Laterisation processes and gold mineralization in the Geita- Kahama gold field, Tanzania

Although gold bearing laterite deposits in the Archaean greenstone belts are becoming an important class of gold deposits, their mode of emplacement is still not well understood, and widely accepted models are generally lacking. The present study, which was conducted in the Geita-Kahama goldfield, some 100 km southwest of Mwanza town, was therefore, aiming at highlighting the geological and geomophological processes involved in the development of the laterite weathering profile in the auriferous Archaean greenstone rocks; and penecontemporaneous supergene gold mineralization. To accomplish this study, literature survey followed by fieldwork and geochemical and mineralogical analyses were done. The acquired data were statistically and geostatistically treated to facilitate the interpretation. Results indicate that, BNBB ferruginous laterite weathering profiles were developed together with supergene gold mineralization, over Archaean auriferous greenstone rocks, during the Cainozoic, under tectonically stable landforms, in a humid, tropical climate, in which the water table was close to the surface, fluctuating, causing alternating acidic to neutral to alkaline pH conditions. Profile development conditions involved systematic leaching of mobile elements like Ag, Mg, Ca, Na, and K from primary minerals (e.g. Amphiboles, Plagioclase) and enrichment of immobile elements like Fe, A1, Au, Cr, Ni, Mn, etc. creating secondary mineral assemblages like goethite, quartz, kaolinite and to some extent hematite, manganite and gibbsite. Laterite formation and gold mineralization was a complex phenomenon involving varying remobilizing media, and transporting ligands. Under acidic and oxidising conditions and high chloride activity gold was complexed and migrated as AuCl4- complex while under neutral to alkaline solution gold migrated as Au(S20s)3-z and Au(HS)2-. Surface gold migration could have been attributed to organic complexing. Through these processes which to some extent agreed with others operating elsewhere, gold grains of various behaviour were enriched throughout the profile with main concentration being in the basal, quartz-pebble pay bed within the depths of 3 to 12 metres at Rukarakata, Buziba and Busolwa areas respectively. This has led into the formation of a workable supergene gold deposit possibly similar to the Boddington and Tefler of Western Australia.