GEOLOGY AND VOLCANIC SETTING OF THE HORNE DEPOSIT, ROUYN-NORANDA, QUéBEC

The Horne deposit in the Noranda mining camp, northwestern Québec, represents one of the largest volcanic-hosted massive sulfide deposits in the world. Between 1927 and 1976, the mine produced 260 t of Au and 1.13 Mt of Cu making it the largest gold producer of its class. Detailed surface mapping has been carried out to constrain the volcanological and stratigraphic setting of the deposit and to understand the geological factors that contributed to its unusual size and gold-rich nature. Field work focused on the Horne West area that contains one of the best exposed and preserved sections of the Horne stratigraphy. Volcanic facies analysis revealed that the Horne West succession is dominated by a proximal facies association comprising coherent rhyolite and associated volcaniclastic rocks that formed by autobrecciation and quench fragmentation. Effusive and shallow intrusive volcanism in the Horne West area occurred broadly contemporaneously with the deposition of mass-flow derived volcanic debris containing pyroclasts generated by explosive eruptions of a felsic volcanic source. In addition to a reconstruction of the volcanic facies architecture, the study highlighted the mineral potential of the Horne West area. Disseminated sulfide mineralization and associated hydrothermal alteration are a conspicuous feature of this portion of the Horne stratigraphy. Elevated gold grades were encountered in the immediate footwall of two paleo-seafloor positions within the volcanic succession that are particularly prospective for seafloor massive sulfide accumulations.