Temporal and spatial variations in characteristics of fluid inclusions in epizonal magmatic-hydrothermal systems: Applications in exploration for porphyry copper deposits

Abstract

Temporal and spatial variations in the room temperature phase relations of fluid inclusions serve as a proxy for the PTX properties of fluids associated with emplacement and crystallization of a shallow epizonal intrusion following the “Burnham Model”. Accordingly, during magma ascent, hydrous magmas approach and achieve volatile saturation in response to decreasing pressure (“first boiling”) and/or as a result of cooling and crystallization of anhydrous mineral phases (“second boiling”). In both cases, a magmatic aqueous fluid phase will exsolve from the volatile-saturated silicate melt. Fluid inclusions that trap the exsolved magmatic fluid show predictable room temperature phase relations, compositions, and microthermometric characteristics indicative of the pressure and temperature conditions of the trapping environment.We model aqueous phase equilibria as a function of temperature, pressure, and fluid composition. The composition and phase behavior of the magmatic aqueous fluid phase is adequately approximated by the system H2O-NaCl, and the PVTX characteristics of the magmatic fluids may be determined at any temperature and pressure using phase equilibrium data for this binary system. Then, PVTX data for the system H2O-NaCl are used to predict the room temperature phase relations and homogenization temperatures of fluid inclusions that trap the magmatic fluid at various times and locations in the magmatic-hydrothermal system. The temperature at any position in the system is based on established numerical models of conductive and convective cooling of shallow plutons, and the pressure at any depth is calculated assuming that hydrostatic conditions apply at temperatures <400 °C, and that lithostatic conditions are applicable at higher temperatures.We apply the model to four different stages during the crystallization of a shallow, intermediate composition magma, beginning in the early stages of crystallization and immediately following initial volatile saturation, and ending with complete crystallization of the melt. Mapping the stable fluid phase fields onto a cross-section of the pluton allows us to predict the characteristics of fluid inclusions trapped at various times and locations in the overall system. The fluid inclusion characteristics are predicted along five depth profiles that extend from the center of the pluton to the periphery. The results show good agreement with numerous studies that describe the distribution of halite-bearing, vapor-rich, and two-phase, liquid-rich inclusions in porphyry copper deposits.The results of this study may be applied in exploration for porphyry copper mineralization, as well as other magmatic-hydrothermal porphyry deposits in which the magmatic fluids can be approximated by the system H2O-NaCl. Importantly, halite-bearing fluid inclusions occur only within the pluton and in the immediately surrounding wallrocks, which represents the part of the system that often hosts mineralization. Similarly, fluid inclusion assemblages comprised of only vapor-rich inclusions occur in the near-surface, sub-volcanic part of the system, whereas coexisting halite-bearing and vapor-rich inclusions characterize the deeper parts of the system. The distribution of fluid inclusion types constrains the vertical and lateral position within the overall magmatic-hydrothermal system and, thus, develops vectors towards the magmatic center of the system that is most likely to host economic mineralization.