Significance of hydrothermal reworking for REE mineralization associated with carbonatite: Constraints from in situ trace element and C-Sr isotope study of calcite and apatite from the Miaoya carbonatite complex (China)

Abstract

A majority of carbonatite-related rare earth element (REE) deposits are found in cratonic margins and orogenic belts, and metasomatic/hydrothermal reworking is common in these deposits; however, the role of metasomatic processes involved in their formation remains unclear. Here, we present a comprehensive in situ chemical and isotopic (C-Sr) investigation of calcite and fluorapatite within the Miaoya carbonatite complex located in the South Qinling orogenic belt, with the aim to better define the role of late-stage metasomatic processes.Carbonatite at Miaoya commonly occurs as stocks and dykes intruded into associated syenite, and can be subdivided into equigranular (Type I) and inequigranular (Type II) calcite carbonatites. Calcite in Type I carbonatite is characterized by the highest Sr (up to ∼22,000 ppm) and REE (195–542 ppm) concentrations with slight LREE-enriched chondrite normalized patterns [(La/Yb)N = 2.1–5.2]. In situ C and Sr isotopic compositions of calcite in Type I carbonatite define a limited range (87Sr/86Sr = 0.70344–0.70365; δ13C = −7.1 to −4.2 ‰) that are consistent with a mantle origin. Calcite in Type II carbonatite has lower Sr (1708–16322 ppm) and REEs (67–311 ppm) and displays variable LREE-depleted chondrite normalized REE patterns [(La/Yb)N = 0.2–3.3; (La/Sm)N = 0.2–2.0]. In situ 87Sr/86Sr and d13C isotopic compositions of Type II calcite are highly variable and range from 0.70350 to 0.70524 and −7.0 to −2.2 ‰, respectively. Fluorapatite in Type I and Type II carbonatites is characterized by similar trace-element and isotopic compositions. Both types of fluorapatite display variable trace element concentrations, especially LREE contents, whereas they exhibit relatively consistent near-chondritic Y/Ho ratios. Fluorapatite is characterized by consistent Sr isotopic compositions with a corresponding average 87Sr/86Sr ratio of 0.70359, which suggests that fluorapatite remained relatively closed in relation to contamination. The combined geochemical and isotopic data for calcite and fluorapatite from the Miaoya complex suggest that carbonatite-exsolved fluids together with possible syenite assimilation during the Mesozoic metasomatism overprinted the original trace-element and isotopic signatures acquired in the early Paleozoic magmatism. Hydrothermal reworking resulted in dissolution-reprecipitation of calcite and fluorapatite, which served as the dominant source of REE mineralization during the much younger metasomatic activity. The results from this study also suggest that carbonatites located in orogenic belts and cratonic edges possess a great potential for forming economic REE deposits, especially those that have undergone late-stage metasomatic reworking.