Carbonaceous matter in sulfide-quartz veins at the Kurultyken base-metal deposit,eastern Transbaikal region,Russia

The carbonaceous matter filling cavities in sulfide-quartz veins at the Kurultyken hydrothermal base-metal deposit in the Khapcheranga ore district, Transbaikal region, was studied using chromatography/mass spectrometry, X-ray diffraction, thermal and isotopic analyses, and IR spectroscopy. It was established that carbonaceous matter was a maltha composed of polycyclic aromatic hydrocarbons (PAHs). Chrysene, pyrene, and benzpyrelene identified among PAHs are evidence for the hydrothermal origin of the initial carbonaceous matter of maltha. The main mass of carbonaceous matter was synthesized under reductive conditions and at a low temperature, i.e., at the final stage of base-metal ore formation. Nevertheless, the thermometric data indicate that part of the carbonic compounds could have formed at 480°C, i.e., at the high-temperature stage of the postmagmatic process. The contribution of host rocks as a source of carbonaceous matter was minimal.     

The Darasun gold deposit,Eastern Transbaikal region: Chemical composition,REE patterns,and stable carbon and oxygen isotopes of carbonates from ore veins

The chemistry, REE patterns, and carbon and oxygen isotopic compositions of carbonates from ore veins of the Darasun deposit are discussed. In addition to the earlier described siderite, calcite, and carbonates of the dolomite-ankerite series, kutnahorite is identified. The total REE content in Fe-Mg carbonates of the dolomite-ankerite series (2.8–73 ppm) is much lower than in later calcite (18–390 ppm). δ¹³C of Fe-Mg carbonates and calcite varies from +1.1 to −6.7‰ and from −0.9 to −4.9‰, respectively. δ¹⁸O of Fe-Mg carbonates and calcite varies from +17.6 to 3.6‰ and from +15.7 to −0.5‰, respectively. The REE sum and carbon and oxygen isotopic compositions reveal zonal distribution relative to the central granodiorite porphyry stock. The correlation between the carbon and oxygen isotopic compositions and REE sum reflects variations in the physicochemical formation conditions and composition of ore-forming fluid. The isotopic composition of fluid is calculated, and possible sources of its components are considered. Earlier established evidence for a magmatic source of ore-forming fluid and participation of meteoric water in ore formation is confirmed. Geochemical evidence for interaction of ore-forming fluid with host rocks is furnished. The relationships between the REE sum, on the one hand, and carbon and oxygen isotopic compositions of hydrothermal ore-forming fluid, on the other, are established.     

Jurassic uraniferous rocks in the eastern Transbaikal region

The paper presents the results of lithological studies of Jurassic rocks in one of the largest basins in the eastern Transbaikal region (Olov depression). These results refined settings of the volcanosedimentary association and indicated that the Olov depression was formed in three stages: stage 1 related to tectonic activation manifested as cataclasis of granitoids in the Transbaikal region and incipience of numerous depressions (Olov included); stage 2 characterized by catastrophic events related to reactivation of tectonic motions, strong volcanism, and intense activity of geysers; and stage 3 marked by termination of volcanic activity and relative tectonic stability of the region that promoted the deposition of rocks of the middle and upper Ukurei subformations in the course of slow synsedimentary subsidence of the depression. Uranium mineralization in the rocks was governed mainly by the following conditions: (1) abundance of cataclased granitoids that delivered terrigenous material to the sedimentation basin; (2) favorable paleogeographic setting for the formation of both sedimentary and postsedimentary hydrothermal U-rich sequences. Field works in the Transbaikal region and analytical results reported in the present paper made it possible to make a videofilm shown in https://youtu.be/UOe9xzSKOEI.     

Oscillatory Zoning in Tennantite-(Fe) at the Darasun Gold Deposit (Eastern Transbaikal Region,Russia)

Geology of Ore Deposits - Oscillatory zoning in tennantite-(Fe) crystals found in carbonate–sulfide aggregates at the Darasun gold deposit has been studied. Similar elements have been...     

