Introduction

One of the critical resources linked to national security and the lifeblood of the national economy, rare earth, also referred to as the “industrial vitamin”, is the essential raw material for the high-tech industry and the national defense research and technology sector1,2. Ion adsorption of rare earth is the most important source of heavy rare earth and yttrium in the world. China, the US, Russia, Egypt, and other nations have discovered substantial rare earth enrichment in phosphorite in recent years. Researchers believe that rare earth deposits of the phosphorite type are distinct from those of the ion adsorption type. Rare earth and phosphate minerals are strongly connected in phosphorite-type rare earth deposits, and the diagenesis of the marine phosphorus deposit has a beneficial impact on the rare earth enrichment process3,4,5. The presence of phosphate minerals influences rare earth migration, but it also encourages rare earth precipitation at the same time6,7. These findings have significantly advanced our knowledge of the phosphorite-type rare earth enrichment mechanism. Nevertheless, comprehensive and systematic studies on the metallogenic environments, migration routes, and material sources of rare earth elements are still lacking. Further data collection of particular deposits is still required to provide a fine description of the metallogenic process of this kind of deposit.

Rare earth resources are abundant in China. The Xinhua phosphorite-type rare earth deposit in Zhijin, Guizhou belongs to a large rare earth- phosphate deposit. It is one of the three major phosphorus ore concentration areas in Guizhou. Rare earth is found in the early Early Cambrian Gezhongwu Formation, a sedimentary formation of siliceous rock and phosphorus-bearing dolomite8. It is especially rich in heavy rare earth elements and has typical geological characteristics. As of 2023, 1.348 billion tons of proven phosphorus ore reserves and 3.5 million tons of rare earth oxides, with significant economic value and research significance, and the Motianchong phosphate mine, which was put into operation on September 4, 2024, has been included in the key projects of new industrialization in Guizhou Province9,10,11. Predecessors on the occurrence of rare earth12,13,14,15,16, and geochemical characteristics of elements17,18,19,20,21,22 conducted a more in-depth study and achieved a more consistent understanding. Regarding the source of rare earths, Zhou Kelin23 believes that rare earths originated from seawater and terrestrial sources through the rare earth partitioning model, while Lou Fangju24 and Xing Jieqi5 argued that rare earths originated from pore water and overlying water through geochemical data and so on. The following are the key concepts regarding the mineralization of rare earth deposits of the phosphorite type: (1) Bioaccumulation theory25,26: The primary mechanism for the creation of phosphorite-type rare earth deposits is REE uptake and ingestion enrichment by live creatures or debris, and the ore contains numerous tiny shell fossils; (2) Upwelling ocean current enrichment theory27: it is found that there is a positive correlation between rare earth content and phosphorus content, but no obvious correlation between rare earth content and the abundance of small shell fossils. The amount of fluorapatite and collophane produced by chemical precipitation in the deep water mass returned from the upwelling ocean current mostly determines the genesis of phosphorite-type rare earth deposits; (3) Hot water enrichment theory28: The whole-rock geochemical characteristics of the ore reflect that the hot water plays a certain role in the mineralization process of phosphorite-type rare earth deposits.

Some knowledge about the multi-element rare earth migration pathway and precipitation mechanism in Zhijin phosphorite-type rare earth deposits has been gained. However, the formation of phosphate agglomerates is the main focus of the research, and there is a lot of disagreement regarding it. Research on the usual particle structure of phosphorite-type rare earth ore is insufficiently deep and has not been thoroughly analyzed. Therefore, based on detailed field investigation, the author carried out the identification of rock and mineral microstructure, in-situ microstructure analysis and test, combined with the geochemical research of main elements, trace elements, and rare earth elements, to describe the source, transportation, and storage process of phosphorite type rare earth enrichment.

Moreover, the enrichment characteristics and controlling factors of rare earth elements in many Cambrian phosphate deposits worldwide are similar to those in the Zhijin phosphorite-type rare earth deposit. For example, in the phosphate deposits of the Georgina Basin in northern Australia, Tongren in Guizhou, Longtan in Yunnan, Shangrao in Jiangxi, Jiangshan in Zhejiang, and Nanjing in Jiangsu, China, the REE concentrations are mainly controlled by seawater composition and sedimentary environment29,30. The sedimentary phosphorite-type rare earth deposits in the shallow marine tidal environment of the ancient Baltic Sea basin are the result of the absorption of rare earth elements by brachiopod debris during the processes of transportation, deposition, and early diagenesis31. Therefore, exploring the enrichment process and mechanism of rare earth elements in the Zhijin phosphorite-type deposit can provide valuable insights for the study of similar deposits worldwide.

Geological background and sample analysis

Geological characteristics of the deposit

Paleogeographical environment

Following the Neoproterozoic Ediacaran, the Early Cambrian represents a significant period in the evolution of lithofacies paleogeography in southern China. The paleogeographic pattern of Guizhou in the early Cambrian was further developed and formed based on the late Ediacaran, which is generally characterized by high in the northwest and low in the southeast. A brief transgression took place in Guizhou during the Meishucun period of the early Cambrian, against the backdrop of convergence, as a result of the contraction of the Nanhua rift trough and the reduction of the oceanic plate. During this time, the maximum flooding surface was reached, with seawater entering from the southeast. The carbonate platforms in the central western and northern parts are gradually submerged, but the overlying water is shallow. The sedimentary environment is phosphorus-containing submerged platform facies25. The Zhijin Xinhua rare earth-phosphorus deposit in central Guizhou is located at the southwestern end of the “Qianzhong Uplift”. The ancient terrain is high and is a bioclastic shoal. The sedimentary formation is limited to the western margin of the Yangtze phosphorus-forming domain of the phosphorus-bearing rock series of the Lower Cambrian Meishucun Formation (Fig. 1), which belongs to the Early Cambrian Sichuan-Yunnan-Guizhou phosphorus-forming belt.

Fig. 1
figure 1

Geological map of the mining area (based on23 revision).

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Orebody geological characteristics

With tectonic lines primarily spreading northeast and tectonic deformation dominated by ruptures and folds, the geological structure of the Zhijin Xinhua research region in central Guizhou is very simple. The Gezhongwu Formation’s phosphorite-type rare earth ore-bearing rock sequence, which is 28 km long, 0.4–4.0 km wide, and 0–33.73 m thick, is primarily found along the northwest wing of the Guohua anticline’s near axis (Fig. 1). Gray, gray-black, thin to medium-thick strata of dolomitic bioclastic, sandy debris, phosphorus-rich rare earth ore, and dolomitic, siliceous phosphorus rare earth ore make up the majority of the ore body. The development of trough cross-bedding, herringbone cross-bedding (Fig. 2a, c), laminar structure (Fig. 2b). Under the polarized light microscope, the lamellar structure is characterized by a rhythmic interbedded layer of dark laminae (composed of cryptocrystalline-amorphous colloidal apatite, organic matter-rich biological debris, and sand-debris phosphorus-rich rare earth ore) and light laminae (phosphorus rare earth ore, biological debris, sand debris are less, intergranular cement are mainly dolomite) (Fig. 2h, i). The black shale of the Niutitang Formation, which lies on top of the Gezhongwu Formation, is in integrated contact with it, whereas the dolomite of the Ediacaran Dengying Formation beneath it is in pseudo-integrated contact.

