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Home , Calaverite, Hessite, Krennerite, Petzite, Silver telluride, Sylvanite

Review Transformations in –() 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) 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 minerals is presented, focusing on the relationships between , , and , 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 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 or a mixture of calaverite, , and . 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 [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 , 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, , 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 , 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 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 (%), 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 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). 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 of the main gold–(silver) tellurides. Composition wt % Mineral Chemical Formula Color Density3 Hardness Composition wt % Mineral Chemical Formula Color (g/cm ) HardnessAu Ag Te (g/cm3) Au Ag Te Calaverite AuTe2 Silver white to brassy yellow 9.04 2.5–3 43.6 0 56.4 Calaverite AuTe2 Silver white to brassy yellow 9.04 2.5–3 43.6 0 56.4 Krennerite (Au1−x,Agx)Te2 Silver white to blackish yellow 8.53 2.5 43.6 0 56.4 Krennerite (Au1−x,Agx)Te2 Silver white to blackish yellow 8.53 2.5 43.6 0 56.4 Sylvanite AuAgTe4 Steely gray to silver gray 7.9–8.3 (8.1) 1.5–2 34.4 6.3 59.4 SylvaniteMuthmannite AuAgTe(Ag,Au)Te4 2 SteelyBlackish gray yellow, to silver grayish gray white 7.9–8.3 - (8.1) 2.5 1.5–2 34.3 34.4 19.2 46.56.3 59.4 MuthmannitePetzite (Ag,Au)Te Ag3AuTe2 2 BlackishBright yellow, gray grayis to ironh white black 8.7–9.14 - 2.5 2.5 25.4 34.341.7 32.9 19.2 46.5 Empressite AgTe Bronze, light bronze 7.5–7.6 3.5 0 46.3 53.7 Petzite Ag3AuTe2 Bright steel gray to black 8.7–9.14 2.5 25.4 41.7 32.9 Stuetzite Ag Te , (x = 0.24–0.36) Gray, dark bronze 8 3.5 0 57.0 43.0 Empressite 5− AgTex 3 Bronze, light bronze 7.5–7.6 3.5 0 46.3 53.7 Hessite Ag2Te gray, steel gray 7.2–7.9 1.5–2 0 62.8 37.2 Stuetzite Ag5−xTe3, (x = 0.24–0.36) Gray, dark bronze 8 3.5 0 57.0 43.0 Note: Data is from [15]. Hessite Ag2Te Lead gray, steel gray 7.2–7.9 1.5–2 0 62.8 37.2 Note: Data is from [15]. 2. Gold–(Silver) Tellurides in Nature and Their Alteration 2. Gold–(Silver)The economic Tellurides importance in ofNature gold–(silver) and Their telluride Alteration minerals in gold deposits has meant that they have received significant attention from geologists and mineralogists. More than 100 occurrences The economic importance of gold–(silver) telluride minerals in gold deposits has meant that they have been reported worldwide. The International Geoscience Programme project IGCP-486 was have received significant attention from geologists and mineralogists. More than 100 occurrences undertaken from 2003 to 2008 and focused on the interplay between and of have been reported worldwide. The International Geoscience Programme project IGCP-486 was telluride minerals [4]. The project directly contributed to a summary of the distribution of gold–(silver) undertaken from 2003 to 2008 and focused on the interplay between mineralogy and ore genesis of telluride-bearing deposits and a better understanding of the formation of these deposits. Gold–(silver) telluride minerals [4]. The project directly contributed to a summary of the distribution of gold– telluride deposits normally contain a dozen or more different telluride and minerals and (silver) telluride-bearing deposits and a better understanding of the formation of these deposits. present complex ore textures. An example is seen in the ores of the recently discovered Sandaowanzi Gold–(silver) telluride deposits normally contain a dozen or more different telluride and selenide gold deposit, where sylvanite is the most abundant gold-bearing mineral and together with petzite minerals and present complex ore textures. An example is seen in the ores of the recently discovered and krennerite accounts for >60% of the total tellurides by volume [7,8]. The mixtures of gold–(silver) Sandaowanzi gold deposit, where sylvanite is the most abundant gold-bearing mineral and together telluride minerals were explored in both vein ores and disseminated ores [7]. As shown in Figure3, with petzite and krennerite accounts for >60% of the total tellurides by volume [7,8]. The mixtures of gold and krennerite coexist with petzite and stuetzite, or form symplectic intergrowths with sylvanite. gold–(silver) telluride minerals were explored in both vein ores and disseminated ores [7]. As shown The size of individual telluride grains at this deposit can be up to 3 cm in diameter. In these in Figure 3, gold and krennerite coexist with petzite and stuetzite, or form symplectic intergrowths textures (Figure3A), stuetzite is irregularly shaped and randomly distributed as patches within with sylvanite. The size of individual telluride grains at this deposit can be up to 3 cm in diameter. petzite symplectites. Native gold and krennerite occur in close association as a mineral pair and are In these textures (Figure 3A), stuetzite is irregularly shaped and randomly distributed as patches often included within petzite–stuetzite symplectites. Native gold also occurs as isolated grains along within petzite symplectites. Native gold and krennerite occur in close association as a mineral pair intragranular cracks in the telluride grains. The various combinations of gold tellurides (Figure3B) and are often included within petzite–stuetzite symplectites. Native gold also occurs as isolated have been attributed to retrograde reactions [21], and Liu et al. [8] suggested the formation of telluride grains along intragranular cracks in the telluride grains. The various combinations of gold tellurides assemblages at Sandaowanzi is related to the breakdown of early telluride phases (e.g., γ- and (Figure 3B) have been attributed to retrograde reactions [21], and Liu et al. [8] suggested the formation χ-phase of Cabri [10]). In this deposit, isolated gold grains occur in a “bamboo shoot-like” morphology of telluride assemblages at Sandaowanzi is related to the breakdown of early telluride phases (e.g., γ-phase and χ-phase of Cabri [10]). In this deposit, isolated gold grains occur in a “bamboo shoot- Minerals 2019, 9, 167 4 of 17 Minerals 2019, 9, x FOR PEER REVIEW 4 of 17 like”(Figure morphology3C), with the (Figure filaments 3C), being with 3 the to5 filamentsµm in diameter being 3 and to 5 10 μ tom 15in µdiameterm in length. and Gold 10 to also 15 occursμm in length.in irregular Gold patches also occurs within in cavitiesirregular in patches gold tellurides within cavities (Figure in3D). gold tellurides (Figure 3D).

