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Volume 31 Issue 3
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Girish Kumar Mayachar, Subhasish Ghosh. Fluid Inclusion Characteristics of Tungsten Mineralization in the Agargaon Area of Sakoli Fold Belt, Central India. Journal of Earth Science, 2020, 31(3): 559-570. doi: 10.1007/s12583-019-1271-4
Citation: Girish Kumar Mayachar, Subhasish Ghosh. Fluid Inclusion Characteristics of Tungsten Mineralization in the Agargaon Area of Sakoli Fold Belt, Central India. Journal of Earth Science, 2020, 31(3): 559-570. doi: 10.1007/s12583-019-1271-4

Fluid Inclusion Characteristics of Tungsten Mineralization in the Agargaon Area of Sakoli Fold Belt, Central India

doi: 10.1007/s12583-019-1271-4
More Information
  • The Lower to Middle Proterozoic Sakoli fold belt in Central India forms a triangular belt with significant mineralization of strategic minerals. The Sakoli fold belt comprises metasediments, felsic and mafic volcanics with metabasalts bounded by the gneissic-migmatitic terrain. The last pulses of granitic activity in the form of quartz lenses intrude the metasediments and are associated with tungsten mineralization. The metasediments are intruded by the quartz veins and tourmaline breccias trending 60°N to 65°E and 60°S to 65°W and are parallel to the regional structural foliations. The tungsten mineralization in this area is restricted to tourmaline-quartz mica greisens and quartz veins. The NE-SW trending foliated contact zones of chlorite mica schist and porphyritic granite/gneisses have served as easy channels for the mineralizing vapours and solutions to percolate, which formed ore bearing greisens and quartz veins. This mineralization is erratic and manifested by sparse and sporadic disseminations of wolframite and scheelite associated with minor amount of molybdenite and chalcopyrite. The fluid inclusion microthermometry on mineralized quartz veins and quartz-tourmaline veins reveals the existence of a metamorphogenic aqueous-gaseous (H2O-CO2+NaCl) fluid that underwent phase separation and gave rise to gaseous (CO2) inclusion. The salinity of tungsten mineralizations varies from low to high (1.32 wt.% to 40.44 wt.% NaCl eq.). The estimated P-T range of tungsten mineralization varies from 1.2 to 2.2 kbar at 280 to 390 ℃. Raman spectroscopy reveals that the fluid inclusions mainly contain H2O and CO2 with rarely H2S and CH4. Stable isotopic data reveal that the sulfur isotope fractions from the deposits δ34S ranging from +3.1‰ to +3.35‰, suggesting the deep crustal source for the sulfur, which can be further interpreted as a single (magmatic) supply of sulfur during magmatic-hydrothermal mineralization. The studies reveal the presence of chlorides such as FeCl2/MgCl2 and CaCl2, indicating the involvement of chloride complexes in transportation of tungsten to the fluid system and the evolution of the ore-forming fluids by mixing or immiscibility of high-temperature, high-salinity magmatic fluids and low-temperature, low-salinity fluids in hydrothermal system, and also representing magmatic-hydrothermal interactions contributed wolframite and scheelite with minor amount of molybdenite and chalcopyrite.
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Fluid Inclusion Characteristics of Tungsten Mineralization in the Agargaon Area of Sakoli Fold Belt, Central India

doi: 10.1007/s12583-019-1271-4

Abstract: The Lower to Middle Proterozoic Sakoli fold belt in Central India forms a triangular belt with significant mineralization of strategic minerals. The Sakoli fold belt comprises metasediments, felsic and mafic volcanics with metabasalts bounded by the gneissic-migmatitic terrain. The last pulses of granitic activity in the form of quartz lenses intrude the metasediments and are associated with tungsten mineralization. The metasediments are intruded by the quartz veins and tourmaline breccias trending 60°N to 65°E and 60°S to 65°W and are parallel to the regional structural foliations. The tungsten mineralization in this area is restricted to tourmaline-quartz mica greisens and quartz veins. The NE-SW trending foliated contact zones of chlorite mica schist and porphyritic granite/gneisses have served as easy channels for the mineralizing vapours and solutions to percolate, which formed ore bearing greisens and quartz veins. This mineralization is erratic and manifested by sparse and sporadic disseminations of wolframite and scheelite associated with minor amount of molybdenite and chalcopyrite. The fluid inclusion microthermometry on mineralized quartz veins and quartz-tourmaline veins reveals the existence of a metamorphogenic aqueous-gaseous (H2O-CO2+NaCl) fluid that underwent phase separation and gave rise to gaseous (CO2) inclusion. The salinity of tungsten mineralizations varies from low to high (1.32 wt.% to 40.44 wt.% NaCl eq.). The estimated P-T range of tungsten mineralization varies from 1.2 to 2.2 kbar at 280 to 390 ℃. Raman spectroscopy reveals that the fluid inclusions mainly contain H2O and CO2 with rarely H2S and CH4. Stable isotopic data reveal that the sulfur isotope fractions from the deposits δ34S ranging from +3.1‰ to +3.35‰, suggesting the deep crustal source for the sulfur, which can be further interpreted as a single (magmatic) supply of sulfur during magmatic-hydrothermal mineralization. The studies reveal the presence of chlorides such as FeCl2/MgCl2 and CaCl2, indicating the involvement of chloride complexes in transportation of tungsten to the fluid system and the evolution of the ore-forming fluids by mixing or immiscibility of high-temperature, high-salinity magmatic fluids and low-temperature, low-salinity fluids in hydrothermal system, and also representing magmatic-hydrothermal interactions contributed wolframite and scheelite with minor amount of molybdenite and chalcopyrite.

