High-alumina titanite in mineral assemblages of the Berezitovy gold-base-metal deposit, Upper Amur Region

Grothite—a rare Al- and F-rich variety of titanite—was identified in two different gold-bearing mineral assemblages of the Berezitovy gold-base-metal deposit, Upper Amur Region, Russian Far East. Grothite is associated with quartz, orthoclase, chlorite, muscovite, tourmaline, almandine-spessartine garnet, ilmenite, pyrophanite, magnetite, fluorapatite, and sulfides. Grothite forms numerous scattered lamellar aggregates 20–100 μm in size with a relatively homogeneous structure. The lamellae grow in chlorite or between chlorite and orthoclase. According to microprobe analyses, variations in major elements of grothite are as follows (wt %): 30.56–34.07 SiO₂, 7.91–12.71 Al₂O₃, 22.83–28.29 TiO₂, 23.55–29.21 CaO, 0.52–4.25 FeO, and 2.19–6.16 F. It is suggested that grothite appeared in the gold-bearing mineral assemblages of the Berezitovy deposit due to the specifics of the primary composition of the host rocks and physicochemical conditions of the fluid regime at the final stage of deposit formation.     

Ore-bearing duplexes at the Malomyr orogenic gold deposit, Amur Region

This study made it possible to reconstruct three stages of the structure formation at the large Malomyr gold deposit in the Amur Region: (1) premineral folding and left-lateral strike-slip faulting; (2) synmineral thrust faulting, and (3) postmineral dike and neotectonic stage. The study has shown that the formation of the Malomyr deposit was related to the system of duplexes as the structural traps that are the most favorable for ore deposition. The left-lateral duplex hosts the major orebodies composed of refractory ore at the Central site of the deposit, whereas the highest-grade and readily processable ore from the Quartzitic site is localized in the right-lateral duplex. The development of both duplexes during the same stage of thrust faulting indicates that readily processable and refractory ores were formed almost simultaneously. The type of mineralization depends on structural conditions of ore deposition. Refractory ore with invisible gold was formed under compression, whereas the readily processable ore with free gold was deposited in the extension setting.     

The Dzhalinda Deposit in the Amur Region,Russia: Genesis and position in the classification of tin deposits

The Dzhalinda wood tin deposit is located at the eastern margin of the Bureya Massif and, according to the Russian classification of tin deposits, is referred to the rhyolite-hosted type. The Dzhalinda deposit is compared with the deposits located in the southwestern United States and in northern Mexico. In spite of some similar features, the Dzhalinda deposit differs significantly from the American deposits in the composition of ore-bearing rhyolite and the type and composition of host rocks. It is suggested that the ore-bearing rhyolitic melt at the Dzhalinda deposit evolved with the formation of a highly silicic residual melt depleted in P and Li and enriched in Sn, being opposed in this respect to the ongonite model typical of such deposits. The drastic change in the physicochemical parameters of the system caused by the evolution of the melt under near-surface conditions of a volcanic vent led to the separation of Si-Sn complexes, which broke down into various silica modifications and oolite-like wood tin.     

Feldspathic metasomatic rocks from the Lermontovo deposit,Primorye, Russia

Drill cores of metasomatic rocks and ores from the Lermontovo skarn scheelite-sulfide deposit (Lermontovo ore field, Primorsky krai) have been studied. Feldspar-apatite-scheelite-sulfide metasomatic rocks containing more than 10% WO₃ have been identified and characterized. The relationships of these altered rocks to skarn, greisen, and sulfide ore are shown and the implications of these rocks for the formation of ore are determined. In addition, some genetic problems of the deposit are discussed.     

