Geology, Mineralization and Development Potential of Rare and Uncommon Earth Ore Deposits in Southwest China

Abstract

The southwestern region of China is tectonically situated within the Tethyan tectonic domain, with the eastern part comprising the Upper Yangtze Block, while the western orogenic belt forms the main part of the Tibetan Plateau. This belt was formed by the subduction of the Paleo-Tethys Ocean and subsequent arc-continent collision, and was later further modified by the India-Asia collision, resulting in complex geological structures such as the Hengduan Mountains. The lithostratigraphy in this region can be divided into six independent units. In terms of mineralization, the area encompasses two first-order metallogenic domains: the Tethyan-Himalayan and the Circum-Pacific. This study synthesizes extensive previous research to systematically investigate representative rare earth element (REE) deposits (e.g., Muchuan and Maoniuping in Sichuan; the Xinhua deposit in Guizhou; the Lincang deposit in Yunnan). Through comparative analysis of regional tectonic-metallogenic settings, we demonstrate that REE distribution in Southwest China is fundamentally controlled by Tethyan tectonic evolution: sedimentary-weathered types dominate in the east, while orogenic magmatism-related types prevail in the west. These findings reveal critical metallogenic patterns, establishing a foundation for cross-regional resource assessment and exploration targeting. The region hosts 32 identified REE occurrences, predominantly light REE (LREE)-enriched, genetically classified as endogenic, exogenic, and metamorphic deposit types. Metallogenic epochs include Precambrian, Paleozoic, and Mesozoic-Cenozoic periods, with the latter being most REE-relevant. Six prospective exploration areas are delineated: Mianning-Dechang, Weining-Zhijin, Long’an, Simao Adebo, Shuiqiao, and the eastern Yunnan-western Guizhou sedimentary-type district. Notably, the discovery of paleo-weathering crust-sedimentary-clay type REE deposits in eastern Yunnan-western Guizhou significantly expands regional exploration potential, opening new avenues for future resource development.

1. Preface

REEs, the 17 elements comprising the lanthanides, scandium, and yttrium, exhibit similar physical and chemical properties. Due to their unique optical, electrical, and magnetic properties, REEs are often used as additives for other compounds and/or metal alloys, forming various novel materials in national defense and military industry, aerospace, clean energy, information technology, industrial catalysis, specialty materials, energy, and agriculture. Therefore, REEs hold critical strategic significance for the national economy, security, and technological development [1,2,3,4,5,6]. REEs have emerged as critical and strategic mineral resources for global economic development and social advancement due to their growing importance in a wide range of industries. However, REE deposits manifest an extremely uneven global distribution, principally concentrated in China, Brazil, Vietnam, Russia, the United States, Australia, India, and Greenland. China has led in both REE reserves and production. Recently, the international community has focused heavily on REE supply, with increased investment from governments and mining industries in exploration and exploitation, challenging China’s dominant position. However, China’s advantages in REE resources are also facing increasingly severe challenges. First, despite holding nearly one-third of global REE reserves, China has accounted for over 80% of world production for over 20 years, resulting in rapid domestic resource depletion. Second, China’s REE resources are primarily light REEs (LREEs, over 90%), while the reserves of heavy REE (HREE) resources with wider application prospects and higher market prices are relatively low.

Based on the metallogenic conditions, Zhang Peishan divided China’s REE deposits into 10 genetic types: (1) Granite, alkaline granite, granodiorite, and albitized granite types; (2) Alkaline rock type; (3) Igneous carbonatite type; (4) Skarn type; (5) Pegmatite type; (6) Metamorphic and sedimentary-metamorphic carbonate rock types; (7) Hydrothermal metasomatism and hydrothermal vein types; (8) Sedimentary rock type; (9) REE placer type; (10) Granitoid weathering-crust type. Endogenetic REE deposits, most notably of the carbonate rock, alkaline rock-alkaline granite, and hydrothermal types, are exemplified by deposits such as Bayan Obo (Inner Mongolia), Maoniuping and Dalucao (Mianning-Dechang area, western Sichuan), Weishan (western Shandong), Miaoya (Hubei), Baerzhe (Inner Mongolia), and Yinachang (Yunnan). These deposits are generally characterized by large scales, high grades, and low contents of radioactive substances, garnering increasing attention in exploration and exploitation in recent years [7,8,9,10,11,12,13]. REE deposits have been found in Sichuan, Yunnan, and Guizhou provinces, but most mineral localities are under exploitation, showing limited proven reserves. LREEs are predominant in industrial deposits. The Mianning-Dechang area in Sichuan Province holds proven REE resources of about 250 × 10⁴ t, making it the most promising area for single bastnaesite deposits [14]. Weathering crust-type REE deposits are distributed in Longchuan and Jinping counties in western and southern Yunnan Province, respectively. Some phosphatic rock-type REE deposits are distributed in northwestern Guizhou Province. In the Sichuan-Guizhou region, a large-scale palaeo-weathered crust-sedimentary-clay-type REE deposit was newly discovered in the Xuanwei Formation, where the underlying claystones generally contain multiple rare elements like REEs, niobium, scandium, gallium, and zirconium, forming a stable enrichment layer. This REE deposit has over 10 sites with prospective resources of over 100 × 10⁴ t [15]. It exhibits enormous potential for REE exploration and the development of large to super-large resource scales [16]. This research addresses the research gap concerning the weak correlation of rare earth element mineralization and the lack of systematic integration in the southwest region of China, particularly regarding the understanding of regional mineralization and prospective evaluation. Focusing on key rare earth deposits such as Maoniuping in Sichuan and Lincang in Muchuan, this study systematically investigates critical scientific issues using mineralization theory. These issues include the temporal and spatial distribution of magmatic activities and hydrothermal alteration mineralization in the mining area, the structural-hydrothermal superposition and enrichment mechanism, the fluid characteristics and evolution of alteration mineralization stages, the source of ore-forming fluids and substances, and the evaluation of mineralization potential. The research results are of great significance for studying the formation mechanism and mineralization model of rare earth deposits in the southwest region, and have important guiding significance for prospecting in the deep and peripheral areas of the southwest region.

