©2008 Society of Economic Geologists, Inc. Economic Geology, v. 103, pp. 801–827
Metamorphism, Fluid Flux, and Fluid Evolution Relative to Gold Mineralization in the Hutti-Maski Greenstone Belt, Eastern Dharwar Craton, India* BISWAJIT MISHRA† Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India AND
NABARUN PAL
Reliance Industries Ltd. Petroleum Business (E & P), RCP 5a 2nd Floor, Ghonsoli, Mumbai 400701, India
Abstract We evaluate the metamorphic conditions (P, T, and fluid composition), resultant fluid flux, and gold mineralization in the Hutti-Maski greenstone belt, eastern Dharwar craton, southern India, from the integrated study of three working mines (Hutti, Uti, and Hira-Buddini). In the observed D1 to D5 deformation sequence at Hutti, the proximal biotite alteration zones, containing isoclinally folded quartz veins (D2) and the laminated fault-fill veins (D3) are auriferous. The absence of extension or extensional shear veins implies that the pore fluid pressure was ≤σ 3 + T, where T is the tensile strength of the greenstones. Important auriferous vein structures of the Hira-Buddini mine are the fault-fill veins that run along the steeply dipping, reverse, brittle-ductile shear zone and shallow-dipping, sigmoidal extension veins (Pf ≥σ 3 + T). Metabasites from Hutti record amphibolite facies conditions (3–5 kbars and ~650ºC) as the probable peak metamorphic P-T. A clockwise P-T-t path could be established for the Uti region, from garnetiferous amphibolite and garnet biotite schist, with peak P-T reaching amphibolite facies conditions (~6 kbars and 650º–700ºC). The deduced path can be explained by a subduction-related compressional to transpressional tectonic setting, invoked for the Dharwar craton. The estimated average enthalpy change for the relevant dehydration and decarbonation is about 75 kJ, which is necessary to release one mol of H2O + CO2. By assuming volatile release between 400° and 600°C, the total heat required to metamorphose a kilogram of an average mafic rock is ~235 kJ. Furthermore, by considering 3 percent volatile loss during metamorphism, the maximum rate of volatile production is 28.98 kg·cm–2my–1. Gold mineralization at Hutti took place on the metamorphic retrograde path beginning with initial alteration (and sulfidation) at upper greenschist facies. After a protracted phase of fluid evolution, the mineralization culminated with the formation of auriferous laminated quartz veins at lower greenschist facies. The ore fluid had a ΣS content ~0.1 m, and a decrease in both fO2 (and pH) caused the precipitation of gold in the proximal biotite zone from a hydrothermal solution containing Au(HS–2). Fluid inclusions in D2 quartz veins within the proximal zone comprise a unique assemblage of five distinct types of carbonic inclusions containing variable proportions of CO2, CH4, graphite daughter products, and H2O. Precipitation of thin films of graphite in the inner walls of carbonic inclusions is interpreted to be the result of reaction between CO2 and CH4 (CO2 + CH4 = 2C + 2H2O) within those inclusions that were trapped at >400ºC and contained sufficient CH4. Entrapment of these carbonic and aqueous inclusions followed phase separation of the initial aqueous-carbonic fluid during a decompression event. Simultaneous entrapment of coeval and cogenetic aqueous and carbonic inclusions in D3 auriferous laminated quartz veins was due to phase separation of fluid of broadly similar composition but at lower temperature. Accordingly, gold precipitation in these veins may have been a result of decrease in ΣS content of the aqueous fluid rather than the wall-rock sulfidation and fO2 decrease, as in the biotite zone. Thus, gold precipitation at Hutti, in the proximal alteration zone and laminated veins occurred over a range of temperatures and by several mechanisms. As in the Hutti deposit, mineralization at Uti occurred on the metamorphic retrograde path. Mass-balance calculations indicate introduction of SiO2, K2O, S, As, and Zr and depletion of CaO in the mineralized portion. Gold at Hira-Buddini is recovered both from the wall-rock mylonites and from the fault-fill and sigmoidal extension veins. The sigmoidal extension veins contain numerous aqueous as well as carbonic inclusions in close association, showing large density variation within single clusters. Such variation points to pressure cycling or operation of a fault-valve during the formation of these extensional veins. Occurrence of gold within these veins implies that phase separation during sudden pressure drops caused gold precipitation. Thus, a striking characteristic of this belt is the variability in the style of mineralization and fluid evolution among the deposits.
