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Vol 70, No 1 (2025)
Articles
LOW Pb ISOTOPIC VARIATIONS IN THE EXTENSIVE CHATKAL–KURAMA ORE PROVINCE, MIDDLE TIEN SHAN, AND SOURCES OF THE LARGE SCALE Au, Ag, AND MULTIMETAL MINERALIZATION: EVIDENCE FROM HIGH-PRECISION Pb ISOTOPE DATA
Abstract
The Chatkal–Kurama region in the central Tien Shan is a superlarge porphyry–epithermal gold ore province. The paleovolcanic area hosts world-class Au, Ag, and base-metal deposits (Kalmakyr, Kochbulak, Kanimansur, etc.). Using the high-precision (±0.02 %) MC-ICP-MS method of lead isotope analysis, we studied a collection of 63 ore samples (47 of them are galena) from 18 deposits, which represent all types of Au–Ag, Au–Ag-base metal, and Cu–Au–Mo deposits known in the region. The same method was applied to study 21 samples of igneous rocks from this region, for which lead isotope composition was determined in monomineralic feldspar separates. The Pb isotope ratios 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb from the ore deposits vary within narrow ranges: 17.9885–18.1598, 15.5897–15.6412, and 38.0385–38.2380, respectively. These variations in relative terms are 0.94, 0.33, and 0.52 %, respectively, and are among the smallest among ore provinces around the world. An even higher (two to five times) degree of homogeneity is typical of the Pb isotopic composition at individual deposits in the region. The lead isotope composition of deposits and ore fields in the Chatkal–Kurama region does not depend on their mineralogical and geochemical features but is instead controlled by the geological settings of the deposits. The iscovered close similarity between ore deposits and Late Paleozoic granitoids in Pb isotope composition provides evidence in support of the hypothesis that genetic connection of the large-scale Au, Ag, and base-metal is genetically related to magmatism, which developed in a subduction environment. An interesting fact is that the Pb isotope composition is identical at the Kalmakyr Cu–Au–Mo porphyry deposit and the neighboring Akturpak Au epithermal deposit, which provides evidence that metals for these deposits (which are different in composition and were formed under different P–T parameters) were derived from a common source. The isotope composition and its evolutionary model characteristics according to the Stacey–Kramers model indicate (in agreement with the data on Sr and Nd) that Pb of the rocks and deposits in the region is mid-crustal, typical of island-arc regions of the Andean type. The mantle component of the source of the regional ore-bearing magmas was the material of mantle lithosphere and oceanic crust that was partially melted in a subduction environment in the mantle wedge zone. The ratio 𝑇ℎ/𝑈 = 3.86–3.99, which is higher than the average crustal value, indicates a significant contribution of Precambrian basement rocks of the Chatkal–Kurama terrane to the petrogenesis of the ore-bearing magmas.
Geochemistry International. 2025;70(1):3-37
3-37
THE ARCHEAN AGE OF GRANITE-GNEISS COMPLEXES FROM THE KAMA-VYATKA ZONE (THE VOLGA-URAL SEGMENT, EAST EUROPEAN CRATON)
Abstract
The formation history of granulite complexes is fundamental to understanding the processes of early continental crust origins. The work presents the results of an isotope-geochronological study of rock samples from the main complexes of the Kama-Vyatka zone (the Volga-Ural segment, the East European craton) — enderbites of the Otradnensky series and quartz diorites of the Tanai plagiogranitoid massif. The Sm-Nd isotopic data were used to calculate model ages of quartz diorites of the Tanaysky plagiogranitoid massif and enderbites of the Otdnenskaya series — 3.2 and 3.0 Ga, respectively. The U–Pb isotope system of zircon was investigated by LA-ICP-MS. The zircon of quartz diorites indicated the Archean age of protolith formation of the plagiogranitoids of the Tanaysky Massif. This time interval — 3.04–2.98 Ga — includes the stage of the earliest granulite metamorphism immediately following the episode of magmatism. There are several generations of zircon grains from a sample of weakly gneissed enderbites of the Otradnensky series in the 3.0–2.8 Ga age interval, which record the following events: the formation of primary enderbites, locally manifested partial melting under granulite facies conditions and regressive stage of metamorphism in the transitional conditions of granulite to amphibolite facies. Considering to model age of enderbites, for the first time, an estimate of the age of the Otradnensky Group of the Kama-Vyatka zone of the Volga-Ural segment was obtained — 3.0 ± 0.1 Ga. In the 2.75–2.60 Ga age interval, zircon from a sample of weakly gneissed enderbites records the most significance episode of granulite metamorphism, widely manifested throughout the Volga-Ural segment and, replacing it, regressive metamorphism with the input of water-bearing fluid and temperature decreasing.
