ETD Collection
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Item Fluid-rock interaction in carbonatite and alkaline composite intrusions and implications for rare earth element mineralization(2019) Ozturk, AnilThe Spitskop Igneous Complex is a carbonatite-alkaline silicate complex located 190 km northeast of Pretoria and 48 km east of Groblersdal in South Africa. It covers an area of 50 km2 and intruded into the Bushveld Complex on the Kaapvaal Craton at 1.3 Ga. It is considered a part of the Pilanesberg Alkaline Province, as it contains similar rock types such as nepheline syenites, ijolites and carbonatites and has a similar age. The carbonatite component of the Spitskop Complex consists primarily of dolomite carbonatite, calcite-dolomite carbonatite and calcite carbonatite with an apatite-rich zone. The outer part of the complex comprises alkaline rocks including ijolite and nepheline syenite, surrounded by the Rustenburg Layered Suite and the Lebowa Granite Suite of the Bushveld Complex. It is a unique complex, where both felsic and mafic fenites occur together. REE mineralization is hosted in carbonatites, however it is not considered an economic mineral deposit. This study characterizes the alteration stages that led to the formation of fenites and alkaline rocks, and the petrology and geochemistry of the Spitskop Complex. It shows that the fluids controlled the rare earth element content of Spitskop and affected the mobility of REE. The Spitskop Complex was mapped and samples were collected from different lithologies. Thin section petrology was used to determine the characteristic features and distribution of minerals. A total of 125 polished thin sections were studied using transmitted-reflected light microscopy and scanning electron microscopy (SEM). XRF and ICP-MS data of rocks have been obtained and the distribution of major-, trace- and rare earth elements of different lithologies were studied. The chemical composition of the fenitized Bushveld rocks have been compared with the unfenitized Bushveld rocks. The carbonatites all have similar major element concentrations except for CaO, MgO and MnO. The CaO and MgO concentrations reflect the type of carbonatite and the carbonatite mineralogy. Trace element and REE patterns of the different carbonatites are similar. The REE content of Spitskop carbonatites is up to 740 ppm. Nepheline syenites show metasomatic REE alteration patterns. Fenites are divided into two groups in the Spitskop Complex, mafic fenites and granite fenites. Mafic fenites represent metasomatized Upper Zone gabbro, whereas granite fenites represent metasomatized Nebo Granite. Moreover, granite fenites are subdivided into feldspar fenite, 4 which contains mostly feldspar; and quartz-feldspar fenite, which contains quartz and feldspar together. Mafic fenites are enriched in Na2O-K2O and P2O5 relative to the likely parental Upper Zone gabbros. Most of the trace element and all of the REE content of the mafic fenites are higher than the Upper Zone gabbros except Sc, V, Ni, Cu and U. Feldspar fenites are enriched in Na2O, K2O and MgO, and depleted in SiO2 compared to the parental Bushveld Granites. There are only some limited locations in the world where mafic and granitic rocks are extensively metasomatized together, therefore the Spitskop Complex is an ideal place to investigate the metasomatic geochemical processes. The mobility of the trace elements changes with increased fenitization. Nebo Granite has the highest trace elements concentrations, rather than in the quartz-feldspar fenite, with the feldspar fenite most depleted. However Cu, Sc and Eu are depleted in Nebo Granite. The REE data shows that the fenites compositionally lie between the unaltered Nebo Granite and the unaltered Upper Zone rocks. Fenitized Nebo Granite is depleted in REE and fenitized Upper Zone rocks are enriched in REE. The unaltered Upper Zone country rocks defines the lower REE boundary and the Nebo Granite defines the upper REE abundances. It is suggested that the metasomatic fluids caused a depletion of REE in the felsic rocks, whereas the same fluids caused an enrichment of REE in the mafic rocks. In mafic rocks the enrichment is dispersed through the whole rock across a broad zone and is therefore not economic. The data from thin section petrography and SEM suggests that the fenitization evolved in multiple steps at the Spitskop Complex. Alteration minerals show that there is a systematic change of minerals, from plagioclase to albite or nepheline, from olivine and orthopyroxene to clinopyroxene, from nepheline to analcite and from clinopyroxene to amphibole, which represent mafic fenites. Geochemical data, particularly REE patterns, suggests that the nepheline syenite at Spitskop is not a magmatic nepheline syenite, rather it is a product of fenitization. The REE patterns of the nepheline syenites are similar to the fenites of the Spitskop Complex and differ from other Pilanesberg nepheline syenites such as those in Pilanesberg, which are not fenitized. REE geochemistry also suggested that the syenites produced from fenitization (feldspar fenites) can be distinguished from magmatic syenite with the same REE patterns.Item Distribution of rare earth elements in the Epembe Carbonatite Dyke, Opuwo Area, Namibia(2019) Kapuka, Ester P.The Epembe carbonatite dyke at the Epembe Carbonatite-Syenite Complex in the Kunene region on the northwestern border of Namibia was emplaced along a northwest-trending fault zone, into syenites and nepheline syenites and extends for approximately 6.5 km in a northwest to southeast direction with a maximum outcrop width of 