Girish Kumar Mayachar, Subhasish Ghosh. Fluid Inclusion Characteristics of Tungsten Mineralization in the Agargaon Area of Sakoli Fold Belt, Central India. Journal of Earth Science, 2020, 31(3): 559-570. doi: 10.1007/s12583-019-1271-4
Citation: Girish Kumar Mayachar, Subhasish Ghosh. Fluid Inclusion Characteristics of Tungsten Mineralization in the Agargaon Area of Sakoli Fold Belt, Central India. Journal of Earth Science, 2020, 31(3): 559-570. doi: 10.1007/s12583-019-1271-4
  • The Sakoli fold belt of Lower to Middle Proterozoic age is significant from its mineralization point of view. A number of metalliferous deposits including those stratiform zinc, vein type copper-gold, quartz-scheelite and greisen type wolframite-scheelite have been reported within the belt (Saha et al., 2001). The interesting feature of these varied mineralizations in the Sakoli folded belt centers around the fact that all of them are housed within the Bhiwanipur Formation (Roy et al., 1996). The stratiform zinc mineralization is closely associated with coticulas (spessartine-almandine rich cherts) interlayered with amphibolites and tuffaceous phyllites. The quartz-sheelite veins occur in the amphibolites in a zone parallel to the zinc mineralization zone.

    The tungsten mineralization is mainly associated with the quartz-veins and greisen veins, which are in the grantic body as well as running parallel to the foliation plane of the metasediments of the Sakoli Group. The dominant units are quartz-muscovite schist with accessory minerals including magnetite, chlorite, and staurolite, a thick pile interlayered amphibolite metachert-phyllite and tuffaceous phyllite. Metamorphosed acid tuffs, quartzites and marble make up a small part of the sequence and poly-phase deformation and metamorphism have affected the formations (Bhoskar et al., 2004; Saha et al., 2001; Mohan and Bhoskar, 1990).

    It was not until the year 1907 that it was known that tungsten mineralization occurs near Agargaon, and Fermor (1908) had described the history, occurrence, mode of tungsten mineralization and its origin in a brief note. Sengupta (1941) recorded the association of scheelite with wolframite in this deposit for the first time. Prospecting operations were conducted by the Geological Survey of India in 1942-1943 and the investigation continued. The Geological Survey of India (GSI) and Indian Bureau of Mines (IBM) have established and summarized the total estimate of about 47 000 tons of ore concentration of 65% WO3 grade (Lokras et al., 1981). The fluid inclusion assemblage (FIA) is one of the most important concepts in fluid inclusion research and refers to the examination of groups of fluid inclusions using petrographic microscopes. Such microscopic inclusions usually occur along growth bands or healed micro-fractures, which temporally entrap them (Goldstein, 2003; Goldstein and Reynolds, 1994). In previous fluid inclusion studies, FIA analyses have offered more accurate and representative microthermometry data (Chi and Lu, 2008).

    In this study, we focus on the primary tungsten mineralization in the central part of India, which can be considered as representative for the quartz-vein type of mineralization in the entire Agargaon area of Sakoli fold belt, since they all have a comparable structural and geological setting, mineralogy and paragenesis (Saha et al., 2001). The spatial and temporal relationship between tungsten deposits and granitoid rocks has been widely documented in the literature (Èerný et al., 2005; Heinrich, 1990; Eugster, 1985), but different opinions exist to what extent these granites were involved in the genesis of the ore deposits, ranging from a direct magmatic origin of the mineralizing fluids to a more limited role as source of heat that caused hydrothermal convection in the surrounding rocks leading to tungsten mineralization in flakes of muscovites reported by us for the first time.

  • The Sakoli fold belt (SFB) of Lower to Middle Proterozoic age forms a triangular outcrop of 3 600 km2 in the eastern districts of Maharashtra, viz., Nagpur, Bhandra and Gadhiroli, occurring at the northern fringe of Bastar Craton (Fig. 1). The Sakoli Basin with the western limb trending NE-SW, the eastern one trending N-S while the southern margin with a general E-W trend. This synclinorium basin deposited over granitic gneisses is considered to be moderate southwesterly plunge. These supracrustal rocks are granitic gneisses which have been considered to be the basement for the Sakoli rocks and designated as Amagaon gneiss (Roy et al., 1994). Regionally, the Sakoli Group comprises fourfold volcanic metasedimentary lithostratigraphic units, named as Gaikhuri, Dhabeketekri, Bhiwapur and Pawni formations. Lithologically, the volcano sedimentaries of Sakoli Basin comprises predominately of metapelitic rocks with minor quartzite, arkose and conglomerate, banded iron formation (BIF) and bimodal volcanics including metamorphosed rhyolite, tuffs, epiclastic rocks and metabasalt. Locally, in the southwestern part of the area, a host of metaexhalites including stratiform and stratabound tourmalites, coticulas and chlorites and units of quartz-magnetite rocks occur in the vicinity of sedex type strata bound copper and stratiform zinc mineralization (Bandyopadhyay, 1992). Stratiform zinc sulphides mineralization attributed a chemogenic exhalative origin, hosted in coticulas, occurs in the lower part of Bhiwapur Formation, while the vein and greisen type mineralization of base metal-gold-platinum and tungsten respectively occur in the upper part of the Bhiwapur Formation (Mohan and Bhoskar, 1990; Seetharam, 1990). Major rock types in the study area are quartz chlorite mica schist with later influx of quartz veins. Migmatitic gneisses, amphibolites and later prophoritic granite intrusives were also noted.

