Conspicuous spherical and subspherical barite nodules occur in Devonian(?)/Mississippian carbonaceous shales and argillaceous dolomitic limestones in Twitya river area (western foothills of the Backbone Ranges, Mackenzie Mountains, N.W.T.).The majority of barite nodules are spherulitic, composed of grey radiating barite crystals. The microscopic study of barite and of sedimentary fabrics in its host rocks has demonstrated that the crystallization of nodules was a late diagenetic event postdating the formation of carbonate concretions in shales, dolomitization, and the early phase of fracturing, but predating the late phase of fracturing and tectonic deformation. The nodules have clearly developed by replacement of their carbonate host.The middle to late Paleozoic, black shale sequence that occurs on both sides of the Yukon – Northwest Territories boundary at the latitude of Macmillan Pass (Canol road) appears to be in a distinct barite province with numerous occurrences of mostly bedded barite recorded. The presently available evidence indicates that the introduction of barium into the sedimentary basin had probably been syndepositional, followed by diagenetic concentration and barite emplacement well before the 'Laramide Orogeny' and the emplacement of Cretaceous quartz monzonite stocks.