- Email: [email protected]
Epigenetic Nature of the BIF-hosted Gold Mineralization at Ajjanahalli, Southern India: Evidence from Ore Petrography and Fluid Inclusion Studies: Reply*
632
Hellmann, A. (2001) Untertagekartierung der Hutti-Mine und Kartierung des Tagebaus der Ajjanahalli-Mine in Sudindien (Diplomkartierung) sowie Fetrographie und Geochemie der Au-Lagerstatte Ajjanahalli (Diplomarbeit): Unpub. M.Sc. thesis, RWTH Aachen, 205p. Hirth, G. and Tullis, J. (1994) The brittle-plastic transition in experimentally deformed quartz aggregates. J. Geophys. Res., v. 99, pp. 11731-11747. Johnson, E.L. and Hollister, L.S. (1995) Syndeformational fluid trapping in quartz: determining the pressure-temperature conditions of deformation from fluid inclusions and the formation of pure CO, fluid inclusions during grain-boundary migration. J. Metam. Geol., v. 13, pp. 239-249. Klemd, R. (1998) Comment on the paper by Schmidt Mumm et al.: High CO, content of fluid inclusions in gold mineralisations in the Ashanti Belt, Ghana: a new category of ore forming fluids? Mineral. Depos., v. 33, pp. 317-319. Kolb, J. and Meyer, EM. (2002) Fluid inclusion record of the hypozonal orogenic Renco gold deposit (Zimbabwe) during the retrograde P-T evolution. Contrib. Mineral. Petrol., v. 143, pp. 495-509. Kolb, J., Meyer, EM. and Hellmann, A. (2002) Lode gold mineralization in a first order shear zone: The Ajjanahalli gold mine (Dharwar Craton, South India). Transactions of the Institution of Mining and Metallurgy (Section B: Applied Earth Sci.), v. 111, pp. 150-151. Kolb, J., Rogers, A., Meyer, F.M. and Hellmann, A. (2001) Structural control of alteration and mineralization in the epigenetic, BIF-hosted Ajjanahalli gold mine (Dharwar Craton, India). Berichte der Deutschen Mineralogischen Gesellschaft, Supp. J. European Mineral., v. 13, p. 98. Pal, N. and Mishra, B. (2003) Epigenetic Nature of the BIF-hosted Gold Mineralization at Ajjanahalli, Southern India: evidence
from ore petrography and fluid inclusion studies. Gondwana. Res., v. 6, pp. 531-540. Ramboz, C., Pichavant, M. and Weisbrod, A. (1982) Fluid immiscibility in natural processes: use and misuse of fluid inclusion data: 11. Interpretation of fluid inclusion data in terms of immiscibility. Chem. Geol., v. 37, pp. 29-48. Ramsay, .J.G. and Huber, M.I. (1987) Folds and fractures. The techniques of modern structural geology, Academic Press Limited, London, 700p. Robert, F. and Poulsen, K.H. (2001) Vein formation and deformation in greenstone gold deposits. In Richards, J.P. and Tosdal, R.M. (Eds.), Structural controls on ore genesis, Rev. Econ. Geol., 14, pp. 111-155. Schmid, S.M. and Handy, M.R. (1991) Towards a genetic classification of fault rocks: geological usage and tectonophysical implications. In: Muller, D.W., McKenzie, J.A. and Weissert, H. (Eds.), Controversies in modern geology, Academic Press, London, v., pp. 339-361. Stipp, M., Stunitz, H., Heilbronner, R. and Schmid, S.M. (2002) The eastern Tonale fault zone: a ‘natural laboratory’ for crystal plastic deformation of quartz over a temperature range from 250 to 700°C. J. Struct. Geol., v. 24, pp. 1861-1884. Touret, J.L.R. (2001) Fluids in metamorphic rocks. Lithos, v. 55, pp. 1-25. Vityk, M.O. and Bodnar, R.J. (1995) Textural evolution of synthetic fluid inclusions in quartz during reequilibration, with applications to tectonic reconstruction. Contrib. Mineral. Petrol., v. 121, pp. 309-323. Watson, E.B. and Brenan, J.M. (1987) Fluids in the lithosphere 1.Experimentally determined wetting characteristics of C0,H,O fluids and their implications for fluid transport, hostrock physical properties and fluid inclusion formation. Earth Planet. Sci. Lett., v. 85, pp. 497-515.
Gondwana Research (Gondwana Newsletter Section) V 7, No. 2, p p . 632-635. 02004 International Association for Gondwana Research, Japan.
GNL
DISCUSSION
Epigenetic Nature of the BIF-hosted Gold Mineralization at Ajjanahalli, Southern India: Evidence from Ore Petrography and Fluid Inclusion Studies: Reply* Nabarun Pall and Biswajit Mishra2* Reliance Industries Ltd. (Oil & Gas),Dhirubhai Ambani Knowledge City, Block H, Koparkhairane, Navi Mumbai - 400 709, lndia Department of Geologyand Geophysics, Indian lnstitute of Technology,Kharagpur - 721 302, lndia, E-mail:[email protected]
We are happy to learn that Dr. Kochen Kolb has taken some interest in reading our above paper. Without any prelude, we are going straight into our response to Kolb’s
comments. Possibly Kolb has not properly gone through our paper, leading to his observation that all ‘Ifluid inclusions are classified as primary”. However, in the
*See Nabarun Pal and Biswajit Mishra (20031, Gondwana Research, v. 6, p p . 531-540.
