The Rivière Noire Intrusion is an alkaline ultramafic intrusion consisting of clinopyroxenite and of syenite, alkali feldspar syenite and quartz syenite veins, veinlets and dykes. These syenitic facies are associated with clinopyroxenite and cannot be mapped at the scale of the mapping (1:50,000 scale) in the Dolbeau-Blondelas project (sheet 32H01). This intrusion is more or less easy to delineate on aeromagnetic maps (Intissar and Benahmed, 2015) and hosts a rare earth element mineralized zone discovered during the work of El Bourki and Moukhsil (2021; Aligas mineralized zone).

The main rock constituting this intrusion is a clinopyroxenite. It has a greenish alteration patina and a greenish-black to pinkish fresh surface (outcrop 2021-GS-2094). It is medium to coarse grained and locally porphyroid. It has a cumulate structure and it is slightly foliated. It consists of clinopyroxene (augite, >72%), titanite (4-8%), apatite (2-4%), potassium feldspar (10-15%), calcite (<1%), olivine (<1%) and traces of orthopyroxene, quartz, magnetite and zircon. The clinopyroxenite is cut by veins, veinlets and dykes of syenite, alkali feldspar syenite and quartz syenite. The clinopyroxenite appears recrystallized and the associated syenitic phases appear foliated and deformed. Some evidence of syenitic and ultramafic magmas mixing is locally highlighted in the intrusion by diffuse contacts between the two magmas.

Clinopyroxenite contains titanite which can reach 2 cm in length and, under the microscope, shows a typical twinned crystal with a strong positive relief. This titanite is observed as crystals between other minerals as well as inclusions within clinopyroxene and potassium feldspar. The size of the apatite is <0.3 cm, while clinopyroxene rods exceed 1.5 cm. X-ray microdiffraction (µXRF) maps show the different chemical elements and the association of minerals forming clinopyroxenite.

Alkali feldspar syenite is pink-green in fresh and altered surfaces, medium to coarse grained and locally porphyroid with potassium feldspar phenocrysts (e.g., outcrop 2021-GS-2093). The syenite consists of greenish clinopyroxene, potassium feldspar, titanite, apatite and opaque minerals along with traces of orthopyroxene. Clinopyroxene (1-5%, up to 1 cm in length) is seen in isolated crystals and in decimetric clusters. Under the microscope, titanite (2-5% of the rock) is seen as isolated crystals here and there in the rock and as inclusions in clinopyroxene. The potassium feldspar is devoid of typical twinning and reaches 5 cm in length. It is generally enriched with several micrometric inclusions of acicular rutile and other unidentified minerals. The alkali feldspar syenite contains centimetric enclaves of clinopyroxenite.

Veins, veinlets and dykes of syenite and quartz syenite are pink in fresh surface and pink-green in alteration patina. The rock is medium grained and porphyroid or granoblastic. In thin sections, the main minerals are potassium feldspar in phenocrysts, locally perthitic and with Scottish kilt type twinning, clinopyroxene and quartz. Clinopyroxene crystals are grouped in irregular clusters throughout the rock. Accessory minerals include apatite, opaque minerals (pyrite and pyrrhotite) and titanite, the latter (<1% of the rock) occurring as isolated crystals and inclusions, mostly in the potassium feldspar.