Oscillatory zoning in skarn garnet: Implications for tungsten ore exploration

The Sangdong and Weondong deposits, Taebaeksan mineral district, South Korea, are tungsten skarn ore deposits. Andradite (Fe-rich) garnet is a characteristic mineral in these skarn deposits, and is characterized by notable trace element contents (W, Mo, Sn, and U) compared to grossular (Al-rich) garnet. We report characteristics of the trace elements including W, in the garnet structure from the vein-skarn system (outermost exoskarn) and identify a relationship of exploration significance between the trace element composition and W-ore. The chemical characteristics of the garnets were determined using Back-Scattered Electron (BSE) imagery, Electron Probe Micro-Analyzer (EPMA) point analyses and X-ray dot-mapping, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) point analyses and mapping. Garnets in the study area are mainly composed of grandite (grossular–andradite) garnets, and exhibit compositional variation as oscillatory zoning. The chondrite-normalized REE patterns of Al-rich garnets are HREE-enriched. In contrast, Fe-rich garnets are LREE-enriched. Tungsten is strongly fractionated into the Fe-rich garnets (average concentration: 618 ppm), compared to the Al-rich rims (average concentration: 19 ppm). In addition, other trace elements (Mo, Sn, and U) are also closely correlated with W and REE signatures. The Fe-rich garnets are attributed to rapid growth by infiltration metasomatism during disequilibrium, which caused the incorporation of these trace elements into the garnet structure that reflect the composition of the ore-forming fluid. Consequently, these results indicate that andradite is a useful indicator mineral in the exploration of tungsten ore deposits.