RARE Au-Cu OCCURRENCES IN THE METASEDIMENTARY ROCKS OF THE INNER LESSER HIMALAYAS

Abstract

Himalayas are the ideal terranes for the precipitation of ore minerals as the belt is marked with complex tectonic and magmatic events which involve collision, subduction, break-up, detachment, accompanied by the obduction of the Eastern Gondwana continent and all these processes develop the optimum conditions for the precipitation of ore mineral deposits. Orogenic gold deposits are reported from the regionally metamorphosed terranes of all ages including Archean greenstone belts and younger phanerozoic metamorphic belts. In Himalayan region, gold, gold-antimony and antimony mineral deposits are reported from the Tethyan Himalayan belt in the Tibetan plateau. There are very few incidences of gold mineralization associated with the Lesser Himalayan metasediments. This study involves investigating the reported gold occurrences in association with sulphide mineralization within the quartz veins in the Deoban Dolostones and in the Mandhali carbonaceous shales and an attempt is made to underline the genesis of the gold occurrences within these rock types. SEM analysis for the polished sections has confirmed the presence of gold in the fracture spaces within the pyrites in the carbonaceous shales and within the chalcopyrite grains in the polished section of dolerite. EPMA studies have revealed the presence of native gold within the dolerite. It is suggested here from the petrographic studies of polished sections and SEM and EPMA studies that the gold precipitated from the ocean in the diagenetic pyrites after the deposition of the carbonaceous shales, and during the prograde metamorphism of the shales it got expelled from the crystal structure of the diagenetic pyrites and then the hydrothermal fluids exsolved from the dolerite intrusion in the later stages, after the deformation, reprecipitated the gold within the fracture spaces of the pyrites along with the coeval precipitation of chalcopyrite and sphalerite. In this case, carbonaceous shales are both the source as well as host rock for the gold deposits. The presence of gold prospects in the dolerite are attributed to be assimilated from the country rocks during its advancement towards the surface, and hence source rock for the gold in the dolerites could not be typified.