tThe Sangan iron skarn deposit is located in the Sabzevar-Dorouneh Magmatic Belt of northeastern Iran.The skarn contains zoned garnet, clinopyroxene and magnetite. Cores and rims of zoned garnets aregenerally homogeneous, having a relatively high REE, low LREE/HREE ratios, and positive Eu anoma-lies. The cores of the zoned clinopyroxenes are exceptionally HREE-rich, with relatively high REEand HREE/LREE ratios, as well as positive Eu anomalies. Clinopyroxene rims are LREE-rich, with rela-tively low REE contents and HREE/LREE ratios, and do not have Eu anomalies. Magnetite grains areenriched in LREEs in comparison with the HREEs and lack Eu anomalies. Variations of fluid composi-tion and physicochemical conditions rather than YAG-type substitution mechanism are considered tohave major control on incorporating trace elements, including REE, into the skarn mineral assemblage.Based on baro-acoustic decrepitation analysis, the calc-silicate and magnetite dominant stages wereformed at similar temperatures, around 350–400◦C. In the Sangan skarns, hydrothermal fluids shiftedfrom near-neutral pH, reduced conditions with relatively high REE, low LREE/HREE ratios, and U-richcharacteristics towards acidic, oxidized conditions with relatively low REE, high LREE/HREE ratios, andU-poor characteristics.

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