Formation of the Huayangchuan (Central China) carbonatite-associated REE-Nb-U polymetallic deposit constrained from monazite mineral chemistry and isotope systematics: HREE enrichment in late-stage monazite

The process of enrichment of middle and heavy rare earth elements (MREE and HREE = Sm-Lu plus Y) in magmatic-hydrothermal systems, especially the carbonatite system, remains unclear. Here, we performed in-situ monazite element and isotopic analysis to investigate the genesis of the Huayangchuan REE-Nb-U polymetallic deposit in central China and the enrichment process of MREE and HREE during the evolution of the magmatic hydrothermal system. The Huayangchuan deposit is spatially associated with HREE-rich calcite carbonatites, which exist as two mineralization stages: the ores of the first stage are hosted by carbonatite itself; in the second stage, the ores are present as veins, dominantly by wall rocks around the carbonatite. The results of secondary ion mass spectrometry (SIMS) monazite U-Pb dating on the thin sections yielded Tera-Wasserburg lower intercept ages of 207 +/- 4 Ma and 206 +/- 5 Ma for carbonatite-hosted monazite and vein-type monazite, respectively. However, some monazite analysis points provide a wide Yanshanian age range (112-182 Ma). The lower intercept age of 206-207 Ma is explained as the mineralization age of the deposit, whereas the wide younger age with higher 238U/206Pb and lower 208Pb/232Th ratios may be ascribed to the loss of radioactive Pb, as the varying degrees of reworking resulted from the later Yanshanian tectono-thermal event. The monazite laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) Sr-Nd isotopes from both carbonatite-hosted and wall-rock-hosted vein-type ores indicate that the Huayangchuan deposit is closely related to enriched mantle-derived (EM1) calcite carbonatite but locally affected by late fluid activation. Combined with the integrated age spectra, such reworking processes associated with new weak mineralization widely exist in other carbonatite-related deposits in the Lesser Qinling, but are not important for carbonatite-related mineral-ization in this area. Controlled by element behaviors, MREE and HREE are more likely to dissolve in hydro -thermal fluids, enabling their long-distance migration and deposition, than light REE (LREE) in some carbonatite systems, resulting in much higher MREE and HREE contents of the late monazite in the vein-type ores than those of the early carbonatite-hosted monazite. Therefore, it could be expected that there might be MREE and HREE enrichment and exploration potential in the periphery of some large carbonatite-related LREE-dominated deposits.

Automated summary

The genesis and enrichment process of middle and heavy rare earth elements in the Huayangchuan deposit were investigated through the analysis of monazite element and isotopic composition. The deposit is associated with HREE-rich calcite carbonatites, with mineralization occurring in two stages. The ages of the mineralization were determined to be 206-207 Ma, with younger ages attributed to reworking from later tectono-thermal events. The deposit is related to enriched mantle-derived calcite carbonatite, but locally affected by late fluid activation.