Geochronology, Fluid Inclusions and Isotopic Characteristics of the Dongjun Pb-Zn-Ag Deposit, Inner Mongolia, NE China

The Dongjun Pb-Zn-Ag deposit in the northern part of the Great Xing'an Range (NE China) consists of quartz-sulfide vein-type and breccia-type mineralization, related to granite porphyry. Hydrothermal alteration is well-developed and includes potassic-silicic-sericitic alteration, phyllic alteration and propylitic alteration. Three stages of mineralization are recognized on the basis of field evidence and petrographic observation, demarcated by assemblages of quartz-pyrite-arsenopyrite (early stage), quartz-polymetallic sulfide (intermediate stage) and quartz-carbonate-pyrite (late stage). Zircon LA-ICP-MS U-Pb dating indicates that the granite porphyry was emplaced at 146.7 +/- 1.2 Ma (Late Jurassic). Microthermometry and laser Raman spectroscopy shows that ore minerals were deposited in conditions of intermediate temperatures (175-359 degrees C), low salinity (0.5-9.3 wt% NaCl eqv.) and low density (0.60-0.91 g/cm(3)). Ore-forming fluids were derived largely from magmatic hydrothermal processes, with late-stage addition of meteoric water, belonging to a H2O-NaCl-CO2 +/- CH4 system. The delta S-34(V-CDT) values range from 0.75 parts per thousand to 4.70 parts per thousand. The Pb-206/Pb-204, Pb-207/Pb-204, and Pb-208/Pb-204 values of the ore minerals are in the ranges of 18.240-18.371, 15.542-15.570, and 38.100-38.178, respectively. Data for the S and Pb isotopic systems indicate that the ore-forming metals and sulfur were derived from Mesozoic magma. Based on the geological characteristics and geochemical signatures documented in this study, we conclude that the Dongjun deposit is a mesothermal magmatic hydrothermal vein-type Pb-Zn-Ag deposit controlled by fractures and related to granite porphyry, in response to Late Jurassic tectonic-magmatic-hydrothermal activity. We further conclude that fluid immiscibility, fluid mixing and fluid-rock interactions were the dominant mechanisms for deposition of the ore-forming materials.