime morphology suggests autigenic catagenetic and/or metamorphic origin. Monazite contains (apfu) Ce (0.27-0.56), Nd (0.10-0.37) and La (0.09-0.33), minor Pr, Sm, Gd, Eu and Dy; ThO 2 up to 9.78 wt. % (0.08 apfu). It is similar with monazite composition from other streams of the east part of the Bashkirian anticlinorium and can be evidence of their similar origin. Xenotime contains major Gd, Dy and Er and minor Tb and Ho. Xenotime from the Bogryashka stream is characterized by the increased concentration of (apfu) Gd (0.10-0.24), Nd (0.01-0.02), Sm (0.03-0.06), Eu (0.02-0.06) and absence of Ho and Yb. Xenotime composition from the Kamenny and Bolshoy Klyuch streams is similar with ones from east part of the Bashkirian anticlinorium. Galena inclusions in REE phosphates, monazite inclusions in goethite and xenotime inclusions in pyrite can be evidence about similar conditions and time of formation gold-sulfide and REE mineralization.

The Ore Mineralogy of the Kedrovskoe Gold Deposit (the Muya Region, the Republic of Buryatia, Russia)

The ore mineralogy of the largest quartz vein, Osinovaya, at the Kedrovskoe gold deposit has been studied. Three stages of mineral formation, namely, marcasite–pyrrhotite–pyrite, gold–polysulfide, and hypergenic stages are identified. Native gold is attributed to the gold–polysulfide stage and is represented by two generations. The earlier high fineness generation (600–870, 780–820 prevails) cements the fragments of pyrite grains or forms inclusions in pyrite, and the later low fineness generation (520–580, 540–580 prevails) is associated with sphalerite–chalcopyrite–galena veinlets in pyrite. The disappearance of arsenious pyrite, the increase in iron content of sphalerite, and the change in pyrite to pyrrhotite with depth is recorded.

ORE MINERALOGY OF THE KEDROVSKOE GOLD DEPOSIT (MUYSKY DISTRICT, REPUBLIC OF BURYATIA, RUSSIA)

Ore mineralogy of the largest quartz vein Osinovaya at the Kedrovskoye gold deposit has been studied. Three stages of mineral formation are identified: marcasite-pyrrhotite-pyrite, gold-polysulphide and hypergenic ones. Native gold belongs to gold-polysulfide stage and is represented by two generations. The earlier high fineness (600–870, prevails 780–820) generation cements fragments of pyrite grains and forms inclusions in pyrite, and the later low fineness (520–580, prevails 540–580) generation associates with sphalerite-chalcopyrite-galena veinlets in pyrite. The disappearance of arsenoan pyrite, the increase in iron content of sphalerite, the change of pyrite to pyrrhotite with depth is noted.

A combined fluid inclusion and S–H–O–He–Ar isotope study of the Devonian Ashele VMS-type copper–zinc deposit in the Altay orogenic belt, northwest China

