Formation conditions and REY enrichment of the 2060 Ma phosphorus mineralization at Schiel (South Africa): geochemical and geochronological constraints

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Rocks of the rare-earth element (REY)-enriched apatite deposit in the eastern part of the Schiel Alkaline Complex (SAC; Southern Marginal Zone, Limpopo Belt) were studied for their whole-rock and mineral chemistry, REY mineral distribution and geochronology. Apart from phoscorite (sensu lato), pyroxenite and various syenitic rock types with quite variable apatite contents display P-REY enrichments. Field observations, mineralogical composition as well as major and trace element chemistry of soils make it possible to constrain the distribution of the hidden P-REY-rich rock types in the apatite deposit. Uranium-lead ages of zircon from phoscorite (sensu lato) and syenite are in the range of 2.06–2.05 Ga. Samarium-neodymium (εNd(t) −8.6 to −6.0) and in part Rb-Sr (87Sr/86Sr(t) 0.70819–0.70859) isotope data for whole-rock samples and mineral separates indicate an origin from an isotopically enriched and slightly variable source. Fluorapatite, early allanite and titanite are the main REY carriers at Schiel. Fluorapatite dominates the REY budget of pyroxenite and phoscorite, whereas early allanite hosts most of the REY in syenite. Three apatite types are distinguished based on their occurrence in the rocks, REYtotal contents and colouration in cathodoluminescence microscopy. Magmatic apatite in pyroxenite and in phoscorite (sensu lato) as well as early stage type I/II apatite in syenitic rocks have moderate to high REYtotal abundances (up to 3.2 wt%) with the mineral enriched in light REE. Early ferriallanite-(Ce) is strongly enriched in light REE and shows very high REYtotal values (13.7–26.4 wt%), while late allanite has lower REYtotal concentrations (6.9–14.9 wt%). Titanite is abundant in most syenitic rocks (REYtotal 1.7–6.4 wt%); chevkinite-(Ce) occurs locally and contributes to an REY enrichment in contact aureoles between syenite and different lithologies. Apatite-enriched rocks in the SAC in part contain significantly higher REYtotal concentrations in apatite grains compared to those in apatite-mineralized pyroxenite, phoscorite and carbonatite from Phalaborwa.

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The permission of the Chief of the area to carry out field campaigns at Schiel is greatly acknowledged. Fabian Kemner, Malte Junge, Andzani Ndhukwani and local field guides are thanked for help during field work in the Schiel Alkaline Complex. The authors are grateful to Oscar Laurent for providing pyroxenite samples and for discussion. Monika Bockrath, Siegrid Gerlach, Christian Wöhrl, Hans Lorenz and Nikola Koglin are acknowledged for analytical assistance at the BGR. Hiltrud Müller-Sigmund (University of Freiburg) and colleagues kindly performed mineral separation. This paper contributes to the project RoStraMet of the BGR. Two anonymous reviewers of Mineralium Deposita and the handling editor Hartwig Frimmel provided useful comments, which considerably improved the manuscript.

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Reiner Klemd thanks the BGR (grant 203-10047988) for financial support.

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Correspondence to Torsten Graupner.

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Editorial handling: H. Frimmel

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ESM Fig. S1

Regional geology and main tectonic features of the Limpopo Mobile Belt (modified after Kramers et al. (2006)). The Schiel Alkaline Complex is situated within the Southern Marginal Zone (SMZ) in close contact to the prominent Hout River Shear Zone (JPEG 11848 kb)

ESM Fig. S2

Nomenclature diagram for the system 2REEPO4–CaTh(PO4)2–2ThSiO4 showing the endmember proportions of the monazite and thorite found in the SAC magmatites. Cation proportions were calculated on the basis of eight atoms of oxygen, H2O-free. Endmember proportions were calculated according to Linthout (2007) (JPEG 2664 kb)

ESM Fig. S3

X-ray diffraction of oriented mounts of clay fractions of air-dry (AD, black) and ethylene glycol solvated (EG, blue) samples. a Sample Sch4. b Sample Sch5 (JPEG 864 kb)

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Graupner, T., Klemd, R., Henjes-Kunst, F. et al. Formation conditions and REY enrichment of the 2060 Ma phosphorus mineralization at Schiel (South Africa): geochemical and geochronological constraints. Miner Deposita 53, 1117–1142 (2018) doi:10.1007/s00126-018-0791-7

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  • Schiel Alkaline Complex
  • Apatite-mineralized rocks
  • Soil geochemistry
  • Rare-earth elements
  • U-Pb, Lu-Hf, Sm-Nd and Rb-Sr isotopic compositions
  • Cathodoluminescence