EFFECT OF HOST MINERAL SPECIES ON POST-TRAPPING CHANGES IN FLUID INCLUSION COMPOSITION – AN EQUILIBRIUM MODELING APPROACH

Prior to trapping, a hydrothermal fluid's composition is controlled by simultaneous equilibria with several species of minerals. Once trapped within a specific host mineral, the inclusion fluid is isolated and can re-equilibrate only with that host species during uplift and cooling of the rock. Its final composition is therefore affected by the solubility behavior of that host species during cooling.

An equilibrium modeling approach is used to predict the effects of different host mineral species on post-trapping chemical changes in fluid inclusions during uplift and cooling of hydrothermal ores. A hydrothermal fluid with a composition typical of Upper Mississippi Valley-type Pb-Zn ores is first equilibrated with a mineral assemblage containing quartz, barite, calcite, galena and sphalerite at 100^oC. The fluid is then isolated and cooled to 25^oC in a stepwise manner in equilibrium with calcite, quartz or sphalerite, respectively. Predicted changes in chemical and isotopic composition of the trapped fluids are derived.

The predicted masses of crystals that precipitate or dissolve in the inclusion during cooling are too small to be resolved optically, but they have dramatic effects on metal concentrations and isotopic compositions of the fluids. The fluid in the calcite host is predicted to dissolve calcite and precipitate quartz, whereas fluids in the quartz and sphalerite hosts become undersaturated in calcite and precipitate quartz and barite. Very small amounts of metal sulfide minerals are also precipitated. Metal concentrations in the calcite-hosted fluid decrease by 4 orders of magnitude during cooling, while those in the quartz- or sphalerite-hosted fluids decrease by 2 orders of magnitude. In calcite hosts, δ¹³C of the bulk fluid decreases by 2 permil and that of CO₂ gas by 7 permil. These effects are caused by calcite dissolution and a slight pH shift when cooling calcite-hosted fluids, making such inclusions the least representative of the original fluid's metal and isotopic composition. These predicted differences in post-trapping behavior among host minerals should be considered when designing schemes for sampling and micro-analysis of fluid inclusions for their metal and isotopic compositions.