The Altay orogenic belt of Kazakhstan and China incorporates one of the world-class volcanogenic massive sulfide (VMS) polymetallic metallogenic belt. More than 12 large and superlarge VMS deposits have been found in this belt. The Ashele Basin is located at the western end of the Chinese Altay bordering Kazakhstan and hosts the large Ashele copper–zinc deposit, which is a typical VMS deposit and the largest one in this basin. The principal orebodies are stratabound and located between basalt and tuff units in the Ashele Formation. The mineralization has a two-layered structure that consists of an upper stratiform, concordant, massive Cu–Zn (or barite) orebody, and a lower veinlet–disseminated and stockwork Cu (or Cu–Pb–Zn–Ag) orebody associated with silicic alteration. Several exhalative rocks, such as barite, hematitic jasper, pyrite and silicified units also occur.Here we investigate the ore-forming fluids of the exhalative–sedimentary units including jaspilte, stratiform barite and Cu–Zn orebodies which carry fluid inclusions that show homogenization temperatures of 100–410 °C, with two peaks around 230 and 150 °C, and low salinity (clustering between 2 and 8 wt% NaCl equiv), corresponding to NaCl–H2O fluids. The ore-forming fluids of the vein orebody and silicified zones are characterized by low to medium temperature (peak homogenization temperatures between 120 and 280 °C), low to medium salinity (0.7–12.3 wt% NaCl equiv), and H2O–CO2 (±CH4/N2)–NaCl fluids. The δ34S values of barite associated with the stratiform barite orebody range from 20.4‰ to 21.4‰, indicating that the sulfur was derived from the seawater. The sulfur isotope values of sulfide (−3.7‰ to 7.7‰ with a mean of 3.1‰) are similar to those of VMS deposits from other parts of the world. The host volcanic rocks are inferred to be the major source from which the mineralizing fluid leached the ore-forming elements. The 3He/4He ratios of fluid inclusions in pyrite are in the range of 0.136–0.260 (R/Ra), broadly similar to the helium ratios of the crust. The 40Ar/36Ar ratios range from 394 to 9515, and are higher than those of atmospheric argon. The 40Ar/4He ratios of the ore fluids range from 0.002 to 0.064 with a mean of 0.034. The helium and argon isotope compositions of fluid inclusions suggest that the ore fluids of the Ashele deposit were mainly derived from the crust and were mixed with a minor amount of mantle component. The δ18OSMOW values of quartz from the silicified zones and vein orebody range from 8.3‰ to 11.1‰, with corresponding δ18Ofluid values of −4.45‰ to 2.24‰, whereas the δD values of fluid inclusions are between −140‰ and −90‰. The δ18OSMOW values of barite from the stratiform barite orebody range from 7.7‰ to 9.1‰, with corresponding δ18Ofluid values of –3.03‰ to 1.79‰, with δD values of fluid inclusions showing a range of –162‰ and –135‰. The combined isotopic data (H–O and He–Ar) suggest that the ore-forming fluids of the Ashele deposit were mainly derived from deep circulation of seawater and mixed with magmatic fluids. The decrease in temperature and pressure, water–rock exchange, and changes in the composition of ore fluids played important roles in the ore-forming processes of the Ashele Cu–Zn deposit.

Genetic implications for the Damajianshan W‐Cu‐As polymetallic deposit in Lvchun, Southwest China: Constraints from H–O, He–Ar, S, and Pb isotopes

The Sanjiang Tethyan domain in SE Asia is one of the most important mineral belts in China. The Damajianshan (DMJS) W‐Cu‐As polymetallic deposit is located in the southern part of Sanjiang Tethyan domain, related to Triassic quartz porphyry. Detailed exploration thereafter shows that the reserves of W, Cu, and As are 0.09, 0.42, and 0.12 Mt, respectively. The W is a typical oxyphile element and always coexisted with Sn–Li–Be–Nb–Ta. The Cu–As are typical sulphophile elements and always coexisted with Au–Ag–Pb–Zn–Sb–Hg. It is rarely reported that a deposit preserves tungsten and copper–arsenic simultaneously in the southern Sanjiang region. In this study, the S–Pb isotopic ratios of sulphides, the H–O isotopic compositions of fluid inclusions in hydrothermal quartz, and the He–Ar isotopic ratios of fluid inclusions in sulphides were analysed to constrain the origin of the DMJS deposit. Studies have shown that the δ34S values of sulphides from ores are mainly between −6.17‰ and +0.02‰; the sulphur isotope compositions indicate that the ore‐forming materials might originate from deep sources. The Pb isotope characteristics indicate that the ore metals might originate from deep sources and be closely related to the quartz porphyry. The δ18Ofluid values of ore‐forming fluids calculated from hydrothermal quartz range from −5.5‰ to +7.5‰, and the δD values of the fluid inclusions in quartz are from −81.7‰ to −50.2‰. The H–O isotope systematics indicate that the ore fluids in the DMJS deposit were probably initially sourced from magmatic water and later gradually mixed with Mesozoic meteoric water. Fluid inclusions in pyrite, chalcopyrite, arsenopyrite, and pyrrhotite possess 3He/4He ratios of 0.02–0.05 Ra, and their 40Ar/36Ar ratios range from 562.47 to 4159.15, indicating a complete crustal fluids and with higher radiogenic 40Ar. The noble gas isotopic data, along with the stable isotopic data, suggest that the ore‐forming fluids have a deep source. The unique geologic structure of the Sanjiang Tethys tectonic metallogenic region and the evolution of ore‐forming fluids both promoted the formation of this rare DMJS deposit.

