THE LOULO GOLD AND TOURMALINE-BEARING DEPOSIT - A POLYMORPH TYPE IN THE EARLY PROTEROZOIC OF MALI (WEST-AFRICA)

The stratabound polymorph Loulo gold deposits and occurrences were dis covered in sedimentary rocks of the Early Proterozoic Birimian Formati on, in the Kenieba region of SW Mali. Early work showed reserves in th e Loulo-0 and Loulo-3 deposits of 28 t of gold in ore averaging 4.43 g /t Au, to depths of 140 m; recent drilling has found mineralization to at least 300 m depth. Early tourmalinization of the sedimentary host rock is indicated by reworked tourmalinite pebbles in non-tourmalinize d hanging-wall rocks. Mineralization, restricted to highly tourmaliniz ed zones, occurs as: (a) pre-D1 (deformation) disseminated pyrite with micron-size inclusions of gold particles; (b) syn-/late-D2 stockworks with minor disseminated sulphides and native gold. Gold in the sulphi des is small (10-40 mum), and occurs at pyrite-crystal edges, filling cracks and/or as inclusions. Post-D2 stock-work mineralization derived from the pre-D1 one that was emplaced with tourmaline, as is indicate d by the small differences between pre-D1 and post-D2 sulphur isotopes , as well as by the type of quartz vein that indicates an almost close d system. In West Africa, the pre-D1 mineralization of Early Birimian age represents the first major introduction of gold into the Proterozo ic. It attributes a major metallogenic role to the tourmalinized sands tone that indicates crustal accretion in West Africa during the Early Proterozoic.