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Multistage Au-As-Sb Mineralization and Crustal-Scale Fluid Evolution in the Kwekwe District, Midlands Greenstone Belt, Zimbabwe: A Combined Geochemical, Mineralogical, Stable Isotope, and Fluid Inclusion Study

Peter Buchholz and Thomas Oberthür

Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, D-30655 Hannover, Germany

Volker Lüders

GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany

Jamie Wilkinson

Department of Earth Science and Engineering, Royal School of Mines, Imperial College London, Exhibition Road, London SW7 2AZ, United Kingdom

Corresponding author: e-mail, peter.buchholz{at}bgr.de

The Kwekwe district comprises the central part of the Archean Midlands greenstone belt of Zimbabwe and represents one of few Archean gold provinces in the world hosting gold deposits associated with massive antimony mineralization. The deposits typically occur along late Archean transcrustal deformation zones. Mineralization is characterized by an early Au-As stage 1, followed by a pronounced Au-Sb stage 2, and a low-grade Sb-bearing stage 3.

Four gold deposits within the Kwekwe district have been studied. The Primrose and Globe and Phoenix gold deposits display typical features of Archean orogenic lode gold systems such as fluid inclusions with low salinity, mixed aqueous-carbonic fluids, formation temperatures between 300° and 400°C, and a common stable isotope composition of fluid and mineral precipitates. Deposits of this type formed in the brittle-ductile crustal transition zone at 1.5 to 3.0 kbars. In contrast, gold mineralization at Jojo and especially the Indarama gold deposits probably formed at lower temperatures (<<300°C) and from dominantly aqueous, early moderate- to late high-salinity fluids. At Indarama, microthermometric studies indicate stibnite-gold mineralization at temperatures between 120° and 230°C at about 0.8 to 1.8 kbars. Low formation temperatures and extremely low ³⁴S values of pyrite (–52) and stibnite (–36) of the latest mineralization stage at Indarama reveal reactivation of the hydrothermal system within a near subsurface, organic-rich epithermal or hot spring-type environment during the late Archean or even early Proterozoic. Such low ³⁴S values have not been reported previously from gold deposits in Archean greenstone belts. We propose that hydrothermal activity and gold mineralization has not only taken place at different crustal levels but also at different times.