Regional setting, structure and descriptive geology of the Middle Proterozoic Syama gold deposit

The Syama gold deposit lies in a Birimian (2.0-2.2 Ga) greenstone belt in southern Mali. Gold mineralization is hosted by a thin (0.5-2.0 km) but regionally extensive sequence of basalt and andesite, interbedded graywacke and argillite, and andesitic-lamprophyric intru-sions. This sequence is bounded on the east by andesitic onglomerate and graywacke and on the west by interbedded graywacke and argillite. In the Syama mine area, this ore-bearing sequence is structurally layered, overturned, and east vergent. Massive competent basalt and conglomerate bound the hanging wall and footwall, respectively, of the mineralized interval. Lateritic weathering developed an oxide orebody that overlies sulfide mineralization. The oxide orebody extends to 35 to 40 m below the surface; sulfide mineralization extends to a depth of at least 500 m. Mineable oxide reserves are 3.0 million tons with an average grade of 3.20 g/ton. Geologic sulfide reserves from the top of sulfides to 320 m below the surface are 21 million tons with an average grade of 4.02 g/ton. The oxide deposit was expressed on the surface as a series of 3- to 15-m-deep ancient mine workings, which extended 800 m along the strike of the deposit, and sparse outcrops o