400 m. The Epembe carbonatite has a Mesoproterozoic age of 1184 ± 10 Ma which is slightly younger than their host nepheline syenites (1216 ± 2.4 Ma). Following the geological data collection and laboratory analysis of whole-rock samples [using optical microscopy, X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS)] the collected data was studied in detail in order to determine the geochemical composition of the Epembe carbonatite dyke. This research therefore presents new geochemical data for the Epembe carbonatite in order to describe the distribution and occurrence of rare earth elements of this dyke. The carbonatite displays a heterogeneous characteristic both texturally and mineralogically highlighting clear successions of at least three magmatic pulses. Irrespective of the changes, all carbonatite phases are inferred to be sourced from the same magma because they are typified by a similar geochemical signature of both major and trace element composition. They are characterised by high concentrations of calcium (CaO: 38.01 - 55.31 wt. %), phosphorus (P) (up to 18076), titanium (Ti) (up to 5122 ppm) strontium (Sr) (up to 12315 ppm) and niobium (Nb) with the (highest value of up to 2022 ppm ) alongside low concentrations of iron (FeO: 0.87 - 9.29 wt. %), magnesium (MgO: 0.19 – 1.33 wt. %) silica (SiO2: 1.30 – 10.89 wt. %) and total alkalis (K2O + Na2O < 2.0 wt. %) , hence they are regarded as one carbonatite dyke. The petrography and whole-rock element compositions of major elements have demonstrated the Epembe carbonatite is primarily made up of course-grained calcite (~92%) with a CaO+MgO+Fe2O3+MnO ratio of 0.93 relative abundances (in wt. %) and thus is classified as calcio or calcite carbonatite. The total REE content of Epembe carbonatite is high (406 – 912 ppm) with high LaN/YbN value (10.19 -28.49) and thus atypical of calcio-carbonatites. Chondrite normalized REE pattern for the carbonatite exhibit a strong steady decrease (negative slope) from LREEs to HREEs with a slight negative Eu anomaly but those are relatively low compared to global average calcio-carbonatites. Even though the Epembe carbonatite is enriched in Rare Earth Elements, there were no REE-bearing minerals observed at Epembe carbonatite except for monazite in trace amounts. Geochemical results show that the REE are either included in several accessory minerals such as apatite and pyrochlore and possibly in gangue minerals (i.e., silicates [including calcite and zircons] and carbonates) through enrichment processes related to fractional crystalisations and chemical substitution.Item Process mineralogy and extraction of rare earth elements from a beach placer deposit(2019) Moila, Awelani VeronicaRare earth elements (REE) are a group of the lanthanides and are significant in the world’s economic growth and modern technology market. This REE technological resource is globally distributed and highly monopolised in China. The global demand in REE led to China – “the leading economic producer of REE”, limiting its export quotas of the commodity, thus reducing the supply of REE. The decline in REE supply opened up opportunities for other countries to explore alternative and additional sources of REE. This research aims to investigate alternative sources of REE and to explore an efficient means of processing REE minerals from an existing beach placer deposit operation, currently being mined for titanium. Mineralogical characterisation and hydrometallurgical testwork were chosen for this study. The sample represented a tailings fraction from heavy mineral concentration. The sample was screened into four size classes namely; +212μm, –212+150μm, –150+106μm and –106μm. Each size class was mineralogically characterised. Mineralogy is an important factor in plant optimisation and process route predictions. In order to process REE efficiently, an upfront mineralogy is a necessity to reduce the rising hefty ore-processing costs. An integration of X-ray diffraction, optical microscopy, scanning electron microscopy (SEM), electron microprobe analysis (EMPA), automated SEM and bulk chemical analysis was employed in defining the mineralogical characteristics of the tailing sample. The mineralogical analysis of the tailing sample showed monazite as the prominent REE- bearing mineral, followed by zircon. Other minerals such as epidote, amphibole, rutile, quartz, leucoxene, titanite and almandine were identified in the sample. The results also revealed that the mineralogy of the sample varies per size fraction. The concentrations of REE in other minerals were confirmed in zircon, leucoxene, titanite and almandine by means of EMPA. The mineralogy findings showed that zircon and monazite are well liberated, with the majority of these minerals distributed in the–150+106μm and –106μm finer fractions. Approximately 50 mass% of the sample, constituting the finer fraction, has concentrated monazite and zircon. The naturally concentrated monazite and zircon in the finer size fractions showed that the fraction does not require ore upgrading and it is amenable to direct leaching. Subsequent to the mineralogical findings, the leaching testwork was carried out on the combined –150+106μm and –106μm finer fractions in three stages: caustic cracking, water leaching and HCl leaching. The leached products and residues were investigated for their REE extraction success. The extraction findings showed a 55% extraction efficiency of rare earth elements extracted from monazite only. The mineral zircon was identified as an alternative source of REE, apart from monazite, although processing of zircon proved to be inefficient.