Fig. 2
figure 2

Ore structure and structural characteristics. Clh = collophane; Ap = apatite; Dol = dolomite; Qtz = quartz; Py = pyrite. (a) Grooved cross-bedding; (b) Laminated structure; (c) Herringbone cross-bedding; (d) Biodebris, sand structure, good orientation of debris, collophane in the sand was replaced by dolomite, 10 ppm (–) ; (e) Collophane sand debris and fibrous apatite cement, 10 × 20 ( +); (f) Collophane biological debris, dolomite, quartz cement, and pyrite, 10 × 20 ( +) ; (g) agglomerate structure, 10 × 5 (–) ; (h) Biodebris, sand debris structure, basement-pore cementation, matrix-particle support, lamellar structure, 10 × 5 ( +) ; (i)Bioclastic structure, pore-contact cementation and basal cementation, particle support and matrix support, lamellar structure, 10 × 5 (+).

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Geological characteristics of ore

The phosphorite-type rare earth ore in the mining area can be classified into two types based on the major element content and the ore structure: gray and grayish-brown phosphorus-rich rare earth ore (P2O5 content > 12%, particle-supported) and gray and dark gray phosphorus rare earth ore (P2O5 content ≤ 12%, matrix-particle support).

In both types of ores, the host minerals of rare earth are apatite (with cryptocrystalline-amorphous colloidal structure or microcrystalline-fine crystal structure), and the gangue minerals are mainly dolomite, quartz, and so on. The most significant of these is collophane, which makes up over 90% of the ore mineral composition. It is formed by cryptocrystalline-amorphous apatite mixed with clay minerals, carbonate minerals, and organic matter (commonly known as collophane). It is widely distributed in ores and can be seen in biological remains, sand debris, and cement (Fig. 2d, e, f). Microcrystalline-fine apatite accounts for a small proportion of ore minerals, which are mostly short columnar, exist in the form of sand debris, or co-exist with collophane in biological debris. Occasionally, ring edges of fibrous apatite cement with an equal thickness that is created by collophane recrystallization are visible (Fig. 2e). The most significant gangue mineral is dolomite, which is primarily found in the ore as cement, which cements particles like sand and biological material (Fig. 2d, f); quartz content of gangue mineral is quite low, and it also exists in the form of cement.

Both types of ores have a granular structure, with the granular debris being mainly bioclastic, followed by sandy debris and agglomerates. The majority of the granular components are collophane, with sparry dolomite making up the majority. Phosphate cement and siliceous cement come next (Fig. 2d, e, f). Small shell fossils, typically in the shape of long nails and strips, ranging in length from 0.1 to 0.5 mm, make up the majority of the bioclasts. Strong hydrodynamic forces may have caused significant damage to some of the small shell fossils, whereas others are directional and others are well preserved. The shape of the sand debris is irregular, the length and diameter are about 0.1–0.4 mm, and the directional arrangement (Fig. 2h). Comprising composite particles wrapped in varying-sized sand chips, the agglomeration structure is held together by fibrous apatite concentric rings that cement the collophane sand chips (Fig. 2g), and the diameter of the agglomerate is mostly 1–3.0 mm. Ores can be cemented in two ways. One is the phosphorus-rich rare earth ore, which is primarily a pore and contact cementer, with particle support as the primary support type (Fig. 2i); the other is the cementation type of phosphorus rare earth ore, which is dominated by pore type and supplemented by basal cementation, and the support type belongs to matrix-particle support (Fig. 2d, h).

Sample and test methods

Sample

From the bottom to the top of the measured portion of the Gezhongwu Formation in Zhijin Xinhua, a total of thirty samples were gathered. Three samples (DY-1-1, DY-1-2, and DY-1-3) of dolomite and siliceous rock, respectively, were taken from the Dengying Formation floor. One sample (NTT-9-1) of the roof Niutitang Formation is carbonaceous shale, whereas the other 26 samples (GZW-2-1 to GZW-8-3) of the Gezhongwu Formation are phosphorus-rich rare earth ore and phosphorus rare earth ore (Fig. 3).

Fig. 3
figure 3

Sedimentary characteristics and sampling location of Zhijin Xinhua section plane.

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Test methods

(1) X-Ray Fluorescence (XRF) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) testing. First, the collected samples were cleaned by removing weathered surfaces, and fresh portions were selected, rinsed with deionized water, and dried in an oven at 100 °C. Subsequently, the samples were crushed and ground to a particle size of < 200 mesh. Precisely 100 mg of the powdered sample was weighed and placed in a digestion vessel, followed by the addition of 1 mL HF and 0.5 mL HNO₃. The mixture was then digested at 190 °C for 12 h. After digestion, an additional 0.5 mL HNO₃ was added, and the solution was reheated at 140 °C for 3 h. Upon cooling, the solution was diluted to 100 mL with deionized water.

The major element composition of the samples was determined by XRF, while trace elements were analyzed by ICP-MS. All analyses were conducted at the Beijing Nuclear Industry Testing and Analysis Center and Guizhou Tongwei Testing Co., Ltd. The measured values exhibited an average standard deviation of < 10% and an average relative standard deviation of < 5%.

(2) Total Organic Carbon (TOC) analysis. After field collection, samples were preserved in insulated containers and immediately transported to the laboratory for pretreatment. The samples were air-dried in the dark, then crushed and ground to < 200 mesh. A measured amount of powdered sample was treated with excess 4 mol/dm3 HCl and reacted for 24 h. Then the acid was removed by rinsing with deionized water until neutral pH was achieved, after which the samples were oven-dried at 60 °C to constant weight. After that, An appropriate amount of the prepared sample was weighed into 5 mm × 9 mm tin capsules and sealed for analysis. Measurements were performed using a Sercon Integra2 elemental analyzer coupled with an isotope ratio mass spectrometer (EA-IRMS). Reference standards included: IAEA-600 (δ13C = -27.71‰), USGS40 (δ13C = − 26.39‰), and Acetanilide #1 (δ13C = − 29.53‰). All analyses were conducted at Guizhou Tongwei Testing Co., Ltd., with an analytical precision deviation of < 0.5%.

