POSSIBLE EARLY OXIDATION OF THE BIWABIK IRON FORMATION, MINNESOTA

Fluids have interacted with banded iron formation of the Paleoproterozoic (~1.85 Ga) Biwabik Fm., exposed in the Mesabi Range of Minnesota, during diagenesis and dissolution/oxidation. Burial diagenesis likely took place shortly after deposition at around 150° – 200° C based on mineral assemblages and published oxygen isotope thermometry (Perry et al., Econ. Geol. 68, 1110), whereas dissolution of silicates and carbonates and formation of hematite and/or goethite (comprising high-grade ‘natural ore’) have long been interpreted as a much younger supergene phenomenon. However, fluid inclusions and bulk geochemical analysis (specifically rare earths) of veins and whole-rock samples from several pits show that oxidation may have taken place over a range of burial depths, hence over a protracted period of time. Quartz vein samples from various paragenetic stages show considerable overlap of fluid inclusion homogenization temperatures (mean = 154° +/- 74° C, n= 467) and salinity (9.5 +/- 5.3 wt% NaCl equivalent, n=166) in the sample set. Notably, fault breccia samples associated with oxidized (red hematitic) iron formation in two sampled pits yield diagenetic temperatures and salinities, not consistent with supergene alteration.

Geochemical analysis of unoxidized and oxidized iron formation and veins from nearby faults indicates that fluids that precipitated quartz in the fault zones were locally rock-buffered, acquiring the rare earth element ‘fingerprint’ of iron formation in terms of positive europium, thulium, and/or cerium anomalies. At one location, ‘natural ore’ and associated quartz / calcite + hematite fault breccia are both characterized by strong cerium anomalies consistent with near-surface oxidation. At a different location, both oxidized iron formation and quartz + hematite fault breccia associated with oxidized ore show no cerium anomaly. Instead, both breccia and iron formation show pronounced Tm anomalies, indicating local control on the chemistry of fluid associated with oxidation and absence of externally-derived supergene fluids during oxidation of iron formation adjacent to the sampled fault. Thus an early oxidation event, separate from late supergene ‘natural ore’ formation, may have tapped a mixture of diagenetic and oxidizing meteoric waters during uplift shortly after burial.