Abstract

The Steeprock pyritic zones are conformable, lenticular bodies in and along the hanging wall of an iron ore-bearing member of a Precambrian volcanic-sedimentary sequence in north western Ontario. The zones contain about 50 percent pyrite. About 90 percent of this pyrite consists of fragments possessing coloform and cryptocrystalline textures. This type of pyrite does not replace any minerals or rocks, but is veined and replaced by later crystalline pyrite, goethite, and hematite. A trace element study reveals a marked paucity in the number and quantity of the trace elements present. This, combined with the low cobalt content, low cobalt-nickel ratios, presence of arsenic in the pyrite, and the negligible selenium content, indicates that, with respect to the trace element content, the Steeprock pyrite is more akin to known sedimentary pyrite than it is to hydrothermal pyrite. The Steeprock pyrite, contrary to other published opinions, is regarded as a syngenetic sedimentary deposit of Precambrian age precipitated in restricted basins under anaerobic conditions. Iron is considered to have been derived through chemical weathering of a nearby ferruginuous limestone-dolomite member of the Steeprock group; sulfur is thought to have been contributed by anaerobic bacteria and volcanic activity that is clearly closely related in time and space to the pyrite.

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