In-situ geochronology and geochemistry of allanite, monazite and calcite in the Yushishan metasedimentary rock-hosted Nb-Ta-REE deposit (NW China)

The newly discovered Yushishan Nb-Ta-REE deposit is hosted by metasedimentary rocks (mainly leptynite) in the South Qilian orogen, northwestern China. However, key aspects of this deposit, including the precise timing of rare earth element (REE) mineralization, its metallogenic history, and the mechanisms driving REE enrichment, remain poorly constrained. This study integrates in-situ U-Pb geochronology of REE-rich minerals (allanite and monazite), in-situ geochemical and C-Sr isotopic analyses of calcite, and whole-rock Sr-Nd-Pb isotopic compositions to elucidate the deposit’s genesis. In-situ U-Pb dating of monazite and allanite constrains the timing of REE mineralization to ∼490 Ma, indicating a genetic connection between REE mineralization and regional alkaline intrusions. Petrological and geochemical characteristics identify four types of calcites (Cal-1 to Cal-4) from aegirine-augite syenite, syenite and mineralized leptynite. Cal-1 and Cal-3, characterized by higher (La/Lu) N ratios, are inferred to derive from magmatic fluids, whereas Cal-2 and Cal-4 likely originate from metamorphic fluids. Additionally, petrological and geochemical evidence confirms a hydrothermal origin for monazite and allanite in mineralized leptynite, highlighting the critical role of hydrothermal fluids in mineralization. Calcite C-Sr isotopic compositions indicate that ore-forming fluids incorporated components from both host rock marble and ancient strata (e.g. Daken Daban Group). Whole-rock Sr-Nd isotopes reveal a shared magma source for alkaline intrusions and leptynite, pointing to a hybridized lithospheric mantle-derived reservoir as the primary source of ore-forming materials. The metallogenic history of the Yushishan deposit is divided into two key stages: (1) a Neoproterozoic pre-enrichment stage (∼830 Ma) involving REE-enriched alkaline volcanism, which laid the foundation for subsequent mineralization; and (2) an Early Paleozoic metallogenic event (∼490 Ma) triggered by the northward subduction of the North Qilian Block beneath the Central Qilian Block. This subduction event induced asthenospheric upwelling, promoting partial melting of the lithospheric mantle to generate Nb-Ta-REE-rich melts. These melts mixed with lower crustal melts and intruded into the Aoyougou Formation leptynites. Subsequent magmatic-hydrothermal fluids then remobilized pre-existing REE reservoirs in the leptynite, culminating in REE mineralization. This multistage model underscores the combined role of magmatic-hydrothermal and metamorphic-hydrothermal systems in the activation, transport, and deposition of REEs in the Yushishan deposit.