The Presence of Carbonic-Dominant Volatiles during the Crystallization of Sulfide-Bearing Mafic Pegmatites in the North Roby Zone, Lac des Iles Complex, Ontario

Mafic pegmatites in the platinum-group element (PGE)-mineralized Roby zone, Lac des Iles Complex, northwest Ontario, Canada, comprise dikes, veins, and irregular pods of coarse-grained magnesiohornblende, pyroxene, and labradorite-andesine with minor biotite, apatite, Fe-Ti oxides and intercumulus quartz that grades into massive quartz or graphic quartz-base metal sulfide-vysotskite [Pd, Ni(S)] intergrowth at their cores. Quartz, apatite, and magnesiohornblende host primary and secondary assemblages of one- or two-phase carbonic fluid (CO2 +/- up to similar to 10% CH4 minor H2O, N-2) inclusions that contain ore metals (Ni, Cu, Pd, Bi, Te, Fe). Rare trails of late-stage, high-salinity, aqueous fluid inclusions are secondary in origin and, therefore, unrelated to the crystallization of the pegmatites. Assemblages of primary carbonic fluid inclusions show considerable variation in mode and temperature of homogenization, reflecting large fluctuations in confining pressure at the time of quartz crystallization of as much as similar to 1 kbar (in single quartz crystals) and similar to 2.8 kbars (all data). Independent thermobarometric methods constrain conditions for the following two stages of pegmatite formation (and carbonic fluid entrapment): (1) the crystallization of magnesiohornblende-plagioclase intergrowth at T similar to 650 degrees to 850 degrees C, and P similar to 1 to 3 kbars; and (2) the crystallization of quartz at T similar to 535 degrees to 650 degrees C, and P similar to 0.4 to 3.2 kbars, setting the maximum depth of emplacement of the Lac des lies Complex North Roby zone magma at 10 to 1.2 km. The results indicate that aqueous-dominant volatile phases were absent during the crystallization of pegmatitic gabbroic rocks at the Lac des Iles Complex and that water-poor, carbonic fluid entrapment persisted to well below solidus conditions. A role played by carbonic fluid as a potential transport medium for ligands involved in the precipitation and remobilization of the PGE and base metals is strongly suggested and warrants further investigation.