Opalisation of the Great Artesian Basin (Central Australia): an Australian Story with a Martian Twist P

Home , Bulldog Shale, Rolling Downs Group, Toolebuc Formation, Wallumbilla Formation

This article was downloaded by: [203.45.115.130] On: 03 June 2013, At: 22:34 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/taje20 Opalisation of the Great Artesian Basin (central Australia): an Australian story with a Martian twist P. F. Rey a a Earthbyte Research Group—School of Geosciences , The University of Sydney , Sydney , NSW , Australia Published online: 22 May 2013.

To cite this article: P. F. Rey (2013): Opalisation of the Great Artesian Basin (central Australia): an Australian story with a Martian twist, Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia, 60:3, 291-314 To link to this article: http://dx.doi.org/10.1080/08120099.2013.784219

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. TAJE_A_784219.3d (TAJE) 03-05-2013 15:56

Australian Journal of Earth Sciences (2013) 60, 291–314, http://dx.doi.org/10.1080/08120099.2013.784219

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

P. F. REY

Earthbyte Research Group—School of Geosciences, The University of Sydney, Sydney NSW 2006, Australia

This paper exposes the unique set of attributes explaining why precious opal has formed in such abun- dance 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.

KEYWORDS: Australian precious opal, Great Artesian Basin, acidic weathering, Mars analogue

INTRODUCTION recognition’ approach is the only avenue to guide opal prospectors (Aracic 1999). A computerised equivalent of According to a much-repeated statement, more than 90% this approach, via the data-mining ‘unsupervised learn- ’ of the world s precious opal comes from the Great Arte- ing approach’ to find hidden patterns in a randomly se- sian Basin (GAB) in central Australia. Regardless of the lected dataset, has been recently proposed (Merdith accuracy of this figure, the GAB has been the main

Downloaded by [203.45.115.130] at 22:34 03 June 2013 et al. 2013). As a gemstone, however, precious opal is well source of gem-quality opal worldwide over the past characterised. In the 1960s, researchers from CSIRO 120 years, and precious opal is one of most recognisable documented the microstructure of precious opal, and ’ Australian icons as well as Australia s National Gem- explained the play of colour as diffraction caused by a stone since 1993. Over the past decade, opal mining has regular three-dimensional array of uniformly sized slowed considerably, accelerating the decline of many amorphous silica spheres 1500–4000 A in diameter (Sand- townships. According to opal miners, this is due in part ers 1964, 1968; Darragh et al. 1966; Sanders & Darragh to a decline in the number of active miners strongly af- 1971; Darragh et al. 1976; Sanders & Murray 1978). Since fected by the rising cost of mining, the lack of discovery these early studies, research has focussed mainly on the of new opal fields and the lack of science to guide explo- physical and chemical properties of opal (McOrist et al. ration. Nevertheless, opal is still an important part of 1994; McOrist & Smallwood 1995, 1997; Smallwood et al. the economic fabric and cultural identity of many town- 1997, 2007, 2008a, 2008b, 2009; Brown et al. 2002, 2003, 2004; ships of central Australia. Thomas et al. 2006, 2007, 2008, 2010; Gaillou et al. 2008a). Despite its iconic status and a long opal mining his- Only a few untested propositions have been put forth to tory, the geological processes behind the formation of explain the formation of precious opal in the GAB. Early precious opal in the GAB are still largely unknown. Ow- on, it was proposed that opal formed during the Ceno- ‘ ing to this lack of knowledge, the empirical pattern zoic, from silica released through the weathering of

[email protected] Ó 2013 Geological Society of Australia TAJE_A_784219.3d (TAJE) 03-05-2013 15:56

292 P. F. Rey

detrital feldspar and volcanic ash into kaolinite, at a time when the aridity of the climate restricted ground water to shallow traps (Darragh et al. 1966; Senior et al. 1977; Senior & Mabbutt 1979). According to this model, precious opal may have formed slowly via dehydration of silica-rich gel trapped in fractures, and cavities resulting from the decomposition and leaching of fossils and minerals (Darragh et al. 1966). In order to understand why there is so much precious opal in the GAB and almost nowhere else on Earth, this paper seeks to document the attributes that make the GAB unique on Earth, and to place the formation of precious opal within its geological context. In particular, this paper emphasises that the sedimentary rocks of the GAB are particularly reactive to oxidative weathering owing to a combination of their exceptional acidification potential and exceptional neutralisation capacity. This paper proposes that opal formed at near-surface conditions, along the redox front where reduced lithologies and groundwater met oxic conditions. According to this model, opal is the product of self-limiting acidic oxidative weathering. This type of regional scale weathering is unusual at the Earth’s surface. Very acidic weathering is locally documented, mainly in association with acid mine drainage (e.g. Jambor et al. 2000), volcanoes (e.g. Morris et al. 1996), and acid sulfate soils around estuaries (e.g. Wor- rall & Clarke 2004). Although Cenozoic to Recent ages cannot be ruled out, peak production of precious opal may have occurred from ca 97 Ma to ca 60 Ma during the denudation of the GAB that followed the inland sea regression and drying out of the GAB landscape. This model, largely based on a review of three decades of re- Figure 1 Simplified geological map of the Lower Cretaceous search on the evolution of the GAB and its surroundings, geology of the Great Artesian Basin (central Australia), and research on weathering processes, explains why showing the location of main opal fields (solid black many opal mines are underneath preserved Cenozoic sil- squares). A, Andamooka; C-P, Coober Pedy; J, Jundah; M, crete caps. It is consistent with the observation that no Mintabie; L, Lambina; L-R, Lightning Ridge; O, Opalton; Q, opal deposit and no opalised fossils have been found in Quilpie; Y, Yowah; W-C, White Cliffs; W,Winton. overlying Cenozoic sandstones and siltstones. Interest- ingly, acidic oxidative weathering has been documented at the surface of Mars (Milliken et al. 2008), and as for Winton Formations (Yowah, Quilpie, Jundah, Opalton, the GAB, this weathering is interpreted as the conse- Winton, Lightning Ridge) in the interior of the GAB. Downloaded by [203.45.115.130] at 22:34 03 June 2013 quence of the drying out of its surface. This paper emphasises that Mars and the GAB share similar surface lithologies, similar weathering history and processes, similar secondary mineralogy and, last but not Opal ﬁelds and deep weathering least, the same colour. Deep weathering, down to 100 m depth, affects Creta- ceous and Cenozoic lithologies of the GAB. A source of THE FORMATION OF OPAL IN THE GAB: PREVIOUS acidity is often invoked to explain this deep weathering WORK (Senior & Mabbutt 1979; Thiry & Milnes 1991; Thiry et al. 1995, 2007; Simon-Coincon¸ et al. 1996; Horton 2002; New- Opal ﬁelds in the stratigraphy of the GAB berry 2005). Geochemical modelling has shown that With the exception of the Mintabie opal field, which is in weathering of fresh shales containing pyrite by a sulfate- Ordovician sandstones and siltstones but within a few rich brine produces sulfuric acid and explains the for- metres beneath the eroded Lower Cretaceous unconfor- mation of kaolinite, alunite, gypsum and amorphous sil- mity, precious opal in the GAB occurs within 50 m of the ica (or common opal) observed in Cenozoic paleosurface surface in sub-concordant levels in deeply weathered (Thiry et al. 1995). Unlike precious opal, common opal is Lower Cretaceous sedimentary rocks (Jones & Segnit a very frequent occurrence in many weathered continen- 1966). Opal fields are distributed in the marine sand- tal sediments, in which it can form in large amounts. For stone and siltstone of the Bulldog Shale and Wallumbilla instance, in bleached Cretaceous and Cenozoic litholo- Formations (Coober Pedy, Andamooka, White Cliffs) gies of the GAB, common opal can make up to 60% of the along the margin of the GAB (Figure 1), and also within rock (e.g. Thiry et al. 1995, 2007; Simon-Coincon¸ et al. fluvial sandstone and siltstone of the Griman Creek and 1996). TAJE_A_784219.3d (TAJE) 03-05-2013 15:56

Opalisation of the Great Artesian Basin 293

However, a low pH is not suitable for the formation of conditions during or after the late Oligocene to Miocene precious opal (Iler 1979). Acidic conditions promote sil- (Horton 2002). ica precipitation and lead to amorphous silica in which silica spheres of uneven size bind together randomly SYN-TECTONIC MODEL into highly disordered arrays to form common opal that does not show the play of colour characteristics of pre- Miners have noted the common close association be- cious opal (Iler 1979). In contrast, precious opal requires tween precious opal deposits and faults. This observa- alkaline conditions. In silica-rich colloid at high pHs, tion has led to the proposition that opal may have been negatively charged mutually repelling silica spheres of produced during episodic syn-tectonic fracturing involv- uniform size develop (Iler 1979). This self-organising col- ing over-pressurised silica-rich fluids (Pecover 1996, loid forms the matrix in which precious opal develops. 1999, 2007). According to this scenario, the source of silica-rich fluid may be local (i.e. fluids from host lithologies mobilised during compression), or more distant The timing of formation of precious opal such as hot artesian waters coming from deep within the ‘ ’ Paleomagnetic dating (e.g. Idnurm & Senior 1978; Smith basin. According to this so-called syn-tectonic model, et al. 2009), oxygen isotope dating (Bird & Chivas 1993) precious opal in veins may have formed through slow po- and K–Ar and Ar–Ar dating of alunite and hollandite lymerisation of dissolved silica, while excess silica es- (Bird et al. 1990; Newberry 2005; Vasconcelos et al. 2008) caping to the surface may have produced Cenozoic suggest that weathering and opalisation may have silcretes, suggesting that opal deposits are Cenozoic in occurred during a few discrete weathering episodes from age (Pecover 1999). The syn-tectonic model is supported – Late Cretaceous to Cenozoic (Jones & Segnit 1966; by studies emphasising the role of upper Oligocene – Wopfner et al. 1974; Senior & Mabbutt 1979; Simon- Lower Miocene long wavelength (20 50 km), low ampli- Coincon¸ et al. 1996). More recently, radiocarbon dating on tude (few tens of metres) doming (Wopfner et al.1974; carbon inclusions in opal from Lightning Ridge indicates Senior & Mabbutt 1979; Horton 2002). Noting that pre- ages <46 ka (e.g. Dowell et al. 2002; Fink 2011) suggesting cious opal deposits are often found at the crest of large continuous formation during the Cenozoic to Recent. domes, whereas Cenozoic silcretes often form along domes flanks (Senior & Mabbutt 1979), Horton (2002) has proposed that the bulk of precious opal formed during or Previous models on the formation of precious soon after the late Oligocene–early Miocene doming opal in the GAB (<24 Ma) via the remobilisation of silica under warm arid alkaline conditions. There is little doubt that opal- Although the physics and chemistry behind the forma- bearing hydraulic fractures are common in many if not tion of common and precious opal are reasonably well all opal fields (e.g. Glass-Van der Beek 2003). However, the understood, the exceptional geological and/or geochemi- syn-tectonic model does not explain the formation of the cal conditions necessary for the production of precious silica-rich fluid, nor does it explain the formation of other opal remain to be explained. Nevertheless, and in addi- types of deposits, in particular opalised fossils. More tion to the weathering model, a few propositions have problematic for this model is the absence of precious opal been made to explain in geological terms the formation in weathered Cenozoic sandstones and mudstones. of precious opal in the GAB.

RADIOACTIVE CATALYST MODEL

Downloaded by [203.45.115.130] at 22:34 03 June 2013 DEEP WEATHERING The natural radioactivity of opal levels often appears Because of its close association with weathered rocks, it higher than background radioactivity (Senior & has been proposed that opal was the product of the deep Chadderton 2007). Based on this observation, a model weathering that shaped the central Australian landscape was proposed in which silica spheres in precious opal (Darragh et al. 1966; Senior et al. 1977). According to this grow around a radioactive catalyst nano-nucleus pro- model (the so-called deep weathering model), silica was moting optimum spherulite development (Senior & released during Cenozoic to Recent weathering (Darragh Chadderton 2007). Indeed, silica spheres making pre- et al. 1966; Senior et al. 1977; Horton 2002) through the cious opals often show growth layering centred on one breakdown of feldspar, volcanic clasts and volcanic ash or a few nano-nucleii (Darragh et al. 1966; Gaillou et al. in Lower Cretaceous sandstones (Exon & Senior 1976; 2008b). These nano-particles could be made of heavy met- Senior & Mabbutt 1979). Silica-rich fluids filled the als such as uranium. However, Australian opals are porosity, cracks and fractures of the host rocks, and known for their low uranium-content compared with cavities resulting from the decomposition and leaching other precious opals in the world. of fossils and minerals (Darragh et al. 1966). Through dehydration, the trapped silica gel would have turned into MICROBIAL MODEL precious opal. Given the role of low pH in the mobilisa- tion of silica and that of high-pH in the formation of reg- Several authors have emphasised the possible role of aer- ular array of silica sphere, it has been proposed that obic microbes through the release of organic acid. Al- formation of precious opal may have had a multi-stage though microbe remains are only rarely observed in history involving release of silica under humid and precious opal, fossilised microbes are frequent in com- acidic conditions during the Late Cretaceous to mon opal, as well as in their host rocks (Behr et al. 2000; Paleocene, and remobilisation under arid and alkaline Watkins et al. 2011). These aerobic microbes point to an TAJE_A_784219.3d (TAJE) 03-05-2013 15:56

294 P. F. Rey

oxic, near-surface environment (T <35C) with near- PALEOGEOGRAPHY, TECTONICS, STRATIGRAPHY neutral pH (Watkins et al. 2011). This model suggests AND WEATHERING OF THE GAB that carbonic and organic acids may have aided the bio- chemical weathering of clay and feldspar to produce sil- This section reviews the geological evolution of the GAB ica-rich solutions. Importantly, and in contrast to other over the past ca 130 Ma. It emphasises the unusual set of hypotheses, this model proposes that opal formed during geologic, paleogeographic and environmental attributes the Early Cretaceous at the time of sediment deposition that led to a regional-scale, high-latitude and long-lived (Watkins et al. 2011). episode of acidic oxidative weathering perhaps unique on Earth.

