Zircon dating of polycyclic volcanism at Sturgeon Lake and implications for base metal mineralization.

The S Sturgeon Lake sequence consists of 4 volcanic cycles. The lower 2 cycles host Cu/Zn massive sulfide mineralization. Trace element and REE analyses indicate that the lower 3 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 7 rocks from the lower 3 cycles in an effort to resolve the ages. All age analyses agree within error, giving an average of 2735.5Ma with a range of 1.5Ma. The uppermost cycle was previously dated at 2717.9^+2.7 _-1.5Ma, 18Ma younger than the lower cycles. Moreover, REE 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 volanism and acted as a heat source driving hydrothermal convection which transported and deposited base metals. -from Authors