Geochemistry International最新文献

The results of geochemical studies of soils and anthropogenic grounds within the territory of the large Pioneer gold ore deposit developed in the Amur Region are presented. Using the enrichment indices for the soil and ground cover of the Pioneer deposit territory, the following pollutant elements were determined: As, Sb, Mo, Bi, W, S, Cd, and Pb. Using the methods of mathematical statistics, the background contents of As, Sb, Bi, Mo, W, S, Cd, and Pb in the technogenically transformed territory at the sampling time accounted for 63, 8.84, 0.69, 3.54, 4.19, 529, 0.11, and 36.5 mg/kg, respectively. The exceedance of background values for As, Sb, Bi, Mo, W, and S is caused by the natural metallogenic features of the territory. It has been established that the sources of metal emissions into the environment are mining facilities and structures of the mining complex: quarries, waste dumps, heap leaching areas, gold extraction plant, and tailings dumps.

The paper presents the first data on individual fluid inclusions hosted in quartz in the ores of three 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 salts in the fluids of polysulfide ores are Na and Mg chlorides, whereas the mineral-forming fluids of the 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 %-equiv. NaCl, 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.

The Ilmen miaskite massif in the Southern Urals remains largely understudied from the mineralogical and geochemical standpoints, and theories of its formation are still debatable. The paper presents the first data on the mineral associations of the miaskite varieties and REE-rich minerals. Microchemical studies determined that the pyroxene–amphibole miaskites are the most promising rock variety for REE mineralization (REE content at ca. 1500 ppm). These rocks show clearly discernible positive Nb anomalies combined with a negative Pb anomaly. The temperatures of feldspar exsolution indicate their following formation sequence within the miaskite varieties (from higher temperature to lower temperature ones): pyroxene–amphibole miaskite → garnet–amphibole miaskite → amphibole miaskite → biotite miaskite

For the first time, a study was conducted at the Russian Far East to assess the rate of accumulation of gross and mobile forms of actinides (U and Th) in a 5000-year-old soil chronosequence 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, the content of the gross form of actinides in the soils of the automorphic series increased from 1 to 2 mg/kg for U and from 4 to 10 mg/kg for Th. In the soils of the hydromorphic series, the increase over a shorter time period (2600 years) 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, and that of Th, from 0.02 to 0.2 mg/kg. In the automorphic soils, the accumulation of U is observed as long as the floodplain is regularly flooded, while Th continues to accumulate even after the floodplain leaves the flood zone. In the 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 occurs mainly due to the weathering of melanocratic granitoid minerals in the alluvium. The mobilization of actinides is affected by pH in automorphic soils and Eh in hydromorphic soils.

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 discovered 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.

The formation history of granulite complexes is fundamental significance for understanding the growth of early continental crust. The work presents the results of an isotope-geochronological study of rock samples from the main lithotectonic complexes of the Kama–Vyatka zone (the Volga–Ural segment of the East European Craton)—enderbites of the Otradnenskaya Group and quartz diorites of the Tanaisky plagiogranitoid massif. The model ages of quartz diorites of the Tanaisky plagiogranitoid massif and enderbites of the Otradnenskaya Groups calculated from Sm–Nd data are 3.2 and 3.0 Ga, respectively. Zircons from the quartz diorites of the Tanaisky plagiogranitoid massif and the enderbites of the Otradnenskaya Group were dated by U–Pb LA-ICP-MS method. Zircon from quartz diorites yielded the Archean age of protolith of the plagiogranitoids of the Tanaisky Massif. This time interval of 3.04–2.98 Ga marks the stage of the oldest granulite metamorphism immediately following the magmatic event. Zircons from weakly gneissose enderbites of the Otradnenskaya Group is subdivided into two age groups: 3.0–2.8 and 2.750–2.60 Ga. Based on the morphology, internal structure of the crystals, and their isotope-geochemical characteristics (Th and U contents, Th/U ratio), each of the indicated age groups includes several zircon generations. Within a time interval of 3.0–2.8 Ga, the identified zircon generations record the following events: the formation of primary enderbites, local partial melting under the granulite-facies conditions, and retrograde metamorphism under transitional granulite–amphibolite facies. With allowance for the model age of the enderbites, the Otradnenskaya Group of the Kama–Vyatka zone of the Volga–Ural segment was dated for the first time at 3.0 ± 0.1 Ga. In the time interval of 2.75–2.60 Ga, zircon from weakly gneissose enderbites records the peak granulite metamorphism, which spanned the entire Volga-Ural segment, and subsequent retrograde metamorphism accompanying by the input of hydrous fluid and temperature decrease.

