BASE METALS WITHIN THE BASEMENT ROCKS IN BARAPAHARPUR, NORTH-WEST BANGLADESH: IMPLICATIONS FOR MINERALIZATION IN THE SHALLOW BASEMENT OF BANGLADESH

In most part of Bangladesh, the crystalline basement complex is overlain by thousands of meters of Late Permian to Quaternary sediments. The shallowest occurrence of the crystalline basement is recorded at a depth of ~150 to 400 meters at the north-western part of Bangladesh. The drillhole GDH-54 has encountered the basement at a depth of ~366 meters. Diorite is the dominant lithounit, identified in the basement, which has been dissected by two closely spaced hornblendite dykes at a depth of 460 meters. The dykes are moderately folded, marked by fine golden streaks in the crestal part and in the zone proximal to the contact with the diorites. XRF analysis of the hornblendite confirms the presence of Au-Ag-Cu and other base metals. From ore petrographic study, a wider presence of pyrite-chalcopyrite-magnetite is revealed. Calcite, epidote, titanite, chlorite are the alteration mineral assemblage associated with the mineralization. The studied portion of the basement probably is most likely represent the part of a large granitoid that has been infiltrated by a late hydrothermal fluid resulting in the mineralization. This is the first report of the base metal occurrence in the basement rocks in Bangladesh. The observation in drillcore, XRF result and ore petrographic study implies the studied part corresponds to a volatile rich segment of a large hydrothermal system; the fluid rich part of which may have been occurred as veins at a greater depth. Study of the mineralized samples from deeper part in and around the GDH-54 depth could unravel the nature and genesis of the Au-Ag-Cu mineralization in the basement at the NW Bangladesh.