The origin of fluids and nature of fluid-rock interaction in mid-crustal auriferous mylonites of the Renco mine, southern Zimbabwe

Geothermometric constraints on auriferous shear zones of the Renco mine in the Northern Marginal Zone of the late-Archaean, granulite-facies Limpopo B elt in southern Zimbabwe indicate that deformation and associated mineraliz ation occurred at temperatures of at least 600 degrees C up to more likely 700 degrees C. Mid- to upper-amphibolite facies conditions during mineraliz ation correspond to the regional-scale retrogression of granulite facies wa ll rocks during the late-Archaean thrusting of high-grade metamorphic rocks of the Northern Marginal Zone onto low- to medium-grade granite-greenstone terrains of the Zimbabwe craton. Mineral assemblages indicate that the ore fluid was moderately oxidized with log f(O2) values between 10(-17) and 10 (-18) bars with high H2S activities of 0.25-0.75. Elements enriched in the shear zones include Au, S, Fe, Cu. Mo, Bi, Te, Ni, Co, and H2O, Au and Cu b eing the most enriched. Geochemically, Au correlates with Cu but not with S , which, together with the fact that gold is only rarely intergrown or in d irect contact with sulfides! possibly indicates a transport of gold as a ch loride complex. The siting of gold along fractures or within implosion brec cias suggests that gold was precipitated due to fluid immiscibility induced by catastrophic fluid pressure drops during seismic slip events. Fluid inc lusions are predominantly CO2 (+/-CH4 +/- N-2)-rich, but petrographic work indicates that fluid inclusions have undergone extensive post-entrapment mo difications due to the pervasive recrystallization of mineral textures in t he high-temperature shear zones. The mineralized shear zones are enriched i n O-18 compared to wall-rock enderbites, which is interpreted to represent an influx of externally derived fluids of probably metamorphic origin. Base d on temporal and spatial relationships between mineralization, late-Archae an overthrusting of the Northern Marginal Zone onto the Zimbabwe craton, an d coeval amphibolite-facies hydration of granulites, we suggest that the Re nco mineralization formed in a mid-crustal environment from metamorphic flu ids that were generated from dehydration of subcreted greenstone terrains o f the Zimbabwe craton.