2. Regional Geological Setting

Southwest China exhibits complex regional geology, superior metallogenic conditions, and rich mineral resources. Tectonically, it is primarily located in the Tethys tectonic domain. Roughly bounded by the Longmenshan fault zone and the Ailaoshan fault, it is divided into the eastern Upper Yangtze block and the western orogenic belt (Figure 1).

The Yangtze Block’s basement formed in the Proterozoic. East-west rift basins developed during the Paleoproterozoic breakup of Columbia, accompanied by submarine volcanism. By the late Mesoproterozoic, its southern margin (Kangdian zone) became an active margin during Rodinia’s assembly. The Sinian period saw rifting and Paleo-Tethys expansion, depositing clastic-carbonate sequences. The Caledonian orogeny closed the rift, unifying the Yangtze Block with the South China Block. In the Late Permian, the Emeishan basalt event caused block fragmentation, resulting in uplift in the east and subsidence in the west, forming mixed continental-marine deposits. Middle Permian transgression transformed the Sichuan Basin into a semi-enclosed inland sea. The Late Triassic Indosinian orogeny closed the Tethys Ocean, thrusting the western margin (Longmenshan-Jinpingshan) and forming the Sichuan foreland basin. By the Early Jurassic, the Yangtze Block became an inland basin with migrating subsidence centers. Cenozoic India-Eurasia collision induced strong differential uplift in the western margin, shrinking the Sichuan Basin and uplifting the Qinghai-Tibet Plateau, ultimately forming the Longmenshan thrust belt and Hengduan Mountains.

The lithostratigraphic framework of Southwest China consists primarily of the Qinling-Qilianshan-Kunlunshan, Qiangtang-Changdu-Simao, Bangonghu-Shuanghu-Nujiang-Changning-Menglian, Gangdise-Himalayan, Indian, and Yangtze stratigraphic region. From the Paleoproterozoic to the Quaternary, volcanic rocks in Southwest China experienced the Changchengian, Nanhuaan, and Middle-Late Permian large-scale rifting-magmatic events, the breakup of the Columbia, Rodinia, and Gondwana supercontinents, and the Early Paleozoic, Late Paleozoic, and Triassic-Cenozoic collisional orogenies. Five tectono-magmatic provinces of volcanic rocks and 19 volcanic zones are identified in Southwest China.

Southwest China possesses excellent metallogenic conditions and rich mineral resources. It can be divided into two first-order metallogenic domains: the Tethyan-Himalayan and the circum-Pacific metallogenic domains. By 2010, Southwest China was known to have 155 mineral types (including 100 types with proven resource reserves), over 12,000 mineral localities, and more than 1200 medium- and large-scale mineral deposits. Among them, dominant mineral types include iron, manganese, chromium, vanadium, titanium, copper, lead, zinc, bauxite, tin, mercury, silver, REEs, phosphorus, sylvite, mirabilite, and barite, with copper, zinc, and chromium bauxite predominating.

The mineral resources in various provinces in Southwest China are summarized as follows:

Tibet is located in the Tethyan-Himalayan metallogenic domain. Statistics indicate that Tibet held over 4000 mineral deposits/ore occurrences/mineralized spots and more than 3000 mineral localities by the end of 2012. Specifically, Tibet contained 125 mineral types (including 41 types with proven resource reserves) and 71, 83, and 175 large, medium, and small deposits, respectively, with the dominant mineral resources including chromium, copper, lead-zinc-silver polymetals, molybdenum, iron, antimony, gold, and high-temperature geothermal energy, as well as lithium, boron, and potassium in salt lakes.

Yunnan, located at the junction of the Nujiang-Lancangjiang-Jinshajiang metallogenic belt in Southwest China, the Yangtze metallogenic region, and the South China metallogenic belt, holds over 2700 various deposits and 1338 mineral localities. It hosts 155 types of metallic and non-metallic mineral resources (including 86 types with proven resource reserves) and 119, 285, and 934 large, medium, and small deposits, respectively. The dominant mineral resources include tin, zinc, indium, thallium, cadmium, phosphorus, crocidolite, lead-titanium-iron placer, platinum-group metals, sylvite, arsenic, wollastonite, copper, nickel, silver, strontium, germanium, and mirabilite.

Sichuan geologically spans three tectonic units: the Yangtze block, the Tibet-Sanjiang orogenic system, and the Qinling-Qilianshan-Kunlunshan orogenic system. Besides oil, natural gas, uranium, geothermal water, and mineral water, Sichuan holds 2149 mining areas and 135 mineral types (including 82 types with proven resource reserves), with dominant mineral resources including vanadium, titanium, lithium, pyrite, mirabilite, LREEs, salt minerals, iron, cobalt, platinum, palladium, cadmium, beryllium, zirconium, and natural gas.

Guizhou, located on the southwestern margin of the Yangtze block, was successively influenced by the Jiangnan orogenic belt, the eastern circum-Pacific metallogenic domain, and the western Tethyan metallogenic domain. It has been found to contain 3000 mineral occurrences, 123 mineral types, and 735 large-scale deposits, with dominant mineral resources including coal, phosphorus, aluminum, mercury, antimony, manganese, gold, lead-zinc polymetal, silver, nickel, molybdenum, vanadium, and barite.

