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.

La région de Conselheiro Pena et Galileia est située dans l'est de l'état de Minas Gerais (sud-est du Brésil). Elle fait partie du nord de la ceinture de cisaillement atlantique et est formée par des gneiss d'âge protérozoique inférieur et par des séquences supracrustales considérées d'âge néoprotérozoique. Deux phases de déformation et un métamorphisme de moyen degré ont affecté la région pendant le cycle Brasiliano. La phase D1 s'associe à des mouvements décrochants dextres. Cette phase a été suivie tardivement par la phase D2, caracterisée par une phase d'extension. La suite Galileia et la suite Urucum ont une ascension et mise en place associées à la déformation D1. Les granitoïdes de la suite Galileia (594±6 Ma) sont constitués par des tonalités, tonalogranodiorites, granodiorites, granites et microgranites. Dans l'ensemble, ces granitoïdes sont métalumineux à peralumineux avec une affinité calco-alcaline. Ils ont eu une origine sous des pressions d'environ 10 kbars et des températures entre 850°C et 700°C. L'étude isotopique indique une origine à partir de la fusion de roches crustales. Les leucogranites de la suite Urucum (582±2 Ma) sont représentés par des granites peralumineux. Elles ont été cristallisées entre 750°C et 600°C et sous des pressions de l'ordre de 4 KBars dans un environnement riche en fluides. L'origine de cette suite à partir de la fusion de roches crustales est indiquée par les études isotopiques, par la composition et typologie des zircons et aussi par la présence commune de curs de zircons hérités. Les observations de terrain et les études minéralogiques et géochimiques montrent qu'il existe une relation étroite entre les leucogranites de la suite Urucum et les pegmatites à éléments rares de la région. Nous pouvons envisager un modèle de genèse évoquant une fusion du manteau et, en suite, de la croûte due à l'épaississement des marges continentales sous un régime tectonique transpressional associé à la fin du cycle Brasiliano.