Coal combustion byproducts are increasingly examined as secondary sources of rare earth elements (REEs) and critical elements (CEs), but resource appraisal based only on total concentration can overstate practical recovery potential and overlook environmental liabilities. This study integrated geochemical enrichment, acid extractability, contamination, and ecological risk to evaluate feed coal, fly ash, bottom ash, and mixed coal combustion residue. Thirty-six composite samples were characterized for elemental composition and mineralogy using standard laboratory techniques. Recovery performance was assessed through standardized hydrochloric-acid leaching, target-element leaching efficiency, matrix dissolution, selectivity, standardized HCl dosage, and a recovery-potential score. Environmental performance was evaluated using contamination factors, the pollution load index, geoaccumulation index, and potential ecological risk index. Fly ash contained the highest mean ΣREY concentration (1205.44 ± 182.95 mg kg − 1), enrichment factor (3.65 ± 0.36), target-element leaching efficiency (66.45 ± 8.20%), and recovery-potential score (0.77 ± 0.06). Mixed combustion residues retained substantial resource value but also had the highest mean pollution load and ecological risk. CRITIC–TOPSIS weighting shifted the strongest priorities toward fly ash and selected bottom-ash samples while reducing the priority of the most environmentally burdened residues. The integrated framework distinguished high-value recoverable streams, pre-treatment-dependent targets, environmental-management priorities, and low-priority materials, providing a defensible basis for screening coal-derived materials before detailed process development.
Trace-element patterns in pyrite (Py) record successive stages of fluid evolution and metal transport and are widely used to reconstruct ore-forming processes. In this study of the Zawar PbZn deposit, Rajasthan, India, an unsupervised machine learning workflow is applied to trace element data by integrating centred log-ratio transformation, Hierarchical Density-Based Spatial Clustering of Applications with Noise, i.e. HDBSCAN clustering, and non-linear dimensionality reduction techniques (t-SNE and UMAP), to discriminate Py generations based on multivariate geochemical signature. In-situ LA-ICP-MS analyses, encompassing framboidal, euhedral and coarse crystalline textures, are conducted on 96 analytical spots, quantifying the concentrations of 22 trace elements. This framework identifies three geochemically distinct pyrite populations Py-1, Py-2, Py-3, without prior labels, consistent with petrographic observations while preserving transitional compositions indicative of fluid mixing. Geochemical ratios (As/Sb, Co/Ni, Cu/Zn, Pb/Zn, Au/Ag) show systematic compositional differences among the machine-learning derived clusters. Weighted modular correlation network analysis (WMCNA) further shows a progressive reorganisation of trace-element coupling from dense and coherent early-stage associations to progressively more fragmented late-stage associations. Elemental enrichments and characteristic inter-element linkages revealed by WMCNA further emphasize systematic geochemical differences corresponding to the discrete stages of Py growth. Py-1 exhibits a distinct multielement geochemical signature characterized by a densely connected trace-element correlation network. Py-2 shows elevated Zn concentrations and relatively higher As/Sb ratios and is petrographically associated with the principal sulphide mineralization stage. Py-3 exhibits weaker and more heterogeneous trace-element associations, reflecting compositional differences among successive pyrite generations. This integrated approach complements petrography and enhances the reconstruction of pyrite paragenesis and ore-forming processes.
Quartz veins in the Danyore Valley prospect of the Kohistan Island Arc, northern Pakistan, host Cu-Au-bearing sulfide assemblages and provide a useful archive of hydrothermal fluid evolution. Previous geochemical, zircon geochronological, and SPb isotopic studies have constrained the emplacement and geodynamic evolution of mineralized Mesozoic granitoid intrusions, but the sources of the fluids and ore-forming components, as well as the evolution of the ore-forming system, remain poorly understood. This study integrates petrography, scanning electron microscope cathodoluminescence (SEM-CL) imaging, and in situ LA-ICP-MS trace-element analysis of quartz veins hosted mainly in Early Cretaceous intrusions at the Danyore Valley CuAu prospect. Four quartz generations (Qz-1 to Qz-4) are identified. The earliest generation (Qz-1) is weakly luminescent and texturally homogeneous; Qz-2 shows oscillatory growth zoning and records renewed quartz precipitation; Qz-3 forms the main vein-filling quartz and is locally cut and sealed by bright, fracture-controlled Qz-4. Chalcopyrite, pyrite, and bornite, and the associated CuAu mineralization, are spatially associated only with Qz-4. Trace-element data indicate that Qz-1 to Qz-3 are generally Ti-poor (medians 0.86, 1.24 and 0.11 ppm, respectively), have low base-metal contents, and have Al/Ti and Ge/Al ratios consistent with precipitation from evolved, low-salinity metamorphic (orogenic-style) fluids. Elevated Cu (median 323 ppm) and high Au (up to 118 ppm) in the Qz-4 domains, together with their bright CL response, sharp cross-cutting relationships, and association with sulfides, indicate that Qz-4 records the principal metal-introducing stage. Fluid cooling and possible boiling, accompanied by the loss of volatile components such as CO2 and H2S, may have destabilized metal-ligand complexes and promoted the precipitation of Cu-Au-bearing sulfides. Combined with published sulfur and lead isotope data, these observations suggest that one or more pulses of metal-bearing hydrothermal fluids overprinted an earlier, barren metamorphic-hydrothermal vein system. We therefore interpret the Danyore Valley prospect as a structurally controlled quartz–sulfide vein system with a late magmatic-hydrothermal, possibly porphyry-related, CuAu overprint; a distal position within a concealed porphyry system cannot be excluded on present evidence.
