Compositional Variations of Rinkite in the Saima Alkaline Complex, Liaoning Province, and Its Implications for Alkaline Magma Evolution

The Saima complex is a typical alkaline complex with uranium mineralization in Liaoning Province, China. In order to explore its REE mineralization mechanism, the optical microscope, SEM and EPMA were used to study the rare earth minerals in the Saima complex. From nepheline syenite through aegirine-bearing nepheline syenite pegmatite to late lujavrite, the REE mineralization is characterized by the enrichment of the representative rare earth mineral, rinkite (Na₂Ca₄REETi(Si₂O₇)₂OF₃), the increases in Nb, Zr, REE (especially for HREE) and other high field strength elements of it, and the compositional zonation caused by compositional variations of Zr, REE and other elements in some rinkite grains. The above proportional and compositional variations of rinkite are closely related to the behavior of incompatible elements, the nature and fractional crystallization process of alkaline magma. In addition, the later series of hydrothermal alteration in the rinkite is represented by the initial enrichment in Ti, Ca, Sr, and Na but depletion in Zr and REE contents in altered parts of crystal, and ultimate pseudomorph after rinkite, which is comprised of residual rinkite+calcite+fluorite+britholite-(Ce). It is likely that an alkali, F and CO₂-rich hydrothermal fluid is involved in the rinkite alteration. Our study suggests that rinkite is an ideal mineral in the aspects of deciphering the fractional crystallization of alkaline magma, fractionation behavior and hydrothermal mobilization of rare earth elements by its compositional variations and secondary alteration mineral assemblage.