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section on inclusion petrography (p. 5351, we have clearly stated that there are distinct trail-bound high saline secondary inclusions. Kolb claims that quartz veins in phyllites are developed as extensional veins “cross-cutting the S, foliations” and are slightly foIded, suggesting that these veins “most probably formed late during 02)). We have seen that these veins parallel S,. Accordingly, we assume that veins in phyllites are syn-D,. In other words, how can the extension veins in phyllites be later than the tension gash veins in the BIF? We firmly believe that these two veining episodes are part of the same deformational continuum. We are well aware of the three not “mo”fundamental criteria of fluid immiscibility as proposed by Ramboz et a1 (1982). However, let us first admit that there are lots of confusions in fluid inclusion literature pertaining to synonymy of the two terms: fluid immiscibility and boiling (see Roedder, 1992; p.15). Boiling is indeed a process of phase separation caused by decrease in pressure and/or temperature of the initially trapped homogeneous fluid, leading to equilibrium entrapment of water (H,O + salt) and steam (Roedder and Bodnar, 1997). Obviously manifestation of boiling in hydrothermal systems is by opposing modes of homogenization (in liquid as well vapor states) at nearly similar temperature (Shepherd et al, 1985). The other mechanism of phase separation is splitting of homogeneous H,O + CO,+NaCl fluid from any ambient temperature (above the H,O-CO, solvus) to an H,O-NaC1- and a CO, (+ H,O)-rich fluid (Roedder and Bodnar, 1997). Such phase separation from a n H,O+CO,+CH,+NaCl fluid is a viable mechanism that promotes necessary changes in fluid chemistry leading to gold precipitation (Robert and Kelly, 1987; Naden and Shepherd, 1989; Bowers, 1991; Wilkinson and Johnston, 1996; Samson et al, 1997). We will deal with fluid immiscibility little later. Kolb writes that we found a density variation for carbonic and aqueous-carbonic inclusions of 0.42-0.94 g m . ~ r nand - ~ no density data was furnished for aqueous inclusions. Further, Kolb stresses that we did not observe opposing modes of homogenization of various inclusion types and he is of the opinion that ‘ffluid inclusions representative of the gas phase of an immisciblefluid should show total homogenization into vapor phase”. We feel that there is a need for a closer look into the process of phase separation of an H,O+CO,+NaCl fluid in a more realistic way. A low saline aqueous-carbonic fluid, during phase separation (due to decrease in T and/or P) may intersect its solvus and split into a C0,-rich (+CH, or N,) fluid and more saline aqueous fluid. Further, depending on the P-T condition and bulk fluid composition, the above two fluids could be of similar density resulting in both retained ~
Gondwana Research, K 7, No. 2,2004
within the system. However, if these two fluids had contrasting densities, then only the C0,-rich phase forms a low-density vapor that can physically separate from the immediate environment, leading to its under-representation and virtual absence in the inclusion assemblage (Wilkinson, 2001). Hence, it boils down the basic factor pertaining to the entrapment condition of the two immiscible fluids, If the V/V+L ratio is high, then there are chances of vapor phase homogenization. Perhaps Kolb can see this point in table 5 of Dugdale and Hagemann (2001), which he has quoted. Explosion textures (c.f. Vityk and Bodnar, 1995), if observed, imply isothermal decompression. That does not mean that if we do not see the same, the possibility of isothermal decompression is ruled out. Based on metamorphic and alteration mineral assemblages, Kolb constrains the peak metamorphic temperature in the range of 300”-350°C. But the greenschist facies assemblage, what Kolb mentions, can definitely persist to high temperature. In any case, how does it eliminate isothermal decompression, in relation to fluid evolution? The corresponding author firmly believes that Kolb needs to have a better scientific insight before making such generalized statement that “there are number of weaknesses’’ in the paper. Addirionally, I am afraid such untoward remarks create apprehensions pertaining to the competency of the GR reviewers. However, the replies to his remarks/comments are listed below. (1) We think Kolb himself admits that the extension veins in phyllites may form contemporaneously with F,. If not, why should they parallel S,? Our Fig. 2a does not indicate a “transected7hinge. It only depicts that the shears operated along axial planes of the F, folds. (2) Yes, we have only seen the oxide facies and carbonate minerals such as siderite locally occurs in small portions in the western side of the main fold, not any facies variation. If Kolb has seen “lateral variation in oxideand carbonate facies”, then let him publish his results. As far as we are concerned Hellmann (2001) is a n unpublished M.Sc. dissertation and Kolb et al. (2001, 2002) are abstracts, not full papers. (3) A primary inclusion does not become secondary (or pseudo-secondary) up on recrystallization, albeit undergoes partial reequilibration. Therefore, should we group them as reequilibrated primary inclusions? Again we have not seen any evidence of grain boundary migration to think about the possibility of selective trapping of aqueous and carbonic inclusions as Kolb suggests. In any case, recognition of coexisting inclusions of contrasting compositions that represent partial mechanical mixtures of two fluids, rather than one fluid that underwent separation is not a trivial matter (Wilkinson, 2001; p. 261).
