Tatiana V. Beliaeva

We used thermodynamic modeling methods to calculate the stability of auroselenide AuSe_(s) in hydrothermal solutions at different temperatures (25–350 °C), pressures (1–165 bar), salinities (0–5 m NaCl), and acidity–alkalinity (0.00001–0.1 m HCl or NaOH). Gold selenide dissolves congruently in near-neutral solutions. In acidic chloride solutions, AuSe_(s) dissolves incongruently to form selenium Se_(s,l), and in alkaline solutions, to form gold Au_(s). Gold selenide has a low solubility at temperatures of 25–200 °C. With increasing temperature, the solubility of AuSe_(s) increases and at 350 °C the concentration of dissolved gold in highly acidic solutions (without NaCl) reaches 10⁻⁶ m, while in near-neutral and alkaline solutions, it varies from 2·10⁻⁷ to 6·10⁻⁷ m. At concentrations of NaCl and HCl higher than 0.01 m, the solubility of AuSe_(s) increases by half an order of magnitude owing to the formation of gold chloride complexes. In low acidic, near-neutral, and alkaline solutions, gold hydroxocomplex is predominant. We constructed diagrams for the Au–Se–H₂O system at various temperatures (25, 100, 200 and 300 °C), which show the stability fields of AuSe_(s), Au_(s) + AuSe_(s), Se_(s,l) + AuSe_(s) and Au_(s) on lg ƒO₂–pH. Gold chalcogenides are characteristic minerals of epithermal deposits. The relationships of auroselenide with native selenium and native gold and other minerals in the Au-Ag ores of the Gaching ore occurrence (Kamchatka Peninsula, Russia) and the Bleïda Far West Au-Pd deposit (Morocco) were studied. It was revealed that auroselenide occurs in the peripheral parts of native gold grains, and, less often, in the form of inclusions and intergrowths with other gold chalcogenides in the core of native gold grains. The presence of solidified microdroplets of composition ranging from Te_0.97Se_0.03 to Te_0.71Se_0.28S_0.01 and Se_0.58Te_0.41S_0.01 in the ore minerals at these and other golddeposits suggests participation of chalcogens existing at temperatures of 217–449 °C. The formation of auroselenide and other gold chalcogenides is likely with a decrease in temperature and neutralization of highly acidic or highly alkaline solutions, or with the participation of melts or chalcogen gas particles. The results of thermodynamic calculations are confirmed by the data on the composition of mineral associations with auroselenide from gold deposits. The presence of auroselenide in the ores from Au-Ag epithermal and other gold deposits with Au–Se–Te–S mineralization is predicted.

Heterogeneous grains in the heavy fractions of the placers of the Olkhovaya-1 river (Kamchatka Cape Peninsula, Eastern Kamchatka, Russia) containing Au-Ag, Au-Cu, and Au-Ag-Cu particles were analyzed using scanning electron microscopy, electron-probe microanalysis, and X-ray powder diffractometry. The analyses showed that auricupride dominates in some grains, whereas in others, the main phases are tetra-auricupride or Ag-rich and (or) Au-rich alloys. It was revealed that in the central parts of some grains of exsolution structures, auricupride Cu_3.04–2.74Au_0.96–1.26 (Ag < 1 wt.%) occurs in intergrowths with low-fineness Ag_0.86–0.67Au_0.09–0.33Cu _{solid solutions (160‰–480‰), and tetra-auricupride Cu1.12–0.87Au0.88–1.11Ag (Ag < 2 wt.%) occurs with higher-fineness Au0.73–0.62Ag0.2–0.38Cu solid solutions (520‰–850‰). We also observed, mainly in the peripheral parts of the grains, insignificant amounts of secondary phases that were compositionally similar to Cu2Au, Cu3Au2, Au3Cu2, Au2Cu, Au3Cu, high-fineness gold with impurities of Cu and occasionally Ag (>850‰), and pure gold. In intergrowths with auricupride and tetra-auricupride, we also observed earlier-formed silicates (garnet, pyroxene, chlorite, epidote, titanite), syngenetic sulfides (pyrrhotite, bornite, galena), and later minerals (chalcocite, covellite, anilite, cuprite, goethite, etc.). The XRD analysis of the peripheral parts of some grains showed the presence of auricupride (Cu3Au), tetra-auricupride CuAu (I) and Cu(Au0.92Ag0.08) (II), and gold. The profiles show the absence of peaks of the Au3Cu phase and other Au-Ag-Cu phases identified by the EMPA, which is likely due to their low concentrations in the samples or their structural similarity to gold. It is assumed that the probable source of Au-Ag-Cu mineralization in the Olkhovaya-1 river placers is located in the upper reaches of watercourses that erode the ultrabasic massif of Mounts Soldatskaya and Golaya (Kamchatka Cape Peninsula).}

