U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting Charleston South Carolina March 8-12,1999--Volume 1 of 3--Contamination From Hard-Rock Mining, Water-Resources Investigation Report 99-4018A

Partitioning of Trace Metals Between Contaminated Stream Waters and Manganese Oxide Minerals, Pinal Creek (AZ)

By Jill E. Best, Katherine E. Geiger, and Peggy A. O'Day

ABSTRACT

Copper mining activities near a perennial stream in central Arizona have produced high trace metal concentrations in addition to cobalt, copper, nickel, and zinc, in solution and ubiquitous precipitation of manganese oxide minerals as coatings on stream sediments. Analyses of coating samples at two sites along the stream indicate bulk manganese concentrations of 7.2 to 29.4 wt. %. At the upstream site (R2b), manganese minerals (identified by XRD) are primarily birnessite and rancieite with minor amounts of rhodochrosite, pyrolusite, cryptomelane, and franklinite. At a site approximately 5 kilometers downstream (9272a), the manganese mineral assemblage is dominantly pyrolusite with smaller amounts of todorokite, birnessite, rhodochrosite, and kutnahorite. Maps of elemental distributions in coatings from site R2b (using SIMS and LA-ICP-MS) show strong correlations of cobalt to manganese and correlation of copper to both manganese and iron. Samples from site 9272a show that trace metal concentrations relative to total manganese are lower than at site R2b and that calcium and iron are associated with manganese. Trace metal distributions in samples from R2b and 9272a are attributed to cation substitutions in birnessite, rancieite, and todorokite. Activities of dissolved metals decrease downstream as pH increases and metals are removed by precipitation. Thermodynamic equilibrium between stream waters and end-member manganese oxides predicts mineralogical conversion from metastable birnessite and todorokite to stable pyrolusite, consistent with observations of the stream sediments. Manganese phases may persist metastably by substitution of trace metals into the mineral structure. Conversion to thermodynamically stable manganese oxide minerals may exclude trace metals and potentially remobilized them.