Western US Phosphate Field - Rasmussen Ridge, Vernal, Smoky Canyon, Dry Valley, Enoch Valley, Gay


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The phosphorite-bearing Phosphoria Formation was deposited over an area in excess of 350 000 sq. km in southwestern Montana, eastern Idaho, northern Utah and western Wyoming in what constitutes the Western US Phosphate Field. From 1906 to 2005, ~229 Mt of marketable rock phosphate have been extracted from 70 mines in the four states, 49 of which were underground, 17 open pit only and 7 employed both types of mining. Idaho was the most significant producer, followed by Montana, Utah and Wyoming. By 1905, production was only in Idaho and Utah at a rate of >5 Mt per annum.

The mines operating, or under development over the last 10 years from 2002 include: Smoky Canyon, Dry Valley, Enoch Valley, Manning Creek, Deer Creek, Sulfur Canyon, Trail Creek and Rasmussen Ridge, all located near Soda Springs in southeast Idaho; Vernal (Uintah County) in northeastern Utah. Important historic mines include the Waterloo mine near Montpelier, the Conda mine and Gay open pits in the Soda Springs district of Idaho.

The Soda Springs district in Idaho is to the east and northeast of the town of Soda Springs, and covers an area of ~25 km radius, centred on #Location: 42° 45'N, 111° 20'W, while Vernal in Utah is at #Location: 40° 39'N, 109° 25'W.

Deposition of the Phosphoria Formation spans one of the most extreme global transitions in the geologic record. The Early Permian experienced widespread glaciation in both hemispheres, and low CO2 levels. In contrast, the Late Permian coincided with the onset of hot and dry climates over most of the middle and low-latitude continents, rapidly increasing CO2 levels (2 to 5 x modern values), widespread desertification accompanied by vast areas of evaporite deposition, and widespread anoxic deep-water conditions in the world's oceans. At the same time boreal forests, coal swamps and reptiles proliferated to within 10° of the South Pole (Hiatt and Budd, 2003 and references quoted therein).

During the Permian the coast of western North America was largely a passive margin. Deposition of the Permian Phosphoria Formation accumulated in the "Phosphoria Sea", which formed in a shallow marginal foreland basin/embayment, centred in eastern Idaho, over an interval of ~900 km along the western margin of North America at a palaeolatitude of about 20°N. The shallow shelf environment of the Phosphoria Sea was partially separated from the Panthalassa Ocean to the west by a string of islands along the margin of the steep continental slope into that ocean. To the east of the island chain, the sea was characterised by a wide (>500 km), shallow water eastern margin, and a wide coastal plain with sand dune fields and common evaporite basins (Hiatt and Budd, 2003).

Hiatt and Budd (2003) suggest that an oxygen-poor, nutrient-rich intermediate water mass was upwelled along a steep continental slope from the anoxic ocean, and flowed into the Phosphoria embayment where it impinged on the mid-ramp area. Seasonal coastal upwelling brought this water to the surface, where it mixed with warm waters flowing seaward from the restricted shallow lagoons to the east, resulting in high palaeoproductivity, organic matter accumulation and oxygen depletion in the water column. The Phosphoria Formation phosphorites formed in outer ramp (<200 m water depth), organically productive mid-ramp, and very shallow and restricted inner-ramp environments. Large amounts of organic matter accumulated on the seafloor where primary productivity was extreme, and anoxic conditions within the sediments and bacterial breakdown of the organic matter released organic-bound phosphorous into the sediments, which in turn led to phosphogenesis.

The Phosphoria Formation is composed of three unconformity bounded stratigraphic sequences. Phosphate (as francolite) occurs throughout all three sequences, but phosphorite beds with >10% francolite are concentrated in the Meade Peak and Retort members of the Phosphoria Formation in the upper two sequences. Both members mark the initial stages of a transgression, unconformably overlying carbonates of the previous cycle, and grading upward into deep-water cherts which, in turn pass eastward towards the shoreline into carbonates and then shales (Hiatt and Budd, 2003).

The phosphorites within these two members primarily occur as phosphatic peloids, with phosphatic ooids and intraclasts, and less common phosphatised skeletal grains. They are often characterised by grain-supported textures, grading, and contain mechanically abraded grains, all of which suggest mechanical reworking in relatively shallow-water subtidal to peritidal depositional environments (Hiatt and Budd, 2003).

The Meade Peak member is the larger of the two high-productivity phosphatic units in the Phosphoria Formation. Deposition of this member was initiated in the Late Leonardian (~275 Ma) and extended through the Roadian stage (to ~268 Ma) of the Guadalupian (i.e., during the mid-Permian). Sedimentation and phosphogenesis occurred on a nearly flat ramp with an estimated shelf depositional angle of <0.25° between the Meade Peak depocentre and the palaeoshoreline. The Meade Peak is a seaward-thickening wedge of sediments whose depocentre was in southeastern Idaho (Hiatt and Budd, 2003).