SUMMARY OF PREVIOUS MODEL Paleogeography and tectonics Each of the above propositions is based on pertinent observations. However, each invokes unexceptional pro- The Eromanga Basin, the Surat Basin and the Carpenta- cesses that contrast with the relatively rare occurrence ria Basin are part of the GAB (Figure 1). They were of precious opal in the world. In addition, none of these formed in the Jurassic–Early Cretaceous (e.g. Exon & models convincingly explain why so much precious opal Senior 1976) as part of retro-arc flexural foreland basin formed across the GAB, and comparatively so little in adjacent to the Zealandia Cordillera. Zealandia was a the rest of the world. The formation of precious opal in major Cordillera Orogen built along the Pacific margin the GAB can only be the fortuitous result of a set of geo- of East Gondwana as a result of a long-lived west dipping logical circumstances perhaps unique on Earth. Since subduction zone (Tulloch et al. 2006). This Cordillera in- the GAB is one of the largest intracontinental basins on cluded a largely andesitic volcanic arc called the Whit- Earth, and since opal is found in many locations in the sunday Volcanic Province (Figure 2a) (Bryan et al. 1997; GAB, the conditions favourable to the formation of pre- Tulloch et al. 2006). This volcanic arc, one of the largest cious opal must have been peculiar yet widespread felsic volcanic provinces on Earth (Bryan et al. 1997), across the basin. To understand these peculiar condi- was active between 132 and 95 Ma and extended along tions one must address a simple question: What makes the Pacific margin of East Gondwana from offshore the GAB unique on Earth? To answer this question, we North Queensland to Antarctica (Figure 2a). Paleogeo- must consider the geological history of the GAB. graphic reconstructions show that from ca 130 to ca 97 Downloaded by [203.45.115.130] at 22:34 03 June 2013

Figure 2 GPlate-derived (Boyden et al. 2011) paleogeographic reconstruction of Australia over the past 130 Ma, and location of the main geological elements mentioned in the text (GAB, Whitsunday Volcanic Province, Ceduna Sub-basin, Winton Forma- tion, Eyre Basin). TAJE_A_784219.3d (TAJE) 03-05-2013 15:56

Opalisation of the Great Artesian Basin 295

Ma (Figure 2a), East Gondwana (i.e. Australia and Ant- et al. 1987; Gallagher & Lambeck 1989; Gurnis et al. 1998; arctica) detached from Gondwanaland and moved to the Campbell & Haig 1999; Matthews et al. 2011). In the strati- southeast at near constant velocity toward the Pacific/ graphic record, this major marine flooding episode cor- Phoenix plates (e.g. Muller€ et al. 2008). At that time, New responds to a change from mature sandstone to very Caledonia, Lord Howe Rise, New Zealand’s South and immature arkose dominated by feldspar and volcanic North Islands, Challenger Plateau, Campbell Plateau, clasts (Exon & Senior 1976). The average content of Chatham Rise and Marie Byrd Land were all part of the quartz (14.5%), feldspar (41%) and lithic clasts (44%, the Zealandia Cordillera (Tulloch et al. 2006). During its east- vast majority of which is of andesitic origin) of the ward motion, East Gondwana moved over a mantle Lower Cretaceous Rolling Downs Group, which includes down-welling zone associated with the subduction of the both marine and fluvial sedimentary rocks, points to the west-dipping Pacific slab. Under the pull of the down- Whitsunday Volcanic Arc as a source (e.g. Exon & Senior welling mantle, the surface of Australia subsided below 1976; Hawlader 1990; Bryan et al. 1997, 2012). River sys- sea-level (e.g. Gurnis et al. 1998; Matthews et al. 2011), tems surrounding the Eromanga Sea filled the GAB with and a marine transgression, well constrained by subsi- iron-rich and organic-rich continental sediments, and dence analysis (e.g. Gallagher & Lambeck 1989; Deighton large volumes of volcaniclastic sediments eroded from & Hill 1998), was initiated in the Barremian (130–125 the 132–95 Ma Whitsunday Volcanic Province (Bryan Ma). This epicontinental sea, called the Eromanga Sea, et al. 1997). The Aptian–Albian Bulldog Shale, and its lasted until the eastward motion of East Gondwana equivalent the Wallumbilla Formation in the central slowed down suddenly during the late Albian and early and northern part of the GAB, and the Doncaster Mem- Cenomanian (100–97 Ma; Exon & Senior 1976). At its ber in the Surat Basin, are widespread and consist of py- peak, during the early–late Albian, the Eromanga Sea rite and carbonaceous-rich marine mudstones and flooded 60% of Australia (Frakes et al. 1987). siltstones interbedded with fine sandstones. From 97 to 60 Ma, East Gondwana remained relatively The Eromanga Sea was too cold and too muddy, to al- stationary and at high latitude, although drifting slowly low for the formation of thick carbonate platforms. Nev- to the northeast and away from Antarctica (Figure 2b), ertheless, thin (<3 cm) beds of limestone can be found in while the Tasman Sea, Ross Sea and West Antarctic rift the thin, yet widespread, Toolebuc Formation (101–98 systems were initiated (Gaina et al. 1998). The sudden Ma), a dominantly organic-rich black shale marking the slowdown of East Gondwana at ca 97 Ma is synchronous maximum deepening of the epicontinental sea in the with a rapid regression during the late Albian and early late–middle to early–late Albian (Haig & Lynch 1993; Cenomanian, and with a switch from marine to fluvial- Henderson 2004). Foraminiferal biofacies suggests sea- deltaic sedimentation during which the Winton Forma- water depth of about 50 m in the Surat Basin, and up to tion was deposited (Figure 2b). This was followed from 150 m in the Eromanga Basin and southern part of the ca 93 to ca 60 Ma by a period of protracted uplift and de- Carpentaria Basin (Haig & Lynch 1993). Importantly, the nudation at high-latitude and a cold climate (Bird & Toolebuc Formation hosts the remains of bivalve colo- Chivas 1988, 1989, 1993). Fluvial sedimentation resumed nies well adapted to oxygen-poor to anoxic environment with the deposition of the Eyre Formation during the (Henderson 2004). This suggests that, at time of maxi- late Paleocene while Australia’s motion switched to the mum basin depth, the redox boundary lay close to or northwest. Fluvial sedimentation of the Eyre Formation above the sediment–water interface (see also Boreham & lasted from ca 60 to 40 Ma (Figure 2c). Finally, from Powell 1987). This is independently confirmed by the de- 40 Ma, Australia’s motion switched to the northeast position, in the Toolebuc Formation, of molybdenum moving rapidly to its present position (Figure 2d). and vanadium typically mobilised from organic matter Downloaded by [203.45.115.130] at 22:34 03 June 2013 in anoxic conditions (Patterson et al. 1986; Lewis et al. 2010). These anoxic conditions, at the time when the epi- Stratigraphy and deposition environment continental sea was at its deepest, suggest poor bottom- The Lower Cretaceous stratigraphy of the GAB records in water circulation over the entire duration of the flooding minute detail the paleogeography and changes in plate episode (Exon & Senior 1976), most likely because the motion of East Gondwana since its breakup from Eromanga Sea was shallow and only poorly connected to Gondwana ca 130 Ma ago. Early Cretaceous sedimenta- the open ocean via a narrow and shallow channel in the tion in the GAB started at around 136–125 Ma ago with Gulf of Carpentaria (Figure 2a). the deposition of the quartz-rich sandstones and conglom- The Surat Siltstone, equivalent in age to the Toolebuc erates of the fluvial-paralic Cadna-owie Formation in the Formation, marks the maximum deepening in the Surat Eromanga Basin, the age-equivalent feldspar-rich sand- Basin (Haig & Lynch 1993), and marginal-marine facies stones of the Bungil Formation in the Surat Basin, and of the base of the overlying Griman Creek Formation in- the George River Formation in the Carpentaria Basin clude disseminated pyrite and glauconitic sandstone (Exon & Senior 1976). At that time, Australia, still at- beds (Kernich et al. 2004). Glauconite is a frequent authi- tached to Antarctica, was at high-latitudes and near freez- genic mineral in cool water marine sandstone. It results ing conditions (e.g. Veevers et al. 1991) as shown in the from interaction between nutrients and siliceous and southern edge of the GAB by diamictite deposits at the carbonate biotas and, as pyrite, it requires reducing con- base of the Cadna-owie Formation (Alley & Frakes 2003). ditions to prevent oxidation into hematite (e.g. Reid et al. From ca 130 to ca 97 Ma, dynamic subsidence, related 2008). This suggests that anoxic conditions extended to to the eastward motion of Gondwana over the subduct- the southeast into the Surat Basin. ing Pacific plate, led to the formation of a high-latitude In the late Albian to early Cenomanian, the onset of shallow epicontinental sea in central Australia (Frakes deposition of the Winton Formation, a thick fluvial- TAJE_A_784219.3d (TAJE) 03-05-2013 15:56

296 P. F. Rey

lacustrine to coastal plain sequence of carbonaceous and De Caritat 2003). These formations include levels of py- pyritic shale, siltstone and coarse volcaniclastic sand- ritic laminated carbonaceous mud (Wopfner 1974; Tonui stone with minor coal layers (Krieg et al. 1995), marks & De Caritat 2003) and host a rich fauna of fossils includ- the regression of the Eromanga Sea (e.g. Dettmann et al. ing gastropods, ostracods, algae, fish, reptiles, birds and 1992, 2009). The presence of disseminated pyrite in the mammals (e.g. Callen 1977; Woodburne et al. 1993; Alley Winton Formation suggests that anoxic conditions were 1998; Roberts et al. 2009). During the Pliocene to Quater- maintained at shallow depth during its deposition. The nary, lacustrine evaporitic sediments including arenite, sudden influx of volcaniclastic sediments at ca 97 Ma calcrete and gypcrete, and aeolian sediments, deposited indicates that the Pacific margin of Gondwana was being in the Eyre Basin (Alley 1998). uplifted at that time in relation to the initiation of the opening of the Tasman Sea (e.g. Persano et al. 2002). De- Erosion and weathering position of the Winton Formation ceased at ca 93–91 Ma, before the early Turonian (Moore & Pitt 1984), and a long Central Australia owes its vibrant yellow to rust-red period of non-deposition followed from ca 93 to 60 Ma. hues and a large part of its geomorphology to a long his- The preserved thickness of the Winton Formation tory of weathering. As such, weathering is a key attri- reaches 1200 m in the centre of the basin, and it over- bute to the geology of the GAB. The rapid deposition of lays about 600 m of Lower Cretaceous parallic to marine the Winton Formation was followed by a protracted pe- sediments (Wallumbilla, Toolebuc, Allaru and Mack- riod of uplift, erosion, denudation and cooling lasting unda formations). until ca 60 Ma (Exon & Senior 1976; Deighton & Hill 1998; From ca 60 Ma to ca 40 Ma, Australia, now separated Raza et al. 2009). No super-Turonian to lower Paleocene from Antarctica, reversed its motion from southeast to (ca 93 to ca 60 Ma) sedimentary rocks were preserved be- northwest. During this time, the fluvial-lacustrine to aeo- fore the deposition of fluvial-lacustrine Cenozoic to Re- lian Cenozoic sediments of the Eyre Basin were deposited cent sediments of the Eyre Basin. over most of the GAB (Figure 2c). The Eyre Basin com- The Jurassic to Cretaceous sedimentary succession of prises the Upper Paleocene to Middle Eocene Eyre For- the GAB has a present maximum thickness of 2.7 km at mation (62–42 Ma) and the uppermost Oligocene to the basin depocentre (e.g. Gallagher & Lambeck 1989; Pliocene Etadunna and Namba formations (26–12 Ma). Passmore 1989). However, at the end of the deposition of The Eyre Formation rests unconformably above weath- the Winton Formation, the sedimentary pile was much ered Lower Cretaceous rocks including the Bulldog Shale, thicker. Studies on paleogeotherms from boreholes the Ooddanatta Formation and the Winton Formation. across the Eromanga Basin (Figure 3a, b) show that the Up to 120 m thick, it is made of fluvial and locally lacus- exposed present-day surface of the GAB was at a temper- trine sedimentary rocks, mainly derived from reworked ature between 50 and 90C during the mid-Cretaceous. Lower Cretaceous lithologies, and deposited by braided This cooling suggests that 1 km of sedimentary se- streams, accompanying epeirogenic uplift of the Olary quence has been eroded from the GAB, increasing to Block and the Barrier Ranges (Wopfner et al. 1974; Alley 3 km on the eastern margin of the basin (e.g. Gallagher 1998; Alley et al. 1999; Wulser€ et al. 2011). This formation et al. 1994; Mavromatidis 1997; Deighton & Hill 1998; Tin- includes conglomerate, pyritic mature quartz sandstone gate & Duddy 2002; Raza et al. 2009). Exhumation and with interbeds of clay lenses (44% kaolinite) and shale cooling may have increased further to the southeast pebbles, carbonaceous and micaceous siltstone, dolomitic where basement rocks along the Australian southeast siltstone and lignite with locally abundant wood frag- coast were at temperatures up to 120C and possibly be- ments, and contains large trunks of fossil trees (Wopfner tween 3 and 4 km depth before exhumation (Moore et al. Downloaded by [203.45.115.130] at 22:34 03 June 2013 et al. 1974). Polished gravels including frequent bleached 1986; Gallagher et al. 1994; Persano et al. 2002; Mavroma- pebbles from the underlying deeply weathered Creta- tidis & Hillis 2005). Products of erosion from central, east ceous sediments (Idnurm & Senior 1978; Alley 1998) mark and southeast Australia, and possibly as far as the Aus- the base of the Eyre Formation (Alley 1998). The top of tralian Eastern Highlands, were transported into the the Eyre Formation was eroded sometime between the Ceduna Sub-basin (offshore South Australia; Figure 3a, late Oligocene to early Miocene before being locally over- c) where 12–15 km of Upper Cretaceous sediments were lain by the fossiliferous Miocene beds of the Etadunna deposited (Totterdell et al. 2000; Veevers 2000; King & and Namba formations (Wopfner et al. 1974). Mee 2004; MacDonald et al. 2013). In a recent paper, Mac- From ca 40 Ma to Recent, as Australia moved faster to Donald et al. (2013) looks in detail at the provenance of the NNE, dolomite, limestone, fine sand and clay of the the sediments in the Ceduna Sub-basin. On the basis of Etadunna and Namba Formations deposited on the Eyre detrital zircon ages from a well at the centre of the Formation and locally on the Cretaceous strata (Alley Ceduna Sub-basin, this study shows that early in the 1998). Up to 250 m thick, the Etadunna and Namba for- Late Cretaceous (100–84 Ma), the Ceduna-River catch- mations consist of largely immature fluvio-lacustrine ment extended over the entire central, east and south- sand, silt and clay deposited in an alkaline and anoxic east Australia, and shrank towards the end of Late environment (e.g. Wopfner 1974; Tonui & De Caritat Cretaceous (84–65 Ma). This study also documents a 2003). Their mineralogy consists of quartz (up to 50%), ‘younging downward’ pattern with 100 to 85 Ma zircon kaolinite (up to 40%), feldspar (up to 15%), mica (up to ages in the deeper section of the well, and 120 to 100 Ma 15%), smectite (up to 5%), carbonate (up to 5%), illite (a ages in the shallowest, consistent with a progressive few %), iron oxide (a few %), palygorskite and dolomite unroofing of the sedimentary pile in the GAB. (characteristics of alkaline lakes), gypsum and alunite Overall, the Ceduna Sub-basin records a near-contin- (Callen 1977; Barnes et al. 1980; Alley et al. 1995; Tonui & uous sedimentation from ca 93 Ma to ca 34 Ma with a TAJE_A_784219.3d (TAJE) 03-05-2013 15:56