Geochemistry International. 2025;70(1):38-60
38-60
DIFFERENTIATION FEATURES OF ALKALINE ROCKS IN ILMEN MIASKITE MASSIF: NEW MINERALOGICAL AND GEOCHEMICAL DATA
Abstract
The Ilmen miaskite massif located in the Southern Urals remains largely understudied from mineralogical and geochemical standpoints, while the theories of its formation are still debatable. The article presents new data on mineral associations of miaskite varieties and REE-rich minerals. Microchemical studies determined pyroxene-amphibole miaskites as the most promising variety on REE mineralization (REE content at ca.1500 μg/g). These rocks showed distinct positive Nb anomalies combined with a negative Pb anomaly. The temperatures of feldspar exsolution indicate their following formation sequence within miaskite varieties (from higher temperature to lower temperature ones): pyroxene-amphibole miaskite → garnet-amphibole miaskite → amphibole miaskite → biotite miaskite.
Geochemistry International. 2025;70(1):61-73
61-73
FORMATION CONDITIONS OF GOLD MINERALIZATION IN THE SPOKOININSKY ORE CLUSTER, ALDAN SHIELD, RUSSIA
Abstract
The article presents the first data on individual fluid inclusions hosted in quartz in the ores of three ore types (polysulfide, gold-silver-telluride and gold-bismuth) of the Spokoininsky ore cluster with gold ore mineralization. The three ore types show differences in the physicochemical parameters and composition of their fluids. The fluid of the Spokoininsky cluster polysulfide ores are characterized by a relatively low initial temperature (180–350 ○C), a higher CO2 density (0.27–0.71 g/cm3) and a higher fluid pressure (0.7–1 kbar) compared to the fluids that formed the gold-silver-telluride ores (temperature 200–260 ○C, CO2 density 0.28–0.56 g/cm3, pressure 0.7 kbar). The dominant salt in the fluids of polysulfide ores are Na and Mg chlorides, whereas the mineral-forming fluids of gold-silver-telluride ores are simpler saline aqueous fluids containing Na chlorides. The fluids that formed the polysulfide ores have a H2O-CO2-N2 composition, whereas the fluid of the gold-silver-telluride ores, is mostly of H2O-CO2 composition. The gold-bismuth ores in the Mayskoe ore field were formed by H2O-CO2-bearing fluids with a salinity concentration of 4.0–6.4 wt. % NaCl eq., a CO2 density of 0.56–0.61 g/cm3, at a temperature of 280–335 ○C and a pressure of 0.7 kbar. The data led us to conclude that the ore-forming fluid of the Spokoininsky ore cluster was similar to the fluids of orogenic gold deposits.
Geochemistry International. 2025;70(1):74-88
74-88
ACTINIDES IN THE SOIL CHRONOSEQUENCE OF THE AMUR RIVER FLOODPLAIN
Abstract
For the first time in the Russian Far East, a study was conducted to assess the rate of accumulation of gross and mobile forms of actinides (U and Th) in a 5000-year-old soil chrono-sequence embedded within the floodplain of the middle reaches of the Amur River. The relationships between actinides and the properties of alluvial and residual alluvial soils are characterized using regression models. It was found that during the evolution of soils, the content of the gross form of actinides in soils of the automorphic series increased for U from 1 to 2 mg/kg, for Th from 4 to 10 mg/kg. In the soils of the hydromorphic series, over a shorter period of time (2600 years), the increase was from 1 to 3 mg/kg for U and from 4 to 12 mg/kg for Th. The content of the mobile U form in automorphic and hydromorphic soils increased on average from 0.1 to 0.4 mg/kg, for Th from 0.02 to 0.2 mg/kg. In automorphic soils, accumulation of U is observed while the flood regime is in effect, Th continues to accumulate even after the floodplain leaves the flood zone. In hydromorphic soils, the accumulation of actinides continues over the entire chronological range. The results obtained show that the main soil properties determining the accumulation of actinides in soils are the content of clay minerals and iron oxides. The intake of actinides into the soils of the Amur River floodplain is carried out mainly due to the weathering of melanocratic granitoid minerals in the composition of alluvium. The mobilization of actinides is influenced by pH in automorphic soils and Eh in hydromorphic soils.
Geochemistry International. 2025;70(1):89-104
89-104