A Low F Pegmatite-Related Mo Skarn from the Southwestern Grenville Province, Ontario, Canada: Phase Equilibria and Petrogenetic Implications

The Hunt Mo skarn deposit is one of the best examples of a late tectonic, granitic pegmatite-related, skarn system in the Grenville province. An Re-Os age of 1069 ± 11 Ma was obtained from a molybdenite crystal (6.4 ppm Re) hosted in proximal skarn (sample 50O) consistent with a contact metasomatic origin related to the granitic body. Although less evolved than other Grenvillian pegmatites, the Condon Lake granitic pegmatite is a low-temperature, A-type intrusion, with moderate redox characteristics. In contrast to many Mo-bearing skarns in the region, this deposit has low U, Th, REE, F, and P contents. The reduced marginal magnesian skarn is well zoned geochemically, mineralogically, and texturally from the contact outward into the dominant graphite-bearing calc-silicate-calcitedolomite marble, which is locally intercalated with clinopyroxenite, orthoamphibolite, and pyroxene-biotite quartzofeldspathic gneiss. A narrow zone of endoskarn (<1 m; scapolite-K feldspar-Ca clinopyroxene) and wider, zoned exoskarn (<10 m; scapolite-Ca clinopyroxene (proximal), Ca clinopyroxene-phlogopite, Ca clinopyroxene-tremolitephlogopite, tremolite-phlogopite (distal), marble), which hosts the bulk of the primary molybdenite (± pyrrhotite) and minor secondary pyrite-pyrrhotite-molybdenite veins. In addition to the obvious Si, Fe, Ti, Mn, S, and Mo addition, Cu, Zn, Y, Nb, and Zr are also slightly enriched. The high f(H2O)f(HF) ratios (138,000-204,000) and moderate f(H2O)f(HF) ratios (2,200-1,000) at the Hunt deposit contrast with the lower fugacity ratios from other Mo-bearing U-Th-REE skarns in the region, indicating that Mo transport and deposition is principally related to a hydroxide complex. The calc-silicate phase equilibria used to describe the zonation sequence have been reinterpreted using quartzundersaturated (silica) activities; this shifts the calcsilicate-forming reaction boundaries to very low X (CO2) (<0.005), consistent with decarbonation reactions induced via dissolved silica infiltration. The modeled fluid/rock in the skarn based on dissolved molybdenum is greater than 20. The carbon and oxygen isotopes of calcite are consistent with infiltrative metasomatic skarns from the pegmatite and a dominantly igneous source for the sulfur (δS = 2-9‰). Overall, the deposition of Mo is probably facilitated by a reaction with carbonate (increase in Ca(OH) 2 , CaCl 2 resulting in complex destabilization), a decrease in fO 2 triggering an increase H 2 S/SO 2 , S in the host rocks (fS 2 ), and a slight decrease in temperature (650°-500°C).