Phosphorite


Phosphorite, phosphate rock or rock phosphate is a non-detrital sedimentary rock that contains high amounts of phosphate minerals. The phosphate content of phosphorite varies greatly, from 4% to 20% phosphorus pentoxide. Marketed phosphate rock is enriched to at least 28%, often more than 30% P2O5. This occurs through washing, screening, deliming, magnetic separation or flotation. By comparison, the average phosphorus content of sedimentary rocks is less than 0.2%.
The phosphate is present as fluorapatite Ca53F typically in cryptocrystalline masses referred to as collophane-sedimentary apatite deposits of uncertain origin. It is also present as hydroxyapatite Ca53OH or Ca1062, which is often dissolved from vertebrate bones and teeth. In contrast, fluorapatite can originate from hydrothermal veins. Other sources also include chemically dissolved phosphate minerals from igneous and metamorphic rocks. Phosphorite deposits often occur in extensive layers, which cumulatively cover tens of thousands of square kilometres of the Earth's crust.
Limestones and mudstones are common phosphate-bearing rocks. Phosphate-rich sedimentary rocks can occur in dark brown to black beds, ranging from centimeter-sized laminae to beds that are several meters thick. Although these thick beds can exist, they are rarely composed only of phosphatic sedimentary rocks. Phosphatic sedimentary rocks are commonly accompanied by or interbedded with shales, cherts, limestone, dolomites and sometimes sandstone. These layers contain the same textures and structures as fine-grained limestones. They may represent diagenetic replacements of carbonate minerals by phosphates. They also can be composed of peloids, ooids, fossils, and clasts that are made up of apatite. Some phosphorites are very small and have no distinctive granular textures. This means that their textures are similar to that of collophane, or fine micrite-like texture. Phosphatic grains may be accompanied by organic matter, clay minerals, silt-sized detrital grains, and pyrite. Peloidal or pelletal phosphorites occur normally, whereas oolitic phosphorites are not common.
Phosphorites are known from Proterozoic banded iron formations in Australia, but are more common from Paleozoic and Cenozoic sediments. The Permian Phosphoria Formation of the western United States represents some 15 million years of sedimentation. It reaches a thickness of 420 metres and covers an area of 350,000 km2. Commercially mined phosphorites occur in France, Belgium, Spain, Morocco, Tunisia, Saudi Arabia and Algeria. In the United States phosphorites have been mined in Florida, Tennessee, Wyoming, Utah, Idaho and Kansas.

Classification of phosphatic sedimentary rocks

Pristine: Phosphates that are in pristine conditions have not undergone bioturbation. In other words, the word pristine is used when phosphatic sediment, phosphatized stromatolites and phosphate hardgrounds have not been disturbed.
Condensed: Phosphatic particles, laminae and beds are considered condensed when they have been concentrated. The extraction and reworking processes of phosphatic particles, as well as bioturbation, help this process.
Allochthonous: Phosphatic particles that were moved by turbulent or gravity-driven flows and deposited by these flows.

Phosphorus cycle, formation and accumulation

The heaviest accumulation of phosphorus is mainly on the ocean floor. Phosphorus accumulation occurs from atmospheric precipitation, dust, glacial runoff, cosmic activity, underground hydrothermal volcanic activity, and deposition of organic material. The primary source of dissolved phosphorus is from continental weathering, which is carried by rivers to the ocean. It is then processed by both micro- and macro-organisms. Diatomaceous plankton, phytoplankton, and zooplankton process and dissolve phosphorus in the water. The bones and teeth of certain fish absorb phosphorus and are later deposited and buried in the marine sediment.
Depending on the pH and salinity levels of the ocean water, organic matter will decay, releasing phosphorus from sediment in shallow basins. Bacteria and enzymes dissolve organic matter on the water–bottom interface, thus returning phosphorus to the beginning of its biogenic cycle. Mineralization of organic matter can also cause the release of phosphorus back into the ocean water.

