Ga- and Ge-bearing minerals and trace element-rich magnetite in the Makeng skarn Fe polymetallic deposit, Eastern China

The Makeng skarn Fe polymetallic deposit is the largest Fe deposit in Eastern China. It hosts substantial Fe and Mo resources alongside potential Ga and Ge by-products. However, the distribution and enrichment mechanism of Ga and Ge in this deposit remain poorly understood. Our study reveals that Ga is primarily hosted in pyroxene, amphibole, and magnetite with median contents of 180 ppm, 24 ppm, and 22 ppm, respectively. Germanium is preferentially concentrated in andradite and amphibole with median contents of 50 ppm and 37 ppm, respectively. Sulfides contain negligible Ga and Ge. Four generations of magnetite (Mag-I to Mag-IV) have been identified based on mineral assemblages. Mag-I is mainly associated with andradite and pyroxene, whereas Mag-II is associated with amphibole and chlorite. Mag-III is replaced or filled by pyrite, whereas Mag-IV is associated with pyrrhotite, pyrite and fluorite. Mag-I to Mag-III have median Ga and Ge contents of 13–20 ppm and 4–15 ppm, respectively. In contrast, Mag-IV shows higher median Ga and Ge contents of 113 ppm and 20 ppm, respectively. Gallium incorporates into silicate minerals by substituting for Fe²⁺/ Fe³⁺ or Mn²⁺, whereas Ge tends to replace Si⁴⁺ or Al³⁺. In magnetite, Ga enters via couple substitution, Ga³⁺ + Al³⁺ ↔ 2Fe³⁺, whereas Ge incorporates through either isomorphic substitution (Ge²⁺ ↔ Fe²⁺) or coupled substitution (Ge²⁺ + Mg²⁺ ↔ 2Fe²⁺). The enrichment of Ga and Ge in silicate minerals is attributed to their crystal structures and high partition coefficients. The elevated Ga and Ge contents in Mag-IV may result from the superposition of Ga- and Ge-rich fluids, along with lower temperature and higher oxygen fugacity.