Multistage W-mineralization and magmatic-hydrothermal fluid evolution: Microtextural and geochemical footprints in scheelite from the Weondong W-skarn deposit, South Korea

Abstract

Weondong deposit is one of the W-skarn deposits in South Korea, and main ore mineral is scheelite (CaWO4). Scheelite occurs in both a massive skarn zone (MSZ; Type I) and a quartz porphyry fracture zone (QPFZ; Type II), forming solid solution with powellite (CaMoO4). We examined the relationship between the evolution of ore-forming fluids and formation of multistage scheelites based on microtextural and geochemical features. Type I scheelites can be divided into Type Ia and Type Ib. Type Ia scheelites are characterized by complex internal textures with oscillatory zoning. The texturally associated clinopyroxene (diopside), garnet (andradite), and magnetite strongly suggest an oxidized prograde skarn origin. They are also characterized by enriched light rare earth elements (LREEs) and a negative Eu anomaly. Type Ib scheelites are W-enriched (74–81 wt% of WO3) with no internal zoning. Relict grossular garnet, calcite, pyrrhotite, serpentine, and vesuvianite indicate its relatively reduced retrograde skarn origin. Their rare earth element (REE) pattern is flat with a negative Eu anomaly. Type II scheelites can further be divided into Types IIa and IIb based on textural and geochemical differences. The former largely exhibit patchy zoning with minor oscillatory zoning that normally occurs along microfractures in association with fluorite. Type IIa scheelites show REE fractionation between W- and Mo-enriched zones (i.e., those containing 60–71 wt% of WO3 and 15–25 wt% of MoO3, respectively). Tungsten enriched scheelites show middle rare earth element (MREE)-enriched patterns with a strong negative Eu anomaly; Mo-rich zones are MREE-depleted and show a weak negative Eu anomaly. Type IIb scheelites are W-rich and rarely have zoned texture, similar to Type Ib scheelites. Type IIb scheelites generally coexists with sulfide minerals, indicating a relatively high sulfur fugacity (fS2) retrograde conditions. The REE pattern is flat with a negative Eu anomaly. These results indicate that Type Ia and Type IIa scheelites were formed during prograde conditions at each stage, followed by Type Ib and Type IIb in retrograde conditions. Type Ia scheelites have LREE-enriched patterns, which mimic the REE profile of juvenile magmatic fluids under open systems. Type IIa scheelites have MREE-enriched fractionation patterns that likely reflect a highly evolved magmatic fluid origin in a closed system. The flat REE pattern in Type Ib and IIb scheelites is due to meteoric water in retrograde conditions. This causes changes in the trace element composition of scheelites that reflect differences in the fluid origin and stages of evolution. The geochemical characteristics of both scheelite types match well with their microtextural features and provide crucial evidence for understanding the multistage formation of the Weondong W-skarn.