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The Paleo-Mesoproterozoic North Singhbhum Mobile Belt (NSMB) is a fold-thrust belt typically found in a collisional setting and is well known for hosting gold mineralisation. A study carried out in the northern part of NSMB depicts varieties of acid volcanic rocks along with mica schist, ferruginous cherty quartzite, black shale, and sheared granite, which are all ubiquitously traversed by quartz veins/reefs. In the study area, gold mineralisation is mostly restricted to brecciated quartz reef occurring ENE–WSW along S1 foliation, which also depicts syn to late kinematic brittle–ductile shearing. The petrographic studies of mineralised cherty variant of quartzite in quartz reef depict the presence of two generations of the quartz vein. The older one is smoky/bluish, altered, recrystallised and consists of disseminated and stringers of pyrite, chalcopyrite, etc., whereas the younger veins are milky white and lack mineralisation. The presence of pyrite–pyrrhotite and chalcopyrite–pyrrhotite associations and conversion of pyrite to pyrrhotite along fracture depicts prograde metamorphism. Trace elemental mineral chemistry depicts significant Au enrichment in pyrite. A positive correlation between Co and Ni concentrations in pyrite indicates hydrothermal origin. Au–As ratio explains the enrichment of gold in pyrite, where gold occurs as a solid solution in pyrite, which is generally referred to as invisible gold. The presence of gold grains/nuggets at the microscopic level and in pan concentrate as well as considerable analytical value in bulk rock coupled with occurrence in sulphide structure, points towards the bimodal nature of gold mineralisation. Mode of occurrence, nature of mineralisation, and surficial, structural and mineralogical characteristics suggest gold mineralisation of the study area is of orogenic type. [ABSTRACT FROM AUTHOR]

The structurally controlled copper–gold (Cu–Au) mineralization in the Suwara area in the southern part of the Bundelkhand craton, Central India, is hosted within quartz reefs traversing older granitoids. Mineralization is distinctly associated with hydrothermal alteration (potassic, chloritic and iron alteration in the ore zone and Kmetasomatism in the wall rocks). Field observations and laboratory evidence confirm the Fe–Cu–Au association in the study area. Sulphide mineralization is represented by pyrite, chalcopyrite, covellite, chalcocite and bornite occurring as disseminations, stringers and specks within the altered quartz veins, with gold occurring as fine isolated disseminations within quartz and iron oxides. Chemical analysis of surface and subsurface samples showed promising values for Cu (up to 1.0%) and Au (up to 450 ppb) content. Fluid inclusion studies indicated the occurrence of saline mineral fluids (Na– K–Cl-bearing) with the presence of H2O–CO2–CH4 in the mineral system. Overall characteristics of the mineralization and alteration patterns of the prospect indicate that it probably belongs to an iron oxide–copper–goldtype set-up for mineralization. The present study on Cu–Au mineralization from the southern Bundelkhand craton will have significant implications for further exploration and research in the area. [ABSTRACT FROM AUTHOR]

The Cenozoic-age Makum coal from northeastern India offers numerous research opportunities because of its diverse geochemical and geological characteristics. Due to its high sulfur content, the coal has been found to be less useful for industrial purposes. It can, however, serve as a hub for ongoing research on coal-based derivative products. The aim of this research work is to investigate the mineralogical and geochemical compositions of the coal, mine overburden (MOB) and shale samples taken from the Makum coal field and also on establishing a mutual relationship between them. To characterize the geochemical controlling factors of the Makum coal field, the study employs coal petrography, FTIR, mineralogical, and geochemical analysis. According to X-ray diffraction analysis, the major minerals like quartz, kaolinite, haematite, illite, pyrite, and calcite are present in coal and MOB. Pyrite is observed by SEM–EDS analysis as cubic-shaped particles that are smaller than a few µm in size. The presence of sulfide minerals represents a phase of pyrite mineralization. The petrography study was used to better understand the environment that existed during the formation of the plant material, which aids us in determining the quantity of detrital mineral sediment contained in the coal. According to the ICP-MS analysis, the samples indicate significant levels of rare earth elements including yttrium. The present study reveals higher concentrations of potential hazardous elements in the coal samples, with V, Cr, Ni, Cu, and Zn content in coal being considerably enriched compared to world-average concentrations. The correlation analysis reveals that the potential hazardous elements like Co, Ni, As, and Cu are associated with pyrite as they have strong affinity towards pyrite. Thus, numerous minerals and rare earth elements (REEs) exist, opening up a fresh avenue for more research in the area. This study also assists researchers in understanding the significance of Makum coal and provides numerous ideas for coal characterization. [ABSTRACT FROM AUTHOR]

The Zn‐Cd‐S critical minerals system is a significant mineralogical indicator of hydrothermal ore‐forming processes. The understanding of greenockite (CdS) formation in natural mineral systems is very restricted due to low crustal abundances (Cd <0.2 ppm). We report the first occurrence of greenockite from the Bagada orogenic gold prospect, Paleoproterozoic Mahakoshal belt, Central Indian Tectonic Zone (CITZ). This provides an ideal opportunity to access the occurrence and its genesis. Greenockite occurs in discrete sheared quartz veins hosted in phyllites. Sulphide mineralization is closely associated with hydrothermal chlorite alterations. Pyrrhotite, arsenopyrite, pyrite, and chalcopyrite assemblages are formed at high temperatures, while sphalerite and galena precipitated when the temperature decreased. Greenockite, native silver and bismuth with gold precipitated during the breakdown of arsenopyrite to scorodite in progressively oxidizing conditions. Greenockite precipitation occurred in a transforming hydrothermal environment from a reducing to an oxidizing one during late‐stage hydrothermal alteration. Sphalerite and chlorite thermometry yielded temperatures in the range from 266 to 366°C for this late‐stage hydrothermal alteration. Textural relations show two modes of occurrence of greenockite, one (Gck‐1) as exsolution within sphalerite (Sp‐2) and the other (Gck‐2) occurring as a discontinuous rim replacing the Cd‐rich sphalerite (Sp‐3). The Gck‐1 contains high Zn and low Cd, while as compared to Gck‐2. Zinc and Cd in greenockite show diadochy relation (Zn2+ ↔ Cd2+). Laser ablation ICPMS analyses of sphalerite (Sp‐2) reveal remarkably high Cd contents (1.9–12 wt%). Cd >0.02 ppm suggests possible Cd derivation from sphalerite (Sp‐2) under variable fS2, T, pH, fO2 conditions, fluid–rock interaction, and change in the Zn/Cd ratio. Sphalerite compositions display a low Zn/Cd (av.22) ratio and high temperature of formation, with characteristics intermediate between volcanogenic massive sulphides (VMS) and magmatic‐hydrothermal Cu‐Pb‐Zn system. [ABSTRACT FROM AUTHOR]

The Gondpipri layered mafic–ultramafic intrusion at the western margin of the Bastar Craton in Central Indiacomprises leucogabbro, gabbronorite, and websterite. The intrusion hosts both magmatic and hydrothermal Ni-platinum group element (PGE)mineralisation. In this study, we use in-situ measured trace element composition of pyrite and magnetite and zircon U–Pb geochronology to elucidate hydrothermal processes and their timing. Secondary platinum group minerals (PGMs) occur as veins and fracture fillings in sulfide and oxide minerals together with hydrothermal zircon clusters within chlorite alteration. Electron microprobe (EPMA) analysis reveals that magmatic PGMs are enriched in Pt, Pd, and Rh, whereas the hydrothermal PGMs are characterized by higher Fe, S, Te, Bi, and Ni. A semi-metal collector model (Bi-Te) is proposed for PGE in the Heti Ni-PGE prospect, where an immiscible Bi-Te melt exsolves and acts as a collector for formation of primary PGM following precipitation of Pd tellurides, tsumoite, melonite and hessite upon cooling of temperature hydrothermal fluids. Two generations of pyrite (Py-I and Py-II) and magnetite (Mag-I and Mag-II) are identified. Py-I and Py-II exhibit distinctive concentrations of Co, Se, and Au, while Mag-I and Mag-II have variable concentrations of REEs, Cr, Ti, Ga, V, Ba, and Sr. Selenium geothermometry of pyrite indicates that hydrothermal mineralisation occurred within a temperature range of 200 °C to 475 °C. The Ni-PGM-Bi-Te mineralisation is associated with an unusual cluster of megacrystic zircons, which are likely hydrothermal origin. Uranium-lead (U–Pb) dating of five zircons using LA-ICPMS yields a concordia age of 2524 ± 7 Ma, interpreted as the age of the hydrothermal sulfide-hosted Ni-Te-Bi-PGE mineralization. [ABSTRACT FROM AUTHOR]

Shallow borehole drilling has been conducted at the Sarni area of Betul Belt, Central India to understand the subsurface geology and nature of mineralization. The predominant lithounits are ferruginous quartzites, felsic volcanic rocks, and with minor intrusions of amphibolites. The encountered felsic volcanic rocks are rhyolites varying from pink (rich in K‐feldspar) to grey (rich in Na‐feldspar and amphiboles). The rhyolitic rocks have undergone intense hydrothermal alteration. The alteration zones are defined by the presence of biotite‐chlorite‐garnet‐gahnite; biotite‐sericite‐chlorite‐anthophyllite and anthophyllite‐actinolite‐tremolite‐carbonate mineral assemblages. Sulphide pits, highly leached zones with occasional bluish tinges, limonitic patches are present in the felsic volcanic rocks, followed by semi‐massive to disseminated sulphide mineralization indicative of Kuroku‐ type Zn–Cu–Pb volcanic‐hosted Massive Sulphide (VHMS) mineralization. Scanning electron microscope and energy‐dispersive x‐ray spectroscopy studies documented sphalerite, pyrite, chalcophyrite, arsenopyrite, pyrrhotite, and minor Au–Ag phases. The Sarni rhyolites exhibit chalcopyrite disease in sphalerite, replacement, and overgrowth ore‐textures. The high HFSE, low Zr/Y ratio, pronounced negative Eu anomalies characterize the felsic volcanic rocks as FIIIb type with high zircon saturation temperatures of 856–931°C. These rhyolites are formed at a back‐arc rift setting in an ensialic island‐arc tectonic regime. The synvolcanic structures facilitated the magma ascent to the surface. The hydrothermal circulation due to adequate heat flow in the back‐arc rift environment, in an episodic multicyclic process provided sufficient time for accumulation of sulphides from the seafloor, which was later deposited to form a VHMS mineralization. We also report tungsten (W) mineralization in the quartz veins of these felsic volcanic rocks. It is suggested that the felsic magma has interacted with metasedimentary sequences and assimilated W‐ore bearing fluids, and during remobilization in the later stages of tectonic activity enabled the deposition of tungsten in these quartz veins. [ABSTRACT FROM AUTHOR]

Felsic volcanic rocks of rhyolite to dacite composition, hosting copper mineralization, are exposed near Mundiyawas-Khera area within the Alwar basin of Proterozoic North Delhi Fold Belt. Distinct variations in grain size, mineralogical composition, texture, sulfide mineral associations, structure, and outcrop patterns of the felsic volcanic rocks are observed in the field. In this study, litho-facies analysis of felsic volcanic rocks around Mundiyawas-Khera copper deposit has been attempted for the first time to understand their spatial distributions and bearing on copper mineralization. Signatures of five distinct facies variations in felsic volcanic rocks are recorded based on the field and laboratory studies. These facies variations recorded in the study area are (a) vitrophyric felsic volcanics; (b) fine grained felsitic fiamme bearing tuff; (c) fiamme-bearing welded ash flow tuff; (d) poorly welded ash flow tuff; and (e) volcanic ash. Presence of glass shards in the study area indicates phreatomagmatic eruptions in an aqueous to sub-aqueous environment. Occurrences of massive sulfide associated with the felsic volcanic unit can also correlate with well-known volcanic hosted massive sulfide (VHMS) deposit and is taken as an additional evidence of felsic volcanic rock occurrences at a submarine environment. [ABSTRACT FROM AUTHOR]

