Background

Description

Blondeau Formation Undivided (nAbl): Volcaniclastic Rock, Feldspathic Sandstone, Rhyodacite, Variolitic Basalt, Andesitic Basalt, Mudrock, Mudstone, Graphitic Clayshale and Conglomerate

Blondeau Formation 1 (nAbl1): Basalt and Andesitic Basalt

The basal part of the unit, in close proximity to contact with mafic volcanic rocks of the Bruneau Formation, is characterized by the presence of basalt and andesitic basalt, which commonly have a variolitic texture. These massive, pillowed and brecciated basalts form unit nAbl1. Light green, they typically contain 1 to 40% millimetric to centimetric varioles. Variolitic lavas have been recognized at several locations including: (a) east of Marianne Lake (sheet 32G16-200-0202); (b) at the base of the reference section exposed on the side of Road 167, along a NE-oriented segment between Cummings Lake and the northern boundary of sheet 32G16 (Duquette, 1970); (c) northeast of Roberge Lake on the UMA Property mineralized stripping (sheet 32H13-200-0201); (d) on an outcrop south of Philippon Lake (sheet 32G15-200-0202); and (e) along Road 113, near the western exit of the town of Chapais (sheet 32G15-200-0101). Unit nAbl1 also has a series of shallow flows (<2 m) of massive, pillowed and brecciated basalt without variolitic texture. In cross-section, the long axis of pillows is 30 to 40 cm in diameter. Higher in the unit, pillows contain vesicles and amygdules evenly distributed throughout the rock (up to 2%). Breccia fragments (up to 15 cm in diameter) are distinguished from the darker chloritic matrix by their beige to light green colour.

Blondeau Formation 2 (nAbl2): Andesite, Amygdaloidal Basalt and Volcaniclastic Rocks

Unit nAbl2 has been identified in the la Trêve Lake area. Here, the rocks consist mainly of volcaniclastic rocks of andesitic composition and calc-alkaline magmatic affinity (Daoudene and Beaudette, 2021). They are variably deformed due to the proximity of the shear zone that marks the contact between the volcanic rocks of the Roy Group and the sedimentary rocks of the Opémiska Group. Despite the deformation, primary structures are preserved. Quartz and feldspar crystals as well as lithic fragments showing a laminar flow banding structure are common in the rhyolitic rocks (outcrop 2021-YD-2082). Fragments range from 1 cm to 20 cm in length and 1 mm to 5 mm in thickness. The proportion of fragments can locally represent up to 10% of the volume of the rock. The proportion of crystals varies across the outcrop, which may reflect the presence of a primary bedding (outcrops 2021-MB-3095 and 2021-MB-3055). 

Polygenic lapilli tuff is present at the edge of the la Trêve Lake. The rock contains two main types of fragments of presumed volcanic origin. The first is aphanitic and whitish grey; the second contains ~30% plagioclase phenocrysts surrounded by chlorite. In total, the lapilli represent 25-30% of the volume of the rock and range in size from 1 to 5 cm in length and 2 mm to 2 cm in thickness. Fragments of massive sulphides (mainly pyrite) are also present, but their proportion does not represent more than 1% of the volume of the rock. The matrix around the lapilli is rich in chlorite and contains 2 to 3% of disseminated millimetre-sized pyrite. Drill cores (80-DAN-D-3, D-4, D-5 and F-1) in this area also show the presence of fine, banded and locally cherty tuff containing 5-10% pyrite and millimetre-sized pyrrhotite grains (Beauregard et al., 1980; Cornet and Girard, 1981).

In thin section, the volcaniclastic rocks show a microlithic texture with ~40% plagioclase crystals. The crystals are tabular and generally fresh, although locally some are sericitized. They are surrounded by a very fine-grained matrix composed mainly of plagioclase (40-50%) and quartz (5-10%). Trace minerals present are white mica, greenish biotite, epidote, chlorite, sphene and apatite (outcrops 2021-YD-2118 and 2021-YD-2119). Locally, porphyritic millimetre-sized fragments are observed and represent ~30% of the rock (outcrop 2021-YD-2116).

Drill core descriptions (80-DAN-D-3 and 80-DAN-D-5) from the la Trêve Lake area also indicate the presence of andesite and amygdaloidal basalt. The amygdules are filled with calcite and the rocks have carbonate veinlets (Beauregard et al., 1980; Cornet and Girard, 1981).

Geophysical Characteristics

In the la Trêve Lake area, the rock level representing the Blondeau Formation is characterized on the aeromagnetic map by low magnetic susceptibility and the absence of textural variations. Electromagnetic anomalies appear to be absent.