Figure 3. 3. MineralogyMineralogy and and microstructures microstructures of ofAu–(Ag) Au–(Ag) tellu tellurideride ores ores at Sandaowanzi at Sandaowanzi depositdeposit [7,8]. [ 7(,A8]). Native(A) Native gold goldalong along intragranular intragranular cracks cracks in the in te thelluride telluride grains grains and andnative native gold–krennerite gold–krennerite pair includedpair included in petzite–stuetzite in petzite–stuetzite symplectite. symplectite. (B) Native (B) gold Native and gold krennerite and krennerite patches contained patches containeda petzite– stuetzitea petzite–stuetzite symplectite symplectite in association in association with sylvanite. with sylvanite. (C) Bamboo (C) Bamboo shoot-like shoot-like native gold native grains gold grainsalong intragranularalong intragranular cracks cracksin telluride in telluride grains.grains. (D) Irregular (D) Irregular shaped shaped native gold native grains gold within grains a within cavity a in cavity gold tellurides.in gold tellurides.

The alteration ofof gold–(silver)gold–(silver) tellurides tellurides to to fine fine wires, wires, or or spongy spongy gold, gold, is wellis well known known [22 ][22] and and the thegold gold product product is sometimes is sometimes called called “mustard “mustard gold” gold” because because of of its its distinctive distinctive appearanceappearance in reflected reflected light (Figure 44))[ [23].23]. The The formation formation of of mustard mustard gold gold at at the the Dongping Dongping Mines Mines (Hebei (Hebei Province, Province, China) China) has been linked to the decomposition of calaverite by selective leaching of tellurium while leaving the gold alloy in the cavitycavity formedformed byby thethe alterationalteration reactionreaction [[24,25].24,25]. This type of pseudomorphic alteration was also documented by Palache et al. [26]. [26]. The occurrence of microporous gold has also been observed under cold climatic conditions, such as at the Aginskoe low-sulfidationlow-sulfidation epithermal deposit in Central Kamchatka, Russia. In this deposit, calaverite is the main Au and it has been partially replaced by porous gold [[2277].]. By comparing the textures of microporous gold from this natural occurrence with those obtained experimentally via via the dealloying of gold–(silver) tellurides [16,17,28,29], [16,17,28,29], Okrugin et al. [30] [30] confirmed confirmed that natural microporous gold can form via the replacement of telluride minerals and assessed the the role that hydrothermal fluids fluids may play in the formation of microporous gold. Minerals 2019, 9, 167 5 of 17 Minerals 2019, 9, x FOR PEER REVIEW 5 of 17

FigureFigure 4. 4.Native Native gold gold from from thethe GachingGaching ore occurrence (Maletoyvayam (Maletoyvayam ore ore field), field), Kamchatka, Kamchatka, Russia Russia (polished(polished sections sections in in reflected reflected light) light) (imaged (imaged by N.by Tolstykh).N. Tolstykh). (A) ( ColorA) Color of mustard of mustard gold gold is brown is brown yellow toyellow brown to under brown reflected under light.reflected (B) Porouslight. (B gold) Porous (mustard gold gold) (mustard is observed gold) is along observed with aalong homogeneous with a goldhomogeneous grain ( gold gold). grain (solid gold).

