Genesis of the Bagenheigeqier Pb-Zn skarn deposit in Inner Mongolia, NE China: constraints from fluid inclusions, isotope systematics and geochronology

Most skarns are found near the pluton or in lithologies containing at least some limestone. However, recent research has shown that neither a pluton nor limestone is necessarily required to form a skarn deposit. The newly discovered Bagenheigeqier Pb-Zn skarn deposit is located in NE China. The skarn and Pb-Zn orebodies occur in volcanic lithologies of the Baiyin'gaolao Formation and are controlled by NE-SW-trending faults. The nearest pluton is a granite porphyry, at a distance of 20-250 m from the orebodies. Five paragenetic stages at Bagenheigeqier are recognized: (I) skarn; (II) oxide; (III) early sulphide; (IV) late sulphide; and (V) late quartz-calcite. The fluid inclusions from stages II to V homogenized at temperatures of 402-452, 360-408, 274-319 and 167-212 degrees C, respectively. The hydrogen and oxygen isotope compositions (delta O-18(H2O), -12.4 parts per thousand to +9.3 parts per thousand; delta D-H2O, -156.5 parts per thousand to -99.1 parts per thousand) indicate that the ore-fluids were primarily of magmatic origin, with the proportion of meteoric water increasing during the progression of ore formation. Sulphur isotope values (delta S-34(VCDT), 1.4-5.5 parts per thousand), lead isotope values (Pb-206/Pb-204, 18.184-18.717; Pb-207/Pb-204, 15.520-15.875; Pb-208/Pb-204, 37.991-38.379) and the initial Os-187/Os-188 ratios of the pyrite (0.307 +/- 0.06) suggest that the ore metals were derived from the granite porphyry and Baiyin'gaolao Formation. Re-Os dating of pyrite intergrown with galena and sphalerite yielded a well-constrained isochron age of 151.2 +/- 4.7 Ma, which is coeval with the laser ablation - inductively coupled plasma - mass spectrometry zircon U-Pb age of 154 +/- 1 Ma for the granite porphyry. The deposit was therefore formed during Late Jurassic time.