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

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.

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