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Volume 31 Issue 3
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Rajagopal Krishnamurthi, Ajit Kumar Sahoo, Rajesh Sharma, Prabhakar Sangurmath. Abundance of Carbonic Fluid Inclusions in Hira-Buddini Gold Deposit, Hutti-Maski Greenstone Belt, India: Inferences from Petrography and Volume Ratio Estimation of Fluid Components. Journal of Earth Science, 2020, 31(3): 492-499. doi: 10.1007/s12583-019-1272-3
Citation: Rajagopal Krishnamurthi, Ajit Kumar Sahoo, Rajesh Sharma, Prabhakar Sangurmath. Abundance of Carbonic Fluid Inclusions in Hira-Buddini Gold Deposit, Hutti-Maski Greenstone Belt, India: Inferences from Petrography and Volume Ratio Estimation of Fluid Components. Journal of Earth Science, 2020, 31(3): 492-499. doi: 10.1007/s12583-019-1272-3

Abundance of Carbonic Fluid Inclusions in Hira-Buddini Gold Deposit, Hutti-Maski Greenstone Belt, India: Inferences from Petrography and Volume Ratio Estimation of Fluid Components

doi: 10.1007/s12583-019-1272-3
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  • Low saline aqueous carbonic fluids are considered to be the ore forming solutions for orogenic lode gold deposits. Phase separation/fluid immiscibility of the ore fluid is quite common and is one of the major reasons for deposition of gold in these deposits. Abundant carbonic fluid inclusions have been observed in quartz grains of Hira-Buddnini Gold Deposit. Theoretical estimation indicates that more volume of H2O compared to CO2 is likely to be trapped in inclusions at different P-T conditions. Preferential loss of H2O from fluid inclusions during ductile deformation of quartz grains have been attributed as the suitable reason for abundance of carbonic fluid inclusions.
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Abundance of Carbonic Fluid Inclusions in Hira-Buddini Gold Deposit, Hutti-Maski Greenstone Belt, India: Inferences from Petrography and Volume Ratio Estimation of Fluid Components

doi: 10.1007/s12583-019-1272-3

Abstract: Low saline aqueous carbonic fluids are considered to be the ore forming solutions for orogenic lode gold deposits. Phase separation/fluid immiscibility of the ore fluid is quite common and is one of the major reasons for deposition of gold in these deposits. Abundant carbonic fluid inclusions have been observed in quartz grains of Hira-Buddnini Gold Deposit. Theoretical estimation indicates that more volume of H2O compared to CO2 is likely to be trapped in inclusions at different P-T conditions. Preferential loss of H2O from fluid inclusions during ductile deformation of quartz grains have been attributed as the suitable reason for abundance of carbonic fluid inclusions.

Rajagopal Krishnamurthi, Ajit Kumar Sahoo, Rajesh Sharma, Prabhakar Sangurmath. Abundance of Carbonic Fluid Inclusions in Hira-Buddini Gold Deposit, Hutti-Maski Greenstone Belt, India: Inferences from Petrography and Volume Ratio Estimation of Fluid Components. Journal of Earth Science, 2020, 31(3): 492-499. doi: 10.1007/s12583-019-1272-3
Citation: Rajagopal Krishnamurthi, Ajit Kumar Sahoo, Rajesh Sharma, Prabhakar Sangurmath. Abundance of Carbonic Fluid Inclusions in Hira-Buddini Gold Deposit, Hutti-Maski Greenstone Belt, India: Inferences from Petrography and Volume Ratio Estimation of Fluid Components. Journal of Earth Science, 2020, 31(3): 492-499. doi: 10.1007/s12583-019-1272-3
  • Orogenic lode gold deposits have spatial, temporal and genetic link with the evolution of host metamorphic terrains (Groves et al., 2003; Goldfarb et al., 2001; McCuaigh and Kerrich, 1998). The temperature and pressure of gold mineralization in these deposits widely range of 180-700 ℃ and < 0.5 to 7 kbar (Groves et al., 1998; Hagemann and Brown, 1996; Gebre-Mariam et al., 1995). The mineral assemblage in the alteration zones is characterized by dominant hydrous mineral phases (such as chlorite, biotite, muscovite and sericite), and the development of particular mineral assemblage depends on mineralogy of host rock and P-T conditions of mineralization (Eilu et al., 1999). Low saline aqueous carbonic fluids have been considered as the ore forming solution in orogenic lode gold deposits (Goldfarb and Groves, 2015; Ridley and Diamond, 2000). Aqueous-carbonic, carbonic and low saline aqueous fluid inclusions have been commonly reported in auriferous quartz veins (Ridley and Diamond, 2000). But, the abundance of only carbonic fluid inclusions is very rare which may mislead to interpret the nature and composition of ore forming fluids. In this paper, we are making an attempt to demonstrate the volumetric abundance of CO2 and H2O that is likely to be trapped in inclusions from a low saline aqueous carbonic ore fluid by virtue of fluid immiscibility. A suitable explanation has been proposed for the abundance of carbonic fluid inclusions with special reference to Hira-Buddini Gold Deposit of Hutti-Maski greenstone belt, Dharwar Craton in India.