(3) Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) testing. First, prepare the sample. Select a typical sample, and after removing the weathered surface layer, cut it into a laser sheet measuring 47 mm in length, 25 mm in width, and 3 mm in thickness. Clean the laser sheet with ultra-pure water (MQ), dry it, and place it in the sample holder. Then, proceed with laser ablation. Select points at the corresponding positions of the sample and standard, and insert the standard into the sample. Pre-ablate all test points with the same size laser beam for 15 s to cause the material on the sample surface to evaporate and vaporize. The laser spot size was 20 µm, with a frequency of 15.21 Hz, an energy of 6.0 mJ, a background time of 15 s, and an ablation time of 20 s. Subsequently, ICP-MS analysis was performed. Helium (He) was selected as the carrier gas. The ablated sample was transported from the sample cell by the He gas, mixed with argon (Ar) gas, and then introduced into the ICP-MS for analysis. Finally, the data were processed. The raw data were corrected using NIST610 as the internal standard. The offline processing of the analytical data was conducted using iolite, followed by further processing with Isoplot.

The analytical instrument used in this study was LA-ICP-MS. The laser ablation system was a RESOlution SE-S155, and the ICP-MS system was a Thermo Fisher iCAP RQ. The LA-ICP-MS analysis was conducted at Guizhou Tongwei Analytical Technology Co., Ltd.

Results

Element geochemical characteristics

Major elements

As shown in Table 1, the content of P2O5 in the phosphorus-rich rare earth ore is 12.01–37.38%, with an average content of 22.75%. The average contents of CaO and SiO2 can reach 37.99% and 14.47%, respectively. MgO and Fe2O3 have average contents of 4.79% and 0.69%, respectively. However, the content of P2O5 in phosphorus rare earth ore is 2.99–11.30%, with an average content of 7.5%. SiO2 and CaO can have average contents of 10.29% and 33.05%, respectively. MgO and Fe2O3 have average contents of 14.20% and 1.22%, respectively. Compared to phosphorus rare earth ore, phosphorus-rich rare earth ore has greater average levels of P2O5, CaO, and SiO2, whereas phosphorus rare earth ore has higher average contents of MgO and Fe2O3. This is because that phosphorus-rich rare earth ore has a higher concentration of phosphate minerals made up of P2O5 and CaO, whereas phosphorus rare earth ore has a higher concentration of gangue minerals like dolomite. Except that the Al2O3 content of the sample GZW-7-4 is 2.81%, the Al2O3 content of other samples is about 1%, with an average of 0.76%. Furthermore, the phosphorite-type rare earth ore has very low levels of terrigenous elements, as seen by the extremely low average contents of K2O and Na2O (0.23% and 0.06%, respectively).

Table 1 The content of major elements in the ore-bearing rock series of Zhijin Gezhongwu Formation (%).
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Trace elements

The sample test results were normalized and projected into a graph using the trace element content of the upper crust (UGG)32 as the standard value in conjunction with the trace element test findings (Table 2). There were notable differences in the trace element content across the samples of various lithologies (Fig. 4).

Table 2 Content of trace elements in ore-bearing strata of Zhijingezhongwu Formation (ppm).
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Fig. 4
figure 4

Trace element spider diagram of ore-bearing strata and its Surrounding rock.

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Pb, Sb, and Sr had average enrichment coefficients of 55.89, 53.10, 32.11, and 17.78, 13.89, and 11.86, respectively, which were significantly higher in phosphorus-rich rare earth ore than in phosphorus rare earth ore. The average enrichment coefficients for Cd, As, U, Ba, and Zn in phosphorus-rich and phosphorus rare earth ores were 4.10, 3.32, 2.46, 1.85, 1.77, and 4.58, 3.53, 1.89, 1.79, 1.32, respectively. With an average enrichment coefficient of 1.40, Cu is marginally enriched in rare earth ore that is rich in phosphorus. Rb, Th, Co, Cr, Ni, V, Sc, Ga, Zr, Nb, Hf, Ta, Li, Cs, Mo, and other elements in the two types of ore are in a state of loss, and the loss degree of phosphorus rare earth ore is higher than that of phosphorus-rich rare earth ore. This can be because the two kinds of rare earth ores have different formation environments and material sources.

Rare earth elements

In the ore-bearing strata of the Xinhua Gezhongwu Formation in Zhijin, the rare earth element content and characteristic value calculations are displayed in Table 3. The rare earth enrichment features in the ore-bearing strata are clearly visible. The total amount of rare earth in the phosphorite-type rare earth ore ∑REY is 187.99–1959.93 ppm, with an average of 1031.34 ppm, which is about 33 times the total amount of rare earth in the dolomite and siliceous rock samples of the floor Dengying Formation (∑REY average content is 31.56 ppm), which is nearly 5.34 times higher than the average total amount of rare earth in the carbonaceous shale of the roof Niutitang Formation (∑REY = 193.14 ppm). The heavy rare earth element yttrium (Y) is generally enriched. The content was 66.7–669.00 ppm, with an average of 280.16 ppm. Among them, the total amount of rare earth ∑REY of phosphorus-rich rare earth ore ranged from 683.92 ppm to 1959.93 ppm (average 1187.16 ppm), and the total amount of rare earth ∑REY of phosphorus rare earth ore ranged from 187.99 ppm to 568.06 ppm (average 376.92 ppm). Compared to phosphorus rare earth ore, phosphorus-rich rare earth ore has a substantially higher rare earth element content.

Table 3 Rare earth element content and characteristic value of ore-bearing strata.
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The correlation between P2O5 and ∑REY in phosphorite-type rare earth ore was analyzed (Fig. 5), and the correlation coefficient was R = 0.97, with a significant positive correlation between the two. This indicates that the relationship between rare earth elements and phosphorus elements was very close, which is because rare earth elements are mainly adsorbed by collophane or exist in apatite lattice defects in the form of isomorphism.

Fig. 5
figure 5

Correlation diagram between P2O5 and ∑REY of phosphorite-type rare earth ore.

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The rare earth elements of ore-bearing strata in Australian shale are standardized and the rare earth element distribution curve is drawn (Fig. 6). It is evident that both phosphorus-rich rare earth ore and phosphorus rare earth ore have "hat-shaped" rare earth element distribution curves that are comparable. The light rare earth is more enriched than the heavy rare earth, and the LREE/HREE values are 5.38–6.20 (average 5.83) and 5.06–5.60 (average 5.33), respectively. There is minimal variation in the δCe values of the two types of ore which are 0.31–0.44 (average 0.36) and 0.32–0.38 (average 0.35), respectively, with obvious negative anomalies. On the contrary, the δEu value changes obviously in the ore-bearing strata. The δEu values of phosphorus-rich rare earth ore range from 0.93 to 1.40 (mean 1.11). The majority of the samples exhibit positive anomalies, except for a small number having weak negative anomalies. The δEu values of phosphorus rare earth ore range from 0.92 to 0.97 (mean 0.94), which are weak negative anomalies. From the bottom to the top of the ore-bearing layers, the δEu values generally exhibit a downward-upward pattern (Fig. 12).

