TWO MAPPABLE THERMOCHEMICAL INDICATORS OF ECONOMIC PORPHYRY COPPER DEPOSITS BASED ON GASEOUS HYDROGEN CONVECTION

While porphyry Cu deposits (PCD) are viewed as giant geochemical anomalies, the fact that they are also thermal anomalies is not appreciated. Subordination of the thermal magnitude of these heat and mass transport engines has meant that the economic ranking of ore deposits is done in terms of Cu grade and tonnage alone with nothing being said about the thermal energy required to drive mineralization. Here a new strategy is described which accommodates both thermal and chemical attributes into an integrated thermochemical framework applicable at the early exploration stage when regional target selection requires making hard choices about which targets deserve drilling. A thermodynamic diagram is described here for redox equilibria in PCD genesis using alteration mineral assemblages to define directly the cooling and heating pathways of convective cells. The H₂ vs 1/T Van’t Hoff reference frame offers an operative geochemical diagram to visualize the scale of ore deposits of different size and hence economics. Confined-flow migration of small H₂ bubbles by protracted magmatic outgassing during thermal advection is evident in two distinct types of advective circulation cells. The largest advective circulation cell defines giant Tier 1 sediment-hosted Bingham-type systems that reach both extreme early oxidative states with chalcopyrite, bornite, and digenite and extreme reductive states with distal orbicular actinolite alteration where H₂ diffusion causes radial mineral growth. In contrast, the batholith-hosted Butte-type cell, also a giant Tier 1 system, has a limited early oxidation range that does not extend up to bornite or digenite and instead consists of only of chalcopyrite. It also lacks a distal orb ring due to restrictive chemical buffering by the influence of granite wall rock minerals. Butte has instead a cupola of biotitized hornblende defining the upper edge of Potassic alteration surrounded by an epidote-bearing propylitic zone. Our exploration method implemented by geological mapping of distal orbicular actinolite may be an innovation that significantly alters established norms for target valuation in business negotiations. Discovery and mapping of an orb ring is a value-added strike by itself and should be considered to be a viable substitute for a fraction of conventional drilling data.