Effect and significance of contamination-related compositional melt variations of the Tweerivier and Bulhoekkop carbonatites, South Africa, with special reference to their REE mineralization
Final Report Abstract
Carbonatites represent important rare earth elements (REE) deposits, which is reflected by the fact that REE exploration projects worldwide are dominated by carbonatites and accompanying rock types. The extraordinary REE enrichment in carbonatitic melts is generally attributed to a combination of parameters, including low-degree partial melting of enriched mantle sources, crystal fractionation, separation of fluid phases and hydrothermal alteration. However, the effect of contamination by crustal and cogenetic rocks, as a potential first order process is only weakly constrained. The funded project combines investigations on the Gross Brukkaros, Dicker Willem, Kieshöhe carbonatite complexes in Namibia and the Bulhoek, Tweerevier, Welgevonden, Nooitgedacht and Palabora carbonatite complexes in the Republic of South Africa. The results clearly provide evidence that the interaction of carbonatite magma with silicate wall rocks and xenoliths can significantly influence the REE mineralization of carbonatites. This concerns both the magmatic mineralisation of discrete REE minerals, which can exhibit systematic changes in the type REE minerals occurring with the intensity of contamination (e.g. the specific occurrence of allanite and britholite), and the magmatic mineralisation of REE-bearing minerals, such as apatite, which specifically incorporates REE through contamination (Si involved britholite substitution) and thus depletes the residual melt in REE. Furthermore, hydrothermal postmagmatic processes can also be strongly influenced by the presence of silicates (formed by prior magmatic xenolith input/contamination) during the redistribution of REE, as silica-enriched fluids in the carbonate-rich system possess different ligand properties. The chosen examples mentioned above are known to bear a large variability of crustal xenoliths (BIF, sedimentary dolomites, granites, anorthosites and gabbros but also alkaline igneous rocks) and contain variable amounts of silicate minerals (e.g., amphibole, clinopyroxene, mica, tremolite) that can be related to the effect of crustal contamination. The case study of Gross Brukkaros, Dicker Willem, Welgevonden, Nooitgedacht and Palabora clearly show the influence of crustal contamination by various chemically contrasting contaminants whereas the Kieshöhe case study show the influence of silica-rich xenoliths on the hydrothermal and supergene stages and related REE enrichments. Hence crustal contamination of carbonatite melts is a first order process that needs to be considered at each stage of mineralization and redistribution.
Publications
-
Silicates in carbonatites – origin and interpretation. Goldschmidt2022 abstracts. European Association of Geochemistry.
Giebel, R. Johannes; Marks, Michael A.W.; Walter, Benjamin & Markl, Gregor
-
The Kieshöhe carbonatites of southwestern Namibia–the post-magmatic role of silicate xenoliths on REE mobilisation. Communications of the Geological Survey of Namibia, 25, 1-31
Walter, B. F., Giebel, J., Marlow, A. G., Siegfried, P. R., Marks, M., Markl, G., Palmer, M. & Kolb, J.
-
Crustal contamination and rare earth element enrichment in the Nooitgedacht Volcano, South Africa: Petrographic, mineralogical, and isotopic insights into carbonatite evolution. GeoBerlin. 03.-07.09.23. Berlin, Germany
Raza, M., Giebel, R.J., Kolb, J. & Walter. B.F.
-
Deep-stuck Ti-rich carbonatites: A link to the composition of primitive carbonatite melts? GeoBerlin. 03.-07.09.23. Berlin, German
Giebel, R.J., Walter, B.F., Marks, M.A.W. & Markl, G.
-
Mineralogy and origin of a silico carbonatite mega xenolith in the alvikites of the Dicker Willem Complex (Namibia). GeoBerlin. 03.-07.09.23. Berlin, Germany
Kemmler, L., Giebel, R.J. & Walter. B.F.
-
The carbonatite melt – granite wall-rock interaction: A case study on the Bulhoek carbonatites, South Africa. GeoBerlin. 03.-07.09.23. Berlin, Germany
Sauter, O., Giebel, R.J. & Walter. B.F.
-
The eruption interface between carbonatitic dykes and diatremes – The Gross Brukkaros volcanic field Namibia. Chemical Geology, 621, 121344.
Walter, Benjamin F.; Giebel, R. Johannes; Siegfried, Pete R.; Gudelius, Dominik & Kolb, Jochen
-
Crustal wall-rock assimilation and metasomatism of carbonatite melts – key processes for HFSE enrichment? EMC 2024 (European Mineralogical Conference). 18-23.08.2024. Dublin, Ireland
Walter, B.F., Marks, M.A.W., Giebel, R.J. & Markl G.
-
Multi-Stage evolution of the Nooitgedacht Carbonatite: Apatite and Pyrochlore Geochronology using In-Situ LA-ICPMS U-Pb Dating. Petrology, Petrophysics and Geochemistry Section Meeting 2024, Karlsruhe
Raza, M., Giebel, R.J., Beranoguirre, A., Kolb, J. & Walter. B.F.
-
Pyrochlore: A window into magmatic and hydrothermal evolution of Nooitgedacht carbonatite, South Africa. 8th GOOD Meeting (Geology of Ore Deposits). 17-18.03.2024. Freiberg, Saxony, Germany
Raza, M., Giebel, R.J., Staude, S., Kolb, J. & Walter, B.F.