Fig. 6
figure 6

Rare earth distribution curve of ore-bearing strata in Gezhongwu Formation.

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TOC characteristics of total organic carbon

Considering the field distribution characteristics, structural features, and mineral composition characteristics of the ore, a total of 19 samples of different types of phosphorite-type rare earth ore samples were selected for TOC testing and analysis (Table 5) from the bottom to the top of the phosphorite system. Only 7 samples, which were very close in stratigraphy and consistent in lithology with the tested samples, were not subjected to repeated testing. The selected samples can represent the ore characteristics. In general, the TOC value of 2% can be used as the boundary of organic matter33. The test results show that the organic matter content of Zhijin phosphorite-type rare earth ore is low (TOC content is 0.01–0.12%), and the average TOC content of phosphorus rare earth ore (0.01%) is lower than that of phosphorus-rich rare earth ore (0.05%).

Characteristics of micro-area structural components

Based on the differences in particle types and cementing material composition of phosphorite-type rare earth ores, combined with variations in rare earth content, representative ore samples from different positions (bottom, lower, middle, upper, top) and types of the phosphorite series were selected for LA-ICP-MS micro-area testing and analysis. A total of 9 samples were analyzed, including 8 phosphorus-rich rare earth ore (GZW-2-1, GZW-3-1, GZW-5-1, GZW-6-1, GZW-6-2, GZW-6-5, GZW-7-2, GZW-7-3), one phosphorus rare earth ore (GZW-8-3). The distribution of rare earth and phosphorus in the center and edge of particles (biological debris, sand debris, and agglomerates) and cement (phosphate minerals, sparry dolomite, and siliceous minerals) in the ore were tested respectively. A total of 65 points (25 biological debris, 8 sand debris, 5 agglomerates, and 27 cements) were tested, and the distribution positions of some points are shown in Fig. 7.

Fig. 7
figure 7

Distribution map of LA-ICP-MS test points for particles and cementing materials in ore structure. SW tinei = Inside the biological debris; SW bY = Biomass edge; SW JJW = Cement; SW = Biomass; Gzw = Cement.

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The results show that the ∑REY and P content varies significantly among different particle types (Table 4). The internal shells of small shelly fossil (SSF) debris exhibit the highest concentrations of ∑REY and P. The ∑REY content ranges from 1849.26 to 2850.06 ppm, with an average value of 2365.70 ppm, while the P content ranges from 172,549.6 to 997,893.51 ppm, with an average value of 319,579.76 ppm. The shell walls of small shelly fossils (SSFs) exhibit the second-highest concentrations of ∑REY and P. The ∑REY content ranges from 759.67 to 2820.45 ppm, with an average value of 2270.13 ppm, while the P content ranges from 58,303.98 to 426,778.54 ppm, with an average value of 233,207.77 ppm. The ∑REY content in the shell walls is roughly comparable to that within the internal shells. The concentrations of ∑REY and P within the sand debris are 926.99 ppm to 2117.71 ppm (with an average value of 1457.10 ppm) and 86,431.63 ppm to 331,929.57 ppm (with an average value of 214,478.72 ppm), respectively. At the edges of the sand debris, the concentrations of ∑REY and P exhibit significant variations. The ∑REY content ranges from 36.73 ppm to 2500.86 ppm, with an average value of 699.24 ppm. The P content ranges from 1422.21 to 420,900.80 ppm, with an average value of 120,775.86 ppm. Microscopic identification revealed that the edges of sand debris are occasionally subject to dolomitization, which may lead to a decrease in the contents of REE and P in these areas. In contrast, the aggregates mainly composed of phosphate sand debris show high contents of ∑REY (751.82–2165.94 ppm, with an average of 1521.94 ppm) and P (114,251.47–304,323.17 ppm, with an average of 193,469.06 ppm).

Table 4 In-situ micro-area phosphorus and rare earth element contents of phosphorite-type rare earth ore (ppm).
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The cements of different compositions show significant variations in the total content of REE and P. In phosphate cementation, the concentrations of ∑REY and P are 325.64–2617.09 ppm (with an average of 1456.29 ppm) and 69,238.05–353,636.01 ppm (with an average of 177,192.69 ppm), respectively. These values are significantly higher than those in bright crystalline dolomite cementation (∑REY contents are 66.49–679.36 ppm, with an average of 308.14 ppm, and P contents are 2589.38–46,630.75 ppm, with an average of 20,893.81 ppm) and siliceous cementation (∑REY contents are 14.15–149.52 ppm, with an average of 46.64 ppm, and P contents are 0.00–10,447.88 ppm, with an average of 998.74 ppm).

Integrating the aforementioned characteristics, it is observed that the contents of P and REY exhibit a highly similar variation trend across different types of particles and cements. This indicates a close association between P and ∑REY, with a correlation coefficient R2 of 0.95 (Fig. 8). They are high in biological debris, inside of sand debris, agglomerates, other particles, and phosphate cement with collophane and apatite as the main components, while the content in the edge of dolomite sand debris, bright crystal dolomite cement, and siliceous cement decreases sharply, which further proves that the rare earth in Zhijin Xinhua phosphorite-type rare earth ore may be adsorbed by collophane or exist in the apatite lattice in the form of isomorphism. On the one hand, the main components of collophane are cryptocrystalline–amorphous apatite and clay minerals. Clay minerals possess strong adsorption properties34, which enable them to adsorb rare earth elements. On the other hand, apatite crystals have an “open” hexagonal columnar structure. The ionic radii of rare earth elements (106–85 pm) are similar to those of Ca2⁺ (100–120 pm), allowing rare earth elements to enter the apatite crystal lattice and substitute for Ca2⁺ ions16,35. Simultaneously, it was also discovered that bioclasts are extremely abundant in phosphorus and rare earth elements, particularly in small shells, which are followed by small shell walls. It is hypothesized that the breakdown of biological death software could be the cause. Under elutriation and wave and tide bumping, the apatite containing rare earth elements is preferentially differentiated and poured into the biological cavity. Furthermore, it is associated with the enrichment of rare earth elements in the shell wall and the absorption of phosphorus by small shell biological life activities to form the shell wall.

Fig. 8
figure 8

Rare earth ore micro-area P and ∑REY covariation diagram.

