AUTOMATED MINERALOGY TO CHARACTERIZE RESIDENCE OF LEAD IN FINE GRAINED SEDIMENT ALONG A RIVER IMPACTED BY HISTORIC LEAD MINING IN SOUTHEAST MISSOURI, USA

The Big River drains historic Pb, Zn, and barite mining districts in Southeast Missouri including the Old Lead Belt (OLB). Underground mining (1864-1972) resulted in large chat piles and tailings impoundments that are a source of widespread environmental impacts to the watershed. We are exploring scanning electron microscopy automated mineralogy techniques for quantification of Pb-bearing minerals that occur at trace concentrations in contaminated sediments to further understand controls on metal mobility and bioaccessibility.

Bed, suspended, and flood drape sediments were collected along 200 km of the Big River from above the OLB to near its mouth at the Meramec River. Fine-grained (<63 μm) material comprised less than ~1 wt. % of sampled riverbed sediments but dominated suspended and flood drape sediments. Lead concentrations were consistently highest in flood drape sediments and lowest in bulk (<2 mm) bed sediments. Concentrations were intermediate in fine-grained bed and suspended sediments. Fine-grained sediments collected within the OLB had Pb concentrations between 800 and 4,500 mg kg^-1 while those collected near the mouth of the river were lower but still of environmental concern (600 to 1,200 mg kg^-1).

Up to 280,000 grains from each sample were analyzed by automated mineralogy. At least 85 wt. % of grains in all samples were quartz, feldspar, and carbonates (dolomite, ankerite, calcite) and major mineral abundances correlated well with bulk chemistry. Carbonate minerals reflect the gangue that dominates tailings and account for up to 20 wt. % of some fine-grained sediments indicating the large extent of contamination. In contrast, up to 20 grains of galena (dominant lead ore mineral) and 74 grains of cerussite (secondary ore mineral and/or galena weathering product) were detected in each sample. These rare grains account for almost all Pb in some samples. We are currently refining sulfate mineral identification to quantify anglesite and plumbojarosite. Ideally, sorbed Pb can be estimated as the difference between Pb in quantified Pb-bearing minerals and bulk Pb. The ability to quantify low abundance minerals and readily locate them for various microanalytical techniques has promise for investigating mine tailings contamination and bioaccessiblity.