SPHERULITIC RHYOLITES OF THE ARCHEAN BLAKE RIVER GROUP, CANADA - IMPLICATIONS FOR STRATIGRAPHIC CORRELATION AND VOLCANOGENIC MASSIVE SULFIDE EXPLORATION

Based on the distinctive textural and chemical character of highly sph erulitic rhyolites in the Late Archean, volcanogenic massive sulfide-r ich, Blake River Group of Quebec, it is proposed that they represent k ey beds which may be used to infer the age relationships between the t hree subgroups (Rouyn-Pelletier, Misema, and Noranda) into which the B lake River Group is divided in this area. These rhyolites occur as flo w and fragmental (volcaniclastic, probably pyroclastic) facies, both o f which contain crystallization spherulites. They occur as three units , a flow facies exhibiting parallel-agglomerated lath-shaped plagiocla se crystals (the Fish-roe rhyolite), a fragmental spherulitic rhyolite which is aphyric, and a second, very local, flow which is quartz-plag ioclase porphyritic. Chemically, however, tile three units are identic al and distinct within the Blake River Group stratigraphy. The Ti, Y, Yb, and Zr contents are higher in the spherulitic rhyolites than in ti le 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 spheruliti c rhyolites and result in their being important stratigraphic marker u nits in the group in Quebec. Statistical analyses of the geochemical d ata from the rhyolites in the Blake River Group show that there is a 9 5 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 spherulit ic rhyolite in the andesitic Misema Subgroup and in the bimodal andesi tic-rhyolitic Noranda Subgroup implies that these two domains formed, at least in part, concurrently. The same conclusion can be drawn for t he 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 t han a single vertically stacked evolutionary system, was responsible f or the formation of tile Blake River Group and its volcanogenic massiv e sulfide deposits.