Ion microprobe analysis of fluorine, water, and oxygen isotopes in garnet from skarn deposits in the Tongling ore district, eastern China: Implications for coupled F-OH substitution mechanism and polymetallic mineralization

Skarn deposits, as integral components of magmatic-hydrothermal systems, represent major global resources of economically critical metals. Volatile constituents such as fluorine (F), chlorine (Cl), and sulfur (S) play fundamental roles in regulating metal complexation, transport efficiency, and precipitation within hydrothermal fluids. Despite this recognized importance, the partitioning behavior of F among melt, minerals, and fluid phases and its influence on metal selectivity across different skarn deposits remain insufficiently constrained. This study investigates representative skarn deposits from the world-class Tongling ore district in eastern China to evaluate the substitution mechanisms and partitioning behavior of F and hydroxyl (OH) in garnet, together with oxygen isotope systematics, using high-precision ion microprobe analysis. The objective is to clarify the metallogenic role of the volatiles in polymetallic skarn mineralization. The principal findings are as follows: (1) F and hydroxyl incorporation in garnet are dominantly coupled, reflecting defect-related substitution mechanisms; (2) a first-order quantitative estimate of the garnet-fluid F partition coefficient (DFGrt−Fluid = ∼0.074) indicates that F is highly mobile in hydrothermal fluids but exhibits limited structural incorporation in garnet; (3) elevated F contents in garnet from skarn Cu-Mo deposits imply the development of hydroxyl-fluoride complexes in ore-forming fluids, potentially enhancing Mo transport and precipitation; and (4) calculated δ18Ofluid values decrease from early skarn stages (6.3‰−11.2‰, Vienna standard mean ocean water) to late quartz-sulfide stages (3.1‰−7.6‰), indicating dominantly magmatic fluid sources with minor input of ancient meteoric water during the later stages of mineralization. Collectively, integrated secondary ion mass spectrometry determinations of F and H2O contents, combined with garnet oxygen isotopes, provide quantitative constraints on F partitioning behavior in skarn systems and elucidate the fundamental role of volatiles in controlling metal transport and enrichment during polymetallic mineralization.