Stable isotope ratios of carbonate and sulfide minerals from the Gunflint Formation: Evidence for the origin of iron formations.
Creators: name: Carrigan, William J.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1990
Subjects: subject: Geology.

description: The $\sim$1.9 Ga Gunflint Formation is a Lake Superior type iron-formation, located in the Thunder Bay district of northwestern Ontario, that was deposited on a shallow shelf analogous to modern carbonate environments. Carbonate minerals in the iron-rich lithofacies of the Gunflint Formation include siderite, ankerite, and calcite. Petrographic evidence indicates that siderite precipitation initiated either within the water column or at the sediment/water interface and continued during very early diagenesis. Ankerite and calcite formed during early to late diagenesis as pore-filling cements and as replacements of other minerals. The iron-poor limestone facies contains very early diagenetic dolomite and early to late diagenetic calcite. $\delta\sp $C values of carbonate minerals from unmetamorphosed rocks range between 0 and $-6\perthous$ (PDB). The values near 0$\perthous,$ which are considered to be representative of the basin water composition, indicate that the primary source of carbon was marine bicarbonate. The lighter values indicate that a minor component of oxidized organic carbon was added during early diagenesis. The heaviest $\delta\sp $O values for unmetamorphosed carbonate minerals range between $-$5 and $-7\perthous$ (PDB), which is the same range of values observed for many early Proterozoic marine carbonates. $\delta\sp $O values of carbonate minerals are the result of isotopic exchange with pore waters, originally of marine composition, at increasing temperatures and/or are the result of isotopic exchange with $\sp $O-depleted meteoric water during early diagenesis. Disseminated fine-grained, very early diagenetic pyrite is widespread throughout the formation, usually in amounts less than about 2%. However, pyrite is locally observed as laminae or thin layers, suggesting that some pyrite may have formed at or above the sediment/water interface. Low S/C ratios indicate that dissolved sulphate was the limiting factor in pyrite formation. $\delta\sp{34}$S values between +5 and +12$\perthous$ (CDT) imply that sulfide formed by bacterial sulphate reduction under closed system conditions. In the lower part of the Gunflint Formation coarse-grained pyrite and pyrite concretions are associated with syndepositional faults. High S/C ratios and highly variable $\delta\sp{34}$S values ($-$33 to +35$\perthous)$ suggest an external source of sulphate was introduced by fluids moving upward along these faults. The Gunflint basin is best characterized by a stratified water column with high concentrations of dissolved ferrous iron below the redox boundary. Volcanic activity or rifting within this basin contributed a high flux of reducing hydrothermal solutions to the seawater. Hydrothermal activity was probably the dominant source of iron, although reduction of detrital ferric iron may have contributed significant amounts of dissolved iron. During periods of increased tectonic activity, the expansion of the redox boundary to shallower water allowed the transport of iron to the shallow shelf. Ferric iron-bearing minerals would have been precipitated on the shelf by oxidation in surface waters whereas ferrous iron-bearing minerals would have been precipitated under more reducing conditions either in deeper water or in sheltered environments. The transition to the iron-poor limestone member resulted from a lowering of the redox boundary. (Abstract shortened by UMI.)