    Figure 1.  Geological map of Sakoli fold belt, Central India (modified after Roy et al., 1996).

  • Agargaon prospect falls on the western margin of the NE-SW trending limb of the Sakoli fold belt (Fig. 2). The schistose sequence that hosts the tungsten is within the Sakoli Group of metasediments, which show low grade metamorphism. Structurally, the deposit is inferred to be associated with a shear zone involving intricately folded and vertically/steeply dipping schistose sequence. The narrow zone of schistose sequence, carrying the tungsten mineralization, sandwiched between migmatite and granite in the north and a thin band of sericite quartzite in the south. The migmatite and the granite at the contact with the schistose sequence show sporadic occurrence of scheelite. The tourmaline-quartz mica greisens traversing the country rocks viz., chlorite mica schist is bearing the wolframite and scheelite mineralization. Tourmaline-quartz mica veins are highly deformed and they are associated with very small crystals of wolframite. The development of secondary scheelite occurs as small coatings and disseminations along the fractures and cleavages of earlier formed wolframite. The mineralized zone extends along NE-SW direction with general dip towards south, which is in conformity with the regional trend.

    Figure 2.  Geological map of Agargaon area (modified after Lokras et al., 1981).

    Tungsten mineral occurrences are spread in patches within the NNE-SSW to ENE-WSW trending linear Agargaon belt. This belt comprises of metasedimentary and meta pelitic rocks exposed on the surface and seen in the borehole inter-sections within the basement granitic gneiss (Mohan and Bhoskar, 1990). The main ore body consists of a zone of S2 sub parallel wolframite-scheelite-molybdenite-tourmaline-quartz veins within the schistose rocks (Raychaudhury and Das, 1981). The wolframite mineralization is of vein type, but it also occurs as disseminated, sheared, stretched and elongated in pockets as ores along with tourmalines, which are sporadically distributed, showing pinch and swell structures at places.

    In Agargaon tungsten prospect, wolframite is confined to narrow zone of E-W trending quartz-chlorite-mica and tourmaline schist. This zone extends over a length of 1.4 km and flanked by migmatites and granites on the north and a thin band of sericite quartzite to the south. The mineralization is characterized by a tourmalinisation of the host rock (Lokras et al., 1981). Wolframite generally occurs as small but occasionally, it forms large wedge shaped crystals. The mineralization is extremely erratic and is manifested by sporadic disseminations of wolframite and scheelite (Narsimhan et al., 1997; Seetharam, 1990). The mineralogical paragenesis of the Agargaon area is examined through field and microscopic observations. The deposits occurring within the porphoritic granite and metasediments. The main ore mineral assemblages of this belt are tungstates (wolframite and scheelite), oxides and sulphides along with various alteration products and other minor amount of ore minerals.

  • Major rock types in the studied area are metasediments of quartz chlorite mica schist with later influx of quartz veins bounded by the migmatitic gneisses, amphibolites and later intrusives porphyritic granite. Schists occupy a major portion of the area and host a major portion of the area of the tungsten mineralization. It is mainly composed of dark to pale green flakes of chlorite, grains of colorless to smoky grains of quartz and varying amounts of biotite, muscovite and sericite. Quartz chlorite schist is the predominant type intercalated with the tourmaline schist, magnetite chlorite schist, sericite chlorite schist, chlorite sericite schist. Both towards north and south, the schistose units near the contact of granitic gneisses are graded into migmatite. The rock section shows fine to medium grained porphyroblastic schistose texture (Figs. 3a and 3b). It includes quartz, feldspar, dark to pale green flakes of muscovite and varying amounts of chlorite. Imperceptibly grades of brownish to reddish colored biotite are observed at some places.

    Figure 3.  (a) Field photograph showing quartz chlorite mica schist intruded by quartz vein at Agargaon; (b) thin section showing biotite, chlorite in the quartz chlorite mica schist; (c) field photograph showing porphyritic granite at Agargaon; (d) porphyritic granite marked by phenocrysts of potash feldspar, quartz; (e) field photograph showing brecciated quartz veinsat at Agargaon; (f) microphotograph showing almandine garnet within quartz veins studied at Agargaon; (g) core sample from borehole core, wolframite grain within quartz vein marked by circle; (h) floats of scheelite within wolframite grain.

