Research status and prospect of the evolution mechanism of ore-forming fluids for carbonatite-hosted REE deposits

Carbonatite-hosted endogenetic rare earth element (REE) deposits related to alkaline rocks are very important in light rare earth resources in China and even in the world. At present, controversies for the evolution mechanism of ore-forming fluids for carbonatite-hosted REE deposits are mainly among crystallization differentiation, liquid immiscibility (melt-melt, melt-fluid and fluid-fluid immiscibility) and hydrothermal metasomatic alteration. Crystallization differentiation can gradually enrich the incompatible REEs in the residual melt phase until the REE minerals are formed. Immiscibility can lead to selective enrichment of REEs in one of the two or multiple-phases formed after immiscibility, resulting in REE mineralization. The hydrothermal fluids formed in the late stage of ore-forming fluids evolutionary process have metasomatic reaction with the early-formed minerals or surrounding rocks and release anions (anion clusters) such as F^-, (CO₃)^2- and (SO₄)^2-, which can form complexes with REEs in the hydrothermal aqueous solution, and finally recrystallize or precipitate REE minerals in appropriate ore-hosting structures under suitable external conditions. Each of the above three viewpoints has its own theoretical basis, but they are more or less inadequate in the explanation of some geological phenomena or experimental geochemical simulation results of carbonatite-hosted REE deposits. The previous study results shown that there are a large number of melt inclusions, melt-fluid inclusions and CO₂-rich fluid inclusions in carbonatite-hosted REE deposits. In the past, most of these inclusions decrepitated or were leaked before reaching the total homogenization status when heated at 101325 Pa by using conventional high temperature heating stages, such as Linkam TS1400XY and Linkam THMS600, which greatly restrict our understanding of the evolutionary process of ore-forming fluids in high temperature magmatic stage and medium-high temperature magmatic-hydrothermal stage for this type of REE deposits. In addition, studies on the contents variation characteristics and geochemical behavior of REEs in the ore-forming fluid evolutionary process is mainly through the Raman spectroscopy analysis of the components of inclusions, as well as the whole rock geochemical analysis of ore bodies and surrounding rocks, and there is still a lack of in-situ microanalysis data about the element contents of individual inclusions. In the future, for melt inclusions, melt-fluid inclusions, and CO₂-rich fluid inclusions that trapped in this type of deposits, in-situ high temperature and high pressure microthermometry experiments by employing hydrothermal diamond-anvil cell together with in-situ LA-ICP-MS microanalysis of trace elements contents in individual inclusions, are supposed crucial to reveal its evolution mechanism of ore-forming fluids.