Iron isotopic constraints on the ore-forming process of the Dashui gold deposit, western Qinling, China

• First report of whole-rock δ 56 Fe values for the Dashui gold deposit. • Iron isotopes explain the influence of strata on gold mineralization. • Decarbonation-driven mineralization undergoes multiple iron isotope fractionations. • Decarbonation is a key process governing iron isotope behavior. Iron isotope fractionation during diagenesis and mineralization serves as an effective tool for tracing both the mineralization processes and the source of mineralized materials. Fluid-strata interactions are a significant factor in causing iron isotope fractionation during gold mineralization. However, the specific behavior of iron during this process remains inadequately explored. The Dashui gold deposit in the West Qinling region is notable for its high-grade ore and unique mineral assemblage. Hematite alteration and silicification are well-developed, with both showing a positive correlation with gold mineralization. Accordingly, this study obtained iron isotope data from plutons, dykes, and carbonate strata of the Dashui gold deposit to comprehensively constrain the behavior of iron isotopes during the metallogenic process. The progressive enrichment of heavy iron isotopes with increasing SiO 2 content in the plutons indicates that they formed during different evolutionary stages of the parent magma during differentiation. In contrast, the wider δ 56 Fe range of the dykes (δ 56 Fe = −0.01 to 0.17 ‰) reflects multiple phases of magmatic activity. During the mineralization stage, hydrothermal alteration caused the migration of iron from dykes into the fluid and led to the initial enrichment of heavy iron isotopes in limestone. Meanwhile, decarbonation of the limestone resulted in the enrichment of light iron isotopes in dykes and further enhanced the enrichment of heavy iron isotopes in the limestone. The negative correlation between Au and δ 56 Fe (R = −0.725), along with the positive correlation between Fe 3+ /ΣFe and δ 56 Fe (R = 0.752) in gold-bearing limestone, indicates that gold deposition is mechanistically linked to the migration of iron species with distinct valence states. This pattern of iron isotope fractionation is primarily governed by silicification and decarbonation processes. Therefore, this study proposes that decarbonation is one of the critical processes controlling iron isotope behavior during gold mineralization.