Units in the Blake River Group: Correlations, geometry and mineral potential Jean Goutier, Claude Dion, Marc Legault (MRNF), Pierre-Simon Ross, Vicki McNicoll, Eric de Kemp, John Percival, Thomas Monecke, Gilles Bellefleur, Patrick Mercier-Langevin, Kathleen Lauzière (GSC), Phil Thurston (LU), John Ayer (OGS)

Seven major formations were recognized and are, from the oldest to the youngest: Hébécourt, Horne, Rouyn-Pelletier, Duprat-Montbray, Noranda, Bousquet, and Reneault-Dufresnoy. The Hébécourt Formation and the lower Rouyn-Pelletier Formation are interpreted as the tholeiitic base upon which all other units developed. The Horne sequence, which hosts the massive sulphide ore deposit of the same name, is characterized by a pile of felsic volcaniclastic rocks and proximal vent facies; geochronology studies reveal it is the oldest felsic unit in the Blake River. The upper Rouyn-Pelletier Formation and the Duprat-Montbray Formation consist of bimodal volcanic assemblages. The Duprat-Montbray Formation extends into Ontario to include rhyolites in Pontiac Township. The Noranda Formation, younger than the Duprat-Montbray, is divided into three sequences:

1. the Mine sequence, a bimodal sequence hosting numerous VMS deposits;
2. the Héré sequence, overlying the latter and hosting the Delbridge and D’Eldona gold-rich VMS ore deposits;
3. the Cyprus sequence, the youngest and showing a strong tholeiitic affinity.

The Reneault-Dufresnoy Formation forms the northern margin of the Blake River in Québec, and is interpreted to extend over much of the Blake River in Ontario, including the volcanic rocks in Ben Nevis Township. It stratigraphically overlies both the Hébécourt and Duprat-Montbray formations. It is characterized by plagioclase-phyric units, by a wide variety of volcanic facies, many types of volcaniclastic rocks, and vesicular layers. This marks a drastic change in Blake River volcanism, and a hiatus can probably be defined at the base of this formation. Although partly coeval with the Reneault-Dufresnoy Formation, the Bousquet Formation is distinctive, due to its volcanic facies and a concentration of gold-rich VMS ore deposits (Bousquet and LaRonde Penna). The structure of the Blake River Group can be represented using a simplified image of a N-S flattened sombrero, where early units outcrop outside and in the centre. Younger units therefore occur along the brim. New potential zones for VMS deposits are:

• the top of the Noranda Formation, which shows a Matagami-style setting with spherulitic rhyolites of tholeiitic affinity (Cyprus);
• the base of the Reneault-Dufresnoy Formation;
• areas to the west of intermediate volcaniclastic rocks adjacent to rhyolites in the Reneault-Dufresnoy Formation (Fabie Bay-Magusi style).