Age and sources of matter for the Kedrovskoe gold deposit,Northern Transbaikal tegion,Republic of Buryatia: Geochronological and isotopic geochemical constraints

The paper presents new geochronological and isotopic geochemical data on gold mineralization of the Kedrovskoe deposit. The deposit is located in the northeastern part of the Transbaikal metallogenic province, Russia’s largest. The Early Permian age (273 ± 4 Ma) of mineralization based on the results of Rb–Sr study of metasomatic rocks is correlated with the age of the final phases of Hercynian magmatism in the Baikal–Muya Foldbelt. The Sr, Nd, and Pb isotopic geochemical characteristics of mineralization show that the host rocks are involved in the formation of the latter. It has been established that ore lead was supplied to the hydrothermal system of the deposit mainly from a geochemical reservoir represented by the Neoproterozoic juvenile continental crust of the Baikal–Muya Foldbelt.     

Age of tin ore from the Solnechnoe quartz-tourmaline-cassiterite deposit, the Khabarovsk krai, Russia from the results of Rb-Sr dating of quartz and adularia

The age of the main productive phase of ore formation at the large Solnechnoe tin deposit has been estimated for the first time based on the study of the Rb-Sr isotopic system of hydrothermal quartz and adularia from ore veins and metasomatic rocks. The Rb-Sr isochron age (84 ± 1 Ma) of mineralization coincides with the age of intrusive rocks pertaining to the third phase of the Silinka Complex, which control tin mineralization. The ⁸⁷Sr/⁸⁶Sr ratios of ore-forming solution and granitic rocks of the final intrusive phase are close to each other, indicating that the granitic melt was most likely one of the main sources of metals. The long and multistage formation history of the deposit could have been caused by complex geodynamic evolution of the Sikhote-Alin accretionary fold region in the Cretaceous.     

Geochronology of eruptions and parental magma sources of Elbrus volcano,the Greater Caucasus: K-Ar and Sr-Nd-Pb isotope data

Complex geochronological and isotope-geochemical studies showed that the Late Quaternary Elbrus volcano (Greater Caucasus) experienced long (approximately 200 ka) discrete evolution, with protracted periods of igneous quiescence (approximately 50 ka) between large-scale eruptions. The volcanic activity of Elbrus is subdivided into three phases: MiddleNeopleistocene (225–170 ka), Late Neopleistocene (110–70 ka), and Late Neopleistocene-Holocene (less than 35 ka). Petrogeochemical and isotope (Sr-Nd-Pb) signatures of Elbrus lavas point to their mantle-crustal origin. It was shown that hybrid parental magmas of the volcano were formed due to mixing and/or contamination of deep-seated mantle melts by Paleozoic upper crustal material of the Greater Caucasus. Mantle reservoir that participated in the genesis of Elbrus lavas as well as most other Neogene-Quaternary magmatic rocks of Caucasus was represented by the lower mantle “Caucasus” source. Primary melts generated by this source in composition corresponded to K-Na subalkali basalts with the following isotopic characteristics: ⁸⁷Sr/⁸⁶Sr = 0.7041 ± 0.0001, ƒ_Nd = +4.1 ± 0.2, ¹⁴⁷Sm/¹⁴⁴Nd = 0.105–0.114, ²⁰⁶Pb/²⁰⁴Pb = 18.72, ²⁰⁷Pb/²⁰⁴Pb = 15.62, and ²⁰⁸Pb/²⁰⁴Pb = 38.78. The temporal evolution of isotope characteristics for lavas of Elbrus volcano is well described by a Sr-Nd mixing hyperbole between “Caucasus” source and estimated average composition of the Paleozoic upper crust of the Greater Caucasus. It was shown that, with time, the proportions of mantle material in the parental magmas of Elbrus gently increased: from ∼60% at the Middle-Neopleistocene phase of activity to ∼80% at the Late Neopleistocene-Holocene phase, which indicates an increase of the activity of deep-seated source at decreasing input of crustal melts or contamination with time. Unraveled evolution of the volcano with discrete eruption events, lacking signs of cessation of the Late Neopleistocene-Holocene phase, increasing contribution of deep-seated mantle source in the genesis of Elbrus lavas with time as deduced from isotope-geochemical data, as well as numerous geophysical and geological evidence indicate that Elbrus is a potentially active volcano and its eruptions may be resumed. Possible scenarios were proposed for evolution of the volcano, if its eruptive activity were to continue.     

Sources and fluid regime of quartz-carbonate veins at the Sukhoi Log gold deposit,Baikal-Patom Highland

Complex (δ¹⁸O, δ¹³C, ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and REE composition) data were obtained on quartz-carbonate veins in metasedimentary rocks to elucidate the material sources and to evaluate fluid regime during low-sulfide gold-quartz ore mineralization at the Sukhoi Log deposit. In order to use an oxygen isotopic thermometry for quartz veins, we calibrated empirical dependence of fractionation factors between vein quartz and altered wall rocks. The temperature range of quartz equilibration with wall rocks was evaluated at 380–190°C. Independent temperatures obtained using this thermometer indicate that the vein ankerite can be both earlier and later than vein quartz. The isotopic systematics (δ¹³C and δ¹⁸O) of ankerite in the quartz-carbonate veins, carbonates in the ore-hosting shales of the Khomolkho and Imnyakh formations both within and outside mineralized zones at the deposit indicate that the ore-hosting rocks and veins in the mineralized zone contain incoming carbonate, which was most probably borrowed from the carbonate rocks of the Imnyakh Formation. REE composition of vein ankerite shows that these elements were transported by fluid as carbonate complexes. The behavior of the Eu/Eu* and (La/Yb)_n ratios and Mn of the vein ankerite suggest that during carbonate crystallization the system was closed with respect to fluid. Sr-Nd isotope systematics indicates that the isotopic parameters of the vein ankerite were formed with the participation of metasedimentary host rocks of both the Imnyakh and Khomolkho formations, which are contrastingly different in Nd isotopic composition. A fluid/rock ratio during metasomatic processes in the wall rocks was calculated for two scenarios of their thermal history: with a continuously operating heat source beneath the Sukhoi Log structure and with a linear cooling of the structure. The effective integral W/R ratios calculated lie within the range of 0.046–0.226 and suggest that the veins were produced with the metamorphic fluid. Low W/R ratios are inconsistent with the mechanism of vein quartz crystallization due to fluid oversaturation with respect to SiO₂ at decreasing temperature. We believe that the main mechanism responsible for the origin of these veins was variations of fluid oversaturation due to pressure variations (pressure solution mechanism). This hypothesis is consistent with the reported isotopic-geochemical characteristics of the wall rocks at the Sukhoi Log deposit.