  • The ore forming fluids of orogenic lode gold deposits are generally low saline (3 wt.%-7 wt.% NaCl equivalents) and contain 5 mol% to 30 mol% of CO2 (Goldfarb and Groves, 2015; Saunders et al., 2014; Elmer et al., 2006; Ridley and Diamond, 2000). Homogeneous entrapment (Diamond, 2003) of such ore forming fluids in quartz grains of auriferous veins may lead to the formation of fluid inclusion assemblage consisting of only aqueous-carbonic fluid inclusions. However, heterogeneous entrapment (Diamond, 2003) of fluid in response to decrease in pressure and temperature may lead to the formation of fluid inclusion assemblages invariably consisting aqueous-carbonic, carbonic and low to moderate saline aqueous inclusions. A brief review on abundance of fluid inclusion assemblages in auriferous quartz veins of orogenic lode gold deposits have been given in Ridley and Diamond (2000). The dominant fluid inclusion assemblages reported in various deposits are (ⅰ) aqueous-carbonic inclusions, (ⅱ) aqueous-carbonic and carbonic inclusions, (ⅲ) aqueous-carbonic, carbonic and low to moderate saline aqueous inclusions, and (ⅳ) aqueous-carbonic and low to moderate saline aqueous inclusions (Ridley and Diamond, 2000). The presence of salt crystal bearing high saline fluid inclusions in these fluid inclusion assemblages have also been found in several deposits and possibly due to the involvement of high saline aqueous fluid of magmatic origin.

  • Hutti-Maski greenstone belt (HMGB) is located in the eastern Dhawar Craton, India (Fig. 1a). The rocks of the greenstone belt formed during ~ 2 670 to 2 570 Ma (Hazarika et al., 2015a; Jayananda et al., 2013; Sarma et al., 2008; Rogers et al., 2007), predominantly erupted as island-arc tholeiitic basalts (Pal and Mishra, 2002; Giritharan and Rajamani, 1998). Subsequent to formation, the rocks were subjected to metamorphism of amphibolite facies, deformation and intruded by several granitoids bodies. The peak temperature and pressure during metamorphism was approximately 620 ℃/6 kbar at ~2 564±12 Ma (Hazarika et al., 2015a) and the age of the granitoids range from 2 570 to 2 540 Ma (Anand et al., 2014).

    Figure 1.  (a) Generalized geological map of Dharwar Craton (modified after Chadwick et al., 2000). (b) Geological map of Hutti-Maski greenstone belt (modified after Srikantia, 1995). WDC. western Dharwar Craton; EDC. eastern Dharwar Craton; HMGB. Hutti-Maski greenstone belt; KSB. Kolar schist belt.

