Abstract

The Ben Nevis volcanic complex occurs in the Misema–Duprat-Montbray formation of the Ontario portion of the Blake River Group. The Misema–Duprat-Montbray formation is considered contemporaneous with the Noranda formation of the Blake River Group although they display varying volcanic styles. The Noranda formation displays bimodal flow-dominated rhyolite-andesite volcanism, pyroclastic rocks being rare; the eastern part of the Misema–Duprat-Montbray is similar but becomes dominantly andesitic with subordinate rhyolites to the west. In both the Noranda and Misema–Duprat-Montbray formations, the andesites are of two end-member affinities (tholeiitic and calc-alkalic) with a third transitional andesite type. The Ben Nevis volcanic complex differs somewhat from the two formations in that it is not bimodal. It exhibits a continuous spectrum of lithologic units from basaltic-andesite to rhyolite; the andesites occur as either flows or as pyroclastic deposits, and the dacites-rhyolites are dominantly pyroclastic. Geochemically, the andesites are of a single, calc-alkalic affinity. The Misema–Duprat-Montbray formation has a published U-Pb age date of 2701 ± 2 Ma from a rhyolite in Pontiac Township in Ontario. A rhyolite in the Ben Nevis volcanic complex yields a new precise age of 2696.6 ± 1.3 Ma, making it younger than the main volcanic phases of the Misema–Duprat-Montbray. The Ben Nevis age overlaps published ages for the youngest formations of the Blake River Group in Québec, the Reneault-Dufresnoy (2696 ± 1.1 Ma) and Bousquet (2698 ± 1 Ma) formations. A new U-Pb age for a second, porphyritic Ben Nevis rhyolite of 2699.8 ± 3.6 Ma, although less precise, is also nominally younger than the earliest phase of the Misema–Duprat-Montbray volcanism.

The Ben Nevis volcanic complex shares other similarities, in addition to age, to the Reneault-Dufresnoy and Bousquet formations. Similar to the Bousquet formation, the Ben Nevis volcanic complex is not bimodal. Pyroclastic rhyolites are also common in the Ben Nevis volcanic complex, as in the Reneault-Dufresnoy and Bousquet formations. The rhyolites of all three late-phase volcanic formations exhibit greater LREE enrichment and larger negative Nb anomalies than the Noranda or Misema–Duprat-Monbray formation rhyolites. The synvolcanic mineral deposits in the Reneault-Dufresnoy and Bousquet formations are polymetallic (gold, silver, and base metal sulfides), and similar but much smaller showings occur in the Ben Nevis volcanic complex. The similarity in age, lithological and geochemical character, and style of mineralization are consistent with the Ben Nevis volcanic complex having been emplaced during the late Blake River Group volcanism that formed the Reneault-Dufresnoy and Bousquet formations.

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