Thesis (M.Sc. (Geology)) - Rhodes University, 2007. / A thesis submitted in partial fulfillment of the requirements for the degree of Masters of Science.

M.Sc. (Geology) / Please refer to full text to view abstract

The Pennian (~ 245 Ma) Jwaneng kimberlite, situated in southern Botswana, comprises three steep-sided pipes that coalesce approximately 100m below the present day surface to fonn a 54ha body. These pipes have been labelled the South, Centre and North Lobes. The kimberlites intruded a thick sequence of Proterozoic shales, dolomites and sandstones and a thin veneer of consolidated to poorly consolidated mudstones and siltstones of the Karoo Supergroup. Although the shapes of these pipes are comparable to other southern African pipes, they are filled predominantly with crater facies volcaniclastic kimberlite. No tuffisitic kimberlite breccia, the characteristic rock type of the diatremes of other southern African pipes, has yet been identified. The Jwaneng kimberlite thus represents an exception to the standard model for southern African kimberlites, implying that different processes need to be invoked to explain its fonnation. The present study involves a detailed volcanological and sedimentological analysis of the volcaniclastic fill of the Jwaneng South Lobe. Two principal and distinct lithofacies have been identified: the quartz-free RVK facies and the quartz-bearing QRVK facies. Both facies include fine to coarse grained, predominantly massive and subordinate chaotically bedded deposits. The volcaniclastic rocks have been classified as resedimented volcaniclastic kimberlite (RVK) , since their deposition is ascribed to mechanisms dominated by mass flow processes. Based on certain characteristics and differences between the two principal facies, and their spatial distribution within the pipe, they are interpreted as being the products of at least two separate eruption episodes. Certain characteristics (e.g. }hape, granularity~ of the juvenile· magma clasts III the volcaniclastic kimberlite suggest complete crystallisation and devolatilisation of the magma at depth prior to explosive fragmentation. A scenario in which this might have occurred, and which led to catastrophic explosive eruption and pipe excavation is proposed. Explosive eruption and associated tuff cone formation is followed by resedimentation of the material back into the pipe by mass flow processes. Mass flow processes are dominated by debris flow, with lesser grain flow, hyperconcentrated flow and subaqueous mud flow and suspension settling of muddy kimberlitic sediments. Geochemical analyses of the latter indicate a high degree of contamination and weathering, and mixing between pristine kimberlite and silicic shale/mud compositions. Failure and collapse of parts of the underlying pipe walls yielded megablocks of poorly consolidated Permian Karoo mudstone in the peripheral zone of the pipe. This source of the megablocks is supported by their bulk chemical composition. Minor phreatic/phreatomagmatic eruptions are suggested by the presence of rare accretionary and armoured lapilli within both the QRVK and RVK facies. Subsidence of the volcaniclastic pipe fill, inferred mainly from the oversteepened dips of the bedded QRVK and RVK facies, may be related to gravity-induced compaction, late-stage phreatomagmatic eruptions or eruption ofthe adjacent Centre Lobe.

No description available.

