2022 Graduate Asbtracts

 

Researcher: Kwame Fynn (Hons, UJ)

Supervisor: Professor Axel Hofmann

Title: Investigation of drill core (NMDD 024) from the Nangodi Greenstone Belt of NW Ghana

 

Abstract

One of the recent gold discoveries in Ghana is the Namdini Gold Project in the northern part of the country situated within the Bole-Nangodi greenstone belt. The Bole-Nangodi greenstone belt consists of deformed rocks comprising a lower sedimentary assemblage of greywacke, shale, conglomerate and siliceous rocks, overlain by a basaltic unit with minor dacitic and felsic rocks and some chert. Mn-rich sedimentary rocks separate the lower and upper units. The belt is flanked on both sides by granodiorite and dioritic/granodioritic gneissic intrusions. RC and diamond drilling in the area has led to the estimation of a 5.1 Moz ore reserve. Studies have been done on 12 samples from one of the drill cores (NMDD024) from the Namdini area. These include petrographic analysis of the rocks, whole rock major and trace element geochemistry and ore mineralogy and mineral chemistry. The results from the various analyses presented in this work show that the gold mineralization is associated with strongly sheared arc-type meta-andesite and meta-dacite/-rhyolite. The general implication is that the gold mineralization is restricted to shear zones characterized by carbonate-quartz veins that may have acted as fluid pathways for the precipitation of sulphides hosting the gold. Compositional zoning in sulphides is interpreted as due to the evolution of mineralizing fluids from As-poor to As-rich (high grade ore), forming early-stage pyrite/arsenopyrite, followed by the deposition of As-poor, late-stage pyrite. Oriented sulphides, pressure shadows and deformed veins all suggest that the mineralization was syn-deformational (coeval to D1 deformation).

 

 

Researcher: Bridgette Murathi (MSc, UJ)

Supervisors: Dr Marvin Moroeng, Professor Nicola Wagner

Title: The effect of a dolerite intrusion on the mineralogy and geochemistry of coals from Manungu Colliery, Witbank Coalfield, South Africa

 

Abstract

South African Permian coals of the Main Karoo Basin are affected by Jurassic-age dolerite intrusions associated with the emplacement of the Drakensburg lavas. This study investigates the effects of a dolerite intrusion on the petrography, mineral, trace, and rare earth element (REE) compositions of dyke-affected, inertinite-rich coals from Manungu Colliery, located in the western limb of the Witbank Coalfield, South Africa. Coal samples were collected from the No. 2 Seam at 0.25 m intervals (to a cumulative distance of 1.25 m) on either side of a 0.40 m thick dolerite dyke. An unaltered coal (UN) was included as a control. In addition to the coals, a sample of the dolerite was also collected. This was done in order to compare the mineralogy and geochemistry of the intruded coals and the dolerite. The samples were analyzed using: (1) coal petrography (for maceral group composition and changes in reflectance); (2) coal quality analyses (proximate, ultimate, and calorific value); (3) X-Ray Diffraction (XRD); (4) X-Ray Fluorescence (XRF); (5) Inductively Coupled Plasma Mass Spectroscopy (ICP-MS); and (6) Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM-EDS). Both petrography and geochemistry appear to vary between the eastern and western side. The intruded samples were dominated by inertinite (between 54.2 vol.% (volume percentage) and 77.5 vol.%). The unaltered sample (UN) was also dominated by inertinite (75.5 vol%). Both mean random total reflectance (%RoTmr) and vitrinite reflectance (%RoVmr) was done. Due to low content of vitrinite, %RoTmr was used to compare the maturity of the intruded coal samples. %RoTmr increased from a background value of 1.65 %RoTmr to 3.82 %RoTmr towards the contact of the dolerite dyke on the western side. In comparison, the eastern side recorded lower %RoTmr values, a finding that was unexpected. Similar to published literature, carbonate minerals dominated the mineralogy of the intruded coals. Specifically, dolomite in the intruded coals occur as a cleat and fracture infilling phase, consistent with an epigenetic mode of occurrence, suggesting the dolerite as the possible source. The western side appears to be more altered compared to the eastern side, indicating differential heavy conditions. Group C elements, associated with carbonate minerals, were enriched next to the dyke contact on the western side, indicating a magmatic source. The REE composition of the carbonate fraction was characterized by M-type enrichment with no distinctive anomaly, consistent with a hydrothermal source. Comparable studies in literature did not consider changes on both sides of the dolerite intrusion.

 

 

Researcher: Phumele Mashele (MSc, Wits)

Supervisor: Professor Judith Kinnaird

Title: The geology of selected sites along the northern margin of the Barberton Greenstone Belt

 

Abstract

An increasing number of UNESCO World Heritage Sites are recognised for their geological significance. One such site forms part of the Barberton Greenstone Belt (BGB), which has been widely acknowledged as an outstanding site for Archean geology research. Though members of the geoscience community are familiar with the outstanding universal value of the Barberton Greenstone Belt, the people who live on it often do not. With the World Heritage Status (2018) comes an obligation to unpack the geological heritage of the Barberton region to local people and visitors, and to promote further research so the true value of the geological importance of the region may be better understood. For this reason, this project set out to interpret and contribute to concepts formulated from studies of BGB rocks to produce a thesis usable for generating various science communication media and topics for further research. Themes discussed in this thesis include granite-greenstone relationships, alteration of ultramafic rocks and the associated economic products, as well as deformation of various lithologies. Ease of access guided the selection of eight mostly roadside outcrops along the R38 secondary road linking Barberton to the southern gate of the Kruger National Park. These sites link up to form a geological trail along the northern margin of the Barberton Greenstone Belt, allowing visitors to easily interact with outcrops on their way to other popular attractions.

 

By driving along the R38 with science communication media produced from this project, locals and visitors will learn how geology shaped their world. They will gain insight on how mineable concentrations of verdite, talc and magnesite occur in certain ultramafic bodies. They will also learn of different ways in which rocks respond to stress as a common perception of rocks among laypersons is that they are solid, stable and unchanging.

 

 

Researcher: Patrick Richards (MSc, UJ)

Title: Petrographical, Mineralogical and Geochemical characterisation of the Leinster Deposit, Kalahari Manganese Field, South Africa.

