Simandl, et al., reported in 2015 on the general aspects of sedimentary phosphate deposits are stratiform or lens-shaped, measuring from less than 1 meter to tens of meters in thickness. They extend for tens to hundreds of kilometres in their longest dimension. Mineralized zones consist of phosphorites (≥18% P2O5) or phosphate rocks (<18% P2O5). These rocks are bedded. They may be primary or reworked (secondary). The main ore mineral is microcrystalline francolite, commonly in form of laminae, pellets, oolites, nodules and fragments of bones or shells. This mineral may be also present within the rock matrix.
TECTONIC SETTING(S)
The most favorable tectonic settings for larger deposits are passive continental shelves and adjacent sag basins; some sedimentary phosphate deposits formed at active continental margins, intracontinental basins and even lacustrine environments.
DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING
Deposition usually occurred in areas of warm paleoclimate, mostly between the 40th parallels. The most common depositional environment for sedimentary phosphate deposits is a marine sedimentary basin with a good connection to the open sea (commonly west-facing at the time of phosphate deposition), and upwelling areas with high plankton productivity.
AGE OF MINERALIZATION
Deposits range in age from Proterozoic to Holocene. Phosphate deposits are particularly abundant in Cambrian, Permian, Jurassic, Cretaceous, Eocene and Miocene times (Cook and McElhinny, 1979). In terms of inferred resources (tonnage), the Eocene, Miocene and Permian are the most important time intervals. In British Columbia, the majority of phosphate occurrences are located in rocks of Jurassic and Triassic age. See Geological Fieldwork 2011, Paper 2012-1 217
HOST / ASSOCIATED ROCK TYPES
Host rocks are phosphorites (≥18% P2O5) and phosphate rocks (<18% P2O5). Associated rock types are typically sedimentary rocks including marl, black shale, chert, limestone, dolostone, and in some cases lava flows, tuffs and diatomite-bearing rocks. Figure 2 shows conceptual vertical section of the platform perpendicular to the shoreline.
DEPOSIT FORM
Phosphate-bearing rocks are generally stratiform; bed thicknesses range from less than 1 meter to tens of metres and may extend for distances up to several hundreds of kilometres in their longest dimension. The thickest deposits are amalgamated/condensed beds (tabular units) reflecting variations in upwelling intensity and storm frequency through time. Individual phosphorite deposits delimited by drilling may measure from a few hundreds of metres to tens of kilometres in their longest dimension. Phosphorite deposits commonly occur in belts.
TEXTURE / STRUCTURE
Phosphorite deposits commonly contain phosphate pellets and nodules as well as phosphatized shells and bones; ooids, intraformational rip-up breccias, clasts, concretions, phosphatic stromatolite mounds (or their fragments), crossbeds, hardgrounds and burrows. Phosphatic minerals may also form the matrix. Gangue mineralogy and textures in phosphorites are determined partially by the depositional environment at the time of phosphogenesis and partially by the environment that prevailed during reworking and winnowing (natural P2O5 upgrading) of the original phosphate rocks and during diagenesis. Phosphorites formed by replacement (during diagenesis) may be paler buff, tan and/or macroscopically featureless.
“Pristine” phosphate accumulations (one cycle of phosphogenesis) are characterized by phosphatized laminae and lenses, coated grains, coprolites, peloids and fossils. Allochthonous phosphate accumulations show sharp and erosive lower boundary, assemblages of phosphatic and non-phosphatic particles and internal grading, accreted grains, minor unconformities, scour marks and bed truncations, and heterogeneous phosphate particles. Condensed phosphates represent an intermediate stage between pristine and allochthonous phosphate deposits (Föllmi, 1996).
ORE MINERALOGY (Principal and subordinate)
Francolite (carbonate-rich fluorapatite), secondary minerals derived by the weathering of francolite: millisite, Fe-pallite, crandalite, wavellite and other Al-phosphates. Secondary minerals are not desirable from the metallurgical point of view.
GANGUE MINERALOGY (Principal and subordinate)
Dolomite, calcite, quartz, montmorillonite or illite±chert, ±halite, ±gypsum, ±iron oxides, ±siderite, ±pyrite, ±carnotite*, ±glauconite,±sphalerite, ±zeolites.
*Carnotite is a potassium uranium vanadate radioactive mineral with chemical formula: K2(UO2)2(VO4)2•3H2O. It is commonly considered as a gangue mineral; however, if present in high concentrations it becomes an ore mineral.
ALTERATION MINERALOGY
Dahllite is believed to form during late diagenesis (Trappe, 1998).
WEATHERING
Lateritic alteration of francolite results in the formation of millisite, Fe-pallite, crandalite, wavellite and other aluminum-phosphates. Turquoise may form if copper is present. Weathering decreases concentrations of pyrite and sphalerite and may result in the release of selenium.
ORE CONTROLS
Phosphorites are stratigraphically and spatially linked to paleodepositional environments favorable for phosphogenesis (high bio-productivity and phosphorus flux, stratification within water/unconsolidated sediment column, and a moderate to low supply of allogenic sediment). Phosphorite deposits are spatially related to multiple cycles of regression-transgression. Phosphate facies commonly rest on, or are associated with, erosional surfaces (unconformities) and/or start with phosphatic lag concentrates. Entrapment basins (zones) characterized by a low influx of continent-derived sediments are required for the deposition of phosphorites.
A case history of phosphate in Tunisia: http://www.i2mconsultants.com/downloads/corp/2010fibreux-Tlili.pdf
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