Search Results (708)

The Galinge deposit in East Kunlun, China is a large Fe-polymetallic skarn system with a significant by-product, Co, but the respective roles of magmatism, skarn evolution, and wall rock interaction in Co enrichment remain incompletely understood. We integrate zircon and garnet U–Pb geochronology, zircon Lu–Hf isotopes and trace elements, whole-rock geochemistry, and SEM-EDS and EPMA mineral chemistry. Granodiorite and diorite porphyry yield zircon U–Pb ages of 230.09 ± 0.91 Ma and 229.4 ± 1.3 Ma, respectively, whereas skarn garnet yields 224.4 ± 9.3 Ma, placing intrusion and skarn formation within a Late Triassic magmatic–hydrothermal system. Both suites are metaluminous, LREE-enriched, and Nb–Ta–Ti-depleted; zircon ε_Hf(t) values of −9.4 to −1.8 indicate the predominant reworking of older crustal material with variable input from a more radiogenic component. Strictly screened Ti-in-zircon temperatures and lattice strain Ce anomalies yield median apparent ΔFMQ values of +3.36 for granodiorite and +3.04 for diorite porphyry, indicating comparably oxidized magmatic conditions. The analyzed intrusions contain 2.12–13.4 ppm Co, whereas cobaltite and Co-bearing arsenopyrite contain 32.83–34.14 wt% and 0.38–4.53 wt% Co, respectively. Spatial and paragenetic relations place Co enrichment after magnetite deposition, during an early sulfide-stage hydrothermal sulfarsenide event within the skarn system. We infer that Late Triassic intrusions supplied heat, fluids, and ligands, whereas structural focusing and cooling, coupled with carbonate wall rock reactions and a reduction in carbonaceous or Fe²⁺-bearing domains, promoted As–S-rich Co precipitation; the leaching of intermediate–mafic wall rocks may have supplemented the Co inventory. Full article

Permian sedimentary manganese carbonate deposits in the Qiling Basin exhibit substantial resource potential, yet their depositional environments and precipitation pathways remain insufficiently constrained. This study integrates petrographic, mineralogical, whole-rock geochemical, and carbonate C–O isotope data from the Shuibutou Mn deposit in southern Hunan. We compare this deposit with the coeval Dongxiangqiao deposit and other Permian marine sedimentary Mn deposits to constrain its depositional setting, Mn-carbonate precipitation mechanisms, and potential Mn sources. The Shuibutou ores contain 10.88–17.18 wt.% MnO and are characterized by spherulitic–oolitic textures, abundant bioclasts, and framboidal pyrite. Mo–U enrichment indicates anoxic bottom waters, whereas the near-marine δ¹³C_carb values of ore carbonates (−0.24‰ to 1.28‰) are consistent with precipitation within a marine carbonate system and suggest direct precipitation of Mn(II) carbonates from dissolved Mn²⁺ under anoxic conditions. In this setting, dissolved Mn²⁺ accumulated below the chemocline. Partial dissolution of platform-derived calcite grains near the chemocline increased local dissolved inorganic carbon and alkalinity, driving Mn-carbonate supersaturation and the authigenic precipitation of manganoan calcite, ultimately forming manganese carbonate ores. The low Al/(Al + Fe + Mn) ratios, high Fe/Ti ratios, and Co–Ni–Zn and REY geochemical characteristics are consistent with a possible contribution from Mn-rich deep fluids to the dissolved Mn²⁺ inventory of the basin waters. Similarities in mineralogy, geochemistry, and depositional setting between Shuibutou and Dongxiangqiao point to a possible regional role for the direct precipitation of Mn(II) carbonates under anoxic conditions in the Middle Permian Qiling Basin. Relatively deep intraplatform basins may therefore represent favorable targets for manganese carbonate exploration and provide a reference for exploration targeting of Permian sedimentary Mn deposits within the Qiling Basin. Full article

