Ore Genetic Significance of Geochemical Trends During Progressive Migmatisation Within Part of the Singhbhum Shear Zone, Bihar

Abstract

Study of the trends of geochemical migration of some major and minor elements during progressive migmatisation within part of the Singhbhum shear zone supports the view that apatite-magnetite, uraninite and copper sulphide deposits and the associated wall rock alterations are related to processes of migmatisation therein. During migmatisation extensive mobilisation of elements took place when Si, Al, Na and small quantities of Ga, U and Zr were introduced into the pre-existing basic metavolcanics within the shear zone, and Fe, Mg, Ca, K, Ti, Mn, P, Ba, Rb, Sr, Li, Ni, Co, Cr, V, Cu, and Y were expelled from these rocks. The addition of the former group of elements brought about transformation of these rocks, into felspathic schists and gneisses (the soda-granites) while the expulsion of the latter group of elements from these rocks and their addition to the migmatising fluids caused progressive changes in the chemical composition of these fluids. At the closing stages of the process these fluids were transformed into mineralising solutions that gave rise to apatite-magnetite, uraninite and copper sulphide deposits and to extensive sericitisation, chloritisation, biotitisation and silicification within the shear zone. The various elements, present in traces, entered the crystal lattices of these minerals, in accordance with their geochemical preferences and in proportion to their concentration in the mineralising solutions. The resulting deposits should be regarded as 'metasomatic hydrothermal'.