Gold mineralization associated with quartz veins and sulfide minerals has been reported in Banded Iron Formation (BIF) at Girar, Bundelkhand Craton, India. The BIF is deformed with the development of tight- isoclinal folds in high strain zones and hosts four types of quartz veins viz., (i) folded veins, (ii) band parallel veins, (iii) low angle cross-cutting veins, and (iv) high angle cross-cutting veins. Folded and band parallel veins are continuous with each other and spatially restricted to zones of tight-isoclinal folds in BIF. Shape of quartz aggregates in the folded and band parallel veins is interlobate, and these veins contain pyrite and chalcopyrite as dominant sulfide minerals along with gold. Microstructure of quartz grains and presence of fragments of BIF in sulfide mineralgold bearing folded and band parallel veins indicate that the formations of these veins were related to a single deformation episode. Spatial association of folded and band parallel veins to intensely folded zones in BIF is attributed to epigenetic nature of sulfide and gold mineralization. [ABSTRACT FROM AUTHOR]

Scapolite occurrences are widely observed in the metasedimentary rocks exposed around the Khetri Copper Belt and adjoining Nim ka Thana copper mineralized area in western India. Amoeboidal to well-developed and rounded/elliptical-shaped marialitic scapolite (Na-rich end-member) rich zones with variable Cl contents ranging from 1.0 wt % to 2.9 wt % have been identified in proximity to the ore-bearing hydrothermal fluid activity zones. Although scapolite is formed as a product of regional metamorphism in many places, in this study, we propose a strong possibility that scapolite was formed by hydrothermal ore-bearing fluid interaction with metasediments. The evidence of hydrothermal activity and Cl sourcing is attributed to (i) the absence of evaporite beds in the area and no Na-rich plagioclase as inclusions within the scapolite suggesting the formation of marialitic scapolite from sodic plagioclase in the metasediments with the interacting hydrothermal fluid; (ii) an epithermal to mesothermal hydrothermal fluid with moderate salinity responsible for the Cu mineralization that is ascribed to be the source of Cl for the formation of marialitic scapolite; (iii) diffusion of SO2 in the scapolite in close association with the sulfide mineral phase (chalcopyrite) supporting the involvement of ore-bearing fluid in the development of scapolite; (iv) the absence of zoned scapolite, the spatial distribution of scapolite in a particular lithology, the occasional incorporation of sulfur into marialitic scapolite and the texture/geometry in the scapolite suggesting a broad hydrothermal linkage instead of a pure metamorphic origin. [ABSTRACT FROM AUTHOR]

The rocks of the Aravalli Protocontinent of NW India are enriched in lead–zinc bearing deposits amongst which the Zawar mineralized belt is one of the famous for base metal deposits and was mined since ancient times. In the present study, an attempt has been made to identify and map the extension of the mineralized belt and base metal prognostic zones using the integration of multi-sensor remote sensing, geological and geochemical data. Remote Sensing studies were carried out using ASTER, AVIRIS-NG, and ASAR datasets to understand the extension and associated structural features of host rocks (dolomite in the present case) of lead–zinc mineralization from the Middle Aravalli Group. Relative band depth (B6 + B9/B8) was used to delineate the dolomite of the region using the ASTER imagery. Mineral map was derived using the AVIRIS-NG dataset with the help of the MTMF algorithm. Multifrequency and multipolarization ASAR datasets demarcated the structural features in the complexly deformed rocks of the extended belt. The obtained results from remote sensing were validated with the help of geological and geochemical studies. Geological studies (field surveys and petrographic studies) confirmed the presence of dolomites and associative mafics. Mineralogical, major oxides and trace elements data further substantiated the presence of dolomite, associated sulfides such as galena, sphalerite, pyrite, and chalcopyrite, and lead, zinc and copper in the Zawar–Dungarpur Belt. Presence of chromium and nickel were observed through the trace element studies of dolomite belt. The trace elements interpolated maps were superimposed by traced structural maps using ASAR datasets. The densely populated E-W lineaments are considered the suitable zones for base metal accumulation. These lineaments carried the base metal bearing fluid along with a low concentration of Chromium and Nickel. Under the finding of this study, the northern head of Hati Magra and near Keekawat are the suitable zones for Pb–Zn sulfide mineralization. [ABSTRACT FROM AUTHOR]

Gold (I) sulphide (Au2S) is an inorganic compound and the principal sulphide of gold. This study aimed to separate gold sulphide from ore samples such as banded hematite quartzite mined from the banded iron formation of Goa, India. The samples were dried, powdered and then concentrated by the panning method. The coarse ferromagnetic material was separated and sieved into different fractions of varying sieve sizes, i.e. 250, 150, 106 and 53 µm. Next, they were subjected to serial washing, and after drying, were tested for the presence of gold by ICP-AES, optical microscopy, phase contrast microscopy, SEM-EDS and total content of the sulphur was analysed by CHNS/O elemental analyzer. [ABSTRACT FROM AUTHOR]

Gahnite occurs in diverse rock types and metamorphosed mineral deposits. The gahnite geochemistry is being used as an exploration tool for zinc sulfide deposits. In the present study, the geochemistry of gahnite and associated minerals viz. sphalerite, garnet, sillimanite, staurolite, anthophyllite, biotite, ilmenite, and chlorite, and their petrography have been used in studying the genesis, metamorphic evolution, deposit characterization, and exploration significance of seven Palaeoproterozoic volcanogenic massive sulfides (VMS) deposits in the Betul belt in the Central Indian Tectonic Zone, India. Different textural types of gahnites are recognized in the Betul belt. Field investigations of textural and geochemical characteristics suggest that the gahnites were formed by multiple geological processes: desulfidation of sphalerite, breakdown of almandine garnet, and zincian staurolite, and chloritization of zincian biotite corresponding with regional metamorphism. The temperature conditions of gahnite formation are estimated to be ~600–650°C using empirical Ti in biotite thermometry. The elemental concentrations in gahnites vary from Zn/Fe ratio 2–4; Co 1–30 ppm; Ga 14–573 ppm and Mn 126–3422 ppm, indicating that bimodal volcanics and volcano‐sedimentary rocks as protolith. High Mn content in gahnite suggests an influx of magmatic‐hydrothermal fluid in the protolith. Variations of trace elements (Mg vs. Al/V, Ti vs. Co/Mn, Co vs. V/Ga, Mn vs. Zn/Fe) and high‐temperature of gahnite formation attest to sensu stricto VMS type mineralization in Betul belt, which is entirely different from SEDEX, Broken Hill‐type, and non‐sulfide deposits. Principal component analysis of trace elements shows the geochemical variations at a few deposits and reveals that gahnites were formed as a result of the competing effect of fS2‐fO2 and governed by bulk rock composition. [ABSTRACT FROM AUTHOR]

The novelty of this study consists, use of the Powder X-Ray Diffraction (PXRD) technique facility at CSIR-NGRI to report the occurrence of platinum and skaergaardite (Pd Cu) in the Hutti underground gold mines. This study utilized a quartz-bearing sample from the Hutti underground gold mines. One of the split samples were analyzed by PXRD. In this study, quartz samples are found to be associated with sulphides composed of pyrite, chalcopyrite, and arseno-pyrite with intense chemical alterations. The mineral phase of skaergaardite shows a strong intensity at 2θ 43.5° and confirmed with database of International Centre for Diffraction Data Powder Diffraction File — ICDD-2021-PDF4+ PDF00-057-0606 and native platinum, syn (Pt) Pt 0.0999 Sn 0.001 matched with PDF 04-001-3300. Further research work is planned on this interesting mineral association with primary gold ore in Hutti underground gold mine. [ABSTRACT FROM AUTHOR]

The importance of light-stable isotopes and their mass-dependent fractionations in understanding past geological processes is enormous. The present research delivers precise, high-resolution, in-situ oxygen (18/16O) and triple sulfur (32/33/34S) isotope ratios from quartz and sulfide minerals found in the Cu Au Kendadih deposit located along the Mesoproterozoic (∼1.6 Ga) Singhbhum Shear Zone (SSZ) in eastern India. Oxygen and sulfur isotope analyses were carried out on quartz and pyrite-chalcopyrite pairs that are in textural equilibrium, using the most advanced Large Geometry-Secondary Ion Mass Spectrometer (LG-SIMS; CAMECA IMS-1300HR3). The results show restricted ranges for δ18O quartz (+6 to +7.7‰; average: 6.9 ± 0.9‰, n = 50) and δ34S chalcopyrite-pyrite (+11.4 to +11.9‰; average: 11.79 ± 0.2‰, n = 62). This isotopic homogeneity suggests a single, uniform fluid source for the ore-forming metals. The combined oxygen, sulfur isotope ratios, and fluid inclusion data are consistent with a hydrothermal fluid derived from an "I-type" granitic melt. This study incorporates existing fluid inclusion, stable, and radiogenic isotope data from temporally similar deposits within the shear zone, reevaluating their implications in light of the new findings. It proposes that, around 1.6 billion years ago, underplating the Singhbhum Craton by the Dalma Plume resulted in the remelting of the lower crust. This remelting is attributed to the plume's introduction of a significantly elevated thermal perturbation. The process is hypothesized to have led to the generation of second-order granitic melts. Upon emplacement in the lower crust, these granitic melts became the potential source for essential metals, ligands, and hydrothermal fluids, contributing to mineralization within the deep-seated Singhbhum Shear Zone. This research attempts to comprehensively describe the "source-to-sink" ore genesis model by proposing potential magma chamber processes that concentrate metals and ligands at the roof zone, followed by the separation of metal-rich hydrothermal fluids, fluid-rock interaction, and the physicochemical conditions governing the deposition of the extensive polymetallic deposit. This work offers novel insights into the Mesoproterozoic metallogenesis along the Singhbhum Shear Zone, challenging existing paradigms that favored evaporite or seawater brine sources. Furthermore, it sheds light on the Mesoproterozoic sub-crustal processes beneath the Indian Craton in this region. [ABSTRACT FROM AUTHOR]

It is well established that the major and minor element contents of chromites are subject to change during greenschist to amphibolite facies metamorphism. During upper amphibolite facies metamorphism, chromite can be completely converted to chrome magnetite. However, not all elements are affected to the same degree, the concentrations of +2 ions (e.g. Zn, Co, Mn) being particularly vulnerable to modification. The degree to which trace elements, particularly the platinum-group elements (PGE), are affected has not been closely examined. The compositions and textures of chromites from komatiites of the Gorumahishani greenstone belt of the Singhbhum Craton (India) have experienced a range of metamorphic conditions from greenschist to amphibolite facies, providing the opportunity to study the changes of trace and platinum-group element composition with metamorphic grade. Five types of altered chromites are identified from the komatiitic suite of rocks in the ~120-km-long greenstone belt. The type-I chromites are non-porous and characterized by the least modified cores. These chromites are mostly present in the northern Maharajgunj-Tua Dungri section where rocks show metamorphism from greenschist to greenschist-amphibolite transition facies. The type-II and type-III chromites are porous and mostly found in the southern Kapili section of the greenstone belt where rocks show metamorphism up to the mid-amphibolite facies. Type-IV and type-V chromites are completely modified to ferritchromit and chrome magnetite, respectively, and are present in the komatiitic rocks from the entire greenstone belt. The central cores of the type-I and type-II grains have relatively higher concentrations of mobile trace elements (e.g. Zn, Co, and Mn) with higher Mg# [Mg/(Mg + Fe2+)], lower Cr# [Cr/(Cr + Al)], and lower Fe3+/R3+ (R3+ = Fe3+ + Cr3+ + Al3+) ratios than their respective rims. Significantly higher concentrations of the immobile trace elements (e.g. Ti and V) in the cores of the type-II grains relative to their chrome magnetite rims from the Kapili section and to the type-I varieties from other sections might be due to the metamorphism of the komatiitic rocks under higher-grade conditions (amphibolite facies). In situ LA-ICPMS analysis for PGE reveals a relatively higher concentration of Ru and Rh in the rims of the type-I chromites than in the cores which is due to the diffusion of these elements from the normal spinel structure of the cores towards the bivalent octahedral sites of the inverse spinel structure of the chrome magnetite rims during metamorphic processes. The lower concentrations of Os, Ir, Ru, and Rh in the cores of the type-II chromites from the Kapili section might be related to the metamorphism of the rocks under higher-grade conditions that facilitated the diffusion of these elements to associated sulphide or platinum-group mineral or alloy phases. The calculated partition coefficients of Sc, Ti, V, Mn, Ni, Ga, Os, Ir, Ru, and Rh from the least altered chromite cores assuming equilibrium with the parental komatiitic melt also suggest the variable effects of metamorphism when compared with global experimental and empirical values of the natural samples. [ABSTRACT FROM AUTHOR]

