Based on detailed mineralogical and geochemical analysis, we determined that the Ga- and V-bearing minerals precipitated subsequent to the main alunite mineralization. The in-situ LA-ICP-MS trace element results reveal that Ga and V are predominantly enriched in dickite/pyrophyllite and Fe-Ti oxides, with peak concentrations of 3543 ppm and 1817 ppm, respectively. The in-situ LA-ICP-MS mapping and strong positive correlations of Ga with Al and V with Fe/Ti demonstrate that their enrichment occurred primarily via isomorphous substitution. Fluid pH is identified as the key controlling factor, where strongly acidic conditions favored V enrichment in the alunite ore layer, while moderately acidic conditions promoted Ga enrichment in proximal alteration halos. This study provides significant insights for the exploration of critical metals in large-scale alunite systems. • Ga enrichment peaks in proximal wall rocks, while V is highest in the alunite ore layers. • Ga (up to 3543 ppm) is primarily hosted in muscovite, coarse-grained alunite, dickite and pyrophyllite, whereas V (up to 1817 ppm) is concentrated in Fe–Ti oxides and alunite. • Isomorphous substitution is the key mechanism that involvs the substitution of Ga and V with ions and atoms in different minerals: Ga replaces Al 3+ in Al-rich minerals, while V substitutes for Fe 3+ /Ti 4+ in Fe–Ti oxides and clay minerals. Gallium (Ga) and vanadium (V) are widely used in strategic emerging industries due to their unique physical and chemical properties, and their modes of occurrence and enrichment mechanisms represent key research focuses in the study of critical metal minerals. The gallium metal resources of the Fanshan deposit are approximately 8000 tonnes, and the vanadium metal resources exceed 43,000 tonnes. The grade of gallium and vanadium are respectively 0.0022%–0.05% and 0.04%–0.14%, which is well above the industrial grade and shows great resource potential. The paper reveals the distribution patterns of Ga and V based on whole-rock and in situ trace element and mapping analyses of minerals within the Fanshan alunite deposit, Ga contents decrease in different zones according to the order of proximal country rock (avg. of 80 ppm) > alunite ore layer (avg. of 54 ppm) > distal country rock (avg. of 26 ppm). Conversely, V 2 O 5 contents decrease in the order of alunite ore layer (avg. of 881 ppm) > proximal country rock (avg. of 293 ppm) > distal country rock (avg. of 18 ppm). Ga and V are enriched in different minerals. Ga contents range from 5 to 3543 ppm, with alunite, dickite, and pyrophyllite serving as the primary Ga host minerals (peak of 1224 ppm; avg. of 393 ppm). Fe‒Ti oxides (specularite, hematite, and ilmenite) and alunite are the dominant carriers of V (peak of 1687 ppm; avg. of 845 ppm). The Ga content is positively correlated with the aluminum content in the host minerals, whereas the V content is positively correlated with the iron and titanium contents. Elemental mapping and time-resolved signal profiles demonstrate that Ga and V are uniformly distributed in Ga/V-enriched minerals. These results indicate that Ga and V form via isomorphous substitution within Al-rich minerals and Fe/Ti-enriched minerals, respectively. The spatial distribution characteristics of Ga and V are primarily controlled by the concentrations of ore-forming elements (Ga, V, etc.) and the pH levels of hydrothermal fluids. In the Fanshan alunite deposit, strongly acidic conditions facilitate V enrichment, whereas moderately acidic environments facilitate Ga enrichment.
Bao et al. (Wed,) studied this question.