Episodic late Triassic and early Cretaceous gold mineralization events and associated tectonothermal processes in the Youjiang basin, SW China: Evidences from hydrothermal rutile and quartz inclusions

Abstract

The Youjiang basin in southwestern China represents a world-class gold province hosting structurally controlled, disseminated gold deposits, such as the pyroclastic-hosted Bieli and Dixu deposits. The high-Ti concentrations in Emeishan basalts and associated ore-hosting pyroclastic rocks facilitated extensive hydrothermal rutile formation, providing critical constraints on regional mineralization age and fluid provenance. High-precision U-Pb dating of syn-ore hydrothermal rutile constrains two discrete mineralization episodes: a late Triassic event (Dixu: 213.4 ± 13.6 Ma) and an early Cretaceous event (Bieli: 138.4 ± 3.5 Ma). These ages establish a unified basin-scale spatiotemporal framework and demonstrate that the late Triassic mineralization represents a regional-scale tectonothermal event rather than a localized hydrothermal process. Integrated hydrothermal rutile geochemistry, syn-ore fluid inclusion petrography and microthermometry, and H-O isotope systematics reveal these mineralizing systems exhibit fundamentally distinct fluid characteristics: main-ore stage fluid inclusions in Bieli show lower δ¹⁸O_H2O values (mean 9.06 ‰) and higher salinity (mean 6.69 wt% NaCl eq.), with W-REE-enriched rutile containing sericite micro-inclusions, whereas Dixu displays higher δ¹⁸O_H2O (mean 10.2 ‰) and lower salinity (mean 5.49 wt% NaCl eq.), with Zr-Nb-enriched rutile containing monazite and zircon micro-inclusions. These results reveal the late Triassic mineralization is resulted from mixing between magmatic fluids and deep-circulating fluids with metamorphic basement contribution, whereas the early Cretaceous mineralization preserving a dominant magmatic signature with subordinate metamorphic component inputs. These differences could be attributed to contrasting geodynamic regimes related to major tectonic processes at the margin or in the far-field of the basin. This study establishes hydrothermal rutile as a reliable chronometer and fluid tracer, and highlighting how the compressional-to-extensional tectonic transition reorganized fluid sources and pathways in the Youjiang basin.