 

Abstract

The world-renowned Paleoproterozoic Kalahari Manganese Field (KMF) is host to the world’s largest land-based manganese (Mn) deposit. The KMF is hosted within the Hotazel Formation, which comprises three chemosedimentary manganese ore beds interbedded with four superiortype iron formation beds. The Leinster Deposit is one of five erosional relics of the HotazelFormation, and the northernmost deposit of the KMF. As a relatively small deposit, comprising low manganese grades, little research has been conducted on the Leinster Deposit compared to the remaining four deposits that lie to the south of this deposit. This study provides a detailed geological investigation of the Leinster Deposit, from which samples were taken from five drill cores that were provided by Anglo American. The nature of the deposit, as well as post-depositional alteration has also been addressed. The samples were petrographically, mineralogically and geochemically characterised into one of four groups, namely: (1) high-grade Banded Iron Formation, (2) BIF, (3) Mn ore and (4) mafic intrusions. The high-grade BIF group exhibits the highest Fe2O3 content, linked to the highest abundance of magnetite. This group also shows clear evidence for fluid alteration, which has resulted in the development of secondary gangue phases, as well as the minor sulphide precipitation (both pyrite and chalcopyrite). Despite being in close proximity to several igneous intrusions within the area, mineralogical constraints within the high-grade BIF group indicate that this group did not experience temperatures exceeding 700°C. The Fe content (44 to 54 wt%), combined with the SiO2 content (up to 34 wt%), restricts these samples from being classified as iron ore, but rather represent slightly upgraded BIFs. Hydrothermal leaching of Si (from an alkaline fluid) resulted in moderate residual enrichment of iron and Rare Earth Elements (REE). Rare Earth Elements and Yttrium (REY) patterns show a high level of similarity to previously reported BIF samples from the Main Kalahari Manganese Deposit (MKMD), which include being Heavy Rare Earth Element (HREE) enriched, show positive Y and La anomalies, as well as having absent Ce anomalies. Together, these REY trends are indicative of a marine environment with a high temperature hydrothermal component. Of importance, is the consistent positive Eu anomaly that present in the samples from this study but is typically absent in Hotazel BIFs.

 

The BIF group has been characterised as having Fe contents less than 43 wt% with lesser abundances of magnetite, together with an increased variety of Fe-, Mg-, and Ca-bearing silicates, compared to the high-grade BIFs. This group of samples is geochemically very similar to the high-grade BIFs, including the positive Eu anomaly. However, these BIF samples are slightly depleted in in their total REE concentration compared to the high-grade BIF samples. The manganese ore samples display Mn metal contents that range between 17 and 35 wt%. The Leinster Deposit has been graphically depicted (in figures or drawings) as being a low-grade jacobsitic ore deposit by previous authors, however, only one of the ten samples that comprise this group was dominated by jacobsite. The remainder of the samples had low-grade manganese-bearing mineral phases such as kutnohorite, manganocalcite, friedelite and tephroite, and cannot be correlated to either of the three Mn beds of the Hotazel Formation. In conjunction with these phases, was the presence of substantial quantities of iron-bearing oxides, silicates and sulphides such as magnetite, andradite, and pyrite. Although the manganese ore group of samples was subjected to development of secondary Si and Fe-bearing phases, the ore has retained significant quantities of manganese, reaching grades as high as 35 wt% Mn. Comparisons of REY patterns of these samples are most similar to Mamatwan-type ore compared to Wessels-type ore; however, they also closely resemble the REY from the two previous BIF groups, suggesting a genetic link. These samples show positive La, Y and Eu anomalies, as well as an overall HREE enrichment. These samples do not show the negative Ce anomaly that is typical for Mn units of the Hotazel Formation. The geochemical resemblance between both manganese ore samples and the BIF groups, together with the absent negative Ce anomaly in the Mn ore samples from this study suggests that these samples are a type of variation between a low-grade Mn ore group and typical Hotazel BIF. The analysis of the intrusion samples is imperative to the characterisation of the Leinster Deposit due to the sheer volume of intrusive material within this deposit. There is a change in the mode of occurrence of these intrusions from the northern Avontuur and Leinster deposits, where they occur as thick dolerite sills, compared to the southern MKMD, where they occur as thin dykes. The study of these samples has resulted in the first detailed textural description of three phases of pyrophanite ([Mn,Fe]TiO3) for the KMF, as well as provide qualitative and semi-quantitative analyses for this group of samples. These samples are completely dominated by clinochlore suggesting complete alteration, and most likely represents the contact zone between the intrusion and country rock, which has acted as a natural fluid pathway. These intrusion samples are heavily depleted in large ion lithophile elements (LILE), once again suggesting fluid alteration by a low temperature, SO4 deficient meteoric fluid. REY patterns of the intrusion samples from the Leinster Deposit are comparable to intrusion samples originating from the Avontuur and MKMD deposits. The deposition of the chemical sedimentary units that comprise the Hotazel Formation, the Leinster Deposit has been subjected to large-scale intrusion, hydrothermal alteration, and lowgrade metamorphism, all of which have contributed to the unique mineralogical and geochemical character on this deposit. The high proportion of intrusive material has displaced, and resulted in the high temperature alteration of the surrounding country rock adjacent to these intrusions. Manganese ore beds are observed to closely resemble BIFs (geochemically), rather than one of the manganese ore types that have been previously characterised for ores of the KMF. The extent of the intrusions would make this deposit an unlikely future target for manganese exploitation.

 

 

Researcher: Floyd Mautle (MSc, UWC)

Supervisor: Dr Elizaveta Kovaleva

Title: Petrographic and geochemical evidence for the Vredefort proximal impact ejecta in the Northern Cape, South Africa

 

Abstract

The advancement of the understanding of hypervelocity impacts lies in discovering impact structures and both proximal and distal impact ejecta. This study uses petrographic and geochemical techniques to investigate the occurrence of Vredefort proximal impact ejecta in the Paleoproterozoic Maremane Dome, South Africa. Anomalous spherical objects ca. 15 mm in apparent diameter have been observed near the top of the Doornfontein Conglomerate Member in the Maremane Dome, South Africa. The objects were previously described as pisolites formed by laterization. The Doornfontein Conglomerate Member was thought to be a conglomerate unit formed by erosion and karstification of the Maremane Dome during the tectonic uplift. However, these interpretations appear to not fully explain most observations in the Doornfontein Conglomerate Member and anomalous objects and are therefore a concern. Through petrographic techniques intergraded with geochemical techniques, this work investigated 24 drill core samples with the anomalous objects from the Doornfontein Conglomerate Member in the Maremane Dome to test the hypothesis that the samples could be related to the Vredefort structure. The drill core samples are clay-rich and ferruginised matrix-supported breccias. The samples occur between the Hartley Formation (1.93 Ga) and Ongeluk Formation (2.22 ± 13 Ga), implying that the samples may occur within a permissible time of formation of the Vredefort structure (2.02 Ga). The samples are <5Rc (Re; distance between the centre of the crater and final rim of the crater) from the centre of the Vredefort structure, suggesting that the samples may occur within an expected distance from the Vredefort structure for Vredefort proximal impact ejecta. The samples appear to be similar to impact breccias in the Stac Fada impact ejecta layer. The anomalous objects in the samples appear to be accretionary lapilli and resemble accretionary lapilli formed by impact processes based on their large size, typically darker crusts and lithic rock fragments aligned typically parallel to the margins. The occurrence of the accretionary lapilli at the top of the Doornfontein Conglomerate Member is also analogous to the occurrence of accretionary lapilli at the top of the impact breccias in the Stac Fada impact ejecta layer.

