Metamorphosed and Hydrothermal Carbonaceous Matter at the Orogenic-Type, Sediment-Hosted Gold Deposits of West Africa
Abstract
Chemical and physical properties of carbonaceous matter (CM) at Paleoproterozoic, shear zone-hosted gold deposits in Burkina Faso (the Inata deposit), Mali (the Syama deposit) and Ghana (the Obuashi and Bogoso deposits) were studied. Two types of CM are present at all studied deposits: metamorphosed and hydrothermal.
Hydrothermal CM is commonly associated with gold mineralization. It forms veinlets in hydrothermally altered rocks and quartz veins, or irregular accumulations parallel or subparallel to C-type cleavage within shear zones.
The isotopic composition of carbon in metamorphosed as well as hydrothermal CM (-26.2 to-33.1%o; PDB) indicates their biogenic origin. Optical properties, XRD and Raman spectra of metamorphosed and hydrothermal CM within a single deposit are similar.
However, the degree of structural ordering and optical properties of CM vary at the individual studied deposits, depending on the metamorphic degree of ambient rocks. This suggests a close relationship between metamorphic temperatures and temperatures of hydrothermal fluids.