The geochemistry of the dykes in the Carletonville Goldfield

Abstract

Numerous dykes traverse the Witwatersrand Supergroup rocks in the Carletonville Goldfield. The aim of this study was to investigate a classification system for the dykes. Samples were obtained from Tau Tona and Mponeng mines as well as from AngloGold Ashanti’s field office. The mineralogical investigation revealed that most dykes, with the exception of the Brazil dyke, are altered. The most abundant minerals are chlorite, actinolite, epidote, quartz and albitised and/or saussuritised feldspar, corresponding to a greenschist metamorphic facies mineral composition. Veins are commonly filled with quartz, calcite, epidote and chlorite, with sulphides and Fe oxides occurring occasionally. However, mineralogical heterogeneity as a result of different degrees of alteration, were found between samples from the same dyke. This heterogeneity may be an important consideration where rock engineering is concerned as it could cause different sections of the same dyke to have different physical properties Geochemical separation of the dykes into different groups was achieved by means of Bowen’s (1984) TiO2 v Zr and Zr/P v P/Ti plots as well as Linton’s (1992) discriminant plot. These same plots were employed in order to classify the dykes according to geochemical data taken from literature for four igneous events, namely, the Ventersdorp Supergroup, Transvaal Supergroup, Bushveld Igneous Complex and Karoo Supergroup, as well as geochemical data for dykes from the East Rand Proprietary Mine. Rare Earth Element patterns from the dykes were compared to literature data for the above-named igneous events in order to obtain a better classification.