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Discussion

Sources of rare earth

Tracing of rare earth elements

Due to their unique physical and chemical characteristics, rare earth elements are frequently used in research on the origins of ore-forming materials and the formation of rocks36. Conventional marine hot water sediments’ rare earth properties are very different from those of conventional seawater sediments37: low LREE/HREE, a left-leaning or horizontal standard distribution curve of Australian shale, and a clear negative Ce anomaly are characteristics of rare earth elements found in marine hydrothermal deposits. However, the standard distribution curve of Australian shale is right-leaning, Ce exhibits the traits of positive anomaly, LREE/HREE is excessively large, and the typical saltwater sediments tend to contain a large total quantity of rare earth. Hot water sediments frequently exhibit an Eu-positive anomaly, which is typically interpreted as an indication of hot water activity38,39,40.

The samples of Zhijin phosphorite-type rare earth ores exhibit a nearly flat REE distribution pattern, resembling a “cap shape” when normalized to Australian shale (Fig. 6). The LREE/HREE ratios are relatively low, ranging from 5.06 to 6.20. A significant negative Ce anomaly is observed, with Ce anomaly values ranging from 0.31 to 0.44. These characteristics are typical of marine hydrothermal sediments. Only six of the 21 phosphorus-rich rare earth ore samples have δEu slightly less than 1, with an average δEu value of 1.11 and mainly positive anomaly. However, the δEu of the phosphorus rare earth ore samples are all slightly less than 1, with an average value of 0.94, and Eu has a weak negative anomaly. These features suggest that throughout the typical marine deposition, Zhijin phosphorite-type rare earth is deposited and mineralized through hydrothermal action.

The La/Yb and ∑REE of the samples were cast (Fig. 9). As can be observed, the top plate (carbonaceous shale of Niutitang Formation) and bottom plate (dolomite and siliceous rock of Dengying Formation) have distinct falling point areas than Zhijin phosphorite type rare earth ore. All of the phosphorus-rich rare earth ore fall in the granite area, most of the phosphorus rare earth ore (those with higher P and ΣREE contents) fall in the granite area, and a few (those with lower P and ΣREE contents) fall in the sedimentary rock-calcareous mudstone area, but close to the granite area, indicating that the phosphorite-type rare earth ore are affected by strong hydrothermal activity while normal marine sedimentation, and the samples with higher P and ΣREE contents have more significant characteristics of hot water sources.

Fig. 9
figure 9

La/Yb-REE diagram of phosphorus (rare earth) rock series (based on37).

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Trace element tracing

The enrichment of trace elements Sb and As is a crucial indicator for differentiating hot water sediments from regular deposits, according to the findings of Michard’s38 characterization study of trace elements in contemporary marine hot water sediments. The Zhijin phosphorite-type rare earth ores show significant enrichment of Sb and As in the trace element spider diagrams (Fig. 4). The Sb contents of Zhijin phosphorus-rich rare earth ore and phosphorus rare earth ore are 1.87–161.00 ppm (average 15.06 ppm) and 2.01–11.20 ppm (average 6.07 ppm), respectively. The average enrichment coefficients are 37.65 and 15.18, respectively, compared with the abundance of trace elements in the upper crust (UCC: Sb = 0.4 ppm). The As content ranged from 9.27 ppm to 74.64 ppm (average 31.80 ppm) and 11.70 ppm to 24.20 ppm (average 16.76 ppm), respectively, and the average enrichment coefficients were 6.63 and 3.49, respectively, compared with the upper crust (UCC: As = 4.8 ppm). It can be seen that Zhijin phosphorite-type rare earth ore is enriched in Sb and As, and phosphorus-rich rare earth ore is enriched to a higher degree than phosphorus rare earth ore, indicating that phosphorite-type rare earth ore was involved in strong hot water when they were deposited. Perhaps the ores with high rare earth content were more obviously affected by hot water.

Co, Ni, and Zn exhibit similar geochemical behaviors. During the sedimentation process, they are adsorbed onto the surfaces of Fe–Mn oxides. As a result, these elements show distinct enrichment patterns in hydrothermal and marine sediments. The ratios of Co/Ni and Co/Zn can be utilized to determine the genesis of sediments41. It is commonly believed that a Co/Ni ratio less than 1 and a Co/Zn ratio less than 0.15 indicate a hydrothermal sedimentary origin. Conversely, when the Co/Ni ratio is greater than 1 and the Co/Zn ratio is greater than 2.5, a non-hydrothermal sedimentary origin is suggested42.

As calculated (Table 5), the Co/Ni ratios of the Zhijin phosphorus-rich rare earth ore range from 0.06 to 1.74 (with an average of 0.28), and all but one sample is less than 1, with the average value also less than 1. The Co/Zn ratios range from 0.01 to 0.09 (with an average of 0.03), and all samples are less than 0.15. For the phosphorus rare earth ore, the Co/Ni ratios range from 0.18 to 1.88 (with an average of 0.65), with only one sample having a ratio greater than 1, while the rest are less than 1, and the average value is also less than 1. The Co/Zn ratios range from 0.01 to 0.05 (with an average of 0.03), all of which are less than 0.15. Given that both types of phosphorite-type rare earth ore have Co/Ni average values less than 1 and Co/Zn ratios less than 0.15, these results indicate that the Zhijin phosphorite-type rare earth ore possesses characteristics of a hydrothermal sedimentary origin.

Table 5 Calculation results of tracer element characteristic value and TOC content of sedimentary environment of phosphorite-type rare earth ore.
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Normally, the Th element is relatively inert in seawater, while the U element is strongly influenced by the redox state of seawater, transporting as U6+ in oxidizing environments and precipitating as U4+ insoluble compounds in reducing environments. In regular seawater sedimentary rocks, the trace element Th has a larger abundance than U, but in hot water sedimentary rocks, the situation is just the opposite. The reason is that hot water deposition has a higher deposition rate, so hot water sediments are often enriched with U. Therefore, U/Th > 1 in hydrothermal sedimentary rocks and U/Th < 1 in normal sedimentary rocks43. Calculated (Table 5) that the U/Th values of phosphorus-rich rare earth ore range from 0.75 to 3.49, with an average value of 2.36, and only one sample has a U/Th value of less than 1. Phosphorus rare earth ore has an average U/Th value of 1.93, with values ranging from 1.23 to 2.46. The ratio of U/Th of phosphorus-rich rare earth ore is larger and the average value of U/Th of both types of phosphorite-type rare earth ore is greater than 1. This suggests that the Zhijin phosphorite-type rare earth ore should be classified as a hot water sedimentary rock and that hot water has a more noticeable effect on phosphorus-rich rare earth ore.

