Pathways of microfossil mineralisation by hematite in the 1878 Ma Gunflint Formation

Abstract

The ~1878 Ma Gunflint Formation contains some of the foremost examples of Palaeoproterozoic life but questions remain regarding the potential metabolic and taphonomic pathways within the microfossil assemblage. Here we report on hematite-mineralised examples of the most abundant Gunflint organisms (Gunflintia and Huroniospora), using correlative light-, Raman- and electron-microscopy to document subtle variations in iron oxide mineralisation styles.Data come from two localities where similar stromatolitic facies are found, namely Schreiber Channel and Mink Mountain, Ontario. At Schreiber Channel, rare hematitic microfossils are found at the margins of small pods of pyritised microorganisms, with some individual cell or sheath walls retaining pyrite cores and hematite rims. Microfossil wall hematite grains form anhedral masses, contain large numbers of quartz inclusions, plus a small number of barium-rich and calcium-rich nano-crystals. Hematitic, pyritic and carbonaceous microfossil preservation styles co-occur within a single petrographic thin section. A sub-set of iron-mineralised fossils at Schreiber have thinner walls, comprising tabular hematite grains plus Fe-Al-silicates and traces of organic material. At Mink Mountain, microfossil walls are ubiquitously preserved as hematite but well-preserved examples occur only in isolated zones of particularly dark brown/purple hematite. Here, hematite grains show a cuspate to anhedral habit with embayment and inclusions of quartz plus rare Ti-rich nano-crystals; nano-particles of iron oxide and rare organic material occur as bridges between some grains.The morphological similarity of microfossil assemblages having walls now preserved with different chemical compositions (carbon, pyrite, Fe-Al-silicate and hematite), and across widely-spaced stromatolitic localities, indicates that iron oxide mineralisation was a taphonomic rather than primary metabolic process. The pyrite and Fe-Al-silicates at Schreiber are interpreted as the earliest stages of iron mineralisation, forming under reducing diagenetic conditions in the presence of excess organic material. Partial replacement of these phases by hematite followed later when oxygen bearing fluids penetrated limited zones within the sediments. At Mink Mountain, we infer that zones of well-preserved microfossils were initially mineralised as pyrite, followed by the complete oxidation of pyrite plus all surrounding organic material to hematite.This study demonstrates the importance of localised micro-environmental conditions on taphonomic processes and, in turn, the role that taphonomy can play in the modification of the morphology of fossilised organisms. This has implications both for the study of early life on Earth and for the assessment of any putative microfossils returned from Mars.