3. Primary Mineralization Types and Typical Deposits

The REE deposits in Southwest China can be genetically categorized into endogenetic, exogenetic, and metamorphic deposits. The endogenetic deposits are primarily alkaline rock-carbonatite types, associated with alkaline rocks and carbonates, and granite types, associated with granites. The exogenetic deposits are principally phosphate and claystone types associated with sedimentation, along with the alluvial-eluvial weathered crust placer and the ion adsorption types, both associated with weathering. The metamorphic deposits are sedimentary-metamorphic rock type, associated with sedimentation and metamorphism. Primary REE deposits in Southwest China include the iREE deposit in the middle section of granites in Lincang City, Yunnan Province; the REE-bearing phosphate deposit in Xinhua Town, Guizhou Province; the Maoniuping REE deposit as well as the newly discovered palaeo-weathered crust-sedimentary- and clay-type REE deposits with considerable exploitation potential.

3.1. Lincang REE Deposit (Ion Adsorption and Clay Types) in Yunnan Province

3.1.1. Strata

According to the Opinions on National Lithostratigraphic Checkup (Yunnan Bureau of Geology and Mineral Resources, 1996), the Lincang area is located in the Lancang stratigraphic region in the Yunnan-Tibet superregion. The strata in the Lincang area are primarily composed of the Lincang complex granite mass, the Chongshan Group, the Damenglong Group, and the Lancang Group metamorphic rock series, including the Proterozoic to the Quaternary rocks, and some Paleozoic and Mesozoic rocks, with the Cenozoic faulted basin overlying the complex rock mass.

The exposed rocks in the Lincang area primarily include the Precambrian Damenglong (Pt₁d), Chongshan (Pt₂cs), and Lancang (Pt₃l) groups, the Devonian-Carboniferous Nanduan Formation (DCn), the Permian Laba Formation (Pl), the Middle Triassic Manghuai Formation (T₂m), the Triassic-Jurassic Xiaodingxi Formation (T-Jxd), the Upper Triassic Sanchahe Formation (T₃sc), the Middle Jurassic Huakai Formation (J₂h), the Neogene Sanhaogou (N₁sh) and Mangbang (N₂m) formations, and the Quaternary (Q) overburden (Figure 2).

3.1.2. Structures

The Lincang REE deposit hosts complex geological structures characterized by folds and NNE- and SN-trending faults. Secondary NWW-trending transpressive faults and NNW-trending transtensional faults are developed on both sides of the dominant faults. The SN-trending Lancangjiang deep fault significantly controls the magmatism during the Indosinian and the Yanshanian and mineralization in the deposit. It strikes SN-NW-SN-NE-NW from north to south in an overall inverted S shape, forming the general structural framework of the Lincang area.

The dominant faults in the Lincang area include the Lancangjiang fault, the Nantinghe fault, the Wuliangshan-Yingpanshan fault, the Xiaojie-Dazhai fault, the Jiufang fault, the Nayuhe fault, and the Qianmaihe fault (also known as the Nanling-Chengzi fault).

The Lancangjiang fault is the most significant fault controlling the structural framework of the Lincang area. It is generally a high-angle thrust transpressive structure with a slightly SW-dipping direction and an overall inverted S-shaped strike in the planar view. It hosts a 100-m-wide compressional fracture zone, where mylonite, fault gouge, schistosity, tectonic breccias, and small folds are developed. Multistage intermediate intrusion masses occur parallel to the faults, showing a zonal distribution along them [20].

Figure 2. Geological sketch map of the middle section of the Lincang granitic batholith [21,22]. 1—Quaternary; 2—Neogene; 3—Middle Jurassic-Cretaceous; 4—Middle Jurassic; 5—Lower Jurassic; 6—Permian; 7—Upper Paleozoic; 8—Lancang Group; 9—Damenglong Group; 10—Paleogene monzogranite; 11—Neogene granodiorite; 12—Triassic monzogranite; 13—Permian granodiorite; 14—Contact boundary; 15—Fault.

Figure 2. Geological sketch map of the middle section of the Lincang granitic batholith [21,22]. 1—Quaternary; 2—Neogene; 3—Middle Jurassic-Cretaceous; 4—Middle Jurassic; 5—Lower Jurassic; 6—Permian; 7—Upper Paleozoic; 8—Lancang Group; 9—Damenglong Group; 10—Paleogene monzogranite; 11—Neogene granodiorite; 12—Triassic monzogranite; 13—Permian granodiorite; 14—Contact boundary; 15—Fault.

3.1.3. Intrusions

The regional magmatism exhibits distinct multiphase characteristics from the Proterozoic to Cenozoic, demonstrating strong structural control by the Lancangjiang deep fault system in association with Paleo-Tethyan evolution [23]. The Indosinian phase represents the most intensive magmatic episode, forming the Lincang batholith (predominantly biotite monzogranite with local porphyritic granite;) accompanied by fault-parallel volcanic eruptions [24]. Subsequent Late Yanshanian magmatism manifested as clustered small intrusions (quartz diorite, plagiogranite, monzonitic granite) along the eastern Lincang-Menghai belt [25], while Himalayan-phase Oligocene-terminal alkaline magmatism completed the tectonic-magmatic cycle. Crucially, all magmatic units display strict spatial adherence to the Lancangjiang fault orientation, with pluton trends perfectly paralleling regional structures, unequivocally demonstrating fault-dominated magma emplacement mechanisms.