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(4) Yes, entrapment P-T conditions of aqueous and carbonic inclusions in the gash veins (in the BIF) are indeed little lower than those trapped in phyllites, although they overlap, as correctly pointed out by Kolb. We have used the method of intersecting isochores (Roedder and Bodnar, 1980). At least, we have honestly mentioned that in the absence of suitable inclusion pairs, strictly adhering to the criteria of coevality (and cogenesis), all the primary aqueous and carbonic inclusions in various samples were tried, leading to the broad P-T fields (Fig. 8, Pal and Mishra, 2003). In the above figure, we have plotted isochores for each sample with minimum and maximum density values, both for aqueous and carbonic inclusions. Had we got suitable inclusion pairs, we must have tightly constrained the P-T values for each sample, not the broad P-T regimes. We believe that there is nothing wrong with such exercise. Touret (2001) writes about the judicious intersection of inclusion isochores with metamorphic P-T boxes. We are well aware of this problem that relates to fluid inclusion studies in metamorphic systems. Here we are dealing with evolution of a metamorphogenic hydrothermal fluid, not metamorphic rocks. (5) We have not ventured into any discussion on diverse mechanism of vein formation in phyllites and the BIF, since this aspect is beyond the scope of our paper. We have only furnished a small description explaining a viable possibility of non-preservation of large-sized aqueouscarbonic inclusions in gash veins within the BIF (p. 538). We are aware of the diverse formation mechanisms of gash veins (in the BIF) and extensional veins (in phyllites), which are rudimentary. (6) Sutured and serrated grain boundaries indicate dynamic recrystallization i.e., recovery proceeds as deformation operates. Such microtextures can result at temperatures above -4OOOC (see Fig. 4d in Stripp et al, 2002 and other references in Kruhl and Peternell, 2002). Hence, Kolb’s temperature constraint of “around 3OOOC” can never be a plausible inference. Kolb apparently perceives a model of “tectonometamorphic evolution” for Ajjanahalli and possibly isothermal decompression does not fit the scenario he envisages. But then, what is the model? He neither mentions about it nor has published anywhere. Before publishing one’s own results and interpretations thereof, accepted by the scientific community, we feel that such comments are irrelevant and out of context.
References Bowers, T. (1991) The deposition of gold and other metals: pressure induced fluid immiscibility and associated stable isotopic signatures. Geochim. Cosmochim. Acta, v. 55, pp. 2417-2434.
Dugdale, A.L. and Hagemann, S.G. (2001) The Bronzewing lode-gold deposit, Western Australia: P-T-X evidence for fluid immiscibility caused by cyclic decompression in gold-bearing quartz-veins. Chem. Geol., v. 173, pp. 59-90. Hellmann, A. (2001) Petrographie und Geochemie der Au-Lagerstatte Ajjanahalli. Unpub. Diplom-Arbeit thesis. Institut fur Mineralogie und Lagerstattenlehre, RWTH Aachen, Germany, 162p. Kolb, J., Meyer, EM. and Hellmann, A. (2002) Lode gold mineralization in a first order shear zone: The Ajjanahalli gold mine (Dharwar Craton, South India). Transactions of the Institution of Mining and Metallurgy (Section B: Applied earth science), v. 111, pp. 150-151. Kolb, J., Rogers, A., Meyer, EM. and Hellmann, A. (2001) Structural control of alteration and mineralization in the epigenetic, BIF-hosted Ajjanahalli gold mine (Dharwar Craton, India). Berichte der Deutschen Mineralogischen Gesellschaft, Supp. J. European Mineral., v. 13, p. 98. Kruhl, J.H. and Peternell, M. (2002) The equilibration of highangle grain boundaries in dynamically recrystallized quartz: the effect of crystallography and temperature. J. Struct. Geol., V. 24, pp. 1125-1137. Naden, J. and Shepherd, T.J.(1989) Role of methane and carbon dioxide in gold deposition. Nature, v. 342, pp. 793-795. Pal, N. and Mishra, B. (2003) Epigenetic nature of the BIF-hosted gold mineralization at Ajjanahalli, southern India: evidence from ore petrography and fluid inclusion studies. Gondwana Res., v. 6, pp. 531-540. Ramboz, C., Pichavant, M. and Weisbrod, A. (1982) Fluid immiscibility in natural processes: use and misuse of fluid inclusion data: 11. Interpretation of fluid inclusion data in terms of immiscibility. Chem. Geol., v. 37, pp. 29-48. Robert, F. and Kelly, W.C. (1987) Ore-forming fluids in Archaean gold-bearing quartz veins at the Sigma mine, Abitibi greenstone belt, Quebec, Canada. Econ. Geol., v. 82, pp. 1464-1482. Roedder, E (1992) Fluid inclusion evidence for immiscibility in magmatic differentiation. Geochim. Cosmochim. Acta, v. 56, pp. 5-20. Roedder, E and Bodnar, R.J. (1997) Fluid inclusion studies of hydrothermal ore deposits. In: Barnes, H.L. (Ed.), Geochemistry of hydrothermal ore