Derdepoort

Outcrops of carbonatite and breccia extend over an area of about one kilometre square, but only in the north, where carbonatite cuts quartzite, can contacts be seen. Verwoerd (1967) considers it to be an irregularly shaped pipe. The most widespread rock type is probably a volcanic breccia consisting of fragments of possible felsite and hornfels, quartzite, diabase, syenite and dolerite set in a carbonate-rich matrix. A distinctive breccia, found in three areas, is characterised by large biotite flakes and broken hornblende crystals, with an extensive suite of rock fragments, generally between 0.5 and 15 cm, but occasionally reaching a metre, in diameter, and including felsite, diabase, hornfels and a range of other igneous and sedimentary rocks in an ankeritic groundmass. At the northern end of the pipe country rock quartzites are in places intensely brecciated with dense ferruginous material sometimes filling interstices. In the southeastern corner a rock of biotite, apatite, magnetite, amphibole and pyroxene occurs an analysis of which Verwoerd (1967) considers to indicate a carbonated alnoite. Areas of somewhat altered syenite and of quartz-hematite rock occur, and there are contact altered rocks including fenitized diabases containing alkali feldspar and sodic amphibole. In the northern part of the pipe an irregular area of carbonatite was mapped by Verwoerd (1967) in which he distinguished three carbonatite types. One variety consists of ankerite with abundant green chlorite and quartz. White beforsite consists of ankerite with calcite, K-feldspar, quartz, apatite, chlorite and magnetite, and red beforsite is characterised by its colour and may have been produced by impregnation of white beforsite by hematite; the principal carbonate is dolomite with only a little iron. 87Sr/86Sr data on three samples of carbonatite are available in Harmer (1985).