 

Based on optical microscopy and scanning electron microscopy, the samples contain potential shocked quartz and zircon grains with possible planar deformation features and planar fractures induced by impacts. Low siderophile element abundances, especially Ni, Co, Cr and Ir, were detected in the samples using instrumental neutron activation analysis, but Ir was only detected in a single sample, measuring 0.58 ppb. Although the siderophile element abundances are low, they may result from an impact because the siderophile element abundances appear to be lowered and diluted by ferruginisation and hydrothermal fluids, which appear to have crystallised rutile and anatase determined by Raman spectroscopy. Due to the absence of sediment reworking, varying thickness and continuous intergradation without a grain size change in the lithofacies identified for the samples and the sample’s similarity to the impact breccias in the Stac Fada impact ejecta layer, the samples are suggested to have been emplaced as a ground-hugging density current ejecta that propagated outwards from the Vredefort impact site, forming the Vredefort proximal impact ejecta.

 

The proposed Vredefort proximal impact ejecta is the first described occurrence of the proximal impact ejecta associated with this impact structure. This is also true for the continuous ejecta blanket of the Vredefort structure. The proposed discovery of the Vredefort proximal impact ejecta and continuous ejecta blanket of the Vredefort structure will provide a glimpse of the Vredefort impact ejecta’s record because a considerable amount of this record is still unknown.

 

 

Researcher: Thapelo Motaung (MSc, UFS)

Supervisor: Professor Frederick Roefolse

Title: The Sr isotopic stratigraphy of the Eastern Limb of the Bushveld Complex

 

Abstract

Data on the modal mineralogy, whole-rock geochemistry, mineral chemistry and Srisotopic compositions of plagioclase from the broadly pyroxenitic Lower Critical Zone to the gabbroic Upper Zone in the Eastern Limb of the Bushveld Complex are presented in this study, covering a stratigraphic interval of approximately 5000 m.

 

The data were determined using transmitted light microscopy, X-Ray Fluorescence Spectrometry (XRF), Electron Probe Micro-Analyzer (EPMA) and Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MC-ICPMS) to produce a continuous and relatively high-resolution profile of variations in mineral chemistry, whole-rock major and trace elements and Sr-isotopic composition of plagioclase.

 

The results show Sr-isotopic trends that are similar to those observed in the Western Limb, with the first isotopically heterogeneous stage named the “Integration Stage”, characterised by fluctuating initial 87Sr/86Sr compositions from the Lower Critical Zone (~0.7051) to the Lower Main Zone (~0.7084). These variations are attributed to repeated influxes of compositionally distinct magmas. The Lower / Upper Critical Zone boundary is characterized by a sharp increase in initial 87Sr/86Sr ratios from 0.7054 to 0.7062 while the Critical Zone / Main Zone boundary is characterized by a sharp increase from 0.7065 to 0.7080.

 

The isotopically homogeneous “Differentiation Stage” includes the Upper Main and Upper zones. This interval is characterized by relatively uniform initial 87Sr/86Sr composition of ~0.7073. Limited variation in Sr-isotopic compositions suggests that this stratigraphic interval was formed through fractional crystallization with limited or no injections of new magma. The last major addition of magma into the Bushveld magma chamber is recorded by fluctuations in plagioclase An% and initial 87Sr/86Sr ratios at the level of the Pyroxenite Marker, which occurs at the boundary between the Lower and Upper Main zones. There is a decrease in initial 87Sr/86Sr ratios at the level of the Pyroxenite Marker, from 0.7079 to 0.7075.

 

Within the “Integration Stage” where significant variations in initial 87Sr/86Sr ratios of plagioclase are recorded, there is limited variation in plagioclase An% compared to the “Differentiation Stage”, which is characterized by a dominantly normal differentiation trend shown by decreasing plagioclase An% with increasing vi stratigraphic height and limited variation in initial 87Sr/86Sr ratios of plagioclase. This trend between plagioclase An% and initial 87Sr/86Sr ratios of plagioclase is also observed in the Western and Northern Limbs of the Bushveld Complex. Inter and intra-crystal initial 87Sr/86Sr disequilibrium is locally observed in the Lower Main and Upper Critical zones and is limited in the Upper Main and Upper Zones.

 

This disequilibrium supports a model of petrogenesis which suggests that the Critical and Lower Main Zones formed through intrusion of variably contaminated crystal mushes derived from a sub-compartmentalized, sub-Bushveld staging chamber that underwent different degrees of contamination with crustal rocks of the Kaapvaal craton. The Upper Main and Upper Zones revealed uniform initial 87Sr/86Sr compositions with a decreasing plagioclase An% trend, suggesting normal fractionation as the main petrogenetic process.

 

 

Researcher: Rudolph Malatji (MSc, UFS)

Supervisor: Professor Frederick Roefolse

Title: The strontium isotopic stratigraphy of the LCZ-UCZ transition in the Western Limb, Bushveld Complex

 

Abstract

Data on the modal mineralogy, whole-rock geochemistry, plagioclase mineral chemistry and Sr isotopic compositions in lithologies covering an interval of ~100 m across the Upper Critical Zone (UCZ) and Lower Critical Zone (LCZ) transition in the Western Limb of the Bushveld Complex are presented in this study. The aims of this study were to (1) investigate the presence or absence of isotopic disequilibrium in plagioclase (2) to investigate differences between the LCZ and UCZ from a geochemical, petrological and Sr isotopic perspective and (3) to refine chromitite formation models using the data obtained over the course of the study. Samples were obtained from the BH7929 drill core donated by Impala platinum to the University of the Free State. Samples were analysed using transmitted light microscopy, X-Ray Fluorescence Spectrometry (XRF), Inductively Coupled Plasma Mass Spectrometry (ICPMS), Electron Probe Micro-Analyzer (EPMA) and Laser Ablation Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA MC-ICP-MS) to produce wholerock major and trace element geochemistry and plagioclase elemental and isotopic compositional profiles across the UCZ-LCZ transition.

 

Results reveal that the LCZ is composed of orthopyroxene-dominated lithologies that display cryptic layering, hosting two chromitite layers (MG1-2), with plagioclase predominantly existing as an intercumulus phase. Plagioclase is predominantly cumulus in the UCZ, dominating the UCZ lithologies that display modal layering. Two chromitite layers were investigated in the UCZ (MG3-4). Compositional breaks in whole-rock major and trace elements are detected at the UCZ-LCZ transition and at the level of the chromitite layers, reflecting variations in the dominant mineral phases.

 

Fractionation indices including whole-rock Mg# and Cr/V ratio reveal little variation throughout the study interval in silicate-dominated lithologies, with variations mostly detected at the level of chromitite layers. Plagioclase An% averages 82.10 ± 1.90% in the UCZ, whereas it averages 73.58 ± 2.60% in the LCZ. Chromitite layers in the LCZ reveal lower An% values in comparison with adjacent silicate lithologies, whereas the UCZ reveals very little to no variations between chromitites and silicate lithologies. Sri values in the UCZ average 0.7059 ± 0.0003, whereas Sri in the LCZ averages 0.7054 ± 0.0004. Decreases in the Sri value of plagioclase are observed at the level of the chromitite layers in the LCZ, whereas the UCZ reveals a constant Sri up the stratigraphy.

