MINERAL AFFINITIES AND DISTRIBUTION OF SELENIUM IN PHOSPHATIC SHALES OF THE MEADE PEAK MEMBER OF THE PERMIAN PHOSPHORIA FORMATION

The release of selenium to the environment from weathering of phosphate rocks has become an increasing concern following incidents of selenium toxicosis in livestock in the southeast Idaho phosphate-mining district. Although Se enrichment is to be expected in phosphorites, given the biogenic origins of P and micronutrients such as Se, there has been little conclusive evidence regarding Se residence in such deposits. Identification of host phases is a prerequisite to modeling weathering and release of Se from phosphate deposits to the environment.

Se residence in Meade Peak rocks was studied in variably weathered samples from three sections by sequential extractions, microprobe, and X-ray absorption spectroscopy (XAS) analyses. The results indicate that the largest fraction of Se in unweathered or minimally-weathered samples is associated with sulfides. Se concentrations > 2,000 ppm were measured in sphalerite, > 5,000 ppm in Cu-V sulfides (sulvanite?), and up to 2% in subhedral pyrite present within the matrix and as inclusions in phosphate pellets. Framboidal pyrite and masses of cryptocrystalline sulfides are ubiquitous in the Meade Peak, particularly within organic-rich matrix material, and may contain even higher concentrations of Se as well as other elements of environmental concern. Most of the non-sulfide Se is associated with organic residues and oxyhydroxides, although a small amount is present in elemental form.

XAS analyses indicate that Se(IV) is the dominant form of Se in more weathered samples, implying oxidation of primary organic and sulfide Se host phases. Although the actual time frame over which oxidation took place is not known, previous laboratory studies (e.g., Losi and Frankenberger, 1997) have shown that up to 60% of elemental and organic-bound Se in contaminated soils and sediments could be oxidized to Se(IV) and Se(VI) over periods of 2 to 900 days. However, in Meade Peak rocks, the occurrence of Se in phases that have variable oxidation rates and particle sizes spanning at least four orders of magnitude, and which may occur as inclusions within more-resistant phosphate pellets, suggest that the release of Se to the environment in southeast Idaho will be variable but long-term.