The mineralogical distribution of the REE in carbonatites: A quantitative evaluation

The LREE enrichment of carbonatites is generally considered to be an intrinsic characteristic of the rocks. Although all carbonatite magmas are inherently LREE-enriched, not all carbonatites host REE minerals, and even fewer are sufficiently enriched in the REE to form economic deposits. The origin of these deposits is still debated, particularly in respect to the relative importance of magmatic and hydrothermal processes in the concentration of the REE. The evidence to support either origin has focussed on the mineralization, with little attention being paid to the contribution of the rock-forming minerals to the overall REE budget. We evaluate the changing REE contents of all minerals (primary and secondary) in the Ashram carbonatite-hosted REE deposit, Canada, and use these data, in combination with the changing proportions of each mineral, to calculate their contributions to the overall REE budget of the carbonatites. Our mass balance calculation traced each REE individually across all minerals from primary magmatic to secondary hydrothermal stages. The results of the calculation showed quantitatively that at Ashram, all the REE were mobilized by hydrothermal fluids and concentrated to varying degrees in REE minerals. The extent to which the individual REE were concentrated in these minerals, however, decreased with increasing atomic number (and/or decreasing ionic radius, e.g., Y), and with decreasing fluid flow (fluid-rock ratio), such that, in all zones, La is more strongly concentrated than Lu, and across zones, La is most strongly concentrated in areas of greatest fluid flow. These trends were the result of the relative thermo-dynamic stability of the various aqueous REE-complexes and REE minerals. Overall, magmatic and non-REE phases host a low proportion of the whole-rock REE concentration, especially of the LREE.