Opalisation of the Great Artesian Basin 297

Figure 3 Uplift, erosion, denudation and cooling of the GAB, and coeval sedimentation in the Ceduna Sub-basin. (a) Location of documented pre-late Paleocene weathering profile (empty stars), location of the Burunga 1 and Burley 2 wells discussed in (b) and location of cross-sections presented in (c). (b) Present-day geotherm and maximum geotherm recorded in the Burunga 1 well document a denudation of 1.5 km (after Raza et al. 2009). This is consistent with the temperature history recorded in the centre of the GAB (Burley 2 well) documenting a rapid cooling owing to denudation soon after the deposition of the Winton Formation. Dark grey shading shows the time window for the deposition of the Winton Formation, and the burial-related in- crease in temperature. The pale grey shading shows the time window for the deposition of the Eyre Formation (after Deighton & Hill 1998). (c) Simplified cross-section across the GAB (after Gallagher et al. 1994) and across the Ceduna Sub-basin (after

Downloaded by [203.45.115.130] at 22:34 03 June 2013 Totterdell & Bradshaw 2004) documenting the Late Cretaceous to early Paleocene denudation in the GAB and coeval sedimentation in the Ceduna Sub-basin.

sudden reduction in sedimentation rate at about 60 Ma downward, is a self-limiting process unless fresh rocks (Norvick & Smith 2001; Totterdell & Bradshaw 2004). Ap- are continuously advected to the weathering front by atite fission track and vitrinite reflectance analyses of erosion. The continuous uplift and erosion of the GAB samples from wells from the GAB, and the stratigraphy until 60 Ma helped weathering by removing altered of the Ceduna Sub-basin, suggest that denudation and rocks and exposing fresh rocks to alteration. This pre- cooling were not uniform but followed a two-stage evolu- late Paleocene weathering is directly confirmed by the tion (Figure 3b, c). The bulk of denudation occurred occurrence of silcrete pebbles in the basal conglomerate from ca 93 to ca 60 Ma, starting with a rapid pulse from of Eyre Formation (Exon & Senior 1976), and by ages of ca 93 to ca 80 Ma, and was followed from 60 Ma onwards weathering profiles in the GAB and its underlying base- by periods of alternating minor deposition and denuda- ment (Figure 3a). Paleomagnetic dating of ironstones tion (Deighton & Hill 1998; Tingate & Duddy 2002; Raza from the deep weathering profile in the Winton Forma- et al. 2009). tion near Windorah (south Queensland, Figure 3a) The period between ca 93 to 60 Ma was also one of reveals a weathering age of 60 10 Ma (Idnurm & Senior deep weathering in a cold environment (e.g. Wopfner 1978). Paleomagnetic dating of ferricrete from the Bull- et al. 1974; Senior et al. 1977; Bird & Chivas 1993; Alexan- dog Shale near Marla (northern South Australia) yields der & Jensen-Schmidt 1996). Weathering, involving a top- ages between 65 and 50 Ma (Pillans 2005). A similar paleo- down alteration front where surface water percolates magnetic age of 60 10 Ma is reported near Armidale to TAJE_A_784219.3d (TAJE) 03-05-2013 15:56

298 P. F. Rey

the east of the Surat Basin (Schmidt & Ollier 1988), as matching in age the duration of the flooding event, pro- well as near Cobar to the south of the GAB (Smith et al. vided a large source of volcaniclastic sediments and vol- 2009). Ar–Ar dating on Mn-oxide from a deeply ferrugi- canic ash. Although disseminated carbonates and thin nised weathering profile in the Mount Isa region reveals limestone layers have been reported in the GAB, the lack an age of ca 67 Ma (Vasconcelos et al. 2008). A K–Ar age of thick carbonate in this shallow continental sea is no- of 62 Ma is reported on an alunite in weathered Permian ticeable. No single factor can explain on its own the lack sandstone near Springsure to the east of the GAB (Bird of carbonate in shallow continental basins. In the GAB, et al. 1990). These geochronological data point to a major the combination of near freezing surface temperature period of deep regional weathering at the scale of the owing to the high-latitude of East Gondwana during the GAB before the late Paleocene (Figure 3a). This period is Cretaceous (documented by the presence of ikaite, dia- often assumed to represent weathering during a warm mictite and striated pebbles; David 1950; Alley & Frakes climatic period, which contrasts with the high latitude 2003), the influx of siliclastic sediments commonly lead- of Australia at that time (e.g. Bird & Chivas 1988; Fujioka ing to muddy and opaque waters, and the anoxic condi- & Chappell 2010). This may suggest a strong temperature tions explain the lack of thick carbonate. Importantly, gradient, with central Australia being warmer than disseminated pyrite is ubiquitous in Lower Cretaceous southern Australia (e.g. Fujioka & Chappell 2010), and/ sediments and has been reported in the widespread Bull- or that acidic water, perhaps in relation to high atmo- dog Shale (e.g. Thiry & Milnes 1991; Thiry et al. 1995), in

spheric CO2 (Schmitt 1999) or another source of acidity, the Wallumbilla Formation (Campbell & Haig 1999), in may have driven deep weathering despite colder the Toolebuc Formation (Boreham & Powell 1987; Hen- conditions. derson 2004), in the Griman Creek Formation (Kernich Post-late Paleocene erosion is limited in magnitude to et al. 2004) and in the fluvial Winton Formation (e.g. Se- a few tens of metres and is geographically localised to nior et al. 1977; Krieg et al. 1995). Along with glauconite, gentle domes and faulting caused by late Oligocene to pyrite suggests that Lower Cretaceous sediments were Miocene tectonic reactivation (Jones & Segnit 1966; buried in a reducing environment. Pyrite in sedimen- Ingram 1968; Hutton et al. 1978; Moore & Pitt 1984; Alley tary rocks is often produced by anaerobic sulfate-reduc- 1998; Moussavi-Harami & Alexander 1998; Alley et al. ing bacteria (e.g. Donald & Southam 1999; Schoonen 1999). The very limited sedimentation rate (a few hun- 2004). These bacteria thrive in anoxic, low-energy envi- dred metres of sediment deposited in 60 Ma), constant ex- ronment such as coastal wetlands, barrier estuaries posure and limited erosion during the Cenozoic are and coastal lakes where sulfate-rich seawater mixes with conditions favourable to slow, continuous and self-limit- Fe-rich and organic matter-rich continental sediments. ing weathering during which older weathering assemb- The enclosed shallow sea and fluvial-lacustrine plains lages are rejuvenated (Taylor & Shirtliff 2003). Oxygen surrounded by continental landmasses would have been isotope dating (Bird & Chivas 1988, 1989, 1993), paleo- an ideal ecosystem for the production of biogenic pyrite. magnetic dating (Idnurm & Senior 1978; Schmidt & Pyrite-rich and feldspar-rich lithologies, and the lack Ollier 1988; McQueen et al. 2002; Pillans 2005, 2007; Smith of thick carbonates, are key attributes of the Lower et al. 2009), and K–Ar and Ar–Ar dating of alunite (Bird Cretaceous stratigraphy of the GAB. Together, these et al. 1990; Newberry 2005) and Mn-oxide (Vasconcelos & attributes make the GAB unique on Earth by providing Conroy 2003; Vasconcelos et al. 2008) largely confirmed a the GAB with both a large acidification potential long and continuous period of weathering from ca 45 Ma (through the oxidation of pyrite, and ferrolysis of fer- to present. Indeed, Cenozoic weathering has profoundly rous iron) and a large pH neutralisation capacity reworked and rejuvenated the Late Cretaceous to Paleo- (through the weathering of feldspar and volcanic ash). Downloaded by [203.45.115.130] at 22:34 03 June 2013 cene weathering (Taylor & Shirtliff 2003). Therefore, These potentials may have been realised, from the Late ages ranging from ca 30 to ca 5 Ma obtained via K–Ar Cretaceous to Recent, when Lower Cretaceous litholo- and Ar–Ar geochronology on secondary minerals, such gies where exposed to the oxic world. The following sec- as alunite and hollandite, in close association with opal tion describes field observations supporting the possible (Bird et al. 1990; Newberry 2005) may not constrain the role of acidic oxidative weathering in the production of formation of precious opal but may instead provide a precious opal. minimum age for the weathering profile (e.g. Vasconce- los et al. 2008). A REDOX FRONT MODEL FOR THE FORMATION OF PRECIOUS OPAL IN THE GAB

WHAT MAKES THE GAB UNIQUE ON EARTH? Field observations The GAB basin is an interesting example of high-latitude Opal miners are keen to emphasise how different their retro-arc flexural foreland basin. The Eromanga Sea that opal fields are from other opal fields in the GAB. Never- flooded central Australia was shallow (<150 m), poorly theless, opal fields across the GAB share many features connected to the open ocean, relatively stagnant and that point to a single precious-opal forming process com- rich in continent-derived organic matter. This led to the mon to all opal fields. With the exception of the Mintabie development of a cold, muddy and anoxic depositional opal field, precious opal in all opal fields is found in environment, in which anaerobic sulfate-reducing bacte- weathered Lower Cretaceous sedimentary rocks (Jones ria, some of which were well adapted to cold environ- & Segnit 1966). When present, Cenozoic rocks from the ments (e.g. Knoblauch et al. 1999; Jørgensen 2006), Eyre Basin are capped with silcretes made of silicified thrived. In the mean time, a major volcanic arc, sandstone and conglomerate. Although precious opal is TAJE_A_784219.3d (TAJE) 03-05-2013 15:56

Opalisation of the Great Artesian Basin 299

often considered of Cenozoic age (Jones & Segnit 1966; Observations of viscous flow structures, frequent in Senior et al. 1977; Bird et al. 1990; Simon-Coincon¸ et al. opal-bearing veins (Figure 7), suggest that precious opal 1996; Horton 2002; Newberry 2005) only common opal has was produced not via solid-state reaction, but during the ever been found in weathered Cenozoic rocks. production of a low viscosity gel, in which both precious In weathered Lower Cretaceous rocks, opalised hori- and common opal domains coexisted (e.g. Glass-Van der zons generally do not exceed 50–80 cm in thickness and Beek 2003). Under local differential pressure, this com- are commonly at the contact between permeable sand- posite gel was able to flow over short distances, while stone and underlying less permeable claystone preserving precious opal bands side by side with com- (Figure 4a, b). Although opal can seal the porosity of the mon opal bands (e.g. Glass-Van der Beek 2003). The silica base of sandstone beds in contact with claystone spheres themselves were soft enough to deform into a (Figure 4c) over a distance of 10–20 cm (known as the hexagonal shape during the early stage of their arrange- ‘steel-band’ in Lightning Ridge), precious opal mainly ment into a regular array (Darragh et al. 1966). occurs in claystone beds. Commonly, sandstone–clay- In summary, opalised levels are closely associated stone contacts are underlined by a <10 cm thick ferrugi- with weathered Lower Cretaceous rocks. These opalised nous band, which includes numerous precious opal- horizons extend laterally and remain at near constant el- bearing micro-fractures (Figure 4a, b). In the opalised evation over many tens of kilometres, often protected horizon, fibrous gypsum is commonly present as centi- from erosion by overlying Cenozoic silcrete caps, and metre-thick sub-horizontal veins discordant on the sedi- strongly suggest that precious opal deposits evolved mentary layering (Figure 4d). In Coober Pedy opal fields, along regional weathering fronts. Precious opal devel- gypsum veins are particularly thick (up to 40 cm) and in- oped in a silica gel and preferentially at the contact be- clude vertical veins contemporaneous with sub-horizon- tween permeable sandstone and mudstone, or in the tal veins. In the opal levels, precious opal occurs as vicinity of permeable faults and fractures, or within the cement in sandstone (known as matrix opal, Figure 4c); pore space of volcaniclasitic sandstone. This suggests it fills microfractures (Figure 4e), desiccation cracks that fluid–rock interactions involved in the development (Figure 4f), veins and cavities in ironstones (boulder of precious opal were favoured in the vicinity of gra- opal, Figure 4g); and it commonly occurs as replacement dients of permeability. Once formed, the silica-rich col- of mineralised fossils and as replacement of clay rip-up loid filled pore space, cracks, fractures and cavities clasts (the so-called Yowah nuts; Figure 4h) in sandstone resulting from the dissolution of mineralised fossils and beds. In all opal fields, precious opal is also found in primary minerals. The silica colloid was able to flow close association with faults (Figure 5). These faults are over a short distance into hydraulic extensional frac- commonly sealed by a coating of smeared clay a few tures while preserving the regular arrangement of silica millimetres thick (Figure 5e). In the vicinity of these spheres necessary for precious opal. faults, pink colouration or alternatively discolouration can be observed, suggesting that these faults have helped Petrographic observations the circulation of oxidising or reducing fluids. Precious opal commonly develops in the footwall of faults as opal- In weathered Lower Cretaceous rocks, secondary miner- bearing veins (Figure 5f), and as boulder opal als include common and precious opal, diagenetic gyp- (Figure 5g). Once coated with clay, faults act as barrier sum, barite, kaolinite, smectite, jarosite, alunite, for lateral fluid flow, contributing to the development of goethite and hollandite (e.g. Jones & Segnit 1966; Exon & local closed-system environments favourable to the for- Senior 1976; Senior et al. 1977; Thiry et al. 1995; Simon- mation of precious opal. Remarkably, precious opal lev- Coincon¸ et al. 1996; Newberry 2005). Because some of Downloaded by [203.45.115.130] at 22:34 03 June 2013 els often remain laterally at a near constant elevation these secondary minerals point to neutral to alkaline over many tens of kilometres, beneath the flat lying Ce- conditions (e.g. hematite, precious opal), whereas others nozoic unconformity (Figure 6). This suggests that opal- require acidic conditions (e.g. jarosite, alunite), it is ised levels may have extended over large distances sub- most likely that weathering involved a multi-stage pro- parallel to the Cenozoic paleosurface, hence pointing to cess with minerals formed at different times, in various a near-surface environment of formation. The associa- redox and pH conditions. A close inspection of mineral tion between precious opal and river paleochannels is of- inclusions trapped in precious opal veins shows that sec- ten mentioned (e.g. Watkins et al. 2011). The Winton ondary minerals specifically associated with precious Formation and the Griman Creek Formation were de- opal consist of amorphous oxyhydroxide and goethite, posited in a fluvial to fluvial-lacustrine environment by manganese oxide, with traces of gypsum and barite braided, anastomosing channels on a flat landscape. (Figure 8). In the pore spaces of sandstone, precious opal This environment typically leads to the deposition of al- and goethite replace clay, feldspar and volcanic frag- ternating lenses of sand, deposited during high-energy ments. Goethite and precious opal are intimately associ- flow, and silt deposited during low-energy conditions. Be- ated in ferruginous bands that develop at the interface tween anastomosing channels, flood plains consist between sandstone and claystone beds (Figure 9). At the mainly of mudstone and siltstone. Because of their per- weathering front, the occurrence of secondary sulfate meability, sandstones are critical to the downward trans- minerals (gypsum, barite and alunite) and the absence port of oxic surface water into reduced lithologies. of pyrite, ubiquitous in unweathered rocks, suggest that Hence, top-down alteration fronts involving the percola- pyrite was, at least in part, the source of the sulfate and tion of surface oxic water are expected to be far more ef- was dissolved by oxidative weathering. ficient in channel deposits rather than the intervening These observations suggest that precious opal was flood plains. formed via redox-dependent and pH-dependent TAJE_A_784219.3d (TAJE) 03-05-2013 15:56

