The nature and composition of hydrothermal fluids related to gold mineralization in parts of Akobo greenstone belt, southwestern Ethiopia

Abstract

Gold mineralization in the Akobo greenstone belt of Southwestern Ethiopia is confined to metamorphosed volcano-sedimentary rocks of greenschist-amphibolite facies and is closely associated with shear/fault zones. Gold is found in quartz veins of the Gindibab-Wolleta area, while altered mafic-ultramafic rocks are the hosts for Chamo-Segele prospects. Microstructures found in vein quartz indicate intra-crystalline ductile deformation followed by fracturing during later brittle deformation episode. The assemblage of fluid inclusions in mineralized quartz veins in Gindibab-Wolleta area has been grouped into three types and inferred to be compositionally and texturally modified during the formation-deformation of quartz veins. Aqueous biphase inclusions are the only fluid inclusions found in the vein quartz of the Chamo-Segele area and retain the primary nature of fluid entrapment. Microthermometric analyses of inclusions indicate a striking difference in the composition of hydrothermal fluids responsible for the quartz vein formation in Gindibab-Wolleta and Chamo-Segele areas. The ore-forming fluids related to vein formation and gold mineralization in the Gindibab-Wolleta area were low saline aqueous carbonic. In contrast, in the case of the Chamo-Segele area, the quartz veins were formed by low to moderate saline aqueous fluid. The results of the fluid inclusion in the Akobo Greenstone Belt indicate an orogenic type of lode gold mineralization in the Gindibab-Wolleta area of the Gambella region, Ethiopia.