The discovery of Sr-rich carbonatite and its significance in Baozishan REE deposit, Mianning, Sichuan Province. Acta

Carbonatites reported in the world are mainly of calcio-, magnesio- and ferro-ones, and subordinately of sodium- and silico-carbonatites, while only very a few belong to Sr-rich carbonatites. Therefore, the petrogenesis, the resource and indicative significances and the magmatic evolution of the Sr-rich carbonatite remain unclear. Here we report a special type of Sr-rich carbonatites with microcrystalline to porphyritic textures in Mianning County, Sichuan Province. The samples were collected in the open pit of the Baozishan REE deposit, about 6km southern from the center of Maoniuping REE deposit. Based on field observation, petrographic, SEM/EDS and Sr-Nd isotopic results, the genesis of the Sr-rich microcrystalline carbonatite and its implication for the magma evolution are discussed. The Sr-rich carbonatites occur as small dikes intruded into a structure breccia. The rock is light purple to purple in color, and with microcrystalline to porphyritic textures. The mineral assemblage includes strontianite, fluorite, phlogopite, calcite, barite, with minor quartz, bastnaesite, parasite, sulfide, ilmenite and rutile. The petrochemical analysis results of the samples show that the carbonatite has a total REE of 3.5% 6. 1%, among which the MREE and HREE take up 1. 14% 1. 77%. It has high Pr (939 x 10 -6 1399 x 10 -6), Nd (2783 x 10 -6 3937 x 10 -6), Gd (237 x 10 -6 320 x 10 -6) contents, which are much higher than the values of La and Ce. Thus, the Sr-rich carbonatites themselves comprise high quality REE ores. In the chondrite-normalized partition pattern, the Sr-rich carbonatites show LREE enriched characteristics, without any Ce or Eu abnormalities. Trace element analysis results show that the Sr-rich carbonatite is enriched in REE, Sr, Ba, and depleted in P, Nb, Ta, Zr and Hf, implying that a mafic mineral fraction crystallization probably occurred during the magmatic evolution. The Sr-Nd isotopic compositions are similar to the rock assemblages in the Maoniuping and Lizhuang deposits, implying they have a similar mantle origin. The chilling texture of the rock may be originated from the shallow emplacement of the carbonite and encounter with infiltrated meteoric water through the fracture zone. The Sr-rich carbonatite represent a late -stage carbonatite magma. The mafic minerals, calcite or dolomite fractional crystallization may lead to REE, Sr, Ba enrichment in the later stage of the carbonatite magma. The quartz phenocryst and low Si02 content in the Srrich carbonatite imply that the carbonatite magma of the late stage is Si02 saturated and has a very low Si02 solubility. The strontianite ( >50%) dominated carbonatite may present a new type carbonatite, the Sr-rich carbonatite, which is obviously different from the known calcio-, magnesio-, ferro-, sodium- and silico-carbonatites.

Automated summary

The Sr-rich carbonatites reported in Mianning County, Sichuan Province have unique microcrystalline to porphyritic textures and a rich mineral assemblage, indicating a special origin related to shallow emplacement and magmatic evolution. The presence of quartz phenocrysts and high-quality ore minerals suggests a significant potential in mineral resources for this type of carbonatite.