Geology, geochronology and isotopic geochemistry of the Xiaoliugou W-Mo ore field in the Qilian Orogen, NW China: case study of a skarn system formed during continental collision

The Xiaoliugou W-Mo ore field lies within the mid-Paleozoic North Qilian Orogen, NW China, and hosts a W resource of 48.8 Mt @ 0.4% and 412.6 Mt of Mo @ 0.075%. It contains five deposits, including Xiaoliugou, Qiqing, Guishan, Qibao and Shiji. The main mineralization styles at Xiaoliugou are skarn and veins in which the mineral sequence is scheelite > molybdenite > chalcopyrite and occurs in the endo- and exo-contact zones of granite intrusions. The scheelite-dominated orebodies are overprinted by molybdenite-dominated quartz veins. Two molybdenite samples yielded Silurian Re-Os model ages of 427.4 ± 6.0 Ma and 428.2 ± 6.0 Ma. Three muscovite samples coexisting with molybdenite yielded Middle Devonian Ar-Ar ages of 392.0 ± 2.7 Ma, 391.1 ± 2.7 Ma and 391.4 ± 2.8 Ma. The Re-Os and Ar-Ar ages indicate that the W-Mo mineralization and alteration occurred at ca. 428–391 Ma, which corresponds with regional continental collision within the Qilian Orogen. δ³⁴S for the sulfides molybdenite and pyrite are 7.70–11.67 ‰ and 4.98–13.17 ‰, respectively. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb of the sulfides are 17.98–21.73, 15.34–18.81, and 37.18–38.63, respectively. The granites yield similar corrected (²⁰⁶Pb/²⁰⁴Pb)_i, (²⁰⁷Pb/²⁰⁴Pb)_i and (208Pb/204Pb)_i, ranging 16.14–19.35, 15.44–15.63, and 37.41–38.31, respectively. Calculated δ¹⁸O of the fluid inclusions in quartz range from -3.38–2.34 ‰, whereas the δD of the hydrothermal fluids ranges from -94 to -47 ‰. The S-, Pb-, O- and D-isotopic data imply that the metals originated from the granite intrusion with a minor component sourced from the host sediments, and that the ore-forming fluids were dominated by magmatic-hydrothermal fluids mixed with minor meteoric water.