Petrographic, Geochemical, and Geochronological Characteristics of the Granite in Yunnan and Its Constraints on Ion-Adsorption Rare Earth Element Mineralization

Abstract

The TuguanZhai rare earth deposit in Tengchong, along with the Longan and Yingpanshan deposits in Longchuan, is a significant ion-adsorption type rare earth (iREE) deposit in Yunnan, China. Previous studies mainly focused on the geochemistry of residual regolith or the migration and enrichment mechanism of rare earth elements (REEs), but lacked systematic analysis of the protoliths. To constrain this deposit and its protolith rock, called Tuguanzhai granite, we systematically integrate petrology features, petrogeochemistry, zircon U-Pb date, and artificial heavy mineral separation (AHMS). Specifically, iREE-host granites include two main periods in this area: the Early Cretaceous (112.13 ± 0.75 Ma) and the Paleocene–Eocene (52.78 ± 0.28 Ma, 48.56 ± 0.19 Ma). The former includes three types of biotite monzogranite with different grain sizes, and the latter is mainly medium-grained biotite monzogranite with local mylonitization. Geochemical features show that these granites generally share high alkalinity compositions (w(K₂O + Na₂O): 7.15 to 12.75 wt%) and potassium contents (w(K₂O): 3.89 to 8.36 wt%). The mineralized granites exhibit significantly higher concentrations of the total REEs than non-mineralized granites, along with a strong enrichment of light REEs. Moreover, the results of AHMS reveal that the REE contents of apatite, allanite, and titanite in mineralized granites are 4.98, 1.29, and 1.90 times more abundant than in non-mineralized granites, respectively. Due to REEs being released from these REE-rich minerals in humid environments, there exists significant potential for iREE formation and exploration in the Early Cretaceous granites in western Yunnan. We innovatively propose the “abundance of easily leachable minerals” as a key indicator for iREE mineralization and exploration, having found it to be better than the traditional total REE contents.

1. Introduction

Ion-adsorption type rare earth (iREE) deposits constitute a significant portion of global rare earth resources and possess considerable economic value, particularly in China, Vietnam, India, etc. [1,2]. This type of deposit is mainly formed by the weathering of granite, rhyolite, and other acidic rocks [3]. Through the weathering process, iREEs commonly accumulate and mineralize at the surface, and thus the migration and enrichment mechanism remains a hot topic in the research of rare earth mineralization [4,5].

The formation of iREE deposits is controlled by both endogenic and exogenic processes. Recent studies have made significant advances in understanding the ages, geochemical characteristics, and weathering processes of REE-bearing protoliths [6]. These plutons can be dated to the Mesozoic, particularly the Jurassic and Cretaceous periods [1]. Geochemical compositions indicate that they are typically enriched in LREEs and exhibit elevated concentrations of alkalies (7~13 wt%), aluminum (11.5~18.6 wt%), and REEs (289 × 10⁻⁶), which can serve as key indicators for understanding their genesis [5]. The mineralizing granites are generally S-type granites derived from crustal anatexis, and partial melting and strong fractional crystallization are key factors controlling the REE concentrations and geochemical behavior in the magma. During the exogenic stage, the migration-enrichment and differentiation mechanisms of REEs are influenced by factors such as pH, Eh, weathering intensity, the contents and properties of clay minerals, and the permeability of the weathered profile [1]. In the early stages, dissociative REE minerals are weathered and broken down, releasing REE³⁺ ions that are subsequently adsorbed by clay minerals. HREE³⁺ ions exhibit stronger mobility than LREE³⁺, leading to the relative enrichment of HREEs in the middle and lower parts of the weathering crust and resulting in LREE-HREE fractionation [7]. These characteristics provide valuable insights into the formation mechanisms of iREEs [8,9].

The iREE deposits in Yunnan are primarily located in the Longchuan-Tengchong, Lincang-Menghai, Jianshui-Gejiu-Jinping, and Yuanmou-Mouding regions [10,11]. Notably, the Tuguanzhai rare earth deposit in Tengchong, along with the Longan and Yingpanshan deposits in Longchuan, constitutes a significant rare earth mineralization area in Yunnan, China [12]. Previous studies have investigated the compositions of the weathering crust and the migration patterns of REEs in the Tuguanahai deposit [7,13]. REEs in the weathering crust are further enriched based on the geochemical inheritance from the protolith. During chemical weathering, Ce³⁺ is readily oxidized to insoluble Ce⁴⁺, resulting in a complex enrichment pattern that deviates from the behavior of other trivalent REEs. However, research on the geochemical compositions and age of host rocks remains limited [14,15]. Based on the analyses of petrology, mineralogy, geochemistry, and geochronology from Tuguanzhai granite, this study aims to address the following questions: (1) to investigate the evolution (ages and compositions) of the Tuguanzhai granite and its influence on iREE mineralization; (2) to identify the dominant REE-rich minerals and their constraints on iREE mineralization.

