Influence of Salinity on T-X(CO2) Stability of Talc and the Possible Role of Fluid Immiscibility In Development of the Talc Zone, Outer Alta Aureole, Utah

In the outermost talc (Tc) zone of the Alta stock aureole, the presence of talc does not define a regular isogradic surface parallel to the inner tremolite, forsterite, and periclase isograds. At a given distance from the stock, the distribution and abundance of talc in the outer Alta aureole is heterogeneous on several scales, and is in part stratigraphically controlled, despite the ubiquitous presence of dolomite + quartz. The implication is that pore fluid X(CO₂) must also have been heterogeneous as a function of stratigraphy at several scales. Possible explanations for these heterogeneities include: multiple, possibly non-contemporary, hydrothermal circulation cells, bedding-controlled variations in fluid flow and reaction progress, and fluid immiscibility. Understanding the P-T-X stability of talc in the CaO-MgO-SiO₂-H₂O-CO₂-NaCl system is important for evaluating these alternative processes. Preliminary calculations made using Perple_X 07 (Connolly, 2005) indicate that the fluid equation of state selected can modify the upper temperature and X(CO₂) limits of the talc stability field by up to 50^oC and 0.2 mol fraction, respectively. Calculations also show that with increasing salinity, the stability field of talc is shifted to higher temperature and lower X(CO₂) with maximum X(CO₂) values of 0.3. Some of the calculated phase equilibria define temperature limits compatible with the thermal window of 375-415^oC defined for the talc zone by applications of calcite-dolomite geothermometry (Cook and Bowman, 1994). Preliminary investigations show that with salinities of ≥20 weight percent, the region of fluid immiscibility overlaps the calculated talc stability field. Microthermometry measurements on fluid inclusions in contact skarns suggest that early skarn-forming fluids (T>575^oC) in the aureole have significant salinity (≥30 weight percent). Therefore, fluid immiscibility must be considered as a possible mechanism for producing the heterogeneity in distribution and abundance of talc in the outer Alta aureole.