New volcanological and geochemical observations from the Blake River Group, Abitibi Greenstone Belt, Quebec: the D'Alembert tuff, the Stadacona unit, and surrounding lavas

Abstract

The Archean Blake River Group hosts a number of volcanogenic massive-sulphide deposits, several of which are world-class. A better understanding of the stratigraphy and volcanic architecture would help exploration for additional mineralization in the area, including the lesser-known areas of the group. To contribute to this objective, the authors present a comparison of two mafi c to intermediate volcaniclastic units on the periphery of the Blake River Group: the D'Alembert tuff in the north, and the Stadacona unit in the south. Both are bedded sequences, several hundred metres in stratigraphic thickness, that were likely emplaced by successions of submarine, water-supported density currents. New fi ndings were that a) explosive basaltic to andesitic eruptions were episodic and separated by pauses long enough to allow the formation of silicic domes and mafi c lava lenses in between the volcaniclastic layers; and b) several chemical subgroups can be identifi ed in both the volcaniclastic rocks and the surrounding lavas. Volcaniclastic units from the periphery of the Blake River Group, including the two discussed here, have been inferred by some previous workers to be correlative and to belong to a caldera-margin setting, with the implication that they were erupted simultaneously or in close succession from a common magma chamber. It is shown that the D'Alembert tuff and the Stadacona unit are geochemically distinct and that they likely have different ages. The authors conclude that these two volcaniclastic units were generated by different eruptive phases, most likely from distinct magma chambers, in different parts of the Blake River Group.