Paragenesis of Barium Silicate Mineralization in Devonian Strata at the Gun Occurrence, the Yukon, Canada

Abstract The Gun locality is a barium silicate mineral occurrence in the east-central Yukon Territory. The skarn hosts an abundance of rare Ba-bearing silicate minerals, several of which have so far been found only at this occurrence. Eight texturally and mineralogically distinct units with Ba silicate mineralization have been identified: (1) pluton contact zone (distinguished only by textural features), (2) relict diopside skarn unit, (3) gillespite skarn unit, (4) ronpetersonite skarn unit, (5) pellyite-sanbornite skarn unit (fine-grained, coarse-grained, and deformed subunits), (6) banded skarn unit (sanbornite-rich veinlet dominant and gillespite-rich nodular dominant subunits), (7) “muirite-Fe” skarn unit, and (8) Ba-Ca-silicate skarn unit. Minerals not previously noted as present at the Gun occurrence but identified during this study include cymrite, kinoshitalite, pabstite, kampfite, and ronpetersonite. The unknown species “notwalstromite” is identified as chemically distinct from walstromite by the presence of CO2 and Cl. The minerals present at the Gun occurrence indicate that two chemically distinct host-rock facies were contacted by the pluton, with one being silica-rich and the other carbonate-rich. Mineral composition and textures within these units provide indicators for the relative degree of exposure to the magmatic fluids and the thermal signature of the pluton. Subsequent hydrothermal alteration partially overprinted the mineral assemblages. The emplacement of the Gun pluton triggered the dissolution and destabilization of baryte and quartz in the contacted host rock to form a suite of predominantly anhydrous barium silicate minerals. Subsequent cooling led to the formation of kampfite and sulfides. The system was then hydrated and carbonated. Very late baryte mineralization is observed in veins that crosscut all other features and is interpreted to result from later fluid activity.