 

The data provide credence to the importance of magma mixing (i.e. Irvine, 1977) as a process operational in the formation of chromitite layers within the LCZ and UCZ and argue against models suggesting variations in intensive parameters or in-situ crystallization as dominant processes in the formation of chromitite layers. It is proposed that the UCZ-LCZ transition displays credible evidence for the repeated intrusion of batches of isotopically distinct magmas, with chromitite layers in the LCZ forming in response to the mixing of newly introduced and resident magma in a manner analogous to that envisaged by Irvine. The MG3 layer in the UCZ also appears to have formed as a direct consequence of mixing between newly intruded UCZ magma and the residual LCZ magma. The MG4 layer does not preserve Sr-isotopic evidence for magma mixing as it has similar Sri as that of adjacent silicate lithologies. In order to account for the mass balance of Cr, it is argued that at the level of chromitite layers, intruded magma pulses were chromite-laden, with additional chromite formation occurring in response to magma mixing. The MG1 chromitite layer provides potential evidence in support of such an argument in the form of multiple isotopically distinct populations of plagioclase that may have been intruded along with suspended chromite crystals.

 

 

Researcher: Anton Viljoen (MSc. UCT)

Supervisor: Dr Geoffrey Howarth

Title: Petrology of the micaceous Koidu and Tongo – Tonguma kimberlites, Man Craton, Sierra Leone

 

Abstract:

Sierra Leone contains two Jurassic-aged diamondiferous volcanic clusters, namely Koidu and Tongo-Tonguma (hereafter referred to as Tongo), consisting of eruptive pipes at Koidu and NE-SW trending dikes at both Koidu and Tongo. Petrographically, the diamondiferous rocks from both clusters have features of both micaceous kimberlites and unevolved Kaapvaal lamproite, making classification ambiguous. To successfully classify and constrain the petrogenesis of these rocks, a combination of detailed petrography, and phlogopite, spinel and olivine chemistry is used in this study.

 

The Koidu rocks are predominantly macrocrystic in texture, whereas the Tongo rocks can broadly be separated into macrocrystic and aphanitic varieties. Olivine is present as macrocrysts and microcrysts, while phlogopite is occasionally present as macrocrysts and abundantly present as groundmass microcrysts (33 vol.% average). Other groundmass minerals include spinel, perovskite, apatite, and calcite set in a base of serpentine possibly containing secondary carbonate. These characteristic features can be present in both micaceous kimberlites and unevolved Kaapvaal lamproites, making classification based on petrography ambiguous.

 

Phlogopite and spinel have similar compositional trends for both Koidu and Tongo, comparable to trends observed for archetypal kimberlites rather than lamproites.  This in combination with previously studied trace element geochemistry and isotopic compositions from rocks within the Tongo cluster, indicate characteristics of an archetypal kimberlite. The overall phlogopite abundances further indicate that these rocks are micaceous kimberlites.

 

Olivine core compositions are divided into Mg-rich (Fo >89) and Fe-rich (Fo <89) endmembers, with Koidu and Tongo cores being dominantly Mg-rich with fewer Fe-rich cores. The overall olivine core compositions range between Fo 84.7 to 94.5 for Koidu and between 83.2 to 94.6 for Tongo. The Mg-rich olivine cores often contain intensely resorbed margins and no groundmass inclusions, consistent with a xenocrystic origin, whereas the Fe-rich population have resorbed margins to a lesser extent and are interpreted to be derived from a Cr-poor megacryst suite. Olivine rim compositions have Fo contents which are relatively homogenous, with compositional overlaps observed between Tongo and Koidu. Olivine rims often contain mineral inclusions of groundmass spinels and are interpreted as having crystallized from a magma.

 

Previous models have suggested that olivine chemistry and groundmass mineralogy in world-wide diamondiferous rocks can be used to constrain their origin and evolution. However, the Koidu-Tongo rocks are the first example where multiple kimberlite clusters do not align with these models. The Koidu-Tongo rocks are highly micaceous and contain Mg-rich olivine core-rim compositions and correspond with Kaapvaal lamproites and so an alternative model has been suggested where the rocks are likely derived from the asthenosphere, like a typical kimberlite, but assimilated phlogopite-rich material in the SCLM.

 

 

Researcher: Vongani Chabalala (PhD, UJ)

Supervisor: Professor Nicola Wagner

Title: The Application of Organic Petrology, Raman Spectroscopy, and Geochemistry to Karoo Basin (South Africa) Shale Gas Samples.

 

Abstract

Shale gas has generated economic interest as a hydrocarbon resource for energy production globally. The shale gas exploration initiatives in the Karoo Basin are encouraged by the successes achieved by the United States of America (USA) with regards to unconventional shale gas exploration. In this study, the Cisuralian shales of the Karoo Basin and the Devonian/Carboniferous Berea Sandstone project samples from the Appalachian Basin, USA, were characterized and compared using geochemistry, organic petrology and Raman spectroscopy characterization techniques. The Devonian/Carboniferous Berea Sandstone project samples were included in order to assess the characterization techniques used in South Africa as compared to those used in USA and also for quality assurance purposes. This assessment aided in strengthening the local analytical competency and bolstered confidence in the researcher’s capability to conduct analysis such as total organic carbon (TOC) and thermal maturity assessments. Horizons from two boreholes (KZF-01 and KWV-01) drilled in the southern and south-eastern Karoo Basin were evaluated. Analytical techniques used to characterize the shale samples included TOC, total sulphur (TS), organic petrology (solid bitumen and vitrinite reflectance, and maceral analysis), X-ray Diffraction (XRD), carbon isotopes, N2 and CO2 adsorption, programmed pyrolysis, and Raman spectroscopy. The TOC data indicates that the Whitehill Formation of the Karoo Basin has a good to excellent shale gas resource potential, with TOC’s averaging at 4.35 wt%. However, thermal maturity data of the Whitehill Formation shows the organic matter to be overmature, with vitrinite reflectance equivalent values greater than 3% RoV. The Devonian/Carboniferous Berea Sandstone project samples are thermally immature, with the abundance of solid bitumen macerals. Comparisons were drawn between the Whitehill Formation data (KZF and KWV results) generated from this study and the literature data from the overmature shales from gas producing areas such as Middle Devonian Marcellus and Silurian Longmaxi marine shales. The analyses results generated from KZF and KWV samples are comparable to the literature data from the Marcellus and Longmaxi Formation shales, in terms of the thermal maturity (all are overmature), TOC (all rich in organic carbon) and also contain major minerals such as quartz and clay. The presence of dolerites sills and pyrrhotite in KWV-01 of the Whitehill shales clearly differentiate the shales from the Marcellus and Longmaxi Formation shales. Total sulphur data for the Whitehill Formation and the Devonian/Carboniferous shales suggests that both formations were influenced by marine environments. The Berea Sandstone project samples contain high amounts of quartz, and clay mineral contents. Carbon isotope analysis reflected the absence of carbonates in both Berea Sandstone project samples and KWV samples. A positive correlation between the volume of micropores as determined by CO2 adsorption and the TOC is observed for the Berea Sandstone project samples, implying that micropores in these shales were mainly developed in the organic matter. However, the poor correlation between micropores volume as determined by CO2 adsorption and the TOC in the KWV samples implies that there are factors other than the organic fraction which influence their micropore structure, and such factors may include inorganic materials such as clay minerals. Programmed pyrolysis data indicated that the KWV samples are overmature, and they fall in the category of poor generation resource potential. Raman spectroscopy demonstrated good potential to predict thermal maturity in shales as the technique provided insights about the molecular structure of the organic matter. In combination with petrographic approaches, Raman spectroscopy can be used as a complementary analytical technique with potential to replace conventional methods that are usually time consuming in terms of sample preparation and performing the analysis. In conclusion, Raman spectroscopy was successfully applied as a complementary analytical technique in organic matter characterization of potential source rocks. Collectively, this research provides valuable data for the enhanced understanding of analytical techniques for shale gas exploration in the Karoo Basin, South Africa.

