Fluid inclusions, H-O, S, Pb, and noble gas isotope studies of the Baiyun gold deposit in the Qingchengzi orefield, NE China

Abstract

Abstract The Baiyun gold deposit is located in the Qingchengzi orefield, NE China. The gold mineralization occurs mainly as auriferous quartz veins and auriferous altered rock. Wall-rock alteration includes silicification and potassic alteration along with minor sericitization, carbonatization, and chloritization. Three stages of mineralization are recognized: an early quartz and coarse-grained pyrite stage (Stage I), a middle quartz–gold and fine-grained pyrite–polymetallic sulfide stage (Stage II), and a late quartz–carbonate stage (Stage III). Four types of inclusions are recognized in the quartz veins: two-phase liquid-rich fluid inclusions (Type I), CO2 three-phase aqueous fluid inclusions (Type II), two-phase vapor-rich fluid inclusions (Type III), and pure liquid fluid inclusions (Type IV). The early stage quartz contains Type I–III fluid inclusions that yield low–medium homogenization temperatures (162\u202f°C–350\u202f°C), low salinities (1.57–10.33\u202fwt% NaClequiv.), and belong to a NaCl–H2O–CH4–CO2\u202f±\u202fH2S hydrothermal fluid system. The middle-stage quartz also contains Type I–III fluid inclusions that yield low–moderate homogenization temperatures (148\u202f°C–290\u202f°C), variable salinities (1.02–16.43\u202fwt% NaClequiv.), and belong to a NaCl–H2O–CO2–H2S\u202f±\u202fN2 system. The late-stage quartz contains Type I, II, and IV fluid inclusions that yield homogenization temperatures and salinities of 150\u202f°C–183\u202f°C and 6.30–13.29\u202fwt% NaClequiv, respectively, and belong to a NaCl–H2O–CO2\u202f±\u202fN2 system. Therefore, the ore-forming fluids of the Baiyun gold deposit are generally characterized by low–moderate homogenization temperatures and low–medium salinities, and belong to a NaCl–H2O–CO2–H2S\u202f±\u202fN2 system. The coexisting Type I and II inclusions of the early and middle stages have similar Th ranges but weakly contrasting salinities, indicating that fluid boiling occurred. The H-O isotopic values (δ18OH2O\u202f=\u202f−0.7‰ to 5.6‰; δD\u202f=\u202f−108.3‰ to −74.0‰) and composition of the fluids suggest that the ore-forming fluids consisted mainly of magmatic water with a small amount of meteoric water and metamorphic fluids. Fluid inclusions in hydrothermal pyrite yield 3He/4He ratios of 0.43–0.52 Ra and 40Ar/36Ar ratios of 4648.7–7685.9, indicating the mixing of fluids with mantle and crustal components, which were mainly crust-derived. The δ34SV-CDT values of pyrite are between −12.5‰ and 1.9‰ (average −5.0‰), while δ34SV-CDT values of wall-rocks are between 7.0‰ and 18.7‰ (average 12.6‰). The 206Pb/204Pb, 207Pb/204Pb, and 207Pb/204Pb values of the pyrites are 17.416–18.908, 15.511–15.714, and 35.541–40.032, respectively, indicating that the ore-forming metals may came from Triassic magma and Gaixian Formation. Based on geological, fluid inclusion, and isotopic data, as well as data from previous studies, we propose that the Baiyun gold deposit is a magmatic–hydrothermal ore deposit.