V. A. Kotlyarov

Noble-metal mineralization in the Semenov-2 hydrothermal field (13°31′N), mid-atlantic ridge

The porous fine-grained to microcrystalline copper-zinc ore of the Semenov-2 hydrothermal field, a site in the Semenov hydrothermal cluster discovered in 2007 (13°31′N, MAR), is anomalously enriched in Au (22–188 ppm) and Ag (127–1787 ppm). Chalcopyrite, isocubanite, würtzite, and opal are major minerals; sphalerite, marcasite, pyrite, and covellite are auxiliary; and galena, pyrrhotite, native gold, silver telluride, barite, and aragonite are sporadic. Gold containing 0.31 to 23.07 wt % Ag occurs as up to 9-μm-sized subhedral, dendritelike, and elongated grains mostly hosted in opal and less frequently in sphalerite and in pores within isocubanite-chalcopyrite aggregate. An elongated grain (2 × 4 μm in size) of the Ag-Te phase was found in a pore. So far only basalts have been dredged from the Semenov-2 field, but anomalous gold and silver concentrations suggest the influence of ultramafic rocks; the latter were found 1.5 km westward, in the Semenov-1 hydrothermal field. Mineral assemblage and morphology of gold particles indicate its primary origin in contrast to the hydrothermal fields hosted in basalts, where gold is a product of remobilization. Zonal gold grains, found on oceanic floor for the first time, are characterized by low Ag content in the cores and high Ag content in the outer rims, reflecting variation in formation conditions.

Chemical evolution of pyrite at the Kopylovsky and Kavkaz black shale-hosted gold deposits, Bodaybo district, Russia: Evidence from EPMA and LA-ICP-MS data

The Kopylovsky and Kavkaz gold deposits in the Artemovskiy ore cluster, Bodaybo district, hosted in terrigenous carbonaceous sequence of the Dogaldyn formation of the Upper Proterozoic Bodaybo group, are localized in cores of high-order anticlines. These deposits contain gold-sulfide and gold-sulfide-quartz ore types. Pyrite is the predominant ore mineral; copper, zinc, nickel, and cobalt sulfides are minor. Native gold is found as free grains, and inclusions and ultrafine stingers in pyrite. A few morphological varieties of pyrite are identified: (1) framboidal and fine-grained pyrite resulting from sedimentation and diagenesis; (2) fine-grained subhedral to euhedral pyrite corresponding to catagenesis, metamorphism, and dynamometamorphism; and (3) coarse-crystalline euhedral gold-free pyrite occurring in the host rocks beyond ore bodies and deposits. According to electron microprobe and LA-ICP-MS data, the contents of Au, Ag, Co, Pb, Sb, Bi, Ba, Mo, and Tl decrease from sedimentary-diagenetic to metamorphic pyrite, while Ni, Cu, As, Se and especially U contents increase in the same direction. Primary gold is suggested to have accumulated during sedimentation as species associated with carbonaceous matter and cogenetic pyrite and redeposited as a result of catagenesis and metamorphism as native species with crystallization of the latest pyrite and associated sulfides of base metals.

Banded sulfide-magnetite ores of Mauk copper massive sulfide deposit, Central Urals: Composition and genesis

The results of investigation of metamorphosed sulfide-magnetite ores from the Mauk deposit located within the Main Ural Fault at the junction of Tagil and Magnitogorsk massive sulfide zones are discussed. The ore-hosting sequence comprises metamorphic rocks formed from basalt, carbonaceous and carbonaceous-cherty siltstone, and lenticular serpentinized ultramafic bodies. The ores of the deposit are represented by banded varieties and less frequent breccia. The clastic origin of the banded ore is indicated by load casts at the bottom of sulfide beds, alternation of sulfide and barren beds, and the truncation of the growth zones of pyrite crystals. Pyrite, pyrrhotite, chalcopyrite, sphalerite, and magnetite are the major minerals of the banded ores. The internal structure of the listed minerals testifies to the deep metamorphic recrystallization of primary hydrothermal-sedimentary ores accompanied with deformation. Cubanite, pyrrhotite, mackinawite, greigite, and gold are enclosed in metacrysts of pyrite, magnetite, and chalcopyrite. The accessory minerals of the Pb-Bi-Te, Bi-Te, and Ag-Te systems as well as uraninite have been found at the Mauk deposit for the first time. Magnetite predominantly replaces pyrite and less frequently chalcopyrite, pyrrhotite, and gangue minerals. It was established that the major carriers of As and Co are crystals of metamorphic pyrite. Chalcopyrite is the major carrier of Zn, Sn, Te, Pb, Bi, and Ag. Admixture of Fe and Cu is typical of sphalerite, and Se and Ni are characteristic of pyrrhotite. Ti, V, Mn, Sb, As, Ba, and U are concentrated in magnetite. The banded ores of the Mauk deposit are suggested as having been transformed in several stages: diagenesis, anadiagenesis, epidiagenesis (t < 300°C), and amphibolite facies metamorphism (t > 500°C).

Native bismuth, tsumoite, and Pb-bearing tsumoite from the Tarn’er copper-zinc massive sulfide deposit, northern Urals

Bismuth mineralization, including native bismuth, tsumoite (Bi1.99–2.03Te2.00), and Pb-bearing tsumoite (Bi1.56–1.88Pb0.45–0.14)2.00–2.03Te2.00, was identified in the Au-enriched disseminated ore at the Tarn’er massive sulfide deposit formed under the effect of a large diorite intrusion. Native bismuth associated with hessite forms idiomorphic inclusions in chalcopyrite. The assemblage of Pb-bearing tsumoite, hessite, and altaite occurs as angular allotriomorphic-granular inclusions in silicates or at the contact between silicate and sulfide aggregates. Tsumoite in allotriomorphic-granular aggregates with galena, hessite, and sphalerite is devoid of lead. Gold (Au0.65Ag0.35) was identified along with bismuth tellurides. The temperature of contact methamorphism (500–800°C) was estimated from the stability of andalusite, sillimanite, and cordierite. The morphology of the bismuth telluride aggregates in silicates and graphic intergrowth of tsumoite with galena suggest possible crystallization from anatectic melt. The positive correlation between Bi, Te, and Au confirms their probable joint transportation in the melt.

