• M Deb

      Articles written in Journal of Earth System Science

    • Petrology, geothermobarometry and C-O-H-S fluid compositions in the environs of Rampura-Agucha Zn-(Pb) ore deposit, Bhilwara District, Rajasthan

      M Deb Uma Sehgal

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      The massive Zn-(Pb) sulfide ore body at Rampura-Agucha in Bhilwara district, Rajasthan, occurs within graphitic metapelites surrounded by garnet-biotite-sillimanite gneiss containing concordant bodies of amphibolite. These rocks and the sulfide ores have been studied to estimate the pressure, temperature and fluid composition associated with upper amphibolite facies metamorphism. Geothermobarometric calculations involving garnet-biotite and garnet-hornblende pairs, as well as sphalerite-hexagonal pyrrhotite-pyrite and garnet-plagioclase-sillimanite-quartz assemblages indicate that the most pervasive P-T condition during peak of regional metamorphism was 650°C and 6 kb, and was attained between the first and second deformations in the region. Some temperature-pressure estimates also cluster around 500°C–5.1 kb which probably represent retrograde cooling during unloading.

      Consideration of devolatilization equilibria in the C-O-H-S system at the pervasive metamorphic conditions mentioned above shows that the metamorphic fluid was H2O-rich ($$X_{H_2 O} = 0.52$$) but also had a substantial component of$$CO_2 (X_{CO_2 } = 0.39)$$.$$H_2 S(X_{H_2 S} = 0.043)$$ and$$CH_4 (X_{CH_4 } = 0.025)$$ were the other important phases in the fluid. CO (XCO = 0.002) and$$H_2 (X_{H_2 } = 0.002)$$ were the minor phases in the fluid. It is probable that a part of this aqueous fluid was consumed by re-/neocrystallization of hydrous silicate phases like chlorite during the retrogressive metamorphic path, so that fluid entrapped in quartz below 450°C was rendered CO2-rich (Holleret al 1996).

    • Deformation and metamorphism of gold-sulphide lodes in the Bhukia–Jagpura gold prospect, Rajasthan: Implications for ore genesis

      S Deol A Chattopadhyay M Deb

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      The role of polyphase deformation in controlling the emplacement of gold-quartz lodes in dilational regimes is demonstrated from the Proterozoic Bhukia–Jagpura gold prospect in south Rajasthan. Earlier researchers deciphered the gold-sulphide mineralisation event as synchronous to the second phase of deformation (D2) without convincing microstructural or metamorphic evidences. In this contribution, we correlate the deformation and metamorphic imprints in the host rocks with those in the gold-sulphide mineralised zone, and present a new interpretation for the relative timing of gold emplacement vis-á-vis deformation. The ore-forming process first involved layer-parallel influx of ore-bearing hydrothermal fluids along $S_1$ schistosity in the host rocks, synkinematic with respect to the first phase of deformation (D1). This initial ore concentration experienced metamorphism isofacially (∼500° C at 5.3 kb) along with its host rocks during D1, and subsequently underwent extensive remobilisation, giving rise to gold-bearing silicified lodes along the hinges and axial surfaces of F2 folds during D2.

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