| Summary: | Multiple occurrences of bedded barite (BaSO4) are preserved in mudstones of the upper-most Canol Formation (Frasnian) of the Selwyn Basin (Canada). Samples of barite were collected from eight sites (seven measured sections), and examined using petrographic and isotopic techniques (87Sr/86Sr, δ34S and δ18O values) to determine whether the barite has a diagenetic or hydrothermal origin. Barite at all locations occurs in two morphologies: (1) laminated barite composed of thin layers of microcrystalline barite with lesser amounts of pyrite and hyalophane (Ba-K-feldspar) interlaminated with mudstone, and (2) nodular barite consisting of barite, hyalophane, cymrite (hydrous Ba-silicate) and minor pyrite. At the Cowan occurrence, one sample has a mineral assemblage comprising barite and nodular barium carbonates (witherite, norsethite). The δ34S and δ18O values of the laminated barite co-vary, and range between +24.1 and +35.3‰ and +14.8 to +18.3‰ respectively, with the exception of one sample with δ34S and δ18O values of +32.9‰ and −0.5‰. The laminated samples have an average 87Sr/86Sr of 0.7085 (n = 8), which is consistent with Frasnian seawater (0.7081) that has undergone some minor water sediment interaction during early diagenesis. The nodular barite has highly variable δ34S (+4.8 to +56.5‰) and δ18O (+9.2 to +19.3‰) values, which developed from more evolved diagenetic pore fluids with limited sulphate re-supply (i.e., as a relatively closed system). In these conditions sulphate reduction likely occurred at a slower rate, and the sulphate in the barite records both kinetic (δ34SSO4) and equilibrium (δ18OSO4) isotope effects. One sample of nodular barite has a 87Sr/86Sr ratio of 0.7086. The single sample with a barite and barium-carbonate assemblage at Cowan has a 87Sr/86Sr ratio of 0.7133, a δ34S value of +23.9‰, and a δ18O value of +10.7‰ and may represent a local hydrothermal event at this site. Our data supports the interpretation that the barite horizon is a stratigraphic marker throughout the Canol Formation. ...
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