2. Geological Background

2.1. Regional Geology

The Sanjiang Orogenic Belt, located along the eastern margin of the Tethyan tectonic domain (Figure 1a) [16,17], comprises the Indochina, Sibumasu (which includes the Baoshan and Tengchong blocks), and West Myanmar terranes [18,19]. This region has experienced a multistage tectonic evolution since the Early Paleozoic, involving oceanic subduction, arc-continent collision, and intracontinental orogenesis [20,21].

The Tengchong terrane is bordered to the west by the Myitkyina suture zone, which is adjacent to the West Myanmar terrane, and to the east by the Nujiang-Ruili fault zone, which separates it from the Baoshan block (Figure 1a). Within this terrane, iREE deposits, including Tuguanzhai, Longan, and Yingpanshan, have been identified [22]. Regional structures are characterized by ductile-brittle faults with a predominant northeast strike. Strata from the Proterozoic, Permian, and Neogene periods are primarily exposed, while strata from other geological periods are preserved only fragmentarily or discontinuously [18,21]. As the crystalline basement of the Tengchong Block, the Proterozoic Gaoligong Group comprises biotite–plagioclase gneiss, gneiss (protolith: granite), mica schist, and quartz schist, all of which have undergone metamorphism ranging from greenschist to amphibolite facies [21]. The Permian Bangdu Formation (P₁b) is characterized by metamorphosed quartz siltstone, silty slate, and sericite slate. Neogene sediments, representing deposits from continental rift lake basins, include the Nanlin (N₁n) and Mangbang (N₂m) formations, which are composed of conglomerate, quartz sandstone, mudstone, and thin coal seams. Quaternary volcanic rocks, found in the Lianghe-Jiucheng area, belong to the Tengchong volcanic suite. Granitic intrusions from the Triassic, Cretaceous, and Paleogene periods are widespread: Triassic granites are typically light gray, medium- to fine-grained two-mica granite; Early Cretaceous granites exhibit compositional diversity (e.g., monzogranite, granodiorite) [23]; Paleogene granites, predominantly medium- to coarse-grained biotite monzogranite, serve as the protolith for iREE mineralization [24].

2.2. Geological Characteristics of the Deposit

In the Tuguanzhai region, Early Cretaceous and Paleogene–Eocene granitic rocks are extensively exposed (Figure 1b). The granites are divided into two sections by the Jiaxiangshi Fault (F10). The western section primarily consists of Paleogene medium-grained biotite monzogranite, which is identified as the primary protolith for iREE mineralization and exhibits localized mylonitization. The eastern section comprises Early Cretaceous biotite monzogranite, including medium- to fine-grained biotite monzogranite (K₁ηγ^a), porphyritic medium-grained biotite monzogranite (K₁ηγ^b), and coarse-grained biotite monzogranite (K₁ηγ^c), with localized REE enrichment (Figure 2).

The Tuguanzhai iREE deposit is located between the Quaternary basin of Xinhua Township and the Paleogene basin of Puchuan in Tengchong City [13]. Topographically, this area features a central uplift surrounded by lower elevations and has experienced a warm, humid climate, which can intense chemical weathering [7]. As a result, a widespread weathering crust and iREE mineralization have developed within the weathered profiles of Paleogene–Eocene granites, while Early Cretaceous granites exhibit localized mineralization. Based on the intensity of weathering, the profile transitions from the surface to the bedrock as follows: humus soil layer (A), sub-clay layer (B), completely weathered layer (C), strongly weathered layer (D), and semi-weathered layer (E), with gradual transitions between these layers. The completely and strongly weathered layers serve as the main iREE-bearing horizons.

The ore body extends approximately 10 km in a north–south direction (Figure 1b) and from 1 to 3 km in an east–west orientation, as delineated by 43 drill holes. The mineralized horizons are primarily hosted within Paleogene–Eocene mylonitized granite, medium-grained granite, and weathered crusts derived from Early Cretaceous granites [7,13]. Based on exploration density and weathering intensity, the deposit is divided into two orebodies: Orebody II1 (2.61 km²) and Orebody II2 (>9.29 km²). Orebody II1 varies in thickness from 2.00 to 21.40 m, with a mean thickness of 11.82 m, and grades ranging from 0.081 to 0.120%, averaging 0.106%. The iREE leaching rate in composite samples ranges from 39.16 to 55.48%, with a mean of 49.64%. Orebody II2 varies in thickness from 0.50 to 31.60 m, with an average thickness of 7.24 m, and grades between 0.080% and 0.218%, averaging 0.094%. The leaching rate of ion-adsorption REE-bearing minerals ranges from 20.42% to 54.49%, with an average of 37.95%.

