Volcanogenic copper-zinc sulphide deposits and their geological setting, in the central portion of the murchison greenstone belt, North eastern Transvaal, South Africa

Abstract

The Murchison greenstone belt is of Archaean age, and occurs within the Kaapvaal Craton of South Africa. The greenstone sequence consists of high-magnesia basalts, overlain by tholeiitic basalts, and, on the northern flank of the Belt, rhyolitic tuffs and lavas form the top of the volcanic pile. All units are overlain by clastic sediments. The entire sequence is intruded by numerous, large, sills of tholeiitic composition; and a large gabbroic complex occupies the base of the rhyolite pile. Collapse and deformation of the sequence produced a tight syncline with a subhorizontal fold axis and vertical axial plane. Low Grade metamorphism accompanied the folding, with temperatures in the range 450-480º and a pressure of about 5.6kbar. Plutons of tonalitic magma were emplaced along the margins of the Belt. Subsequent cooling of these superimposed a thermal metamorphism onto the existing fabric. Massive copper-zinc sulphide deposits were formed immediately following the ryolitic volcanism. Two main, and three minor, deposits were studied in the central part of the Murchison Belt. At the larger deposit, the massive sulphide body is underlain by altered tuff. This alteration comprises an inner core of chlorite with pyrrhotite and chalcopyrite, and an outer, ill-defined sericitic zone containing pyrite and sphalerite. Bulk chemical analyses show an increase in total iron, and decrease in A1(_2)O(_3) and K(_2)O towards the core of the alteration zone. The massive sulphides form a stratiform lens of sphalerite, pyrite, pyrrhotite, and chalcopyrite. Relict, original textures suggest formation as unconsolidated sulphidic sediments on the sea-floor. The lower sulphide horizons were originally rich in chalcopyrite and silica, with only minor sphalerite. As the sulphides accumulated, sphalerite replaced the silica. The upper sulphide horizons show no such replacement, and are generally rich in sphalerite. The other main deposit is similar, but has no underlying alteration zone, A model is presented to account for these, and other, copper-zinc deposits in the Murchison Belt. Following rhyolitic volcanism, sea-water percolated through the underlying rocks, became heated, and set up a system of circulating, hot brines, capable of leaching metals from the rocks. Discharge along pre-existing fractures, precipitated sulphides at the rock/sea-water interface. Initial solutions were hot, with a high Cu/Zn ratio, whilst later, cooler solutions were zinc rich, and deficient in copper.