A mineralogical investigation of the Permian Phosphoria Formation, southeastern Idaho: Characterization, environmental concerns, and weathering

The Permian Phosphoria Formation of southeastern Idaho is one of the world's largest marine phosphorites, containing six times more phosphate than the total budget in today's oceans. Subsequently, phosphate mining is the largest mineral industry in the state of Idaho. The Phosphoria Formation's size makes it interesting scientifically and important economically. Recently, however, selenium and other potentially hazardous trace elements in mine wastes have drawn attention to the environmental concerns of mining these rocks. In an effort to better understand these rocks, and the related environmental threat, work has been directed to produce a more detailed geological characterization, including a mineralogical quantification and characterization using the Rietveld method. The U.S. Geological Survey collected 475 samples from nine stratigraphic sections from four active phosphate mines in southeastern Idaho. Seven of these sections can be considered in a “least-,” “less-,” “more-weathered” series analyzed to consider the effects of weathering on the mineralogy, geology, and the environmental issues.; Mineral characterization included both quantification of the major minerals and determination of the amount of CO₃2− substituted for PO₄3− in the carbonate-fluorapatite (CFA) structure. The carbonate substitution in CFA has long been the source of both academic debate and curiosity, particularly the question of what controls the carbonate substitution. Contrasting theories attribute variability in the CO₃2− content to either deposition, or post-depositional forces such as weathering and diagenesis. Our results show a clear link between the measured carbonate content and lithology, however there is also the suggestion that weathering may destabilize the CFA structure, and result in a non-diffracting phosphate component.; The quantified mineralogical data has been combined with analytical chemical data collected by the U.S.G.S. for the samples in order to better understand the nature of Se residency. Statistical analyses, including factor analysis, is used to search for relationships between Se residency, mineralogy, geochemistry, stratigraphy, and lithology. The geochemical diversity, and generally low concentrations (ppm range) make Se difficult to trace directly. However, statistical analyses, particularly factor analysis, gives clues to some of the many underlying forces which determined where Se was originally deposited, and how and to where it was subsequently mobilized.