This information mainly pertains to Lightning Ridge opals, but the basis is applicable to the formation of opals found worldwide
Opal Formation
Opal formation
Lunatic Hill open cut mine, Lightning Ridge.
In 1986, a mining syndicate mined an opal called Halley`s Comet, the largest black opal nobby on record. At the time it was valued at $6 million.
It came from an area almost in the middle of this picture on the far side of the cut.
As the exact science of formation of opals has more than one theory that is not easy to put into my own words I would like to thank the following;
The author of the text below is Carrol Cummings from Cartoscope Pty Ltd and is unedited by myself.
Lightning Ridge black opals are the most valuable in the world. Lightning Ridge is one of the few places in the world where the precious and highly prized black opal is found. Unlike other opal, the black opal contains carbon and iron oxide trace elements, producing a very dark stone which has hints of blue, green and red play of colour.
Geology
Lightning Ridge lies in a large geological feature called the Surat Basin, which is part of the vast Great Australian Basin. The Great Australian Basin covers 1.7 million square kilometres of eastern Australia. It was formed during the Jurassic to Cretaceous, when dinosaurs walked the Earth. About 140 million years ago, the basin contained a large inland sea which accumulated sediments that later hosted the formation of precious opal. The sedimentary host rocks are essentially horizontal because they were deposited on the seafloor and haven't been deformed much since. The rocks which host the opal at Lightning Ridge were deposited in shallow water near the edge of the basin, probably in an estuary.
Overlying the Cretaceous sedimentary rocks are sandstones and conglomerates that were deposited by streams and rivers in the Tertiary period, about 15 to 5 million years ago. Many of these younger rocks have hardened due to weathering processes to form silcrete and are commonly quarried for road materials.
Opal occurs at Lightning Ridge either as irregular nodules ('nobbies') or in seams and thin layers within vertical or horizontal joint planes, in a characteristic soft, grey to buff-coloured claystone that hardens and whitens on drying. This claystone is commonly called 'opal dirt'. In some parts of the Lightning Ridge field, a thin, hard, silicified band, known as the 'steel band', is found immediately above the topmost opal dirt lens.
Opal is irregularly distributed, but it is commonly found within, and close to the top of, the various opal dirt lenses. In some places it may extend into the steel band. Most of the levels of opal dirt occur between 6 and 18 metres below the surface in the local fields, but are not necessarily horizontal or persistent. In the newer fields, shaft depths of up to 30 metres are common.
Opalised shells, wood, fish, bird, mammal and reptilian bones of Cretaceous age are found in parts of the field.
Precious opal is unique among gemstones. Each stone has its own individual pattern and range of colours, and its colours and patterns can change with the angle of view. The ever-changing rolls and flashes of colour make precious opal a gem of infinite interest.
Australia produces over 90% of the world's supply of precious opal, and probably all of the highest quality gems. The ex-mine value of Australia's opal output was estimated at over $A100 million in 1991, and more than half came from New South Wales in eastern Australia. Over 99% of the most valuable black opal comes from only two localities - Lightning Ridge and Mintabie. Around $30 million worth of opals was produced from Lightning Ridge in 2005/06. On April 10, 2008 the NSW Premier announced the government would designate the black opal as the state's official gemstone.
The New South Wales opal fields are all situated in arid inland areas, on ridges surrounded by black soil plains. The sedimentary opal deposits occur in deeply weathered Cretaceous rocks formed 65 to 135 million years ago within the Great Australian Basin. The opal is found at shallow depths in sedimentary rocks through which there has been considerable silica seepage into cavities and other structures. Gradual loss of water from the silica gel has resulted in hardening of the material and the formation of opal.
Opal Formation
Host rocks contained a variety of voids formed by the weathering process, which leached carbonate from boulders, nodules, fossils, cracks, hollow centres of ironstone nodules and horizontal seams. Most opaline silica deposited is common opal (or potch). It does not show a play of colour. Opal also fills pore space in sand-size sediments, cementing the grains to form deposits known as matrix or opalised sandstone. Opal is often associated with lineaments or faults which break the rock, providing conduits for the movement of ground water. These have been found useful in locating opal at Lightning Ridge and sought in other states. In addition, opal has been found associated with Cretaceous palaeochannels in Queensland and Lightning Ridge. Variations in the types of opal depend on a number of factors. Firstly the climate provides alternating wet and dry periods, creating a rising or falling water table which concentrates silica in solution. Deep weathering of Cretaceous clay-rich sedimentary rock produces both silica and kaolin. Silica spheres are deposited in a regular array in voids from a receding water table forming precious opal in a variety of host materials. The orderly arrangement of the spheres in a regular three-dimensional array creates diffraction and interference, producing the brilliant colours of precious opal. The range of colours is controlled by the size of the silica spheres and by refraction at the surface of the opal.
