Regional characterization of ultramafic to mafic intrusions in the La Grande Rivière and Eastmain domains, Superior Province, Quebec

Abstract

The Eeyou Istchee Baie-James region is dominantly underlain by Archean rocks belonging to the Superior Province, which is mainly composed of juxtaposed sequences of predominantly sedimentary, volcano-plutonic and plutonic rocks. The distribution of mafic to ultramafic intrusions and ultramafic volcanism reveals a general abundance at the regional scale, with the greatest volume in the La Grande Riviere and Eastmain domains of the La Grande Subprovince in the central-eastern part of the Superior Province flanked by the Opinaca Subprovince to the south, the Ashuanipi Subprovince to the east, and the Minto Subprovince to the north. Preliminary geochronological results in the Eeyou Istchee Baie-James region indicate that the ultramafic-mafic magmatism extended over a period of more than 200 Ma, from ca. 2.88 Ga to 2.63 Ga. Various types of ultramafic to mafic magmatism have been documented in the La Grande Riviere and Eastmain domains and their vicinity: 1) ultramafic lavas and ultramafic intrusions of komatiitic affinity; 2) dominantly ultramafic intrusions; 3) dominantly mafic intrusions; 4) ultramafic to mafic intrusions of alkaline affinity; and 5) lesser volumes of ultramafic lamprophyres. Several of these ultramafic to mafic intrusion types also contain Cr-(PGE), Ni-Cu-(PGE), and Fe-Ti-(V) mineralization. The most important examples of these types of mineralization include the Menarik Complex (Cr?(PGE)), the Lac des Montagnes intrusions Cr-(PGE), the Lac Gayot intrusions (Ni-Cu-(PGE)), the Nisk intrusion (Ni-Cu-(PGE)), and the baie Chapus Pyroxenite (Fe-Ti-(V)). As part of the Targeted Geoscience Initiative Phase 4 (TGI-4), the Geological Survey of Canada and the Ministere de lmp;gt;'Energie et des Ressources naturelles (MERN) have undertaken reconnaissance work to better characterize these types of intrusions in terms of their distribution, geochemical and mineralogical characteristics, and prospectivity for the above-mentioned mineralization types. Studies are ongoing for intrusions in the Lac Yasinski, Lac Pelletan, Lac Gayot, Lac des Montagnes, and Lac Fed areas, in addition to a more regional study underway in the area south of the La Grande 4 Reservoir (Lac Richardie). New geochronological work is also in progress, which will allow us to better understand the temporal distribution of the ultramafic to mafic magmatism in this portion of the Superior Province. A better understanding of these ultramafic and mafic intrusions will also reveal whether they demonstrate similar characteristics to ultramafic to mafic intrusions recently discovered in the McFaulds Lake area of Ontario (a.k.a. "Ring of Fire"), which contains major Cr-(PGE) and Ni-Cu-(PGE) deposits.