Introduction LATE ARCHEAN granite-greenstone terranes, worldwide, characteristically host orogenic gold deposits, amazingly efficient † Corresponding
author: e-mail,
[email protected] *A digital supplement to this paper is available at or, for members and subscribers, on the SEG website, . 0361-0128/08/3752/801-27
products of crustal-chemical processes of that time. Genetic models of these deposits must address the following key components: (1) structural styles and tectonic setting; (2) the relationship between metamorphism of host (and associated) rocks and mineralization; (3) the alteration geochemistry including gains and/or losses of elements and changes in mass and/or volume of the shear zones; and (4) the fluid evolution, with emphasis on transport and deposition of gold (Goldfarb
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et al., 2001; Groves et al., 2003). Few quantitative P-T-t paths have been established for Archean greenstone terranes, mainly due to the absence of suitable mineral assemblages (Wilkins, 1997). Some attempts have been made for the Big Komati Formation, Barberton greenstone belt (Cloete, 1990) and the Southern Cross province, Yilgarn craton (Dalstra et al., 1999). Several authors (Walther and Orville, 1982; Peacock, 1987, 1989; Brady 1989; Connolly, 1997) have worked on various problems related to rates of metamorphism, fluid production, and fluid flux during regional metamorphism. However, questions pertaining to the amount and flux of fluid that can be released by metamorphic devolatilization of geologically realistic volumes of mafic greenstones have not been addressed. The Late Archean Hutti-Maski greenstone belt, located in the eastern Dharwar craton (Fig. 1a), hosts the Hutti gold mine (16°12' N, 76°43' E), currently the largest operational gold mine in India. At present, the mine produces ≥3 metric tons (t) of Au per year. It has produced 66 t of Au and has a proven reserve of another 50 t to a depth of 732 m. The first phase of mining is planned at least up to depth of 915 m.
Following the criteria of Groves et al. (2003), Hutti is a worldclass deposit (>100 t Au), with an average grade of 4.42 g/t Au. The Hutti-Maski greenstone belt is a hook-shaped, 100km-long belt (Fig. 1b) that along with other north-south- or northwest-southeast–trending greenstone belts, such as the Raichur, Pennukonda-Ramagiri, Kadiri, and Kolar belts, occurs within the surrounding Dharwar batholith (Chadwick et al., 2000). Metamorphosed basaltic rocks constitute the dominant lithologic unit and are followed in abundance by felsic to intermediate volcanic rocks and quartz and/or rhyolite porphyry. The inner margin of the belt is made up of metavolcanic rocks with intercalations of quartzite, quartz-sericite, and biotite schist. Younger intrusive granitoids, namely the Kavital (biotitehornblende granite-granodiorite, locally tonalite) and the Yelagatti (coarse- to medium-grained pink granite), surround the schist belt, on the northern and eastern parts (Fig. 1b). The Yelagatti granite has been assigned an age of 2532 ± 3 Ma by U-Pb dating of titanite (Anand et al., 2005). The regional structural pattern is a consequence of polyphase superposed folding (Roy, 1979, 1991). The structural setting,
Raichur
FIG. 1. (a). Regional geologic map of part of the Dharwar craton in the southern peninsular India, showing the major greenstone belts in the eastern part (modified after Chadwick et al., 2000). (b). Geologic map of the Hutti-Maski greenstone belt, eastern Dharwar craton (redrawn from Srikantia, 1995). 0361-0128/98/000/000-00 $6.00
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mineralization, and fluid compositions of the gold deposits in the Hutti-Maski greenstone belt exhibit many similarities to typical greenstone-hosted orogenic gold deposits (Pal and Mishra, 2002; Pandalai et al., 2003). Kolb et al. (2004a) distinguished two deformation events (i.e., D2, at upper greenschist to lower amphibolite facies, and D3, at lower to middle greenschist facies), both on the retrograde path. Whereas the D2 event was typified by ductile deformation, D3 was a brittle-ductile event, characterized by quartz veining associated with a crack-seal-slip mechanism. Metamorphism of the greenstones has been examined by Kolb et al. (2004a, b). Low-salinity (3.9–13.5 wt % NaCl equiv) aqueous inclusions, characteristic of orogenic gold deposits, were observed to coexist with carbonic (CO2 ± CH4) inclusions at pressures and temperatures of 1.0 to1.7 kbars and 280° to 320°C in the laminated gold veins (D3) at Hutti (Pal and Mishra, 2002). Pandalai et al. (2003), on the other hand, identified five compositional inclusion types and recorded salinities up to 40 wt percent NaCl equiv and widely varying P-T conditions (1.6–3.6 kbars and 240°–360°C). Since there is no mention of the nature of veins (Kolb et al., 2004b), we interpret the halite-bearing inclusions (Pandalai et al., 2003) as coming