300 P. F. Rey Downloaded by [203.45.115.130] at 22:34 03 June 2013

Figure 4 Example of precious opal typology in central Australia. (a) Precious opal is often associated with ferruginous bands that develop either at the contact between sandstone (top) and claystone (bottom), or slightly above as a sub-horizontal bar in the sandstone suggesting the position of a paleo water-table (Opalton opal field, central Queensland). (b) Detail showing a ferruginous band made of precious opal in goethite at the contact between sandstone (top) and claystone (bottom). (c) Details of the sandstone immediately in contact with the claystone. Precious opal and goethite (in red) fill the pore space between altered volcanic clasts and quartz grains. (d) Discordant diagenetic gypsum vein in a ferruginous band developed at the contact between sandstone (top) and claystone (bottom). (e) Opal-bearing extensional fractures in a claystone from the Griman Creek Formation (Lightning Ridge opal field, northern New South Wales). (f) Opal-bearing desiccation cracks in a sandstone from the Winton Formation (Quilpie opal fields, south Queensland). (g) Opal-bearing fractures in a boulder opal (Quilpie opal fields). Note the gradient of colour from top to bottom, and the internal layering. (h) Opalised rip-up mud clasts in a coarse sandstone (Quilpie opal fields). Note the oxide shell forming around the clasts. TAJE_A_784219.3d (TAJE) 03-05-2013 15:57

Opalisation of the Great Artesian Basin 301 Downloaded by [203.45.115.130] at 22:34 03 June 2013

Figure 5 Fault and fault-related opal in central Australia. (a) Network of conjugated normal faults in the Griman Creek Forma- tion (Lightning Ridge, NSW), and (b) a rare reverse fault at the same location. (c) Conjugated normal faults in the Bulldog Shale Formation (Coober Pedy, SA). (d) Normal fault in the Winton Formation in Opalton (central Queensland). (e) Clay coating along a normal fault (Lightning Ridge, NSW). (f) Opal-bearing dilatant shear fractures. Such dilatant fractures require a small differential stress and a small maximum effective principal stress consistent with sub-surface formation. (g) Boulder opal at the footwall of a clay-coated normal fault. TAJE_A_784219.3d (TAJE) 03-05-2013 15:58

302 P. F. Rey

Figure 6 Opalised sub-upper Paleocene paleosurface: Alaric opal mine (a) and Hayrick opal mine (b) (south Queensland) are lo-

Downloaded by [203.45.115.130] at 22:34 03 June 2013 cated within the weathered sandstone and mudstone of the Winton Formation. In both cases, the Winton Formation is capped by Cenozoic silcretes that have protected the underlying rocks from erosion. (c) Although these two mines are 18 km apart (image from Google Earth), they stand at the same elevation (from 270 to 290 m) within a flat lying weathered horizon that par- allels the local stratigraphy.This suggests that the opalised horizon corresponds to a flat pre-late Paleocene paleosurface, buried underneath Cenozoic sediments.

weathering processes when reduced pyritic lithologies (Barth-Wirsching et al. 1990; Burns 1993; McAdam et al. were progressively exposed to the oxic world. Acidity, 2007, 2008), and on the direct leaching of kaolinite and via oxidation of pyrite and ferrolysis of ferrous iron, has smectite via sulfuric acid (e.g. Komadel et al. 1996; Gates been invoked to explain Cenozoic deep weathering et al. 2002; Metz et al. 2005; Mako et al. 2006; Shaw & Hen- (Thiry & Milnes 1991; Thiry et al. 1995, 2007; Simon-Coin- dry 2009; Panda et al. 2010). These studies allow explora- con¸ et al. 1996; Newberry 2005). However, following tion of the range of possible acidic oxidative reaction acidic weathering, precious opal requires a shift to alka- paths leading to the observed secondary minerals, in line conditions (Iler 1979). particular the co-precipitation of precious opal and goethite at the expense of clay,and the development of a conceptual geochemical model for the formation of precious A geochemical conceptual model opal in the GAB. Numerous recent studies provide details on the oxida- Following uplift and denudation of the GAB, lowering tive weathering of anoxic pyritic marine sediments in a of the water-table and the downward percolation of sur- cold environment (e.g. Pirlet et al. 2010), on the oxidative face water through porous sandstones and faults led to weathering of volcanic rocks in acid-sulfate water the oxidation of pyrite disseminated in reduced TAJE_A_784219.3d (TAJE) 03-05-2013 15:58

Opalisation of the Great Artesian Basin 303

Figure 7 Viscous flow microstructures and strain fabrics in opal-bearing veins. (a) Shear folds in an opal vein involving streaks of precious, semi-precious and common opal. (b) Shear fabric in an opal vein. The flow banding diverges from a small opening, most likely a point of entrance of the opal-bearing fluid into the fracture. The flow banding rapidly rotates to become sub-parallel to the fracture walls, producing shear folds in the process. (c) Example of CS shear fabric within one of the limb of the fold shown in (a). Dashed line shows the orientation of the flattening plane (S plane); arrows show a top to the left sense of shear along shear planes (C plane). This low-T microstructure is analogous to that formed at much higher temperature

Downloaded by [203.45.115.130] at 22:34 03 June 2013 (T>600C) in quartz-rich rocks. (d) Deformation lamella in gem opal. Note the curvature in the silica sphere lattice (ondulose colour) as well as the curved micro-extensional fractures.

sediments. Oxidation of pyrite with oxygen or ferric iron (Fe8O8(OH)6SO4) via reaction 5: 3þ 2 2þ (Fe ) releases sulfate (SO4 ), ferric or ferrous (Fe ) iron, and protons (Hþ) via the following reactions 2þ þ þ : ! þ þ ð Þ (Williamson & Rimstidt 1994): 2Fe 3H2O 0 5O2 2FeOOH 4H 3a

3þ þ Fe þ 2H O ! FeOOH þ 3H ð3bÞ 3þ 2 þ 2 FeS2 þ 3:75O2 þ 0:5H2O ! Fe þ 2SO4 þ H ð1Þ 2þ þ þ þ : ! ð Þ þ ð Þ 3þ 2þ 2 þ 2Fe 5H2O 0 5O2 2Fe OH 3 4H 4a FeS2 þ 14Fe þ 8H2O ! 15Fe þ 2SO4 þ H ð2aÞ

3þ þ þ þ þ þ ! ð Þ þ ð Þ 2 3 Fe 3H2O Fe OH 3 3H 4b 15Fe þ 15H þ 3:75O2 ! 15Fe þ 7:5H2O ð2bÞ

8Fe3þ þ SO 2 þ 14H O ! Fe O ðOHÞ SO þ 22Hþ ð5Þ In the mean time, oxidation and the rapid hydrolysis of 4 2 8 8 6 4 ferrous and ferric iron present in solution in reduced sediments release more acidity and lead to the formation Ferrihydrite, commonly the first iron oxyhydroxide of oxyhydroxides (e.g. Raiswell et al. 2009) such as goe- formed, is unstable and transforms into either goethite thite (FeOOH) via reactions 3a and 3b, ferrihydrite (Fe or hematite depending on the pH (e.g. Schwertmann &

(OH)3) via reactions 4a and 4b, and schwertmannite Murad 1983). TAJE_A_784219.3d (TAJE) 03-05-2013 15:59

304 P. F. Rey Downloaded by [203.45.115.130] at 22:34 03 June 2013

Figure 8 Mineralogy of an opal vein from central Queensland. (a) Hyperspectral imaging of a precious opal veins. This image was acquired using HySpex hyperspectral camera SWIR-320m-e covering a near infrared spectral range from 0.96 to 2.5 mm. The walls of the opal vein are covered with a bright orange poorly crystallised to amorphous iron oxide. The NIR spectrum matches the spectrum of ferrihydrite C1JB130B (dashed spectrum) from the spectral library of the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). (b1, 2) Microscope and SEM imaging, and (b3, 4) elemental maps (Si and Fe) of the wall of a precious opal vein showing trigonal mineral casts, possibly alunite or jarosite, pseudomorphosed into iron oxide, probably goethite, and silica. TAJE_A_784219.3d (TAJE) 03-05-2013 15:59

Opalisation of the Great Artesian Basin 305

Figure 9 Hyperspectral imaging of a ferruginous band at the contact between sandstone and mudstone (sample from Opalton, central Queensland). The NIR spectra of both the sandstone (A) and the mudstone (C) show absorption at

Downloaded by [203.45.115.130] at 22:34 03 June 2013 1.9 mm and two absorption doublets at around 1.4 and 2.2 mm, characteristics of well-crystallised halloysite (i.e., hydrated kaolinite). The strong crystallinity of kaolinite suggests it is the product of low temperature very acidic weathering. Kaolinite is lost in the NIR spectrum of the ferruginous band (B) whose mineralogy is dominated by visible precious opal and goethite clearly identifiable in the Visible to NIR range (0.4 to 1 mm). This suggests that kaolinite dissolved into goethite and amorphous silica. The pH in the silica-rich colloid evolved towards alkaline conditions to allow for the formation of precious opal. TAJE_A_784219.3d (TAJE) 03-05-2013 15:59

306 P. F. Rey

Mn oxides such as hollandite can precipitate directly goethite (Trolard & Tardy 1989; Tardy 1997). from the solution (e.g. Vasconcelos & Conroy 2003):

þ 2þ þ þ Al Si O ðOHÞ þ 4Fe3 þ 5H O þ 3O 8Mn þ K þ 8H2O þ OH þ 4O2 ! KMn8O16ðOHÞþ16H 2 2 5 4 2 2 ð6Þ

! 3ðFe Al : Þ O :H O þ 2SiðOHÞ ð12aÞ Acidity released by reactions 1–6 induces the dissolution 0:67 0 33 2 3 2 4 of carbonates (including carbonate from mineralised fossils) following: ð Þ þ 3þ þ þ Al2Si4O10 OH 2 4Fe 10H2O 3O2

þ 2þ 4CaCO3 þ 4H ! 4Ca þ 4HCO3 ð7Þ ! ð Þ þ ð Þ ð Þ 3 Fe0:67Al0:33 2O3:H2O 4Si OH 4 12b At low pH, minerals such as jarosite (KFe3(SO4)2(OH)6), alunite (KAl3(SO4)2(OH)6) and gypsum (CaSO4.2H2O) may also precipitate directly from the solution: Reaction 12a explains remarkably well the reaction texture documented in Figure 9, in which kaolinite 3þ þ 2 þ þ þ ! ð Þ ð Þ ð Þ 3Fe 2SO4 K 6OH KFe3 SO4 2 OH 6 8a transforms into an association of precious opal and þ þ goethite. 3Al3 þ 2SO 2 þ K þ 6OH ! KAl ðSO Þ ðOHÞ ð8bÞ 4 3 4 2 6 Hence, weathering reactions in pyrite-bearing and 2þ 2 Ca þ SO4 þ 2H2O ! CaSO42H2O ð8cÞ carbonate-poor volcaniclastic lithologies lead to acid leaching of primary assemblages and to the formation of Acidity also accelerates the rate of weathering of feld- clays, iron oxyhydroxides, iron oxides, sulfate minerals spar and volcanic ash into kaolinite (Al2Si2O5(OH)4) and and amorphous silica. Reactions involved in the dissolu- smectite (Al2Si4O10(OH)2), respectively, whereas smectite tion of pyrite, and those involved in the oxidation and transforms into kaolinite releasing silica via solid-state hydrolysis of iron and manganese (reactions 1–6), lower stripping of tetrahedral sheet (Amouric & Olives 1998; the pH of the solution, allowing for the precipitation of Ryan & Huertas 2005). The overall weathering reaction sulfate mineral such as jarosite, alunite and gypsum (re- of feldspar into kaolinite is well known (e.g. Faure 1998): action 8a–c). In contrast, most reactions involved in the breakdown of feldspar, clays and volcanic ash (reactions ð ; Þ þ þ þ ! ð þ; þÞ 2 K Na AlSi3O8 9H2O 2H 2 Na K 9, 10b, 10c, 11a, 11b) consume protons and therefore in- crease the pH of the solution. This may induce the disso- þ4SiðOHÞ þ Al2Si2O5ðOHÞ ð9Þ 4 4 lution of pH-sensitive sulfate such as jarosite and alunite, while at the same time releasing silica, the poly- If the activity of soluble silica is low enough, kaolinite merisation of which leads to a silica-rich colloid. In addi- decomposes further into gibbsite (Al(OH) , via reaction 3 tion, the formation of clay and the precipitation of 10a), or alunite (KAl (SO ) (OH) , via reaction 10b) at low 3 4 2 6 sulfate minerals, iron oxide and silica contribute to the pH (Barth-Wirsching et al. 1990), or alunogen collapse of the rock porosity. This promotes a reduction (Al2(SO4)3.17H2O, via reaction 10c) when the activity of þ in the permeability in sandstone, and could explain hy- soluble K is low (e.g. Mako et al. 2006; Panda et al. 2010). draulic fracturing in claystone. Precipitation of hy- Importantly,each of these reactions releases silica: drated minerals sealing the porosity would drive the Downloaded by [203.45.115.130] at 22:34 03 June 2013 formation of isolated closed systems. In these closed sys- Al Si O ðOHÞ þ 5H O ! 2SiðOHÞ þ 2AlðOHÞ ð10aÞ 2 2 5 4 2 4 3 tems, which may include ironstones, clay pebbles and ð Þ þ þ 2 þ þ þ þ 3Al2Si2O5 OH 4 9H2O 4SO4 2K 6H clay lenses embedded in sandstone, as well as carbonate ! ð Þ þ ð Þ ð Þ ð Þ fossils, low pH conditions produced during the early 6Si OH 4 2KAl3 SO4 2 OH 6 10b stage of weathering may progressively shift to alkaline ð Þ þ þ 2 þ þ Al2Si2O5 OH 4 16H2O 3SO4 6H conditions following alteration of feldspar, volcanic ash ! ð Þ þ ð Þ : ð Þ 2Si OH 4 Al2 SO4 3 17H2O 10c and clay.A shift to alkaline conditions at a time when kaolinite ultimately dissolves into Al-goethite and silica If, following dissolution of carbonate, feldspar and volca- may explain the production of precious opal in ferrugi- nic ash acidic conditions are maintained, kaolinite and nous bands (Figures 4b, 9) and other closed systems. smectite can collapse into amorphous silica as alumin- Hence, the final mineralogical assemblage and the type ium is released via reaction 11a (e.g. Trolard & Tardy of opal (common or precious) reflect the balance between 1989; Tardy 1997) and reaction 11b (e.g. Komadel et al. the acidification potential (AP) and the neutralisation 1996; Gates et al. 2002; Metz et al. 2005; Shaw & Hendry capacity (NC) of the host rock, and more precisely the 2009; Vazquez et al. 2010). balance in the closed system in which opal evolves. In unweathered lithologies, this balance depends on the 2þ þ þ one hand on the pyrite content and Fe content in solu- Al Si O ðOHÞ þ 6H ! 2SiðOHÞ þ 2Al3 ð11aÞ 2 2 5 4 4 tion, and on the other hand on the amount of carbonate, ð Þ þ þ þ ! ð Þ þ 3þ ð Þ Al2Si4O10 OH 2 6H 4H2O 4Si OH 4 2Al 11b feldspar and volcanic ash. Hence, precious opal requires an environment with a significantly large AP to release Kaolinite and smectite can also react with Fe3þ (soluble silica, yet one in which NC >AP so the silica-rich colloid

in acid solution) and O2 in solution to lead to Al-bearing can evolve locally under alkaline conditions. TAJE_A_784219.3d (TAJE) 03-05-2013 15:59