Genesis of gold and silver sulfides at the Ulakhan deposit (northeastern Russia)

Geology and mineralogy of the Ulakhan Au-Ag epithermal deposit (northeastern Russia, Magadan Region) are considered. A four-stage scheme of mineral formation sequence is proposed. Concentrations of Au and Ag in minerals of early and late parageneses were determined. It has been established that uytenbogaardtite is associated with native gold and hypergenesis stage minerals — goethite, hydrogoethite, or limonite replacing pyrite. The compositions of uytenbogaardtite (Ag₃AuS₂), acanthite (Ag₂S), and native gold were studied. The composition of the Ulakhan uytenbogaardtite is compared with those of Au and Ag sulfides from other deposits. Thermodynamic calculations in the system H₂O–Fe–Au–Ag–S–C–Na–Cl were carried out, which simulate the interaction of native gold and silver with O₂- and CO₂-saturated surface waters (carbonaceous, sulfide-carbonaceous, and chloride-sodium-carbonaceous) in the presence and absence of acanthite and pyrite at 25 °C and 1 bar. In closed pyrite-including systems, native silver and kustelite are replaced by acanthite; electrum, by uytenbogaardtite, acanthite, and pure gold; and native gold with a fineness of 700–900‰, by pure gold and uytenbogaardtite. Under the interaction with surface waters in the presence of Ag₂S and pyrite, Au-Ag alloys form equilibrium assemblages with petrovskaite or uytenbogaardtite and pure gold. The calculation results confirmed that Au and Ag sulfides can form after native gold in systems involving sulfide-carbon dioxide solutions (H₂Saq > 10^–4 m). The modeling results support the possible formation of uytenbogaardtite and petrovskaite with the participation of native gold in the hypergenesis zone of epithermal Au-Ag deposits during the oxidation of Au(Ag)-containing pyrite, acanthite, or other sulfides.     

Calcic hydrosilicate metasomatic rocks at the Gumeshevsk skarn-porphyry copper deposit in the central Urals,Russia

The body of hydroxylellestadite metasomatic rock penetrated by a borehole drilled at the Gumeshevsk deposit at depths of 530–534 m includes a thin interval of younger lower temperature tobermorite-plombierite metasomatic rock with subordinate amounts of Ca-Si gel, tacherenite, cubic lime, and thaumasite. Hydroxylellestadite has never before been found in calc skarns. The hydroxylellestadite metasomatic rock is cut by gypsum and fukalite veinlets, and the tobermorite-plombierite metasomatic rock is intersected by thaumasite veinlets. The pristine rock of the metasomatics was marble, and the metasomatic rock replaced andradite-bearing wollastonite skarn (with wollastonite replaced by foshagite). The ore minerals (chalcopyrite, valleriite, sphalerite, and others) were formed after the hydroxylellestadite metasomatite but most probably before the tobermorite-plombierite metasomatic rock and the veinlets of calcic minerals. The metasomatic rock was produced at significant variations in the oxygen, sulfur, and carbon dioxide fugacities. The composition of the hydroxylellestadite is, according to its microprobe analysis, as follows (wt %): SiO₂ 17.10, TiO₂ 0.01, Al₂O₃ 0.02, FeO 0.20, MnO 0.00, MgO 0.04, CaO 55.40, Na₂O 0.14, K₂O 0.09, P₂O₅ 0.12, CO₂ 1.90 (chemical analysis), SO₃ 21.60, F 0.16, Cl 0.14, total 96.92. The plombierite (SiO₂ 43.8–44.1 wt %, CaO 30.5–31.1 wt %) in the metasomatic rock notably differs from rare plombierite (SiO₂ 48.18 wt %, CaO 39.19 wt %) contained in the veinlets of thaumasite (SiO₂ 12.70 wt %, CaO 30.69 wt %, SO₃ 17.78 wt %).     

Oxygen isotopic composition of quartz veins and host rocks at the Sukhoi Log deposit,Russia