from postmineralization smoky quartz veins or quartz-calcite (±chlorite) veins. Mishra et al. (2005) described the evolution of unusual CO2-free mineralizing fluid at Uti. There has been no attempt to characterize the fluids in the D2 quartz veins within the biotite zone and to link those to the main auriferous zone (D3) at Hutti. In this communication, we present results of thermodynamic analyses pertinent to metamorphism, fluid flux, and hydrothermal alteration in the deposits and describe unusual fluid-deposited graphite and almost pure CH4 inclusions in Hutti D2 quartz veins. In addition, we present new fluid inclusion data on the mineralized veins of the Hira-Buddini deposit. These results are integrated into a genetic model of gold metallogeny in the Hutti-Maski greenstone belt. Geologic Setting Hutti gold mine The Hutti gold mine and the other two mines are operated by Hutti Gold Mines Ltd. The structure of the deposit is dominated by a series of parallel, ~ 1-km long and 2- to 10-mthick, steep shear zones, locally referred to as reefs (Fig. 2). Each reef comprises quartz veins along with intensely altered and mylonitized wall rocks and is separated by ~200-m-thick unaltered amphibolites and felsic metavolcanic rocks. Shear bands that closely resemble S-C fabrics are observed frequently in the mylonites (Fig. 3a). Lenticular, unsheared fragments of unaltered amphibolite (lithons) and isoclinally folded quartz veins (Fig. 3b, c) occur within the shear bands in the auriferous proximal alteration zone. The central portion of the reefs is occupied by a large number of parallel thin quartz veins, separated by millimeter-scale slivers of altered wall rocks, mostly chlorite. These auriferous veins (~10 m thick) can be classified as fault-fill veins (Robert and Poulsen, 2001), which have sharp contact with the wall rocks and are mostly parallel to the penetrative reef foliation. Careful underground field study has revealed the presence of at least four types of veins. These are (1) isoclinally folded, 0361-0128/98/000/000-00 $6.00
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FIG. 2. Simplified geologic map of the Hutti gold mine, showing different mineralized reefs projected to the surface (redrawn after Curtis and Radhakrishna, 1995).
early quartz veins within the biotite zone (Fig. 3c), (2) laminated quartz veins (Fig. 3b), (3) open folded and boudins of smoky quartz veins, and (4) late quartz-calcite (±chlorite) veins cutting across the reef structures (Pal and Mishra, 2002). Kolb et al. (2004a, 2005a) studied the relationship of these vein types, associated microstructures, and the mineral paragenesis, and proposed five stages of deformation (D1 through D5) at the mine scale. The work of Kolb et al. (2004a, 2005a) and our own observations indicate that quartz veins first formed by metamorphic segregation possibly at syn- and/or post-peak amphibolite facies conditions during the earliest stage of deformation (D1), which corresponds to the F1 regional folding episode of Roy (1979). The D2 stage that predated the regional F2 event (Roy, 1979) marked the formation of the north-northwest– south-southeast–trending regional-scale mineralized shear zones. The D1 quartz veins were folded with the formation of prominent S2 mylonitic foliation in the high strain zones. Shear bands and lithons were also produced in relatively low strain zones alongside emplacement of more quartz veins inside the reefs that are oriented parallel to the dominant shear foliation and hydrothermal alteration of the host-rock amphibolites, possibly at upper greenschist facies conditions. The D3 event reactivated the D2 shear zones, which possibly corresponds to the regional F2 folding of Roy (1979). This event resulted in (1) oblique shearing with respect to the D2 structures, (2) development of S3 mylonitic foliation, and (3) formation of auriferous laminated quartz veins, parallel to the S3 foliation, with repeated fracturing, possibly a consequence of a crack-seal mechanism (Kolb et al., 2004a; 2005a). The absence of any other vein orientations, such as extension and/or extensional shear veins, implies that the pore fluid pressure never exceeded σ3 + T, where T is the tensile strength of the surrounding rocks (Sibson et al., 1988; Robert and Poulsen
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FIG. 3. Underground field photographs and photomicrographs of samples from auriferous veins in the Hutti mine. (a). “Shear bands” in the mylonites, resembling S-C fabric. (b). Laminated quartz vein with symmetrically distributed biotite zone (Bt-Z) on either side. Note an earlier folded quartz vein. (c). A sharp contact of biotite (Bt-Z) and chlorite (Chl-Z) alteration zones. (d). Undulose extinction and core-mantle structure in mineralized quartz vein. (e) and (f). Strain fringes of quartz against rigid arsenopyrite (Asp) in the biotite alteration zone. Photomicrographs (d through f) were taken in cross polars.
