Background

Description

Bruneau Formation 1 (nAbnu1): Basalt to Andesitic Basalt, Amphibolite

Bruneau Formation 1b (nAbnu1b): Ferriferous Tholeiitic Basalt and Amphibolite

The ferriferous tholeiitic basalt is distinguished from the magnesian tholeiitic basalt by a slightly higher concentration of Al, Fe, Ti, rare earths and trace elements, and a lower concentration of MgO, between 4 and 7% (Daoudene and Beaudette, 2022). 

In the la Trêve Lake area, the magnetic map shows that the Bruneau Formation corresponds to a zone of low magnetic susceptibility without distinctive textures. The strong linear magnetic anomaly of the Abitibi Dyke somewhat obscures the pattern of the Bruneau Formation. Numerous electromagnetic anomalies are present within the unit.

Bruneau Formation 2 (nAbnu2): Mafic to Felsic Volcaniclastic Rocks, Rhyodacite, Rhyolite and Iron Formation

The Bruneau Formation contains a small proportion (1%) of volcaniclastic rocks (coarse to fine-lapilli tuff) and rhyodacite grouped in unit nAbnu2. These rocks consist of thin lenses (usually <10 m wide) with limited lateral extent (<500 m long), interstratified with mafic volcanic rocks. At the Bruneau Mine, one of these lenses is cut at the base by a gabbroic sill where the medium-lapilli tuff outcrops over an estimated thickness of 5 m. In thin section, glass shards are preserved in the chlorite ± sericite matrix. In outcrop, the white colour of lapilli suggests an intermediate composition. The tuff is overlaid by two beds of iron formation (95% magnetite, 5% pyrite, chalcopyrite traces) 40 and 80 cm thick respectively, separated by 2 m of bedded chert. These ferriferous layers can be tracked laterally over two outcrops located a few hundred metres apart, but the presence of gabbro sills precludes an estimate of their regional extent. The top of the Bruneau Mine sequence is characterized by the reoccurrence of pillow mafic volcanic rocks. To the west of the Gwillim Mine (sheet 32G16-200-0201), a medium-lapilli tuff lens measuring a few tens of metres long is also associated with an exhalative pyritic chert layer (Bouchard, 1986). In the Dolomieu Lake (sheet 32G14-200-0102) and Deux Orignaux Lake (sheet 32G14-200-0101) areas, the quartz-plagioclase-epidote-chlorite matrix lapilli tuff forms decametre-thick lenses. The lapilli essentially consist of plagioclase crystals elongated in the regional schistosity plane. Black chlorite replaces some of devitrified glass shards.

Thickness and Distribution

The Bruneau Formation thickness ranges between 2 and 3 km. This unit appears on the flanks of the Waconichi Syncline, Chibougamau Syncline and Chibougamau Anticline, from west of Chapais (sheets 32G14 and 32J03) to the Grenville Front Tectonic Zone (sheets 32H13 and 32I04).

Dating

A sample of medium to coarse-lapilli tuff collected in unit nAbnu2 at the emergency exit of the Bruneau Mine (outcrop 2006-FL-6129) sets the age of volcanism at 2724.4 ±1.2 Ma (Davis et al., 2014). 

Stratigraphic Relationship(s)

On the northern flank of the Chibougamau Anticline (northern part of sheet 32G16), the Bruneau Formation base consists of pillowed volcanic rocks that overlie lapilli tuffs and rhyodacites of the Waconichi Formation‘s Allard Member. The unit’s upper contact is usually obscured by a shear zone located at the base of the Roberge and Ventures sills. However, west of Blondeau Lake, along Road 167 (sheet 32G16-200-0202), massive and pillowed flows of the Bruneau Formation are overlaid by variolitic lava forming the base of the Blondeau Formation. On the southern flank of the Chibougamau Anticline (sheets 32G09-200-0201, 32G09-200-0202 and 32G10-200-0202), effusive rocks of the Bruneau Formation conformably lie on volcaniclastic rocks of the Waconichi Formation (Queylus, Andy, Chevrier, and Îles members). In the same area, the unit’s sommital contact is truncated by the Kapunapotagen Shear Zone, except in the Dollier Lake area (sheet 32G09-200-0202) where mafic volcanic rocks are overlain by volcanic and sedimentary rocks of the Blondeau Formation (Daigneault, 1986; Roy et al., 2007). However, the low density of outcrops does not indicate the nature of contact at this location.

