Geochronology and fluid evolution of the Qulong porphyry system: Implications for porphyry deposit formation

Abstract

Understanding the metal enrichment process and rate in porphyry Cu systems (PCS) is critical to underpin the genetic model of PCS and refine the template for exploration, of which robust temporal constrain is the key. In addition, fluid evolution paths constrained by bulk analysis potentially suffer problems of contamination. Based on detailed field geology and petrographic study, this PhD thesis addresses the timescales and fluid evolution process of the world class Qulong porphyry Cu-Mo deposit, Tibet, China, by applying high precision geochronology and high spatial resolution isotope analysis.
A fluid inclusion study indicates that the bulk mineralization at Qulong was deposited between 425 and 280 oC under hydrostatic pressure conditions. The depth of formation of the Qulong porphyry Cu-Mo system is estimated at ~2.7 km, which implies ~2.3 km of erosion has occurred since its formation. Zircon CA-ID-TIMS U-Pb geochronology constrains the emplacement ages of the Rongmucuola pluton, the P porphyry and quartz aplite to 17.142 ± 0.014/0.014/0.023 (analytical/plus tracer/plus decay constant uncertainty), 16.009 ± 0.016/0.017/0.024 and 15.166 ± 0.010/0.011/0.020 Ma, respectively. Molybdenite ID-NTIMS Re-Os geochronology suggests that the bulk mineralization at Qulong was deposited through multiple shorted lived pulses (~ tens of kyrs) between 16.126 ± 0.008/0.060/0.077 and 15.860 ± 0.010/0.058/0.075 Ma, with a duration of 266 ± 13 kyrs. Quartz SIMS oxygen isotope analysis indicates a periodic interplay between meteoric and magmatic fluids and continuing increase of meteoric water from ~10 to ~25 % volume percent during the ore-forming process. As a result meteoric water is invoked as the main trigger for metal deposition at Qulong.
The major conclusions of this study from Qulong are supported by numerical modelling, titanium diffusion and high precision studies, and have implications for understanding porphyry systems worldwide, for example, multiple and cyclic magmatic-hydrothermal fluid pulses cooled by meteoric water are fundamental for ore formation.