2001). The microtextures observed in these veins include sweeping undulose extinction, subgrain formation and irregular grain boundaries, and core-mantle structures (Fig. 3d), indicating varying extents of dynamic recrystallization of the vein quartz. Spectacular development of antitaxial quartz “strain fringes” (Passchier and Trouw, 1998) within the mylonites (Fig. 3e, f) was possibly a result of perturbations in noncoaxial flow by the rigid arsenopyrite grains. The overall 0361-0128/98/000/000-00 $6.00
suture orientation and the curvature of the fibers point toward a dextral noncoaxial flow, supporting analogous evidence of the shear sense. Presence of chlorite along S3 and the development of the strain fringes indicate deformation at low temperature under high fluid pressure. The overall geometry and disposition of various units are summarized in a block diagram (Fig. 4). Overprinting of the D2 event with its characteristic textures and alteration by the D3 event can be
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FIG. 4. Schematic block diagram showing the geometry and disposition of different reef-related structures in Hutti, demonstrating dextral strike-slip shearing.
explained by progressive deformation and not by two separate episodes. However, it was observed that the P-T regime of the D2 event is slightly higher than that of the D3, which necessitated the existence of the above two phases of deformations at different crustal levels. Kink bands and quartzcalcite-chlorite veins crosscutting the reef structures comprise the D4 event (~F3 folding episode of Roy, 1979). The D5 event is represented by numerous regional- to local-scale faults, which strike east-west or southeast-northwest and have steep dips ranging from 60° to 75°, either toward north or to the south. Uti gold mine The Uti gold deposit is located in the northern tip of the Hutti-Maski greenstone belt (Fig. 1b). Seventeen lodes, with individual strike extensions of about 50 to 720 m and widths from 0.3 to 24 m, have been delineated (Curtis and Radhakrishna, 1995). Among these, only lode 4 is currently productive. A total reserve of 6.51 Mt of ore is estimated down to 60m depth at an average grade of 2.64 g/t Au. The Uti area consists of coarse- and fine-grained amphibolites, metamorphosed felsic volcanic rocks, and garnetiferous amphibolite as the major lithologic units, with subordinate schistose rocks (Fig. 5). Dolerite dikes, granite, pegmatite, and quartz veins intrude these rocks. Gold lodes are mostly associated with small north-northeast–south-southwest–trending shear zones, 50 to 700 m along strike, with varying widths of 2 to 10 m, and are best developed in coarse-grained amphibolites (Mishra et al., 2005). The local tectonic trend of the area is cut by small east-west–trending post-tectonic faults; a mappable one is shown near lodes 6 and 7 (Fig. 5). Hira-Buddini gold mine The Hira-Buddini gold mine, located 19 km east of Hutti (Fig. 1b), is the latest development of the Hutti Gold Mines Ltd. where underground operations started in 2001. Earlier 0361-0128/98/000/000-00 $6.00
FIG. 5. Surface geologic map of the Uti mine showing the disposition of the mineralized zones. (modified from maps of the Geological Survey of India and Hutti Gold Mines Ltd.).
