Background

Description 

Mistassini Dyke Swarm (nAmib): Diabase

Mistassini Dyke Swarm 1 (nAmib1): Aphyric Diabase

Unit nAmib1 is a package of near-vertical, NNW-SSE to N-S oriented mafic intrusions of gabbronoritic composition. These dykes are traced from outcrops, aerial photos, satellite images, magnetic field vertical gradient map, petrography and geochemistry. They are distinguished from other mafic intrusions of the same type by their orientation and petrographic, paleomagnetic and geochemical characteristics. Mistassini dykes are characterized by a high magnetic susceptibility highlighted by positive linear aeromagnetic anomalies (NW-SE) that can be tracked over kilometric distances (interactive map). Near Mistassini Lake, these dykes are uncommon, but farther north they form large ridges.

The dykes vary in thickness from 0.5 to 100 m and have a chill margin a few centimetres thick in direct contact with the host rocks. The colour is gray to dark green in fresh exposure and grey to brown in altered surface. Locally, the rock is highly hematitized and has a reddish stain (Goutier et al., 2001, Photo 9). The diabase is massive, homogeneous, magnetic and weakly deformed. Fracture networks are observed with local filling of quartz, carbonate or barite-hematite. The center of the dykes is typically medium-grained, with an equignanular to heterogranular structure, locally subophitic to ophitic (nAmib1). These dykes are mainly composed of plagioclase prisms partially to strongly altered in very fine-grained white mica (damourite), weakly or partially uralitized augite and orthopyroxene pseudomorphs replaced by an assemblage of chlorite, acicular amphibole and talc. Biotite and ilmenite-magnetite are also observed. In thin section, orthoclase and quartz (microgranophyric texture) associated with rutile and thin rods of apatite form interstitial areas with large crystals of plagioclase and pyroxene. Epidote and carbonates are also present in accessory quantities. Orthoclase also forms rims around plagioclase prisms (Fahrig et al., 1986; Gigon and Goutier, 2017). Particular geochemistry, presence of biotite, K-feldspar, quartz and abundance of apatite are distinctive characteristics of the Mistassini Dyke Swarm. Excluding K-feldspar, these dykes are gabbronoritic in composition. Mistassini dykes are characterized by TiO₂+K₂O+P₂O₅ contents generally greater than 5 wt.% and average ratios Fe₂O₃v/Feo = 0.4, Zr/Y>8 and La/Yb>4.5.

Mistassini Dyake Swarm 1a (nAmib1a): Porphyritic Diabase with Plagioclase Phenocrystals

The diabase dykes of subunit nAmib1a are distinguished from unit nAmib1 by their porphyritic texture with plagioclase phenocrystals. In the Mistassini Lake area, these phenocrystals are abundant (up to 15% in the centre of a dyke), 1-19 cm long and have a trachytic texture. On contact and at the edges of the dykes, centimetric crystals are usually absent or few in number. Further north, phenocrystals are less abundant and smaller, such as in the Robert-Bourassa Reservoir area (e.g., 1999-JG-1190, 7%, 1-8 cm) or Pelletan Lake (e.g., 2012-JG-1008, <1%, 0.5-3 cm). 

Mistassini Dyke Swarm 2 (nAmib2): Magnesium-Rich Diabase

The nAmib2 diabase dykes are most abundant in the Mistassini Lake area, where they were recognized by Chown and Caty (1973). They referred to them as unit 12c (uralitic gabbro). According to Fahrig et al. (1986), the most magnetic dykes are strongly altered and primary minerals are not preserved; it would be peridotite and pyroxenite. Olivine is completely replaced by serpentine and magnetite, while pyroxenes are uralitized. Phlogopite was observed in a metapyroxenite. Mg-rich dykes (7-17% MgO) would be olivine gabbronorites. The coarser dykes have a similar appearance to that of unit nAmib1, with a high proportion of mafic minerals. The latter are composed of plagioclase prisms, pyroxene and olivine (5-10%) forming a subophitic to ophitic texture. Plagioclase is saussuritized, pyroxenes are partly uralitized, and olivine is replaced by an assemblage of serpentine, talc and amphibole. Apatite and magnetite account for almost 5% of the rock. Magnetite grains are surrounded by fine-grained biotite and leucoxene. The immobile trace element ratios (Zr/Y, La/Yb) of the magnetic dykes are comparable to those of units nAmib1 and nAmib1a, supporting their assignment to this swarm.

The NW-SE to N-S dykes observed in the Ashuanipi Subprovince may represent smaller distal intrusions related to the magmatic event responsible for formation the Mistassini Dyke Swarm. Ashuanipi dykes are located more than 100 km east of the most easterly dykes of the Mistassini Dyke Swarm. Their assignment to the Mistassini Dyke Swarm is therefore uncertain and based solely on their orientation. This system is poorly developed in Ashuanipi, which may explain the limited number of dykes with this orientation (Simard et al., 2015). Geochemical analyses would be required to confirm their belongingness in this swarm.

Thickness and Distribution

The Mistassini Dyke Swarm consists of many NW-SE to N-S oriented dykes, which are almost vertical and extend from Mistassini Lake for a few kilometres to more than 375 km. They are distributed over an area of approximately 300,000 km₂, forming a range that converges to a point 150 km southeast of Mistassini Lake. Dykes ranging in thickness from 1 m to 133 m are locally nearly continuous for up to 375 km (interactive map). Dykes in unit nAmib2 are narrower (<10 m) and have a smaller extension (<15 km near Mistassini Lake and about 80 km in sheets 23E03 to 23D06). They are present on maps of the Wabisinane River area (unit 12c of Chown and Caty, 1973).

Dating

K-Ar dating has estimated a Mesoproterozoic to Paleoproterozoic age for the Mistassini Dyke Swarm (Wanless et al., 1968, 1972). An new age of 2.47 Ga (TIMS, U-Pb on baddeleyite) was proposed by Heaman (1994) following analysis of Fahrig et al. (1986) FA65-55 sample. The summary of Heaman (2004) suggests that an age of 2.51 Ga was also obtained for this same sample. The FA65-56 diabase sample (UTM NAD83 zone 18: 600299 mE, 5711169 mN) from the Fahrig et al. (1986) collection determined the crystallization age of the Mistassini Dyke Swarm at 2515 ±3 Ma (TIMS, U-Pb on delebadyite; Hamilton, 2009). A sample of a dyke further north (outcrop 2012-JG-1008) yielded an age of 2503 ±2 Ma (TIMS, U-Pb on baddeleyite; Davis et al., 2018), indicating that these dykes were intruded from 2515 Ma to 2503 Ma.

Stratigraphic Relationship(s)

The Mistassini Dyke Swarm cuts Archean rocks of the Superior Province. However, these dykes do not pass through the overlying sedimentary rocks of the Mistassini Group and Sakami Formation. The age of the Mistassini Dyke Swarm is therefore the maximum sedimentation age of the Mistassini Group and the Sakami Formation.

Paleontology

Does not apply.

References