Tweerivier

The Tweerivier carbonatite complex is located at the confluence of the Crocodile and Elands Rivers. The northern and southern halves of the complex (Tweerivier North and Tweerivier South of Verwoerd, 1967), are geologically very different, being separated by the rivers, which obscure contacts between them. The complex is surrounded by Bushveld granite which is fenitized around the northern part. Tweerivier North is composed predominantly of dolomitic carbonatite which has a well developed planar structure that strikes approximately parallel to the outer contacts and has steep to vertical dips. The planar structure is defined by the orientation of tabular carbonate crystals, intercalations of calcite and dolomite carbonatite and orientated tremolite. Dolomite bands are predominant and contain magnetite aggregates, phlogopite and ovoid apatite crystals; a little quartz is present in some varieties. The less abundant calcite bands contain magnetite, minor phlogopite and tremolite and rare zircon, but apatite is notable by its absence. Two areas of tremolite-bearing carbonatite were mapped by Verwoerd (1967). One irregular area, close to the northwestern margin, comprises a schistose rock in which carbonate is subordinate to tremolite and there is evidence of this rock replacing the undifferentiated dolomitic carbonatite. Magnetite and 'augen' of phlogopite are present and dolomite forms lenticular areas. The other, much more extensive, area of tremolite-bearing carbonatite grades outwards into the tremolite-free carbonatite. Large tabular inclusions of rocks, which are probably derived from the Transvaal Sequence, lie conformably within the carbonatites and include metamorphosed banded ironstone, recrystallized quartzite and garnet-sillimanite-andalusite hornfels. The banded ironstones have been metamorphosed to rocks consisting of aegirine, layers of hematite and a riebeckitic-arfvedsonitic amphibole. Smaller quartzite xenoliths are fractured with the development of eckermannitic amphibole along the fractures. Bodies of iron ore, which occur throughout the undifferentiated carbonatite, consist of hematite and magnetite and are considered by Verwoerd (1967) to be segregations rather than xenoliths. A series of bifurcating and coalescing ankeritic beforsite dykes is concentrated close to the northern margin of the complex, some of which extend out into the fenites close to the eastern margin, and define a circular structure in the south just north of the Elands River, which Verwoerd interprets as an eruptive centre. The carbonate of these rocks is ankerite; apatite is abundant, quartz is interstitial and there is sparse pyrite and a little chlorite. The marginal fenites are brick red rocks which are heavily brecciated. The feldspar is deeply turbid and near the carbonatite contact there is no quartz; they are free of dark minerals. Tweerivier South is poorly exposed but drilling enabled Verwoerd (1967) to map this area, which contrasts remarkably with the northern half of the complex. The dominant rocks are gabbros and anorthositic gabbros, cross-cutting sovite sheets and a radioactive, silicified ferruginous rock. The sovite sheets exhibit flow structure, as defined by accessory phlogopite, apatite and magnetite, and consist predominantly of calcite, sometimes with isolated dolomite crystals; pyrite, rare olivine and baddeleyite are accessory. Magnetite crystals up to a centimetre in diameter are present and layers 0.5 m thick, or more, have been encountered in boreholes, often in association with patches of glimmerite rock and aggregates of apatite; the last constitutes 50% of some rocks. The gabbros consist of diallage, sometimes enclosing orthopyroxene, and in least altered specimens may be surrounded by hornblende, plagioclase and magnetite. Alteration adjacent to sovite leads to replacement of pyroxene by hornblende and development of apatite prisms and biotite. The genesis of the gabbros is discussed at some length by Verwoerd (1967) who concludes that they were probably brought up as a breccia by the sovite from Bushveld Complex gabbros lying beneath the granite. Although less extensive than at Tweerivier North fenitization is more intensive and in many rocks all quartz has been replaced by up to 40% of aegirine and zoned arfvedsonitic amphibole. Three whole rock carbonatite analyses are given by Verwoerd (1967). 750 m southwest of the Tweerivier South body is a circular, 300 m-diameter breccia plug which forms a conical hill called Spitzkoppie. It is composed essentially of brecciated granite traversed by thin veinlets of quartz and finely comminuted material. There is little sign of fenitization. However, a borehole encountered not only similar breccias but also granitic rocks in which primary hornblende and biotite had developed rims of 'soda hornblende'. At greater depths a volcanic breccia was encountered including fragments of iron ore, quartzite and quartz beforsite containing sphalerite and with veinlets of quartz and dolomite.