DECIPHERING PRIMARY AND DIAGENETIC CONTROLS ON MINERALOGY IN THE ~1.9 GA BIWABIK IRON FORMATION, MN USING PAIRED TEXTURAL AND GEOCHEMICAL ANALYSES

The primary mineral inventory of Precambrian Iron Formations (IF) records the evolving composition of seawater. However, post-depositional changes during diagenesis and later metamorphism often obscure the original mineralogy. Here, we investigate texturally early silicate and carbonate minerals of the well-preserved (sub-greenschist) ~1.9 Ga Biwabik IF in Minnesota to infer potential primary phases present in Paleoproterozoic oceans. We examined 53 samples from drill core LWD-99-1 for sedimentary characteristics as well as textural and mineralogical data using transmitted and reflected light microscopy. We focused sampling for detailed scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA) work at contacts between granular and banded horizons, screening samples for geochemical analyses based on textural indicators. Here, we report novel mineral data for greenalite granules and a range of carbonate phases reported as average compositions. Based on SEM and EPMA imaging, greenalite granules display cuspate fractures interpreted to result from dewatering of a precursor Fe-Si gel composition. Such grains appear as an intergrowth of 2 phases at the sub-μm scale with an average composition of (Fe_5.22Mn_0.03Mg_0.56)Al_0.14Si_4.06O₁₀(OH)₈ (9 points on 3 granules). The same sample contains a thin layer of fine-grained (~5-10 μm) siderite ((Ca_0.04Mg_0.16Mn_0.06Fe_0.75)CO₃; 23 points) displaying a mottled texture. Within other carbonate horizons, we identify 2 potentially primary textures: (1) a ~9 mm putative crystal fan of calcite composition ((Ca_0.99(Mg_0.01)CO₃); 17 points) associated with fine-grained calcite of the same composition (40 points); and (2) a separate fine-grained (~10-30 μm) ferroan-dolomite horizon (Ca_1.06(Mg_0.63Fe_0.29Mn_0.02)(CO₃)₂ averaged from 17 points) containing a putative microbial mat morphology. Both features are distinct from secondary euhedral crystals (~1-5mm) which possess thick rims of ankerite composition (Ca_1.08(Mg_0.38Fe_0.51Mn_0.03)(CO₃)₂; 5 points). Broadly, our results identify a range of near-primary to diagenetic textures and compositions from different horizons of the Biwabik IF, and highlight the importance of pairing quantitative geochemical analyses with spatial relationships.