COMPLEX MULTIPHASE FLUID INCLUSIONS IN WOLLASTONITE FROM THE MERIDA CONTACT-METAMORPHIC DEPOSIT, SPAIN - EVIDENCE FOR ROCK HCL-RICH FLUID INTERACTION/

Primary multiphase fluid inclusions are abundant in skarn wollastonite from a contact-metamorphic aureole near Merida, Spain. Wollastonite i s found along the boundaries between marble and interbedded metachert layers and nodules. At room temperature, fluid inclusions consist of a n aqueous liquid, a bubble and several solids (wollastonite, calcite, apatite, quartz, halite and sylvite have been identified). Wollastonit e and calcite are always present and show a fairly uniform phase ratio suggesting that they are daughter minerals. The solid assemblage, exc ept the chlorides, does not dissolve appreciably, even at 550 degrees C. Two types of fluid inclusions are found: Type I, which contains no visible halite and sylvite, and Spe II, which contains halite (+/- syl vite). The first type seems to be ubiquitous within the aureole, where as the second is restricted to the proximity to the granite contact. A n estimation of the composition of the trapped fluid has been made for two cases of Types I and II fluid inclusions. They were highly saline brines, especially the Type II fluids. Total salinity is 52 wt% in th e case of Type I fluids, and rises to ca. 69 wt% in Type II fluids. Th e CaCl2 content is remarkably high (ca. 39 and 45 wt% respectively); s ilica content (as SiO20) is also high in both cases (11.2 and 7.4 wt% respectively) and CO2 molar fraction is close to 0.05 in both fluids. These fluids are interpreted as exhaust fluids resulting from the woll astonite-forming reaction. Taking into account the inability of H2O-Na Cl fluids to incorporate significant amounts of Ca into solution, we s uggest that HCl of granitic origin was an important component in the p rimary fluid entering the marble-metachert sequence, thus explaining t he high solubility of wollastonite and calcite (and probably also quar tz) at the expected temperatures of trapping of the fluid inclusions. This primary fluid was a moderately saline brine with H2O-NaCl-(KCl-Mg Cl2)-HCl and aqueous SiO2, as major components, and minor amounts of C O2 and P. The range of fluid compositions recorded by fluid inclusions can be explained as a result of mixing of either the primary fluid, o r the exhaust fluid, with a low-salinity brine. Precipitation of wolla stonite (and probably also in part, calcite and quartz) inside fluid i nclusions was probably favoured by outward diffusion of hydrogen throu gh the wollastonite host.