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.
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).
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
(wt.% NaCl eq.)
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.