Opalisation of the Great Artesian Basin 307

DISCUSSION absence of precious opal deposits in Cenozoic rocks requires an explanation. Since precious opal can be Constraints on the age of opalisation found in marine (Bulldog Shale), brackish (Griman Creek) and fluvio-lacustrine (Winton) lithologies, the de- The timing of precious opal formation is poorly con- positional environment does not seem to have any strained. For some authors, opal was formed during the major bearing on the formation of precious opal. One deposition of Lower Cretaceous sediments (Watkins possible explanation is that mid-Cenozoic to Recent et al. 2011); for others, precious opal formed during the acidic weathering mainly affected lithologies derived development of mid-Cenozoic silcretes (Jones & Segnit from reworked weathered Cretaceous lithologies (e.g. 1966; Senior et al. 1977; Bird et al. 1990; Simon-Coincon¸ Wulser€ et al. 2011). Cenozoic lithologies may have lacked et al. 1996; Horton 2002; Newberry 2005), and some a significant unweathered volcaniclastic component, authors (e.g. Dowell et al. 2002) suggest continuous for- and therefore their AP may have exceeded their NC. mation during the Cenozoic to Recent based on very These acidic conditions may explain the formation of sil- young 14C ages (46 000 to 1000 BP) on carbon inclusions crete and the absence of precious opal in Cenozoic in opal from Lightning Ridge (e.g. Dowell et al. 2002; lithologies. Fink 2011). A number of observations can help to clarify One question remains: Could Cenozoic weathering this debate. lead to the formation of precious opal in deeper Creta- The very young 14C ages obtained on carbon inclu- ceous lithologies? For this to happen, surface oxic waters sions are applicable to the timing of the formation of must have reached unweathered Cretaceous lithologies opal only if the carbon inclusions were formed before, and therefore first travelled through the Cenozoic cover, or were cogenetic with, opal. However, because opal is a and then through weathered Lower Cretaceous rocks, porous material and because the remains of microbes while maintaining their oxidation potential. This is pos- have been found in inclusion in opal, as well as their sible when surface waters travel downward along perme- host rocks (Behr et al. 2000; Watkins et al. 2011), it is pos- able faults, because in this setting, the water-to-rock sible that these carbon inclusions are the remains of ratio is very large, and fluids keep their oxidation poten- younger bacteria contaminating much older opals. In tial, whereas rocks along faults are altered. In contrast, such a case, 14C ages would range from present up to ca when porous flow through sandstone is involved, the wa- 50 ka (the upper limit for radiocarbon dating), which is ter-to-rock ratio is much smaller, and oxic waters loose what is being observed (Fink 2011). Hence, the existing their oxidation potential faster. Thus, in the absence of radiocarbon dataset is compatible with a contamina- erosion, regional oxidative weathering via downward tion scenario. percolation of surface water is a self-limiting process. When exposed to temperature between 50 and 100C This explains why the maximum thickness of the weath- (the temperatures reached at about 1000–1500 m in the ering profile in the GAB does not exceed 100 m, despite GAB) opal turns into microcrystalline quartz (e.g. Behl over 60 Ma of continuous weathering. Hence, bearing in 2011) within about 40 kyr to 4 Myr (Blatt et al. 1980). mind that the Cenozoic cover consists largely of imma- Hence, assuming that opal is primary (i.e. syn-sedimen- ture reduced lithologies able to reduce oxic water, only tary), it would have transformed into microcrystalline where the Cenozoic cover was less than 100 m thick quartz upon burial to depth greater than 1000 m. On could oxidising waters have reached, via porous flow, un- the other hand, syn-sedimentary opal buried at a depth derlying Cretaceous lithologies. In addition, because of <1000 m would have been eroded sometime between 97 their strong AP and strong NC, Lower Cretaceous rocks Ma and 60 Ma, since, as discussed earlier, 1–3kmofover- were already deeply weathered before the deposition of

Downloaded by [203.45.115.130] at 22:34 03 June 2013 burden has been eroded in the GAB. A recent study of Cenozoic rocks. Aqueous weathering strongly reduces opalised fossils has shown that opalisation is a second- the permeability of rocks because of the precipitation of ary process occurring after the original fossilisation pro- secondary minerals into pore space, faults and fractures. cess (Pewkliang et al. 2008). Hence, precious opal now at Hence, reduced permeability in the Upper Cretaceous to the surface of the GAB is most likely the product of sec- Paleocene weathering profile would have acted as a ondary processes and could only have formed during or poorly permeable to impermeable barrier protecting after the erosion of the GAB upper section, in any case deeper rocks. Thus, at the scale of the GAB, deeper un- not before ca 100 Myr (Figure 10). weathered Lower Cretaceous rocks may have limited Ce- Assemblages including kaolinite, goethite, gypsum, nozoic weathering. This is consistent with the barite and alunite have been described in the Miocene observation that many weathering profiles below the Namba Formation east of the Flinders Ranges (Wulser€ Eyre Formation have preserved their pre-late Paleocene et al. 2011). This suggests that, at least locally along faults ages. where strong uplift and denudation were involved, To summarise, precious opal in the GAB may have acidic oxidative weathering and associated silcrete formed from the Late Cretaceous to Recent. However, it resulted from exposure of pyritic mudstone of Miocene is more probable that the bulk of precious opal formed age (Wulser€ et al. 2011). Interestingly, Cenozoic acidic ox- in the Late Cretaceous to Paleocene when unweathered idative weathering seems to have produced only silcrete Lower Cretaceous rocks were slowly brought to the sur- caps and common opal. No precious opal deposit and no face by erosion and exposed to weathering. If this were opalised fossils have been found in Cenozoic to Recent correct, the bulk of precious opal formation could be formations of the GAB. If one assumes that, at the scale linked to the drying out of the surface of the GAB follow- of the GAB, precious opal formation were synchronous ing the regression of the Eromanga Sea, continuous up- with the development of Cenozoic silcretes, then the lift and erosion from 97 to 60 Ma. TAJE_A_784219.3d (TAJE) 03-05-2013 15:59

308 P. F. Rey Downloaded by [203.45.115.130] at 22:34 03 June 2013

Figure 10 Opalisation of the GAB in the context of its geological history of the past 130 Ma. A period of dominantly marine sedimentation unfolded between 130 and 97 Ma, triggered by a negative dynamic topography as East Gondwana moved eastward towards and above a west dipping subduction zone. The subduction-related Whitsunday Volcanic Arc was the source of volcaniclastic sediments. Continental iron and organic-rich sediments were transported into the shallow, cold and stagnant continental sea. Reducing environment at and below the sea floor favoured anaerobic bacteria and the production of biogenic pyrite. From 97 to ca 93 Ma, the eastward motion stopped leading to a sudden uplift of East Gondwana Pacific margin, and the regression of the inland sea. Enhanced erosion of the Whitsunday Arc shed large volume of volcaniclastic sediments (Winton Formation) into the GAB. These sediments were buried in a reducing environment. A slow and continuous period of protracted uplift, erosion, denudation and cooling followed from 93 to 60 Ma. Erosion advected fresh reduced sediments into the oxic environment, triggering the oxidation of pyrite and ferrolysis of ferrous iron, in turn driving—at the scale of the GAB— acidic weathering releasing amorphous silica. Most precious opals, formed at the near surface redox front (weathering profile) at that time, were lost owing to erosion. From 60 Ma, uplift stopped and a period of sedimentation followed covering the GAB with a thin veneer of Cenozoic fluvial-lacustrine sediments most deriving from weathered lower Cretaceous rocks. These Cenozoic sediments protected the pre-late Paleocene opalised redox front from erosion. In the central part of the GAB, mild post mid-Cenozoic tectonic activity produced gentle doming, local uplift and erosion, whereas around the Flinders Ranges and possibly the Great Dividing Ranges stronger uplift, faulting and erosion drove more acidic weathering. In contrast to the Late Cretaceous to Paleocene regional acidic weathering, mid-Cenozoic to recent acidic weathering affected sediments mainly derived from pre-weathered lithologies, and therefore with a lower neutralisation capacity compared with that of fresh Lower Cretaceous sediments. TAJE_A_784219.3d (TAJE) 03-05-2013 15:59

Opalisation of the Great Artesian Basin 309

One opal-forming process but multiple opal extend, in Lower Cretaceous lithologies, to a few concentrating mechanisms tens of metres below the Cenozoic unconformity. The redox-front model suggests that the percolation of Precious opal occurs in a variety of habitus (Figure 4) oxidised surface fluids into deeper reduced lithologies including: replacement of clay in sandstone pore space led to acidic weathering. In the GAB, faults and fractures (matrix opal), hydraulic extensional fractures (seam were able to channel oxic surface fluids deeper into re- opal), desiccation cracks, open cavities in ironstone duced lithologies, allowing the redox-front to migrate (boulder opal), replacement of clay pebbles (Yowah faster along faults and fractures. Hence, the redox model nuts), and replacement of mineralised fossils. However, explains the occurrence of precious opal deposits in the variety of habitus does not mean variety of processes. vicinity of the faults and fractures, and therefore At the microscopic scale, precious opal is frequently accounts for the so-called ‘syn-tectonic model’ (Pecover found in pore spaces of preserved opalised redox front 1996). Faults may have contributed to the formation of in many parts of the GAB. The so-called ‘matrix-opal,’ precious opal in other ways. First, fracturing is a process which refers to play of colour in the pore space of sand- able to concentrate, into veins, fluids trapped into the stone, is common to all opal fields in the GAB. The con- rock porosity. In addition, faults and fractures smeared centration of precious opal into visible economic with clay are known to act as efficient fluid barriers (e.g. deposits can occur via a number of processes, which Caine et al. 1996), and therefore are able to form local can explain the different variety of precious opal found fluid traps in which opal may evolve in a closed system in the GAB. These processes include the trapping of sil- (e.g. Figure 5f). Late Oligocene to Miocene gentle doming ica-rich solution in open fractures and cavities (e.g. in (Wopfner et al. 1974) and focussed erosion along crest ironstone boulders prominent in iron-rich lithologies domes explain the presence of opalised levels close to the of central Queensland, and desiccation cracks), the de- surface around dome crest (Horton 2002). However, the velopment of high pore pressure and hydraulic fractur- exhumed opalised levels most likely pre-date doming. ing (e.g. the so-called seam opal prominent in Lightning Uranium deposits in fluvial to marginal marine sand- Ridge claystone), and the replacement of carbonate fos- stones constitute an important class of uranium depos- sils via coupled dissolution precipitation mechanism its. They form when oxic surface fluids, carrying soluble (Pewkliang et al. 2008) particularly well represented in oxidised uranium, flow into reduced sedimentary rocks. fossiliferous Bulldog Shale around Coober Pedy. Frac- Indeed, uranium is a redox-sensitive element. In oxic turing was possibly owing to mild tectonic activity in þ surface water, oxidised uranium (U(VI)O 2 ) is soluble relation to the opening of the Tasman Sea from ca 97 2 and therefore mobile. At the redox front, uranium pre- Ma to 60 Ma (Korsch et al. 2009; Rey & Muller€ 2010; cipitates as its redox state switches from oxidised to re- Muller€ et al. 2012), uplift of the Flinders Ranges from duced (U(IV)O ) under the action of reducing agents mid-Cenozoic to Recent (Foster et al. 1994) and other 2 such as carbonaceous material and sulfides. Thus, the fracturing processes including: (1) chemical cementa- redox front model also accounts for the hypothesis pro- tion of pore space during weathering; (2) swelling of posed by Senior & Chadderton (2007) and predicts the clay during hydration; and (3) volume reduction owing higher-than-background radioactivity observed in some to dehydration, and/or pH and redox changes in rela- opal levels. tion to the dissolution of clays (e.g. Yowah nuts, in Microbial metabolic processes are a major contribu- South Queensland). Therefore, it is likely that the dif- tor to weathering. Not surprisingly, microbe remains ferences in habitus of precious opal between the vari- are not only found in amorphous silica but also perva- ous opal fields are related to different mechanisms sive throughout their weathered host rocks (Watkins

Downloaded by [203.45.115.130] at 22:34 03 June 2013 controlling the concentration of silica-rich solution et al. 2011). The biogenic model proposed by Watkins into economic traps, not differences in opal-forming et al. (2011) suggests a syn-sedimentary origin for opal. processes. However, as discussed earlier, it is very unlikely that opal is syn-sedimentary, since syn-sedimentary opal would have been destabilised during burial at depths >1000 m, while those at depths shallower than 1000 m Comparison with former models would have been eroded sometime between 97 Ma and Compared with the weathering model proposed by 60 Ma. Darragh et al. (1966) and Senior et al. (1977), the model The acidic oxidative weathering model explains proposed here defines more precisely the conditions of many key observations on which previous models have weathering, the timing and the geological context that been built. As such, the redox front model offers a unify- led to the formation of precious opal in the GAB. The ing framework to explain the formation of precious opal weathering was acidic, oxidative and affected in chemi- in the GAB. cally closed systems strongly reducing lithologies. Al- though precious-opal of mid-Cenozoic to Recent ages cannot be ruled out, the redox model suggests that the OPAL-BEARING MINERAL ASSEMBLAGES, ACIDIC main period of opal formation occurred between ca 97 OXIDATIVE WEATHERING AND THE MARS and ca 60 Ma, and proposes that most opal deposits now CONNECTION preserved at the surface of the GAB were those formed towards the end of the uplift and erosion period, as older Although precious opal is rare, common opal is a fre- ones were most likely lost to erosion. The redox model quent form of hydrated amorphous silica often associ- predicts that the most prospective levels for precious ated with near-surface acidic weathering of feldspar-rich TAJE_A_784219.3d (TAJE) 03-05-2013 15:59

