Au Tellurides Research Articles

Maletoyvayamite, Au3Se4Te6, a new mineral from Maletoyvayam deposit, Kamchatka peninsula, Russia

AbstractMaletoyvayamite, Au3Se4Te6, is a new mineral discovered in a heavy-mineral concentrate from the Gaching occurrence of the Maletoyvayam deposit, Kamchatka, Russia (60°19′51.87″N, 164°46′25.65″E). It forms anhedral grains (10 to 50 μm in size) and is found in intergrowths with native gold (Au–Ag), Au tellurides (calaverite), unnamed phases (AuSe, Au2TeSe and Au oxide), native tellurium, sulfosalts (tennantite, tetrahedrite, goldfieldite and watanabeite) and supergene tripuhyite. Maletoyvayamite has a good cleavage on {010} and {001}. In plane-polarised light, maletoyvayamite is grey, has strong bireflectance (grey to bluish grey), and strong anisotropy; it exhibits no internal reflections. Reflectance values for maletoyvayamite in air (Rmin,Rmaxin %) are: 38.9, 39.1 at 470 nm; 39.3, 39.5 at 546 nm; 39.3, 39.6 at 589 nm; and 39.4, 39.7 at 650 nm. Sixteen electron-microprobe analyses of maletoyvayamite gave an average composition: Au 34.46, Se 16.76, Te 47.23 and S 0.84, total 99.29 wt.%, corresponding to the formula Au2.90(Se3.52S0.44)Σ3.96Te6.14based on 13 atoms; the average of eleven analyses on synthetic analogue is: Au 34.20, Se 19.68 and Te 45.42, total 99.30 wt.%, corresponding to Au2.90Se4.16Te5.94. The calculated density is 7.98 g/cm3. The mineral is triclinic, space groupP1, witha= 8.901(2),b= 9.0451(14),c= 9.265(4) Å, α = 97.66(3), β = 106.70(2), γ = 101.399(14)°,V= 685.9(4) Å3andZ= 2. The crystal structure of maletoyvayamite represents a unique structure type resembling a molecular structure. There are cube-like [Au6Se8Te12] clusters linkedviavan der Waals interactions. The structural identity of maletoyvayamite with the synthetic Au3Se4Te6was confirmed by powder X-ray diffraction and Raman spectroscopy.

Linkage of distal vein–type Au mineralization in carbonate rocks with Cu–Au skarn mineralization in the Fengshan area, Eastern China: Mineralogy and stable isotope geochemistry

Numerous limestone–hosted vein–type Au deposits (12 million tonnes at 5.0 g/t Au) are located peripheral to Cu–Au skarn deposits around the Late Jurassic granodiorite intrusions in the Fengshan area, Middle–Lower Yangtze River metallogenic belt, Eastern China. Both the Cu–Au skarn and distal vein–type Au deposits have a complex mineral assemblage, consisting of base metal sulfides, sulfosalts and Au–Ag–(Tl) telluride minerals. In the Cu–Au skarn deposits, grains of electrum, Au–Ag tellurides (hessite and petzite) coexist with Bi–Te minerals (native bismuth, tetradymite, Phase C, and aikinite), sulfides (chalcopyrite, sphalerite, and galena), occurring as grain aggregates or veinlets. In the distal vein–type Au deposits, fine–grained Au–Ag–Tl tellurides (calaverite, petzite, hessite, and Tl–bearing Au telluride) occur as ore minerals in assemblages of sulfides, and carbonate minerals. The mineralogical Bi–Te signature characterizes the skarn assemblages, while the Te–Tl signature is typical of the distal vein–type Au mineralization.Textural relations and mineral assemblages allow the recognition of five generations of hydrothermal carbonate: (1) Cu sulfides and tellurides–bearing calcite assemblages; (2) sphalerite, galena and tellurides–bearing dolomite assemblages; (3) sericite, quartz and tellurides–bearing carbonate veinlets; (4) realgar and fluorite–bearing carbonate veinlets; (5) pure calcite veinlets. Electron microprobe analysis indicates that the tellurides–bearing, ore–related carbonate contains variable Mn content (up to 80 mol.% MnCO3).The C–O isotope compositions of carbonate from the Cu–Au skarn mineralization, to the carbonate–replacement Pb–Zn and distal vein–type Au mineralization, and to the post–ore, realgar–bearing veinlets in the country rocks display a tendency towards heavier isotopic compositions with time, which could be best interpreted as the effect of fluid–rock interaction between magmatic fluid and the Triassic carbonate host rocks. New S isotope data on sulfate minerals (anhydrite, gypsum, and celestite), and C–O isotopic data suggest a common magmatic fluid source for both skarn and distal vein–type mineralization.

