This study describes the structural and stratigraphic setting of Devonian bedded barite and sedimentary exhalative lead-zinc-silver-barite deposits near MacMillan Pass in eastern Yukon. It shows that the deposits occur within MacMillan Fold Belt, an anomalous west-trending feature made up of three parallel elongate domains called the North, Central and South Blocks. Each is characterized by distinctive styles of deformation and Devonian strata.
The north Block is characterized by a thick Early and Middle Devonian chert and shale sequence included in the Lower Earn Group and by an intricate array of southerly directed thrust faults. The Central Block includes unusually thick Early and Middle Devonian silty limestone intercalated with volcanic and volcaniclastic rocks within the upper part of the Road River Group. The Tom and Jason sedimentary exhalative zinc-lead-silver-barite deposits are associated with a thick sequence of coarse clastic rocks thought to belong to a submarine fan complex within the Lower Earn Group. Tight upright folds, high angle reverse faults and irregularly oriented faults are characteristic.
In the South Block, the Lower Earn Group comprises a thin Devonian shale sequence. Open upright folds and few faults are the structural style. The differences in Devonian strata and contrasting style of deformation between blocks may reflect older (Devonian?) fault control to depositional patterns, but structures of that age have not been identified.