3.3. Mineral Mineral Replacement Replacement Reactions Reactions ofof Gold–(Silver)Gold–(Silver) Tellurides Tellurides in in the the Presence Presence of of Fluids Fluids ThereThere are are limited limited reliable reliable thermodynamic thermodynamic data fordata gold–(silver) for gold–(silver) tellurides tellurides due to the compositionaldue to the overlapcompositional and structural overlap complexity and structural of the maincomplexity mineral of phases the main and mineral therefore phases the difficulty and therefore in calculating the meaningfuldifficulty in phase calculating diagrams meaningful that represent phase observeddiagrams assemblagesthat represent in observed Au–Ag–Te assemblages systems. Manyin Au–Ag– studies onTe tellurium-bearing systems. Many studies systems on tellurium-bearing have focused only syst onems the have binary focused subsystems only on the of binary Au–Te subsystems and Ag–Te. Sinceof Au–Te the 1960s, and severalAg–Te. experimentalSince the 1960s, [10 ,several31,32] andexpe theoreticalrimental [10,31,32] studies (e.g., and [theoretical33]) have been studies conducted (e.g., on[33]) the have Au–Ag–Te been conducted system. Cabri on the [ 10Au–Ag–Te] conducted system. a systematic Cabri [10] investigation conducted a systematic in the Au–Ag–Te investigation ternary system,in the Au–Ag–Te to determine ternary the equilibriumsystem, to determine phase relations the equilibrium in the mineralogicallyphase relations in important the mineralogically area of the ternaryimportant system area and of phase the ternary changes system in the assemblages and phase overchanges a range in the of temperatures. assemblages However,over a range it should of betemperatures. noted that Cabri’s However, study it should was performed be noted that using Cabri’s traditional study was dry sealedperformed tube using methods traditional rather dry than sealed tube methods rather than under hydrothermal conditions. Zhang et al. [34] evaluated the under hydrothermal conditions. Zhang et al. [34] evaluated the stability of calaverite and hessite and stability of calaverite and hessite and discussed it in the context of the stability of other minerals in discussed it in the context of the stability of other minerals in the Au–Ag–Te system. The calculated the Au–Ag–Te system. The calculated stability of hessite and calaverite were used to explain the stability of hessite and calaverite were used to explain the physicochemical conditions of formation physicochemical conditions of formation of the Gies and Golden Sunlight gold–(silver) telluride of the Gies and Golden Sunlight gold–(silver) telluride deposits in Montana, USA. Wang et al. [35] deposits in Montana, USA. Wang et al. [35] contributed new thermodynamic data for the Au–Te contributed new thermodynamic data for the Au–Te system, while McPhail [36] and Grundler et system, while McPhail [36] and Grundler et al. [37–39] studied the complexation and transport of al.tellurium [37–39] studied in hydrothermal the complexation fluids. and transport of tellurium in hydrothermal fluids. TheThe mineral mineral replacementreplacement reactions reactions of of gold–(silver) gold–(silver) tellurides tellurides in the in presence the presence of fluids of have fluids been have beenexplored explored in recent in recent years. years. In a study In a of study the kinetics of the and kinetics mechanism and mechanism of mineral ofreplacement mineral replacement reactions, reactions,Zhao et al. Zhao [28] et investigated al. [28] investigated the replacement the replacement of calaverite of calaveriteby porous bygold porous over a gold wide over range a wideof rangehydrothermal of hydrothermal conditions. conditions. The transformation The transformation proceeds in proceeds a pseudomorphic in a pseudomorphic manner via a manner coupled via a coupleddissolution-reprecipitation dissolution-reprecipitation (CDR) reaction (CDR) reactionmechanism. mechanism. While the While gold the precipitates gold precipitates locally locallyand andpreserves preserves the the shape shape of ofthe the original original calaverite calaverite grain, grain, the the tellurium tellurium is selectively is selectively removed removed and and lost lost to to thethe bulk bulk solution. solution. Zhao Zhao et et al.al. [[16]16] furtherfurther investig investigatedated the the transformation transformation of of sylvanite sylvanite to to Au–Ag Au–Ag alloy alloy byby exploring exploring the the roles roles of of temperature temperature andand fluidfluid composition. The The reaction reaction follows follows a acomplex complex path, path, wherewhere CDR CDR reactions reactions interact interact withwith solid-statesolid-state diff diffusionusion processes, processes, and and results results in in complex complex textures. textures. ThisThis complexity complexity is dueis due to theto factthe thatfact sylvanitethat sylvanite has a has higher a higher Ag content, Ag content, which resultswhich inresults the formation in the offormation a metastable of a Ag-rich, metastable Te-depleted Ag-rich, calaverite Te-depleted I phase. calaverite To achieve I phase. equilibrium, To achieve the equilibrium, metastable phasethe breaksmetastable down phase to stable breaks calaverite down to stable II plus calaverite phase- χII. plus Phase- phase-χ subsequentlyχ. Phase-χ subsequently breaks down breaks to down hessite andto hessite petzite. and To petzite. further To investigate further investigate the effects the of effects Ag in of the Ag parent in the crystalparent crystal for the for reaction the reaction path of Au–Agpath of tellurides Au–Ag duringtellurides replacement, during replacement, Xu et al. [Xu17] et designed al. [17] adesigned set of hydrothermal a set of hydrothermal experiments usingexperiments krennerite using under krennerite similar under conditions similar conditions to those used to those by used Zhao by et Zhao al. [16 et, al.24 ].[16,24]. The resultsThe results show thatshow krennerite that krennerite transformed transformed to Au–Ag to alloyAu–Ag in a alloy pseudomorphic in a pseudomorphic manner manner very similar very tosimilar calaverite to andcalaverite distinct and from distinct sylvanite. from Thesylvanite. reaction The paths reacti ofon thesepaths threeof these reactions three reactions are summarized are summarized in Figure in 5. Figure 5. In the next section we will review in detail these three comprehensive studies of the In the next section we will review in detail these three comprehensive studies of the transformation of transformation of calaverite, krennerite, and sylvanite to gold–silver alloys by CDR reactions, Minerals 2019, 9, 167 6 of 17 Minerals 2019, 9, x FOR PEER REVIEW 6 of 17 calaverite,focusing krennerite, on the product and sylvanitetextures, reaction to gold–silver mechanis alloysm, and by CDRthe kinetics reactions, of the focusing oxidative on leaching the product of textures,Te. reaction mechanism, and the kinetics of the oxidative leaching of Te.

FigureFigure 5. Overview5. Overview of of the the proposed proposed reaction reaction paths paths ofof thethe hydrothermalhydrothermal reaction reaction for for calaverite calaverite (A (A), ), sylvanitesylvanite (B) ( andB) and krennerite krennerite (C ().C CDR). CDR stands stands for for coupled coupled dissolutiondissolution reprecipitation and and SSD SSD stands stands for solidfor solid state state diffusion. diffusion.

3.1.3.1 Product Product Textures Textures

◦ WhenWhen calaverite calaverite grains grains are are heated heated in in a seriesa series of of 0.2 0.2 M M buffer buffer solutions solutions (ranging(ranging from pH 2525 °C C2 2to to ◦ 12) at12) 220 at 220C, °C, Te isTe selectively is selectively removed removed from from the the calaverite, calaverite, leaving leaving a rim a rim of porousof porous gold gold (Figure (Figure6A) 6A) [ 28 ]. The[28]. gold The filaments gold filaments produced produced grow perpendicular grow perpendicula to ther to surface the surface of calaverite. of calaverite. The goldThe gold filaments filaments have diametershave diameters ranging ranging from 200 from to 500200 to nm, 500 with nm, lengthswith lengths up to up ~25 to µ~25m μ (Figurem (Figure6B). 6B). Texturally, Texturally, they they are randomly-orientedare randomly-oriented gold ,gold crystals, forming forming generally generally dendritic dendritic aggregates aggregates (Figure (Figure6C). 6C). This This texture texture is mostis most likely likely due todue repeated to repeated twinning twinning on on {111}, {111}, which which is is common commonin in reticulatedreticulated and and dendritic dendritic gold gold aggregatesaggregates [26 [26].]. The The morphology morphology of of the the gold gold sponge sponge does does notnot vary significantly significantly with with solution solution pH pH and and temperature,temperature, but but the the extent extent of of the the reaction reaction depends depends on on thethe solubilitysolubility of Te4+ inin solution, solution, and and this this is is pH-dependentpH-dependent (Figure (Figure7)[ 7)28 [28,39].,39]. The The textural textural features features ofof thethe replacementreplacement of of calaverite calaverite by by gold gold are are consistentconsistent with with a pseudomorphic a pseudomorphic replacement replacement reaction re proceedingaction proceeding via an interface-coupled via an interface-coupled dissolution reprecipitationdissolution reprecipitation (ICDR) process (ICDR) [40–42 process]. [40–42]. Minerals 2019, 9, 167 7 of 17 Minerals 2019, 9, x FOR PEER REVIEW 7 of 17 Minerals 2019, 9, x FOR PEER REVIEW 7 of 17

FigureFigure 6. 6.( A((A))) Back-scattered Back-scatteredBack-scattered electronelectron imageimage image of of of cross cross cross sectio sectio sectionnn of of partially-reacted partially-reacted of partially-reacted calaverite calaverite calaverite showing showing showing the the thephase phase boundary boundary between between thethe porous theporous porous goldgold product goldproduct product andand thethe and parentparent the calaverite parentcalaverite calaverite (solid(solid grain). grain). (solid High- High- grain). High-magnificationmagnification images images ofof goldgold of gold showingshowing showing three-dimensionalthree-dimensional three-dimensional structure structure structure of of gold gold of filaments, goldfilaments, filaments, which which whichwere were cut werecut cutperpendicular perpendicular (B) () B andand) and parallelparallel parallel ((CC)) ( toCto) thethe to the longlong long axisaxis axisof of the the of gold thegold gold filaments. filaments. filaments.

