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

Rajesh K. Srivastava ¹, R. P. Hall ¹, R. Verma ², R. K. Singh ²

Affiliations
1 Department of Geology, University of Portsmouth, Portsmouth POI 3QL, GB
2 Department of Geology, Banaras Hindu University, Varanasi - 221 005, IN

Abstract

Numerous mafic dykes occur within the Archacan-granite-gneisses of the Bastar (Bhandara) craton of central India. Width of these dykes varies from about 20 to 200 metres and are 1 to 20 kilometers long. The dykes trend mainly NW-SE, following the foliation of the granite gneiss and major regional structural trend. They mainly comprise two different types, one, amphibolitic and the other doleritic. The amphibolite dykes have a granoblastic texture whereas the unaltered dolerite dykes are ophitic or sub-ophitic. All of the dykes are sub-alkaline Fe-rich tholeiites. However, variations in the incompatible elements such as Nb, P, Ti, Zr, Y and the light rare earth elements (LREE), suggest that the Bastar dykes infact comprise two different sets. The amphibolite dykes were derived from an earlier relatively high-Mg, low-Ti olivine tholeiite magma. These dykes have high field strength element (HFSE) values only around six times higher than those of primordial mantle. In contrast, the doleritic dykes have fractionated HFSE levels 10 to 30 times higher than primitive mantle values; they are also characterised by a marked negative Sr anomaly not present in the older dykes. One subset of amphibolitic dykes corresponds geochemically to the predominantly doleritic swarm. These latcr dykes are derived from a low-Mg, high-Ti quartz-tholeiitic magma. The two different Bastar dyke sets are recognized here as the older, BD1 amphibolitic dykes and younger, BD2 doleritic dykes. Age constraints for these dykes are set by the ages of the enveloping host gneisses and discordant granitoids and hence they are placed at or near the end-Archaean.

Keywords

Petrology, Mafic Dykes, Precambrian, Bastar Craton, Central India.

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Tapan K. Baidya ¹, S. K. Mondal ², V. Balaram ³, R. Parthasarathy ⁴, R. Verma ⁴, P. K. Mathur ⁴

Affiliations
1 Department of Geological Sciences, Jadavpur University, Calcutta 700 032, IN
2 Refractory Division, Central Glass & Ceramic Research Institute, Calcutta 700 032, IN
3 Geochemistry Group, National Geophysical Research Institute, Hyderabad 500 007, IN
4 Analytical Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, IN

Abstract

The Precambrian ultramafic-mafic plutonic complex of Nuasahi, Orissa in the Eastern Indian Shield represents at least three suites of magmatic intrusion: (i) an interlayered sequence of enstatitite, websterite (± olivine), dunite, chromitite, harzburgite and orthopyroxenite; (ii) massive gabbroic suite with minor bodies of norite, anorthosite and bands of titaniferous magnetite and (iii) dykes and sills of dolerite and clinopyroxenite. A prominent breccia zone is developed near the interface of the first and second magmatic suites in the eastern part of the complex including the Shankar chromite lode and the ultramafic host of the first suite, and the gabbro-noritic rocks of the second suite. This breccia zone is enriched in Cu-Fe-Ni sulphides and has become the principal site for concentration of PGE, gold and silver. On a whole, Pd dominates over Pt in the sulphide-bearing Shankar chromite lode and the adjacent orthopyroxenite as well as gabbro in this breccia zone. The chromite-sulphide association is the most enriched part for PGE, gold and silver. Ag is positively correlated with Pd. The breccia zone which was developed prior to the gabbroic emplacement into the chromiferous ultramafics. represents a sulphide-rich metasomatic zone leading to maximum concentrations of PGE, Au, Ag in this zone alongwith formation of various sulphides, oxides and silicates. Both Pt and Pd are considerably higher and Ir is lower in the present area than in chromite-sulphide associations from ophiolitic complexes and Alpine-type massifs. Rather, Pt, Pd and Ir values compare better with the chromite-sulphide associations from layered intrusions.

Keywords

Economic Geology, Ultramafic-Mafic Complex, Breccia Zone, Chromite-Sulphide Assemblage, PGE-Ag-Au, Nuasahi, Orissa.

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