AN OPERATIVE PORPHYRY COPPER MODEL BASED ON THE REDOX THERMODYNAMICS AND TRANSPORT OF GASEOUS HYDROGEN

A thermodynamic diagram is developed here for redox equilibria in porphyry copper genesis using H₂ gas instead of O₂ which is not present. The H₂ vs 1/T Van’t Hoff reference frame at water saturation works as a powerful operative geochemical diagram because confined-flow migration of H₂ by magmatic outgassing and thermal advection causes oxidation by dehydrogenation and reduction in the regions where H₂ concentrates along the return flow heating path approaching the pluton. Expulsion of highly oxidized early high temperature aqueous fluids continue to lose H₂ and oxidize along a cooling path defined by the Mg-rich (X_Fe=0.2), Ti-poor hydrothermal biotite-magnetite-K-feldspar Potassic alteration mineral assemblage line which serves as definitive trajectory of the cooling pathway. The return heating pathway is demarked by Propylitic alteration and distal actinolite orbicular alteration. Two concentric advective circulation cells emerge in the log H₂ vs. 1/T space. The largest advective circulation cell defines sediment-hosted Bingham-type PCD systems which reach both extreme early oxidative states with chalcopyrite, bornite and digenite (chalcocite) and extreme reductive states with distal actinolite orbicular alteration. In contrast, the batholith-hosted Butte-type PCD cell has a narrow limited early oxidation range which does not extend up to bornite or digenite and consists of chalcopyrite-pyrite. It also lacks a distal orb ring due to restrictive chemical buffering by the influence of granite wall rocks with red-colored high-Ti (X_Fe=0.5) biotite terminating in the advanced argillic assemblage with covellite and chalcocite on the oxidized end. Occurrences of actinolite orbs at Bingham, Escondida, El Hueso, Cajamarca, Morenci, Fortitude, Cananea, and Oyu Tolgoi imply that an actinolite orb ring may be a definitive indicator of large PCD systems. Given the necessity of electron conservation, a large actinolite orb ring portends a large highly-oxidized Potassic alteration zone with chalcopyrite, bornite and digenite. This exploration target size indicator supported by H₂ dynamics and redox thermochemistry may provide focus and help guide discovery of new deep confined PCDs of considerable size and is being tested at the Clementine prospect in southwest Montana which has a 3 by 5 km orb ring.