Fluid evolution of the Ermi porphyry Cu system, southern Jilin Province, northeastern China

• Porphyry-style mineralization at Ermi shows low to intermediate sulfidation state. • The mineralizing fluids were predominantly magmatic water with addition of meteoric water. • Metal deposition resulted from cooling of fluids at of 205 ± 56 bars. The Ermi Cu deposit of northeastern China, is a “reduced” porphyry-type Cu deposit associated with emplacement of dioritic to granitic intrusions of Late Cretaceous. Mineralization style at Ermi is characterized by sequenced sulfides, from early pyrrhotite–pyrite to chalcopyrite to late arsenopyrite, which is related to the sericite alteration. These sulfide phases suggest an intermediate to low sulfidation state of Ermi ore system. Their slightly heavier S isotopic compositions, combined with the systematic variation of H and O compositions of hydrothermal fluids, have a vector towards a magmatic origin of mineralizing fluids that were increasingly mixed with meteoric water. A detailed microthermometric study of fluid inclusions hosted in quartz of different fluid stages also indicates a cyclic dilution of meteoric water, with cooling down by 200 ℃ (from 385℃ to 185℃). The cooling of mineralizing fluids in the Ermi porphyry Cu system is considered to have a first-order control on the Cu precipitation by decreasing the solubility of Cu complex in fluids. The Cu mineralization formed at pressure of 205 ± 56 bars under hydrostatic conditions estimated using temperature and salinity of fluid inclusions of boiling assemblage, corresponding to a depth of emplacement of 2.1 ± 0.6 km beneath the paleowater table.