Compositional characteristics and enrichment mechanisms of dispersed elements in sphalerite from Taolin Pb-Zn deposit, Hunan Province

Dispersed metals including Cd, Ga, In, Ge, Tl, Tc, Se, and Te are classified as national strategic critical mineral resources globally, which are irreplaceable raw materials for new energy equipment, semiconductor manufacturing, and information high-tech industries. Most dispersed elements are characterized by ultra-low crustal abundance and extremely scattered distribution, and they rarely form independent industrial deposits. Instead, they are dominantly hosted in sphalerite within hydrothermal Pb-Zn deposits, making sphalerite an ideal mineral carrier to decode the super-enrichment mechanisms of Cd, Ga, In, and Ge. The Taolin Pb-Zn deposit, located in Linxiang City, northeastern Hunan Province, lies in the central segment of the Jiangnan orogenic belt on the southeast margin of the Yangtze block. It is a large-scale medium-low temperature hydrothermal deposit with total proven Pb+Zn metal reserves of up to 0.98 million tons. Previous research on this deposit mainly focuses on regional tectonic controls, magmatic evolution, and general metallogenic patterns, while systematic microscale constraints on the occurrence and enrichment mechanisms of associated dispersed metals remain lacking, which restricts the comprehensive resource evaluation of co-existing critical metals. To fill this research gap and improve the metallogenic theory of polymetallic deposits in northeastern Hunan, this study takes zoned sphalerite from Taolin deposit as the research object and conducts integrated petrographic and in-situ geochemical analyses.

A total of seven weakly altered drill-core ore samples were collected from multiple mining segments of the deposit. Through hand-specimen observation and polished thin-section microscopic identification, two generations of sphalerite were distinguished: Early-stage SphⅠ formed in early hydrothermal veins and late-stage SphⅡ filling later veinlets that crosscut early mineralized zones. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) microscale testing was conducted based on a 193 nm ArF excimer laser coupled with an Agilent 7700e mass spectrometer, including 100 quantitative trace element point analyses (69 spots for SphⅠ, 31 spots for SphⅡ) and full elemental mapping for representative sphalerite grains. NIST 610, NIST 612 glass standards and MASS-1 sulfide standard were adopted for signal calibration to ensure data reliability.

The analytical results showed distinct differentiation of dispersed elements in Taolin sphalerite: The mineral was highly enriched in Cd and Ga, moderately enriched in In, and strongly depleted in Ge, while Tl and Te were not detected in all testing points. The mass fraction of Cd ranged from 945.9×10⁻⁶ to 11734.8×10⁻⁶ with an average value of 2978.0×10⁻⁶. Ga varied between 0.7×10⁻⁶ and 3331.7×10⁻⁶, averaging 310.7×10⁻⁶. In had a mean concentration of only 27.1×10⁻⁶, with a maximum of 322.4×10⁻⁶, while Ge averaged merely 6.7×10⁻⁶, and over half of the testing points fell below the limit. Element mapping demonstrated that Cd, Ga, and In were unevenly distributed inside single sphalerite crystals, and Ga displayed an extremely strong positive correlation with Cu (R²=0.93), as did the sum of Ga+In against Cu (R²=0.92). These correlations indicated two isomorphic substitution pathways inside the sphalerite lattice. The coupled charge-balancing reactions were Cu⁺+(Ga+In)³⁺↔2Zn²⁺ and Cd²⁺↔Zn²⁺. Notably, Cd presented a weak positive correlation with Fe in the samples, which contradicted the widely accepted negative correlation between Cd and Fe in high-temperature Fe-rich sphalerite. This discrepancy was interpreted as the limitation of crystal lattice capacity under different formation temperatures. Source tracing indicated that Cd, Ga, and In had mixed material supplies. The primary metal reservoir was late magmatic-hydrothermal fluids derived from the Mufushan Yanshanian biotite monzogranite, whose zircon U-Pb age (136±0.8 Ma) is consistent with the sphalerite Rb-Sr mineralization age (135.4±2.6 Ma). Secondary dispersed metals were extracted by ore-forming fluids when migrating through Neoproterozoic Lengjiaxi Group and Sinian siliceous-carbonaceous slates, containing abundant organic matter and adsorbed trace sulfides. Sulfur isotopic data of ores provided solid evidence for stratigraphic material contribution. During fluid migration, Cd, Ga, and In formed stable fluoride and organic humic complexes for long-distance transportation. As hydrothermal fluids ascended to shallow structural fractures, continuous temperature reduction and progressive mixing of meteoric water increased fluid oxidation intensity. Under high oxidation conditions, Cu⁺ converted to Cu²⁺, eliminating the monovalent cation required for coupled isomorphic substitution, explaining the evidently lower Ga content in late-generation SphⅡ compared with early SphⅠ.

This study proposes a complete multi-source metallogenic model for dispersed elements in medium-low temperature Pb-Zn deposits within the Jiangnan orogenic belt. It innovatively reveals the temperature and redox controls on Cd-Ga-In enrichment in sphalerite, supplements geochemical discrimination criteria for hydrothermal mineralization, and offers significant mineralogical references for prospecting and comprehensive utilization of associated critical metals in analogous deposits across northeastern Hunan and the whole Jiangnan metallogenic belt.