Fluid inclusions, H-O-S-Pb isotopes and metallogenic implications of Triassic Hua'naote Ag-Pb-Zn deposit (Inner Mongolia, China) in the eastern Central Asian Orogenic Belt

Abstract

The Hua'naote Ag-Pb-Zn deposit (Inner Mongolia, northern China) is a hydrothermal vein-type deposit in the Erlianhot-Dongwuqi Metallogenic Belt (EDMB), which is situated in the eastern Central Asian Orogenic Belt (CAOB). Mineralization occurs in the contact between monzogranite and the Devonian Angeer Yinwula Formation (Fm.) silty slate, and is also closely related to syenogranite. The mineralization can be divided into three stages: (I) arsenopyrite + pyrite + quartz, (II) pyrite + galena + sphalerite + chalcopyrite + quartz, and (III) galena + sphalerite + calcite + quartz. Fluid inclusions (FIs) from Hua'naote are liquid-rich (L-type), vapor-rich (V-type), CH4 + CO2 (C1-type), CO2-bearing (C2-type), and daughter-mineral-bearing three-phase (S-type). Laser Raman spectral analysis shows that the gas phase in Stage I and II C2-type FIs is mainly composed of H2O, CO2, N2 and CH4, while that in Stage III FIs contains predominantly H2O. Stage I FIs are of L-, V-, C1-, C2- and S-types. These FIs homogenized mainly at 300–380 °C, and their salinities are bimodal (1.7–8.4 wt% and 35.2–45.8 wt% NaCl eqv.). Our results suggest that Stage I fluids belong to a NaCl–H2O–CO2 ± CH4 system characterized by medium-to-high temperature and salinity. Stage II FIs are of L-, V- and C2-types, with homogenization temperatures and salinities of 180.4–302.8 °C and 2.23–5.70 wt% NaCl eqv., respectively. Stage II fluids belong to a NaCl–H2O–CO2 system featured by medium-to-low temperature and low salinity. Stage III FIs contain only L-type, with homogenization temperatures and salinities of 125.5–244.6 °C and 2.56–4.94 wt% NaCl eqv., respectively. Stage III fluids belong to a NaCl-H2O system characterized by low temperature and salinity. The δ18Ofluid and δDfluid values of quartz and calcite are of −4.70 to 5.50‰ and − 144.8 to −121.8‰, respectively, indicating that the Hua'naote early-ore fluids were mainly derived from the magma with minor meteoric water input. The Hua'naote sulfides have δ34SCDT = −0.3 to 8.2‰ (mainly 0–2‰ and 5–8‰), and 206Pb/204Pb = 18.304–18.483, 207Pb/204Pb = 15.552–15.683 and 208Pb/204Pb = 38.104–38.350. The sulfur and lead isotope evidence suggests that the ore metals were sourced primarily from deep-seated magma with minor upper crustal input. We suggest that the Hua'naote Ag-Pb-Zn deposit is best classified as intrusion-related, fractures-controlled medium-temperature hydrothermal vein-type deposit.