This work presents the occurrence of sulphide mineralization for the first time in the Middle Porcellanite Member belonging to the Porcellanite Formation of the Semri Group in and around Amrora-Barwari-Kupa areas, Garhwa district, Jharkhand. The Porcellanite Formation has been categorised into four mappable units i.e. lower Porcellanite Member; middle Porcellanite Member; Amrora Member, and upper Porcellanite Member. Out of the four members, sulphide mineralization has been mostly observed in the middle Porcellanite Member which is represented by grey to black, fine-grained, thinly laminated porcellanite and made up of quartz and few grains of feldspar. Petrographic study shows that it comprises of devitrified glass, quartz shards, K-feldspar, plagioclase, and perthite phenocrysts as essential minerals and accessories are sericite, calcite, chlorite, opaque, and devitrified groundmass. It consists of sulphide minerals like chalcopyrite, pyrite, pyrrhotite, and grains of arsenopyrite and gold. Based on the field observations and ore petrographic study, three zones of sulphide mineralization have been marked within this unit. The longest sulphide mineralization zone exposed to the northeast of Amroa village is 750m long and 300m wide having an E-W trend. In this mineralization zone, specks of pyrite, chalcopyrite, and gold are observed which occur as dissemination. The second and third mineralization zones are 360m long and 150m wide having N-S trend and 300m long and 100m wide having NW-SE trend respectively which have mainly been noticed to the southeast of Barwari and northeast of Kupa village. The sulphide mineralization is mainly associated with shearing and faulting which has been corroborated by the petrography and field evidences. [ABSTRACT FROM AUTHOR]

Singhbhum craton of eastern India is bestowed with several metal deposits such as iron, manganese, chromite, and platinum group of elements (PGE). Iron and manganese ores are associated with banded iron formations of Paleoarchean greenstone belts, and chromite and PGEs are associated with mafic-ultramafic rocks of Mesoarchean Baula- Nuasahi and Sukinda belts. Earlier studies indicate a hydrothermal origin of PGE mineralisation. Here, the occurrence of micron-sized gold grain and base metal sulfides in the Mesoarchean Mayurbhanj Gabrro Anorthosite Complex (GAC) are reported. These gabbros are considered coeval with Mayurbhanj granites and comprise of plagioclase, clinopyroxene and orthopyroxene, amphiboles, minor quartz, chlorite, and accessory opaque mineral phases. Scanning Electron Microscopic study shows the occurrence of a ∼3 µm sized sub-rounded gold spherule in feldspar, along with disseminated grains of sulphides of varying sizes. Geochemical analysis reveals that the Sarat-Ghat exposure of Mayurbhanj GAC contains an appreciable amount of gold and PGE (∑PGEs up to 1094 ppb and Au up to 396.5 ppb). The presence of sulphide minerals and intense alteration of feldspars and pyroxenes in the mineralised zone points towards a hydrothermal origin. This lithounit warrants a detailed appraisal to understand the nature and extent of gold and PGE mineralisation. [ABSTRACT FROM AUTHOR]

Appreciable amount of copper mineralisation dominated by chalcopyrite along with incidence of uranium within the Mayurbhanj granite batholith in Kesharpur-Madansahi-Dudhiasol sector has been established through preliminary exploration (UNFC-G3). Chalcopyrite-pyrite-pyrrhotite assemblages along with minor pentlandite seems to be preferably controlled through secondary structural shear fabric and vein selvages along the contact zones of granite and country rocks viz. hornblende biotite schist and metabasics. Hydrothermal alteration is notably marked by potassic, propyllitic and phyllic zones besides wide chloritisation and silicification in close proximity and within mineralised zone. Three potential prospects of significant copper mineralisation in terms of thickness and grade have been brought out at Kesharpur, Madansahi and Dudhiasol. These prospects show a linear lensoidal synformal disposition of the ore body trending NE-SW in eastern part to E-W in central part and NW-SE in the western part with moderate to steep northerly dipping. Out of these three prospects, Madansahi is quite promising where thickness of copper lodes varying from 2.00 m to 18.35 m and grade varies from 0.20% Cu to 1.76% with maximum nickel concentration of up to 0.13% Occurrence of copper mineralisation along with nickel concentration and anomalous uranium values up to 0.045% U3O8 is a significant achievement in establishing polymetallic mineralisation and augmenting the mineral resources in eastern India. [ABSTRACT FROM AUTHOR]

The Palaeo- to Meso-Proterozoic (1.0–2.4 Ga) North Singhbhum Mobile Belt (NSMB) is an assembly of multiphase folded, low- to medium-grade meta-sedimentary and meta-igneous rocks, lying within the north Singhbhum crustal province. The mobile belt is situated between the Archean Singhbhum craton in the south and the Meso/Neo-Proterozoic (0.9–1.7 Ga) Chotanagpur Gneissic Complex in the north. Gold mineralisation of moderate concentration is reported from different parts of NSMB along shear /fracture zones within the volcano-sedimentary and meta-sedimentary rocks sequence. The mineralisation is structurally controlled and is associated with higher order shear zones. Gold occurs in association with sulfides like pyrite, arsenopyrite, pyrrhotite, chalcopyrite, sphalerite, etc. This work attempts to map the auriferous shear/fracture zone along the Babaikundi-Birgaon axis (BBA), which lies within the NSMB. In the present study, combined geophysical approach comprising self-potential (SP), electrical resistivity tomography (ERT) and induced polarisation (IP) methods were used for mapping of subsurface gold prospects. SP data were collected along 7 profiles at 10 m station interval with approximate profile separation of 100 m. The anomalies of possible ore bodies were found at depth of about 18–25 m using Euler deconvolution of SP data. The ERT and IP data were collected along four profiles at 10 m electrode spacing with approximate profile separation of 100 m. The inverted resistivity sections indicate very low resistivity value in the depth range of ∼20 m inferring highly conducting, while relatively high chargeability are also observed at the depth range of ∼25 m. The possible existence of Babaikundi-Birgaon lineation and the occurrences of sulfide minerals in quartz reef associated with gold mineralisation were mapped based on the horizontal locations and extensions of negative anomaly in SP data, low resistivity in 2-dimensional (2D)-inverted resistivity section and high chargeability in 2D-inverted IP section. Previous studies based on electron probe micro-analyzer (EPMA) and scanning electron microscope (SEM) analysis over the study area substantiate our combined geophysical interpretations for the presence of gold prospective zones along the BBA. Highlights Integrated geophysical approach comprising SP, ERT and IP methods Delineation of BBA Mapping of subsurface gold prospects/ mineralisation Validation of results using borehole data, EPMA and SEM–SED analysis of rock samples (a) Map showing profiles AA/, BB/, CC/ and DD/across the Babaikundi-Birgaon Axis (BBA). (b) Upward continued (60 m) self-potential (SP) anomaly map showing possible extension of a part of BBA. Schematic resistivity (c) and chargeability (d) distribution models showing possible extension of a part of the BBA. (e) mineralised sample with sulfide hosted quartz reef in BBA. The possible existence of BBA and the occurrences of gold-bearing sulfide minerals in quartz reef are mapped based on the horizontal locations and extensions of negative SP, high conducting / low resistivity in electrical resistivity tomography (ERT) and high chargeability in Induced Polarization (IP). Note that profiles C-C/ and D-D/ are not parallel to each other; their field orientation is shown in Figure 1(c) and Figure 2. [ABSTRACT FROM AUTHOR]

Coking/metallurgical coal is one of the essential raw materials for steel industries. The metallurgical coal in India contains high ash content (mineral matters), is rich in inertinite, and requires optimization before its proper utilization. Distribution of mineral matter in coking coal plays a significant role in achieving the desired product for blast furnace and coke-making operations. The present paper reports a case study on the distribution and nature of mineral matter in Indian coking coals at varying densities with respect to the coal's beneficiation behavior. The maceral analysis indicated the maximum liberation of reactives at the relative density (RD) of 1.2–1.3 g/cm3 while maximum inerts concentrated at the highest density (RD > 1.8 g/cm3). The most dominating mineral matter found in the coal is argillaceous minerals containing substantial amounts of quartz followed by carbonate and sulfides existing as cavity fillings or in disseminated form. X-ray diffraction (XRD), X-ray fluorescence (XRF), and electron-probe microanalysis (EPMA) techniques were used to study the mineral distribution and transformation behavior of coke produced from the coking coal during carbonization process. A case study on the reactivity of dispersed mineral matter in coke and the implication of mineral transformation behavior on coke is also discussed in this paper. [ABSTRACT FROM AUTHOR]

One atmosphere experimental studies have been conducted on two amygdule‐free representative samples from Khandwa, Madhya Pradesh, Central India, belonging to Eastern Deccan Volcanic province in order to understand mineral petrogenesis, crystallization history of the parent magma and role of liquid immiscibility. One atm experiments were carried out on these two samples at specified temperatures on designated time in order to understand crystallization history. The run products were identified both by optical means and electron microprobe analysis. The identified run products include pyroxenes, feldspar, opaques phase (oxide and sulphide), silica phases, and glass (both brown and colourless variety). The run product pyroxenes belong to quadrilateral pyroxene, and species‐wise correspond to augite, clinoenstatite, and pigeonite. Feldspars in majority are found to be anorthoclase (which are interpreted to have been formed during sudden crystallization in contact with water at shallow depth during experiments) and less commonly calcic plagioclase (in the range of labradorite to anorthite). Opaque phases have both oxide and sulphide, namely hematite, magnetite, martite, and pyrite, respectively. The zoned nature between the oxide and sulphide phases is suggestive of oscillation of oxygen fugacity during crystallization. The run product silica phase corresponds to tridymite. The silica‐rich glass (colourless) is overwhelmingly dominant, whereas silica‐poor glass (brown glass) is a manifestation of liquid immiscibility. The silica‐rich glasses give rise to direct crystallization of tridymite, whereas silica‐poor glasses act as the main driving liquid (or parent liquid) to further the crystallization towards lower temperature. The crystallization behaviour of the parent magma, and liquid immiscibility can be well documented with reference to forsterite‐silica phase diagram; in this phase diagram, the parent liquid corresponds to normative Qtz: 2.542, Fo: 0.098, and En: 0.34. The ambient parent liquid plots very close to Ab apex with reference to the Di‐Ab‐An phase diagram. The clear and distinct zonation pattern present in the opaque phases signifies that crystallization straddles between both oxidizing and reducing condition yielding crystallization of both oxide and sulphide phases. Our study indicates a unique role of liquid immiscibility of the parent magma supplemented by oscillation of oxidizing and reducing conditions giving rise to ensemble of variety of glass compositions, silicate, and opaque phases. [ABSTRACT FROM AUTHOR]

