Acta Geochimica最新文献

By studying the light isotopic compositions of carbon, oxygen, and hydrogen, combined with previous research results on the ore-forming source of the deposit, the authors try to uncover its metallogenic origin. The δ¹⁸O and δ¹³C isotope signatures of dolomite samples vary between 10.2 and 13.0‰, and between −7.2 and −5.2‰, respectively, implying that the carbon derives from the upper mantle. δD and δ¹⁸O of quartz, biotite, and muscovite from different ore veins of the deposit vary between −82 and −59‰, and between 11.6 and 12.4‰, respectively, implying that the metallogenic solutions are mainly magmatic. According to the relevant research results of many isotope geologists, the fractionation degree of hydrogen isotopes increases as the depth to the Earth’s core increases, and the more differentiated the hydrogen isotopes are, the lower their values will be. In other words, mantle-derived solutions can have extremely low hydrogen isotope values. This means that the δD‰ value − 134 of the pyrrhotite sample numbered SD-34 in this article may indicate mantle-derived ore-forming fluid of the deposit. The formation of the Dashuigou tellurium deposit occurred between 91.71 and 80.19 Ma.

The Gangdese belt in Xizang has experienced both Jurassic subduction and Cenozoic continental collision processes, making it a globally renowned region for magmatic rocks and porphyry copper deposits. Numerous Jurassic intrusions have been identified in the belt. Apart from the quartz diorite porphyry in the large Xietongmen deposit, the Cu mineralization potential of other Jurassic intrusions in this belt remains unclear. This study presents zircon U–Pb dating and trace elements, apatite major and trace elements as well as published whole-rock geochemical and isotopic data of the Dongga tonalite in the central part of the Gangdese belt, aiming to reveal the petrogenesis, oxidation state, volatile content, and Cu mineralization potential of this intrusion. The Dongga tonalite has a zircon U–Pb age of 179.4 ± 0.9 Ma. It exhibits high whole-rock V/Sc values (8.76–14.6), relatively low apatite Ce_N/Ce_N^* ratios (1.04–1.28), elevated zircon (Eu/Eu^*)_N values (an average of 0.44), high Ce⁴⁺/Ce³⁺ values (205–1896), and high ∆FMQ values (1.3–3.7), collectively suggesting a high magmatic oxygen fugacity. The Dongga tonalite features amphibole phenocrysts, relatively high whole-rock Sr/Y ratios (20.3–58.9), and lower zircon Ti temperatures (502–740 °C), reflecting a high magmatic water content. Estimation of magmatic sulfur content (0.002–0.024 wt%) based on apatite SO₃ contents indicates an enriched magma sulfur content. Combined with previous studies and the collected Sr–Nd–Hf isotopes, the Dongga tonalite is derived from juvenile lower crust related with subduction of the NeoTethys oceanic slab. When compared with Xietongmen ore-bearing porphyries, the Dongga tonalite exhibits remarkable similarities with the Xietongmen ore-bearing porphyries in terms of magma source, tectonic background, magmatic redox state, and volatile components, which indicates that the Dongga tonalite has a high porphyry Cu mineralization potential, and therefore, provides important guidance for the future mineralization exploration.

