FORMATION OF SYNGENETIC AND EARLY DIAGENETIC IRON MINERALS IN THE LATE ARCHEAN MT. MCRAE SHALE: PALEOCEANOGRAPHIC IMPLICATIONS

The late Archean Mt. McRae Shale demonstrates mineralogical and geochemical variations arising from a transient or secular increase in ocean/atmospheric oxygenation and the episodic enrichment of iron from hydrothermal sources. The lower shale interval (LSI), banded iron formation (BIF), upper shale (USI) and a carbonate (C1) have been analyzed for total Fe and Al and authigenic Fe phases by combining quantitative XRD diffraction with a modified extraction methodology that quantifies sideritic Fe and silicate Fe, plus Fe present as oxides and sulfides. This approach stresses the need for careful method validation to enable fine-scale environmental changes to be deciphered. The LSI and BIF have mean Fe_T/Al values of 2 and 5, respectively, which record iron enrichment from hydrothermal sources. Iron was precipitated primarily as siderite accompanied by Fe-rich chlorite from anoxic bottom waters rich in dissolved Fe. Pyrite formation was probably limited by the availability of sulfate, which was present at low concentrations and became rapidly depleted. The USI has generally lower Fe_T/Al values (0.6 to 1.3), similar to those found in Paleozoic shales, with the exception of one interval where enrichment may reflect either a weak hydrothermal source or the operation of an iron shuttle as observed in the Phanerozoic ocean. This interval contains authigenic Fe predominantly as pyrite, where high values for DOP (> 0.8) indicate the existence of a water column that briefly became rich in dissolved sulfide (euxinic) when sulfate concentrations increased due to a transient or secular increase in ocean/atmosphere oxygenation. High concentrations of dissolved sulfide maintained low concentrations of dissolved Fe, which allowed only minor amounts of Fe to be precipitated as carbonates and silicates. The carbonate C1 represents a basinal chemistry where sulfide is absent and Fe_T/Al values are ~ 1 indicating that weak hydrothermal activity returned to produce bottom waters with low concentrations of dissolved Fe. The methodologies described here illuminate the fine details of the balance between Fe and S delivery and availability, and the paleoenvironmental implications of these controls as tied to hydrothermal Fe inputs and atmospheric oxygenation.