Chemistry and mineralogy of Precambrian paleosols at Elliot Lake, Ontario, Canada

Abstract

Two profiles from the paleosol underlying the Huronian Supergroup at Elliot Lake, Ontario were studied to determine conditions present at the time of their formation. (1) One paleosol profile (> 10.5 m thick), developed on greenstone, is exposed within the Denison Mine. It is tentatively classified as an intrazonal gley soil formed in an area of poor drainage and reducing groundwater conditions. It is characterized by extensive loss of Fe, Mn, Cu, Ni, and Mo, as well as Na, Ca, and Mg. An increase in K is attributed to diagenetic K metasomatism. Paleosol structures indicate clay translocation forming an illuvial B_g horizon and suggests a fluctuating water table to depths of 6m. (2) The second paleosol profile (> 5.5 m thick), developed on granite, is exposed near the Pronto Mine. This profile exhibits iron oxidation and enrichment. This indicates that free oxygen was present in the mid-Precambrian atmosphere, although a precise determination of pO₂ is not possible from evidence from the paleosols. This paleosol exhibits many characteristics of modern spodosols. The soils probably formed in a temperate humid climate during tectonically stable conditions. The difference in oxidation and behavior of iron in these soils is inferred to result from differences in paleotopography and drainage. The preponderance of gley paleosols found underlying the Huronian Supergroup might result either from lower mid-Precambrian pO₂ or from preferential preservation. Evidence suggests that by 2.3 Ga/ago, soil forming processes were capable of producing soils similar to Recent types.