Date of Award
Secondary manganese oxides (MnOx) are common weathering products of polymetallic vein deposits, and are also locally abundant in streams draining abandoned mine lands. MnOx are scavengers of trace metals, and the trace metal composition of MnOx could potentially be used as a vector to locate mineral anomalies or point sources of pollution in mining-impacted streams. This study is examining the chemistry of MnOx in weathered rock samples and partially flooded underground mine tunnels in the Butte Cu-Ag-Pb-Zn deposit, as well as MnOx crusts on boulders in small streams draining the nearby Basin-Boulder Superfund Site, Montana. Data obtained using a portable XRF on MnOx deposits in situ are compared to analysis of the same samples after crushing, drying, and homogenization. These results show that direct analysis of mineral crusts in the field give inaccurate concentration data, although when expressed as metal ratios (e.g., Mn/Zn, Mn/Cu, Mn/Fe) the agreement is improved. Some of the factors that lead to inaccurate concentrations from direct field measurement include variable water content of the solids, sample inhomogeneity on a mm to cm scale, and, in some cases, penetration of the X-rays through the MnOx crusts to the rock beneath the crust. Comparison of XRF results of powdered samples to ICP analysis of acid-digests of the same samples shows generally good agreement for most of the elements of interest, both in terms of absolute concentration and when expressed as ratios.
MnOx crusts in the Butte district show a zonation in trace element makeup that crudely parallels the known metal zonation in the primary deposit. MnOx crusts in the Central and Intermediate zones are more enriched in Cu and As, whereas crusts from the Peripheral zone are more enriched in Zn, Pb, and Ag. Analyses of freshly forming MnOx and FeOx crusts in the Lexington Tunnel of Butte show that Cu and Pb are preferentially incorporated into the Ferich crusts whereas Zn, Cd, and thallium (Tl) are preferentially incorporated into the Mn-rich crust. In the Basin-Boulder field site, the distribution of trace elements in MnOx crusts varied considerably in the streams closest to the abandoned mine sites, where pH also varied due to localized acid mine drainage. However, the lower reaches of the streams with pH > 7 generally showed consistent trends of decreasing Mn/Zn ratio with distance downstream. The Mn/Zn ratio of the MnOx crusts was 10 to 100 times greater than the aqueous Mn/Zn ratio in the streams themselves. A mass balance model was developed that helps to explain the observed trends of decreasing Mn/Zn ratio with distance downstream, and the model may be applicable to other metal ratio systems. This approach may be useful to help locate sources of metal loading in watersheds due to weathering of natural or mining-related metal anomalies.
Jensen, Dustin, "Rapid Analysis of Secondary Manganese Oxides Using Portable XRF: Applications to Environmental Geochemistry and Mineral Exploration" (2015). Graduate Theses & Non-Theses. 18.