Acta Geochimica最新文献

Scheelite is the principal ore mineral in skarn‑type W deposits, with its texture and geochemical signatures directly recording the metal source, mineralization conditions, and fluid evolution. The Maka W–Pb–Zn–Sn–Be polymetallic deposit in southeastern Yunnan has been studied primarily for its Pb–Zn or Be mineralization, whereas W mineralization has received limited attention, hindering a systematic understanding of regional W potential and ore‑forming processes. Based on integrated field and petrographic analyses, four texturally and compositionally distinct scheelite types (Sch I–Sch IV) have been identified. In-situ textural, geochemical, and Sr‑isotopic analyses of these scheelite varieties were performed using scanning electron microscopy-cathodoluminescence (SEM‑CL) and LA‑(MC)‑ICP‑MS to constrain the W source and mineralization conditions at Maka. Results indicate that Sch I, disseminated within epidote–diopside skarn, exhibits uniform CL brightness, low ΣREE (average 18 ppm), low ⁸⁷Sr/⁸⁶Sr (0.71279–0.71482), moderate Mo (average 206 ppm), Y/Ho ratios of 14–50, and LREE‑enriched patterns. Sch II, occurring in fluorite–quartz veins, displays oscillatory zoning, higher ΣREE (average 96 ppm), elevated ⁸⁷Sr/⁸⁶Sr (0.72662–0.73025), Mo content (average 720 ppm), and Y/Ho ratios (19–101). Sch III, hosted in fluorite–feldspar–quartz veins, shows moderately lower ΣREE (average 65 ppm), further increased ⁸⁷Sr/⁸⁶Sr (0.73137–0.73643), equivalent Mo (average 556 ppm), and reduced Y/Ho ratios (2–29). Sch IV forms fine grains in chalcedony veins, with ΣREE near 79 ppm, a distinctive “wing‑shaped” MREE‑enriched pattern, lower ⁸⁷Sr/⁸⁶Sr (0.71761–0.71841), sharply reduced Mo (average 2.7 ppm), and Y/Ho ratios of 12–40. Systematic shifts in Eu anomalies across the scheelite generations trace an evolving redox trajectory: from high fO₂ (Sch I) to low fO₂ (Sch II) and back to high fO₂ (Sch III and Sch IV). Sch I–III exhibit LREE‑enriched profiles, Y/Ho ratios, and Sr‑isotope compositions consistent with a magmatic‑fluid origin exsolved from the Yanshanian Laojunshan granite. In contrast, Sch IV shows a metamorphic‑fluid affinity, indicated by its MREE‑enriched “wing‑shaped” pattern, low Mo content, high Y and Ho concentration, and elevated Sr/Mo ratios. Integrated with regional tungsten metallogeny, this study proposes that Yanshanian Laojunshan‑related W systems typically comprise "inner quartz-wolframite-vein-type + middle skarn-type + outer fluorite-scheelite-vein-type" mineralization. Scheelite that occurs in fluorite‑bearing veins and exhibits high Sr isotopic ratios with well‑developed oscillatory zoning can serve as a key macro‑ to microscopic indicator for W exploration in late Yanshanian granite.

The Guidong complex in northern Guangdong constitutes a key segment of the Nanling Metallogenic Belt and hosts significant granite-related uranium deposits. However, the petrogenesis of the Yanshanian intrusions in its western part remains poorly constrained because of insufficient high-precision geochronological and mineralogical data. This limitation hinders a comprehensive understanding of regional metallogenesis. To address this, we conducted an integrated study of the Siqian two-mica granite and the Changping biotite granite, incorporating zircon U–Pb geochronology, whole-rock geochemistry, Hf–Nd isotope analysis, and mineral chemistry. Our objectives were to determine their emplacement ages and to investigate their petrogenesis and relationship with uranium mineralization. Zircon U–Pb dating indicates that the Siqian and Changping granites formed at 160 ± 1 Ma and 156 ± 1 Ma, respectively, suggesting both were emplaced during the Early Yanshanian magmatic event. These high-K calc-alkaline rocks are peraluminous (A/CNK = 1.04–1.44), enriched in large-ion lithophile elements (LILEs; e.g., Rb, Th, U), and depleted in high-field-strength elements (HFSEs; e.g., Ba, Nb, Ta). They exhibit negative zircon ε_Hf(t) values ranging from – 15.4 to – 8.4, with corresponding crustal model ages of 1668–2142 Ma. Whole-rock samples show ε_Nd(t) values from –11.2 to –9.1 and Nd model ages of 1693–1860 Ma. These isotopic features indicate that both plutons represent moderately differentiated S-type granites derived from partial melting of Paleo- to Mesoproterozoic metasedimentary sources (e.g., pelitic and psammitic rocks), accompanied by fractional crystallization of minerals such as ilmenite and apatite. Integrated geochemical and mineralogical data reveal that the Siqian and Changping granites are characterized by low Th/U ratios, high F contents, and low magmatic oxygen fugacity—signatures typical of U-rich granites. These attributes provided a favorable lithogeochemical environment for uranium enrichment and acted as primary sources for uranium-bearing, high-temperature hydrothermal fluids, thereby playing a critical role in regional uranium mineralization.

