Composition,formation conditions,and genesis of the Talatui gold deposit,the Eastern Transbaikal Region,Russia

The mineral composition of the Talatui gold deposit has been studied with modern methods. Previously unknown minerals (ilmenite, siegenite, glaucodot, wittichenite, matildite, hessite, pilsenite, zircon, tremolite, cummingtonite, hercynite, and goethite) have been identified in the ore. A high Re content has been detected in molybdenite. The spatiotemporal separation of Au and Ag is caused by different mineral species of these elements and their diachronous precipitation during the ore-forming process. Gold crystallized along with early mineral assemblages, beginning from virtually pure gold (the fineness is 996). Silver precipitated largely at the end of the process as hessite (Ag₂Te) and matildite (AgBiS₂). The temperature of ore deposition varied from 610 to 145°C, the pressure was 3370–110 bar, and the salt concentration ranged from 56.3 to 0.4 wt % NaCl equiv. The heterogeneous state (boiling) of fluid at the early stages has been documented. The chemical and isotopic compositions of the fluid testify to its magmatic nature and the participation of meteoric water at late stages in the ore-forming process. Thermodynamic modeling reproduces the main specific features of ore formation, including separation of Au and Ag. A physicochemical model of the gold mineralization in the Darasun ore district has been proposed. On the basis of several attributes, the Talatui deposit has been referred to the prophyry gold-copper economic type.     

Mineralogy and metamorphism conditions of manganese ore at the South Faizulino deposit, the southern Urals, Russia

The mineralogy of slightly metamorphosed manganese ore at the South Faizulino hydrothermalsedimentary deposit in the southern Urals has been studied; 32 minerals were identified. Quartz, hausmannite, rhodochrosite, tephroite, ribbeite, pyroxmangite, and caryopilite are major minerals; calcite, kutnahorite, alleghanyite, spessartine, rhodonite, clinochlore, and parsettensite are second in abundance. This mineralic composition was formed in the process of gradual burial of ore beneath the sequence of Middle Devonian-Lower Carboniferous rocks. The highest parameters of metamorphism are T ≈ 250°C and P ≈ 2.5 kbar. The relationships between minerals and their assemblages made it possible to reconstruct the succession of ore transformation with gradually increasing temperature and pressure. Manganese accumulated in the initial sediments as oxides and a gel-like Mn-Si phase. Rhodochrosite and neotocite were formed at the diagenetic stage. In the course of a further increase in temperature and pressure, neotocite was replaced with caryopilite; ribbeite, tephroite, pyroxmangite, and other silicates crystallized afterwards. In addition to the PT parameters, the formation of various metamorphic mineral assemblages was controlled by the Mn/(Mn + Si) ratio in ore and X _CO2 in pore solution. The latter parameter was determined by the occurrence of organic matter in the ore-bearing rocks. Ore veinlets as products of local hydrothermal redistribution of Mn, Si, and CO₂ were formed during tectonic deformations in the Middle Carboniferous and Permian.     

Mineralogy and PT formation conditions of lead-zinc ore at the Dzhimidon deposit, north Ossetia

The mineralogy and PT formation conditions of the Dzhimidon Pb-Zn deposit in the Sadon ore district are considered. The deposit is localized in metamorphic rocks of the Buron Formation, which pertain to the pre-Jurassic basement (lower structural stage) and are cut through by Upper Paleozoic granitoids, and in the Lower Jurassic terrigenous sequence (upper structural stage). Orebodies as quartz-sulfide veins are mainly hosted in the metamorphic rocks. Galena, sphalerite, chalcopyrite, pyrite, pyrrhotite, and arsenopyrite are the most abundant sulfides, while quartz, carbonates, chlorite, sericite, and feldspar are gangue minerals. The bismuth mineralization identified at this deposit for the first time is represented by diverse phases of the Ag-Pb-Bi-S system. Five stages of the ore deposit formation are recognized: a premineral stage (quartz-feldspar), three ore-bearing stages (pyrite-arsenopyrite, pyrrhotite-chalcopyrite-sphalerite, and arsenopyrite-sphalerite-galena), and a postmineral stage (quartz-calcite); each stage comprises one or several mineral assemblages. The study of fluid inclusions in quartz, calcite, and sphalerite of the premineral, ore-forming, and postmineral stages has shown that the ore was deposited mainly from Na chloride solution with a salinity varying from >22 to <1.0 wt % NaCl equiv at a temperature from 460 to ～120°C and 430–290 bars pressure. The third stage was characterized by an abrupt increase in temperature and by the appearance of Mg(Fe,Ca) chloride solutions equally with Na chloride fluids, presumably owing to the emplacement of granite porphyry.     

