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Temperature Concentration of Tellurium and Gold in Continental Red Bed Successions

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Temperature Concentration of Tellurium and Gold in Continental Red Bed Successions doi: 10.1111/ter.12213 Low-temperature concentration of tellurium and gold in continental red bed successions John Parnell,1 Samuel Spinks1 and David Bellis2 1School of Geosciences, University of Aberdeen, Aberdeen AB24 3UE, UK; 2School of Natural and Computing Sciences, University of Aberdeen, Aberdeen AB24 3UE, UK ABSTRACT There is very little understanding of tellurium (Te) distribution telluride minerals were recorded. The data show that Te is and behaviour in sedimentary rocks. A suite of 15 samples of mobile and can be concentrated in low-temperature sedimen- reduction spheroids (centimetre-scale pale spheroids in other- tary environments, controlled by redox variations. The consis- wise red rock), including samples from eight localities in Tri- tency in enrichment across widely separate localities implies assic red beds across the British Isles, were mapped for Te that the enrichment is a normal aspect of red bed diagenesis using Laser Ablation–Inductively Coupled Plasma–Mass Spec- and so likely to be controlled by a ubiquitous process, such trometry. Almost all showed enrichment in Te in the cores of as microbial activity. the spheroids relative to background red bed concentrations, by up to four orders of magnitude. Some were also enriched Terra Nova, 28: 221–227, 2016 over background in gold and/or mercury. In one case, discrete and in many cases, the spheroids are metals at low temperatures (Shep- Introduction also enriched in Au. herd et al., 2005). This raises the The mobilization and concentration Tellurium can fix Au within tel- possibility that there may be of gold (Au) are attributed particu- luride minerals in Au ore deposits in widespread mobility of Au in sedi- larly to high-temperature processes a range of settings and can be a ments with variable redox conditions, in magmatic, metamorphic and pathfinder element in Au exploration in contrast to its purported inert hydrothermal environments (Afifi (Boyle, 1979; Ciobanu et al., 2006). behaviour. et al., 1988; Cook and Ciobanu, Conventionally, Au deposition is Continental red beds have a his- 2005). However, there is a growing attributed to medium- to high-tem- tory back to the early Proterozoic, recognition that reduction, com- perature environments, where Te is when sedimentation in oxidizing con- monly microbially mediated, can regarded as mobile and available ditions first occurred. However, there cause the solubilization and redistri- (Afifi et al., 1988; Cook and Cio- were times when their occurrence bution of Au and associated elements banu, 2005). By contrast, almost was especially widespread, including including tellurium (Te) at low tem- nothing is known about the beha- the Mesoproterozoic, the Devonian peratures. Tellurium is of increasing viour of Te in sedimentary rocks, (Old Red Sandstone) and the Permo- technological importance (Zweibel, and except for some exceptional Triassic (New Red Sandstone). Red 2010; Turner et al., 2012; Zepf et al., ocean floor deposits (Hein et al., beds from each of these times con- 2014) and is a critical metal for the 2003), anomalous sedimentary con- tain abundant reduction spheroids, development of low-carbon energy centrations of Te are unknown. Mer- i.e. centimetre-scale pale spheroids in technologies in Europe (Moss et al., cury concentrations in siliciclastic otherwise red rock, attributed a 2011). In this study, we analyse rocks are also extremely low microbiological origin by many reduction spheroids developed ubiq- (McNeal and Rose, 1974). Accord- workers, and containing metal-rich uitously in red bed successions under ingly, Te and Hg are not known con- cores (e.g. Harrison, 1975; Hofmann, possible microbial mediation, to test trols on Au distribution in 1990; Spinks et al., 2010; Zhang for Te enrichment, and find that sedimentary environments. et al., 2014). This study assesses almost all examples exhibit the con- Gold mineralization can occur in whether Te, Hg and Au become con- centration of Te over background or adjacent to continental red beds centrated in these low-temperature levels by up to several orders of mag- (red sandstones and mudrocks), red bed features, focusing on Triassic nitude. Many similarly exhibit con- implying the mobilization and fixing rocks that have never experienced centrations of mercury (Hg). of Au by low-temperature processes high (>150 °C) temperatures. Tellurium and Hg, among others, (Shepherd et al., 2005; Spinks et al., can be pathfinder elements for Au, 2016). To date, such deposits are Material and methods exceptional and have been linked to anomalous Au availability through A set of 10 samples (eight localities, volcanic activity (Colman and two duplicates to test consistency) of Correspondence: John Parnell, School of Cooper, 2000). However, there is a Triassic age over 900 km of the Bri- Geosciences, University of Aberdeen, growing recognition that reduction tish Isles were analysed