Mantle derived rock and mineral fragments from the Koffiefontein Kimberlite Pipe have been studied in the form of xenoliths, megacrysts, concentrate minerals and inclusions in diamonds. Mantle derived xenoliths are unusuaLly scarce. The predominant xenoliths are pyroxenites. Peridotites are rare and eclogites are very rare. The pyroxenites have been assigned to five categories. In general, they show a clear metamorphic textural history with widespread evidence of exsolution, polygonisation and deformation. Some of these pyroxenites are interpreted to represent the crystallisation products of basaltic magma at depth. Some have been metasomatised. The pyroxene chemistries suggest that more than one pyroxenite body has been sampled. The peridotites are very similar to the common peridotites of N. Lesotho and are thought to represent depleted mantle. The Koffiefontein diamonds contain both eclogitic and peridotitic inclusions. The inclusions show evidence for lack of equilibration, and one appears to link tog~ther the two parageneses mentioned above. The presence of (MgFe)O as an inclusion is reported. The megacrysts studied consisted only of garnets, and a few pyroxenes. They are interpreted to have crystallised from a magma which cooled slowly and was relatively deficient in Tio2 to account for the scarcity of ilmenite. The heavy mineral concentrate contained garnet, orthopyroxene, opaque minerals and clinopyroxene in order of abundance. The opaque minerals were chromite and ilmenite. The chemical compositions of most of the concentrate grains do not match the mineral compositions found in xenolith s, or megacrysts. A small number of concentrate grains have compositions ma tched by the diamond inclusions. Attempts to a pply geothe r mobarometry to the pyroxenites and to the diamond inclusions gave conflicting results which areascribe d to lack of equilibration between co-existing mineral phases and/or within individual mineral grains. The Koffiefontein xenolith suite is unusual for the relative abundance of pyroxenite and the scarcity of peridotite. The concentrate is unusual for the relative abundance of orthopyroxene particularly and of chromi te.

No description available.

187Re-187Os analysis of magnetite and other oxide minerals from various well dated geological settings (kimberlites, lamprophyres, Manicouagan impact melt rock, Great Bear magmatic zone iron ore deposits and Laramie Anorthosite complex) worldwide have been used to investigate the potential for precise geochronology. 187Re-187Os isotopes in groundmass oxide minerals from kimberlites and lamprophyres are not suitable for emplacement geochronology due to low Re contents and large quantities of un-radiogenic Os. TRD and TMA ages can be calculated however, and correlate well with previously published depletion ages and large magmatic events in the overlying crust, indicating the effectiveness of this technique. Geochronology using magnetite from the remaining settings is im-precise due to very low Re and Os contents but the ages produced are in the correct area. Young ages produced from the Laramie anorthosite complex indicate that magnetite may have a low closure temperature to Os diffusion (~200-300C).

Thesis (Ph.D.)(Metallurgical Engineering)--University of Pretoria, 2006. / Includes summary. Includes bibliographical references. Available on the Internet via the World Wide Web.

Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: Insets of large anhedral minerals in kimberlites are proposed to mainly have their origin from the disaggregation of mantle-derived xenoliths through mechanical abrasion by the ascending kimberlite magma. Garnet, despite being an important constituent of both mantle-derived peridotite and eclogite, forms only a minor constituent of kimberlites. This suggests that a large proportion of garnet liberated into the kimberlite through the disaggregation of such xenoliths, is consumed before the emplacement of kimberlite. This study records the breakdown mechanism of garnet by the ascending kimberlite magma by conducting experiments between pressures of 2 and 4 GPa and temperatures between 1100 and 1300°C. The starting material used in the experiments was prepared from a natural hypabyssal kimberlite that closely resembles the composition of a proposed primary Group I kimberlite magma. To the kimberlite material 5 wt % garnet, sourced from a natural peridotite, was added. It was found that garnet is not a stable equilibrium phase within the kimberlite magma under the investigated temperature and pressure conditions. Based on large volumes of phlogopite in the experiments it is concluded that garnet melts incongruently in the kimberlite magma to form phlogopite as a peritectic product and melts transitional in composition between silicate and carbonate melts. This is in contrast to more SiO2 -rich melt compositions produced as a consequence of the incongruent breakdown of both orthopyroxene and omphacite within kimberlite magmas. The consequence of the melt compositions produced in the experiments is increased solubility of CO2 in the form of carbonate (CO32-). This finding has implications for both kimberlite ascent mechanisms and the solubility of diamond transported within kimberlite magmas. / AFRIKAANSE OPSOMMING: Dit word voorgestel dat groot oneenvormige mineraalinsluitsels in kimberliet hoofsaaklik afkomstig is van xenolitiese mantelmateriaal wat verbrokkel weens die meganiese werking van die stygende kimberliet magma. Ten spyte daarvan dat granaat ‘n belangrike komponent is van peridotiet en eklogiet xenoliete afkomstig uit die mantel, vorm granaat slegs ‘n geringe deel van die kimberliet mineraalsamestelling. Dit dui daarop dat ‘n beduidende gedeelte van die granaat wat vrygstel word in die kimberliet magma deur die verbrokkeling van xenolitiese materiaal, afgebreek word deur die stygende kimberliet magma voordat dit stol. Hierdie studie ondersoek die afbreekmeganisme van granaat afkomstig van die mantel in die stygende kimberliet magma deur eksperimente uit te voer by drukke tussen 2 en 4GPa en temperature tussen 1100 en 1300°C. Die materiaal met die aanvang van die eksperimente was voorberei uit ‘n natuurlike hipabissale kimberliet wat soortgelyk is in samestelling as ‘n voorgestelde primêre Groep I kimberliet magma se samesteling. Vyf gewigspresent (5 wt %) granaat, verkry van n natuurlike peridotiet, is bygevoeg tot die kimberlietmateriaal. Daar is gevind dat granaat nie ‘n stabiele ewewigsfase is in die kimberliet magma binne die temperatuur-en drukstoestande ondersoek nie. Groot volumes flogopiet teenwoordig in die eksperimente dui daarop dat granaat inkongruent smelt in die kimberliet magma om flogopiet as ‘n peritektiese produk te vorm te same met ‘n smelt wat neig na ‘n karbonatitese smeltsamestelling. Die bevinding is in teenstelling met meer SiO2-ryke smeltsamestellings as gevolg van die inkongruente afbreek van beide ortopirokseen en omfasiet in die kimberliet magma. Die gevolg van die meer karbonatitese smeltsamestellings, is ‘n toename in die oplosbaarheid van CO2 in die smelt in die vorm van karbonaat (CO32-). Die bevinding het nagevolge vir beide kimberliet stygingsmeganismes asook die oplosbaarheid van diamant tydens die vervoer van diamant deur kimberliet magmas.

