Opalisation of the Great Artesian Basin (central Australia): an Australian story with a Martian twist

Abstract

This paper exposes the unique set of attributes explaining why precious opal has formed in such abundance in central Australia, and almost nowhere else on Earth. The Early Cretaceous history of the Great Artesian Basin is that of a high-latitude flexural foreland basin associated with a Cordillera Orogen built along the Pacific margin of Gondwana. The basin, flooded by the Eromanga Sea, acted as a sink for volcaniclastic sediments eroded from the Cordillera's volcanic arc. The Eromanga Sea was shallow, cold, poorly connected to the open ocean, muddy and stagnant, which explains the absence of significant carbonates. Iron-rich and organic matter-rich sediments contributed to the development of an anoxic sub-seafloor in which anaerobic, pyrite-producing bacteria thrived. Rich in pyrite, ferrous iron, feldspar, volcanic fragments and volcanic ash, Lower Cretaceous lithologies have an exceptionally large acidification potential and pH neutralisation capacity. This makes Lower Cretaceous lithologies particularly reactive to oxidative weathering. From 97 to 60 Ma, Australia remained at high latitude, and a protracted period of uplift, erosion, denudation and crustal cooling unfolded. It is possibly during this period that the bulk of precious opal was formed via acidic oxidative weathering. When uplift stopped at ca 60 Ma, the opalised redox front was preserved by the widespread deposition of a veneer of Cenozoic sediments. On Earth, regional acidic weathering is rare. Interestingly, acidic oxidative weathering has been documented at the surface of Mars, which shares an intriguing set of attributes with the Great Artesian Basin including: (i) volcaniclastic lithologies; (ii) absence of significant carbonate; (iii) similar secondary assemblages including opaline silica; (iv) similar acidic oxidative weathering driven by very similar surface drying out; and, not surprisingly, (v) the same colour. This suggests that the Australian red centre could well be the best regional terrestrial analogue for the surface of the red planet.

本文发表的一系列因素可用来解释为什么珍贵蛋白石在澳洲中部形成量如此之大，全球几乎没有一个地方有类似的状况。Great Artesian 盆地的早白垩世历史是高纬度弯曲前陆盆地，与沿冈瓦纳大陆太平洋边缘的Cordillera造山带相关。这个盆地受Eromanga海水侵入，成为剥蚀于Cordillera火山弧的火山碎屑沉积物的沉积地。Eromanga海浅而冷，由于与开放海洋的连通不佳，导致含泥量高且水流不畅。这点可以解释碳酸盐的缺失。富铁和富有机体的沉积物导致缺氧海底的形成，这里厌氧的成黄铁矿细菌繁盛。下白垩统岩石富含黄铁矿、铁、长石、火山碎块和火山灰，具有特别大的酸化作用潜力和pH中和作用能力。这使得下白垩统岩石对氧化性风化作用的反应性特别强。在97Ma至60Ma期间，澳大利亚一直处于高纬度，经受长期的抬升、侵蚀、剥蚀和地壳冷却。可能就在这段时期，大量珍贵蛋白石由于酸性氧化风化作用而形成。当抬升在60Ma停止时，蛋白石化的氧化带因一层新生代沉积物的大幅度沉积而得以保存。在地球上，区域性的酸性风化作用少见。有趣的是，火星上有酸性氧化风化作用的记录。它与Great Artesian盆地有相同的一系列特性：（i）火山碎屑的岩石特征；（ii）明显碳酸盐的缺失；（iii）相似的次生组合包括硅质蛋白石；（iv）相似的酸性氧化风化作用，由于很相似的表面干燥过程而导致；以及意料中的（v）相同的颜色。这表明澳大利亚红色中部区可以作为火星表面最好的区域地质类比。

Keywords:

• Australian precious opal
• Great Artesian Basin
• acidic weathering
• Mars analogue

ACKNOWLEDGEMENTS

The research presented here was supported by the Australian Research Council through a Discovery Grant (DP0987604). Reviews and insights from Jonathan Clarke and Colin Pain greatly improved the manuscript. Many thanks to the Lightning Ridge Miners Association and in particular to Maxine O’Brien, its executive secretary, for getting me involved in exploring the formation of precious opal in the GAB. A long list of opal miners offered access to their mines, providing countless samples, and most importantly sharing their knowledge on opal formation. Many thanks are due to all of them with a particular mention to Bob Barrett (Lightning Ridge), George Kountouris (Coober Pedy), Col Duff (Winton) and Eric Stelzer (Quilpie) who spent days guiding us across their opal fields. Many colleagues, who shared their intimate knowledge on geochemistry, gemmology, micro-analysis, hyperspectral techniques, geodynamics, paleogeography and paleontology, were involved directly or indirectly in the shaping of some of the ideas presented here. I acknowledge here the critical input of Stephen Aracic, Jenni Brammall, Joel Brugger, Elizabeth Carter, Adriana Dutkiewicz, Nicolas Flament, Emmanuel Fritsch, Thomas Landgrebe, Nicolas Mangold, Dietmar Müller, Simon Pecover, Erick Ramanaidou, Benjamin Rondeau, Balwant Singh, Hallvard Skjerping, Tony Smallwood, Elizabeth Smith, Paul Thomas, Patrick Trimby and Lew Whitbourn, to name a few. Many students have contributed to maintain the opal project alive; I thank in particular Ingrid Van der Beek, Roel Verbene, Andrew Merdith and Gemma Roberts.