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Mineral Transformations in Gold–(Silver) Tellurides in the Presence of Fluids: Nature and Experiment

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Mineral Transformations in Gold–(Silver) Tellurides in the Presence of Fluids: Nature and Experiment minerals Review Mineral Transformations in Gold–(Silver) Tellurides in the Presence of Fluids: Nature and Experiment Jing Zhao * and Allan Pring Chemical and Physical Sciences, College of Science and Engineering, Flinders University, Bedford Park, Adelaide, SA 5042, Australia; allan.pring@flinders.edu.au * Correspondence: jing.zhao@flinders.edu.au Received: 16 January 2019; Accepted: 4 March 2019; Published: 9 March 2019 Abstract: Gold–(silver) telluride minerals constitute a major part of the gold endowment at a number of important deposits across the globe. A brief overview of the chemistry and structure of the main gold and silver telluride minerals is presented, focusing on the relationships between calaverite, krennerite, and sylvanite, which have overlapping compositions. These three minerals are replaced by gold–silver alloys when subjected to the actions of hydrothermal fluids under mild hydrothermal conditions (≤220 ◦C). An overview of the product textures, reaction mechanisms, and kinetics of the oxidative leaching of tellurium from gold–(silver) tellurides is presented. For calaverite and krennerite, the replacement reactions are relatively simple interface-coupled dissolution-reprecipitation reactions. In these reactions, the telluride minerals dissolve at the reaction interface and gold immediately precipitates and grows as gold filaments; the tellurium is oxidized to Te(IV) and is lost to the bulk solution. The replacement of sylvanite is more complex and involves two competing pathways leading to either a gold spongy alloy or a mixture of calaverite, hessite, and petzite. This work highlights the substantial progress that has been made in recent years towards understanding the mineralization processes of natural gold–(silver) telluride minerals and mustard gold under hydrothermal conditions. The results of these studies have potential implications for the industrial treatment of gold-bearing telluride minerals. Keywords: gold–(silver) tellurides; natural porous gold; interface-coupled dissolution–reprecipitation; hydrothermal method; calaverite; krennerite; sylvanite 1. Introduction Gold–(silver) tellurides are important accessory minerals, carrying a significant proportion of the gold endowment in some low to medium temperature hydrothermal vein deposits. Gold–(silver) telluride minerals have become one of the most important sources of gold in the world. The Golden Mile deposit in Kalgoorlie, Western Australia, has been an economically important gold–(silver) telluride deposit for over a century; it contained approximately 1450 tons gold, of which approximately 20% was in the form of tellurides [1]. Other notable modern and historic gold deposits carrying significant amounts of the gold as tellurides include Cripple Creek, Colorado (~875 tons gold) [2]; Emperor, Fiji (~360 tons of gold, 10–50% occurring as tellurides) [3,4]; and Sacˇ arîmb,ˇ Romania [5]. Another important example is the recently discovered Sandaowanzi gold deposit on the northeastern edge of the Great Xing’an Range, Heilongjiang Province, North East China, with a total reserve of ≥25 tons of gold and an average grade of 15 g/t [6–9]. We believe that this is the first case of a major gold deposit in which the gold telluride minerals are the dominant ore, with more than 95% of recovered gold occurring as tellurides. Eight gold–(silver) tellurides have been described and are currently recognized as valid minerals: calaverite, krennerite, sylvanite, petzite, muthmannite, empressite, hessite, and stuetzite. A summary Minerals 2019, 9, 167; doi:10.3390/min9030167 www.mdpi.com/journal/minerals Minerals 2019, 9, 167 2 of 17 of the characteristics and physical properties of the main gold (and/or silver) telluride minerals is Minerals 2019, 9, x FOR PEER REVIEW 2 of 17 presented in Table1 and the compositions of these minerals are shown in Figure1. The gold-rich telluridesummary species—calaverite, of the characteristics krennerite and physical and sylvanite—areproperties of the the main most gold common (and/or silver) and economicallytelluride importantminerals minerals is presented of the in group, Table 1 with and the a chemical compositions composition of these minerals of Au1− arexAg shownxTe2. in Cabri Figure [10 1.] gaveThe the followinggold-rich compositional telluride species—calaverite, fields for these minerals: krennerite Calaverite and sylvanite—are 0 to 2.8 wt % Agthe (0most≤ x ≤common0.11); krennerite and 3.4 toeconomically 6.2 wt % Ag important (0.14 ≤ x minerals≤ 0.25); andof the sylvanite group, with 6.7 toa 13.2chemical wt % composition Ag (0.27 ≤ ofx ≤Au0.50).1−xAgxTe A2 more. Cabri recent work[10] byBindi