Geochemical Behaviors and Constraints on REE Enrichment in Weathered Crust of Shallow Metamorphic Rocks: Insights from the Getengzui Ion-Adsorption REE Deposit, South China

Abstract

Ion-adsorption rare earth element (REE) deposits represent strategic critical resources in China, which were traditionally considered to be predominantly hosted in granite weathering crusts. However, the recent discovery of new deposit types within the weathering crusts of shallow metamorphic rocks in South China has opened up novel exploration frontiers, while research on their metallogenic mechanisms remains insufficient. To elucidate the REE enrichment mechanisms in shallow metamorphic rock weathering crusts, this study focuses on the Getengzui ion-adsorption REE deposit in southern Jiangxi Province. Twenty-four samples were collected from the weathering crust profiles of the Qingbaikouan Shenshan and Kuli Formations. Multiple analytical approaches were employed, including major and trace element analysis, Chemical Index of Alteration (CIA), Base Leaching Index (BA), and quantitative evaluation of element mass transfer coefficients (τ). Trace element spider diagrams, REE distribution patterns, and A-CN-K diagram analysis were also utilized. The results reveal that the weathering crusts have progressed to the middle–late stage of chemical weathering. The average CIA value is 83 for the middle-upper part of the completely weathered horizon in the Kuli Formation. In contrast, for the completely weathered horizon in the Shenshan Formation, the value is 86. Intense chemical weathering has resulted in the near-complete decomposition of primary silicate minerals and extensive leaching of base cations. This progress has created an acidic pore water environment, which is critical for REE mobilization. REEs exhibit characteristics of in situ secondary enrichment, with significant enrichment of ΣREE in the middle-upper part of the completely weathered horizon. The peak τ(ΣREE) values reach 0.78 and 2.43 for the Kuli and Shenshan Formations, respectively. Apatite dissolution is identified as the primary source of REE ions. Differences exist in the geochemical mobility sequences of elements between the two formations. REE enrichment is controlled by multi-stage geochemical barriers, including an oxidation barrier and a clay adsorption barrier. The oxidation barrier preferentially fixes Ce⁴⁺, whereas the clay adsorption barrier serves as the dominant mechanism for large-scale REE enrichment. Parent rock lithology is the primary factor governing the efficiency, scale, and fractionation characteristics of REE enrichment. The Kuli Formation is favorable for forming the thick, large-scale orebodies enriched in light rare earth elements (LREEs). In the contrast, the Shenshan Formation tends to host higher-grade orebodies, characterized by a relatively balanced ratio of LREEs and heavy rare earth elements (HREEs). This study clarifies the main controlling factors for ion-adsorption REE mineralization in two shallow metamorphic rocks. It thereby provides a theoretical basis for future exploration. This framework is applicable to analogous REE resources within shallow metamorphic rock distributions across South China and nationwide.

1. Introduction

Rare earth elements (REEs) consist of the 15 lanthanide elements (from La to Lu), together with Sc and Y. These 17 metallic elements share similar chemical properties [1,2,3]. Based on systematic differences in atomic electron configuration, ionic radius, and physicochemical properties, REEs are divided into Light Rare Earth Elements (LREEs: La, Ce, Pr, Nd, Pm, Sm, Eu) and Heavy Rare Earth Elements (HREEs: Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, Sc) [4]. Known as “industrial vitamins,” “war metals,” and “the mother of new materials,” REEs possess an irreplaceable core value. This value derives from their exceptional optical, electrical, magnetic, and catalytic properties, which stem from the unfilled 4f electron shells. As such, REEs serve as strategic cornerstones supporting the modern green energy revolution, information industry upgrading, and advanced national defense development [5,6,7,8,9,10,11].

Global REE distribution is highly uneven. China, Vietnam, Brazil, and Russia account for over 85% of the world’s REE resources. This highly concentrated distribution has made REEs a focal point of strategic competition among major powers [12,13]. This stability of REE supply chains is closely linked to a nation’s industrial competitiveness, scientific and technological innovation capacity, and national defense security baseline. China’s REE resources exhibit a distinct macro pattern of “abundant light REEs in the north and rich heavy REEs in the south”. The northern region, represented by the Bayan Obo Fe-Nb-REE deposit in Inner Mongolia, is the world’s largest light REE resource base. The southern region, particularly the Nanling metallogenic belt spanning the Jiangxi, Guangdong, Hunan, and Guangxi provinces, is renowned for its weathering crust ion-adsorption REE deposits [14,15,16,17,18,19]. These deposits supply over 90% of the global medium and heavy REEs, making them virtually the only practical source of HREEs required by current high-tech industries [20,21].

REE deposits have diverse genetic types. Based on previous studies [22,23,24,25,26,27], they can be broadly categorized into the following four types. First, the magmatic type, especially super-large deposits associated with alkali rock–carbonatite complexes, which form the main body of global light REE resources. Second, the hydrothermal type, including skarn-type, quartz-vein-type deposits, and those related to alkaline granites, with significant variations in scale. Third, the sedimentary type, encompassing coastal placer deposits, with recent major breakthroughs in continental sedimentary-type REE deposits. Fourth, the ion-adsorption type, formed by near-surface weathering and enrichment of REE-rich parent rocks.

Ion-adsorption REE deposits are a unique type that were first discovered and named by Chinese geologists in the late 1960s. They were initially found in granite weathering crusts in areas such as Zudong, Longnan, and Heling, Xunwu, Jiangxi Province. This discovery marked a major breakthrough in REE resource exploration [11,28,29,30,31]. In recent years, with advancing exploration, industrially significant ion-adsorption REE deposits have been discovered for the first time in shallow metamorphic rock weathering crusts. The Getengzui deposit is a typical example of this new type. Its discovery enriched the parent rock types for ion-adsorption REE deposits and opened up a promising new direction for REE exploration [16,32,33]. However, research on weathering crust ion-adsorption REE deposits has long focused primarily on granite weathering crusts [34,35,36,37,38,39,40]. Research on ion-adsorption REE deposits in shallow metamorphic rocks remains relatively underdeveloped. In particular, studies on element migration characteristics in weathering crust profiles are still in their infancy. Their metallogenic mechanisms remain unclear, leaving a series of key scientific questions unresolved.

Currently, the global energy transition and technological revolution are advancing at an unprecedented pace. The explosive growth of industries such as new energy vehicles, wind power generation, industrial robots, and energy-saving frequency conversion has driven a surge in demand for high-performance Nd-Fe-B permanent magnets. This, in turn, imposes sustained and enormous pressure on REEs such as Pr, Nd, Tb, and Dy, especially HREEs, which have a highly concentrated supply and strong scarcity [41,42,43]. Major economies including the EU, the United States, and Japan have listed REEs (especially HREEs) as the top-tier “critical raw materials” or “critical minerals”. They are also vigorously promoting supply chain diversification strategies [44]. Against this global backdrop, China, as the dominant player in REE resources and supply, faces dual challenges [45]. On one hand, it needs to safeguard its strategic resource advantage. On the other, it must address the declining resource potential and increasing exploration difficulty in traditional high-quality granite-type mining areas after decades of extraction.

In response to these issues, this study selects the Getengzui ion-adsorption REE deposit in southern Jiangxi Province as a key case. It focuses on two mineralized shallow metamorphic rock weathering crusts of the Qingbaikouan Kuli and Shenshan Formations, with the collection of twenty-four vertical weathering crust profile samples. The research aims to investigate the spatial distribution characteristics and evolutionary sequences of major and trace elements. It further aims to quantitatively analyze weathering processes and element behaviors. It also explores the elemental composition and migration dynamics between REE source rocks and weathering crust profiles. Ultimately, it seeks to clarify the patterns of REE enrichment and pinpoint the primary controlling factors governing REE mineralization. The findings are expected to provide direct and reliable geochemical theoretical guidance for the investigation, evaluation, and exploration of similar REE resources in other shallow metamorphic rock distribution areas across South China and the entire country. This study addresses the gap in shallow metamorphic rock-hosted REE deposit research. It clarifies metallogenic mechanisms and enrichment controls. The findings boost the exploration efficiency of similar resources.

