GOLD MINERALIZATION IN THE ASHANTI BELT OF GHANA - GENETIC CONSTRAINTS OF THE STABLE-ISOTOPE GEOCHEMISTRY

The Ashanti belt of Ghana is the key district of gold mineralization i n 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 Gr oup which were jointly folded and metamorphosed under greenschist faci es conditions during the Eburnean tectonothermal event at about 2.1 Ga . Regional foliation and subparallel shear zones hosting mesothermal g old mineralization developed during deformation coeval with metamorphi sm. 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 g ranitoids, and (4) paleoplacers of tile Tarkwaian Group. This study co ncentrates on types (1) and (2) of the hydrothermal gold mineralizatio n. Stable isotope analyses of host-rock and ore components were perfor med with the aim of obtaining parameters relevant to the origin and ev olution of the fluids that produced gold mineralization. Carbonaceous matter in the Birimian metasediments displays delta(13)C values rangin g from -11.4 to -28.3 per mil relative to PDB, indicating an organogen ic origin. Carbonates display a unimodal distribution of delta(13)C va lues ranging from -9.9 to -17.0 per mil relative to PDB. CO2 extracted from fluid inclusions in tile auriferous quartz veins has delta(13)C values ranging from -9.5 to -15.7 per mil relative to PDB. It is propo sed that these carbon isotope compositions of carbonates and CO2 refle ct extensive interaction of the CO2-rich hydrothermal fluids with redu ced carbon in Birimian sediments in the deeper parts of the hydrotherm al systems. Carbonates and auriferous vein quartz have delta(18)O valu es ranging from 12.9 to 22.2 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 con ditions during ore formation, from fluids of Inetarnorphic or magmatic origin. Such an origin is corroborated by delta D values of water ext racted from fluid inclusions in vein quartz (-37 to -53 parts per thou sand 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. -10 parts per thousand relative to CDT) . Similar compositions and nide ranges are usually attributed to sulfi de generation by bacterial sulfate reduction from seawater. Arsenopyri te 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, homogen eous fluid reservoir. The low delta(34)S values are interpreted as sou rce-inherited, not related to unusual pH, Eh, temperature, or depositi onal conditions. Sulfides in Birimian sediments represent the most lik ely sulfur reservoir tapped by the fluid systems. The C, O, H, and S i sotope compositions of ore related hydrothermal minerals and fluid inc lusion components indicate that the mineralizing fluids interacted ext ensively with the Paleoproterozoic rocks, especially Birimian sediment s, at deeper crustal levels and at high temperatures. The isotopic com positions are most compatible with the formation of fluids from devola tilization reactions involving Birimian strata during prograde metamor phism at depth (metamorphic fluids).