The Tancredia Intrusive Suite was introduced by Bilodeau (2022) during geological mapping of the Île-du-Grand-Calumet area in 2021. The rocks in this unit were first described by Osborne (1944) as dykes and sills of fine-grained pink granite. The term « pink granite » was dropped in subsequent reports. Madore et al. (1994) then grouped these rocks together in a lithological unit of charnockitic gneiss. Finally, the individualization of the « Tancredia intrusion » associated with the « Obwondiag Complex » (now the Grand-Calumet Complex, mPgrc2) is mentioned in Adair’s (2018) prospecting report.

Tancredia Intrusive Suite 1 (mPtcd1): Quartz syenite to alkali feldspar granite, monzonite

Depending on the composition observed, the rocks in this unit varies in colour from pinkish-grey to greenish-grey in fresh surface and from pinkish-grey to brownish in altered patina. They are composed of 10% to 30% quartz, a variable amount of plagioclase and up to 55% K-feldspar. Secondary minerals include 2% to 10% biotite, 1% to 5% hornblende, 5% to 7% opaque minerals dominated by magnetite, 1% to 2% clinopyroxene in some samples and possibly a few orthopyroxene crystals replaced by hornblende and biotite. Accessory minerals include apatite, titanite, garnet and muscovite. The rock is characterized by a heterogeneous distribution of ferromagnesian minerals. Overall, the unit has a very strong magnetic susceptibility.

The rocks are generally highly foliated, to the extent that some outcrops display a well-developed gneissosity. Foliation is determined by the alignment of fine biotite crystals, giving the rock a granolepidoblastic texture (Madore et al., 1994). Numerous granite injections concordant with foliation are present. Madore et al. (1994) hypothesized that the injections represent mobilisates that have acquired an orientation parallel to the foliation.

The mPtcd2 unit is composed of granitic to granodioritic rocks, with a light to dark grey fresh surface and a light greyish-beige alteration patina. The minerals are generally fine grained, with the exception of quartz which is also present in medium-sized crystals and small clusters a few millimetres in size. The rock is composed of 20% to 45% quartz, up to 50% slightly damouritized plagioclase and a generally low content of ferromagnesian minerals. The only ferromagnesian phase present is a fine biotite defining the foliation, totalling 3% to 8% of the rock. Some samples are hololeucocratic and contain no biotite. The rock contains 1% to 3% homogeneously distributed magnetite, up to 1% fine muscovite present in all samples, and accessory garnet and apatite. This unit has a high magnetic susceptibility.

The rocks of unit mPtcd2 are generally foliated and highly recrystallized, displaying a well-developed granoblastic texcture. Quartz is particularly highly deformed, as seen under the microscope with undulatory extinction, chessboard patterns and triple junctions between crystals. The unit includes concordant granite injections whose foliation contains bronze-brown orthopyroxene phenocrysts with a greenish glint (Madore et al., 1994), as well as discontinuous or boudinaged quartz veins. Enclaves of magnetic hornblende-biotite amphibolite have been observed locally in some decimetric to metric layers.

Thickness and Distribution

Unit mPtcd1 consists of two folded sills, each no more than 150 m thick. They outcrop in the southern part of the Grand-Calumet Complex, near the bridge connecting the municipality of Bryson. Unit mPtcd2 comprises several intrusions—probably sills—which occur exclusively within the Grand-Calumet Complex. The largest intrusion, located in the central-northern part of the unit, outcrops over a width of just over 2 km and has an apparent thickness of ~300 m. Several smaller intrusions are located near fault contacts of the Grand Calumet Complex with the Litchfield Intrusive Suite in the Campbell’s Bay municipality area, or with the Outaouais Supracrustal Sequence in the New Calumet mineralized zone area.

Dating

Isotopic dating of a sample taken from outcrop 2021-CB-1063 revealed a Mesoproterozoic crystallization age.