2. Geological Setting

2.1. Regional Geological Setting

The study area, located in southern Jiangxi Province, is tectonically located between the Yangtze Block and the Cathaysia Block. It is situated at the intersection of the east–west (E–W)-trending Nanling tectonic belt and the northeast (NE)-trending Wuyishan tectonic belt [46,47,48]. Its northern boundary is defined by the Jiangshan–Shaoxing Fault Zone, adjacent to the Yangtze Block. To the south, it is bounded by the Zhenghe–Dabu Fault Zone and the late Mesozoic volcanic rock belt along the southeastern coast (Figure 1a) [49,50].

Extensive research has confirmed that the Yangtze and Cathaysia Blocks collided and amalgamated during the Neoproterozoic (1000–800 Ma), forming the complex South China Block. This block subsequently experienced multi-phase tectono-magmatic overprinting, which shaped its present tectonic framework [52,53,54]. The stratigraphy in the region is relatively complete, with strata from the Proterozoic to the Quaternary. The only exception is the Silurian strata, which are absent (Figure 1b). The basement consists of Neoproterozoic Qingbaikouan shallow metamorphic rocks, mainly phyllite of the Shenshan Formation and metatuff of the Kuli Formation. These rocks serve as the parent rocks for the REE deposits in the area [32,55,56].

Conformably overlying the basement is a thick sequence of marine flysch, consisting of shallow metamorphic clastic rocks from the Nanhua to Cambrian periods. Paleozoic to Mesozoic marine to paralic carbonate and clastic rocks are well-developed. The Meso-Cenozoic strata are dominated by continental basin red clastic sediments and volcanic rock series [57,58]. The regional tectonic framework is complex, characterized primarily by multiple sets of structural systems, including near E-W, NE-trending (Cathaysian, Neocathaysian), and NW-trending structures. The junctions and composite zones of these structures control the distribution of sedimentary basins, magmatic belts, and mineral resources, including tungsten (tin), copper, lead, zinc, silver, cobalt, iron, rare earth elements (REE), as well as potassium feldspar, kaolin, and geothermal resources [59].

Folds and faults are well-developed, providing pathways and space for fluid migration and weathering crust development. Magmatic activity in the region is multi-phased, with the Caledonian and Yanshanian periods being the most intense. Caledonian granites mostly occur as batholiths forming composite plutons. Yanshanian magmatic activity was large-scale, generating widely distributed intermediate-felsic granite bodies. This period represents an important rock- and ore-forming stage in the region. Additionally, minor magmatic activities occurred during the Hercynian and Indosinian periods [60].

2.2. Deposit Geological Characteristics

The Getengzui ion-adsorption REE deposit is hosted in the weathering crust of the Qingbaikouan shallow metamorphic rocks. It is the first discovered weathering crust-type REE deposit hosted in such rocks [55]. The topography of the mining area is dominated by intermontane basins and low-relief hills. The weathering crust is distributed areally, with a thickness ranging from 1.4 to 20.3 m (average 9.53 m) and an outcrop elevation of 230–410 m. From top to bottom, the weathering crust profile comprises the hilltop soil layer, completely weathered horizon, semi-weathered horizon, and bedrock [61]. The total content of clay minerals in the weathering crust is about 60%–80%, dominated by illite (40%–60%) and kaolinite (20%) [62].

The mining area mainly exposes strata including the Qingbaikouan Kuli Formation, the Shenshan Formation, and the Nanhua, Sinian, and Cambrian strata (Figure 2). The Kuli Formation, predominantly composed of metatuff, is distributed in the southern part of the area. It lies in angular unconformity with the overlying Mesozoic strata. To the north, the Shenshan Formation is distributed in a belt and consists mainly of phyllite. The parent rocks for mineralization are the shallow metamorphic rocks of the Qingbaikouan Kuli and Shenshan Formations [32,55,56]. Magmatic rocks in the area mainly include large-scale Caledonian granites exposed in the northwest, as well as small outcrops of Jurassic granite and quartz diorite in the north. Faults are primarily NE-trending, NW-trending, and near E-W trending, controlling the strata distribution [16,63].

Ore bodies are strictly controlled by the weathering crust, mainly occurring on the completely weathered horizon, on gentle hilltops and slopes. They are stratiform-like and lenticular in shape, extending along the topography, and are mostly elliptical in plain view. Engineering statistics show that the ore bodies are shallowly buried. Approximately 93% of the drilling projects encountered ore within a depth of 15 m, with an average thickness of about 5.66 m. Ore bodies are thicker at hilltops and ridges, thinning gradually towards slopes and foot slopes. The average leachable REE grade of the ore bodies is 0.07%. Vertically, the ore bodies often exhibit a “funnel-shaped” distribution with enrichment in the middle and poorer grades at the top and bottom. This distribution pattern indicates intense chemical weathering and secondary enrichment processes [61,64].

3. Methodology

3.1. Sampling Method

Samples were collected from two complete weathering crust profiles of the Kuli and Shenshan Formations within the Getengzui deposit. The Kuli Formation weathering crust profile was exposed by house construction, while the Shenshan Formation profile was uncovered by road excavation. Continuous vertical sampling was conducted from top to bottom, covering the hilltop soil layer, completely weathered horizon, semi-weathered horizon, and bedrock, with a sampling interval of 1 m. A total of 24 samples were obtained: 15 from the Kuli Formation profile and 9 from the Shenshan Formation profile. The Kuli Formation samples included 1 hilltop soil sample, 11 completely weathered horizon samples, 2 semi-weathered horizon samples, and 1 bedrock sample. The Shenshan Formation samples consisted of 1 hilltop soil sample, 5 completely weathered horizon samples, 2 semi-weathered horizon samples, and 1 bedrock sample (Figure 3).

3.2. Analytical Methods

Major and trace element geochemical analyses were all performed at Nanjing Hongchuang Geological Exploration Technology Service Co., Ltd. (Nanjing, China).

Major elements were determined using a JY-ULTIMA2C inductively coupled plasma optical emission spectrometer (ICP-OES) manufactured by HORIBA, Ltd., Kyoto, Japan. Analysis followed the lithium metaborate fusion-ICP-OES internal standard method, with Li as the internal standard, achieving an analytical precision (RSD) of <3%. The analytical procedure was as follows. We accurately weighed 100 mg of the sample, which was crushed to −200 mesh (<0.075 mm) and dried at 105 °C, and 400 mg of anhydrous lithium metaborate. The mixture was then ground thoroughly, transferred to a graphite crucible, and fused in a muffle furnace at 1000 °C for 15 min. After forming a glass bead, we allowed it to cool to room temperature. Place the cooled glass bead into a 250 mL beaker containing 50 mL of 5% aqua regia. Heat and stir the solution in an 80 °C water bath using a magnetic hot plate for 30 min. Once fully dissolved and cooled, dilute the solution to 100 mL with 5% aqua regia, mix well, and let it stand before testing.

The trace elements, including REEs, were analyzed using an Agilent 7900 inductively coupled plasma mass spectrometer (ICP-MS) manufactured by Agilent Technologies, Inc., Santa Clara, CA, USA. The acid digestion-ICP-MS internal standard method was employed, using Rh as the internal standard with five scans per element. The precision (RSD) was less than 3% for all elements. The procedure consisted of the following steps. Prior to analysis, 50 mg of the prepared sample (−200 mesh, dried at 105 °C) was weighed into a PTFE digestion vessel. A mixed acid solution (1 mL HNO₃, 3 mL HCl, 1.5 mL HF) was added, and the sealed vessel was heated at 190 °C for no less than 36 h to ensure complete digestion. Following digestion, the solution was evaporated to moist salts at 160 °C. The addition of 1 mL HNO₃ was repeated twice, and then re-dissolved with 1.5 mL HNO₃, 2 mL H₂O for a secondary 12 h heating at 190 °C. Finally, the cooled digestate was diluted to 50 mL with high-purity deionized water (18 MΩ·cm) for quantitative ICP-MS.

