RARE-EARTH ELEMENTS AND HYDROTHERMAL ORE FORMATION PROCESSES

The geochemical behaviour of REE is influenced by nearly all important hydrothermal ore formation processes including fluid-rock interactions, fluid precipitations, adsorption and scavenging onto particles, and changes in fluid temperature, pressure, pH, Eh, alkalinity and ligand concentration. Destabilization of REE complexes in response to these physicochemical changes and possible chemical-crystallographic controls determine the concentration and distribution of REE within hydrothermal minerals, mineraloids and amorphous phases. Alteration assemblages of intrusive related mineralisations may exhibit a wide range of REE distributions and REE fractionation trends. The REE distribution within these hydrothermally altered lithologies will depend on the REE concentration in the rock and the fluid, the partitioning behaviour of the REE between the rock phases and the fluid, and the types of alteration reactions which take place. Reactive rocks such as carbonate-rich lithologies of skarn deposits may gain or loose significant amounts of REE during fluid-rock interactions. REE geochemical investigations combined with fluid inclusion and stable isotope studies may point to the origin of alteration products and the source (s) of ore fluids. Ancient massive sulphide ores commonly exhibit anomalous Eu concentrations similar to many recent submarine hydrothermal precipitates. The recognition of hydrothermal fluid, seawater, sedimentary or volcanically derived REE distributions within ores, chemical sediments and hydrothermally altered lithologies may help to constrain genetic modelling of massive sulphide deposits. Care should be taken with the interpretation of REE distributions from metamorphosed hydrothermal ore deposits because metamorphic and diagenetic alteration reactions may change the REE distributions. Mobility of REE may occur during diagenesis of carbonate-rich rocks and during the development of shear zones and migmatites. In contrast, very large fluid-rock ratios are necessary to cause significant changes in REE patterns of silicate-rich rocks during diagenesis and regional metamorphism.