We present the findings on in situ trace element geochemistry of sulphides by LA‐ICPMS, sulphur isotope, and hydrothermal fluid evolution of copper ± gold mineralization at Dubarpeth, Central India. Mineralization is associated with chloritized‐silicified alteration zones, structurally localized along the terrain boundary fault between Archean TTG quartz diorite (2.5 Ga) of Western Bastar Craton and Paleo‐Mesoproterozoic cover sediments of Pranhita–Godavari (PG) Valley. Studies reveal that the Cu ± Au mineralization occurs in the form of dissemination, stringers, and veins along the microfaults and fractures. We infer three stages of mineralization based on the field observations and petrography studies. The early stage is characterized by the quartz–pyrite–chalcopyrite assemblage, while the middle stage is represented by quartz–chlorite–chalcopyrite ± pyrite. Calcite ± epidote is conspicuous in the late stage. Fluid inclusion microthermometry and Raman spectroscopy studies indicate that the early‐stage veins are mainly aqueous rich, mixed carbonic‐aqueous, and minor vapour‐rich aqueous inclusions. FIs in the early stage displayed evidence of vein formation during episode of fluid immiscibility with the homogenization temperatures and salinities up to 259°C and 27 wt.% NaCl equiv., respectively, indicating initial ore‐forming fluids of moderate temperature, moderate to high salinity in H2O–CO2–NaCl systems. Copper mineralization stage is liquid‐ and vapour‐rich (H2O–CO2–CH4 + N2) aqueous inclusions, with homogenization temperatures of 174–221°C and salinities of 6.37 to 13.9 wt.% NaCl equiv. This temperature range has been further supported by empirical chlorite thermometry based on Fe/Mg, AlIV, and AlV. The sulphur isotope values of pyrite and chalcopyrite showed δ34S interval of 2.29‰ to −10.85‰, characteristic of highly oxidized nature of the fluids and attributing sulphur source to magmatic origin. The ore‐forming fluids gradually became SO2−4 enriched and relatively oxidized during Cu mineralization. High Co–Ni trace element signatures of pyrite by LA‐ICPMS substantiate this observation, further indicating remobilization from a magmatic‐hydrothermal fluid. The presence of CH4 favours either a source of deep crustal nature or a mantle for the ore‐bearing fluid. We suggest that initial ore‐forming fluids are magmatic in origin that has been further cooled and diluted by mixing with meteoric water. Fluid boiling and mixing facilitated hydrothermal alteration and mineralization in the vicinity of the fault zone, which is perhaps activated during initiation of Paleo‐Mesoproterozoic segment of the Godavari Rift. The tectono‐magmatic setting, mineral assemblage, hydrothermal alteration, FIs, and trace element behaviour of pyrite (As–Co–Ni) suggest that the Dubarpeth Cu deposit is a high sulphidation epithermal vein type of deposit that is similar to those described for the shear/fault‐controlled epithermal deposits found elsewhere in the world. [ABSTRACT FROM AUTHOR]

Chemical erosion in river basins influence the climate by both consuming (via silicate erosion and carbon burial) and releasing (via H 2 SO 4 -mediated carbonate weathering and organic oxidation) atmospheric CO 2. Net erosional effect of a basin on carbon cycle, therefore, requires precise rate assessment of all these pathways; combined data for the four processes for a given basin are sparse. In this contribution, we have quantified these process intensities for a large tropical basin (Mahanadi river) from the eastern part of India using new time-series (weekly) data for dissolved major ions and sulfur isotopes, and compiled data for rhenium concentrations and particulate carbon isotopes for the basin. The discharge-weighted silica concentrations (∼215 μM) of these samples is higher than that reported as global average for rivers (∼145 μM), indicating intense silicate erosion in the basin. The δ34S of the riverine sulfates vary between 8.8 ‰ and 17 ‰, with a discharge-weighted average of 12 ± 4 ‰ (global riverine value ∼4.4 ± 4.5‰). Temporal changes in concentrations for most of the elements (and δ34S) show relatively higher (enriched) values during lean-flow stages. Mass balance calculations involving Ca, Mg and Si values confirm that these seasonal changes are mainly due to increase (by 18 ± 9%) of groundwater supply to the stream during non-monsoon seasons. The silicate-derived cations (Cat silicate) for the Mahanadi show minimal changes during monsoon (36 ± 5%) and non-monsoon (33 ± 8%) seasons. These data correspond to a CO 2 consumption rate of 2.4 ± 1.6 tC/km2/yr, which is about two times higher than that reported for global rivers. Comparison of δ34S values of riverine sulfate with its possible sources computes the relative supply of sulfates from atmospheric deposition (38%), gypsum (46%), and sulfides (16% (a lower limit)) to the river. The minimal contribution via sulfide oxidation confirms low CO 2 release (0.1 ± 0.1 tC/km2/yr) in this basin through sulfuric-acid mediated carbonate weathering. Average rhenium for this river is 5 ± 3 pmol/kg. The source apportionment using S/Re and Na/Re ratios indicates that 85 ± 10% of the Re is derived through organic oxidation. The rhenium data are used to estimate the CO 2 release rate (0.50 ± 0.57 tC/km2/yr) by oxidation of petrogenic carbon, which is higher than the estimated organic burial rate (0.27 ± 0.12) for the basin. The estimated four erosion rates confirm a net negative (∼ − 2.1 ± 1.7 tC/km2/yr) effect of chemical erosion in the Mahanadi basin on the CO 2 budget. Low sulfide-oxidation in this supply-limited regime is possibly linked to limited exposure of fresher minerals, whereas high silicate erosion in the basin is consistent with its silicate-dominated lithology and tropical climate. • Time-series study of major ions, and sulfur isotopes of a large tropical river (Mahanadi river, India) was conducted. • Increased concentrations and δ34S values indicate ∼20% increase in groundwater supply during the lean-flow stages. • Minimum one-sixth of riverine SO 4 is sulfide derived, indicating lower oxidation rate for this supply-limited basin. • Chemical erosion in this basin is a net CO 2 sink with higher intensities for silicate weathering and organic oxidation. [ABSTRACT FROM AUTHOR]

Magnetotelluric (MT), Electrical Resistivity Tomography (ERT), Time-Domain Induced Polarization (TDIP), Geochemical and Geological studies followed by drilling and down-hole logging were undertaken with in the Betul Fold Belt (BFB) in Central India, to demarcate zones of magmatic Ni-Cu-PGE sulfide mineralization. The BFB is predominantly composed of circular to elongate gabbro bodies of the Padhar Mafic-Ultramafic Complex, intruded into a sequence of bimodal volcanic rocks and quartzites. Near-surface samples of ultramafic rocks were subjected to precise geochemical analysis and scanned by an electron microscopy with an energy dispersive spectrometer (SEM-EDS). This work indicated the presence of pyrite, pyrrhotite, chalcopyrite, pentlandite, and minor amounts of W-Cd bearing boweiite and palladenite assemblage. These minerals are regarded as favorable to the occurrence of Ni-Cu-PGE sulfide mineralization. MT data derived from two profiles were analyzed and modeled using 2D and 3D inversion algorithms. The robust conductivity anomalies obtained from the MT model have been interpreted coupled with electrical tomography, geology, and geochemistry data. The near-surface shallow depth conductors observed in the ERT sections are interpreted as the sulfide mineralized zones. They corroborate the MT results. These conductive zones reflect the occurrence of the magmatic Ni-Cu-PGE bearing sulfide mineralization associated with rocks of the mantle-derived Padhar Mafic Ultramafic Complex. This geophysical data, in conjunction with petrological and geochemical analysis of drill core samples have allowed the identification of the origin and paragenesis of sulphide mineralization in the study area. Geochemical studies suggest that the parental magma was generated from a subduction modified, metasomatized and an enriched mantle source that was subsequently emplaced in a magmatic continental arc setting. The interpreted conductors, observed at shallow depths (∼200-300 m), have been generated by secondary hydrothermal fluid circulation leading to vein formation in the host Padhar Mafic-Ultramafic Complex. The MT and electrical tomography models delineate the geological boundaries of the sulfide-bearing mineralized deposits in the BFB. [Display omitted] • Integration of MT, ERT, TDIP results delineated magmatic sulphide mineralized zones. • Geophysical results corroborate with geological and geochemical and drill hole results. • Geochemical study suggest subduction modified, metasomatized, enriched mantle source. [ABSTRACT FROM AUTHOR]

The current work was off-shoot of an attempt to isolate a thermophilic pyritic sulfur degrading bacteria in selective medium from a coal mine dump. This thermophilic bacteria (archaea) classified as Sulfolobus was tested for its feasibility to bioleach copper from a low grade Indian chalcopyrite ore (0.3% copper). With its prevalent ability to preferentially attack pyrite (a major phase in the ore), bioleaching was investigated at various parameters of pH, pulp density (PD), particle size, and temperature. A reflective high 85% copper recovery was obtained using <50 μm particles at 20% (w/w) pulp density, pH 2.0, 75 ?C in 30 days. The copper dissolution was facilitated by iron (III) available in the leach liquor because of bacterial oxidation of pyrite present in the ore under acidic conditions. The biogenically generated Fe (III) ions enhanced copper dissolution from the chalcopyrite ore. The bioleaching of copper appeared to follow chemical control kinetic model with the reaction of lixiviant-Fe (III) and acid on the surface of the solid. Phase identification by XRD and SEM study corroborated the above mechanism of copper leaching. [ABSTRACT FROM AUTHOR]

Banded iron formation ( BIF) of the Gorumahisani-Sulaipat-Badampahar ( GSB) belt in Singhbhum Craton, India, consists predominantly of magnetite. This BIF is intruded by a magnetite dyke. The magnetite dyke is massive and compact with minor sulphide minerals while the host banded magnetite ore, a component of the BIF, shows thin lamination. The magnetite ore of the dyke is fine to medium grained and exhibits interlocking texture with sharp grain boundaries, which is different from the banded magnetite that is medium to coarse grained and show irregular martitised and goethitised grain boundaries. Relics of Fe- Ca- Mn- Mg-carbonate and iron silicates (grunerite and cummingtonite) are observed in the banded magnetite. The intrusive magnetite is distinctly different in minor, trace and REE geochemistry from the banded magnetite. The banded magnetite contains higher amounts of Si, Al, Mn, Ca, Mg, Sc, Ga, Nb, Zr, Hf, Co, Rb and Cu. In contrast, the massive magnetite is enriched in Cr, Zn, V, Ni, Sr, Pb, Y, Ta, Cs and U with higher abundance of HREE. In the chondrite normalized plot, the massive magnetite shows a slight positive Eu anomaly while the banded ore does not show any Eu anomaly. Field disposition, morphology, mineralogy and chemistry show that the intrusive magnetite dyke is of igneous origin, while magnetite in BIF formed from a carbonate protolith through the process of sedimentation. [ABSTRACT FROM AUTHOR]

The distributions of dissolved and particulate Mo and their isotope composition (δ 98 Mo) have been measured in the Narmada and the Tapi estuaries draining into the Arabian Sea. During monsoon, the δ 98 Mo of dissolved Mo in the Narmada estuary ranges from 0.49‰ to 2.19‰ in the salinity range 0–17.2 practical salinity unit (psu) quite similar to that in the Tapi estuary, 0.99–2.36‰, in the salinity range 0–20.3 psu. Mo concentration in suspended sediments of the Narmada estuary collected during monsoon average 512 ± 44 ng/g (range 459–602 ng/g) similar to that measured in one sample from the Tapi estuary 560 ng/g Mo. δ 98 Mo of particulate Mo in the Narmada ranges from −0.21‰ to 0.48‰ with an average −0.03 ± 0.2‰. Dissolved Mo in the Narmada and the Tapi rivers display isotopically heavier Mο compared to that in basalts, the major lithology of their drainage. This could result from a variety of processes, preferential weathering of Mo rich sulphide minerals dispersed in the basalts, preferential removal of isotopically lighter Mo during transport or contribution from marine cyclic salts supplied via rain or chemical weathering of organic rich shales in the basins. The distribution of δ 98 Mo in the Narmada and the Tapi estuaries with salinity does not follow the theoretical mixing line between river and seawater endmembers suggesting its non-conservative behavior. Particulate Mo and δ 98 Mo show concomitant increase with salinity in the Narmada estuary indicating loss of dissolved Mo by adsorption onto Fe–Mn oxyhydroxide. Balancing the Mo budget along the course of these estuaries using inverse model suggests that in the Narmada estuary there could be loss up to 8% of the dissolved Mo and that in the Tapi supply from anthropogenic sources could be up to 27%. The results obtained in this study bring out the processes modifying riverine input of Mo and its δ 98 Mo in the estuaries, oxic sink in the Narmada and anthropogenic input in the Tapi. Repetitive adsorption and desorption of Mo in the Narmada estuary can modify the supply of dissolved Mo and its δ 98 Mo relative to riverine supply by up to 40%, this can significantly impact the Mo isotope budget of the oceans. In contrast, in the Tapi estuary there is enhancement in the dissolved supply of Mo relative to that from river due to anthropogenic input of Mo. The investigations in these two estuaries underscore the importance of solute particle interactions and anthropogenic input in determining the Mo flux and its δ 98 Mo to the open Arabian Sea. [ABSTRACT FROM AUTHOR]

