Formation of 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.
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.
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.
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.