deposits, New York, John Wiley, pp. 657-698. Roedder, E. and Bodnar, R.J. (1980) Geologic pressure determination from fluid inclusion studies. Ann. Rev. Earth Planet. Sci. v. 8, pp. 263-301. Samson, I.M., Bas, B. and Holm, P.E. (1997) Hydrothermal evolution of auriferous shear zones, Wawa, Ontario, Econ. Geol., v. 92, pp. 325-342. Shepherd, T.J., Rankin, A.H. and Alderdon, D.H.M. (1985) A practical guide to fluid inclusion studies. Glasgow, Blackie, 239p. Stipp, M., Stunitz, H., Heilbronner, R. and Schmid, S.M. (2002) The eastern Tonale fault zone: a ‘natural laboratory’ for crystal plastic deformation of quartz over a temperature range from 250 to 700°C. J. Struct. Geol., v. 24, pp. 1861-1884. Touret, J.L.R. (2001) Fluids in metamorphic rocks. Lithos, V. 55, pp. 1-25. Vityk, M.O. and Bodnar, R.J. (1995) Textural evolution of synthetic fluid inclusions in quartz during reequilibration, Gondwana Research, V. 7, No. 2,2004
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with applications to tectonic reconstruction. Contrib. Mineral. Petrol., v. 121, pp. 309-323. Wilkinson, J.J. (2001) Fluid inclusions in hydrothermal ore deposits. Lithos, v. 55, pp. 229-272.
Wilkinson, J.J. and Johnston, J.D. (1996) Pressure fluctuation, phase separation and gold precipitation during seismic fracture propagation. Geology, v. 24, pp. 395-398.
Gondwana Research (Gondwana Newsletter Section) V 7, No. 2, p p . 635-636. 0 2004 International Association for Gondwana Research, Japan.
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CONFERENCE REPORT
Himalayan Tectonics (The HIMPROBE Results) Sandeep Singh Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee - 247 667
A workshop on ‘Himalayan Tectonics (The HIMPROBE Results)’ was organized by Dr. Sandeep Singh, from October 16-17, 2003, at the Department of Earth Sciences, Indian Institute of Technology Roorkee, sponsored by Deep Continental Studies Programme of the Department of Science and Technology (DST), New Delhi. There were about 40 participants who are actively involved in the Himalayan tectonics. The workshop was inaugurated by Dr. H.K. Gupta, Secretary, Department of Ocean Development. There were five main sessions and a final concluding session. Each session was conducted by a Chairperson and a Reporter. The first session was on Karakoram Mountains, which contained six presentations. Chairperson (O.N. Bhargava) and Reporter (S. Singh) were of the opinion that all the studies carried out under different disciplines may be synthesized to present an integrated picture and similar multidisciplinary - multiinstitutional progammes should be launched in other sectors of the Himalaya. Further, there was a recommendation to acquire MT data between Mandi-Baralachala for a complete picture of the geotransect. The second session targeted presentation from the Himalayan Suture Zone with five presentations. Chairperson (M. Joshi) and Reporter (M.L. Sharma) were of opinion that there should be some via media to resolve the controversy of one suture or different sutures along with more age data on other units of Ladakh Batholith. He also mentioned that a detailed gravity modeling is required along its integration with TM and seismic profiling. The third session, dedicated to Himalayan UHP Terrain, had five papers on Tso-Morari Crystallines, the UHP terrain in Ladakh and incorporated latest investigations on metamorphic petrology, P-T Gondwana Research, V. 7, No. 2, 2004
estimates, geochronology and one AMS study in Ladakh. Chairperson (A.K. Jain) and Reporter (T Ahmad) specified the need for more sample collection from different terrains for constraining the age by fossil records as well as much constrained geochronological data on zircon. They also posed few questions like: what controls exhumation uplift of the Indian continental lithosphere from =lo0 km, role of serpentinization and how they are related with other parts of the Himalaya, when did eclogitization taken place and what is the temporal relationship with collision tectonics in the Himalayan are they concomittant or not, if it is a possible to draw a more precise PT-t path for the Tso-Moarari Crystallines and the nature of subduction, like how fast and what is the residence time, eclogiteblue schist connection and if there is exhumation is that single stage or multiple stages. The fourth session was on Tectonics of Great Himalaya with three presentations. Chairperson (P.K. Verma) and Reporter ( S . Bajpai) indicated that there is need of studies for more realistic structures to constrain the structural sequence for modeling reversal of shear indicator and also combined channel flow and ductile shear model. They also mentioned that there is lot of scope for looking back at Himachal Himalaya with a new approach. The fifth session on Deep Crustal Structure had five presentations during the pre-lunch session and two presentations during the post-lunch session. The Chairperson of the pre-lunch session (B.R. Arora) and Reporter (R.G.S. Sastry) were of the opinion that precaution should be taken in comparison on a one-to-one basis. There is also need for revisiting the DSS data along with borehole data of ONGC. B.R. Arora mentioned that the conductance of the zone is estimated to be >20,000 s, much higher than found
