A research project submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of Science. / Rb-Sr emplacement ages of nineteen kimberlites from the Prieska Province vary from 74 to 174 Ma, Their isotopic, whole-rock geochemical signatures and perovskite REE distributions were also determined. Non-micaceous and micaceous kimberlites from the area show similar petrographic, geochemical and isotopic compositions relative to cratonic kimberlites, indicating similar sub-continental mantle source compositions in the two tectonic environments. Transitional varieties of kimberlite, which can be defined petrographically, geochemically and isotopically are also recognised from the area, but are not prevalent in the cratonic environment. The Prieska Province kimberlites are possibly derived by partial melting processes within a subcontinental reservoir characterised by a spectrum of compositions from time-averaged depleted (HllMU) to enriched (Group II). The occurrence of these components in both on- and off-craton settings indicates that the kimberlite source area is not strictly linked to the suberatonic lithosphere. Mantle' plume sources are not favoured because of the variable source compositions and distribution of emplacement ages within the Province. Transitional kimberlites were derived from a source component with mixed character, not yet noted from within the Kaapvaal Craton. This is the first documented example of isotopically transitional kimberlites from southern Africa. / AC2017

A comprehensive study has been made of a large collection of metasomatised peridotite, and MARIO (Mica-Amphibole-ButileIlmenite-Qiopside) xenoliths from the Kimberley pipes, with the aim of obtaining an improved understanding of enrichment processes operating in the sub-continental lithospheric mantle. The metasomatised peridotite suite is divided into garnet peridotites (GP), which contain no texturally equilibrated or primary phlogopite, garnet phlogopite peridotites (GPP), which constitute the most abundant peridotites, phlogopite peridotites (PP) and phlogopite K-richterite peridotites (PKP). Diopside can be present in all four groups. PKP rocks may also contain exotic incompatible element-rich Crtitanates such as lindsleyite and mathiasite, Nb-Cr rutile, ilmenite and armalcolite. Petrographic and chemical evidence presented here suggest that metasomatism increases progressively in the sequence GPGPP-PP-PKP, with the PKP group being richest in Fe, Ti and incompatible elements such as K, Na, Rb, Ba, Sr, Zr, Nb and the LR.EE. Mineral compositions change progressively from the garnet-bearing rocks to the PP and PKP rocks, showing decreases in Al 2o 3 and cr2o 3 contents, and increases in FeO and Tio2 contents. Data from this study show that most PKP and some PP rocks were derived from Al-deficient harzburgites. Other PP rocks probably had garnet-bearing precursors, as they display chemical similarities to GPP rocks, and typically contain aggregates of phlogopitediopside-spinel which are interpreted and modelled as being garnet replacement textures. Pressures and temperatures of equilibration of the peridotites in this study, combined with published experimental mineral stability data, suggest that metasomatism increases in intensity upwards in the subcontinental lithospheric mantle (SCLM) over a considerable depth interval from _170 km to Sr isotope data suggest that the metasomatism is young (1 Ga) have been recorded by Nd isotopes. New and published peridotite mineral and whole-rock Nd and Sr isotopic compositions range from moderately depleted to highly enriched and are interpreted in terms of mixing between variably enriched "ancient" SCLM and young metasomatic fluids with isotopic compositions close to Bulk Earth values. A rnineralogic expression of the ancient enrichments which might suggest that they resulted from older metasornatic events, cannot be clearly identified, but is best represented by enriched Nd-isotopic compositions of nonmetasornatic garnet. Rare phlogopites with low 1 43 Nd/144Nd ratios are interpreted as the product of complex mixing between the young metasomatic fluids and old enriched SCLM, rather than as older phlogopite. A group of unusually diopside-rich peridotites (±Phlogopite, ±ilmenite), are described and discussed. They are probably not directly related to the main group of metasomatised peridotites. Their chemical differences and greater pressures and