Mineralogy and formation conditions of ores in the Bereznyakovskoe ore field,the Southern Urals,Russia

The Bereznyakovskoe ore field is situated in the Birgil’da-Tomino ore district of the East Ural volcanic zone. The ore field comprises several centers of hydrothermal mineralization, including the Central Bereznyakovskoe and Southeastern Bereznyakovskoe deposits, which are characterized in this paper. The disseminated and stringer-disseminated orebodies at these deposits are hosted in Upper Devonian-Lower Carboniferous dacitic-andesitic tuff and are accompanied by quartz-sericite hydrothermal alteration. Three ore stages are recognized: early ore (pyrite); main ore (telluride-base-metal, with enargite, fahlore-telluride, and gold telluride substages); and late ore (galena-sphalerite). The early and the main ore stages covered temperature intervals of 320–380 to 180°C and 280–300 to 170°C, respectively; the ore precipitated from fluids with a predominance of NaCl. The mineral zoning of the ore field is expressed in the following change of prevalent mineral assemblages from the Central Bereznyakovskoe deposit toward the Southeastern Bereznyakovskoe deposit: enargite, tennantite, native tellurium, tellurides, and selenides → tennantite-tetrahedrite, tellurides, and sulfoselenides (galenoclausthalite) → tetrahedrite, tellurides, native gold, galena, and sphalerite. The established trend of mineral assemblages was controlled by a decrease in $ f_{S_2 } $ f_{S_2 } , $ f_{Te_2 } $ f_{Te_2 } and $ f_{O_2 } $ f_{O_2 } and an increase in pH of mineral-forming fluids from early to late assemblages and from the Central Bereznyakovskoe deposit toward the Southeastern Bereznyakovskoe deposit. Thus, the Central Bereznyakovskoe deposit was located in the center of an epithermal high-sulfidation ore-forming system. As follows from widespread enargite and digenite, a high Au/Ag ratio, and Au-Cu specialization of this deposit, it is rather deeply eroded. The ore mineralization at the Southeastern Bereznyakovskoe deposit fits the intermediate- or low-sulfidation type and is distinguished by development of tennantite, a low Au/Ag ratio, and enrichment in base metals against a lowered copper content. In general, the Bereznyakovskoe ore field is a hydrothermal system with a wide spectrum of epithermal mineralization styles.     

Mineralogy and formation conditions of ore from the Biksizak silver-base-metal occurrence in the South Ural,Russia

The Biksizak silver-base-metal occurrence is situated in the Birgil’da-Tomino ore cluster of the East Ural Zone. The mineralization is hosted in the Silurian marble (Eastern site) and limestone interbeds in andesitic dacite (Western site). Four mineral assemblages have been established: the earliest hematite-magnetite, the subsequent pyrite-arsenopyrite and chalcopyrite-sphalerite occurring only in the Eastern site, and fahlore-chalcopyrite known only from the Western site. The closest positive correlation links Cu-Zn-Ag, Zn-Pb, Cu-Ag, and Zn-Au. The correlation between chemical elements varies depending on the localization of the ore. Correlation pairs Au-Ag, Au-Cu, Pb-Ag, and Pb-Cu are characteristic of ore from the Eastern site and are not established in the Western site. In the Eastern site Cu/(Zn + Pb) in ore is < 1, whereas in the Western site this ratio is markedly higher than unity. As follows from fluid inclusion study and mineral geothermometry, ore minerals at the Bilsizak occurrence were formed at a temperature of 300 to 150°C from low- and moderate-saline chloride fluids with 1–9 wt % NaCl equiv. The data obtained show that the Biksizak occurrence was localized at the margin of a porphyry system characterized by hydrothermal and skarn processes.     