Hellmann, A. (2001) Untertagekartierung der Hutti-Mine und Kartierung des Tagebaus der Ajjanahalli-Mine in Sudindien (Diplomkartierung) sowie Fetrographie und Geochemie der Au-Lagerstatte Ajjanahalli (Diplomarbeit): Unpub. M.Sc. thesis, RWTH Aachen, 205p. Hirth, G. and Tullis, J. (1994) The brittle-plastic transition in experimentally deformed quartz aggregates. J. Geophys. Res., v. 99, pp. 11731-11747. Johnson, E.L. and Hollister, L.S. (1995) Syndeformational fluid trapping in quartz: determining the pressure-temperature conditions of deformation from fluid inclusions and the formation of pure CO, fluid inclusions during grain-boundary migration. J. Metam. Geol., v. 13, pp. 239-249. Klemd, R. (1998) Comment on the paper by Schmidt Mumm et al.: High CO, content of fluid inclusions in gold mineralisations in the Ashanti Belt, Ghana: a new category of ore forming fluids? Mineral. Depos., v. 33, pp. 317-319. Kolb, J. and Meyer, EM. (2002) Fluid inclusion record of the hypozonal orogenic Renco gold deposit (Zimbabwe) during the retrograde P-T evolution. Contrib. Mineral. Petrol., v. 143, pp. 495-509. Kolb, J., Meyer, EM. and Hellmann, A. (2002) Lode gold mineralization in a first order shear zone: The Ajjanahalli gold mine (Dharwar Craton, South India). Transactions of the Institution of Mining and Metallurgy (Section B: Applied Earth Sci.), v. 111, pp. 150-151. Kolb, J., Rogers, A., Meyer, F.M. and Hellmann, A. (2001) Structural control of alteration and mineralization in the epigenetic, BIF-hosted Ajjanahalli gold mine (Dharwar Craton, India). Berichte der Deutschen Mineralogischen Gesellschaft, Supp. J. European Mineral., v. 13, p. 98. Pal, N. and Mishra, B. (2003) Epigenetic Nature of the BIF-hosted Gold Mineralization at Ajjanahalli, Southern India: evidence
from ore petrography and fluid inclusion studies. Gondwana. Res., v. 6, pp. 531-540. Ramboz, C., Pichavant, M. and Weisbrod, A. (1982) Fluid immiscibility in natural processes: use and misuse of fluid inclusion data: 11. Interpretation of fluid inclusion data in terms of immiscibility. Chem. Geol., v. 37, pp. 29-48. Ramsay, .J.G. and Huber, M.I. (1987) Folds and fractures. The techniques of modern structural geology, Academic Press Limited, London, 700p. Robert, F. and Poulsen, K.H. (2001) Vein formation and deformation in greenstone gold deposits. In Richards, J.P. and Tosdal, R.M. (Eds.), Structural controls on ore genesis, Rev. Econ. Geol., 14, pp. 111-155. Schmid, S.M. and Handy, M.R. (1991) Towards a genetic classification of fault rocks: geological usage and tectonophysical implications. In: Muller, D.W., McKenzie, J.A. and Weissert, H. (Eds.), Controversies in modern geology, Academic Press, London, v., pp. 339-361. Stipp, M., Stunitz, H., Heilbronner, R. and Schmid, S.M. (2002) The eastern Tonale fault zone: a ‘natural laboratory’ for crystal plastic deformation of quartz over a temperature range from 250 to 700°C. J. Struct. Geol., v. 24, pp. 1861-1884. Touret, J.L.R. (2001) Fluids in metamorphic rocks. Lithos, v. 55, pp. 1-25. Vityk, M.O. and Bodnar, R.J. (1995) Textural evolution of synthetic fluid inclusions in quartz during reequilibration, with applications to tectonic reconstruction. Contrib. Mineral. Petrol., v. 121, pp. 309-323. Watson, E.B. and Brenan, J.M. (1987) Fluids in the lithosphere 1.Experimentally determined wetting characteristics of C0,H,O fluids and their implications for fluid transport, hostrock physical properties and fluid inclusion formation. Earth Planet. Sci. Lett., v. 85, pp. 497-515.
Gondwana Research (Gondwana Newsletter Section) V 7, No. 2, p p . 632-635. 02004 International Association for Gondwana Research, Japan.
GNL
DISCUSSION
Epigenetic Nature of the BIF-hosted Gold Mineralization at Ajjanahalli, Southern India: Evidence from Ore Petrography and Fluid Inclusion Studies: Reply* Nabarun Pall and Biswajit Mishra2* Reliance Industries Ltd. (Oil & Gas),Dhirubhai Ambani Knowledge City, Block H, Koparkhairane, Navi Mumbai - 400 709, lndia Department of Geologyand Geophysics, Indian lnstitute of Technology,Kharagpur - 721 302, lndia, E-mail:[email protected]
We are happy to learn that Dr. Kochen Kolb has taken some interest in reading our above paper. Without any prelude, we are going straight into our response to Kolb’s
comments. Possibly Kolb has not properly gone through our paper, leading to his observation that all ‘Ifluid inclusions are classified as primary”. However, in the
*See Nabarun Pal and Biswajit Mishra (20031, Gondwana Research, v. 6, p p . 531-540.