FigureFigure 7. 7.The The curvecurve of of estimatedestimated solubilitysolubility solubility ofof of Te(IV)Te(IV) Te(IV) in in inwater water at at 220 220 at °C 220°C is is◦ shown Cshown is shown as as pink pink as dashed dashed pink dashed line line line(data (data from from referencereference reference [28,38,39]).[28,38,39]). [28,38,39 SolidSolid]). Solid circlescircles circles stanstand standd forfor thethe for reaction thereaction reaction extentextent extent ofof thethe of replacementreplacement the replacement of of ofcalaverite calaverite [28]. [28 Hollow].Hollow Hollow circlescircles circles andand squaressquares and squares standstand forfor stand the the replacement forreplacement the replacement of of krennerite krennerite of krennerite[17] [17] and and sylvanite sylvanite [17] and sylvanite[16], respectively. [16], respectively. ErrorsErrors ofof thethe Errors reactionreaction of the extentextent reaction (3(3 −− σ σ; extent ;± ± 6%) 6%) are (3are −plotted plottedσ; ± at 6%)at each each are point. point. plotted Reaction Reaction at each extent extent point. Reactionobserved extent experimentally observed experimentally correspondedcorresponded wellwell corresponded toto thethe solubility solubility well of toof tellurium. thetellurium. solubility of tellurium.

TheThe replacement replacement of ofof krennerite krenneritekrennerite isisis similarsimilarsimilar to to ca calaveritecalaveritelaverite [17],[17], [17], proceedingproceeding proceeding viavia via the the the ICDR ICDR ICDR reaction reaction reaction mechanism.mechanism. An An Au–Ag Au–Ag alloy alloyalloy of ofof wormlike wormlikewormlike filaments filamentsfilaments wawa wasss producedproduced due due due to to to higher higher higher silver silver silver contents contents contents in in in krenneritekrennerite (Figure (Figure8). 8). Natural NaturalNatural krennerite krenneritekrennerite normally normallynormally containscontains 3.4 3.43.4 to toto 6.2 6.26.2 wt wtwt % %% Ag AgAg (0.14 (0.14(0.14 ≤ ≤ ≤x x ≤ x≤ 0.25), ≤0.25),0.25), comparedcompared to to calaverite calaverite which whichwhich contains containscontains 0 0 to to 2.8 2.8 wt wt % % AgAg (0(0 ≤≤≤ xx x≤ ≤ 0.11) ≤0.11)0.11) [10]. [10]. [10 The The]. The krennerite krennerite krennerite used used used in in a a in study by Xu et al. [17][17] hadhad aa compositioncomposition AuAu0.820.82AgAg0.180.18TeTe2.002.00, , andand anan Au:AgAu:Ag ratio ratio of of 4.6. 4.6. The The average average a study by Xu et al. [17] had a composition Au0.82Ag0.18Te2.00, and an Au:Ag ratio of 4.6. The average composition of the product is Au0.85Ag0.15, and Au:Ag is ~5.7, which is slightly higher than that of the compositioncomposition of of the the product product isis AuAu0.85AgAg0.15, and, and Au:Ag Au:Ag is ~5.7, is ~5.7, which which is slig ishtly slightly higher higher than thanthat of that the of parent krennerite. The increase of0.85 the Au:Ag0.15 ratio is due to the dissolution of Ag in the reaction fluid theparent parent krennerite. krennerite. The The increase increase of the of theAu:Ag Au:Ag ratio ratio is due is to due the to dissolution the dissolution of Ag ofin the Ag reaction in the reaction fluid and in textural terms forfor thethe Au–AgAu–Ag alloyalloy filamentsfilaments have have diameters diameters ranging ranging from from 200 200 to to 1000 1000 nm. nm. As As fluid and in textural terms for the Au–Ag alloy filaments have diameters ranging from 200 to 1000 nm. the reaction proceeds,proceeds, Au–AgAu–Ag alloyalloy wireswires alsoalso developdevelop locally,locally, havinghaving diametersdiameters up up to to 5 5 μ μmm and and As the reaction proceeds, Au–Ag alloy wires also develop locally, having diameters up to 5 µm and lengths ranging fromfrom 2525 μμmm toto 200200 μμmm andand longer.longer. lengths ranging from 25 µm to 200 µm and longer. Minerals 2019, 9, 167 8 of 17 Minerals 2019, 9, x FOR PEER REVIEW 8 of 17

Figure 8. ((AA)) Secondary Secondary electron electron image image showing showing the the high highlyly porous Au–Ag alloy in the shape of filaments.filaments. ( (BB)) Backscattered Backscattered electron electron image image of of cross section of partially-reactedpartially-reacted krenneritekrennerite grainsgrains showing larger Au–Ag alloy particles coexisting with fine-grainedfine-grained Au–Ag alloy in the resultant gold rim (imaged by W. Xu).Xu).