The relationships between the δ¹⁸O of quartz veins and veinlets pertaining to the main stage of gold mineralization at the Sukhoi Log deposit and metasomatically altered host slates are estimated. The oxygen isotopic composition of veined quartz and host slates is not uniform. The δ¹⁸O of quartz veins from the Western, Central, and Sukhoi Log areas of the deposit vary from +16 to + 18 ‰. The δ¹⁸O range of metasomatically altered slates in the Western and Sukhoi Log areas attains 6 ‰. The δ¹⁸O of quartz veins are always higher than those of host slates by 3–7‰. The regular difference in the δ¹⁸O between quartz veins and host slates indicates that the oxygen isotopic composition of the ore-bearing fluid forming the system of quartz veins and veinlets at the Sukhoi Log deposit could have formed as a result of interaction with silicate rocks, for instance, terrigenous slates enriched in δ¹⁸O. Such interaction, however, took place at deeper levels of the Sukhoi Log deposit. It is suggested that the fluid phase participating in the formation of the vein and veinlet system had initially high δ¹⁸O(>+10‰) due to interaction with the rocks enriched in δ¹⁸O at a low fluid/rock ratio. The oxygen isotope data indicate that the fluid participating in the formation of gold mineralization at the Sukhoi Log deposit was not in equilibrium with igneous rocks at high temperatures.     

Geoarchaeological investigations at Studenoe,an Upper Paleolithic site in the Transbaikal Region,Russia

In 1998, geoarchaeological research was carried out at the Upper Paleolithic site of Studenoe in the Transbaikal Region of Russia. The site is divided into three loci situated on two terraces overlooking the Chikoi River. Alluvial sediments beneath the oldest terrace (T₂) consist of two depositional units. Radiocarbon ages indicate that aggradation of the T₂ fill began before 18,000 yr B.P. Alluvial sediments beneath the lower terrace (T₁) range in age from 13,000 to 10,000 yr B.P. and are divided into three depositional units. Both terraces are overlain by Holocene colluvium. Archaeological materials at Studenoe 1/1, 1/2, and 2 include dwellings, hearths, and thousands of bone and stone artifacts assigned to the late Upper Paleolithic through the Bronze Age. Evidence of microblade technology is present in all components of the site. Material from recent excavations of Paleolithic levels in the T₂ fill at Studenoe includes mobiliary art, bone needles, and a large dwelling with four hearth features. © 2003 Wiley Periodicals, Inc.     

Specific genesis of gold and silver sulfides at the Yunoe deposit (Magadan Region,Russia)

Hypogene uytenbogaardtite, acanthite, and native gold parageneses have been revealed at the epithermal Yunoe gold-silver deposit, Magadan Region, Russia. Thermodynamic calculations in the system Si–Al–Mg–Ca–Na–K–Fe–Pb–Zn–Cu–Ag–Au–S–C–Cl–H₂O were carried out at 25–400 °C and 1–1000 bars to elucidate the role of hydrothermal solutions in the formation of gold and silver sulfides. Several most probable scenarios for ore-forming processes in the deposit are considered: (1) interaction between cold and heated meteoric waters percolating along cracks from surface to depth and reacting with the host rock—rhyolite; (2) evolution of ascending postmagmatic fluid resulting in chloride–carbonic acid solution, which interacts with rhyolite at 100–400 °C; (3) stepwise cooling of hydrothermal ore-bearing solutions; (4) rapid cooling of ore-bearing hydrotherms on their mixing with cold surface waters. Rhyolite with Pb, Zn, Cu, Cl, S, Ag, and Au clarke contents was taken as an initial host rock. Calculations by model 3 showed the possible formation of uytenbogaardtite and petrovskaite at low-temperature stages. Gold and silver sulfides can be deposited during the mixing of ore-bearing acid chloride–carbonic acid hydrothermal solutions with surface alkaline waters.     

The Yermakovsky deposit,Western Transbaikal Region,Russia: Isotopic and geochemical parameters and sources of beryllium-bearing granitoids and other rocks

