The Morley Pyrite Occurrence is an Algoma Type sulfide-facies iron formation, located southeast of the town of Schreiber, in the Abitibi-Wawa Subprovince of the Superior Structural Province. The iron formation consists of colloform pyritic structures, horizontally laminated pyrite, veins and massive pyrite in a chert/carbonaceous slate assemblage. This is overlain by fine grained metasediments and mafic volcanics, and underlain by intermediate volcanics.
Colloform structures, up to 5.2 centimeters in diameter, are comprised of alternating, millimeter thick laminae consisting of pyrite or chert/carbonaceous slate. The colloform structures have distinctive morphologies and display variable angular relationships with underlying strata. Where several structures have developed adjacent to one another, the resulting structure appears stromatolitic.
Carbon, while present in pyritic laminae, was a major constituent in the siliceous bands, with magnetite, hematite, greenalite and stilpnomelane also being identified. Magnetite and hematite, present at the upper chert and lower pyritic laminae contacts, indicate cyclical variations in Eh conditions during the formation of the laminae. Stilpnomelane, indicative of greenschist metamorphism, is also associated with brecciated fragments of colloform structures. Pyrite rims have developed on these fragments, which in turn are locally mantled by sphalerite. Stilpnomelane is associated as rinds on the sphalerite. Stilpnomelane is associated as rinds on the sphalerite, indicating post breccia development and possible association with higher temperature hydrothermal fluids.
The geochemistry of the occurrence, the fine grained nature of the sediment, the abundance of pyrite and the presence of tuffaceous material incorporated into the laminae, suggests a distal, anoxic basin in which reducing conditions dominated and episodic, low temperature hydrothermal input played a significant role. The presence of carbon in the laminae and the regular morphology of the colloform structures suggests an at least partial biogenic origin in which Archean organic mats accumulated sulfidic and siliceous sediment.