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 (⁸⁷Sr/⁸⁶Sr)_{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 (⁸⁷Sr/⁸⁶Sr)_{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.