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 clastic sedimentary Tarkwaian Group which were jointly folded and metamorphosed under greenschist facies conditions during the Eburnean tectonothermal event at about 2.1 Ga. Regional foliation and subparallel shear zones hosting mesothermal gold mineralization developed during deformation coeval with metamorphism.Four major types 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, (2) sulfide ores with auriferous arsenopyrite and pyrite, spatially closely associated with the quartz veins, (3) sulfide disseminations and stockworks in granitoids, and (4) paleoplacers of the Tarkwaian Group.This study concentrates on types (1) and (2) of the hydrothermal gold mineralization. Stable isotope analyses of host-rock and ore components were performed with the aim of obtaining parameters relevant to the origin and evolution of the fluids that produced gold mineralization.Carbonaceous matter in the Birimian metasediments displays delta 13 C values ranging from -11.4 to -28.3 per mil relative to PDB, indicating an organogenie origin. Carbonates display a unimodal distribution of delta 13 C values ranging from -9.9 to -17.0 per mil relative to PDB. CO 2 extracted from fluid inclusions in the auriferous quartz veins has delta 13 C values ranging from -9.5 to -15.7 per mil relative to PDB. It is proposed that these carbon isotope compositions of carbonates and CO 2 reflect extensive interaction of the CO 2 -rich hydrothermal fluids with reduced carbon in Birimian sediments in the deeper parts of the hydrothermal systems.Carbonates and auriferous vein quartz have delta 18 O values ranging from 12.9 to 22.9 and 12.8 to 15.6 per mil relative to SMOW, respectively. Carbonates and quartz were deposited in near isotopic equilibrium with respect to delta 18 O, indicating fluid-dominated conditions during ore formation, from fluids of metamorphic or magmatic origin. Such an origin is corroborated by delta D values of water extracted from fluid inclusions in vein quartz (-37 to -53ppm relative to SMOW).Pyrite of synsedimentary-diagenetic origin in Birimian schists displays sulfur isotope compositions ranging from +7.3 to -20.9 per mil (median ca. -10ppm relative to CDT). Similar compositions and wide ranges are usually attributed to sulfide generation by bacterial sulfate reduction from seawater.Arsenopyrite and cogenetic pyrite from the sulfide ores generally have delta 34 S values in the range -5.3 to -10.2 per mil relative to CDT. The tight unimodal distribution of delta 34 S values indicates a large, homogeneous fluid reservoir. The low delta 34 S 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 components indicate that the mineralizing fluids interacted extensively with the Paleoproterozoic 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 involving Birimian strata during prograde metamorphism at depth (metamorphic fluids).

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