Geochemical exploration for copper-nickel deposits in the cool-humid climate of northeastern Minnesota

Abstract

Water was used as a medium for geochemical exploration to detect copper-nickel mineralization along the basal zone of the Duluth Complex. Ni 2+ is the most important pathfinder for the detection of the mineralized rocks, followed by Cu 2+ and SO 4 2− and to a lesser extent Mg 2+ and SiO 2. A normalized sum plot using these species defines the mineralization more consistently than a single-element plot, mainly because the absence of one variable does not significantly influence the normalized sum value. A hydrogeochemical survey was conducted in an area of known copper-nickel mineralization in the cool-humid climate of northeastern Minnesota. The area is covered with glacial drift, and wetlands are abundant. Modeling of the chemistry of waters indicates that the waters are oxidizing and have a pH of 7 or less. The most important pathfinder species in the waters, Cu 2+, Ni 2+, and SO 4 2−, are derived from the simple weathering of sulfide minerals and are mobile in the waters in this environment. Plots of Cu and Ni concentrations in soils show that Cu followed by Ni are the most useful indicator elements for delineating copper-nickel mineralization. The ability of soils and water to delineate the mineralization supports the use of both media for geochemical exploration in this cool-humid environment. In the wetlands, abundant water is available and soils are scarce or absent; where soils are abundant, waters are generally scarce or absent. The use of both media is recommended for geochemical exploration in this environment.