In addition, elements such as Zn, Ni, Co, and Cu can also indicate the genesis of sediments. Under hydrothermal conditions, Co content is usually higher, Ni and Cu coexist with sulfide minerals, while in sedimentary environments, their content is relatively lower, and Zn content is higher44. Besides, in deep-sea sediments, most of the Cr comes from terrigenous clastic material, and Cr has a significant correlation with Zr45. In the Co/Zn-(Co + Ni + Cu) correlation diagram (Fig. 10a), all test samples within the region fall into the hot water deposition area, and from the Cr-Zr correlation diagram (Fig. 10b), it can be seen that the measured sample values in the study area are mostly distributed in hydrothermal sediments zone, with only one sample close to deep sea sedimentary area, indicating that rare earth elements in the study area major originate from hydrothermal sources.

Fig. 10
figure 10

Co/Zn-(Co + Ni + Cu) correlation diagram (a) and Cr-Zr correlation diagram (b) (base map 42).

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Tracing of major elements

SiO2 is generally closely related to hot water or biogenic, while Al2O3、K2O、Na2O are the main component of clay minerals of terrestrial origin. Therefore, the SiO2/Al2O3 ratio and w(SiO2)-w(K2O + Na2O) discrimination diagram can be used to judge the provenance characteristics of rocks46. Hot water or biological action is thought to be the cause if the SiO2/Al2O3 ratio is higher than 3.60. A ratio of approximately 3.60 suggests that the rock’s provenance is skewed toward terrestrial sources47. Calculation results show (Table 5) that 21 samples of phosphorus-rich rare earth ore have SiO2/Al2O3 ranging from 2.49 to 103.82 (average 18.10), and only 2 samples are slightly less than 3.60. All five phosphorus rare earth ore samples have SiO2/Al2O3 values above 3.60, ranging from 11.11 to 27.30 (average 18.95). It indicates that there are significant hydrothermal or biological effects on rare earths of the phosphorite type.

In the w(SiO2)-w(K2O + Na2O) discrimination diagram, the majority of the samples plot within the hot water genesis area field (Fig. 11), indicating that the lithification process was generally influenced by hydrothermal activity. A small number of samples fall within the biogenetic field, suggesting that biological processes also played a certain role during lithification.

Fig. 11
figure 11

w(SiO2)-w(K2O + Na2O) discrimination diagram (base map 46).

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Considering the characteristics of rare earth, trace, and major elements, the main material source of Zhijin phosphorite-type rare earth should be a hot water source. For some ores with low phosphorus and rare earth content, normal seawater also provides a partial source. In addition, a small amount of rare earth may also come from terrestrial sources. At the same time, it is assumed that the source of phosphorus and rare earth mineralization should be the same because of the very close association between rare earth and phosphorus and the highly consistent in situ micro-area distribution.

Factors affecting the migration and enrichment of rare earths

Paleogeographic conditions

The ore minerals of the Zhijin phosphorite-type rare earth ore are mainly apatite, and the gangue minerals are mainly dolomite. The mineral assemblage is the result of carbonate platform deposition in shallow water. The granular structure is generally developed, and the long strip biological debris with directional arrangement characteristics in the particles is mostly, and some biological debris is not completely preserved and destroyed. The ore cementation is mainly contact type, pore type, a small amount of basement type, and the support type is mainly particle support. All of these traits show that the ore-bearing rock series’ sedimentary environment is a shallow body of water with a highly directed hydrodynamic environment.

When the sea level rises, the ocean current rich in phosphorus-rare earth flows from the southeast ocean to the shelf area and reaches the Zhijin area. The phosphate saturation is high and the hydrodynamic force is substantial because of the submerged platform edge and the presence of a beach of biological debris. After continuous elutriation, the phosphorus-rare earth can be deposited to form a considerable scale of phosphorite-type rare earth deposits.

Oxidation–reduction environment

In an oxidizing environment, the trace elements U, Ni, and Co dissolve readily, while in a reducing environment, they are difficult to dissolve. They can be enriched in an oxygen-poor environment and are not unaffected by the subsequent diagenesis48. Therefore, Ni/Co and authigenic uranium (AU = Utotal-Th/3) are often used as indicators for restoring marine paleo-water media. When Ni/Co > 7 and AU > 12, it is an anoxic environment; when Ni/Co is between 5–7 and AU is between 5–7, it is an oxygen-poor environment; when oxidized environment, Ni/Co < 5 and AU < 1249.

The table (Table 5) displays the Ni/Co and AU values of the Zhijin-Gezhongwu Formation’s phosphorite-type rare earth ore. Except for four samples, it is evident that the Ni/Co ratios of 21 phosphorus-rich rare earth ores are higher than 10. The Ni/Co values of the remaining 17 samples are around 5, ranging from 0.58 to 18.07, with an average value of 6.21, and the authigenic uranium AU is 2.93–9.88, with an average value of 6.12, all of which show an oxidizing to oxygen-poor environment. The Ni/Co ratios of the four phosphorous rare earth ores, however, are less than five, except for one sample that has a ratio greater than five. The five samples have ratios that range from 0.53 to 5.46, with an average of 2.86. The authigenic uranium AU is 1.07–3.18, with an average value of 2.05, which is generally characterized by an oxidizing environment. The sedimentary environment of phosphorite-type rare earth ore in the Zhijin-Gezhongwu Formation is mainly an oxidation-poor oxygen environment, and a hypoxia environment occasionally occurs. When the environment changes from oxidation to poor oxygen, it is more conducive to the enrichment of rare earth in phosphorite (Fig. 12).

Fig. 12
figure 12

Vertical variation diagram of characteristic element content and sedimentary characteristic element ratio of rare earth ore.

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The total rare earth (∑REY) of phosphorus-rich rare earth ore and phosphorus rare earth ore in the ore-bearing strata of the Zhijin-Gezhongwu Formation were analyzed with the redox index Ni/Co ratio and authigenic uranium AU, respectively (Fig. 13). There is a strong association between the Ni/Co ratio, authigenic uranium (AU), and the total amount of rare earth (∑REY). The phosphorus-rich rare earth ore has correlation coefficients of 0.50 and 0.88, respectively. However, the correlation coefficients of phosphorus rare earth ore are 0.47 and 0.68, respectively, indicating that although the metallogenic environment of the ore-bearing strata of the Gezhongwu Formation is limited by the partial oxidation environment, it is more conducive to the formation of high-grade ore when the environment changes to the relatively oxygen-poor environment. The further enrichment of phosphorus and rare earth may be related to the fact that a large number of organisms in the relatively oxygen-poor environment provide enrichment sites.

Fig. 13
figure 13

Rare earth ore redox condition index and ∑REY covariant diagram.

