Geochemistry of shallow and deep water archean meta-iron formations and their post-depositional alteration in western Superior Province, Canada

Abstract

One purpose for studying banded meta-iron formations is to determine the chemical composition of seawater in the Archean ocean and the oxygen content of the Archean oceanic-atmospheric system. Geologists use the geochemistry of meta-iron formations to make interpretations on the chemical conditions in the Archean. However, post-depositional alteration can affect the element geochemistry preserved in the meta-iron formations. This thesis explores the role of post-depositional mechanisms and determines element provenance in four Archean banded meta-iron formations. The four different locations hosting Archean metamorphosed meta-iron formations chosen for this study are: meta-iron formations from the Beardmore/Geraldton greenstone belt of the Eastern Wabigoon Domain, Lake St. Joseph greenstone belt of the Uchi Domain, North Caribou greenstone belt of the North Caribou Terrane and Shebandowan greenstone belt of the Wawa Subprovince. The metairon formations from the Beardmore/Geraldton and Lake St. Joseph greenstone belts are interpreted to have been deposited in a shallow water setting, while meta-iron formations from the North Caribou and Shebandowan greenstone belts are interpreted to be deposited in deeper water environments. This thesis also investigated element and oxygen ocean stratification by comparing the geochemistry of shallow and deep meta-iron formations. The main source of iron and silica to the oceans was hydrothermal venting fluids. Iron and silica precipitated out of seawater as iron oxyhydroxides and amorphous silica. Elements dissolved in the Archean ocean were adsorbed onto iron oxyhydroxides and silica during deposition. Crystallization of quartz, magnetite and hematite occurred during diagenesis and magnetite continued to grow during progressive metamorphism. The lack of cerium anomalies, absence of significant Y/Ho anomalies and deficiency of authigenic chromium supplied to the ancient suggests that the oceans were anoxic. Therefore, oxygen stratification did not occur between shallow and deeper water environments in the Archean. Significantly most of the elements were derived from multiple sources, including the siliciclastic phase, seawater or hydrothermal venting fluids, at various proportions. Al2O3, TiO2, Th, V, Nb, U, REEs and Y were determined immobile during post-depositional alteration. Mobility during diagenesis is clearly exhibited by sodium and potassium in the meta-iron formation samples from the Beardmore/Geraldton, Lake St. Joseph and North Caribou greenstone belts. Diagenetic modification mobilized sodium in the hematite-, jasper- and chert-dominated samples, while potassium was mobilized in the magnetite-dominated samples. Element stratification occurred in the Archean due to the source provenance. Deeper oceans were more enriched in Cs, Na2O, CaO, MnO, Cr and HREEs relative to shallow waters. Shallow oceans were more enriched in K2O, Rb and LREEs relative to deeper waters. This indicates that the Archean oceans were heterogeneous.