Trace element contamination represents a persistent environmental issue, particularly in industrialized areas where anthropogenic emissions overlap with natural geochemical backgrounds. This study investigates the atmospheric deposition of trace elements in the Milazzo district (Italy), which is characterized by intense industrial activity. Pinus pinea L. needles were used as biomonitors to assess the spatial distribution and sources of trace elements, combined with lead isotopic analysis for source apportionment. Forty needle samples were analyzed by ICP-OES and ICP-MS for Ca, K, Mg, Na, P, Al, As, Ba, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Sb, Ti, V, Zn, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu, while 25 samples were selected for Pb isotope ratio determination (²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁶Pb). Multivariate statistical analyses identified source groups related to industrial and petrochemical emissions, vehicular traffic, crustal resuspension, and mixed combustion processes. Elevated concentrations of As, Cr, Mo, Ni, Pb, Sb, V, and Zn ranged from 16.6 μg g⁻¹ (Zn) to 0.09 μg g⁻¹ (Sb), with the following order of abundance: Zn > Cr > Ni > Pb > Mo > V > As > Sb; these elements were found near industrial facilities and urban areas. Enrichment Factor calculations indicated strong anthropogenic contributions to Cd, Cu, Mo, Sb, V, and Zn, with EF > 10, ranging from 10 (Cd) to 60 (Zn), whereas Al, Fe, and Ti exhibited EF values between 0.5 and 2, reflecting geogenic origins. Pb isotopic ratios (²⁰⁶Pb/²⁰⁷Pb = 1.153–1.192 and ²⁰⁸Pb/²⁰⁶Pb = 2.063–2.108) revealed mixing between industrial emissions and the local geological background, with limited influence from historical gasoline-derived Pb. This integrated geochemical and isotopic approach can effectively identify contamination sources in complex industrial environments. Full article

The Dafosi well field is a typical Huang-Long Jurassic low-rank coalbed methane (CBM) field. Clarifying its CBM geochemical characteristics and the mechanisms of its formation is of significant importance for deepening the understanding of the formation mechanisms of low-rank CBM in China and for the scientific assessment of its resource potential. A total of eight gas emission samples from six coalbed methane wells in the Dafosi coalfield were collected, along with 22 coal samples from the 4^# coal seam. Detailed analyses of microscopic coal petrographic components, gas chemical compositions, and carbon isotopes were performed. By integrating data from the 20 relevant literature sources on coalbed gas composition and isotopic characteristics within the study area, a comprehensive dataset comprising 28 sets was utilized to examine the carbon isotope characteristics and genesis types of both CH₄ and CO₂ in the coalbeds, as well as elucidate the mechanism behind CH₄ carbon isotope depletion. The findings indicate that in the primary 4^# coal seam’s microscopic petrographic composition, the organic matter content is considerably higher, averaging 93.2%. Among these, the inertinite group is dominant, averaging 68.2%; the vitrinite group is the next most abundant, averaging 22.8%. The CBM composition is predominantly CH₄, with concentrations varying from 68.753% to 98.006%, averaging 80.276%. N₂ concentrations range from 1.259% to 29.926%, averaging 17.476%. CO₂ concentrations vary from 0.04% to 2.380%, averaging 1.032%. The average concentration of heavier hydrocarbons C₂ and above is less than 0.078%, indicative of typical dry gas characteristics, C₁/C_1~n > 0.999. The concentration of CH₄ and N₂ was negatively correlated. δ¹³C₁ ranges from −87.200‰ to −62.400‰, averaging −75.802‰. CH₄ is composed of secondary biogenic gas with dominant content and a small amount of thermogenic gas. δ¹³C_CO2 ranges from −41.693‰ to −7.065‰, averaging −20.016‰. CO₂ is an organic gas, mainly derived from thermal degradation and microbial degradation of organic matter. The mechanism responsible for the light carbon isotopic composition of δ¹³C₁ lies in the fact that most of CH₄ is produced by CO₂ reduction, and a small amount is produced by acetic acid fermentation. In the gas generation process of these two pathways, biogenic methane will eventually enrich light carbon isotopes, resulting in light δ¹³C₁. Full article

The Çıralı (Yanartaş) gas seepage system in the Tekirova Peridotite, SW Türkiye, is an ophiolitic H₂–CH₄ system associated with serpentinized ultramafic rocks and fault-controlled pathways. This study evaluates molecular compositions, C₁–C₃ compound-specific carbon isotope data, and methane hydrogen isotope compositions from four newly sampled gas outlets together with previously published data for which the full parameter set is available. The new samples contain 85.5–91.6% CH₄ and 2.6–5.3% H₂, with δ¹³C-CH₄ values of −7.3 to −4.4‰ and δ²H-CH₄ values of −121 to −104‰. The C₁–C₂ isotope reversal index (RI₁₂) ranges from +13.6 to +14.5‰, whereas the C₂–C₃ isotope proximity index (PI₂₃) ranges from 0.2 to 1.3‰. These values indicate the clear isotopic decoupling of methane from ethane and propane, whereas the C₂–C₃ pair shows comparatively small isotopic differences. The same relative pattern occurs at all four outlets, indicating consistency between samples. The results are consistent with a multistage model in which C₂+ components are compatible with a thermogenic contribution, whereas methane may have been affected by serpentinization-related H₂ input, fracture-controlled flow, and shallow ophiolitic modification. Catalytic CO₂ hydrogenation is evaluated not as a demonstrated field-scale process but as a hypothesis requiring testing in chromitite–serpentinite-related microenvironments. Full article