temperatures of equilibration are consistent with formation by interaction between a diapir of asthenospheric melt and surrounding peridotitic mantle at the base of the SCLM. Comparison with chemical and isotopic data from the literature for Cr-rich "granny-smith" diopside rnegacrysts and glimmerites (phlogopite-diopside-ilmenite aggregates), suggests that they may have close genetic links with the diopside-rich suite. MARIO nodules are modally dominated by phlogopite, Krichterite and diopside, with lesser ilmenite and rutile, and accessory minerals such as calcite, barite and apatite. Olivine is absent, as is (with one exception) orthopyroxene, which serves to distinguish MARIO rocks from highly metasornatised peridotites. Relative modal proportions, textures and grain-size of MARIO rocks vary considerably, both within and between nodules. Textures suggest that they are igneous cumulates. The maximum stability depth of Krichterite restricts their depths of crystallisation to <120km. Data obtained in this study show that MARIO mineral compositions are Fe and Ti rich compared to most peridotites, and are much poorer in Cr, Al, Mg and Ni. Bulk compositions are alkali- and magnesian-rich (K20=4-9 weight%, Mg0=20-25 weight%), with moderate to high concentrations of i ncompatible trace elements. New and published MARIO mineral Nd and Sr isotopic measurements ranges from slightly depleted to highly enriched compositions. They are interpreted in terms of mixing of recent (phlogopite metasomatism is presented. The model proposes that there was recent input of metasomatic fluids at two distinct levels in the sub-continental mantle lithosphere. (1) Deep level fluids were generated at the base of the subcontinental lithosphere between 170-200 km depth - l l I ) possibly evolved from failed asthenospheric melts. The intensity of metasomatism progressively increased as these fluids percolated upwards to _100 km depth, and the GPP and some of the PP rocks were formed. (2) Shallow level metasomatic fluids were evolved during the crystallisation of MA.RID magmas ponded at 60-100 km depth. They were released into the surrounding SCLM, and formed the PKP and some PP rocks. These MA.RID-derived fluids appear to have overprinted the deep-derived metasomatism in places; some PP rocks have metasomatic signatures with characteristics of both events, which is interpreted as evidence for the passage of both types of fluid through them. Various lines of evidence including variations in mineral and whole-rock chemistry, suggest that both types of fluids contained Si, Al, Fetotal, Fe 3+, Ti, ±Ca, incompatible elements such as K, Na, Rb, Ba, Sr, Zr, Nb, V, LREE, s, and possibly F and Cl. The MARIO-derived fluids may have been generally richer in these elements, as the PKP rocks are more intensely metasomatised than even the most metasomatised GPP. u and related PP rocks. The behaviqr of Ba appears to have been different in the two sources/fluids - the deeper-derived fluids produced Ba-rich phlogopites, whereas the MARID magmas and MARID-derived fluids resulted in the formation of Ba-poor phlogopites in MA.RID and PKP rocks. Modelling of fluid compositions using published crystal/melt and crystal/fluid partition coefficients and PKP mineral trace element abundances is inconclusive but suggests that the MARIDderived fluids (and possibly the deep-derived fluids as well), were strongly LREE-enriched with REE patterns comparable to lamproites or kimberlites, and with moderately high Rb and Ba contents. However, inferred concentrations of Sr in the metasomatic fluids are of the order of 1% or more. The PKP rocks have attracted more attention in the literature than the GPP and PP groups, due to their spectacular J metasomatic assemblages. However, it is worth noting that they are relatively rare (~10% of peridotites from the Kimberley pipes). The ¥.LA.RID-derived metasomatism was probably intense, but strongly localised. In contrast the GPP and PP rocks are extremely abundant at Kimberley (_50% and _30% respectively) and were equilibrated over a large depth interval (170-100 km). The deep-derived metasomatism therefore appears to have been much more pervasive, and although less intense (most GPP rocks have 1-2% phlogopite) , it is considered to be a more significant phenomenon in the context of mantle metasomatic processes, especially as it appears to predate host kimberlite formation and emplacement.

