AN INVESTIGATION OF THE MINERALOGY, PETROLOGY AND GEOCHEMISTRY OF BANDED IRON FORMATIONS FROM THE NUVVUAGITTUQ SUPRACRUSTAL BELT, CANADA

The Nuvvuagittuq Supracrustal Belt (NSB) in northwestern Québec is one of very few Eoarchean volcano-sedimentary rock assemblages that has been dated to be >3.75 Ga. Metasedimentary rocks from the NSB include banded iron formations (BIFs), which may preserve evidence of early life and thus need to be documented geologically. We present a systematic study of the petrography, mineral associations and geochemistry of BIFs from the NSB. These BIFs were divided into four and possibly five distinct groups based on the relative abundances of quartz, magnetite, amphibole, and hematite. Petrographic analyses suggest that amphiboles in these BIFs were likely formed by high temperature reactions between quartz and carbonate and between quartz and magnetite under metasomatic conditions. Apatite grains were mapped in several thin sections and found to occur mainly in quartz layers, although in BIFs with little to no quartz, apatites occur as inclusions in amphibole. In the latter BIFs, apatite grains occur within networks of magnetite and amphibole veins that lead to the apatite, which suggests a metasomatic origin. In a few amphibole + quartz + magnetite BIFs, a few grains of apatite associated with graphite were found. For all BIF types, geochemical analyses revealed low concentrations of elements typically associated with detrital minerals such as Zr, Cr, TiO₂, and Al₂O₃, which suggest that the BIFs are essentially devoid of detrital phases. The lack of correlation between P and Fe-oxides abundances suggests insignificant contributions of P from adsorption onto Fe-oxide particles in the water column during deposition. These results collectively suggest that apatite grains in the NSB BIFs were formed during metasomatism or diagenesis. Carbon isotope analyses on bulk rock powders indicate that small amounts of organic C occur in these rocks (typically less than 0.05 wt %) and δ¹³C values range from -13‰ to -30‰, which may reflect microscopic heterogeneities and/or C losses and ¹³C enrichments during metamorphism. Overall results suggest a strong influence of metamorphism and metasomatism on the mineralogy of these BIFs, but there may be mineral associations that survived since diagenesis.