Comprehensive research has been implemented to raise the efficiency of the geochemical survey of stream sediments (SSs) that formed under the cryolithogenesis conditions. The authors analysed the composition, structure and specific features of the formation of exogenous anomalous geochemical fields (AGFs) identified through SSs of large river valleys of IV order. In our case, these were the valleys of Maly Ken, Ken and Tap Rivers. These rivers are located in the central and southern parts of the Balygychan–Sugoy trough enclosed in the Magadan region, North–East of Russia. The authors proposed a new technique to sample loose alluvium of SSs in the large river valleys along the profiles. The profiles were located across the valleys. The AGFs of Au, Ag, Pb, Zn, Sn, Bi, Mo and W were studied. Correlations between elements have been established. These elements are the main indicator elements of Au–Ag, Ag–Pb, Sn–Ag, Mo–W and Sn–W mineralization occurring on the sites under study. The results obtained were compared with the results of geochemical surveys of SSs. It is concluded that the AGFs recognized along the profiles reflect the composition and structure of eroded and drained ore zones, uncover completely and precisely the pattern of element distribution in loose sediments of large water flows. The alluvium fraction < 0.25 mm seems to be most significant in a practical sense, as it concentrated numerous ore elements. Sampling of this fraction in the river valleys of IV order does not cause any difficulty, for this kind of material is plentiful. The developed technique of alluvium sampling within large river valleys is efficient in searching for diverse mineralization at all stages of prognostic prospecting. It is applicable for geochemical survey of SSs performed at different scales both in the North–East of Russia, as well as other regions with similar climatic conditions, where the SSs are formed under the cryolithogenesis conditions.

Bulk geochemistry, Sr, Nd, and O–H isotope systematics are reported for the first time on banded iron formation (BIF)-hosted high-grade iron ore at the north-western segment of Congo Craton (CC). Located in Mbalam iron ore district, Southern Cameroon, Metzimevin iron ore deposit is a hematite-magnetite BIF system, dominated by SiO₂ + Fe₂O₃ (97.1 to 99.84 wt%), with low concentrations of clastic elements e.g., Al₂O₃, TiO₂, and HFSE, depicting a nearly pure chemical precipitate. The REE + Y signature of the iron deposit displays strong positive Eu anomaly, strong negative Ce anomaly, and chondritic to superchondritic Y/Ho ratios, suggestive of formation by mixed seawater-high temperature hydrothermal fluids in oxidising environment. The ⁸⁷Sr/⁸⁶Sr ratios of the BIF are higher than the maximum ⁸⁷Sr/⁸⁶Sr evolution curves for all Archean reservoirs (bulk silicate earth, Archean crust and Archean seawater), indicating involvement of continentally-derived components during BIF formation and alteration. The Ɛ_Nd(t) (+ 2.26 to + 3.77) and Nd model age indicate that chemical constituents for the BIF were derived from undifferentiated crustal source, between 3.002 and 2.88 Ga. The variable and diverse O and H isotope data (− 1.9‰ to 17.3‰ and − 57‰ to 136‰ respectively) indicate that the Metzimevin iron ore formed initially from magmatic plumes and later enriched by magmatic-metamorphic-modified meteoric fluids. Mass balance calculations indicate mineralisation by combined leaching and precipitation, with an average iron enrichment factor of > 2.67 and SiO₂ depletion factor of > 0.99. This is associated with an overall volume reduction of 28.27%, reflecting net leaching and volume collapse of the BIF protholith.

The Yueguang gold deposit is located in Fengjia, Xinhua County, Hunan Province, South China. It represents a recently discovered small-scale gold deposit situated in the southwestern region of the Jiangnan Orogenic Belt, west of the Baimashan granitic batholith. In order to discern the characteristics of the ore-forming fluids, the underlying mineralization processes, and establish a foundation for the origin of the Yueguang gold deposit fluid inclusion microthermometry, as well as quartz hydrogen and oxygen isotope analysis, have been carried out on samples obtained from various stages of mineralization. The hydrothermal mineralization stages within the Yueguang gold deposit can be categorized into three stages: (i) the barren, pre-ore quartz-pyrite stage (Stage I), the quartz-pyrite-gold stage (Stage II), and the post-ore quartz-carbonate stage (Stage III), with the second stage being the main mineralization stage. The fluid inclusions identified in samples from the main mineralization stage can predominantly be described with the NaCl–H₂O and CO₂–NaCl–H₂O systems. These inclusions display homogenization temperatures ranging from 158.8 to 334.9 °C, and the fluid salinity ranges from 0.3% to 4.0% (wt.% NaCl equiv.). Laser Raman spectroscopy analysis of individual inclusions further reveals the presence of gas-phases such as CO₂, CH₄, and N₂. Isotopic analysis indicates δ¹⁸O fluid values ranging from 3.95 to 6.7 ‰ and δD_H2O values ranging from − 71.9 to − 55.7 ‰. These results indicate that the ore-forming fluid of the Yueguang gold deposit belongs to metamorphic hydrothermal fluids of middle-low temperature and low salinity. In the process of ore formation, gold is transported in the form of Au (HS)²⁻ complexes, with gold deposition being driven by fluid immiscibility. Therefore, the Yueguang gold deposit is categorized as an orogenic gold deposit dominated by metamorphic hydrothermal fluid. It may become a new target for gold exploration in the Baimashan region, central Hunan Province.

