Gypsum global production in 2013 was 160 Mtonne. In 2013 gypsum production in South Australia was estimated at 4.4 Mtonne, which was 80% of the total Australian production.
Gypsum is naturally occurring hydrated calcium sulfate (CaSO42H2O). Its main use is in the manufacture of plaster products including wall and ceiling boards, mouldings, and blocks for construction use, as well as sculpturing plasters, medical and dental uses and pottery. These uses rely on the relative ease with which gypsum loses and then regains its water of crystallisation. When heated to 300–350ºC, gypsum loses 75% of its H2O to form the hemi-hydrate form of calcium sulfate known as Plaster of Paris (CaSO4½H2O). When water is added, this material can be spread, cast or moulded prior to regaining its water of crystallisation and setting. Gypsum used for plaster manufacture usually contains at least 90% CaSO42H2O, a maximum of 0.02% NaCl (salt), and approximately 2% acid insolubles.
Another major use of gypsum is in the cement industry, where it is added at the rate of 2–5% to cement clinker prior to final grinding to retard the setting rate of concrete. Cement-grade gypsum tolerates higher NaCl (0.5–0.6% maximum) and insolubles (6% has been used).
Agricultural uses
Agricultural gypsum is being increasingly used to treat sodic soils, symptoms of which are waterlogging, increased runoff, poor water storage, surface crusting, and problems with cultivation and erosion. Sodium causes swelling and dispersion of clay. The clay particles may then move through the soil, clogging pores and reducing infiltration and drainage. The calcium from gypsum displaces the sodium which can then be leached deeper into the soil. South Australian production of agricultural gypsum exceeded 300,000 tonnes in 2002 representing a 400% increase over 8 years.
Amended regulations to the Agricultural Chemical Act 1955 became effective on 30 September 1999, such that South Australian gypsum fertiliser products must now be labelled according to minimum gypsum, calcium and sulphur contents, and size grading. Four grades of fertiliser gypsum (Premium, and Grades 1, 2 and 3) have now been specified according to sulphur content. A 15% moisture content upper limit is specified, and products containing over 0.8% sodium must have an appropriate warning. Results of a state-wide program of sampling agricultural gypsum stockpiles are presented in Keeling et al. (2001).
The gypsum deposits described briefly below are the most significant in South Australia.
Geological controls
Gypsum has been deposited in two distinct evaporitic environments in arid or semi-arid areas of the State — coastal salinas in interdunal corridors of the Quaternary beach dune system, and continental playas underlain by relatively impermeable sandy clay in enclosed inland depressions (Warren, 1982).
Coastal salinas
At the end of the last ice age, ~12,000 years BP, melting of the polar ice cap caused a rise in sea level. This rise stabilised at ~6000 years BP and evaporation began from interdunal depressions which had filled with seawater either by direct connection to the ocean or by seepage through the porous coastal dunes of the Pleistocene Bridgewater Formation. As salinity increased, first aragonite then gypsum and salt were deposited.
In the deeper parts of lakes such as Lake MacDonnell on the far west coast of South Australia, with a large initial volume, selenite domes containing aragonite pelletoids evenly distributed between large gypsum prisms were deposited from a stable gypsum-saturated brine. Laminated selenite and aragonite followed. As the salina filled with sediment and the volume of the brine pond decreased, seasonal salinity changes became more rapid and new sand-sized prisms were deposited each summer, forming laminated gypsarenite. Lamination decreased with increasing subaerial exposure.
In deposits such as Spider Lake on southern Yorke Peninsula, only gypsarenite formed, the rate of salinity change having always been too rapid and the bottom brine salinity too unstable for the formation of selenite. The exposed gypsarenite was eventually blown by prevailing winds into lunettes (gypsum dunes) at the salina margins (predominantly leeward). Surficial gypsum dissolved by rainwater is reprecipitated in the zone of soil moisture as silt-sized gypsum (gypsite).
Continental playas
Gypsum in these inland lakes is older than that in coastal salinas, with deposition thought to have ceased ~16,000 years BP. The lakes are all in enclosed depressions in relatively impermeable sandy clay, and at the time of deposition were closed basins subject to rapid seasonal variation in volume and salinity of the brine pool. As for coastal salinas, these are the conditions favouring seasonal development of new sand-sized prisms and deposition of laminated gypsarenite. The playas are not connected to an infinite reservoir of saline water and the source of the gypsum is uncertain. It is likely that the salts were flushed from gypsic soils which are common in surrounding areas. The largest of these playas is at Blanchetown in the Murray Basin, and is mined predominantly for plaster and cement manufacture. Lunettes developed on the leeward side of continental playas are the sources of much of the State’s agricultural gypsum.
Coastal salina deposits
Lake MacDonnell
Lake MacDonnell, with a resource of ~500 Mt, is Australia’s largest gypsum mine (Warren, 1983). Operations commenced in 1919, and production in 2013 was ~ 3.5 Mtonne. The deposit has been operated by Gypsum Resources of Australia Pty Ltd since 1984 when CSR Ltd and Boral Ltd combined their separate operations. The deposit comprises ~1 m of gypsarenite at 93% CaSO4.2H2O overlying ~5 m of selenite at 94–96% CaSO4.2H2O. Gypsum as mined is too high in salt content for plaster manufacture, so it is stockpiled on site for several years to allow leaching by rainwater. When the salt content has been sufficiently reduced, the gypsum is railed 64 km to a 160,000 t stockpile at Thevenard for loading on ships by conveyor.