This study investigates the geochemical evolution of highly acidic mine tailings from the abandoned Ouixane iron mine (northeastern Morocco) through kinetic leaching experiments and PHREEQC geochemical modeling. During 85-day weathering cell experiments, leachates remained persistently acidic (pH 2.20–3.95), with cumulative sulfate releases reaching 74,800–90,600 mg/kg, while iron and aluminum releases ranged between 14,900–17,700 mg/kg and 963–3070 mg/kg, respectively. Significant mobilization of trace elements was also observed, with Zn, Cu, and As reaching maximum values of 52.5, 108, and 20.8 mg/kg, respectively. Column tests conducted over 12 months confirmed sustained acid generation throughout the experimental period, with pH values remaining extremely low (1.48–2.48). Sulfate releases ranged from 74,000 to 187,000 mg/kg, while iron and Al releases varied between 12,600–13,800 mg/kg and 58.0–2250 mg/kg, respectively. Maximum releases of trace elements reached 19.8 mg/kg for Zn, 120 mg/kg for Cu, and 21.1 mg/kg for As. Geochemical modeling showed that dissolved iron occurred predominantly as Fe2+ under strongly acidic conditions, whereas Fe(III)-bearing secondary minerals, including schwertmannite, ferrihydrite, goethite, and jarosite, remained undersaturated, limiting metal attenuation. Gypsum was slightly supersaturated during the early stages, indicating limited secondary precipitation. Sulfate speciation was dominated by SO42−, with additional contributions from HSO4− and metal–sulfate complexes that enhanced trace-element mobility. These findings demonstrate the persistent acid-generation and metal(loid)-release potential of the Ouixane tailings and provide new insights into the geochemical processes governing the evolution of abandoned mine wastes, supporting appropriate rehabilitation and environmental management strategies.
Cobalt (Co), a strategic critical metal, is significantly sourced from magmatic CuNi sulfide deposits. The Jinchuan deposit, the world's third-largest magmatic CuNi sulfide deposit, provides an ideal setting for studying Co enrichment. Previous studies on Co enrichment have focused on isolated stages without quantitative comparison of their relative contributions, leaving the dominant mechanism unresolved. This study analyzed ore types (massive, net-textured, and disseminated) from drill core ZK50–3 in Segment II, employing petrography, SEM-EDS, and EPMA to elucidate the distribution, occurrence states, and enrichment mechanisms of Co. Results show that Co primarily resides in pentlandite via isomorphous substitution for Fe2+ and Ni2+, with selective fluid alteration causing anomalous enrichment in secondary magnetite and pyrite. Two key enrichment stages are identified: sulfide liquid segregation (enrichment factor: 39–43) and fractional crystallization of the sulfide melt (enrichment factor: 172–189), with the latter playing a dominant role. This study provides the first quantitative comparison of their relative contributions to Co enrichment. The high-Mg tholeiitic parental magma, relatively enriched in Ni but depleted in Co, was generated by low-degree partial melting of peridotite within a metasomatized, enriched subcontinental lithospheric mantle. During cooling of the sulfide melt, Co preferentially entered the monosulfide solid solution (MSS) and subsequently migrated into pentlandite at 450–550 °C. The presence of pyrite and magnetite with elevated Co concentrations can provide useful exploration clues for CoNi mineralization. In particular, Co-rich hydrothermal magnetite acts as a diagnostic indicator of intensive fluid overprinting. This work elucidates the occurrence and enrichment of Co, offering insights for both ore genesis and mineral exploration.