Dhanjori formation, a significant member of Singhbhum Craton, Eastern India, is well-known for its complex mineralogical character and huge content of Au-mineralization. This 2.1 Ga old Paleoproterozoic formation is composed of siliciclastic metasedimentary rocks interlayered with mafic/ultramafic volcanic rocks. Traces of gold has been spotted by many researchers in the sulfide-hosted mafic-ultramafic volcanic rocks and quartz pebble conglomerate units by different studies. Regional Audio Magnetotelluric (AMT) survey was conducted in 10 kHz–10 Hz frequency range over various lithological units of Dhanjori and adjacent Iron Ore Group. The key challenge faced during the analysis of magnetotelluric (MT) data is due to the presence of near-surface heterogeneities. The amplitude of the MT response gets distorted by galvanic effects which further create difficulties in modelling and inversion. If the effect is not removed in advance, it might lead to erroneous subsurface modelling and interpretations. On the contrary, the phase tensor data does not show any change in values even in the presence of galvanic distortion. Though phase tensors are the computed values from impedance tensor, still the response is free from any shallow level ambiguities unlike impedance data. We have performed 3-D inversion of phase tensor data to generate a conductivity model of Dhanjori basin. The objective was to verify whether the inclusion of phase tensor components in the inversion routine was able to provide some extra information that might add value to the analysis. Model from phase tensor inversion indicates only relative variation in the subsurface conductivity structure due to lack of amplitude information in the data. Hence, we also performed 3D joint inversion including both impedance and phase tensor components for obtaining the final conductivity model of Dhanjori to recover the absolute subsurface conductivity structure unaffected by galvanic distortion. Correlation of the conductivity model with available geoscientific information in Dhanjori and adjacent geological units helped us to relate the conductors to the sulfide mineralization prospect. • Study area has complex mineralogical character and Au-mineralization content. • 3-D inversion of MT phase tensor data performed. • Joint inversion of MT impedance and phase tensor components conducted. • Conductivity model with available geology identifies sulfide mineralization. [ABSTRACT FROM AUTHOR]

Semra komatiites that occur at the NW fringe of the Chotanagpur Gneissic Complex represent high PGE content, associated with the pyrrhotite, pentlandite, chalcopyrite and pyrite. They display wide variation in the IPGE concentration and characterized by Pd/Pt values (> 1) similar to that of primitive mantle. The negative Ir anomaly is attributed to temperature dependant solubility limit, therefore, not a reflection of bulk mantle IPGE content. Wide variation in the Au content indicates its highly mobile nature. Metasomatism and serpentinisation possibly have played significant role in the Au enrichment. [ABSTRACT FROM AUTHOR]

Abstract: Sediments from a core retrieved during installation of a shallow drinking water well in Ambikanagar (West Bengal, India) were analyzed for various physical and chemical parameters. The geochemical analyses included: (1) a 4-step sequential extraction scheme to determine the distribution of As between different fractions, (2) As speciation (As3+ vs. As5+), and (3) C, N and S isotopes. The sediments have a low percentage of organic C and N (0.10–0.56% and 0.01–0.05%, respectively). Arsenic concentration is between 2 and 7mgkg−1, and it is mainly associated with the residual fraction, less susceptible to chemical weathering. The proportion of As3+ in these sediments is high and ranges from 24% to 74%. Arsenic in the second fraction (reducible) correlates well with Mn, and in the residual fraction As correlates well with several transition elements. The stable isotope results indicate microbial oxidation of organic matter involving SO4 reduction. Oxidation of primary sulfide minerals and release of As from reduction of Fe-(oxy)hydroxides do not seem important mechanisms in As mobilization. Instead, the dominance of As3+ and presence of As5+ reducing microorganisms in this shallow aquifer imply As remobilization involving microbial processes that needs further investigations. [Copyright &y& Elsevier]

Abstract  The ∼200-km-long intensely deformed Singhbhum Shear Zone (SSZ) in eastern India hosts India’s largest U and Cu deposits and related Fe mineralization. The SSZ separates an Archaean cratonic nucleus to the south from a Mesoproterozoic fold belt in the North and has a complex geologic history that obscures the origin of the contained iron-oxide-rich mineral deposits. This study investigates aspects of the history of mineralization in the SSZ by utilizing new petrographic and electron microprobe observations of pyrite textures and zoning in the Turamdih U–Cu(–Fe) deposit. Mineralization at Turamdih is hosted in intensively deformed quartz–chlorite schist. Sulfides and oxides include, in inferred order of development: (a) magmatic Fe(–Ti–Cr) oxide and Fe–Cu(–Ni) sulfide minerals inferred to be magmatic (?) in origin; followed by (b) uranium, Fe-oxide, and Fe–Cu(–Co) sulfide minerals that predate most or all ductile deformation, and are inferred to be of hydrothermal origin; and (c) Fe–Cu sulfides that were generated during and postdating ductile deformation. These features are associated with the formation of three compositionally and texturally distinct pyrites. Pyrite (type-A), typically in globular–semiglobular composite inclusions of pyrite plus chalcopyrite in magnetite, is characterized by very high Ni content (up to 30,700 ppm) and low Co to Ni ratios (0.01–0.61). The textural and compositional characteristics of associated chalcopyrite and rare pyrrhotite suggest that this pyrite could be linked to the magmatic event via selective replacement of magmatic pyrrhotite. Alternatively, this pyrite and associated sulfide inclusions might be cogenetic with hydrothermal Fe-oxide. Type-B pyrite that forms elongate grains and irregular relics and cores of pyrite with high Co contents (up to 23,630 ppm) and high Co to Ni ratios (7.2–140.9) are interpreted to be related to hydrothermal mineralization predating ductile deformation. A third generation of pyrite (type C) with low Co, low Ni, and moderate Co to Ni ratios (0.19–13.93) formed during and postdating the ductile deformation stage overgrowing, replacing, and surrounding type-B pyrite. The textural evolution of pyrite parallels the tectonometamorphic evolution of the shear zone demonstrating grain elongation during progressive ductile deformation and prograde metamorphism, annealing at the peak metamorphic condition, porphyroblastic growth at the retrograde path and cataclasis following porphyroblastic growth. Compositional characteristics of hydrothermal pyrite and available geological information suggest that the U–Cu(–Fe) deposit at Turamdih might be a variant of the Fe oxide (–Cu–U–rare earth elements) family of deposits. [ABSTRACT FROM AUTHOR]

During an investigation of some high-sulphur coals from Meghalaya, Northeast India, with respect to their trace metal content, we recorded the occurrence of gold in iron sulphides (pyrites/marcasite) associated with such coals. A maximum of 6 mg/l gold has been recorded in the pyrite concentrates. Gold is lattice-bound and occurs in 'elemental form', forming solid solutions within iron sulphides. It is inferred that the iron sulphides are mostly syngenetic with coal and gold is concentrated in iron pyrites by biological activity during their precipitation and by inorganic adsorption process during recrystallization in a reducing environment. [ABSTRACT FROM AUTHOR]

The article deals with the sulphur isotope composition of galena from Bandalmottu area, Guntur District in Andhra Pradesh, India. A location map of Bandalamotta deposit in Cuddapah Basin is given. The Bandalamottu mineralization through isotope data are presented. It is noted that there is a restricted range that is characteristic of magmatic-hydrothermal-type deposits.

Coal from northeastern India is considered to be a very good quality coking coal owing to its low ash content (<8%). It has an excellent caking property, and after carbonisation it can provide a very good CSR (coke strength, after reaction with CO[sub 2]). On the other hand, its metallurgical application is not so encouraging as a result of its high sulphur content (3-7%). Normally the acceptable sulphur content in metallurgical coke is 0·7% maximum. About 90% of the sulphur is present in organic form, and the remaining 10% as pyrites and sulphates. Removal of sulphur from the organic types is somewhat complicated as it cannot be done by any physical process such as benificiation or flotation, whereas sulphur in pyrites and sulphates can be removed by physical means. With a view to lowering the sulphur content of the coal, desulphurisation studies were carried out at elevated temperatures ranging from 400 to 950°C under the flow of various reducing gases such as coke oven gas, water gas, steam and pure hydrogen. The retention time at each temperature of experimentation was usually varied at 15, 30 and 45 min. It was observed that the sulphur content in the coal was decreased in every experiment with the various reducing gases. The extent of sulphur removal in treatment of the coal with coke oven gas was up to 78·1%, that with steam 83·1% and that with water gas 85·7%. The minimum sulphur content found in desulphurised coal was 0·86%, which could not be used for metallurgical purposes unblended. In the case of experiments with steam, a severe loss of carbon (15-20%) was observed because of reaction with the steam, whereas no such losses were observed in experiments with the other two reducing gases. Hydrogen lowered the sulphur content at a faster rate in comparison with the other gases, but could not increase the degree of reduction. The present study may help to establish the technoeconomic feasibility of the processes; however, large scale experimental studies are required for confirmation. I&S/1721 [ABSTRACT FROM AUTHOR]

[Display omitted] • Biotite and uraninite related to IOCG mineralization are associated with redox sensitive minerals such as magnetite, ilmenite, and graphite. • Calculation of Fe3+ in biotite using methods of Dymek (1983) and Guidotti and Dyar (1991) • Mineral association, Fe3+ content of biotite, and U/Th ratio of uraninite point to redox conditions reaching below the Fayalite-magnetite-quartz (FMQ) buffer. • The presented method could be useful for magnetite-poor IOCG systems. Key constraints related to the nature of alteration and mineralization in geologically complex and economically important iron oxide copper–gold (IOCG) systems remain highly speculative, especially, the physicochemical characteristics of the hydrothermal fluids. This study was conducted on samples from the Khetri Copper Belt (KCB) in western India that hosts several IOCG-type deposits. The study aims to constrain qualitative oxygen fugacity conditions during alteration-mineralization events using biotite and uraninite chemistry. The inferred redox conditions are backed up by the presence or absence of redox-sensitive minerals including magnetite, ilmenite, and graphite. Biotite is present in association with REE ± Th ± U and Cu-Co-U-REE mineralization, which has a genetic connection with IOCG-style K-Fe-Mg alteration. The first type of mineralization is mostly sulfide-free and is characterized by the presence of REE-silicate, REE-phosphate, U-Th-silicate, and U-oxides with minor ilmenite, magnetite, and pyrite. The other type is characterized by the presence of Cu-Co-Fe sulfides, U-oxides, REE-silicates and -phosphates with significant volume of graphite. The Fe3+ in biotite was calculated from electron probe data by two different methods, and U and Th concentrations in uraninite were measured by an electron probe microanalyzer (EPMA). Stability of redox-sensitive phases (magnetite, ilmenite, and graphite), Fe3+-Fe2+-Mg content of biotite, and U/Th ratio of uraninite collectively suggest redox conditions essentially below the haematite-magnetite buffer often reaching the fayalite-quartz-magnetite buffer. We conclude that the studied alteration-mineralization assemblages represent the deeper and reduced facies of the Khetri IOCG system. We suggest that biotite and uraninite chemistry can be used as an indicator of redox conditions for alteration/mineralization assemblages in IOCG systems where redox-sensitive minerals particularly magnetite is absent. [ABSTRACT FROM AUTHOR]