3.1.4. Alterations and Mineralization

(1) Ore body characteristics

The ore body in the Lincang area occurs primarily within the completely weathered layer of biotite monzogranites, with weathered-layer thicknesses ranging from 7 to 22 m. It displays primarily kaolin, sericite, clay, chlorite, limonite, and argillic alterations. It shows ore grades ranging from 0.05% to 0.31% and high continuity [23,26,27]. Its scale and morphology are strictly controlled by the development degree of the weathered crust and the microtopography (Figure 3) and characterized by a horizontal distribution along regional tectonic lines and watersheds. The ore body is primarily distributed in the middle and lower parts of the completely weathered layer, presenting as an overall stratiform/stratoid ore body. The ore bodies exhibit concealed or semi-concealed occurrence with relatively high grades beneath gentle ridges or flat hilltops. On ridge flanks and hilltop peripheries, they appear as exposed or residual outcrops showing significant variations in thickness and grade. In longitudinal profiles along the main ridge trend, the ore bodies demonstrate characteristic changes with increasing slope gradient: the burial depth becomes shallower, thickness decreases progressively, and grades show a gradual reduction trend. The ore body manifests undulating changes in the direction perpendicular to the ridge, the maximum thicknesses on the hillside and top of low and gentle hills, and gradually decreased thicknesses towards the gullies on both sides. The ore body in deeper areas is cut off by Quaternary gullies (Figure 4a,b; [28]).

(2) Ore mineral characteristics

The host rocks in the middle section of the Lincang granites are Late Triassic biotite monzogranites consisting of medium to fine-grained, medium- to coarse-grained, and porphyraceous biotite monzogranites. Their primary mineral assemblage is composed of K-feldspar (25%~30%), plagioclase (30%–35%), quartz (25%), and biotite (8%), with occasional muscovite, opaque minerals, apatite, and altered feldspar due to the metasomatism by muscovite (Figure 5).

The petrographic analysis reveals: K-feldspar occurs as subhedral to anhedral grains with sericitization and argillization surfaces, containing quartz and biotite inclusions; plagioclase exhibits polysynthetic twinning and subhedral-anhedral morphology with pervasive sericitic alteration; anhedral quartz (1st-order gray-yellow interference colors) fills intergranular spaces among feldspars; biotite displays reddish-brown pleochroism in subhedral flakes, hosting opaque minerals and apatite inclusions [27,29].

3.1.5. Mineralization Stages

The iREE deposits are typical endogenetic-supergene mineral deposits. Their ore-forming materials occur in magmatic or metamorphic rocks and special sedimentary rocks, but the enrichment and mineralization of REEs occurred under the weathering and leaching during the Quaternary [30,31,32]. According to previous studies on the petrography and structural evolution of the Lincang area [27,28,30,33,34], the mineralization in the Lincang area can be roughly divided into an early magmatic crystallization differentiation stage and a late weathering-crust leaching and adsorption stage.

In the early mineralization stage, the high-degree evolution and differentiation of magmas in the Lincang REE deposit created a precondition for the fractionation and crystallization of REEs in the bedrocks. The Indosinian biotite monzogranites are the primary intrusions in the Lincang REE deposit and also highly differentiated peraluminous calc-alkaline rocks. Early in this stage, monazite, apatite, sphene, and other REE-rich accessory minerals were precipitated before the crystallization of rock-forming minerals, providing sufficient material sources for the extraordinary enrichment and mineralization of REEs under weathering and leaching during the Quaternary [27].

In the late mineralization stage, biotite monzogranites rich in REEs were weathered and decomposed under hot-humid climatic conditions. Due to weathering, rock-forming minerals like feldspar and biotite in rock masses were altered into clay minerals such as kaolinite and montmorillonite, serving as excellent REE adsorbents. Minerals including sphene, allanite, monazite, xenotime, and apatite in rock masses were prone to break and decompose under supergene conditions due to their intrinsic properties. Among them, accessory minerals like monazite, xenotime, and zircon were principally retained in situ as heavy minerals, thereby enriching LREEs in the deposit [30,35,36,37]. Free REEs existed in the form of exchangeable hydrated cations. They flowed and infiltrated with surface water and groundwater under weakly acidic conditions. They migrated downward in the vertical profile of the weathered crust. Under suitable pH conditions, clay minerals exhibited enhanced adsorption capacity, leading to a REE enrichment zone in the completely weathered layer of the weathered crust [33,34].

3.1.6. Metallogenic Epoch

Integrated studies [20,25,26,27,38,39,40,41] demonstrate that the Lincang composite batholith, with biotite monzogranite as the predominant ore-hosting lithology, was emplaced during 211~237 Ma based on systematic LA-ICP-MS zircon U-Pb geochronology (

Table 1. Statistics of chronological data of magmatic rocks and ores in the Lincang deposit.

Deposit/Area	Rock/Mineral	Dated Mineral/Rock	Dating Method	Age (Ma)	Date Source
Beside the Mengku Highway, Lincang	Biotite monzogranite	Zircon	LA-ICP-MS U-Pb	237.7 ± 0.8	Shi et al., 2006 [38]
Western Fengqing rock mass	Biotite monzogranite	Zircon	LA-ICP-MS U-Pb	214 ± 2	Kong, 2011 [25]
Western Manghuai, Lincang	Biotite monzogranite	Zircon	LA-ICP-MS U-Pb	211.9 ± 1.8	Kong, 2011 [25]
Western Jinghong, Lincang	Biotite monzogranite	Zircon	LA-ICP-MS U-Pb	227.00 ± 0.80	Kong, 2011 [25]
Eastern Lancang, Lincang	Biotite monzogranite	Zircon	LA-ICP-MS U-Pb	219. 69 ± 0. 67	Kong et al., 2012 [39]
Eastern Menghai, Lincang	Biotite monzogranite	Zircon	LA-ICP-MS U-Pb	219. 19 ± 0. 99	Kong et al., 2012 [39]
Eastern Lancang	Biotite monzogranite	Zircon	LA-ICP-MS U-Pb	227 ± 0.8	Dong et al.,2013 [40]
Beside the Lancang Simao Highway	Biotite monzogranite	Zircon	LA-ICP-MS U-Pb	223.8 ± 1.3	Wang et al., 2014 [41]
Beside the Lancang Simao Highway	Biotite monzogranite	Zircon	LA-ICP-MS U-Pb	216.6 ± 1.6	Wang et al., 2014 [41]
Lincang	Biotite monzogranite	Zircon	LA-ICP-MS U-Pb	224.3 ± 2.3	Sun, 2018 [20]
Lincang	Biotite monzogranite	Zircon	LA-ICP-MS U-Pb	213 ± 2.0	Zhang et al., 2020 [26]
Lincang	Biotite monzogranite	Zircon	LA-ICP-MS U-Pb	226 ± 1.6	Zhang et al., 2024 [27]

