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Magnesite

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

state_magnesite_sm

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 m 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 is situated 5km by station track west of Copley. The first recorded mining was in 1919 but the main period of activity was from 1940 to 1955 when F.H. Faulding & Co. Ltd extracted 6000 t for pharmaceutical and chemical use.

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
At the Myrtle Springs deposit, 30 km by road northwest of Leigh Creek (Figure 1, opens in new window), magnesite has been mined over a strike length of 400 m from four magnesite beds of ~1 m thickness dipping 65 east.

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.

magnesite_myrtle_springs.jpg (10246 bytes)

Magnesite and dolomite interbeds exposed in a quarry at Myrtle Springs. The magnesite bed at right is ~1 m thick

magnesite_hutton_mine.jpg (8020 bytes)Northeasterly view of the Mount Hutton magnesite deposit in the northern Flinders Ranges. The deposit lies entirely in the unvegetated valley floor.

magnesite_mt_hutton.jpg (10667 bytes)Magnesite and dolomite outcrop in a creek bed, Mount Hutton deposit.

Pug Hill
A diamond-drilling program undertaken by Pima Mining in 1998 defined a resource with a strike length of 2.5 km comprising a 240 m thick sequence with 47 magnesite beds 0.1–2.1 m thick interbedded with dolomite. The average magnesite grade is 42.7% MgO.

Termination Hill
A diamond-drilling program undertaken by Pima Mining in 1998 defined a resource with a strike length of 6.9 km comprising a 124 m thick sequence with 45 magnesite beds 0.1–2.9 m thick interbedded with dolomite. The average magnesite grade is 42.8% MgO, and the measured resource is 4 Mt.

Witchelina
The deposit is 35 km westerly along station tracks from Farina. Leases were held by F.H. Faulding & Co. Ltd from 1964 to 1980, when a total of 5000 t was produced. The deposit occupies a shallow synclinal basin 4.3 km long and 2.0 km wide. Outcrop is generally subdued and the sequence is not fully exposed. A diamond-drilling program undertaken by Pima Mining in 1998 defined a resource comprising a sequence up to 350 m thick with 23 magnesite beds 0.3–11.0 m thick interbedded with dolomite. The average magnesite grade is 40.0% MgO, and the measured resource is 23.7 Mt.

Screechowl Creek – West Mount Hut
A 93 m thick sequence, with 25 magnesite beds 0.2–3.9 m thick interbedded with dolomite, is exposed in the deeply incised banks of Screechowl Creek, 105 km northwest of Leigh Creek. Detailed mapping has established continuity of the sedimentary sequence for a total strike length of 18 km.

Southern Flinders Ranges
Deposits hosted by Skillogalee Dolomite in the southern Flinders Ranges (Mundallio, Port Germein Gorge) are thinner and pose greater mining problems than those in the northern Flinders. BHP produced 4500 t from shallow open cuts and underground workings in these deposits in the 1940s and 50s.

Flinders Ranges replacement deposits

Balcanoona
Access is via 110 km of unsealed road easterly from Copley. The deposit is now within the Gammon Ranges National Park. A total of 20 Mt is inferred in four orebodies in the Weetootla Gorge area. The only production has been a 660 t trial parcel from exploratory adits in 1956. The parcel was analysed as 95.7% MgCO3, 0.7% CaCO3, 0.8% SiO2, 1.8% Fe2O3 and 0.5% Al2O3.

Mount Fitton and Mount Livingston
Small occurrences of material similar to the Balcanoona deposit are known at Mount Fitton and Mount Livingston, ~60 km to the north.

Residual deposits

Robertstown
The Robertstown deposits, 115 km east-northeast of Adelaide, are the most significant deposits of this type. Recorded production since 1916 is ~20 000 t. The deposits are discontinuous and of variable grade, with a surficial cap of nodular magnesite to 1 m deep overlying Skillogalee Dolomite containing fracture infills extending 2–3 m below the surface.

 

Products and uses

Magnesite
Used as a filler in plastics and paints and as an agricultural fertiliser.

Caustic calcined magnesia (caustic magnesia)
Formed by calcining magnesite at 700–1000C to produce a form of magnesia with high specific surface area and high reactivity.

Variety of applications including:

  • Animal feeds and fertilisers
  • Magnesia cements, flooring compounds and other building materials
  • Environmental (water and effluent treatment, and flue gas desulphurisation)
  • Metallurgical flux
  • Flame retardants
  • Source of magnesium chemicals for industries, including the pharmaceutical industry.

Dead burned magnesia (a sintered magnesia)
Formed when magnesia is calcined at temperatures of 1600–2000C. Main use is in heat resistant linings.

Electrofused magnesia
A strong, abrasion-resistant material used in premium grade refractories and ceramics.

Magnesium metal
Can be produced from magnesite by electolysis of magnesium chloride. World production of magnesium metal in 2006 was estimated at 726 000t.

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). 


Additional reading

Crettenden, P.P., 1985. Magnesite in South Australia — a historical review 1915–1984. South Australia. Department of Mines and Energy. Report Book, 85/62.

Horn, C.M., 2000. The South Australian magnesium metal project. MESA Journal, 16:5-10.

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.