Geology, fluid inclusions and sulphur isotopes of the Zhifang Mo deposit in Qinling Orogen, central China: a case study of orogenic-type Mo deposits

• Published in: Geological journal (Chichester, England) England), 2014-07, Vol.49 (4-5), p.515-533
• Main Authors: Deng, Xiao-Hua, Santosh, M., Yao, Jun-Ming, Chen, Yan-Jing
• Format: Article
• Language: English
• Subjects: China; East Qinling Mo belt; fluid inclusion; orogenic Mo deposit; sulphur isotope; Zhifang Mo deposit
• ISSN: 0072-1050; 1099-1034
• DOI: 10.1002/gj.2559

The East Qinling region in central China, hosting several tens of Mesozoic magmatic‐hydrothermal Mo deposits, is one of the largest molybdenum belts in the world. The Zhifang Mo deposit is hosted in volcanic rocks of the Xiong'er Group in the Waifangshan area, Qinling Orogen. Previous studies variously correlated the mineralization in this deposit with Yanshanian magmatism or Palaeo‐Mesoproterozoic volcanic‐hydrothermal events. The orebodies are associated with quartz veins and controlled by subsidiary faults of the Machaoying Fault. The ore‐forming process can be divided into the early, middle and late stages and is characterized by quartz‐pyrite, quartz‐polymetallic sulphide and quartz‐carbonate veins, respectively. The early‐stage quartz is structurally deformed, suggesting a compressional tectonic regime; the middle‐stage sulphides fill the fractures of the early‐stage assemblages and show no deformation, suggesting a tensional setting; the late‐stage veins mostly infill the open‐space fissures. Three types of fluid inclusions (FIs) are identified at the Zhifang deposit: H2O‐NaCl (W‐type), CO2‐rich (C‐type) and daughter mineral‐bearing inclusions (S‐type). Fluid inclusions of early‐stage quartz homogenize between 380 and 470 °C, with salinities ranging from 0.4 to 9.6 wt.% NaCl equiv., whereas the late‐stage calcite contains only the W‐type FIs with homogenization temperatures of lower than 240 °C, and salinities of 0.4–8.7 wt.% NaCl equiv. This indicates that the ore fluid system evolved from CO2‐rich, probably metamorphic hydrothermal to CO2‐poor, meteoric fluid. All three types of FIs can be observed in the middle‐stage quartz, and even in the microscopic domain of a crystal, suggesting that this heterogeneous association was trapped from a boiling fluid system. These FIs homogenized at temperatures ranging from 250 to 360 °C and display two salinity clusters of