Thesis (M.Sc.)--University of the Witwatersrand, Science Faculty (Geology), 1997. / The Sterkspruit Intrusion, in the south-western portion of the Barberton greenstone belt, is a sill-like body containing rocks of gabbroic to dioritic composition. It is hosted by a sequence of komatiitic basalts and komatiites of the Lower Onverwacht Group. The intrusion is considered unique in this area in that it lacks ultramafic components and has no affinities with the surrounding mafic- to- ultramafic lavas. The gabbroic suite also contains an unusual abundance of quartz, and the chill margin shows an evolved quartz-normative, tholeiitic parental magma. Based on petrographic and geochemical evidence, the intrusion can be subdivided into four gabbroic zones and a quartz diorite, which is an end product of a differentiating magma. The chill margin records an MgO content of 4.8%, an Mg# of 42, an Si02 value of 52.5% and a normative plagioclase composition of An44. The sill-like nature of the body, indicated by geochemical trends, and the steep sub-vertical layering, point to a body that has been tilted along with the surrounding lavas. This constrains the relative age of the body to be older than the emplacement of the Kaap Valley and Nelshoogte Plutons, which caused the regional deformation observed in the Nelshoogte Schist Belt. A sequence of tholeiitic to andesitic basalts from the Kromberg Formation in the Upper Onverwacht Group, have compositions which can be correlated with the Sterkspruit Intrusion. It is possible that this body represents a subvolcanic magma chamber, which acted as a feeder to tholeiitic lavas higher up in the volcanic sequence. The Sterkspruit Mafic Dyke Swarm intruded the southern Nelshoogte Schist Belt and displays intra-dyke chemical variation. / AC2017

A dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. / The Sterkspruit Intrusion, in the south-western portion of the Barberton greenstone belt, is a sill-like body containing rocks of gabbroic to dioritic composition, It is hosted by a sequence of komatilitic basalts and komatilites of the Lower Onverwacht Group. The intrusion is considered unique in this area in that it lacks ultramafic components and has no affinities with the surrounding mafic- to- ultramafic lavas, The gabbroic suite also contains an unusual abundance of quartz, and the chill margin shows an evolved quartz-normative, tholelitic parental magma. ( Abbreviation abstract ) / AC2017

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

D.Phil. (Geology) / Please refer to full text to view abstract

The 2.9 Ga old Usushwana Complex in the Piet Retief-Amsterdam area (south-eastern Transvaal) represents an exposed segment of a layered intrusion. It has the form of a dyke-like body elongated in a northwesterly direction, and extends to an estimated depth of 3000 -5500 m. Lithologically, the Complex consists of a cumulate succession of mafic rocks capped by granitoids and has intruded along the contact between the basement and the supracrustal sequences of the Kaapvaal Craton. Differentiation of an already contaminated gabbroic magma resulted in an ordered stratigraphic sequence comprising progressively more evolved lithotypes, with at least two imperfect cyclic units developed over a stratigraphic thickness of about 700 metres (Hlelo River Section). Meso- to orthocumulate textured gabbros and quartz gabbros grade upwards into magnetite- and apatite-bearing quartz gabbros, interlayered with discontinuous magnetitite horizons. The gabbros in turn grade into hornblende-rich, granophyric granodiorites. The differentiation process is regarded as having been considerably enhanced by the assimilation of acidic material, derived by partial melting of the felsic country rocks at the roof of the magma chamber. Recrystallisation of these rocks gave rise to the microgranites that locally overlie the granodiorites. Mineralogical, textural and geochemical features indicate a relatively advanced fractionation stage, suggesting that the exposed sequence of the Usushwana Complex in the study area represents the upper portion of the intrusion. No significant mineralised occurrences were identified. However, on the basis of similarities between the Usushwana Complex and other mafic layered intrusions which host significant ore deposits, it is suggested that economic concentrations of base metal(Cu-Ni) sulphides, PGE and chromitites are likely to be developed at lower stratigraphic levels.

Bibliography: pages 100-109. / The Kaap Valley Tonalite (KVT) was a homogenous hornblende+ biotite tonalite that had been subjected to overail propylitic and potassic alteration. Petrographic studies reveal that carbonate alteration and increased proportions of hydrous minerals occur along the KVT and Barberton greenstone belt contact zone, and along major shear zones within the KVT.