3.3. Data Processing Methods

To assess the weathering intensity, the Chemical Index of Alteration (CIA) and the Base Leaching Index (BA) were selected as indicators. CIA quantitatively characterizes the degree of conversion of silicate minerals to clay minerals, calculated as follows: CIA = [Al₂O₃/(Al₂O₃ + CaO* + K₂O + Na₂O)] × 100 [65]. BA evaluates the leaching intensity of alkaline elements (Ca, Na, K, Mg), thereby reflecting the maturity of the weathering crust and the pH evolution of the mineralizing fluid. It is calculated as follows: BA = (K₂O + Na₂O + CaO + MgO)/Al₂O₃ [66]. A lower BA value indicates a higher chemical weathering intensity. In the above formulas, element contents are expressed in molar proportions, and CaO* specifically refers to silicate-bound calcium [66].

To study element mobility, the mass transfer coefficient (τ) was selected as an indicator to quantitatively characterize the degree of migration (loss or gain) of a specific element, relative to an immobile reference element during weathering. The formula is as follows: τ_i,j = (C_{i, w}/C_i,p) × (C_ref,p/C_ref,w) − 1. Where τ_i,j is the mass transfer coefficient of element “i” in weathered sample “j”, C_{i, w} is the concentration of element “i” in the weathered sample, C_i,p is the concentration of element “i” in the bedrock, C_ref,p is the concentration of the immobile reference element in the bedrock, and C_ref,w is the concentration of the immobile reference element in the weathered sample [67].

To construct the trace element spider diagrams, sample trace element concentrations were normalized to primitive mantle values [68]. For REE distribution patterns and calculations of anomalies δCe and δEu, sample REE concentrations were normalized to chondrite values [69]. The formulas are as follows: $\mathsf{\delta }\mathrm{C}\mathrm{e}={\mathrm{C}\mathrm{e}}_N/\sqrt{{\mathrm{L}\mathrm{a}}_N\times {\mathrm{P}\mathrm{r}}_N},\mathsf{\delta }\mathrm{E}\mathrm{u}={\mathrm{E}\mathrm{u}}_N/\sqrt{{\mathrm{S}\mathrm{m}}_N\times {\mathrm{G}\mathrm{d}}_N}$, and La_N, Ce_N, Pr_N, Sm_N, Eu_N, Gd_N are chondrite-normalized values. For A-CN-K ternary diagrams, the parameters are defined as A = Al₂O₃/(Al₂O₃ + CaO + K₂O + Na₂O), CN = (CaO + K₂O)/(Al₂O₃ + CaO + K₂O + Na₂O), and K = K₂O/(Al₂O₃ + CaO + K₂O + Na₂O). The element contents are in weight percent, and CaO* refers specifically to silicate-bound calcium [70].

For the statistics of light rare earth elements (LREEs) and heavy rare earth elements (HREEs), the definitions are as follows: LREEs = La + Ce + Pr + Nd + Pm + Sm + Eu, HREEs = Gd + Tb + Dy + Ho + Er + Tm + Yb + Lu + Y + Sc, and REEs (total rare earth elements) = LREEs + HREEs [4].

4. Results

Concentrations of and variations in the major elements, trace elements, and rare earth elements (REEs) are presented in

Table 1. Major element concentration of Kuli and Shenshan Formations’ weathering crust profile samples (%).

Formation	Samples	Depth	SiO2	Al2O3	TFe2O3	TiO2	CaO	MgO	Na2O	K2O	P2O5	MnO	LOI	CIA
Kuli Formation	GK-1	0.3–1 m	57.9	21.0	6.3	0.7	0.1	0.8	0.1	2.7	0.03	0.04	10.2	88.0
	GK-2	1–2 m	65.4	18.4	4.0	0.5	0.04	0.5	0.1	3.1	0.02	0.1	7.5	85.0
	GK-3	2–3 m	66.0	17.8	4.0	0.5	0.1	0.6	0.1	3.4	0.02	0.1	7.2	83.0
	GK-4	3–4 m	66.0	17.7	4.1	0.5	0.04	0.8	0.2	3.6	0.02	0.1	6.7	82.0
	GK-5	4–5 m	66.2	17.4	4.1	0.5	0.04	0.8	0.3	3.8	0.02	0.1	6.5	81.0
	GK-6	5–6 m	67.2	16.6	4.0	0.5	0.04	0.6	0.3	3.9	0.02	0.1	6.3	79.0
	GK-7	6–7 m	61.5	20.0	4.7	0.6	0.1	0.8	0.3	4.4	0.03	0.1	7.1	81.0
	GK-8	7–8 m	62.6	19.8	3.9	0.5	0.1	0.9	0.9	4.0	0.03	0.1	6.7	80.0
	GK-9	8–9 m	66.1	17.9	3.2	0.5	0.1	0.7	1.4	4.2	0.02	0.1	5.4	76.0
	GK-10	9–10 m	68.6	16.4	3.1	0.4	0.1	0.6	1.8	3.9	0.02	0.1	4.6	74.0
	GK-11	10–11 m	69.3	15.9	3.2	0.4	0.3	0.7	2.1	3.5	0.02	0.1	4.3	73.0
	GK-12	11–12 m	69.6	15.5	3.1	0.4	0.3	0.7	2.3	3.7	0.02	0.1	4.1	71.0
	GK-13	12–13 m	68.4	16.0	3.3	0.5	0.4	0.7	2.6	3.9	0.02	0.1	3.6	70.0
	GK-14	13–14 m	68.1	15.9	3.5	0.5	0.5	0.8	2.4	3.8	0.02	0.1	4.2	71.0
	GK-15	14–15 m	70.5	14.4	3.4	0.5	0.2	0.8	3.6	3.7	0.1	0.04	2.5	66.0
Shenshan Formation	GS-1	0.3–1 m	65.5	16.9	5.1	0.6	0.04	1.2	0.1	2.8	0.02	0.1	7.3	86.0
	GS-2	1–2 m	66.8	15.9	5.5	0.7	0.1	1.3	0.1	2.8	0.01	0.2	6.4	85.0
	GS-3	2–3 m	66.8	16.1	5.4	0.7	0.04	1.2	0.1	2.8	0.02	0.1	6.6	85.0
	GS-4	3–4 m	66.7	16.5	5.3	0.7	0.04	1.0	0.1	2.6	0.01	0.1	6.7	86.0
	GS-5	4–5 m	67.3	16.4	5.1	0.7	0.04	1.0	0.1	2.4	0.02	0.1	6.7	87.0
	GS-6	5–6 m	67.2	16.3	5.1	0.7	0.1	0.9	0.1	2.2	0.02	0.1	6.9	88.0
	GS-7	6–7 m	66.4	16.4	5.4	0.7	0.04	1.2	0.1	3.1	0.02	0.1	6.2	84.0
	GS-8	7–8 m	67.4	15.9	5.3	0.7	0.1	1.1	0.1	2.9	0.02	0.1	6.2	84.0
	GS-9	8–9 m	65.2	17.0	5.0	0.6	0.2	1.7	1.2	6.2	0.1	0.1	2.4	69.0

,

Table 2. Trace element concentration of Kuli and Shenshan Formations’ weathering crust profile samples (ppm).

