RECONSTRUCTING FE-SILICATE PARAGENESIS USING PAIRED TEXTURAL AND COMPOSITIONAL DATA IN THE ~1.9 GA BIWABIK IRON FORMATION, MN

Primary Fe-mineral phases in Precambrian iron formations (IF) potentially record the redox conditions of ancient seawater. However, diagenesis and metamorphism commonly obscure the original mineralogy, muddling the seawater chemistry archive. To better constrain IF paragenesis and gain insight into Paleoproterzoic seawater chemistry, we investigated Fe-silicate phases of the well preserved (sub-greenschist) ~1.9 Ga Biwabik Iron Formation in Minnesota. We examined 70 samples from drill cores LWD-99-1 and LWD-99-2 for mineral and textural attributes among quartz-cemented granular and non quartz-cemented banded horizons. We utilized transmitted and reflected light microscopy to identify spatial relationships between mineral phases for scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Greenalite and chamosite form sub-μm crystals within granules, and are often cross-cut by minnesotaite and stilpnomelane, which form larger (~5-30 μm) crystals interpreted as diagenetic phases. Magnetite, when present, cuts across all mineral phases as euhedral crystals. Within non-quartz cemented horizons, granules often appear elongate in one direction, in direct contrast to their ovoid or near-spherical shape in quartz-cemented horizons. We therefore interpret greenalite and chamosite within quartz cemented horizons as the texturally earliest Fe-silicate phases, and targeted these for EPMA analysis. Greenalite granules (average composition Fe_4.49Mg_1.04Al_0.33Si_4.14O₁₀(OH)₈, 10 points) within quartz cemented horizons display non-compaction fractures or appear as sub-angular ~200 μm granule fragments, both interpreted as dewatering of a precursor Fe-Si gel phase. Quartz cement also preserves chamosite granules (compositional range Fe_2.69-3.99Mg_0.99-2.56Al_1.46-2.30Si_2.75-3.59O₁₀(OH)₈, 40 points). Minnesotaite and stilpnomelane are also present in granules, but form as crystal lathes that cut across greenalite and chamosite phases, as well as granule-cement contacts. To summarize, the paragenetic sequence described here indicates greenalite and chamosite are the texturally earliest Fe-silicate minerals in the Biwabik IF. The next step is to determine if and/or how the composition of these phases changed during early diagenesis.