This paper reviews and summarizes the available information on the composition of palladian gold with various contents and sets of isomorphic impurities (Ag, Cu, Hg) at 50 deposits and ore occurrences with Au-Pd mineralization. It is revealed that Palladian gold is represented by the systems Au–Pd, Au–Pd–Hg, Au–Pd–Cu, and Au–Pd–Ag–Hg, but more frequently corresponds to Au–Pd–Ag, Au–Pd–Ag–Cu, and Au–Pd–Ag–Cu–Hg. Objects with palladian gold belong to different types of gold deposits and to the deposits at which the main components of ores are PGE, Cr, Cu, Ni, V, and Ti. We propose a classification of the types of deposits with palladian gold: (1) PGE ore deposits related to mafic–ultramafic magmatic complexes (two subtypes—(a) low-sulfide-grade (less than 2%–5% sulfides) Alaskan, and (b) high-sulfide-grade (more than 5% sulfides) Norilsk); (2) orogenic gold deposits (OG); (3) epithermal (porphyry) gold–copper deposits (EPGC); (4) iron oxide copper gold deposits (IOCG); (5) ferruginous quartzite deposits; (6) volcanic exhalation; and (7) gold-PGE placers of five subtypes corresponding to the types of 1–5 primary sources. Physicochemical conditions of the formation of palladian gold at some deposits of type 1 cover two areas—magmatic high-temperature and hydrothermal low-temperature. At the majority of deposits of types 2–4, its formation proceeds with the participation of hydrothermal fluids (300–60 °C) of various salinities (0.2–30 wt.% NaCl eq.). Palladian gold is mainly high-fineness (910‰–990‰), is less frequently medium-fineness, and contains Ag and Cu, but does not contain Hg at the deposits of types 1, 3, and 4. The only exception is the Au-Pd-Hg Itchayvayam ore occurrence (Kamchatka, Russia), for which two varieties of Pd,Hg-bearing native gold (fineness 816‰–960‰ and 580‰–660‰) are determined. Low-fineness palladian gold with the major content of Ag is typical of OGD deposits. Medium-fineness palladian gold occurs at ferruginous quartzite deposits and in volcanic exhalations. Hg, Ag, Cu-bearing high-fineness palladian gold is present mainly in placer deposits (type 7). The most common minerals in association with palladian gold are arsenides, stibioarsenides, sulfides, stannides, bismuthides, tellurides, and selenides of Pd and Pt. These are typical of deposit types 1 and 7. The minerals of Au, Ag, and Cu (tetra-auricupride, aurostibite, chalcopyrite, bornite, chalcocite, eucairite, etc.) are in association with palladian gold at OG, EPGC, and IOCG deposits. Hg minerals (cinnabar, tiemannite, coloradoite, potarite) are at some deposits (types 1, 2, 7-1, 7-4). Cu, Fe, and Pd oxides (tenorite, hematite, magnetite, PdO, (Pd,Cu)O) and Fe and Pd hydroxides (goethite, (Fe,Pd)OOH) occur at the deposits of the 3, 4, and 7 groups and indicate the highly oxidizing conditions of ore formation. The most common minerals among host minerals are quartz and muscovite, including fuchsite (Cr-Ms), chlorite, albite, K-feldspar, hornblende, and carbonates (calcite, siderite, etc.). The fineness, content, and set of impurities in palladian gold and minerals in association with it reflect the mineralogy of Au-Pd ores and allow them to be used as indicators for the deposit types.