It is a challenge to make thorough but efficient experimental designs for the coupled mineral dissolution and precipitation studies in a multi-mineral system, because it is difficult to speculate the best experimental duration, optimal sampling schedule, effects of different experimental conditions, and how to maximize the experimental outputs prior to the actual experiments. Geochemical modeling is an efficient and effective tool to assist the experimental design by virtually running all scenarios of interest for the studied system and predicting the experimental outcomes. Here we demonstrated an example of geochemical modeling assisted experimental design of coupled labradorite dissolution and calcite and clayey mineral precipitation using multiple isotope tracers. In this study, labradorite (plagioclase) was chosen as the reactant because it is both a major component and one of the most reactive minerals in basalt. Following our isotope doping studies of single minerals in the last ten years, initial solutions in the simulations were doped with multiple isotopes (e.g., Ca and Si). Geochemical modeling results show that the use of isotope tracers gives us orders of magnitude more sensitivity than the conventional method based on concentrations and allows us to decouple dissolution and precipitation reactions at near-equilibrium condition. The simulations suggest that the precise unidirectional dissolution rates can inform us which rate laws plagioclase dissolution has followed. Calcite precipitation occurred at near-equilibrium and the multiple isotope tracer experiments would provide near-equilibrium precipitation rates, which was a challenge for the conventional concentration-based experiments. In addition, whether the precipitation of clayey phases is the rate-limiting step in some multi-mineral systems will be revealed. Overall, the modeling results of multi-mineral reaction kinetics will improve the understanding of the coupled dissolution–precipitation in the multi-mineral systems and the quality of geochemical modeling prediction of CO₂ removal and storage efficacy in the basalt systems.

The regular hydrochemical monitoring of groundwater in the Mila basin over an extended period has provided valuable insights into the origin of dissolved salts and the hydrogeochemical processes controlling water salinization. The data reveals that the shallow Karst aquifer shows an increase in TDS of 162 mg L⁻¹ while the thermal carbonate aquifer that is also used for drinking water supply exhibits an increase of 178 mg L⁻¹. Additionally, significant temperature variations are recorded at the surface in the shallow aquifers and the waters are carbogaseous. Analysis of dissolved major and minor elements has identified several processes influencing the chemical composition namely: dissolution of evaporitic minerals, reduction of sulphates, congruent and incongruent carbonates’ dissolution, dedolomitization and silicates’ weathering. The hydrogeochemical and geothermometric results show a mixing of saline thermal water with recharge water of meteoric origin. Two main geothermal fields have been identified, a partially evolved water reservoir and a water reservoir whose fluid interacts with sulphuric acid (H₂S) of magmatic origin. These hot waters that are characterized by a strong hydrothermal alteration do ascend through faults and fractures and contribute to the contamination of shallower aquifers. Understanding the geothermometry and the hydrogeochemistry of waters is crucial for managing and protecting the quality of groundwater resources in the Mila basin, in order to ensure sustainable water supply for the region. A conceptual model for groundwater circulation and mineralization acquisition has been established to further enhance understanding in this regard.

