GOLD IN KAROO SUPERGROUP HEAVY-MINERAL DEPOSITS, KWAZULU-NATAL, SOUTH AFRICA

Abstract

To date, all sites where gold has been recorded in KwaZulu-Natal have been in rocks of Archaean and Proterozoic Age. The site described is a heavy-mineral-rich sandstone bed of the Permian Vryheid Formation (Ecca Group) near Muden in the KwaZulu-Natal midlands. Sedimentary structures in the sandstone units of the Vryheid Formation indicate that these deltaic deposits were produced by rivers flowing from the northeast, with minor longshore drift towards the southeast. The deltaic deposits consist of three units: a basal unit (Unit A) of shale and mudstone, which coarsen upward into siltstones with lenses of fine-grained sandstone; a middle unit (Unit B) of ferruginous sandstone and immature greywacke (argillaceous sandstone) beds that coarsen upwards; and an upper unit (Unit C) of medium- to coarse-grained lithic sandstones with lenses of grit and sparse small pebbles of white vein quartz. The sand grains are physically and chemically very immature, suggesting the possibility that most of the sand grains were reworked from the then unlithified glaciogenic Dwyka Group sediments; the primary source of which was the Transvaal Supergroup and the granite - greenstone belts of the provenance area presently located in northern and eastern South Africa. The identification of gold-bearing grains in sandstones of the Vryheid Formation (Karoo Supergroup) from near Muden lends strong support to the concept that the gold-bearing horizons of similar Permian age in the Beaufort Group near Prince Albert, Western Cape Province were formed as natural placer deposits. The heavy-minerals take the form of both individual mineral grains and inclusions in quartz and feldspar with gold occurring only as inclusions. The majority of heavy-minerals occur as thin lenses and stringers in sandstone of the upper part of Unit B, where sorting was poor and flow rates were normally insufficient to winnow out the lighter minerals. Less abundant, but moderately well sorted, heavy-mineral lenses occur on the upper surface of Unit C in the troughs of megaripples, where deposition was rapid prior to subsequent preservation by an overlying organic-rich mud of Unit A.