Gold mineralization at the Morila Mine, Mali, West Africa

Abstract

The Morila Gold Mine (7 Moz) is located in southwest Mali, approximately 280 kilometres southeast of the capital city, Bamako, within the West African Craton. The ore body is situated in a shallowly dipping shear zone hosted within Birimian metasediments, and surrounded by Doubalakoro-type granites to the west, and Massigui-type granites to the east. Several smaller granodiorite and tonalite intrusive rocks have been noted at Morila as well, and likely play an important role in gold mineralization. The tonalite observed in REG019, 2 km NW of the open pit, is relatively undeformed and only very weakly mineralized within altered hornblende glomerocrysts. Some intersections of metasediment in this hole have abundant sulphide mineralization, particularly along the margins of hydrothermal brecciated fragments, but no gold was observed in the polished thin sections. A sample from the hybrid zone exhibits sulphide mineralization along the margin between metasediment and tonalitic domains, containing bismuth-tellurides and lollingite within the metasediment enclave. DDH SAN297, 5OOm west of the deposit, intersects weakly deformed granodiorite and mineralized metasediment. The granodiorite with stockworked veins is mineralized with sulphides, scheelite, native bismuth, lollingite, and native gold. The metasediment in this suite contains several generations of veins and Au mineralization locally occurring along vein margins. These observations may account for a model that involves beginning spatially and temporally associated with a sulphur-undersaturated intermediate melt that is rich in Au. The dykes coming off of the main pluton intruded the sulphur-rich sediments and, in part, assimilated them, providing an environment suitable for gold precipitation.