3. Sample Collection and Testing Methods

3.1. Sample Collection

In this study, two granite samples were selected for U–Pb zircon dating (Figure 1b): (1) D2133-0-1, a medium-grained biotite granite from the Early Cretaceous period; and (2) D2652-TW1, a fractured medium-grained biotite granite from the Paleogene period.

Nineteen granite samples were collected for whole-rock geochemical analysis, including nine non-mineralized granite samples and ten mineralized granite samples enriched in iREE. All granite samples were unaltered and derived from both Cretaceous and Paleogene granites (Figure 1b).

Seventeen samples were collected for the heavy mineral analysis (Figure 1b), including three non-mineralized biotite monzogranite samples, two weathered crust samples (drill core) above non-mineralized granites, seven mineralized granite samples, and five weathered crust samples (drill core) above mineralized granites. All mineralized and non-mineralized granite samples were identified based on field observations and subsequent confirmations.

3.2. Analytical Methods

Samples D2133-0-1 and D2652-TW1 were crushed, and zircons were separated from the minerals using a magnet in warm water. The selected zircons were manually optimized under a binocular microscope. We chose well-formed, larger zircons that exhibited no obvious fractures or inclusions. These zircons were then mounted for cathodoluminescence (CL) imaging to observe their detailed internal structures and zoning.

U-Pb zircon dating was performed using a 193 nm ArF excimer laser (Photon-Machines, Bozeman, MT, USA) coupled with an inductively coupled plasma mass spectrometer (ICP-MS) (Agilent 7900, Agilent Technologies, Santa Clara, CA, USA) at Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. The laser spot diameter was 24 μm, with an ablation depth ranging from 20 to 40 μm. Each analysis began with a background signal lasting from 20 to 30 s, followed by a 50 s ablation signal. Other analytical procedures and data processing followed the protocols described in the previous studies [25,26].

Major element analysis of whole-rock samples was conducted at Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. X-ray fluorescence (XRF) analysis was performed using a Rigaku ZSX Primus II wavelength-dispersive spectrometer (Rigaku Corporation, Tokyo, Japan). The X-ray tube employed was a 4.0 kW end-window Rh target, operated at 50 kV and 60 mA. Calibration was carried out using national standard reference materials, and data correction was applied using the theoretical α-coefficient method. The relative standard deviation (RSD) was less than 2%. Loss on ignition (LOI) was determined by a gravimetric method.

Trace element analysis was performed using an Agilent 7700e ICP-MS (Agilent Technologies, Santa Clara, CA, USA) at Wuhan Sample Solution Analytical Technology Co., Ltd. The digestion procedure was as follows: (1) 200-mesh sample powder was dried at 105 °C for 12 h; (2) 50 mg powder was weighed into a Teflon bomb; (3) 1 mL HNO₃ and 1 mL HF were added; (4) the sealed bomb was placed in a stainless-steel jacket and heated at 190 °C for >24 h; (5) the residue was evaporated to at 140 °C; (6) 1 mL HNO₃, 1 mL Milli-Q water, and 1 mL internal standard solution (1 ppm In) were added, and the bomb resealed and heated resealed at 190 °C for >12 h; (7) the final solution was diluted to 100 g with 2% HNO₃. The analytical precision for most elements was better than ±5% [27].

Heavy mineral separation was conducted at Langfang Yantuo Geological Services Co., Ltd. (Langfang, China). The samples, ranging from 40 to 140 mesh, were processed by shaking table concentration, followed by several fine-washing steps. The concentrate then underwent magnetic separation, leading to strongly and weakly magnetic fractions. The weakly magnetic fraction was further subdivided into strongly electromagnetic, weakly electromagnetic, and non-magnetic components. Each fraction was examined under a microscope, and mineral abundances were calculated based on modal percentages.

4. Results

4.1. Chronological Characteristics

Zircon U–Pb data of the medium-grained biotite granite were collected (sample D2133-0-1, K₁ηγ^a in Figure 1b). These zircons are subhedral to euhedral, typically short-prismatic with few long-prismatic grains, from 50 to 280 μm in length, and length/width ratios of from 1:1 to 3.5:1. Cathodoluminescence (CL) images show typical oscillatory magmatic zoning (Figure 3a,b). U concentrations range from 92 × 10⁻⁶ to 1182 × 10⁻⁶, Th from 69 × 10⁻⁶ to 896 × 10⁻⁶, and Th/U ratios from 0.61 to 2.37 (Table S1). All the above features indicate that the zircons belong to a magmatic origin [28,29]. Twenty-two analytical spots on zircon rims form a tight cluster on concordia and yield a weighted mean ²⁰⁶Pb/²³⁸U age of 112.13 ± 0.75 Ma (n = 22, MSWD = 0.103) (Figure 3a), interpreted as the crystallization age of the biotite granite, which belongs to the early Cretaceous. The tight cluster along the concordia line indicates a closed system for these zircons since formation and no significant Pb or U disturbance after post-crystallization [30].

