Mineralizations of Nb-Ta-Li and rare-earth elements in the Jiabusi rare-metal deposit, Inner Mongolia, China: Constraints from monazite, cassiterite, and bastnasite U-Pb geochronology and geochemistry

The Jiabusi Nb-Ta-Li deposit is a medium-sized rare-metal deposit discovered in central Inner Mongolia in recent years, and it is accompanied by Rb-Cs-REEs mineralization. Its magmatic and metallogenetic ages, magmatic source attributes, and mechanisms of magmatic evolution and related mineralization are not known. To identify these characteristics, with multi-stage granitic intrusions of the Jiabusi deposit as study objects, investigations relating to petrography, lithogeochemistry, monazite-cassiterite-hydrothermal bastnasite LA-ICP-MS U-Pb geochronology, and Nd isotopes were systematically conducted. The following results were obtained: The LA-ICP-MS monazite U-Pb ages of the metallogenetic Li-mica albite granite and granitic pegmatite are 148 +/- 7 and 141 +/- 2 Ma and 141 +/- 3, 137 +/- 2, 136 +/- 3, and 132 +/- 2 Ma, respectively; the cassiterite U-Pb ages of the Ta-Rb ore bodies are 148 +/- 2 and 146 +/- 1 Ma; and the bastnasite U-Pb age of the magmatic hydrothermal fluid is 116 +/- 1 Ma. These ages indicate that there were two episodes of granitic magmatism and related rare (rare-earth) metal mineralization in the Jiabusi area during the Yanshanian period (Jurassic-Cretaceous). Geochemical analysis results reveal that the barren biotite granite is characterized by high SiO2 (75.89 wt%) and high alkali (Na2O + K2O = 8.48 wt%) and a relatively high FeOT/(MgO + FeOT) value (0.92) and a relatively high Zr + Nb + Ce + Y content (393x10-6), whereas Al2O3 (12.35 wt%), CaO (0.51 wt%), and MgO (0.13 wt%) contents are all relatively low. These results indicate that the biotite granite is A-type granite, with characteristics similar to those of A1-type granite. The Li-mica albite granite is characterized by high F (0.95%-2.76%) and strongly peraluminous nature (A/CNK = 1.26-2.34). Meanwhile, it is strongly enriched in Cs, Li, Rb, Nb, and Ta but extremely depleted in Ba, Sr, P, Ti, Zr, and light REEs, with quite a strong negative Eu anomaly (delta Eu <0.01), an evident lanthanide tetrad effect (TE1,3 = 1.36-1.44), and Zr/Hf (3.0-4.0) and Nb/Ta (0.5-1.0) ratios deviating from those of a normal magmatic system. X-ray fluorescence and electron probe microanalysis surface scanning imaging analysis revealed the activity of F-REE-rich hydrothermal fluid and related REE mineralization in the uppermost part of altered granites. These features indicate that the metallogenetic ore bodies underwent higher degrees of magmatic fractionation and F-rich fluid-present melt-hydrothermal interaction. The Li-mica albite granite and multi-stage granitic pegmatite have consistent Nd isotope characteristics (epsilon Nd(t) =-3.4 to-3.9) and relatively concentrated two-stage model ages (TDM,2 = 1174-1247 Ma), suggesting that they are quite likely syngeneic multi-stage products with different degrees of fractional crystallization. Through comprehensive analysis of the results in this study and from previous research, the Yanshanian (Jurassic-Cretaceous) granite in the Jiabusi area is inferred to quite likely have originated from the partial melting of the ancient lower crust caused by lithospheric thinning and asthenospheric upwelling along the northern margin of the North China Craton, and it is revealed that a large amount of juvenile crustal materials had contributed to this process.