[Display omitted] • Shankaraghatta belt is constituted by serpentinites and mafic schists. • Serpentinites host disseminated magmatic sulfide phases. • AFC-process is responsible for inducing sulfide saturation in komatiitic melt. • Komatiitic plume was derived from a majorite garnet source (>400 km). • Magmatic sulfide phases experienced distinct hydrothermal overprinting. The Shankaraghatta ultramafic–mafic suite of rocks (12 × 0.3 km) within the late Archean Shimoga greenstone belt (Western Dharwar Craton, southern India) is constituted of sulfide-bearing serpentinized dunite flanked by metabasaltic schist rocks, which are probable analogues of the adcumulate-mesocumulate sheet flows in a komatiitic sequence, flanked by komatiitic basalts. Owing to limited field exposures in this study, the bulk concentrations of Ni, Cr, Ti, Fe, and Mg were used to classify the rocks into different volcanogenic facies observed in a komatiitic flow. The mineralized mesocumulate (serpentinized dunites) flanking its S-poor adcumulate counterpart host 5–8 modal % disseminated sulfide globules formed by an assemblage of pentlandite-millerite-pyrite-chalcopyrite. Millerite is the dominant sulfide mineral, whereas coarse euhedral pyrite grains are abundant in the serpentinites. The sulfide-bearing serpentinites have higher platinum-group element (PGE) abundances than those lacking visible sulfides (∑PGE sulfide-bearing samples = 440–1848 ppb; ∑PGE sulfide-lacking samples = 4.8–69 ppb) and show a strong PPGE (Pd, Pt, Rh) enriched trend with distinct negative Pt-anomalies in the primitive-mantle normalized PGE diagram. The mineralized serpentinites have higher Ni and Cu concentrations (0.5–2.2 wt% and 29–201 ppm, respectively) compared to their unmineralized counterparts (0.1–0.6 wt% and 22–72 ppm, respectively). Major element data indicate a komatiitic composition for the serpentinites, whereas the schists are komatiitic basalts. Chondrite-normalized REE plots for the serpentinites and schists show LREE-enriched patterns (La/Sm N serp = 1.8–3.5; La/Sm N schist = 2.5), with almost flat to slightly depleted HREEs (Gd/Yb N serp = 1.1–1.7; Gd/Yb N schist = 1.8). The rocks show prominent negative anomalies of Zr and Hf, which along with the subchondritic Al 2 O 3 /TiO 2 ratios for serpentinites (~9.9), indicate derivation of the komatiitic magma from a plume that originated from a deep mantle source (>400 km) with the mantle residue retaining a majorite garnet phase. The komatiites and komatiitic basalts from which the serpentinites and schists were derived have a common mantle source. They are connected by fractional crystallization as understood from collinear and coherent patterns in bivariate plots between major oxides such as MgO, and alteration resistant immobile trace elements such as Nb, that are plotted with other major and trace elements, as well as from the coherent patterns observed in the chondrite-normalized REE and multi-element spider diagrams. The serpentinites and schists show signatures of crustal contamination as defined by negative anomalies of Nb, Ta, and Ti, high La/Sm ratios, along with higher concentrations of trace elements like La and Th, and lower Nb/Th ratios (Nb/Th avg serp = 6.6; Nb/Th avg schist = 3.7). AFC (Assimilation Fractional Crystallization) models indicate that the schists underwent a greater extent of crustal contamination (50% AFC) relative to the serpentinites (12% AFC). Sulfide saturation was achieved late, after emplacement of the komatiitic magma and crystallization of the olivine adcumulates, when the melt was slightly more differentiated to form the mesocumulates. The sulfides are later overprinted by low-temperature alteration processes, as evident from the significant enrichments in serpentinites for Au (Au N avg: 2.1), As (As N avg: 20), Bi (Bi N avg: 8.2), and Te (Te N avg: 1.5), which occurred perhaps during the replacement of pentlandites by millerites. [ABSTRACT FROM AUTHOR]

[Display omitted] • Co/Ni ratio of pyrite characterized submarine-magmatic hydrothermal ore process. • High-temperature VMS system > 300 °C formed from multiple sulfur sources. • Geochemical trends in sphalerite and pyrite by LA-ICP-MS suggest VMS-type deposits in the Betul belt. • PCA on pyrite and sphalerite helps in the reconstruction of the metallogeny and exploration prospectivity. • Chalcopyrite disease in sphalerite reflect high-temperature ore fluid and metamorphism. The Paleoproterozoic volcanogenic massive sulfide (VMS) deposits in the Betul belt (BB) (ca.1.7 Ga) in central India are well-known for their low-grade Zn-Cu-Pb sulfide mineralization. The physicochemical conditions of ore formation, size, grade, extent, and nature of mineralization are the ongoing debates and concerns to the exploration agencies for the last two decades. Field examination, drill core analysis, petrography, and scanning electron microscopic studies reveal the occurrence of chalcopyrite, sphalerite, pyrite, galena with minor pyrrhotite in the form of dissemination, stringers, and semi-massive sulfide veins. We present the minor-trace element contents in sphalerite and pyrite from drill core samples of four deposits (Banskhapa, Jangaldehri, Biskhan, and Bhuyari), analyzed by electron probe microanalyzer (EPMA) and in-situ laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS) for determining ore-forming temperature, ore genesis and exploration prospectivity. The EPMA and LA-ICP-MS studies of sphalerite display a slightly high concentration of trace elements like Mn, In with a mean value of 3533 ppm and 33 ppm, respectively. On the other hand, Ga, Ge, and Ag content are low in abundance along with a low concentration of Bi, Pb, and Sb in all four deposits. Pyrites show low Ni and high Co/Ni (mostly > 1) ratios and variable Ti, Se, As, and Mn. Multivariate statistical analysis, especially the principal component analysis (PCA) of these trace elements, defines the geochemical variations among the Betul belt deposits. The ore-forming temperature is estimated using sphalerite thermometry (374 °C to 402 °C) and pyrite thermometry (225 °C to 484 °C). The geochemical discrepancies occur due to magmatic-hydrothermal activities caused by the sub-volcanic intrusions, followed by recrystallization and remobilization of the sub-microscopic inclusions during subsequent metamorphism. Additionally, sulfur isotope analysis provides insights into the sulfur source variations, metals, and mineralization. Carbon and Oxygen isotopes are used to understand the physicochemical conditions of the ore-forming environment. Sphalerite, pyrite, and chalcopyrite from Banskhapa deposit suggest a broad range of δ34S values (7.27–7.79‰, 8.37–8.61‰, and 6.58–6.72‰ respectively, n = 7), which advocate sulfur mostly derived by thermochemical reduction (TSR) of seawater sulfate with some input of magmatic sulfur leaching from the igneous basement. Carbon and oxygen isotope studies of carbonates from Jangaldehri and Bhuyari deposits (δ 13C: −6.0 to −13.45‰ with an average value of −9.51‰, n = 7; δ 18O: 7.32‰ to 20.58‰ with an average value of 15.65‰, n = 7) indicate that the carbonates formed from a hydrothermal fluid (seawater with an insignificant magmatic input), the mixing of hydrothermal fluid with ingressing seawater, or both in the mineralized zone. These processes collectively led to Zn-Cu and Zn-Cu-Pb mineralization in the Betul belt. High temperature, strong isotopic and trace element geochemical variations in sphalerites (Ag-Mn, In/Cd-Mn, In/Cd-Fe, Mn-In/Ge, Fe-In/Ge, Fe-In) and pyrite (Ni-Co, As-Cu, Ag-Au, and Se-Te) attest to the mineralization as being volcanogenic massive sulfide (VMS) deposits in Betul belt and differentiated from other genetic types like Mississippi Valley-type (MVT), sedimentary-exhalative (SEDEX), skarn, porphyry, and epithermal hydrothermal deposits. [ABSTRACT FROM AUTHOR]

[Display omitted] • Structurally oriented massive lenses of Fe-Ti oxide ores within gabbronorite. • An unusual mode of occurrence, mineralogy and geochemistry of the ore bodies. • Fe-Ti oxide ore and gabbronorite formed from an enriched basaltic melt. • The parent melt generated by low degree partial melting of enriched mantle source. • Silicate fractionation increased Fe-Ti content and f O 2 of the residual melt. Magmatic Fe-Ti oxide ore bodies of the eastern Chotanagpur Granite Gneiss Complex (CGGC), India occur as massive conformable lenses within a gabbronorite sill. The occurrence, lithological association, mineralogy, and geochemistry of the Fe-Ti oxide ore bodies are significantly different from the typical Fe-Ti oxide deposits associated with anorthosite complexes and mafic- ultramafic complexes. Textural evidence and mineral chemistry suggest mostly simultaneous fractional crystallization of the comprising minerals e.g. titanomagnetite, ilmenite and magnesio-hercynite. The temperature and f O 2 conditions of re-equilibration are calculated using the compositions of ilmenite and pre-exsolved titanomagnetite pairs. Extrapolation of the data along ulvospinel isopleths reveals the probable original magmatic conditions e.g. temperature ~ 900 °C and f O 2 ~ ΔFMQ + 2. The ore forming melt calculated by 'equilibrium distribution method' shows no crustal contamination and has a trace elemental pattern similar to typical enriched basaltic melts e.g. ferropicrites, alkali basalts, OIB and EMORB. High Ti and Zr contents of the melt suggest a possible within plate tectonic setting and enrichment in HFSEs and LREEs suggest that the parent melt could be generated from an enriched mantle source by low degree partial melting of garnet-peridotite layer of the upper mantle. The 'quantitative fractional crystallization modelling' indicates that early crystallization of olivine, pyroxenes, plagioclase and ilmenite from an enriched basaltic melt similar to EMORB (most fit melt) led to a condition of high oxygen fugacity and iron enrichment in the residual melt. The residual Fe-Ti rich melt moved to low pressure zones within the gabbronorite sill during late stage of D 2 deformation, and later crystallized in the form of present ore bodies. [ABSTRACT FROM AUTHOR]

