The role of magmatic and post-magmatic hydrothermal processes on rare-earth element mineralization: A study of the Bachu carbonatites from the Tarim Large Igneous Province, NW China

Abstract

The contribution of magmatic and hydrothermal processes to rare earth element (REE) mineralization of carbonatites remains an area of considerable interest. With the aim of better understanding REE mineralization mechanisms, we conducted a detailed study on the petrology, mineralogy and C-O isotopes of the Bachu carbonatites, NW China. The Bachu carbonatites are composed predominantly of magnesiocarbonatite with minor calciocarbonatite. The two types of carbonatite have primarily holocrystalline textures dominated by dolomite and calcite, respectively. Monazite-(Ce) and bastnäsite-(Ce), the major REE minerals, occur as euhedral grains and interstitial phases in the carbonatites. Melt inclusions in the dolomite partially rehomogenize at temperatures above 800 °C, and those in apatite have homogenization temperatures (Th) ranging from 645 to 785 °C. Oxygen isotope ratios of the calciocarbonatite intrusions (δ18OV-SMOW = 6.4‰ to 8.3‰), similar to the magnesiocarbonatites, indicate the parental magma is mantle-derived, and that they may derive from a more evolved stage of carbonatite fractionation. The magnesiocarbonatites are slightly enriched in LREE whereas calciocarbonatites have higher HREE concentrations. Both dolomite and calcite have low total REE (TREE) contents ranging from 112 to 436 ppm and 88 to 336 ppm, respectively, much lower than the bulk rock composition of the carbonatites (371 to 36,965 ppm). Hence, the fractional crystallization of carbonates is expected to elevate REE concentrations in the residual magma.Rocks from the Bachu deposit with the highest TREE concentration (up to 20 wt%) occur as small size (2 mm to 3 cm) red rare earth-rich veins (RRV) with barite + celestine + fluorapatite + monazite–(Ce) associations. These rocks are interpreted to have a hydrothermal origin, confirmed by the fluid inclusions in barite with Th in the range 198–267 °C. Hydrothermal processes may also explain the existence of interstitial textures in the carbonatites with similar mineral assemblages. The C-O isotopic compositions of the RRV (δ13CV-PDB = −3.6 to −4.3‰, δ18OV-SMOW = 7.6 to 9.8‰) are consistent with an origin resulting from fluid exsolution at the end of the high temperature fractionation trend. A two-stage model involving fractional crystallization and hydrothermal fluids is proposed for the mineralization of the Bachu REE deposit.