The Eau Claire gold deposit is located in the Eastmain River Archean volcano-sedimentary belt, James Bay. The deposit is hosted dominantly by amphibolitized mafic volcanics intercalated with horizons of quartz-feldspar rock (sediments or felsic volcanics) intruded by several tonalitic to granitic plutons. The Eau Claire gold deposit has many features similar to amphibolite grade gold deposits of Western Australia.

        The gold mineralization is hosted by upper greenschist to lower amphibolite grade rocks and comprises fault-filled quartz-tourmaline-calcite (QTC) veins and actinolite-tourmaline-biotite-calcite alteration zones at veins selvages. The veins are boudinaged with variable thickness (average 25cm) and are enclosed in sheared alteration zones up to 4m wide. Native gold in QTC veins occurs in micro-fractures in quartz and tourmaline or in association with Bi- and Ni-tellurides. Pyrite, pyrrhotite and chalcopyrite are rare and are not associated with gold. The alteration zones, characterized by biotite, actinolite, tourmaline and calcite in various proportions, are commonly coarser grained than unaltered host rocks. Microprobe analyses show that amphiboles in the host rocks are tschermakite and magnesio-hornblende, whereas actinolite and tremolite characterize the alteration zones and veins. Biotite in the alteration zones is enriched in Mg and depleted in Ti compared to the host rocks.

        The deposit is located in an open E-W F₂ fold that deforms an earlier S₀-S₁ fabric. In the hinge zone, auriferous QTC veins are E-W and dip at 40^oS. These veins make a high angle with S₀-S₁ and are not axial planar to the F₂ fold. In this part of the deposit, the S₀-S₁ foliation is transposed by the schistosity surrounding the veins, indicating that gold mineralization is post S₀-S₁. A folded QTC vein on the south limb of the F₂ fold indicates that the mineralizing event is pre- to early-D₂.