Abstract
AbstractThe Cretaceous Okorusu carbonatite, Namibia, includes diopside-bearing and pegmatitic calcite carbonatites, both exhibiting hydrothermally altered mineral assemblages. In unaltered carbonatite, Sr, Ba and rare earth elements (REE) are hosted principally by calcite and fluorapatite. However, in hydrothermally altered carbonatites, small (<50 µm) parisite-(Ce) grains are the dominantREEhost, while Ba and Sr are hosted in baryte, celestine, strontianite and witherite. Hydrothermal calcite has a much lower trace-element content than the original, magmatic calcite. Regardless of the lowREEcontents of the hydrothermal calcite, theREEpatterns are similar to those of parisite-(Ce), magmatic minerals and mafic rocks associated with the carbonatites. These similarities suggest that hydrothermal alteration remobilisedREEfrom magmatic minerals, predominantly calcite, without significant fractionation or addition from an external source. Barium and Sr released during alteration were mainly reprecipitated as sulfates. The breakdown of magmatic pyrite into iron hydroxide is inferred to be the main source of sulfate. The behaviour of sulfur suggests that the hydrothermal fluid was somewhat oxidising and it may have been part of a geothermal circulation system. Late hydrothermal massive fluorite replaced the calcite carbonatites at Okorusu and resulted in extensive chemical change, suggesting continued magmatic contributions to the fluid system.
Highlights
The rare earth elements (REE) are well known as ‘critical metals’ – metals which are sourced from restricted regions and have low substitutability and recyclability (Wall, 2014; European Commission, 2017; US Geological Survey, 2018)
In this study we investigate the mineralogy of the Okorusu Carbonatite Complex, Namibia, in order to understand the mechanism and scale of hydrothermal reworking of REE in a carbonatite
We have argued above that the growth of parisite and other hydrothermal stage 3a phases in the calcite carbonatites resulted from breakdown of magmatic minerals, mainly calcite, releasing REE, Ba and Sr
Summary
The rare earth elements (REE) are well known as ‘critical metals’ – metals which are sourced from restricted regions and have low substitutability and recyclability (Wall, 2014; European Commission, 2017; US Geological Survey, 2018). The largest and highest-grade REE deposits are associated with carbonatite complexes (Chakhmouradian and Wall, 2012; Chakhmouradian and Zaitsev, 2012; Wall, 2014; Verplanck et al, 2016) While this association demonstrates that magmatic processes are required for REE concentration, the REE as a group are mobile in some hydrothermal fluids (Linnen et al, 2014), and hydrothermal activity can play a major role in the development of economic concentrations of REE The REE are believed to be transported predominantly as Cl– complexes, reflecting the dominance of Cl– over other potential ligands in most natural fluids (Banks et al, 1994), as well as the greater solubility of REE-chloride complexes relative to fluoride, phosphate and carbonate complexes (Williams-Jones et al, 2012)
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