Located 30 km south–southeast of Kabul in Logar province, Aynak is the largest and best-known copper orebody in Afghanistan. The deposit is hosted by the Loy Khwar Formation, a Neoproterozoic–Cambrian metasedimentary sequence of dolomite marble, carbonaceous quartz schist and quartz–biotite–dolomite schist. The primary ore minerals are chalcopyrite and bornite, with less abundant pyrite and minor cobaltite, chalcocite, pyrrhotite, sphalerite and molybdenite. Sulphides occur as bedded laminae and disseminations, in metamorphic segregations, and in syn- to post-metamorphic cross-cutting veins. Building on the mineralogical, lithogeochemical and sulphur isotope framework established by Waizy et al. (2020), ICP-MS analyses of sulphide-rich separates from Central and Western Aynak (n = 31) were undertaken to characterise trace-element distributions, evaluate possible metal sources, and further constrain the genetic model of the deposit. Co and As enrichment in chalcopyrite-dominant samples is consistent with cobaltite, whereas Co enrichment in the absence of arsenic suggests the possible presence of carrollite. Fluid inclusion analyses of secondary quartz-hosted inclusions indicate interaction between the Aynak deposits and saline aqueous fluids (32 to 47 equivalent wt% NaCl) at minimum P-T conditions of ~100–200 MPa and 300 °C. It is uncertain whether these fluid parameters relate to primary copper transport and deposition, or to remobilisation during metamorphism. Nevertheless, comparison with analogous sediment-hosted copper deposits suggests that highly saline basinal brines played an important role in the formation and evolution of the deposit. Occurrences of scapolite provide additional evidence for a model of brine-related mineralisation. Together with previously published mineralogical, lithogeochemical and sulphur isotope evidence, these findings support a sedimentary–diagenetic origin for the Aynak copper deposit that is broadly comparable with sediment-hosted stratiform copper systems of the Central African Copperbelt. Full article

The Xiaoyinuogaigou (XYN) gold deposit is located in the central Erguna Metallogenic Belt, northeastern China, and represents a characteristic example of Mesozoic gold mineralization in the Mongol–Okhotsk orogenic province. Despite its economic significance, the timing of mineralization and the nature of the ore-forming fluid system have not been well constrained. To address these questions, a systematic investigation of fluid inclusions, isotopes, geochemistry, and geochronology was conducted. The study reveals that: (i) four types of fluid inclusions are identified in auriferous quartz veins, with co-existing aqueous liquid-vapor and NaCl daughter-mineral three-phase inclusions, indicating the involvement of at least two chemically distinct fluids; (ii) the ore-forming fluids are characterized by moderate temperatures (163.2–357.6 °C), low to moderate salinities (3.06–11.34 wt% NaCleqv.), and H–O isotope compositions (δD = −133.3 to −112.3‰; δ¹⁸O fluid (SMOW) = +1.74 to +4.06‰) consistent with a hydrothermal system incorporating a substantial non-magmatic water component; and (iii) REE geochemistry and sulfur isotopes (δ³⁴S = +4.47 to +11.75‰, mean +8.29‰, n = 4) indicate that ore-forming materials were derived at least in part from the ore-hosting granite porphyry. Zircon U–Pb geochronology constrains the granite porphyry crystallization to 180 ± 3 Ma, and hydrothermal monazite U–Pb dating yields an age of 162.3 ± 3.6 Ma (MSWD = 0.95), for a Middle Jurassic hydrothermal event. We therefore conclude that XYN is an epizonal orogenic gold deposit formed during the late compressional to post-collisional transition of the Mongol–Okhotsk orogeny. Full article