 

Researcher: Samuel Nunoo (PhD, UJ)

Supervisor: Professor Axel Hofmann

Title: Origin and age of gold mineralization in NW Ghana ˗ a case study of selected gold deposits from the Julie and Wa-Lawra belts

 

Abstract

Gold mining in Ghana has a long history that dates back to the then Gold Coast. The exploration for gold keeps expanding across the nation, as mining forms an integral part of the country’s economic growth. The gold is hosted within Paleoproterozoic rocks of the Birimian Supergroup, Tarkwaian Group and associated granitoids. Gold mineralization types include the Birimian mesothermal quartz vein and quartz vein-associated sulphide types, and the Tarkwaian placer gold type. The sources of the Birimian and Tarkwaian gold remain debated.

 

Mafic rocks or carbonaceous sedimentary rocks are regarded as the potential metallogenic source rocks. This study focused on the evaluation of potential source rocks for gold in NW Ghana, especially the shale-hosted Collette and Kjersti Au deposits within the Julie greenstone belt. The Julie belt is part of the NW Birimian terrain of which the geology remains poorly constrained. A fundamental issue relates to linking various lithologies from this belt to the established Birimian and Tarkwaian stratigraphy in SW Ghana.

 

Field work within the Julie belt indicated that the lithologies (Chapter 3) share similarities to Paleoproterozoic terrains in the SW. Stratigraphically, the Julie belt is characterized by metavolcanic rocks (Sefwi Group) consisting mainly of basalts with minor rhyolite (and agglomerate), and metasedimentary rocks (Kumasi Group) of interbedded shales and siltstones, and greywackes. Both units are unconformably overlain by a clastic sequence (Tarkwaian Group) of minor conglomerate and predominately cross-bedded sandstone with the ubiquitous presence of magnetite. These groups are intruded by granitoids of different generations. Shales and greywackes are sulphidic and rich in carbonaceous matter (CM). Provenance and tectonic setting of these rocks (Chapter 4) indicated their derivation from erosion of a granitoidgreenstone terrain containing basalt, rhyolite and granitoid rocks compositionally similar to rocks from adjacent Birimian volcanic and plutonic terrains. The composition of shales and greywackes suggests their derivation largely from a Paleoproterozoic volcanic arc setting. They were possibly deposited in forearc and/or back-arc basins prior to the Eburnean orogeny and collision within the West African Craton (WAC). The sedimentary rocks, especially greywackes, are part of the rocks whose zircon U-Pb ages and Lu-Hf isotopes have been determined (Chapter 5). U-Pb ages from both detrital and igneous zircons on eight samples (1 greywacke, 3 sandstones, 1 rhyolite and 3 granitoids) show that all zircons analyzed indicated Paleoproterozoic ages in the range of 2112 ± 5 to 2337 ± 11 Ma. This age range corroborates with existing zircon ages obtained from volcanic and plutonic rocks in various parts of Ghana, and other Birimian and Tarkwaian terrains within the WAC. The ɛHf values of zircon grains VIII from analyzed rocks are in the range of -1.78 to + 6.37. These values agree with most Hf isotopic signatures currently available from the Paleoproterozoic Birimian or Baoulé-Mossi domain, which indicate reworking of an initially juvenile crust that was extracted from depleted mantle at ~2.33 Ga.

 

Regarding the source (s) of shale-hosted gold of the Collette and Kjersti deposits (Chapter 6), integrating textural evidence on sulphidic and CM-bearing shales and greywackes, their δ13C and δ34S isotopic signatures, and content of gold and other elements (e.g. As and Pb), suggest that microbial activities likely facilitated the formation of precursor sulphides that incorporated Au. Gold was later transported as complexes of hydrosulphide and/or organic-ligands along favourable structural channels, and became trapped/deposited into recrystallized euhedral sulphides largely of arsenopyrite and pyrite, and moderate chalcopyrite. CM present in these rocks may have played multiple roles especially during pyrite formation, and in the precipitation, transport and deposition of gold. In addition, Raman spectroscopy of carbonaceous matter (Chapter seven) indicates tentative peak metamorphic temperatures of 410 to 510 °C (± 50 °C) for these CM-rich rocks. Estimated temperatures constrain the degree of heat that generated hydrothermal fluids, which leached precursor gold associated with CMrich sediments and deposited it in secondary sulphides along structurally controlled channels either during or after deformational episodes.

 

 

Researcher: René Booysen (PhD, Wits)

Supervisor: Professor Paul Nex

Title: The potential of drones in multi-scale hyperspectral imaging for mineral exploration: Examples from Southern Africa

 

Abstract

The transition towards a green economy is paradoxically the main driver to the increased demand for resources. Recycling alone is not enough to sustain a purely circular economy, thus mineral exploration will still be required. Traditional exploration techniques are primarily based on extensive field work that are supported by geophysical surveying and drilling. These techniques can be restricted by field accessibility, financial status, area size and climate. Furthermore, these methods typically have a considerable footprint on the environment, upsetting the surrounding community and resulting in mistrust in the exploration sector. This PhD introduces a novel multi-scale remote sensing approach that incorporates state-of-the-art methods focused on the applicability of uncrewed aerial vehicles (UAVs)- and ground-based hyperspectral imaging (HSI) for mineral exploration.

 

The aim of this approach is to improve efficiency, reduce costs and increase the safety of field personnel. The work also shows the benefits by promoting non-invasive, innovative remote sensing methods to foster social acceptability.

