Effects of host mineral re‐equilibration during uplift and cooling on the fidelity of primary hydrothermal fluid inclusions: a theoretical example using Mississippi Valley‐type ore fluids

Abstract

AbstractAt the moment of its trapping as a primary fluid inclusion, a hydrothermal fluid is typically at or near equilibrium with multiple mineral species at depth and temperature. After trapping, however, the isolated inclusion fluid can re‐equilibrate only with its own host mineral species during later uplift and cooling to surface conditions. Because the solubility versus temperature behavior is unique for each host mineral species, identical inclusions trapped at the same time within different species may re‐equilibrate in a disparate manner upon cooling and become variably less representative of the original trapped fluid once they reach ambient temperature. To test the significance of this effect, a series of theoretical equilibrium reaction models was constructed in which a trapped hydrothermal fluid characteristic of Mississippi Valley‐type ore deposits is cooled in contact with silicate, sulfide and carbonate hosts, respectively, from 100 to 25°C. Dissolved base metal concentrations are predicted to decline by two to four orders of magnitude in inclusions in all hosts, due to the precipitation of optically undetectable masses of sulfide daughter minerals. Fluids in the calcite host show the greatest decline in dissolved base metals upon cooling, due to its retrograde solubility and consequent shift in the pH and aqueous C speciation of the fluid. δ13C values for CO2 in all hosts become depleted by 2–7‰ relative to the original trapped fluid, with depletions again being the greatest for the calcite host due to its retrograde dissolution. Analytical techniques that extract and analyze the complete contents of fluid inclusions at room temperature can account for the predicted precipitation of microscopic daughter minerals during cooling, but may not compensate for chemical changes caused by the retrograde dissolution of calcite. Such solubility effects are another reason to be cautious in using carbonate minerals for fluid inclusion studies, in addition to their undesirable physical properties of softness, deformability and perfect cleavage.