Pilanesberg (Pilansberg, Pilaan’S Berg)

This circular complex of 24x28 km covers 530 km2 and as such is the most extensive alkaline complex in South Africa. It lies towards the centre of a major province of alkaline rocks, to which it gives its name (Ferguson, 1973), and includes occurrences numbered 7 to 24. The complex consists of a volcanic series, much depleted by erosion, intruded by several rings around a central core, the whole being reflected topographically by concentric arcuate lines of hills. The volcanic rocks dip mainly inwards and comprise lavas, which are the dominant form, tuffs, breccias and agglomerates. The lavas are much altered, with secondary calcite and fluorite, but apparently range from trachytes to phonolites (Lurie, 1973) with aegirine sometimes recognisable, alkali feldspar phenocrysts widespread, and altered, and occasionally fresh, nepheline, rare biotite and amphibole (Shand, 1928). The intrusive rocks are mainly foyaites of which four units are distinguished with lesser amounts of tinguaite and an outer syenite. The core and much of the central area is underlain by 'red foyaite', which is generally pink, coarse-grained and deeply weathered. It consists dominantly of microcline (50-70%) with altered nepheline, occasional albite, magnetite, and in some areas fluorite, which has been mined. The 'white foyaite' is the most extensive and forms an irregular ring around the core, arcuate bodies in the southwest and north, and smaller areas elsewhere. This rock is diverse in texture and, to a lesser extent, in mineralogy. Texturally it varies from medium- to coarse-grained with pegmatitic and vein facies common. The feldspar is generally microcline, which is often perthitic, and nepheline is abundant. Aegirine-augite is predominant but aegirine may be present, while arfvedsonite or katophorite are also usual. Biotite and magnetite are sporadic with zeolites, cancrinite and sodalite sometimes present. Accessory minerals include titanite, zircon, aenigmatite, eucolite, astrophyllite, mosandrite and pectolite (Retief, 1963). The 'green foyaite’ probably forms a complete ring around the complex but is much obscured by volcanic rocks. The rock is, indeed, generally dark green and commonly porphyritic and trachytic, and usually medium- to coarse-grained in texture. Retief (1963) subdivided the green foyaite, on the basis of texture and mineralogy, into seven types of which one, containing eudialyte, is the lujavrite of earlier writers. In general these foyaites consist of 25-40% microcline and 10-45% aegirine/aegirine-augite with minor and accessory zeolites, sodalite, arfvedsonite, lamprophyllite, mosandrite, catapleite, eudialyte, zircon, titanite, calcite, fluorite and magnetite. Retief (1963) distinguished a fourth foyaite type, the Ledig foyaite, which forms an arcuate sheet near the southern margin of the complex and has some similarities with both the green and white foyaites, but is distinguished by the presence of radial clusters of aegirine prisms. The Ledig foyaite is somewhat variable with either microcline or perthite and nepheline present or absent, together with aegirine, arfvedsonite, zeolites, cancrinite and sodalite and generally the greatest abundance and variety in the Pilanesberg of rarer minerals including eucolite, aenigmatite, astrophyllite, lamprophyllite, catapleite and a number of unidentified species. The syenite, known as the Red syenite, forms the outermost unit for about half the complex, a mapped arcuate body in the southwest, and fragments and boulders in the northeast indicating that an almost complete ring may exist; other extensive areas are known within the complex. The syenite consists of perthite and albite, biotite, aegirine and opaque phases. Small bodies of quartz-bearing syenite, including alkali granite, have been identified (Lurie, 1973 and 1986). Ijolite is also reported to occur in the western part of the complex and other rock types encountered include tinguaite, nepheline-feldspar porphyry and dykes of syenite and microfoyaite. Chemical analyses of rocks will be found in Retief (1963) and Shand (1928), while Lurie (1986) has investigated the distribution of the rarer elements. A detailed study of the feldspars has been made by Retief (1962).