Multiple pulses of fluids involved in the formation of carbonatite-related REE deposits as revealed by fluorite

Abstract

Formation of carbonatite-related REE deposits generally associated with hydrothermal fluids exsolved from carbonatite melts. Deciphering the nature and evolution of the fluids is critical for establishing ore-formation models, and was mostly benefited from multi-analytical investigation of specific minerals presenting in multi-generation veins. However, there is a risk that the minerals in specific-generation veins not always precipitated solely from the single-stage fluids particularly for a dynamic mineralizing system. In order to evaluate this potential risk, this study conducted a combined textural, in situ trace elemental and fluid inclusion investigation of fluorite from multiple generations of veins in the Muluozhai carbonatite-related REE deposit, SW China. The veins dominate the mineralization system of the deposit, and include four generations/stages: fenitization (Stage I), pre-REE (Stage II), early REE (Stage III) and late REE (Stage IV) stages in which the ore minerals are all dominated by bastnäsite. Fluorite grains are widely present in these generations of veins, but high-resolution CL imaging reveal that most of them display complex internal textures composed commonly of core, mantle and rim domains from center outward. These domains exhibit different CL-intensities that are well correlated with chemical compositions and even types of fluid inclusions they hosted. This study confirmed that the “core-mantle-rim” textures of fluorite are essentially responses to interaction of early grains (i.e., represented by cores) with multi-stage fluids through dissolution-reprecipitation processes. Trace elemental and fluid-inclusion microthermometric results show that the cores (also for mantles or rims) in fluorite from Stages I to III veins have comparable REE concentrations and patterns and homogenization temperatures of fluid inclusions, strongly suggesting that the core-mantle-rim fluorite grains in these veins are originally formed at Stage I, and were variably captured or metasomatized by fluids of Stage II or III during development of these generations of veins. Similar core-mantle-rim fluorite grains are also present in Stage IV veins, and thus are constrained to be also “xenocrysts” formed in a similar manner. They are surrounded or even locally disturbed by a number of euhedral and/or slightly oscillatory fluorite grains which are supposed to be formed from a different manner. On the basis of their close association with bastnäsite and relatively high homogenization temperatures of fluid inclusions they host, we proposed that these euhedral fluorite grains are products of Stage IV fluids.Clear understanding of the internal textures and paragenesis of fluorite allows us to propose a genetic model for the Muluozhai deposit that the REE mineralization in Stages III and IV involves likely different pulses of carbonatite-related REE-rich fluids, and REE deposition was possibly mainly triggered by incursions of external fluids (e.g., meteoric water). This study also highlights that the phenomenon regarding presences of early-stage “xenocrysts” in late-generation veins of carbonatite-related vein systems may be more common than previously thought, and thus investigation in a textural context is critical for deciphering ore genesis.