MAPPING MINERAL REACTIONS IN A MID-PROTEROZOIC IRON FORMATION NEAR BARABOO, WI, USA

Drill cores from Baraboo, Wisconsin, USA contain the Freedom Formation, an iron-rich chemical sedimentary unit that provides new insights into reconstructing geochemical attributes of Mid-Proterozoic surface environments. The Freedom Formation conformably overlies the classic Baraboo quartzite and Seeley slate. Combining data from drill cores with petrographic, mineralogical, and geochemical datasets we identify depositional attributes by categorizing phases into depositional vs. post-depositional mineral associations. We determined the following mineral formation sequence: (1) if a field of view contains quartz, quartz is the texturally earliest phase; (2) if quartz is absent in a field of view, chamosite is the texturally earliest phase; (3) if chamosite is absent or rare, then carbonate is the texturally earliest phase; (4) nano-scale hematite exists at boundaries of chamosite, quartz, and stilpnomelane crystals, and within veinlets; (5) euhedral magnetite cross-cuts all other phases, and is often associated directly with chamosite; and (6) large hematite sometimes crosscuts small magnetite, or forms oxidized rims on euhedral magnetite crystals. Placing this mineral information in a stratigraphic context, mineralogy shifts from a chamosite, quartz, and magnetite assemblage at the base, to Mn-carbonate, hematite dominant assemblage towards the top of the sections. The mineral changes are present across a stratigraphic transition to carbonate accompanied by an increase in sand-sized grains and a decrease in mud-sized material. The transition from reduced fine-grained, texturally-early Fe²⁺ containing minerals to Mn-carbonate is accompanied by anoxic geochemical signatures and may indicate minor changes in oxygen conditions during the formation of the mineral phases preserved in the Freedom Formation. Veining occurs throughout the cores, indicating past fluid flow, which may link directly to the observed redox changes, including the post-formational oxidation observed as hematite rims on large, euhedral magnetite crystals. This work emphasizes the importance of detailed petrographic investigation to determine mineral formation histories that inform the interpretation of redox geochemistry data.