The geological factors associated with opal formation continue to be the subject of research and active discussion by geologists, opal prospectors and miners.
Three main models for opal formation presently exist, and these are briefly outlined below. Although the models have been put forward as stand-alone concepts, further research may show some linkages between them.
Weathering Model
In the Lightning Ridge opal fields, most opal is recovered from near the top of the various Finch clay facies lenses (opal dirt), immediately below the overlying portion of the Wallangulla Sandstone Member. Geological mapping of the opal fields has shown that the sandstone is thickest in areas that have produced large amounts of opal. Opal is rarely produced from areas in which the sandstone is thin or absent.
This data supports a view that the weathered sandstone at Lightning Ridge is the source of silica for the formation of opal. The occurrence of opal in the uppermost portions of the Finch clay facies is consistent with the concept that siliceous solutions (groundwater + silica) percolated down from their source rock (Wallangulla Sandstone Member) and were trapped by the impervious barrier presented by the clay fades (opal dirt).
Interpretation of aerial photographs and satellite images shows numerous lineaments (representing faults or joints) throughout the Lightning Ridge area. The lineaments identified are commonly several kilometres in length and may be expressed at the surface by a number of large box or belah trees following the line of lineament. There is a good relationship between the occurrence of lineaments, particularly where they intersect, and the occurrence of opal deposits. It is possible that these major lineaments were passageways that allowed deeper weathering and initial groundwater movement, and therefore controlled the sites of the development of the smaller-scale structural features, such as joints, faults, and subvertical breccia zones, which are observed in most of the workings in the Lightning Ridge area. Collectively, these features have created zones that allowed the passage of groundwater down from the surface, resulting in today's opal fields.
An additional factor necessary for the formation of opal is the appropriate chemical environment. This process is not well understood, but may involve: a change from an alkaline to an acidic environment; the presence of aluminium oxide, ferric oxide or magnesium oxide; and the presence of sodium chloride or sodium sulphate.
Fundamental concepts associated with the weathering model include:
opal developed by weathering at some time after the rocks in which it is found;
a source of silica, predominantly sandstone, is required for opal to form;
a claystone permeability barrier trapped water carrying silica, which allowed the opal to form; and
blows and faults generally enhanced the permeability of rock strata and increased the potential for opal to form in association with sandstone and claystone.
Syntectonic Model
This model proposes that opal formed from mineral-bearing waters rising towards the surface under pressure along fault and breccia pipes. Deformation of sandstone and claystone layers by large-scale, tectonic, geological processes caused water to be forced hydraulically into fractures and faults. As the water dissipated into areas of lower pressure, the opal was deposited as veins. Some of the silica-rich water that formed opal also escaped to the surface as hot springs. Where this water infiltrated Tertiary sands and gravels, silcrete has been formed as a result. Accordingly, there may be an association between areas of silcrete and opal deposits.
Fundamental concepts associated with the syntectonic model include: opal was developed after the rocks in which it is found, opal was formed relatively quickly; and faults and blows are essential for opal to have formed in a given area.
Microbe Model
The opal-bearing claystone commonly contains substantial amounts of fine, fossilised organic matter. Various types of microbe fossils, primarily aerobic (air-dependent) bacteria, have been identified within samples of opal from the Lightning Ridge area. Although none of these fossil organisms are visible to the naked eye, microscopic studies have shown them to be quite abundant.
At the time the Cretaceous sediments were deposited, abundant organic matter and montmorillonite (smectite) clay within some sediments provided an ideal habitat for the microbes to feed and breed. Waste acids and enzymes excreted by the microbes caused the chemical weathering of clay minerals and feldspars in the surrounding rocks. Ultimately, the ongoing feeding and waste production processes of the microbes created favourable physical and chemical conditions for the formation of opal.
Fundamental concepts associated with the microbe model include:
opal developed at the same time as the rocks in which it is found;
microbes tended to be most abundant where there is an abundance of montmorillonite (smectite); and
the biological activity of microbes created a chemical environment that promoted the weathering of clay minerals and feldspar.
My additional theory;
Opals fields in Australia are all in areas that were round the perimeter of the Great Australian Basin, the large inland sea that existed millions of years ago.
If you look at a map of Australia now and the areas of the opal fields, compared to when the sea was there, it is fairly obvious.
Much of this area is still flooded ocasionally, allowing more water to pick up silica and leach through the bedrock over time.
With the vast areas involved in the centre of Australia there is a reasonable chance that more undiscovered (as yet) opal fields exist.