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy, / The Ventersdorp Contact Reef(VCR) is a major gold-bearing reef in the Witwatersrand Basin. It occurs between the overlying Klipriviersberg Group lavas and the underlying Central Rand Group sediments, and was strongly altered by hydrothermal fluids circulating in the Witwatersrand Basin. A detailed study of the mineralogy, geochemistry of rocks and minerals, physicochemical conditions, stable isotopes and ages of hydrothermal alteration zones associated with the VCR were carried out at Western Deep Levels South Mine, South Africa. ( Abbreviation abstract) / Andrew Chakane 2019

Submitted in fulfilment of the requirements for a Master of Science degree in Geology, in the Department of Geosciences, University of the Witwatersrand, Johannesburg, South Africa. 2015 / This study undertook to re-evaluate the conventional historic interpretation that accepted the Marginal Zone as representative of the chill phase to the earliest emplacement of Lower Zone magmas. The Clapham Trough preserves a thick sequence of the Marginal Zone rocks, at least 220 m thick. Poor exposures and incomplete stratigraphy of the rock succession that occurs between the floor and the Marginal Zone rocks presented great limitations to earlier studies, and led earlier workers to accepting that the base of the Bushveld Complex is the Marginal Zone norite. This study presents results from the 692 m CH6 drilled core, which intersects the Marginal-Lower Zone boundary in the Clapham Trough. The base of the CH6 drill core consists of melanorite (with less than 40 % cumulus plagioclase), which is conformable with the underlying, thick Basal Ultramafic Sequence (BUS, described in Wilson and Chunnett, 2010; and Wilson, 2012) separating the Marginal Zone rocks with the floor rocks of the Magaliesberg Formation. The amount of cumulus plagioclase in the Marginal Zone increases with increasing stratigraphic height such that the top units of the Marginal Zone are norite-leuconorites (typically 45-65 % cumulus plagioclase), bordering on anorthosite. The progressive changes in the modal variations led to the subdivision of the Marginal Zone norite to a basal Mafic Norite and a xenolith-bearing Shelter Norite. The latter is deemed a correlative of the Xenolithic Norite described at Olifants River Trough. Coupled with the increasing amount of cumulus plagioclase, the An# increases with stratigraphic height. The An# fractionation trend is reversed from that of the co-existing orthopyroxene observed in the same interval (An63-74 vs. En81-70). The reversed An# compositions are an abnormal differentiation trend. The compositional disequilibrium between co-existing orthopyroxene and plagioclase formed from in-situ crystallization with floatation of plagioclase, through convection, separating the cotectic phases. All the data in the Marginal Zone show that these rocks have continuous fractionation trends with no interruptions. The Marginal Zone rocks are cumulus rocks that formed through fractional crystallization in a temporarily closed magma chamber. The present work showed unequivocally that the Marginal Zone is a product of differentiation of earlier emplacement of B1-magma, and cannot be representative of either a chill zone or composite sills. The appropriate (parental) liquid composition of the Marginal Zone formed after 30 % crystallization of the B1-magma. The postulated liquid composition is 6 wt. % MgO and 56.7 wt. % SiO2. The entire Marginal Zone succession would have formed from about 30-54 % crystallization of the B1-magma. The crystallization of the Marginal Zone was ended abruptly by the emplacement of a new batch of B1-magma, which must have mixed with the residual magma that must have ponded atop Marginal Zone cumulates after 54 % crystallization. The mixing of the evolved residual magma and the primitive B1-magma formed the liquid postulated to be parental to the Lower Zone A (10.59 wt. % MgO and 57.10 wt. % SiO2). The Transitional Pyroxenite bears all the evidence of mixing between magmas of contrasting compositions, forming the 10-30 m gradational boundary unit between the Marginal Zone and Lower Zone A (correlative of the Lower Orthopyroxenite Subzone described at Olifants River Trough). The Lower Orthopyroxenite Subzone at the Clapham Trough is almost a mono-mineralic rock succession with generally constant orthopyroxene composition (En87-86), with the exceptions at marker norite horizons (En84-82; An83-81). The constant compositions observed in Lower Zone A are attributed to contemporaneous emplacement of new magma and differentiation, which maintained the composition of the parental magma.