Geochemistry, alteration, and genesis of gold mineralization in the Okote area, southern Ethiopia

Abstract

Ductile shear zone hosted mesothermal gold mineralization of the Okote area is located in southern Ethiopia. Three N-S striking ductile shear zones, with different intensity of shearing and hydrothermal alteration, cut the mafic rocks. The gold-mineralized parts of these shear zones reveal zonings: slightly altered but not sheared protolith at shear boundaries, transitional zone, and mylonite zones. Auriferous quartz-carbonate-tourmaline veins occur mainly in the mylonite zone. The ore minerals of the veins and their wall rocks are pyrite, chalcopyrite, pyrrhotite, gold, and accessory chalcocite, covellite, galena, and melonite (NiTe2). The textural relationships among minerals in these alteration zones indicate that epidote, ferroamphiboles, and magnetite were formed first, followed by chlorite, ankerite, pyrrhotite, chalcopyrite I, and K-feldspar, and, finally, calcite, chlorite, biotite, tourmaline, gold, and galena. Primary fluid inclusion data from auriferous type 2 quartz veins (qv2) indicate aqueous-carbonic inclusions with low salinity (<6.59 wt % NaCl equivalent), 0.38 to 0.90 g/cm3 in density that homogenized at 218°C to 345°C. Most of the inclusions decrepitate at 220°C to 388°C before or immediately after homogenization. Chlorite geothermometry gives temperatures of formation that range from 230°C to 410°C with modes at 250°C and 380°C, in good agreement with fluid inclusion data. Chemical mass balance studies, using samples from meta-gabbro and alteration products, reveal addition of K2O, P2O5, volatile, Ba, Sr, V, and Cu into wall rock and loss of MgO, CaO, and SiO2 from the wall rock to the veins accompanying gold mineralization. Chondrite-normalized REE patterns of samples show HREE enrichments in meta-gabbro, a flat pattern with a positive Eu anomaly in the epidote-amphibole-magnetite rich transitional zone, and HREE enrichment with a negative Cc anomaly in the mylonite zone. Stable isotope ratios of sulfur, carbon, and oxygen indicate a predominance of deep-seated fluids of metamorphic and magmatic signatures. Considering the combined structural and spatial association of gold with greenschist facies, the mineral and wall rock chemistry, fluid inclusion data, and isotopic data presented here, we conclude that the Okote gold mineralization formed by interaction of structurally focused hydrothermal fluids with mafic rocks.