BACKGROUND In Liaoning Province, there are limited known types of rare earth deposits, and historically, exploration and evaluation efforts have mainly focused on monazite placer and alkaline rock type rare earth deposits. Less attention has been given to sedimentary metamorphic rare earth primary deposits. Previous studies have shown that the Jixiangyu rare earth deposit is categorized as a sedimentary metamorphic remodeling deposit related to ancient volcanic structures. The analysis of existing data and previous exploration results prove that this type of rare earth deposit has a considerable rare earth content with monazite and allanite. However, unresolved issues remain, such as the occurrence status of rare earth minerals and the feasibility of extracting and utilizing rare earth minerals.

OBJECTIVES To explore the metallogenic mechanism and unveil the formation process of rare earth deposits by identifying the types of rare earth minerals, investigating the occurrence and distribution of rare earth elements within minerals, and analyzing the composition, structure, and distribution characteristics of rare earth minerals and associated minerals.

METHODS The experimental testing was conducted at the Henan Provincial Rock and Mineral Testing Center. The experimental instruments used in this study included an ultra-high resolution field emission scanning electron microscope (Zeiss Sigma500), an electric cooling energy spectrometer (Bruker XFlash6610), and a set of AMICS automatic mineral identification and characterization systems. During the experiments, a high vacuum environment was maintained, with an accelerating voltage of 20kV and a beam current of 5nA. The working distance was set at 11.8mm, and the point analysis acquisition time reached 250kcps before stopping automatically.

RESULTS The primary rare earth minerals in the Jixiangyu rare earth deposit were identified as allanite, monazite, and cerianite, constituting 6.25%, 0.73%, and 0.25% of the total mineral content, respectively. Light rare earth elements such as La, Ce, Pr, and Nd were predominantly present, enriched in monazite, xenotime, and bastnaesite, with a minor occurrence of rare earth elements in the form of solid solutions within apatite. Gangue minerals include actinolite, quartz, plagioclase, potassium feldspar, sphene, and biotite.　　The rare earth minerals, including allanite, monazite, cerianite, and apatite exhibited interlocking structures with magnetite, occurring as single particles or clustered structures, and were distributed along the edges and gaps of magnetite, forming complex symbiotic relationships.

CONCLUSIONS The rare earth formation process can be attributed to the following factors: (1) Sedimentary enrichment: The Jixiangyu rare earth deposit is situated in the core area of the Liao Ji Rift, providing favorable sedimentary conditions for ore deposition. Over time, sediments accumulated, compacted, and underwent alteration, gradually releasing and enriching rare earth and iron elements, leading to formation of the ore deposit. (2) Magmatic modification: Magmatic hydrothermal fluids may have interacted with variolitic rocks from the Lieryu section, facilitating the simultaneous or interweaving mineralization of rare earth minerals and magnetite in the same geological environment, resulting in the activation and in situ enrichment of high background values of rare earth and iron elements. (3) Tectonic control: The Jixiangyu rare earth deposit is situated on the anticlinal structure of the Liaoning—Jilin Paleoproterozoic rift core where fault zones and folds may have acted as channels for mineral enrichment, facilitating the migration of minerals from deeper to shallower crustal regions.