Low-TOC saline lacustrine shales are widespread in Cenozoic basins of northwestern China and Central Asia, yet their organic matter accumulation (OMA) mechanisms under arid, saline conditions remain poorly understood. We analyzed TOC, major and trace elements, and C–O isotopes in 64 core samples from eight wells in the western Qaidam Basin, and reconstructed terrigenous input, weathering intensity, paleoproductivity, redox conditions, and salinity. TOC ranges from 0.04% to 1.10% (mean 0.32%), with a statistically significant upward trend from the Shangganchaigou (mean 0.25%) to the Shangyoushashan Formation (mean 0.43%) (Kruskal–Wallis, p = 0.010). The climate was dry–cold with progressive aridification (increasing δ¹⁸O and Sr/Ba). The water body evolved from closed, strongly reducing, and unstable to more open, weakly reducing, and increasingly saline. We propose that limited nutrient supply set a low baseline for organic enrichment. Weakened terrigenous dilution and enhanced water-column stability then drove the gradual TOC increase. This reveals that OMA can occur without concurrent productivity rise, sustained solely by long-term preservation improvement, offering new constraints for source rock evaluation in arid lacustrine basins. Full article

Carbonate jade has emerged as a recently recognized commercial variety in the Chinese gemstone market. Systematic gemological and mineralogical investigations on carbonate jade, however, remain very scarce. Three Xinjiang calcite-dominated jade samples were investigated by using Fourier-transform infrared (FTIR) spectroscopy, Raman spectroscopy, ultraviolet-visible (UV-Vis) absorption spectroscopy, microbeam X-ray fluorescence (Micro-XRF) spectrometry, trace element analysis, in situ U-Pb dating, and C-O isotope analysis. The orange-red color originates from staining by hematite and magnetite inclusions, while the green color is produced by d-d electronic transitions of lattice-bound Fe³⁺ and Mn²⁺. The provenance comparison reveals systematic differences in trace element compositions between the Xinjiang carbonate jade and Pakistani Lvwen stone: the Xinjiang samples are characterized by extremely low Cu and Sr contents, whereas the Pakistani Lvwen stone has high Cu, Mn and Sr contents, and low Fe content. The U-Pb age obtained for the Xinjiang carbonate jade sample coincides with a Late Cretaceous rapid cooling event. Enriched light rare earth element (LREE) and C-O isotope (δ¹³C_V-PDB = −1.19–2.21‰, δ¹⁸O_V-SMOW = 15.00–20.01‰) signatures indicate that the carbonate-precipitating fluids were derived from marine carbonate wall rocks. These findings provide new mineralogical and geochemical constraints on the coloration mechanism, provenance, and fluid evolution of carbonate jade from Xinjiang. Full article

Groundwater is a critical resource for domestic, agricultural, and industrial use in the Gilgel Gibe and Dhidhessa catchments of southwestern Ethiopia, where volcanic aquifer systems are the main sources. However, groundwater quality in these catchments has been under pressure from anthropogenic activities such as population growth, land-use changes, and pollution driven by rapid development and poor resource management. This study investigates hydrogeochemical processes and evaluates groundwater quality in volcanic aquifers using hydrochemical analyses and a stable isotope approach applied to 115 water samples. The spatial distribution of various physicochemical and hydrogeochemical parameters shows a distinct contrast between the highland and lowland regions, indicating topography-driven variations in water quality and geochemical processes. In hand-dug wells, springs, and surface waters, the ionic order is Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ and HCO₃⁻ > NO₃⁻ > Cl⁻ > SO₄²⁻, whereas deep wells show Na⁺ > Ca²⁺ > Mg²⁺ > K⁺ and HCO₃⁻ > Cl⁻ > SO₄²⁻ > NO₃⁻. The predominant groundwater type is Ca-HCO₃, followed by Na-HCO₃ and Ca-NO₃, with other types including Ca-Mg-HCO₃, Ca-Na-HCO₃, and Na-Ca-HCO₃. Water types of Ca-HCO₃ and Ca-Mg-HCO₃ dominate the upland areas, indicating relatively young groundwater with moderate total dissolved solids (TDSs) and enrichment in δ¹⁸O and δ²H, where highly mineralized Na-HCO₃ water types prevail in the deep aquifers of the lowland regions, where δ¹⁸O and δ²H are relatively depleted. Principal component analysis, cross-plots of major cations versus HCO₃⁻, and mineral stability diagrams indicate that aluminosilicate weathering and dissolution are the dominant processes controlling groundwater chemistry in the study area. The higher saturation index values observed in the deep wells indicate water closer to mineral equilibrium, suggesting more extended water–rock interaction relative to the shallow wells. The CO₂ partial pressures calculated using PHREEQC exceed atmospheric levels (~10^−3.5 atm), indicating sources from atmospheric influx, soil, or biogenic activity for most samples, and deeper sources such as mantle degassing may be found in a few deep wells. Scatter plots of Cl⁻ vs. SO₄²⁻ and Cl⁻ vs. NO₃⁻, associated with Ca(NO3)₂, NaNO₃, and CaCl₂ water types, suggest that anthropogenic inputs are the second major factor influencing the area’s water chemistry. Stable isotope analyses and hydrochemical data indicate that groundwater in the area primarily originates from local precipitation, with isotopic signatures reflecting strong groundwater–surface water interaction. These findings improve understanding of regional hydrogeochemistry and groundwater quality and help identify promising zones for sustainable groundwater development. This study provides valuable insights into groundwater resource management both in the study area and in regions sharing comparable geological contexts. Full article

