Geochemical and petrographical investigation of the Waaikraal gold deposit, northwest of Brits, Transvaal

Abstract

The Waaikraal Gold Deposit is situited approximately 25 km northwest of Brits and 30 km northeast of Rustenburg. The gold occurs in a sill-like body consisting of alternating layers and lenses of pyroxenite and a magnetite-rich rock, hosted in sediments belonging to the Rayton Formation of the Transvaal Sequence. The sediments are situated between the magnetite gabbros of the upper zone of the Bushveld Complex and the granites of the Lebowa Granite Suite. The pyroxenite is composed of clinopyroxene (augite and hedenbergite), plagioclase (mainly andesine), orthoclase, quartz, amphibole (hastingsite and hastingsitic hornblende) and some magnetite and apatite. The magnetite-rich rock consists of magnetite, clinopyroxene (augite and hedenbergite) and plagioclase (mainly andesine). Olivine (fayalite), apatite and ilvaite are accessory constituents in the magnetite-rich rock. The gold occurs in three different phase’s viz., very pure native gold, with Ag-content < 1.3 weight per cent, is the most prominent phase, in maldonite (Au2Bi) and in a gold-copper compound (CuAu3). It is associated with cobaltite (CoAsS), arsenopyrite (FeAsS), safflorite ((Co,Fe)As2 ), native bismuth, bismuthinite (Bi2S3), joseite-B (Bi4Te2S) and chalcopyrite. Other ore minerals include pyrrhotite, pyrite and sphalerite. The composition of silicates and the presence of high amounts of titanium in the magnetite grains suggest that the pyroxenite is related to the upper zone of the Bushveld Complex and that it crystallised from a highly differentiated upper zone type liquid, which was injected into the host rocks during the final stages of consolidation of the upper zone. The magnetite-rich rocks, on the other hand, are considered to be of sedimentary origin. Derivation of this rock from an iron-rich metamorphosed sediment is supported by its low titanium content, but comparison with iron-rich sediments shows that this possibility is unlikely. The presence of typical skarn minerals such as hedenbergite, magnetite and ilvaite, as well as the typical association of Cu, Co, As, Bi and Au with Fe, suggest that this rock could also be a calcic iron-skarn derived from sediments through metasomatism by iron-rich fluids. The "original sediment" is most likely is a marl with a composition equivalent to equal parts of shale and limestone. The temperature range of formation of the ore minerals associated with the gold and the mode of occurrence of the gold suggests a possibly Bushveld related, hydrothermal origin for the gold, where the gold may have been transported as sulfide complexes.