Chapter 21 Western phosphate field - Depositional and economic deposit models

The Western Phosphate Field (WPF), composed of Permian marine sedimentary strata that cover over 300,000 km² in the middle Rocky Mountains of Idaho, Montana, Utah, and Wyoming in the United States, contains vast resources of phosphate mined for fertilizer and a range of other industrial applications. The richest deposits of phosphate in the WPF occur in the Meade Peak Phosphatic Shale Member of the Phosphoria Formation in southeast Idaho.

Phosphate is an essential and even limiting nutrient of algal production, which occurs at the bottom of the marine food web in the oceanic photic zone. The high concentrations of phosphate and trace elements in the Phosphoria Formation reflect a low accumulation rate of diluting phases, such as terrigenous siliciclastic debris and carbonate, rather than an unusually high level of primary productivity at the time of deposition. Indeed, the mean rate of accumulation of PO₄³ required a continuous flux of PO₄³ into the basin and the photic zone of the water column, but only at a moderate level. This flux was maintained by upwelling of nutrient-rich seawater, imported at depth from the open ocean. Although only a fraction of the organic matter that hosted the PO₄³ and other nutrients (NO₃, Cd, Cu, Mo, Ni, and Zn) actually escaped oxidation in the water column, their rate of accu- mulation on the sea floor defined the basin hydrography.

Rates of accumulation of Cr, U, V, and rare-earth elements by precipitation and adsorp- tion reactions identify redox conditions in the bottom water as having been denitrifying, maintained by a balance between the rate of oxidation of organic matter settling through the water column and the flux of open-ocean seawater at depth. Atmospheric mixing maintained oxygen respiration in the uppermost several tens of meters of the water column. This hydrography and seawater chemistry are present in several sedimentary envi- ronments in the ocean today.

In the WPF, there is an estimated surface mineable reserve base and subeconomic resource of 7.6 billion mt, at an average grade of 24% P₂O₅; a subeconomic underground- mineable resource of 17 billion mt, at a grade of 28%; and 507 billion mt of subresource- grade phosphatic material that underlie the WPF at a depth greater than 305 m. The relationship between phosphate-ore specifications and weathering suggests that significant changes in processing, with associated cost increases, will be required to extend recovery of ore below the relatively strongly weathered zone near the surface.

Four open pit mines currently extract phosphate from two moderately to steeply dipping ore zones that typically contain between 20% and 35% P₂O₅. Although the shales are enriched in trace elements, especially As, Cd, Cr, Cu, Mo, Se, U, V, Zn, and rare-earth elements, the relative concentration of organic carbon and selected major element oxides determines the suitability of phosphate-rich rock for feed to processing plants and its other applications. Selected specifications from the four operating mines include the following: minimum P₂O₅ of 18-20% and average of 26-27%; maximum A1₂O₃ of 1.6-5.0%; maximum MgO of 0.3-0.6%; a CaO/P₂O₅ ratio of 1.5-1.6; and total carbon content of 4%-5%. Weathering to a depth of as much as 100 m significantly enhances ore quality by decreasing the proportions of calcite, dolomite, and organic matter relative to carbonate fluorapatite, the primary ore mineral