Stukpan

This intrusive carbonatite complex was discovered by drilling of a prominent, circular aeromagnetic anomaly, with which is associated a positive gravity anomaly, on the farm Stukpan 435. The complex is not exposed at the surface but overlain by Karoo strata and a dolerite sill. Detailed ground magnetic and gravity surveys indicate an oval, pipe-like body of 3.1x1.7 km. Drilling close to the northern margin penetrated 182 m of Ecca shale, arkosic sandstone and dolerite followed by alternating bands of carbonatite and amphibolite down to the bottom of the hole at 400 m. The amphibolite is considered to be part of the country rock sequence and the irregular alternation of carbonatite and amphibolite interpreted as a contact zone (Verwoerd et al., 1986). The carbonatite is calcitic with minor dolomite bands. A streaky appearance is caused by lenticles of fibrous blue sodic amphibole; phlogopite, magnetite, as discrete crystals up to 1 cm in diameter, and apatite, as sugary aggregates and rounded grains, are plentiful (Verwoerd et al., 1986 and 1993). Fenitization of the amphibolite is manifested by veins of blue amphibole, while fenitization in quartzites takes the form of brecciation and infiltration of iron oxides and K-feldspar, now altered. Preliminary geochemical work indicates consistently high Sr values with some samples giving high Ba, Nb and Zr. A further borehole 1.8 km east of the main complex intersected a carbonatite sill between 294 and 388 m and thinner carbonatite veins, red fenite and brecciated quartzite.