Gold mineralization in the Ashanti Belt of Ghana; genetic constraints of the stable isotope geochemistry

Abstract

The Ashanti belt of Ghana is the key district of gold mineralization in the Paleoproterozoic terrane of West Africa. The area considered in southwest Ghana is covered by lithologies of the volcanic-sedimentary Birimian Supergroup and the overlying elastic sedimentary Tarkwaian Group which were jointly folded and metamor- phosed under greenschist facies conditions during the Eburnean teetonothermal event at about 9..1 Ga. Regional fotiation md subparallel shear zones hosting mesothermal gold mineralization developed during deformation coeval with metamorphism. Four major tsoes of primary gold mineralization are present in the Ashanti belt: (1) mesothermal, generally steeply dipping quartz veins in shear zones mainly in Birimian sedimentary rocks, (9,) sulfide ores with auriferous arsenopyrite and pyrite, spatialty dosely associated with the quartz veins, (3) sulfide disseminations and stockworks in granitoids, and (4) paleoplaeers of the Tarkwaian Group. This study concentrates on types (1) and (9,) of the hydrotherlnal gold mineralization. Stable isotope analyses of host-rock and ore components were performed with the ailn of obtaining parameters relevant to the origin and evolution of the fluids that produced gold mineralization. Carbonaeeous matter in the Birimian metasediments displays 93C values ranging from -11.4 to -9,8.3 per mil relative to PDB, indicating an organogenie origin. Carbonates display a unimodal distribution of 9aC values ranging from -9.9 to -17.0 per rail relative to PDB. COz extracted from fluid inclusions in the auriferous quartz veins has 93C values ranging from -9.5 to -15.7 per mil relative to PDB. It is proposed that these carbon isotope compositions of carbonates and COz reflect extensive interaction of the CO,2-rieh hydrothermal fluids with reduced carbon in Birimian sediments in the deeper parts of the hydrothermal systems. Carbonates and auriferous vein quartz have 6So values ranging from 19,.9 to 9,9,.9, and 19,.8 to 15.6 per mil relative to SMOW, respectively. Carbonates and quartz were deposited in near isotopic equilibrium with respect to 6So, indicating fluid-dominated conditions during ore formation, from fluids of metamorphic or magmatie origin. Such an origin is corroborated bySD values of water extracted from fluid inclusions in vein quartz (-37 to -53% relative to SMOXV). Pyrite of synsedimentary-diagenetie origin in Birimian schists displays sulfur isotope compositions ranging from +7.3 to -9,0.9 per rail (median ca. -10% relative to CDT). Similar compositions and wide ranges are usually attributed to sulfide generation by bacterial sulfate reduction from seawater. Arsenopyrite and eogenetie pyrite from the sulfide ores generally have 534S values in the range -5.3 to -10.9, per rail relative to CDT. The tight unimodal distribution of 534S values indicates a large, homogeneous fluid reservoir. The low (534S values are interpreted as source-inherited, not related to unusual pH, Eh, temperature, or depositional conditions. Sulfides in Birimian sediments represent the most likely sulfur reservoir tapped by the fluid systems. The C, O, H, and S isotope compositions of ore-related hydrothermal minerals and fluid inclusion compo- nents indicate that the mineralizing fluids interacted extensively with the Paleoproterozoie rocks, especially Birimian sediments, at deeper crustal levels and at high temperatures. The isotopic compositions are most compatible with the formation of fluids from devolatilization reactions invoMng Birimian strata during pro- grade metamorphism at depth (metamorphic fluids).