[Display omitted] • Vein type magmatic epithermal deposit hosted in mafic dykes of Eastern Dharwar Craton, India. • Zoned alteration pattern with narrow central potassic zone surrounded by wide propylitic halo. • Ore precipitation caused by increase of fluid pH via fluid-rock interaction. • Main sulfide ores emplaced in low temperature (180–200 °C) and pressure (1.2–2 kbar) • Carbon and sulfur isotope data suggest magmatic source of fluid. The Archaean Dharwar Craton was intruded by mafic dyke swarms during early Paleoproterozoic and some of the E-W trending dykes show evidence of copper mineralisation. In the present study, we have conducted detailed mineralogical, fluid inclusion and C-O isotopic study of the Tintini copper deposit (TCD), which is hosted by a mafic dyke (ca. 2.21 Ga) in the Yelagatti granitoid near the Hutti-Maski greenstone belt. The main copper mineralisation is hosted in steeply dipping quartz-calcite veins that cut across N85°E trending diabase dyke. The TCD shows a typically zoned alteration pattern akin to magmatic-hydrothermal deposits, with a narrow potassic alteration zone at the centre and a peripheral wide propylitic alteration halo. Formation of hydrothermal K-feldspar and amphiboles indicate high temperature (>450 °C) of alteration during potassic alteration stage. The proximal central alteration zone was affected by later pulses of cooler fluids that caused extensive propylitisation of the early potassic alteration assemblage. Geothermometry based on mineral chemistry of hydrothermal chlorites (type-I trioctahedral) of propylitic stage show 260–300 °C temperature of formation. The main sulfide ores were precipitated during a later stage after propylitisation, at a temperature (180–200 °C), much lower than the formation temperature of the early chlorite (260–300 °C). The pressure of emplacement of ores is calculated as 1.2–2 kbar. It is inferred that copper was carried as aqueous chloride complex and that ore precipitation took place because of change in fluid pH due to fluid-rock interaction. The δ13C fluid(VPDB) (–6.38 to –6.03‰) and δ34S H2S(VCDT) (1.0 to 3.7‰) compositions of hydrothermal fluid are consistent with a magmatic source for the hydrothermal fluid. The alteration assemblage and sulfide mineralisation of TCD show evolution and cooling of hydrothermal system from high temperature (>450 °C) potassic alteration stage, to intermediate temperature (260–300 °C) chlorite stage, to low temperature (180–200 °C) main sulfide stage. The oxygen fugacity of ambient fluid shows wide variation. TCD can be classified as intrusion related, fracture-controlled, vein-type epithermal deposit. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mayurbhanj mafic complex in India contains Fe-Ti-(V) oxide ore deposit within gabbro-norite-anorthosites. • Higher chalcophile element concentrations in magnetite indicate equilibration with co-magmatic sulfide minerals. • Enrichment of Pb, Ag, Bi, As, Sb in few pyrites suggest a hydrothermal alteration of primary magmatic pyrites. The Archean Mayurbhanj mafic complex in the Singhbhum Craton (eastern India) contains a magmatic Fe-Ti-(V) oxide ore deposit within a gabbro-norite-anorthosite suite of rocks. Immediate country rocks belong to the early Archean Gorumahisani greenstone belt and 3.1 Ga granitic complex. The gabbro-noritic rocks of the Mayurbhanj complex consist of plagioclase (An 26−69), clinopyroxene (En 42−54 Fs 4−36 Wo 22−42), and orthopyroxene (En 67−81 Fs 15−28 Wo 4−5) as cumulus phases with Fe-Ti-oxides (magnetite and ilmenite) and sulfides (pyrite, pyrrhotite, chalcopyrite) as disseminated phases. Two-pyroxene thermometry yields temperatures of 1199 °C − 956 °C which are close to the primary liquidus temperature of pyroxene from a fractionated basaltic magma. The co-existing magnetite and ilmenite pairs in gabbro and magnetitite yield equilibrium temperatures of 498–797 °C and oxygen fugacities of 11.38–27.1 (log f O 2) reflecting various stages of cooling and subsolidus re-equilibration of the mineral phases. Disseminated sulfides are mainly present along the grain boundaries of accessory Fe-Ti oxide grains, within interstitial spaces of silicate minerals, as inclusions within the silicate minerals and along altered fracture zones of the silicate host whereas the magnetitite bodies are devoid of such sulfide mineralization. In-situ trace element analysis of oxide and sulfide minerals from the gabbro-noritic rocks and magnetitite ores reveals a higher concentration of Cr (22301–67433 ppm) and Ni (896–3880 ppm) in the magnetite of gabbro than in the magnetitite bodies. This is due to early fractionation of the accessory magnetite in the gabbro from a Fe-rich basaltic magma. The lower concentration of V (30211–34608 ppm) in the accessory magnetite of the gabbro-noritic rocks than in the magnetitite (48534–64855 ppm) suggests partitioning of vanadium into clinopyroxene within the gabbroic rocks. Higher chalcophile element (Pb ≈ 2–11 ppm, Zn ≈ 6773–8139 ppm, Mo ≈ 25–68 ppm) concentrations in magnetite of the gabbro-noritic rocks indicate equilibration with co-magmatic sulfide mineral assemblages. The partitioning behavior of HFSE (Zr, Hf, Ta, Nb) in the magnetite and ilmenite is mainly dependent on the Ti4+ content of the minerals. The rim of iron-oxide minerals around sulfide grains and negative Ni anomaly in the trace element pattern of pyrite suggest that these pyrite grains have formed after pyrrhotite which is magmatic in origin. Enrichment of Pb (≈ 27–3120 ppm), Ag (≈ 8–15 ppm), Bi (≈ 0.2–9 ppm), As (≈ 48–6561 ppm), and Sb (≈ 0.4–18 ppm) in a few pyrites suggests hydrothermal alteration of primary magmatic pyrite grains. [ABSTRACT FROM AUTHOR]

[Display omitted] • Zn-rich mineralization is hosted by intensely altered quartz-porphyritic rhyolite. • A-type, high-temperature rhyolite contains high HFSE and REE contents. • Zoned metamorphosed alteration show high-Mg core and outer high-K envelop. • VMS-type mineralization formed in a rifted continental back-arc setting. • Pb-model ages of galena constrain VMS mineralization at ~1800 Ma. The Palaeoproterozoic Betul Belt in central India contains several small base metal (Zn-Cu and Zn-Pb-Cu) deposits that are hosted by felsic volcanics which form part of a bimodal volcanic suite. Although a volcanogenic massive sulfide (VMS) origin has been attributed to these deposits, they are not well-defined based on modern classification criteria. Regionally, the volcanic facies association around the Bhuyari zinc prospect includes massive rhyolite, pillowed basalt, rhyolitic autobreccia, hyaloclastite and peperite that indicate volcanism and sedimentation in relatively deep water. At the deposit-scale, the mineralization is hosted by intensely altered massive quartz-porphyritic rhyolite and associated tremolite-carbonate rock. Alteration facies mapping show a zoned alteration system with an inner phlogopite-biotite-chlorite-garnet-rich core (BPG schist; interpreted as having a chlorite-rich protolith) and an outer quartz-muscovite-K-feldspar zone (QMK schist; sericite-rich protolith). Metamorphism was largely isochemical and the mineral assemblages are amphibolite facies equivalents of hydrothermally altered protoliths. Tremolite-carbonate rocks are spatially close to mineralization and are interpreted either as metaexhalites or intensely altered rhyolite and probably had talc-dolomite protoliths. Carbon and oxygen isotope signatures of carbonate show derivation from seawater-dominant hydrothermal solutions. Tremolite-carbonate and some BPG schist show enrichment in HREE indicating hydrothermal mobilization of REE. Geochemically, the least-altered rhyolite has transitional magmatic affinities with moderate negative Nb anomaly and strong negative Sr, and Ti anomalies. Mantle upwelling and the associated high-heat source generated high-temperature, high-silica rhyolites with elevated HSFE and REE contents. Emplacement of felsic melts at depth provided heat that drove hydrothermal fluid circulation and formation of zinc-rich mineralization and associated hydrothermal alteration zones. Pb-model ages of galena give ~1800 Ma using the Stacey and Kramers (1975) model for VMS mineralization in Bhuyari. The Bhuyari deposit possibly formed in a continental back-arc rift environment during the amalgamation of Bastar and Bundelkhand cratons along the Central Indian Tectonic Zone. The high HFSE-REE contents of Bhuyari rhyolites are similar to other highly prospective VMS districts formed by rifting of evolved continental crust. The recognition of metamorphosed VMS deposit at Bhuyari has implications for further exploration of such deposits along the Paleoproterozoic Betul Belt and similar-aged deformed terrains in India and elsewhere. [ABSTRACT FROM AUTHOR]

• Ore petrography and EPMA studies reveal chalcopyrite disease in Fe2+ rich sphalerite at Biskhan and Jangeldehri. • The absence of chalcopyrite disease in Fe2+ poor sphalerite at Ghisi is due to variable conditions of the reduced hydrothermal fluid, and different depth of mineralization. • The significant and wider variation of δ 34S VCDT of sulfide from three VHMS deposit of Betul Belt suggestive of multiple sources. • Different depth S reduction leads to different assemblages of mineralization (Zn–Cu–Pb to Zn–Cu). This work provides an overview of the geological, petrological, and geochemical data available on the three volcanic-hosted massive sulfides (VHMS) deposits (Ghisi, Biskhan, and Jangaldehri) in Betul Belt, Central Indian Tectonic Zone. Sulfur isotope geochemistry was used for the first time from the Betul Belt to constrain the source of sulfur for a better understanding of the ore genesis process. Mineralization is hosted in a felsic dominant bimodal volcano-sedimentary sequence associated with syn-genetic hydrothermal alteration, i.e., intense Mg–Ca alteration (actinolite-tremolite-anthophyllite) and Al–Fe alteration zone (gahnite-garnet-biotite-staurolite) followed by regional metamorphism. Field observations, drill core logging, and petrography studies reveal different modes of mineralization, i.e., dissemination, stringers, and semi-massive sulfides vein, comprises of chalcopyrite, sphalerite, pyrite, galena with lesser pyrrhotite, are paragenetically distinct in three deposits. The petrography, SEM-EPMA studies show the presence of chalcopyrite disease in Fe-rich sphalerite at Biskhan and Jangaldehri in the east and absence of the same texture at Ghisi in the west. This difference reveals the variable physio-chemical nature of the hydrothermal fluid and partial melting of sulfides during subsequent metamorphism. Cu–Fe mineralization was deposited from high-temperature hydrothermal fluids with less sulfur activity at Biskhan-Jangaldehri. Later Zn–Cu–Pb mineralization was formed from lower temperature hydrothermal fluids with higher sulfur activity. The δ 34S values of sulfides (sphalerite, chalcopyrite, and pyrite) from three deposits show a wide range of sulfur values (δ 34S) from −12.87 to + 19.31‰ (n = 27), consistent with heterogeneous sourcing of S, probably combining magmatic source along with the reduction of seawater sulfate. Variation in the sulfur isotopic compositions of sulfide was the result of dilution and cooling of the metalliferous fluid at different stages after interaction with meteoric, seawater and hydrothermal fluids, which caused the deposition of base metal sulfides. Sulfides from Ghisi in the west display both +ve and −ve values (−12.88 to + 0.38‰; n = 10), suggest sulfur predominantly derived by thermochemical reduction of seawater sulfate with minor input of magmatic sulfur reduction lead to polymetallic Zn–Cu–Pb mineralization. The gradual increasing in the δ 34S values (more + ve values +18.65 to +19.31‰; n = 8) from west to east at Biskhan and Jangaldehri (+ 5.01 to + 8.3‰, n = 9) may be due to involvement of both the leaching of igneous basement rocks and chemical process of seawater sulfates by thermochemical reduction (TSR) from deep water levels lead to Zn–Cu mineralization. This δ 34S variation occurs in three deposits of the Betul Belt due to modifications of the primary mineralization during subsequent stages of ore formation and metamorphism, indicating a complex fluid evolution history for VHMS deposits in Betul Belt, Central India, where S derived from a heterogeneous source by multiple geological processes. This situation is more akin to many established VHMS deposits of ancient and modern submarine hydrothermal systems. [ABSTRACT FROM AUTHOR]

• Significant melting possible at 500 °C in the system PbS-Cu 2 S-Sb 2 S 3 -Ag 2 S. • All Cu, Sb and Ag essential for abundant melting. • 21 to 35 mol % of PbS can be incorporated in this melt. • S-deficiency in melt may give rise to S-enriched stable phases such as famatinite. • Ni in galena and Ag-Pb-Sb-S phases may be used as a sign of sulfide partial melting. In the present study, we have documented hitherto unreported Ag-Pb-S, Ag-Sb-S and Sb-Ag phases associated with galena, pyrrhotite, sphalerite, freibergite, argentite, gudmundite, nisbite and breithauptite from the remobilized ore at the Sindesar Khurd base metal sulfide deposit, Rajasthan, India. These Ag-Pb-Sb-(±Cu)-S phases display pronounced compositional variability and intimate reaction relationship with coexisting sulfides and probably signify crystallization from a sulfide partial melt. To examine the possibility and nature of such melts, we have conducted evacuated silica tube experiments at 500 °C in the system Cu 2 S-Sb 2 S 3 -PbS-Ag 2 S at 10 mol % Ag 2 S. The melts obtained in these experiments are S-deficient, with varying concentrations of Cu (14.65–21.01 atom %), Ag (7.03–20.32 atom %), Sb (9.32–19.92 atom %), Pb (6.35–11.32 atom %) and S (44.16–48.41 atom %). The solid phases obtained are galena ss , Ag-tetrahedrite and famatinite. Non-stoichiometric S-enrichment is observed in the solid phases with a complementary S-deficiency in the melt produced, in which Ag and Sb are enriched. The results of this study indicate that significant amounts of Pb-, Cu-, Sb- and Ag-enriched sulfide melts can be produced even at 500 °C and fractionation of Ag and Sb into sulfide partial melt may concentrate Ag and Sb during metamorphism of base metal sulfide deposits. The various Ag-Pb-Sb-(±Cu)-S phases observed at Sindesar Khurd perhaps are the products of late-stage crystallization and testify to the presence of sulfide partial melt. [ABSTRACT FROM AUTHOR]