The unique minerals of the Au-Pd-Hg system in gold grains from heavy concentrates of the Itchayvayam placers and watercourses draining and ore samples of the Barany outcrop at the Itchayvayam mafic–ultramafic complex (Kamchatka, Russia) were investigated. Gold grains from watercourses draining and heavy concentrates of the Itchayvayam placers contain substitution structures formed by Pd,Hg-rich low-fineness gold (Au_0.59–0.52Pd_0.24–0.25Hg_0.17–0.23, 580‰–660‰) and Pd,Hg-poor high-fineness gold (Au_0.94–0.90Pd_0.02–0.04Hg_0.03, 910‰–960‰). Potarite (PdHg) without and with impurities (Au < 7.9, Cu < 3.5, Ag < 1.2 wt.%), Ag-poor high-fineness gold (Au_0.91Ag_0.09, 950‰), Ag,Pd,Hg-bearing middle-fineness gold (Au_0.75Ag_0.08Pd_0.09Hg_0.08—Au_0.88Ag_0.09Pd_0.02Hg_0.01, 820‰–930‰), and Pd,Hg-rich low-fineness gold with minor contents Ag and Cd (Au_0.51–0.55Pd_0.25–0.22Hg_0.21–0.16Ag_0.03–0.06Cd_0.01, fineness 580‰–630‰) were observed as individual microinclusions in the ore samples of the Barany outcrop. XRD and EBSD study results show that the Pd,Hg-rich low-fineness gold is isotypic to gold and has the same structure type, but different cell dimensions. According to data obtained for the Itchayvayam and some deposits and ore occurrences with Pd,Hg-bearing gold, the stable ternary phases and solid solutions of the following compositions in the Au-Pd-Hg system have been identified: Pd,Hg-poor gold (Au_0.94–0.90Pd_0.02–0.04Hg_0.03), Pd,Hg-rich gold (Au_0.59–0.52Pd_0.24–0.25Hg_0.17–0.23), Au-potarite (PdHg_0.62Au_0.38—Pd_1.04Hg_0.96—Au_0.80Pd_0.68Hg_0.52), and Au,Hg-bearing palladium (Pd_0.7Au_0.3Hg_0.1). The genesis of Pd,Hg-rich gold is insufficiently studied. We supposed that the meteoric waters or low-temperature hydrotherms rich in Pd and Hg could lead to the replacement Pd,Hg-poor gold by Pd,Hg-rich gold. High concentrations of Pd in Pd,Hg-bearing gold indicate a genetic relationship with mafic–ultramafic rocks.

Mineral associations and compositions of sulfoselenides and other silver chalcogenides from epithermal deposits of the Okhotsk–Chukotka volcanic belt (Northeast Russia)—Rogovik, Lunnoe, Julietta, Valunistoe, Corrida, and Pepenveem—have been studied. Different types of relationships between S–naumannite and Se-acanthite in the ores of the study deposits have been established. It is shown that the replacement of S–naumannite and Se-bearing minerals (Se–galena, Se–stephanite) of early parageneses by Se-acanthite or acanthite at later stages is a general pattern in the sequence of formation of silver chalcogenides. Based on the results of thermodynamic calculations and data on the composition of the main productive mineral associations, the physicochemical parameters (T, ƒS₂, ƒSe₂, ƒO₂, pH) of ore formation at these deposits were reconstructed.