 

This approach is exemplified by the exploration of economically important commodities that contribute to the development of green technologies. Deposits and geological bodies containing tin (Sn), lithium (Li) and rare earth elements (REEs) were used as case studies in order to test the multi-scale approach. These sites comprise the Li-bearing pegmatites of the Uis tin mine in Namibia, the REE-bearing carbonatite complexes of Marinkas Quellen and Lofdal in Namibia and finally the Zaaiplaats historic tin mine in South Africa. In the proposed scheme, satellite and aeroplane-based data constitute the first level of exploration. The second level consists of the improvement of field work by making use of hyperspectral imaging, both with ground-based and UAVs surveys. Each level of data acquisition comes with its own set of advantages and limitations; i.e., satellites can cover a large area extent, but the data are usually at a relatively low spatial resolution, while UAV-based data can have a high spatial resolution, UAVs themselves can only cover a limited area. By using a multi-scale approach, we can minimize each platform’s limitation and exploit their advantages. As this approach is innovative and yet untested, the core of the project focuses on the numerous aspects of the acquisition, processing and validation of UAV-based hyperspectral data. In order to adequately demonstrate its relevance, this work highlights specific aspects of the multi-source, multi-scale approach adapted to the peculiarities of the case studies.

 

Spectral tools and machine learning techniques were adapted to process satellite and plane-base data in order to locate areas of interests. In the present case, these methodswere used to map the regional geology and identify potential zones of mineralisation for further investigation at the Sn-hosted granites of Zaaiplaats and the REE-bearing carbonatites of Lofdal. Following this, UAV-based hyperspectral data can vastly improve the accuracy of field mapping in mineral exploration. UAV-based measurements can supplement and direct geological observation immediately in the field and therefore allow better integration with in-situ ground investigations. A hyperspectral camera was attached to a multi-copter to acquire data from the visible (VIS) to the near-infrared (NIR) range of the electromagnetic spectrum. The acquired data was then corrected for radiometric and geometric distortions.

 

In addition, high resolution digital surface models (DSM) and orthomosaics were generated using photogrammetry. The corrected data provides information on the spectral signatures of outcropping lithologies to the field geologists and the exploration teams. In the cases of Marinkas Quellen and Lofdal, this was achieved by using end-member modelling and classification techniques such as non-linear machine learning algorithms, e.g., spectral angle mapper (SAM) or minimum wavelength mapping (MWM). Ground truth points, which were collected during field work, were used as referencing.

 

Furthermore, this work also shows a novel approach whereby 3D virtual outcrops are used to map vertical mine walls. Heavy sensors that cannot be mounted to a UAV such as short-wave infrared (SWIR) or long-wave infrared (LWIR) sensors can be used to image outcropping rocks in the field, or as validation tools in the laboratory. In the case study of the Li-bearing pegmatites at Uis, ground-based hyperspectral SWIR data was used in unison with photogrammetric 3D models to produce georeferenced 3D virtual models with hyperspectral attributes in order to map Li mineralisation by using MWM. To verify the ground-based data from Uis, decision trees allowed for the determination of the mineralogy and mineral associations in hand samples and drill-cores. Lastly, to validate all the remote sensing data, in-situ measurements were taken with portable devices such as a handheld X-ray fluorescence (XRF), laser-induced breakdown spectroscopy (LIBS) and spectroradiometer. Geochemical analyses such as XRF, X-ray diffraction (XRD) and inductively coupled plasma mass spectrometry (ICP-MS) were performed on field samples. Additional validation was performed in laboratories such as thin section microscopy, and advanced spectroscopic techniques such as mineral liberation analysis (MLA) and laser induced fluorescence (LiF).

 

The results indicate that UAV-based surveying has a very high potential in fundamentally lowering the acquisition costs and increasing the amount of valuable information captured in the field. Together with ground-based hyperspectral imaging, these techniques can be seamlessly used in a multi-scale approach in the exploration of other deposits as well. Furthermore, innovative, non-invasive and more sustainable techniques in exploration, such as the one proposed in this work, encourage social acceptability in the community at large and promote the use of greener technology.

 

Finally, this work has laid the foundation for future research in the integration of UAVbased hyperspectral and geophysical data. This would allow us to gain information not just on the surface, but fuse surficial with sub-surface information. Additionally, by incorporating a time component with repeated acquisitions, 4D models can be made that would allow to monitor the evolution of industrial or environmental targets.

 

Keywords: Multi-scale remote sensing, critical minerals, mineral exploration, sustainability, noninvasive, hyperspectral imaging, UAVs, 3D models

 

 

Researcher: Dora Paprika (PhD, UJ)

Supervisor: Professor Axel Hofmann

Title:  Mesoarchaean volcanism and associated epithermal activity of the Dominion Group, South Africa – implications for Witwatersrand Basin gold

 

Abstract

Although many carbonatite occurrences world-wide show temporal and spatial association with silica under-saturated alkaline rocks (e.g. nephelinites, ijolites, nepheline syenites) their petrogenetic relationship is still a matter of debate. In the Epembe area of northwestern Namibia, there is an occurrence of a carbonatite associated with alkaline rocks. Such an occurrence provides an ideal opportunity to investigate their petrogenesis and the petrogenetic relationship between them if any. The rocks were studied using whole rock geochemistry, U-Pb geochronology, mineral and isotope geochemistry.

 

The Epembe Alkaline Carbonatite Complex (EACC) was emplaced along a fault zone into medium- to high-grade Palaeoproterozoic basement rocks of the Epupa Metamorphic Complex (EMC) and extends over a distance of 9 km in a south-easterly direction with a width of 1 km. Alkaline rocks constitute the main lithologies and are cross-cut by a calcite-carbonatite dyke. The alkaline rocks can be classified as syenite and nepheline-bearing syenites, with alkalic, metaluminous and ferroan affinities. The syenite occurs as a discontinuous intrusion which separates the nepheline syenite from the EMC, while the carbonatite cross-cuts the nepheline syenite body along strike. The syenite comprises alkali feldspar and clinopyroxene with accessory apatite and magnetite. The nepheline syenite comprises major to minor alkali feldspar, nepheline, biotite and cancrinite, while plagioclase, apatite, sphene, zircon, calcite and magnetite occur as accessory phases. The carbonatite consists of calcite, apatite, pyrochlore, pyroxene (aegirine), biotite, zircon, alkali feldspars and plagioclase in varying amounts. Based on zircon U-Pb dating, a 206Pb/238U weighted mean age of 1220 ± 3 Ma (2SE, MSWD = 1.3) was determined for the syenite emplacement, whereas two nepheline syenite samples give identical magmatic ages of 1209 ± 3 (206Pb/238U weighted mean age, 2SE, MSWD = 1.1) and 1205 ± 13 Ma (concordia intercept age, 2SE, MSWD = 2). The nepheline syenite ages correspond with the concordia age of 1198 ± 5 Ma (2SE, MSWD = 1.1) of the carbonatite interpreted as magmatic. The nepheline-normative syenites define broadly linear trends in Harker plots consistent with evolution by fractional crystallization involving pyroxene and apatite. The rare earth element (REE) pattern of the syenite shows a negative Eu anomaly, consistent with plagioclase fractionation. The syenite is characterized by the absence of a negative Nb anomaly on a Primitive Mantle (PRIMA)-normalized diagram with Ce/Pb and Nb/U ratios of 12 and 19, respectively, suggesting that it was not affected by crustal contamination during ascent and emplacement. The absence of a crustal signature indicates that the syenite is not related to the nepheline syenite by combined assimilation and fractional crystallization, and was likely emplaced as a distinct magma batch derived from the mantle. REE patterns of the nepheline syenite and carbonatite are coherent showing LREE enrichment relative to HREE. On a PRIMA-normalized multi-element diagram both the carbonatite and syenite display relative depletions of Zr, Hf and Pb and relative enrichment of Sr. Low Mg, Cr and Ni contents of the carbonatite and nepheline syenite suggest that these rocks crystallized from an evolved parental magma rather than a primitive mantle-derived melt. Geochemical similarities between the carbonatite and nepheline syenite also suggest that the Epembe carbonatite is not a product of immiscible separation between a carbonate and silicate melt. Instead, the carbonate may represent a melt fraction that formed after partial crystallization of the nepheline syenite.

