On the E side of Hudson Bay, the NW margin of the Archaean Superior Structural Province is overlain by an unmetamorphosed, little deformed, Early Proterozoic supracrustal sequence. The sequence is divisible into three parts, possible tectosomes, each characterized by a distinct style of mineralization. The arkosic, alluvial Richmond Gulf Group was shed eastward from a thermal uplift and has been preserved only in a later aulacogen, the Richmond Gulf Graben. It contains an apatite-cemented redbed. Secondly, a widespread post-aulacogen, carbonate-rich, W-facing miogeocline blanketed the region. A pyritic zone developed at its basal contact is particularly thick and hosts disseminated copper mineralization where formed on the arkose in the graben. A pyritic, brecciated, stromatolite-rich horizon within the carbonate contains galena and sphalerite. Thirdly, widespread iron formation and a local 60 cm thick manganese carbonate bed, overlying W-derived basalt flows, flysch and arkosic molasse, all of uncertain tectonic affinity, terminate the sequence.

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