The Sadiola deposit is located in the north central section of the window and is hosted by sediments of the Kofi Formation, which have been intruded by numerous felsic intrusives. The sediments consist of fine-grained greywacke, probably distal turbidites, and impure carbonates with minor tuffs and acid volcanics.
The deposit occurs along the 010º striking Sadiola Fracture Zone (“SFZ”), which is interpreted as a brittle-ductile splay off the Senegalo-Mali Shear Zone at a sinistral releasing bend. The SFZ follows the steeply west dipping contact between greywacke to the west and impure carbonate to the east. The SFZ and its wallrock are intruded by discontinuous diorite dykes, which may contain a weak mineral foliation and rarely intense ductile deformation. Quartz-feldspar-porphyry (QFP) dykes intrude younger, 020º striking and steeply west dipping faults and preserve more brittle deformational features.
Gold mineralisation occurs in all of the four major rock types (marble, greywacke, diorite and quartz-feldspar porphyry), and is spatially associated with a complex alteration pattern. Alteration assemblages identified to date include calcsilicate, potassic, chlorite–calcite and carbonate and point to a mesothermal origin for gold mineralisation. Gold is associated with both arsenic and antimony dominated sulphide assemblages including arsenopyrite, pyrrhotite, pyrite, stibnite and gudmundite. Primary gold is extremely fine grained, dominantly less than 15 microns, with rare grains approaching 50 microns.
The Sadiola deposit has been intensely weathered to depths of up to 220 metres. The operation has mainly exploited soft oxide ore since startup with the current known oxide reserves expected to be mined out by 2016. A significant mineral resource of hard sulphide ore occurs below the final Sadiola pit design and is currently the target of an expansion project pending final approvals.