Mineralogical and Geochemical Features of Oolitic Ironstones from the Sinara–Techa Deposit, Kurgan District, Russia

The paper discusses the mineralogical and geochemical features of oolitic ironstones from the Sinara–Techa deposit, Transural region, Kurgan district. The ore unit is localized in the lower part of a thick Mesozoic–Cenozoic sequence of sedimentary rocks that fill the West Siberian Basin beneath calcareous clay and overlying beds enriched in glauconite and clinoptilolite. The ironstone consists of goethite ooids in smectite–opal cement. Accessory minerals are pyrite, galena, sphalerite, and monazite. The texture and structure make it possible to suggest the formation of sediments enriched in iron as a result of colloid coagulation. The most probable source of iron is related to inland drift. Deposition of iron took place in the estuaries of subtropical rivers due to mixing of colloidal solution of river water with seawater electrolyte. The chemical features of rocks are controlled by the composition of the adsorbed iron oxi/hydroxide complex.

Chrome spinels and accessory mineralization in the weathering crust of the Vladimir deposit, Varshavsky ultramafic massif, southern Urals

The paper presents the characteristics of chrome spinels from an ore-bearing packet of the Vladimir chromite deposit. Three main types of chrome spinels are distinguished by morphology and chemical composition: medium-chrome ore-forming, high-chrome transformed, and low-chrome relict accessory. The significant role of weathering conditions is expressed in alteration of accessory chrome spinel. The formation of high-chrome spinels is explained by the hydrothermal effect of the Varshavsky granitoid massif with accompanying dikes and talc–carbonate metasomatic rocks. Characteristic accessory minerals are represented by native gold and nickel, millerite, pentlandite, chalcopyrite, maucherite, PGE sulfides, and picroilmenite.

New data on REE and rare-metal mineralization in pegmatites of the Slyudyanogorsk muscovite deposit in the Southern Urals

The Slyudyangorsk muscovite deposit in the southern Urals was explored and mined in 1926–1957. By the mid-1950s, 104 veins of quartz–feldspar pegmatites including 21 muscovite-bearing veins have been found. Pegmatites with giant black Y-bearing epidote crystals are crosscut by veins with giant muscovite crystals, which, in turn, are intersected by veins of two-mica–quartz–two-feldspar pegmatites with rare-metal and REE mineralization. Microprobe data on compositions of complex Ti–Ta–Nb oxides [fergusonite-(Y), samarskite-(Y), euxenite-(Y), polycrase-(Y), columbite-(Fe), pyrochlore supergroup] are characterized, as well as of uraninite, ilmenorutile, scheelite, Y-bearing epidote, certain sulfides and rock-forming minerals from the Slyudyanogorsk deposit. The morphology and interrelation of minerals indicate that they are the result of crystal growth in cavities rather than of metasomatic replacement of gneisses, as has been suggested earlier. Thus, it is more promising for rare-metal and REE minerals in the Slyudorudnik area to be found in igneous rocks (granitic muscovite–quartz–feldspar pegmatites with the Nb–Ta–Ti–Y–U–W–Mo mineralization) than in metasomatic rocks.

Silver sulfoselenide from ore of the Valunisty Au-Ag deposit, Chukchi Peninsula

Silver sulfoselenide (Ag,Cu)9Se2S4 from ore of the Valunisty Au-Ag deposit on the Chukchi Peninsula is described for the first time. The mineral occurs in mineralized quartz-adularia veins, where it associates with chalcopyrite, sphalerite, galena, and electrum and replaces arsenpolybasite. It forms anhedral grains up to 0.2 mm in size. The reflectivity of sulfoselenide is moderate, no anisotropy is observed, and the microhardness is very low. The chemical composition of the mineral differs from other known members of the Ag-Se-Se-S system: the S/Se ratio of the mineral is 2/1 and the (Ag + Cu)/(Se + S) ratio is about 1.5.

Mineralogy of oxidized ores at the Ik-Davlyat gold-base-metal deposit, the southern Urals

Three types of oxidized ores are identified in the Ik-Davlyat gold-base-metal deposit in the southern Urals: (1) carbonate-sericite-chlorite mineralized rock, (2) vein-shaped quartz-goethite-illite clay, and (3) limonitized rock related to veins. Heavy concentrate of the first type of ore is composed of goethite, rutile, native gold Au0.91Ag0.08Cu0.01, and chalcophanite Zn1.02Mn2.98O4 · 3H2O. The second type of ore contains goethite, rutile, Pb-bearing jarosite, native gold Au0.90−0.93Ag0.06−0.08Cu0−0.01Fe0−0.01, silver amalgamide (schachnerite) Ag0.75Hg0.97Au0.98-Ag0.75Hg0.97Au0.28, coronadite (Pb1.72Mn7.51Fe0.41Cu0.36)8O16, a chalcophanite-hydrohetaerolite mixture, and cerussite. Gold of the highest fineness (Au0.98Ag0.01Cu0.01) is associated with silver amalgamide. The third type of ore is quite similar to the first variety but contains a jarosite impurity. The composition of oxidized ores indicates a difference in composition of primary ores, in particular, the presence of lead minerals in primary veins. The first finding of chalcophanite in Russia is confirmed by chemical, optical, and X-ray data.