An isotopic study of igneous and metamorphic rocks has been carried out at the Yermakovsky bertrandite-phenakite-fluorite deposit. It has been established that the model age of the schists pertaining to the Zun-Morino Formation is 1360–1260 Ma. In Nd and Sr isotopic composition, these schists deviate from the isotopic composition of the continental crust and are close in this respect to the enriched mantle reservoir (EM-II). The model age of carbonate rocks of the Zun-Morino Formation is 1330–1020 Ma. The Middle Riphean model age of the Zun-Morino Formation is interpreted as the age of its protolith. According to the Sr and Nd isotopic data, all preore igneous rocks (granitic dikes, gabbroic rocks, and gneissose granite of the Tsagan Complex) were formed with the participation of continental crustal material. Synore basic dikes, alkali leucogranite stock, and syenite intrusion are considered to be mixtures of mantle components (DM+HIMU) and various continental crustal components (Tsagan gneissose granite, crystalline schists, the mean composition of granitoids of the Angara-Vitim batholith as an estimate of average composition of the regional continental crust). Synore igneous rocks are genetically cognate and related to the magmatic activity in the Western Transbaikal Rift Zone presumably formed in the Triassic under effect of a mantle plume.     

Physicochemical crystallization conditions of Al-F sphene in metasomatic rocks with ore mineralization at the Berezitovoe Deposit

Al-F sphene (grothite) was found in mineralized rocks at the Berezitovoe Deposit in the Russian Far East. The paper is devoted to the mineral assemblages and composition of the mineral and its thermodynamic crystallization conditions. The average Al and F concentrations (p.f.u., microprobe data) in the grothite are 0.45 and 0.42 in sample 1374, 0.32 and 0.32 in sample 1306, and 0.35 and 0.33 in sample 96. Grothite was found in the rocks in association with chlorite, ilmenite (pyrophanite), and magnetite, and this mineral assemblage was obviously overprinted on the primary garnet-biotite assemblages. We estimated the temperature of grothite crystallization at 400–500°C. With regard for available experimental data on the mineral equilibrium between Al-F sphene, fluid, and anorthite, a tool is proposed for evaluating F concentrations in fluids by the equilibrium of Al-F sphene with plagioclase, rutile, and F-bearing aqueous fluid. Our model simulations indicate that the maximum F concentration in fluid during the crystallization of Al-F sphene richest in F at the temperatures and pressures of metasomatic rocks at the Berezitovoe deposit could reach 300–500 mg per kg of the aqueous solution. The level of F concentration in the fluid during the crystallization of Al-F sphene at the deposit is comparable with the F concentration in fluid during the development of greisens and rare-metal pegmatites, but these high F concentrations were reached only during the final evolutionary stages of the deposit.     

Mineral composition of uranium ore at the Dalmatovo deposit,Russia

The mineral composition of hydrogenic uranium ore of the Dalmatovo deposit was studied with analytical scanning electron microscopy. The results correspond to earlier known data only in general terms. Phosphosilicate uranium mineralization, which is predominant in the samples, is similar to P-bearing coffinite in elemental composition but differs in morphology. The quantitative analysis of microcrystals corresponds to the formula (U,Ca)[(Si,P)O₄]₂, where U/Si ratio is twice as low as in coffinite. The occurrence of oxide pitchblende mineralization has been confirmed. The initial stage of the formation of uranyl minerals has been revealed. The mineral species of Ti-U substance that determines geochemical attributes of the Dalmatovo deposit is considered.     

Wall-Rock alteration of biotite hornfels at the Tyrnyauz deposit,Russia

A new zonation type of W–Mo-bearing altered biotite hornfels at the Tyrnyauz deposit is reported. The review of zonation indicates a subsequent transition into the mobile state of CaO, MgO, FeO, and Al₂O₃ and retention of volume owing to dissolution or deposition of quartz as an excess mobile mineral. The main features of zonation are similar to those in acid leaching columns; the input of strong CaO base into the outer zone is unusual.     

The Yermakovsky beryllium deposit,Western Transbaikal region,Russia: Geochronology of igneous rocks