Zircon U–Pb data of the medium-grained biotite granite were collected (sample D2652-TW1, Eηγ in Figure 1b). These zircons exhibit typically subhedral to euhedral, short-prismatic with few long-prismatic grains, from 60 to 230 μm in length, and length/width ratios of from 1:1 to 3:1. Cathodoluminescence (CL) images show clearly oscillatory magmatic zoning (Figure 4b). U concentrations range from 42 × 10⁻⁶ to 1089 × 10⁻⁶, Th from 58 × 10⁻⁶ to 1052 × 10⁻⁶, and Th/U ratios of from 0.61 to 2.37 (Table S1). All features reveal a typical magmatic zircon [28,29]. Twenty-five analytical spots on zircon rims form a tight cluster on concordia and yield a weighted mean ²⁰⁶Pb/²³⁸U age of 48.56 ± 0.19 Ma (n = 25, MSWD = 2.1) (Figure 4a), corresponding to the Paleogene–Eocene period. These characteristics indicate that the zircons have remained since formation, and the age can be interpreted as the crystallization age of the biotite granite [30].

4.2. Geochemical Compositions

The major element concentrations of the Tuguanzhai granite are listed in Table S2. The Early Cretaceous non-mineralized granite has SiO₂ contents from 70.90 to 77.61 wt%, K₂O from 4.99 to 5.84 wt%, Na₂O from 4.08 to 6.73 wt%, TFeO from 0.62 wt% to 2.47 wt%, and MgO from 0.14 wt% to 0.57 wt%. The Paleogene–Eocene mineralized granite has SiO₂ from 61.02 to 74.29 wt%, K₂O from 3.89 to 8.36 wt%, Na₂O from 2.22 to 4.40 wt%, TFeO from 1.05 to 4.62 wt%, and MgO from 0.27 wt% to 1.10 wt%. Although some mineralized samples exhibit relatively low silica and elevated iron and Mg (magnesium) contents due to deformation and alteration, they are still classified as granites based on field observations, petrographic analysis, and modal mineralogy. To better illustrate the geochemical variations, the granite compositions were plotted on a TAS (Total Alkali–Silica) diagram (Figure 5). Although this diagram is primarily designed for volcanic rocks, it is used here solely as an auxiliary geochemical tool to visualize trends in alkali and silica contents, especially for altered or mylonitized granites. In this diagram, the Early Cretaceous samples fall within the granite field, while most Paleogene–Eocene samples also fall in the granite field, with a few plotting in the quartz monzonite or syenite fields, likely due to secondary alteration or chemical modification during deformation. Formal classification of these plutonic rocks follows the IUGS QAP (Quartz–Alkali feldspar–Plagioclase) system, based on thin section identification. The geochemical variation trends for both granite suites are consistent: as SiO₂ increases, FeO, MgO, TiO₂, MnO, CaO, and P₂O₅ decrease (Figure 6), reflecting magmatic differentiation trends [31].

The Early Cretaceous non-mineralized granite shows a differentiation index (DI) ranging from 86.52 to 96.61, calculated based on the CIPW normative compositions as the sum of normative quartz, orthoclase, albite, and nepheline. This reflects the degree of magmatic differentiation. The CIPW normative corundum index ranges from 0.66 to 2.23, indicating a weakly peraluminous nature. K₂O + Na₂O contents range from 7.23 to 9.83 wt%, with K₂O/Na₂O ratios between 1.29 and 2.23. The alkalinity coefficient (K₂O + Na₂O)/Al₂O₃ ranges from 0.57 to 0.67, and the sodium coefficient Na₂O/(K₂O + Na₂O) varies from 0.31 to 0.44. The Fe₂O₃/(Fe₂O₃ + FeO) ratios, estimated based on total iron (TFeO) data from XRF, range from 0.52 to 0.78. The aluminum saturation index (A/CNK = molar Al₂O₃/(CaO + Na₂O + K₂O)) varies from 1.05 to 1.22, indicating a peraluminous composition. Magmatic differentiation is reflected in the decoupling behavior of elements: as the DI increases, SiO₂ and Na₂O increase, while FeO, MgO, CaO, and K₂O decrease. This trend likely reflects the fractional crystallization of mafic minerals (e.g., hornblende, biotite) and plagioclase, which preferentially remove Fe, Mg, Ca, and K from the melt. In the K₂O versus SiO₂ and Na₂O versus K₂O diagrams (Figure 7), most samples fall within the high-K calc-alkaline field, suggesting that the granites are peraluminous, high-K calc-alkaline in affinity [33,34].

