|Type area:||An outcrop on the edge of Road 167, on the north side of the Chibougamau Syncline, exposes variolitic basalts characterizing the Blondeau Formation base (northern half of NTS sheet 32G16, UTM NAD83, Zone 18: 557923E, 5537601N). Further south along the same road, other outcrops show volcaniclastic rocks (lapilli tuff) and sedimentary rocks (arkose, graphitic mudstone with pyrite laminae). Sedimentary figures (sorting, channels) indicating a southward polarity are consistent with the position on the north flank of the Chibougamau Syncline|
|Geological province:||Superior Province|
|Geological subdivision:||Abitibi Subprovince|
|Lithology:||Volcaniclastic and sedimentary rocks|
The Blondeau Formation was established by Duquette (1964) following geological mapping in the Roy Township’s NW quarter. The unit’s name originates from Blondeau Lake (sheet 32G16-200-0202).
The Blondeau Formation (Duquette, 1964) comprises intermediate to felsic volcaniclastic rocks, sedimentary rocks (arkose to mudstone), rhyodacite and variolitic basalt (Duquette, 1982; Lefebvre, 1991).
Mapping of the Blondeau Formation in the Geoff Lake area (sheet 32G16-200-0202) identified six facies with transitional contacts consisting of volcaniclastic and sedimentary rocks (Lefebvre, 1991). At this time, mapping details do not allow to specify the lateral extension of these six facies which, however, characterize the entire Blondeau Formation in association with variolitic basalt. These different lithologies are therefore grouped in unit nAbl.
Facies 1 includes rhyolitic breccia, massive sulphide lenses and felsic coarse debris flow. At Marianne Lake (sheet 32G16-200-0202), breccia and coarse lapilli tuffs contain bombs and blocks of rhyolite and pyrrhotite-pyrite ± chalcopyrite massive sulphides.
Facies 2 consists of finely bedded volcaniclastic rocks. This facies is characterized by decimatric to metric, light-grey to dark-grey sorted beds with parallel, oblique and cross-bedded laminations, as well as convoluted beds typical of the lower portion of a Bouma sequence (Ta-Tb-Tc-Td). The coarser parts of debris flows are composed of subangular to rounded fragments of medium-grey to beige intermediate to felsic volcanic rocks. Larger fragments commonly have a lighter rim that could indicate a high-temperature emplacement.
Facies 3 consists of wacke, arkose, volcanogenic conglomerate and coarse volcaniclastic rocks. In the Marianne Lake area and Road 167 section west of Cummings Lake, arkoses and more locally volcanogenic conglomerates form decimetric beds. Some of these beds are sorted with a finer-grained upper section of siltstone. In the Roy Township’s NW quarter and in the Blondeau Lake area (sheet 32G16-200-0202), Duquette (1964) noted the presence of thick lenses of unsorted volcanogenic conglomerates similar to conglomerates observed in the Cummings Lake Road 167 section. Stones and cobbles consist of porphyritic rhyolite, sandstone and siltstone in a grey sandy matrix.
Facies 4 groups turbidites consisting of alternating millimetric to decimetric beds of volcanogenic arkose, siltstone and mudstone. In the Cummings Lake section, these turbidites have oblique or parallel laminations, current wrinkles and convolute beds. Stratification is affected by synsedimentary folds and faults. Beige to white siltstone beds are dismembered and folded inside rusted black mudstone horizons that also show dysharmonic folds, suggesting an unstable depositional environment.
Facies 5 consists of pumiceous tuffs, interpreted as plinian fallouts, volcanogenic arkoses and smaller amounts of graphitic ash tuffs. This massive yellowish-white patina unit forms centimetric to decimetric beds with multiple normal to reverse sorting that indicate continuous, but pulsed eruptive activity (Lefebvre, 1991).
Facies 6 is a unit of siltstone, mudstone and ash tuff. This unit outcrops particularly well along Road 167 in the Cummings Lake section, as well as on type locality outcrops on the north side of the Chibougamau Syncline. It consists of centimetric to millimetric, dark-grey to black graphite-rich beds, and locally contains beds of euhedral, nodular or framboidal pyrite. This facies has also been recognized over an E-W distance of more than 2 km northwest of Dolomieu Lake (sheet 32G14-200-0102) as discontinuous lenses a few tens of metres thick located in contact with gabbroic sills (I3Ac).
The area between Cooke Mine and Laura Lake (sheet 32G15-200-0102) is characterized by massive flows of grey rhyolite and rhyodacite containing up to 5% plagioclase phenocrystals, interstratified with volcaniclastic rocks (Morin, 1994). These felsic rocks were not recognized by Lefebvre (1991) in the Chibougamau area.
Detailed study of the facies characterizing the Blondeau Formation was made possible through stratigraphic drilling carried out in 1979 by the Ministère in the Barlow Lake area northeast of Chapais (sheet 32G15-200-0202; Morin et al., 1996). The different facies recognized by Archer (1983) during analysis of drillings correspond to those defined by Lefebvre (1991) in the Marianne Lake area north of Chibougamau. These correlations are illustrated in the table below.
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 lava has been recognized at several locations including: (a) east of Marianne Lake (sheet 32G16-200-0202); (b) at the base of the reference section exposed at the edge 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 section, the long axis of pillows is 30 to 40 cm in diameter. Upper 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.
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 southeast 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 approximately 500 m (Daigneault, 1986).
At Marianne Lake (NTS 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). 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).
|Isotopic System||Mineral||Crystallization Age (Ma)||(+)||(-)||Reference(s)|
|U-Pb||Zircon||<2721||3||3||Leclerc et al., 2012|
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 masked 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 is represented by an erosional discordance. In the Dollier Lake area (sheet 32G09-200-0202), discordance between the Blondeau and Haüy formations is obliterated by the Kapunapotagen Shear Zone (Daigneault, 1986). East of Waconichi Lake (sheet 32J01), small-lapilli grey tuffs of the Blondeau Formation are interstratified with grey-green arenites of the Bordeleau Formation (Caty, 1978; Moisan, 1992).
West of Chapais, rocks of the Blondeau Formation form three strips (sheet 32G14): 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 sheet 32G14-200-0202), the basal contact with rocks of the Bruneau Formation is masked by intrusion of kilometre-thick gabbroic sills. West of Armada Lake (NE of sheet 32G14-200-0102), mafic to intermediate lapilli and block tuffs at the base of the Blondeau Formation (unit nAbl) are in sharp contact with mafic volcanic rocks at the top of the Bruneau Formation (unit nAbnu1). The area west of Deux Orignaux Lake (sheet 32G14-200-0201) does not outcrop and exposed outcrops on islands do not allow direct observation of the base of the Blondeau Formation. Since they directly overlie on rocks of the Waconichi Formation’s Deux Orignaux Member, 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, less than 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 (Opemisca Group; Durocher, 1979).
Does not apply.