Spheroidal and filamentous organic-walled microfossils have been detected in ca. 1.7 Ga old cherts of the Hornby Bay Group, Northwest Territories, Canada. The majority of the spheroidal forms range from 1 to 4 μm in diameter, are referable to the genus Sphaerophycus, and probably represent the preserved sheaths of chroococcacean cyanophytes. A single, robust-walled, 27 μm diameter, spheroidal microfossil of undetermined affinity is also present. The filamentous forms are tubular, unbranched, and range from 1 to 8 μm in diameter. They appear to represent the preserved sheaths of nostocalean cyanophytes or filamentous bacteria. The filaments illustrate the relationship between matrix mineralogy and the fidelity of preservation of organic-walled microfossils. Where they occur in silica the filaments are preserved as three-dimensional tubular micro-structures, which are readily recognized as microfossils. In contrast, where they extend from silica into adjacent dolomite they are highly compressed and not readily discernable as microfossils. This compression appears to have been caused by aggrading neomorphism and pressure dissolution of the carbonate minerals, and it illustrates the contribution of diagenesis, in addition to decomposition of organic material, in causing the paucity of microfossils in Precambrian carbonate rocks.

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Deerite has been found in layers of Fe-rich metasediments, metamorphosed under conditions of blueschist to eclogite facies (T--ca. 480 ~ Geochemical comparisons with other deerites described in the literature show that the Ile de Groix deerites most closely resemble the Mn-poor, alpine type, and Sifnos deerites. The P-T conditions of the Ile de Groix metamorphism do not conform with the stability field of deerite. In the assemblages examined here deerite seems to be a relic from earlier stages of prograde metamorphism.

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