Geochemical characteristics and origin of Yangchuling porphyry W-Mo deposit

The Yangchuling porphyry W-Mo deposit is a new-type tungsten deposit discovered recently in southern China. Being of certain economic significance, this type of deposits promises some new prospects for the availability of increasing tungsten resource. The ore-bearing rock mass, intermediate to acid in composition, is a composite rock body resulting from comagmatic, multiple-stage intrusion during Middle-Late Yenshanian Period. Spatially, the rock mass is controlled by the Guangji—Anqing—Nanjing deep fault. As a typical porphyry deposit, the ore-bearing rocks (granodiorite porphyry, mozonitic granite porphyry and camouflage breccia) belong to sub-volcanic facies characteristic of near-surface environment. The rocks have suffered strong hydrothermal alteration which is closely related to mineralization. The deposit is classified as meso-epithermal deposit and our study shows that W and Mo come from the granodiorite, granodiorite porphyry and mozonitic granodiorite porphyry, which are believed to be derived through remelting from some W-rick basement rocks contaminated with minor amounts of upper mantle materials.     

Magmatism and formation conditions of the Urma helvite-bertrandite deposit,West Transbaikal berillium province

The relationships between mineralization and magmatism during the formation of the Early Mesozoic West Transbaikal beryllium province are exemplified in the Urma helvite-bertrandite deposit. The deposit is drawn toward granitoids of elevated alkalinity, which belong to the Tashir Complex. Mineralization is related to leucogranite and characterized by patched distribution controlled by localization of metasomatic alteration. The latter is identified owing to replacement of feldspar with microcline and albite followed by silicification related to fracture zones. Helvite and bertrandite are the major Be minerals at the deposit. The Be grade of the ore is nonuniform and varies from 740 to 25000 ppm. Zircon, malacon, monazite, allanite, bastnaesite, columbite, and xenotime occur in metasomatic rocks together with Be minerals. Geochemical characteristics of alkali granites and metasomatic rocks are similar in a wide range of incompatible elements. Both are characterized by lowered Ba, Sr, P, and Eu contents and enriched in Th, U, Pb, Zr, and Hf. The degree of enrichment is the highest in the ore. The Be content in the ore correlates with concentrations of a number of other rare metals typical of host granite, which form their own mineralization against the background of metasomatic alteration, including Zr and REE minerals. Similarity in geochemistry of granitic rocks and Be ore indicates that the Urma deposit was related to the evolution of magmatic melt. Regional correlation shows that the ore-magmatic system of the Urma deposit is close to that of the Orot deposit, one of the largest in the central segment of the West Transbaikal metallogenic province. Both deposits are characterized by a similar composition of granitoids and comparable localization of ore zones in the structure of plutons. This similarity supports the high ore resource potential of Early Mesozoic alkali granites in the western Transbaikal region. Taking into account that these granitoids are widespread in the West Transbaikal Rift Zone that controls the metallogenic province, one can expect the discovery of new deposits therein.     

Mineralogy of metamorphosed carbonatite of the Vesely occurrence, Northern Transbaikal region, Russia

Metamorphism of carbonatite is exemplified in the Vesely occurrence. According to available data, the age of the carbonatite is 596 ± 3.5Ma, whereas metamorphism is dated at 550 ± 14 Ma. The rocks at the Vesely occurrence were metamorphosed under conditions of greenschist facies (epidote-muscovite-chlorite subfacies) under elevated pressure. Microthermometry of fluid inclusions in minerals indicates that the temperature of metamorphism is 377−450°C and the pressure estimated from phengite geobarometer is 6−8 kbar. The low-grade metamorphism led to the partial recrystallization of carbonates and apatite with removal of trace elements. This process resulted in a change of the oxygen isotopic composition of the studied minerals. Metamorphism was accompanied by formation of talc, phengite, chlorite, quartz, tremolite-actinolite, and anthophyllite, which are not typical of carbonatite. The data obtained show that the metamorphism exerted an effect on the mineralogical, isotopic, geochemical, and technological properties of the carbonatite. The effect of metamorphism should be taken into account in determination of the nature of ore mineralization and estimation of ore quality and perspective of the occurrence.     

