Geological, structural and mineralogical constraints on the evolution of the Morila gold deposit, South Mali, West Africa

Morila is an 8.1 Moz world-class gold deposit located in southern Mali, West Africa. It is hosted in 2,103 Ma Palaeoproterozoic Birimian metagreywackes which were deformed during the polyphase Eburnean orogeny from ~2,100 to 2,070 Ma, and intruded during a prolonged phase of arc magmatism from 2,098 to 2,065 Ma. Initial deformation during D[sub]1 involved northwest-directed isoclinal recumbent folding and thrust faulting during a phase of compressional tectonics. This was followed by the onset of transcurrent tectonics during D[sub]2-3, which formed tight to open steeply inclined folding, and 2nd or 3rd order fault splays off the Banifin Shear Zone that later defined discrete domain boundaries. Late dextral reverse movement along these domain boundaries resulted in the juxtaposition of proximally-derived, isoclinal-recumbent folded sediments of the Morila and Sirakoro domains against distally-derived, upright folded sediments of the Northwest domain across sharp structural breaks that define discontinuities in lithology, structure, metamorphism and mineralisation. Early gold mineralisation is associated with the intrusion of the composite Doubalakoro pluton under prograde contact metamorphic conditions. Mineralisation is hosted in variably deformed polymineralic veins containing sulphides and bismuth-antimony assemblages, and in hornfelsed metasediments around the carapace and shoulders of small, cylindrical granodiorite plutons. This early intrusion-related gold mineralisation was subsequently overprinted by a later phase of orogenic gold mineralisation characterised by idioblastic arsenopyrite crystals containing löllingite cores. The latter is spatially restricted to the northeast-trending high-grade axis of the Morila orebody where it overprints the earlier phase of mineralisation, contributing to the exceptional grades and ore thicknesses characteristic of Morila. Exploration criteria for Morila-like deposits need to consider controls on both intrusion-related and orogenic gold mineralisation. At a regional scale, airborne electromagnetic surveys may aid in the identification of buried plutons proximal to orogenic belt margins. At the target scale, mapping should aim to identify hornfelsed aureoles surrounding small plutons, and 2nd order shears capable of transporting mineralised fluids. Geochemical analyses for Au, As, Bi, Sb, Te and W may be carried out at surface, and on subsurface drilling samples.