Geology and metallogeny of the Opinaca area – Phase 2/3 (33C10, 33C11 and 33C15)
Daniel Bandyayera, Pierre Lacoste, Alexandra Fliszár and Charles Maurice (MRNF)

The Opinaca project covers the northern part of map sheets 33C and 33B. This area was selected based on:

1. the paucity of geological knowledge;
2. its strategic position to understand links between the La Grande and Opinaca subprovinces;
3. its great mineral potential.

During the first phase of the project, mapping in sheets 33C09 and 33C16 outlined the Kasak volcano-sedimentary formation (>2709 Ma) draped around the Opinaca Reservoir Pluton, a multiphase intrusive dome (2703-2710 Ma) composed of diorite-granodiorite- porphyritic tonalite.

This second phase of the project consists in a mapping survey at 1:50,000 scale of NTS sheets 33C10, 33C11 and 33C15, to the west of Opinaca Reservoir. Our work traced the extension of the Kasak Formation to the west of Opinaca Reservoir, where sedimentary units locally show aluminosilicate alteration zones similar to those observed on the Eleonore property. In the south part of map sheet 33C11 and the west part of 33C10, another important volcano-sedimentary sequence, some 32 km long by 7 km wide, is defined and bounded to the south by a tonalite-quartz diorite pluton, and to the north by a granodiorite pluton. This sequence is formed of massive to pillowed, locally glomerophyric, basalts at the base, with crystal, lapilli and/or blocky volcaniclastic rocks at the top. Polygenic conglomerates, overlain by bedded or massive wacke and sandstone units, unconformably overlie the volcanic sequence. Remnants of Algoma-type iron formations are also observed at the top of the basalt sequence or intercalated with the sedimentary sequence.

North of map sheet 33C10, the metamorphic grade rapidly increases from the amphibolite facies to reach the granulite facies in map sheet 33C15. The current boundary between the La Grande and Opinaca subprovinces corresponds to the path of this metamorphic isograd. The boundary is locally characterized by late injections of pegmatite or massive to weakly foliated granodiorite. In the map area, the Opinaca Subprovince consists of bands of moderately to strongly migmatized paragneiss injected with pegmatitic migmatites.

Our work indicates promising potential for:

1. porphyry-type occurrences;
2. gold-rich volcanogenic massive sulphides;
3. gold associated with deformation zones or contact zones between sedimentary and volcanic sequences;
4. epigenetic occurrences associated with metasomatic vein networks;
5. iron formation-hosted occurrences;
6. rare elements in white pegmatites.