Carbonatitic fluid infiltration and REE remobilization from magmatic calcite at the Maoniuping deposit, China: constraints from trace-element and Sr–Pb isotopic variations in mineralized and barren veins

The Maoniuping deposit (∼3.17 Mt REO) in the Mianning–Dechang REE belt, western China, consists of a system of carbonatitic veins intruding a nordmarkite (syenite) complex. Despite extensive study, the mechanisms responsible for REE mineralization in one of the largest REE deposits currently mined remain disputed. Here, we present new petrological, trace-element, and in situ Sr–Pb isotopic data for calcite and silicate minerals from mineralized and barren veins to trace the composition and origin of melts and fluids forming these veins, constrain the role of wall-rock reaction, and investigate the processes responsible for REE deposition. Aegirine-augite crystallizes along the margins of carbonatite veins and also occurs as the major phase in discrete clinopyroxene-dominated veins. The Pb isotopic compositions of aegirine-augite and co-crystallizing K-feldspar resemble those of the nordmarkite host, whereas the Sr isotopic compositions of aegirine-augite match those of calcite from the interiors of carbonatite veins. This decoupling records localized carbonatite melt–wall rock interaction during vein emplacement—an antiskarn process. The texture and geochemistry of calcite in the carbonatite veins record a continuum from primary magmatic to fully recrystallized during fluid overprinting. Recrystallized calcite is characterized by polygonal mosaic textures, lower Sr and REE contents and La/Yb values, and elevated Fe and Mn contents relative to coarse-grained, generally deformed, primary magmatic calcite. Calcite Sr–Pb isotopic compositions are variable across the deposit. Binary mixing modelling indicates that this variability was inherited from the infiltrating melts and fluids rather than acquired through interaction with the nordmarkite at the level of emplacement. Progressive recrystallization of magmatic calcite was therefore driven by the infiltration of compositionally heterogeneous carbonatitic fluids. Bastnäsite occurs exclusively in association with veins containing recrystallized calcite, fluorite, and baryte, and never with primary magmatic calcite. The decrease in REE content from magmatic to fully recrystallized, metasomatic calcite indicates that REE were released from magmatic calcite during fluid interaction. These observations support a genetic model in which REE mineralization was controlled by late-stage carbonate-rich fluids (or brines), with remobilized REE from magmatic calcite contributing to the REE budget carried by the carbonatitic liquids. A paucity of P was a necessary condition for generating such high levels of bastnäsite enrichment, suppressing the crystallization of REE-bearing apatite and thereby favoring REE fluorocarbonate formation, although the reason for this P paucity is presently unknown.