The Grand-Calumet Complex was introduced by Bilodeau (2022) to group all the metavolcanic rocks and other associated lithologies in the Île-du-Grand-Calumet area. The unit has been defined and mapped on several occasions over more than a century, since it hosts a number of mineralized zones, including the former New Calumet Mine. The first comprehensive and adequate description of the unit comes from the work of Osborne (1944), who described alternating facies of mafic gneiss cut by a mafic rock. Subsequent work has strengthened the hypothesis proposed by Bishop (1987) and Gauthier (2015) suggesting that the unit is of volcanic origin.

Grand-Calumet Complex 1 (mPgrc1): Amphibolite derived from hornblende-biotite-magnetite basalt

Unit mPgrc1 consists of different facies of hornblende ± biotite ± magnetite ± garnet amphibolite. The unit is typically characterized by a rock with alternating bands of amphibolite containing biotite, garnet and small hornblende phenocrysts, and bands of magnetic amphibolite containing hornblende but not biotite. A pyroclastic facies has also been observed in the area of the New Calumet mineralized zone (Gauthier, 2015). The rocks are generally highly recrystallized with an increase in crystal size, giving them the appearance of very fine-grained gabbro in places.

The amphibolite’s alteration patina is variable in colour, generally light brownish-grey to brownish-grey to very dark greenish-brown and locally slightly light grey or rusty. The fresh surface is grey to very dark grey, and locally brownish-grey. The unit is generally banded, with a fine to medium grain size and is fairly homogeneous overall. Bands vary in grain size and composition over centimetric thicknesses. The rock is typically composed of 20% to 40% fine-grained euhedral dark olive-green hornblende and 40% to 45% recrystallized and locally antiperthitic plagioclase. Slight late damouritization is locally visible along the cleavages. Up to 20% fine epidote is present in clusters, indicating saussuritization of the plagioclase, which varies from slight to complete depending on the sample. The distribution of hornblende and plagioclase is generally homogeneous in each band.

Bands contain up to 15% orthopyroxene and 10% clinopyroxene, fine-grained or in medium-grained relicts. Bands also contain a highly variable proportion (5-25%) of fairly fine biotite, homogeneously distributed and generally associated with hornblende. The biotite varies in size in some samples. The amphibolite is considered slightly or not uralitized. However, some bands contain phenocrysts a few millimetres in size of poikilitic hornblende, possibly replacing pyroxene, which contains numerous inclusions of plagioclase and clinopyroxene. Foliation is marked by hornblende and biotite. The proportion of sulphides and magnetite varies from 0% to 8%, reflecting differences in the intensity of magnetic susceptibility in outcrop, a property estimated to be regionally lower than that of the surrounding units. Up to 3% quartz in isolated coarse crystals and <1% garnet, tremolite, carbonate chlorite, and epidote occur locally.

Unit mPgrc2 is a heterogeneous mafic intrusive subunit consisting of two main facies: 1) fine-grained gabbro, and 2) gabbronorite.

Fine-grained gabbro facies

Gabbro is typically very fine grained, massive, homogeneous and recrystallized. The rocks are locally heterogeneous, heterogranular and foliated. This heterogeneous variety is characterized by a slight compositional or granulometric banding interpreted as being of magmatic origin.

This gabbro has a grey to dark grey, locally black, patina and a dark to very dark grey altered surface. The rock is composed of 50% to 60% fine to medium plagioclase grains. Some samples are slightly damouritized. The microgabbro contains 35% to 45% fine hornblende very homogeneously distributed and defining a slight foliation. The presence of hornblende clusters indicates an almost complete replacement of clinopyroxene due to uralitization. Clinopyroxene relicts represent ~5% of the volume of the rock. Gabbro contains 3% to 5% biotite without preferential orientation, a phase considered to be secondary and occurring later. The rock contains up to 1% pyrite and very little magnetite, as evidenced by its low magnetic susceptibility. Small enclaves, only a few centimetres in size, of fine-grained mafic composition have been observed locally.

Gabbronorite facies

Gabbronorite is mesocratic, homogeneous, massive and locally granoblastic. It has a brown to dark grey alteration patina and a brownish-grey or very dark grey fresh surface. It is composed of 60% fine to medium unaltered plagioclase grains and 15% to 25% fine euhedral hornblende. The rock contains approximately 12% to 15% fine unaltered orthopyroxene, which is fairly homogeneously distributed and appears to have crystallized later than hornblende, and 3% clinopyroxene heterogeneously distributed. Pyroxene crowns are locally replaced by hornblende, and replacement is complete in some samples (up to 15% of the volume of the rock). The rock contains up to 5% subhedral to anhedral garnet of variable grain size and heterogeneous distribution. Accessory minerals include traces of quartz and up to 5% opaque minerals, including 1% sulphides. Unlike other lithologies in the Grand-Calumet Complex, the gabbronorite does not contain biotite. Magnetic susceptibility varies from very low to high.

Thickness and Distribution

The rocks of the Grand-Calumet Complex occupy almost the entire southern half of the Île-du-Grand-Calumet municipality. They are the only known example of this type of rock in the area. Similar metavolcanic rock units have been identified in other parts of the Central Metasedimentary Belt (CMB), in Ontario (Rivers, 1989). The thickness of the unit is unknown.

Dating

A sample of fragmental metavolcanic rock from the Grand-Calumet Complex was dated by J. David (in Gauthier, 2015). The zircons gave a U-Pb crystallization age of 1232.8 +3.9/-2.7 Ma.