    The later phase of porphyritic granites are exposed as small irregular-shaped bodies in the northern part of the mineralized zone and they show feebly developed gneissic banding with phenocrysts of potash-feldspars oriented along the gneissose bands. These phenocrysts are measured up to 4 cm. The granites usually consist of orthoclase feldspar and quartz and are often relatively poor in mafic constituents of biotites. Other minerals are muscovite, sericite and magnetite. It is only occasionally and sporadically that a tiny needle of tourmaline is seen within these porphyritic granites. In the thin section, granites are fine to medium grained and are essentially composed of potash feldspar and quartz with varying amounts of microcline and perthite (Figs. 3c and 3d). Potash feldspars are often clouded and are altered to kaolin and sericite. Phenocrysts of potash feldspars are perthitic and contain small inclusions of quartz and plagioclase feldspars. The muscovite mica shows two generations, in which the brownish color mica is the older and the bluish to greenish color mica is the younger and it is perpendicular or parallel to the older mica foliation.

    The quartz veins in the area are mostly lenticular and of highly varying dimensions and these veins are often seen in meta sediments and granites. The quartz veins which carry tungsten mineralization have a brownish tinge and to a certain extent are micaceous (Fig. 3e). The usual thickness of the quartz veins varies between 15 and 25 cm. Veins as much as one and a half meter thick are also rarely seen. With an increase in the proportion of muscovite, as noticed in the proximity of the granite-schist contact the rock simulates a greisen. In the thin sections, brecciated quartz vein shows that the grains indicate strain effects. Flakes of muscovite mica and chlorite are present in minor proportions and almandine garnet is also present (Fig. 3f). The ore minerals are chalcopyrite with intergrowths of sphalerite showing emulsion texture, wolframite, stringers and specks of pyrrhotite, pyrite and hematite are observed.

    The important ore mineral assemblages of this belt are tungstates (wolframite and sheelite), oxides and sulphides along various alteration products. Wolframite and scheelite occupy less than 1% of volume of the total ore assemblages and the sulphides, muscovite, tourmaline and carbonates are the principle accessory minerals (Pophare et al., 2006). The wolframite is metallic black to brown in color with a brownish black streak. It occurs as medium to coarse grained and occupy the position in between fractures of brecciated quartz and in greisens or at the periphery of the quartz vein lenses in contact with tourmaline mica schist (Fig. 3g). Wolframite displays grayish silver color with weak pleochroism; showing less reflectance than that of sphalerite or magnetite (Fig. 3h). Scheelites are also associated with the wolframite crystals and form two generations. Scheelites are distinguished in the mineralized zone viz., primary scheelite and secondary scheelite in the area. The primary scheelite occurs within the interstices of quartz grains and it is rare in the mineralized zone. The secondary scheelite occurs as pseudomorphs preserved as relics of amoeboid formed due to replacement of earlier phase of grains of wolframite which are partially replaced by scheelite. Scheelite occurs as micro pods (of 500 μm) with distinct boundaries displaying shades of grey colored grains within the bigger grains of wolframite. These scheelite grains of minor nature and is in close association with the major phase, revealing an alteration product.

    The oxide phases are mainly of magnetite which occurs as euhedral crystal aggregates and fine exsolved lamellae of hematite are found in some magnetite crystals. These crystals contain inclusions of wolframite and they occur as intergrowths with muscovite at places, which was deposited later than wolframite. The sulphide phases present in the study area are pyrrhotite, pyrite, chalcopyrite and molybdenite. Chalcopyrite occurs as medium to coarse grained anhedral aggregates, disseminations and fine veins exhibiting golden yellow to brassy yellow pleochroic color with weak anisotrophism and these sulphides are also associated with the wolframite. In addition, it is observed that it preserves rounded patch of earlier phase of cuprite within itself.

  • Different analytical techniques were judiciously issued by analyizing these samples along with international standards as mentioned in GSI's SOPs (standard operating procedure). The fresh samples collected from the field were cut to 3 mm width for subsequent preparation of thin sections, polish sections and wafers of 0.03 mm thickness. The polished thin sections were analyzed with the help of a four-WDS Cameca SX-100 electron probe micro analyzer (EPMA) at the NEGR, GSI, Bengaluru. Analyses were performed with 15 kV acceleration voltage, 12 nA sample current, and a beam diameter of 1 μm. The counting time was 10 and 5 s for the peaks and the background, respectively. Natural standards were chosen for calibration and ZAF corrections were made by the program PAP (Pouchou and Pichoir, 1984).

    Images of fluid inclusions in wolframite and coexisting quartz were displayed on a monitor for observation, using Olympus BX51 microscope manufactured by Fluid Inc., USA, and a high-resolution camera. Based on their petrographic character, various fluid inclusions in the mineralized quartz veins were selected for microthermometric measurements. The fluid inclusions were analyzed with LINKAM MDSG-600 heating-freezing stage, fitted on Olympus BX-51 petrological microscope housed in NCEGR, GSI, Bengaluru. The unit operates in the temperature range of -195 to 600 ℃, and is periodically calibrated by synthetic pure H2O inclusions (0 ℃) and H2O-CO2 inclusions (-56.6 ℃). The accuracy of the measured temperature is about ±0.2 ℃ during cooling and about ±2 ℃ between 100 and 600 ℃. The salinities of NaCl-H2O inclusions were calculated using the final melting temperatures of ice (Bodnar, 1993). The salinities of three-phase NaCl-CO2-H2O fluid inclusions were calculated using the melting temperatures of clathrate (Collins, 1979). Calculations of fluid density, salinity and construction of isochores were done using PVTX (version 2.0) software of M/S Likam, UK. Subsequently FLUIDS 2003 package (Bakker, 2003), AQSO, BULK, Q2 and ISCO programs were utilized. FLUIDS software incorporates the equations of state (EOS) of Thiery et al. (1994), Duan et al. (1992) for CO2±CH4 system (carbonic inclusions) and those of Archer (1992) and Zhang and Frantz (1987) for H2O-NaCl system (aqueous inclusion) and H2O-NaCl-CO2(V)-CO2(L)±CH4 (aqueous carbonic inclusions).

