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Title: The geology and genesis of the Syama gold deposit, Mali, West Africa
Author: Diarra, Pobanou Hugues
ISNI:       0000 0001 3423 5009
Awarding Body: University of Southampton
Current Institution: University of Southampton
Date of Award: 1996
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Proterozoic; Mineralisation