The genesis of carbonate minerals in the Dong Pao REE deposit, northwest Vietnam

Abstract

The Dong Pao REE deposit in northwest Vietnam contains REE-fluorocarbonates, barite and fluorite. It has been proposed that this deposit is carbonatite-related, but carbonatite has not yet been properly identified, described and analysed. In this study, a magmatic-hydrothermal assemblage comprising calcite-barite- celestine±tainiolite with an accessory amount of apatite and pyrochlore has been identified. Calcite has elevated SrO (2.50 wt. % on average), MnO (0.54 wt. % on average), and REE (1250 ppm on average) and displays enrichment of LREE over HREE. Calcite has δ13C values of -4.7–-2.7‰VPDB and δ18O values of +9.3–+12.5‰VSMOW, with half of the analyses falling within the field of primary igneous carbonatite. Apatite in the assemblage also has an elevated Sr/Mn ratio (>100:1), indicative of a carbonatitic origin. REE-fluorocarbonates in a hydrothermal assemblage contain calcite, synchysite, celestine and fluorite. The hydrothermal calcite was formed subsequent to synchysite, celestine and fluorite. It is typified by MnO>SrO, elevated REE concentrations (total REE ≥500 ppm) and a flat chondrite-normalized REE pattern. Geochemical signatures of apatite and calcite C- O isotopic compositions reflect a common signature with the magmatic-hydrothermal assemblage. This is interpreted as a product of the evolving carbonatitic fluid after sequestration of LREE by REE-fluorocarbonates. Calcite with such an enriched and flat REE signature could be a pathfinder of potential REE deposits in carbonatitic systems. Being 100- times enriched over chondrite, this hydrothermal calcite might have a potential to be exploited for HREE. In-situ LA-ICP-MS analyses on Sr isotope systematics reveal a high 87Sr/86Sr ratio, 0.7078–0.7081, for all carbonatitic and carbo-hydrothermal calcite and apatite. Enrichment of Li, as indicated by the presence of tainiolite (KLiMg2Si4O10F2), Ba, Sr and REE characterises the nature of the Dong Pao carbonatite. Such a nature is consistent with the REE-mineralizing carbonatites in the eastern Indo-Asian collision zone (EIACZ). These geochemical signatures suggest an involvement of enriched mantle component. Subduction of marine sediments could have fertilized the sub-continental lithospheric mantle at the edge of the South China Block and laid the foundation for regionwide REE mineralization in the EIACZ.