The metamorphic and alteration history of West Rand Group shales from distal portions of the Witwatersrand Basin

The maximum metamorphic grade in West Rand Group shales from three borehole s to the east and south of the Vredefort Dome is lower greenschist facies. In most samples the peak metamorphic assemblage consists of quartz + muscov ite + chlorite, with the phyllosilicates defining a tectonic cleavage. In m any of the samples more coarsely crystalline alteration zones that clearly crosscut both bedding and the tectonic cleavage overprint this assemblage. The alteration zones are localized around minor faults and shears where flu id ingress has occurred. Muscovite was not observed in these zones and the cleaved matrix assemblages has been replaced by assemblages consisting of p yrite + quartz +/- chlorite +/- stilpnomelane +/- calcite +/- carbonaceous matter +/- Mn-garnet +/- epidote +/- chalcopyrite. In some of the alteratio n zones stilpnomelane is a very common product and appears to arise through the breakdown of chlorite + muscovite. K and Fe metasomatism are indicated during this reaction, as well as the obvious mobility of S, Cu and C. Assa ys on small samples indicate gold introduction into the shales during the a lteration event. Chlorite thermometry was used to estimate the equilibratio n temperature of chlorite from the cleaved matrix and the alteration zones. Temperatures close to 300 degrees C are indicated for both chlorite variet ies, and for all three boreholes. These results, and the petrographic data, imply that fluid influx post-dated the formation of the tectonic cleavage, but occurred close to the peak of metamorphism. Comparison between the chl orite thermometry data from this study with those from similar studies in t he goldfields indicates very similar peak metamorphic grades in both areas, suggesting that these metamorphic conditions may be typical of almost the entire basin. The one exception is obviously the amphibolite facies metamor phism exposed in the northwestern sector of the collar of the Vredefort Dom e. Several recent studies have suggested that this metamorphism is represen tative of a widespread zone of higher grade metamorphism within the central portions of the basin. The results of this study indicate that this is unl ikely to be the case and suggest that these amphibolite facies rocks are pr obably unique to the northern portions of the dome.