EFFECTS OF MINING THE MISSISSIPPI VALLEY-TYPE DEPOSITS IN THE OZARK REGION ON HEAVY-METAL CONCENTRATIONS AS INFERRED FROM GEOCHEMICAL AND LEAD ISOTOPE DATA

The Ozark region of the U.S. midcontinent is one of the world’s most important provinces of MVT mineralization, hosting world-class ore deposits. The late Permian ores in the Tri-State (TS) and Northern Arkansas (NA) mining districts, dominated by sphalerite, are mainly hosted by platform carbonate rocks and vary in age from Ordovician and Mississippian for NA and Mississippian for TS. The TS mining region was designated as a Superfund site in 1983.

Current geochemical studies of ore samples from the TS and NA districts and of stream-sediment samples collected from selected sites from the mid-Arkansas River (the Neosho and Spring Rivers, and Tar Creek) and upper White River (Crooked Creek, W and E Sugarloaf Creeks) drainage basins show that the effect of mining activity on Pb concentrations in the river can be readily documented. Lead isotopic ratios of leaches of stream sediments from the Tar Creek collected downstream from Picher, OK, correlate well with those of ore samples from the Tri-State Mining District, suggesting a common source. Lead isotope measurements of leaches delimit the extent of high Pb, Zn, and As concentrations in the Tar Creek caused by mineralization at Picher, OK. The effects of mining are noticed to a lesser extent in the Spring and Neosho rivers. Leaches of stream-sediment samples collected from the above-mentioned two rivers show lower Pb isotopic ratios than the Tar Creek leaches and plot outside the field defined by ore samples from the Tri-State Mining District. This suggests a different source of Pb, possibly from other Zn-Pb mining districts located near-by or anthropogenic Pb. Further research is required to fingerprint this source.

There is a gradual decrease in the Zn, Pb, As, and other heavy-metals concentrations in the stream sediment leaches with distance from the mines. The Zn concentration reaches a maximum of 1592 ppm in the Picher area, about 20 times normal crustal abundance. About 15 miles downstream from Picher, the Zn concentrations decrease substantially to values lower than normal crustal abundance. Similarly, the Pb concentration is highest in the Picher area (326 ppm), about 30 times normal crustal abundance, and it drops to much lower values downstream. Even though the highest As concentration is seen in the Picher area (1.44 ppm), it is lower than normal crustal abundance.