Ar/Ar geochronology of hydrothermal K-feldspar from the Mowana Cu Mine and implications for geotectonothermal evolution and Cu mineralisation in the Archean Matsitama Greenstone Belt, Zimbabwe Craton, northeastern Botswana

Abstract

Unlike numerous Archean-Proterozoic greenstone belts worldwide, which are known to host orogenic and intrusion-related-type gold deposits, the Archean Matsitama Greenstone Belt (MGB) of NE Botswana hosts numerous base metal deposits/occurrences, including the Bushman Lineament (a regional brittle-ductile shear-hosted) Mowana Cu deposit. Economic copper mineralisation therein occurs as hypogene (chalcopyrite), supergene (chalcocite mainly), and oxidised (malachite, cuprite, tenorite, and chrysocolla) ores within quartz-K-feldspar-calcite veins. Due to the insufficiency of radiochronologic data, the geotectonic setting and evolution of the MGB are unclear, and the time of Cu introduction in the MGB remains contentious and unconstrained. In this study, hydrothermal K-feldspar spatially associated with Cu mineral phases in the ore veins was dated by ⁴⁰Ar/³⁹Ar to shed light on the window time of Cu mineralisation and understand the geological context of the MGB. K-feldspar from the hypogene ore-dominated and the oxidised ore-dominated areas yields similar integrated total fusion ⁴⁰Ar/³⁹Ar ages of ca. 550 Ma. Our new age is at odds with the Archean geological setting of the MGB and the ca. 2.2-1.9 Ga galena Pb-Pb age previously assigned to the Cu mineralisation within the MGB. This new age, however, reveals for the first time that the Pan-African thermal rejuvenation that was previously recognised in other portions of the Zimbabwe Craton also affected the MGB, therefore refining the spatial extent to which the Zimbabwe Craton underwent thermal rejuvenation. Our ca. 550 Ma age also highlights the possibility of a Neoproterozoic Cu mineralisation event at Mowana Copper Mine, in addition to the known Paleoproterozoic initial Cu introduction. This possible Neoproterozoic Cu mineralisation event at the Mowana Copper Mine is coincident with Cu events within the Kalahari Copper Belt as well as within the Central African and Zambian Copper belts, therefore highlighting the importance of the Pan-African Orogenesis in the Cu endowment of south-central Africa.