Mineralogical characterization of weathered and less weathered strata of the Meade Peak phosphatic shale member of the Permian Phosphoria Formation: Measured sections E and F, Rasmussen Ridge, and measured sections G and H, Sage Creek area of the Webster Range, Caribou County, Idaho

Abstract

The Permian Phosphoria Formation of the western U.S. includes one of the largest phosphate deposits in the world. Despite the economic significance of this formation, its fine-grained nature has discouraged detailed mineralogical characterization and quantitative studies. Recently, selenium and other potentially hazardous trace elements in mine wastes have drawn increased attention to these rocks and motivated more extensive study. Part of this effort has focused on a more detailed geological and mineralogical characterization of the rocks. This study uses powder X-ray diffraction (XRD) with Rietveld quantification software to characterize the mineralogy of channel samples from stratigraphic sections measured by the U.S. Geological Survey in the Meade Peak Phosphatic Shale Member of the Permian Phosphoria Formation. Measured sections are at the Rasmussen Ridge mine and at the Smoky Canyon mine approximately 15 and 25 miles, respectively, northeast of Soda Springs, Idaho. The dominant minerals present in these samples are carbonate-fluorapatite, which is the most common phosphatic ore mineral in this and other marine phosphorites, quartz, muscovite, albite, orthoclase, the ammonium feldspar buddingtonite (NH4AlSi3O8), dolomite, and calcite. Because of their potential for hosting trace elements such as Se, the presence of minor pyrite and sphalerite is also noteworthy. Analysis of the carbonate content in the carbonate-fluorapatite by Rietveld refinement shows relatively low carbonate contents, generally between 2 ? 3% (wt.) CO3 2- in the apatite structure, compared to other marine phosphorites.