 

Apatite grains from one syenite, six nepheline syenite and five carbonatite samples were examined using cathodoluminescence (CL) imaging, trace element and Sr-Nd isotope compositions as well as U-Pb geochronology. Syenite-hosted apatite is homogenous in CL and contains the highest concentration of REE (9189-44100 ppm) with light (L) REE enrichment (LaN/YbN = 4-91) relative to heavy (H) REE and negative Eu anomalies (Eu/Eu* = 0.4-0.9). These features are attributed to the formation of apatite in an evolved mantle-derived melt associated with plagioclase fractionation. Nepheline syenite-hosted apatite is also generally homogeneous in CL, while core-rim zoning and patchy textures are only observed occasionally. Both texturally homogeneous and core-rim zoned apatite are enriched in LREE (LaN/YbN = 32-94) relative to HREE, consistent with a magmatic origin. Core-rim zoned apatite is characterized by a rim-ward increase in REE concentrations accompanied by uniform Sr and Nd isotopic compositions, which can be attributed to the re-equilibration of early-formed apatite (core) with later infiltrating melt enriched in REE, causing the formation of apatite overgrowths (rims). Patchy apatite is depleted in Na, Y and REE, particularly the LREE (LaN/YbN = 4-19) and is enriched in Sr relative to apatite from other nepheline syenite, reflecting interaction with fluids (metasomatism). The strontium isotope composition of metasomatic apatite and magmatic apatite is indistinct suggesting a magmatic origin of the fluids responsible for alteration. Carbonatite apatite is LREE-enriched (LaN/YbN = 24-161) relative to HREE and displays core-rim zoning in CL accompanied by a rim-ward increase in REE, attributed to mineral fractionation. No Eu anomalies (Eu/Eu* = 1) in chondrite-normalized REE patterns are observed in any apatite hosted by nepheline syenite and carbonatite. A LA-ICPMS U-Pb age of 1216 ± 11 Ma (MSWD = 4.3, 2 SE) for syenite apatite constrains emplacement of the syenite, while magmatic nepheline syenite apatite ages of 1193 ± 14 Ma, 1197 ± 17 Ma and 1194 ± 16 Ma (MSWDs < 4.0, 2 SE) have been determined. The Sr and Nd isotopic composition of apatite in syenite (87Sr/86Sr(i) = 0.7035-0.7048; ƐNd(t) = +2.5 to +3.2), nepheline syenites (87Sr/86Sr(i) = 0.7031-0.7037; ƐNd(t) = +1.5 to +4.4) and carbonatite (87Sr/86Sr(i) = 0.7031-0.7033; ƐNd(t) = 0 to +3.3) overlap, pointing to a common but heterogeneous mantle source, possibly involving HIMU and EMI mantle endmembers.

 

Lastly, cathodoluminescence (CL) imaging combined with trace elements (including REE) as well as Hf isotope compositions of zircon grains extracted from one syenite, five nepheline syenite samples and one carbonatite sample are presented. Carbonatite and syenite zircons are generally unaltered and characterized by steeply rising REE patterns in chondrite-normalized diagrams, with positive Ce anomalies (Ce/Ce* = 1-8). Syenite zircon further displays significant negative Eu anomalies (Eu/Eu* = 0.2-0.4) attributed to earlier plagioclase formation and fractionation. These features are consistent with zircon formation in a magmatic environment. In the nepheline syenite samples, two zircon types are recognized. Type 1 zircon is magmatic, with homogeneous-grey, unzoned and oscillatory-zoned domains in CL, while type 2 zircon is hydrothermally altered and displays a cloudy appearance in CL. Type 2 zircon is characterized by enrichment in LREE (101-853 ppm), Nb (97-339) and Ti (63-140 ppm) when compared to magmatic type 1 zircon (LREE = 17-93 ppm, Nb = 7-59 and Ti = 2-15 ppm). The Hf isotope composition of type 1 and type 2 zircon is indistinct suggesting that the fluids involved in zircon alteration were sourced from within the complex. The similarity of ƐHf(t) values in zircon from syenite (+0.5 ± 0.4 to +1.5 ± 0.4), nepheline syenite (+1.6 ± 0.3 to +2.7 ± 0.5) and carbonatite (+0.5 ± 0.4 to +1.9 ± 0.2) is consistent with the melts having been derived from a moderately depleted mantle.

 

The data presented in this study are best explained by partial melting of a heterogeneous mantle at ca. 1200 Ma, which resulted in the formation of an alkali basalt and a carbonated-alkali basaltic melt. The alkali basalt melt ascended first utilizing an existing fault system and evolved to form the syenite. Subsequently, the carbonated-alkali basaltic melt percolated upwards and differentiated to form the nepheline syenite and calcite-carbonatite.

 

Finally, metasomatic interaction occurred between magmatic derived fluids and primary assemblages.

 

 

Researcher: Mbili Tshiningayamwe (PhD, Wits)

Supervisor: Professor Robert Bohlar

Title: Petrogenesis and detailed geochemical and isotopic studies of the Epembe carbonatite and syenite rocks, NW Namibia

 

Abstract

Although many carbonatite occurrences world-wide show temporal and spatial association with silica under-saturated alkaline rocks (e.g. nephelinites, ijolites, nepheline syenites) their petrogenetic relationship is still a matter of debate. In the Epembe area of northwestern Namibia, there is an occurrence of a carbonatite associated with alkaline rocks. Such an occurrence provides an ideal opportunity to investigate their petrogenesis and the petrogenetic relationship between them if any. The rocks were studied using whole rock geochemistry, U-Pb geochronology, mineral and isotope geochemistry.

