Browsing by Author "Bester, Andeon"

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    Controls of magma emplacement and 3d modelling of cupriferous basic bodies of the Kuperberg suite at Narrap mine, Okiep copper district, Namaqualand
    (Stellenbosch : Stellenbosch University, 2023-12) Bester, Andeon; Kisters, Alexander; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The Okiep Copper District hosts more than 1800 copper-mineralised basic bodies that form part of the Koperberg Suite. These mafic magmas intruded the high-grade granite gneiss terrain of the Bushmanland Subprovince during a period of E-W extension (1060-1030 Ma) and peak, lower- granulite facies metamorphism. Basic bodies have different, commonly highly irregular geometries ranging from massive, steeply-plunging, carrot-shaped intrusions to thin stringers or foliation-parallel sills in the gneiss sequence. For the most part, however, basic bodies show subvertical E-W trending blade- or dyke-like geometries hosted by localised high-strain 'steep structures'. The close spatial relationship between basic bodies and steep structures highlights the importance of structural anisotropies as conduits facilitating mafic magma ascent through the stratified mid-crustal OCD. The 3D model of Narrap Mine's wall rocks and Koperberg Suite intrusions revealed three distinct structures: 1) a central cluster of vertical basic bodies cutting across various lithologies and emplacing along the vertical foliation of a prominent steep structure, 2) a subvertically orientated, laterally extensive collection of basic bodies emplaced along the upwarped lithological contact of the overlying Moddefontein Gneiss and underlying Wolfram Schist, and 3) subhorizontal penny-shaped sill-like bodies following the shallowly dipping regional gneissic layering. A commonality between all these structures is their discontinuous nature and small aspect ratios, indicative of buoyancy-driven, self- contained hydrofractures. Furthermore, these structures are congruent with mechanical anisotropies, be it lithological contacts or foliation, signifying the importance of anisotropies as magma propagation pathways. Additionally, the presence of multiple generations of Koperberg Suite magmas within a particular steep structure highlights the effectiveness of utilising steep structure fabrics as repeated pathways. This demonstrates that emplacement was independent of the regional stress field and occurred under low deviatoric stresses in the high-grade metamorphic, partially molten mid-crustal section of the OCD. In this case, structural anisotropies (foliations, lithological contacts, steep structure fabrics) determine the emplacement and orientation of basic bodies. The 3D model further indicates how the mechanically layered gneiss and interlayered metasediments impede the buoyancy-driven ascent of the Koperberg Suite magmas and modify their geometry. Basic bodies typically thicken and accumulate in schist and gneiss units as a result of the lower wall-rock rigidity. In contrast, the refractory Springbok Quartzite resists fracture dilation, yielding stringer-like geometries and the likely ponding of magmas below this unit. All these observations, including the three aforementioned structures, bear striking similarities to what is seen at the rest of the mines in the OCD, with the softer metasediment and gneisses above the Springbok Quartzite hosting the majority of the basic bodies mined historically. This stratigraphic position hosts the laterally branching structures, which have commonly emplaced along the underlying Wolfram Schist and overlying gneisses, and the subvertical clustered basic bodies that pinch and seemingly terminate against the upper contact of the Springbok Quartzite. Thus, future exploration should focus on the gneisses and schists hosting the branching structure and central cluster, in addition to following the down-plunge continuation of the central cluster below the quartzite as ponding is likely.