Distribution of rare earth elements in the Epembe Carbonatite Dyke, Opuwo Area, Namibia
Date
2019
Authors
Kapuka, Ester P.
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Abstract
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.
Description
A research report submitted in partial fulfillment of the requirements for the degree of Masters of Science in Economic Geology (Course Work & Research Report) to the Faculty of Science, University of Witwatersrand, Johannesburg, 2019
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Citation
Kapuka, Ester Panduleni (2019) Distribution of Rare Earth Elements in the Epembe carbonatite dyke, Opuwo area, Namibia, University of the Witwatersrand, Johannesburg, <http://hdl.handle.net/10539/29428>