Abstract: Gold mineralization at the Ashanti Mine occurs in shear zones which are second-order components of a major transcrustal shear zone. However, gold-rich telluride mineralization in veins in the hangingwall of the Obuasi ore zone at Ashanti appears to have post-dated development of the major gold-bearing shear zone. Complex assemblages of goethite, chalcocite, coloradoite, calaverite, sylvanite, kostovite, petzite, stutzite, hessite, altaite, rickardite, weissite and henryite were succeeded by a relatively simple assemblage of altaite-petzite-hessite ± sylvanite ± coloradoite. Precipitation occurred over a temperature range of approximately 220°C to 165°C from a CO2-bearing, low salinity (<5 wt. % NaCl equiv.) fluid under conditions of high ƒo2 and high ƒTe2/ƒS2. Replacement of earlier gold-sulphide mineralization is indicated.
The fluids were introduced during late activation of the Obuasi shear zone, which evolved during the c.2000 Ma Eburnean tectonothermal event that marked the onset of cratonization of the West African shield. The localized and time-specific distribution of the telluride mineralization may be indicative of a late influx of tellurium-rich fluid linked to emplacement of a synorogenic, Cape Coast-type granitoid intrusion.