New 40Ar/39Ar and (U-Th)/He dating for the Zhunuo porphyry Cu deposit, Gangdese belt, southern Tibet: implications for pulsed magmatic-hydrothermal processes and ore exhumation and preservation

Abstract

Understanding the magmatic-hydrothermal cooling and exhumation history is fundamental for investigating porphyry deposit formation and preservation. In the Zhunuo porphyry copper deposit of southern Tibet, hydrothermal biotite related to early potassic alteration likely formed from 14.7 ± 0.3 Ma (zircon U-Pb age) to 14.23 ± 0.13 Ma (molybdenite Re-Os age) and cooled below the closure temperature for 40Ar/39Ar system at 13.91 ± 0.05 Ma (hydrothermal biotite 40Ar/39Ar plateau age). The 40Ar/39Ar spectra of another hydrothermal biotite sample that was selectively replaced by chlorite record the timing of late phyllic alteration at 13.53 ± 0.08 Ma. Zhunuo hydrothermal alteration and mineralization likely ceased at 13.09 ± 0.07 Ma based on the 40Ar/39Ar plateau age of the fresh magmatic biotite crystals. The biotite 40Ar/39Ar systematics have not been reset by ~ 12.0 Ma post-mineralization granite porphyry within ore district, which is likely due to rapid cooling and/or absence of fluid exsolution for the granite porphyry. In combination with previous zircon U-Pb and molybdenite Re-Os data, two-pulses of magmatic-hydrothermal activity from 14.78 ± 0.11 to 14.44 ± 0.05 Ma and from 14.23 ± 0.13 to 13.53 ± 0.08 Ma can be recognized at Zhunuo. (U-Th)/He thermochronological data on zircon and apatite from the inter-mineralization monzogranite porphyry suggest three cooling events characterized by first-stage magmatic cooling at a rate of ~ 1850 °C/m.y., and two later phases of uplift and exhumation at ~ 70 °C/m.y. and ~ 10 °C/m.y. Approximately 1.4-km thickness of materials have been removed from the Zhunuo Miocene monzogranite porphyry during uplift and erosion, including a large volume of ore.