D.Phil. (Geology) / A specific geological event has been characterized with the aid of an integrated metamorphic and fluid inclusion study of data obtained from syn-tectonic vein-quartz associated with thrusting and bedding-parallel shear along the northern margin of the Witwatersrand Basin. The vein-quartz associated with this event occurs as boudin-shaped bodies with their long and intermediate axes orientated within the foliation-, bedding- or fault-planes. The length of the quartz lenses which are spatially confined to shear zones often exceeds the thickness of the shear zones. These phenomena and the fact that quartz-fibres are orientated parallel to and not at right angles to the foliation confirms the syn-tectonic nature of the quartz veins. Heterogeneous P-T condition is indicative of imbrication, i.e. crustal thickening which is also substantiated by the random growth of pyrophylite and kyanite in shear zone assemblages, indicating that metamorphism outlasted deformation. Metamorphic studies of aluminous schists and vein-quartz with pyrophylliteand pyrophyllite - kyanite selvages established the development of two critical mineral assemblages: 1 Kaolinite + 2 Quartz = 1 Pyrophyllite + 1 H20 ... (1) and at higher P-T conditions 1 Pyrophyllite = 1 Kyanite + 3 Quartz + 1 H20 ... (2). The schists and quartz vein assemblages are quartz-oversaturated in contrast to the study material of Wallmach and Meyer (1990) which is quartz-undersaturated. Peak metamorphic conditions, therefore, are closely constrained by the position of the reaction curve (2) in P-T space, as is also substantiated by the presence of coexisting kyanite and pyrophyllite which are closely associated with syn-tectonic vein-quartz at the Florida Lake, Monarch Shaft and Krugersdorp localities. The nature of and circumstances under which the equilibrium aSsemblage pyrophyllite + kyanite + quartz has formed support an univariant situation, i.e. this assemblage can only coexist along the pyrophyllite kyanite isograd. The mineral assemblages that equilibrated during peak metamorphism are still present in the rocks of the shear zones, and show only incipient rehydration. The quartz-oversaturated nature of the rocks in the shear zones and the fact that kyanite formation is ascribed to reaction (2), cannot explain the abundance of quartz veins. Accordingly it is concluded that there must have been an external source from which Si02 was imported into shear zone to give rise to the formation of the large quantities of vein-quartz.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:11306 |
| Date | 02 June 2014 |
| Creators | Coetzee, Dirk Stephanus |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Johannesburg |
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