Incursion of meteoric water triggers molybdenite precipitation in porphyry Mo deposits: A case study of the Chalukou giant Mo deposit

Abstract

There has been a long-term debate about the timing of the incursion of meteoric water into porphyry systems and whether it plays an important role in ore precipitation. To evaluate the role of meteoric water in the mineralization in porphyry Mo deposits, typical hydrothermal veins of different stages from the Chalukou giant porphyry Mo deposit are selected for combined scanning electron microscope cathodoluminescence (SEM-CL), fluid inclusion microthermometry and high-resolution secondary ion mass spectroscopy (SIMS) oxygen isotope analyses. Multiple generations of quartz are identified in almost all of the veins. The barren quartz veins and quartz-magnetite veins of the early Mo mineralization stage are dominated by granular CL-bright quartz (EMQ1), which usually shows dissolution and re-deposition textures and are overprinted by CL-dark quartz (EMQ2)-filled healed fractures. The molybdenite precipitation in the quartz-molybdenite veins of the main Mo mineralization stage is synchronous with small volumes (<15%) of CL-dark quartz (MMQ2) and postdates the large volume (>85%) of CL-bright quartz (MMQ1). Similar to the quartz-molybdenite veins, the precipitation of pyrite in the quartz-pyrite veins during the transitional stage and sphalerite in the quartz-sphalerite-pyrite veins during the Zn-Pb mineralization stage are also directly associated with CL-dark quartz (TSQ2, ZPQ2). The SIMS oxygen isotopic values of EMQ1 and MMQ1 fall into the range of 5.42–9.79‰ with calculated equilibrium fluid compositions of 2.07–6.45‰, indicating a magma-dominated fluid composition, whereas MMQ2, TSQ2 and ZPQ2 have very low oxygen isotopic values from −4.68 to 4.29‰ with corresponding fluid compositions of −13.64 to −5.94‰, implying that the fluid formed by the mixing of magmatic water and a large proportion of meteoric water. The contrasting SEM-CL features and oxygen isotopic compositions of the two generations of quartz in the quartz-molybdenite veins demonstrate that the incursion of meteoric water into the hydrothermal system occurred earlier than the molybdenite precipitation. Based on these results and the crosscutting relationships and spatial distribution of the veins, we reconstructed the quartz evolutionary sequence during the vein development process. After the early precipitation of barren quartz, the reduction in fluid pressure from lithostatic to hydrostatic conditions allowed the incursion of convecting cool meteoric water into the relatively hot hydrothermal system, and the temperature of the evolved Mo-rich fluid decreased rapidly from >500 °C to <400 °C, leading to concentrated molybdenite precipitation with small volumes of quartz. Fluid cooling resulting from the incursion of meteoric water may have played a greater role in the metal precipitation in the Chalukou Mo deposit than wallrock interactions and fluid boiling. Moreover, this process may have also occurred in other porphyry Mo deposits.