We studied the mineralization and sulfur isotopic composition of sulfides of gold–palladium ores in olivine clinopyroxenites from the Dzelyatyshor massif made up of a continuous layered series of rocks: olivine-free clinopyroxenite–olivine clinopyroxenite–wehrlite. The primary igneous layering of rocks, manifested as different quantitative ratios of clinopyroxene and olivine in them, controls the local trends of variability in the chemistry of mineral-forming medium and the concentrations of ore components, including noble metals, and sulfur in each separate layer during its cooling. The replacement of primary rock-forming minerals by secondary minerals, when the temperature decreases, is a characteristic trend for pyroxenites: (a) olivine → serpentine, secondary magnetite, and (b) clinopyroxene → amphibole, secondary magnetite → chlorite. The deposition of native gold in parageneses with PGM and sulfides at the Ozernoe occurrence took place during the replacement of earlier rock-forming minerals by chlorite. This process completed mineral formation at the deposit and took place at temperatures 150–250 °C and at the high activity of S, Te, Sb, and As of fluid. The variability of mineral formation conditions during chloritization is reflected in the change of native-sulfide forms of Pd by arsenide-antimonide forms and the sulfur isotopic composition of sulfides. The Pd content in native gold increases in the series—Au-Ag solid solution (<1.5 wt.% Pd)—Au-Cu intermetallides (to 6 wt.% Pd)—Cu-Au-Pd solid solutions (16.2–16.9 wt.% Pd). The sulfur isotopic composition of pyrite, chalcopyrite, and bornite varies from −2.1 to −2.9‰. It is assumed that a deep-seated magmatic basic melt was the source of fluid, ore components, and sulfur.

Electron microprobe analysis (EMPA), X‐ray powder diffraction (XRD), and Raman spectroscopy (RS) were applied to characterize the synthetic gold chalcogenides of the Au–Te–Se–S system and natural analogs from the Gaching deposit (Central Kamchatka, Russia). The EPMA results showed that the synthetic chalcogenides have different Te/Se/S and Au/X (X = Te + Se + S) ratios: AuX₂, Au₃X₁₀, and AuX. They are similar in composition to natural compounds — calaverite (AuTe₂), maletoyvayamite (Au₃Te₆Se₄), and unnamed minerals AuTe_0.7Se_0.3 and AuSe_0.7S_0.3. It was established that chalcogenides AuX, Au₃X₁₀, and AuX₂ have a specific Raman spectra with characteristic peaks. The position of the peaks and the character of the spectra of the synthetic phases and their natural analogs from the Gaching deposit coincide within the limits of accuracy. Different ratios of chalcogenes Te/Se/S in compounds influence the Raman peak positions. The positions of the peaks for natural compounds AuX differ depending on the predominance of Te (AuTe_0.7Se_0.3) or Se (AuSe_0.7S_0.3). AuSe synthetic phases consist of a mixture of α‐ and β‐polymorphs. Raman spectroscopy can be used for the identification of natural gold chalcogenides worldwide, which are difficult to diagnose by other methods due to the microscopic grain sizes and close intergrowths with other ore minerals. The similarity of the Raman spectra upon changing the concentrations of Se and S suggests identical structures and possible isomorphism in the composition range of AuTe_0.7Se_0.3– AuTe_0.7Se_0.2S_0.1, AuTe_1.9Se_0.1– AuTe_1.8Se_0.1S_0.1, and Au₃Te₆Se₄– Au₃Te₆S₃Se.

—We have studied the mineral composition of ores from the Pepenveem epithermal Au–Ag deposit, which is a promising new object of the Chukchi Peninsula. It has been found that the ore formation process was developed in the following sequence: Pyrite, arsenopyrite, and marcasite were deposited at the early stage, next were Pb, Zn, and Cu sulfides; at the late stage, native gold, pyrargyrite, stephanite, proustite, minerals of the pearceite–polybasite series, acanthite, and other Ag minerals were deposited. The results of fluid inclusion studies indicate that the Au–Ag mineralization formed from low-temperature (236–137 °C) low-concentration chloride hydrotherms (0.18–1.57 wt.% NaCl eq.). The results of calculation of thermodynamic equilibria have shown that in the temperature range from 200 to 100 °C, there were a decrease in the fugacity of sulfur (lg fS₂ from –10 to –21) and oxygen (lg fO₂ from <–36 to <–48) and a change from near-neutral to acidic solutions. Compared to other Au–Ag deposits on the Chukchi Peninsula (Corrida and Valunistoe), which are characterized by wide distribution of Se- and Te-bearing Au–Ag chalcogenides (naumannite, cervelleite, and hessite), ore formation with gold–silver–sulfosalt mineralization at the Pepenveem deposit took place at lower temperatures and lower selenium, tellurium, and oxygen fugacity. The data obtained permit us to refer the Pepenveem deposit to the group of epithermal low-sulfidation (LS) deposits.