Depositional environments

Phosphates are known to be deposited in a wide range of depositional environments. Commonly, phosphates are deposited in very shallow, near-shore marine or low-energy environments. This includes environments such as supratidal zones, littoral or intertidal zones, and most importantly, estuarine. Currently, areas of oceanic upwelling cause the formation of phosphates. This is due to the constant influx of phosphate from the large, deep ocean reservoir. This cycle allows the continuous growth of organisms.
Supratidal zones: Supratidal environments are part of the tidal flat system where the presence of vigorous wave activity is non-existent. Tidal flat systems are formed along open coasts and in relatively low-wave-energy environments. They can also develop on high-energy coasts behind barrier islands, where they are sheltered from the high-energy wave action. Within the tidal flat system, the supratidal zone lies at a very high tide level. However, it can be flooded by extreme tides and cut across by tidal channels. This is also subaerially exposed, but is flooded twice a month by spring tides.
Littoral environments/intertidal zones: Intertidal zones are also part of the tidal flat system. The intertidal zone is located within the mean high and low tide levels. It is subject to tidal shifts, meaning it is subaerially exposed once or twice a day. It is not exposed long enough to support the growth of most vegetation. The zone contains both suspension sedimentation and bed load.
Estuarine environments: Estuarine environments, or estuaries, are located at the lower parts of rivers that flow into the open sea. Since they are in the seaward section of the drowned valley system, they receive sediment from both marine and fluvial sources. These contain facies that are affected by tide and wave fluvial processes. An estuary is considered to stretch from the landward limit of tidal facies to the seaward limit of coastal facies. Phosphorites are often deposited in fjords within estuarine environments. These are estuaries with shallow sill constrictions. During the Holocene, sea-level rise led to the formation of fjord estuaries through the drowning of glacially eroded U-shaped valleys.
The most common occurrence of phosphorites is related to strong marine upwelling of sediments. Upwelling is caused by deep water currents that are brought to the surface near the coast, where a large deposition of phosphorites may occur. This type of environment is the main reason why phosphorites are commonly associated with silica and chert. Estuaries are also known as a phosphorus “trap”. This is because coastal estuaries contain a high productivity of phosphorus from marsh grass and benthic algae, which allows an equilibrium exchange between living and dead organisms.

Types of phosphorite deposition

  • Phosphate nodules: These are spherical concentrations that are randomly distributed along the floor of continental shelves. Most phosphorite grains are sand size, although particles greater than 2 mm may be present. These larger grains, referred to as nodules, can range up to several tens of centimeters in size. Phosphate nodules are known to occur in significant quantities offshore northern Chile.
  • Bioclastic phosphates or bone beds: Bone beds are bedded phosphate deposits that contain concentrations of small skeletal particles and coprolites. Some also contain invertebrate fossils like brachiopods and become more enriched in P2O5 after diagenetic processes have occurred. Phosphate minerals can also cement bioclastic phosphates.
  • Phosphatization: Phosphatization is a type of rare diagenetic process. It occurs when fluids that are rich in phosphate are leached from guano. These are then concentrated and reprecipitated in limestone. Phosphatized fossils or fragments of original phosphatic shells are important components within some of these deposits.

    Tectonic and oceanographic settings of marine phosphorites

  • Epeiric sea phosphorites: Epeiric sea phosphorites are within marine shelf environments. These are in a broad and shallow cratonic setting. This is where granular phosphorites, phosphorite hardgrounds, and nodules occur.
  • Continental margin phosphorites: Convergent, passive, upwelling, non-upwelling. This environment accumulates phosphorites in the form of hardgrounds, nodules and granular beds. These accumulate by carbonate fluorapatite precipitation during early diagenesis in the upper few tens of centimeters of sediment. There are two different environmental conditions in which phosphorites are produced within continental margins. Continental margins can consist of organic-rich sedimentation, strong coastal upwelling, and pronounced low-oxygen zones. They can also form in conditions such as oxygen-rich bottom waters and organic-poor sediments.
  • Seamount phosphorites: These are phosphorites that occur in seamounts, guyots, or flat-topped seamounts, seamount ridges. These phosphorites are produced in association with iron and magnesium-bearing crusts. In this setting, the productivity of phosphorus is recycled within an iron oxidation-reduction phosphorus cycle. This cycle can also form glauconite, which is usually associated with modern and ancient phosphorites.
  • Insular phosphorites: Insular phosphorites are located in carbonate islands, plateaus, coral islands consisting of a reef surrounding a lagoon or an atoll lagoon, and marine lakes. The phosphorite here originates from guano. Replacement of deep-sea sediments precipitates that has been formed in place on the ocean floor.