Processes of ore genesis at the world-class Yuchiling molybdenum deposit, Henan province, China

The Yuchiling molybdenum deposit is one of the most significant porphyry molybdenum systems in the eastern Qinling of central China. The mineralization is mainly hosted by a porphyritic granite and associated cryptoexplosive breccia. Hydrothermal alteration minerals include K-feldspar, sericite, pyrite, chlorite, epidote, carbonate, kaolinite, fluorite, and gypsum. Ore minerals are dominated by molybdenite and pyrite, with lesser amounts of chalcopyrite, galena, scheelite, wolframite, ilmenite, leucoxene, native gold, sphalerite, and hematite. The δ34S compositions of sulfide minerals range from −6.0‰ to +4.0‰. The deposit is characterized by four hydrothermal stages: quartz-K-feldspar (stage I), molybdenite-quartz (stage II), pyrite-sericite-quartz (stage III), and quartz-carbonate (stage IV). Microthermometric studies of fluid inclusions show that the fluids evolved gradually during the ore-forming process. Homogenization temperatures, salinities, and minimum pressure estimates for the inclusions from each mineralization stage evolved as follows: (1) stage I: homogenization temperatures = 203.7-525.8 °C, salinities = 2.96-10.49 and 29.66 wt.% NaCl equiv., and minimum pressures = 101.9-196.2 MPa; (2) stage II: homogenization temperatures = 173.6-448.6 °C, salinities = 1.81-9.74 wt.% NaCl equiv., and minimum pressures = 93.1-172.0 MPa; (3) stage III: homogenization temperatures = 130.1-386.0 °C, salinities = 1.40-9.73 and 34.07 wt.% NaCl equiv., and minimum pressures = 95.5-142.5 MPa; (4) stage IV: homogenization temperatures = 170-230 °C and salinities = 0.18-5.71 wt.% NaCl equiv. Various fluid inclusions were observed to contain H2O, CO2, CH4, SO2, C2H2, C2H4, C2H6, and (or) H2S, as well as solids that include halite, sylvite, anhydrite, chalcopyrite, hematite, molybdenite, and jamesonite. The δ18O and δD of the hydrothermal fluids vary from −4.4‰ to +8.5‰ and −81‰ to −61‰, respectively. Microthermometric and stable isotope data indicate that the ore-forming fluids for the Yuchiling molybdenum deposit evolved from early magmatic, to a mixture of meteoric and magmatic water, and finally to a principally late stage meteoric water. A molybdenite Re-Os age of 131.2 ± 1.4 Ma coincides with the early part of the 131-112 Ma period of magmatic and related ore-forming events that are widely recognized in eastern Qinling.