deep drilling by the Geological Survey of India confirmed about 200,000 t of potentially high grade ore (6–10 g/t) to a depth of 145 m (Curtis and Radhakrishna, 1995). From drill core data and exploratory mining, the lode has been identified as a linear zone of 600 m in length and 1.0 and 3.5 m in thickness that trends ~east-northeast–west-southwest (with a steep northward dip) and runs along the lithologic contact of amphibolite and felsic metavolcanic rocks (Fig. 6a). The shear zone shows reverse and locally oblique sense of displacement, with both ductile as well as brittle structures, wherein the shear sense is mostly inferred from the S-C fabric and the nature of rotation of the sigmoidal extension veins (Pal, 2003). Quartz-rich (+biotite and/or sulfides), auriferous fault-fill veins of 25 cm maximum thickness occur within the shear zone (Fig. 6b). Subhorizontal sigmoidal extensional veins, containing quartz (+ minor calcite + high gold values; Fig. 6b) occur with rare conjugate shear veins occurring at an angle of 90º to 120º (Fig. 6c). A schematic block diagram of the deposit is shown in Figure 7. Unlike Hutti, the sigmoidal veins at Hira-Buddini represent hydraulic extension fractures
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FIG. 6. (a). Surface geologic map of the Hira-Buddini deposit (Hutti Gold Mines Ltd.). Underground field photographs in the Hira-Buddini mine. (b). Sigmoidal extensional vein arrays of quartz (+ minor calcite), associated with oblique reverse faults. (c). Conjugate shear veins in felsic metavolcanic rocks.
that opened during the vein filling episodes by fluid pressures (Pf) in excess of the lithostatic load (i.e., Pf ± σ3 + T; cf. Robert and Brown, 1986). Analytical Methods Mineral phases in selected thin sections were analyzed with a JEOL superprobe (JXA-8900R) at the Institüt fur Mineralogie und Lagerstattenlehre, RWTH Aachen, Germany. Operating conditions were 15 kV acceleration potential, 15 nA beam current, 20- to 30-s counting time, and beam diameter of ~1 µm. A slightly defocused beam was used for analysis of feldspars and mica. Appropriate natural and synthetic minerals were used as standards, and the raw data were corrected
FIG. 7. Schematic block diagram showing the orientation of the veins and their relationship with the shear zone at the Hira-Buddini mine. 0361-0128/98/000/000-00 $6.00
with the help of a built-in ZAF correction program. All analyses are provided in a digital supplement to this paper at at (or, for members and subscribers, on the SEG website, ). Throughout the text, mineral abbreviations are after Kretz (1983). In addition, major and trace elements in whole rocks of selected samples from in and around the Uti mine have been analyzed by XRF, RWTH Aachen (Phillips, PW 1400 Xray fluorescence spectrometer). Operating conditions were 45 kV and 35 mA. Analyses were performed using fused glass disks and powder pellets for major and trace elements, respectively, with rhodium X-ray excitation. Precision is estimated to be within ±2 per cent of the amount present for major elements and ±5 per cent for the trace elements. Heating–freezing experiments on fluid inclusions were performed with the help of a Fluid Inc.-adopted USGS gas flow heating-freezing system (Reynolds stage) on a LEITZ Laborlux petrological microscope. For details about temperature calibration, see Mishra and Panigrahi (1999). Reduction of microthermometric data was carried out with the FLINCOR computer program (Brown, 1989), using the equation of state of Zhang and Franz (1987) for the H2O-NaCl system. For CO2 ± CH4 inclusions the program MAC FLINCOR (Brown and Hagemann, 1995) was used along with the equation of state of Kerrick and Jacobs (1981). The fluid inclusions were analyzed by a Renishaw RM1000B laser Raman probe, attached to a Leica microscope. The system is equipped with edge filters to block the Rayleigh lines, confocal configuration, thermoelectrically cooled CCD detector, air-cooled laser, and software to acquire and evaluate the spectral data. Irradiation was by the 514.5-nm line of a continuous wave Ar ion laser, which delivered ~8 mW laser power at the sample surface. The acquisition time was 60 s. With several scans on the same inclusion, the reproducibility of the Raman wave number was found to be ±1 cm–1. Metamorphism Petrography and mineral chemistry Amphibolites of the Hutti mine area show crude foliation and contain amphibole, plagioclase, quartz, chlorite, calcite, epidote, ilmenite, sphene (titanite), and pyrite (Fig. 8a). Chlorite (
