The geology and mineralogy of the Loulo mining district, Mali, West Africa: evidence for two distinct styles of orogenic gold mineralisation.

Loulo is a world-class orogenic gold mining district in the Birimian terrane of western Mali. Orebodies are located along second- or higher-order shears associated with the Senegal-Mali shear zone, with gold mineralisation largely linked to a transtensional event. Two distinct styles of gold deposit on the basis of their differing geological characteristics are the Gara deposit and the Yalea deposit, both hosted by similar rock types, calcareous graywackes and calcitic-dolomitic marbles. Gara-style orebodies occur as sulphide disseminations or ankerite-rich shear vein stockworks, hosted in folded tourmalinised sandstones and breccias. Gold lodes are Fe-rich, dominated by nickeloan pyrite, contain Cu-Ni +/-Co minor and trace sulphides, and show consistently high levels of P-REE-W-bearing phases apatite, monazite, xenotime and scheelite. Base metal concentrations show a marked increase in marble host rocks, with the formation of nickeloan pyrite-cobaltite-clausthalite ores. In contrast, Yalea-style deposits are associated with quartz +/-ankerite vein lodes and disseminated sulphide stringer zones. Ore paragenesis is enriched in As, mainly as multistage growth of arsenopyrite and arsenian pyrite. Base metal sulphides, scheelite and (REE-)phosphates are extremely rare. The diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region.
Loulo is a world-class orogenic gold mining district in the Birimian terrane of western Mali. Orebodies are located along second- or higher-order shears associated with the Senegal-Mali shear zone, with gold mineralisation largely linked to a transtensional event. Two distinct styles of gold deposit on the basis of their differing geological characteristics are the Gara deposit and the Yalea deposit, both hosted by similar rock types, calcareous graywackes and calcitic-dolomitic marbles. Gara-style orebodies occur as sulphide disseminations or ankerite-rich shear vein stockworks, hosted in folded tourmalinised sandstones and breccias. Gold lodes are Fe-rich, dominated by nickeloan pyrite, contain Cu-Ni +/-Co minor and trace sulphides, and show consistently high levels of P-REE-W-bearing phases apatite, monazite, xenotime and scheelite. Base metal concentrations show a marked increase in marble host rocks, with the formation of nickeloan pyrite-cobaltite-clausthalite ores. In contrast, Yalea-style deposits are associated with quartz +/-ankerite vein lodes and disseminated sulphide stringer zones. Ore paragenesis is enriched in As, mainly as multistage growth of arsenopyrite and arsenian pyrite. Base metal sulphides, scheelite and (REE-)phosphates are extremely rare. The diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region.