Stable isotope geochemistry of cherts and carbonates from the 2.0 Ga gunflint iron formation: implications for the depositional setting, and the effects of diagenesis and metamorphism

Abstract δ 18 O values for all but three of 97 samples of chert along a ∼220 km transect through the 2.0 Ga Gunflint Iron Formation of the Animikie Basin, North America, range from 21.3 to 24.7%o. There is no correlation between δ 18 O and the type of chert (i.e., deeper-water lutitic chert, shallower water arenitic chert, and peritidal algal chert). The fact that the Gunflint Iron Formation has been exposed to thermal conditions only slightly above burial diagenesis suggests that the δ values of the chert have not been significantly affected by metamorphism. Field and petrographic observations, and the small δ 18 O range of all the chert types suggests that silicification, lithification, and any conversion to quartz chert from a hydrous silica precursor occurred close to the sediment-water interface. The uniform δ 18 O values of all the different chert types suggest that all cherts precipitated over a relatively common temperature interval (∼20°C) from a common parent water having a relatively uniform δ 18 O. The isotopic data suggest silica saturation and precipitation was not the result of the mixing of seawater and meteoric water, or the result of evaporative processes. Siderite in the deep-water, banded facies of the Gunflint displays a δ 18 O range of −6.0 to −2.6%o, whereas siderite in the more shoreward, organic-rich, shale facies is significantly enriched in 13 C ( δ 13 C=−2.5 to +0.5%o). δ 18 C of siderite does not appear to have been altered as a result of neomorphic recrystallization, and authigenic microsparitic siderite along stylolites displays the same δ 13 C range as primary and neomorphosed siderite. However, neomorphic and authigenic siderite are depleted in 18 O by up to 7%o relative to primary, unaltered microspheroidal siderite. The lower δ 13 C of banded facies siderite (relative to the shale facies siderite) does not seem to be the result of thermal decarboxylation or anaerobic oxidation (abiotic or bacterial) of organic matter, but rather is better explained by primary precipitation in an ocean system that was layered with respect to carbon isotopes. The more basinward banded facies siderite precipitated from a 13 C depleted, Fe 2+ -rich, hydrothermal water mass that was moved by advective upwelling from submarine regions of magmatic/volcanic-hydrothermal activity onto the continental shelf. The siderite of the shale facies precipitated from a more near-shore, shallow seawater mass that was comparatively enriched in 13 C ( δ 13 C≈0±2% o ). Ankerite in the Gunflint Iron Formation occurs as a replacement of primary siderite in the banded facies, and as a replacement of greenalite and chert in the arenite facies. Many ankerite samples have δ 13 C values that are similar to those of the shale facies siderite which suggests that shallow seawater was a major component of the diagenetic fluid. Later neomorphism of the replacement ankerite at higher burial temperatures and probably in the presence of 18 O depleted waters resulted in a wide range of δ 18 O values (13.8–22.5%o). The unique mineralogy of the upper most Gunflint Iron Formation (calcite, serpentine and magnetite) compared to the majority of the Formation, the very low δ values, and overlying diabase sills suggest that the non-ferroan nature of the carbonate in the upper Gunflint is the result of contact metamorphism.