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Gold mineralization in Palaeoproterozoic granitoids at Obuasi, Ashanti region, Ghana: Ore geology, geochemistry and fluid characteristics

Department of Geology, University of the Witwatersrand, Private Bag 3, WITS 2050, Johannesburg, South Africa. E-mail: 065yyao{at}cosmos.wits.ac.za

Granitoid-hosted gold deposits at Obuasi are located along the eastern margin of the Kumasi basin, about 5 km west of the Ashanti goldfields, and comprise the auriferous Nhyiaso, Ayankyerim, Yamensakrom, and Kunka plutons. These deposits represent a relatively new style of gold mineralization in the Ghanaian Birimian. The mineralization consists of quartz veins/stockworks and pervasive alteration zones within the granitoids in brittle structures. The ore mineral assemblage is mainly composed of pyrite and arsenopyrite, with minor chalcopyrite, sphalerite, and rutile. Hydrothermal alteration minerals are dominated by quartz, sericite (muscovite), sulphides (mainly pyrite, arsenopyrite) and carbonates. Gold is closely associated with the sulphides in both quartz veins and alteration zones. Chemical mass balance calculations show that major and trace element contents in alteration zones were probably buffered by the original granitoid composition.

Fluid inclusions in vein quartz are composed of aqueous H₂O-CO₂±NaCl and gaseous CO₂-N₂±CH₄ types, with minor (<5%) aqueous H₂O±NaCl types. The mineralizing fluids typically have salinities <6 wt.% NaCl equivalent with bulk densities in the range 0.65 to 0.95 g/cm³. The petrographic and microthermometric data suggest that the H₂O- and CO₂-rich fluid inclusions resulted from phase separation of an initial homogeneous fluid and were trapped at around 180 to 300–350°C and 1 to 3 kbar. Gaseous compositions of fluid inclusions, obtained from Raman spectroscopy, are dominated by CO₂ (80 to 95 mol%), with significant amounts of N₂ (2 to 20 mol%) and CH₄ (0 to 10 mol%). Thermodynamic calculations show that the initial homogeneous H₂O-CO₂-rich fluid contains 50 to 80 mol% H₂O at 300–350°C and 2 kbar. Oxygen fugacities (O₂) of fluids were estimated to be equal to QFM + 1.4–2.2 and coincide with the NNO buffer in the pyrite field, indicating relatively reduced conditions of fluids during ore formation. Gold deposition within the granitoids may be mainly induced by fluid immiscibility and sulphidization of host rocks during alteration. Based on the geological setting, ore geology and geochemistry, and fluid inclusion characteristics, it is suggested that the granitoid-hosted gold deposits at Obuasi are not intrusion-related, but were formed by metamorphic low-salinity H₂O-CO₂-N₂±CH₄ fluids related to the waning stages of the regional Birimian orogeny at about 2100 Ma. The host granitoids acted as favourable sites for fluid flow due to their brittle lithological characteristics.