The origin and evolution of REE enrichment in the Dashigou Mo-REE deposit, China: Insights from mineralogy, geochemistry and C-O-Sr-Nd isotope systematics

Abstract

The Dashigou deposit is a carbonatite-hosted REE-Mo deposit within the Huanglonpu metallogenic district of the Qinling belt in central China. This work aims to study origin of heavy REE enrichment using the mineralogy, mineral chemistry and C-O-Sr-Nd isotope signatures of the carbonates. Two stages of REE mineralization are distinguished, along with two generations of calcite (Cal-I and Cal-II). Monazite, bastnäsite, and xenotime are the primary REE minerals at the early carbonatite stage, with Cal-I having a high REE content of 1257–1745 ppm. The late-stage veins are characterized by abundant REE minerals, sulfates, and sulfides, including monazite, allanite, betafite group minerals, celestine, baryte, molybdenite, pyrite and sphalerite. Early magmatic minerals show signs of secondary replacement and metasomatism, with particularly abundant hydrothermal calcite and allanite. This indicates that REEs were remobilized hydrothermally into a secondary REE-rich mineral assemblage, possibly in large part through the dissolution of early REE-rich calcite. This is also supported by Cal-II having lower REE contents compared to CalI. In situ trace element compositions display a flat REE pattern with HREE enrichment compared to bulk rock compositions. The Cal-I contains a higher HREE content of 768–1124 ppm with (La/Lu) N ratios of 0.84–1.37, making it the dominant mineral that controls the HREE budget in Dashigou. The CO isotopic compositions of calcite have a narrow range from −7.27 to −6.65 ‰, and from 9.10 to 9.28 ‰, respectively, falling within the primary igneous carbonatite field. This indicates it is mantle-derived. In situ Sr isotopic values of calcite vary between 0.70502 and 0.70572, which are comparable to bulk rock values for the Huanglongpu carbonatites. Monazite grains from the Dashigou carbonatites have ¹⁴³Nd/¹⁴⁴Nd ratios between 0.51190 and 0.51205, and their εNd (t) values vary from −10.0 to −7.1, with an average value of −8.5. Our data suggests that both enriched mantle and crustal components contributed to the formation of REE-Mo mineralization in the Dashigou carbonatites.