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

G. V. Ananta Iyer ¹, V. N. Vasudev ²

Affiliations
1 Inorganic & Physical Chemistry Dept., Indian Institute of Science, Bangalore 560012, IN
2 Chitradurga Copper Co. Ltd., 16/1, Ali Asker Road, Bangalore 560052, IN

Abstract

Geochemical data on Archaean goldfields metavolcanics (2.7 b. yrs) have been plotted on major and minor element discriminant diagrams to define the magma type and to formulate the tectonic environment model for the origin of these ancient volcanic rocks. The meta basalts of Kolar and Hutti (Karnataka, South India), Norseman (Eastern Goldfields, Western Australia). Steynsdorp (Barberton, South Africa) and Yellowknife (Canada), plot within the field of mid-ocean ridge basalts (MORB). The marginal basin basalts (MBB) of South Sandwich Islands of Scotia Arc Region of Phanerozoic age also plot within the field of MORB. Unlike the depleted MORB, the MBB are enriched in certain large ion lithophile (LIL) elements with higher Rb/Sr, Ba/Sr and lower K/Ba ratios. Further, in contrast to MORB, they exhibit slightly enriched LREE patterns. The LIL element enriched goldfield volcanics of Kolar, Hutti, Norseman, Steynsdorp and Yellowknife, with their slightly enriched LREE patterns, compare more favourably with MBB than with MORB. Based on geochemical features and geological setting, marginal basin model is proposed as the most favourable tectonic environment for the generation of gold bearing Archaean greenstone belts of Kolar and Hutti.

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P. K. Ramam ¹, G. V. Ananta Iyer ², T. R. Narayanan Kutty ², M. V. N. Murthy ³

Affiliations
1 33/4 RT, Barkatpura, Hyderabad-500027, IN
2 Dept. of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore - 560012, IN
3 A/3, Himanshu Bhawan, Sawai Jai Singh Highway, Jaipur-302006, IN

Abstract

The field observation that chrysotile asbestos near Pulivendla is localised along stylolites in the Proterozoic carbonate sediments of the Vempalli Formation (Cuddapah Supergroup) proved crucial and paved the way for a re-thinking on the origin of Pulivendla asbestos. The length of the fibre, nowhere, transgresses the amplitude of the stylolite. Further, the lateral termination of the chrysotile seams invariably coincides with that of the enclosing stylolite. The stylolite development and fibre growth appear to be a sequel to directional pressure. X-ray and infra-red studies of the massive serpentinous zones revealed dioctahedral dewelyite to be the main serpentine mineral. It is suggested that the chemical pre-requisites (e.g. magnesia, silica and hydroxyl molecules) necessary for serpentinisation and pari passu chrysotilisation, were present in the carbonate sediment itself at the loci of stylolitisation and that the intrusive dolerite merely provided the desired thermal energy. The emplacement took place, possibly, when the carbonate sediment was in a semi-consolidated stage. Stylolitisation and chrysotilisation appear to have ceased with the complete consolidation of the enclosing sediment.

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G. V. Ananta Iyer ¹, V. N. Vasudev ², S. Jayaram ²

Affiliations
1 Inorganic & Physical Chemistry Dept., Indian Institute of Science, Bangalore 560012, IN
2 Department of Mines & Geology, Bangalore 560001, IN

Abstract

No Abstract.

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A. G. Menon ¹, V. S. Venkatasubramanian ¹, V. N. Vasudev ², G. V. Ananta Iyer ¹

Affiliations
1 Department of Physics, Indian Institute of Science, Bangalore 560012, IN
2 Department of Mines & Geology, Bangalore 560001, IN

Abstract

The δ³⁴S values of pyrrhotite and arsenopyrite samples from the gold bearing quartz-sulphide reefs of Hutti have been measured. A narrow range of mantle-meteoritic values is observed indicating a magmatic-hydrothermal origin of the ores. An attempt is made to evaluate the solution parameters by the Ohmoto procedure, as outlined in Parts I and II.

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A. G. Menon ¹, V. S. Venkatasubramanian ¹, G. V. Ananta Iyer ¹

Affiliations
1 Indian Institute of Science, Bangalore 560 012, IN

Abstract

Sulphur isotope abundance variations in chalcopyrite, pyrite and pyrrhotite samples from the sulphide deposits of Kalyadi have been measured with a double-collector mass spectrometer provided with digital ratio recording. The results fall into two groups - one with a near-zero per-mil value and another with a small positive value ∼5‰. Data on coexisting sulphide minerals enable temperatures of deposition to be calculated, these being low ∼250° C. The Ohmoto-procedure is adopted, and the range of δ³⁴S values of the minerals explained by assuming an initial total sulphur δ³⁴S_TS value and a range of pH and f_O2 values that are plotted on an f_S2 - f_O2 diagram consistent with the stability fields of the minerals.

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V. S. Venkatasubramanian ¹, A. G. Menon ¹, G. V. Ananta Iyer ¹

Affiliations
1 Indian Institute of Science, Bangalore 560 012, IN

Abstract

In this sequel to the previous note, we describe the results of δ³⁴S measurements on sulphides of the Ingaldhal deposit. A component with near-zero permit values is superposed on a possibly larger component with positive δ³⁴S values ∼+10‰. The spread can be explained in terms of change in the solution parameters (T, pH and f_O2) of the source-fluid, these being represented by f_s2-f_O2 diagrams. An area on this diagram, sets constraints on the solution parameters during deposition, as indicated.

Keywords

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