Studies of Metallic and Trace Minerals of the Tiegelongnan Cu‐Au Deposit, Central Tibet, China

Electron probe micro‐analysis (EPMA) and scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS) have been used to investigate the principal ore minerals and coexisting metallic mineral inclusions in polished thin sections from the Tiegelongnan deposit, which consists of a high‐sulfidation epithermal system (HSES) and a porphyry system (PS). Molybdenite, chalcopyrite, bornite, tennantite, enargite, digenite, anilite, covellite, and tetrahedrite have been identified by EPMA. Intergrowth, cross‐cutting and replacement relationships between the metallic minerals suggest that molybdenite formed first (stage 1), followed by chalcopyrite ± bornite ± hematite (stage 2), then bornite ± Cu‐sulfides ± Cu‐Fe‐sulfoarsenides (stage 3), and lastly Cu‐Fe‐ sulfoarsenides ± Cu‐ sulfides (stage 4). Pyrite is developed throughout all the stages. Droplet‐like inclusions of Au‐Te minerals commonly occur in tennantite but not in the other major sulfides (molybdenite, chalcopyrite and bornite), implying that tennantite is the most important Au telluride carrier. The pervasive binary equilibrium phases of calaverite and altaite constrain fTe2 in the range from ∼−6.5 to ∼−8 and fS2 < −11. The intergrowth of bornite and chalcopyrite and the conversion from bornite to digenite suggest fluctuated and relatively low precipitation temperature conditions in the HSES relative to the PS. Contrastingly, the dominance of chalcopyrite in the PS, with minor bornite, suggests relatively high temperature conditions. These new results are important for further understanding the mineral formation processes superimposed by HSES and PS systems.

Conditions of formation of Au–Se–Te mineralization in the Gaching ore occurrence (Maletoyvayam ore field), Kamchatka, Russia

ABSTRACTThe Gaching high-sulfidation epithermal deposit in the Maletoyvayam ore field features a wide range of Se-containing minerals and selenides, as well as complex gold oxides, Au tellurides (calaverite, krennerite) and native gold typical for epithermal deposits. Pyrite included in quartzites and quartz-alunite rocks was probably formed during an early stage of the ore-forming process. During the following Au-rich stage, the $f_{{\rm S}{\rm e}_{\rm 2}}$/$f_{{\rm S}_{\rm 2}}$ increased with $f_{{\rm O}_{\rm 2}}$ being relatively high, resulting in the formation of very rare compounds that have not been previously described in nature. These include Au2Te4(Se,S)3, Se3Te2, AuSe and Au(Te,Se,S) phases. The Au2Te4(Se,S)3 compounds have some variations in composition: the complete isomorphic series between Au2Te4Se3 and Au2Te4S3 was observed. The gold and Au-minerals at the main ore stage can be stable within a range of log$f_{{\rm O}_{\rm 2}}$ of −27.3 and atmospheric oxygen (?); log$f_{{\rm S}{\rm e}_{\rm 2}}$ between −12.4 and −5.7; log$f_{{\rm T}{\rm e}_{\rm 2}}$ between −10.5 and −7.8; and log$f_{{\rm S}_{\rm 2}}$ between −12.8 and −6.8 (at 250°C). The increasing oxygen fugacity during the final stage of mineralization resulted in the formation of complex Sb,As,Te,S-bearing Au oxides. Gold-oxide formation occurs due to oxidation of Au-tellurides. The final products of this process are newly-formed secondary mustard gold and Te–Se solid solutions.