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 and ore formation conditions of the Bugdaya Au-Bearing W-Mo porphyry deposit, Eastern Transbaikal Region, Russia

The Bugdaya Au-bearing W-Mo porphyry deposit, Eastern Transbaikal Region, Russia, is located in the central part of volcanic dome and hosted in the large Variscan granitic pluton. In its characteristics, this is a Climax-type deposit, or an Mo porphyry deposit of rhyolitic subclass. The enrichment in gold is related to the relatively widespread vein and veinlet gold-base-metal mineralization. More than 70 minerals (native metals, sulfides, sulfosalts, tellurides, oxides, molybdates, wolframates, carbonates, and sulfates) have been identified in stockwork and vein ores, including dzhalindite, greenockite, Mo-bearing stolzite, Ag and Au amalgams, stromeyerite, cervelleite, and berryite identified here for the first time. Four stages of mineral formation are recognized. The earliest preore stage in form of potassic alteration and intense silicification developed after emplacement of subvolcanic rhyolite (granite) porphyry stock. The stockwork and vein W-Mo mineralization of the quartz-molybdenite stage was the next. Sericite alteration, pyritization, and the subsequent quartz-sulfide veins and veinlets with native gold, base-metal sulfides, and various Ag-Cu-Pb-Bi-Sb sulfosalts of the gold-base-metal stage were formed after the rearrangement of regional pattern of tectonic deformation. The hydrothermal process was completed by argillic (kaolinite-smectite) assemblage of the postore stage. The fluid inclusion study (microthermometry and Raman spectroscopy) allowed us to establish that the stockwork W-Mo mineralization was formed at 550–380°C from both the highly concentrated Mg-Na chloride solution (brine) and the low-density gas with significant N2 and H2S contents. The Pb-Zn vein ore of the gold-base-metal stage enriched in Au, Ag, Bi, and other rare metals was deposited at 360–140°C from a homogeneous Na-K chloride (hydrocarbonate, sulfate) hydrothermal solution of medium salinity.     

Fahlore and Sphalerite from the Darasun Gold Deposit in the Eastern Transbaikal Region,Russia: I. Mineral Assemblages and Intergrowths,Chemical Composition,and Its Evolution

The mineral assemblages, mode of occurrence, and chemical compositions of coexisting fahlore and sphalerite from the Darasun gold deposit have been described. Three generations of fahlore and three generations of sphalerite have been recognized. The FeS content in sphalerite coexisting with fahlore ranges from 0.8 to 9.4 mol %. The complete solid solution series Fe-tetrahedrite–Zn-tetrahedrite–Fe-tennantite–Zn-tennantite reflected in Sb/(Sb + As) and Fe/(Fe + Zn) ratios ranging from 0 to 0.97 and from 0.07 to 1.00, respectively, with a predominant negative relationship between these ratios has been identified for the first time at the deposit. Stepped, oscillatory, and combined stepped-oscillatory growth zonings within fahlore grains and heterogeneous aggregates of fahlore have been found. Fahlore is enriched in As with respect to Sb, and Zn-tetrahedrite is followed by Fe- and Zn-tennantite from early to late generation; Zn-tetrahedrite is followed by Fe-tennantite in zoned grains and overgrown rims; sphalerite crystallized at decreased temperature and sulfur fugacity. The evolution of the chemical composition of fahlores was caused by the evolving temperature, fluid salinity, and conditions of metal migration.     

Fahlore and Sphalerite from the Darasun Gold Deposit in the Eastern Transbaikal Region,Russia: II. Fe and Zn Partitioning,Fluid Inclusions,and Formation Conditions

The partitioning of Fe and Zn between coexisting fahlore and sphalerite and fluid inclusions in sphalerite from the Darasun gold deposit have been studied. These data were used to estimate the formation temperature of the minerals by the sphalerite–fahlore geothermometer. The calculated crystallization temperature of 175–355°С is close to the homogenization temperature of fluid inclusions in sphalerite of 225–385°С.The estimated pressure for fluid inclusion trapping ranged from 340 to 1420 bar. The sulfur fugacity obtained from the FeS content in sphalerite associated with pyrite and the calculated temperature ranges from 10^–5.5 to 10^–11 bar.     

