Geological, fluid inclusion and isotopic studies of the Yinshan Cu–Au–Pb–Zn–Ag deposit, South China: Implications for ore genesis and exploration

Abstract

The Yinshan Cu–Au–Pb–Zn–Ag deposit is located in Dexing, South China. Ore bodies are primarily hosted in low-grade phyllite of the Neoproterozoic Shuangqiaoshan Group along EW- and NNW-striking fault zones. Pb–Zn–Ag mineralization is dictated by Jurassic rhyolitic quartz porphyries (ca. 172Ma), whereas Cu–Au mineralization is associated with Jurassic dacite porphyries (ca. 170Ma). The main ore minerals are pyrite, chalcopyrite, galena, sphalerite, tetrahedrite–tennatite, gold, silver, and silver sulphosalt, and the principal gangue minerals are quartz, sericite, calcite, and chlorite. Two-phase liquid-rich (type I), two-phase vapor-rich (type II), and halite-bearing (type III) fluid inclusions can be observed in the hydrothermal quartz-sulfides veins. Type I inclusions are widespread and have homogenization temperatures of 187–303°C and salinities of 4.2–9.5wt.% NaCl equivalent in the Pb–Zn–Ag mineralization, and homogenization temperatures of 196–362°C and salinities of 3.5–9.9wt.% NaCl equivalent in the Cu–Au mineralization. The pervasive occurrence of type I fluid inclusions with low-moderate temperatures and salinities implies that the mineralizing fluids formed in epithermal environments. The type II and coexisting type III inclusions, from deeper levels below the Cu–Au ore bodies, share similar homogenization temperatures of 317–448°C and contrasting salinities of 0.2–4.2 and 30.9–36.8wt.% NaCl equivalent, respectively, which indicates that boiling processes occurred. The sulfur isotopic compositions of sulfides (δ34S=−1.7‰ to +3.2‰) suggest a homogeneous magmatic sulfur source. The lead isotopes of sulfides (206Pb/204Pb=18.01–18.07; 207Pb/204Pb=15.55–15.57; and 208Pb/204Pb=38.03–38.12) are consistent with those of volcanic–subvolcanic rocks (206Pb/204Pb=18.03–18.10; 207Pb/204Pb=15.56–15.57; and 208Pb/204Pb=38.02–38.21), indicating a magmatic origin for lead in the ore. The oxygen and hydrogen isotope compositions (δ18O=+7.8‰ to +10.5‰, δD=−66‰ to −42‰) of inclusion water in quartz imply that ore-forming fluids were mainly derived from magmatic sources. The local boiling process beneath the epithermal Cu–Au ore-forming system indicates the possibility that porphyry-style ore bodies may exist at even deeper zones.