310 P. F. Rey

lithologies. The recent discovery on the surface of Mars gel into veins. Once formed, and following the sealing of of hydrated secondary minerals (e.g. Squyres et al. 2004; pore space by secondary minerals, the amorphous silica Bibring et al. 2005; Poulet et al. 2005) including opal-bear- gel evolved in closed environments in which pH evolved ing mineral assemblages (Squyres et al. 2008; Milliken to alkaline conditions favourable for the production of et al. 2008) and their implication for the presence of liq- homogeneous silica spheres necessary for the formation uid water, offers a new perspective on the link between of precious opal. the formation of opal in the GAB and the dehydration of This scenario predicts that, although local Cenozoic its surface in the Late Cretaceous. Not unlike the GAB, opalisation may have been possible, peak conditions for the surface of Mars is largely covered by sedimentary opalisation prevailed for as long as reduced sediments volcaniclastic rocks derived from erosion of Mars basal- were advected into the oxic environment during the Late tic crust. Not unlike the GAB, the surface of Mars is re- Cretaceous to early Paleocene uplift and erosion (i.e. markable for the lack of significant amounts of from ca 97 to ca 60 Ma). Unfortunately, it predicts that carbonate (e.g. Chevrier & Mathe 2007). Not unlike the most of the precious opal has been transported and bur- GAB, the surface of Mars went through a major and final ied into the Ceduna Sub-basin, although it may not have period of dehydration. Not unlike the GAB, the reddish survived erosion and transportation. It also predicts colour of Mars suggests that oxidative weathering played that most opal deposits now at the surface of the GAB an important role during and after the dehydration of were formed by ca 60 Ma (i.e. at the end of uplift and ero- Mars’ surface (e.g. Hartman & McKay 1995). The visible- sion period), and were protected from erosion by the de- near infrared spectrometers on board ESA’s Mars Ex- position of the widespread Eyre Formation in the late press and NASA’s Mars Reconnaissance Orbiter detected Paleocene to Eocene. This explains the location of most GAB-like hydrated mineralogical assemblages including opal mining districts in the GAB often just below the Ce- amorphous silica, kaolinite, smectite, jarosite, ferrihy- nozoic unconformity. This scenario explains why pre- drite, goethite and gypsum (Poulet et al. 2005; Bibring cious opal deposits in sedimentary basins are rare on et al. 2005; Milliken et al. 2008; McAdam et al. 2007, 2008). Earth, as they require the following conditions: (1) large Geochemical modelling shows that these assemblages volume of organic matter-rich, iron-rich volcaniclastic can be obtained through aqueous weathering of basaltic sediments; (2) an anoxic deposition environment; (3) lit- lithologies by sulfate-rich acidic groundwater, and/or tle to no carbonates (efficient acidity sink); (4) sea re- via interaction with an atmospheric acid-fog (e.g. Burns gression to drive oxidative weathering; and (5) a stable 1993; Banin et al. 1997, Hurowitz et al. 2006; McAdam landscape to preserve opal from erosion and to prevent et al. 2007, 2008). The dehydration of the Mars’ surface its transformation into quartz via burial. Finally, the sometime during the Hesperian (ca 3.7 to 2 Ga) is often proposed model draws a parallel between the landscape proposed as one of the main drivers for the acidic oxida- evolution and environment evolution of the GAB during tive weathering of the Mars surface (e.g. Burns 1988, the Late Cretaceous to Paleocene and that of Mars dur- 1993; Bibring et al. 2005; Poulet et al. 2005; Chevrier & ing the Hesperian. Mathe 2007; Davila et al. 2008). Hence, it is possible that on Mars and on the GAB, opal-bearing assemblages developed via the acidic oxidative weathering of volcanic ACKNOWLEDGEMENTS and volcaniclastic rocks, under evaporitic conditions during the drying out of their respective surface. If this The research presented here was supported by the Aus- hypothesis were correct, then the GAB would be one of tralian Research Council through a Discovery Grant the best terrestrial analogues of the Martian surface. (DP0987604). Reviews and insights from Jonathan Clarke Downloaded by [203.45.115.130] at 22:34 03 June 2013 and Colin Pain greatly improved the manuscript. Many thanks to the Lightning Ridge Miners Association and CONCLUSIONS in particular to Maxine O’Brien, its executive secretary, for getting me involved in exploring the formation of Continental volcaniclastic sediments accumulated in the precious opal in the GAB. A long list of opal miners of- GAB between ca 130 and ca 93 Ma, in a cold, poorly oxy- fered access to their mines, providing countless samples, genated to anoxic marine to fluvial-deltaic environment and most importantly sharing their knowledge on opal (Figure 10). Between 93 Ma and 60 Ma, slow uplift and formation. Many thanks are due to all of them with a erosion of the GAB sedimentary sequence drove the oxi- particular mention to Bob Barrett (Lightning Ridge), dation of organic-rich, Fe-rich Lower Cretaceous re- George Kountouris (Coober Pedy), Col Duff (Winton) and duced volcaniclastic sediments. This oxidation led to the Eric Stelzer (Quilpie) who spent days guiding us across production of a large volume of sulfuric acid via oxida- their opal fields. Many colleagues, who shared their inti- tion of biogenic pyrite, and acid produced via ferrolysis mate knowledge on geochemistry, gemmology, micro- of reduced iron. The quasi-absence of carbonate, owing analysis, hyperspectral techniques, geodynamics, paleo- to the high-latitude position of Australia until the early geography and paleontology, were involved directly or Paleocene, was key to the development of very acidic indirectly in the shaping of some of the ideas presented conditions across the GAB. The acidic oxidative weath- here. I acknowledge here the critical input of Stephen ering of volcaniclastic rocks led to the formation of large Aracic, Jenni Brammall, Joel Brugger, Elizabeth Carter, volumes of amorphous silica, through the weathering of Adriana Dutkiewicz, Nicolas Flament, Emmanuel feldspars and volcanic ash into clay, and clay into amor- Fritsch, Thomas Landgrebe, Nicolas Mangold, Dietmar phous silica and goethite. Locally, occasional hydraulic Muller,€ Simon Pecover, Erick Ramanaidou, Benjamin fracturing was able to concentrate the amorphous silica Rondeau, Balwant Singh, Hallvard Skjerping, Tony TAJE_A_784219.3d (TAJE) 03-05-2013 15:59

Opalisation of the Great Artesian Basin 311

Smallwood, Elizabeth Smith, Paul Thomas, Patrick BOYDEN J., Mu€LLER R. D., GURNIS M., TORSVIK T. , C LARK J., TURNER M., IVEY- Trimby and Lew Whitbourn, to name a few. Many stu- LAW H., FARROW J. & WATSON R. 2011. Next-generation plate- dents have contributed to maintain the opal project tectonic reconstructions using GPlates. In: Keller G. R. & Baru C. eds. Geoinformatics: cyberinfrastructure for the solid alive; I thank in particular Ingrid Van der Beek, Roel earth sciences,pp.94–114. Cambridge University Press, Cambridge. Verbene, Andrew Merdith and Gemma Roberts. BROWN L. D., RAY A. S., THOMAS P. & G UERBOIS J. L. 2002. Thermal Char- acteristics of Australian Sedimentary Opals. Journal of Thermal Analysis and Calorimetry 68,31–36. 29 27 BROWN L. D., RAY A. S. & THOMAS P. 2003. Si and Al NMR study of amorphous and paracrystalline opals from Australia. Journal of REFERENCES Non-crystalline Solids 332, 242–248. BROWN L. D., RAY A. S. & THOMAS P. 2004. Elemental analysis of Austra- ALEXANDER E. M. & JENSEN-SCHMIDT B. 1996. Structural and tectonic lian amorphous banded opals by laser-ablation ICP-MS. Neues history. In: Alexander E. M. & Hibburt J. E. eds. The petroleum ge- Jahrbuch fur Mineralogie Monotshefte 9,411–424. – ology of South Australia, Volume 2: Eromanga Basin,pp.3347. BRYAN S. E., CONSTANTINE A. E., STEPHENS C. J., EWART A., SCHoN€ R. W. & South Australia Department of Mines and Energy, Report Book PARIANOS J. 1997. Early Cretaceous volcano-sedimentary succes- 96/20, Adelaide. sions along the eastern Australian continental margin: implica- ALLEY N. F. 1998. Cainozoic stratigraphy, palaeoenvironments and tions for the break-up of eastern Gondwana. Earth and Planetary geological evolution of the Lake Eyre Basin. Palaeogeography, Science Letters 153,85–102. 144 – Palaeoclimatology, Palaeoecology , 239 263. BRYAN S. E., COOK A. G., ALLEN C. M., SIEGEL C., PURDY D. J., GREENTREE J. ALLEY N. F & FRAKES L. A. 2003. First known Cretaceous glaciations: S. & UYSAL I. T. 2012. Early–mid Cretaceous tectonic evolution of Livingstone Tillite Member of the Cadna-Owie Formation, South eastern Gondwana: from silicic LIP magmatism to continental Australia. Australian Journal of Earth Sciences 50, 139–144. rupture. Episodes 35, 142–152. ALLEY N. F., CLARKE J. D. A., MACPHAIL M. & TRUSWELL E. M. T. 1999. Sed- BURNS R. G. 1988. Gossans on Mars. Proceedings of the 18th Lunar and imentary infillings and development of major Tertiary paleo- Planetary Science, Cambridge University Press and The Lunar drainage systems of south-central Australia. In: Thiry M. & and Planetary Institute, p. 713–721. ¸ Simon-Coincon R. eds. Palaeoweathering, palaeosurfaces, and re- BURNS R. G. 1993. Oxidation of dissolved iron under warmer, wetter lated continental deposits. Special Publication of the International conditions on Mars: Transitions to present-day arid environ- Association of Sedimentologists 27,337–366. ments. In: Squyres S. & Kasting J. eds. Workshop on early Mars: ALLEY N. F., LINDSAY J. M., BARNETT S. R., BENBOW M. C., CALLEN R. A., how warm and how wet?, pp. 3–4. LPI Tech. Rpt. 93–03, Lunar and COWLEY W. M . , G REENWOOD D., KWITKO G., LABLACK K. L., LINDSAY J. Planetary Institute, Houston, Part 1. M., ROGERS P. A . , S MITH P. C . & W HITE M. R. 1995. Tertiary.In: Drexel CAINE J. S., EVANS J. P. & FORSTER C. B. 1996. Fault zone architecture J. F. & Preiss W. V. eds. The geology of South Australia. Volume 2. and permeability structure. Geology 24,1025–1028. The Phanerozoic. Bulletin of the Geological Survey of South Aus- CALLEN R. A. 1977. Late Cainozoic environments of part of northeast- tralia 54,151–218. ern South Australia. Journal of the Geological Society of Australia AMOURIC M. & OLIVES J. 1998. Transformation mechanism and inter- 24, 151–169. stratification in conversion of smectite to kaolinite: an HRTEM CAMPBELL R. J. & HAIG D. W.1999. Bathymetric change during Early Cre- study. Clays and Clay Minerals 46, 521–527. taceous intracratonic marine transgression across the northeast- ARACIC S. 1999. Rediscover opal in Australia. Stephen Aracic and Mary ern Eromanga Basin, Australia. Cretaceous Research 20,403–446. Aracic, Lightning Ridge, NSW. CHEVRIER V. & M ATHE P. E. 2007. Mineralogy and evolution of the sur- BANIN A., KAN H. I. & CICELSKY A. 1997. Acidic volatiles and the Mars face of Mars: A review. Planetary and Space Sciences 55,289–314. 102 – soil. Journal of Geophysical Research , 13341 13356. DARRAGH P. J. , G ASKIN A. J. & SANDERS J. V.1976. Opals. Scientific Ameri- BARNES L. C., OLLIVER J. G. & SPENCER W. G. 1980. Reconnaissance sam- can 234,84–95. pling of palygorskite deposits, Lake Frome area, South Australia. DARRAGH P. J. , G ASKIN A. J., TERRELL B. C. & SANDERS J. V.1966. Origin of Mineral Resources Review, South Australia 149,22–31. precious opal. Nature 209,13–16. € BARTH-WIRSCHING U., E HN R., HoLLER H., KLAMMER D. & SITTE W. 1990. DAVID T. W. E. 1950. The Geology of the Commonwealth of Australia. Studies of hydrothermal alteration by acid solutions dominated Volume 1. Edward Arnold, London. 2 by SO4 : Formation of the alteration products of the Gleichen- DAVILA A. F., FAIREN A. G., GAGO-DUPORT L., STOKER C., AMILS R., BONAC- — berg latitic rocks (Styria, Austria) Experimental evidence. CORSI R., ZAVALETA J., LIM D., SCHULZE-MAKUCH D. & MCKAY C. P. 2008. Mineralogy and Petrology 41,81–103. Subsurface formation of oxidants on Mars and implications for Downloaded by [203.45.115.130] at 22:34 03 June 2013 BEHL R. J. 2011. Chert spheroids of the Monterey Formation, Califor- the preservation of organic biosignatures. Earth and Planetary nia (USA): early-diagenetic structures of bedded siliceous depos- Science Letters 272, 456–463. 58 – its. Sedimentology ,325 351. DEIGHTON I. & HILL A. J. 1998. Thermal and Burial History. In: Grave- BEHR H. J., BEHR K. & WATKINS J. J. 2000. Cretaceous microbes— stock D. I., Hibburt J. & Drexel J. F. eds. Petroleum geology of producer of black opal at Lightning Ridge, NSW, Australia. 15th South Australia Volume 4—Cooper Basin,pp.143–155. PIRSA, Australian Geological Convention, Geological Society of Australia Adelaide. Abstracts p. 59. DETTMANN M. E., CLIFFORD H. T. & PETERS M. 2009. Lovellea wintonensis BIBRING J-P., LANGEVIN Y. , G ENDRIN A., GONDET B., POULET F. , B ERTHE M., gen. et sp. nov. Early Cretaceous (late Albian), anatomically pre- SOUFFLOT A., ARVIDSON R., MANGOLD N., MUSTARD J., DROSSART P. served, angiospermous flowers and fruits from the Winton For- AND THE OMEGA TEAM. 2005. Surface diversity as revealed by mation, western Queensland, Australia. Cretaceous Research 30, the OMEGA/Mars Express observations. Science 307, 1576–1581. 339–355. BIRD M. I. & CHIVAS A. R. 1988. Oxygen isotope dating of the Australian DETTMANN M. E., MOLNAR R. E., DOUGLAS J. G., BURGER H. D., FIELDING C., 331 – regolith. Nature ,513 516. CLIFFORD H. T., FRANCIS J., JELL P. , R ICH T., WADE M., RICH P. V. , BIRD M. I. & CHIVAS A. R. 1989. Stable isotope geochronology of the Aus- PLEDGE N., KEMP A. & ROZEFELDS A. 1992. Australian Cretaceous ter- tralian regolith. Geochimica et Cosmochimica Acta 53,3239–3256. restrial faunas and floras: biostratigraphic and biogeographic BIRD M. I. & CHIVAS A. R. 1993. Geomorphic and palaeoclimatic implications. Cretaceous Research 13,207–262. implications of an oxygen-isotope chronology for Australian DONALD R. & SOUTHAM G. 1999. Low temperature anaerobic bacterial deeply weathered profiles. Australian Journal of Earth Sciences diagenesis of ferrous monosulfide to pyrite. Geochimica et Cosmo- 40, 345–358. chimica Acta 63, 2019–2033. BIRD M. I., CHIVAS A. R. & MCDOUGALL I. 1990. An isotopic study of surfi- DOWELL K., MAVROGENES J., MCPHAIL D. C. & WATKINS J. 2002. Origin and cial alunite in Australia. 2. Potassium–argon geochronology. timing of formation of precious black opal nobbies at Lightning Chemical Geology 80, 133–146. Ridge. In: Roach I. C. ed. Regolith and landscapes in Eastern Aus- BLATT H., MIDDLETON G. & MURRAY R. 1980. Origin of sedimentary rocks, tralia,pp.18–20. CRC LEME. second edition. Prentice-Hall, Englewood Cliffs, NJ. EXON N. F. & SENIOR B. R. 1976. The Cretaceous of the Eromanga and BOREHAM C. J. & POWELL T. G. 1987. Sources and preservation of or- Surat Basins. BMR Journal of Australian Geology and Geophysics ganic matter in the Cretaceous Toolebuc Formation, eastern Aus- 1,33–50. tralia. Organic Geochemistry 11,433–449. TAJE_A_784219.3d (TAJE) 03-05-2013 15:59

