The Michipicoten group, of Precambrian age, comprises flows and pyroclastic rocks of the andesite-rhyolite association together with conformable zones of clastic sediments and banded iron formations. Volcanic rocks display great heterogeneity, typically exhibiting marked variation in lithology, structure, and thickness over short intervals. The typical volcanic cycle progressed from (1) widespread and prolonged outpouring of andesite-basalt flows and pyroclastic material, through (2) brief but violent expulsion of rhyolite-dacite pyroclastic rocks, to (3) large-scale hot-spring and fumarolic activity. Clastic sediments are interpreted as products of contemporaneous erosion of expanding volcanic piles. Development of the Michipicoten group is viewed as a continuous process which, once initiated, proceeded through explosive, erosional, and chemical phases to produce a complex volcanic-sedimentary family group in which the members, although each possessing unique characteristics, are related by common volcanic heritage.
Volcanic divisions are described in terms of distribution, lithology, relationship to adjacent formations, and environment of deposition. Average chemical analyses of volcanic rocks are presented. In contrast to the typical andesite-rhyolite suite, Michipicoten volcanic rocks are deficient in CaO, Na2O, K2O, and SiO2, and enriched in Fe, MnO, and CO2.
Banded iron formations contain two mutually gradational facies associated respectively with sedimentary and volcanic rocks. The latter type contains large bodies of siderite and pyrite. Banded iron formations are ascribed genetically to large-scale hot-spring and fumarolic activity. Iron carbonate and sulfur components appear to be of subvolcanic, presumably magnatic, derivation, whereas silica was largely derived by chemical leaching of the volcanic rocks. Average chemical compositions of banded chert, pyrite, and carbonate members of the iron formation are presented.
The clastic sediments are like true volcanic sediments in that rock and mineral components have their counterparts in subjacent volcanic rocks in either extrusive or intrusive form and appear to have been derived from them by rapid, contemporaneous erosion. Granite detritus in coarse clastic sediments is inferred to reflect contemporaneous erosion of near-surface granite stocks and apophyses emplaced under conditions of high geothermal gradient.
Michipicoten rocks have been folded about east-west-trending primary axes, themselves cross-folded about northwest-trending axes. As a result, fold axes describe undulating patterns in longitudinal and cross sections. Prominent, northerly striking vertical faults with left-hand horizontal components of displacement have sliced the area into large parallel blocks.