- © 2002 Geological Society of South Africa
The Allanridge Formation of the Ventersdorp Supergroup in the Northern Cape Province consists of andesite to basaltic andesite. The properties of these rocks are described by aid of detail from two localities near Douglas (Kalkdam and Katlani), where seven lava flows can be discerned. Greenschist metamorphism has affected all of the lava flows and sporadic sulphide mineralization has also occurred. The intensity of sulphide mineralization is controlled by porosity so that the amygdaloidal bases and tops of flows are more affected than the massive parts, hence producing a conspicuous bleaching. Thus light amygdaloidal lavas (LA) that are sporadically mineralised by sphalerite, galena and chalcopyrite can be distinguished from less altered dark amygdaloidal (DA) lavas.
Sulphides are present in altered amygdales of the LA together with quartz, chlorite and calcite. These minerals were introduced by means of hydraulic fracturing and brecciation features are common. Sphalerite tends to dominate over galena in the amygdaloidal lava flows, while the opposite is true in the breccia zones. Mass transfer calculations reveal that the net mass loss for LA and DA was 7 to 20 % and 3 to 5% respectively. Enrichment/depletion diagrams illustrate that LA is relatively enriched in SiO2, Al2O3, K2O, P2O5, Rb, Ba, Cr and Zn, while the DA is relatively enriched in MnO, Fe2O3, Nb, Cr, V, Co and Zn with respect to unmineralized lava flows.
The alteration of DA may be regarded as propylitic, while changes in LA involved potassium metasomatism also. Two generations of mineralization can be discerned. The source of the saline fluids capable of transporting metals is probably in the banded iron formations of the Griquatown and Kuruman Formations. Channelways for fluid migration were provided during extensive deformation.