The origin of secondary heavy rare earth element enrichment in carbonatites: Constraints from the evolution of the Huanglongpu district, China

The silico-carbonatite dykes of the Huanglongpu area, Lesser Qinling, China, are unusual in that they are quartzbearing, Mo-mineralised and enriched in the heavy rare earth elements (HREE) relative to typical carbonatites. The textures of REE minerals indicate crystallisation of monazite-(Ce), bastnäsite-(Ce), parisite-(Ce) and aeschynite-(Ce) as magmatic phases. Burbankite was also potentially an early crystallising phase. Monazite-(Ce) was subsequently altered to produce a second generation of apatite, which was in turn replaced and overgrown by britholite-(Ce), accompanied by the formation of allanite-(Ce). Bastnäsite and parisite where replaced by synchysite-(Ce) and röntgenite-(Ce). Aeschynite-(Ce) was altered to uranopyrochlore and then pyrochlore with uraninite inclusions. The mineralogical evolution re carbonatite, to more silica-rich conditions during early hydrothermal processes, to fully hydrothermal conditions accompanied by the formation of sulphateminerals. Each alteration stage resulted in the preferential leaching of the LREE and enrichment in the HREE.Mass balance considerations indicate hydrothermal HREE to themineralisation. The evolution of the in may have been the HREE source. Leaching in the presence of strong, LREE-selective ligands (Cl for the depletion in late stage minerals in the LREE, but cannot account for subsequent preferentialHREE addition. Fluid inclusion data indicate the presence of sulphate-rich brines during alteration, and hence sulphate complexationmay have been important for preferential HREE transport. AlongsideHREE-enrichedmagmatic sources, and enrichment during magmatic processes, late stage alteration with non-LREE-selective ligands may be critical in forming HREE-enriched carbonatites.flects the evolution from magmaticfluids must have contributedfluorcarbonate mineral assemblage requires an increaseaCa2+ and aCO3 2− in the metasomatic fluid (where a is activity), and breakdown of HREE-enriched calcite−) may account