Mineral chemistry, bulk rock geochemistry, and S‐isotope signature of lode‐gold mineralization in the Bétaré Oya gold district, south‐east Cameroon

Lode‐gold mineralization in the Bétaré Oya gold district is related to a series of quartz–sulphide veins that define a steeply dipping NNE–SSW‐trending brittle–ductile shear zone. The veins transect metasedimentary sequences in the vicinity of small granitic intrusions. Quartz veins range from barren (massive quartz) to mineralized with textures that vary from foliated, sheared, vuggy, fractured, and sugary to brecciated with inclusions of altered wall rock. They contain gold, pyrite, and galena in addition to chalcopyrite and both hypogene and supergene haematite and covellite. Parameters that consistently define primary gold mineralization in the area include silicification, sulphidation, sericitization, K‐feldspar alteration, haematitization, and carbonatization. Gold grains recovered from heavy mineral concentrate and grains that occur as inclusions in pyrite were characterized by microchemical methods. The gold grains are zoned and exhibit a simple internal structure of partly deformed and leached rims and high‐purity intragranular veinlets. Gold is alloyed with Ag; gold fineness varies between 859 and 877 in the core and from 958 to 997 in the leached rims. Inclusions of gold in pyrite cluster between 875 and 900. Pyrite has up to 0.59 wt% Au, and other trace elements occur in low quantities but for Pb (0.13 wt%), Ag (1.36 wt%), and Sb (1.63 wt%). The bulk trace element signature of the veins is characterized by Au, Ag, As, Bi, Ba, Cr, Cu, Mn, Pb, Sb, Sr, V, W, Zn, and Zr. This depicts a geochemical association of Au ± Cu ± Bi ± As ± Pb. The occurrence of Au as well as elevated Pb and Zn could suggest a granitic source for the ore‐bearing fluid. The auriferous quartz veins are characterized by a widespread in δ34S from 2.8 to 14.9‰ pointing to multiple sources of sulphur in the system, perhaps a mixture of sulphur from the metasedimentary and the granitic rocks.

Granitoid-associated gold mineralization in Egypt: a case study from the Atalla mine

