Shihong Tian

Formation of carbonatite-related giant rare-earth-element deposits by the recycling of marine sediments

Carbonatite-associated rare-earth-element (REE) deposits are the most significant source of the world’s REEs; however, their genesis remains unclear. Here, we present new Sr-Nd-Pb and C-O isotopic data for Cenozoic carbonatite-hosted giant REE deposits in southwest China. These REE deposits are located along the western margin of the Yangtze Craton that experienced Proterozoic lithospheric accretion, and controlled by Cenozoic strike-slip faults related to Indo-Asian continental collision. The Cenozoic carbonatites were emplaced as stocks or dykes with associated syenites, and tend to be extremely enriched in Ba, Sr, and REEs and have high 87Sr/86Sr ratios (>0.7055). These carbonatites were likely formed by melting of the sub-continental lithospheric mantle (SCLM), which had been previously metasomatized by high-flux REE- and CO2-rich fluids derived from subducted marine sediments. The fertility of these carbonatites depends on the release of REEs from recycled marine sediments and on the intensity of metasomatic REE refertilization of the SCLM. We suggest that cratonic edges, particularly along ancient convergent margins, possess the optimal configuration for generating giant REE deposits; therefore, areas of metamorphic basement bounded or cut by translithospheric faults along cratonic edges have a high potential for such deposits.

Lithium isotope traces magmatic fluid in a seafloor hydrothermal system.

Lithium isotopic compositions of fluid inclusions and hosted gangue quartz from a giant volcanogenic massive sulfide deposit in China provide robust evidence for inputting of magmatic fluids into a Triassic submarine hydrothermal system. The δ7Li results vary from +4.5‰ to +13.8‰ for fluid inclusions and from +6.7‰ to +21.0‰ for the hosted gangue quartz(9 gangue quartz samples containing primary fluid inclusions). These data confirm the temperature-dependent Li isotopic fractionation between hydrothermal quartz and fluid (i.e., Δδ7Liquartz-fluid = –8.9382 × (1000/T) + 22.22(R2 = 0.98; 175 °C–340 °C)), which suggests that the fluid inclusions are in equilibrium with their hosted quartz, thus allowing to determine the composition of the fluids by using δ7Liquartz data. Accordingly, we estimate that the ore-forming fluids have a δ7Li range from −0.7‰ to +18.4‰ at temperatures of 175–340 °C. This δ7Li range, together with Li–O modeling , suggest that magmatic fluid played a significant role in the ore formation. This study demonstrates that Li isotope can be effectively used to trace magmatic fluids in a seafloor hydrothermal system and has the potential to monitor fluid mixing and ore-forming process.

Muluozhai REE deposit in Sichuan Province, China: Stable isotope data and their implications on the dynamics of mineralization

The Muluozhai REE deposit, located about 60 km to the southwest of the Mianning county, Sichuan Province, is the third largest light REE deposit in Sichuan. The Muluozhai REE deposit is tectonically located on the northern Jinpingshan Mountains, a Cenozoic intracontinental orogenic belt in southwestern China. The authors analyzed the essential ores from two tunnels for their carbon, hydrogen, oxygen, and sulfur isotopic compositions of the mineralizing fluid. The δ13CV-PDB values of fluid from fluid inclusions of the quartz and fluorite vary from −2.5 to −9.0 per mil, which show characteristics of mantle-derived carbon. The δDV-SMOW values of fluid from fluid inclusions of the calcite, quartz, fluorite, and bastnaesite range from −63 to −87 per mil, which are characteristics of mantle-derived hydrogen. The δ34SV-CDT values of barite and galena vary in a narrow range of +0.1 to +2.2 per mil and −8.6 to −9.3 per mil, respectively, showing the isotopic characteristics of mantle-derived sulfur. The δ13CV-PDB values and the δ18OV-SMOW values of calcite range from −6.6 to −6.8 per mil and from +8.4 to +9.1 per mil, respectively, which are fallen into the range of the “primary carbonatites”, showing the carbon and oxygen in the ores of the Muluozhai ore veins were mainly derived from depths. The stable isotopic data suggest a mantle source for the rare earth elements mineralization and a dynamic process involving mantle materials and tectonics.