312 P. F. Rey

FAURE G. 1998. Principles and Applications of Geochemistry. Prentice- JONES J. B. & SEGNIT E. R. 1966. The occurrence and formation of opal Hall, Upper Saddle River, N.J. at Coober Pedy and Andamooka. Australian Journal of Science FINK A. 2011. Unearth the potch opal from Lightning Ridge. 7th Na- 29, 129–133. tional Opal Symposium, Lightning Ridge. JøRGENSEN B. B. 2006. Bacteria and marine biogeochemistry. In: FOSTER D. A., MURPHY J. M. & GLEADOW A. J. W.1994. Middle Tertiary hy- Schulz H. D. & Zabel M. eds. Marine geochemistry, pp. 169–206. drothermal activity and uplift of the northern Flinders Ranges, Springer, Berlin. South Australia: Insights from the apatite fission-track thermo- KERNICH A., PAIN C., KILGOUR P. & M ALLY B. 2004. Regolith landforms in chronology. Australian Journal of Earth Sciences 41,11–17. the Lower Balonne area, Southern Queensland, Australian. CRC FRAKES L.A, BURGER D., APTHORPE M., WISEMAN J., DETTMANN M., ALLEY LEME open file report 161,59pp. N., FLINT R., GRAVESTOCK D., LUDBROOK N., BACKHOUSE J., SKWARKO S., KING S. J. & MEE B. C. 2004. The seismic stratigraphy and petroleum SCHEIBNEROVA V. , M CMINN A., MOORE P. S . , B OLTON B. R., DOUGLAS J. potential of the Late Cretaceous Ceduna Delta, Ceduna Sub-ba- G., CHRIST R., WADE M., MOLNAR R. E., MCGOWRAN B., BALME B. E. & sin, Great Australian Bight. In: Boult P. J., Johns D. R. & Lang S. DAY R. A. 1987. Australian Cretaceous shorelines, stage by stage. C. eds. Eastern Australasian Basins Symposium II,pp.63–73. Pe- Palaeogeography, Palaeoclimatology and Palaeoecology 59,31–48. troleum Exploration Society of Australia, Special Publication. FUJIOKA T. & C HAPPELL J. 2010. History of Australian aridity: chronol- KNOBLAUCH C., SAHM K. & JøRGENSEN B. B. 1999. Psychrophilic sulphate ogy in the evolution of arid landscapes. Geological Society, Lon- reducing bacteria isolated from permanently cold Arctic marine don, Special Publications 346,121–139. sediments: description of Desulfofrigus oceanense gen. nov., sp. GAILLOU E., DELAUNAY A., RONDEAU B., BOUHNIK-LE COZ M., FRITSCH E., nov., Desulfofrigus fragile sp. nov., Desulfofaba gelida gen. nov., sp. CORNEN G. & MONNIER C. 2008a. The geochemistry of opals as evi- nov., Desulfotalea psychrophilia gen. nov.,sp. nov.,and Desulfotalea dence of their origin. Ore Geology Reviews 34, 113–126. arctica sp. nov. Journal of Systematic Bacteriology 49,1631–1643. GAILLOU E., FRITSCH E., AGUILAR-REYES B., RONDEAU B., POST J., BARREAU KOMADEL P. , M ADEJOVA J., JANET M., GATES W. P. , K IRKPATRICK R. J. & A. & OSTROUMOV M. 2008b. Common gem opal: An investigation of STUCKI J. W.1996. Dissolution of hectorite in inorganic acids. Clays micro- to nano-structure. American Mineralogist 93, 1865–1873. and Clay Minerals 44, 228–236. GAINA C., Mu€LLER R. D., ROYERAND J-Y. & SYMONDS P. A. 1998. The tec- KORSCH R. J., TOTTERDELL J. M., FOMIN T. & N ICOLL M. G. 2009. Contrac- tonic history of the Tasman Sea: a puzzle with 13 pieces. Journal tional structures and deformational events in the Bowen, Gunne- of Geophysical Reserarch 103, 12413–12423. dah and Surat Basins, eastern Australia. Australian Journal of GALLAGHER K. & LAMBECK K. 1989. Subsidence sedimentation and sea- Earth Sciences 56,477–499. level changes in the Eromanga Basin, Australia. Basin Research KRIEG G. W., ALEXANDER E. M. & ROGERS P. A. 1995. Mesozoic: Jurassic– 2,115–131. Cretaceous epicratonic basins: Eromanga Basin. In: Drexel J. F. GALLAGHER K., DUMITRU T. A . & G LEADOW A. J. W. 1994. Constraints on & Preiss W. V. eds. The geology of South Australia, pp. 101–127. the vertical motion of eastern Australia during the Mesozoic. Ba- South Australia Geological Survey,Adelaide. sin Research 6,77–94. LEWIS S. E., HENDERSON R. A., DICKENS G. R., SHIELDS G. A. & COXHELL S. GATES W. P. , A NDERSON J. S., RAVEN M. D. & CHURCHMAN G. J. 2002. Miner- 2010. The geochemistry of primary and weathered oil shale and alogy of a bentonite from Miles. Queensland, Australia and char- coquina across the Julia Creek vanadium deposit (Queensland, acterization of its acid activation products. Applied Clay Science Australia). Mineralium Deposita 45,599–620. 20, 189–197. MACDONALD J. D., HOLFORD S. P., GREEN P. F. , D UDDY I. R., KING R. C. & GLASS-VAN DER BEEK I. 2003. Structural and lithological relationships BACKe G. 2013. Detrital zircon data reveal the origin of Australia’s to opal deposition at Lightning Ridge, NSW. Honours thesis, The largest delta system. Journal of the Geological Society of London University of Sydney. 170,3–6, doi:10.1144/jgs2012-093 ̈ GURNIS M., MULLER R. D. & MORESI L. 1998. Cretaceous vertical motion MAKO E., SENKAR Z., KRISTOF J. & VAGV OLGYI€ V. 2006. Surface modifica- of Australia and the Australian Antarctic Discordance. Science tion of mechanochemically activated kaolinites by selective 279,1499–1504. leaching. Journal of Colloid and Interface Science 294,362–370. HAIG D. W. & LYNCH D. A. 1993. A late early Albian marine transgres- MATTHEWS K. J., HALE A. J., GURNIS M., MULLER€ R. D. & DICAPRIO L. 2011. sive pulse over northeastern Australia, precursor to epeiric ba- Dynamic subsidence of Eastern Australia during the Cretaceous. sin anoxia: foraminiferal evidence. Marine Micropaleontology 22, Gondwana Research 19, 372–383. 311–362. MAVROMATIDIS A. 1997. Quantification of exhumation in the Cooper- HARTMAN H. & MCKAY C. P. 1995. Oxygenic photosynthesis and the oxi- Eromanga Basins, Australia. PhD thesis, University of Adelaide. dation state of Mars. Planetary and Space Science 43, 123–128. MAVROMATIDIS A. & HILLIS R. 2005. Quantification of exhumation in the HAWLADER H. M. 1990. Diagenesis and reservoir potential of volcano- Eromanga Basin and its implications for hydrocarbon explora- genic sandstones Cretaceous of the Surat Basin, Australia. tion. Petroleum Geoscience 11,79–92. Downloaded by [203.45.115.130] at 22:34 03 June 2013 Sedimentary Geology 66,181–195. MCADAM A. C., ZOLOTOV M. Y., MIRONENKO M. V.& SHARP T. G. 2007. Acid HENDERSON R. A. 2004. A mid-Cretaceous association of Shell beds or- weathering of basaltic lithologies: equilibrium modeling and ap- ganic-rich shale: Bivalve exploitation of a nutrient-rich, anoxic plication to Mars. Lunar and Planetary Science XXXVIII, ab- sea-floor environment. Palaios 19, 156–169. stract 2198. HORTON D. 2002. Australian sedimentary opal—Why is Australia MCADAM A. C., ZOLOTOV M. Y., MIRONENKO M. V. & SHARP T. G. 2008. For- unique? Australian Gemmologist 21, 287–294. mation of silica by low-temperature acid alteration of Martian HUROWITZ J. A., MCLENNAN S. M., TOSCA N. J., ARVIDSON R. E., MICHALSKI rocks: Physical-chemical constraints. Journal of Geophysical Re- J. R., MING D. W., SCHRODER€ C. & SQUYRES S. W. 2006. In situ and ex- search 113, E08003, doi:10.1029/2007JE003056. perimental evidence for acidic weathering of rocks and soils on MCORIST G. D. & SMALLWOOD A. 1995. Trace elements in coloured opals Mars. Journal of Geophysical Research 111, E02S19, doi:10.1029/ using neutron activation analysis. Journal of Radioanalytical 2005JE002515. and Nuclear Chemistry 198,499–510. HUTTON J. T., TWIDALE C. R. & MILNES A. R. 1978. Characteristics and ori- MCORIST G. D. & SMALLWOOD A. 1997. Trace elements in precious and gin of some Australian silcretes. In: Langford-Smith T. ed. Silcrete common opals using neutron activation analysis. Journal of in Australia,pp.18–39. University of New England, Armidale. Radioanalytical and Nuclear Chemistry 223,9–15. IDNURM M. & SENIOR B. R. 1978. Palaeomagnetic ages of Late Creta- MCORIST G. D., SMALLWOOD A. & FARDY J. J. 1994. Trace elements in Aus- ceous and Tertiary weathered profiles in the Eromanga Basin, tralian opals using neutron activation analysis. Journal of Radio- Queensland. Palaeogeography, Palaeoclimatology and Palaeoecol- analytical and Nuclear Chemistry 185,293–303. ogy 24, 263–277. MCQUEEN K. G., PILLANS B. J. & SMITH M. I. 2002. Constraining the ILER R. K. 1979. The chemistry of silica: solubility, polymerization, col- weathering history of the Cobar region, western NSW. In: Preiss loid and surface properties, and biochemistry. Wiley,New York. V. P. ed. Geoscience 2002: Expanding Horizons. 16th Australian INGRAM J. A. 1968. Notes on opalisation in south-western Queensland. Geological Convention, Adelaide SA, Geological Society of Aus- Bureau of Mineral Resources, Geology and Geophysics Record tralia, Abstract, p. 426. 1968/47. MERDITH A. S., LANDGREBE T. C . W. , D UTKIEWICZ A. & Mu€LLER R. D. 2013. JAMBOR J. L., NORDSTROM D. K. & ALPERS C. N. 2000. Metal-sulfate salts Towards a predictive model for opal exploration using a spatio- from sulfide mineral oxidation. Reviews in Mineralogy and Geo- temporal data mining approach. Australian Journal of Earth Sci- chemistry 40,303–350. ences 60,x–x. TAJE_A_784219.3d (TAJE) 03-05-2013 15:59

Opalisation of the Great Artesian Basin 313

METZ V. , A MRAM K. & GANOR J. 2005. Stoichiometry of smectite dissolu- Mars and implications for early martian climate. Nature 438, tion reaction. Geochimica et Cosmochimica Acta 69, 1755–1772. 623–627. MILLIKEN R. E., SWAYZE G. A., ARVIDSON R. E., BISHOP J. L., CLARK R. N., RAISWELL R., BENNING L. G., DAVIDSON L., TRANTER M. & TULACZYK EHLMANN B. L., GREEN R. O., GROTZINGER J. P., MORRIS R. V., MURCHIE S. 2009. Schwertmannite in wet, acid, and oxic microenviron- S. L., MUSTARD J. F., WEITZ C. 2008. Opaline silica in young deposits ments beneath polar and polythermal glaciers. Geology 37, on Mars. Geology 36,847–850. 431–434. MOORE M. E., GLEADOW A. J. W.& LOVERING J. F.1986. Thermal evolution RAZA A., HILL K. C. & KORSCH R. J. 2009. Mid-Cretaceous uplift and de- of rifted continental margins: new evidence form fission tracks nudation of the Bowen and Surat Basins, eastern Australia: Rela- in basement apatites from southeastern Australia. Earth Plane- tionship to Tasman Sea rifting from apatite fission-track and tary Science Letters 78,255–270. vitrine-reflectance data. Australian Journal of Earth Sciences 56, MOORE P. S . & P ITT G. M. 1984. Cretaceous of the Eromanga Basin— 501–531. implications for hydrocarbon exploration. Australian Petroleum REID C. M., NOEL P. J. , K YSER T. K. & B EAUCHAMP B. 2008: Diagenetic Cy- Exploration Association 24, 358–376. cling of Nutrients in Seafloor Sediments and the Carbonate–Sil- MORRIS R. V., MING D. W., GOLDEN D. C. & BELL J. F. 1996. An occurrence ica Balance in a Paleozoic Cool-Water Carbonate System, of jarositic tephra on Mauna Kea, Hawaii: Implications for the Sverdrup Basin, Canadian Arctic Archipelago. Journal of Sedi- ferric mineralogy of the Martian surface. In: Dyer M. D., et al. mentary Research 78, 562–578. eds. Mineral spectroscopy: A tribute to Roger G. Burns,p.327–336. REY P. F. & M u€LLER R. D. 2010. Fragmentation of active continental Geochemical Society Special Publication 5, Houston, TX. plate margins owing to the buoyancy of the mantle wedge. Nature MOUSSAVI-HARAMI R. & ALEXANDER E. 1998. Tertiary stratigraphy and Geoscience 3, 257–261 tectonics, Eromanga Basin region. MESA Journal 8,32–36. ROBERTS K. K., ARCHER M., HAND S. J. & GODTHELP H. 2009. New Austra- Mu€LLER R. D., DYKSTERHUIS S. & REY P. F. 2012. Australian paleo-stress lian Oligocene to Miocene Ringtail possums (pseudocheiridae) fields and tectonic reactivation over the past 100 Ma. The Austra- and revision of the genus Marlu. Palaeontology 52,441–456. lian Journal of Earth Sciences 59,13–28. doi:10.1080/ RYAN P. C . & H UERTAS F. J. 2005. The temporal evolution of pedogenic 08120099.2011.605801. Fe-smectite to Fe-kaolin via interstratified kaolin-smectite in a Mu€LLER R. D., SDROLIAS M., GAINA C., STEINBERGER B. & HEINE C. 2008. moist tropical soil chronosequence. Geoderma 151,1–15. Long-term sea-level fluctuations driven by ocean basin dynamics. SANDERS J. V.1964. Colour of precious opal. Nature 204,1151–1153. Science 319, 1357–1362. SANDERS J. V. 1968. Diffraction of light by opals. Acta Crystallograph- NEWBERRY T. L. 2005. Weathering geochemistry and geochronology of ica A24, 427–434. the Australian sedimentary-hosted opal deposits. PhD thesis, The SANDERS J. V. & DARRAGH P. J. 1971. The microstructure of precious University of Queensland. opal. The Mineralogical Record 2,261–268. NORVICK M. & SMITH M. 2001. Southeast Australia-Mapping the plate SANDERS J. V. & MURRAY M. J. 1978. Ordered arrangements of spheres of tectonic reconstruction of southern and southeastern Australia two different sizes in opal. Nature 275,201–203. and implications for petroleum systems. APPEA Journal-Austra- SCHMIDT P. W. & O LLIER C. D. 1988. Palaeomagnetic dating of Late Creta- lian Petroleum Production and Exploration Association 41,15–36. ceous to Early Tertiary weathering in New England, N.S.W. Aus- PANDA A. K., MISHRA B. G., MISHRA D. K. & SINGH R. K. 2010. Effect of sul- tralia. Earth-Science Reviews 25,363–371. phuric acid treatment on the physico-chemical characteristics of SCHMITT J-M. 1999. Weathering, rainwater and atmosphere chemistry: kaolin clay. Colloids and Surfaces A: Physicochemical and Engi- example and modelling of granite weathering in present condi-