Some secondary ore formation features of the Sar-Cheshmeh porphyry copper-molybdenum deposit,Kerman, Iran

New secondary mineral features and the control of secondary molybdenum enrichment have now been recognized at the Sar-Cheshmeh copper deposit. In part of the deposit, copper is locally concentrated within a subvolcanic unit — the so-called Late Fine Porphyry in amounts that seem too great to have come from the primary phase of the same unit. Clay minerals are believed to have influenced the deposition of copper by through-flowing oxidized zone solutions within this otherwise subeconomic unit. These solutions have caused argillic alteration and copper concentration within the plagioclase phenocrysts. Fractures are filled with chrysocolla, but little chrysocolla was observed within the altered plagioclase phenocrysts. The scattered layers of sedimentary unroofing breccia — horizontal layers of Quaternary age accumulated within the depressions at the top of the deposit — contain minor amounts of native copper. The breccia consists of angular to subangular clasts of local derivation cemented by a matrix of hematite, limonite, geothite, and small rock fragments. Native copper, the only copper mineral associated with the breccia, mainly adhers to the cavity walls in the cementing materials. The amount of molybdenum within the leached capping of the Sar-Cheshmeh deposit is generally controlled by the amount of pyrite. Abundant pyrite has caused the retention of molybdenum within the leached capping of the pyritic halo as a result of the more acidic solution during weathering, in contrast to the leached zone of the pyrite-poor stock, where molybdenum is partially mobile.     

Geology and genesis of the Cheremshanka silica deposit, western Transbaikal region, Russia

The geology and genesis of a large high-grade silica deposit is considered. It occurs in the form of a quartzite layer, 20–50 m thick, extending for 8 km in conformity with the host Upper Proterozoic silicate-carbonate metasedimentary rocks. The average content of SiO₂ is 99.2%. It has been established that quartzite was formed by metasomatic silicification of sandstone during metamorphism of the carbonate-silicate sequence. The rocks were silicified by infiltration acid leaching, whereas long-term refinement of quartzite was provided by diffusion in finely dispersed capillary-porous systems, where the energy of the solution-solid phase interface was important. In the course of metasomatic migration of components, Au, Ag, Pb, Zn, Fe, and other elements were removed from quartzite and formed gold-sulfide mineralization in contact zones of the quartzite body. This opens up opportunities for discovering economic Au-Ag and Pb-Zn ores in the ore field.     

Mineralogy,geochemistry, and ore formation of Nuwaifa bauxite deposit,western desert of Iraq

Nuwaifa Formation is a part of sequence stratigraphy that belongs to the Jurassic system exposed in the western desert of Iraq. The Jurassic system consists of Ubaid, Hussainiyat, Amij, Muhaiwir, and Najmah formations. Each formation is composed of basal clastic unit overlain by upper carbonate unit. Nuwaifa karst bauxite was developed in fossil karsts within the Ubaid Formation in areas where maximum intersection of fractures and faults exist. This bauxitization process affected the upper surface of the Ubaid limestone formation, which directly underlies the Nuwaifa bauxite Formation. Nuwaifa Formation represents karst-filling deposit that consists of a mixture of allochthonous (sandstone, claystone, and mudstone) and autochthonous lithofacies (bauxite kaolinite, kaolinitic bauxite, iron-rich bauxite, and flint clay). Most bauxite bodies occur within the autochthonous lithofacies and are lenticular in shape with maximum thickness ranges from few meters to 35 m and in some place up to 100 m. Petrographically, the bauxite deposit exhibits collomorphic-fluidal, pisolitic, oolitic, nodular, brecciated, and skeletal textures indicative of authigenic origin. Mineralogy boehmite and gibbsite are the only bauxite minerals; the former is dominant in the upper parts of the bauxite profiles, whereas the latter is dominant throughout the lower and middle part of the bauxite. Kaolinite, hematite, goethite, calcite, and anatase occur to a lesser extent. The study bauxites are mainly composed of Al₂O₃ (33–69.6 wt.%), SiO₂ (8.4–42 wt.%), Fe₂O₃ (0.5–15.9 wt.%), and TiO₂ (0.7–6.1 wt.%) with LOI ranging from 13.5 to 19.1 wt.%. Geochemical investigations indicate that the immobile elements like Al₂O₃, TiO₂, Cr, Zr, and Ni were obviously enriched, while SiO₂, Fe₂O₃, CaO, MgO, Zn, Co, Ba, Mn, Cu, and Sr were depleted during bauxitization process. The results of this study strongly suggest that the bauxite deposits of the Nuwaifa Formation are derived from the kaolinite of the Lower Hussainiyat Formation.     

