The origin and composition of carbonatite-derived carbonate-bearing fluorapatite deposits
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The origin and composition of carbonatite-derived carbonate-bearing fluorapatite deposits Sam Broom-Fendley 1 & Pete R. Siegfried 1,2 & Frances Wall 1 & Mary O’Neill 1 & Richard A. Brooker 3 & Emily K. Fallon 3 & Jonathan R. Pickles 1 & David A. Banks 4 Received: 5 November 2019 / Accepted: 29 July 2020 # The Author(s) 2020
Abstract Carbonate-bearing fluorapatite rocks occur at over 30 globally distributed carbonatite complexes and represent a substantial potential supply of phosphorus for the fertiliser industry. However, the process(es) involved in forming carbonate-bearing fluorapatite at some carbonatites remain equivocal, with both hydrothermal and weathering mechanisms inferred. In this contribution, we compare the paragenesis and trace element contents of carbonate-bearing fluorapatite rocks from the Kovdor, Sokli, Bukusu, Catalão I and Glenover carbonatites in order to further understand their origin, as well as to comment upon the concentration of elements that may be deleterious to fertiliser production. The paragenesis of apatite from each deposit is broadly equivalent, comprising residual magmatic grains overgrown by several different stages of carbonate-bearing fluorapatite. The first forms epitactic overgrowths on residual magmatic grains, followed by the formation of massive apatite which, in turn, is cross-cut by late euhedral and colloform apatite generations. Compositionally, the paragenetic sequence corresponds to a substantial decrease in the concentration of rare earth elements (REE), Sr, Na and Th, with an increase in U and Cd. The carbonate-bearing fluorapatite exhibits a negative Ce anomaly, attributed to oxic conditions in a surficial environment and, in combination with the textural and compositional commonality, supports a weathering origin for these rocks. Carbonate-bearing fluorapatite has Th contents which are several orders of magnitude lower than magmatic apatite grains, potentially making such apatite a more environmentally attractive feedstock for the fertiliser industry. Uranium and cadmium contents are higher in carbonate-bearing fluorapatite than magmatic carbonatite apatite, but are much lower than most marine phosphorites. Keywords Carbonate-fluorapatite . Staffelite . Francolite . Weathering . Phosphate resources . Uranium
Introduction Editorial handling: R. Linnen Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00126-020-01010-7) contains supplementary material, which is available to authorized users. * Sam Broom-Fendley [email protected] 1
Camborne School of Mines and Environment and Sustainability Institute, University of Exeter, Penryn Campus, Cornwall TR10 9FE, UK
2
GeoAfrica Prospecting Services CC, PO Box 24218, Windhoek, Namibia
3
School of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS8 1RJ, UK
4
School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK
Phosphorus is essential to all life and is a limiting nutrient for plant growth (Cordell and White 2
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