Genesis of the Dongguashan skarn Cu-(Au) deposit in Tongling, Eastern China: Evidence from fluid inclusions and H-O-S-Pb isotopes

Abstract

The Dongguashan copper-(gold) deposit is located in the Eastern Tongling ore district of the Middle-Lower Yangtze River Valley Metallogenic Belt (MLYRB), Eastern China. The Cu-(Au) mineralization is associated with the Qingshanjiao quartz monzodiorite and consists of various types including stratiform, porphyry, vein-like and skarn. There are three deposit models, including: 1) stratabound skarn-type; 2) exhalative sedimentary (SEDEX)-hydrothermal superimposition-type; and 3) exhalative sedimentary (SEDEX) type. The ore-forming processes can be divided into the prograde skarn alteration (Stage I), retrograde alteration (Stage II) and quartz-carbonate alteration (Stage III). Four types of fluid inclusions (FIs) are recognized at Dongguashan: Vapor-rich two-phase aqueous (Type I), liquid-rich two-phase aqueous (Type II), daughter-mineral-bearing three-phase (homogenized to liquid by the disappearance of vapor phase; Type III), and daughter-mineral-bearing three-phase (homogenized to liquid by the disappearance of daughter minerals; Type IV). All four types of FIs are present in Stage II. Stage I does not contain Type IV FIs, and only Type II FIs are found in Stage III. For Stage I, Type I, II and III FIs homogenized at 379–548 °C, 319–522 °C and 450–512 °C, respectively, with salinities of 10.1–19.6, 5.9–19.4 and 36.7–43.3 wt% NaCl equivalent. For Stage II, Type I, II, III and IV FIs homogenized at 286–378 °C, 235–372 °C, 265–349 °C and 324–350 °C, respectively, with salinities of 1.4–17.0, 4.3–16.1, 34.7–40.6 and 34.4–41.0 wt% NaCl equivalent. The Type I to III FIs in Stage I and II have similar homogenization temperatures but different salinities, which indicates the occurrence of fluid immiscibility. Hydrogen-oxygen isotopes of the quartz (δD = −84.0 to −71.5‰, δ18OH2O = +4.51 to +8.97‰) and calcite (δD = −70.7 to −60.95‰, δ18OH2O = +4.41 to +4.91‰) suggest that the ore-forming fluids were principally magmatic-derived. δ34S values of the stratiform orebodies (+4.1 to +5.3‰), vein-like orebodies (+4.2 to +5.7‰), skarn orebodies (+4.9‰), porphyry orebodies (+5.1 to +5.7‰) and the Qingshanjiao pluton (−2.2 to +5.2‰) are different from those of the wall rocks (−29.5 to −4.6‰), suggesting that wall rocks were unlikely to be a major sulfur source. Lead isotopes of the sulfides from the different orebody types are similar to those of the plagioclase from the Mesozoic intermediate-felsic rocks in the MLYRB, suggesting that they have had similar Pb source. Sulfur and lead isotopes suggest that the ore-forming materials were derived from a deep-seated magmatic source. The ore-forming fluid immiscibility during Stage I and II may have altered the physicochemical conditions of the hydrothermal fluid system, leading to the Cu-(Au) sulfide precipitation. Based on the ore geological and alteration/mineralization features, mineral assemblages, ore-forming material source and fluid evolution, we consider that Dongguashan is best classified as a porphyry–skarn Cu-(Au) deposit.