by laser Aberdeen AB24 3UE, UK. Tel.: can cause the solubilization and ablation–inductively coupled plasma– +44 1224 273464; fax: +44 1224 272785; e-mail: [email protected] redistribution of Au and other rare mass spectrometry (LA-ICP-MS) to © 2016 The Authors. Terra Nova Published by John Wiley & Sons Ltd. 221 This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Low-temperature tellurium and gold in red beds • J. Parnell et al. Terra Nova, Vol 28, No. 3, 221–227 ............................................................................................................................................................. produce maps showing patterns of Bantycock, Cropwell Bishop, Gipsy as abundant micron-scale grains in enrichment against background Lane) are all in active or former gyp- the spheroids, as they were in the (Fig. 1). Samples were collected from sum workings (Fig. 2), as are the Devonian spheroids from Millport bedding surfaces in mudrocks depos- sites in Ireland. In the East Mid- (Spinks et al., 2014). ited in low-energy fluvial successions lands, Elliott (1961) noted that the in the Upper Triassic Mercia Mud- spheroids in mudstones ‘may form Discussion stone Group. For comparison, reduc- local marker bands where they occur tion spheroids of Mesoproterozoic in profusion, as they do for instance Conditions of concentration age from Culkein, Scotland (Spinks in the roof of the ... gypsum mines’. et al., 2010), Devonian age from The Triassic samples have never Millport, Scotland (Spinks et al., Results experienced temperatures exceeding 2014), Carboniferous age from Car 100 °C. They have very shallow bur- Rocks, Scotland and Cultra, Ireland, The LA-ICP-MS maps show Te ial histories, typically no more than and Triassic age from Utah were enrichment in 9 of 10 Triassic sam- 2 km, and the maximum tempera- treated in the same manner. Typi- ples (Fig. 1) by up to four orders of tures reached at two of the sites in cally, rocks contained 50– magnitude (Table 1), from back- central England and northwest Scot- 100 spheroids mÀ3. Samples were ground values of <1 p.p.m. to maxi- land are estimated at about 80 °C chosen with a core about 2 mm in mum values commonly in the range (Kilenyi and Standley, 1985; McKin- diameter, suitable for seven or eight 103–104 p.p.m. Background values ley et al., 2012). At these near-sur- laser transects. Analysis was per- are all <0.1 p.p.m., consistent with face temperatures and the high formed using a UP213 laser ablation previous measurements in sediments oxygen levels implied by red beds (LA) system (New Wave) coupled to (Belzile and Chen, 2015). Whole core and neutral to alkaline conditions, an Agilent 7500ce inductively cou- values of 2.4 and 4.5 p.p.m. Te oxyions predominate (McPhail, pled plasma–mass spectrometer (Table 1) confirm enrichment over 1995), so Te should be mobile until (ICP-MS). LA-ICP-MS was tuned background by orders of magnitude. it reaches a redox boundary, where it for maximum sensitivity and stability Seven of 10 spheroids also show Hg, could become concentrated by pro- using standard SRM 612 for trace and all show some degree of Au gressive precipitation. The predomi- elements in glass (NIST), optimizing enrichment, up to five orders of mag- nance of Te concentration over Se is the energy fluence to about 2 J/cm2. nitude (Fig. 3). Comparison of data consistent with the strongly oxidizing A semi-quantitative calibration was from multiple laser transects, and environment (Schirmer et al., 2014). provided using MASS-1 Synthetic >100 individual analyses, on each The data (Figs 1, 3 and and Polymetal Sulfide (USGS). Samples sample, which produce sensible maps Table 1) show that there is consistent and the standard were analysed using (Fig. 3), was used to conclude that enrichment of Te, Hg and Au in the a 100-lm diameter round spot mov- measurements were consistent and reduction spheroids, including spher- ing in a straight line at 50 lmsÀ1.A reliable. We emphasize the pattern of oids from other geological periods. 15-s laser warm-up preceded 30 s of concentration at the cores, rather The degree of enrichment varies ablation (1.5 mm) and a 15-s delay. than the precise level of enrichment. between spheroids, including between 82Se and 125Te were monitored for Different elements show different paired spheroids from a single local- 0.1-s dwell time each. The average enrichment patterns (Fig. 2), just as ity, but enrichment occurs regardless count signal from three lines over they are zoned variably in roll-front of locality or age. The geological his- 20 s of the ablation was calculated deposits, due to variable response to tory of the samples chosen constrains for each element. The standard was redox changes (Reynolds and Gold- the enrichment process to a low-tem- used to calculate the concentration haber, 1983). The degree of enrich- perature mechanism. The reduction (lggÀ1)/counts ratio, which was ment will vary between spheroids at features on which the enrichments multiplied by the sample counts to the same locality, as demonstrated at are based are a widespread feature of estimate the concentration.
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