Structural deformation and hydrothermal fluid migration in the niobium enrichment of the Yushishan deposit, eastern Altyn, northwestern China

The role of tectonic deformation in controlling hydrothermal fluid migration and niobium (Nb) enrichment in orogenic rare-metal deposits is still not well understood. This study explores the interplay of solid-state deformation, hydrothermal fluid flow, and Nb mineralization in the Yushishan deposit, located in the Altyn Mountains, northwestern China. Integrated structural, mineral microtextural, electron backscatter diffraction, fluid inclusion, and H-O isotope analyses reveal that Nb was pre-enriched in Neoproterozoic alkaline volcanic rocks and was subsequently modified and re-enriched by early Paleozoic tectono-hydrothermal events. Ductile shear zones provided permeable pathways for deep-derived F−-Cl−-CO2-bearing fluids, as evidenced by foliation-parallel fluid inclusion planes. Syn-tectonic alteration of amphibole, pyroxene, and titanite released Nb, Y, and rare earth elements (REEs), which were transported as soluble complexes. Fluid alkali consumption and CO2 input lowered fluid pH, enhancing Nb solubility. Although the complex stability initially inhibited precipitation, tectonic focusing of fluids facilitated extensive water-rock interaction and localized Nb enrichment, particularly along the margins of altered minerals, where spikes in Fe2+ destabilized the complexes. Orebody formation occurred in brittle-ductile to brittle domains, controlled by the coupling of deformation-induced permeability and chemical feedbacks. Shifts in the strain regime enhanced fluid pathways, enabling mixing between deep CO2− and NbF6−-YF63−−rich fluids and external alkaline or Ca-rich magmatic or hydrothermal fluids. This fluid mixing triggered abrupt changes in pH and/or redox conditions, leading to complex destabilization and precipitation of Nb minerals. These minerals occur as inclusions hosted within magnetite and ilmenite, or as microveins, providing evidence of strain-fluid pulsation. Furthermore, fault-valve pressure cycling induced fluid immiscibility and boiling, further disrupting complex stability. Our findings highlight that tectonically driven fluid migration is crucial for Nb enrichment, offering a structural-geochemical framework for the exploration of orogenic rare-metal deposits.