Primary carbonate mineralogy of Lower Algal Chert Member ooids and stromatolites, Paleoproterozoic Gunflint Formation, Ontario, Canada

The chert members of the Paleoproterozoic Gunflint Formation, Ontario, Canada, are commonly regarded as examples of primary silica chemical sediments. This interpretation is founded upon the high fidelity of preservation of microfossils in the chert, the ubiquitous nature of silica, and a lack of observed primary carbonate. Previous arguments for silica replacement of limestone (carbonate) are based on indirect evidence and are largely dismissed. Gunflint microfossils are thus regarded as having lived in a silica-rich environment, which makes them unusual compared to most other pre-Phanerozoic microfossils that are known from silicified carbonates.;Direct evidence that carbonate was a primary mineral species of the Lower Algal Chert Member (where most of the microbiota are found) has been discovered at Whitefish Falls, Nolalu, Ontario. Iron-stained, low-Mg calcite ooids containing (1) well-preserved growth laminae, (2) textures indicating a disrupted tangential fabric of growth laminae, and (3) quartz cross-cutting the growth laminae indicate the initial mineral of the ooids was a carbonate, probably aragonite. Stromatolite columns contain large areas of low-Mg calcite, with specular hematite crystals that display cross-cutting relationships with calcite and quartz that demonstrate calcite was primary, hematite secondary, and quartz tertiary. The cross-cutting relations also show that silicification took place in at least two stages. One of the silicification events took place very early in diagenesis.;Thus the Gunflint microbiota may not have existed in a radically different environment (silica precipitating) than those of most other known Proterozoic microbiotas. High fidelity of preservation of microfossils does not always indicate that the mineral entombing them was the primary mineral precipitated from aqueous solution.