Skarn classification and element mobility in the Yeshan Iron Deposit, Eastern China: Insight from lithogeochemistry

Skarns form by significant fluid-mediated exchange of mass and heat between igneous rocks and their surrounding wall rocks into which they intruded. Quantification of the mass exchange associated with skarn alteration requires knowledge of the skarn protoliths, which are often masked by metamorphic recrystallization and intense calc-silicate metasomatism. To overcome this challenge in characterizing the Yeshan skarn Fe deposit in Eastern China, a cross-section through the complete rock sequence, from the carbonate wall rock to the pluton, was systematically sampled, and analyzed for bulk-rock major and trace elements. Underpinned by the skarn zonation model, ln(SiO2/Al2O3), ln(SiO2/TiO2), and REE + Y values in the skarns were used to distinguish the various skarn protoliths. The effectiveness of the ln(SiO2/Al2O3) and ln(SiO2/TiO2) is supported by the variable mobility of Si, Al, and Ti during magma-derived fluid infiltration into the carbonate wall rocks at Yeshan. The effectiveness of REE + Y is based on their significant concentration differences in the carbonate wall rocks and igneous rocks at Yeshan. These geochemical indexes may be applicable to the characterization of protoliths and mass transfer in skarn deposits where igneous rocks intruded carbonate wall rocks.