Formation	Samples	Depth	Th	Nb	Ta	Zr	Hf	Sn	W	Rb	Sr	Ba	Cs	V	Cr
Kuli Formation	GK-1	0.3–1 m	25.0	18.1	1.2	395.1	10.6	3.8	2.3	120.1	13.0	625.1	7.9	70.4	23.3
	GK-2	1–2 m	19.5	16.3	1.3	294.6	8.1	3.1	1.6	93.1	12.9	866.1	5.0	40.6	13.4
	GK-3	2–3 m	19.6	16.0	1.3	292.3	7.8	3.0	2.0	109.7	15.5	918.4	6.3	40.3	14.8
	GK-4	3–4 m	19.5	15.3	1.2	281.2	7.6	3.1	1.7	144.4	17.7	1059.0	9.2	41.5	16.4
	GK-5	4–5 m	19.9	15.6	1.3	293.8	8.0	3.2	1.8	153.0	18.8	1136.0	8.9	40.9	16.7
	GK-6	5–6 m	20.7	14.9	1.3	275.8	7.9	3.1	1.8	151.6	21.2	1223.0	8.8	42.7	15.9
	GK-7	6–7 m	23.6	18.6	1.6	358.1	10.8	3.5	4.0	183.7	16.7	1641.0	7.4	55.6	16.2
	GK-8	7–8 m	24.6	18.9	1.6	336.4	10.2	3.4	2.2	178.3	14.6	1546.0	5.3	41.2	13.6
	GK-9	8–9 m	19.9	14.5	1.2	251.7	7.5	3.0	2.1	161.8	19.1	1569.0	4.8	32.1	13.8
	GK-10	9–10 m	18.5	13.6	1.1	234.9	6.9	2.8	1.9	157.1	24.1	1405.0	4.7	32.6	12.7
	GK-11	10–11 m	18.0	13.6	1.1	235.6	6.8	2.8	1.6	137.8	50.9	1251.0	2.9	31.1	12.5
	GK-12	11–12 m	16.5	12.7	1.0	213.8	6.0	2.8	1.8	137.1	50.3	1250.0	4.5	33.3	12.5
	GK-13	12–13 m	17.9	14.0	1.1	241.6	6.8	2.8	1.7	134.0	79.5	1337.0	3.9	34.9	13.0
	GK-14	13–14 m	17.9	14.1	1.2	261.5	7.2	2.8	1.7	144.4	88.8	1303.0	3.6	37.1	13.0
	GK-15	14–15 m	17.3	13.0	1.0	273.0	7.1	2.9	1.5	120.4	33.1	970.7	10.8	36.6	15.3
Shenshan Formation	GS-1	0.3–1 m	17.9	16.0	1.2	339.1	9.2	3.6	2.5	169.2	11.2	493.5	11.9	71.4	42.9
	GS-2	1–2 m	14.0	14.2	1.1	313.8	7.9	3.5	2.0	170.1	13.2	651.7	12.6	77.5	43.8
	GS-3	2–3 m	14.9	14.9	1.1	335.0	8.8	3.4	2.2	164.6	18.0	702.9	10.8	80.5	46.4
	GS-4	3–4 m	14.7	14.6	1.1	314.3	8.7	3.2	2.1	129.3	13.0	588.8	9.0	75.5	40.2
	GS-5	4–5 m	14.3	14.0	1.1	285.5	8.1	3.1	2.0	131.6	13.2	621.2	8.0	74.1	39.8
	GS-6	5–6 m	14.1	13.9	1.1	302.4	8.2	3.3	2.2	130.3	12.5	616.3	7.5	74.0	37.8
	GS-7	6–7 m	15.1	14.8	1.2	309.7	8.5	3.4	2.7	198.0	16.3	608.4	13.8	76.4	41.8
	GS-8	7–8 m	15.2	14.8	1.1	306.5	8.6	3.4	2.8	216.2	16.6	581.9	14.1	76.9	41.9
	GS-9	8–9 m	22.0	16.4	1.5	309.1	9.5	4.2	3.5	312.1	108.4	1137.0	10.0	48.0	33.3
Formation	Samples	Depth	Co	Ni	Mo	Ce	U	Li	Cu	Zn	Pb	Bi	Ga	Be	Tl
Kuli Formation	GK-1	0.3–1 m	9.5	9.1	0.8	194.4	4.2	91.5	9.8	91.3	35.5	0.2	28.9	2.4	0.7
	GK-2	1–2 m	20.6	8.0	0.7	132.6	4.8	67.7	6.8	111.8	30.5	0.2	21.7	2.5	0.5
	GK-3	2–3 m	11.0	5.9	0.6	133.0	4.1	84.4	5.4	96.4	24.0	0.1	20.4	2.4	0.7
	GK-4	3–4 m	7.3	6.4	0.4	142.7	3.6	66.6	6.0	104.7	29.4	0.1	21.8	2.4	0.9
	GK-5	4–5 m	6.8	6.8	0.4	129.0	3.5	65.4	5.7	101.7	26.5	0.1	21.3	2.6	1.0
	GK-6	5–6 m	6.6	5.2	0.5	131.1	3.6	118.8	6.3	76.7	23.2	0.1	20.4	2.7	1.0
	GK-7	6–7 m	6.9	6.3	0.3	170.1	5.1	74.6	6.6	101.4	23.9	0.2	26.7	3.6	1.1
	GK-8	7–8 m	5.8	5.9	0.2	164.6	5.3	46.0	8.5	104.7	25.2	0.2	25.3	3.1	0.8
	GK-9	8–9 m	4.8	5.0	0.5	118.5	3.4	47.3	6.2	75.9	21.0	0.2	24.3	2.9	0.7
	GK-10	9–10 m	4.6	4.0	0.3	102.7	2.9	77.3	6.0	60.7	19.4	0.1	22.3	2.9	0.6
	GK-11	10–11 m	5.3	4.6	0.3	112.7	3.6	26.1	5.3	70.0	24.2	0.1	20.3	3.0	0.5
	GK-12	11–12 m	5.3	4.6	0.3	117.8	2.7	62.0	5.3	65.7	24.3	0.1	20.3	3.1	0.5
	GK-13	12–13 m	5.1	4.5	0.3	117.1	3.4	44.6	5.0	65.6	23.5	0.1	20.2	3.2	0.5
	GK-14	13–14 m	5.9	5.2	0.2	136.4	3.4	43.2	4.0	69.9	23.8	0.1	20.2	3.5	0.5
	GK-15	14–15 m	5.4	5.9	0.1	100.3	2.0	180.9	6.2	63.7	10.5	0.1	17.7	2.7	0.6
Shenshan Formation	GS-1	0.3–1 m	19.3	22.2	0.3	122.1	2.9	43.1	20.8	110.4	41.8	0.2	21.5	2.7	0.9
	GS-2	1–2 m	21.6	17.6	0.3	120.2	3.1	47.0	13.7	113.1	22.2	0.1	20.6	2.4	0.9
	GS-3	2–3 m	16.9	17.4	0.2	92.2	3.9	44.6	14.1	109.4	25.4	0.2	21.4	2.2	0.8
	GS-4	3–4 m	14.0	15.2	0.3	105.2	4.5	38.9	12.9	101.4	22.8	0.1	19.7	2.2	0.6
	GS-5	4–5 m	13.7	14.7	0.3	82.8	4.3	35.2	13.4	99.7	20.9	0.2	18.8	2.0	0.6
	GS-6	5–6 m	20.7	15.4	0.4	120.9	3.7	34.8	13.3	104.0	27.5	0.2	19.8	1.8	0.6
	GS-7	6–7 m	14.1	23.3	0.2	102.1	2.8	57.8	16.9	149.3	37.5	0.2	21.3	2.5	1.0
	GS-8	7–8 m	11.8	19.7	0.2	99.4	2.8	58.5	16.7	147.5	20.3	0.2	19.4	2.4	1.0
	GS-9	8–9 m	6.8	14.4	0.0	49.2	2.8	42.8	10.4	101.8	15.6	0.1	26.9	4.2	1.3

and

Table 3. Rare earth element contents of Kuli and Shenshan Formations’ weathering crust profile samples (ppm).