Copper–arsenic decoupling in an active geothermal system: A link between pyrite and fluid composition

Over the past few decades several studies have reported that pyrite hosts appreciable amounts of trace elements which commonly occur forming complex zoning patterns within a single mineral grain. These chemical zonations in pyrite have been recognized in a variety of hydrothermal ore deposit types (e.g., porphyry Cu–Mo–Au, epithermal Au deposits, iron oxide–copper–gold, Carlin-type and Archean lode Au deposits, among others), showing, in some cases, marked oscillatory alternation of metals and metalloids in pyrite growth zones (e.g., of Cu-rich, As–(Au, Ag)-depleted zones and As–(Au, Ag)-rich, Cu-depleted zones). This decoupled geochemical behavior of Cu and As has been interpreted as a result of chemical changes in ore-forming fluids, although direct evidence connecting fluctuations in hydrothermal fluid composition with metal partitioning into pyrite growth zones is still lacking. In this study, we report a comprehensive trace element database of pyrite from the Tolhuaca Geothermal System (TGS) in southern Chile, a young and active hydrothermal system where fewer pyrite growth rims and mineralization events are present and the reservoir fluid (i.e. ore-forming fluid) is accessible. We combined the high-spatial resolution and X-ray mapping capabilities of electron microprobe analysis (EMPA) with low detection limits and depth-profiling capacity of secondary-ion mass spectrometry (SIMS) in a suite of pyrite samples retrieved from a ∼1km drill hole that crosses the argillic (20–450m) and propylitic (650–1000m) alteration zones of the geothermal system. We show that the concentrations of precious metals (e.g., Au, Ag), metalloids (e.g., As, Sb, Se, Te), and base and heavy metals (e.g., Cu, Co, Ni, Pb) in pyrite at the TGS are significant. Among the elements analyzed, As and Cu are the most abundant with concentrations that vary from sub-ppm levels to a few wt.% (i.e., up to ∼5wt.% As, ∼1.5wt.% Cu). Detailed wavelength-dispersive spectrometry (WDS) X-ray maps and SIMS depth vs. isotope concentration profiles reveal that pyrites from the TGS are characterized by chemical zoning where the studied elements occur in different mineralogical forms. Arsenic and Co occur as structurally bound elements in pyrite, Cu and Au in pyrite can occur as both solid solution and submicron-sized particles of chalcopyrite and native Au (or Au tellurides), respectively. Pyrites from the deeper propylitic zone do not show significant zonation and high Cu–(Co)–As concentrations correlate with each other. In contrast, well-developed zonations were detected in pyrite from the shallow argillic alteration zone, where Cu(Co)-rich, As-depleted cores alternate with Cu(Co)-depleted, As-rich rims. These microanalytical data were contrasted with chemical data of fluid inclusions in quartz and calcite veins (high Cu/As ratios) and borehole fluid (low Cu/As ratios) reported at the TGS, showing a clear correspondence between Cu and As concentrations in pyrite-forming fluids and chemical zonation in pyrite. These observations provide direct evidence supporting the selective partitioning of metals into pyrite as a result of changes in ore-forming fluid composition, most likely due to separation of a single-phase fluid into a low-density vapor and a denser brine, capable of fractionating Cu and As.

Pyrite as a record of hydrothermal fluid evolution in a porphyry copper system: A SIMS/EMPA trace element study

