Synthetic Fluid Inclusions. XVII. PVTX Properties of High Salinity H2O-NaCl Solutions (>30 wt % NaCl): Application to Fluid Inclusions that Homogenize by Halite Disappearance from Porphyry Copper and Other Hydrothermal Ore Deposits

Abstract

Fluid inclusions that homogenize by disappearance of the halite crystal after the vapor bubble has disappeared have been reported from a wide range of ore-forming systems. While this inclusion type is most commonly observed in porphyry copper and similar magmatichydrothermal ore deposits, they have also been reported from iron oxide copper-gold (IOCG), Mississippi Valley-type, orogenic (lode, greenstone) gold, massive sulfide, unconformity U, and mantotype deposits, as well as from non-ore associated granitoids and oceanic crust. The lack of experimental data to interpret microthermometric data from these inclusions, combined with poor petrographic characterization of inclusion origin, has led to a wide range of inferred PT trapping conditions for the inclusions. The relationship between liquid-vapor homogenization temperature (ThL-V), halite dissolution temperature (Tmhalite) and pressure has been determined using synthetic H2O-NaCl fluid inclusions that homogenize by halite disappearance. The experimental data cover the range ThL-V ≈ 150° to 500°C and Tmhalite ≈ 275° to 550°C. An empirical equation describing the relationship between pressure, ThL-V, and Tmhalite has been developed to estimate formation pressures from microthermometric data and is valid from pressures along the liquid + vapor + halite curve to 300 MPa. A detailed literature review reveals that the results of this study cannot be applied retroactively to estimate pressures of previously reported inclusions that homogenize by halite dissolution for several reasons. First, most workers have not collected and reported the data in a manner that allows one to determine if the inclusions have trapped a single, homogeneous phase and have not leaked or changed volume following trapping. Thus, it is not possible to determine if the inclusions show consistent microthermometric behavior within a group of coeval inclusions. Additionally, many published studies provide only summaries of results from numerous samples or present the data graphically, precluding application of our results to individual inclusions. However, based on our review, it appears that much of the published data for inclusions that homogenize by halite dissolution represent inclusions that have either trapped a halite crystal along with the liquid or have reequilibrated by necking and/or stretching. Results from this study have been used to estimate minimum formation pressures for inclusions from the Ditrau Alkaline Massif, Transylvania, Romania, the Musoshi stratiform copper deposit, Zaire, the Bismark skarn deposit, northern Mexico, the Naica chimneymanto deposit, Mexico, the Questa porphyry molybdenum deposit, New Mexico, and the Bingham Canyon porphyry Cu-Mo deposit, Utah. In each case, pressures estimated using results from the present study are in good agreement with previous pressure estimates. However, in some cases estimated pressures (both our estimates as well as those of other authors) appear to be unreasonably high, based on the geologic setting at the time of formation. These results highlight the currently limited understanding concerning pressures (depths) of ore formation.