Fluid inclusion evidence for hydrothermal evolution of the Sadaigoumen porphyry Mo deposit on the northern margin of the north China Craton

Abstract

The Sadaigoumen porphyry Mo deposit is located on the northern margin of the North China Craton (NCC). In this study, in situ fluid-inclusion analyses including microthermometry, laser Raman spectroscopy, and LA–ICP–MS microanalysis, as well as Quartz H–O isotopes are present to precisely constrain the hydrothermal evolution of the Sadaigoumen deposit. Five stages of hydrothermal activity are identified, with Stage 1 characterized by the development of magnetite with rare sulfide minerals; Stage 2 by K-feldspar as the typical vein mineral; Stage 3 by molybdenite mainly in veins; Stage 4 by the development of fluorite–quartz and quartz–pyrite veins; and Stage 5 by barren carbonate–quartz veins. Fluid inclusion types were distinguished based on petrographic observations and microthermometric analysis, and fluid compositions were detected by LA–ICP–MS. Microthermometric analyses indicate decreasing homogenization temperatures in primary fluid inclusions from Stage 1 to Stage 3. Intermediate-density fluids of stages 1 and 2 are characterized by high homogenization temperatures (420 °C–470 °C), moderate salinities (6.3–11.2 wt%), and high Mo contents (52–541 ppm). Hydrothermal evolution during Mo mineralization in Stage 3 is characterized by immiscible inclusion assemblages that formed through phase separation from intermediate-density fluid. Fluid inclusions of Stage 3 were trapped at pressures fluctuating around the two-phase fluid surface, leading to at least two fluid immiscible phases (373 °C–414 °C and 311 °C–341 °C). The Mo contents of fluid inclusions vary greatly in Stage 3, with Mo tending to be enriched in brine. In stages 4 and 5, fluids display characteristics that indicate mixing with meteoric water, with homogenization temperatures (<299 °C) and salinities (<5.8 wt%) decreasing together with Mo content. The Cs/(Na + K + Mn + Fe) ratios of fluid inclusions are stable from stages 1 to 4, indicating a constant supply of ore-forming fluids from an underlying, geochemically uniform magma chamber. The fluid inclusion data suggest that Mo and Cu in the initial exsolved fluids of the Sadaigoumen porphyry Mo deposit display different migration and precipitation progresses, which is of great significance to the study of the delicate ore-forming progress of porphyry Mo (–Cu) deposits in the northern NCC.