A comparison of the Horne volcanogenic massive sulfide deposit and intracauldron deposits of the Mine Sequence, Noranda, Quebec

Most of the massive sulfide deposits of the Rouyn-Noranda area are associated with rhyolite and andesite flows of the Mine sequence and occur within the Noranda cauldron, a volcanic subsidence structure related to partial evacuation of an underlying magma chamber (the Flavrian pluton). The Horne massive sulfide orebodies lie within a sequence of rhyolitic breccias located just south of the inferred margin of the subsidence structure and are separated from the Mine sequence by the Horne Creek fault. Traditionally, the Horne deposits and the intracauldron deposits have been interpreted to occur within the same stratigraphic interval. Even though they share some characteristics such as mineral zoning patterns, alteration assemblages, and an association with synvolcanic faults, the two deposit groups are clearly distinguishable.Intracauldron massive sulfide deposits are small (<5 Mt), Cu-Zn-rich, concordant mounds associated with interflow horizons and are closely linked to vent zones where synvolcanic faulting provided hydrothermal fluids with cross-stratal structural permeability. The three major sulfide deposits at the Horne mine are much larger (20-150 Mt) and formed in a fault-bounded, sediment-filled graben on the flank of a rhyolitic vent complex. The orebodies are Au-Cu rich but Zn poor and have bowl-shaped morphologies which resulted from extensive subsea-floor sulfide precipitation. Whereas the intracauldron sulfide deposits lie within a bimodal, flow-dominated succession consisting mainly of andesite-basalt and high silica rhyolite, with volumetrically insignificant pyroclastic rocks, the Horne orebodies occur in felsic fragmental rocks and rhyolite flows which are part of a stratigraphic sequence which contains virtually no mafic volcanic rocks. Also, compared to all intracauldron rhyolites, the Horne rhyolites show a fundamental depletion in incompatible trace elements such as the high field strength and rare earth elements, which implies that the Horne rhyolites may have followed a different magmatic evolutionary path.Such differences in deposit characteristics, their paleoenvironments of formation, and the physical volcanology and geochemistry of their host volcanic successions suggest that, contrary to previous interpretations, the massive sulfide deposits of the intracauldron Mine sequence and those of the Horne sequence may have formed at different stages in the evolution of the Noranda cauldron.