The Paleogene–Eocene mineralized granite shows a differentiation index (DI) ranging from 79.46 to 93.41, calculated based on the sum of CIPW normative quartz, orthoclase, albite, and nepheline (DI = Q + Or + Ab + Ne), reflecting the degree of magmatic differentiation. The CIPW normative corundum index ranges from 1.67 to 4.62, suggesting a strongly peraluminous nature. K₂O + Na₂O contents vary from 7.15 to 12.75 wt%, and the K₂O/Na₂O ratio ranges from 1.08 to 2.93. The alkalinity coefficient ((K₂O + Na₂O)/Al₂O₃) ranges from 0.46 to 0.66, and the sodium coefficient Na₂O/(K₂O + Na₂O) from 0.25 to 0.48. The iron oxidation ratio Fe₂O₃/(Fe₂O₃ + FeO), estimated based on XRF total iron data (TFeO) and assumed Fe³⁺/Fe²⁺ proportions, varies between 0.59 and 0.86. The aluminum saturation index (A/CNK = molar Al₂O₃/(CaO + Na₂O + K₂O)) ranges from 1.10 to 1.40. Similar to the Early Cretaceous granite, magmatic differentiation in the Paleogene–Eocene granite is reflected in the systematic geochemical trends: as the DI increases, SiO₂ and Na₂O contents increase, whereas FeO, MgO, CaO, and K₂O decrease. These patterns can be attributed to the fractional crystallization of mafic minerals (e.g., hornblende, biotite) and plagioclase, which preferentially incorporate Fe, Mg, Ca, and K into the early-formed mineral phases, thereby depleting these elements in the residual melt. In the diagrams of K₂O versus SiO₂ and Na₂O versus K₂O (Figure 7), most samples plot within the high-K calc-alkaline series field, indicating that the mineralized granites are high-K, peraluminous, and calc-alkaline in affinity [33].

The Early Cretaceous non-mineralized granites have ΣREE contents of from 82.86 × 10⁻⁶ to 241.99 × 10⁻⁶ (average: 177.32 × 10⁻⁶) with LREE/HREE ratios of from 4.43 to 23.22 (average: 13.11), showing LREE enrichment and pronounced light-to-heavy REE fractionation. In contrast, Paleogene Eocene mineralized granites exhibit higher ΣREE (from 95.92 × 10⁻⁶ to 794.95 × 10⁻⁶; average: 528.81 × 10⁻⁶) and LREE/HREE ratios (from 5.18 to 52.66; average: 25.65). Both suites display moderate Eu anomalies (δEu: from 0.20 to 0.91 and from 0.32 to 0.99) and subtle Ce anomalies (δCe: from 1.00 to 1.05 and from 0.92 to 1.15), with δEu and δCe values inversely correlating with magma acidity (Figure 8a,c). Trace element patterns are characterized by Rb-Th enrichment and distinct depletion in Ba-Nb-Sr-Zr-Hf-Ti (Figure 8b,d)

4.3. Characteristics of REE-Bearing Minerals

Previous studies have shown that REEs in granitic rocks are primarily hosted in accessory minerals such as zircon, monazite, allanite, titanite, apatite, and, to a lesser extent, ilmenite and anatase, depending on the rock’s geochemical composition and crystallization conditions [35]. These minerals serve as the main reservoirs for REEs in both fresh and weathered granites, and their stability during weathering significantly influences the REE enrichment in the weathering crust.

The primary REE-bearing minerals in the Tuguanzhai granite include zircon, apatite, ilmenite, anatase, rutile, and titanite. The rare earth mineral content in both granite and weathered crust is shown in

Table 1. REE-bearing mineral content in Tuguanzhai granite.