    Absolute pure fractions molybdenite samples were analyzed for sulfur isotopes in continuous flow mode using Isotope Ratio Mass Spectrometer (Make: SerCon, Model: Geo 20-20), in NCEGR, GSI, Bengaluru, using the ANCA GSL (automated nitrogen and carbon analyser for gas solids and liquid) peripheral system, in sequential steps as follows: (i) About 35 to 50 µg of pure fraction of molybdenite sample was weighed into tin capsules. (ii) The samples containing tin capsules were packed manually and are placed within the auto-sampler unit above the ANCA and positions were marked. (iii) During analyses each tin capsule is dropped one by one into a furnace at 1 050 ℃ with an additional oxygen pulse. The analytical techniques and fractionation mechanism are described in detail by de Groot(2009, 2004) and Hoefs (2009). Each sample was analyzed thrice along with international reference (NBS) and internal laboratory standards in a batch. Measurements of all sulfur-bearing samples are expressed on the VCDT scale.

  • The EPMA study of six samples of wolframite and scheelite minerals within the quartz veins reveals inclusions of scheelite and galena with bismuth within the wolframite. The wolframite has 63 wt.% to 68 wt.% of WO3, 18 wt.% to 22 wt.% FeO, 4 wt.% to 5 wt.% MnO and 2 wt.% to 3 wt.% SiO2 and is the Fe-rich ferberite variety (Fig. 4a). Ferberite crystals assume fairly large dimensions and some of the crystals are euhedral in outline and occasionally anhedral grains occur. Sheelite mostly occur in two generations, primary and secondary. The secondary scheelite has resulted from the alteration of wolframite, though some crystals of free scheelite also occur in the area (Fig. 4b). In addition, wolframite grains are also found in the boundaries of the muscovite mica grains and these tiny specks or grains of wolframite crystallized during cooling. The presence of wedge shaped crystals of wolframite of 1.5 mm in length and 0.20 mm in width parallel to the cleavage planes of the muscovites in the late pulses of granitic activity has been reported for the first time and implies late stage of coeval crystallization of muscovite with wolframite during cooling phase.

    Figure 4.  BSE images showing (a) wolframite (ferberite variety) and (b) scheelite within wolframite.

  • Fluid inclusion parameters in the mineralized quartz veins were studied and the identification of inclusions was based on genetic and phase types. A large number of fluid inclusions, including primary, pseudo secondary and secondary fluid inclusions were identified using detailed petrographic observations and FIA methods. In accordance with the classification principles and techniques outlined by Roedder (1984) and Lu et al. (2004). In addition, abundance of fluid inclusions was noted following the methods of Touret (2001). An attempt was made in the light of new experimental studies by Bodnar et al. (1989), on the morphology of fluid inclusions to correlate the different textural features of fluid inclusions with equilibration path. Oriented samples of mineralized quartz veins from trenches/abandoned mines were collected for the detailed studies of nature and composition of trapped fluids which were responsible for mineralization in the area. The size of the inclusions varies from 1.7 to 70 µm. The size of vapour phase of water vapour/CO2 bubbles varies from 0.39 to 5.86 µm. Based on the phases present, they are grouped into four types: type-Ⅰ primary monophase carbonic inclusions, type-Ⅱ primary aqueous carbonic inclusions and type-Ⅲ primary aqueous bi-phase inclusions. Type-Ⅳ multiphase inclusions contain the halite crystal with cubes in nature. Type-Ⅰ inclusions are common in both tungsten mineralized quartz veins. These inclusions contain only one phase and some are two phases at room temperature. The CO2 vapour is perfectly circular, spherical (Fig. 5a) and sometimes oval in shape and few of the CO2 vapour bubble shows a dark rim in the periphery at room temperature which is due to the presence of a thin film of liquid CO2 over vapour CO2 and are homogenized into the liquid phase. The type-Ⅱ inclusions are less abundant and not so common, these inclusions occur in isolated pattern and contain CO2 (liquid)+CO2 (gas)+H2O (liquid)+NaCl (Fig. 5b). The aqueous carbonic inclusions often show varying degrees of fill. The vapour content generally ranges from 25% to 60% by volume. Type-Ⅲ inclusions are comparatively more abundant than any other inclusions and these inclusions occur as isolated and form fluid inclusion assemblages (FIA) (Fig. 5c). They are generally small and rounded as well as irregular and contain two phases, liquid (H2O+NaCl) and a vapour bubble H2O (vapour), that is homogenized to liquid upon heating. The proportion of the vapour phase is typically about 10% to 15% by volume. The type-Ⅳ inclusions contain aqueous liquid, carbonic liquid and/or vapour, and one more solids. The liquid to vapour ratios are similar to those in type-Ⅲ inclusions. The solids are very small in size and anistropic and generally have high birefringence, although some solids are weakly birefringent or non birefringent (Fig. 5d).

