Why is Australian Opal Unique?

Australian black and Boulder Opal

Australia currently produces about 95 per cent of the world’s precious opal from widely scattered fields throughout central Australia. No other country on Earth has such an abundance of this rare precious gemstone.

The sedimentary opal deposits of central Australia occur along generally flat-lying horizontal layers within 30 metres of the earth’s surface “What is Opal?“. They are a product of a unique set of geological events which occurred over a 100 million year period. These events can be summarised as follows:

  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 Magave rise to subtle extremely long wavelength surface folds which facilitated bothlateral 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 “Types of Opal“.

Reference:
AUSTRALIAN SEDIMENTARY OPAL – WHY IS AUSTRALIA UNIQUE? David Horton, Managing Director, Opal Horizon Limited [LINK]

Note: The above post is reprinted from materials provided by Opal Horizon. The original article was written by David Horton.

 

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