PALEOENVIRONMENTAL CONTROLS ON SYNGENETIC DEPOSITION OF PHOSPHORITES AND ASSOCIATED METALS IN THE PERMIAN PHOSPHORIA FORMATION, WESTERN UNITED STATES

The Phosphoria Rock Complex (PRC) contains one of the largest sedimentary phosphate deposits known. These Permian phosphorite-siltstone-chert-carbonate-evaporite successions were deposited in the Phosphoria Sea on the western margin of North America. The abundance of phosphate, organic matter and chert in the PRC has long been cited as evidence for marine upwelling. Although upwelling is critical to supply nutrient and metal-rich waters, the regional and stratigraphic distribution of phosphate, organic carbon, and syngenetic metals requires an understanding of the paleoceanography of this marginal sea. In fact, the stratigraphic and regional distribution of phosphorite, and metals, such as U, V, Cu, Zn, Cd, Mo, Ni and Cr, is independent of lithofacies. Phosphate was fixed in bottom sediments where phosphorus was delivered via organic matter; however, preservation of organic matter and deposition of metals was controlled by oxygen levels, both in the water column and in the sediment. To delineate syngenetic phosphorite-metal associations, the chemofacies classification of Hiatt (1997) was used: dysoxic (< 1.5 wt % TOC); anoxic (>1.5 TOC and TOC/TS >2); and euxinic (TOC > 1.5 wt% and TOC/TS <2). Stratigraphically, chemofacies show varying degrees of intercalation, except in inner ramp settings, which are marked exclusively by the dysoxic chemofacies. In general, the dysoxic facies dominates the inner ramp, whereas the anoxic chemofacies dominates both the outer ramp section and the mid-ramp sections. The euxinic facies is found only in a few beds of the mid-ramp settings. Phoshorite occurs in each chemofacies, but metals vary with paleoredox conditions. For example, Cd has a mean concentration of 13, 85, and 104 ppm while Cr only ranges from 395, 505, and 939 ppm in the dysoxic, anoxic and euxinic chemofacies of the Meade Peak member, respectively. The euxinic chemofacies represents extreme hypereutrophic, sulfate-reducing conditions where metals associated with sulfides and organic matter were concentrated. A tripartite classification based on phosphate, sulfide, or organic compound association will be presented that provides a means to predict relative metal concentration levels with major element composition, and can be applied to other syngenetic phosphorite-metal deposits.