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.
University of Minnesota M.S. thesis. November 2020. Major: Earth Sciences. Advisor: Latisha Brengman. 1 computer file (PDF); vii, 111 pages.
Identification of primary and diagenetic mineralogy preserved in silica-cemented horizons of the Biwabik Iron Formation, Minnesota, using petrography and electron microprobe analysis.
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