    Figure 5.  Microphotographs showing (a) primary carbonic inclusions; (b) primary vapour rich aqueous carbonic inclusions and aqueous carbonic inclusions; (c) primary aqueous inclusions are in the cluster and isolated in nature; (d) isolated primary multiphase inclusions.

  • The type-Ⅰ and type-Ⅲ inclusions are more dominant in veins type minerals and are present in both tungsten mineralized veins. Microthermometric measurements of fluid inclusions in mineralized quartz veins are summarized in Table 1.

    Host mineral FI type N Size Tm, CO2
    (℃)
    Th, CO2
    (℃)
    Tm, cal
    (℃)
    Te

    (℃)
    Tm, ice
    (℃)
    Th, total
    (℃)
    Salinity
    (wt.% NaCl eq.)
    Density
    (g/cm3)
    Mineralized vein quartz Type-Ⅰ 35 3.5 to 42.36 -56.6 to -59.1 8.0 to 28.6 5.6 to 6.7 0.64 to 0.87
    Type-Ⅱ 20 3.5 to 70.26 -56.6 to -57.1 22 to 30 6.7 to 8.2 -50.3 to -35 -13.2 to -1.5 200 to 323 2.96 to 16.98 0.79 to 1.0
    Type-Ⅲ 94 1.72 to 58.9 -65 to -20 -0.8 to -20.2 175 to 285 1.32 to 22.49 0.81 to 1.04
    Type-Ⅳ 10 15.9 to 48.5 190.8 to 260(halite melting 215 to 328) 32.65 to 40.44 1.12 to 1.16

    Table 1.  Microthermometric measurements of fluid inclusions in mineralized quartz veins

    The type-Ⅰ inclusions are primary carbonic inclusions formed solid CO2 upon cooling. The solid CO2 in quartz in both tungsten and molybdenite hosted veins are melt at the temperature between -56.6 and -59.1 ℃. The melting temperature lower than of the pure CO2 inclusions (-56.6 ℃), indicating an admixtures of other gases, most probably that of CH4 (Brown and Lamb, 1989; van den Kerkhof, 1988) and the melting temperature (Tm, CO2) are graphically illustrated in histogram Fig. 6a. The temperature of homogenized of CO2 into vapor phase at temperature 8.0 to 28.6 ℃ (density varies 0.64 to 0.87 g/cm3), which are graphically plotted in histogram Fig. 6b. The primary aqueous carbonic inclusions (Type-Ⅱ), which are comparatively less abundant and not so common like the aqueous inclusions, occur as isolated and also cluster type patterns. These inclusions are characterized by the nucleation of a vapour bubble on cooling, consistent with the composition, CO2 (liquid)+CO2 (gas)+H2O (liquid)+NaCl. These aqueous carbonic inclusions often show varying degrees of fill and coexist with aqueous rich inclusions, suggesting that the fluid immiscibility might have occurred towards the stage of crystallization (Roedder, 1984). The range of homogenization temperature varies from 200 to 323 ℃. The initial ice melting eutectic temperatures (Te) range from -50.3 to -35 ℃ with an average of -39.43 ℃ and this suggests that the major component in aqueous phase is NaCl±FeCl2 in the fluid system. The final melting temperature of ice ranges from -13.2 to -1.5 ℃ (average -5.89 ℃) corresponding with salinities of 2.96 wt.% to 16.98 wt.% NaCl eq. (average 8.80 wt.% NaCl eq.) following the equations of Bodnar(1993, 1983). The CO2 density of carbonic aqueous varies from 0.79 to 1.0 g/cm3 with an average of 0.90 g/cm3.

    Figure 6.  Histograms of the Tm, CO2 (a), Th, CO2 (b), Th, total (c), and salinity (d) in quartz from Agargaon.

    The type-Ⅲ inclusions are aqueous inclusions and are frozen at temperatures mainly between -55 to -69 ℃. The range of homogenization temperature varies from 175 to 285 ℃ (Fig. 6c) and during the heating runs the first melting (eutectic) temperature (Teu) has been observed, ranging from -65 to -20 ℃ with an average of -35.5 ℃ and suggesting that the major component in aqueous phase is NaCl±FeCl2 in the fluid system. The maximum eutectic temperature -61 ℃ may indicate the presence of CaCl2±FeCl2 with NaCl and H2O (Shepherd et al., 1985). The final melting temperature of ice (Tm, ice) ranges from -0.8 to -20.2 ℃ (average -7.05 ℃) corresponding salinities of 1.32 wt.% to 22.49 wt.% NaCl eq. (Fig. 6d). The density of aqueous varies from 0.81 to 1.04 g/cm3 with an average of 0.93 g/cm3. The type-Ⅳ inclusions are the aqueous multiphase inclusions contain aqueous liquid, a vapor bubble and a halite crystal and these halite crystals show characteristic feature of cubic outline. The size of the halite crystals varies from 2.5 to 6 μm (in terms of the largest edge) and 2.5 to 5 μm (diameter), respectively. The homogenization temperature varies from 190.8 to 260 ℃ and with a halite melting temperature ranges from 215 to 328 ℃. The solid halite crystals melt at very high temperature at 328 ℃, this may due to they are accidentental trapping and are insignificant in numbers. The salinity ranges from 32.65 wt.% to 40.44 wt.% NaCl eq.

