FLUID EVOLUTION ASSOCIATED WITH EPIZONAL SILICIC PLUTONS

Porphyry copper deposits (PCDs) are associated with arc magmas and form in subduction environments where calc-alkaline magmas are generated above the subducting slab. The formation of economic accumulations of Cu to form PCDs involves many physical and chemical processes that must work in concert to produce economic mineralization. First, the source magma must contain sufficient amounts of Cu to form a deposit, and the metal must be efficiently extracted from the melt and transferred to a magmatic aqueous fluid during crystallization of the melt. Moreover, the magmatic fluid must be focused into a relatively small volume of rock where processes are operating that can quantitatively remove metal from the fluid to produce ore, or later hydrothermal activity may serve to concentrate low-grade mineralization to produce higher grades. During and following emplacement, hydrous magmas reach volatile saturation due to pressure decrease (“first-boiling”) and crystallization (“second boiling”) and exsolve a magmatic aqueous phase. Fluid inclusions that trap the magmatic fluid show room temperature phase relations, compositions, and microthermometric characteristics indicative of their trapping environment. The distribution of fluid inclusion types can be related to the distribution of metals and provide vectors towards the portion of the system most likely to contain economic mineralization. In this study, temporal and spatial variations in fluid properties during the crystallization of a hydrous magma associated with PCDs has been modeled as a function of temperature, pressure, and fluid composition, and fluid inclusion characteristics are predicted using the PVTX properties of the system H₂O-NaCl. The results of this study have implications in exploration for porphyry copper mineralization because some types of fluid inclusions are spatially related with Cu mineralization. Importantly, rocks containing halite-bearing fluid inclusions are predicted to occur only within the pluton and in the immediately adjacent wallrocks, and this also corresponds to the mineralized portion of the system. Fluid inclusions thus provide a tool to direct the explorationist to that part of the magmatic-hydrothermal system that is most likely to host mineralization.