Fluid evolution and metallogenic processes of the Xie′ertala Fe-Zn deposit, Chenbaerhu Banner, Inner Mongolia

Located in the Variscan and Yanshanian metallogenic belt in the middle of the Da Hinggan Mountains, the Xie'ertala Fe-Zn deposit is a large-medium-size Fe-Zn deposit of volcanic eruption sedimentation-hydrothermal enrichment type discovered in recent years. Based on an analysis of material composition, the authors investigated emphatically the fluid inclusions and isotopes so as to study the ore-forming fluid evolution and mineralization. Salinity had negative correlation with fluid temperature from early to middle stage, indicating a boiling event. From middle to late stage, however, salinity was positively correlated with fluid temperatures, suggesting a mixing event. S, C, Pb, H and O isotopic compositions show that the ore-forming materials were derived mainly from country rocks and underlying rocks and partly from deeper sources. The data obtained suggest that the ore-forming fluids were derived predominantly from an active meteoric groundwater system and magmatic water. Because of meteoric supplement in the late stage, strong isotope exchange occurred between ore-forming fluids and surrounding rocks. The amount of the hydrothermal fluids in the ore-forming and alteration system was relatively insignificant. All these data demonstrate a combination of ore-forming fluids with moderate to medium-low temperature, low salinity and low density. Structural thermal effect, geothermal gradient and magmatic exhalation heat might have played the dominant role in Fe and Zn activation, whereas structural transition led to the Fe and Zn precipitation at a favorable position through changing the conditions of the primary well-evolved fluids.