Lake sediments are key reservoirs for sulfur (S), iron (Fe), and their associated compounds, playing a central role in the coupled Fe–S biogeochemical cycle. However, the dynamics of this cycle vary greatly among lakes with different pollution histories and redox conditions. To clarify how Fe–S cycling responds to varying geochemical environments, this study investigates the vertical distribution and speciation of multiple sulfur and iron forms, acid-volatile sulfide (AVS), elemental sulfur, Fe(III), and Fe(II), in sediments from three lakes in Guizhou, China—Maoshitou, Aha, and Hongfeng—which are characterized by distinct pollution and redox histories. We combined sequential extraction with sediment geochemical analysis to quantify the reactivity of Fe(III) and explore its role in the coupled Fe–S processes. The results show that the three lakes exhibit contrasting redox environments, leading to markedly different vertical profiles of S and Fe species. In particular, Maoshitou Lake displays unusually high concentrations of dissolved elemental sulfur, reflecting unique redox transitions in its sediments. Elevated AVS concentrations in deeper, more reducing sediments reflect active sulfate reduction and the subsequent formation of iron sulfide minerals. In these environments, solid-phase Fe(III), mainly as iron (oxyhydr)oxides, is reduced to Fe(II), which then reacts with sulfide to form FeS minerals. These results highlight that differences in lake pollution history and oxidative conditions strongly influence Fe–S coupling and the transformation pathways of reactive Fe(III) and reduced S species. The enhanced Fe(III) reactivity observed in Maoshitou Lake underscores its pivotal role in regulating Fe–S biogeochemical cycling under varying redox regimes.

Nitrate salts are critical resources for both agriculture and industry, so understanding the genesis of large deposits is of strategic importance. Nitrate deposits are typically classified into two main types based on their depositional environment: salt flat and salt lake. Previous isotopic studies have established that nitrate in the Atacama deposit, Chile, the world’s largest, and in salt flat-type deposits in Xinjiang, China, is of predominantly atmospheric origin. However, the nitrogen and oxygen isotopic signatures of the Wuzongbulake and Wuyongbulake niter deposits (salt lake type) in Xinjiang differ significantly from those of the salt flat type, suggesting an additional nitrate source. This study systematically analyzed the δ¹⁵N and δ¹⁸O values of nitrate from niter deposits and associated aqueous samples (snowmelt, fracture water, groundwater, brine, and spring water) from the Kumishi Basin, Xinjiang, to constrain these sources. The obtained results reveal that nitrate deposits (salt flat type) derive nitrate solely from atmospheric deposition. In contrast, niter deposits (salt lake type) incorporate nitrate from aqueous pathways in addition to the atmospheric contribution. This dual-source mixing model explains their contrasting isotopic signatures and provides a framework for differentiating niter deposit types in arid endorheic basins.

Karst water serves as a crucial source of water supply in many regions worldwide, yet it remains environmentally sensitive and highly vulnerable to pollution, especially from mining activities. Understanding the spatial–temporal variations in water quality and identifying the key geochemical processes in mining-impacted karst rivers are essential for ensuring regional water security and effective environmental management. This study undertakes a karst river affected by acid mine drainage (AMD), where typical mining activities generated wastewater, using a combination of Integrated Pollution Index (P), irrigation suitability assessment, health risk assessment, and multivariate statistical analysis to elucidate the impact of mining on water quality dynamics. The results show that seasonal variation of P and irrigation suitability follows the trend: dry season > flat season > wet season, but health risk assessments did not show significant differences in time. Spatially, streams influenced by upstream AMD exhibited higher P and lower irrigation suitability, indicating poor water quality unsuitable for both drinking and agricultural use. In contrast, downstream water quality improved due to dilution processes. Health risk assessments identified Mn as the primary non-carcinogenic risk factor in the study area. Multivariate statistical analysis revealed that water quality variations were predominantly controlled by pH, TN, NH₃-N, SO₄²⁻, Ca²⁺, Mg²⁺, and heavy metals, all of which are closely associated with enhanced mineral weathering impacted by mining activities. These findings provide a scientific basis for targeted environmental policy-making and sustainable management strategies aimed at protecting the ecological integrity of karst river systems under mining pressure.

The Dongxi-Nanguanling gold deposit, at the southeastern margin of the Xiaotian-Mozitan volcanic basin (XMVB), is the largest gold deposit in the eastern Beihuaiyang tectonic belt. The XMVB hosts multiple low-sulfidation epithermal gold deposits, but recent resource depletion underscores the need for new exploration indicators. Based on detailed geological observations, as well as mineralogical and geochemical analyses of hydrothermal chlorite and pyrite, we show that sericitization and carbonatization are the main hydrothermal alteration types, with alteration intensity positively correlated with ore grade. Chlorite occurs as Mg-rich varieties in andesitic host rocks and Fe-rich varieties in andesitic lithic tuffs, with Fe/Mg ratios mainly reflecting precursor rock composition. The spatial variations in Ga within Fe-rich chlorite may serve as a useful indicator for identifying mineralization centers. Chlorite geothermometry yields a formed temperature range of 227–252 °C, consistent with the variation of chlorite and pyrite geochemical features, confirming a low-temperature hydrothermal fluid system. Pyrite in ore zones shows high As and Sb contents, variable Co/Ni ratios, and H–O isotope compositions of the ore-forming fluids suggesting contributions from both magmatic-hydrothermal and sediment-derived fluids. The involvement of sediment-derived fluids in the hydrothermal system likely buffered redox state and temperature, contributing to more efficient gold deposition. Overall, the mineralogy and geochemistry of chlorite and pyrite provide robust constraints on mineralization conditions and can serve as reliable indicators for future gold exploration in the XMVB.

