REFINEMENT OF THE EVALUATION OF THE ROLE OF CO2 IN MODIFYING ESTIMATES OF THE PRESSURE OF EPITHERMAL MINERALIZATION

Pressure is the most important of the intensive parameters for relating epithermal mineralization to the geologic setting. This paper describes the limitations on pressure (and therefore depth) of mineralization that may reasonably be derived from simple observations of the behavior of fluid inclusions (i.e., the existence of ice or CO2 clathrate on the liquidus, the amount of expansion or contraction of the bubble as the host inclusion is crushed in oil on the microscope stage, and the freezing and homogenization temperatures for the inclusion). It is based on the reasonable model that mineralization occurs from a hydrostatically pressured NaCl-CO2-H2O fluid, consistent with the probability that H2O and CO2 are the only gases contributing significantly to the total pressure. The pressure of CO2 is, of course, a function of CO2 content, but, from 100-degrees to 300-degrees-C, it is a surprisingly minor function of either temperature or salinity. The presence of the clathrate in freezing studies of fluid inclusions indicates pressures of CO2 that add at least 1 km to the probable depth of inclusion trapping compared to that estimated from CO2-free water. Thus undetected (i.e., no clathrates on cooling) CO2 in fluid inclusions can nonetheless contribute very significantly to the possible depth of epithermal mineralization. On the other hand, the observation that fluid inclusions crushed in oil have bubbles that do not expand (i.e., <1 atm P(CO2) at 25-degrees-C), demonstrates CO2 contents that could add at most a few tens of meters to the depth of mineralization.