Stable isotope and fluid inclusion constraints on the source and evolution of ore fluids in the Xiuwacu W-Mo granite-related quartz-vein deposit, Yunnan Province, China

Abstract

The Xiuwacu granite-related quartz-vein type W-Mo deposit (9.16 million metric tons at 0.26% WO3 and 0.27% Mo) is situated in the South Yidun terrane (SYT) in SW China. Orebodies, together with aplite veins, occur mainly along the NW-SE-trending fault systems within the Xiuwacu complex intrusions. However, the metallogenic mechanism of this deposit is still ambiguous. In this study, three stages for the ore-bearing quartz veins have been recognized based on the field and petrographic observations: (1) pre-ore fluorite-quartz; (2) syn-ore scheelite-quartz; and (3) post-ore calcite-quartz assemblage. The average δ34S value is 3.84‰, combination with the molybdenite Re values (4.3–18.6 × 10−6), indicating that the ore-forming fluid derived from lower crust magmatic fluid. The δD and δ18Ofluid values of the ore-forming fluids in the Xiuwacu deposit vary broadly from −104.9‰ to −75.0‰ and from −2.5‰ to +5.9‰, respectively, indicating a mixture of magmatic and meteoric water or seawater during the ore-forming process. The coexisting quartz with scheelite contains two-phase aqueous inclusions (type Ia), minor two-phase vapor inclusions (type Ib) and two- or three-phase CO2-bearing inclusions (type II), while scheelite only contains two-phase aqueous inclusions (type Ia). The homogenization temperatures for fluid inclusions in scheelite are very similar in both mineralized zones, which are slightly higher than those in quartz in the East mineralized zone, but are slightly lower than those in quartz in the West mineralized zone. These indicate that fluid cooling is predominant in the formation of scheelite. As the homogenized temperatures in the fluid inclusions of scheelites have a normal distribution, together with the medium-coarse grained signatures of scheelite, it could be concluded that Ca supply was insufficient during the tungsten precipitation process. Thus, the ore-forming mechanism of the Xiuwacu deposit is controlled by both the simple fluid cooling and addition of Ca during fluid mixing and/or fluid-rock interaction.