A fluid inclusion and stable isotope study of the Pebble porphyry copper-gold-molybdenum deposit, Alaska

- The Cretaceous-aged Pebble porphyry Cu-Au-Mo deposit is a world-class mineralized system.
- This work investigates the magmatic-hydrothermal fluid evolution using fluid inclusions and stable isotopic data.
- A magmatic intermediate-density parental fluid underwent phase separation during cooling and depressurization.
- Two brines of different salinity plus vapor are related to two variably mineralized hydrothermal alteration assemblages.
- A second pulse of magmatic fluid formed an advanced argillic alteration overprint and a zone of high grade mineralization.

The hydrothermal fluid evolution in the supergiant Pebble porphyry Cu-Au-Mo deposit in southwest Alaska has been constrained using fluid inclusion and light stable isotope data. The deposit is related to ~ 90 Ma granodiorite porphyry intrusions and mineralization occurs primarily in sodic-potassic, potassic, and advanced argillic alteration zones.Throughout the deposit vein quartz hosts intermediate-density fluid inclusion assemblages with an average salinity of 9.5 wt% NaCl equivalent and homogenization temperatures of 377 to 506 °C. Pressure corrections result in trapping temperatures with a maximum of 610 °C. This early fluid exsolved from a crystallizing magma at depth and underwent phase separation under varying conditions during cooling and depressurization forming two distinct fluid compositions. The two fluids formed two hydrothermal alteration assemblages which have different metal signatures. In the hydrothermal core in the eastern part of the system, high-density quartz veins, potassic alteration and the most economically significant mineralization are related to ~ 52 wt% NaCl equivalent brines that coexist with very low-density vapor. In the western part of the system peripheral to the hydrothermal core, lower quartz vein densities accompanied by sodic-potassic alteration and lower grade mineralization are associated with ~ 38 wt% NaCl equivalent brines and low-density vapor. Sulfide mineralization precipitated late in this early alteration event at temperatures between 375 and 330 °C.Fluids related to sodic-potassic alteration minerals have δ18O signatures from + 6.0 to + 10.5‰ VSMOW and δD values from − 70 to − 41‰ VSMOW, which are consistent with a magmatic fluid source. Fluids related to potassic alteration assemblages have δ18O of − 1.7 to + 2.3‰ VSMOW and δD of − 68 to − 59‰ VSMOW, which suggests that the magmatic fluids evolved to lighter oxygen isotope signatures due to quartz precipitation during alteration.Lower temperature (~ 280 °C) sericite and illite alteration followed the high-temperature magmatic stage and formed from the rising vapor plume as it mixed with varying amounts of meteoric fluid. Sericite in quartz-sericite-pyrite alteration on the periphery of the deposit and illite from areas of illite ± kaolinite alteration which overprints sodic-potassic alteration and forms quartz-illite-pyrite alteration within the deposit are related to low-salinity aqueous fluids that have a significant magmatic component (+ 2.1 to + 4.1‰ δ18O and − 76 to − 67‰ δD). Illite alteration which overprints both potassic alteration and quartz-sericite-pyrite alteration formed from fluids with a large meteoric component (− 9.1 to − 4.8‰ δ18O and − 101 to − 90‰ δD). The higher vein density and therefore higher permeability in the core of the deposit facilitated ingress of larger amounts of meteoric fluid compared with the more peripheral region.Advanced argillic alteration and associated high-grade mineralization are related to a second pulse of magmatic fluid which did not intersect the two phase field during cooling and depressurization. The fluid contracted to form a 3 wt% NaCl equivalent aqueous fluid that resulted in a sericite-pyrite-bornite-digenite bearing assemblage with a magmatic signature (4.6 to 6.5‰ δ18O and − 108 to − 102‰ δD) at 340 °C. Cooling and mixing of this fluid with meteoric water resulted in a fluid that formed a pyrophyllite-quartz-sericite-chalcopyrite bearing assemblage (− 8.9 to − 0.6‰ δ18O and − 110 to − 96‰ δD) at 300 °C. Compared to older potassic and sodic-potassic alteration, the advanced argillic alteration has a highly depleted δD signature which is attributed to degassing of a crystallizing intrusive fluid source.