    The Hira-Buddini Gold Deposit is located in the eastern part of the HMGB (Fig. 1b). Gold mineralization took place during a brittle-ductile deformation episode. The temperature was around 500 ℃ and pressure values varied from 2.05 to 4.36 kbar (Sahoo et al., 2018, 2016). Quartz grains of the auriferous veins are highly deformed and exhibit grain boundary migration (Fig. 2; Sahoo et al., 2018). Two types of alteration zones have been found in the altered felsic volcanic rock: (ⅰ) muscovite-calcite rich zone represented by muscovite-sericite-calcite-chlorite assemblage, and (ⅱ) albite-biotite rich zone represented by albitebiotite-calcite (±muscovite) assemblage. Altered amphibolite and altered felsic volcanic rocks are associated with abundant sulphide minerals (Sahoo et al., 2018). Isotopic analyses and mineralogical studies of unaltered amphibolite and altered amphibolite indicated that the auriferous lode is enriched in SiO2, K, Pb, loss on ignition, and S due to rock-fluid interaction during gold mineralization (Sahoo et al., 2018).

    Figure 2.  Photomicrograph showing grain boundary migration in vein quartz (a) and schematic drawing of inter-lobate grain boundaries of quartz grains (b).

  • The fluid inclusion assemblage related to gold mineralization present in the quartz grains consists of carbonic, aqueouscarbonic, aqueous and salt crystal bearing aqueous polyphase inclusions. Carbonic fluid inclusions are the most abundant among these inclusions (Sahoo et al., 2018). The most salient features related to the distribution of fluid inclusions are: (i) Some portion of the quartz grains are entirely devoid of fluid inclusions. The areas lacking fluid inclusions are significantly more in most of the quartz grains and even some grains are entirely devoid of fluid inclusions (Fig. 3). (ii) Areas having highly irregular shaped fluid inclusions indicating textural re-equilibration (Figs. 4a-4b). (iii) Fluid inclusions along grain boundaries (Figs. 4c-4d).

    Figure 3.  Photomicrographs illustrating areas devoid of fluid inclusions indicated by arrows (Sahoo et al., 2018).

    Figure 4.  Photomicrographs showing highly re-equilibrated fluid inclusions in quartz. (b) Magnified part of the inset (rectangular outline) in (a). (c), (d) Fluid inclusions along grain boundaries (Sahoo et al., 2018).

    Low saline aqueous carbonic fluids have been attributed as the major ore forming fluids in the deposit (Sahoo et al., 2018; Mishra and Pal, 2008). Such low saline aqueous carbonic fluids are released from basaltic rocks during prograde metamorphism at the transition from greenschist to amphibolite facies condition (~500-550 ℃) (Phillips and Powell, 2010; Elmer et al., 2006). These fluids contain appreciable amount of sulfur to make gold soluble as sulfide complexes (Tomkins, 2010).

  • Fluid immiscibility/phase separation is one of the major reasons for deposition of gold in orogenic deposits (Williams-Jones et al., 2009; Ridley et al., 1996). Phase separation of low saline aqueous carbonic fluids will lead to formation of two end member phases, i.e., (ⅰ) low saline aqueous and (ⅱ) carbon dioxide. The volume estimation of H2O and CO2 after phase separation may be useful to know the amount of these phases likely to be present in inclusions. Hence, an attempt was made to estimate the volume of H2O and CO2 formed as a result of phase separation of low saline aqueous carbonic fluid based on the following assumptions.

    Assumption 1: The salinity of the ore fluid is relatively less, so it is assumed that aqueous phase is pure H2O.

    Assumption 2: The CO2 content of aqueous carbonic ore fluid is 5 mol%-30 mol%. The calculation was performed for 30 mol%. The maximum limit was chosen to know the maximum volume of CO2 that will be formed and trapped.

