Typically mineralization on the Ashanti, Sefwi and Kibi-Winneba Belts of SW Ghana occurs in NE trending structures hosting quartz lodes and associated disseminated sulphide (pyrite +/- arsenopyrite) in altered and deformed wallrocks. The wall rocks are typically Birimian sediments including phyllites and greywackes as well as conformable Birimian volcanics and/or diabase intrusives. Structural observations at Prestea, Bogosu and Obuasi suggest a late transpressive deformational event focussed on pre-existing regional structures, including early thrust and wrench features. In detail, structural fabrics such as late crenulation cleavage development and associated kink and crenulation folds, intersection lineations, rodding and boudinage support these observations. In many cases the orientation of these late structures and fabrics corresponds to the orientation of known pay-shoots and lodes at producing mines.
The relative competency of granitoids caught up in these structural corridors is considered to provide the competency contrast that can focus mineralization and provided the brittle-ductile regime conducive to the deposition of gold from auriferous fluids streaming up these structural conduits. Several similar small deposits are located west of the main Ashanti workings at Obuasi. The Tarkwaian epiclastics are largely confined to elongate north-northeast trending basins, believed to represent intra-cratonic rifts. The margins of these basins commonly coincide with the major (frequently mineralized) structures representing the contact between Upper and Lower Birimian sequences. Gold mineralization is developed within conglomerates and quartzites termed the Banket Formation.