Petrogenesis of gold-hosting Neoproterozoic syenite from the Tulu Kapi area, western Ethiopia

Abstract

The Tulu Kapi syenite-hosted gold mineralization in the Western Ethiopian Shield has been under exploration during the last few decades. The geochemical and petrological characterization of the intrusive rocks and their alterations including the gold hosting syenites are crucial in appraising the economic potential of the mineralized belt on a larger scale. The present study concerns with the geochemical and petrogenetic behaviour of the gold-hosting syenites and other associated intrusions using an integrated field, geochemical and petrological investigation. The Tulu Kapi area is situated within a strongly sheared and NE-SW foliated, low-grade (lower greenschist to lower amphibolite facies) volcano-sedimentary belt (the “Birbir belt”) that is intruded by mafic-ultramafic and granitic intrusives (chlorite-serpintine-talc schist, birbirite, metagabbro, metadiorite, metagranite and metasyenite). Major and trace element geochemistry revealed that the gold mineralization hosting syenites, associated diorites and granites are ferroan, calc-alkaline, metaluminous to slightly peraluminous bodies formed in a volcanic arc subduction to syn-collisional tectonic setting by the fractionation of underplated, LREE-enriched basaltic magma with substantial crustal input to possible crustal melting. The underplated basaltic magma was sourced from a LREE-enriched mantle by reworking of sediment laden crustal slabs at depth in a subduction zone. Fractional crystallization resulted in the formation of Ca-rich plagioclase, which continually reacted with hydrothermal fluids to more fractionated Na-rich plagioclase, forming the gold mineralization hosting altered syenites viz. albitization.