Abstract:

This study focuses on major and rare earth element compositions of garnets from the Langdu skarn copper deposit. Electron microprobe analyses show that the garnets from the Langdu deposit belong to grossularandradite solid solution ranging in composition from Ad87Gr13 to Ad92Gr8. The garnets from the Langdu deposit are characterized by much lower ∑REE content and LREEenriched and HREEdepleted pattern, with a significantly positive Eu anomaly, which is obviously different from those of the majority of skarn deposits worldwide. The present study indicates that the andradites from the Langdu deposit grew rapidly and were not in complete equilibrium with the oreforming fluids, indicative of high W/R ratios. Under such conditions the incorporation of REE in andradites was controlled by crystal kinetics (surface adsorption and occlusion) and the REE pattern of the magmatic fluid.  Eu2+(r=125 ) has a bigger ionic radius than the other REE3+, making it easier to be adsorbed by garnet crystals, which could have been the dominant control on the strongly positive Eu anomaly in garnets of the Langdu deposit.

Key words: skarn copper deposit, garnet, REE, positive Eu anomaly, Langdu, Zhongdian island arc