The determination of trace elements in Fe–Mn oxide coatings on pebbles using LA-ICP-MS

Iron–manganese oxide coatings form on a wide range of geologic samples where they have the ability to adsorb elements and potentially act as a mineral exploration/environmental monitoring tool. In this study, Fe–Mn oxide coatings on stream pebbles were collected from streams in four study areas located across the province of Newfoundland and Labrador, Canada. The study locations were in areas of former copper mines (Tilt Cove and Betts Cove), carbonate geology (Robinsons River), and a metropolitan area (Rennies River). Collected pebbles underwent a simple sample preparation procedure and were then analyzed for a wide range of elements by LA-ICP-MS after optimization of the operating conditions. Water samples accompanied the pebbles, and these were analyzed for pH, dissolved oxygen, conductivity, and a large selection of elements by ICP-MS. Multivariate statistics, in the form of Principal Component Factor Analysis (PCFA) was performed on both data sets. Graphs of the factor scores from the PCFA produced groupings of the samples that were related to geologic/environmental inputs. The loading of variables in each factor was related to the adsorption of the element either to the MnO₂ or Fe₂O₃ phase with most elements except Cr and Cu displaying preferential adsorption to MnO₂. Elemental Fe–Mn oxide coating concentrations were a result of the element's affinity (chalcophile, lithophile, or siderophile), pH of the environment, stream water concentration, and amount of each oxide phase present. Even with these complications, LA-ICP-MS analysis of Fe–Mn oxides was able to identify areas of heavy metal pollution and locate geologic inputs.