Compared to calaverite and krennerite,krennerite, sylvanite generally contains significantlysignificantly higher Ag contents (6.7 (6.7 to to 13.2 13.2 wt wt % %Ag, Ag, illustrated illustrated by Cabri by Cabri [10]). [ 10In]). the Instudy the by study Zhao by et Zhaoal. [16] et the al. sylvanite [16] the sylvanitehad a composition had a composition of Au0.63Ag of Au0.36Te0.632.00Ag which0.36Te2.00 correspondswhich corresponds to 9.2 wt to % 9.2 Ag. wt In % Ag.contrast In contrast to the toreplacement the replacement of calaverite of calaverite and krennerite, and krennerite, sylvanite sylvanite was replaced was replaced by an by assemblage an assemblage of products of products and andthe resulting the resulting textures textures are complex. are complex. In addition In addition to Au–Ag to Au–Ag alloy alloy(Au0.87 (AuAg0.130.87),Ag a range0.13), a of range other of phases other phasesformed formed as intermediate as intermediate products, products, including including petzite petzite ((Au0.92 ((AuAg3.150.92)TeAg2),3.15 hessite)Te2), (Ag hessite1.89Au (Ag0.071.89Te),Au and0.07 twoTe), andcompositions two compositions of calaverite. of calaverite. The calaverite The calaverite I phase I phase has hasan anAg-rich, Ag-rich, Te-depleted Te-depleted composition, (Au(Au0.78AgAg0.220.22)Te)Te1.741.74, which, which is issimilar similar to to natural natural krennerite, krennerite, but but its its XRD XRD pattern pattern is is close to natural calaverite. Calaverite Calaverite II II has has a anormal normal calaverite calaverite composition composition of (Au of (Au0.93Ag0.930.07Ag)Te0.072. )TeThe2 .calaverite The calaverite I phase I phaseis porous is porous while calaverite while calaverite II lacks obvious II lacks obvioussigns of porosity signs of in porosity SEM images. in SEM The images. texture The of a texture partially- of areacted partially-reacted sylvanite grain sylvanite is shown grain in is shownFigure in9. The Figure Au–Ag9. The alloy Au–Ag rim alloy is composed rim is composed of wormlike of wormlike Au–Ag Au–Agalloy particles alloy particles (Figure (Figure 9A), with9A), withdiameters diameters rang ranginging from from 200 200 to to1000 1000 nm. nm. Wire Wire gold gold has alsoalso developed locally (up to 5 μµm in diameter, 25 µμm in length; Figure9 9A).A). TheThe rimrim ofof thethe graingrain isis highlyhighly porous, with the Au–Ag alloy growing loosely on the surface and along cracks within the sylvanite (Figure(Figure9 9B).B).Relatively Relativelylarge largegaps gapswere were observed observed between between the the alloy alloy rim rim and and thethe particle.particle. UnderneathUnderneath the Au–Ag alloy rim (Figure9 9C,D),C,D), sylvanitesylvanite isis replacedreplacedby by assemblagesassemblagesof of calaveritecalaveriteI I andand aa mixturemixture of petzite and hessite. Petzite and hessite occur intimately mixed either as small patches or inclusions within calaverite I, or adjacent to grains of calaveritecalaverite II. Au–Ag alloy and calaverite II are observedobserved together within petzite-hessite lamellae, which which is si similarmilar to the textures of natural tellurides at the Sandaowanzi deposit. Minerals 2019, 9, 167 9 of 17 Minerals 2019, 9, x FOR PEER REVIEW 9 of 17

Figure 9. 9. (A(A) Secondary) Secondary electron electron images images showing showing the micro the micro Au–Ag Au–Ag alloy wires alloy growing wires growing on the surface on the surfaceof a partially-reacted of a partially-reacted sylvanite sylvanitegrain. (B, grain.C) Backscattered (B,C) Backscattered electron images electron of cross images section of cross of partially- section ofreacted partially-reacted sylvanite grains sylvanite showing grains a range showing of products a range after of productsthe replacement after the reaction. replacement (D) Zoomed reaction. in (imageD) Zoomed of Figure in image 9C, showing of Figure the9C, textures showing ofthe the textures calaverite of theII, petzite, calaverite hessite, II, petzite, and Au–Ag hessite, alloy; and Au–Ag petzite alloy;and hessite petzite occur and hessiteintimately occur mixed intimately either mixed as smal eitherl patches as small or inclusions patches or inclusionswithin calaverite within calaveriteI. I.

3.2.3.2 Reaction Reaction Mechanism Mechanism Under oxidizing conditions, gold–(silver) tellurides are ultimately replaced by gold or Au–Ag alloy, while the Te is eventually lost to bulk solution and some is precipitated in the form of alloy, while the Te is eventually lost to bulk solution and some is precipitated in the form of TeO2(s) TeOparticles2(s) particleson the outer on thesurface outer of surface gold/Au–Ag of gold/Au–Ag alloy. The selective alloy. The removal selective of Te removal from gold–(silver) of Te from gold–(silver)tellurides is often tellurides referred is often to as referred leaching, to a as process leaching, conventionally a process conventionally considered as considered a solid-state as adiffusion-driven solid-state diffusion-driven mechanism. In mechanism. this case, it proceeds In this case, in a itpseudomorphic proceeds in a pseudomorphicmanner via an interface- manner viacoupled an interface-coupled dissolution–reprecipitation dissolution–reprecipitation (ICDR) mechanism (ICDR) (summarized mechanism in (summarizedFigure 10). The in distinctive Figure 10). Thetextural distinctive outcome textural of a CDR outcome reaction of is a that CDR the reaction product is phase thatthe of go productld or Au–Ag phase alloy of gold preserves or Au–Ag the alloy preserves the external dimension of the parent mineral. The scale of pseudomorphism in the external dimension of the parent mineral. The scale of pseudomorphism in the replacement of gold– replacement of gold–(silver) tellurides by gold or Au–Ag alloy varies from nanometer scale (e.g., (silver) tellurides by gold or Au–Ag alloy varies from nanometer scale (e.g., the replacement of the replacement of calaverite) to a few micrometers (e.g., the replacement of sylvanite). The textural calaverite) to a few micrometers (e.g., the replacement of sylvanite). The textural features indicate features indicate that the dissolution of gold–(silver) telluride is the rate-controlling step, which is that the dissolution of gold–(silver) telluride is the rate-controlling step, which is closely coupled with closely coupled with the precipitation rate of the products in both space and time scales [40]. the precipitation rate of the products in both space and time scales [40]. The coupling between parent The coupling between parent and product minerals is controlled by the solution chemistry at the and product minerals is controlled by the solution chemistry at the reaction front. The porosity is reaction front. The porosity is strong textural evidence for a CDR reaction. The reaction is sustained by strong textural evidence for a CDR reaction. The reaction is sustained by continuous mass transport continuous mass transport through open pathways for the influx of fluid and solutes (e.g., the oxidant) through open pathways for the influx of fluid and solutes (e.g., the oxidant) to the reaction interface to the reaction interface and the removal of dissolved Te and Ag from the reaction interface (e.g., [43,44]). and the removal of dissolved Te and Ag from the reaction interface (e.g., [43,44]). The abundant The abundant porosity of the product phases is associated with negative volume changes; although porosity of the product phases is associated with negative volume changes; although systems with systems with positive volume changes still exhibit porosity, it is often very fine grained [45]. The overall positive volume changes still exhibit porosity, it is often very fine grained [45]. The overall volume volume change is determined by the changes in molar volume as well as the solubility of the parent change is determined by the changes in molar volume as well as the solubility of the parent and and product phases within a given solution [46]. The former parameter plays a role in the extent of product phases within a given solution [46]. The former parameter plays a role in the extent of the the volume change, but the latter determines the sign of the volume change [41,47]. The solubility of volume change, but the latter determines the sign of the volume change [41,47]. The solubility of each each phase is a function of the grain size, fluid composition, temperature, and pressure, among other phase is a function of the grain size, fluid composition, temperature, and pressure, among other variables, and hence will likely evolve as the replacement reaction proceeds [47]. Pollok et al. [46] variables, and hence will likely evolve as the replacement reaction proceeds [47]. Pollok et al. [46] defined the change in volume by considering not only molar volumes but the relative solubilities of defined the change in volume by considering not only molar volumes but the relative solubilities of the parent and product: the parent and product:

