Background

Description

Anatacau-Pivert Formation 1 (nAnp1): Amphibolitized Basalt and Amphibolite

Unit nAnp2 is interstratified with the nAnp1 unit. It outcrops in the Kaministikuch Lake area (sheet 33C01) where it was described by Franconi (1978). The unit consists of amphibolite with 10% garnet porphyroblast 1 to 3 cm in diameter. Remains of pillows are still recognizable. Under the microscope, the rock contains 70-80% hornblende, 10% garnet, 10% plagioclase and 2% quartz. Accessory minerals are biotite and tourmaline. 

Anatacau-Pivert Formation 3 (nAnp3): Andesite

Unit nAnp3 consists of sparse horizons of andesitic lava, locally amygdaloidal. In the field, it is difficult to differentiate between basaltic lava (unit nAnp1) and andesitic lava (unit nAnp2) due to metamorphism and strong alteration of the rock. The composition of these volcanics was confirmed by lithogeochemical analyses. Andesite contains up to 15% magnetite and is associated with horizons of iron formation at the oxide facies (unit nAnp5). Under the microscope, andesite shows a trachytic texture associated with the preferential orientation of plagioclase microlites.

Anatacau-Pivert Formation 4 (nAnp4): Rhyolite and Rhyodacite

Unit nAnp4 consists of massive rhyolite and rhyodacite that outcrop on sheets 33C01 and 33C02. These rocks are silicified, chloritized and sericitized.

Anatacau-Pivert Formation 5 (nAnp5): Iron Formation

Unit nAnp5 consists mainly of iron formations at the oxide facies and, locally, at the silicate facies. Sulphidic chert horizons have also been integrated into this unit. Aeromagnetic maps highlight this unit with high magnetic susceptibility. Unit nAnp5 is interbedded in mafic volcanics of units Anp1 and Anp2. The thickness of these units varies from a few metres to about 30 metres. They display banding materialized by alternating magnetite beds, recrystallized brecciated chert ribbons or centimetric bands of metasediment.

Anatacau-Pivert Formation 6 (nAnp6): Felsic, Intermediate and Mafic Tuffs

Unit nAnp6 forms discontinuous horizons within the nAnp1 unit or in contact between the nAnp1 unit and the sedimentary sequence of the nAnp7 unit. It is composed of lapilli or block volcanoclastics (tuffs) of felsic to intermediate composition, locally mafic. Some horizons consist of only andesitic tuffs. Felsic tuffs contain about 60% felsic fragments and show strong alteration in chlorite and sericite. Some felsic horizons have quartz and plagioclase phenocrystals 0.3 to 0.7 mm in diameter. Lithic fragments are recrystallized and generally stretched and flattened parallel to schistosity. Intermediate tuffs contain horizons of trachytic texture andesite that are characterized by the presence of plagioclase phenocrystals. These rocks are granoblastic to lepidoblastic, locally banded.

Anatacau-Pivert Formation 7 (nAnp7):  Siltslate, Mudslate and Conglomerate Horizons

Unit nAnp7 was added to the Anatacau-Pivert Formation, and not to the Auclair Formation of the Nemiscau Subprovince, due to its close association with volcanic rocks, exceptional preservation of primary structures and low metamorphic facies (Moukhsil, 2000). It consists of siltslate, mudstone and conglomerate horizons. Siltslate and mudslate show an interlocked hemigranoblastic texture. Some of the outcrops in this unit show well-preserved S₀ stratification, characterized by alternating more or less fine beds with biotite, greenish chlorite, quartz and muscovite.

Microscopic analysis shows that the rock is composed of quartz (up to 55%), plagioclase (11%), biotite (23%) and muscovite (15%). Accessory minerals are hornblende, chlorite, apatite, zircon, sericite, tourmaline, carbonates and opaque minerals. In places, carbonates form areas 0.20 to 0.50 mm in diameter.

Thickness and distribution

The Anatacau-Pivert Formation is the largest volcano-sedimentary sequence in the central part of the LERVB. Its lateral extension exceeds 45 km in the NE-SW direction, while its width varies from a few hundred metres in the periphery to 8 km in the central part. 

Datation

Isotopic dating of a rhyolite sample (nAnp4) gave an age of 2723 Ma (Moukhsil et al., 2001). A plagioclase porphyritic diorite dyke that cuts the basalts (nAnp1) at the Réservoir Grid C-52 mineralized zone has been dated to 2713 Ma (Moukhsil et al., 2001), providing a minimum age for unit nAnp1. Finally, the dating of a felsic tuff level inside a sequence of amphibolitized basalts (nAnp1) yielded an age of 2716 Ma. These ages allow placing the Anatacau-Pivert Formation in the third volcanic cycle of the LERVB (2720-2705 Ma; Moukhsil et al., 2003).

Stratigraphic Relationship(s)

The Anatacau-Pivert Formation is one of the five volcano-sedimentary units of the LERVB (Kauputauch, Natel, Anatacau-Pivert, Kasak and Komo formations) that Mukhsil et al. (2002) have combined into the Eastmain Group. It is located in the central part of the Eastmain Group, between the Natel Formation at the east and Komo Formation at the west.  

Paleontology

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References

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