Spitskop

This is an oval-shaped complex of about 5x4 km with an ill-defined northern margin. Within the complex the distribution of the silicate rocks is not well known in places, because of poor exposure. Ijolite, within which is an area of biotite pyroxenite, is the most abundant rock type and is intruded by two bodies of foyaite that might be ring-dykes. A heart-shaped multiple plug of carbonatite is younger than the silicate rocks. There is an extensive fenite aureole and Strauss and Truter (1950a) considered that certain nepheline-bearing rocks inside the obviously fenitized basement rocks may also have been generated by metasomatism. The ijolites are of two main types: an earlier fine-grained black ijolite which is intruded as dykes and apophyses by a coarse-grained red ijolite which has local urtite facies. There are also melteigites and jacupirangites which map as a distinct mass on the eastern side of the complex. An area of biotite pyroxenite within the ijolites is older, but its exact relationship is not clear. The coarse red ijolite consists of anhedral nepheline, which encloses poikilitically prismatic pyroxene zoned outwards to aegirine, abundant cancrinite and titanite, zeolite, calcite, occasional alkali feldspar, apatite and locally melanite and pectolite. The fine-grained ijolites are mineralogically similar but with less cancrinite. The melteigites and jacupirangites of the southeastern part of the complex are characterised, according to Strauss and Truter (1950a), by the presence of biotite and magnetite, and they consider these rocks to be partially, at least, the result of metasomatic changes. Near the biotite pyroxenite body biotite and magnetite are found in the ijolites and appear to grade into nepheline-bearing pyroxenite that Strauss and Truter (1950a) interpret as being the result of nephelinisation. The pyroxenite consists of diopside-hedenbergite, biotite, calcite, cancrinite, titanite and altered anhedral nepheline. The pyroxenites are intensely cut by veins of fine-grained, dark ijolite marginal to which the pyroxenites are converted to biotite-nepheline-titanite-cancrinite rocks. Foyaite forms an oval ring on the eastern side of the complex which is cut on its western side by carbonatite. It encloses large rafts and blocks of ijolite and in places forms an injection breccia. Locally it is banded or foliated and this structure is either vertical or steeply outward dipping. Foyaite also forms an arcuate body on the western side of the complex and a small dyke-like body in the northeast. The foyaites include porphyritic, banded to schistose and massive varieties and are variably coarse- or medium-grained. They consist of alkali feldspar, albite, zoned aegirine-augite/aegirine, nepheline, cancrinite, sodalite, which is locally abundant, accessory titanite, apatite and zeolites and rare sodic amphibole, wollastonite and pectolite. Strauss and Truter (1950a) mapped and described a suite of rocks described as fayalite diorite, theralite, anorthositic theralite and magnetite theralite, some of which they considered to be fenite but others they thought were magmatic. Verwoerd (1966b) re-investigated these rocks and concluded that they were all fenites and represented xenoliths within the ijolite derived from the Main Zone and Upper Zone of the nearby Bushveld Complex. Fenites produced by metasomatism of Bushveld granite and granophyre occur all around the complex the most extensive type, called white umptekite by Strauss and Truter (1950a), extending to nearly a kilometre in places. Quartz is replaced and blue sodic amphibole, microcline perthite and albite develop, but near the igneous contact amphibole is replaced by aegirine-augite. A distinctive red fenite, which contains both sodic pyroxene and amphibole, lies outside the white umptekite on the eastern margin. The carbonatite body is probably surrounded by ijolite, except in the north where it is in contact with foyaite, but intrusive contacts are nowhere exposed (Verwoerd, 1967, Folder 3). The carbonatite has a more or less vertical, concentric structure. Four carbonatite types have been mapped by Verwoerd (1967). White sovite forms a remnant along all but the western margin and is a massive, coarse calcite rock with abundant blue amphibole, aggregates of apatite, chlorite, magnetite and pyrite. A discontinuous zone of grey dolomite sovite lies inside the white sovite, in which calcite forms remnants in a groundmass of dolomite. Accessories include aegirine, blue sodic amphibole, apatite, magnetite, phlogopite and monazite. Beforsite forms the main area of the carbonatite body. It is a fine- to medium-grained rock of 'parankerite' with subordinate dolomite, late calcite and similar accessories to the dolomite sovite. The fourth type is an apatite-rich beforsite that forms zones with gradational boundaries in the beforsite and dolomite sovite. In a central, circular zone of this rock P2O5 is about 8%. Accessories are similar to those of the other carbonatites. Inclusions of ijolite and foyaite up to nearly 200 m in diameter occur in the beforsite. A suite of dykes, mainly of foyaite, are distributed radially around the complex and occur within it cutting the silicate, but not carbonatitic, rocks. Analyses of all the principal rock types, but lacking trace elements, are given by Strauss and Truter (1950a) who also give numerous photomicrographs and field photographs. Harmer (1999) gives numerous rock analyses, with much trace element data, including full suites of rare earth elements, together with Sr, Nd, Pb, C and O isotope data; he also gives a Mg-(Fe+Mn)-Na diagram for extensive data on pyroxenes from the silicate rocks. Carbon and oxygen isotope analyses for nine carbonates from carbonatites were made by Suwa et al. (1975).