THE MCDOUGALL-DESPINA FAULT SET, NORANDA, QUEBEC - EVIDENCE FOR FAULT-CONTROLLED VOLCANISM AND HYDROTHERMAL FLUID-FLOW

The McDougall-Despina fault set in the Noranda district of northwester n Quebec defines the eastern margin of the Despina cauldron, nested wi thin the district-scale Noranda cauldron. The faults are stratabound i n that they displace stratigraphy of the cauldron fill, but do not off set the cauldron cover. The faults are occupied by dikes that acted as eruptive centers to volcanic members of the cauldron-filling sequence . Measurable displacement across the McDougall-Despina fault set is gr eater than 750 m, and coincident with post-cauldron volcanism. Feeder dikes cross-cut each other and are typically depleted in Na2O and CaO, and enriched in MgO, K2O, Fe2O3 and MnO relative to fresh flows. Econ omic and subeconomic massive sulfide deposits occur adjacent to the Mc Dougall-Despina fault set at different levels over a stratigraphic thi ckness of 3000 m. These include the Corbet deposit, the subeconomic D- 68 Zone deposits, and the subeconomic stringer zones of South Rusty Hi ll and Bedford Hill. Numerous copper occurrences and abundant hydrothe rmal alteration occur in and immediately adjacent to the fault set. Th e McDougall, Despina, and adjacent, parallel, ancillary faults define a synvolcanic fault set which controlled the location of magmatism, hy drothermal alteration and mineral deposition during the early evolutio n of the Noranda cauldron. Significant sulfide mineral deposition comm only occurred in proximity to the intersection of the McDougall-Despin a fault set with northeast-trending synvolcanic faults. Recognition of synvolcanic faults is therefore a major objective of exploration prog rams for volcanic-associated massive sulfide deposits.