 

 

The Epembe Alkaline Carbonatite Complex (EACC) was emplaced along a fault zone into medium- to high-grade Palaeoproterozoic basement rocks of the Epupa Metamorphic Complex (EMC) and extends over a distance of 9 km in a south-easterly direction with a width of 1 km. Alkaline rocks constitute the main lithologies and are cross-cut by a calcite-carbonatite dyke. The alkaline rocks can be classified as syenite and nepheline-bearing syenites, with alkalic, metaluminous and ferroan affinities. The syenite occurs as a discontinuous intrusion which separates the nepheline syenite from the EMC, while the carbonatite cross-cuts the nepheline syenite body along strike. The syenite comprises alkali feldspar and clinopyroxene with accessory apatite and magnetite. The nepheline syenite comprises major to minor alkali feldspar, nepheline, biotite and cancrinite, while plagioclase, apatite, sphene, zircon, calcite and magnetite occur as accessory phases. The carbonatite consists of calcite, apatite, pyrochlore, pyroxene (aegirine), biotite, zircon, alkali feldspars and plagioclase in varying amounts. Based on zircon U-Pb dating, a 206Pb/238U weighted mean age of 1220 ± 3 Ma (2SE, MSWD = 1.3) was determined for the syenite emplacement, whereas two nepheline syenite samples give identical magmatic ages of 1209 ± 3 (206Pb/238U weighted mean age, 2SE, MSWD = 1.1) and 1205 ± 13 Ma (concordia intercept age, 2SE, MSWD = 2). The nepheline syenite ages correspond with the concordia age of 1198 ± 5 Ma (2SE, MSWD = 1.1) of the carbonatite interpreted as magmatic. The nepheline-normative syenites define broadly linear trends in Harker plots consistent with evolution by fractional crystallization involving pyroxene and apatite. The rare earth element (REE) pattern of the syenite shows a negative Eu anomaly, consistent with plagioclase fractionation. The syenite is characterized by the absence of a negative Nb anomaly on a Primitive Mantle (PRIMA)-normalized diagram with Ce/Pb and Nb/U ratios of 12 and 19, respectively, suggesting that it was not affected by crustal contamination during ascent and emplacement. The absence of a crustal signature indicates that the syenite is not related to the nepheline syenite by combined assimilation and fractional crystallization, and was likely emplaced as a distinct magma batch derived from the mantle. REE patterns of the nepheline syenite and carbonatite are coherent showing LREE enrichment relative to HREE. On a PRIMA-normalized multi-element diagram both the carbonatite and syenite display relative depletions of Zr, Hf and Pb and relative enrichment of Sr. Low Mg, Cr and Ni contents of the carbonatite and nepheline syenite suggest that these rocks crystallized from an evolved parental magma rather than a primitive mantle-derived melt. Geochemical similarities between the carbonatite and nepheline syenite also suggest that the Epembe carbonatite is not a product of immiscible separation between a carbonate and silicate melt. Instead, the carbonate may represent a melt fraction that formed after partial crystallization of the nepheline syenite.

 

 

Apatite grains from one syenite, six nepheline syenite and five carbonatite samples were examined using cathodoluminescence (CL) imaging, trace element and Sr-Nd isotope compositions as well as U-Pb geochronology. Syenite-hosted apatite is homogenous in CL and contains the highest concentration of REE (9189-44100 ppm) with light (L) REE enrichment (LaN/YbN = 4-91) relative to heavy (H) REE and negative Eu anomalies (Eu/Eu* = 0.4-0.9). These features are attributed to the formation of apatite in an evolved mantle-derived melt associated with plagioclase fractionation. Nepheline syenite-hosted apatite is also generally homogeneous in CL, while core-rim zoning and patchy textures are only observed occasionally. Both texturally homogeneous and core-rim zoned apatite are enriched in LREE (LaN/YbN = 32-94) relative to HREE, consistent with a magmatic origin. Core-rim zoned apatite is characterized by a rim-ward increase in REE concentrations accompanied by uniform Sr and Nd isotopic compositions, which can be attributed to the re-equilibration of early-formed apatite (core) with later infiltrating melt enriched in REE, causing the formation of apatite overgrowths (rims). Patchy apatite is depleted in Na, Y and REE, particularly the LREE (LaN/YbN = 4-19) and is enriched in Sr relative to apatite from other nepheline syenite, reflecting interaction with fluids (metasomatism). The strontium isotope composition of metasomatic apatite and magmatic apatite is indistinct suggesting a magmatic origin of the fluids responsible for alteration. Carbonatite apatite is LREE-enriched (LaN/YbN = 24-161) relative to HREE and displays core-rim zoning in CL accompanied by a rim-ward increase in REE,

 

attributed to mineral fractionation. No Eu anomalies (Eu/Eu* = 1) in chondrite-normalized REE patterns are observed in any apatite hosted by nepheline syenite and carbonatite. A LA-ICPMS U-Pb age of 1216 ± 11 Ma (MSWD = 4.3, 2 SE) for syenite apatite constrains emplacement of the syenite, while magmatic nepheline syenite apatite ages of 1193 ± 14 Ma, 1197 ± 17 Ma and 1194 ± 16 Ma (MSWDs < 4.0, 2 SE) have been determined. The Sr and Nd isotopic composition of apatite in syenite (87Sr/86Sr(i) = 0.7035-0.7048; ƐNd(t) = +2.5 to +3.2), nepheline syenites (87Sr/86Sr(i) = 0.7031-0.7037; ƐNd(t) = +1.5 to +4.4) and carbonatite (87Sr/86Sr(i) = 0.7031-0.7033; ƐNd(t) = 0 to +3.3) overlap, pointing to a common but heterogeneous mantle source, possibly involving HIMU and EMI mantle endmembers.

 

Lastly, cathodoluminescence (CL) imaging combined with trace elements (including REE) as well as Hf isotope compositions of zircon grains extracted from one syenite, five nepheline syenite samples and one carbonatite sample are presented. Carbonatite and syenite zircons are generally unaltered and characterized by steeply rising REE patterns in chondrite-normalized diagrams, with positive Ce anomalies (Ce/Ce* = 1-8). Syenite zircon further displays significant negative Eu anomalies (Eu/Eu* = 0.2-0.4) attributed to earlier plagioclase formation and fractionation. These features are consistent with zircon formation in a magmatic environment. In the nepheline syenite samples, two zircon types are recognized. Type 1 zircon is magmatic, with homogeneous-grey, unzoned and oscillatory-zoned domains in CL, while type 2 zircon is hydrothermally altered and displays a cloudy appearance in CL. Type 2 zircon is characterized by enrichment in LREE (101-853 ppm), Nb (97-339) and Ti (63-140 ppm) when compared to magmatic type 1 zircon (LREE = 17-93 ppm, Nb = 7-59 and Ti = 2-15 ppm). The Hf isotope composition of type 1 and type 2 zircon is indistinct suggesting that the fluids involved in zircon alteration were sourced from within the complex. The similarity of ƐHf(t) values in zircon from syenite (+0.5 ± 0.4 to +1.5 ± 0.4), nepheline syenite (+1.6 ± 0.3 to +2.7 ± 0.5) and carbonatite (+0.5 ± 0.4 to +1.9 ± 0.2) is consistent with the melts having been derived from a moderately depleted mantle.

 

The data presented in this study are best explained by partial melting of a heterogeneous mantle at ca. 1200 Ma, which resulted in the formation of an alkali basalt and a carbonated-alkali basaltic melt. The alkali basalt melt ascended first utilizing an existing fault system and evolved to form the syenite. Subsequently, the carbonated-alkali basaltic melt percolated upwards and differentiated to form the nepheline syenite and calcite-carbonatite. Finally, metasomatic interaction occurred between magmatic derived fluids and primary assemblages.