Yufei Wang

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.

Ion-adsorption rare earth deposits (IADs) constitute the primary global source of medium-to-heavy rare earth elements. The sustainable exploitation of these resources hinges on the precise characterization of the weathered crust architecture and its hydrogeological properties. Conventional drilling often fails to resolve the complex vertical heterogeneity of regolith profiles, thereby limiting orebody delineation and the optimization of in situ leaching (ISL). To address this, this study integrates Electrical Resistivity Tomography (ERT) with Archie’s Law and the Kozeny–Carman equation. Focusing on the YZK1 and YZK2 survey lines in the Dabu mining area, southern Jiangxi, we derived 2D distributions of porosity and permeability from resistivity inversions. Our results delineate the weathered crust into four distinct vertical strata: a surface accumulation layer, a completely-to-highly weathered granite layer, a moderately-to-slightly weathered layer, and fresh bedrock. Quantitatively, the completely-to-highly weathered layer exhibits a “low resistivity (10–700 Ω·m)–high porosity (8.7%–17%)–high permeability (0.04–1.2 mD)” mode, serving as the primary leachable reservoir. In contrast, the surface and moderately weathered layers display “high resistivity (>800 Ω·m)–low porosity (3.4%–6.4%)–low permeability (0.01–0.04 mD)” characteristics. Crucially, although the fractured bedrock zones on both YZK1 and YZK2 profiles exhibit similar absolute permeability values (~0.016 mD), they represent two distinct hydraulic architectures. On the YZK1 profile, the fractured zone acts as a relatively homogeneous potential leakage pathway. Conversely, the YZK2 profile displays significant vertical segmentation: the upper and lower margins function as permeable pathways, while the central core, likely infilled with fault gouge, acts as a sealing zone that impedes fluid flow. Consequently, resistivity data alone are insufficient to accurately assess hydraulic conductivity; joint interpretation incorporating both permeability and porosity is essential to definitively determine whether a fault zone acts as a “conduit” or a “barrier.” By transitioning from qualitative imaging to quantitative parameter evaluation, this study provides a robust technical paradigm for fine-scale exploration and ISL optimization in IADs.

Ion-adsorption rare earth deposits are mainly formed by the weathering and leaching of granite ore-forming parent rocks, and heavy rare earth elements (HREEs) are predominantly hosted in this type of deposit. In this study, we focused on the Late Jurassic REE mineralization parent rock, specifically the Shuitou pluton. We employed chronology, petrogeochemistry, and isotope geochemistry to elucidate the REE enrichment process in the granite. The results show that the zircon U–Pb age of the Shuitou pluton is ~150 Ma, and the monazite U–Pb age is ~145 Ma, suggesting that the pluton was formed in the Yanshan Stage. The rocks have high SiO₂ (72.85–75.55 wt%), Al₂O₃ (12.85–14.63 wt%), and K₂O (4.46–5.27 wt%) content, with A/CNK values of 1.05–1.19, differentiation index (DI) values of 87.48–95.59, zircon saturation temperature values of 689–746 °C, Nb/Ta ratios of 2.72–9.54, and Zr/Hf ratios of 7.12–26.11. In addition, the rocks also contain peraluminous minerals such as muscovite and garnet. These characteristics indicate that these rocks belong to highly fractionated S-type granite. The ε_Hf(t) values of zircon and monazite range from −10.04 to −6.78 and from −9.3 to −8.2, respectively, indicating that the magma was primarily derived from Proterozoic metamorphosed sedimentary rocks of crustal origin. In the extensional tectonic setting of South China, a high temperature promotes the melting of REE-enriched accessory minerals, and a higher content of F increases the solubility of REEs in the molten mass. The presence of heavy rare earth minerals, such as garnet, in these rocks contributes to a high content of heavy rare earth elements (HREEs). Additionally, REE-enriched minerals like titanite, bastnaesite, and allanite create the necessary material conditions for the formation of ion-adsorption REE deposits.

Ion-adsorption rare earth deposits are mainly formed by the weathering and leaching of the granite ore-forming parent rocks, and the heavy rare earth resources in the world dominantly occur within this type of deposits. In this study, we take the Shuitou pluton, the Late Jurassic rare earth element (REE) ore-forming parent rocks, as the research object, and elucidate the REE enrichment process in the granites through the chronology, rock geochemistry and isotope geochemistry analyses‌. The results show that the zircon U–Pb age of the Shuitou pluton is ~150 Ma, and the monazite U–Pb age is ~145 Ma, both indicating the pluton was formed in the Yanshan Stage. The rocks have high content values of SiO2 (72.85–75.55 wt%), Al2O3 (12.85–14.63 wt%), and K2O (4.46–5.27 wt%), with the A/CNK values of 1.05–1.19, the differentiation index (DI) values of 87.48–95.59, the zircon saturation temperature values of 689–746 ℃, the Nb/Ta ratios of 2.72–9.54, and the Zr/Hf ratios of 7.12–26.11; besides, the rocks also contain peraluminous minerals muscovite and garnet. All these indicate that rocks belong to highly fractionated S-type granite. The εHf(t) values of zircon and monazite range from –10.04 to –6.78 and –9.3 to –8.2, respectively, indicating that the magma mainly originated from the Proterozoic crustal metamorphic sedimentary rocks. In the extensional tectonic setting of South China, high temperature promotes the melting of the REE-enriched accessory minerals, and the higher content value of F increases the solubility of the REEs in the molten mass. The presence of the heavy rare earth minerals such as garnet in the rocks makes the rocks have a high heavy rare earth element (HREE) content, and the REE-enriched minerals such as titanite, bastnäsite, and allanite provide the material conditions for the formation of the ion-adsorption REE deposits.