Trace Element Geochemistry and Sr‐Nd Isotopic Characteristics of Scheelite from the Gejiu Sn‐Cu‐(W) Ore District: Constraints on Tungsten Sources and Evolution of W‐bearing Hydrothermal Fluids

The Gejiu tin‐copper‐(tungsten) (Sn‐Cu‐(W)) polymetallic district is located in the southwest of the W‐Sn metallogenic belt in the western Youjiang Basin, Yunnan, Southwest China. Abundant W minerals have been identified in the region via exploration. However, metallogenic sources and evolution of W remain unclear, and the existing metallogenic model has to be updated to guide further ore prospecting. Elemental and Sr‐Nd isotopic data for scheelites assist in the determination of sources and evolution of the W‐mineralizing fluids and metals in the district. Based on field geological survey, the scheelites in the Gejiu district can be categorized into three types: altered granite (Type I), quartz vein (Type II) from the Laochang deposit, and skarn (Type III) from the Kafang deposit. Types I and II scheelites have low molybdenum (Mo) and strontium (Sr) contents, and Type II scheelite has lower Sr contents than Type I as well as higher Mo and Sr contents than Type III scheelites. Varying Mo contents across the scheelite types suggests that the oxygen fugacity varied during ore accumulation. Type I and Type II scheelites exhibit similar rare earth elements (REE) patterns; Type III scheelite contains lower REE content, particularly HREE, compared with the other scheelites. All scheelites exhibit negative Eu anomalies in the chondrite‐normalized REE patterns. As the W‐mineralization and two‐mica granite share close spatial and temporal relationships, the negative Eu anomalies were likely inherited from the two‐mica granite. Type I and Type II scheelites display varied ( 87 Sr/ 86 Sr) 82 Ma (0.7090–0.7141) and ε Nd (82 Ma) (from –9.9 to –5.4) values, similar to those of granite. However, Type III scheelite exhibits lower ( 87 Sr/ 86 Sr) 82 Ma (0.7083–0.7087) and lower ε Nd (82 Ma) (from –10.5 to –6.9) values than the two‐mica granite. This indicates that the two‐mica granite alone did not provide the ore‐forming fluids and metals and that the Type III scheelite ore‐forming fluids likely involved external fluids that were probably derived from carbonate rocks. The implication is that highly differentiated two‐mica granites were the source of primary W‐bearing metals and fluids, which is consistent with earlier research on the origin of Sn ore‐forming materials.