Organic carbon content combined with organic carbon isotope composition have been applied for the study of organic matter transformation in marine sediments during upward gas migration at seep areas of the Laptev Sea. Organic matter extracted from marine sediments was separated into five fractions (hexane, hexane-benzene, benzene, benzene-methanol, asphaltenes) using solvents of increasing polarity. It has been shown that in the seep the destruction of asphaltenes fractions leads to enrichment of benzene-methanol fraction by isotope-light components. δ¹³C values of benzene-methanol fractions were much lower than δ¹³C values of asphaltenes fractions and were associated with the accumulation of bacterial biomass in the sediment core through which the upward methane flow was passed. The organic matter of seep area sediment cores can be classified by two clusters, according to δ¹³C values of benzene fractions of organic matter. The first cluster center was in the surface layer (about 10 cm) of marine sediments. The second cluster center was in a deeper sediment layer. The difference in carbon isotope composition between the cluster centers was 2–3‰. The use of carbon isotope type-curves for different horizons of a sediment core has enabled a better understanding of the biological effects related to upward gas migration in seep areas of the Arctic Seas.

Cobalt bearing metasedimentary rocks from the Nasibpur and surrounding areas of North Delhi Fold Belt, NW India has been studied to elucidate their provenance, palaeoweathering, palaeoclimatological conditions and depositional environment. The investigated area is characterized by metasedimentary formations such as quartzites, mica schists, garnet mica schist, phyllites, gneiss, amphibolites and magmatic phases like granites and pegmatites. Petrographic analysis affirmed that quartzites are mineralogical mature, fine to medium grained. On the basis of modal compositions these quartzites range from Fe-sand and quartz arenite in composition. The calculated chemical index of alteration (CIA) values (31.98 to 75.87 with an average of 48.60 indicates low to moderate weathering under semiarid to humid climatic conditions. Most of the quartzites samples have SiO₂/Al₂O₃ (>10, avg. 43) which indicates high maturity. Cobalt concentration in these quartzites, mica schist, pegmatites range from 166 to 3657, 62–166, and 167–519 ppm respectively. Quartz dilution during hydraulic sorting has led to low concentrations of rare earth elements in the quartzites. The rocks exhibit enrichment in LREE with (La/Sm)_n (1.2–7.41 avg. 4.56) with noticeable negative Eu anomalies (Eu/Eu* = 0.34–0.74). Evaluation of mineral and whole rock geochemistry collectively indicate their predominant derivation from a felsic source and deposition in an active to passive continental setting.

CaSiO₃ inclusions in diamonds from the Juina area in Brazil have low Fe (0.08–0.53 wt % FeO) and Al (0–1.52 wt % Al₂O₃) concentrations; they belong to the ultramafic association. Two different types exist among CaSiO₃ grains. Type I has a normal REE_n pattern, while type II has a sinusoidal REE_n pattern. Type I CaSiO₃ associates with high-Mg–high-Ni protogenetic ferropericlase, and type II associates with high-Fe–low-Ni syngenetic ferropericlase. Thus, type I CaSiO₃ grains are protogenetic, formed, like high-Mg–high-Ni ferropericlase, in the upper part of the lower mantle as davemaoite (CaSi-perovskite), and type II CaSiO₃ were formed in the transition zone as breyite. The enrichment of CaSiO₃ in REE, particularly in LREE, corresponds to high values of their partition coefficient CaSiO₃/melt and shows the CaSiO₃’s origin from a mantle material under high pressures. The isotope characteristics of the studied CaSiO₃ demonstrate strong geochemical heterogeneity in the inclusions. The ⁸⁷Rb/⁸⁶Sr ratios in type II CaSiO₃ (0.127–3.23) are 3–4 orders higher than in type I (0.0008). Even within a single diamond, different CaSiO₃ grains have ⁸⁷Rb/⁸⁶Sr ratios varying from 0.014 to 3.23. The same is true for U/Pb isotope systematics (e.g., ²³⁸U/²⁰⁶Pb varies in one sample in an order of magnitude from 0.031 to 0.312) and, to some extent, for Sm/Nd ratios. This implies the geochemical heterogeneity in Deep Earth on a very small scale.