The geology and genesis of the Syama gold deposit, Mali, West Africa

Regional greenschist facies metamorphism accompanied the initial D1 phase of deformation, generating a selectively penetrative S1 foliation. Subsequent D2 deformation was focused along the contacts between the sedimentary rocks and the volcanic rocks or along highly strained D1 zones and generated N-S to NE-SW trending ductile shear zones oriented subparallel and at low oblique angles to bedding planes. Granitoid intrusions were synchronous with the D2 event. The introduction of the granitoid bodies probably induced hydrothermal activities. Post-alteration D3 and D4 deformation phases led to the development of SZ3 shear-zones and numerous brittle fracture sets most of which are reverse. These shear zones and fractures were the channelways for the mineralising fluids.In the absence of fluid inclusion and stable isotope data, the fluids characteristics were estimated from mineral assemblages and mineral stability diagrams. This suggests that gold was deposited from reduced H2O-CO2 fluids of salinity ≤ 26.3 wt% NaCl equivalent (but could be < 6 wt% NaCl equivalent), under P-T conditions of 292-337°C, and 2.5-5 kb, and was derived from both magmatic and metamorphic sources.Extensive fluid-rock interaction at Syama accompanied the D2 shearing, and resulted in the formation of a zoned alteration envelope, the extent of which is unknown due to the weathering but could be about 200-300 m wide. Four main alteration zones were identified: an outer selvage of chlorite + carbonate, through sericite or fuchsite + carbonate, albite + carbonate, to quartz + albite zone comprising massive silica in places. The albite-carbonate, and the sericite and albite transition zones host the pyrite which is accompanied by the gold mineralization.Element behaviour corresponding to wallrock alteration was highly variable, but gold introduction was characterised by significant volume increase of the host rocks and a significant enrichment in K2O, Na2O Al2O3 and S. An early gold phase Au 1 accompanied this wallrock alteration.