The origin of glimmerite and its significance to rare earth element mineralization: Insights from the Longbaoshan deposit in North China Craton

Glimmerite is often associated with critical metal mineralization; however, its origin remains controversial, and the significance of glimmerite in rare earth mineralization is unclear. At Longbaoshan, glimmerite exhibits significant rare earth mineralization. We conducted mineralogical, morphological, and mineral geochemical analysis on the glimmerite REE orebodies in the Longbaoshan deposit in the Luxi Block of China. Micro X-Ray Fluorescence Spectrometry (μ-XRF) elemental mapping combined with an ImageJ image analysis was used to evaluate the mineralization of the glimmerite. We identified that the glimmerite experienced three stages of hydrothermal metasomatism, all of which led to REE enrichment in the glimmerite. The stage-Ⅰ metasomatism formed the glimmerite, during which phlogopite (∼80 %) and chevkinite-(Ce) crystals (1–2 % in volume) were formed. Titanite was replaced and enclosed by chevkinite-(Ce), suggesting a metasomatic origin of the latter. Because the phlogopite and chevkinite-(Ce) crystals share equilibrium contacts with each other, indicating their common, metasomatic origin. This is also supported by the lower Fe/Mg value and higher concentrations of Na, Ba, and Sr of unaltered phlogopite compared to the alteration halos surrounding unaltered phlogopite. This alteration halo is interpreted as alteration by a stage-II metasomatism, which is, in nature, a siliceous alteration that added Si and F in phlogopite and formed quartz veins and fluorbritholite-(Ce) which occupied 15 vol% and 24 vol% of glimmerite, respectively. The fluorbritholite-(Ce) crystals are formed through the replacement of fluorapatite in the glimmerite. The halogen fugacity during stage-II metasomatism is lower than that during the formation of glimmerite, with the rate of decrease in F fugacity faster than that of Cl fugacity. We use this to conclude that the primary factor driving the precipitation of rare earth minerals is the dominant decrease of F relative to Cl fugacity. The stage-III metasomatism was brought about by a carbonate fluid that altered fluorbritholite-(Ce) into aggregates of parisite-(Ce) and synchysite-(Ce) (these two together occupy 29 % of the volume of glimmerite). Mass balance calculations reveal that the second stage of siliceous metasomatism resulted in 15 times of REE enrichment in the glimmerite compared to the initial mineralogy of glimmerite, while the contribution of carbonate fluid metasomatism (Stage III) to REE enrichment was minimal.