Dehai Wu

Ion-adsorption-type heavy rare earth element (HREE) deposits are predominantly hosted in the weathering crusts of Yanshanian granites, whereas HREE deposits developed from Caledonian granites have rarely been reported. In this study, zircon and monazite U–Pb geochronology, whole-rock geochemistry, zircon Lu–Hf isotope analyses, and scanning electron microscopy (SEM) of REE-bearing minerals were conducted on the Caledonian Shangyou pluton, the parent rock of HREE mineralization, to investigate the mechanism of HREE pre-enrichment in the parent rock. Zircon U–Pb dating of two samples yielded weighted mean ages of 442 ± 3 Ma (MSWD = 0.20, n = 20) and 442 ± 4 Ma (MSWD = 1.15, n = 16), whereas monazite U–Pb dating yielded a Tera–Wasserburg lower-intercept age of 452 ± 7 Ma (MSWD = 2.30, n = 40), indicating that the pluton is Late Ordovician–Early Silurian (Katian–Rhuddanian) granite. The rocks are characterized by high SiO₂ (75.90–78.49 wt.%), Al₂O₃ (11.91–12.88 wt.%), and K₂O (4.04–4.95 wt.%) contents, A/CNK ratios of 1.13–1.25, differentiation indices (DIs) of 95–96, and zircon saturation temperatures of 716–740 °C. Their Nb/Ta and Zr/Hf ratios range from 3.70 to 5.91 and from 19.69 to 23.83, respectively, indicating that the Shangyou pluton is a highly fractionated S-type granite. Zircons from the two samples have ε_Hf(t) values of −15.28 to 0.86 and −9.33 to 1.49, corresponding to two-stage Hf model ages (T_DM2) of 1375–2392 Ma and 1335–2019 Ma, respectively. These isotopic characteristics suggest that the magma was derived predominantly from Proterozoic metasedimentary crust, with a minor contribution from mantle-derived material. The Shangyou pluton has ΣREY contents of 91.1–208.1 ppm and ΣLREE/ΣHREY ratios of 0.57–1.39, indicating favorable parent-rock conditions for the formation of ion-adsorption-type HREE deposits through weathering. HREE enrichment occurred during the late-magmatic and/or hydrothermal stages following extensive magmatic differentiation. During late magmatic–hydrothermal evolution, F-rich fluids metasomatized relatively weathering-resistant minerals crystallized during the late magmatic stage, producing weathering-prone REE-bearing minerals such as synchysite-(Y). This mineralogical transformation provided a favorable material basis for the subsequent formation of ion-adsorption-type REE deposits during weathering.

The Dashigou deposit is one of the most representative carbonatite-type Mo-REE deposits in the East Qinling metallogenic belt of China, with a molybdenum resource of more than 180 kt and a rare earth resource of 37.8 kt. Recent exploration has revealed a considerable scale of uranium mineralization within this deposit. Therefore, this study conducted detailed mineralogical and EPMA U-Th-Pb chemical dating on the uranium mineralization in the Dashigou deposit. The results indicate that the U-ore body in the Dashigou deposit mainly consists in carbonatite veins, and principally as anhedral, mesh-like uraninite. The mineral assemblage is characterized by uraninite + rutile + bastnasite + parisite or brannerite. The uraninite displays geochemical compositions of high Y and Ce and low Si, Ti, and Mg. The EPMA U-Th-Pb chemical dating is 144 ± 3.1 Ma, representing the Yanshanian uranium mineralization age in the region. The newly discovered uranium mineralization age indicates that the deposit experienced a uranium remobilization event during the Cretaceous and was formed in an intracontinental orogenic and extensional environment post-collision orogeny.