Genesis of the Late Jurassic Shizitou Mo deposit, South China: Evidences from fluid inclusion, H[sbnd]O isotope and Re[sbnd]Os geochronology

Abstract

The Shizitou deposit is a middle-sized Mo deposit in the Qin-Hang metallogenic belt, South China. Three stages of mineralization are recognized at the Shizitou, including the early stage characterized by barren quartz veins with potassic alteration, followed by the main ore stage represented by quartz-molybdenite veins with strong phyllic alteration, and the late stage featured by quartz-pyrite veins with weak propylitization. Two-phase liquid-rich aqueous (type I), two-phase gas-rich aqueous (type II), and halite-bearing (type III) inclusions are present in the main ore stage veins, but only type I inclusions occur in the early and late stages of veins. Type I inclusions in the early stage veins show homogenization temperatures (Th) of 291–376°C, with salinities of 2.2–9.5wt.% NaCl equivalent. Type II and III inclusions in the main ore stage veins display Th of 266–351°C and 245–336°C, and salinities of 0.6–3.5 and 28.0–36.2wt.% NaCl equivalent, respectively. Type I inclusions in the late stage quartz-pyrite veins have Th of 209–288°C and salinities of 0.5–5.0wt.% NaCl equivalent. Fluid boiling took place in the main ore stage and promoted the quick precipitation of molybdenite from ore-forming solutions. Raman analysis of fluid inclusions implies that mineralizing fluids contain little CO2 and N2. Hydrogen and oxygen isotopes indicate that initial ore-forming fluids were magmatic in origin and shifted gradually to meteoric regime. ReOs dating of molybdenite gives a 187Re/187Os isochron age of 162.3±2.4Ma, in response to a localized extensional setting resulting from the far-field stresses of the initial subduction of the Paleo-Pacific plate beneath the margins of the South China Block.