On the geochemical fractionation of rare earth elements during the formation of Ca-minerals and its application to problems of the genesis of ore deposits.

Relating petrological observations to geochemical results of distribution of REE in fluorite and calcite led to the construction of variation diagrams for each mineral; specific fields can be defined which are characteristic for various modes of formation. In the Tb/Ca and Tb/La diagram, fields can be outlined for fluorite formed under pegmatitic, hydrothermal or sedimentary conditions. In such a graph the arrangement of the analytical data-points for any mineral occurrence additionally displays primary or secondary crystallization as well as assimilation of Ca-rich rocks by the fluorite-bearing solutions or the interactions of such solutions with clay-rich sediments. In the Yb/Ca-Yb/La diagram, magmatic and hydrothermal calcites can be distinguished and clearly separated from those of biogenic origin. Similar diagrams have been considered for other carbonates (dolomite, magnesite) and apatite/phosphorite. The petrogenetic information obtained from fluorite or calcite can be extended to other minerals in paragenesis. This greatly enlarges the applicability of the variation diagram. The fractionation of REE during formation of Ca minerals is discussed, including the behaviour of REE complexes in solutions. A simple closed box model for the fractionation of REE is described and its results are transferred to open systems such as hydrothermal vein mineralization processes. Cerium and Eu anomalies quite often observed in Ca-minerals might serve as useful indicators for the origin of the ore-bearing solutions, and the Eh conditions during deposition, respectively. -J.M.H.