Fluid evolution and ore genesis of the Weilasituo Li-Sn-Cu-Zn polymetallic deposit in Inner Mongolia: Evidence from fluid inclusion and C-H-O-Li isotopes

Abstract

The Weilasituo Li-Sn-Cu-Zn polymetallic deposit is located in the southwestern Great Xing'an Range (GXR), and crypto-explosive breccia-type and hydrothermal vein-type mineralization are developed in the mining area. With the Early Cretaceous amazonite granite as the mineralization center, the greisen-type Li(-Rb) mineralization is developed in the crypto-explosive breccia pipe above the granite, and the quartz vein-type Sn(-W-Mo) and Cu-Zn(-Ag) mineralization is successively developed in the surrounding Paleoproterozoic Xilinguole complex, forming mineralization zoning. The polymetallic metallogeny comprises early Li (Rb) through Sn(-W-Mo) to late Cu-Zn(-Ag) mineralization, and can be divided into six stages: (I) zinnwaldite–topaz–fluorite, (II) quartz–arsenopyrite–cassiterite–sphalerite–wolframite, (III) quartz–molybdenite–polymetallic sulfides, (IV) quartz–arsenopyrite–pyrite–chalcopyrite, (V) quartz–chalcopyrite–pyrrhotite–sphalerite–galena ± pyrite, and (VI) quartz–calcite–fluorite.Fluid inclusions (FIs) in the zinnwaldite, topaz and fluorite of Li(-Rb) ore phase contain high-temperature medium-salinity (348–404℃; 5.09–9.60 wt%) vapor–liquid two–phase aqueous type (vapor–liquid ratio: 35–70 %), similar to intermediate-density (ID) type FIs, and represent a single-phase fluid directly exsolved from the magma; FIs in the Sn(-W-Mo) ore phase quartz contain daughter mineral-bearing three-phase (SVL), vapor (V), vapor-rich two-phase aqueous (LV) and liquid-rich two-phase aqueous (VL) type, reflecting fluid boiling; FIs in the Cu-Zn(-Ag) ore phase quartz contain CH4 ± CO2 two-phase FIs and VL-type FIs, indicating fluid immiscibility. Lithium isotope data show that the Li ore-forming material was crustal derived, while H-O isotope data indicate that the ore fluid was derived from magmatic water and mixed with meteoric water in the late Cu-Zn(-Ag) ore phase; Carbon isotope data suggest that the CH4 and CO2 in the FIs were sourced from the fluid-rock reaction between the hydrothermal fluid and with the Xilinguole Complex graphite-bearing wallrocks. Raman spectroscopy shows that stage Ⅱ-Ⅴ FIs contain CH4 and CO2, whose content increased markedly from the Sn(-W-Mo) to Cu-Zn(-Ag) stage, indicating that CH4 and CO2 are enriched continuously with fluid migration and fluid-rock reaction.We conclude tha the initial single-phase ID Li-Rb-W-Sn-rich fluid was exsolved from the magma, and formed the Li(-Rb) mineralization in the crypto-explosive breccia pipe as the temperature dropped. Subsequently, the fluid entered the faults and the pressure dropped, causing fluid boiling, and triggered the proximal Sn(-W-Mo) vein-type mineralization. With the fluid migration and more water–rock reaction, CH4 and CO2 are continuously enriched and fluid immiscibility occurred, forming the distal Cu-Zn(-Ag) vein-type mineralization.