). After excluding minor inherited zircons, the robust age clustering unequivocally constrains the granitic magmatism to the Indosinian orogenic episode, consistent with regional tectonic evolution.

3.1.7. Sources of Ore-Forming Fluids and Minerals

The ore-forming parent rock in the study area is Indosinian biotite monzogranite, which represents the most crucial metallogenic material source. Zircon CL images exhibit distinct oscillatory zoning (Figure 6), with U = 765–6190 ppm, Th = 305–2247 ppm contents yielding Th/U ratios of 0.15–0.65, confirming their magmatic origin [42,43]. Geochemically, the granite is classified as strongly peraluminous S-type, with SiO₂ = 64.26–69.54%, K₂O = 3.92–6.14%, Na₂O = 2.80–3.50% (avg. K₂O + Na₂O = 6.12%), and high differentiation index (DI = 73.62–82.35). Elevated Al₂O₃ (12.04–13.42%) and corundum molecule index (C = 1.53–2.49) further demonstrate its peraluminous nature [44]. Samples plot within the peraluminous field in A/NK-A/CNK diagrams and exhibit calc-alkaline affinity (σ = 0.89–1.66; AFM diagram), consistent with syn-collisional granites [39,40,43,45,46,47,48,49]. REE patterns show LREE enrichment with pronounced Eu anomalies (Figure 7), indicative of upper crustal partial melting [39,50]. High Rb/Sr ratios (0.77–4.88, avg. 1.55) and εHf(t) values below the depleted mantle line (Figure 8), coupled with Nb depletion, confirm derivation from ancient crustal materials without mantle contribution. Source discrimination using CaO/Na₂O ratios (0.60–2.01, avg. 0.94) and CaO/Na₂O-Al₂O₃/TiO₂ systematics (Figure 9) reveals a psammite-dominated source, analogous to the Bethanga pluton in the Lachlan Fold Belt [51,52]. These features collectively suggest that the Late Triassic syn-collisional granites formed through high-degree differentiation of psammite-derived, peraluminous calc-alkaline magmas, with feldspar fractionation (evidenced by Eu anomalies) facilitating REE enrichment [26,30,39].

3.1.8. Genesis of the Deposit

Previous studies indicate that the evolutionary process of the Paleotethys Ocean in the Lancangjiang area experienced six stages: (1) the initial rifting of the main Paleotethyan oceanic basin during the Early Devonian; (2) the emergence of the oceanic crust in the main Paleotethyan oceanic basin during the Middle Devonian; (3) the spreading of the Paleotethyan oceanic basin from the Late Devonian to the Late Carboniferous; (4) the subduction of the Paleotethyan oceanic basin from the Early Permian to the Late Permian; (5) the continent-arc and continent-continent collision from the Late Permian to the early stage of the Middle Triassic; (6) the post-collision from the Middle Triassic to the Late Triassic [54].

The Lincang complex granitic batholith is primarily composed of granites formed by the Indosinian intrusive activity (γ1.5). In the late stage of the Middle Triassic, the Paleotethyan subducting slab began to break off, transitioning to the extensional transformation stage (characterized by strike-slip movement). The upwelling asthenosphere, triggered by the detachment of the subducted slab, generated massive magma chambers through both its tremendous thermal energy and decompression melting of the thickened crust. These processes induced extensive anatexis of crustal wall rocks, ultimately forming the main body of the Lincang granite batholith [54,55,56,57,58,59,60,61].

The Lincang composite granite batholith, influenced by its structure, tectonics, meteoric water, and topography, has developed a five-layered weathering profile consisting of humus, sub-clay, fully weathered, semi-weathered, and weakly weathered zones. The weathering crust is an open, heterogeneous, and dynamic system characterized by repeated adsorption and desorption processes, leading to rare earth element (REE) fractionation under weathering and leaching (Figure 10; [28,62,63,64]). Early meteoric water, acidified by humic substances, acted as a transport medium for REEs, with flow direction determining their migration and enrichment zones. During the migration process, REEs are released from accessory minerals in the granite and ultimately deposited in either the upper fully weathered layer or the lower sub-clay layer. Intense weathering transformed most granite-forming minerals into kaolinite-dominated clays, enabling acidic fluids to adsorb and concentrate REEs (especially light REEs) in the fully weathered layer. Heavy REEs, owing to their higher atomic numbers, migrated faster and enriched below light REEs in the middle-lower fully weathered zone. In contrast, the semi- and weakly weathered layers, with increasing internal stress and reduced clay content, adsorbed minimal REEs and thus were not major enrichment zones [26].

Concerning exploration and production, the climatic and geomorphic conditions in the Lincang area create natural conditions for the enrichment of REEs. In terms of material source, the granite weathered crust in the Lincang area is extremely developed and characterized by considerable thicknesses, an extensive area, and high contents of original REEs in rocks, holding the geologic conditions for forming the iREE deposit and favorable supergene weathering conditions. Especially in the Mengwang-Manmai area, the preliminary control shows TREO resources of 11,400 tons and 60,500 tons in the industrial and low-grade ore bodies, respectively, and zones with a TREO grade of 0.14% and LREE cut-off grade exceeding 60%, suggesting the potential for prospecting for a large REE deposit.

