ARCHEAN IRON FORMATIONS, A COMPARISON FROM THE SINGHBHUM AND WYOMING CRATONS

Archean iron formations (IFs) geochemical character, spatio-temporal distribution, and correlation with other rock assemblages provide insight into their depositional setting, cratonic formation, and the development of global modern plate tectonics (Bekker et al., 2010). Here we compare geochemical data (whole-rock major and trace element, Sm-Nd and Pb-Pb isotopic data) from banded iron formation (BIF) of the Western Iron Ore Group (WIOG) of the Singhbhum Craton and BIF + IF from the Beartooth Mountains of the Wyoming Craton, which have similar Archean histories (i.e. tonalite-trondhjemite-granodiorite (TTG) + granite development from 3.4 to 3.2 Ga, and mobile belt formation at ~2.9 Ga or younger). A stark contrast exists between the two cratons regarding the exposure and/or preservation of their IF deposits. Whereas the large geographic extent of the WIOG resembles an open-water Superior-type environment (carbonate absent), the Beartooth IF is only preserved in limited outcrop(s) with amphibolite, quartzite, and gneisses in a shear zone. A 3.39 Ga Rb-Sr isochron represents either a minimum age (age of isotopic homogenization during granulite facies metamorphism) or average age (if no isotopic reconstitution) for Beartooth IFs + other supracrustals (Henry et al., 1988). The WIOG has not been directly dated, but ~3.5 Ga U-Pb zircon ages of southern IOG and eastern IOG volcanics and IOG intrusive relations with the ~3.4-3.2 Ga Singhbhum Suite hint that WIOG is also ~3.5 Ga.

New geochemical data from the Singhbhum and Wyoming Cratons may highlight key differences in the formation and evolution of the IF protoliths from craton inception (~3.5-3.4 Ga) to stabilization (~2.7 Ga) and until present. Both the datasets show (PAAS-normalized) LREE depletion, positive Eu anomalies, and weakly negative to no Ce anomalies indicating an influential hydrothermal input prior to global oxygenation. High Cr and Ni abundances indicate an authigenic source for the Beartooth dataset. Isotope systematics are complicated, Sm-Nd T_dm for some samples indicate separation from the mantle at ~2.8 Ga for the WIOG and ~3.8 Ga for the Beartooth IF, indicating complexities in isotope systematics for the WIOG. Negative ϵNd values from indicate a terrestrial (evolved) Nd source for the Beartooth IF. In contrast, positive ϵNd values for the WIOG (for any reasonable age from 3.5-2.7 Ga) suggest a hydrothermal (juvenile) Nd source.