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

P. Venkataramana ¹

Affiliations
1 Kumud Villa, Nongrim Hills, Shillong 793003, IN

Abstract

Magnesite occurs as a network of veins and bands within serpentinised peridotite and cap-rock (birbirite). Three stages by which the original peridotite is altered to give rise to magnesite and birbirite are recognised. Geochemical studies indicate that by the action of weakly acidic (with dissolved CO₂) water vapour, the original ferromagnesian minerals in peridotite broke down releasing Mg²⁺ and to a minor extent Si⁴⁺ to be precipitated as magnesite and quartz in favourable locales. Ca²⁺, Al and Fe²⁺ were retained in the silicate matrix. Al and Fe²⁺ precipitated as hydroxides while Ca²⁺ formed CaCO₃(calcite and aragonite).

The approximate P-T conditions at which magnesite crystallised is suggested to be between 500 to 1000 bars and 150 to 250°C respectively with XCO₂ in the fluid phase around 0.05. Magnesite formation is related to late-stage hydrothermal activity associated with ultramafic-mafic magmatism in the area.

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P. Venkataramana ¹, T. R. N. Kutty ¹, G. V. Anantha Iyer ¹

Affiliations
1 Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, IN

Abstract

Previous experimental studies on magnesite formation which are confined to temperatures greater than 250°C, do not explain all the field and petrographic observations in natural situations where magnesite is associated with ultramafic complexes, as at Dodkanya, Mysore district, Karnataka.

A series of experiments were conducted using natural samples (rock and mineral powders) as starting materials. The experimental temperatures were varied from 35° to 300°C at intervals of approximately 50°C and normal and elevated pressures (500 and 1000 bars). At higher pressures, the X_CO2 in the fluid phase was also varied.

These studies have shown that magnesite and quartz can be formed directly by the interaction of CO₂-bearing water vapour and ferromagnesian minerals (olivine, orthopyroxene) without passing through the intermediate serpentine stage. The temperature conditions at which these reactions take place is between 150 and 250 °C at 500 bars total pressure with X_CO2 in the fluid phase greater than 0.015. The presence of Na⁺ salts enhances these reactions.

The presence of calcite-aragonite-quartz-aluminous hydroxide gel-limonite (birbirite) cap rock intersected by a network of magnesite veins in Dodkanya and surrounding areas has been explained on the basis of experiments with natural hornblende, the major Ca²⁺-bearing mineral in hornblende-peridotites. Hornblende, under hydrothermal conditions, breaks down to calcium silicate hydrate (CSH). CSH, being unstable in the presence of CO₂, inverts to CaCO₃ (calcite/aragonite) and quartz. Mg²⁺ in trace quantities stabilises aragonite.

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P. Venkataramana , A. K. Datta ¹

Affiliations
1 Geological Survey of India, 2 Church Street, Bangalore 560001, IN

Abstract

The 200 km long Naga Hills opiliolite belt which forms the northern part of the Indo-Burman ophiolites is made up of two contrasting volcanic suites. viz.; (1) high-Mg basalts (Mg-number 52 to 71) comparable to mid-oceanic ridge basalts and (2) low-Mg (Mg-number 23 to 49) akin to seamount type volcanism.

Based on geological and geochemica 1 evidences, a three stage model comprising (1) marginal basin spreading event, (2) seamount event and (3) collision event, is presented to explain the origin and evolution of the Naga Hills ophiolites.

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S. V. Srikantia ¹, P. Venkataramana ¹

Affiliations
1 A. M. S. E. Wing, Geological Survey of India, No.2, Church Street, Bangalore 560001, IN

Abstract

The meta-ultramafites of the Archaean Nagamangala belt of Karnataka craton display pillow structures and have kornatiitic chemistry. This is in conformity with other ancient supracrustals of Karnataka craton having komatiitic association.

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