Characterization of heavy minerals in the Superior, Green Bay, and Lake Michigan glacial lobes and reconnaissance kimberlitic garnet search in northern Wisconsin

Glacial till has been sampled all along the late Wisconsinan glacial margin in Wisconsin and Illinois. Heavy minerals extracted from the fine sand fractions of all till samples were grain counted to examine potential differences in the heavy mineral suites between and within the Superior, Green Bay, and Lake Michigan Lobes. Results of this work indicate that the three major glacial lobes can be distinguished by their heavy mineral populations. Heavy mineral suites in both the Superior and Green Bay Lobe tills contain large amounts of pyroxene and magnetic minerals and small amounts of garnet and epidote group minerals, whereas Lake Michigan Lobe tills contain significantly less pyroxene and magnetic minerals and relatively large amounts of garnet, epidote group minerals, accessory minerals (zircon, rutile, tourmaline, sphene, and monazite), and hornblende. Unusually high abundances of opaque heavy minerals in the Superior Lobe tills serve to separate them from all Green Bay Lobe tills. Differences in the abundances of various heavy minerals also exist within each glacial lobe, but these differences are much smaller than differences between the major lobes.; Heavy minerals were also extracted from the combined fine and medium sand fractions of eight samples of glaciofluvial sediment collected along the late Wisconsinan margin of the Superior and northern Green Bay Lobes. Potential kimberlitic garnets as well as regional garnets representative of each color group present then were selected and their chemistries determined by electron microprobe analysis. One garnet picked from northern Green Bay Lobe outwash was found to be chemically similar to G5 (magnesian almandine) garnets found in kimberlite and lamproite, but a possible crustal source can not be ruled out. No other possible kimberlitic garnets were found in this limited reconnaissance study. The regional garnets divided chemically into two distinct groups: iron-rich purple and pink garnets, and red and orange garnets containing larger amounts of manganese, calcium, or both.