633
section on inclusion petrography (p. 5351, we have clearly stated that there are distinct trail-bound high saline secondary inclusions. Kolb claims that quartz veins in phyllites are developed as extensional veins “cross-cutting the S, foliations” and are slightly foIded, suggesting that these veins “most probably formed late during 02)). We have seen that these veins parallel S,. Accordingly, we assume that veins in phyllites are syn-D,. In other words, how can the extension veins in phyllites be later than the tension gash veins in the BIF? We firmly believe that these two veining episodes are part of the same deformational continuum. We are well aware of the three not “mo”fundamental criteria of fluid immiscibility as proposed by Ramboz et a1 (1982). However, let us first admit that there are lots of confusions in fluid inclusion literature pertaining to synonymy of the two terms: fluid immiscibility and boiling (see Roedder, 1992; p.15). Boiling is indeed a process of phase separation caused by decrease in pressure and/or temperature of the initially trapped homogeneous fluid, leading to equilibrium entrapment of water (H,O + salt) and steam (Roedder and Bodnar, 1997). Obviously manifestation of boiling in hydrothermal systems is by opposing modes of homogenization (in liquid as well vapor states) at nearly similar temperature (Shepherd et al, 1985). The other mechanism of phase separation is splitting of homogeneous H,O + CO,+NaCl fluid from any ambient temperature (above the H,O-CO, solvus) to an H,O-NaC1- and a CO, (+ H,O)-rich fluid (Roedder and Bodnar, 1997). Such phase separation from a n H,O+CO,+CH,+NaCl fluid is a viable mechanism that promotes necessary changes in fluid chemistry leading to gold precipitation (Robert and Kelly, 1987; Naden and Shepherd, 1989; Bowers, 1991; Wilkinson and Johnston, 1996; Samson et al, 1997). We will deal with fluid immiscibility little later. Kolb writes that we found a density variation for carbonic and aqueous-carbonic inclusions of 0.42-0.94 g m . ~ r nand - ~ no density data was furnished for aqueous inclusions. Further, Kolb stresses that we did not observe opposing modes of homogenization of various inclusion types and he is of the opinion that ‘ffluid inclusions representative of the gas phase of an immisciblefluid should show total homogenization into vapor phase”. We feel that there is a need for a closer look into the process of phase separation of an H,O+CO,+NaCl fluid in a more realistic way. A low saline aqueous-carbonic fluid, during phase separation (due to decrease in T and/or P) may intersect its solvus and split into a C0,-rich (+CH, or N,) fluid and more saline aqueous fluid. Further, depending on the P-T condition and bulk fluid composition, the above two fluids could be of similar density resulting in both retained ~
Gondwana Research, K 7, No. 2,2004
within the system. However, if these two fluids had contrasting densities, then only the C0,-rich phase forms a low-density vapor that can physically separate from the immediate environment, leading to its under-representation and virtual absence in the inclusion assemblage (Wilkinson, 2001). Hence, it boils down the basic factor pertaining to the entrapment condition of the two immiscible fluids, If the V/V+L ratio is high, then there are chances of vapor phase homogenization. Perhaps Kolb can see this point in table 5 of Dugdale and Hagemann (2001), which he has quoted. Explosion textures (c.f. Vityk and Bodnar, 1995), if observed, imply isothermal decompression. That does not mean that if we do not see the same, the possibility of isothermal decompression is ruled out. Based on metamorphic and alteration mineral assemblages, Kolb constrains the peak metamorphic temperature in the range of 300”-350°C. But the greenschist facies assemblage, what Kolb mentions, can definitely persist to high temperature. In any case, how does it eliminate isothermal decompression, in relation to fluid evolution? The corresponding author firmly believes that Kolb needs to have a better scientific insight before making such generalized statement that “there are number of weaknesses’’ in the paper. Addirionally, I am afraid such untoward remarks create apprehensions pertaining to the competency of the GR reviewers. However, the replies to his remarks/comments are listed below. (1) We think Kolb himself admits that the extension veins in phyllites may form contemporaneously with F,. If not, why should they parallel S,? Our Fig. 2a does not indicate a “transected7hinge. It only depicts that the shears operated along axial planes of the F, folds. (2) Yes, we have only seen the oxide facies and carbonate minerals such as siderite locally occurs in small portions in the western side of the main fold, not any facies variation. If Kolb has seen “lateral variation in oxideand carbonate facies”, then let him publish his results. As far as we are concerned Hellmann (2001) is a n unpublished M.Sc. dissertation and Kolb et al. (2001, 2002) are abstracts, not full papers. (3) A primary inclusion does not become secondary (or pseudo-secondary) up on recrystallization, albeit undergoes partial reequilibration. Therefore, should we group them as reequilibrated primary inclusions? Again we have not seen any evidence of grain boundary migration to think about the possibility of selective trapping of aqueous and carbonic inclusions as Kolb suggests. In any case, recognition of coexisting inclusions of contrasting compositions that represent partial mechanical mixtures of two fluids, rather than one fluid that underwent separation is not a trivial matter (Wilkinson, 2001; p. 261).
