Fault-fluid evolution in the Xitian W–Sn ore field (South China): Constraints from scheelite texture and composition

Abstract

Understanding fault-fluid evolutionary processes is crucial for understanding the genesis of vein-type deposits. Scheelite and quartz are common hydrothermal minerals in W–Sn ore field faults. In the Xitan ore field (Nanling Range, South China), scheelite textural and compositional features were studied and two types of scheelite were identified: Type I (represented by scheelite in the Xihu fault) is intergrown with quartz, has oscillatory zonation, variable zonation band width and is homogeneous light grey color in cathodoluminescence images; and Type II (represented by scheelite in the Goudalan fault) is intergrown with quartz that shows multi-stage growth, has oscillatory zonation and each zonation band is the same width. Scheelite grains of both types have homogeneous major element distribution (e.g., W and Ca), however trace element contents (e.g., Mg, Nb) are slightly variable, and rare earth element (REE) and Y contents gradually decrease by an order of magnitude from the core to the rim in both types of scheelite. From core to rim, the chondrite-normalized REE patterns of Type I scheelite change from flat curves with no or slightly positive Eu anomalies to intense concave shapes with intense positive Eu anomalies. The middle REEs gradually become depleted, indicating the MREE fractionation associated with early scheelite precipitation in a relatively closed fault-fluid system. In contrast, the chondrite-normalized REE patterns of Type II scheelite grains are similar, all showing weak MREE depletion from the core to rim, implying recurrent fluid pulses in an open fault-fluid system. The REE patterns in the cores of the Type I scheelite are similar to those of the scheelite from Jurassic granites related to the W–Sn mineralization, suggesting that the initial ore-forming fluids were exsolved from the Jurassic magma. The REE patterns in the rims of Type II scheelite are consistent with those of scheelite from the Goudalan W deposit, suggesting that the fault-fluid system experienced a similar evolution to the ore-forming fluid system. Combining the results of this work with previous quartz H-O isotopic and fluid inclusion studies, it can be concluded that fluid mixing occurred in both the fault-fluid system and the ore-forming fluid system, triggering large-scale W precipitation.