Volcanology and geochemistry of the Monsabrais area, Blake River Group, Abitibi Greenstone Belt, Quebec: implications for volcanogenic massive sulphide exploration

Abstract

The ca. 2.70 Ga Blake River Group in the southern Abitibi Greenstone Belt (Quebec and Ontario) hosts numerous volcanogenic massive sulphide deposits. The peripheral areas of this group contain a greater proportion of volcaniclastic rocks than the traditional 'central camp', and also host some of the largest volcanogenic massive sulphide mines, including LaRonde-Penna and Bouchard-Hébert. These represent deposits formed in part by subseafl oor replacement in permeable fragmental rocks. Many peripheral sectors of the Blake River Group have been less intensely explored, yet hold potential for volcanogenic massive sulphide mineralization, especially if volcanic centres can be identifi ed. Two basemetal deposits, Magusi River and Fabie Bay, are found just south of the Monsabrais area. Both are partly hosted by andesitic fragmental rocks. In this report, the authors examine the lithofacies and geochemistry of the Monsabrais area, interpreted as a distinct volcanic centre. It includes a synvolcanic intrusion, the Monsabrais Pluton, newly dated at 2696.2 ± 0.9 Ma. The Monsabrais volcaniclastic rocks - many of which lack bedding or grading and are intimately intermixed with coherent subaqueous lava fl ows - are dominantly interpreted as hyaloclastite. Observations supporting this interpretation also include the monomictic character of many facies, jigsaw-fi t textures, curviplanar clast outlines, and gradations into massive lava. Pyroclastic fl ow deposits do not represent a signifi cant volume in the Monsabrais area. This has implications for the recently proposed Blake River Group megacaldera model, which relies in part on a semicontinuous belt of 'ignimbrites' (pyroclastic fl ow deposits) to trace the caldera margin.