Geology and Geochemical Aspects of Ore Formation at the Prestea Mesothermal Vein Gold Deposit in the Birimian System of Ghana

Abstract

The Prestea lode gold deposit occurs in a graphitic shear zone in the Birimian system of Ghana. The Birimian is an Early Proterozoic greenstone assemblage (≈ 2100 Ma) with large gold deposits, consisting of a lower unit predominantly of metasediments and an upper unit of metavolcanics with interbedded sediments. The metamorphic stage is of greenschist facies grade. The gold generally occurs as free gold or closely associated with sulfides, particularly arsenopyrite and sulfosalts. It is usually coarse grained and occurs along grain boundaries, as inclusions and in fractures with the ore minerals. The gold is nearly pure, with analyzed grains containing 95-96 wt% Au, 4-5 wt% Ag, and less than 0.1 wt% Sb, Bi, Zn, or Cu. Associated metallic phases consist of pyrite, arsenopyrite, chalcopyrite, pyrrhotite, sphalerite, galena, and sulfosalts such as tetrahedrite, boulangerite, bournonite, and jamesonite. The fluid associated with the gold mineralization was H2O-CO2-NaCl in composition, with a mean salinity = 3.6 wt% NaCl equivalent, CO2 density (mean) = 0.84 ± 0.09 g/cm3, and XCO2 = 13 to 33 mole %. Total homogenization temperatures range from 250° to 380° C. The ore fluid exhibits features suggesting contamination of volatiles such as N2 ± CH4 ± H2S, or post-depositional leakages. Arsenopyrite geothermometry indicates a temperature of mineralization from 325° to 450° C. Estimated pressures of trapping range from 1.0 to 2.2 kbars, with corresponding depths from 3.5 to 7.7 km, assuming the pressure is lithostatic. The gold deposition occurred from fluids with δ18O water = +9.6 to +13.9o and δDfluid irclusion = −29 to −65%0, suggesting a crustal origin for the ore-forming fluid, including a metamorphic and a magmatic source. A δ34Spyrite value of −7.1 to −11.7%o and δ34SΣS = −9.6%o suggests a sedimentary origin for the sulfur. The gold deposition is attributed to destabilization of the bisulfide complex as a result of ore fluid reaction with host rock, resulting in a probable reduction by the carbonaceous matter in the host rock, and/or a decrease in the total sulfur concentration in the co-precipitation of sulfides.