Samples	Depth	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Sc	Y	ΣREE	L/H
Kuli Formation
GK-1	0.3–1 m	119.3	194.4	25.1	90.5	14.7	2.8	13.0	1.8	10.5	2.0	5.9	0.8	5.4	0.8	13.5	52.8	553.4	5.7
GK-2	1–2 m	183.5	132.6	36.5	127.6	21.1	4.2	20.0	3.3	19.3	3.7	9.8	1.2	7.2	1.0	8.3	95.0	674.3	3.7
GK-3	2–3 m	153.0	133.0	30.1	102.1	16.9	3.6	17.4	2.8	16.3	3.2	8.4	1.1	6.8	0.9	8.3	85.5	589.0	3.7
GK-4	3–4 m	91.9	142.7	18.2	63.3	10.1	1.9	9.7	1.5	8.4	1.6	4.7	0.7	3.9	0.6	8.7	43.3	411.0	5.2
GK-5	4–5 m	75.0	129.0	15.2	53.0	8.3	1.6	7.9	1.1	6.4	1.3	3.7	0.6	3.6	0.5	8.9	32.2	348.3	5.9
GK-6	5–6 m	90.0	131.1	18.2	60.8	10.0	2.0	8.3	1.3	7.9	1.5	4.2	0.6	4.2	0.7	8.8	37.1	386.1	5.6
GK-7	6–7 m	71.9	170.1	15.3	53.5	8.4	1.6	7.3	1.1	6.6	1.3	4.0	0.6	4.4	0.7	9.7	30.4	387.0	6.7
GK-8	7–8 m	88.4	164.6	18.7	65.3	10.0	2.0	8.9	1.3	7.1	1.5	4.5	0.6	4.5	0.8	9.1	35.3	422.0	6.5
GK-9	8–9 m	68.2	118.5	14.5	49.8	7.8	1.5	6.4	1.0	5.5	1.1	3.4	0.5	3.3	0.6	7.6	27.7	317.3	6.2
GK-10	9–10 m	72.7	102.7	15.0	50.6	8.0	1.7	7.6	1.1	5.8	1.2	3.6	0.5	3.5	0.6	7.5	29.1	310.9	5.7
GK-11	10–11 m	54.6	112.7	11.6	40.3	6.0	1.2	5.3	0.8	4.5	0.8	2.7	0.4	2.8	0.5	7.2	22.3	273.4	6.7
GK-12	11–12 m	58.4	117.8	12.1	41.3	6.7	1.3	5.5	0.9	4.7	1.0	3.0	0.4	3.0	0.5	7.3	24.0	287.7	6.5
GK-13	12–13 m	56.8	117.1	11.5	39.9	6.5	1.4	5.3	0.8	4.3	0.9	2.7	0.4	2.7	0.5	7.6	23.1	281.5	6.7
GK-14	13–14 m	63.5	136.4	13.3	45.5	7.3	1.5	6.0	0.9	5.3	1.0	3.0	0.4	2.8	0.5	7.8	26.3	321.5	6.8
GK-15	14–15 m	86.7	100.3	16.1	59.3	9.4	1.7	9.1	1.3	7.3	1.4	4.3	0.6	3.6	0.5	6.2	42.6	350.3	4.6
Shenshan Formation
GS-1	0.3–1 m	62.2	122.1	15.1	57.3	8.4	1.4	8.1	1.1	6.5	1.3	4.1	0.6	4.0	0.8	11.4	38.2	342.3	4.9
GS-2	1–2 m	132.7	120.2	28.5	101.8	16.8	2.8	13.5	1.9	10.4	2.2	5.9	0.8	4.9	0.8	10.6	54.6	508.3	5.1
GS-3	2–3 m	186.8	92.2	36.7	126.2	22.0	4.3	22.5	3.6	20.1	3.8	10.5	1.4	8.3	1.2	11.1	108.4	659.3	3.1
GS-4	3–4 m	163.9	105.2	31.5	108.6	19.4	3.8	21.6	3.6	20.6	4.0	10.5	1.4	8.0	1.1	10.0	113.5	626.4	2.8
GS-5	4–5 m	136.1	82.8	26.3	89.5	16.9	3.4	18.8	3.2	19.2	3.6	9.6	1.3	7.6	1.0	10.0	105.7	535.0	2.5
GS-6	5–6 m	124.6	120.9	24.5	85.4	15.6	3.1	18.3	3.4	19.6	3.7	9.9	1.3	7.2	1.1	10.5	107.3	556.1	2.6
GS-7	6–7 m	68.4	102.1	16.0	57.5	9.6	1.7	9.0	1.3	8.0	1.6	4.6	0. 7	4.4	0.7	10.6	43.3	339.3	4.1
GS-8	7–8 m	66.3	99.4	15. 5	54.8	8.9	1.6	8.5	1.2	7.2	1.5	4.4	0.6	4.2	0.6	10.1	37.5	322.2	4.5
GS-9	8–9 m	23.7	49.2	5.2	20.3	3.5	0.6	4.3	0.8	5.8	1.4	5.2	0.8	5.4	0.9	12.8	39.7	179.0	1.8

Note: “L/H” represents the ratio of the total light rare earth element (LREE) contents to the total heavy rare earth element, including element Y (HREE) contents.

and Figure 4, Figure 5 and Figure 6, respectively.

4.1. Major Elements

For the Kuli Formation, the average sum of SiO₂, Al₂O₃, and TFe₂O₃ contents is close to 90%. These three elements constitute the main components of the weathering crust. SiO₂ has the highest content, ranging from 57.92% to 70.54%. The Al₂O₃ content varies from 14.41% to 21.04% and TFe₂O₃ ranges from 3.05% to 6.25%. The MgO and TiO₂ contents are relatively low, with ranges of 0.53%–0.88% and 0.43%–0.68%, respectively. The K₂O, Na₂O, and CaO contents in the weathering crust are also low. Specifically, K₂O ranges from 2.73% to 4.44%, Na₂O from 0.08% to 3.57%, and CaO from 0.04% to 0.46%. Their contents generally decrease from bedrock to hilltop soil. This indicates the rapid and extensive decomposition of plagioclase, as well as the leaching of Na and Ca during the weathering process. The MnO content ranges from 0.04% to 0.12%, and P₂O₅ from 0.02% to 0.08%. The loss on ignition (LOI) increases gradually from 2.53% (bedrock) to 10.21% (hilltop soil), with an average of 5.79%. This increase reflects two key trends. First, secondary minerals containing water of crystallization and volatiles (CO₂, SO₂, etc.) accumulate across various weathered horizons as the weathering intensifies. Second, plant roots and humus become enriched within the hilltop soil of the upper weathering crust. The Chemical Index of Alteration (CIA) values range from 66% to 88%, with an average of 77%. They generally increase from bedrock to hilltop soil, showing a high consistency with the LOI trend. Fedo and Nesbitt [70] proposed that CIA values of 50–60 represent incipient chemical weathering, 60–80 represent intermediate chemical weathering, and values above 80 correspond to extreme chemical weathering. Thus, the Kuli Formation weathering crust at the Getengzui REE deposit has entered the intermediate to extreme stage of weathering, with the middle-upper part of the completely weathered horizon basically being in the extreme stage (Table 1 and Figure 4).

For the Shenshan Formation, SiO₂, Al₂O₃, and TFe₂O₃ are also the main components, with their average sum being similarly close to 90%. K₂O, Na₂O, CaO, MgO, and P₂O₅ have low contents and exhibit highly consistent variation trends. These elements remain stable from hilltop soil to the semi-weathered horizon, then increase sharply at the bedrock contact. LOI and CIA values show a highly consistent trend: they change slightly from hilltop soil to the semi-weathered horizon, then decrease abruptly at the bedrock contact. From hilltop soil to the semi-weathered horizon, all CIA values for the Shenshan Formation profile exceed 80%, indicating that the weathering crust has entered the extreme stage of weathering (Table 1 and Figure 4).