The sequence of rock and ore formation at the Yermakovsky beryllium deposit is established on the basis of geological relationships and Rb-Sr and U-Pb isotopic dating. The Rb-Sr age of amphibolitefacies regional metamorphism is determined for quartz-biotite-plagioclase schist (266 ± 18 Ma) and dolomitized limestone (271 ± 12 Ma) of the Zun-Morino Formation. The U-Pb zircon age of premineral gabbro is 332 ± 1 Ma. The Rb-Sr age of gabbro is somewhat younger (316 ± 8.3 Ma), probably owing to the effect of Hercynian metamorphism on sedimentary rocks of the Zun-Morino Formation and gabbroic intrusion that cuts through it. The U-Pb zircon age of gneissose granite of the Tsagan Complex at the Yermakovsky deposit is 316 ± 2 Ma, i.e., close to the age of metamorphism superimposed on gabbro rocks. The U-Pb zircon age of preore granitic dikes, estimated at 325 ± 3 and 333 ± 10 Ma, is close to the age of gabbro. The Ar/Ar age of amphibole from a granitic dike (302.5 ± 0.9 Ma) probably displays a later closure of this isotopic system or the effect of superimposed processes. The Rb-Sr age of alkali syenite intrusion is 227 ± 1.9 Ma. The U-Pb zircon age of alkali leucogranite stock pertaining to the Lesser Kunalei Complex is 226 ± 1 Ma, while the Rb-Sr age of beryllium ore is 225.9 ± 1.2 Ma. These data indicate that beryllium ore mineralization is closely related in space and time to igneous rocks of the Lesser Kunalei Complex dated at 224 ± 5 Ma and varying from gabbro to alkali granite in composition. Thus, the preore Hercynian magmatism at the Yermakovsky deposit took place ∼330 Ma ago and was completed by metamorphism dated at 271–266 Ma. The ore-forming magmatism and beryllium ore mineralization are dated at 224 ± 5 Ma. Postore magmatic activity is scarce and probably correlated with tectonic melange of host rocks.     

Relict sulfides in terrigenous rocks of the Upper Kolyma Region

In terrigenous flysch strata of the Upper Kolyma Region in northeastern Russia, microaggregates of sulfides with spherical form are widespread. During dynamic metamorphism, spherical sulfide units were segregated into lenticular aggregates along the cleavage planes, were as seed, or were recrystallized into faceted metacrystals. Further stress on the rock sometimes led to a morphologically expressed particular dissolution of pyrite crystals along the cleavage directions, and to their depletion with admixture elements. A more substantial effect led to transformation of pyrite into pyrrotine with inclusions of chalcopyrite and Fe-Ni-Co-sulfoarsenides. Relict sulfides reflect, to a certain degree, the primary geochemical condition of the sedimentation period and its further evolution. When studying the numerous sulfidization zones, the composition of relict sulfides allows us to predict the geochemical specialization and the degree of inheritance of post-sedimentation mineralization.     

弓长岭铁矿蚀变岩及富矿成因

弓长岭铁矿床是鞍山本溪地区最典型的BIF型铁矿床之一,而且是该地区最大的富铁矿产区。从野外产出关系来看,弓长岭矿区的富铁矿与蚀变岩密切相关,蚀变岩与富铁矿基本上是形影相随。蚀变岩具有分带性,由富铁矿向外依次为镁铁闪石岩石榴石岩绿泥石岩弱蚀变斜长角闪岩斜长角闪岩。弱蚀变岩保留了蚀变原岩的岩貌特征,矿物的蚀变并不完全,可见残余的原生矿物。强蚀变岩的蚀变较彻底,基本无原生矿物残留。将蚀变岩与斜长角闪岩、磁铁石英岩的地球化学特征进行对比可以发现弱蚀变岩、石榴石岩、绿泥石岩与斜长角闪岩的痕量元素特征基本一致,而镁铁闪石岩的痕量元素特征更接近磁铁石英岩。再结合镜下特征、野外接触关系、主量元素特征等证据,认为除了镁铁闪石岩是由磁铁石英岩蚀变形成,其余蚀变岩都是由斜长角闪岩蚀变形成。根据各类蚀变岩中主要矿物的(Fe+Mg)/Si值以及蚀变岩的SiO2和Fe2OT3含量变化规律可以发现,在蚀变岩和富矿形成过程中发生了Mg、Fe以及Si的迁移。对本次取样的样品进行原岩恢复和构造环境判别投图,投图结果表明,绿泥石岩和弱蚀变岩的最初原岩都是形成于弧后盆地的玄武岩。