Schurwedraai And Baviaanskranz (Vaal River Bosses)

Two intrusions of peralkaline granite lie across the Vaal River and cut rocks of the Witwatersrand Supergroup while nepheline syenite forms dykes cutting both intrusions and develops as larger intrusions to the east of Schurwedraai. The latter intrusion occupies an area of 3.9 km2 and the Baviaanskranz intrusion 2.6 km2; there are also a few small peralkaline granite outcrops at Koedoesfontein 6 km to the northeast. All the granites consist of microcline, albite, quartz, arfvedsonite, aegirine and biotite with minor opaques, titanite, zircon and apatite. Seven granite analyses are collected in Bisschoff (1973). The nepheline syenites were called canadite and litchfieldite by Hall and Molengraaff (1925) while Tilley (1960) and Bisschoff (1973) referred to them as mariupolite. In most of the dykes the nepheline syenite is of two textural types: fine- to medium-grained and coarse-grained to pegmatitic. The pegmatitic parts form narrow bands and wavy or irregular patches, as illustrated in Bisschoff (1973), and the fine-grained is generally trachytic in texture. They are composed of albite, a little microcline, which is more prominent in the pegmatitic rocks, nepheline, which varies from <1% by volume in some fine-grained rocks to pure veins and patches in the pegmatites and may be replaced by cancrinite, aegirine and accessory biotite, titanite and an opaque phase. Analyses of two nepheline syenites from southeast of the Schurwedraai intrusion, and nepheline and microcline from one of them, are given and discussed by Tilley (1960); three further analyses will be found in Hall and Molengraaff (1925).