    Assumption 3: The temperature range of mineralization in orogenic gold deposits is 180 to 700 ℃ (Hagemann and Brown, 1996; Ridley et al., 1996) and the fluid inclusions are generally observed at room temperature. The molar volume of H2O and CO2 was deduced using the Compensated-Redlich-Kwong Equation (Holland and Powell, 1991) in the software CHO. The Compensated-Redlich-Kwong Equation is not valid below 100 ℃. So, the molar volume of H2O and CO2 at room temperature has been assumed to be that of 100 ℃.

    The following steps have been used to estimate volume of H2O and CO2 formed after phase separation of aqueous carbonic fluid having 30 mol% of CO2 is as follows.

    Step 1: Molar volume (expressed in joule/bar) of H2O and CO2 were calculated at different temperature and pressure using Compensated-Redlich-Kwong Equation (Holland and Powell, 1991) in the software CHO. The unit of molar volume was converted to cm3 (Tables 1 and 2) from joule/bar.

    T (℃) 100 200 300 400 500 600 700
    P (kbar)
    1 18.17 19.61 22.01 25.34 33.24 47.5 63.43
    2 17.71 18.85 20.52 22.49 25.86 30.41 36.01
    3 17.31 18.26 19.57 21.02 23.24 25.98 29.19
    4 16.85 17.67 18.76 19.92 21.61 23.59 25.83
    5 16.48 17.21 18.14 19.12 20.49 22.04 23.78
    6 16.17 16.82 17.64 18.5 19.65 20.94 22.35
    7 15.9 16.49 17.23 17.99 18.98 20.08 21.28

    Table 1.  Molar volume of H2O expressed in cm3 at different temperature (T) and pressure (P)

    T (℃) 100 200 300 400 500 600 700
    P (kbar)
    1 46.6 55.16 65 75.45 86.08 96.68 107.16
    2 40.34 44.36 48.78 53.52 58.46 63.54 68.7
    3 37.78 40.43 43.29 46.31 49.47 52.74 56.08
    4 36.33 38.32 40.43 42.65 44.96 47.35 49.8
    5 35.38 36.98 38.66 40.41 42.22 44.1 46.02
    6 33.07 34.43 35.84 37.31 38.83 40.39 41.99
    7 31.92 33.09 34.31 35.58 36.88 38.21 39.58

    Table 2.  Molar volume of CO2 expressed in cm3 at different temperature (T) and pressure (P)

    Step 2: Density of H2O and CO2 at different temperature and pressure was calculated by dividing the molar mass of H2O (i.e., 18.015 28 g) and CO2 (i.e., 44.01 g) with the corresponding molar volume given in Tables 3 and 4 (density=mass/volume).

    T (℃) 100 200 300 400 500 600 700
    P (kbar)
    1 0.991 48 0.918 678 0.818 504 0.711 0.541 976 0.379 27 0.284 018
    2 1.017 24 0.955 718 0.877 938 0.801 0.696 647 0.592 41 0.500 285
    3 1.040 74 0.986 598 0.920 556 0.857 0.775 184 0.693 43 0.617 173
    4 1.069 16 1.019 54 0.960 303 0.904 0.833 655 0.763 68 0.697 456
    5 1.093 16 1.046 791 0.993 125 0.942 0.879 223 0.817 39 0.757 581
    6 1.114 12 1.071 063 1.021 274 0.974 0.916 808 0.860 33 0.806 053
    7 1.133 04 1.092 497 1.045 576 1.001 0.949 172 0.897 18 0.846 583

    Table 3.  Density of H2O (g/cm3) at different temperature (T) and pressure (P)

    T (℃) 100 200 300 400 500 600 700
    P (kbar)
    1 0.944 421 0.797 86 0.677 1 0.583 3 0.511 3 0.455 21 0.410 694
    2 1.090 977 0.992 11 0.902 2 0.822 3 0.752 8 0.692 63 0.640 611
    3 1.164 902 1.088 55 1.016 6 0.950 3 0.889 6 0.834 47 0.784 772
    4 1.211 396 1.148 49 1.088 5 1.031 9 0.978 9 0.929 46 0.883 735
    5 1.243 923 1.190 1 1.138 4 1.089 1 1.042 4 0.997 96 0.956 323
    6 1.330 813 1.278 25 1.228 1.179 6 1.133 4 1.089 63 1.048 107
    7 1.378 759 1.330 01 1.282 7 1.236 9 1.193 3 1.151 79 1.111 925