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(4) Yes, entrapment P-T conditions of aqueous and carbonic inclusions in the gash veins (in the BIF) are indeed little lower than those trapped in phyllites, although they overlap, as correctly pointed out by Kolb. We have used the method of intersecting isochores (Roedder and Bodnar, 1980). At least, we have honestly mentioned that in the absence of suitable inclusion pairs, strictly adhering to the criteria of coevality (and cogenesis), all the primary aqueous and carbonic inclusions in various samples were tried, leading to the broad P-T fields (Fig. 8, Pal and Mishra, 2003). In the above figure, we have plotted isochores for each sample with minimum and maximum density values, both for aqueous and carbonic inclusions. Had we got suitable inclusion pairs, we must have tightly constrained the P-T values for each sample, not the broad P-T regimes. We believe that there is nothing wrong with such exercise. Touret (2001) writes about the judicious intersection of inclusion isochores with metamorphic P-T boxes. We are well aware of this problem that relates to fluid inclusion studies in metamorphic systems. Here we are dealing with evolution of a metamorphogenic hydrothermal fluid, not metamorphic rocks. (5) We have not ventured into any discussion on diverse mechanism of vein formation in phyllites and the BIF, since this aspect is beyond the scope of our paper. We have only furnished a small description explaining a viable possibility of non-preservation of large-sized aqueouscarbonic inclusions in gash veins within the BIF (p. 538). We are aware of the diverse formation mechanisms of gash veins (in the BIF) and extensional veins (in phyllites), which are rudimentary. (6) Sutured and serrated grain boundaries indicate dynamic recrystallization i.e., recovery proceeds as deformation operates. Such microtextures can result at temperatures above -4OOOC (see Fig. 4d in Stripp et al, 2002 and other references in Kruhl and Peternell, 2002). Hence, Kolb’s temperature constraint of “around 3OOOC” can never be a plausible inference. Kolb apparently perceives a model of “tectonometamorphic evolution” for Ajjanahalli and possibly isothermal decompression does not fit the scenario he envisages. But then, what is the model? He neither mentions about it nor has published anywhere. Before publishing one’s own results and interpretations thereof, accepted by the scientific community, we feel that such comments are irrelevant and out of context.
References Bowers, T. (1991) The deposition of gold and other metals: pressure induced fluid immiscibility and associated stable isotopic signatures. Geochim. Cosmochim. Acta, v. 55, pp. 2417-2434.
Dugdale, A.L. and Hagemann, S.G. (2001) The Bronzewing lode-gold deposit, Western Australia: P-T-X evidence for fluid immiscibility caused by cyclic decompression in gold-bearing quartz-veins. Chem. Geol., v. 173, pp. 59-90. Hellmann, A. (2001) Petrographie und Geochemie der Au-Lagerstatte Ajjanahalli. Unpub. Diplom-Arbeit thesis. Institut fur Mineralogie und Lagerstattenlehre, RWTH Aachen, Germany, 162p. Kolb, J., Meyer, EM. and Hellmann, A. (2002) Lode gold mineralization in a first order shear zone: The Ajjanahalli gold mine (Dharwar Craton, South India). Transactions of the Institution of Mining and Metallurgy (Section B: Applied earth science), v. 111, pp. 150-151. Kolb, J., Rogers, A., Meyer, EM. and Hellmann, A. (2001) Structural control of alteration and mineralization in the epigenetic, BIF-hosted Ajjanahalli gold mine (Dharwar Craton, India). Berichte der Deutschen Mineralogischen Gesellschaft, Supp. J. European Mineral., v. 13, p. 98. Kruhl, J.H. and Peternell, M. (2002) The equilibration of highangle grain boundaries in dynamically recrystallized quartz: the effect of crystallography and temperature. J. Struct. Geol., V. 24, pp. 1125-1137. Naden, J. and Shepherd, T.J.(1989) Role of methane and carbon dioxide in gold deposition. Nature, v. 342, pp. 793-795. Pal, N. and Mishra, B. (2003) Epigenetic nature of the BIF-hosted gold mineralization at Ajjanahalli, southern India: evidence from ore petrography and fluid inclusion studies. Gondwana Res., v. 6, pp. 531-540. Ramboz, C., Pichavant, M. and Weisbrod, A. (1982) Fluid immiscibility in natural processes: use and misuse of fluid inclusion data: 11. Interpretation of fluid inclusion data in terms of immiscibility. Chem. Geol., v. 37, pp. 29-48. Robert, F. and Kelly, W.C. (1987) Ore-forming fluids in Archaean gold-bearing quartz veins at the Sigma mine, Abitibi greenstone belt, Quebec, Canada. Econ. Geol., v. 82, pp. 1464-1482. Roedder, E (1992) Fluid inclusion evidence for immiscibility in magmatic differentiation. Geochim. Cosmochim. Acta, v. 56, pp. 5-20. Roedder, E and Bodnar, R.J. (1997) Fluid inclusion studies of hydrothermal ore deposits. In: Barnes, H.L. (Ed.), Geochemistry of hydrothermal ore deposits, New York, John Wiley, pp. 657-698. Roedder, E. and Bodnar, R.J. (1980) Geologic pressure determination from fluid inclusion studies. Ann. Rev. Earth Planet. Sci. v. 8, pp. 263-301. Samson, I.M., Bas, B. and Holm, P.E. (1997) Hydrothermal evolution of auriferous shear zones, Wawa, Ontario, Econ. Geol., v. 92, pp. 325-342. Shepherd, T.J., Rankin, A.H. and Alderdon, D.H.M. (1985) A practical guide to fluid inclusion studies. Glasgow, Blackie, 239p. Stipp, M., Stunitz, H., Heilbronner, R. and Schmid, S.M. (2002) The eastern Tonale fault zone: a ‘natural laboratory’ for crystal plastic deformation of quartz over a temperature range from 250 to 700°C. J. Struct. Geol., v. 24, pp. 1861-1884. Touret, J.L.R. (2001) Fluids in metamorphic rocks. Lithos, V. 55, pp. 1-25. Vityk, M.O. and Bodnar, R.J. (1995) Textural evolution of synthetic fluid inclusions in quartz during reequilibration, Gondwana Research, V. 7, No. 2,2004
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with applications to tectonic reconstruction. Contrib. Mineral. Petrol., v. 121, pp. 309-323. Wilkinson, J.J. (2001) Fluid inclusions in hydrothermal ore deposits. Lithos, v. 55, pp. 229-272.
