Eskers and Heavy Mineral Prospecting, Northeastern Minnesota

Abstract

Mineral exploration in heavily glaciated terrain is difficult because a mantle of glacial drift generally covers the bedrock. Eskers and the heavy minerals they contain were studied to (1) determine their potential use as a prospecting tool, and (2) better understand the dynamics of esker formation. Studies emphasized paleohydraulics, origin of esker sediment, lithology, and mineralogy. 'IWo main types of eskers are recognized in northeastern Minnesota on the basis of their morphology, sedimentology, and origin. Continuous eskers form as a single, continuous segment when an ice sheet is stagnant, or nearly stagnant. Meltwater flowing through an ice tunnel deposits sediment in both the tunnel and onto an adjacent outwash plain. Melting of the tunnel walls induces inflow of the ambient ice which leads to the influx of rock debris. Seasonal fluctuations of meltwater discharge leave a sedimentary sequence characterized by a wide variety of grain sizes (medium-grained sand to boulders) and bedforms (climbing ripples, massive beds, and planar, trough, and graded cross beds). Be2ded eskers are a series of consecutive segments, or beads, that are separated by swamps or lakes, and whose trends follow the low ground between the drumlins of the Toimi Drumlin Field. A limited range of bedforms (massive beds and trough cross beds) and grain sizes (mostly bouuders) are exposed in the beaded eskers. Other sedimentary features may be present, but they are covered by the surrounding swamp deposits and not exposed. Individual segments of the beaded esker system probably form at the margin of an actively retreating ice sheet as an annual deposition event. Average distances between successive bead crests imply an annual retreat rate of 1200 feet (365 m) per year. Stone counts, X-ray diffraction, and petrographic examinations of heavy mineral mounts were evaluated as drift prospecting tools. Similar results between stone counts in eskers and in the surrounding till indicate that the esker material is most likely derived from the basal ice debris. Concentration of the heavy mineral population by fluvial processes makes eskers and outwash better locations for sampling than till. A small split from each of the 97 bulk samples was analyzed by X-ray diffraction. As no more than 5 or 6 minerals could be identified from the charts, it does not appear to be an efficient method for prospecting. In contrast, petrographic examination of heavy mineral mounts gave a much better indication of minerals present. However, no economic indicator minerals were identified with either method. The petrographic studies were useful in differentiating glaciofluvial material deposited by the various lobes of ice. The St. Louis sublobe deposits, with a higher percentage of garnets, can be distinguished from the Rainy lobe and Superior lobe deposits, whose heavy mineral populations reflect the mafic bedrock over which the ice moved.