Hydrothermal evolution from porphyry- to epithermal-style mineralization in the Naozhi deposit, NE China

Abstract

The Naozhi deposit in the Yanbian district of northeastern China contains polymetallic epithermal veins and subeconomic porphyry-style Cu mineralization within 2 km of each other. These two mineralization styles are genetically associated with 130–126 Ma andesites and dioritic–granitic intrusions. Isotopic ages determined on sericite (128.2 ± 2.9 Ma; 40Ar–39Ar plateau age) and sulfide (124.6 ± 2.7 Ma; Rb–Sr isochron) associated with the epithermal mineralization constrain the hydrothermal activity to at approximately 128 Ma. Three types of fluid inclusions, including halite-bearing, vapor-rich, and liquid-rich inclusions were trapped in quartz from porphyry-style veinlets associated with an assemblage of K-feldspar–biotite–magnetite. Based on microthermometry and the homogenization behavior of halite-bearing inclusions, they were trapped at lithostatic pressures of > 500 bars and at temperatures of ≥ 350 °C, whereas the vapor- and liquid-rich inclusions were trapped at pressures and temperatures of 110 ± 35 bars and ≤ 350 °C.Epithermal mineralization comprises four vein stages, which, from oldest to youngest, are quartz–pyrite, quartz–base metal sulfide, quartz–precious metal, and quartz–carbonate. The ore assemblage precipitated from low-salinity fluids (<10 wt% NaCl equiv.) at temperatures and pressures of ≤ 350 °C and ≤ 100 bars. The δ18OH2O (2.5 to −2.4 ‰) and δDH2O (−85 to −114 ‰) compositions of fluids related to porphyry- and epithermal-style mineralization indicate a mixture of magmatic and meteoric waters. The δ34SH2S (−3.1 to 4.5 ‰) values of fluids become isotopically heavier from the quartz–pyrite stage to the quartz–base metal sulfide stage, reflecting continuous reduction during epithermal mineralization. The mineral assemblage and chemical composition of sulfide minerals indicate a transition from oxidizing, high sulfidation porphyry-style fluids to reducing, intermediate–low sulfidation epithermal fluids, with log ƒO2 values ranging from −31 to > −33, and log ƒS2 values ranging from −7.0 to ≤ −13.4 during the epithermal veining stage. Metal deposition in the Naozhi magmatic–hydrothermal system resulted from cooling, sulfidation, neutralization, and boiling of predominantly magmatically sourced hydrothermal fluids that mixed with meteoric water.