The Rocher Intrusion first designated an informal ultramafic to mafic intrusive unit described by Bandyayera and Sharma (2001), following the discovery of the Rocher Lake Ni-Cu mineralization by Nuinsco Resources in 1997. Leclerc and Caron-Côté (2017) formalized this lithodeme and included it into the Rocher-Quénonisca Intrusive Suite, which also includes the Whitefish Intrusion dominated by purple gabbro and monzonite.

The Rocher Intrusion is an ovoid-shaped body about 1 km long (E-W axis) for 600 m wide. It is divided into four units: purple gabbro (nAlrc1), pyroxenite (nAlrc2), peridotite (nAlrc3) and plagioclase peridotite (nAlrc3a).

Descriptions come mainly from megascopic and microscopic observations made on Nuinsco Resources drill cores. These drills were designed to evaluate the Rocher Lake Ni-Cu±PGE deposit. Mafic and ultramafic intrusive rocks are little or not deformed and primary structures are preserved. Pseudomorphized primary minerals are usually identifiable. All ferromagnesian minerals are hydrated and replaced by amphibole (tremolite-actinolite), chlorite and epidote. Olivine is mainly replaced by serpentine, talc and magnetite. Plagioclase is partially altered to sericite ± epidote.

The gabbroic unit occupies the peripheral zone of the intrusion. It is medium to coarse-grained and mesocratic with a subophitic structure. Gabbro is composed of clinopyroxene replaced by tremolite-actinolite, highly sericitized and epidotized purple-colored plagioclase and 2-3% titanite. Amphibole appears in aggregates of pale green crystals, usually elongated, rarely tabular. This mineral is partially or completely replaced by the assemblage chlorite ± epidote ± sericite ± muscovite. Gabbro also includes nearly 4% interstitial primary biotite. The unit is generally not mineralized except for the bottom part, where some disseminated sulphides (up to 10% of sulphides) are observed.

Peridotite (nAlrc3) occupies the eastern part of the intrusion. It is a lherzolite composed of olivine, orthopyroxene, clinopyroxene, plagioclase, traces of biotite, phlogopite and interstitial sulphides. Secondary minerals include amphibole, chlorite, serpentine and magnetite. Olivine is a cumulus phase and sometimes includes brown spinel grains. Orthopyroxene, clinopyroxene, plagioclase and phlogopite fill interstitial spaces (intercumulus phases).

Plagioclase peridotite (lherzolite) (nAlrc3a) occurs locally in the edge region south-east and north of the intrusion. It is composed of olivine, orthopyroxene, clinopyroxene, biotite, plagioclase, magnetite, chromite and traces of sulphides. Serpentinized olivine contains small grains of chromite and intercumulus plagioclase.