The Mistamisk Complex outcrops within the Lac Romanet Horst, a large NW-SE oriented structure bounded by steep or dipping faults (sheets 24B04, 24B05 and 24C08). Prior to the formation of the horst, basalt of the Swampy Bay Group and sedimentary rocks of the Seward Group covered the horst rocks in continuity (Dimroth, 1978).

The Mistamisk Complex comprises various sodic lithofacies of hydrothermal origin: albitic or chloritic matrix breccia, albitic clusters and albitic veins (pPmik1); carbonate matrix breccia (pPmik2); laminated albitic rocks (pPmik3); albitic sodic diorite clusters (pPmik4); and various undivided albitic rocks (pPmik) (Kearvell, 1985; Clark, 1986; Kearvell and Clark, 1988). Kearvell (1985) proposed that albitic matrix breccias are of intrusive and explosive origin. Based on the texture of fragments in albitic matrix breccias, albitization of fragments occurred during metasomatism of the protolith, prior to the breccia’s intrusion. To explain the presence of slate cleavage in some fragments, Kearvell (1985) proposed that breccias were emplaced after part of regional deformation, but before the final phase. Citing several field evidences, the same author suggested that carbonate matrix breccias are also of intrusive and explosive origin and formed by rebrecciation of albite matrix breccia.

The albitization phenomenon seems to be related to an episode of syntectonic metasomatism during which sodic fluids would have circulated through the various lithofacies of the Lac Romanet Horst (Kish and Cuney, 1981; Clark, 1986). The presence of finely disseminated hematite in association with albitic alteration would indicate that fluids were also oxidizing. Sodic alteration is possibly related to the presence of evaporitic sedimentary rocks in the lower part of the first volcano-sedimentary cycle. Indeed, the presence of albite-sericite sodic schist in the Lace Lake Formation adjacent to copper and radioactive mineralization on the Romanet River shoreline was reported by Kish and Cuney (1981). The presence of evaporites was also proposed by Dimroth (1978).

The Mistamisk Complex is associated with an impressive number of known mineralized zones (Clark, 1986; Clark and Wares, 2004). These are both uranium-bearing (Eagle, Simon, GM) and copper-bearing (Delhi Pacific, Lomer Copper, Lac Taché), while some display high gold and silver contents (e.g. Eagle, Kish, GM, Delhi Pacific, Lac Taché). Copper-bearing sulphides and uranium minerals are thought to have precipitated during the sodic metasomatism event. This alteration would have resulted from the passage of hydrothermal fluids possibly derived from evaporitic sediments during the Hudsonian orogenesis (Kish and Cuney, 1981). The oxidizing and carbonate composition of hydrothermal fluids would have played an important role in the transport of uranium in solution (Kish and Cuney, 1981). A link to iron oxide-copper-gold (IOCG) mineralization was proposed (Clark and Wares, 2004; Conliffe et al., 2019).

Mistamisk Complex 3 (pPmik3): Laminated Beige, Grey, Pink or Black Albitic Rock, Locally Sulphide-Bearing

​Laminated (or banded), fine-grained, locally sulphide-rich beige, grey, pinkish or black albitic rocks occur in the escarpment along the Romanet River at the Delhi Pacific mineralized site (sheet 24B05). The following description and interpretations are from Clark (1986). The rock, with Na₂O contents ranging from 7 to 11%, forms a strip ~800 m long, and ranges in width from a few metres to ~45 m. The rock is interbedded with basalt and mudstone layers belonging to the Bacchus Formation and with thin gabbro sills. Traces of at least two major faults follow the escarpment. Albitic laminae (or bands) are millimetre to centimetre thick and strongly folded; they locally contain a significant amount of chalcopyrite, accompanied by pyrrhotite and pyrite. Sulphides are located in thin veins and small (millimetric) irregular clusters of relatively coarse-grained albitic rock compared to the surrounding albitic rocks. The sodic composition of laminae is interpreted as being of metasomatic origin and caused by the circulation of sodic hydrothermal fluids. The structures suggest that albite and sulphides were recrystallized. The original albitic rock was possibly mudstone or tuff. The initial development of sulphides was probably contemporaneous with sodic metasomatism. Structural and textural evidence suggests that metasomatism occurred during various folding episodes.