• Chemical data of Surda sulfides build a significant hydrothermal genetic model. • Sulfide minerals are formed prior to the closing of deformation and metamorphism. • The source of sulfur for sulfides might be seawater sulfate and modified seawater. • The MDF signatures suggest that the mineralization event took place after the GOE. The present study combines major and trace element composition, and sulfur (S) isotope data of pyrite and chalcopyrite from the Surda copper sulfide deposit in the Singhbhum Shear Zone, the most important copper and uranium producing belt (Singhbhum Cu-U Belt) in India. Three textural and compositional types of both pyrite and chalcopyrite were distinguished; unzoned to partially zoned Pyrite IA with high Co (up to 54900 ppm) and low Ni content is earliest, followed by oscillatory zoned Pyrite IB with high As (up to 25600 ppm) and Co (up to 46800 ppm), both occurring in pyrite I + chalcopyrite I + pyrrhotite + magnetite + apatite vein; Cobaltite-type substitution (Fe 1−x Co x)(S 1−x As x) 2 is suggested for Pyrite IB. Gold occurs in Pyrite I as minor "invisible" gold and as electrum inclusions. It also occurs along with Cu, Mn, Ni, Hg, Ag, Pb, Sb, Zn, Ce, Y, U, and Th in micro-fractures that transgress the primary zoning pattern defined by As, Co, and Ni in Pyrite IB. The early inclusion-rich chalcopyrite generation (Chalcopyrite I) contains high concentration of Zn and Se, and minor to trace amounts of Co, Ni, Hg, Pb, Sb, Te, and Bi, appeared in between Pyrite I and II. Low Co, high Ni (up to 37700 ppm) content Pyrite II, and inclusion-free Chalcopyrite II enriched in Co, Ni, Hg, Bi, Mn, Ag, Sb, V, and Pb are cogenetic, and occurring in pyrite II + pyrrhotite + pentlandite + chalcopyrite II ± violarite vein. Low Co and Ni containing Pyrite III + Chalcopyrite III occur mainly as disseminated grains. The relative timing of formation of Pyrite II + Chalcopyrite II with Pyrite III + Chalcopyrite III remains uncertain. Pyrite + chalcopyrite textures indicate that all pyrite + chalcopyrite formed at some time prior to the end of deformation and metamorphism. Both in situ and mineral separates of all pyrite types and associated chalcopyrite yield a narrow range of positive δ34S values (between +3.8 and +6.9‰) suggesting sulfur being derived from a similar source. Consistent positive δ34S values and other circumstantial evidence indicate that most sulfur was derived from seawater sulfate or modified seawater (brine/evaporite). Δ33S values revealed mass dependent fractionation (MDF) signature. It is proposed that incorporation of MDF sulfur of the mineralization event in Paleoproterozoic Singhbhum Cu-U Belt took place after the great oxidation event. The high Se concentrations (260–400 ppm) and ∑Se/∑S ratios for both Pyrite I and II from Surda deposit (4.4–5.7 × 10−4) suggest a low temperature of the Cu–rich ores (250°–350 °C), and precipitation from a metalliferous fluid with a high ∑Se/∑S ratio (10−4 to 10−3) consistent with igneous input of these elements. [ABSTRACT FROM AUTHOR]

• The study area represents a Proterozoic bornite-rich copper deposit in western India. • Iron and barite mineralization are observed apart from Cu mineralization in the area. • Ore petrography, ore fluid and isotopic signatures have similarity to IOCG deposits. The copper mineralization at the southeastern fringe of the Khetri copper belt in western India is primarily hosted within the metapelites and dolomite units of the Mesoproterozoic Ajabgarh Group of rocks. The copper mineralization is well exposed along a NE-SW trending basinal part around Nim ka Thana area where a few of the prospects namely Dokan, Baniwala-ki-Dhani, Dariba, and Nanagwas primarily consist of abundant bornite, chalcopyrite, covellite, digenite, and chalcocite as disseminated phases hosted within a wide-ranging litho units and along quartz-calcite-barite veins. An integrated approach has been adopted to understand the metallogenetic evolution of this bornite dominated copper mineralization. δ13C V-PDB and δ18O V-SMOW results of the carbonate veins ranging from 0.57‰ to −3.75‰ and δ18O range from 15.01‰ to 20.74‰ respectively suggest the influx of hydrothermal fluid in shaping up the deposit. δ13C (CO 2) of the ore-bearing fluid ranges from −1.67‰ to 2.65‰ and δ18O (H 2 O) from 7.11‰ to 12.84‰ respectively also suggest the input of hydrothermal source. The S-isotopic values of Cu-mineralized barite samples range between 16‰ and 20‰, which are lower than that of seawater (δ34S‰ = 20–22‰). This provides the evidence that the barite mineralization could have formed from the hydrothermal fluid that is cogenetic with the late-stage mineralization in a phase-wise separation as an associated mineral assemblage in the system progressively enriching the bornite-rich copper deposit. Based on the field data and evidences, petrographic characteristics, mineral chemistry, fluid inclusion, and stable isotopic signatures, a three-stage mineralization event is proposed as 1) syngenetic precipitation of dispersed bornite-pyrite in a euxinic environment; 2) hydrothermal vein-type mineralization dominated by chalcopyrite-bornite phase, and 3) supergene enrichment of the vein filled mineralization with the development of wide-scale chalcocite-digenetite-covellite mineral phases. Fluid inclusion microthermometry suggests a temperature range from 225°-320 °C with moderate salinity fluid representing epithermal mineralization which is coherent with the stable isotope data (C, O and S isotopes) of the mineralized carbonate and barite veins suggesting the involvement of large scale hydrothermal fluid influx. Te content of various Cu-bearing sulfide phases shows a gradual change in the concentration from chalcopyrite (2.21% to 2.31%), bornite (2.31% to 4.50%), chalcocite (2.77% to 2.87%), digenite (2.57% to 3.70%) and covellite (2.53% to 3.13%) suggesting their evolution from hydrothermal to supergene process. The studied area has experienced wide-scale alterations and Na-metasomatism with the development of associated minerals such as scapolite, tremolite, and sericite. Occurrence of primary and secondary copper sulfides in the metasedimentary sequence, alteration assemblages affected by Na-metasomatism, association of magnetite and specularite in the study area is affiliated to the hydrothermal system akin to IOCG clan as noted from the adjoining Khetri Copper Deposit. [ABSTRACT FROM AUTHOR]

Mafic granulites from key localities of the Eastern Ghats Province preserve Fe–Ti oxides, Cu–Fe sulphides and traces of sulphate minerals along with silicate phases. Two different varieties of mafic granulite exhibit slightly contrasting mineral assemblages. While the massive type of mafic granulite contains minerals assemblage orthopyroxene + clinopyroxene + plagioclase + magnetite + ilmenite + pyrite + pyrrhotite, the migmatitic variety contains garnet as an additional phase. Both oxide and sulphide minerals show contrasting textural characters. Textural analysis and construed mineral reactions imply that the variation of oxide–silicate, oxide–sulphide and sulphate relations is linked to variation of f O 2 during the pre-peak, peak and post-peak stages of metamorphism. The calculated f O 2 values range up to +4 log units relative to the QFM (quartz-fayalite-magnetite) buffer among the samples, except for one sample which shows lower values (−10 log units relative to the FMQ (fayalite-magnetite-quartz) buffer). The consistently high f O 2 condition at the lower crust could result from several factors, but the role of the externally derived fluid appears to be plausible. Hot brine solution with CaCl 2 species can explain the oxidation as well as local metasomatism of the mafic lower crust even though its presence is not verified from direct characterisation like fluid inclusion analysis. [ABSTRACT FROM AUTHOR]

• Chigargunta and Bisanatham–promising gold prospects in South Kolar Greenstone Belt. • Upper greenschist facies conditions of hydrothermal alteration and mineralization. • Low salinity, slightly alkaline and reduced metamorphogenic aqueous-gaseous ore fluid. • Mineralizing P–T conditions show high temperature and huge pressure fluctuation. • Carbon and sulfur isotopes indicate crustally derived fluid of metamorphic source. There are many Neoarchaean orogenic gold hosting greenstone belts in the Dharwar Craton (DC), India; among them the southern extension of the Kolar greenstone belt (SKGB) contain promising gold deposits such as Chigargunta and Bisanatham. Auriferous laminated quartz-calcite veins are hosted mainly within the sheared and altered Champion gneiss and amphibolite in Chigargunta and Bisanatham respectively. The hydrothermal alteration mineral assemblages show three main alteration zones at Chigargunta. However, only two alteration zones were identified at Bisanatham. The hydrothermal mineral assemblage at Chigargunta and Bisanatham indicate a similar mineralogy that consists of tourmaline, muscovite, biotite, chlorite, epidote, sericite, calcite and quartz along with sulfide and gold. Sulfides associated with ore are predominantly pyrrhotite, pyrite, arsenopyrite, chalcopyrite, ±molybdenite in both the places. Native gold is mainly associated with pyrrhotite in the inner and proximal zone of both the deposits. Fluid inclusion petrography and microthermometry from quartz-calcite veins within the mineralized zone at Chigargunta reveal a low to medium salinity (0.5 to 13.3 wt% NaCl equiv.) H 2 O-NaCl-CO 2 -CH 4 ± N 2 bearing fluid. However, at Bisanatham only H 2 O-NaCl fluid with low salinity (1.56 to 6.44 wt% NaCl equiv.) was observed. P-T conditions of the gold mineralization at the Chigargunta and Bisanatham deposits range between 1.7 and 3.5 kbar pressure/285 °C to 378 °C temperature and 0.8 to 1.2 kbar pressure/365 °C to 405 °C temperature respectively. Alteration mineralogy, ore mineral assemblages, fluid compositions and P-T conditions from the SKGB confirm that the near neutral to slightly alkaline mineralizing fluid transported gold as Au(HS) 2 − complexes with low ƒ O 2 conditions. The observed drastic pressure and temperature fluctuation suggests that fault-valve mechanism/pressure cycling occurred at the time of gold precipitation. Carbon (δ13C CO2) isotopic composition of ore fluid deduced from calcite varies from −2.4 to +3.3‰ (average: −0.3‰) and −2.1 to +1.4‰ (average: −0. 8‰) for Chigargunta and Bisanatham respectively. Oxygen isotopic (δ18O H2O) values for both the deposits vary from +7.2 to +14.6‰ (average: +9.3) and +7.3 to +8.9‰ (average: +8.3‰) respectively. Ore-forming fluid could have possibly been derived from the decarbonation of marine carbonates during the metamorphic devolatilization of greenstones. The slight intra-deposit variation of the carbon isotopic value implies the Rayleigh fractionation with progressive consumption of the CO 2 as a result of interaction between hydrothermal fluid and wall rock as well as immiscibility of CH 4 and CO 2 fluid. Ore fluid sulfur (δ34S H2S) isotopic composition, derived from δ34S values of sulfides, varies from −0.42 to +2.43‰ and +0.25 to +2.34‰ for Chigargunta and Bisanatham respectively. Both the sulfur and carbon isotopic values, from these deposits are comparable with other orogenic gold deposits in the DC and elsewhere in the world. Although carbon isotopic values possibly indicate the metamorphic fluid source, the sulfur isotopes signify average crustal sulfur composition that is from crustally derived/modified fluid. Based on the field-geological, mineralogical, fluid, and isotopic compositions, the possible fluid source could be from the metamorphic devolatilization process of the thick greenstone sequence in the SKGB. However, a magmatic source could be further tested with geochronological studies on intrusives in the area. [ABSTRACT FROM AUTHOR]