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The Michipicoten group of older volcanic and sedimentary rocks comprises flows and pyroclastic rocks of andesite-rhyolite association together with conformable zones of clastic sediments and banded iron formation. Later intrusive rocks consist of dacite, granite, and diabase.
The typical volcanic cycle progressed from (1) widespread and prolonged extrusion of andesite-basalt flows, through (2) violent ejection of rhyolite-dacite pyroclastics, to (3) extensive hot-spring and fumarolic activity. Banded iron formations are considered to represent chemical products of this last stage. Development of the Michipicoten group is viewed as a continuous process which, once initiated, proceeded through explosive, erosional, chemical, and intrusive phases to produce a complex family group of which the members, although each possesses unique characteristics, are related by common volcanic heritage.
Iron, gold, and base metal deposits occur within, or marginal to, the principal acid volcanic zones. In general, mineralized centers coincide with what may be reasonably interpreted as centers of maximum explosive volcanic discharge. In this manner, siderite-pyrite members of banded iron formation overlie coarse acid pyroclastic zones; gold and base metal deposits occur within, or marginal to, nearby porphyry intrusive stocks. Acid extrusive rocks, porphyry intrusions, and mineral deposits are considered to have a common, sub-volcanic derivation and to owe their present stratigraphic association to common generative volcanic processes.
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