Metallogeny of the Powell Block, Rouyn-Noranda Mining District, Québec

Abstract

The Rouyn-Noranda mining district of Québec is located along the southern margin of the Abitibi greenstone belt of the Archean Superior Province. It hosts 22 Cu-Zn-Au-Ag volcanogenic massive sulfide (VMS) deposits, distributed amongst four fault blocks (Hunter, Flavrian, Powell and Horne). The Powell Block hosts the Au-rich Quemont VMS deposit and separates the Horne deposit, a world class Au-rich VMS deposit, from conventional VMS deposits (≤ 1 g/t Au) north of the Beauchastel fault. The structural and stratigraphic evolution of the Powell Block is poorly constrained and as a result, the time-stratigraphic position of the Quemont deposit relative to the Horne deposit and deposits of the Flavrain Block is uncertain, as are the processes responsible for the anomalous gold content of the Horne and Quemont deposits. Gold-quartz-carbonate and quartz-sulfide (Cu-Ag-Zn) epigenetic veins are also present, but their origin and relative timing remain enigmatic. The Powell Block can be subdivided into two distinctive domains, the Brownlee and Joliet domains, representing two overlapping volcanic centres. These two domains both consist of lower Blake River Group volcanic strata that differ slightly in age and volcanic history. The Brownlee domain is characterized by mafic dominated, bimodal volcanism (ca. 2701 Ma), whereas the Joliet domain is characterized by a thick sequence of felsic coherent and volcaniclastic units (≥2702 Ma). Two base metal mineralizing hydrothermal events are recognized. An early event is related to voluminous felsic volcanism, concomitant subsidence and the formation of the Quemont and Horne deposits in the Joliet domain and Horne block, respectively. These two deposits formed within the district’s largest monogenic felsic volcanic centre in a subaqueous setting that was locally emergent. Given this setting, the Au-rich Quemont and Horne deposits may have had a more direct magmatic input resulting in higher Au grades than the Cu-Zn VMS deposits of the Flavrian Block, which formed in a mafic dominated, bimodal, effusive and perhaps, a deeper water volcanic setting. A later hydrothermal event at ca. 2699~2695 Ma is related to the formation of disseminated, vein and breccia hosted (e.g. Joliet Breccia) quartz-sulfide (Cu-Ag-Zn) mineral occurrences, with variations in deposit characteristics reflecting differences in depth of formation and proximity to a magmatic source. The lateral zoning of alteration types, compositional gains and losses, and metal tenor are similar to VMS footwall alteration zones within the district and elsewhere, and these breccias and veins are part of a broad continuum of subsurface, cross-stratal magmatic-hydrothermal mineralization to seafloor, stratiform, VMS mineralization, a spectrum akin to the porphyry-epithermal continuum of modern subaerial volcanic arcs. Approximately 30–40 m.y. later, gold-quartz-carbonate veins were emplaced during N-S shortening of the volcanic rocks. Superposition of alteration related to these younger veins perturbed and masked regional alteration patterns related to the Quemont and Horne deposits, but due to their restricted distribution and comparatively low-grade, did not substantially affect the original gold tenor of these VMS deposits.