Some ultramafic intrusions had already been mapped by Hocq in NTS sheet 33A02 (Hocq, 1985), but the Sorbier Mafic-Ultramafic Suite was not introduced until 2018, following a geological mapping campaign by the Ministère (Beauchamp, 2020).

The Sorbier Mafic-Ultramafic Suite consists of a series of intrusions whose composition varies from peridotite, pyroxenite, hornblendite to gabbro. On geophysical maps, these intrusions are generally associated with strong, generally positive, magnetic anomalies. They form elongated and lenticular bodies that cut migmatized sedimentary rocks of the Laguiche Complex. This string of mafic-ultramafic rocks, included in the Eastmain-Sorbier Shear Zone, Ayr Structural Domain and Mabille Structural Domain, forms small deformed, discontinuous and boudinaged entities.

Peridotite outcrops of the Sorbier Mafic-Ultramafic Suite can be easily identified in the field because of their significant positive relief. They form round hummocks a few metres high. Rocks are characterized by a rough and irregular orange-brown altered surface. In fresh exposure, they range from dark green to black. Peridotite is fine to medium grained, homogeneous, foliated, granoblastic and typically magnetic. It is composed of green to colourless amphiboles (actinolite-tremolite and anthophyllite) in thin section, olivine, orthopyroxene, serpentine, talc, chlorite and magnetite. Accessory minerals observed locally are phlogopite, spinel, calcite and epidote. An igneous primary cumulate texture, defined by the presence of olivine crystals (relics) and orthopyroxene porphyroblasts, is common within this unit. Olivine is replaced by serpentine and magnetite. Deformed poikilitic crystals are interpreted as relics of orthophyroxene oikocrystals. Pyroxene was replaced by tremolite during the retrogressive metamorphic event. Tremolite is syntectonic to post-tectonic and is oriented at a certain angle relative to the S_n foliation. The outcrops’ brownish surface is covered by fractures arranged in diamond patterns. These negative relief fractures contain serpentine, talc, tremolite and magnetite. Magnetite and serpentine veinlets are commonly observed within the peridotite facies.