LAUSR

Abstract The Au–polymetallic deposit in the Sonchon district is located in the western parts of North Pyongan Province, DPR Korea. Here, we present a detailed study of the petrogenesis, magma source of the Mesozoic granitoids, and the sources of ore–forming materials and fluids. Mesozoic granitoids consist of diorite, granodiorite and biotite granite. These rocks are alkali–enriched (K2O + Na2O = 5.02–7.75 wt%) and classified as high–K calc–alkaline. The diorite is metaluminous, while the granodiorite and biotite granite are weakly peraluminous and peraluminous, respectively. The samples show high CaO/Na2O ratios of 1.66–2.16 for the diorite and 0.74–1.35 for the granodiorite, indicating a derivation from clay–poor psammite–derived melt source, and the biotite granite has low CaO/Na2O ratios of 0.14–0.38, indicating a derivation from clay–rich pelite–derived melt source. The biotite granite belongs to fractionated M–, I–, and S–type granites (FG), whereas the granodiorite and the diorite belong to unfractionated M–, I–, and S–type granites (OGT). These rocks are typically arc-related. The similar REE distribution patterns of these rocks show that they are relatively uniform with partial melting processes playing an important role in their generation. The REE distribution patterns of ore–bearing quartz are similar to those of granodiorite rather than diorite, biotite granite and wall rocks, suggesting that they were mainly inherited from the granodiorite. The barren quartz has more negative Eu anomalies than the ore–bearing quartz. The histogram of sulfur isotopic composition shows that the δ34S values are mainly distributed around +5 to +10‰ and indicate the sulfur within sulfide was derived from a magmatic source. The absence of sulfates in ore bodies indicates that the low fO2 and high pH of ore–forming fluid system. Fluid inclusions in quartz have only aqueous–gas phases without CO2–rich phase and daughter minerals, indicating that ore–forming fluids was homogenous magmatic and had low salinity. According to the values of X (Na+)/X (K+) ratios, X (Na+)/X (Ca2+ + Mg2+) ratios, and X (F−)/X (Cl−) ratios, the ore–forming fluids could be interpreted as a mixture of magmatic and meteoric water. Microthermometric analyses indicate that these inclusions have medium temperature (223.5–286.3 °C) and low salinity (1.39–4.01 wt% NaCl) and are NaCl + H2O or NaCl + KF + H2O systems. All these data in this study suggest that the Sonchon Au–polymetallic deposit is mesothermal and related to the granodiorite.