3.2. Maoniuping Deposit in Sichuan Province (Alkaline Granite, Carbonate Rock, Magmatic-Hydrothermal Metasomatism, and Hydrothermal Vein Types)

3.2.1. Rock Type

The Maoniuping deposit is situated in the northern segment of the Panzhihua-Xichang rift zone on the western margin of the Kangdian platform uplift in the Yangtze platform. The rock in the deposit, intruded by Jurassic Yanshanian alkaline granite masses, predominately comprise the epimetamorphic carbonate strata derived from the Devonian-Lower Permian neritic/volcanic clastic rocks, and the Quaternary loess layer. The primary rocks are composed of sandy grayish-yellow sericite phyllites with metamorphic fine-grained sandstones, dolomitic marbles or epimetamorphic marbleized dolomitic limestones, and dark-gray sericite phyllites with thinly laminated metamorphic sandstones in the lower member of the Middle Devonian, the Permian neritic clastic rocks and carbonate rocks, the Permian basalts, the Triassic carbonaceous sedimentary stratum, and the Quaternary loess layer (Figure 11; [65,66,67]).

3.2.2. Structures

The Maoniuping deposit, located in the central Mianxi area, lies in the south-central part that projects westward of the Haha fault zone. Regionally, the Haha fault zone is a secondary NNE-trending strike-slip fault within the Xianshuihe-Anninghe strike-slip fault zone. Rock masses along this fault zone show a banded distribution along tight en echelon structures, showing multistage intrusion gradually younger inward from both sides of the fault zone. The Haha fault zone runs through the Maioniuping deposit in an overall NNE direction, with a strike of nearly NE and a dip direction of NW. This fault zone primarily exhibits NNE-NE-directed fracture planes on its east and west sides. These fracture planes intertwine with NW-NNW-directed counterparts, forming a reticular tectonic fissure zone, which provides a space for the formation of the stockwork zone. The parallel-vein zone adjacent to the Haha fault zone primarily develops two sets of joints in the NE-NNE and NW-NNW directions (Figure 11). The development degree of various joints decreases in the order of NE-NNE-, NW-NNW-, SN-, and EW-oriented joints. The Anninghe, Xiaojiang, and Ganluo faults are developed near the deposit. The Anninghe fault, trending in the N-S direction, is recognized as a significant boundary fault for regional mineralization zoning, with small-scale syenites exposed in the nearby Cida and Baima areas. The carbonatite-alkaline complex zone, extending in the N-S direction, is distributed on the west side of the Anninghe River. Due to the influence of compressive and torsional stresses, the Anninghe fault zone has cut most rock units in the deposit.

3.2.3. Intrusions

The Maoniuping deposit strata have been intruded by Jurassic Yanshanian alkaline granites, reflecting frequent regional magmatic activity manifested by Indosinian-Yanshanian granitoids (including the Mianxi composite granite batholith), Himalayan carbonatites, nepheline syenite, alkaline granite porphyry, as well as various-aged rhyolites and minor lamprophyre, diabase, and granite porphyry dikes [69]. The intrusive rocks in this area are primarily classified into carbonatite-alkaline complexes and alkali-feldspar granitoids (Figure 12). The country rock of the carbonate complex is alkali-feldspar granite, and the contact metasomatic aureole between the carbonate complex and the country rock is mainly fenitization (Figure 13). These alkaline complexes play dual geological roles: serving as both the main host rocks for REE orebodies and the parent rocks providing essential material sources for mineralization, while their internally developed stockwork-disseminated mineralization can form economic ore deposits [65,66]. Composed dominantly of nepheline syenite, carbonatite, granite porphyry and compositionally complex alkaline pegmatite veins, they mainly occur as plutonic columns with carbonatite and nepheline syenite being the principal ore-hosting rocks, whereas the granite porphyry intruded post-mineralization and shows no direct genetic relationship with ore formation.

Carbonatite is predominantly composed of calcite, phlogopite, quartz, and albite, with apatite as the main accessory mineral. Calcite occurs as xenomorphic granular crystals with triple-junction texture and two sets of cleavages. Phlogopite exhibits subhedral platy morphology with one set of perfect cleavage. Quartz and albite appear as anhedral granular aggregates.

Syenite consists mainly of albite, K-feldspar, and quartz, with minor accessory minerals such as apatite and monazite. Albite shows subhedral prismatic habit with fine polysynthetic twinning. K-feldspar is predominantly microcline (displaying grid twinning) and perthite, often exhibiting poikilitic texture. Quartz occurs as anhedral granular crystals.

3.2.4. Alterations and Mineralization

(1) Characteristics of ore bodies

The ore bodies in the deposit are principally distributed along the main body of the Haha fault zone. They converge in the NE direction and spread in the SW direction, presenting an en-echelon distribution within the mineralized zone. Small-scale ore bodies are developed on the east and west sides of the Haha fault zone. The ore bodies are primarily covered by the Quaternary alluvial and proluvial sedimentary layers and man-made accumulations, with burial depths ranging from 30 to 50 m, exhibiting semi-concealed and concealed characteristics. They manifest complex branches and diversified morphologies, occurring primarily as veined, banded, and lentoid structures underground, along with minor irregular dendritic, lentoid, and sac-shaped structures. Furthermore, they are characterized by branching and recombination, confluence, intermittent pinch-out and reoccurrence, and lateral pinch-out development (Figure 13; [65]). The ore bodies in the deposit are principally enriched in the turning part of the Haha fault zone within the range of exploration lines 45 to 53. The average grades of the ore bodies (represented by REOs) range from 1.09% to 5.59%. The coefficients of variation in REE grade within the same ore body range from 30% to 129% but from 84% to 129% in the case of the TREO value exceeding 2%, indicating a more balanced distribution [68].

