The opal is almost exclusively located within Cretaceous sedimentary rocks of the Great Artesian Basin, which experienced a major phase of uplift in the Late Cretaceous with subsequent erosion removing a package of sedimentary rock up to 3 km in thickness.
|Why Is Australian Opal Unique?|
Intense weathering resulted in extensive silicification at relatively shallow levels within the Tertiary regolith. However, despite a billion dollar industry and a well-constrained geological history of the basin, the formation of sedimentary opal and its uniqueness to the Australian continent are still very poorly understood.
1- Between about 122 million years ago (Ma) and 91 Ma, central Australia was covered by a vast shallow epicontinental sea. The sedimentary rocks which were deposited in this sea were derived from volcanic rocks and were organic-rich. These formed the principal host rocks for opal deposits in central Australia.
2-Following surface exposure through lowering of the sea level, these host rocks were subject to a prolonged sub-tropical weathering regime until about 40 Ma. Central Australia probably looked not unlike today’s Amazon Basin. During this time, the water table was close to the surface and was acidic releasing silica and iron from weathering of the host rocks.
3-The climate became more arid from about 40 Ma and, as a result, water table levels gradually lowered and the groundwater became alkaline. Mild tectonism at 24 Ma
gave rise to subtle extremely long wavelength surface folds which facilitated both lateral and vertical migration under arid conditions of the earlier-released silica. Opal was preserved in the weathered profiles beneath the crests of the developing surface folds as water tables here lowered more rapidly due to tectonic uplift. Siliceous cap rocks discouraged erosion.
4- Over the last 10 million years, dissection and scarp erosion exposed the weathering profiles containing the opal.
Geologists believe that the volume of gems that have been produced over the past 150 years in Australia is but a minute fraction of the amount yet to be discovered.
|Precious Opal Deposits of the World. credit: David Horton|
A unique combination of geological events appear to have taken place in central Australia over the last 100 million years to form precious opal. These are:
- Deposition of volcanic-derived organic-rich sediments over a 30 million year period during the Cretaceous.
- Weathering under warm wet acidic conditions from the Late Cretaceous to the mid-
Eocene which released silica and iron.
- Remobilisation of the silica under warm arid alkaline conditions during a period of
tectonic instability during the Late Oligocene-Early Miocene.
- Preservation of the opal forming in the weathering profile through lowering of the watertable.
Deposition of precious opal occurs mostly by replacement of layer-silicate clays, gypsum, calcite, goethite, fossils and organic material as well as by infilling of voids particularly in ironstone.
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Read also: Types of opal with photos
- Australian Sedimentary Opal – Why Is Australia Unique? by David Horton, Managing Director, Opal Horizon Limited
- Elemental Characteristics of Australian Sedimentary Opals and their Implications for Opal Formation and Gemstone Fingerprinting