A look back and a step forward: Fruitfull ends and new beginnings in the geological understanding of the Eeyou Istchee Baie-James area

In the Eeyou Istchee James Bay region, geological mapping campaigns conducted by the MRNF teams have been going on year after year. Over the past 30 years, these efforts have covered more than half of the region’s surface area, including the Eastmain and Yasinski groups, the Bienville Domain, the Ashuanipi Subprovince, the northeastern part of the La Grande Subprovince, the boundaries of the Opinaca Subprovince, and, most recently, the Lac des Montagnes belt.

All of this work, combined with the dedicated efforts of the mining exploration industry, has highlighted the tremendous economic potential of the greater Eeyou Istchee James Bay region. However, the task of mapping the entire region remains colossal, and the context continues to evolve. Markets are changing, new resources are gaining prominence, geological and exploration tools are becoming increasingly sophisticated, and the territory is opening up more and more to exploration activities.

Many areas remain to be explored, and there are undoubtedly numerous discoveries still waiting to be made. The question that remains is: where should we begin?

The Comeau Lake region, located in the heart of the Opatica Subprovince, is predominantly composed of the Théodat Complex. The results of this geological mapping project, conducted during the summer of 2025, have refined the boundaries between the various intrusive subunits of this intermediate to felsic composition complex. A sector associated with these intrusions in the western part of the map, exhibits favourable potential for porphyry-type Cu ± Au ± Mo ± Ag ± Zn mineralization.

The survey also led to the identification of several new amphibolite bands derived from basaltic rocks of the Michaux Group. These units, which locally reach several hundred metres in thickness, include a metre-scale banded iron formation horizon. This horizon contains significant copper (286 to 415 ppm Cu) and zinc (178 to 195 ppm Zn) concentrations.

These volcano-sedimentary sequences are closely associated with peridotite intrusions belonging to the Comeau Mafic-Ultramafic Suite. One of these intrusions returned anomalous chromium values (1.11% Cr₂O₃), leading to the definition of the new Hérisson mineralized zone.

The mapped area records a polyphase tectonic history characterized by a dominant NE-SW structural grain. Integration of geochronological data and field observations has led to the recognition of at least five deformation phases. The D1 deformation phase, at approximately 2,760 Ma, is responsible for the dome-and-basin architecture characteristic of the core of the Opatica Subprovince, whereas the subsequent phases (D2 to D5) accommodated NW-SE compression through various mechanisms, producing structural fabrics with distinct orientations.

The geological survey of the Holton Lake area covers NTS sheets 32P10, 32P15 and 32P16 and focuses essentially on mapping the Archean basement.

The northern part of the area consists of variably migmatized Mesoarchean plutono-gneissic masses of the Maingault Complex, on which volcano-sedimentary rocks of the Mistamiquechamic Formation were deposited. The whole is intruded by kilometric intrusions of the Chamic Mafic-Ultramafic Suite.

The west and southwest parts of the area represent the eastern end of the Lac des Montagnes Belt (LMB), which rests in structural contact on the Hutte Complex. The LMB consists of volcanic rocks of the Tichégami Group and the metasedimentary sequence of the Voirdye Formation. The whole is cut by white pegmatite intrusions of the Senay Suite and intrusions of the Nasacauso Mafic-Ultramafic Suite.

The southwest part of the area is covered by Paleoproterozoic rocks of the Papaskwasati Formation, which unconformably overlie the Hutte Complex and the Tichégami Group.

The central part of the area exposes the Mantouchiche plutono-gneissic complex.

The eastern part of the area consists of Paleoproterozoic rocks of the Otish Basin that unconformably overlie the Hippocampe greenstone belt, which exhibits well-preserved primary structures.

The area presents several metallogenic contexts:

- Cu-Zn-Au-Ag massive sulphide mineralization associated with volcanic rocks of the Tichégami and Hippocampe groups;

- synvolcanic Cu-Au-Ag quartz-sulphides veins associated with volcanic rocks of the Tichégami and Hippocampe groups;

- Ni-Cu (± Co ± PGE) magmatic mineralization associated with intrusive rocks of the Nasacauso and Chamic mafic-ultramafic suites;

- Li-Cs-Ta mineralization associated with granitic pegmatites of the Senay Suite.

The Lac des Montagnes Belt (CLM), located at the contact between the La Grande and Opatica subprovinces, is of particular interest for minerals exploration as it hosts two critical and strategic minerals deposits: Whabouchi (Li) and Nisk (Ni-Cu-PGE). In recent years, work conducted by the Ministry has significantly improved geoscientific knowledge of this region. However, the current stratigraphy brings together Mesoarchean and Neoarchean rocks within the same units. This study proposes a comprehensive stratigraphic revision of the Mesoarchean to early Neoarchean units in a portion of the CLM. The study suggests that the CLM comprises two distinct volcanic episodes and two distinct sedimentary episodes and highlights the mineral potential of the Mesoarchean rocks of the Théodat Complex. Interpretation of crystallization ages indicates that a portion of this vast complex (Athe1) is likely synchronous with the deposition of the Mesoarchean volcano-sedimentary rocks. These rocks are affected by diverse and substantial metasomatism. The nature, extent, and intensity of this metasomatism suggest the potential for discovering previously unrecognized mineralization types in the area—specifically porphyry-type or acid-epithermal-type mineralization within the TTG-type intrusive basement of the Opatica Subprovince. These interpretations open new avenues for mineral exploration, both along the margins of the CLM—at the contact between the Opatica and La Grande subprovinces—and elsewhere in the Eeyou Istchee James Bay territory, where portions of synvolcanic sialic basement are adjacent to Mesoarchean volcano-sedimentary sequences.

A mapping campaign was carried out in the summer of 2025 in the Turgeon River area (southern half of NTS sheet 32E14). This area is located in the northwestern part of the Abitibi Subprovince, near the Selbaie deposit (53 Mt at 1.0% Cu, 2.0% Zn, 0.6 g/t Au and 41 g/t Ag). The lithostratigraphic map for the area was updated based on work by CONSOREM and using a compilation of a large number of drill holes (571) and new data acquired during the summer 2025 field campaign.

The area consists of a weakly metamorphosed volcano-sedimentary sequence and a few synvolcanic to late-tectonic intrusions. Most of the study area is occupied by volcanic rocks of the Brouillan-Fénelon Group (2729 Ma), the disposition of which reveals an E-W-trending anticlinal structure that contains, in its core, synvolcanic intrusive rocks of the Brouillan Pluton (2729 Ma). To the south, volcanics of the Brouillan-Fénelon Group are in contact with the volcano-sedimentary sequence of the Enjalran-Bapst Group and metasedimentary rocks of the Taibi Group (<2697.2 Ma), which form the core of a synclinal basin.

With more than 30 mineralized sites exceeding the showing threshold in metals, the area is recognized for its volcanogenic massive sulphide and mesothermal quartz-carbonate gangue gold vein mineralization. The metals targeted by exploration work since the 1950s are, in order of importance, gold, copper, zinc, silver, lead and graphite.