Wilkinson, J.J. and Johnston, J.D. (1996) Pressure fluctuation, phase separation and gold precipitation during seismic fracture propagation. Geology, v. 24, pp. 395-398.
Gondwana Research (Gondwana Newsletter Section) V 7, No. 2, p p . 635-636. 0 2004 International Association for Gondwana Research, Japan.
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CONFERENCE REPORT
Himalayan Tectonics (The HIMPROBE Results) Sandeep Singh Department of Earth Sciences, Indian Institute of Technology Roorkee, Roorkee - 247 667
A workshop on ‘Himalayan Tectonics (The HIMPROBE Results)’ was organized by Dr. Sandeep Singh, from October 16-17, 2003, at the Department of Earth Sciences, Indian Institute of Technology Roorkee, sponsored by Deep Continental Studies Programme of the Department of Science and Technology (DST), New Delhi. There were about 40 participants who are actively involved in the Himalayan tectonics. The workshop was inaugurated by Dr. H.K. Gupta, Secretary, Department of Ocean Development. There were five main sessions and a final concluding session. Each session was conducted by a Chairperson and a Reporter. The first session was on Karakoram Mountains, which contained six presentations. Chairperson (O.N. Bhargava) and Reporter (S. Singh) were of the opinion that all the studies carried out under different disciplines may be synthesized to present an integrated picture and similar multidisciplinary - multiinstitutional progammes should be launched in other sectors of the Himalaya. Further, there was a recommendation to acquire MT data between Mandi-Baralachala for a complete picture of the geotransect. The second session targeted presentation from the Himalayan Suture Zone with five presentations. Chairperson (M. Joshi) and Reporter (M.L. Sharma) were of opinion that there should be some via media to resolve the controversy of one suture or different sutures along with more age data on other units of Ladakh Batholith. He also mentioned that a detailed gravity modeling is required along its integration with TM and seismic profiling. The third session, dedicated to Himalayan UHP Terrain, had five papers on Tso-Morari Crystallines, the UHP terrain in Ladakh and incorporated latest investigations on metamorphic petrology, P-T Gondwana Research, V. 7, No. 2, 2004
estimates, geochronology and one AMS study in Ladakh. Chairperson (A.K. Jain) and Reporter (T Ahmad) specified the need for more sample collection from different terrains for constraining the age by fossil records as well as much constrained geochronological data on zircon. They also posed few questions like: what controls exhumation uplift of the Indian continental lithosphere from =lo0 km, role of serpentinization and how they are related with other parts of the Himalaya, when did eclogitization taken place and what is the temporal relationship with collision tectonics in the Himalayan are they concomittant or not, if it is a possible to draw a more precise PT-t path for the Tso-Moarari Crystallines and the nature of subduction, like how fast and what is the residence time, eclogiteblue schist connection and if there is exhumation is that single stage or multiple stages. The fourth session was on Tectonics of Great Himalaya with three presentations. Chairperson (P.K. Verma) and Reporter ( S . Bajpai) indicated that there is need of studies for more realistic structures to constrain the structural sequence for modeling reversal of shear indicator and also combined channel flow and ductile shear model. They also mentioned that there is lot of scope for looking back at Himachal Himalaya with a new approach. The fifth session on Deep Crustal Structure had five presentations during the pre-lunch session and two presentations during the post-lunch session. The Chairperson of the pre-lunch session (B.R. Arora) and Reporter (R.G.S. Sastry) were of the opinion that precaution should be taken in comparison on a one-to-one basis. There is also need for revisiting the DSS data along with borehole data of ONGC. B.R. Arora mentioned that the conductance of the zone is estimated to be >20,000 s, much higher than found