Genesis of the Taoguanping molybdenum deposit in the Northern Qinling Belt, Central China: Constraints from geology, fluid inclusions, and H-O-S isotopes

The Taoguanping molybdenum deposit is located in the west of the Mangling pluton in the Northern Qinling Belt, but its fluid characteristics and ore-forming process are unclear, hindering the understanding of its ore genesis and genetic model. The paragenetic sequence of the Taoguanping molybdenum deposit can be divided into: quartz-K-feldspar-sulfide stage (Stage I), quartz-polymetallic sulfide stage (Stage II), and quartz-calcite stage (Stage III). Stage I is characterized by disseminated molybdenite mineralization associated with siliceous and Kfeldspar alteration in the medium- to fine-grained monzogranite, representing the early mineralization. Stage II is the main mineralization featured by coarse-grained quartz-sulfide veins, e.g., quartz-molybdenite and quartz-biotite-molybdenite +/- pyrite +/- chalcopyrite veins, crosscutting monzogranite and host rocks. Stage III is late hydrothermal veins to crosscut former minerals. Detailed fluid inclusion study shows that temperature and salinity of fluids decreased from Stage I (peaks at ca. 320-380 degrees C and 3.7-13.6 wt% NaCl eqv.), through Stage II (peaks at ca. 260-300 degrees C and 1.2-11.8 wt% NaCl eqv.) to Stage III (peaks at ca. 180-220 degrees C and 0.4-8.9 wt% NaCl eqv.), accompanied by fluid mixing and boiling. The H-O isotopes of quartz (delta Dfluid = -72%o to -53%o and delta 18Ofluid = 9.1-10.9%o) suggest Stage II fluids are magmatic-hydrothermal origin mixed with minor meteoric water. The in-situ sulfur isotope of pyrite (-33.4%o to -13.4%o and 0%o to 7.8%o) indicates Stage II fluids being mainly magmatic-hydrothermal origin, with participation of strata materials inferred by presence of Stage II euhedral to anhedral pyrite with inclusion-rich domains and CH4 in Stage II fluid inclusions during fluid-rock interaction. In combination of this study and regional tectonic setting, we propose that the Taoguanping molybdenum deposit underwent porphyry and hydrothermal vein-type mineralization, contributed by medium- and fine-grained monzogranite and possible granite porphyry, respectively, which should be ascribed to a porphyry mineralization system and formed in a post-collisional setting during the Late Mesozoic.