Preliminary Map 62 presents the geology of the Mount Forster map-area (82K/9, 10, 15) consists of a geological map at 1:25 000 scale covering approximately 500 square kilometres along and to the west of the Rocky Mountain Trench in southeastern British Columbia. The map sheet is bounded by Horsethief Creek to the south and Frances Creek to the north and includes exposures on Steamboat Mountain in the trench. A second sheet shows cross-sections, and accompanying notes briefly describe the geology.
The area is underlain by the Purcell Supergroup and unconformably overlying Windermere Group and lower Paleozoic strata. It is structurally complex, transected by the Mount Forster-Steamboat fault. Numerous silver-lead-zinc and copper showings occur throughout the area, including Grotto, Steamboat, Puzzle and others, and the Brisco barite mine is located in the northeast corner.
Rocks of the Belt-Purcell Supergroup are the oldest strata exposed in the area. The lowest unit consists of quartzites, shales, and laminated dolomites of the Dutch Creek Formation. Above are varicoloured quartzites, shales and dolomites of the Mount Nelson Formation. The Frances Creek Formation overlies the Mount Nelson with sharp discordance and locally with angular unconformity. The Frances Creek consists of thick-bedded white orthoquartzites, grey dolomites and thinly interbedded sandstone and shale. The basal member is a grey, thick-bedded algal dolomite with halite and gypsum pseudomorphs.
Windermere Supergroup components exposed are the Toby Formation and the Horsethief Creek Group. The Toby is a highly variable sequence of diamictites, conglomerates, sandstones, shales, dolomites, breccias, and volcanic rocks. It rests unconformably on the Belt-Purcell rocks. Volcanic rocks comprise the base of the Toby Formation. The base of the Horsethief Creek Group is marked by a black, detrital limestone. The Group comprises a series of interbedded grey shales, grits, quartzites, limestones and dolomites.
Paleozoic strata unconformably overlie the Windermere rocks. The basal Cranbrook Formation consists of mature quartzites and quartz grits. The Middle to Upper Cambrian Jubilee Formation is massive dolomite and unconformably overlies the Cranbrook strata. Base metal mineralization occurs in solution breccias beneath the Devonian and Ordovician unconformities. The Upper Cambrian to Middle Ordovician McKay Group conformably overlies the Jubilee Formation and is unconformably overlain by the Upper Ordovician Beaverfoot Formation. The McKay consists of recessive shales, sandstones and dolomitic biowackestones. The Beaverfoot has basal white quartzite overlain mainly by biowackestones and peloidal mudstones. The Middle Devonian Mount Forster Formation unconformably overlies the Beaverfoot and karsting and related features, including base metal mineralization, are developed adjacent to the unconformity. The Upper Devonian Starbird Formation, which consists mainly of fossiliferous dolomitic biowackestones, unconformably overlies the Mount Forster strata.
Syenitic sills and dykes intrude the Proterozoic sequence and were sheared during Mesozoic deformation. Narrow minette dykes also intrude the Proterozoic section. The Cretaceous Horsethief Creek batholith is largely coarse-grained quartz monzonite. The metamorphic aureole overprints the tectonic fabric in the country rocks.
The structural evolution of the area was episodic. Early basin formation and extension from Proterozoic through to Upper Paleozoic was followed by thrusting and folding in the Mesozoic Columbian-Laramide orogeny.