Complex multiphase fluid inclusions in wollastonite from the Mérida contact-metamorphic deposit, Spain: evidence for rock/HCl-rich fluid interaction

Abstract

Primary multiphase fluid inclusions are abundant in skarn wollastonite from a contact-metamorphic ureole near Merida, Spain. Wollastonite is found along the boundaries between marble and interbedded metachert layers and nodules. At room temperature, fluid inclusions consist of an aqueous liquid, a bubble and several solids (wollastonite, calcite, apatite, quartz, halite and sylvite have been identified). Wollastonite and calcite are always present and show a fairly uniform phase ratio suggesting that they are daughter minerals. The solid assemblage, except the chlorides, does not dissolve appreciably, even at 5509C. Two types of fluidinclusions are found: Type I, which contains no visible halite and sylvite, and lype II, which contains halite(± sylvite). The first type seems to be ubiquitous within the aureole, whereas the second is restricted to theproximity to the granite contact. An 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 the case of Type I fluids, and rises to ca. 69 wt% in Type II fluids. The CaCl2 content is remarkably high (ca. 39 and 45 t% respectively); silica content (as SiO2°) is also high in both cases (11.2 and 7.4 wt% respectively) and CO2 olar fraction is close to 0.05 in both fluids. These fluids are interpreted as exhaust fluids resulting from the wollastonite-forming reaction. aking into account the inability of H2O-NaCl fluids to incorporate significant amounts of Ca into solution, we suggest that HC1 of granitic origin was an important component in the primary fluid entering the arble-metachert sequence, thus explaining the high solubility of wollastonite and calcite (and probably also uartz) at the expected temperatures of trapping of the fluid inclusions. This primary fluid was a moderately aline brine with H2O-NaCl-(KCl-MgCl2)-HCl and aqueous SiO2, as major components, and minor amounts of CO2 and P. The range of fluid compositions recorded by fluid inclusions can be explained as a result of mixing of either the primary fluid, or the exhaust fluid, with a low-salinity brine. Precipitation of wollastonite (and probably also in part, calcite and quartz) inside fluid inclusions was probably favoured by outward diffusion of hydrogen through the wollastonite host.