neering Aspects 363,98–104. tions in a CO2-rich, and in an anoxic palaeoatmosphere. In: Thiry PASSMORE V. L. 1989. Petroleum accumulations of the Eromanga Ba- M. & Simon-Coincon¸ R. eds. Palaeoweathering, palaeosurfaces, sin: a comparison with other Australian Mesozoic accumula- and related continental deposits,pp.21–41. Special Publication of tions. In:O’Neil B. J. ed. The Cooper and Eromanga Basins, the International Association of Sedimentologists 27. Australia, pp. 371–390. Petroleum Exploration Society of Aus- SCHOONEN M. A. A. 2004. Mechanism of sedimentary pyrite formation. tralia, Society of Petroleum Engineers and Australian Society of In: Amend J. P., Ewards K. J. & Lyons T. W.eds. Sulfur biogeochem- Exploration Geophysicists, Adelaide. istry—past and present, pp. 117–134. Geological Society of America PATTERSON J. H., RAMSDEN A. R., DALE L. S. & FARDY J. J. 1986: Geochem- Special Paper 379. istry and mineralogical residences of trace elements in oil shales SCHWERTMANN U. & M URAD E. 1983. Effect of pH on the formation of goe- from Julia Creek, Queensland, Australia. Chemical Geology 55,1– thite and hematite from ferrihydrite. Clays and Clay Minerals 31, 16. 277–283. PECOVER S. R. 1996. A new genetic model for the origin of opal in the SENIOR B. R. & MABBUTT J. A. 1979. A proposed method of defining Great Australian Basin. Geological Society of Australia Conference deeply weathered rock units based on regional geological map- Downloaded by [203.45.115.130] at 22:34 03 June 2013 Extended Abstracts 43, pp. 450–454. ping in southwest Queensland. Australian Journal of Earth Scien- PECOVER S. R. 1999. The new syntectonic model of origin to explain the ces 26,237–254. formation of opal veins (Seams and Nobbies), breccia pipes SENIOR B. R. & CHADDERTON L. T. 2007. Natural gamma radioactivity (Blows) and faults (Slides) at Lightning Ridge. 1st National Opal and exploration for precious opal in Australia. The Australian Symposium, Lightning Ridge, extended abstracts, p. 1–8. Gemmologist 23, 160–176. PECOVER S. R. 2007. Australian Opal Resources: Outback Spectral Fire. SENIOR B. R., MCCOLL D. H., LONG B. E. & WHITELEY R. J. 1977. The geol- The Australian Gemmologist 82, 103–115. ogy and magnetic characteristics of precious opal deposits, south- PERSANO C., STUART F. M . , B ISHOP P. & B ARFOLD D. N. 2002. Apatite west Queensland. BMR Journal of Australian Geology and (U–Th)/He age constraints on the development of the Great Geophysics 2, 241–251. Escarpment on the southeastern Australian passive margin. SHAW S. A. & HENDRY M. J. 2009. Geochemical and mineralogical

Earth and Planetary Science Letters 200,79–90. impacts of H2SO4 on clays between pH 5.0 and 3.0. Applied Geo- PEWKLIANG B., PRING A. & BRUGGER J. 2008. The formation of precious chemistry 24, 333–345. opal: Clues from the opalisation of bone. The Canadian Mineralo- SIMON-COINcON¸ R., MILNES A. R., THIRY M. & WRIGHT M. J. 1996. Evolu- gist 46,139–149. tion of landscapes in northern South Australia in relation to the PILLANS B. 2005. Geochronology of the Australian regolith. In: Anand distribution and formation of silcretes. Journal of the Geological R. R. & de Broekert P. eds. Regolith-landscape evolution across Society London 153, 467–480. Australia,pp.41–61. CRC LEME, Perth. SMALLWOOD A. G., THOMAS P. & R AY A. S. 1997. Characterisation of sedi- PILLANS B. 2007. Pre-Quaternary landscape inheritance in Australia. mentary opals by Fourier transform Raman spectroscopy. Spec- Journal of Quaternary Science 22,439–447. trochimica Acta Part A 53, 2341–2345. PIRLET H., WEHRMANN L. M., BRUNNER B., FRANK N., DEWANCKELE J., Van SMALLWOOD A. G., THOMAS P. & R AY A. S. 2008a. Characterisation of the ROOIJ D., FOUBERT A., SWENNEN R., NAUDTS L., BOONE M., CNUDDE V. & dehydration of Australian sedimentary and volcanic precious HENRIET J-P. 2010. Diagenetic formation of gypsum and dolomite opal by thermal methods. Journal of Thermal Analysis and Calo- in a cold-water coral mound in the Porcupine Seabight, off Ire- rimetry 92,91–95. land. Sedimentology 57,786–805. SMALLWOOD A. G., THOMAS P. & R AY A. S. 2008b. Comparative analysis of POULET F. , B IBRING J. P., MUSTARD J. F., GENDRIN A., MANGOLD N., LANGEVIN sedimentary and volcanic precious opals from Australia. Journal Y. , A RVIDSON R. E., GONDET B. & GOMEZ C. 2005. Phyllosilicates on of Australasian Ceramic Society 44,17–22. TAJE_A_784219.3d (TAJE) 03-05-2013 15:59

314 P. F. Rey

SMALLWOOD A. G., THOMAS P. , R AY A. S. & SIMON P. 2007. TMA and SEM TOTTERDELL J. M., BLEVIN J. E., STRUCKMEYER H. I. M., BRADSHAW B. E., characterization of the thermal dehydration of Australian sedimen- COLWELL J. B. & KENNARD J. M. 2000. A new sequence framework tary opal. Journal of Thermal Analysis and Calorimetry 88, 185–188. for the Great Australian Bight: starting with a clean slate. Austra- SMALLWOOD A. G., THOMAS P. , R AY A. S. & SIMON P. 2009. Application of a lian Petroleum Production and Exploration Association Journal Fickian model of diffusion to the dehydration of graded specimens 40,95–117. 3þ 3þ of a precious Australian sedimentary opal derived from Coober TROLARD F. & T ARDY Y. 1989. A model of Fe -Kaolinite, Al -Goethite, Pedy. Journal of Thermal Analysis and Calorimetry 97, 685–688. Al3þ-Hematite equilibria in laterites. Clay Minerals 24,1–24. SMITH M. L., PILLANS B. J. & MCQUEEN K. G. 2009. Paleomagnetic evi- TULLOCH A. J., BEGGS M., KULA J. L., SPELL T. L . & M ORTIMER N. 2006. Cor- dence for periods of intense oxidative weathering, McKinnons dillera Zealandia, the Sisters Shear Zone and their influence on mine, Cobar, New South Wales. Australian Journal of Earth Scien- the early development of the Great South Basin. In: 2006 New Zea- ces 56,201–212. land Petroleum Conference proceedings, 11 pp. Ministry of Eco- SQUYRES S. W., ARVIDSON E. R., RUFF S., GELLERT R., MORRIS R. V., et al. nomic Development, Wellington. 2004. In situ evidence for an ancient aqueous environment at Mer- VASCONCELOS P. M . & C ONROY M. 2003. Geochronology of weathering idiani Planum, Mars. Science 306, 1709–1714. and landscape evolution, Dugald River valley, NW Queensland, SQUYRES S. W. & 17 others. 2008. Detection of silica-rich deposits on Australia. Geochimica et Cosmochimica Acta 67, 2913–2930. Mars. Science 320, 1063–1067. VASCONCELOS P. M . , K NESEL K. M., COHEN B. E. & HEIM J. A. 2008. Geo- TARDY Y. 1997. Petrology of laterites and tropical soils. Masson edition, chronology of the Australian Cainozoic: a history of tectonic and Paris, 408 p. igneous activity, weathering, erosion, and sedimentation. Austra- TAYLOR G. & SHIRTLIFF G. 2003. Weathering: cyclical or continuous? An lian Journal of Earth Sciences 55,865–914. Australian perspective. Australian Journal of Earth Sciences 50, VAZQUEZ O., PU X., MONNELL J. & NEUFELD R. 2010. Release of aluminum 9–17. from clays in an acid rock drainage environment. Mine Water THIRY M. & MILNES R. 1991. Pedogenic and groundwater silcretes at and the Environment 29, 270–276. Stuart Creek opal field, South Australia. Journal of Sedimentary VEEVERS J. J., POWELL C. MCA. & ROOTS S. R. 1991. Review of seafloor Petrology 61,111–127. spreading around Australia. I. Synthesis of the patterns of THIRY M., MILNES R., RAYO T V. & S IMON-COINcON¸ R. 2007. Interpretation of spreading. Australian Journal of Earth Sciences 38,373–389. palaeoweathering features and successive silicification in the VEEVERS J. J. ED. 2000. Billion-Year Earth History of Australia and Tertiary regolith of inland Australia. Journal of the Geological So- Neighbours in Gondwanaland. Geochemical Evolution and Metal- ciety of London 163, 723–736. logeny of Continents (GEMOC) Press, Macquarie University, THIRY M., SCHMITT J-M., RAY O T V. & M ILNES R. 1995. Geochimie des alte- Sydney. rations des profils blanchis du regolithe tertiaire de l’interieur WATKINS J. J., BEHR H. J. & BEHR K. 2011. Fossil microbes in opal from de l’Australie. Comptes Rendus de l’Academie des Sciences, Serie Lightning Ridge—Implications for the formation of opal. Quar- IIa, 230, 279–285. terly Notes, Geological Survey of New South Wales 136,1–21. THOMAS P. , B ROWN L. D., RAY A. S. & PRINCE K. 2006. A SIMS study of the WILLIAMSON M. A. & RIMSTIDT J. D. 1994. The kinetics and electrochemi- transition elemental distribution between bands in banded Aus- cal rate determining step of aqueous pyrite oxidation. Geochi- tralian sedimentary opal from the Lightning Ridge locality. Neues mica et Cosmochimica Acta 58,5443–5454. Jahrbuch Fur Mineralogie-Abhandlungen 182,193–199. WOODBURNE M. O., MACFADDEN B. J., CASE J. A., SPRINGER M. S., PLEDGE THOMAS P. , S ESTAK J., HEIDE K., FUEGLEIN E. & SIMON P. 2010. Thermal N. S., POWER J. D., WOODBURNE J. M. & SPRINGER K. B. 1993. Land properties of Australian sedimentary opals and Czech molda- mammal biostratigraphy and magnetostratigraphy of the Eta- vites. Journal of Thermal Analysis and Calorimetry 99, 861–867. dunna Formation (Late Oligocene) of South Australia. Journal of THOMAS P. , S IMON P. , S MALLWOOD A. G. & RAY A. S. 2007. Estimation of the Vertebrate Paleontology 13, 483–515. diffusion coefficient of water evolved during the non-isothermal WOPFNER H. 1974. Post-Eocene history and stratigraphy of north-east- dehydration of Australian sedimentary opal. Journal of Thermal ern South Australia. Transaction of the Royal Society of South Analysis and Calorimetry 88,231–235. Australia 98,1–12. THOMAS P. , S MALLWOOD A. G., RAY A. S., BRISCOE B. J. & PARSONAGE D. 2008. WOPFNER H., CALLEN R. & HARRIS W. K. 1974. The lower tertiary Eyre Nanoindentation hardness of banded Australian sedimentary Formation of the Southwestern great Artesian basin. Australian opal. Journal of Physics D: Applied Physics 41,1–6. Journal of Earth Sciences 21,17–51. TINGATE P. R . & D UDDY I. R. 2002. The thermal history of the eastern Of- WORRALL L. & CLARKE J. D. A. 2004. The effect of Middle to Late Ter- ficer Basin (South Australia): evidence from apatite fission track tiary fluctuations of sea level on the geochemical evolution of analysis and organic maturity data. Tectonophysics 349, 251–275. West Australian regolith. In: Fabel D. ed. Abstracts of the 11th Aus- TONUI E. & DE CARITAT P. 2003. Composition, diagenesis, and weather- tralian and New Zealand Geomorphology Group,p.80. Downloaded by [203.45.115.130] at 22:34 03 June 2013 ing of the sediments and basement of the Callabonna sub-basin, Wu€LSER P. R . , B RUGGER J., FODEN J. & PFEIFER H-R. 2011. The sandstone- central Australia: Implication for landscape evolution. Journal of hosted Beverley uranium deposit, Lake Frome basin, South Aus- Sedimentary Research 73, 1036–1050. tralia: Mineralogy, geochemistry, and a time-constrained model TOTTERDELL J. M. & BRADSHAW B. E. 2004. The structural framework for its formation. Economic Geology 106,835–867. and tectonic evolution of the Bight Basin. In: Boult P. J., Johns D. R. & Lang S. C. eds. Eastern Australasian Basins Symposium II, pp. 41–61. Petroleum Exploration Society of Australia, Special Publication. Received 12 October 2012; accepted 27 January 2013