Yellowknife Geoscience Forum

Barite (BaSO₄) is a critical raw material widely used in the drilling mud and a key component in radiative cooling applications. 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 (Canada), bedded barite deposits are hosted by black mudstones of the Late Devonian Canol Formation, where barite comprises disseminated, laminated, and nodular styles that are common in an assemblage with pyrite. The origin of the pyrite and barite, and whether it formed via diagenetic or hydrothermal processes, has been the subject of extensive debate. This study describes barite and pyrite in samples collected from multiple sections of Late Devonian stratigraphy (Northwest Territories), which are broadly correlative with the unit that hosts CD-type deposits at Macmillan Pass (Yukon). Detailed high-resolution scanning electron microscopy (SEM) has been coupled with in-situ secondary ion mass spectrometry (SIMS) isotopic analyses (n= 1032) of pyrite (δ³⁴S) and barite (δ³⁴S and δ¹⁸O). The δ³⁴S_barite (+37.1‰ to +67.9‰) and δ¹⁸O_barite (+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 ‘superheavy’ pyrite (δ³⁴S_Pyrite > δ³⁴S_{SO4(Seawater)}) also demonstrates coprecipitation under progressively sulfate-limited conditions. The replacement of diagenetic barite by Ba-bearing phases (e.g., witherite, cymrite, and hyalophane) provides further evidence of severe sulfate depletion and conditions under which barite was soluble. The diagenetic assemblage formed at the sulfate methane transition zone (SMTZ) where opposing diffusional fluxes of methane (+ barium) and sulfate mixed and resulted in sulfate reduction coupled to anaerobic methane oxidation (SR-AOM). The regional extent of this authigenic mineral assemblage in the Selwyn Basin indicates the important role of SR-AOM in the Late Devonian sulfur cycle.