Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

D. C. Pal ¹, M. K. Panigrahi ¹, B. Mishra ¹

Affiliations
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721 302, IN

Abstract

Stanniferous pegmatites of Malkangiri occur both in metabasics and metasediments showing signatures of deformation but very little wall rock alteration. Fluid inclusion microthermometric studies in pegmatitic quartz and quartz from quartz veins reveal the following three types of inclusions: aqueous biphase, metastable (?) monophase aqueous inclusions which do not show any thermal response, and rare halite-bearing inclusions. The CO₂-free nature of the inclusions is noteworthy. Isochores of halitebearing inclusions (where T_{s Nacl} > T_h) intersect at P-T values of 2.65 kb and 337°C, implying preservation of the magmatic fluid that separated during the late stage of felsic magmatism in the area. Additionally, the above P-T values compare reasonably well with those obtained from the intersection of halite liquidi with the corresponding isochores.

Tin-bearing pegmatites in metabasics are characterized by their low temperature of homogenization and isothermal variation in salinity. This is explained by mixing of a Sn-bearing, granite affiliated, presumably reduced fluid with a heated, less evolved oxygenated meteoric water that led to cassiterite precipitation. Interestingly, inclusions in cassiterite-bearing pegmatites within metasediments record a low temperature comparable with those from within metabasics. In contrast, barren pegmatites within the schistose metasediments preserve high temperature signature and indicate a simple cooling trend. The study reveals different styles of evolution of stanniferous and barren pegmatites at Malkangiri.

Keywords

Economic Geology, Pegmatites, Tin, Fluid Inclusion, Malkangiri, Orissa.

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M. K. Panigrahi ¹, B. Mishra ¹, A. Mookherjee ¹

Affiliations
1 Department of Geology and Geophysics Indian Institute of Technology, Kharagpur 721302, IN

Abstract

Textural and paragenetic study of primary minerals (chalcopyrite, pyrite, magnetite, hematite, sphalerite, bornite, chalcocite and molybdenite) reveal intermittent fracturing, precipitation and dissolution phenomena with at least two recognizable stages: magnetite(I) - pyritc(I) - molybdenite (?) followed by pyrite(II) - chalcopyrite - sphalerite - magnetite(II) - hematite - bornite - chalcocite. Oxidation and supergene enrichment effects producing cuprite-malachite-native copper and chalcocite-covellite are moderate. Difference in the mineralogy of the oxidation zone and restricted occurrence of supergene enrichment are due to the variation in the primaty mineralogy of the ore.

The ranges of fluid inclusion homogenization temperature (maxima 160°-180°C) and fluid densities in the reef quartz and ore bearing vein quartz are more or less the same, with a wider range of salinity in the vein quartz ores. However, mixed aqueous carbonic inclusions are restricted to the vein quartz ores and hence points towards a difference in the fluid regime. T_h vs. salinity and T_h vs. density plottings rule out simple cooling and/or mixing processes in the evolution of the fluid but rather hint at immiscible two-fluid phase separation. The suspected presence of low temperature Iss-phases and the results of fluid inclusion microthermometty point towards an unexpectedly low temperature environment of ore deposition. The paragenetic sequence is suggestive of an increase in the Cuffe ratio and fluctuating f_O2 conditions in the fluid during the main stage of mineralization.

Keywords

Ore Mineralogy, Fluid Inclusion Study, Malanjkhand, Copper, Madhya Pradesh

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