Mica compositional constraints on the petrogenesis and mineralization of the syenite-carbonatite complex in the Maoniuping REE deposit, SW China

Mica, as a long-lasting mafic mineral in the magmatic-hydrothermal evolution of alkaline-carbonatite complexes, is an ideal monitor of petrogenesis and mineralization processes. Here, the compositions of mica in the bulk syenite, carbonatite and their related hydrothermal veins from the Maoniuping rare earth elements (REE) deposit (SW China) were analyzed in detail, in order to reveal the petrogenesis and REE mineralization processes of the Maoniuping syenite-carbonatite complex. Micas in the bulk syenite and carbonatite are common phlogopite with indistinguishable compositions, while micas in the hydrothermal veins of syenite and carbonatite are quite different from that in the bulk syenite and carbonatite and compositionally close to ideal tetraferriphlogopite. The common phlogopite is interpreted as orthomagmatic xenocryst crystallizing from parent magma system, whereas tetraferriphlogopite precipitates in the REE-enriched hydrothermal fluids. The compositional variations of mica show that Fe and F contents increase, whereas Al and Ti contents and Nb/Ta ratios decrease evolving from common phlogopite to tetraferriphlogopite. Compared to common phlogopite, tetraferriphlogopite has higher levels of REE (up to 77 ppm), Ba (up to 5000 ppm), Sr (up to 700 ppm), Li (up to 14000 ppm), Rb (up to 1700 ppm), and Na (up to 1200 ppm). The overlapped chemical compositions of common phlogopite in the bulk syenite and carbonatite support that the liquid immiscibility of syenitic and carbonatitic melts from a common parent magma, which may happen due to moderate crystal fractionation induced by decreasing temperature/pressure in the crustal magma chamber. Meanwhile, the presentation of tetraferriphlogopite in the hydrothermal veins of syenite and carbonatite indicates that syenitic and carbonatitic melts subsequently experienced independent magmatic-hydrothermal evolution path after separation, leading to REE mineralization of syenite and carbonatite respectively. The tetraferriphlogopite in the hydrothermal veins of carbonatite has higher oxygen fugacity and higher contents of incompatible elements (e.g., Li, Rb, Na, F, Sr, Ba, and REE) than that in the hydrothermal veins of syenite, which indicates that the carbonatite-derived fluids have higher REE mineralization potential than the syenite-derived fluids. Therefore, we conclude that liquid immiscibility and separated magmatic-hydrothermal evolution may be the key processes controlling the formation of the giant Maoniuping REE deposit.