Some ultramafic intrusions were first mapped by Hocq (1985) in NTS sheet 33A02, but the Sorbier Mafic-Ultramafic Suite was not introduced until 2018, after the geological mapping campaign in the Lake Cadieux area (sheets 33A02 and 33A07; Beauchamp, 2020). Following the mapping campaign in the Lake Caulincourt area, Chartier and Saint-Laurent (2024) extended the Sorbier Mafic-Ultramafic Suite to the west (sheet 33A03).

The Sorbier Mafic-Ultramafic Suite is named after the eponymous lake located in the NW corner of sheet 33A02.

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

Peridotite and pyroxenite 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 and pyroxenite are fine to medium grained and typically magnetic. These rocks may locally exhibit pronounced foliation on some outcrops and be rather massive on others. They are 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.

Two typical facies were identified in thin section. The most common facies has an igneous cumulate texture, defined by the recurrent presence of olivine crystals (relics) and orthopyroxene porphyroblasts. Olivine is generally replaced by serpentine and magnetite. Orthopyroxene may also be altered and replaced by chlorite and sericite assemblages. In some cases, deformed poikilitic crystals are interpreted as relics of orthophyroxene oikocrystals. The matrix of these rocks is generally granoblastic and composed of clinopyroxene, which is commonly amphibolitized to actinolite-tremolite. The second, less common, facies has little to no cumulate texture. The mineralogical composition is generally more heterogeneous and the mineralogical proportions vary greatly in thin section. A matrix rich in amphibole is predominantly observed, often accompanied by biotite. Amphibole in this facies also exhibits a rather variable relief in thin sections, suggesting a variation in amphibole composition. The characteristics of these two facies have been repeatedly observed within the same outcrop or on neighbouring outcrops (<50 m).

In these rocks, pyroxene was replaced by an actinolite-tremolite assemblage is thought to have occurred during the retrogressive metamorphic event. Tremolite is a syntectonic to post-tectonic mineral and is oriented at a certain angle relative to the S_n foliation. The outcrops’ brownish surface is covered by negative relief fractures containing serpentine, talc, tremolite and magnetite. These fractures are arranged in diamond patterns. Magnetite and serpentine veinlets are also visible within the peridotite facies.