M.Sc. / This thesis involves a study ofthe geology surrounding the —530 Ma to —519 Ma Venetia kimberlite pipes situated between AIldays and Messina (now renamed Musina) in the Beit Bridge Terrane of the Limpopo Belt, South Africa. The Limpopo Belt is an eastnortheast trending high grade metamorphic terrane thought until recently to be the result ofa collisional event between the Kaapv_aal and Zimbabwe Cratons between 2.7 and 2.65 Ga. However, recent studies have challenged this concept and suggest that the assembly was more complex and took place over an extended period of time ending at —2.04 Ga. This study involved surface mapping of the Farms Rugen (south) and Ostrolenca, providing additional information to help with mine planning, grade control and exploration. It forms a portion of a project initiated between Venetia Mine, the Venetia- Limpopo Nature Reserve and Professor Jay Barton of RAU to geologically map in detail the area around the pipes (scale < 1:10 000) and to study various aspects of the regional geology. The rock types into which the Venetia kimberlite pipes intruded belong to the Venetia klippe, an east-west trending synclinal structure with the axial plane dipping steeply northwards. Lithologically, the Venetia klippe comprises four layered units in which interlayered granitic or arkosic quartzofeldspathic gneisses, with and without biotite and garnet, and para and ortho-amphibolite, quartzite and meta-carbonate rocks (marble and limestone), banded iron formation and calc-silicate rock occur. Geochemical analysis (SEM and electron microprobe) of the meta-carbonates (re-crystallised magnesian limestone, coarse-grained marble and fine-grained foliated marble), indicate the precursors to be magnesian limestone, dolomite and limestone. Several events have been identified during the structural evolution of the area. They include: formation of gneissic metamorphic layering, tectonic suturing between different lithologies, formation of a syncline and east-west strike-slip faulting, north-south trending folds and northeast-southwest dextral strike-slip faulting, tourmaline bearing pegmatite emplacement, dolerite intrusion, tourmaline absent pegmatite emplacement, kimberlite emplacement and reactivation of pre-existing structures. Depositional structures only in the fine-grained foliated marble are preserved, e.g. graded bedding, cross-bedding, rip-up clasts and channels. These structures suggest deposition of the carbonates in two main depositional environments; peritidal (channels and rip-up clasts) and subtidal shelf (graded bedding and cross-bedding). A study of pegmatites in the area shows two main pegmatite types: tourmaline bearing and tormaline absent, each locally displaying a zonation. Mineralogically, the pegmaties are rich in quartz and albite and lack K-feldspar so they are classified as sodic-rich or plagio-pegmatites. Step heating 40Ar/39Ar analyses of muscovite from undeformed pegmatite yields an age of —2.0 Ga, which is interpreted to represent the time of pegmatite emplacement into the Venetia klippe rocks. Structurally, the pegmatites are sheet-like bodies cross-cutting compositional layering, joints, faults, folds and the dolerite, except for the older tourmaline bearing pegmatite that has intruded along east-west faults, but not northeast-southwest trending faults. By applying the principles of a dike propagation model, the source of the Venetia pegmatites should be greater than 5X5X5 km in volume and a maximum of 10km away. An appropriate granitic source has been recognized on the farm Gotha to the south of the mine by Martina Barnett. Leucocratic granodiorite, tonalite and granite with minor xenoliths of amphibolite, quartzite and magnetite quartzite define the Gotha Granitic Complex and pegmatite decreases in abundance away from it to the north and east. Deposition of Unit 3 lithologies into a rifted basin and an ancient epeiric sea is possible. However, there is more evidence (peritidal and shelf environments of the metacarbonates) and clean mature quartzites to suggest deposition into a passive continental margin or epeiric sea similar to the Malmani dolomites of the Transvaal Supergroup.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:9473 |
| Date | 16 August 2012 |
| Creators | Twiggs, C. |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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