Lakes near Streaky Bay
The largest known undeveloped deposit in South Australia is 26 km south of Streaky Bay. Gypsarenite resources of 49 Mt at 87.9% 44 CaSO4.2H2O and averaging 2.5 m in thickness at Lake Purdilla, and 10 Mt at 85.9% CaSO4.2H2O and averaging 2.1 m at Lake Toorna, have been measured at a cut-off thickness of 1 m. Additional resources of 4 Mt are indicated at each lake, and adjacent dunes contain high-grade gypsum (Olliver et al. 1988).
Bielamah (Davenport Creek)
A resource of 5 Mt of selenite and gypsarenite up to 5 m thick and containing up to 97% CaSO4.2H2O has been delineated by Pioneer Plasterboard Pty Ltd for use in wallboard manufacture in Sydney and Melbourne. Gypsum has been stockpiled to leach salt prior to shipment through the port of Thevenard.
Stenhouse Bay deposits
In the 1890s, a tramline, jetty and loading facility were erected at Marion Bay on southern Yorke Peninsula to handle gypsum mined from Lake Inneston, and continuous production began in 1913. Marion Lake was brought into production shortly after, and together these deposits provided the bulk of South Australian output until the end of World War II. In 1993, operations were transferred to Spider Lake which contains a resource of 2.4 Mt of laminated gypsarenite averaging 1.2 m in thickness. Waratah Gypsum Pty Ltd produced 4600 t of plaster grade gypsum in 2002 production having declined significantly in the period 2000–2002 following suspension of operations at the Gillman plasterboard plant. The plant resumed production in September 2003. Nearby Snow Lake contains an unworked 2.8 Mt resource with an average depth of 0.9 m, but the average grade of 76.3% is well below that required for plaster manufacture (Olliver and Warren, 1979).
Lake Fowler
Lake Fowler is a lunette deposit of gypsite and gypsarenite averaging 93% CaSO4.2H2O which is used by Adelaide Brighton Cement Ltd in cement manufacture. Production is variable from a small resource (250,000 t inferred). 2002 production of 10,000 t was the largest since 1988.
Kangaroo Island
Mining operations were initiated in 1956 by Ingham Plaster Co. Ltd at Salt Lake near Flour Cask Bay, 14 km southwest of Penneshaw. Crude gypsum was trucked to American River for shipment by ketch to a plaster factory at Port Adelaide. CSR Ltd took over the operations in 1959, erected a crusher and washing plant at the lake, and a ship-loading facility at Ballast Head. When Salt Lake deposit was exhausted in 1981, operations transferred to Pelican Lagoon (New Lake deposit), 8 km southwest of Penneshaw, and continued until 1986. These deposits contained an average of 4 m of laminated selenite which was mined by excavator. Total production was 4.8 Mt.
Inland playa deposits
Blanchetown
David Linke Contractor Pty Ltd produced 216 500 t in 2002 from an indicated 9 Mt contained within a substantial additional undefined resource (Barnes and Warren 1983). The deposit comprises up to 3.6 m of bedded gypsarenite which becomes increasingly clayey with depth. Various grades are produced at the Nuriootpa screening plant and marketed for cement and plaster manufacture and for agricultural uses (Valentine 1989, pp.67–69).
Cooke Plains
The Cooke Plains deposit is South Australia’s largest producer of agricultural gypsum, which is used both in this State and the eastern States. The indicated resource of 2 Mt is contained within a lunette on the eastern side of a samphire swamp 5 km east of Cooke plains. Paterson Bulk Transport produced 72,700 t in 2002.
Ashville
Drilling of the Warnes and Lihou deposits ~35 km south of Tailem Bend by Olliver Geological Services has outlined 5 Mt of gypsum at 73–88% CaSO4.2H2O.
Meningie
Push-tube coring of 15 lakes to the south of Meningie has indicated a gypsum resource totalling 1.3 Mt, almost 60% of which is of plaster grade. Approximately 1 Mt of this resource is contained in the Clanto deposit 4 km south of the town. Production commenced at Gemlake, 6 km south of Meningie, in 1998. Elephant Lake deposit, 12 km northeast of Meningie, contains 1.4 Mt at 91.8% gypsum. Meningie Gypsum Ltd produced a total of 6600 t from both deposits in 2002.
Everard
Variable tonnages (35,000 t in 2002) of agricultural gypsum are mined by G.J. Mills and McArdle Pty Ltd from lunette deposits near Everard Central, 15 km east of Lochiel in the State’s Mid North.
Gordon Lagoon
Lunette dunes flanking the eastern and southern margins of Gordon Lagoon, 29 km east-southeast of Burra, are mined for agricultural gypsum. Two separate parts of the deposit are mined; in 2002, M.E. Neindorf produced 25,800 t and G.G. Strachan produced 8140 t.
Lake Malata
Elongate lunette dunes near the eastern margin of Lake Malata, 20 km west-northwest of Cummins, are an increasingly important source of agricultural gypsum for southern Eyre Peninsula, with 21,700 t being mined by Malata Nominees Pty Ltd in 2000.
Additional Reading
Warren JK 1982. The hydrological setting, occurrence and significance of gypsum in late Quaternary salt lakes in South Australia. Sedimentology 29(5):609-637.
