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

Abstract

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 reflects the evolution from magmatic carbonatite, to more silica-rich conditions during early hydrothermal processes, to fully hydrothermal conditions accompanied by the formation of sulphate minerals. Each alteration stage resulted in the preferential leaching of the LREE and enrichment in the HREE. Mass balance considerations indicate hydrothermal fluids must have contributed HREE to the mineralisation. The evolution of the fluorcarbonate mineral assemblage requires an increase in a Ca 2+ and a CO 3 2− in the metasomatic fluid (where a is activity), and breakdown of HREE-enriched calcite may have been the HREE source. Leaching in the presence of strong, LREE-selective ligands (Cl − ) may account for the depletion in late stage minerals in the LREE, but cannot account for subsequent preferential HREE addition. Fluid inclusion data indicate the presence of sulphate-rich brines during alteration, and hence sulphate complexation may have been important for preferential HREE transport. Alongside HREE-enriched magmatic sources, and enrichment during magmatic processes, late stage alteration with non-LREE-selective ligands may be critical in forming HREE-enriched carbonatites.

The Petyayan-Vara Carbonatite-Hosted Rare Earth Deposit (Vuoriyarvi, NW Russia): Mineralogy and Geochemistry

“…However, high-Ti carbonatites and especially their apatite-rich areas are noticeably enriched in HREE compared to all other rocks of the Petyayan-Vara field, including Ba-Sr-REE carbonatites. Late carbonatites with HREE mineralization associated with late-stage hydrothermal phenomena are found in many complexes (e.g., Tundulu, Malawi [80]; Songwe-Hill, Malawi [47]; Bear Lodge, USA [68]; Lesser Qinling, China [34]; Fen, Norway [58]). Researchers have presented several hypotheses for the HREE enrichment of late carbonatites.…”

“…As a result of these mechanisms, xenotime-(Y) [34,69,133], brockite-(Y) [69], mckelveyite-(Y), ewaldite, donnayite-(Y) [78], and other HREE mineral phases were formed in these rocks. In carbonatites of the Petyayan-Vara and a number of other complexes (e.g., [47,80,81]), the accumulation of HREE is related to fluorapatite (see Section 4.2).…”

“…Therefore, rare earth carbonatites are a major focus of critical metal research. Over the past five years, several dozens of papers have been published on the subject of rare earth carbonatites (RE carbonatites) in Angola [5,6], Australia [7][8][9], Brazil [10,11], Canada [12][13][14][15][16][17], China [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37], Finland [38], India [39][40][41][42], Italy [43,44], Malawi [45][46][47][48][49][50][51], Mongolia [52,53], Namibia…”

Röentgenite-(CE) and other ree fluorcarbonates from vein no. 35, Vishnevye mountains, Southern Urals

“…Агрегаты фторкарбонатов Рзэ развиваются по более ранним фазам -кальциту, монациту, алланиту, Рзэ-обогащенному апатиту и другим минералам. Все минералы диагностированы по химическому составу (Smith et al, 2018). Упоминание о находке рентгенита-(Се) на месторождении Мяньнинь (Mianning) в китайской провинции Сычуань не сопровождается какой-либо аналитикой (Anthony et al, 2003).…”

Critical metal mineralogy and ore genesis revisited: thematic set arising from the Third International Critical Metals Meeting, Edinburgh

“…Such breakdown would release sulfate into a hydrothermal fluid, which the authors suggest is responsible for the alteration of primary REE-bearing phases and the subsequent re-distribution of REE into newlyformed REE minerals. Continuing the theme of the importance of sulfate, Cangelosi et al (2020b) present work on the Dashigou carbonatite, China, where early-formed monazite is hydrothermally replaced by a relatively HREE-rich mineral assemblage (Smith et al, 2018). In this second contribution, Cangelosi et al (2020b) demonstrate that the fluid responsible for the hydrothermal replacement is preserved in fluid inclusions trapped in quartz lenses in the carbonatite.…”