Mineralization and time scales of the Yulong porphyry Cu-Mo deposit in eastern Tibet, China: Insights derived from hydrothermal quartz and tungsten-bearing rutile

Hydrothermal minerals intimately associated with mineralization processes are pivotal in deciphering the genesis of porphyry deposits. A meticulous examination of these hydrothermal minerals serves to quantitatively delineate the progression of ore-forming hydrothermal fluids and to scrutinize the myriad factors influencing porphyry Cu-Mo mineralization. In this study, we employed a multifaceted approach encompassing mineral geochemistry, geochronology, and diffusion chronology to probe into the time scales of mineralization, as well as to characterize and trace the evolution of ore-forming fluids within the Yulong porphyry Cu-Mo deposits located in eastern Tibet.

Quartz extracted from hydrothermal veins displays cathodoluminescence (CL) images characterized by diverse brightness and textures. Trace element analyses reveal a robust correlation between the intensity of CL and the titanium (Ti) content within the quartz. This study takes advantage of diffusion chronology to determine time scales of multistage magma-related hydrothermal events. The pronounced Ti concentration gradients observed in CL images, in conjunction with Ti diffusion modeling for distinct quartz generations, suggest that the majority of mineralization at the Yulong deposit occurred within a relatively brief interval ranging from 880,000 to 16,000 years. The complex, multi-stage hydrothermal stockwork veins and the relatively short time scales indicate that fluid pulses at Yulong developed rapidly, within spans of tens of thousands of years. These research outcomes underscore that delineating the time scale of a singular mineralization pulse is instrumental in constructing a more precise chronological framework for porphyry deposits, thus facilitating the quantification of extensive metal enrichment processes. In comparison with other globally recognized giant porphyry deposits, this study identifies the mineralization rate, magma injection rate, and fluid flux as critical determinants influencing the magnitude of porphyry mineralization.

Hydrothermal rutile (TiO₂), a common accessory mineral found in hydrothermal veins and alteration assemblages of porphyry deposits, offers significant insights into the characteristics of hydrothermal fluids. In the Yulong deposit, TiO₂ polymorphs have been identified through Raman spectroscopy, textural analysis, and chemical characterization. Brookite and anatase pseudomorphs are indicative of low-temperature hydrothermal fluids that destabilize primary Ti-bearing minerals during argillic alteration processes. Rutile intergrown with sulfides in veins exhibits well-defined patchy and sector zoning, with notable tungsten enrichment in the backscattered bright patches and sector zones. The enrichment of tungsten is effectively facilitated by halogen-rich (F, Cl) aqueous fluids during the hydrothermal mineralization. Consequently, the chemical and isotopic compositions preserved in rutile provide comprehensive information that enhances our understanding of the hydrothermal fluids active during the formation of porphyry deposits. This enhanced understanding may potentially aid in delineating vectors that lead to the localization of porphyry deposits. Moreover, precise identification of TiO₂ polymorphs is crucial for a deeper comprehension of hydrothermal processes, especially when employing rutile geochemistry as an indicator for mineralization.