Occurrence of sulphide minerals in marbles associated with granitic rocks around Chando, Palamau District, Bihar

Abstract

Thin stringers and lenslets of sulphide minerals replacing marbles along cracks and fissures are found around Chando (23° 58′ 36″ N, 84° 6′ 12″ E) in Palamau District, Bihar. Marbles and the paraschists represent the oldest rocks of the area. The marbles occur as more or less circular pockets within granite gneiss country. The minerals are mostly simple sulphides of Fe, Cu, Zn and Pb. The most abundant minerals are bornite and chalcopyrite. Two types of association are common—(a) chalcopyrite-bornite solid solution and (b) bornite-chalcocite solid solution. Myrmekite-like texture in bornite-chalcocite is noteworthy where chalcocite has largely replaced pyrite leaving behind minute remnants. In bornite, chalcopyrite has exsolved as blades along (111).

A mineral is detected which closely resembles ‘Orange Bornite’ (Orcel, 1937, 1943—supposed composition Cu₅ FeS₆) with yellowish orange colour and a higher reflectivity than bornite. It does not extinguish completely, but shows a greenish-pink colour and a mixed±reaction with KCN and HNO₃.

The ore minerals show little or no secondary alterations. The source of the solution probably seems to be that from which the granite was derived and the solution might have supplied the metals along the fissures and cracks of the marble occurring within the granitic country. In course of replacement the sulphide solution produces beautiful ‘delta textures’.