Metamorphism of Sulfides of the Singhbhum Copper Belt, India; The Evidence from the Ore Fabric

Rich copper mineralization has taken place intermittently along the 150-km Singhbhum copper belt, Bihar, India, the highest concentration being noted in a 50-km section from Badia to Turamdih. Derivatives of basic volcanics host the orebodies in most places; in a few others "soda granites" that formed metasomatically principally from the basic volcanics contain the ores. Orebodies are peneconcordant with their host rocks.Chalcopyrite is the dominant ore mineral at most places, only locally yielding the position to pyrite. Other minerals present are pyrrhotite, pentlandite, millerite, magnetite, ilmenite, molybdenite, tetradymite, wehrlite, cubanite, sphalerite, galena, skutterudite, bismuthinite, mackinawite, valleriite, uraninite and rutile. Supergene phases include marcasite, pyrite, violarite, cuprite, native copper, chalcocite, covellite. Quartz, chlorite, biotite, and sericite in varying proportions constitute the gangue.The ores have been subjected to dynamothermal-dynamic metamorphism. Effects of thermal metamorphism are noted locally. Textural features such as polygonization, equiangular triple-junction points in polycrystalline aggregates, annealing twins, poikiloblastic growths, etc., in the ores may be attributed to dynamothermal metamorphism. This metamorphic event affecting the ores is correlated with the retrogressive metamorphism along the belt. Later deformation crushed and bent some of the ore minerals and produced deformation twins in others.Late dolerite dikes, from less than 2 m to 7 m thick, thermally affected the ores at Badia-Mosaboni Mines, producing exsolved lamellae of chalcopyrite + pyrite in a talnakhitic matrix close to the intrusives. In addition, valleriite mackinawite, and an unknown mineral developed, and chalcopyrite diffused out from the original grains. Ores collected 27 cm from the contact of a dike 7 m thick do not show lamellae of chalcopyrite, but pyrite has exsolved and chalcopyrite diffused out. A sample collected 1 m from the same contact shows only diffusion of chalcopyrite. Ilmenite in the sulfide ores close to the intrusive contact lost iron along grain boundaries.