Veined ore bodies are primarily categorized into the large pegmatite veins composed of alkaline mafic pegmatites or calcite carbonatites, parallel vein zones, and the reticular vein system consisting of barite-aegirine-augite veinlets and fluorite-quartz-feldspar veinlets or calcite veinlets. Specifically, large pegmatite veins and parallel vein zones are predominantly distributed in the northern portion of the deposit, with minerals in the large veins exhibiting zoning characteristics. In contrast, the reticular veins are principally situated on the east and west sides of the deposit [71]. The remaining irregular ore bodies predominantly occur in the transitional zone between nepheline syenite and large vein-type ore bodies. Their typical characteristics include early-formed syenite host rock breccias cemented by later hydrothermal veins, forming distinct brecciated textures. A complete alteration halo develops around the ore bodies, exhibiting concentric zoning patterns with an inner fenitization alteration zone adjacent to the ore bodies and an outer carbonatization alteration zone.

(2) Characteristics of ore minerals

The Maoniuping deposit contains 85 ore minerals, including over 10 REE minerals like bastnaesite, parisite, chevkinite, britholite, cerianite, sphene, and monazite (Figure 14). The REE ores in the deposit show an average grade of about 4.73%, with most REOs occurring as independent REE minerals and a minority dispersed in weathered soil and gangue. Bastnäsite exhibits banded, veinlet, and drusy textures, with replacement and pegmatitic structures. The crystals are euhedral/subhedral tabular or anhedral granular, with relatively complete crystal forms. Bastnäsite primarily occurs in carbonatites, syenites, and granites. In carbonatite-type ores, it mainly exists as massive or mottled mineral aggregates or in disseminated form, associated with minerals such as barite, aegirine-augite, and fluorite. Replacement and inclusion textures are well-developed, often enclosing minerals like chevkinite within calcite, barite, or quartz crystals. Aegirine-augite and orthoclase are replaced by bastnäsite, while bastnäsite and chevkinite are in turn replaced by calcite and barite. In syenite-type ores, bastnäsite mainly occurs as drusy, banded, or massive coarse-crystal aggregates, intergrown with euhedral, subhedral, or anhedral coarse minerals such as barite, aegirine-augite, fluorite, and orthoclase, forming aggregates or mineral bands. In fine stockwork ores, it appears as disseminated or micro-veinlet forms [72,73].

3.2.5. Alterations of Surrounding Rocks

The wall-rock alteration is predominantly characterized by fenitization, featuring fractured country rocks permeated by a stockwork of veinlets filled with rare earth minerals, fluorite, and aegirine (Figure 13). The fenite halo displays a length of 1800 m and widths ranging from 100 to over 600 m. From both sides towards the center, the fenite halo is roughly zoned into the cataclastic zone and the aegirine, arfvedsonite, and biotite alteration zones.

(1) The cataclastic zone is characterized by reticulated REE-containing metasomatic veinlets. In the cataclastic zone, the hydrothermal metasomatism of surrounding rocks is manifested as albite alteration. Specifically, minerals like microperthite, perthite, and quartz are metasomatized by albite along crystal edges and microfractures or cleavages within crystals.

(2) The aegirine alteration zone is most significantly characterized by the spotted aegirine or aegirine-augite and albite distributed in a mosaic pattern in surrounding rocks. The K-feldspar and quartz metasomatized by aegirine or aegirine-augite and the surrounding rocks develop finely veined aegirine or aegirine-augite. The significantly enhanced albite alteration in the aegirine alteration zone leads to the formation of albite due to the metasomatism in partial surrounding rocks. The intensity of metasomatism is associated with the alkaline mafic pegmatites, evidenced by the most intense alteration on both sides of the dike.

(3) The arfvedsonite alteration occurs primarily within and beside alkaline mafic pegmatite and carbonatite dikes, characterized by the significant metasomatism of aegirine-augite or aegirine and bastnaesite by arfvedsonite.

(4) The biotite alteration is confined to local areas within and surrounding the alkaline mafic pegmatite dikes, representing the potassic trend of fenitization, that is, the metasomatism between biotite and aegirine/aegirine-augite.

In summary, throughout the process of fenitization in the Maoniuping deposit, K-feldspar and quartz in surrounding rocks are metasomatized by albite in the outer circle of contact metasomatism. The inner zone is metasomatized by albite and aegirine/aegirine-augite and possibly by microcline and microperthite. The final product in the core zone is the alkaline mafic pegmatite dikes containing significant hydrothermal minerals like barite, fluorite, bastnaesite, and quartz.

3.2.6. Mineralization Stages

The Maoniuping deposit primarily hosts carbonatite-syenite complexes, showing weak REE mineralization. The Dagudao ore body in the deposit serves as the primary REE mineralization zone. The mineralization of the Maoniuping deposit can be divided into the magmatic rock, pegmatite, hydrothermal, and REE mineralization stages (

Table 2. Mineralization stages and mineral paragenesis in the Maoniuping deposit [65].

Deposit	Evolutionary Stages
	Magmatic Rock Stage	Pegmatite Stage	Hydrothermal Stage	REE Mineralization Stage
	Mineral Paragenesis
Maoniuping	K-feldspar, biotite, quartz, muscovite, and albite	Calcite, barite, and fluorite	Aegirine-augite, arfvedsonite, phlogopite, and calcite	Barite, fluorite, bastnaesite, and calcite

; [65,67,74]).

(1) Magmatic stage: this stage represents the magma emplacement and the early fluid evolutionary stage. Significant carbonatite-syenite complexes, exemplified by the Guangtoushan complex, formed in this stage. This stage contributes primarily to the evolution of ore-forming fluids, providing the site for the occurrence of veins.

