A melt inclusion study of rhyolitic volcanics in the Bousquet Formation, Doyon-Bousquet-LaRonde district, Abitibi Subprovince, Québec : insight into Archean magmatic processes

Abstract

Silicate melt inclusions (SMI) from 2697 – 2699 Ma Bousquet rhyolites (Doyon-Bousquet-LaRonde district, Québec) were characterized to elucidate magmatic contributions to VMS deposit tenor in the region. The key results are as follows: (i) Early rhyolite is of dominantly ocean ridge (tholeiitic) to transitional affinity, whereas later rhyolite has dominantly volcanic arc (calc-alkaline) affinity; (ii) SMI record a wide compositional range compared to host (bulk) rocks, reflecting extensive plagioclase fractionation at depth and inconsistent with TTG classifications previously proposed for the melts; (iii) Ore/accessory metal concentrations in SMI (e.g., Auavg = 33 to 120 ppb) are much higher than crustal abundances, and reflect pre-eruptive Bousquet Formation magma metal tenor; and (iv) melts were saturated in liquid CO2 but undersaturated in H2O at depth. The study is one of only a few studies of SMI from Archean rocks and the first to present trace element/ore metal data for SMI of this age.