gave the et al. following [11] showed compositional that calaverite fields andfor these sylvanite minerals: can haveCalaverite overlapping 0 to 2.8 wt compositional % Ag (0 ≤ x ≤ fields, 0.11); krennerite 3.4 to 6.2 wt % Ag (0.14 ≤ x ≤ 0.25); and sylvanite 6.7 to 13.2 wt % Ag (0.27 ≤ x ≤ 0.50). and share a similar layered structural topology (as shown in Figure2). The Ag content of calaverite, A more recent work by Bindi et al. [11] showed that calaverite and sylvanite can have overlapping sylvanite,compositional and krennerite fields, and has share been a linkedsimilar layered to its substitution structural topology for Au (as and shown stabilization in Figure of2). the The complex Ag modulatedcontent structures of calaverite, adopted sylvanite, by these and mineralskrennerite [11 has,12 ].been The linked incommensurately to its substitution modulated for Au structureand of calaveritestabilization was of determined the complex by Bindimodulated et al. [struct11] andures its adopted modulations by these are relatedminerals to [11,12]. the distribution The of Auincommensurately3+ and Au+ and modulated the substitution structur ofe of Ag calaverite+ for Au was+. Indetermined krennerite, by AgBindi and et Aual. [11] are and ordered its to avoidmodulations Ag–Te–Ag are linkages related [ 12to ].the Sylvanite distribution occurs of Au in3+ twoand Au forms,+ and one the issubstitution a commensurately of Ag+ for Au modulated+. In superstructurekrennerite, Ag based and onAu theare ordered calaverite to avoid sub-cell Ag–Te–Ag and the linkages other [12]. is an Sylvanite incommensurately occurs in two modulatedforms, formone [13]. is Ona commensurately a historical note, modulated calaverite superstructure was the first ba mineral,sed on the or calaverite compound, sub-cell to be and recognized the other to is have an incommensuratelyan incommensurately modulated modulated structure. form [13].It On was a histor identifiedical note, by calaverite morphological was the crystallographers first mineral, or in compound, to be recognized to have an incommensurately modulated structure. It was identified by 1901 as their attempts to index crystal faces required a model which had intergrowing lattices [14]. morphological crystallographers in 1901 as their attempts to index crystal faces required a model The otherwhich five had telluride intergrowing minerals lattices listed [14]. inThe Table other1 arefive muchtelluride less minerals important listed in in gold Table production 1 are much and less four of themimportant contain in more gold production silver than and gold. four of them contain more silver than gold. FigureFigure 1. Ternary 1. Ternary diagrams diagrams of of Au–(Ag)–Te Au–(Ag)–Te system system (atom%), showing showing compositions compositions of gold–(silver) of gold–(silver) telluridestellurides from from mineral mineral database database [15] [15] and and references references [1,3,8,11,16,17]. [1,3,8,11,16 Compositions,17]. Compositions of synthetic of gold– synthetic (silver) tellurides [16,17] are shown as small colored dots. gold–(silver) tellurides [16,17] are shown as small colored dots. Gold–(silver) telluride minerals in gold deposits are considered refractory ores from a mineral Gold–(silver) telluride minerals in gold deposits are considered refractory ores from a mineral processing perspective, as they are not efficiently leachable in cyanide solutions. Therefore, processingadditional perspective, processing as steps they are are required not efficiently to improve leachable gold recovery in cyanide when solutions. tellurides Therefore,are present additionalin the processingore (e.g., steps [18,19]). are Fine required grinding to and improve pretreatments gold recovery(normally whenroasting tellurides gold tellurides are at present temperatures in the ore (e.g.,≥ [80018,19 °C)]). are Fine generally grinding utilized and to pretreatments improve gold (normallyrecovery. These roasting methods gold are tellurides energy-intensive at temperatures and ≥800raise◦C) areenvironmental generally utilizedissues due to improveto the release gold of recovery. Te species These into methodsthe atmosphere. are energy-intensive An alternative and raisestrategy environmental for gold issuesrecovery due from to thetelluride release ores of is Te need speciesed for into deposits the atmosphere. rich in these refractory An alternative gold ores. strategy for gold recovery from telluride ores is needed for deposits rich in these refractory gold ores. Minerals 2019, 9, 167 3 of 17 Minerals 2019, 9, x FOR PEER REVIEW 3 of 17 Figure 2. Projections of the crystal structures of sylvanite (A) and calaverite (B). Crystal structure data forfor thethe mineralsminerals areare fromfrom referencesreferences [[11,20].11,20]. Table 1. Characteristics and physical properties of the main gold–(silver) tellurides. Table 1. Characteristics and physical properties
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