Petrology of the Pre-Cambrian Kolhan formations of Jhinkpani,Singhbhum district,Bihar

Pre-Cambrian Geology of an area of about 14 sq. miles has been investigated in this paper. The oldest rock is a very small patch of serpentinite, tentatively correlated with the huge ultrabasic body at Jojohatu, some 10 miles north. Granite and Newer dolerite are intrusive rocks of the area. Kolhan series, resting unconformably on Granite and showing facies variation from basal sandstone conglomerate to phyllitic shale through sandstone and impersistent limestone in a thickness of only a few tens of metres, is a shallow epicontinental deposit. Kolhan sandstone, rich in hematitic cement, is an argillaceous orthoquartzite practically devoid of feldspar and is very poor in heavy and accessory minerals. It is richly current bedded, immature, bimodal, fine grained, positively skewed and well sorted. Kolhan limestone is recrystallized micrite, its lower horizon is mostly high grade and contains pink calcite and quartz crystals in cavities. Its upper horizon is low grade, phyllitic and argillaceous and is partly metasomatic and exhibits contemporaneous deformation structures like hassock bedding and slickenside (slickolites?). Kolhan phyllitic shale exhibits effects of Singhbhum orogeny in development of cleavage and crumpling of foliation. Dirty green vug quartz contains abundant oriented microscopic chlorite inclusions. Non-metallic and metallic mineralization in Kolhans is represented by limestones, and local hematite-molybdenite and iron-rich manganese ore respectively.