Outer- and mid-ramp facies of the Meade Peak member include thinly bedded, finely laminated fine- to medium-grained sandstone, dolomitic siltstone and carbonate mudstone, with rare true shales. To the east, it pinches out into green siltstones and carbonates, which, in turn, grade into red beds and eventually interbedded red siltstones and evaporites in central Wyoming (Hiatt and Budd, 2003).

The Meade Peak member unconformably overlies dolostones and sandy dolostones of the Lower Permian Grandeur Formation, a very fine-grained, medium to thick-bedded, white weathering, light to dark-grey dolomite and sandy dolomite, with bioclastic facies low in the sequence. In the areas mined, the Meade Peak member itself is typically composed of siltstone, phosphorite and claystone, and is very thin-bedded and laminated, containing well sorted phosphate peloids and ooliths. Many beds are carbonaceous, nodular or bioclastic (bivalves and gastropods). Phosphorites are defined as containing >50% phosphatic minerals (carbonate fluorapatite) in the form of peloids, nodules, phosphatic bioclasts or other phosphatic allochems. Much of the unit is classified as 'phosphate rock' with >16% P
2O5 (Moore and Hoveland, 1990).

The Meade Peak member is symmetrical, with economic upper and lower phosphate-rich zones. In the main depocentre in the Soda Springs district of eastern Idaho, it is typically ~50 m thick, with a Lower Phosphate interval of ~12.5 m, composed of three phosphate rock intervals, separated by two 1.5 to 2 m thick waste bands of claystone and siltstone, which in some localities include thin-bedded, non-fossiliferous, organic carbon–rich silty dolomite mudstones and siltstones. The Upper Phosphate interval is generally a single ~7.5 m thick band of phosphate rock without significant waste interbands, and is near the top of the member, overlain by several metres of upper waste. The Upper and Lower Phosphate intervals are separated by ~24 m of 'waste' to low grade claystone and siltstone with minor phosphatic bands. Overall the two phosphate intervals comprise a cumulative thickness of ~15 m of ~27.2% P
2O5 (Moore and Hoveland, 1990).

In addtion to its P
2O5, the criteria for a phosphorite being economically recoverable include (i) a CaO:P2O5 ratio of <1.55; (ii)  Fe2O3 + Al2O3 content of < 5%; and (iii) MgO of <1.5%. In general, the more unweathered the phosphate rock, the less likely it is to be ore, as leaching associated with the weathering process enhances the ore characteristics, as well as removing excess organic matter which is deleterious to processing. In the mined areas of Idaho, the Meade Peak member dips at 45 to 70°. Vanadium is a by-product of most mines, with average contents in southern Idaho of 800 ppm V2O5 (Moore and Hoveland, 1990).

In the more landward Vernal-Brush Creek section of the basin in Utah to the SE, the Meade Peak member is only ~7 m thick, and is grey to tan in colour throughout with phosphorite beds comprising 60% of the section that are individually only a few tens of cm thick. None of these phosphorites show any visible evidence of cross bedding or fining-upward textures. All are planar bedded and consist of either well-sorted fine- to medium-grained phosphatic peloids or mixtures of peloids and small intraclasts. Intercalated lithologies are thin to very thin beds of dolomitic phosphatic wackestone and silty dolomite. Ore minerals are collophane (the most abundant), dahllite and francolite. Collophane occurs as 75 to 500, averaging 150 µm pellets cemented by calcite and dolomite with some chert, clay and massive collophane. Local fracturing has promoted weathering, with the concomitant oxidation of the minor sulphide content promoting breakdown of the carbonate cement. The phosphatic beds assay from 16 to 22% P
2O5 and 1.9 to 3.3% MgO (Zhang et al., 2006).

The Rex Chert member gradationally overlies the Meade Peak member in much of the basin, and is a thin-bedded black and grey chert in its lower section, which gives way up section to thick-bedded massive grey chert. It is predominantly a spicularite, composed of granular and mosaic quartz and sponge spicules, with various but minor amounts of other fossils and detrital grains. The cherty shale member and transition rocks between the Meade Peak and Rex Chert are siliceous siltstones and argillaceous cherts with ghosts of sponge spicules and somewhat more detrital grains than the chert. It passes eastward into carbonate facies onlapping onto basement.

Published reserves and resources include:
    Resources within the Western Phosphate Field (Northolt et al., 1989) - 7.6 Gt @ 24% P
    Reserves within the Western Phosphate Field (Zhang et al., 2006) - 1.6 Gt @ 24% P
    Resources, Southeast Idaho and Montana (USGS, 2002) - 9.8 Gt @ 27% P
    Reserves, Southeast Idaho and Montana (USGS, 2002) - 1.038 Gt @ 24.5% P
    Resources, Vernal mine (USGS, 2002) - 640 Mt @ 20% P

The most recent source geological information used to prepare this summary was dated: 2012.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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