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Biological organic matter conditions

Pb and Sr can be strongly absorbed by marine creatures, and many types of algae and plankton can enrich rare earth elements48,49. Organic matter contributes to the breakdown of organic matter following plankton death, releasing rare earth elements that raise the concentration of rare earth elements in sediment pore water. Compared with the content of trace elements in the upper crust, the enrichment coefficients of Pb elements in Zhijin phosphorus-rich rare earth ore and phosphorus rare earth ore are 2.46–175.29 ppm (average 55.89 ppm), and 2.06–34.35 ppm (average 17.78 ppm), respectively, and the enrichment coefficients of Sr elements are 15.38–50.48 ppm (average 32.11 ppm) and 5.95–17.86 ppm (average 11.86 ppm), respectively. Both types of phosphorite ores are rich in Pb and Sr elements, and the enrichment degree of phosphorus-rich rare earth ore is higher. It shows that biological organic matter is involved in the enrichment of rare earths. Living organisms may capture and absorb rare earths and phosphorus to build shell walls during the process of rare earth enrichment. Dead organisms provide enrichment sites for the preferential filling of rare earths and phosphorus. The breakdown and release of rare earth elements by organic matter is linked to the high concentration of rare earths in sediment pore water that some cements reflect.

Microscopic studies have found that the small shell fossils in the Zhijin phosphorite-type rare earth ore are highly developed, and the biogenic debris content in the phosphorus-rich rare earth ore is much higher than that in the phosphorus rare earth ore. Previous research also suggests that shallow water environments can support the breeding of small shelled animals51. The results of rare earth element (REE) analysis indicate that the REE content in phosphorus-rich rare earth ore (683.92–1959.93 ppm, with an average of 1187.16 ppm) is significantly higher than that in phosphorus rare earth ore (187.99–568.057 ppm, with an average of 376.9226 ppm). This suggests that the REE content is related to the degree of development of small shelly fauna.

Regardless of whether the Zhijin phosphorite-type rare earth ore is of the phosphorus-rich rare earth ore or the phosphorus rare earth ore, their rare earth element distribution curves in Australian shale are both cap-shaped and nearly horizontal. This cap-shaped rare earth distribution pattern, characterized by the enrichment of middle rare earth elements, is considered to have primary sedimentary characteristics and is the result of the involvement of biological or organic matter in the phosphorization process52. Consequently, the REE distribution patterns also reveal characteristics of small shelly fauna involvement in the mineralization process.

The LA-ICP-MS analysis results show that in different types of ore particles and cement of varying compositions, the highest total contents of P and ∑REY are found within the small shelly organisms (with average contents of 2365.70 ppm for ∑REY and 319,579.76 ppm for P). The shell walls of these organisms exhibit the second-highest content (with average contents of 2270.13 ppm for ∑REY and 233,207.77 ppm for P), which are higher than those in sand debris, aggregates, and phosphate cements. These values are significantly higher than those in bright crystalline dolomite cements (with average contents of 308.14 ppm for ∑REY and 20,893.81 ppm for P) and siliceous cement (with average contents of 46.64 ppm for ∑REY and 998.74 ppm for P). This suggests that small shelly organisms are an important factor in the formation of phosphorite-hosted rare earth mineralization.

In addition, the correlation between TOC content and total rare earth (∑REY) of phosphorite-type rare earth ore samples was compared and analyzed (Fig. 14). It was determined that there was a moderately positive association (correlation coefficient R = 0.46) between the TOC content of phosphorite-type rare earth ore and ∑REY. It is evident that there is a connection between biological organic matter and the enrichment of rare earth elements and phosphorus.

Fig. 14
figure 14

Ore TOC and ∑REY covariation diagram.

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Based on the above data, it can be demonstrated that the small shelly organisms participated in the mineralization process of the Guizhou Zhijin Xinhua rare earth-phosphate deposit during the rare earth enrichment process. Mainly manifested as, due to the needs of life activities, living small shelled organisms may capture and ingest rare earth elements and phosphates to build their shell walls. This portion of rare earth elements and phosphates is retained in the outer wall of the biogenic sediment during burial. After the death of small shell organisms, the soft tissue decomposes, and their cavities provide a preferential filling site for the enrichment of rare earth elements and phosphates. The high concentration of rare earth elements in the pore water of sediments, as reflected by some cemented materials (phosphate cemented materials), is related to the release of rare earth elements during the decomposition of organic matter.

Rare earth enrichment process

In the bioclastic shoal environment of overall oxidation, the enrichment of phosphorus-rare earth in the early Cambrian Zhijin Xinhua super-large rare earth-phosphorus deposit in Guizhou mainly experienced the four stages of submarine jet + seawater mixing and the formation of phosphorus-rich-rare earth water mass; sea level rise, phosphorus-rare earth homogeneous migration stage; environmental changes, phosphorus-rare earth co-liberation stage; diagenesis, phosphorus-rare earth coprecipitation stage (Fig. 15).

  1. 1.

    Submarine jet + seawater mixing, phosphorus-rich-rare earth water mass formation stage: Strong extensional tectonics caused the South China Ocean Basin to be stretched and rifted in the early Early Cambrian, which also saw fast basin subsidence and a high frequency of hot water activity. Along the synsedimentary fault, a hot water jet carried the rare earth and phosphorus from the deep crust to the surface. This is consistent with the previous view of hydrothermal enrichment in sedimentation28. During the same period, the ocean was in a state of strong redox stratification. The ocean was oxidized above the typical storm wave base surface. Below the normal storm wave base surface, the ocean was in a state of anoxic iron (water hypoxia and free Fe2+)53, and intermittent anoxic sulfidation environment (water hypoxia and free H2S) is widespread from the continental shelf to the slope area54. In oxidized ~ suboxidized fluids, iron hydroxide (FeOOH) also has strong adsorption capabilities for phosphorus and REY, converting into iron-phosphorus complexes that contain REY (FeOOH·PO43−). At this time, the early Cambrian Ocean was teeming with life, and plankton, bacteria, algae, and shelled animals were all thriving. The absorption of rare earth and phosphorus by marine organisms led to the eutrophication of surface water and a significant increase in marine paleoproductivity. This aligns with researchers’ views on the bioconcentration theory of rare earth elements24,26. A water mass rich in Fe + Org + P + REY is formed by the convergence of the phosphorus and rare earth brought by the hot water jet, the phosphorus and rare earth released by the sinking and decomposition of these biological deaths, and the phosphorus and REY adsorbed by the reduction and release of the iron-phosphorus complex. It is evident that, unlike the "hydrothermal enrichment theory" and the "biological enrichment theory," the phosphorus-rare earth mineralization materials do not have a single source; their sources are mainly provided by hydrothermal activities and normal seawater. In addition, some substances originating from land also flow into the ocean.

  2. 2.

