The Purcell Supergroup comprises a thick accumulation of clastic and carbonate rocks that are interpreted to have been deposited in a large intracratonic basin. The northeastern margin of the basin is exposed in the northern part of the Fernie west-half sheet. It developed by growth faulting and is characterized by rapid and pronounced facies and thickness changes, in contrast with more subtle changes that occur elsewhere in the basin.
Moyie sills form an extensive suite of basaltic rocks that intruded the Aldridge and Fort Steele formations, the lower part of the Purcell Supergroup. Many of these sills were intruded at very shallow depths in unconsolidated, water-saturated sediments. Hence a Middle Proterozoic date of 1445 Ma from one of the sills defines the minimum age of deposition of lower and basal middle Aldridge, and of stratiform mineral deposits such as the Sullivan orebody.
Preliminary Map 54 describes the geology of the Cranbrook map-sheet and the Sullivan Mine area (82G/12; 82F/9) in southeastern British Columbia. The 1:50 000 scale geology map includes a legend, geologic sections, a list of mineral deposits and references.
In the Kootenay Ranges, the oldest strata in the area are quartzite, siltstone and argillite of the Fort Steele Formation of the Helikian Purcell Supergroup. Overlying Aldridge Formation strata are subdivided into middle and upper units. Middle Aldridge strata include lower siltstone and argillite; silty dolomite, dolomitic siltstone and minor limestone; siltstone and argillite; thick bedded quartzite with minor conglomerate; siltstone and argillite; and upper quartzite and quartz wacke. Upper Aldridge strata are undivided. Moyie sills and minor dikes are present. Overlying Creston Formation lithologies are a green, grey and purple siltstone unit and a siltstone, quartzite, green and mauve siltstone and dolomitic siltstone unit. Kitchener Formation comprises dolomitic siltstone, siltstone and dolomitic quartzite. Other Purcell Supergroup formations present are the Van Creek, Nicol Creek and Gateway. Paleozoic strata include the Cambrian Cranbrook and Eager Formations, and the Devonian Burnais and Harrogate Formations of shaly limestone, dolomite and gypsum.
In the Purcell Mountains, the oldest strata Purcell Supergroup strata are lower Aldridge rusty weathering siltstone, quartzite or quartz wacke and silty argillite, middle Aldridge quartzite, quartz wacke, with mainly siltstone and rusty weathering argillite near the top, and upper Aldridge argillite and siltstone. Moyie sills and minor dikes are gabbro and diorite. Creston Formation comprises mainly green, grey and mauve siltstone and quartzite. Overlying Kitchener Formation strata are dolomite and limestone that are in part argillaceous and silty, argillite and siltite; Van Creek Formation consists of green and mauve siltstone, argillite and silty quartzite; Nicol Creek Formation is dominated by amygdaloidal and vesicular basalt, but includes volcaniclastic siltstone and sandstone intervals; Gateway Formation strata are green and mauve siltstone, argillite, quartzite, stromatolitic dolomite and silty dolomite; and gabbro or diorite sills. Cambrian rocks include the Cranbrook Formation, with quartzite, conglomerate, limestone and magnesite; and the Eager Formation with shale, siltstone, limestone and quartzite. Cretaceous quartz monzonite and granodiorite intrusions cut the older rocks.
Structurally, the area is divided by the north-northwest trending Rocky Mountain Trench fault. Northeast of the fault, in the Kootenay Ranges, east-northeast trending high angle faults and easterly-directed thrust faults dominate the structural fabric. Strata are deformed into overturned folds with axial surfaces that dip at low angles westerly. A broad area of alluvium marks the Trench. To the west, in the Purcell Mountains northeast to east trending high angle faults are cut by north-northeast trending high angle faults. Thrust and reverse faults are cut by the high angle faults. Thrust and reverse faults with easterly movement are cut by faults, like the St. Mary fault, with northeast to easterly trend and southerly-directed movement. Strata are deformed into gentle, upright folds to tight overturned folds.
In the Purcell Mountains, the oldest rocks exposed are components of the Helikian Aldridge Formation of the Purcell Supergroup. Lower Aldridge strata host the Sullivan deposit and are a prime exploration target in the area. They comprise rusty siltstone, quartzite and silty argillite. In the Kootenay Ranges, age equivalent rocks of the Fort Steele Formation are cross-bedded quartzites and mud-cracked siltstone and argillite.
Other components of the Purcell Supergroup are, from older to younger, the Creston, Kitchener, Van Creek, Nicol Creek, and Gateway formations. In the Purcell Mountains, the Creston Formation is dominantly varicoloured siltstone and quartzite; in the Kootenay Ranges, the Formation also contains an upper unit of coarse-grained siltstone and quartzite.
Cambrian rocks in the area comprise the Cranbrook and Eager formations. Magnesite occurs locally in the Cranbrook Formation in the Purcell Mountains. In the Kootenay Ranges, Devonian strata of the Burnais Formation consist of limestone, dolomite and gypsum.
Mesozoic quartz monzonite and granodiorite intrusions cut older rocks in the rocks in the Purcell Mountains, and Cenozoic glacial and alluvial deposits occur throughout the mapsheet.
Sullivan, a massive sulphide lead-zinc-silver deposit in Aldridge Formation turbidites, is the largest mineral deposit in the area. It has produced in excess of 125 million tonnes of ore from an original deposit of about 160 million tonnes. Other deposits in the Purcell Supergroup include lead-zinc replacement deposits in Upper Purcell carbonates and numerous lead-zinc-silver and copper veins. In the Paleozoic rocks, potential exists for the industrial mineral products magnesite and gypsum.