Elemental Characteristics of Australian Sedimentary Opals and their Implications for Opal Formation and Gemstone Fingerprinting

Opal consists of amorphous SiO₂.nH₂O comprising a network of silica spheres, which in precious opal are of similar size and form an ordered network allowing light to diffract into an array of colors. Common opal, which is often associated with precious opal, lacks this play of color as it is composed of silica spheres of variable sizes. Australia supplies over 95% of the world's precious opal. 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. 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. In this study we have used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) on precious and common opal from key opal mining areas in order to constrain the possible sources of silica fluids involved in opal genesis and to assess whether any major or trace elements could be used to determine the provenance of opal with respect to a particular mining area. A total of 123 spots, each comprising 59 elements, including rare earth elements were analyzed. Globally, volcanic and sedimentary opals can be distinguished on the basis of Ba and Ca concentrations. Although the opals from the Great Artesian Basin are all sedimentary, some show Ba concentrations consistent with volcanic opals suggesting that silica fluids from which they formed were derived from a volcanic province. The most likely source is the Cretaceous volcanic-plutonic province of central Queensland, which supplied vast amounts of volcanogenic material into the Great Artesian Basin. The weathering of feldspars from the volcaniclastic rocks would have provided ample silica-rich fluids for the formation of opal as well as kaolinite, which is abundant within the opal host rocks. Multivariate feature-selection analysis (using a signal-to-noise criterion) of elemental concentrations in opal bands from different locations indicates that the mining region from which the opals originate can best be discriminated using a combination of K, Se and Hf. The best independent provenance discriminators are K, Rb, Ba, Cs and Hf. Precious and common opal can be discriminated using a combination of Th, Hf and Cs with the best independent discriminators being Th, Bi and La. Overall, Th is the best element by which to discriminate between precious and common opals as these are characterized by relatively low and relatively high concentrations of Th, respectively. In general, major and trace element concentrations are significantly higher in common opals than in precious ones. Precious opal color is very difficult to discriminate based on elemental concentrations as it depends on the size and ordering of spheres. However, brown common opal can be distinguished from grey common opal based on concentrations of Pb, Ba and Fe. Opal fingerprinting is therefore possible using just a small number of elements.