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Opal is Australia's national gemstone with a significant fraction of the global supply mined from highly weathered Cretaceous sedimentary rocks within the Great Artesian Basin. Surprisingly, relatively little is known about the petrography and trace elemental composition of opal and its host rocks and consequently about the source of silica that underpins its formation. Using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) of precious and common opal from key opal mining areas in the Great Artesian Basin coupled with multivariate analyses of 59 detectable elements in opal, we show that a mining region from which an opal originates can be constrained by using a combination of Hf, Ba, Zr and Gd with a high degree of confidence. Likewise, precious opal can be distinguished from common (non-precious opal) using a combination of Bi, Ta, Sn and Ca as these particular elements are especially low in concentration in precious opal. Although the opal from the Great Artesian Basin is sedimentary, the Ba content of opals from the eastern part of the basin suggests a volcanic origin. The most likely source of Ba and hence of silica for these opals are feldspars, now altered to kaolinite, sourced as volcaniclastic sediment from the Cretaceous Whitsunday Volcanic Province that marked the rifting and breakup of eastern Gondwana. The alteration of detrital feldspars to kaolinite and their replacement by void-filling opal confirms that weathering has played a critical role in the formation of Australian opal. The opal host rocks are severely weathered with a chemical index of alteration (CIA) up to 92. For the majority of opals studied, the silica is most likely derived locally from the opal host rocks, which impart a unique elemental signature on the opal at any particular locality. Mintabie opal, however, has very low Zr/Hf ratio, which is decoupled from its host rock, suggesting that the silica source is different from all the other opals, or that the silica fluid has experienced intense trace element fractionation, or both. The combination of analytical and statistical methods used here provides a powerful tool for a wide range of provenance studies, not just gemstones, where relationships between a large number of major and trace elements are difficult to unravel. |
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