Sample	Rock Name	Remarks	Zircon	Apatite	Ilmenite	Allanite	Epidote	Magnetite	Limonite	Sphene	Rutile	Others
			mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg	mg/kg
D4002 (B1–B4)	Medium–Fine-Grained Granite	Non-mineralized	0.97	minor	0.00	0.00	0.00	80.10	17.28	0.00	0.11	10.78
D4004 (B1–B3)	Medium-Grained Granite	Non-mineralized	124.86	3.24	0.00	353.08	39.54	1620.89	13.41	112.61	1.15	43.47
D4003 (B1–B3)	Medium–Coarse-Grained Granite	Non-mineralized	130.78	8.72	0.00	379.27	13.95	2216.02	46.79	64.52	4.36	41.85
D2615-0-1	Intensely Weathered Medium–Coarse-Grained Granite	Non-mineralized	254.16	minor	0.00	0.00	137.67	975.70	3.11	0.00	0.00	50.46
D2110-0-1	Intensely Weathered Medium–Fine-Grained Granite	Non-mineralized	160.51	0.00	0.00	0.00	6.21	17.91	543.43	2.66	8.51	69.08
D2518-0-3	Medium-Grained Granite	Mineralized	199.61	129.88	17.43	412.30	59.27	7239.19	35.74	296.37	0.39	162.13
D2133-0-(2-6)	Fragmented Medium-Grained Granite	Mineralized	256.83	minor	25.47	290.48	minor	1150.36	0.00	0.00	0.00	44.95
D8229-(B1–B2)	Fragmented Medium-Grained Granite	Mineralized	79.49	12.49	0.57	233.93	27.25	4768.36	116.97	152.74	0.00	285.03
HLZ (B1–B4)	Fragmented Medium-Grained Granite	Mineralized	97.65	2.76	34.08	583.10	343.59	7081.91	86.59	158.44	0.92	746.14
D2653-0-1	Mylonitized Medium-Grained Granite	Mineralized	75.12	0.35	3.53	526.91	328.00	2138.70	47.26	137.55	0.00	269.10
PM004-B2	Granitic Mylonite	Mineralized	31.04	3.52	73.94	6.82	21.14	1396.55	486.80	42.05	0.22	39.41
PM004-B1	Granitic Ultra-mylonite	Mineralized	1.42	minor	3.52	154.93	17.59	49.28	1.75	0.00	0.71	20.76
HCZ008-(H14/H16)	Semi-Weathered Granite	Mineralized	17.81	0.00	0.00	0.00	977.61	6025.94	674.14	35.61	3.24	322.91
TCZ-025-H6	Semi-Weathered Granite	Mineralized	56.79	0.00	55.93	0.00	5.83	1441.18	93.17	0.01	0.00	62.63
HCZ008-(H7/H9, H11/H13)	Intensely Weathered Granite	Mineralized	286.00	0.00	11.88	9.33	9.33	7654.92	193.50	0.00	4.25	316.55
TGZ-H1	Intensely Weathered Granite	Mineralized	229.85	0.00	149.95	0.00	12.04	10,080.35	187.16	0.00	2.19	264.87
HCZ008-(H3/H5)	Completely Weathered Granite	Mineralized	363.49	minor	0.00	0.00	11.55	9540.26	355.09	0.00	0.00	225.87

and Figure 9 [15]. In the Early Cretaceous non-mineralized granite, zircon content ranges from 0.97 to 130.78 mg/kg, with an average of 85.54 mg/kg. In the weathered crust, zircon content ranges from 160.51 to 254.16 mg/kg, with an average of 207.34 mg/kg. Anatase content ranges from 0.11 to 4.36 mg/kg, with an average of 1.87 mg/kg, and, in the weathered crust, it ranges from 0 to 8.51 mg/kg, with an average of 4.25 mg/kg. Ilmenite was not detected in the samples. In the Paleogene–Eocene mineralized granite, zircon content ranges from 1.42 to 256.83 mg/kg, with an average of 105.88 mg/kg. In the weathered crust, zircon content ranges from 17.81 to 363.49 mg/kg, with an average of 190.79 mg/kg. Anatase content ranges from 0 to 0.92 mg/kg, with an average of 0.32 mg/kg, and, in the weathered crust, it ranges from 0 to 4.25 mg/kg, with an average of 1.93 mg/kg. Ilmenite content ranges from 0.57 to 73.94 mg/kg, with an average of 22.65 mg/kg, and, in the weathered crust, it ranges from 0 to 149.95 mg/kg, with an average of 43.55 mg/kg. This indicates that mineralized granite is relatively enriched in zircon and ilmenite (1.23 and 1.92 times, respectively), while non-mineralized granite is relatively enriched in anatase (13.28 times). REE-bearing minerals like zircon, ilmenite, and anatase are less likely to be dissociated during weathering and are concentrated in the weathered crust of the granite (enrichment coefficient: from 1.92 to 6.03) [15,36].