Thickness and Distribution

The Blondeau Formation mainly outcrops along the Chibougamau Syncline axis, where its thickness is estimated to be approximately 1000 m (Duquette, 1982; Daigneault and Allard, 1990). The Blondeau Formation also appears on the western edge of the France Pluton in the Waconichi Tectonic Zone (sheet 32I04-200-0101). It is found on the sides of the Waconichi Syncline, on both sides of the Chebistuan Formation’s sedimentary rock unit. Within this synclinal structure, rocks of the Blondeau Formation also outcrop along second-order E-W anticlinal structures. To the SE of Chibougamau Lake (Dollier Lake area, sheets 32G09-200-0202 and 32G16-200-0102), in the Chapais Syncline, rocks of the Blondeau Formation outcrop over a thickness of ~500 m (Daigneault, 1986).

Dating

At Marianne Lake (sheet 32G16-200-0202), peperitic textures indicate that sedimentary rocks of the Blondeau Formation were not yet consolidated during intrusion of the Roberge Sill, which is the basal unit of the Cummings Intrusive Suite (Lefebvre, 1991). The emplacement of these two units can therefore be considered synchronous. This field relationship is confirmed by the maximum sedimentation age of an arkosic arenite of the Blondeau Formation (<2721 ±3 Ma; Leclerc et al., 2012), similar to the crystallization age of a quartz ferrodiorite of the Bourbeau Sill (2716.7 +1.0/-0.4 Ma; Mortensen, 1993). 

Stratigraphic Relationship(s)

The basal contact of the Blondeau Formation with massive, pillowed and brecciated mafic flows of the Bruneau Formation is characterized by the appearance of variolitic basalts. Elsewhere, contact between the Bruneau and Blondeau formations is generally hidden by ultramafic to mafic intrusions of the Cummings Intrusive Suite.

In the Chapais Syncline (sheets 32G09-200-0202, 32G15-200-0101 and 32G16-200-0102), the upper contact of the Blondeau Formation with sedimentary rocks of the Haüy Formation (Opémisca Group) is represented by an erosional discordance. In the Dollier Lake area (sheet 32G09-200-0202), the discordance between the Blondeau and Haüy formations is obliterated by the Kapunapotagen Shear Zone (Daigneault, 1986). East of Waconichi Lake (sheet 32J01), the small-lapilli grey tuff of the Blondeau Formation is interstratified with a grey-green arenite of the Bordeleau Formation (Caty, 1978; Moisan, 1992).

West of Chapais (sheet 32G14), rocks of the Blondeau Formation form three strips: the South Kapunapotagen Strip, the Sunset Lake Strip and the Chaleur Lake Strip. The South Kapunapotagen Strip forms a lenticular unit limited by the Kapunapotagen Shear Zone, to the north, and the Lac des Misérables Shear Zone, to the south. It is cut to the west by the Lamarck Shear Zone.

The Sunset Lake Strip is also restricted to the west by the Lamarck Shear Zone and extends eastward to the north of Chapais. These rocks are located on the south side of the Chibougamau Anticline and have a polarity to the south. To the east of Deux Orignaux Lake (western part of sheet 32G14-200-0202), the basal contact with rocks of the Bruneau Formation is hidden by the intrusion of kilometre-thick gabbroic sills. West of Armada Lake (NE part of sheet 32G14-200-0102), the mafic to intermediate lapilli and block tuff at the base of the Blondeau Formation (nAbl) is in sharp contact with mafic volcanic rocks at the top of the Bruneau Formation (nAbnu1). The area west of Deux Orignaux Lake (sheet 32G14-200-0201) does not outcrop and the rocks exposed on the islands do not allow direct observation of the base of the Blondeau Formation. Since they directly overlie on rocks of the Deux Orignaux Member (Waconichi Formation), the most plausible hypothesis seems to imply a structural contact between these two units. The Blondeau Formation’s sommital contact with the base of the overlying Scorpion Formation is sharp and characterized by an E-W deformation zone dominated by flattening.

The Chaleur Lake Strip, <500 m wide, extends from the south of the lake of the same name to the east of the Chibougamau River (NE of sheet 32G14-200-0202). In this area, rocks of the Blondeau Formation are in sharp contact with underlying units of the Bruneau Formation. The sommital part is cut by an E-W shear zone that obliterates the discordance between volcaniclastic rocks of the Blondeau Formation (Roy Group) and sedimentary rocks of the Stella Formation (Opémisca Group; Durocher, 1979).

The la Trêve Lake band is between 500 m and 2 km wide. It extends over ~12 km through Geneviève Bay of the la Trêve Lake and through Gisèle Lake. The rocks are in contact, to the north, with volcanic rocks of the Bruneau Formation. The nature of the contact has not been observed in the field. To the south, the rocks of the la Trêve Lake band are in faulted contact with sedimentary rocks of the Opémiska Group (Daoudene and Beaudette, 2021).

Paleontology

Does not apply.

References

Publications available through SIGÉOM Examine

Other publications

Suggested Citation

Contributors