In summary, the major element compositions of the Kuli and Shenshan Formations are similar, suggesting that they may share a common material source. The overall weathering degree of the Shenshan Formation is slightly higher than that of the Kuli Formation.

4.2. Trace Elements

Relative to the primitive mantle, almost all samples from both profiles show enrichment in various trace elements, with the primitive mantle-normalized ratio of >1. This indicates an overall background trend of trace element enrichment in the Getengzui REE mining area. Most samples, especially those from the completely weathered horizon and hilltop soil, exhibit primitive mantle-normalized ratios of over or approaching 100 for trace elements such as Rb, Ba, Th, U, La, Ce, and Nd. This reflects a highly enriched geochemical background. The maximum contents (in ppm) of these elements in the Kuli profile reach 183.7 (Rb), 1641 (Ba), 24.97 (Th), 5.31 (U), 4.44 (La), 183.5 (Ce), 194.4 (Nd), and 127.6 (Sm). In the Shenshan profile, the maximum contents (in ppm) are 312.1 (Rb), 1137 (Ba), 22.01 (Th), 4.47 (U), 6.21 (La), 186.8 (Ce), 122.1 (Nd), and 126.2 (Sm) (Table 2 and Figure 5).

Comparing the two profiles, their bedrock trace elements show highly consistent distribution patterns. For example, both are relatively depleted in Ba, Nb, Ta, Sr, P, and Ti, and relatively enriched in Th, Zr, Hf, and REE. The Kuli Formation is mainly composed of metatuff and metasandstone, whereas the Shenshan Formation is dominated by phyllite. Despite these lithological differences, this finding strongly confirms that the two formations share a common original material source. Moreover, they have undergone similar geochemical differentiation processes and were likely formed in the context of a unified regional tectono-magmatic event.

For both the Kuli and Shenshan profiles, the primitive mantle-normalized trace element diagrams display parallel curve shapes across successive layers: from bedrock, through semi-weathered horizon, to completely weathered horizon, to hilltop soil. Particularly, high field strength elements (Nb, Ta, Zr, Hf) and REEs (La, Ce, Nd, Sm, Tb, Y, Yb, Lu) show highly consistent trends. This consistency is observed between each weathered horizon and the underlying bedrock. This preliminarily indicates a high degree of inheritance of trace element distribution patterns in the weathering crust from those of the bedrock. Although the weathering process has strongly modified the mineral composition and chemical makeup of the rocks, it has not introduced significant exogenous material or completely disrupted the original element distribution relationships. Thus, the weathering crusts of the Kuli and Shenshan formations are primarily formed through an “in situ weathering-inheritance enrichment process”, rather than by distal hydrothermal superimposition.

4.3. Rare Earth Elements

4.3.1. Vertical Fractionation Characteristics

Overall, the total rare earth element (ΣREE) contents of samples from both the Kuli and Shenshan profiles show a gradual increase from bedrock upward through the completely weathered horizons (Table 3 and Figure 7). The ΣREE contents reach their peaks in the middle-upper part of the completely weathered horizon. ΣREE contents for Kuli profile samples range from 273.36 ppm to 674.26 ppm, averaging 375.60 ppm. This average is higher than that of REE-mineralized granites in the Nanling region (289 ppm) [16]. The LREE/HREE ratio is >1, ranging from 3.65 to 6.81, indicating that LREEs are more enriched than HREEs. The bedrock sample has a ΣREE content of 350.28 ppm, which is 2.34 times the threshold content (150 ppm) considered necessary for REE mineralization from parent rocks in the Nanling region. However, it is lower than the ΣREE content in hilltop soil and middle-upper completely weathered horizon samples. The bedrock sample has an LREE/HREE ratio of 5.81, which is lower than that of samples from hilltop soil, the middle-lower completely weathered horizon, and the semi-weathered horizon.

For Shenshan profile, ΣREE content samples range from 179.35 ppm to 659.33 ppm, with an average of 446.07 ppm. The LREE/HREE ratio is also greater than 1 (1.78–5.11), indicating predominant LREE enrichment. The bedrock sample has a ΣREE content of 179.35 ppm, which is significantly lower than that in hilltop soil, completely weathered horizon, and semi-weathered horizon samples. Its LREE/HREE ratio is 1.78, which is also significantly lower than that of samples from other layers. Comparing the two profiles, both exhibit high REE contents and obvious LREE enrichment. Due to the significant difference in the LREE/HREE ratio of the bedrock between the two profiles, their weathering crusts also show distinct LREE/HREE ratios. This indicates that different parent rocks constrain the degree of REE enrichment, with the Kuli Formation weathering crust profile being more enriched in LREEs than the Shenshan Formation profile.

4.3.2. Distribution Patterns and Anomalies

The chondrite-normalized REE distribution patterns (Figure 6) clearly display the complete weathering–mineralization sequence of REEs in the Kuli and Shenshan Formation weathering crust profiles.

From bedrock to hilltop soil, the distribution curves of both profiles illustrate a high degree of morphological inheritance, indicating in situ inheritance of the ore-forming materials. Specifically, the curves of samples from various weathered horizons are roughly parallel to the curve of the underlying bedrock. As weathering intensifies, the slopes of the LREE segment (La-Sm) and HREE segment (Gd-Lu) of the Kuli Formation curve change slightly from bedrock to hilltop soil, remaining essentially stable.

For the Shenshan Formation, the slope of the LREE segment curve gradually steepens from bedrock to the completely weathered horizon, reaching its maximum in the completely weathered horizon, while the HREE segment curve slope increases slightly. Both profiles exhibit pronounced negative Eu anomalies (δEu < 1). The δEu value range for the Kuli profile is 0.55–0.70, and for the Shenshan profile is 0.50–0.59, with the lowest values appearing in the bedrock for both. Distinct negative Ce anomalies are observed in the upper part of the completely weathered horizon of both profiles, with the minimum values occurring in this specific layer. Additionally, weak negative Ce anomalies are detected in the bedrock of the Kuli profile and the semi-weathered horizon of the Shenshan profile. In contrast, no obvious Ce anomalies (δCe ≈ 1) are found in any other layers of the two weathering crust profiles.

Overall, the Kuli and Shenshan weathering crust profiles, from bedrock, the semi-weathered horizon, and the completely weathered horizon to hilltop soil, exhibit consistent “right-inclined” curves, characterized by LREE enrichment and relatively flat HREE patterns. The occurrences of negative Eu and negative Ce anomalies are essentially consistent between the two profiles.

5. Discussion

5.1. Weathering Intensity and Processes

The Chemical Index of Alteration (CIA) is widely used to quantify the weathering intensity, with higher values indicating more intense chemical weathering [71]. Geochemical indicators from this study clearly demonstrate that the Kuli Formation deposit has entered the intermediate–extreme stage of chemical weathering. Specifically, the middle-upper part of the completely weathered horizon is essentially in the extreme weathering stage, while the Shenshan Formation weathering crust is fully in the extreme weathering stage (Figure 8).

CIA values for the Kuli profile range from 66 to 88, with a distinct peak (averaging 83) in the middle-upper completely weathered horizon. For the Shenshan profile, CIA values vary from 69 to 88, peaking at an average of 86 in the completely weathered horizon. Such intense chemical weathering implies near-complete decomposition of primary silicate minerals, with secondary aluminous clay minerals (e.g., kaolinite) dominating the mineral assemblage. This is consistent with the interpretation that the Getengzui REE deposit area has progressed to the desilication–alitization stage of weathering [72].