, ∆ = 100 × ,, (1) , Minerals 2019, 9, 167 10 of 17

  npVm,p − ndVm d ∆V = 100 × , , (1) ndVm,d where np and nd are the number of moles of the product precipitated and the parent dissolved, and Vm,p and Vm,d are the molar volumes of the precipitating and dissolving phases. Considering that metallic gold or Au–Ag alloy is the final product of the replacement reaction, the molar volume changes (∆V) in the three replacement reactions range from −79% to −85% (details are listed in Table2). The processes include (i) the dissolution of gold–(silver) tellurides, (ii) the oxidation of the Te to a soluble Te(IV) complex and the transportation of Te from the reaction front to the bulk hydros solution, and (iii) the precipitation of gold/Au–Ag alloy. When natural gold–(silver) tellurides are treated with solution, the dissolution of the mineral occurs at the reaction front, resulting in the formation of aqueous Au, Ag, and Te complexes. To decide the nature of the predominant aqueous species of Au, Ag, and Te, and discuss the relative solubilities of each element as a function of pH, Zhao et al. [16] calculated simple diagrams of log f O2(g) vs. pH for the system containing the same amounts of Au, Ag, and Te added to the solution by the dissolution of sylvanite at 200 ◦C for a solution containing 0.01 M chloride. The results illustrate that the dominant Te aqueous species is H2TeO3(aq) under acidic to slightly basic − (pH200 ◦C 2–7) conditions and HTeO3 under more basic conditions. Ag is mainly present as AgCl(aq) − under acidic conditions, but the dominant Ag aqueous species is Ag(OH)2 under basic conditions. Taking O2(aq) as the oxidant and assuming Au immobility, the overall reaction of gold tellurides to Au–Ag alloy can be described as below:

A(s) + O2(aq.) + H2O → B(s) + H2TeO3(aq.) (acidic conditions) (2)

− − A(s) + O2(aq.) + OH → B(s) + HTeO3 (aq.) (basic conditions) (3) where A is the parent phase of gold–(silver) tellurides, and B is the solid product phase or phases. In the replacements of calaverite and krennerite, B represents the single product of gold or Au–Ag alloy. In transformation of sylvanite, B represents both calaverite I and the Au–Ag alloy. Once the concentrations of Te and Ag in solution reach a critical state, the reaction switches and sylvanite dissolution is coupled to the precipitation of calaverite I. This indicates that this reaction is controlled by the amount of Te and Ag in solution. Calaverite I is an unstable phase, which further breaks down to calaverite II and phase χ (Ag3+xAu1−xTe2, 0.1 < x < 0.55) via exsolution which may be fluid-catalyzed [48]. Both products of calaverite and phase χ subsequently transform to Au–Ag alloy by Reactions 2 or 3. Phase χ breaks down to a fine intergrowth of petzite and hessite during the quenching of the autoclaves from the reaction temperature (160 to 220 ◦C) to room temperature [16]. Cabri [10] reported that phase χ breaks down to a mixture of petzite and hessite at 105 ◦C. It is unclear whether the breakdown of calaverite I to calaverite II plus phase χ is really a solid-state diffusion-controlled reaction, or a fluid-catalyzed breakdown reaction in which the highly porous calaverite I undergoes a recrystallization and unmixing reaction driven by a reduction of internal surface area. The lack of porosity in the calaverite II points to solid-state exsolution. Such reactions have recently been studied in the breakdown of the bornite–digenite solid solution [48,49]. Zhao et al. [50] synthesized bornite–digenite solid solution (bdss) by replacing chalcopyrite under hydrothermal conditions. The results demonstrated that its composition principally depended on the temperature of the reaction rather than solution composition. Upon quenching, the unquenchable nanoscale porosity within the bdss system coalesces into fluid inclusions, specifically along grain boundaries, which catalyze the breakdown of the unstable bdss to exsolve digenite [48] or chalcopyrite [49], depending on the solution condition. The transformation of sylvanite proceeds by a complex pathway combining dissolution–reprecipitation, fluid-catalyzed unmixing, and solid-state processes, which all compete during different stages of the reaction. The interplay of different reaction mechanisms results in complex textures, which could easily be misinterpreted in terms of complex multi-episodic geological evolution. Minerals 2019, 9, 167 11 of 17 Minerals 2019, 9, x FOR PEER REVIEW 11 of 17

FigureFigure 10. AA diagrammatic diagrammatic representation representation of interface-coupled interface-coupled dissolution–reprecipitation. ( (AA)) Fluid Fluid + 3+3+ containingcontaining C andand D D ionsions goes goes through through cracks cracks within within the the parent parent phase phase of ABX2.. ABX ABX22 phasephase dissolves dissolves atat the the reaction reaction surface surface and and CDX CDX2 product2 product forms forms on onthe the surface. surface. (B) Fluid (B) Fluid transports transports through through pores pores and cracksand cracks to reaction to reaction interface interface adding adding C+ and C+ Dand3+. ( DC3+) Curved.(C) Curved arrows arrows show show direction direction of fluid of fluidflow. flow. (D) Porosity(D) Porosity anneals anneals out over out time over and time fluid and inclusions fluid inclusions form from form the from solu thetion solution trapped trapped within the within crystal. the Unstablecrystal. Unstable product productphase of phase CDX of2 exsolves CDX2 exsolves into two into new two phases new phases upon uponquenching, quenching, catalyzed catalyzed by the by fluidthe fluid within within the porosity the porosity and andalong along the grain the grain boundaries. boundaries. Minerals 2019, 9, 167 12 of 17

Table 2. Summary of experimental replacement reactions of gold–(silver) tellurides under hydrothermal conditions.