The Tan–Lu Fault Zone (TLFZ) is a lithosphere-scale fault system in eastern Asia with pronounced along-strike heterogeneity in structure and fluid activity. Previous geochemical studies have largely focused on individual segments, sample media, or single tracers, leaving the fault-zone-scale organization of fluid geochemical signals unresolved. Here, published helium-isotope data and cross-fault soil-gas CO₂ and Rn observations are integrated within a common fault-parallel coordinate framework to compare source-sensitive proxies with those more responsive to transport and near-surface processes. The three proxy groups define contrasting spatial patterns. Mantle-derived He is strongest and spatially focused in Liaoning, intermediate but comparatively persistent in Anhui–Jiangsu, and generally subdued in the available Shandong dataset. Soil-gas CO₂ and Rn anomalies, by contrast, are most continuous in Anhui–Jiangsu, locally enhanced in Liaoning, and subdued to intermediate in the available Shandong transects. Combining the mantle-derived He state with the soil-gas response identifies four proxy-association types. The most laterally continuous Type I interval extends for approximately 200 km in the Anhui–Jiangsu segment, whereas southern Liaoning is characterized by spatially focused, adjacent Type II and Type I intervals. These contrasting associations show that the near-surface expression of deep-derived fluids depends not only on volatile availability, but also on crustal pathway connectivity, groundwater circulation, and modification during transport and release. The integrated comparison reveals where deep-source and shallow-response signals are coupled or decoupled along the TLFZ, providing a basis for evaluating fault-zone fluid segmentation. Full article

The newly discovered Caijiagou gold deposit is hosted in metasedimentary rocks of the Silurian Taiyangsi Formation in the West Qinling Orogen (WQO), central China. Three mineralization stages are recognized: (1) the quartz–sericite–pyrite stage (Stage I), characterized by fine-grained, disseminated pyrite (Py1) and arsenopyrite (Apy1) with trace amounts of native gold; (2) the main ore-stage, the quartz–polymetallic sulfide–ankerite stage (Stage II), which contains coarse-grained euhedral pyrite (Py2) and arsenopyrite (Apy2) intergrown with ankerite and native gold; and (3) the post-ore calcite stage (Stage III), which is barren. Hydrothermal alteration assemblages include silicification, sericitization, pyritization, and carbonatization. In situ trace element analyses show that gold is mainly hosted in Py2 (avg. 2.86 ppm Au) and Apy2 (avg. 1.95 ppm Au), with consistently low Co/Ni ratios (<0.6) in ore-stage sulfides, supporting a metamorphic fluid origin. Sulfur isotope values (δ³⁴S = +6.47‰ to +11.96‰) point to sulfur derived from thermochemical reduction of marine sulfate. Apatite from the main ore stage (Stage II) yields a U–Pb lower intercept age of 237 ± 4.3 Ma (MSWD = 0.89), placing the mineralization in the Middle Triassic (Indosinian orogeny). These results are consistent with a sediment-hosted orogenic gold model, and the geochemical and geochronological signatures provide insights into the ore-forming process and regional metallogeny of the WQO. Full article

