Petrological and stable isotope studies of carbonate and sulfide minerals from the Gunflint Formation, Ontario: evidence for the origin of early Proterozoic iron-formation

The ∼ 1.9 Ga Gunflint Formation, in the Thunder Bay district of Ontario, principally comprises iron-formation, overlain by an iron-poor limestone/dolostone member (the Upper Limestone Member). Rocks are virtually unmetamorphosed, retaining primary textures, except locally around diabase sills. The Gunflint presents an opportunity to study both unmetamorphosed and metamorphosed iron-formation and to study the changes from iron-rich to iron-poor sedimentation. Of the carbonate minerals in iron-rich units, siderite formed at or near the sediment/water interface, with ankerite and calcite forming later during diagenesis and during metamorphism around sills. Heaviest δ13C values of siderite, and of dolomite in the Upper Limestone Member, are consistently near 0ℵ throughout the stratigraphic section, indicating that marine bicarbonate was the source of carbon. A spread of values between 0 and −7ℵ for unmetamorphosed rocks is the result of the incorporation of oxidized organic matter during diagenesis, whereas δ13C values more negative than −7ℵ are the result of metamorphism. The heaviest δ18O values from unmetamorphosed rocks, −5.3ℵ (PDB) for siderite, −6.1ℵ for ankerite, and −6.7ℵ for calcite, are considered to represent the original marine composition. Lighter values (to −17ℵ) represent isotopic exchange reactions with pore fluids at higher temperatures and/or isotopic exchange with 18O-depleted meteoric water. Low S/C ratios and a relatively narrow range of positive δ34S values in pyrite throughout most of the formation are consistent with bacterial sulphate reduction of seawater containing sulphate at a significantly lower concentration than the modern ocean. Locally, higher sulfide contents and a wider range of δ34S values were the product of the introduction of sulfur after deposition. It is proposed that the Gunflint Formation was deposited in a stratified water column with anoxic bottom water. The depositional basin had restricted communication with the open ocean and was affected by distal volcanism. Hydrothermal activity associated with the volcanism provided a large source of dissolved iron, and possibly silica, which helped buffer O2 and sulphate in water to low levels. Low concentrations of sulphate limited the generation of H2S that would otherwise have restricted the solubility of iron. During periods of increased hydrothermal activity, the anoxic/oxic water boundary moved upwards, permitting transport of iron to the shallow shelf, where it was precipitated as siderite, iron hydroxides, iron-silicates, or pyrite depending on physico-chemical conditions. The transition to the overlying limestone marks a decrease in hydrothermal activity with a contraction of the redox boundary.