RAPID ANALYSIS OF SECONDARY MANGANESE OXIDES USING PORTABLE XRF: APPLICATIONS TO ENVIRONMENTAL GEOCHEMISTRY AND MINERAL EXPLORATION

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 in situ are compared to analysis of the same samples after crushing and homogenization. Multiple readings are taken to assess homogeneity. The XRF results are then compared to ICP analyses of acid digests of the same MnOx crusts and/or powders.

Whereas the agreement between the absolute concentrations of metals and metalloids (e.g., Mn, Fe, Cu, Zn, As) determined by XRF (in situ or powdered) and ICP is highly variable, the agreement is much improved when the data are expressed as metal ratios, such as Mn/Zn, Mn/Cu, and Mn/Fe. The environmental samples show systematic changes in metal ratio with location in many of the data sets that are a function of several variables, including proximity to mineralization and changes in stream/mine water chemistry. Mn/Cu and Mn/Fe ratios are positively correlated whereas Mn/Zn and Mn/Fe are negatively correlated, consistent with copper’s greater affinity to adsorb and/or coprecipitate with FeOx compared to MnOx, and v. versa for zinc. In partly flooded mine portals, MnOx and FeOx are often delicately interstratified, indicating subtle changes in mine pool chemistry. In streams, MnOx is typically found on the bottom of boulders in the stream bed, whereas FeOx is more abundant on the tops. The thickness of MnOx crusts themselves can be highly irregular and often the movement of boulders in the stream beds in addition to shifting point sources of AMD pollution over time can add to this variability. Partition coefficients for Mn/Zn, Mn/Cu between the aqueous and MnOx phases are being determined based on our empirical data, and will be compared to published experimental data.