Abstract

The South Sturgeon Lake sequence consists of four volcanic cycles. The lower two cycles host Cu-Zn massive sulfide mineralization. Economic mineralization has not been found in the third and fourth cycles. Trace element and rare earth analyses performed previously and in the present study indicate that the lower three cycles have a similar petrogenetic history and are cogenetic with the Beidelman Bay pluton, a subvolcanic trondhjemite sill which intrudes the base of the lower cycle. The upper cycle is distinctly different. U-Pb zircon analyses have been performed on seven rocks from the lower three cycles in an effort to resolve the ages. All age analyses agree within error, giving an average of 2,735.5 m.y., with a range of 1.5 m.y. A previously determined age of 2,733.8 (super +1.4) (sub -1.3) m.y. on the subvolcanic intrusion is marginally younger. The uppermost cycle was previously dated at 2717.9 (super +2.7) (sub -1.5) m.y., 18 m.y. younger than the lower cycles. Moreover, rare earth element data indicate that this upper cycle is not comagmatic with the lower cycles. The data indicate rapid evolution of a volcanic pile followed by a long period of quiescence and later reactivation from a different magmatic source.The younger age of the Beidelman Bay pluton indicates that it probably is not directly related to volcanism and ore-forming hydrothermal activity. However, it may occupy the magma chamber which previously fed rhyolitic volcanism and acted as a heat source driving hydrothermal convection which transported and deposited base metals. This would require the subvolcanic magma chamber to be coupled to a much larger magma system which fed eruptive and high level intrusive activity, remaining molten for about 1.7 m.y.

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