Chemical and textural overprinting of ancient stromatolites: Timing, processes, and implications for their use as paleoenvironmental proxies

Shale-normalized Rare Earth Element (REESN) patterns and transition metal abundances in banded iron formations (BIF) and shales have long been used as paleoenvironmental proxies to investigate the evolving geochemistry of Precambrian oceans. Apparent similarities between the REESN of modern stromatolites and contemporary seawater values also mean that some ancient stromatolites could record paleo-ocean chemistry. To test this hypothesis, we present comparative mineralogical, textural, and spectral analyses of multifurcate, silicified and iron oxide/carbonate-rich, stromatolites from the ca.1.88 Ga Gunflint Formation (Ontario, Canada). Previous studies show that these organo-sedimentary structures represent the initiation of shallow marine sedimentation in the Animikie Basin on the southern tip of the Superior Province. We present evidence that these structures were originally stabilized by aragonite and Mg-bearing carbonates. Fabrics indicative of stromatolite accretion under local hypersaline conditions are also present, with La anomalies suggestive of laterally variable freshwater runoff. The admixture of partially evaporated seawater and silica-oversaturated deep seawater led to pervasive silicification of metastable carbonates and preservation of less soluble early diagenetic carbonate cements. Bulk rock transition metal contents point to lateral variability and dissimilar partitioning and fractionation during mineral formation, yet Ce anomalies amongst samples are similar. Considering the pronounced differences between the REE partitioning between Fe oxyhydroxide precipitates and carbonates, our results point to hematite and siderite as secondary minerals that postdate the stabilization of these structures by carbonates and silica. Stratabound relations of the Fe-rich stromatolites with diabase sills and dikes indicate that the pore water fluids evolved towards a localized state of hematite and siderite oversaturation towards the Mesoproterozoic. If hematite and siderite are both early diagenetic phases, then the Ce anomalies of these stromatolites may be a poor measure of the redox conditions of the shallow areas of the Animikie Basin at the time of accretion.