Origin and age of gold mineralization in NW Ghana : a case study of selected gold deposits from the Julie and Wa-Lawra belts

Abstract: Gold mining in Ghana has a long history that dates back to the then Gold Coast. The exploration for gold keeps expanding across the nation, as mining forms an integral part of the country’s economic growth. The gold is hosted within Paleoproterozoic rocks of the Birimian Supergroup, Tarkwaian Group and associated granitoids. Gold mineralization types include the Birimian mesothermal quartz vein and quartz vein-associated sulphide types, and the Tarkwaian placer gold type. The sources of the Birimian and Tarkwaian gold remain debated. Mafic rocks or carbonaceous sedimentary rocks are regarded as the potential metallogenic source rocks. This study focused on the evaluation of potential source rocks for gold in NW Ghana, especially the shale-hosted Collette and Kjersti Au deposits within the Julie greenstone belt. The Julie belt is part of the NW Birimian terrain of which the geology remains poorly constrained. A fundamental issue relates to linking various lithologies from this belt to the established Birimian and Tarkwaian stratigraphy in SW Ghana. Field work within the Julie belt indicated that the lithologies (Chapter 3) share similarities to Paleoproterozoic terrains in the SW. Stratigraphically, the Julie belt is characterized by metavolcanic rocks (Sefwi Group) consisting mainly of basalts with minor rhyolite (and agglomerate), and metasedimentary rocks (Kumasi Group) of interbedded shales and siltstones, and greywackes. Both units are unconformably overlain by a clastic sequence (Tarkwaian Group) of minor conglomerate and predominately cross-bedded sandstone with the ubiquitous presence of magnetite. These groups are intruded by granitoids of different generations. Shales and greywackes are sulphidic and rich in carbonaceous matter (CM). Provenance and tectonic setting of these rocks (Chapter 4) indicated their derivation from erosion of a granitoidgreenstone terrain containing basalt, rhyolite and granitoid rocks compositionally similar to rocks from adjacent Birimian volcanic and plutonic terrains. The composition of shales and greywackes suggests their derivation largely from a Paleoproterozoic volcanic arc setting. They were possibly deposited in forearc and/or back-arc basins prior to the Eburnean orogeny and collision within the West African Craton (WAC). The sedimentary rocks, especially greywackes, are part of the rocks whose zircon U-Pb ages and Lu-Hf isotopes have been determined (Chapter 5). U-Pb ages from both detrital and igneous zircons on eight samples (1 greywacke, 3 sandstones, 1 rhyolite and 3 granitoids) show that all zircons analyzed indicated Paleoproterozoic ages in the range of 2112 ± 5 to 2337 ± 11 Ma. This age range corroborates with existing zircon ages obtained from volcanic and plutonic rocks in various parts of Ghana, and other Birimian and Tarkwaian terrains within the WAC...Ph.D. (Geology