    Table 4.  Density of CO2 (g/cm3) at different temperature (T) and pressure (P)

    Step 3: Volume of H2O and CO2 exsolved from one mole of homogeneous aqueous carbonic fluid having 30 mol% of CO2 (i.e., 0.3 mole of CO2 per mole of aqueous carbonic fluid) was calculated (Tables 5 and 6). The formulae for calculation of volume of 0.7 mole H2O and 0.3 mole of CO2 are 12.610 696/ ρH2O and 13.230/ρCO2, respectively. 12.610 696 and 13.230 are mass of 0.7 mole H2O and 0.3 mole of CO2 whereas ρH2O and ρCO2 are corresponding density of the phases.

    T (℃) 100 200 300 400 500 600 700
    P (kbar)
    1 12.719 13.727 15.407 17.738 23.268 33.249 99 44.400 99
    2 12.397 13.195 14.364 15.743 18.102 21.287 25.207
    3 12.117 12.782 13.699 14.714 16.268 18.186 20.433
    4 11.795 12.369 13.132 13.944 15.127 16.513 18.081
    5 11.536 12.047 12.698 13.384 14.343 15.428 16.646
    6 11.319 11.774 12.348 12.95 13.755 14.658 15.645
    7 11.13 11.543 12.061 12.593 13.286 14.056 14.896

    Table 5.  Volume of 0.7 mole of H2O (cm3) at different temperature (T) and pressure (P)

    T (℃) 100 200 300 400 500 600 700
    P (kbar)
    1 13.98 16.548 19.5 22.635 25.824 29.004 32.148
    2 12.102 13.308 14.634 16.056 17.538 19.062 20.61
    3 11.334 12.129 12.987 13.893 14.841 15.822 16.824
    4 10.899 11.496 12.129 12.795 13.488 14.205 14.94
    5 10.614 11.094 11.598 12.123 12.666 13.23 13.806
    6 9.921 10.329 10.752 11.193 11.649 12.117 12.597
    7 9.576 9.927 10.293 10.674 11.064 11.463 11.874

    Table 6.  Volume of 0.3 mole of CO2 (cm3) at different temperature (T) and pressure (P)

    Step 4: The ratio of volume of 0.7 mole of H2O to that of 0.3 mole of CO2 was calculated and plotted on Fig. 5.

    Figure 5.  Temperature vs. Vratio (ratio of volume of 0.7 mole of H2O to that of 0.3 mole of CO2) at different pressure.

  • Phase separation of low saline aqueous carbonic fluid is a common phenomenon in orogenic lode gold deposits. Estimation of volume ratio in the present work (Fig. 5) indicates that more volume of H2O compared to volume of CO2 is likely to be trapped at different P-T conditions in inclusions. However, the abundance of CO2 bearing fluid inclusions (in terms of population) is not in accordance with the present calculation. This may lead to the interpretation that CO2 rich ore fluid is responsible for gold mineralization in such circumstances. But, CO2 rich ore fluid can not be attributed due to the following reasons:

    (ⅰ) The solubility of gold and other chemical components such as SiO2, Na2O, K2O, etc., in CO2 is questionable. CO2 is a polar molecule having very less dielectric constant (Michels and Michels, 1933). It cannot dissolve components like SiO2, Na2O, K2O, etc., having ionic character (Gill, 2014).

    (ⅱ) The associated hydrothermal alteration zones of orogenic lode gold deposits are characterized by significant amount of hydrous mineral phases like biotite, muscovite, chlorite, etc. (Eilu et al., 1999). Appreciable amount of H2O is needed in the ore fluid for formation of these hydrous mineral phases.

