Die geologie en petrologie van die wonderfonteingang

Abstract

The Wonderfontein dyke is one of the longest composite dykes of the Pilanesberg dyke-swarm. It can be traced from Pilanesberg where it cuts the outer rings of syenite and nepheline syenite, over the Magaliesberg to northwest of Parys, a total distance of 190 km. This dyke was studied along its entire length and to a depth of 997 min the North Shaft and in the underground workings on West Driefontein Mine. The dyke is composed of a core of nepheline syenite (± 30 m wide) and walls of dolerite (± 2 m wide). These two rock types are separated from one another by a thin zone of altered hybrid rock (± 20 cm wide). The symmetrical disposition of the dolerite in relation to the central nepheline syenite is a striking feature which persists both along the length of the dyke and also in depth, even where the felsic core becomes a minor part of the dyke. A detailed study of the joints in the dyke in North Shaft (sunk in the Wonderfontein dykel revealed that vertical joints are the most abundant over the total vertical depth of 997 m. These joints could be classified as cross joints, diagonal joints, and longitudinal joints. Certain joint concentrations in the dyke correspond to some of the regional joints in the adjacent country rocks. The study includes the petrography and major element chemistry of the dyke rocks. The nepheline syenite is composed essentially of orthoclase, aegirine-augite, nepheline, plagioclase, and biotite. There is little variation in its mineralogical compqsition both horizontally and vertically. The nepheline syenite contains abundant elongated inclusions which differ cqnsiderably in composition from the host rock. Inclusions of nepheline syenite and phonolite are abundant. Along the contact with the dolerite on either side of the nephelirie syenite, i~clusions of dolerite are quite common, but decreq,se in number towards the core of the dyke. A few nepheline syenite pegmatite veins occur in both parts of the composite dyke. rhe transition from nepheline syenite to dolerite on either side of the felsic core of the dyke is represented by a thin zone of hybrid rock which is usually highly altered and contains recognizable inclusions of dolerite but not of nepheline syenite or phonolite. The X-ray fluorescence analyses of samples taken both along the length and breadth, and in depth, of the dyke shows that there is very little variation in the major element composition. The nepheline syenite inclusions, however, show more variation and differ considerably in composition from the nepheline syenite host. On petrographic grounds the nepheline syenite may be classified as peralkaline, but the norm contains no acmite. The norms of the pegmatite veins, however, do contain acmite and this indicates a trend towards peralkalinity with progressive crystallization which is substantiated by the modal increase in albite and the presence of aegirine in place of aegirine-augite. The dolerite is composed of labradorite and augite. The field relationships show that the dolerite was emplaced first and chilled against the country rocks. The chilled dolerite contains vertically orientated microphenocrysts of piagioclase and olivine but, towards the nepheline syenite, the dolerite becomes coarser grained and subophitic. The presence of inclusions of dolerite in the nepheline syenite and in the hybrid zone shows that the dolerite was still partly liquid when the nepheline syenite was intruded. This is substantiated by the linear variation of Ti0 2 , Fe2o3 , MgO, Cao, Na2o, and K2o from the dolerite to the nepheline syenite through the hybrid zone. The nepheline syenite adjacent to the hybrid zone is finer grained with smaller phenocrysts than the central part of the dyke. The presence of composite inclusions of nepheline syenite within nepheline syenite in the core of the dyke indicates that the intrusion of the nepheline syenite must have been multiple in at least three stages. The magmas of Pilanesberg and its dykes were most probably developed by fractional differentiation of primary alkaline olivine basalt or fractional melting of mantle material.