  • The molybdenite (MoS2) collected from mineralized quartz veins in Agargoan, Sakoli fold belt, Nagpur, Maharashtra. The data generated by using Geo 20-20 for five molybdenite (MoS2) samples and International Standards NBS123 (sphalerite) were analyzed along with samples to ensure the accuracy and precision of the analyses. The reported δ34SVCDT (‰) value for NBS123 is +17.44‰ (Beaudoin et al., 1994). Repeated measurements of NBS123 results in an average of +17.33‰ with a standard deviation 0.57. The analyzed values of the δ34SVCDT (‰) of molybdenite (MoS2) range from +3.34‰ to +3.36‰ with an average of +3.35‰.

  • Sulfur in magmas usually has δ34S values near zero (Ohmoto and Rye, 1979), but magmatic fluids derived from them may be reduced or oxidized depending on the oxidation state of the magmas and extensive open-system degassing can lead to local enrichment of 34S (Marini et al., 1998). Hoefs (2009) documented the δ34S data values for some of the geologically important sulfur reservoirs which are used as reference data. There is a clear overlap of δ34SVCDT (‰) fields of evaporite sulfate, ocean water, heavier sulfur isotope enriched sedimentary rocks and heavier sulfur isotope enriched metamorphic rocks. Understanding the range of sulfur data from the large number of analytical results from a given area is the main clue to resolving this issue. The δ34SVCDT (‰) values of molybdenite (MoS2) range from +3.34‰ to +3.36‰ with an average of +3.35‰. Earlier studies carried out in this area also show a similar average δ34SVCDT (‰) value of +3.31‰ (Saha et al., 2001).

    The present δ34S value (+3.35‰) indicates that the sulfur source is enriched in 34S sulfur isotope and could be a reduced ore fluid. Hence the primary sulfur source is inferred to be a lower crustal reservoir and can be further interpreted that a single (magmatic) supply of sulfur during magmatic-hydrothermal mineralization, i.e., granitoid melts sequestered this sulfur, which was later transferred to the upper crust by the fluids through shear/ fracture zones. The range of sulfur isotopic values from the Sakoli Basin clearly reveals about the mineralization as well as sulfur isotopic signatures of molybdenite (MoS2) mineral, that it conclusively proves that the crustal materials were the source of its genesis. Further it may be indicated that magmatic-hydrothermal fluid activity, either by immiscibility or mixing of fluids, which corroborates the salinity of the fluids preserved as inclusions.

  • Triangular in outline, the Sakoli fold belt ranging in age from Lower to Middle Proterozoic hosts major tungsten and basemetal mineralization. Studies carried with an integrated approach starting from field work to applications of various modern analytical techniques to generate precise analytical data, to throw light on the genesis and source of the tungsten deposits. Genetically majority of the tungsten occurrences are attributed to the granite related pegmatitic to pneumatolytic hydrothermal ore deposits (Bhoskar, 2004, 2001; Bhoskar, 1998; Narsimhan et al., 1997; Dekate, 1967). The tungsten mineralization is confined in brecciated quartz veins and tourmaline-mica-quartz veins (greisens) as lenticular bodies in metasediments, the tungsten mineralization occurs within the granitoids and also closer to granite contacts in the Agargaon area (Bhoskar et al., 2004). Porphyritic granite is also found within the borehole intersections in the Agargaon region (Lokras et al., 1981). Coarse wolframite crystals are not uniformly distributed, and scheelite is an alteration product. Lately, sulphides and molybdenum deposited in quartz veins and while at Kuhi the bismuth occurs as fracture fillings along with cuprite.

    Tungsten mineralization is controlled by lithology and structure. It is observed that rich tungsten concentration is restricted to tourmaline-quartz mica greisens and quartz core of the greisens. Tourmaline-quartz mica-greisens/veins emplaced in the tourmalinised schist and chlorite mica schist are the locations of tungsten mineralization. The NE-SW trending foliated contact zones of chlorite mica schist and gneisses have served as easy channels for the mineralizing vapours and solutions, which formed ore bearing greisens and quartz veins emplaced along the foliation planes in the schists.

  • EPMA analyses show that the occurrences of lenticular shape wolframite within the foliation planes of muscovites in the quartz veins are reported for the first time in this study. These lenticular wolframite grains were crystallized syn to crystallization of muscovites. In addition, wolframite grains are also found in the boundaries of the muscovite grains and these tiny specks of wolframite were bought by the magma and deposited along with boundaries of muscovite (Figs. 7a, 7b). Textural features indicate that muscovites acted as host during crystallization of tungsten (this is firstly reported in Sakoli fold belt).

    Figure 7.  BSE image showing wolframite grains (a) within the cleavage plane of muscovite and (b) along the boundary of muscovite.

