Geochronology and fluid inclusion study of the Aoiunhua porphyry Cu-Mo deposit in Arhorqin Area, Inner Mongolia

The Aolunhua Cu-Mo deposit in Arhorqin Area, Inner Mongolia, located in the Xilamulun Mo-metallogenic belt in the northern margin of North China Craton, is a typical porphyry Cu-Mo deposit. Re-Os isotope dating for six molybdenite samples gives an isochron age of 129.4±3.4Ma, that indicates the deposit formed in the early Cretaceous. The hydrothermal ore-forming process could be divided into three stages: the early stage is characterized by mineral assemblages of quartz-pyrite veins while the most important stage, the middle stage, is characterized by mineral assemblages of quartz-polymetallic sulfides veins. Fluid inclusions in minerals formed in early and middle stages yield homogeneous temperatures of 330～430℃ and 250～350℃, respectively, with salinities of 1.06 wt%～58.41 wt% NaCl eqv and 0.88 wt%～48.21 wt% NaCl eqv. Some fluid inclusions with contrasting salinities are homogenized to divergent phases at similar temperatures, and daughter mineral-bearing inclusions coexist with the vapor-and liquid-rich fluid inclusions, which strongly suggests that fluid-boiling have occurred in the early and middle stage. The late stage is characterized by quartz-carbonate-pyrite veinlets. In late-stage minerals, only liquid-rich fluid inclusions can be observed with homogeneous temperatures mainly below 270℃ and salinities between 0.71 wt% ～8.41 wt% NaCl eqv. Laser Raman spectroscopy indicate that the principal gas components of the fluid inclusions trapped in early and middle stages are H_2O and CO_2 whereas in those formed late stage are only H_2O. Hence it is believed that the original fluid, which is characterized by high temperature, high salinity, high oxygen-fugacity and CO_2-rich, have be sourced from magma or magma chamber. The fluid boiled at least twice when the primary magmatic fluid cooled down to 430℃ and 350℃ and resulted in rapid ore-metal precipitation, oxygen-fugacity decrease and CO_2-release. The late-stage fluid inclusions are lack of daughter mineral and poor in CO_2.