Abstract:

The Horne deposit in the Noranda mining camp, northwestern Quebec, represents one of the largest volcanic-hosted massive-sulphide 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. A new mapping program has been initiated 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. Initial fi eldwork 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 this volcanic 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-fl ow?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 highlights the mineral potential of the Horne West area. Disseminated sulphide 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 paleoseafl oor positions within the volcanic succession that are particularly prospective for seafl oor massive-sulphide accumulations. The research contributed to the initiation of a new exploration program involving about 3700 m of diamond drilling.