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

R. K. Sharma ¹, M. K. Pandit ²

Affiliations
1 GSI Colony, Malviya Nagar, Jaipur - 302 017, IN
2 Department of Geology, University of Rajasthan, Jaipur, IN

Abstract

The Sasunavghar-Juchundra area in the western part of Deccan Flood Basalt Province exposes repetitive acidic pyroclastic sequences overlying intermediate to basic pyroclastic basal flows. These acidic tuffs conform to typical 'ignimbrite' eruptive and depositional environment. The explosive activity has probably resulted from a localised collapse of eruptive column.

Keywords

Ignimbrite, Deccan Flood Basalt Province, Maharashtra.

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R. K. Sharma ¹, M. K. Pandit ², S. Warrier ³

Affiliations
1 Geological Survey of India (WR), Jhalana Dungari, Jaipur 302 017, IN
2 Department of Geology, University of Rajasthan, Jaipur 302 004, IN
3 Geological Survey of India, Pune 411 006, IN

Abstract

The Deccan flood basalt in Pune - Ahmednagar region, western India, is intruded by NE-SW. NW-SE and E-W trending dolerite dyke swarms, post-dating the main eruptive phase. The NE-SW trending dykes host a number of large (∼1 to ›5 m across) ellipsoidal rhyolitic and granitic xenoliths that show effects of hybridization along their rims. Granitic xenoliths contain fractured and partially assimilated crystals. whereas in the fine-grained rhyolites hybridization is manifested in the development of flow structures and chlorophaeite-filled amygdules. Hybridization of the xenoliths has resulted in wide variation in silica abundance, whereas the REE patterns show remarkable similarity. This indicates that hybridization was limited only to the readjustment of relatively mobile elements, resulting in alkali exchange across the interface. The rhyolitic xenoliths might have resulted from the fusion of the crust by basic magma, while the granitic xenoliths possibly represent the basement component. Incomplete assimilation of xenoliths by the melt as evidenced by limited reaction, further corroborates the incorporation of xenoliths at a shallow level, when the uprising basic melt had considerably fractionated and cooled. The fractionated nature of host dolerite dyke is also confirmed by moderate MgO, high FeO and LREE enrichment.

Keywords

Petrology, Acid Xenoliths, Dolerite Dykes, Deccan Traps, Maharashtra.

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R. K. Sharma ¹

Affiliations
1 Geological Survey of India, Jhalana Doongn, Jaipur-302025, IN

Abstract

The Dhambasn Cu-Au prospect Dausa district, Northeastern Rajasthan, is the only auriferous copper deposit in the country after the KGF, which is hosted within the basement granite gneisses of the Mangalwar complex of Archaean age. The epigenetic mineralization is associated with the secondary quartz veins along the brittle ductile shear zone that are reactivated during post mineralisation deformational episodes. The fact that the Dhambasn shear passes and cuts through the much younger Raialo and Alwar group of rocks exposed north of the prospect is indicative of its reactivation. The detailed petro-Minerography and sulphur isotope studies suggest a poly phase mineralisation in terms of an early phase demarcated by pyrite rich mineralisation followed by main copper-Gold deposition during advance stage of mineralisation. This paper presents detailed geology, petrography, fluid inclusion and sulphur isotope data of Dhambasn prospect to classify the fluid type and mode of emplacement of copper-Gold mineralisation.

Keywords

Gold, Copper, Mineralisation, Fluid Inclusion, Sulphur Isotopes, Dausa, Rajasthan.

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R. K. Sharma ¹, Saurabh Ray ¹

Affiliations
1 Geological Survey of India, Jaipur - 302 004, IN

Abstract

The auriferous copper mineralisation at Dhani Basri, Dausa district, Rajasthan, occurring in the Archaean basement rocks (Mangalwar complex of Gupta at el. 1980), discovered and explored recently, is a significant and sale occurrence in North Rajasthan. This find assumes significance as no such Cu-Au mineralisation has been found hosted in the oldest rocks of Rajasthan east northeast of Jaipur.

The mineralisation is localized within the silicified hybrid granite gneisses and controlled by shear zones trending N-S to N 10° W-S 10° E, which are sympathetic to the N-S and NNW-SSE trending faults cutting through the cover sequence, namely Alwar Group of Delhi Supergroup and Raialo metasediments.

The principal sulphides are chalcopyrite, pyrite, pyrrhotite, pentlandite and cubanite. Magnetite occurs as oxide phase. Free gold grains embedded in the chalcopyrite and gangue is also associated with this basemetal mineralisation. The gangue minerals are quartz, sericite, chlorite, biotite and amphiboles. Occurrence of pyrrhotite-cubanite exsolution suggests the temperature of mineralisation to be of the order of 350°C. The coarsely crystalline nature, straight grain boundaries of the are minerals, temperature of formation and fluid homogenization temperature collectively suggest this mineralisation to be of hydrothermal, remobilized nature of epigenetic type and suggests acidic environment of deposition.

Keywords

Cu-Au Mineralisation, Ore Mineralogy, Fluid Inclusions, Genesis, Dhani Basri Prospect, Dausa District, Rajasthan.

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