In the Waconichi Lake area (sheets 32J01 and 32I04), rocks of the Bruneau Formation form two strips on the flanks of the Waconichi Syncline. On the northern flank of the syncline, basal contact of the Bruneau Formation with rocks of the Obatogamau Formation is sharp. The top of the unit is cut by an E-W shear zone which hides the stratigraphic contact with sedimentary rocks of the Chebistuan Formation. South of the Waconichi Syncline, the Bruneau Formation overlies volcaniclastic rocks of the Waconichi Formation’s Allard Member (Bélanger, 1979; Marchand, 1990).

In the Deux Orignaux Lake area west of Chapais (sheet 32G14), the Bruneau Formation forms three distinct strips: from south to north, the Dolomieu Lake strip, Chaleur Lake strip and Julien Lake strip. The Dolomieu Lake strip, on the south side of the Chapais Syncline, is not more than 2 km wide and extends laterally over all of sheets 32G14-200-0101 and 32G14-200-0102, south of the Misérables and Dolomieu lakes. The base of the unit, south of Dolomieu Lake, is an E-W reverse shear zone located on the top of Queylus Member rocks (Waconichi Formation). To the west of Misérables Lake, the basal contact of the Bruneau Formation is hidden by the Houghton Pluton intrusive rocks. The Bruneau Formation’s sommital contact with the Blondeau Formation corresponds to an E-W south-dipping reverse shear zone that passes through Misérables and Dolomieu lakes.

Northeast of Deux Orignaux Lake (sheet 32G14-200-0202), in the axial part of the Chibougamau Anticline, the Bruneau Formation’s basal contact with volcaniclastic rocks of the Deux Orignaux Member (Waconichi Formation) is hidden by the intrusion of a kilometre-thick gabbroic sill. West of Armada Lake (NE of sheet 32G14-200-0102), mafic volcanic rocks at the top of the Bruneau Formation (unit nAbnu1) are in sharp contact with mafic to intermediate lapilli and block tuffs of the Blondeau Formation (unit nAbl). East of Chaleur Lake, on the north side of the Chibougamau Anticline and north of the Opémisca Pluton (NE of sheet 32G14-200-0202), the Bruneau Formation is in sharp contact with the Blondeau Formation. South of Chaleur Lake, the unit’s upper contact corresponds locally to an angular discordance overlain by a thin lens of sedimentary rocks attributed to the Lac la Trêve Formation (Durocher, 1979).

The Julien Lake strip, located in the Waconichi Syncline, extends from the north of Trêve Lake (sheet 32G14-200-0201) to the south of Eau Noire Lake (sheet 32J03-200-0102). To the north, a thick differentiated gabbro sill and the Moraine Stock syenite intrude into the contact between rocks of the Obatogamau and Bruneau formations. East of la Trêve Lake, the summital contact of the Bruneau Formation with the sedimentary rocks of the Lac la Trêve Formation is also a thick differentiated sill and is taken up by a north-dipping reverse shear zone whose trace has been interpreted using aeromagnetic data and a few outcrops that indicate an increase in the intensity of the deformation towards the contact. The Bruneau and Lac la Trêve formations are in direct contact only over a very short distance SE of Julien Lake.

In sheet 32G13, the Bruneau Formation forms an E-W band that extends 34 km from the central part of la Trêve Lake to ~8 km west of Bransat Lake. The width of this band varies from 1.5 km to 4.5 km, but increases towards the west and reaches 6.5 km in the NW corner of sheet 32G13. Contacts to the north with the Waconichi (nAwa) and Obatogamau (nAob1b, nAob1c, nAob2) formations, and to the south with the Blondeau Formation (nAnl2), were not observed in the field. To the east and SW, the Bruneau Formation is bounded by the Dussault Shear Zone.

In the eastern part of sheet 32J03, the contact between the Bruneau and Chebistuan formations is interpreted as an unconformity, but this was not observed in the field due to a lack of outcrop.

Paleontology

Does not apply.

References

Publications available through SIGÉOM Examine

Other publications

Suggested Citation

Contributors