Phosphate rock is a general term referring to rock with high concentrations of phosphate minerals, most commonly those of the apatite family with the general formula Ca₅(PO₄CO₃)₃(F,OH,Cl). Ninety percent of phosphate mined is used to produce chemical fertilisers. Phosphorus is one of the three major nutrients required by plants, controlling the transfer and storage of energy at the cellular level and playing an important role in metabolic processes.

For general use in the fertiliser industry, phosphate rock or its concentrates preferably have levels of approximately 30% phosphorus pentoxide (P₂O₅), reasonable amounts of calcium carbonate (5%), and less than 4% combined iron and aluminium oxides. Worldwide, the resources of high-grade ore are declining, and the beneficiation of lower grade ore by washing, flotation and calcining is becoming more widespread.

Continued growth in world population translates into continued demand for phosphate fertilisers for use in agriculture. Phosphate products are also used in animal feeds, as a leavening agent in baking powder and flour, as an additive to beverages and in pharmaceuticals. Industrial uses include water softening, rust proofing, fire proofing, in insecticides and detergents, and for the manufacture of elemental phosphorus.

Phosphate deposits can be classified into three main types:

• marine sedimentary phosphorite - the most economically significant
• apatite-rich igneous rock
• ancient and modern guano accumulations.

World production in 2013 was 224 Mt, with the main producers being the USA, the former Soviet Union, Morocco and China. Since recorded production began in Suffolk, England, in 1847, more than 2 billion tonne of phosphate rock has been mined worldwide. World resources of phosphate rock are greater than 300 billion tonne. United States Geological Survey (USGS) commodity statistics predict a 20% increase in production from 215Mt in 2011 to 260Mt in 2017.

In Australia the Middle Cambrian phosphorite deposits of the Georgina Basin, Queensland and Northern Territory (NT) host 90% of inferred phosphate rock resource of which the total in 2011 was 1813Mt. Australia’s largest deposit is Wonarah in the NT, a sedimentary phosphorite with a 2011 resource of 1552Mt @ 11% P₂O₅. Other significant resources include the Mount Weld deposit in Western Australian (WA) (carbonatite-hosted apatite), and Christmas Island (phosphatic laterite developed on volcanics). World price of phosphate rock peaked in 2008 at approximately US$430/tonne, crashed to a low of less than US$100/tonne in 2010 before recovering to approximately US$175/tonne by June 2013.

South Australian deposits

Production

Phosphate ore in South Australia was discovered at Clinton on Yorke Peninsula in 1899. Subsequently some 90 deposits and occurrences have been identified. South Australia has a long history of production of phosphate rock, mainly for use as a fertiliser. Estimated total production is in the order of 500,000 tonne, with production in the last decade sitting around the 1-3000 tonne per annum mark. Over the years local production has shown steep fluctuations, being influenced by the availability of cheaper sources of ore from interstate and overseas, including Nauru Island in the western Pacific Ocean and Christmas Island in the Indian Ocean.

Grade of the local product has been, and continues to be low. It contains excessive levels of aluminium and iron oxide, which cannot be removed by washing and flotation techniques. Consequently the bulk of the local product has been used in fertiliser preparation without chemical treatment. Only selected local ore has been used in superphosphate manufacture. Production is recorded from 23 deposits, with most production coming from four deposits in the Kapunda–Moculta district. Only small resources remain in these historic quarries.

Currently the main production is from Catfords Cut mine located approximately 4 km due east of Tarcowie township in the mid-north. It produces a low-grade product of approximately 6% phosphorus (P), but with desirable levels of trace elements giving it added value as a fertiliser for use locally in agriculture. Annual production is in the order of 1000tpa.

Geology and resources

South Australian deposits are dominantly residual deposits developed on low-grade phosphorite. The phosphorite is largely restricted to distinct stratigraphic levels within the sedimentary sequence of the Adelaide Geosyncline, i.e.

• Early Cambrian at or near to the Cambrian–Proterozoic boundary)
• Burra Group (Skillogalee Dolomite association)
• Umberatana Group (uppermost Tapley Hill Formation - Brighton Limestone association).

There are minor deposits of guano that have been mined from inland caves, and from coastal areas on islands.

Residual deposits on host phosphorite

Early Cambrian deposits

Kapunda–Moculta district

Phosphate deposits in the Kapunda–Moculta area have been worked intermittently since 1903 from numerous pits and quarries, generally associated with host early Cambrian marble, in some cases with a particular horizon, the Koonunga Phosphorite Member. The deposits are of a residual type, described as gossanous, rich in iron, aluminium, manganese oxide, and the alumino-phosphates wavellite and crendullite. The gossan grades into zones of hard, solution-channelled rock with comparatively high P₂O₅ content, which in turn give way to friable phosphatic clay, silt and sedimentary breccia. Primary zones below the level of weathering are low grade.