    Sea level rise, phosphorus-rare earth homogeneous migration stage: With the decline of the oceanic plate and the contraction of the Nanhua Rift Trough, a brief transgression took place against the backdrop of convergence in the early Early Cambrian, reaching the maximum flooding surface during the sedimentary period of the Gezhongwu Formation. At this time, the Zhijin Xinhua study area in Guizhou was in a shallow water platform environment, and the water body gradually deepened from northwest to southeast. Under the influence of the continental monsoon, the water mass rich in Fe + Org + P + REY reaches the shallow water platform as it rises along the southeast-northwest terrain with the ascending current. Resolved the issue of the unclear properties of the deep water masses brought back by the upwelling currents in the "Upwelling Current Enrichment Theory"27.

  3. 3.

    Environmental changes, phosphorus-rare earth co-liberation stage: The concentration of phosphate in the ocean is frequently difficult to attain saturation under typical seawater geochemical circumstances, making it challenging for apatite minerals to be directly formed in the form of inorganic deposition in saltwater55. Apatite self-deposition is created by the breakdown and release of organic matter, biochemical aggregation, and microbial adsorption close to the sediment–water interface with alternate redox cycles, and phosphorite formation occurs when the phosphate concentration reaches saturation56. The predominant enrichment mode is the "Fe-redox pump" mode, which is followed by the organic matter enrichment mode57. In the early Cambrian, there was a global hypoxia event. Enrichment of phosphorus and REY under the redox interface of layered seawater was facilitated by the reduction-sulfidation of bacteria, which caused the REY-containing iron-phosphorus complex (FeOOH·PO43− + REY) in the reduced (or sulfided) bottom water sinking to the seabed to directly reduce and release the adsorbed phosphorus and REY58. Adsorbed phosphorus and REY enter the pore water as a result of the iron hydroxide on the redox interface continuously forming and breaking down, enriching the pore water with REY59. Additionally, plankton, algae, and other lower aquatic species perished as a result of the Phanerozoic ocean’s mutation, and biological debris including rare earths and phosphorus sunk into the seabed. The breakdown of organic matter and the decrease in sulfate during the shallow burial of organic matter will also encourage the solubility of phosphorus into pore water and the co-liberation of rare earth and phosphorus55.

  4. 4.

    Diagenesis, phosphorus-rare earth coprecipitation stage: The amount of phosphorus and rare earth elements was significantly correlated. Adsorption, which is primarily related to crystal surface control and primarily occurs in the early diagenetic stage, and isomorphism, which is primarily related to the control of crystal structure (REE3+ replaces Ca2+) and typically occurs in the late diagenetic stage, are the two main mechanisms for rare earth entering phosphorite, according to previous research60. Adsorption will preferentially take up LREE to produce higher (La/Yb)N ratios, while isomorphism will preferentially absorb MREE and produce lower (La/Sm)N ratios. Thus, the (La/Sm)N and (La/Yb)N diagrams can be used to differentiate between the many ways that rare earth elements enter apatite61. The (La/Sm)N and (La/Yb)N of Zhijin phosphorite-type rare earth ore are plotted (Fig. 16). It is obvious that the ore samples are mainly located above the modern seawater, indicating that they are affected by strong early diagenesis. In the oxic-anoxic water bodies of the continental shelf, small shelled organisms proliferate abundantly, taking up phosphorus to build their shells while also absorbing REY. When these small shelled organisms die, their remains (shells) become initial phosphorus-rich bodies. Phosphate and Ca2+ in the pore water combine to form apatite, and then REY is adsorbed by apatite or collophane. Weakly consolidated phosphorus-rich rare earth sediments are frequently washed and elutriated in the relatively small bioclastic beach environment, primarily filled into the biological shell or wrapped into sand debris, and partially formed intergranular cement as a result of the strong hydrodynamic force. The recrystallized fibrous apatite cement was observed under the microscope, indicating that the rare earth element was fully adsorbed by apatite or collophane in the long-term late diagenesis, and then the phosphorite-type rare earth deposit with high grade, high reserves, and developed grain structure was formed in the platform facies area.

Fig. 15
figure 15

Phosphorite-type rare earth metallogenic model diagram. (Software used to generate the image: CorelDRAW, version number: CorelDRAW 2024, URL link: https://www.coreldraw.com).

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Fig. 16
figure 16

(La/Sm)N and (La/ Yb)N input point diagram of rare earth ore (based on61).

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Conclusions

  1. 1.

    The phosphorite-type rare earth ore has a typical particle structure. The particle composition is mainly bioclastic, followed by sand debris, and agglomerates, and the composition of the particles is mainly collophane. The cements are mostly sparry dolomite, followed by phosphate cement, and siliceous cement. There are two types of cementation and support, one is contact-pore cementation, particle support; the other is base-pore cementation, matrix-particle support.

  2. 2.

    Based on structural variations and phosphorus content, Zhijin phosphorite-type rare earth ore is classified as either phosphorus-rich rare earth ore or phosphorus rare earth ore. They are all enriched in Y, P, and rare earth elements; the phosphorus-rich rare earth ore has a higher degree of enrichment. Rare earth elements may be mainly adsorbed by collophane or exist in apatite lattice defects in the form of isomorphism.

  3. 3.

    Rare earth elements are mostly concentrated in the small shell and shell wall, followed by the inside of the sand debris, the border of the sand debris, and the phosphate mass. They are also enriched in the phosphate cement, according to the structural micro-area’s LA-ICP-MS analysis. The reason is that the cavity after the death of small shell organisms provides the best space for the enrichment of rare earth elements. Phosphorus is absorbed by living things to build shell walls, which enriches rare earth elements. In addition, sand debris and some later pore water produced by waves and tides are also important carriers of rare earth elements.

  4. 4.

    Conditions related to paleogeography, redox, and biological organic matter influence the enrichment of phosphorus-rare earth. The phosphorite-type rare earth is the sedimentary product of a carbonate platform with shallow water and strong hydrodynamics. It is formed in an oxidation-poor oxygen environment, and when the environment changes from oxidation to poor oxygen, it is more conducive to the enrichment of phosphorus and rare earth.

  5. 5.

    The ore-forming process of the phosphorite-type rare earth ore has gone through four stages, namely, submarine jet + seawater mixing, phosphorus-rich-rare earth water mass formation stage; sea level rise, phosphorus-rare earth homogeneous migration stage; environmental changes, phosphorus-rare earth co-liberation stage; and diagenesis, phosphorus-rare earth coprecipitation stage.

In conclusion, this study develops a rare earth mineralization model for phosphorite, identifies the governing factors in the material sources, migration, and enrichment processes of phosphorus-rare earth elements, and proposes that rare earth elements are primarily enriched within the structures of small shell bioclastic debris, shell walls, sand grains, and phosphate cement. In essence, rare earth elements are adsorbed by collophane or exist in the lattice defects of apat; little shell creatures are just one of the carrier structures. The separation and extraction technology of REY in phosphorite-type rare earth ores should fully consider its occurrence state in phosphorite minerals.