Age of granodiorite porphyry and beresite from the Darasun gold field,eastern Transbaikal region,Russia

The Darasun ore field situated in the southern West Stanovoi Terrane near the Mongolia-Okhotsk Suture comprises the Darasun (>100 t Au), Talatui (～38.2 t Au), and Teremki (3 t Au) lode gold deposits. In the opinion of many researchers, the Darasun deposit is spatially and paragenetically linked to granodiorite porphyry of the Amudzhikan Complex and related metasomatic rocks (beresites). Whole-rock samples of granodiorite porphyry, monomineralic fractions of plagioclase, K-feldspar, and biotite, as well as sericite from beresite (26 samples in total), were analyzed by the Rb-Sr method. Eight biotite and sericite samples were analyzed by the K-Ar method. The Rb-Sr mineral isochrons obtained for individual granodiorite porphyry samples yielded initial ⁸⁷Sr/⁸⁶Sr ratios varying from 0.70560 to 0.70591. The consistent results of both methods allowed us to accept the ages of granodiorite porphyry and beresite as 160.5 ± 0.4 and 159.6 ± 1.5 Ma, respectively. The age of granodiorite porphyry of the Amudzhikan Complex of 160.5 ± 0.4 Ma corresponds to the boundary between the Early and Middle Jurassic and marks the completion of collision between the East Siberian and Mongolia-China continents and related orogeny. Since that time, the eastern Transbaikal region has been involved in the postorogenic (within-plate) stage of evolution, characterized by the formation of large gold, uranium, and other ore deposits.     

Geochemical peculiarities of the formation of the Novoshirokinskii gold-polymetallic deposit,Eastern Transbaikalia,Russia

The fluid inclusions in minerals and isotope composition of sulfur in sulfides and carbon and oxygen in carbonates are studied for the Novoshirokinskii gold-polymetallic deposit. The ore-forming fluids are characterized by the following physico-chemical and isotope-geochemical parameters: temperature of 290–100°C, salinity of 13–2.5 wt % NaCl-equiv., δ¹⁸O from +8 to 0‰, δ¹³C of 2.5 ± 0.5‰, and δ³⁴S of 10.5 ± 1.0‰. It is concluded that the Late Proterozoic-Early Cambrian carbonaceous-terrigenous and carbonate rocks were involved in the Late Jurassic ore-magmatic system.     

Gold-telluride mineralization of the Western Chukchi Peninsula, Russia: Mineralogy, geochemistry, and formation conditions

Mineralogy, geochemistry, and formation conditions of the Sentyabr’sky prospect—the first economic occurrence of Au-Te mineralization in the Chukchi Peninsula—was studied. Gold occurs in native form and as telluride compound (petzite). Petzite and hessite are the major ore minerals of the gold-telluride assemblage; native gold is superimposed on them. Altaite, coloradoite, and paratellurite have been also identified. The study of fluid inclusions in sphalerite and quartz associated with ore minerals shows that the Au-Te mineralization of the Sentyabr’sky prospect and the low-sulfide Au-Ag ore of the adjacent Dvoinoi deposit were formed from different fluids and under different conditions. The multistage hydrothermal process developed in the frames of long-living porphyry-epithermal system functioned in the Ilirnei district. The Au-Te mineralization at the Sentyabr’sky prospect is related to alkaline magmatism. The high-salinity (above 5 wt % NaCl equiv) fluid inclusions in hydrothermal quartz can be indicators of such mineralization. Mineralogy and geochemistry of ore at the Sentyabr’sky prospect provide evidence for its deposition at the middle level of porphyry-epithermal system and testify to prospectivity of deep levels.