Identification of primary and diagenetic mineralogy preserved in silica-cemented horizons of the Biwabik Iron Formation, Minnesota, using petrography and electron microprobe analysis

Abstract

The primary mineralogy of iron formations, iron and silica-rich chemicalsedimentary rocks, are crucial archives of Precambrian seawater chemistry. Post-depositional alteration from diagenesis and metamorphism commonly obscure the original mineralogy in many iron formations. Recent studies of well-preserved iron formations have identified putative primary mineral phases preserved in silica-cemented horizons. Silica cement aids in mineral preservation by sealing pore space with quartz, a stable mineral on the Earth’s surface. These previous studies focus on iron formation precipitation during the initial rise of oxygen in Earth’s atmosphere and oceans from ~2.5 - 2.3 Ga. Following this rise, ocean oxygenation remains poorly understood. The ~1.9 Ga Biwabik Iron Formation in northeastern Minnesota provides an opportunity to study well preserved (sub-greenschist facies) iron formation following the ~2.5 Ga rise in oxygen. Minerals were identified in silica-cemented horizons and non-silica-cemented horizons with petrography and electron microscopy. Cross-cutting relationships and mineral compositional data inform a paragenetic sequence and distinguish diagenetic minerals from texturally earlier minerals. Observations from petrography and electron microscopy suggest silica-cementation preserves textures not present in adjacent banded horizons. Diagenetic mineral compositions are influenced by their relative spatial proximity between silica-cemented and banded horizons. Within different silica-cemented horizons, the texturally earliest mineral phases were greenalite or <5 μm hematite. These two minerals suggest the initial sediment of the Biwabik Iron formation was a Fe(II)-Si greenalite-like gel and/or an oxidized hematite precursor.