Geological Characterization And Genesis Of DIAKHA Gold Deposit,Mali,West Africa

The Diakha gold deposit is located in the Kédougou-Kiénéba inlier at about 1 km east of the Senegal-Mali Shear Zone which is the most important gold production area in the Republic of Mali.The deposit has inferred resources of 14.8 Mt grading 1.81 g/t Au(0.86 Moz)according to the initial mineral resource estimation.The Diakha deposit is hosted by NNE shear zones within a metamorphosed volcano-sedimentary sequence of quartz-feldspar metagreywacke,sandstone,metapelite and carbonate and few volcanic units.Several subparallel elongated ore bodies occur over a wide area at significant widths and grades,in association with albite-carbonate-hematite-chlorite alteration within brecciated sandstone rocks with weak finely disseminated sulfides and rare quartz veining.The mineralization is mainly hosted within D2-related shear zones at the NNE strike extensions.Pervasive albitization,carbonatization and silicification overprint the host and wall rocks of the ore deposit.The phases of hydrothermal alteration include:(i)sodic alteration characterized by a pervasive albitization;(ii)cal-carbonate characterized by the coexistence of potassic alteration and carbonatization,(iii)chloritization-hematitization,(iv)silicification--pyritization and(v)the associated iron oxide minerals(magnetite and hematite)closely associated with gold mineralization.Diakha field data and laboratory analysis suggest the existence of two main stages of hydrothermal mineralization,namely stage I and stage II which both display ductile to ductile-brittle fabrics.Stage I is of breccia types of mineralization and represents the major gold bearing stage.It links to hematite + calcite + ankerite/dolomite and quartz pervasive alteration,controlled by the predominantly brittle-ductile NNE to NE-shearing structures of D2 deformation.Stage II is represented by the second minor hydrothermal episode,showing local quartz-tourmaline and tremolite alteration which is structurally controlled by the dipping S3 cleavage formed during D3 deformations.Petrographic and microthermometric studies of fluid inclusions from quartz veins reveal the presence of early dominant carbonic CO2-pure and aqueous-carbonic CO2 rich(LCO2 + VCO2 ± H2O)and aqueous L-type in stage I.The exclusively aqueous H2 O + Na Cl ± CO2 fluid inclusions(L-type)is the only type of fluid inclusion observed in stage II.Interpretation shows that the ore-forming fluids originated from a low-to-H2 CO2 fluid and sourced from metamorphic water with trapping temperatures ranging from 250 to 280.The estimation indicates a trapping pressure on the deposit between 600 and 800 bars.The results of hydrogen and oxygen isotope analyses indicate that ore-forming fluids are 2O 42 to depo derived from the strata.These results suggest that the ore-forming fluids/materials are mainly of metamorphic hydrothermal origin,derived from sediment.Combination of the characteristics of ore-forming fluids and the ore deposit geology indicate that the Diakha gold deposit has counterparts in the metamorphic hydrothermal gold deposits type and was formed during the D2 deformation of the Birimian orogeny.