This study predicts favorable oil and gas source-rock formation conditions in the Aryskum Depression of the South Turgay Basin, Kazakhstan. This study assesses the thermal maturity and characteristics of organic matter by determining its environmental conditions using data from geochemical analysis of core (pyrolysis) and oil (biomarkers and carbon isotopic compositions) samples. According to the geochemical parameters obtained by pyrolysis, the oil generation potential of the original rocks of most studied samples varies from poor to rich. The facies–genetic organic matter is predominantly humic and less frequently humus–sapropel, indicating organic matter accumulation in the studied samples were under moderately reducing conditions (kerogen III and II types) and coastal–marine environments (kerogen type I). The carbon isotopic compositions of oils derived from the Jurassic deposits of the Aryskum Depression also indicate the sapropelic and mixed humic–sapropelic type of organic matter (kerogen II and I). Biomarker analysis of oils indicates original organic matter formation in an anoxic environment.

The post-collisional Cenozoic basic volcanic rocks in NE Turkey show temporal variations in whole-rock lithophile element and highly siderophile element (HSE) systematics that are mainly associated with the nature of sub-continental lithospheric mantle (SCLM) sources and parental melt generation. So far, the traditional whole-rock lithophile geochemical data of these basic volcanic rocks have provided important constraints on the nature of SCLM sources. Integrated lithophile element and HSE geochemical data of these basic volcanic rocks also reveal the heterogeneity of the SCLM source, which is principally related to variable metasomatism resulting from previous subduction(s) and post-collisional mantle-crust interactions in an extensional setting. Lithophile element geochemical features suggest that the parental magmas have derived from metasomatized spinel- to garnet-bearing SCLM sources for Eocene and Miocene basic volcanic rocks with subduction signatures whereas originated from spinel- to garnet-bearing SCLM sources for Mio-Pliocene and Plio-Quaternary basaltic volcanic rocks without the subduction signature. Lithophile element and HSE geochemistry also reveal that Eocene and Miocene basic volcanic rocks were affected by more pronounced crustal contamination than the basaltic volcanic rocks of Mio-Pliocene and Quaternary. Furthermore, the integrated lithophile element and HSE compositions of these basic volcanic rocks, together with the regional asymmetric lithospheric delamination model, reveal that the compositional variation (especially due to metasomatism) was significant temporally in the heterogeneity of the SCLM sources from which parental magmas formed during the Cenozoic era.

Meso-Neoarchean fuchsite quartzites are present in different stratigraphic positions of Dharwar Craton including the oldest (~ 3.3 Ga) Sargur Group of western Dharwar Craton. The present study deals with the petrographic and geochemical characteristics of the fuchsite quartzites from the Ghattihosahalli belt to evaluate their genesis, depositional setting and the enigma involved in the ancient sedimentation history. Their major mineral assemblages include quartz, fuchsite, and feldspars along with accessory kyanite and rutile. The geochemical compositions are characterized by high SiO₂, Al₂O₃, low MgO, CaO, strongly enriched Cr (1326–6899 ppm), Ba (1165–3653 ppm), Sr (46–210 ppm), V (107–868 ppm) and Zn (11–158 ppm) contents compared to the upper continental crust (UCC). The UCC normalized rare earth element (REE) patterns are characterized by depleted light REE [(La/Sm)_UCC = 0.33–0.95] compared to heavy REE [(Gd/Yb)_UCC = 0.42– 1.65)] with conspicuous positive Eu-anomalies (Eu/Eu* = 1.35–18.27) characteristic of hydrothermal solutions evidenced through the interlayered barites. The overall major and trace element systematics reflect a combined mafic-felsic provenance and suggest their deposition at a passive continental margin environment. The comprehensive field, petrographic, and geochemical studies indicate that these quartzites are infiltrated by Cr-rich fluids released during high-grade metamorphism of associated ultramafic rocks. The Sargur and the subsequent Dharwar orogeny amalgamated diverse lithounits from different tectonic settings, possibly leading to the release of Cr-rich fluids and the formation of fuchsite quartzite during or after the orogeny. These findings suggest a pre-existing stable crust prior to the Sargur Group and the link between orogenic events and various mineral deposits in the Dharwar Craton.