Master of Science in Engineering - Engineering / The most significant benefit of co-disposal of tailings based on the “Paste and Thickened Tailings Disposal” concept is the improved ability to “design” the properties of the co-disposed tailings material to suit the surrounding environment. The overall aim of this study was to gain a better understanding of the relationship between the key slurry material characteristics and the flow behaviour of co-disposed tailings upon deposition, for the case of montmorillonite clay-based kimberlite tailings. A fundamental understanding of this relationship will enable the successful manipulation and exploitation of the co-disposed tailings rheology for optimal tailings disposal and the minimisation of associated financial, environmental and social risks. The key material characteristics selected for investigation were the vehicle solids concentration, suspension pH and vehicle to load ratio. The yield stress was selected as the key rheological property representing both the vehicle component rheology and co-disposed tailings rheology. Two yield stress measurement techniques were used, namely (1) direct yield stress measurement with the vane method and (2) indirect yield stress measurement with the slump test method. The correlation between these two methods was investigated as a secondary objective of this study. It was concluded that the suspension pH strongly influences the degree of microscopic particle interaction of the vehicle component and that manipulation of the suspension pH could move the material between interactive and noninteractive states. It was further concluded that increasing load mass percentage leads to a significant increase in the co-disposed material yield stress. It is believed that the load component mainly affects the co-disposed material yield stress through its contribution to the total solids concentration, which in turn results in an exponential increase in the material yield stress. The findings of this study showed remarkable flexibility in the manipulation of the various input parameters to produce the same yield stress value. It is therefore now possible to maintain a constant yield stress value as required by the environmental depositional requirements through various combinations of the input parameters and so keep the integrity of the deposition site intact. The correlation obtained in this study between the vane and slump test yield stress measurement techniques was fairly poor. The slump test only provided an accurate prediction of the yield stress when the material was in a highly interactive state. It is recommended that future research focuses on the thixotropic nature of the vehicle component as a function of suspension pH, the accuracy of the correlation between the vane and slump measured yield stress and the effect of the load on the bulk rheology of the co-disposed material.