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Abstract:
Barite (BaSO4) is a critical raw material with the major application as a weighing agent in drilling mud. Some of the largest barite deposits in the geological record are spatially associated with large clastic dominant (CD-type) Zn-Pb massive sulfide deposits. In the Selwyn Basin (Yukon, Canada), bedded barite deposits are hosted by black mudstones of the Late Devonian Canol Formation, where barite occurs in the matrix, laminations and nodules commonly in an assemblage with pyrite. The Late Devonian bedded barite also forms part of a larger province extending along western North America’s ancient continental margin. The bedded barite deposits have been variably interpreted, with genesis linked to either lower temperature diagenetic or high-temperature hydrothermal processes. In this study, in-situ isotopic microanalyses by secondary ion mass spectrometry (SIMS) have been used to determine the isotopic composition of sulfur (δ34S) and oxygen (δ18O) in barite and sulfur (δ34S) in pyrite. The δ34Sbarite (+37.1‰ to +67.9‰) and δ18Obarite (+8.8‰ and +23.9‰) values represent a substantial offset from Late Devonian seawater sulfate, consistent with precipitation from modified diagenetic pore fluids. The coexistence of the barite with the highly 34S-positive pyrite also demonstrates coprecipitation under progressively sulfate-limited conditions. A similar assemblage containing diagenetic pyrite and barite has been described in the pre-ore assemblage of Late Devonian CD-type deposits nearby in the Selwyn Basin, demonstrating this is the expression of a regional diagenetic process. The diagenetic assemblage formed at the sulfate methane transition zone (SMTZ) where opposing diffusional fluxes of methane (+ barium) and sulfate interact, leading to sulfate reduction coupled to anaerobic methane oxidation (SR-AOM). The replacement of diagenetic barite by other Ba-bearing minerals (e.g. witherite, cymrite and hyalophane) provides further evidence of severe sulfate depletion and conditions under which barite was soluble.