In the Early Cretaceous non-mineralized granite, the apatite contents range from 0 to 8.72 mg/kg, with an average of 5.98 mg/kg. Only a small amount of apatite is found in the weathered crust. The titanite contents range from 0 to 379.27 mg/kg, with an average of 244.12 mg/kg, and titanite is absent in the weathered crust. Rutile contents vary from 0 to 112.61 mg/kg, with an average of 59.04 mg/kg, while the weathered crust exhibits a range of from 0 to 2.66 mg/kg, with an average of 1.33 mg/kg. In the Paleogene–Eocene mineralized granite, apatite contents range from 0.35 to 129.88 mg/kg, with an average of 29.80 mg/kg; only one sample in the weathered crust contains 7.31 mg/kg, while the others show no apatite. The titanite contents range from 6.82 to 583.10 mg/kg, with an average of 315.50 mg/kg, but titanite is absent in the weathered crust. Rutile contents range from 0 to 296.37 mg/kg, with an average of 112.45 mg/kg, and rutile is present in the weathered crust of the mineralized granite only in samples HCZ008-(H14/H16) and TCZ-025-H6, at 35.61 mg/kg and 0.01 mg/kg, respectively. These findings suggest that REE-bearing minerals, such as apatite, titanite, and rutile, are prone to dissociation during granite weathering, releasing rare earth element (REE) ions that are subsequently leached by rainwater and groundwater. Furthermore, the apatite, titanite, and rutile contents in the mineralized granite are 4.98, 1.29, and 1.90 times higher, respectively, than those in the non-mineralized granite [35].

5. Discussion

5.1. Chronological Characteristics and iREE Mineralization

In South China, iREE-host granites mainly formed during the Caledonian (from 461 to 384 Ma), Indosinian (from 242 to 228 Ma), and Yanshanian (from 198 to 94 Ma) periods [15]. Based on age distributions, Caledonian granites show the greatest metallogenic potential (26.5%), followed by the Yanshanian granites (16.7%). In southern Jiangxi Province, China, a total of 184 iREE-host intrusions have been identified, of which 139 (75.5%) plutons belong to the Yanshanian, mainly Early Yanshanian, while only 30 (16.3%) intrusions are Caledonian.

Metallogenic granites in southwestern Yunnan exhibit distinct temporal distributions. In the Tengchong–Lianghe area, granite ages primarily range from 80 Ma to 52 Ma, whereas those in the Lincang region reveal a cluster between 240 Ma and 208 Ma. In this study, zircon U–Pb dating of the Tuguanzhai granite yielded ages of 48.56 ± 0.19 Ma and 112.13 ± 0.75 Ma. Notably, the latter is the first report of the occurrence of iREE-host Early Cretaceous granites in this region.

These results suggest that the formation of iREE deposits is controlled by factors beyond the formation age of the host granite [15,37].

5.2. Genetic Relationships Between Whole-Rock Geochemical Compositions and iREE Mineralization

Zhang Lian et al. (2015) summarized the geochemical characteristics of the protoliths for iREE-host granites [38], which are typically high in silica (from 70 to 75 wt%) and high in alkalis (w(K₂O + Na₂O) > 8 wt%), particularly enriched in K₂O, while relatively low in Al, Ti, and Ca. Zhao Zhi et al. (2024) analyzed major element data from 79 protolith samples across 13 iREE-bearing bodies in the eastern Nanling region, including both light and heavy REE types [39]. Their results revealed moderately enriched SiO₂ contents (from 61 to 77 wt%, average 72 wt%), low TiO₂ (from 0.01 to 0.70 wt%, average 0.30 wt%), and relatively high Al₂O₃ (from 11.5 to 18.6 wt%, average 13.7 wt%). The A/CNK values were all greater than 1.1. CaO contents ranged from 0.2 to 3.2 wt%, with low Fe, Mg, and Mn concentrations, and high alkalinity (Na₂O + K₂O = 7 wt% to 13 wt%), with K₂O exceeding Na₂O (K₂O/Na₂O > 1).

Although mylonitization or cataclasis has occurred in the adjacent region, the primary mineral assemblages and textures have been preserved. Coupled with extremely limited alterations, we suggest that the major element compositions can largely reflect the original igneous signature. The distributions of coherent major and trace elements, along with well-defined REE fractionations exhibiting negative Eu anomalies, are typical characteristics of magmatic differentiation. These features are compared to those observed in unaltered granites from nearby regions. Therefore, we consider that the geochemical data are reliable for interpreting the petrogenesis of these rocks.

The Early Cretaceous non-mineralized granites at Tuguanzhai exhibit significant alkalinity (w(Na₂O + K₂O) > 7 wt%, mostly >8 wt%, K₂O/Na₂O > 1), along with low Ti (titanium) and Ca (calcium) contents, and relatively low Al (aluminum) levels (generally <15 wt%). In contrast, the Paleogene–Eocene mineralized granites display similar geochemical characteristics, but slightly lower SiO₂ contents (typically <70 wt%). These observations indicate that a granitic protolith characterized by high Na₂O + K₂O, high K, and low Ti and Ca contents is favorable for iREE mineralization [40,41,42].

