A rare gold–telluride montbrayite from the large Svetlinsk gold–telluride deposit (South Urals, Russia) was comprehensively studied using optical microscopy, scanning electron microscopy, electron ...microprobe analysis, reflectance measurements, electron backscatter diffraction, and Raman spectroscopy. Significant variations in the composition of the mineral were revealed (in wt%): Au 36.98–48.66, Te 43.35–56.53, Sb 2.49–8.10, Ag up to 4.56, Pb up to 2.04, Bi up to 0.33, Cu up to 1.42. There are two distinct groups with much more-limited variation within the observed compositional interval (in wt%): (1) Au 36.98–41.22, Te 49.35–56.53, Sb 2.49–5.57; (2) Au 47.86–48.66, Te 43.35–44.92, Sb 7.15–8.10. The empirical formula calculated on the basis of 61 apfu is Au16.43–23.28Sb1.79–6.09Te32.01–38.89Ag0–3.69Bi0–0.14Pb0–0.90Cu0–1.96. Two substitution mechanisms for antimony are proposed in the studied montbrayite grains: Sb→Au (2.5–5.6 wt% Sb) and Sb→Te (7–8 wt% Sb). The dependence of the reflection spectra and Raman spectra on the antimony content and its substitution mechanism, respectively, was found in the mineral. The slope of the reflectance spectra decreases and the curve in the blue–green region of the spectrum disappears with increasing Sb content in montbrayite. Raman spectra are reported for the first time for this mineral. The average positions of the peak with high-intensity are ~64 cm−1 and ~90 cm−1 for montbrayite with Sb→Te and Sb→Au, respectively. Two grains of montbrayite demonstrate decomposition according to two schemes: (1) montbrayite (7 wt% Sb) → native gold + calaverite ± altaite, and (2) montbrayite (5 wt% Sb) → native gold + tellurantimony ± altaite. A combination of melting and dissolution–precipitation processes may be responsible for the formation of these decomposition textures.
We report on the application of the U-Th-He method for the direct dating of pyrite and provide an original methodological approach for measurement of U, Th and He in single grains without loss of ...parent nuclides during thermal extraction of He. The U-Th-He age of ten samples of high-crystalline stoichiometric pyrite from unoxidized massive ores of the Uzelga volcanogenic massive sulfide (VMS) deposit, South Urals, is 382 ± 12 Ma (2σ) (U concentrations ~1–5 ppm; 4He ~10−4 cm3 STP g−1). This age is consistent with independent (biostratigraphic) estimations of the age of ore formation (ca, 389–380 Ma) and is remarkably older than the probable age of the regional prehnite-pumpellyite facies metamorphism (~340–345 Ma). Our results indicate that the U-Th-He dating of ~1 mg weight pyrite sample is possible and open new perspectives for the dating of ore deposits. The relative simplicity of U-Th-He dating in comparison with other geochronological methods makes this approach interesting for further application.
Pampaloite AuSbTe, a rare gold-antimony telluride that was first described in 2019 from the Pampalo gold mine, Finland, was found in samples from the large Svetlinsk gold-telluride deposit, South ...Urals, Russia. Optical microscopy, scanning electron microscopy, electron microprobe analysis, reflectance measurements, electron backscatter diffraction and Raman spectroscopy were used to study eight grains of pampaloite. Pampaloite forms inclusions (5–30 μm) in quartz together with other tellurides (typically petzite), native gold and, less often, sulfides. In reflected light, pampaloite is white or creamy white in color with weak anisotropism and without internal reflections. The empirical formula calculated on the basis of 3 apfu is Au0.97–1.07Ag0–0.02Sb0.96–1.04Te0.96–1.04 (n = 18). The holotype pampaloite structure was used as a reference and provided the perfect match for an experimental EBSD pattern (12 bands out of 12, mean angle deviation 0.19°). Raman spectra are reported for the first time for this mineral. All studied pampaloite grains exhibit vibrational modes in the range 60–180 cm−1. Average peak positions are 71, 108, 125, 147 and 159 cm−1. According to experimental data for the Au-Sb-Te system, we estimate the upper temperature range of pampaloite crystallization at the Svetlinsk deposit to be 350–430 °C.
