POSSIBLE ROLE OF IMMISCIBLE H2O-CO2 FLUIDS IN DEVELOPMENT OF THE OUTER (TALC ZONE) ALTA AUREOLE, UTAH

Field and petrologic observations on several scales indicate that the outermost talc (Tc) zone in the Alta stock aureole differs significantly from the inner tremolite, forsterite, and periclase zones. The Tc isograd is not parallel to either the igneous contact or to the other three isograds. Detailed mapping indicates that the Tc reaction front is a quite irregular surface, and appears to be stratigraphically controlled. Significant development of talc extends 100's of meters farther within restricted stratigraphic intervals than in adjacent dolomite (Dol) + quartz (Qz)-bearing strata. The Tc-bearing intervals appear to contain more abundant carbonate (Dol, calcite) and Qz veining. In thin section, some of these veins originate in Tc reaction rims surrounding chert nodules, and may record fluid escape pathways during contact metamorphism. In contrast to the inner aureole, the progress of the talc-forming reaction is incomplete; significant Qz + Dol always persists, regardless of whether quartz is disseminated within dolostone or in the form of chert nodules. Finally, talc reaction progress is very heterogeneous even within restricted stratigraphic intervals. Quantitative estimates of reaction progress require detailed mapping of the distribution of Tc-bearing and Tc-absent nodules on outcrop scale. Initial results indicate that reaction progress is heterogeneous (> factor of 100 different), can be correlated to stratigraphy, and is generally much less (<10%) than previously measured in the tremolite, forsterite, and periclase zones (Cook and Bowman, 2000). These heterogeneities in the distribution and abundance of talc suggest significant heterogeneity in the X(CO₂) values of pore fluid within the Tc zone. Such heterogeneity may develop from a number of processes: 1) layer-controlled variations in progress of higher grade reactions could induce variations in X(CO₂) values of fluids flowing out from the igneous contact and into the Tc zone (down-temperature flow); 2) layer-controlled variations in permeability could allow influx of water-rich fluids flowing toward the igneous contact (up-T flow); and 3) generation of water-rich fluids by fluid immiscibility as high X(CO₂) fluids generated in the inner aureole flow away from the igneous contact and cool and/or decrease in fluid pressure.