Sapphire, a variety of the mineral corundum, is one of Queensland's most important gemstones.
Sapphires have been mined commercially for more than 100 years on the Anakie field in central Queensland. Most of Queensland's production is from the Anakie field, west of Emerald. Some production has come from the smaller Lava Plains field in north Queensland.
Gem-quality zircon and diamond (rare) are recovered with the sapphires. A diverse range of colours, including blue, green, yellow, gold, parti-colour (blue/green/yellow), orange, pink, mauve and purple are found.
Star stones are mainly dark blue or black, but brown and bronze colours are not uncommon. Rare colour-change stones have been found also. A large proportion of sapphire mined commercially is medium to dark blue, due to the high iron content.
Nearly all of this material is routinely heat treated to improve clarity and colour.
The sapphires are of volcanic origin and occur in alluvial deposits in either present day or fossil drainage systems that drain volcanic terrains composed of alkali-volcanic rocks, mainly basaltic lavas and pyroclastics of Tertiary and Quaternary age.
The occurrence of sapphire in Queensland is associated with Tertiary and Quaternary alluvial deposits derived from the weathering and erosion of alkali-volcanic rocks, mainly basaltic lavas, pyroclastics and volcaniclastics of Tertiary age and underlying Palaeozoic basement rocks.
Similar rocks and volcanic terrains are found throughout the highlands of eastern Australia, and extend from Cooktown to Tasmania.
In Queensland, the most important of these volcanic terrains containing significant deposits of sapphire is the Hoy Basalt Province at Anakie and the McBride Basalt Province at Lava Plains.
On the Anakie field basaltic lavas, pyroclastics and underlying granitic and metamorphic rocks were weathered and eroded to form alluvial deposits in a Tertiary palaeodrainage system.
Consequent erosion reworked some of this alluvium, redepositing the materials as more recent deposits in present day drainage systems.
Remnants of the older alluvium occur in many areas as primary, high-level gravels (known as wash by the miners) on elevated ridges between or adjacent to present day drainages.
Deposits may be at the surface or may be covered by up to 20m of overburden.
Sheets of secondary (low-level) wash resulted from erosion of primary wash (eluvium/colluvium) in some areas.
Some younger reworked wash deposits occur in present day drainages as well.
The sapphires and other heavy minerals were transported and deposited in layers of wash, tending to be concentrated in 'runs' along particular channels.
The character and size of sapphire grains and the composition of associated heavy minerals and detritus comprised in the wash varies considerably.
Typically the wash consists of 'billy' (quartzite) boulders, quartz and rock fragments of basaltic lavas, pyroclastic and volcanicalstic rocks and older basement rocks set in a silty or clayey matrix.
The gravel layers are often interlayered with coarse and fine sediments and vary from a few centimetres to over a metre thick and may rest on basement rocks.
Associated minerals found in the sapphire concentrate include zircon, quartz, ilmenite, pleonaste (black spinel), spinel (red), garnet, topaz, tourmaline, diamond (rare), magnesite, hematite, magnetite and limonite.
The smaller Lava Plains field differs markedly from the Anakie field in that the basalts that host the sapphires are very much in evidence on the field.
The field lies within one of several major basalt provinces in northeastern Queensland, the McBride Basalt Province.
Sapphires occur in eruptive volcanics from a limited number of vents and are recovered from the eluvium, colluvium and alluvium within and adjacent to present day watercourses in the vicinity of the vents.
The sapphires occur in brown and black clayey soils containing vesicular basalt rock fragments and basalt cobbles and boulders.
Associated minerals found in the wash include zircon, ilmenite, olivine, hematite and feldspar.
The main areas mined are centred on Wyandotte Creek and Mines Hill.
Mining, production and outlook
Mining of sapphires varies from simple hand mining methods suitable for working surface and shallow wash deposits and deeper ground by digging shafts, to large-scale open-cut mining lease operations involving the use of heavy earthmoving equipment.
Processing wash to concentrate the sapphires and the heavy minerals also varies from rudimentary hand sieving and washing to more sophisticated mechanised plants.
Queensland's production of sapphire is mostly from the Anakie field in central Queensland. There has been no commercial mining undertaken on the Lava Plains field since 1993.
Sapphires mined are a mixture of dark blue, parti-colours and fancy stones. Only the blue stone is sold in volume to the world market.
A significant proportion of production is made up of smaller commercial size stone suitable for cutting into 2-3mm calibrated stones for the mass jewellery market.
The future of Australia's sapphire industry remains uncertain with almost all commercial sapphire destined for the export market, mainly to Thailand.
The bulk of sapphires produced are exported as rough stone. Very little value adding to product is done in Australia.
In recent years, Australia's position has been further disadvantaged by increased production from other low-cost sources such as Madagascar, Nigeria and Tanzania.
The commercial production of sapphires in Queensland has been in general decline since the late 1970s and only intermittent improvements to prices and markets has occurred since then through to the present time.
However, there are opportunities for further exploration and mining of sapphires in Queensland, and mining tenures remain over the main gem mining areas.
Last updated 22 June 2010