This high weathering intensity is further corroborated by low values on the Base Leaching Index (BA). In both profiles, high CIA values correlate negatively with low BA values. BA values in the middle-upper completely weathered horizon and hilltop soil of the Kuli profile, as well as in the semi-weathered horizon, completely weathered horizon, and hilltop soil of the Shenshan profile, are consistently below 0.3 (Figure 8). This reflects a warm and humid subtropical–tropical paleoclimate [73] and signifies severe, nearly complete leaching of mobile base cations (Ca²⁺, Na⁺, K⁺, Mg²⁺). This leading process lowers the pore water pH significantly, gradually generating an acidic geochemical environment that lays the foundation for REE mobilization and migration from their primary host minerals.

Weathering pathways are clearly delineated in the A-CN-K ternary diagrams (Figure 9). Based on the sequence of element mobility, Nesbitt et al. [69] divided the chemical weathering process into three stages: an incipient Na, Ca-removal stage, an intermediate K-removal stage, and an extreme Si-removal stage. Li et al. [74] noted that Ca mainly hosted in easily weathered minerals such as plagioclase and ferromagnesian minerals (e.g., pyroxene), undergoing intense leaching in the incipient stage of chemical weathering. Sodium resides in feldspars (mainly plagioclase) and micas, and is leached as these minerals decompose during weathering.

All samples from the two profiles show a clear evolutionary trend from the inferred composition of unweathered metamorphic rocks toward the A apex (alumina-rich endmember) in the A-CN-K diagrams. For the Kuli profile, samples collected from the middle-lower completely weathered horizon and semi-weathered horizon exhibit a weathering trend that is parallel to the A-CN side. This observed trend is indicative of an incipient weathering stage, which is primarily dominated by the dissolution of plagioclase. Importantly, this incipient weathering process is accompanied by the preferential leaching and removal of Ca and Na ions. In contrast, Kuli upper completely weathered horizon and hilltop soil samples, as well as all Shenshan profile samples, exhibit a trend aligned with or close to the A-K side. This differs distinctly from the Kuli middle-lower horizon trend. Furthermore, this trend approaches the A apex, reflecting a more advanced weathering state.

This indicates near-complete plagioclase weathering, intense depletion of Ca and Na and relative enrichment of Al, marking the subsequent decomposition of K-feldspar and intense desilication–alitization, as observed in Table 1 and Figure 4. The weathering products are dominated by kaolinite and gibbsite, confirming a high degree of weathering. Notably, Kuli samples cluster along a trajectory reflecting more efficient removal of plagioclase-derived components compared to Shenshan samples. This divergence highlights that parent rock mineralogical composition exerts a fundamental control for the efficiency of clay mineral neoformation and specific weathering pathway, thereby regulating REE enrichment processes.

5.2. Element Mobility and Mass Balance

Primitive mantle-normalized trace element spider diagrams (Figure 5) show that Zr normalized values are highly stable and enriched, consistent with its immobility during chemical weathering. Thus, Zr was selected as the immobile reference element for mass balance calculations [77], quantitatively revealing significant differences in element behavior during weathering at the Getengzui deposit (Figure 8). In the middle-upper completely weathered horizon and hilltop soil of the Kuli profile, base cations are intensely leached. τ values for CaO and Na₂O approach −1, indicating their near-complete removal via plagioclase dissolution.

This is followed by depletion of K₂O, MgO, and SiO₂ (τ < 0, some approaching −0.2), reflecting the progressive breakdown of K-feldspar and ferromagnesian minerals. In contrast, Al₂O₃, Fe₂O₃, and MnO are conservatively enriched (τ > 0) as residual and secondary phases, marking the transition to a clay- and oxide-dominated weathering crust. TiO₂ retains τ values close to 0, confirming its inert behavior during weathering. P₂O₅ is severely depleted throughout the weathering crust (τ ≈ −0.7), indicating the complete dissolution of apatite, identified as the primary source of mobilized REE ions. τ values for ΣREE, ΣLREE, and ΣHREE exhibit consistent trends, indicating coupled migration of LREEs and HREEs. Meanwhile, the τ(ΣREE) values in the upper completely weathered horizon remain above 0.15, peaking at 0.78, confirming true secondary enrichment, rather than passive residual concentration.

In the Shenshan profile (the semi-weathered horizon, completely weathered horizon, and hilltop soil), base cations are also intensely leached. τ values for Na₂O approach −1, and for CaO, they are consistently around −0.8, indicating near-complete removal via plagioclase dissolution. K₂O and MgO are severely depleted, with τ values between −0.3 and −0.65, reflecting the progressive breakdown of K-feldspar and ferromagnesian minerals. SiO₂ remains relatively stable, with τ values around 0. It suggests a dynamic equilibrium: silicon released by primary silicate decomposition (as soluble silicic acid) is partially consumed by in situ or short-distance formation neoformation of secondary clay minerals (e.g., kaolinite).

This is a typical feature of the extreme siallitization stage, indicating a high degree of chemical maturity in the weathering system. τ values for Al₂O₃, Fe₂O₃, MnO, TiO₂, and P₂O₅ are consistent with those of the Kuli profile. τ values for ΣREE, ΣLREE, and ΣHREE show coupled trends, with τ(ΣREE) exceeding 0.7 throughout the completely weathered horizon and peaking at 2.43, indicating intense secondary enrichment.

Combined with CIA and BA data, the geochemical mobility sequences of elements are established for both profiles. The Kuli Formation is Na₂O > CaO > P₂O₅ > MgO > SiO₂ > K₂O > REE > TiO₂ > Fe₂O₃ > Al₂O₃ > MnO, while the Shenshan Formation is Na₂O > P₂O₅ > CaO > K₂O > MgO > Al₂O₃ > SiO₂ > Fe₂O₃ > TiO₂ > MnO > REE.

5.3. Mechanisms and Controlling Factors of REE Enrichment

Highly consistent REE distribution patterns between the Kuli and Shenshan weathering crusts and their respective underlying bedrock (Figure 6) provide definitive evidence for dominant source inheritance. Additionally, primitive mantle-normalized trace element spider diagrams (Figure 5) illustrate congruent patterns between weathering crusts and bedrocks, including consistent negative anomalies for Ba, Nb, Ta, Sr, P, and Ti and positive anomalies for Th, Zr, Hf, and REE. These lines of evidence collectively rule out significant contributions from exogenous materials to REE enrichment.

Intense chemical weathering is the primary driver of REE enrichment, as evidenced by the positive correlation between CIA and ΣREE (Figure 8). This correlation encapsulates a core geochemical chain: intense leaching of base cations (τ(Ca, Na) ≈ −1) lowers the percolating fluid pH, promoting dissolution of REE-bearing accessory minerals (e.g., apatite, τ(P₂O₅) ≈ −0.8). Concurrently, advanced argillic alteration (CIA > 80) generated abundant secondary clay minerals, creating conditions for REE adsorption.

Redox-sensitive processes are highlighted by the coupled behavior of Mn and Ce (Figure 8). In the middle-upper completely weathered horizon of the Kuli profile and the semi-weathered horizon, the completely weathered horizon, and hilltop soil of the Shenshan profile, the enrichment of Mn (τ > 0.2) coincides with negative Ce anomalies (δCe < 1). This jointly indicates an oxidizing environment in these horizons. Under oxidizing conditions, Ce³⁺ is oxidized to Ce⁴⁺, which is more prone to hydrolysis than Ce³⁺ [78]. The hydrolysis products of Ce⁴⁺ form hydrogen bonds with anion groups on the surfaces of clay minerals (e.g., kaolinite). These products are preferentially adsorbed or coprecipitated by newly formed Fe-Mn oxide colloids. This process leads to relative Ce depletion in REE-enriched horizons, due to leaching.

