Intermediate to mafic volcaniclastic units in the peripheral Blake River Group, Abitibi Greenstone Belt, Quebec: origin and implications for volcanogenic massive sulphide exploration

Abstract

Three regionally mappable, plagioclase-rich, intermediate to mafic volcaniclastic units from the peripheral parts of the Archean Blake River Group show intriguing similarities, including a lack of an intimate association with lava flows; and a bedded aspect, with thick, coarse beds displaying a typical sequence of internal features, suggesting deposition from high-density mass flows. The bounding facies and the great volume of variably amygdaloidal clasts and plagioclase crystals contained in the deposits supports major submarine explosive eruptions as the source of the bulk of the material. The physical resemblance between the different volcaniclastic units initially suggested a stratigraphic correlation, but preliminary geochemical data argues against this: the D'Alembert tuff consists of basaltic andesite and andesite of transitional to calc-alkaline affinity, whereas the Stadacona unit essentially comprises tholeiitic basalt, with very different profiles on multi-element plots. Any hypothesis involving a megacaldera and a huge underlying magma chamber to explain the origin of these volcaniclastic units would need to justify such chemical differences. The authors favour a simpler scenario in which the D'Alembert tuff and the Stadacona unit were derived from much smaller, geographically and chemically distinct magma chambers. The explosive eruptions could have taken place at different times and no stratigraphic correlation between the volcaniclastic units is necessary. Peripheral areas of the Blake River Group contain more fragmental material than the traditional Noranda camp, and therefore offer exploration potential for large volcanogenic massive sulphide deposits formed in part by subseafloor replacement, which are preferentially hosted in volcaniclastic rocks (e.g. Bouchard-Hébert, LaRonde Penna).