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.
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