Title: The Kieshöhe carbonatites in SW-Namibia – the role of silicatic xenoliths for REE exploration

Benjamin Florian Walter1, R. Johannes Giebel2, Alan Marlow3, Michael Marks4, Gregor Markl4, Jochen Kolb1

1Karlsruhe Institute of Technology, Germany; 2Technische Universität Berlin, Germany; University of the Free State, Bloemfontein , South Africa; 3Shali Group, Windhoek, Namibia; 4University of Tübingen, Tübingen, Germany

Event: GeoKarlsruhe 2021

Date: 2021

DOI: 10.48380/dggv-28fg-gr68

Exploration of carbonatite-associated REE-deposits is challenging due to heterogeneous ore distribution and variable and often complex ore mineralogy. The Kieshöhe carbonatite in SW Namibia represents a subvolcanic occurrence hosting dolomite, calcite and ankerite carbonatite dykes, ring dykes and diatremes. Petrography, whole rock geochemistry and microXRF imaging provide insights into the role of silicate xenoliths for the REE mineralization in a subvolcanic environment. Xenolith-rich carbonatites are relatively REE-poor with only minor monazite mineralization, whereas xenolith-free carbonatites show high REE-contents incorporated primarily into REE-F-carbonates. Moreover, the additional presence of barite and pseudomorphic replacement of hexagonal precursor minerals suggest the former presence of burbankite, which might represent the potential REE source. The strong association of REE-minerals, barite and quartz furthermore indicates the simultaneous hydrothermal transport of REE, Ba, S and Si. In particular, Si-saturated hydrothermal fluids promote the potential of sulfate complexes to mobilize REE [1]. Since an enhanced xenolith resorption in xenolith-rich parts of the complex increases the Si content of the hydrothermal fluid and thus the capacity of REE transport, REE precipitation is inhibited in such sections but supported in xenolith-free sections. This demonstrates that xenolith entrainment not only has a strong influence on REE mineralization in synmagmatic stages [2], but may also have strong effects in post-magmatic hydrothermal stages of the carbonatitic system. This observation can potentially be used as a first-order field-based exploration indicator for REE-mineralization in carbonatites.

[1] Cui et al. (2020). Geology, 48(2), 145-148.

[2] Giebel et al. (2019). Journal of Petrology, 60(6), 1163-1194.

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