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Oxide-facies BIF in this area is hosted by ultramafic and calc-alkaline volcanic rocks and related sedimentary rocks. At least two phases of metamorphism/metasomatism are recorded. Greenschist-facies metamorphism has affected all rocks on a regional scale while later mineralization and alteration resulted from metasomatic fluids carrying Mg, K, Ca, S, As, Cu and Au. Metasomatism produced cummingtonite + biotite-bearing assemblages which are associated with strongly mineralized iron formation. Pyrrhotite is the dominant sulphide phase and occurs within ductile-deformed iron formation where it pervasively replaces magnetite. Minor fine-grained chalcopyrite and arsenopyrite occur as inclusions in pyrrhotite and recrystallized magnetite. Pyrite is restricted to brittle-deformed iron formation, and to the contact between iron formation and altered ultramafic rocks, where it is associated with quartz veins. Pyrite typically replaces pyrrhotite. Native Au is associated with both pyrrhotite and pyrite. Grunerite/Ca-amphibole + stilpnomelane and sparse pyrrhotite and pyrite characterize weakly mineralized iron formation. Within adjacent ultlramafic rocks, metasomatism produced assemblages of interlayered Ca-amphibole + biotite, and talc + Ca-amphibole + carbonate + biotite. K-Ar geochronology on metasomatic biotite from mineralized BIF yielded an age of approximately 1791 m.y. An epigenetic model is favoured for the emplacement of the Au, the age of which is uncertain but may be Proterozoic.

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GeoRef, Copyright 2018, American Geological Institute.<br/>1996-077736<br/>Third Portage Lake<br/>Woodburn Lake Group