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.     

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.     

Distribution of rare earth elements in waters and bottom sediments of mineralized lakes in the eastern Transbaikal region (Russia)

Contents of rare earth elements in waters and bottom sediments are maximum in the most mineralized soda lakes. It is shown that REE occur in waters mainly as carbonate (LnСО₃)⁺ and oxyhydroxide LnO₂H, LnO⁺, (LnO₂)^– complexes, whose activity in the La → Lu series changes in opposite directions. It has been determined that increase of mineralization leads mainly to higher concentrations of the dissolved HREE. Prevalence of the absolute values of MREE and HREE is recorded in basins with the development of bacterial processes. Geochemical barrier for the accumulation of LREE in waters can be represented by fluorcarbonates, whose saturation degree in the soda lake waters can be several orders of magnitude higher than the solubility products (SP). Oxidative and reductive settings in lakes are favorable for the formation of Ce(OH)₄ and Ce(OH)₃, respectively.     

黑龙江省乌拉嘎金矿赋矿花岗闪长斑岩锆石 U-Pb年龄、岩石成因及其地质意义

黑龙江省是我国著名金矿产区之一,发育多个浅成低温热液型金矿,它们与中生代陆相火山-次火山岩有密切的关系,但对这些陆相火山-次火山岩的成因缺乏系统研究,制约了本区金矿的成因认识和矿床勘查。本次研究采用先进的LA-ICP-MS锆石定年法、地球化学Sr-Nd-Pb同位素示踪等方法,对与乌拉嘎浅成低温热液金矿存在密切成因联系的次火山岩(花岗闪长斑岩)进行深入研究。LA-ICP-MS锆石定年法获得乌拉嘎矿区葡萄沟岩体及其南部含矿岩枝的成岩年龄分别为108.2±1.2Ma和106±1.1Ma,与区内宁远村组火山岩成岩时间基本相近,推断金矿成矿时代为早白垩世晚期,与东安金矿和高松山金矿为同期。岩石地球化学确定该岩体为高钾钙碱性(σ=1.83~2.18)偏铝质I型花岗岩特征,结合微量元素和Sr-Nd-Pb研究显示其具有活动陆缘弧岩浆岩特点,进一步得出岩浆源区和成矿物质具有来源于新元古代形成的镁铁质下地壳的部分熔融的属性。早白垩世时,中国东部处于伸展应力体制下,尤其黑龙江构造活动强烈,起源于新生下地壳重融的岩浆活动频繁,结合已有的浅成低温热液金矿床资料显示,推断区内具有巨大的成矿潜力。     

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.     

俄罗斯阿尔泰-Sayan地区Aksug斑岩Cu-Mo体系的地质与地球化学特征

The Aksug deposit, located in Altay-Sayan region of Russia, is one of the largest porphyry Cu-Mo deposits in Southern Siberia. The ore-bearing porphyries of the Aksug porphyry Cu-Mo system were formed in post-collisional environment. Geochemically they belong to calk-alkaline and high K-calk-alkaline series. Rocks are characterized by enrichment of LILE and depletion of HSFE and HREE, showing the importance of subduction-related components in magma generation. Large plutonic intrusions that host porphyry systems have been formed during collision. The origin of porphyritic rocks is dominantly the mantle with lower crustal contribution. The mainly economically important Cu-Mo mineralization is closely related to a porphyry series in time and space, being emplaced towards the end of magmatic activity. Though the emplacement of plutonic and ore-bearing porphyry complexes took place in different geodynamic environments, both complexes are characterized by certain similarity in geochemical composition, alkalinity, trace element content, Sr isotopic composition. This fact evidently indicates a common deep-seated magmatic source (at the lower crust-upper mantle level). Low initial ⁸⁷ Sr/⁸⁶ Sr, sulfur isotopic characteristics and presence of PGE-Co-Ni mineralization in associated pyrite-chalcopyrite ores suggest that mantle source of chalcophile elements was of high importance in porphyry Cu-Mo mineralization of the Aksug deposit.