Abstract

The Blake River Group of the Abitibi greenstone belt is host to the well-known volcanogenic massive sulfide (VMS) deposits of the Noranda camp, Québec, Canada.

In the Noranda camp massive sulfide deposits are spatially associated with the coeval Flavrian-Powell intrusive complex, which is interpreted to have driven hydrothermal circulation that formed the deposits in the camp. In the western Blake River Group in Ontario there are also abundant intrusive rocks, yet relatively insignificant accumulations of VMS deposits. These intrusive rocks are younger (2686.9 ± 1.2 and 2688.5 ± 2.3 Ma) and are associated with porphyry-style Cu-Mo-Au mineralization with Re-Os molybdenite ages of 2682.4 ± 5 Ma. The intrusions are also smaller in areal extent and do not have a temporally extended, polyphase history like the Flavrian-Powell intrusive complex.

The intrusions of the western Blake River Group can also be distinguished by lithogeochemical attributes and a distinct petrological history compared to the Flavrian-Powell intrusive complex. They have lower total Y, Yb, Th, HFSE, and REE, with higher Zr/Y, La/Yb, Sm/Yb, Al2O3/Yb, and La/SmUCN ratios, and lower Zr/Nb, Zr/Th, Zr/La, Zr/TiO2, and Nb/ThUCN ratios. Petrologic modeling is consistent with the western Blake River Group intrusive rocks being generated as relatively low temperature melts (<800°C) at depths >40 km in the crust where garnet is stable in the residue (e.g., garnet amphibolite residue). In contrast, modeling of the Flavrian-Powell intrusive complex is consistent with melting at shallower levels in the crust (e.g., <40 km) in the amphibolite stability field and at temperatures >800° to 850°C. The occurrence of high-temperature magmatism at high levels within this crust that is coeval with volcanic activity explains the occurrence of numerous VMS deposits with the Flavrian-Powell intrusive complex, as this intrusive complex would have generated abundant heat at high levels in the crust to drive hydrothermal circulation. In contrast, the intrusions of the western Blake River Group were generated at depth and at lower temperature, increasing the probability of heat loss upon emplacement from depth, hence, decreasing the probability of hydrothermal circulation at high levels in the crust and likely explaining the paucity of VMS mineralization associated with these intrusive rocks. The petrological attributes of the Flavrian-Powell intrusive complex and western Blake River Group may be useful in delineating potentially fertile versus less prospective intrusive complexes in greenfields exploration areas.

Notably, the western Blake River Group intrusions have geochemical features similar to some Phanerozoic porphyry Cu-Mo-Au–associated intrusive systems consistent with the observed mineralization and highlighting the potential of porphyry Cu-Mo-Au systems of Late Archean age elsewhere in the Abitibi.

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