In the middle-lower completely weathered horizon and semi-weathered horizon of the Kuli profile and the semi-weathered horizon of the Shenshan profile, strong Mn enrichment (τ(MnO₂) > 1) is coupled with moderate REE enrichment. This indicates early, selective capture of REEs by reactive Fe-Mn oxide colloids at the redox front, though limited adsorption capacity constrains the total REE enrichment.

In the completely weathered horizon, peak weathering intensity (CIA > 80) generates abundant clay minerals, providing orders of magnitude more adsorption sites and driving large-scale ion-adsorption REE enrichment (τ(REE) peaks). This mechanism confirms that REE mineralization at Getengzui is controlled by sequential geochemical barriers: an upper oxidation–precipitation barrier (dominant for Ce fixation) and a lower clay adsorption barrier (dominant for large-scale REE enrichment). Collectively, they ensure efficient interception and enrichment of REEs within the weathering profile.

In summary, Getengzui REE deposit formation is attributed to in situ weathering and ion-adsorption processes. Intense base cation leaching generates an acidic, silica-undersaturated fluid environment. This environment facilitates REE release from primary minerals. Meanwhile, secondary clay mineral formation provides abundant adsorption sites, effectively capturing and enriching mobilized REEs from percolating solutions.

5.4. Contrast Between the Two Profiles and Implications for Metallogeny

A comparison of the Shenshan (phyllite) and Kuli (metatuff) reveals that, despite identical warm and humid paleoclimatic conditions, parent rock lithology exerts a first-order control on the REE enrichment efficiency, scale, and fractionation characteristics.

Differences in the material basis and enrichment scale are evident. The Kuli parent rock has an average initial ΣREE content (350.28 ppm) that is approximately twice that of the Shenshan parent rock (179.35 ppm), providing superior source conditions. Correspondingly, the Kuli weathering crust exhibits a thicker REE-enriched layer. The thickness of its mineralization zone (with ΣREE > 280 ppm) is about 1.6 times that of Shenshan. A distinct REE peak also occurs in the middle-upper part of the completely weathered horizon, where ΣREE reaches 674.26 ppm in the upper completely weathered horizon. In contrast, the overall thickness of the Shenshan weathering crust is slightly smaller. However, its average REE content in the completely weathered horizon (577 ppm) is significantly higher than that in the corresponding horizon of the Kuli Formation (400.67 ppm) (Table 3). This indicates a stronger enrichment intensity at Shenshan, which may be attributed to more thorough weathering (reflected by overall higher CIA values) and more efficient clay adsorption.

Regarding REE distribution patterns (Figure 6), both weathering profiles show LREE enrichment but to different degrees. This fractionation feature directly inherits the composition of their parent rocks. The parent rock of Kuli has a higher initial ΣLREE/ΣHREE ratio (4.58), resulting in stronger LREE enrichment throughout its weathering profile (average ΣLREE/ΣHREE = 5.83). The parent rock of Shenshan shows weaker LREE-HREE fractionation (ΣLREE/ΣHREE = 1.78), and its weathering crust accordingly maintains a relatively gentle distribution pattern (average ΣLREE/ΣHREE = 3.68). These patterns suggest that the Kuli Formation trends to form LREE-dominant orebodies, while the Shenshan Formation may yield REE products with a more balanced composition.

These differences originate from fundamental variations in parent-rock mineralogy and weathering dynamics. Therefore, the parent-rock lithology acts not merely as a passive material source but as a “genetic template” that predetermines the scale, grade, and elemental assemblage of the weathering-crust-type REE deposits.

In summary, the parent rock of the Kuli profile favors the formation of thick, large-scale orebodies with a clear LREE advantage. In comparison, the parent rock of the Shenshan profile may form higher-grade orebodies with relatively weaker fractionation. Both have important exploration value, but their mineralization “fingerprints” are distinct.

6. Conclusions

This study was conducted at the Getengzui ion-adsorption REE deposit in South China. It involved systematic geochemical analysis of weathering crust profiles from the Qingbaikouan Shenshan (phyllite) and Kuli (metatuff) Formations. Through this analysis, the study clarified REE geochemical behaviors and enrichment constraints in shallow metamorphic rock weathering crusts.

The shallow metamorphic rock weathering crust at Getengzui has undergone significant intense chemical weathering, reaching the intermediate–extreme stage through desilication and alitization. The average CIA value is 83 for the middle-upper completely weathered horizon of the Kuli Formation and 86 for the Shenshan Formation’s completely weathered horizon. BA values are generally below 0.3. Such intense weathering has led to the nearly complete decomposition of primary silicate minerals, particularly plagioclase. It has also resulted in extensive leaching of base cations, including Ca, Na, and K. These processes collectively generate an acidic pore water environment that is critical for the mobilization and migration of rare earth elements (REE) from their primary host minerals.

Significant in situ secondary REE enrichment occurs in the weathering crust. Mass transfer coefficient (τ) calculations confirm that ΣREE values in the middle-upper completely weathered horizon, with peak values of 0.78 (Kuli) and 2.43 (Shenshan), indicate true mobilization–adsorption enrichment, rather than a passive residual concentration. Severe P₂O₅ depletion (τ ≈ −0.8) identifies apatite dissolution as the primary source of REE ions. The geochemical mobility sequences of elements differ slightly between the two profiles. The Kuli Formation is Na₂O > CaO > P₂O₅ > MgO > SiO₂ > K₂O > REE > TiO₂ > Fe₂O₃ > Al₂O₃ > MnO, while the Shenshan Formation is Na₂O > P₂O₅ > CaO > K₂O > MgO > Al₂O₃ > SiO₂ > Fe₂O₃ > TiO₂ > MnO > REE.

REE enrichment is controlled by multi-stage geochemical barriers. An oxidation barrier characterized by Mn enrichment and pronounced negative Ce anomalies is distributed in the middle-upper completely weathered horizon, preferentially fixing Ce⁴⁺ and possibly capturing some early-migrating REEs. A clay adsorption barrier marked by the abundant secondary clay minerals such as kaolinite dominates large-scale REE enrichment in the completely weathered horizon. A-CN-K ternary diagrams illustrate the weathering trends of the Kuli and Shenshan Formations. The Kuli Formation displays a weathering path that is parallel to the A-CN edge (plagioclase-dominated) in the middle-lower completely weathered and semi-weathered horizons, whereas the Shenshan Formation follows a trend along the A-K edge (K-feldspar-dominated). These distinct patterns reflect the differences in weathering processes controlled by their parent-rock compositions.

Parent rock lithology is the primary factor controlling REE enrichment efficiency, scale, and fractionation. The Kuli Formation (metatuff) has a higher initial REE content (~350 ppm vs. Shenshan ~179 ppm), hosting a thicker mineralization layer and inheriting stronger LREE enrichment (average LREE/HREE = 5.83). The Shenshan Formation (phyllite) undergoes more thorough weathering, with a higher average REE content in the completely weathered horizon (577 ppm vs. Kuli 401 ppm) but weaker LREE-HREE fractionation (average LREE/HREE = 3.68). Metatuffs are thus conducive to larger-scale LREE-dominated orebodies, while phyllites favor higher-grade orebodies with more balanced REE distributions.

Ion-adsorption REE mineralization in shallow metamorphic terrains is governed by four coupled core controlling factors. Shallow metamorphic rocks with relatively high initial REE contents (suggested > 200 ppm), particularly volcanoclastic-rich metatuffs, provide an indispensable material prerequisite. Intense, sustained chemical weathering (CIA > 80) driven by a warm and humid paleoclimate generates acidic pore water, serving as the dynamic engine for mineralization. Abundant secondary clay minerals (e.g., kaolinite) provide efficient adsorption sites, acting as the final hosts for in situ REE enrichment. Parent-rock lithology and mineralogy act as a “genetic template”, directly determining the deposit scale (thick/large vs. high-grade) and resource type (LREE-dominant vs. balanced REE distribution type).