Main mines include:

• Moculta, residual deposit high in iron, and developed on an undulating karst surface within Angaston Marble. Production 1905–1985 68,000t @ approximately 20% P₂O₅. No remaining resource figure published.
• St Johns, residual deposit on drag-folded segment of host phosphorite 20m thick. Production from 1904–1959 36,000t @ 55% P₂O₅.  Estimated remaining resource 1959 60,000t at 20–30% P₂O₅.
• Toms–Sobeys, residual deposit, highly ferruginous, and operated by BHP for specialty pig iron requirements during 1941–1944. Significant production from 1904–1948, although the figure of 105,300t includes production from St Kitts, grade unknown. Minor production of 1250 t in 1999. No remaining resource figure published.
• St Kitts, residual deposit of phosphatic sedimentary breccia, no reliable production figure, but in the order of greater than 60,000t at 10–15% P₂O₅, and with considerable iron and earthy manganese wad. A small tonnage remains in old stockpiles.

Clinton district

The Clinton deposits produced approximately 10 000 t of rock from residual phosphate rock developed in enlarged joints, solution cavities, and partial replacement of carbonate host within an undulating karst surface etched into lower Kulpara Limestone. Similar deposits occur near Ardrossan. Source of the phosphorus was the host limestone.

Myponga

Near Myponga, lenticular masses of phosphate rock occur within siltstone of the Lower Cambrian Mount Terrible Formation. BHP mined 900 tonne in 1941–1942 for the production of high phosphorus pig iron. Three diamond holes identified a 1975 resource of 12,000 tonne at 22.5% P₂O₅.

Adelaidean–Burra Group (Skillogalee Dolomite association)

The Fairview deposit, 26 km south of Burra, yielded 100 tonne of hand-picked ore grading 25% P₂O₅ in 1903. Similar phosphorite has been located at this stratigraphic level north of Burra, and more than 100km south at Lower Hermitage in the Mount Lofty Ranges.

Adelaidean–Tapley Hill Formation–Brighton Limestone association

As many as five separate horizons of lenticular phosphorite beds to 3m thick occur over a relatively confined stratigraphic interval of near and on the uppermost Tapley Hill Formation–Brighton Limestone contact, and within the Brighton Limestone. The phosphorite beds have been traced intermittently from a southern limit of Olivers deposit near Noarlunga then north to the Napa Napa deposits north-west of Orroroo, a distance of approximately 360km. Economic deposits were generally of the residual type, and developed on host low-grade phosphorite.

The most significant deposits were located within a major syncline, with dimensions 60 km north-south by 5km wide. Main areas of production were from residual deposits immediately south-east of Orroroo, and east of Tarcowie. Catfords Cut, the current main producer, is located approximately 4km east of Tarcowie. Resource estimate in 2006 at 4% P cutoff and to a depth of 10 metres was 60 000 tonne at 6.2% P.

In 2009 Flinders Mines completed a grid soil sampling survey over the prospective horizon, and within the aforementioned regional syncline. The survey identified prospective areas on both limbs of the northern 15km of the regional syncline, and for the eastern limb for a distance of approximately 40km from Orroroo to south of Terowie.

In the early 1960s exploration by Conzinc Riotinto included wide spaced geochemical traverses in the Quorn, Marree, Leigh Creek, and Northern Flinders districts failed to identify any significant phosphatic horizons at the prospective stratigraphic intervals.

Other deposit types

Chemical analysis of volcanic rock from the south-east has identified elevated phosphorus levels. There may be some potential for secondary (lateritic) phosphate deposits of similar type to that recorded for Christmas Island.

Prospectivity

The Geological Survey completed a reconnaissance level assessment for phosphate rock resources in the 1960s to early 1970s. Much information can be found in report books completed during this assessment. A model for the formation of phosphorite is that it formed by chemical precipitation from phosphate-bearing sea-water from upwelling currents during a period of high-stand sea level that followed a major glaciation event. This scenario occurs with widespread deposition of sediment of the Tapley Hill Formation during a high-stand sea level that was post Sturtian glaciation. This would suggest that there is potential for further discoveries at this stratigraphic level given there remain considerable strike lengths of the prospective units that remain untested.

Additional reading

Brook, W.A., 1962. Geology of the phosphorite deposits of the Orroroo district and notes on phosphorite deposits at Kapunda and Noarlunga, South Australia. South Australia. Department of Primary Industries and Resources. Open file envelope 00265.  

Johns, R.K., 1962. South Australian rock phosphate deposits. Mining Review, Adelaide, 114:22-30.
Lockhart Jack, R., 1919. The phosphate deposits of South Australia. South Australia. Geological Survey. Bulletin, 7.