Why do most opal deposits occur in Australia?
A recent study published in the Australian Journal of Earth Sciences, Opalisation of the Great Artesian Basin (central Australia): an Australian story with a Martian twist (Rey, 2013). The study makes the assertion that a unique set of geological conditions existed during the Cretaceous era that led to the relative abundance of opals in Central Australia. Rey proposes a 'redox model' of opal formation to account for the great relative abundance of the precious stone in Australia.

An important aspect to note is that just about all opal deposits occur either in or on the periphery of the Great Artesian Basin, as can be seen in the map below:



Image Source and brief information of opal mining centres: The National Opal Collection

The conditions around the opal field part of the Artesian Basin during the Cretaceous, according to Rey (2013) were:


From about 130 to 93 million years ago, The Great Artesian Basin was flooded by what is known as the Eromanga Sea, this was a cold, shallow, largely stagnant waterway that was poorly connected to the open ocean.
The Eromanga Sea served as a 'sink' for volcaniclastic sediments eroded from orogenic volcanic sources on the Pacific margin of Gondwana.


Rey describes how the iron- and organic- rich sediments contributed to an anoxic seafloor environment that became rich in pyrite, ferrous iron, feldspar and volcaniclasts, but virtually no carbonates. 

At the end of the Cretaceous and into the early Palaeogene (from about 93 to 60 million ears ago), the region experienced slow uplift, erosion and denudation, as well as subsequent sea regression; this, according to Rey, caused an acidic oxidation of the Eromanga Sea deposits as oxic fluids perculated through faults, some trapped by clay deposits - these are where the opal deposits have been found.  Further deposition in the Palaeocene and Eocene provided protection from further erosion.

In summary, Rey notes 5 aspects unique to the geological history of the Great Artesian Basin that has resulted in the great relative abundance of opal:


Large scale organic matter-rich, iron-rich volcaniclastic sediments
The depositional environment to be anoxic
negligible carbonates
sea regression driving oxidative weathering
stable landscape to preserve opal from erosion and to prevent its transformation into quartz via burial


The connection to Mars in the title is based on recent observations by NASA of opaline minerals found on the Martian surface, Rey suggests that there could be a link with the conditions with the dehydration of Eromanga Sea in the Cretaceous and processes that may have occurred on Mars.