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The geology of Steele, Bonis, and Scapa townships, District of Cochrane, Ontario Lumbers, Sydney Blake

Abstract

Steele, Bonis, and Scapa townships, in the District of Cochrane, Ontario, are underlain by a steeply inclined Precambrian metavolcanic-metasediment assemblage that is intruded by sills, dikes, stocks, and batholiths. The metamorphosed Bonis volcanics and Steele Lake volcanics are chiefly intermediate to basic lavas. The Scapa and Steele metasediments are composed mainly of metamorphosed greywacke, calc-silicate rocks, and iron-formation. Metamorphosed ultrabasic and basic intrusions occur in the Bonis volcanics and Scapa metasediments. The metasediments and Steele Lake volcanics show a regional metamorphic zoning relative to the Case batholith on the north margin of the area. Chlorite, biotite, garnet, and staurolite zones are distinguished over an area up to eight miles wide south of the batholith. The regional metamorphic zoning is attributed to intrusion of the Case batholith. The Bonis volcanics have been metamorphosed at their contact with the Sargeant batholith that underlies the southeast part of the area. The Bonis volcanics have acted as a "resistor" in protecting the metasediments from metamorphism by the Sargeant batholith. A narrow contact aureole occurs in the Scapa metasediments adjacent to the Scapa stock. Potash metasomatism and high water pressures have prevented the formation of alumina-rich minerals within the aureole. Diabase dikes of two ages cut all other rocks in the area. The easterly trending rocks of the metavolcanic-metasediment assemblage are locally deflected around the western end of the concordant Sargeant batholith. A spodumene-bearing pegmatite dike found in the Case batholith is of economic interest.

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