ABSTRACT
The crystalline basement in northwest Bangladesh occurs as subcrop underlain by ~130–1,000 m of Cenozoic deposits. A review of the published works classifies the basement into four rock suites: (1) diorite, tonalite granodiorite (DTG); (2) charnockite, monzogranite gneiss, granite (CMGn); (3) alkaline mafic-ultramafic (AMU); and (4) dykes. The DTG suite represents the largest portion of the basement, which were dated at 1.73-1.72 Ga at Maddhapara. The ~1.14 Ga CMGn suite lithologies have been identified in Gaibandha, whereas the AMU suite members were identified in the Madarpur area. Numerous felsic and mafic dykes (suite 4) dissect through the DTG and AMU suites. No cross-cutting relationship is observed among the DTG, CMGn and AMU suites in the drillcores. The felsic suites (1 and 2) of the basement do not show evidence of regional metamorphism and are devoid of strong deformation and xenoliths/mafic enclaves. The relationship between the basement in NW Bangladesh and nearby Chotanagpur Granite Gneissic Complex and Shillong Massif is yet to be constrained. A dissemination style of copper and gold mineralisation has been identified in the Barapaharpur area, where gold occurs in the chalcopyrite, associated with pyrite, galena and magnetite in the hornblendite dyke dissecting diorite.
