THE ROLE OF SILICA IN IRON FORMATIONS: TEXTURAL, MINERALOGICAL, AND GEOCHEMICAL COMPARISONS FROM LOW METAMORPHIC GRADE NEOARCHEAN AND PALEOPROTEROZOIC EXAMPLES

The marine Precambrian silica cycle differs fundamentally from the modern in that the dominant precipitation mechanism is likely chemical, rather than biological. Much of the sedimentary record of Precambrian silica precipitation preserves in siliceous chemical sedimentary rocks, namely chert and iron formation. Recognizable textural attributes allow the separation of iron formations into two broad groups – those that consist predominantly of mud-sized material within distinct micro-bands, and those that contain sand-sized grains, which alternate with layers of mud-sized material. The original mineralogy of these rocks however, is often debated. To discern which textural and mineralogical features link to deposition or early diagenesis, and distinguish these from later diagenetic features, we sampled 10 Archean and Paleoproterozoic iron formations from three, low metamorphic grade (greenschist to sub-greenschist) regions. We analyzed and directly compare stratigraphic setting, petrographic characteristics, mineralogy, and major- and trace-element geochemistry of 97 samples (8 iron formations) from the ~2.7 Ga Abitibi Greenstone Belt, Canada, 70 samples (2 drill cores) from the ~1.9 Ga Biwabik iron formation, Minnesota, and 27 samples (4 historic drill cores), from the <1.71 Ga Freedom Formation, Wisconsin. From this dataset we determined: (1) iron formations from each region possess unique geochemical and textural attributes that do not link to metamorphism; (2) Paleoproterozoic iron formations studied here are mineralogically more diverse and contain more detrital phases than the Archean examples; (3) samples from all three regions possess roughly similar weight percent SiO₂, but it distributes as quartz and Fe-Mg silicate phases in different proportions. Silica predominantly forms chert layers in Archean examples. In Paleoproterozoic samples, silica exists predominantly as quartz cement and sand-sized grains of both SiO₂ and Fe-Mg-Al silicate composition (Biwabik), while nearly all Si is in Fe-Mg-Al-silicate phases in the Freedom Formation. We interpret that these fundamental differences link to mechanisms of silica precipitation within each basin, rather than late diagenetic or metamorphic changes.