phosphate, page-14

The first discovery of phosphate ore in South Australia was in 1899 at Clinton on Yorke Peninsula, and further exploration subsequently identified over 50 deposits (not including guano). Estimated State production totals 451 000 t, averaging 3400 t/year over the last decade. Production history has fluctuated, being influenced by the availability of cheaper sources of high-grade ore from Nauru Island in the western Pacific Ocean and Christmas Island in the Indian Ocean.

Generally the local product is low grade, containing excessive levels of aluminium and iron oxides which cannot be removed by washing and flotation techniques. Consequently only selected local ore has been used in superphosphate manufacture, the bulk of the local product having been used in fertiliser preparation without chemical treatment. Currently producing quarries are nearing exhaustion of their reserves, reflected by declining production in the 1990s.

South Australian deposits are generally restricted to local occurrences at three stratigraphic levels within the sedimentary sequence of the Adelaide Geosyncline ? Early Cambrian (mainly the Cambrian?Proterozoic boundary), Burra Group (Skillogalee Dolomite association), and Umberatana Group (Brighton Limestone association). The deposits are low-grade phosphorites comprising phosphatic shale, siltstone and, typically, intraformational breccia locally developed at surfaces of disconformity below limestone. Subsequent peneplanation and lateritisation of the land surface during the early Tertiary has resulted in secondarily enriched deposits, generally of limited areal and depth extent, often developed in sink holes on karst surfaces and by partial replacement of nearby carbonate rocks.