A METEORIC WATER COMPONENT TO ORE FORMING FLUIDS IN PORPHYRY COPPER SYSTEMS: EVIDENCE FROM SIMS OXYGEN ISOTOPE ANALYSIS

Using SIMS δ¹⁸O analysis of magmatic and vein quartz from the giant Qulong Cu-Mo deposit (Tibet, China, 11 Mt Cu and 0.5 Mt Mo) we demonstrate that ore fluids associated with porphyry ore formation contain ≥10 % meteoric water. Magmatic quartz from pre- and syn-ore intrusion possess typical magmatic water δ¹⁸O values (7.8-8.9‰). A quartz vein formed at the magmatic-hydrothermal transition stage (~650 ^oC) suggests a minimum of 15 % contribution from meteoric water is essential to yield the observed isotopic composition (5.4 ‰).

Coupled with high-precision molybdenite Re-Os dates, meteoric-magmatic water interaction is consistent across bulk Cu-Mo deposition process. We invoke that a temperature drop caused by mixing between meteoric and magmatic water is responsible for the deposition of metals in Qulong.

Our data indicates surface water is incorporated into the system as early as the magmatic-hydrothermal transition stage, and occupies a significant component of the total hydrothermal fluid budget which is associated with the formation of the porphyry deposits. This study highlights the importance of meteoric water involvement in porphyry hydrothermal ore formation processes as a possible mechanism to activate metal deposition.