RARE-EARTH ELEMENT FRACTIONATION IN METAMORPHOGENIC HYDROTHERMAL CALCITE, MAGNESITE AND SIDERITE

Normalized REE patterns of aqueous solutions and their precipitates bear information on the physico-chemical environments a fluid experienced during REE mobilization, fluid migration and minerogenesis. Positive Eu and Yb anomalies indicate REE mobilization by a F--, OH-- and CO32--poor fluid in a high-temperature regime, but are only retained by a precipitating mineral if precipitation occurs in a low-temperature environment. Negative Ce anomalies are typical of oxidizing conditions and are unlikely to develop during siderite precipitation. LREE/HREE fractionation is controlled by fluid composition and "mineralogical control". REE patterns of Ca minerals allow to class the reacting fluids in "normal" (Ca/ligand much greater than 1) and "ligand-enriched" (Ca/ligand almost-equal-to 1), the latter being characteristic for remobilization processes. The Radenthein magnesite and Huttenberg siderite deposits, both Carinthia, Austria, are discussed and shown to be of non-sedimentary, non-metamorphic, but metamorphogenic metasomatic origin.