Hydrothermal synthesis experiments were performed in order to quantify the states of Au and As in pyrite and marcasite. The experiments were performed at 350 °C/500 bar and 490 °C/1000 bar ...(pyrite–pyrrhotite buffer, C(NaCl) = 15 and 35 wt.%). The synthesis products were studied by EPMA, LA-ICP-MS, and EBSD. The EPMA was applied for simultaneous determinations of Au, As, Fe, and S, with a Au detection limit of 45–48 ppm (3σ). The analyses were performed along profiles across zonal grains. The concentrations of As and Au up to 5 wt.% and 8000 ppm, respectively, were determined in pyrite and up to 6 wt.% and 1300 ppm in marcasite. In pyrite, the Au concentration decreases with fluid salinity and temperature increases. Strong positive Au–As correlation and strong negative Au–Fe and As–S correlation were identified in pyrite. Comparison of the correlations with theoretical lines implies Au–As clustering. The cluster stoichiometry is inferred to be AuAs10. Most probably, As in pyrite presents in the form of clusters and in the As→S solid solution. Incorporation of Au in As-rich pyrite can be controlled by the reductive deposition mechanism. In marcasite, the concentrations of Au are not correlated with the As content. The AuAs10 clusters enrich the {210}, {113}, and {111} pyrite faces, where the former exhibits the highest affinity to Au and As. The affinity of {110} and {100} forms to Au and As is lower. Implication of the experimental results to data for natural auriferous pyrite shows that the increase of Au content at C(As) > 0.5–1 wt.% is caused by the incorporation of the Au-As clusters, but not because of the formation of Au→Fe solid solution. Therefore, the concentration of “invisible” gold in pyrite is dictated solely by the hydrothermal fluid chemistry and subsequent ore transformations.
We report on the application of the U-Th-He method for the direct dating of pyrite from the alteration halo of the Novogodnee-Monto Au-Fe-skarn deposit, Polar Urals. The deposit is genetically ...related to the formation of volcanogenic complexes of the Ural Paleozoic belt. A modification of the original methodology for measuring U, Th and He isotopes in a single grain allowed us to determine a U-Th-He age of 382 ± 8 Ma (2σ) based on six pyrite samples from the altered rocks of the deposit (U mass fraction ~0.2 mg/kg; Th/U ~3.5; 4He specific volume ~10−5 cm3·STP·g−1). This age is consistent with estimates of the age of ore formation and coeval with the end of the period of island arc magmatic activity. Our results indicate that U-Th-He dating for pyrite samples of ~1 mg in weight from the hydrothermal-metasomatic halo of ore bodies is possible, providing a crucial next step in the development of U-Th-He pyrite geochronology.
The large gold-telluride Svetlinsk deposit (~135 t Au) is considered to be a nontraditional one in the Urals and its origin is debated. A specific feature of the deposit is the abundance of various ...tellurides, such as tellurides of Fe, Ni, Pb, Sb, Bi, Ag, and Au. The new data of microthermometry, Raman spectroscopy, LA-ICP-MS, and crush-leach analysis (gas and ion chromatography, ICP-MS) for fluid inclusions as well as O-isotope data for quartz were obtained for the construction of PTX parameters of ore-formation and fluid sources in the deposit. Mineralisation was formed at a wide range of temperature and pressure (200–400 °C, 1–4 kbar) and from contrasting fluids with multiple sources. At the early stages, the magmatic fluid evolved during its ascent and phase separation and the fluid derived from the host rock decarbonation and dehydration were involved in the hydrothermal system. In addition, mantle-derived fluid might be involved in the ore-forming process during gold-telluride precipitation as well as heated meteoric waters during the late stages. Early fluids were rich in H2S, S0, and CH4, while the Au-Te mineralisation was formed from N2-rich fluid.
A significant part of the primary gold reserves in the world is contained in sulphide ores, many types of which are refractory in gold processing. The deposits of refractory sulphide ores will be the ...main potential source of gold production in the future. The refractory gold and silver in sulphide ores can be associated with micro- and nano-sized inclusions of Au and Ag minerals as well as isomorphous, adsorbed and other species of noble metals (NM) not thoroughly investigated. For gold and gold-bearing deposits of the Urals, distribution and forms of NM were studied in base metal sulphides by laser ablation-inductively coupled plasma mass spectrometry and by neutron activation analysis. Composition of arsenopyrite and As-pyrite, proper Au and Ag minerals were identified using electron probe microanalysis. The ratio of various forms of invisible gold—which includes nanoparticles and chemically bound gold—in sulphides is discussed. Observations were also performed on about 120 synthetic crystals of NM-doped sphalerite and greenockite. In VMS ores with increasing metamorphism, CAu and CAg in the major sulphides (sphalerite, chalcopyrite, pyrite) generally decrease. A portion of invisible gold also decreases —from ~65–85% to ~35–60% of the total Au. As a result of recrystallisation of ores, the invisible gold is enlarged and passes into the visible state as native gold, Au-Ag tellurides and sulphides. In the gold deposits of the Urals, the portion of invisible gold is usually <30% of the bulk Au.
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•X-ray absorption spectroscopy revealed the state of gold in Cu-Fe sulfides.•Intermediate solid solution iss and bornite solid solution bnss contain “chemically bound” gold.•Au in iss ...and bnss is CuAuS clusters with stromeyerite-like and petrovskaite-like structures.•Au-bearing cluster stability increases with increasing temperature.