In South China, the total REE contents (ΣREE) in the iREE deposits are relatively low, averaging 227 × 10⁻⁶, with significant variation observed among different granites. The highest concentration is found in the alkali-feldspar granite porphyry of the Guanzi intrusion (926 × 10⁻⁶), while the lowest occurs in the coarse-grained biotite granite of the Guikeng intrusion (40 × 10⁻⁶).

Regarding the Early Cretaceous non-mineralized granites at Tuguanzhai, the ΣREE values range from 82.86 × 10⁻⁶ to 241.99 × 10⁻⁶ (average 177.32 × 10⁻⁶), while the Paleogene–Eocene mineralized granites exhibit slightly higher ΣREE contents, ranging from 95.92 × 10⁻⁶ to 794.95 × 10⁻⁶. Typically, granites with REE concentrations exceeding 150 × 10⁻⁶ are considered to have potential for iREE mineralization [39,42,43]. In the Tuguanzhai area, both non-mineralized and mineralized granites have ΣREE values above 150 × 10⁻⁶, suggesting that higher REE contents in protoliths may be more conducive to the formation of iREE deposits, although this is not a controlling factor [41].

The SiO₂ versus ΣREE diagram shows no significant correlation between SiO₂ and REE contents (Figure 10a). This suggests that the overall magmatic evolution had a limited impact on bulk REE concentration, despite involving the crystallization and removal of feldspars and ferromagnesian minerals. However, these fractional minerals can accommodate specific REE groups, particularly the LREEs and HREEs, thereby altering REE distributions during fractional crystallization. Additionally, the δEu versus ΣREE diagram reveals a positive correlation (Figure 10b). This is because Eu anomalies primarily result from the crystallization of plagioclase, reflecting its fractional crystallization, which may further influence REE distributions and concentrations. Therefore, the degree of fractionation and crystallization conditions of plagioclase may be effective controlling factors in iREE formation.

5.3. Genetic Relationships Between REE-Bearing Minerals and iREE Mineralization

As magma evolves towards more acidic compositions, accessory minerals, particularly REE-bearing minerals, play a crucial role in determining the total REE contents in the bulk rock [41]. In most acidic plutons, accessory minerals predominantly contribute to the overall REE contents [35]. In the Tuguanzhai granite, the total REE contents are closely related to the presence of REE-bearing accessory minerals. The concentration of REE-bearing minerals in the Paleogene–Eocene mineralized granites (from 157.60 mg/kg to 1055.98 mg/kg) is significantly higher than that in the Early Cretaceous non-mineralized granites (from 1.08 mg/kg to 594.94 mg/kg), which aligns with the total REE contents. The Early Cretaceous non-mineralized granites are relatively enriched in rutile, whereas the Paleogene–Eocene mineralized granites are comparatively enriched in apatite, titanite, zircon, and ilmenite.

Compared to the Tuguanzhai granitic protolith, the weathered crust is notably enriched in zircon, ilmenite, and rutile, with enrichment factors from 1.92 to 6.03. These minerals share strong chemical bonds and remain stable under natural weathering conditions, largely retaining their integrity as heavy minerals [3]. In contrast, apatite, titanite, and zircon are either absent or present in minimal quantities within the weathered crust [3]. These minerals are highly dissociative and gradually decompose during weathering, releasing REE³⁺ ions. Subsequently, these ions are leached by meteoric and groundwater, and then adsorbed by clay minerals such as kaolinite, illite, and montmorillonite, ultimately leading to iREE mineralization. Dissociative REE-bearing minerals, including apatite, titanite, and zircon, play a crucial role in iREE mineralization in the Tuguanzhai area.

6. Conclusions

(1)

The zircon U-Pb ages of the mineralized granites in the Tuguanzhai area are 112.13 ± 0.75 Ma (n = 22, MSWD = 0.103), 52.78 ± 0.28 Ma (n = 28, MSWD = 12), and 48.56 ± 0.19 Ma (n = 25, MSWD = 2.1). We first confirm that the Early Cretaceous granites have potential for iREE mineralization in the Tengchong-Lianghe region of western Yunnan, China.

(2)

Both the non-mineralized and mineralized granites of Tuguanzhai exhibit high K₂O + Na₂O and K contents, along with low Ti and Ca contents. Magmatic differentiation does not significantly influence the variation of REE contents in these rocks.

(3)

The type and abundance of REE-bearing minerals, including zircon, rutile, ilmenite, titanite, apatite, and monazite, influence the REE distribution in granites. Among these minerals, the high concentration of easily dissociable REE minerals, such as titanite, apatite, and monazite, is a critical factor for mineralization in this region.