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

摘要

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. 172 Ma), whereas Cu-Au mineralization is associated with Jurassic dacite porphyries (ca. 170 Ma). 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 ℃ and salinities of 4.2-9.5 wt.% NaCl equivalent in the Pb-Zn-Ag mineralization, and homogenization temperatures of 196-362 ℃ and salinities of 3.5-9.9 wt.% 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 ℃ and contrasting salinities of 0.2-4.2 and 30.9-36.8 wt.% NaCl equivalent, respectively, which indicates that boiling processes occurred. The sulfur isotopic compositions of sulfides (δ~(34)S = 1.7‰ to +3.2‰) suggest a homogeneous magmatic sulfur source. The lead isotopes of sulfides (~(206)Pb/~(204)Pb = 18.01-18.07; ~(207)Pb/~(204)Pb= 15.55-15.57; and ~(208)Pb/~(204)Pb = 38.03-38.12) are consistent with those of volcanic-subvolcanic rocks (~(206)Pb/~(204)Pb = 18.03-18.10; ~(207)Pb/~(204)Pb= 15.56-15.57; and ~(208)Pb/~(204)Pb = 38.02-38.21), indicating a magmatic origin for lead in the ore. The oxygen and hydrogen isotope compositions (δ~(18)O = +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.