While geogenic processes are believed to be an important contributor to heavy metals in topsoil, notably cadmium (Cd), an understanding of heavy metal constraints from geological formations is hindered due to a lack of large-scale soil geochemical data. Addressing this, our study analyzed 15,181 topsoil samples (0–20 cm) collected from the Sichuan Basin, southwestern China, covering ~25,500 km² of land. The results indicate that geogenic Cd is more concentrated in the topsoil of the Middle-Upper Permian (P_2-3, ~1.07 mg/kg) despite regional outcrops from Cambrian (Ꞓ_2-3, 0.52 mg/kg) to Quaternary (Q_p, 0.29 mg/kg) strata. We assume that the Emeishan basaltic lava that erupted in the Late Permian (P₃e) in southwest China is an understudied factor. This erupted lava probably provided essential elements and heat necessary for heavy metal accumulation in a preferred sea–continental and marine environment, eventually facilitating leaching and diffusion of the metals. Ultimately, geological exposure and pedogenesis led to elevated metals in the topsoil of P_2-3 strata.

Spring systems in tectonically active evaporite basins provide critical insights into groundwater circulation, solute sources, and geothermal potential. However, the processes controlling the hydrogeochemical evolution of springs in the Nangqian Basin, located in the southeastern Qinghai–Tibetan Plateau, remain insufficiently understood. In this study, we collected a comprehensive dataset of 20 spring and river water samples during both wet and dry seasons in 2022, encompassing saline, hot, and cold springs. Major and trace elements were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS), while hydrogen, oxygen, tritium, and boron isotopes were determined through isotope ratio mass spectrometry and liquid scintillation counting. The results indicate that saline springs are characterized by high concentrations of Na⁺, Cl⁻, Ca²⁺, and SO₄²⁻, which suggest dominant evaporite dissolution. In contrast, hot springs exhibit significantly elevated silica contents and reservoir temperatures exceeding 130 °C. The stable isotopic compositions plot near the local meteoric water line, and tritium activity (3.6–26.4 TU) indicate modern meteoric recharge, which is generally considered to have residence times of less than 60 years. Most boron isotope ratios (−8.12‰ to intermediate values) suggest continental depositional characteristics, whereas isolated high ratios, such as +28‰, likely record marine evaporite influences; these variations also reflect isotopic fractionation during deep groundwater circulation. Estimated circulation depths range from approximately 1300 m in cold springs to over 4200 m in hot springs, reflecting contrasting flow paths and structural controls. Integrated geochemical and isotopic evidence supports a conceptual model wherein meteoric water infiltrates along faults and fractures, undergoes extensive water–rock interaction with evaporitic and silicate strata, and variably mixes with ascending geothermal fluids before discharge. These findings enhance the understanding of hydrogeochemical processes in high-altitude evaporite basins and provide a scientific basis for groundwater resource management and geothermal exploration in the Nangqian Basin.

This study evaluates the Se concentration and examines the geologic controls on Se concentration in Upper Cretaceous and Palaeogene mudrocks in parts of Southern Nigeria. Geochemical analysis [(major and trace elements, total organic and inorganic carbon (TOC and TIC)] and pH measurements were conducted on seventy-three mudrock outcrop samples from the estuarine Late Campanian to Mid-Maastrichtian Mamu Formation and ditch cuttings from water wells that penetrated the marine Palaeogene Imo Formation from the Anambra basin and Niger Delta basin in the Southern Benue Trough. Selenium concentration in the Palaeogene calcareous marine mudrock samples reaches up to 21 ppm, significantly exceeding data observed in the Upper Cretaceous estuarine mudrock samples. Covariation charts suggest that Se adsorption onto clay, Fe-minerals, and limited sorption by organic matter in the estuarine mudrock samples are compared to adsorption onto clays, calcite, and very limited sequestration by Fe-minerals observed in the calcareous marine mudrocks. Principal component analysis and redundancy analysis reveal a dominant influence of carbonate minerals on selenium sequestration. Additionally, the Palaeogene calcareous marine mudrock samples are less acidic (mean pH 5.92) than the Upper Cretaceous estuarine mudrocks with a mean pH of 5.02. Therefore, it is hypothesized that higher pH may slightly promote the bioavailability of Se, resulting in higher Se intake by plants in areas underlain by the Palaeogene calcareous mudrocks.