A petrochemical investigation of the Spitskop Alkaline Complex, Eastern Transvaal

Abstract

This study comprises a petrographical and mineralogical description of the rocks of the Spitskop Complex. Data on the chemical composition of the rock types, as well as on some of the minerals, are presented. The complex consists of a pyroxenite pipe near the perimeter of which is situated a composite ring-dyke of foyaite. Ijolite is present as dykes and sills in the pyroxenite, and the carbonatite occurs as a plug intrusive into the pyroxenite. The complex is surrounded by an aureole of umptekite, and xenoliths of gabbroic rocks and magnetitite are present within the complex. A plug and sill of latite inside the complex, as well as dykes and cone sheets of latite and trachyte in the country rocks, were encountered. Profiles of eight core boreholes, drilled to depths between 30 and 40 metres into the complex, are presented, together with thirty-two rock analyses, fifteen mineral analyses, and variation diagrams showing the chemistry of the clinopyroxenes. The borehole profiles indicate that the ijolite is intrusive into the pyroxenite in the form of sills and dykes. This feature is further confirmed by the nature of the outcrops to the west of the carbonatite. The foyaite is subdivided into porphyritic, hypidiomorphic, and trachytoidal textural varieties, and the ijolite into hypidiomorphic and porphyritic varieties. The chemical variation diagrams indicate that the clinopyroxenes of the alkaline rocks follow differentiation trends typical of alkaline complexes, whereas those of the theralites show trends similar to those usually found in layered intrusions. From the geochemistry, the following order of emplacement may be derived: pyroxenite – ijolite – foyaite, in cases where a low CO₂ pressure prevailed in the magma; and pyroxenite – ijolite – carbonatite, in cases of a high CO₂ pressure. The Upper Zone of the layered sequence of the Bushveld Complex is postulated as the source of the theralites, which are here interpreted as xenoliths within the Spitskop Alkaline Complex. A vertical displacement of not more than a few hundred metres is suggested.