REE-enriched skarns in collisional settings: The example of Xanthi's Fe-skarn, Rhodope Metallogenetic Massif, Northern Greece

Abstract

The garnet-epidote zone in the Xanthi Fe-skarn at the Rhodope Metallogenetic Massif, northern Greece, includes noteworthy cerium mineralization manifested by the presence of epidote-(Ce) containing up to ~130,000 ppm ΣREE and ~ 87,000 ppm Ce. The REE skarn represents a late evolutionary phase of the Xanthi's Fe-skarn in a collisional setting.- The epidote-(Ce) was deposited under pressures of ~1000 bars, at temperatures of ~345° to ~400 °C, from an almost pH neutral, saline, Ce-bearing, heterogeneous trapped, skarn-forming fluids with elevated log[αREE3+/(αH+)3] values. Thermodynamic two-stage modeling suggests that Ca metasomatism (e.g., decrease of the logαCa2+(aq) values) coupled with Ce fractionation (e.g., increase of the logαCe(CO3)+(aq) values) of the skarn-forming fluids led to the formation of epidote-(Ce). The major mechanism for the Ce-enrichment in the metasomatic fluids was the remobilization of the REE from the I-type granodiorite of Xanthi, in a back-arc setting due to subduction retreat.