Evolution of the Kibali Granite-Greenstone Belt, North East Democratic Republic of the Congo, and controls on gold mineralisation at the Kibali Gold Deposit

The Kibali Granite-Greenstone Belt is located in the Northeast Democratic Republic of the Congo and is host to significant gold mineralisation in the form of the Kagaraba-Chaffeur-Durba (17 Moz Au), Pakaka (1.2 Moz Au), Mengu ( 0.7 Moz Au) and Pamao (0.7 Moz Au) ore bodies. The greenstone belt is composed of variably metamorphosed metasediments, banded iron formations and basalts intruded by 2.63-2.64 Ga igneous plutons ranging in composition from gabbroic to granitic. The greenstone belt is bounded to the south by the Upper Congo Granitic Massif, a polyphase intrusive complex dated to 2.62-2.64 Ga, and to the north by the West Nile Gneiss, a granitic gneiss complex dated to 985 Ma. The Kibali Granite-Greenstone Belt and Upper Granitic Massif are thought to have formed as part of an arc/back-arc complex that was active along the northern margin of the Proto-Congo Craton during the Neoarchean. The lithologies of the Kibali Granite-Greenstone Belt are thought to have been accreted to the northern margin of the craton at approximately 2.63 Ga. The West Nile Gneiss is hypothesized to have collided with, and over thrust, the Kibali Granite-Greenstone Belt in the period 600-400 Ma. This event is thought to have occurred as part of the Oubanguide orogeny in which the Saharan Metacraton and Congo Craton came together. This event is hypothesized to have reactivated old structures in the Kibali Granite-Greenstone Belt and resulted in largescale fluid flow which has been recorded in the U:Pb zircon record as a regional lead loss event, and by U:Pb monazite in the ore systems. Mineralisation in the Kibali Granite-Greenstone Belt is characterised by a pyrite(±gold)+arsenopyrite+ chalcopyrite+ pyrrhotite (±marcasite) assemblage occurring as both disseminated and vein-style mineralisation, hosted in deformed and altered volcano-sedimentary conglomerates, basalts and banded iron formation. Localised deformation of the host lithologies during regional metamorphism is thought to have created high permeability Fe-phyllosilicate-rich zones into which ascending CO2-rich fluids were focused. Interaction of these fluids with the Fe-rich host lithologies resulted in the widespread development of an Fecarbonate (ankerite±siderite)+quartz±aluminoceladonite alteration assemblage. Gold transporting fluids are inferred to have been H2S-rich, interacting with the Fe-rich host and alteration phases to form the Fesulphide-rich assemblage and deposit gold. Re:Os pyrite data indicate that mineralisation formed at approximately 2 Ga with a reactivation of the mineralising system identified through U:Pb monazite dating, occurring at 600-500 Ma.