Typomorphic features of supergene gold in karst cavities were studied in the recently discovered Au–Te–Sb–Tl deposit within the Khokhoy gold ore field of the Aldan-Stanovoy auriferous province (Aldan shield, East Russia). Two morphological types of supergene gold, massive and porous, are recognized there. The first type is represented by gold crystals and irregular mass, with the fineness ranging from 835 to 1000‰. They are closely associated with goethite, siderite, unnamed Fe, Te, and Tl carbonates, Tl tellurites/tellurates and antimonates, as well as avicennite with a Te impurity. The second type is represented by mustard gold of two types with different internal structure: microporous and dendritic. The supergene gold is characterized by persistently high fineness. Along with Ag, it invariably contains Hg (up to 5.78 wt%) and Bi, and, rarely, Pb, Cu, and Fe. The supergene gold is chemically homogeneous, and its particles are all marked by high fineness, without any rims or margins. The obtained characteristics made it possible to prove the existence of two genetic types of supergene gold. Mustard microporous gold is the result of the decomposition of the associated minerals—goethite, Tl oxides, tellurium, Fe, Mn and Tl carbonates and antimonates, containing microinclusions of gold. Massive gold and dendrites are newly formed. The decomposition, remobilization, and reprecipitation of residual gold nanoparticles and their aggregation led to the formation of dendrites, and with further crystal growth and filling of pores, to gold of massive morphology. In terms of morphology, internal structure, fineness, and trace element composition, supergene gold of the Khokhoy gold ore field is comparable to gold from the Kuranakh deposit (Russia) and the Carlin-type gold deposits. It also is similar to spungy and mustard gold from Au–Te and Au–Sb deposits, weathering crusts, and placers. Its main characteristic feature is a close paragenesis with Tl minerals.

The new mineral amgaite was discovered at the Khokhoyskoe gold deposit, 120 km W of Aldan town, Aldanskiy District, Sakha Republic (Yakutia), Eastern Siberia, Russia. Amgaite forms fine-grained colloform aggregates up to 0.05 mm across, and is often intimately intergrown with avicennite, unidentified carbonates and antimonates of Tl. Other associated minerals include gold, silver, acanthite, arsenopyrite, pyrite, berthierite, chalcocite, weissbergite, chlorargyrite, calcite, quartz, goethite etc. Amgaite is dark reddish brown to black. It has submetallic luster, black streak, brittle tenacity and conchoidal fracture. Its density calculated from the empirical formula and powder XRD data is 8.358 g/cm3. Its Mohs’ hardness is ca. 1.5–2. Optically, amgaite is uniaxial. In reflected light, it is gray with a bluish shade, very weakly anisotropic with rare brownish red internal reflections. Reflectance values for the four COM wavelengths [Rmin, Rmax (%)(λ in nm)] are: 13.5, 14.2 (470); 12.7, 13.2 (546); 12.3, 12.7 (589); and 11.7, 12.3 (650). The Raman spectrum shows bands of Te–O and Tl–O bonds and confirms the absence in amgaite of H2O, OH–, CO32– groups and B–O bonds. The chemical composition is (electron microprobe, wt.%): MgO 0.43, CaO 1.62, Fe2O3 0.36, Tl2O3 66.27, Sb2O5 3.48, TeO3 27.31, total 99.47. The empirical formula based on 6 O apfu is Tl3+1.74Ca0.17Mg0.06Fe3+0.03Te6+0.93Sb5+0.13O6. Amgaite is trigonal, space group P321; unit-cell parameters are as follows: a = 9.0600(9), c = 4.9913(11) Å, V = 354.82(8) Å3, Z = 3. The strongest lines of the powder X-ray diffraction pattern [dobs, Å (I, %) (hkl)] are as follows: 3.352 (100) (111), 3.063 (15) (201), 2.619 (49) (300), 2.065 (18) (221), 1.804 (28) (302), 1.697 (8) (321), 1.625 (9) (411). The crystal structure of amgaite is the same as of synthetic Tl3+2Te6+O6. The new mineral is named after the Amga River, the basin of which hosts the type locality, Khokhoyskoe occurrence. The type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with the registration number 5773/1.

The ore mineralogy of a new promising target of the Aldan–Stanovoy gold province—the Spokoininsky cluster—is considered. Gold mineralization is represented by a volumetric, nonlinear type, unconventional for the region; it is related to elements of fold structures and reverse fault in the enclosing metamorphic basement rocks. Vein-disseminated sulfide–(pyrite)–quartz ores build up deposit-like bodies in beresites from gneisses and granite gneisses and are associated with Mesozoic igneous rocks of subalkaline formations. Mineralization is characterized by polysulfide (Fe-Cu-Pb); gold–bismuth (Au-Bi) and gold–silver–telluride (Au-Ag-Te) mineral types. Different mineral types have their own typomorphic minerals and typochemistry (fineness and impurities) of native gold. The widespread distribution of telluride mineralization and its great importance in the formation of gold mineralization on the Aldan shield is confirmed. The distribution area of bismuth (including tellurium–bismuth) mineralization in the southern part of the Aldan shield, in the zone of influence of the Stanovoy deep fault, has been identified.

The published and original data on the tellurium mineralization of gold ore deposits of the Aldan Shield are systematized and generalized. The gold content is related to hydrothermal-metasomatic processes caused by Mesozoic igneous activity of the region. The formation of tellurides occurred at the very late stages of the generation of gold mineralization of all existing types of metasomatic formations. 29 tellurium minerals, including 16 tellurides, 5 sulfotellurides and 8 tellurates have been identified. Tellurium minerals of two systems predominate: Au-Bi-Te and Au-Ag-Te. Gold is not only in an invisible state in sulfides and in the form of native gold of different fineness, but also is part of a variety of compounds: montbrayite, calaverite, sylvanite, krennerite and petzite. In the gold deposits of the Aldan Shield, three mineral types are distinguished: Au-Ag-Te, Au-Bi-Te, and also a mixed one, which combines the mineralization of both systems. The decrease in the fineness of native gold is consistent with the sequence and temperatures of the formation of Te minerals and associated mineral paragenesis from the epithermal–mesothermal Au-Bi-Te to epithermal Au-Ag-Te. The conducted studies allowed us to determine a wide variety of mineral species and significantly expand the area of distribution of Au-Te mineralization that indicates its large-scale regional occurrence in the Aldan Shield.

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