Parent Mineral Average Composition Au:Ag:Te Overall Reaction Mechanism Products ∆Vm *

Calaverite Au0.94Ag0.05Te2.00 47:2.5:100 Calaverite + 2 O2(aq.) + 2 H2O → Au + 2 H2TeO3(aq.) CDR Porous gold −79.32% Porous Au–Ag alloy, Au0.87Ag0.13 + − Petzite, (Au0.92Ag3.15)Te2 Sylvanite + 2.07 O2(aq.) + 1.87 H2O + 0.27 H (aq.) + 0.27 Cl Sylvanite Au0.63Ag0.36Te2.00 31.5:17.75:100 CDR + exsolution Hessite, Ag1.89Au0.07Te −84.92% → 0.72 Au0.87Ag0.13 + 0.27 AgCl(aq) + 2 H2TeO3(aq.) Calaverite I, (Au0.78Ag0.22)Te1.74 Calaverite II, (Au0.93Ag0.07)Te2 + − Krennerite + 2.01 O2(aq.) + 1.98 H2O + 0.04 H (aq.) + 0.04 Cl Krennerite Au0.82Ag0.18Te2.00 41:9:100 CDR Porous Au–Ag alloy, Au0.85Ag0.15 −80.19% → 0.96 Au0.85Ag0.15 + 0.04 AgCl(aq.) + 2 H2TeO3(aq.)

 npVm,p−ndVm,d  Note: * Volume change for each reaction was calculated using the Equation ∆V = 100· , where np and nd are the number of moles of product precipitated and parent ndVm,d dissolved, and Vm,p and Vm,d are the molar volumes of the precipitating and dissolving phases [46]. Molar volume of each phase equals the molar mass (M) divided by the mass density (ρ). The density of the starting minerals is listed in Table1, and the average density of sylvanite is 8.1 g/cm 3. The density of gold–silver alloy was calculated using the data of gold density 3 3 3 3 (19.32 g/cm ) and silver density (10.49 g/cm ). The calculated density of Au0.87Ag0.13 alloy is 17.41 g/cm and 17.15 g/cm for Au0.85Ag0.15. Minerals 2019, 9, 167 13 of 17 Minerals 2019, 9, x FOR PEER REVIEW 13 of 17

4. Applications and Implications 4. Applications and Implications The morphology of the nanoscale spongy gold wire produced by the replacement of calaverite under hydrothermalThe morphology conditions of the nanoscale is remarkably spongy gold similar wire to produced the mustard by the gold replacement samples [24of ,calaverite25] and the microporousunder hydrothermal gold samples conditions [30] found is remarkably in nature (Figure similar 11 to). the Although mustard the gold fluids samples used in [24,25] the experiments and the aremicroporous probably more gold aggressive samples than[30] thosefound foundin nature in nature, (Figure the 11). similarity Although in the the textures fluids ofused porous in the gold mayexperiments reflect similar are processesprobably more of formation. aggressive The than natural those microporous found in nature, gold the found similarity at the Aginskoe in the textures deposit, Centralof porous Kamchatka gold may epithermal reflect similar district processes [30], has of fo remarkablyrmation. The similar natural textures microporous to these gold synthetic found at spongy the goldAginskoe filaments deposit, in terms Central of both Kamchatk morphologya epithermal and size. district As shown [30], in has Figure remarkably 11, each grainsimilar of textures microporous to goldthese from synthetic Aginskoe spongy consists gold filame of aggregatesnts in terms of fineof both fibers morphology that are about and size. 30–300 As shown nm in in diameter Figure 11, and each grain of microporous gold from Aginskoe consists of aggregates of fine fibers that are about 30– ≥5 µm in length. Andreeva et al. [27] indicated calaverite is the main Au telluride at Aginskoe and 300 nm in diameter and ≥5 μm in length. Andreeva et al. [27] indicated calaverite is the main Au it is a likely precursor in this case. It often displays partial alteration to porous gold. The mustard telluride at Aginskoe and it is a likely precursor in this case. It often displays partial alteration to gold found at the Dongping Mines (Hebei Province, China) is microporous gold aggregate [24] porous gold. The mustard gold found at the Dongping Mines (Hebei Province, China) is microporous with slightly coarser textures. Mustard gold typically has the same Au:Ag ratio as the calaverite gold aggregate [24] with slightly coarser textures. Mustard gold typically has the same Au:Ag ratio inas the the deposit, calaverite and in the the formation deposit, and corresponds the formation to the corresponds selective leachingto the selective of tellurium leaching from of tellurium calaverite. Thefrom recrystallization calaverite. The of recrystallization gold may occur duringof gold ormay after occur the decompositionduring or after of the calaverite, decomposition resulting of in porouscalaverite, gold filaments.resulting in The porous textures gold offilaments. an ICDR The product textures are of normally an ICDR product related toare the normally chemistry related of the fluid,to the as chemistry demonstrated of the in fluid, the replacement as demonstrated of leucite in the (KAlSireplacement2O6) by of analcimeleucite (KAlSi (NaAlSi2O6) 2byO6 analcime·H2O) [51 ]. The(NaAlSi coarsening2O6·H2O) of the[51]. structure The coarsening may also of the occur structure upon ma they completionalso occur upon of the the replacement completion reaction,of the whichreplacement is driven byreaction, surface which energy is reduction driven toby create surface self-similar energy microstructuresreduction to create with ever-increasing self-similar filamentmicrostructures size. with ever-increasing filament size.

FigureFigure 11. 11.The The natural natural microporous microporous gold foundgold found at the Aginskoeat the Aginskoe deposit, deposit, Central KamchatkaCentral Kamchatka epithermal districtepithermal (imaged district by Barbara (imaged Etschmann). by Barbara (A Etschmann).,B) Microporous (A–B gold) Microporous grain consists gold of aggregatesgrain consists of smallof fibers.aggregates The diameters of small fibers. of the goldThe diameters fibers vary of inthe a gold single fibers grain. vary in a single grain.