To elucidate the response characteristics of radial growth and water use strategies in coniferous forests of cold-arid regions to drought stress, this study focused on Larix sibirica in different forestry areas of the Altai Mountains. Using dendrochronology and stable isotope techniques, we calculated the basal area increment (BAI) and intrinsic water-use efficiency (iWUE), and combined these with the standardized precipitation–evapotranspiration index (SPEI) to identify drought events, to investigate tree growth and water-use efficiency responses to climate variability. The results showed that drought years were characterized by reduced radial growth and increased iWUE in Larix sibirica across both forest regions, and tree-ring-derived intercellular CO₂ concentration (Ci) increased with rising atmospheric CO₂ concentrations, whereas the Ci/Ca ratio remained relatively stable throughout the study period. Scenario analysis revealed that, prior to 1980, the long-term trend in iWUE was more consistent with the constant Ci scenario, suggesting relatively strong stomatal regulation. After 1980, iWUE trends became more closely aligned with the constant Ci/Ca scenario, indicating that trees maintained a relatively stable Ci/Ca ratio to balance carbon assimilation and water loss. With increasing drought severity, drought resistance declined in both forest regions; however, substantial spatial differences were observed in drought responses. Under moderate drought conditions, trees in the Haba-River forest area exhibited higher resistance, whereas trees in the Hanaslin forest area showed greater recovery capacity and ecological resilience. Winter temperature, growing-season temperature, late-season temperature, and water availability were identified as key climatic factors influencing variations in the radial growth and iWUE of Larix sibirica. Overall, under the combined influences of rising atmospheric CO₂ concentrations and increasing water limitations, Larix sibirica exhibited adaptive adjustments in carbon–water regulation; however, enhanced iWUE did not fully compensate for the negative effects of drought on radial growth. These findings provide valuable insights into the responses and adaptive strategies of cold-arid forest ecosystems under ongoing climate change and offer scientific support for the conservation and sustainable management of Larix sibiric forests. Full article

The Hailijin uranium (U) deposit is one of the super-large U-deposits recently discovered in the Qianjiadian ore field, southwestern Songliao Basin. The ore bodies are mainly hosted in the lower member sandstones of the Upper Cretaceous Yaojia Formation and occur as multilayered tabular bodies. The nature and source of the ore-forming fluids remain unclear, limiting the understanding of the genetic type of the deposit. This study integrated drill-core observation, mineralogy, whole-rock geochemistry, in situ pyrite trace elements and sulfur isotopes, fluid-inclusion, and Raman spectroscopy to constrain ore-forming fluids. The host sandstones experienced hematitization, limonitization, carbonate cementation, clay alteration, sulfidation and bleaching. Pitchblende and coffinite occur as submicron grains in dissolution pores of quartz and feldspar, on clay-mineral surfaces and within mobile organic matter (OM), commonly associated with pyrite and sphalerite. The ores and gray mineralized sandstones are enriched in U, Mo, Re, Co, Ni, Zn and Pb, and syn-ore pyrite shows positive correlations between U and As, Mo, Cu, Zn, Se and Sb. Mineralization-related fluid-inclusion assemblages occur mainly in syn-ore dolomite/ankerite cements and in secondary trails along microfractures in detrital quartz; they yield homogenization temperatures of 130–190 °C and salinities of 3–8 wt.% NaCl eq., higher than the normal burial temperature of the basin (80–90 °C), especially meteoric fluid. Raman and gas-chromatographic analyses indicate carbonaceous matter, CH₄, CO₂, H₂ and minor O₂. Pyrite δ³⁴S values of −49.24‰ to −23.1‰ indicate isotopically light reduced sulfur ultimately related to microbial sulfate reduction and/or thermal decomposition of sulfur-bearing OM, whereas thermochemical sulfate reduction was unlikely to be dominant. Therefore, the ore-forming fluid is interpreted as a low-temperature, low-salinity organic-rich fluid, most likely derived from U-enriched source rocks at depth, and the uranium mineralization is closely associated with the exudation of such deep-derived organic fluids. Full article

The origin and metallogenic role of pyrite in sandstone-hosted uranium deposits remain difficult to constrain because pyrite may record both early diagenetic reduction and later ore-fluid overprinting. Here, pyrite and associated uranium minerals from mineralized and barren sandstones of the Middle Jurassic Zhiluo Formation in the Yahewan uranium deposit, southern Ordos Basin, were investigated using petrography, SEM-BSE imaging, EPMA, LA-ICP-MS trace-element analysis, and in situ sulfur isotope analysis. Pyrite occurs mainly as framboidal, colloidal, pore-filling, fracture-filling, and massive aggregates and is commonly associated with organic matter and coffinite or coffinite-like U-silicate minerals. Most pyrite domains show low Co/Ni ratios, suggesting a predominantly authigenic to sedimentary–diagenetic origin. High U, Mo, V, W, Se and As occur mainly in pyrite-rich microdomains, but high-U analyses are interpreted cautiously because some signals may include contributions from adjacent coffinite or coffinite-like U-silicate phases, fracture-hosted uranium minerals, or mixed ablation. Together, the petrographic, trace-element and sulfur isotope data support a two-stage model in which early organic matter and authigenic/biogenic pyrite created local reducing microenvironments that were later overprinted by U-bearing basinal fluids. Full article