Background

Description

Chibougamau Pluton 1 (nAchi1): Hornblende Diorite and Quartz Diorite with Lesser Amounts of Biotite-Hornblende Tonalite and Trondhjemite, Tonalite and Porphyritic Diorite Dykes

Chibougamau Pluton 2 (nAchi2): Tonalite with Lesser Amounts of Hornblende Diorite and Quartz Diorite, Tonalite and Porphyritic Diorite Dykes

According to Leclerc et al. (2017, page 28): “The central part of the Chibougamau Pluton is dominated by an even-grained to porphyritic biotite tonalite unit. This heterogeneous unit contains subangular to rounded, generally decimetric to metric, enclaves of hornblende diorite and quartz diorite. […] This unit hosts the Queylus Breccia.”

Chibougamau Pluton 3 (nAchi3): Trondhjemite and Porphyritic Trondhjemite (Leucotonalite)

According to Leclerc et al. (2017, page 28): “The biotite trondhjemite and porphyritic trondhjemite unit, also known as leucotonalite, outcrops south of the Merrill Mine in the Devlin and Corner Bay deposits’ area (SW of NTS sheet 32G16), and in the Lamothe area (east of sheet 32G16). It is characterized by a small amount of mafic minerals (<5%), an abundance of quartz and a porphyroclastic texture.”

Chibougamau Pluton 4 (nAchi4): Intrusion Breccia Consisting of Tonalite with Diorite, Basalt and Quartz Gabbro Enclaves

According to Leclerc and Houle (2013, page 9): “The southwestern edge of the Chibougamau Pluton is characterized by an intrusion breccia zone marking contact between volcanic rocks of the Andy Member (Waconichi Formation) and unit nAchi1. This breccia has a matrix of foliated heterogeneous tonalite. It contains fine-grained diorite enclaves, highly rounded, fractured and cut by veinlets of tonalite, aphyric basalt and quartz gabbro with plagioclase crystals (<5 mm).”

Chibougamau Pluton 5 (nAchi5): Gabbro and Diorite Dyke (Mafic and Intermediate Dyke)

Gabbro and diorite dykes (nAchi5) correspond to diorite and meladiorite dykes of Maillet (1978). They are particularly abundant in the NW portion of Caché Lake, where they cut intrusive rocks of the Lac Doré Intrusive Suite (Houle, 1990; Leclerc et al., 2017).

Chibougamau Pluton 6 (nAchi6): Quartz Diorite Dyke, Porphyritic Tonalite (Grey Dyke)

Quartz diorite dykes (unit nAchi6) correspond to the very fine-grained grey dykes of Maillet (1978) “generally consisting of the chilled margin of porphyritic dykes and tonalite dykes” (Houle, 1990; Leclerc et al., 2017).

Chibougamau Pluton 7 (nAchi7): Sericitized Tonalite Dyke (Acidic Dyke)

“Acidic dykes” include porphyritic felsic dykes (quartz ± plagioclase ± hornblende phenocrystals) and tonalite and porphyric tonalite dykes. They are generally associated with NW-SE shear zones; they are completely sericitized and contain disseminated pyrite (Houle, 1990; Leclerc et al., 2017).

Thickness and Distribution

The Chibougamau Pluton covers an area of 450 km² and follows the Chibougamau Anticline’s axial trace, whose E-W orientation shifts to NE-SW as it approaches the Grenville Front Tectonic Zone (Daigneault and Allard, 1990, 1994).

Dating

Samples of hornblende meladiorite (nAchi1) returned a crystallization ages of 2718 ±2 Ma (Krogh, 1982) and 2716 +3/-2 Ma (Pilote et al., 1997; Joanisse, 1998). A sample of porphyritic quartz tonalite dyke from Clark Lake (nAchi7) yielded a crystallization age of 2715 ±1 Ma (Pilote et al., 1996). At the Merrill Mine pit, a sample of diorite dyke (nAchi5) cut by mineralization and a sample of porphyritic tonalite dyke (nAchi7) cutting mineralization (nAchi7) yielded an identical crystallization age of 2714 +3/-2 Ma (Pilote et al., 1997; Joanisse, 1998). A sample from the tonalitic stock at the Merrill Mine pit also yielded an age of 2714 +3/-2 Ma (Pilote et al., 1997; Joanisse, 1998). At Copper Rand, a sample of quartz diorite dyke (nAchi6) yielded a crystallization age of 2705.1 +1.7/-1.2 Ma (David et al., 2011; reanalysis by Joanisse et al., 1998). A sample of porphyritic leucotonalite in the Queylus Bay area was dated 2701.7 ±2.9 Ma (unpublished data, McNicoll et al., 2008).

Stratigraphic Relationship(s)

The Chibougamau Pluton cuts volcano-sedimentary rocks at the base of the Roy Group (Obatogamau and Waconichi formations), as well as mafic to ultramafic rocks of the Lac Doré Intrusive Suite. The contact of the pluton’s southern part is characterized by a discordance above which lie sedimentary rocks of the Opemisca Group (Stella and Haüy formations). The eastern part of the Chibougamau Pluton is cut, on the one hand, by granodiorite intruding into the Mistassini Fault (oriented NNE-SSW) and, on the other, locally by biotite tonalites characterizing the Parautochton.

Paleontology

Does not apply.

References