Magnesite is an important industrial mineral composed of magnesium carbonate (MgCO3).
Pure magnesite is theoretically 47.8% magnesia (MgO) and 52.2% carbon dioxide (CO2).
It is the source of two-thirds of the world's magnesia (MgO); 25% is extracted from sea water, with the balance coming largely from brines.
Magnesite deposits in South Australia
Three main types of magnesite deposits occur in South Australia:
Sedimentary deposits: Occur as interbeds within the Skillogalee Dolomite of Adelaidean age. This formation extends from the Torrens Gorge near Adelaide to the Leigh Creek–Marree area in the northern Flinders Ranges.
The magnesite was deposited as a chemical precipitate in shallow, marginal marine lagoons and mudflats, and occurs predominantly as cryptocrystalline particles 1–5 µm in size. Much was reworked by storm and tidal action into intraformational conglomerates thinly interbedded with dolomite. The thickest development of magnesite is along a strike length of 120 km, to the northwest of Leigh Creek (Figure 1, opens in new window). Here, the calcium content of the magnesite beds is relatively high, ranging from 2% at Mount Hutton to 4.5% at Screechowl Creek, with calcium being present as dolomite or magnesian calcite.
Talc and quartz are present in minor amounts.
Replacement deposits: Irregular bodies of coarsely crystalline sparry magnesite have been formed by metasomatic replacement of Balcanoona Formation dolomite near Balcanoona and in the Mount Fitton–Mount Livingston area.
Residual deposits: Small deposits of surficial magnesite have developed on magnesium-rich dolomite of the Hutchison Group on Eyre Peninsula, and on Skillogalee Dolomite near Robertstown. Although some nodules are relatively pure (94–97% MgCO3), the deposits are thin and discontinuous.
Flinders Ranges sedimentary deposits
Copley (also known as Camel Flat)
The deposit comprises 60 magnesite beds, 0.05–3.0m thick, over a strike length of 1.5km within the upper 300m of the Skillogalee Dolomite. The average chemical composition of the central zone (21 beds totalling 13.4m is 89.4% MgCO3 (42.7% MgO).
Myrtle Springs–Mount Hutton
Approximately 30 000 t were mined during 1983–84 for water filtration in a Queensland aluminium refinery, but annual production since 1990 has averaged 700 t, principally for agricultural purposes and rockwool manufacture. This deposit, and unworked leases at Mount Hutton along strike to the southeast, were acquired by SAMAG in 2000. Detailed mapping and subsequent drilling has established continuity of the sedimentary sequence for a total strike length of 14 km between the two groups of leases.
The 120 m thick sequence comprises 52 magnesite beds 0.1–2.4 m thick averaging 42.9% MgO interbedded with dolomite. The measured resource is 18.3 Mt.
Screechowl Creek – West Mount Hut
Southern Flinders Ranges
Flinders Ranges replacement deposits
Mount Fitton and Mount Livingston
Products and uses
Caustic calcined magnesia (caustic magnesia)
Variety of applications including:
Dead burned magnesia (a sintered magnesia)
A strong lightweight metal used in the automotive industry. Predictions for strong growth in demand in has resulted in feasibility studies for magnesium production in many parts of the world, including South Australia.
SAMAG Ltd invstigated magnesite deposits northwest of Leigh Creek as possible feedstock for a proposed magnesium metal production facility in the upper Spencer Gulf region (Horn, 2000).
Crettenden, P.P., 1985. Magnesite in South Australia — a historical review 1915–1984. South Australia. Department of Mines and Energy. Report Book, 85/62.
McCallum, W.S., 1986. Camel Flat magnesite deposit near Copley, northwestern Flinders Ranges. Geological investigations 1984 and 1985. South Australia. Department of Mines and Energy. Report Book, 86/17.
McCallum, W.S., 1990. Magnesite deposits in South Australia. In: Hughes, F.E. (Ed.), Geology of the mineral deposits of Australia and Papua New Guinea. Australasian Institute of Mining and Metallurgy. Monograph Series, 14:1151-1154.