Porphyry copper deposits are currently the world’s largest source of copper and molybdenum, and are also among the largest reservoirs of gold in the upper crust. Despite the fact that pyrite is a ubiquitous mineral phase in these deposits and secondary Cu enrichment processes are commonly controlled by the abundance of this sulfide, the major and trace element chemistry of pyrite from porphyry systems remains unconstrained. In this study, we report the first comprehensive trace element database of pyrite from the Dexing deposit, China’s largest porphyry Cu deposit. By combining high-spatial resolution and X-ray mapping capabilities of electron microprobe analysis (EMPA) with low detection limits and depth-profiling capabilities of secondary-ion mass spectrometry (SIMS) in a suite of samples from the Dexing deposit, we show that the concentrations of precious metals (e.g., Au, Ag), metalloids (e.g., As, Sb, Se, Te) and heavy metals (e.g., Cu, Co, Ni, Zn, Hg) in pyrite from porphyry systems are more significant than previously thought. Among the elements analyzed, Cu, As, Au and Ni are the most abundant with concentrations that vary from sub-ppm levels to a few wt.% (i.e., ∼6wt.% Cu, ∼3wt.% As, ∼0.25wt.% Au, and ∼0.2wt.% Ni). Detailed wavelength-dispersive spectrometry (WDS) X-ray maps and SIMS depth vs. isotope concentration profiles reveal that pyrite from the Dexing deposit is characterized by complex chemical zoning where the studied elements occur in different mineralogical forms. While As occurs as a structurally bound element in pyrite, Cu and Au can occur as both solid solution and micro- to nano-sized particles of chalcopyrite and native Au (or Au tellurides), respectively, indicating that pyrite can control metal speciation and partitioning during porphyry Cu mineralization. The well-developed oscillatory zoning detected in pyrite, where Cu-rich, As-depleted growth zones alternate with Cu-depleted, As-rich layers, indicates that Cu is geochemically decoupled from As, suggesting that this selective partitioning of metals into pyrite is most likely the result of changes in hydrothermal fluid composition.

Mineralogy and Formation of Black Smoker Chimneys from Brothers Submarine Volcano, Kermadec Arc

Brothers caldera volcano is a submarine volcano of dacitic composition, located on the Kermadec arc, New Zealand. It hosts the NW caldera vent field perched on the steep slope of the caldera walls and includes numerous, active, high-temperature (max 302°C) chimneys and a greater amount of dead, sulfide-rich spires. Petrographic studies of these chimneys show that three main zones can occur within the chimneys: a chalcopyrite-rich core, surrounded by a sulfate-dominated zone, which is in turn mantled by an external rind of Fe oxides, calcite, and silicates. Four chimney types are identified based on the relative proportions of the chalcopyrite and sulfate layers and the presence or absence of anhydrite. Two are Cu rich, i.e., chalcopyrite-sulfate and chalcopyrite-bornite chimneys, and two are Zn rich, i.e., sphalerite-barite and sphalerite-chalcopyrite. Chimney growth begins with the formation of a sulfate wall upon which sulfides precipitate. Later, zone refining results in a chalcopyrite-rich core with pyrite/marcasite and sphalerite occurring predominantly near the outer margins. In chalcopyrite-bornite chimneys, the chalcopyrite core rapidly loses permeability and limits the thickness of the surrounding sulfate layer. In these chimneys, bornite, chalcocite, and covellite form along the outer margin of the chalcopyrite zone as a result of oxidation by seawater. Zinc-rich chimneys display a more vertical zonation and their growth involves an upward-advancing barite cap followed by chalcopyrite deposition (if present) nearer the base. The vertical zonation and lack of anhydrite in these chimneys also implies that larger chalcopyrite and anhydrite deposits may exist subsea floor. The different chimney types are related to subsea-floor permeability, the amount of fluid mixing that occurs prior to venting, and heterogeneous fluid compositions. The occurrence of specular hematite and Bi or Au tellurides associated with chalcopyrite are consistent with magmatic contributions to the NW caldera vent site. These tellurides are the first gold-bearing phase to be identified in these chimneys, and the Bi-Au association suggests that gold enrichment up to 91 ppm is due to scavenging by liquid bismuth. The presence of tellurides in Brothers chimneys have implications for other telluride-bearing deposits, such those in the Urals. Likewise, other aspects of the mineralogy (i.e., textures) and zonation, including the implied subsea-floor deposition, presented here from an active, undeformed environment can aid in understanding ancient volcanogenic massive sulfide (VMS) deposits that have undergone various degrees of metamorphism.

