HEAVY MINERALOGY OF BEDLOAD SEDIMENT IN THE UPPER MISSISSIPPI RIVER AND THE INFLUENCE OF TRIBUTARIES

The minerals present in the bedload of streams reflect their upstream sources, particularly in the non-quartz fractions. This research was conducted in order to test the hypothesis that the heavy mineral assemblage (the non-quartz fractions) and chemistry in the bedload of large drainage systems vary according to upstream bedrock sources. Five samples were collected: two directly upstream from the confluence of the Wisconsin and Mississippi Rivers, and three locations downstream along a 113-km reach through the Driftless Area. Sediment was collected using a dredge. The 350µm and finer fraction was separated using lithium heteropolytungstate (LST) heavy liquid and was then analyzed using x-ray fluorescence spectroscopy (XRF), scanning electron microscope energy-dispersive x-ray spectroscopy (SEM-EDS), and x-ray mapping. SEM-EDS was repeated ~40 times per sample site to determine the mineralogy. X-ray mapping of sand particles helped discern mineral percentages. Geochemical results from XRF analysis indicate that Fe, Ca, Ti, Si, and Al are remarkably consistent, varying only ~5% in overall composition. In contrast, heavy mineral assemblages, identified using SEM-EDS, show different mineralogy between the Mississippi and Wisconsin Rivers. All samples included: ilmenite, hematite, tschermakite, actinolite, prehnite, chloritoid, and apatite. The Wisconsin River contained significant almandine, which was not found (nor any other garnet variety) in the downstream samples. Other minerals identified exclusively in the Mississippi River included: pumpellyite, staurolite, chamosite, rutile, and edenite. Further research will attempt to identify the bedrock provenance of some of the minerals.