Abstract

Based on the distinctive textural and chemical character of highly spherulitic rhyolites in the Late Archean, volcanogenic massive sulfide-rich, Blake River Group of Quebec, it is proposed that they represent key beds which may be used to infer the age relationships between the three subgroups (Rouyn-Pelletier, Misema, and Noranda) into which the Blake River Group is divided in this area. These rhyolites occur as flow and fragmental (volcaniclastic, probably pyroclastic) facies, both of which contain crystallization spherulites. They occur as three units, a flow facies exhibiting parallel-agglomerated lath-shaped plagioclase crystals (the Fish-roe rhyolite), a fragmental spherulitic rhyolite which is aphyric, and a second, very local, flow which is quartz-plagioclase porphyritic. Chemically, however, the three units are identical and distinct within the Blake River Group stratigraphy.The Ti, Y, Yb, and Zr contents are higher in the spherulitic rhyolites than in the other rhyolites in the Blake River Group, whereas the La content is similar between the two rhyolite populations. These values translate into higher Ti/Zr, and lower Zr/Y and La/Yb, ratios for the spherulitic rhyolites and result in their being important stratigraphic marker units in the group in Quebec. Statistical analyses of the geochemical data from the rhyolites in the Blake River Group show that there is a 95 percent probability that the spherulitic rhyolite analyses represent a single genetically related population and may, in fact, be a single stratigraphic unit.The occurrence of the fragmental highly spherulitic rhyolite in the andesitic Misema Subgroup and in the bimodal andesitic-rhyolitic Noranda Subgroup implies that these two domains formed, at least in part, concurrently. The same conclusion can be drawn for the basaltic Rouyn-Pelletier Subgroup and the Noranda Subgroup, as the Fish-roe rhyolite occurs in both these domains. This in turn suggests that a complex interplay of contemporaneous magmatic systems, rather than a single vertically stacked evolutionary system, was responsible for the formation of the Blake River Group and its volcanogenic massive sulfide deposits.

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