Abdelali Moukhsil, Fabien Solgadi et Patrice Roy
Ministère des Ressources naturelles
From 2010 to 2013, the Bureau de l’exploration géologique du Québec (BEGQ) carried out geoscience knowledge acquisition projects in the Daniel-Johnson (Manic 5) reservoir region, north of the city of Baie-Comeau (Figure 1). In all, $5,1M has been invested in this region to develop its mineral potential.
The region was covered by aeromagnetic geophysical surveys with a line spacing of 300 m. The surveys, which were conducted in 2012 and 2013, provided new detailed geological information to support prospecting and regional mapping (Figure 2).
From 2010 to 2013, geological surveys at a scale of 1:50,000 covered a surface area of 28,000 km2, corresponding to the area covered by 23 NTS map sheets. This broad-scale mapping work deepened our understanding of the regional geology in terms of lithology, stratigraphy and geochronology, and led to the discovery of many new mineral showings, mineral targets and sites for dimension stone and ornamental stone (46). The MRN work uncovered new showings for a wide range of mineral substances, thereby increasing the mineral potential of the region. These new discoveries are presented in Figure 3 and in tables 1 and 2.
The Ni-Cu mineralization is associated with mafic to ultramafic rocks, related or not to anorthositic suites. The host rock is either a dyke or a small intrusion of plagioclase peridotite, olivine pyroxenite, gabbronorite or gabbro. These rocks contain 1 to 3% disseminated to semi-massive sulphides (pyrrhotite, chalcopyrite, pentlandite and pyrite). The host rocks were injected into, or are in contact with, paragneiss.
PGE mineralization was discovered in a 1-m-thick boudin of ultramafic rock, hosted in amphibolitized garnet-gabbronorite. The boudin is composed of fine-grained, foliated and magnetic pyroxenite. Mineralization is characterized by traces of malachite on altered surface and fine dissemination (3 to 5%) of chalcopyrite, pyrrhotite and pyrite.
Magmatic-type Ni-Cu mineralization consisting of approximately 5% sulphides (pyrite, chalcopyrite, pyrrhotite) was discovered in an erratic boulder of clinopyroxenite. The mineralization is present as disseminations or as fine veinlets ranging in length from centimetres to metres.
Hydrothermal (epigenetic) mineralization of lesser importance is associated with late fractures in the host rock (gabbronorite). Sulphide mineralization (2 % chalcopyrite, 3% pyrrhotite and 1% pyrite) is present in disseminated form, or as thin veinlets along small fractures. The extent and number of these fractures have not yet been documented.
The newly discovered iron-titanium showings are of magmatic origin and hosted in anorthositic suites as centimetre- to metre-scale layers. They are also associated with clinopyroxene-, amphibole- and garnet-bearing gabbros, and with foliated porphyroclastic garnet- and magnetite-bearing norites. Vanadium (vanadiferous magnetite) and phosphate (apatite) may also be associated with iron and titanium in these showings. The main oxides are hemo-ilmenite and magnetite. Inclusion-free ilmenite and traces of sulphides (pyrite, chalcopyrite, pyrrhotite) and hematite locally accompany the oxide mineralization.
Rare earth showings were discovered during the mapping programs. The demand for these elements continues to grow thanks to their many uses (hybrid and electric cars, smart phones, plasma screens, the wind turbine industry, electromagnets, etc.).
In Québec, rare earths are most commonly found in alkaline or peralkaline intrusive complexes (carbonatite, granite, syenite, pegmatite). They are also associated with polymetallic iron oxide mineralization and skarns, and rarely with secondary environments (placers or paleoplacers). In the Manicouagan region (central Grenville Province, RG 2003-01, RG 2003-03, RG 2012-01), rare earth mineralization is found mainly in thorium-enriched granite pegmatites. A large volume of pegmatitic dykes has been observed in this region. These granitic pegmatite dykes (photos 1 and 2) range in thickness from 10 cm to 15 m across.
These rare earth pegmatitic dykes were injected into intrusions and metasedimentary rocks. Mineralization is magmatic in nature and the rare earth minerals are monazite, allanite, parisite, synchysite, apatite and zircon.
The region is well known for its graphite and sillimanite potential. Several economically important deposits have been discovered in the Manicouagan region and are still being evaluated to determine their reserves. We discovered new graphite showings associated with rusty paragneiss containing graphite-rich (2 to 20%) lenticular layers. Graphite is present as small laminated flakes, stacked and easily exfoliated, and also as small disseminated rods.
Sillimanite is also known to occur in this region. During our work, several showings were discovered, typically in the same geological units containing the graphite-rich lenses (paragneiss). Good mineral potential for sillimanite (SM) was noted in the Grand Lac du Nord region to the east of the Manicouagan Reservoir. The observed sillimanite zones range from several metres wide (1 to 15 m) to several kilometres long (0.5 to 4 km). Sillimanite accounts for up to 30% of the rock, with grades up to 21% Al2O3, and it occurs in fibrous form or as fibrolite sprays.
Several sites with dimension stone potential were identified during our work. All the sites have easy access and contain a significant volume of stone with locally acceptable quality. The targets represent intrusive rocks, such as greenish quartz mangerite, charcoal grey porphyritic to porphyroblastic garnet-bearing granite, and greenish porphyritic mangerite. The information on these targets is available on the MRN website via E-Sigeom à la carte, under the entry “Construction materials and industrial stones”.
A target for ornamental stone was discovered, which may be of interest for its quality. It represents a dyke of undeformed, massive megaporphyritic leuconorite, several tens of metres thick, with large labradorite phenocrysts. It could be suitable as an ornamental stone due to the play of colours in its labradorite crystals, which range from 2 to 20 cm long and account for up to 33% of the rock. The crystals display primarily blue iridescence and are characterized by colour zoning (vivid iridescent colours with shades of green, blue, or green and orange), which varies depending on the angle of the light hitting the rock’s surface. The dyke is of sufficient volume to warrant interest and the zone containing iridescent labradorite is at least 20 m long over a height of roughly 1 to 2 metres with a moderate slope. The labradorite crystals appear be of semi-precious stone quality, but further study is required.
Zinc mineralization, mainly found in the western part of the Grenville Province, is associated with Mississippi Valley-type (SEDEX-type) carbonate rocks and metasedimentary rocks. In the Manicouagan region, a zinc showing was discovered in rusty biotite-, graphite- and sphalerite-bearing paragneisses associated with marbles.
Gold mineralization is less well known in the Grenville Province. Showings have been reported, among other places, in the small greenstone belt in the Escoumins region (Côte-Nord). A gold showing was discovered in the Manicouagan region on the eastern shore of the Outardes 4 Reservoir. The host rock is an olivine pyroxenite, associated with a locally layered norite, containing plagioclase xenocrysts. The analyzed sample from this showing yielded grades of 0.76 ppm Au (Actlabs laboratory) and 0 ppm Au (ALS Minerals laboratory). This discrepancy may be explained by the nugget effect, which is common to this type of mineralization, or by sample contamination.
Molybdenum mineralization has been discovered along the margins of a white pegmatite dyke of granitic composition (1 to 3 cm thick) at the contact with a gabbronorite. The mineralization consists of large molybdenite crystals (1 to 3 cm) with lamellar structure. Molybdenite mineralization associated with granite pegmatites has been documented in the southwest part of the Grenville Province. This type of mineralization is considered to be post-tectonic and to have crystallized at low temperatures from water-undersaturated liquid.
All this information will soon be synthesized in a geological study. The study will be multidisciplinary, covering several subjects that will assist in understanding the geology and economic potential of this region.