Gold-bearing sulfide-quartz veins cutting mainly through the Atalla monzogranite intrusion in the Eastern Desert of Egypt are controlled by subparallel NE-trending brittle shear zones. These veins are associated with pervasive sericite-altered, silicified, and ferruginated rocks. The hosting shear zones are presumed as high-order structures of the Najd-style faults in the Central Eastern Desert (~ 615–585 Ma). Ore minerals include an early pyrite-arsenopyrite (±pyrrhotite) mineralization, partly replaced by a late pyrite-galena-sphalerite-chalcopyrite (±gold/electrum ± tetrahedrite ± hessite) assemblage. Gold occurs as small inclusions in pyrite and arsenopyrite, or more commonly as intergrowths with galena and sphalerite/tetrahedrite in microfractures. Arsenopyrite geothermometry suggests formation of the early Fe-As-sulfide mineralization at 380–340 °C, while conditions of deposition of the late base metal-gold assemblage are assumed to be below 300 °C. Rare hessite, electrum, and Bi-galena are associated with sphalerite and gold in the late assemblage. The early and late sulfide minerals show consistently a narrow range of δ34S ‰ (3.4–6.5) that overlaps with sulfur isotopic values in ophiolitic rocks. The Au-quartz veins are characterized by abundant CO2 and H2O ± CO2 ± NaCl inclusions, where three-dimensional clusters of inclusions show variable aqueous/carbonic proportions and broad range of total (bimodal) homogenization temperatures. Heterogeneous entrapment of immiscible fluids is interpreted to be caused by unmixing of an originally homogenous, low salinity (~ 2 eq. mass % NaCl) aqueous-carbonic fluid, during transition from lithostatic to hydrostatic conditions. Gold deposition occurred generally under mesothermal conditions, i.e., 1.3 kbar and ~ 280 °C, and continued during system cooling to < 200 °C and pressure decrease to ~ 0.1 kbar. Based on the vein textures, sulfur isotope values, composition of ore fluids, and conditions of ore formation, we suggest that the Atalla monzogranite intrusion acted only as a competent structural host for ore deposition from shear-related, metal-rich fluids migrated up from depth. This model is also presumed for most granitoid-associated Au deposits in the region, considering the similarity in their structural control, alteration pattern and mineralogy, and chemistry of the ore fluids.

Origins of ore-forming fluid and material of the quartz–vein type Mo deposits in the East Qinling–Dabie molybdenum belt: A case study of the Qianfanling Mo deposit

The Qianfanling quartz–vein type molybdenum deposit is located in the East Qinling–Dabie Mo belt. The Mo mineralization in the deposit is mainly present as structurally controlled Mo-bearing quartz veins. Based on chromatography analysis of the Qianfanling Mo deposit, the gas compositions of fluid inclusions in quartz dominantly contain H2O and relatively high concentrations of CO2 (11.019 to 18.554mol%). Cations and anions of liquid compositions of fluid inclusions are mainly Na+ and Ca2+, and SO42− and Cl−, respectively. The characteristics of gas and liquid compositions of fluid inclusions in quartz indicate that the Qianfanling Mo deposit formed under conditions of high oxygen fugacity. Analytical results of He-Ar isotopes of fluid inclusions in pyrite from the Qianfanling Mo deposit show that 3He/4He ratios vary from 0.02438 to 0.0796 Ra (Ra is the 3He/4He ratio of air=1.40×10−6), with F4He values of 252,509–885,224. The calculated Hemantle values range from 0.18 to 0.87%, with an average of 0.39%. 40Ar/36Ar ratios are 694.4–1473.4, with the estimated 40Ar* (radiogenic 40Ar) proportion of 79.9%. These He-Ar isotopic data indicate that most of the He in the fluid inclusions was derived from the crust, with negligible atmospheric He and only a very small input from the mantle probably. Furthermore, the Ar in the ore-forming fluid was mainly derived from the crustal source, with minor atmospheric Ar involved. 17 Re–Os model ages of 15 molybdenite samples from the 4 quartz-vein type Mo deposits (Qianfanling, Daxigou, Maogou, and Zhifang) range from 215.0 to 248.2Ma, with a mean value of 235.6Ma, which is close to the age of peak collision between the North China and Yangtze Cratons. The Re contents in molybdenite from the 4 deposits range from 0.19 to 39.16ppm, with an average of 17.41ppm, indicating a crust-dominated source. Re contents in molybdenite from Mesozoic Mo deposits in different ages and types in the East Qinling–Dabie Mo belt show different characteristics of ore sources. Combined with gas and liquid compositions and He-Ar isotopic characteristics of fluid inclusions, as well as Re contents in molybdenite of this paper and previous studies, we suggest that the ore-forming fluid and material of the Qianfanling Mo deposit were mainly derived from crustal magmatic source. This magmatic source could have formed by partial melting of deep crustal rocks under a certain condition during the collision between the North China and Yangtze Cratons.