    Gold mineralization in orogenic lode gold deposits is associated with deformation and the style of deformation may vary from brittle, ductile or brittle-ductile in a deposit (Witt, 1993; Colvine et al., 1988). Therefore, the quartz grains may be subjected to deformation by virtue of which textural and chemical modification of fluid inclusions (Bodnar, 2003) is possible. Brittle deformation of quartz takes place at lower temperature at relatively less depth and fractures are common characteristics of it. The fluid inclusions are amenable to bulk leakage if the fractures run along the inclusions. Hence, both H2O and CO2 are likely to be lost from fluid inclusions. Above ~300 ℃, ductile deformation mechanisms dominate in case of quartz grains (Owona et al., 2013; Passchier and Trouw, 2005; Stipp et al., 2002a, b). The fluid inclusions present in quartz grains are subjected to sweeping out, stretching and preferential loss of H2O due to ductile deformation (Dijkstra et al., 2011; Bodnar, 2003; Vityk et al., 2000; Sterner et al., 1995; Vityk and Bodnar, 1995; Hall and Sterner, 1993; Bakker and Jansen, 1990; Kerrich, 1976). The sweeping out of inclusions may be manifested by (i) significant areas devoid of fluid inclusions or absence of fluid inclusions in the deformed quartz grain and (ii) fluid inclusions along the grain boundaries. Stretching may modify the original inclusions to highly irregular shape. Fluid inclusions are considered as major crystal defects and act as the source of dislocation multiplication during ductile deformation (Bakker and Jansen, 1990). The dislocation density is significantly more around fluid inclusions (Vityk et al., 2000; Bakker and Jansen, 1994; Hall and Sterner, 1993). Preferential loss of water takes place by diffusion through the dislocations. The reasons for preferential loss of H2O have been mentioned in Bakker and Jansen (1990) and supported by experimental investigation by Sterner et al. (1995), Bakker and Jansen (1994), Vityk et al. (2000) and others.

    The auriferous quartz veins in Hira-Buddni were formed during an episode of brittle-ductile deformation at ~500 ℃ (Sahoo et al., 2018). The involvement of low saline aqueous carbonic fluid of metamorphic origin has been inferred (Sahoo et al., 2018; Hazarika et al., 2015b; Krienitz et al., 2008). Grain boundary migration was the dominant deformation mechanism of ductile deformation of quartz grains. Besides, the quartz grains also have (ⅰ) significant areas devoid of fluid inclusions, (ⅱ) fluid inclusions along the grain boundaries and (ⅲ) presence of highly irregular fluid inclusions (Sahoo et al., 2018). In such circumstances, the preferential loss of H2O from fluid inclusions may lead to the formation of CO2 bearing fluid inclusions.

  • Low saline aqueous carbonic fluids are considered as the ore forming fluids in orogenic lode gold deposits. Phase separation/ fluid immiscibility of the ore fluid is quite common in these deposits that would have resulted in entrapment of more volume of H2O compared to CO2 in inclusions of vein quartz at different P-T conditions. Gold mineralization in such deposits was associated with deformation episodes of the host metamorphic belts. If the temperature was relatively high during deformation, the auriferous quartz veins might be subjected ductile deformation. The fluid inclusions present in the quartz grains of the auriferous veins may undergo textural and chemical re-equilibration. Preferential loss of H2O from fluid inclusions during ductile deformation could be responsible for abundance of carbonic fluid inclusions in auriferous quartz veins of Hira-Buddni Gold Deposit, Hutti-Maski greenstone belt, India.

  • The authors would like to thank the Department of Earth Sciences, IIT Roorkee for necessary facilities for the present work. Organizing committee of Asian Current Research on Fluid Inclusions VII (ACROFI VII), IGGCAS, Beijing, are highly acknowledged for the invitation to write this paper. The help extended by authorities of M/S Hutti Gold Mines Company Limited during sample collection at Hira-Buddini Gold Deposit is highly appreciated. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1272-3.

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