  • Fluid inclusion studies of different mineralized veins from Agargaon areas reveal four types of fluids. The homogenization temperature (Th, total) of type-Ⅱ (200 to 323 ℃) and type-Ⅲ (175 to 285 ℃) and the estimated P-T condition of both types Ⅱ and Ⅲ range from 280 to 390 ℃ and pressure from 1 250 to 2 250 bar (Fig. 8), which corroborate the theoretical calculations of Heinrich (1990), suggesting that the amount of soluble tungsten in hydrothermal ore fluids is in equilibrium with a granitic rock with cooling temperature around 300 ℃ and most of the tungsten deposits are formed within the temperature around 300 ℃ (Jaireth et al., 1990). The salinity measured for tungsten shows high to low salinity ranging from 1.32 wt.% to 40.1 wt.% NaCl eq. and form two clusters (high 12 wt.% to 40.1 wt.% NaCl eq., low 1.32 wt.% to 12 wt.% NaCl eq.). The type-Ⅱ and type-Ⅲ are homogenized in different ways, but inclusions of the latter show higher homogenisation temperatures and lower salinities in type-Ⅱ compared with the type-Ⅲ, where the high salinity with less homogenisation temperature suggests that fluid immiscibility occurred before trapping (Shepherd et al., 1985) and numerous studies show that fluid mixing and fluid immiscibility are the dominant mechanisms for the precipitations of tungsten complexes (Wang et al., 2012). In the present study, it is proved that aqueous carbonic inclusions coexist with aqueous rich inclusions (type-Ⅲ) and also might have occurred towards the stage of crystallization (Roedder, 1984). This can also be seen in the plot of salinity vs. homogenisation temperature for the fluid inclusions (Fig. 9), where the salinities of liquid rich aqueous inclusions are of higher than those of the CO2-rich inclusions, which may have resulted from CO2 separated from the fluid as the temperature and pressure decreased and this is due to escape of gas and also it can lead to an increase in the salinity of the residual fluids (Wang et al., 2012). The eutectic temperatures (Teu) ranging from -30 to -65 ℃ suggests the presence of FeCl2/MgCl2 and more likely CaCl2 in the fluid system. The lower eutectic temperature suggests the possible presence of MgCl2/FeCl2 in the brines or complicated mixtures of fluid salt is estimated. The solubility of W increases with the increasing salinity and chloride complexing may be extremely important to transport other elements such as Fe, Ca, Mn which are common constituents of W minerals. Wolframite occurs mainly within the vein system, the wall rock reaction is not necessary for the precipitation of wolframite by cooling of Fe-W bearing fluid (Heinrich, 1990).

    Figure 8.  P-T estimation by the intersecting isochores of type-Ⅰ (CO2 inclusions density) and type-Ⅲ (aqueous inclusions density) inclusions of mineralised quartz veins in Agargaon area.

    Figure 9.  Salinity vs. temperature of homogenisation (Th, total) for the inclusions of mineralised quartz veins in Agargaon area.

  • The δ34S value (+3.35‰) of molybdenite samples indicate that the sulfur source is enriched in 34S sulfur isotope and could be a reduced ore fluid. Hence the primary sulfur source is inferred to be a lower crustal reservoir and can be further interpreted that a single (magmatic) supply of sulfur during magmatic-hydrothermal mineralization, i.e., granitoid melts sequestered this sulfur, which was later transferred to the upper crust along lineaments/shear zones.

  • The NE-SW trending metasediments of quartz-chlorite-mica schist belonging to Bhiwapur Formation of Sakoli Group hosts the tungsten mineralization. These pelitic metasediments have been intruded by pulses of granitic activity. The intense pneumatolytic and hydrothermal processes related to granite intrusion and the last pulses of granitic activity in the form of quartz lenses intrude the metasediments and are associated with tungsten mineralization. Fluid inclusion and sulfur isotope studies reveal that ore fluids with high and low salinities in aqueous media with less CO2 component and the presence of chlorides such as FeCl2/MgCl2 and CaCl2 indicate the involvement of chlorine in transportation of tungsten to the fluid system, and also evolution of the ore-forming fluids either by immiscibility or by mixing between a high-temperature, high-salinity magmatic water and a low-temperature, low-salinity fluid such as meteoric water. The tungsten (wolframite and scheelite) has relatively high saline and high temperature in the earlier stage and due to intense pneumolytic activity of the hydrothermal system, decrease in salinity and temperature, molybdenite and bismuth precipitate in the later stage. Thus these vein-type wolframite deposits in the study area represent a vein system of magmatic-hydrothermal interactions have contributed to the formation of wolframite and scheelite with minor molybdenum, sulphides and bismuth.

  • The authors would like to express their gratitude and sincere thanks to Shri Raju from Geological Survey of India, Kolkata, Shri S. Dayanand, and Dr. Bijay Kumar Sahu from Remote Sensing and Aerial Survey, Geological Survey of India, Bangalore, for constant guidance. In addition, sincere thanks to officials of the National Centre for Excellence in Geoscience Researchand the Remote Sensing and Aerial Survey for their continuous support, motivation and encouragement to carry out this work. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1271-4.

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