Gold and silver tellurides in the Shirokinsky ore-and-placer cluster, the Sette-Daban Ridge, Yakutia

The first findings of Au and Ag tellurides (sylvanite and petzite) in sulfide-quartz ore of the Shirokinsky ore and placer cluster located in the Sette-Daban Horst-Anticlinorium are described. These minerals were found for the first time at the gold deposits of East Yakutia. The chemical compositions (wt %) of sylvanite (23.65–24.61 Au, 12.7–13.13 Ag, 59.3–59.97 Te, 96.26–97.97 in total) and petzite (23.17–25.24 Au, 42.27–44.40 Ag, 31.26–33.37 Te, 98.19–102.55 in total) are reported. Galena as a host mineral is associated with native gold, electrum, hessite, and stutzite. The finding of Au-Ag and Ag tellurides provides evidence for the development of Au-telluride mineralization in the Sette-Daban Horst-Anticlinorium.

Comparison of three analytical methods in the determination of gold in six Finnish gold ores, including a study on sample preparation and sampling

We used samples from six Finnish ore deposits to evaluate the efficiency of sample pretreatment procedures — crushing, splitting and grinding — and to compare three analytical methods based on the atomic absorption determination of gold following: (1) classical lead fire assay (FA); (2) the aqua regia leach (AR) followed by Hg coprecipitation of Au; and (3) the sodium cyanide (NaCN) leach. Sample size used for the method comparison is 20 g. The Au deposits and ore types were: Suurikuusikko and Osikonmäki, refractory ores in which Au is associated with arsenopyrite and pyrite; Pampalo and Kutemajärvi ores with metallic Au and Au tellurides; and Jokisivu and Pahtavaara ores containing coarse-grained metallic Au. After crushing, the samples were split into three parts, one of which was put aside into storage. Two splits were further divided into two subsamples which were ground to two grades of fineness (<0.03 and <0.06 mm). The four subsamples thus obtained were analysed for Au using the three analytical methods. Each determination was performed five times on each of the four subsamples. According to t-tests on the FA results of the two splits, crushing and splitting produced samples of equal Au content in all six cases. Grinding to a finer grain size gave a significant difference in Au results only for the Pahtavaara ore sample. If the FA results are assumed to represent 100% recovery of Au, we obtained greater than 95% recoveries for all but the Suurikuusikko sample (87% recovery) by the AR leach method. We also obtained recoveries of over 95% by the NaCN leach method for the Pampalo, Kutemajärvi and Pahtavaara samples, whereas recoveries for the other three samples varied between 73 to 92%. The AR leach was also performed on 1-g samples and the NaCN leach on 250-g samples. For three of the ore samples, decreasing sample size from 20 g to 1 g did not cause a significant difference in the variance of the Au results. Increasing the sample size from 20 g to 250 g significantly improves the representativity of only the Pahtavaara sample. For the Kutemajärvi, Pahtavaara and Jokisivu ores, a sample larger than 250 g is needed in order to obtain a precision equivalent to that for reference samples.

CdTe(110) interface formation with reactive and nonreactive overlayers: Al, Ti, Pd, Ag, Au, In, and Ce

High resolution synchrotron radiation photoemission has been used to investigate room temperature interface formation involving CdTe(110) and transition metals (Ti), noble and near-noble metals (Ag, Au, Pd), lanthanide metals (Ce), and simple metals (Al, In). Core level and valence band results reveal substrate interactions with quite different chemical behavior and morphologies. Deposition of Ag and In at 300 K produces a nondisrupted substrate on which clusters form spontaneously. Deposition of Al, Ti, and Ce produces metal–anion reactions and substrate disruption at low coverage, with metal clusters that grow on the reacted layer after reaction has been curtailed by kinetic considerations. For Pd and Au, there is substrate disruption, even though the heats of formation for Pd and Au tellurides are smaller than CdTe, and there is segregation to the surface for both Cd and Te atoms. Studies of Au overlayers formed on reacted Ti interlayers on CdTe show changes in Cd atom distribution and give insight into the stability of these interfaces.