Formation and Diagenesis of a Sub-Huronian Saprolith: Comparison with a Modern Weathering Profile

An Early Proterozoic paleosol that preserves Archean granite protolith textures occurs beneath the Lorrain Formation in the Early Proterozoic Huronian Supergroup near Ville Marie, Québec. Textural, mineralogical, and geochemical comparisons between this "saprolith" and a modern weathering profile developed on the Devonian Toorongo granodiorite in eastern Australia indicate similarities that can be attributed to the effects of primary weathering reactions. The comparison also reveals anomalous features at Ville Marie that are caused by diagenetic reactions between the weathered granite and meteoric waters. K-metasomatism, in the form of sericite replacement of kandites, is ubiquitous in the Ville Marie saprolith zone and is ascribed to reaction of low Na/K meteoric waters with the weathered profile as they moved along a relatively permeable unconformity surface. Earlier diagenetic reactions may have produced calcite alteration occurring in a thin zone at the base of the profile, thought to represent the position of the paleowater Table. Elevated Fe³⁺ /Fe²⁺ in the saprolith zone suggests that weathering fluids were oxidizing, although the total iron content of the granite protolith is relatively low (1.36%). An increase in total iron at the top of the Ville Marie profile may be due, in part, to groundwater addition. A shortcoming of many studies of Precambrian paleosols is that they do not adequately address the significance of these diagenetic and metamorphic effects, particularly as they pertain to observed geochemical trends and assumptions concerning the composition of the earth's early atmosphere.