Genesis of the Bailugou Vein-Type Zinc-Lead-Silver Deposit, Eastern Qinling Orogen, China: Constraints from Ore Geology and Fluid Inclusions

The Bailugou vein-type zinc-lead-silver deposit is located in the Eastern Qinling Orogen, China. There has been a long-standing debate about whether its formation is related to magmatism or metamorphism. To determine the origin of ore-forming materials and fluids, we conducted a geological and fluid inclusion investigation of the Bailugou. Field surveys show that the vein-type orebodies are controlled by faults in the dolomitic marbles of the Mesoproterozoic Guandaokou Group, and they are distal to the regional Yanshanian intrusions. Four ore stages, i.e., quartz–pyrite ± sphalerite (Stage 1), quartz–polymetallic sulfides (Stage 2), dolomite–polymetallic sulfides (Stage 3), and calcite (Stage 4), are identified through microscopic observation. The homogenization temperatures of measured fluid inclusions vary in the range of 100 °C to 400 °C, with the dominating concentration at 350 °C to 400 °C, displaying a descending trend from early to late stages. The estimated formation depth of the Bailugou deposit varies from 2 km to 12 km, which is deeper than the metallogenic limit of the epithermal hydrothermal deposit but conforms to the typical characteristics of a fault-controlled deposit. The ore-forming fluid in Stage 1 originates from a fluid mixture and experiences a phase separation (or fluid immiscibility) between the metamorphic-sourced fluid and the fluids associated with ore-bearing carbonate-shale-chert association (CSC) strata. This process results in the transition to metamorphic hydrothermal fluid due to water–rock interactions in Stage 2, culminating in gradual weakening and potential fluid boiling during the mineralization of Stage 3. Collectively, the Bailugou lead-zinc-silver mineralization resembles an orogenic-type deposit formed by metamorphic fluids in the Qinling Yanshanian intracontinental orogeny.