Geochronology and fluid inclusion study of the Shabutai porphyry Mo deposit, Inner Mongolia

Abstract

The Shabutai molybdenum deposit is a newly discovered deposit located in the northern part of the Xilamulun metallogenic belt. Molybdenum mineralization is distributed in granite porphyry and its exocontact zone is in the shape of quartz veins. The LA-ICP-MS zircon U-Pb age of ore-bearing granite porphyry is 140.1±0.7Ma with a MSDW of 0.078. The granite belongs to the high-K calc-alkaline series characterized by relatively high LREE, low HREE, depletion of Ti, Ba, and Nb, and a negative Eu anomaly. Mineralization evolution is divided into three stages: early, middle and late. The stages are characterized by potassium feldspar veins, quartz-potassium feldspar veins, and quartz veins in the early stage, quartz-molybdenite veins in the middle stage, and quartz-carbonate veins in the late stage. Ore minerals were mainly precipitated in the middle stage. Pure-vapor (PV), vapor-rich (V), liquid-rich (L), and daughter mineral-bearing (S) fluid inclusions were observed in the hydrothermal quartz. Homogeneous temperatures of fluid inclusions in the early stage ranged from 240°C to 420°C, with salinities ranging from 4.0 to 57.2wt.% NaCl eqv. The fluid inclusions contained hematite daughter minerals, indicating high oxygen fugacity conditions in the early stage. The temperatures of the fluid inclusions at the middle stage ranged predominantly from 200 to 300°C with a salinity range of 0.5–48.5wt.% NaCl eqv. The inclusions also showed high K ion content characteristics, both in the early and middle stage fluid. A drop in potassium concentration caused by potassic alteration created a favorable condition for Mo precipitation. The temperature range of the late-stage fluid inclusions was 160–307°C with salinities below 10.7wt.% NaCl eqv. According to Laser Raman microprobe data, CO2 occurred only in a few gas fluid inclusions, while most gases were H2O. Microthermometric analysis results suggested that fluid boiling was the main factor triggering molybdenite precipitate and forming the Shabutai molybdenum deposit.