Chalcopyrite and bornite are the main Au-bearing minerals at Cu porphyry deposits, volcanogenic massive sulfide (VMS) deposits, Cu-Ni deposits of the mafic magmatic complexes, and ores of submarine sulfide edifices. Bornite and intermediate solid solutions with wide compositional variations (bnss and iss – high-temperature chalcopyrite, correspondingly), which can scavenge economic concentrations of Au, appear in the Cu-Fe-S system at ore-forming conditions. However, the state of Au in bnss and iss is yet unknown. To solve this conundrum, we synthesized samples with net chemical composition of bnss and iss, studied them by in situ X–ray absorption spectroscopy (XAS), and used the experimental data to explain the Au distribution among natural ore-forming minerals. The sulfide samples were obtained at 495–700 °C in Au-saturated system by means of salt flux method. The bnss contained ∼1.2–1.6 log units more Au than iss: up to 18 wt.% Au in bnss vs 0.4 wt.% Au in iss at 700 °C. An increase of temperature resulted in the sharp increase of Au concentration in both phases, ∼1 log unit per 100 °C at f(S2) close to S(l) saturation. Analysis of Au L3-edge spectra recorded at 25–675 °C revealed that at 25 °C Au exists mainly in the metallic state. At t > 500 °C the spectral features of Au° disappear, and “chemically bound” Au predominates. The Au form of occurrence in the iss field is interpreted as Au-bearing clusters with a stromeyerite-like (CuAgS) structure. Digenite Cu2–xS and bnss contain Au in a mixture of stromeyerite-like and petrovskaite-like (Au0.8Ag1.2S) clusters. The chemical composition of both forms is close to CuAuS, where the nearest Au neighbors are two S atoms at RAu-S = 2.34–2.36 Å. Results of the present study allow to determine the state of Au and its concentration in the main Cu-bearing minerals of sulfide ores as a function of the T-f(S2)-compositional parameters. Due to the sharp increase of the CuAuS clusters stability with increasing temperature, in high-temperature ores formed at t > 350 °C Au enriches Cu-bearing minerals in comparison with Cu-free or Cu-deficient ones. As a result, in these ores native gold, being a product of decomposition of the Au-bearing clusters, is associated with Cu-rich minerals – chalcopyrite, bornite, digenite, chalcocite.
Arsenian pyrite is an abundant mineral occurring in many geological settings at the Earth’s surface, including hydrothermal ore deposits which are the main source of Au. So-called “invisible” (or ...refractory) form of Au is present in pyrites in all types of these deposits, and its concentration is often directly correlated with As content. Here we report results of the investigation of the local atomic structure of Au in natural (Cu-Au-porphyry) and synthetic (450°C/ 1 kbar, 300°C/ Psat) As-free and As-bearing pyrites by means of X-ray absorption spectroscopy (XAS). In addition, the state of As was determined in pyrite samples from Carlin-type deposit. XANES/EXAFS measurements, compiled with previously published data, revealed the chemical state (valence state, local atomic environment) of Au and As in arsenian pyrites. Au is present in the solid solution state (Au
1+
in the Fe position, octahedrally coordinated by S atoms), as well as in Au
1+
2
S clusters (Au
1+
linearly coordinated by 2 S atoms). The admixture of As has no effect on the Au valence state and Au-S interatomic distance, except one synthetic sample containing a minor amount of FeAsS. Arsenic mostly incorporates into the anion site in pyrite lattice (S
1-
↔As
1-
). Our data demonstrate that pyrites of hydrothermal origin can host up to ~300 ppm of structurally bound “invisible” Au independently of As content.
In this study, a series of synthetic mercury-bearing sphalerite and zinc-bearing metacinnabar crystals was grown using the salt flux method at 605-650 °C at the hot and cold ends of the ampoule, ...respectively, in a composition range from 2 to 82 mol.% of the HgS component. According to the XAFS (X-ray Absorption Fine Structure) measurements, mercury in the sphalerite-metacinnabar crystals exists in the form of a solid solution. The EXAFS (Extended X-ray Absorption Fine Structure) spectra confirm the increase in the ionic radius of all coordination shells in the crystal structure of the Hg-sphalerite and Zn-metacinnabar on 0.14–0.28 Å, depending on the calculated model compared to the distances in pure sphalerite. However, the crystal structure does not change. The established nominal oxidation state of mercury in the (Zn, Hg)S is 2+.
•Continuous series of solid solutions exists in the ZnS–HgS system at 605–650 °C.•Hg substitutes Zn and generates lattice distortions in the sphalerite structure.•Rising of the Hg–S distance results in increasing the lattice parameter.•The nominal oxidation state of mercury in ZnS–HgS mineral system is 2+.•Extended solid solution series of the sphalerite-type structure exists between ZnS–HgS at 605–650 °C.
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