TheThe increasing increasing knowledge knowledge base base ofof thethe controlscontrols of interface-coupled dissolution–reprecipitation dissolution–reprecipitation reactionsreactions over over recent recent years years isis leading to to an an impr improvedoved understanding understanding of mineralization of mineralization processes processes of ofnatural natural systems. systems. As Asshown shown in Figure in Figure 3, 3the, the typi typicalcal textures textures of gold–(silver) of gold–(silver) tellurides tellurides from fromthe theSandaowanzi Sandaowanzi gold gold deposit deposit are are krennerite–gold krennerite–gold intergrowths intergrowths imbedded imbedded within within stuetzite–petzite stuetzite–petzite symplectites.symplectites. Here, Here, the the natural natural krennerite krennerite hashas aa similarsimilar composition composition to to synthetic synthetic calaverite calaverite I, and I, and the the composition of stuetzite is similar to that of synthetic hessite. According to the calculated bulk composition of stuetzite is similar to that of synthetic hessite. According to the calculated bulk composition of the complex texture in Figure 3A, the precursor composition might have been a more composition of the complex texture in Figure3A, the precursor composition might have been a more Ag-rich but Te-depleted sylvanite than that used by Zhao et al. [16]. The complex textures observed Ag-rich but Te-depleted sylvanite than that used by Zhao et al. [16]. The complex textures observed at at the Sandaowanzi deposit are remarkably similar to those synthesized (Figure 9) by Zhao et al. [16] the Sandaowanzi deposit are remarkably similar to those synthesized (Figure9) by Zhao et al. [ 16] by replacing sylvanite under hydrothermal conditions, implying broad similarities in the formation byconditions. replacing sylvaniteThe experimental under hydrothermal studies on the conditions, replacemen implyingt of sylvanite broad by similaritiesgold [17] indicate in the formationthat the conditions.formation Theof the experimental two/three-phase studies symplectites on the replacement at Sandaowanzi of sylvanite gold deposit, by gold are [ 17related] indicate to the that the formation of the two/three-phase symplectites at Sandaowanzi gold deposit, are related to the Minerals 2019, 9, 167 14 of 17 replacement of the gold–(silver) telluride precursor via an interface-coupled dissolution–reprecipitation. The precursor of the mineralization was formed by the upwelling of early mineralization fluids from a deep sub-alkaline magmatic source and the mineral precipitation at the near-surface faulting during the cooling [7,8]. The precursor reacts with meteoric water infiltrating within fractures along the boundaries at mild temperatures, leading to the formation of gold, the precipitation of different gold–(silver) telluride mineral associations, and the different types of solid solutions (e.g., phase χ). The results of sylvanite replacement reaction directly explain the generation sequence of gold–(silver) tellurides in nature. Native gold, Au–Ag tellurides, and Ag tellurides at Sandaowanzi deposit are all the products of precursor replacement reactions, which were formed at the same time-scale but distributed in different layers of quartz matrix formed at different stages. The results also explain that some of the later mineral assemblages (e.g., hessite–petzite and stuetzite–petzite) represent the breakdown of metastable solid solutions during cooling rather than the initial reaction conditions of ore formation. In the experiments, the products are layered by the sequence of the reactions. From the surface of the sylvanite grain to the core, the layers of products show a general trend of increasing Ag telluride abundances together with a decrease in the abundance of Au-dominant tellurides. At the Sandaowanzi gold deposit, the high-grade vein ores are mainly distributed at the +130 m level, which is in the center of two low-grade disseminated mineralization zones along the margins of the orebody. According to the experimental results, the +130 m level could be the starting surface of reaction. The majority of the Te and Ag were dissolved into solution during alteration processes and transferred to the low-grade disseminated mineralization zone by fluids, eventually forming small particles of silver tellurides and other tellurides (e.g., HgTe and PbTe) within the matrix of very fine-grained quartz (µm scale). The recent studies on gold–(silver) tellurides under hydrothermal conditions show that these minerals can be transformed to gold/Au–Ag alloy relatively rapidly (within hours) under all conditions (even in Milli-Q water) at moderately elevated temperatures (~200 ◦C). This process could be added as a preliminary treatment in ore processing before the traditional cyanide process. For gold-bearing tellurides, the overall reactions provide an efficient and less toxic alternative to pretreatment by roasting.

5. Outlook Porous gold is a form of gold with significant technological potential due to its low density, high strength and large specific surface area. The dramatic increase in attention to this material over the last two decades is due to its many potential applications in areas such as catalysis, energy storage, and sensor technology. A number of methods for synthesizing this material have been developed—for example, de-alloying, templating, electrochemical, and self-assembling. De-alloying of gold alloys is currently the most widely-used method. This approach fabricates the porous gold structure by selectively dissolving the less noble components from a gold alloy. To make a porous gold sponge by de-alloying, Au–Ag alloy is firstly synthesized and then the Au–Ag alloy is treated with a high-concentration nitrate solution [52]. This is a two-step process, excluding any purification of the gold source. However, a similar structure of porous gold can be produced by replacing natural gold–(silver) tellurides using a hydrothermal method over a wide range of solutions under mild conditions, the reaction being completed within days, depending on the solution composition. This single-step method appears to have the advantage of allowing fine-tuning of the nature of the porous gold, as the dissolution of a gold telluride occurs over a much wider range of solution conditions than does that of a simple Au–Ag alloy. It is also possible to use natural gold telluride minerals as well as synthetic gold tellurides. A comprehensive experimental study of the controls on the texture of the porous gold obtained via such a route is required to optimize the reaction conditions, manipulate the morphology of the porous gold sponge, and test this porous gold as a functional material in terms of catalysis, energy storage, and sensor technology. Minerals 2019, 9, 167 15 of 17

Author Contributions: Writing—Original Draft Preparation, J.Z.; Writing—Review & Editing, A.P. Funding: This work has been made possible by the financial support of the Australian Research Council (grants DP140102765, DP1095069, and DP170101893). Acknowledgments: We thank Joel Brugger and Barbara Etschmann for their input into the original work. We thank Junlai Liu from China University of Geosciences for his contributions to Figure3 and Nadezhda Tolstykh from VS Sobolev Institute of Geology and Mineralogy of Siberian Branch of Russian Academy of Sciences (SB RAS) for his contributions to Figure4. We also thank all the editors and three anonymous referees. Grateful acknowledgements also to Philippa Horton, who helped edit the revision. Conflicts of Interest: The authors declare no conflict of interest.

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