Abstract

The Roy Group in the Chibougamau area of the Abitibi subprovince, Southeastern Superior craton, Quebec, exemplifies Archean volcanic evolution. The Roy Group formed in two volcanic cycles, with voluminous basal mafic tholeiitic volcanic rocks being capped by a thin succession of calc-alkaline felsic volcanic rocks, banded iron formations, and cherts that represent regional stratigraphic markers. The Gilman Formation is thought to represent the mafic base of the 2nd Roy Group volcanic cycle. Our new mapping reveals that the Gilman Formation (North of the Lac Doré Layered intrusive complex) can be subdivided into a tholeiitic lower part (David member), a calc-alkaline extrusive sequence (Allard member), and a tholeiitic, more mafic upper part (Bruneau Formation, newly defined here). New U–Pb zircon ages on Allard member tuffs (2726.6 ± 0.7 Ma) are within error of those obtained from rhyolites of the Waconichi Formation (2728.2–2728.7 Ma), which underlies the Gilman Formation. These ages, along with geochemical and lithological similarities, imply that the Allard member belongs to the Waconichi Formation and that the David member occupies the same stratigraphic position as the Obatogamau Formation. The upper part of the former Gilman Formation is reassigned to the new Bruneau Formation. A thin transitional calc-alkaline basalt unit at the base of the Allard member is geochemically similar to basalts occurring at the top of the Lemoine member of the Waconichi Formation near the Lemoine mine, suggesting that the Allard member is slightly younger than the Lemoine member, and that its base (also associated with iron formation, breccia, and chert) represents a regional stratigraphic break favourable for volcanogenic-hosted massive sulphide (VHMS) exploration.

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