(2) Pegmatite stage: this stage is characterized by copious pegmatoidal minerals like barite, fluorite, calcite, and quartz, which occur as veins principally in the Guangtoushan complex within the Maoniuping deposit. In this stage, RREs, especially LREEs, began to be enriched in minerals.

(3) Hydrothermal stage: this stage represents the primary fluid evolutionary stage and the formation stage of significant gangue minerals (Table 2) and the vein-veinlet-fine stockwork pattern. The formation of significant gangue minerals provides a material basis for the subsequent formation of REE minerals. The contents of LREEs and HREEs in this stage were reduced compared to the pegmatite stage, with minor differentiation between LREEs and HREEs.

(4) REE mineralization stage: this stage represents the primary mineralization, during which significant REEs in fluids occurred as REE minerals. A major amount of bastnaesite and minor amounts of barite and fluorite formed in this stage were superimposed on other pre-existing minerals (Table 2). This stage, combined with the previous stage, produced complete coarse veins and mineral zoning structures, with elevated REE content and pronounced differentiation between LREEs and HREEs.

3.2.7. Metallogenic Epoch

Integrated geochronological studies of alkaline granites, carbonatites, and syenites in the Maoniuping mining district [65,68,75,76,77,78,79] demonstrate that the magmatic rocks in the district and adjacent areas yield ages clustered between 22–41 Ma (

Table 3. Chronological data of magmatic rocks and ores in the Maoniuping deposit and its vicinity.

Deposit/Area	Rock/Mineral	Dated Mineral/Rock	Test Method	Age (Ma)	Date Source
Maoniuping	Aegirine-augite vein	Phlogopite	Ar40-Ar39	26.2 ± 2.3	Li et al., 2018 [65]
Maoniuping	Alkaline granite	Zircon	U-Pb	22.4	Yuan et al., 1995 [68]
Maoniuping	Aegirine-augite/ barite/bastnaesite	Magnesio-arfvedsonite	K-Ar	31.8 ± 0.7	Yuan et al., 1995 [68]
Maoniuping	Aegirine-augite/ barite/bastnaesite	Biotite	K-Ar	40.3 ± 0.7	Yuan et al., 1995 [68]
Maoniuping	Carbonatite	Magnesio-arfvedsonite	K-Ar	31.7 ± 0.7	Yuan et al., 1995 [68]
Maoniuping	Carbonatite	Biotite	K-Ar	27.8 ± 0.5	Yuan et al., 1995 [68]
Maoniuping	Bastnaesite	Arfvedsonite	Ar40-Ar39	27.6 ± 2.0	Liu and Hou, 2017 [76]
Maoniuping	Bastnaesite	Phlogopite	Ar40-Ar39	26.4 ± 1.2	Liu et al., 2018 [77]
Maoniuping	Carbonatite	Bastnaesite	Th-Pb	25.7 ± 0.2	Ling et al., 2016 [78]
Maoniuping	Carbonatite	Calcite	Sm-Nd	29.9 ± 1.7	Hu et al., 2012 [79]

). These data suggest that the intrusive events were contemporaneous with rare earth element mineralization, corresponding to the Indosinian orogenic episode [71,76].

3.2.8. Sources of Ore-Forming Fluids and Materials

(1) Characteristics of ore-forming fluids

Previous studies on ore-forming fluids in the Maoniuping deposit focus primarily on minerals like barite, calcite, fluorite, quartz, bastnaesite, K-feldspar, aegirine-augite, arfvedsonite, and calcite [65,70,80,81,82,83]. Through the petrographic analysis of fluid inclusions, Li, posited that aegirine-augite, K-feldspar, and arfvedsonite are enriched in aqueous inclusions (V-L) and daughter mineral-bearing multiphase inclusions (V-L + S) [65]. The aqueous inclusions typically show liquid and vapor phases (LH₂O) at room temperature, with a gas volume percentage of 10% in aegirine-augite and gas volume percentages ranging from 5% to 20% in K-feldspar and arfvedsonite. The inclusions in aegirine-augite, K-feldspar, and arfvedsonite showed homogenization temperatures ranging from 230 to 290 °C, 161 to 376 °C, and 177 to 416 °C, respectively (Figure 15a–c). Calcite develops aqueous inclusions with diameters ranging from 3 to 17 μm and homogenization temperatures ranging from 130 to 170 °C (Figure 15d). Fluorite contains primarily daughter mineral-bearing multiphase inclusions (V-L + S), displaying similar sizes and homogenization temperatures to calcite (Figure 15e,d). Bastnaesite is enriched in aqueous inclusions (V-L), with homogenization temperatures ranging from 150 to 191 °C (Figure 15f).

Hou et al. categorized the above inclusions into four groups [70]. The inclusion group I—the melt inclusion assemblage is predominately found in magmatic calcite and also in pegmatoidal fluorite and quartz. It is typically associated with solid mineral inclusions and occasionally with melt-fluid inclusions, representing the products of the melt stage. The inclusion group II—the melt-fluid inclusion assemblage is dominated by melt-fluid inclusions, associated with melt and CO₂-rich inclusions, representing the products of the melt-fluid transition stage. It is principally found in pegmatoidal fluorite and quartz as well as fluorite and quartz in hydrothermal veins. The inclusion group III—the CO₂-rich fluid inclusion assemblage is predominately composed of AC_L, ADC_L, and C_L inclusions. This inclusion group occurs primarily in barite and REE minerals and also as secondary inclusions in early fluorite and quartz. It shows relatively high homogenization temperatures, with a possible indication of the immiscibility between aqueous solution and CO₂. The inclusion group IV—the water-rich inclusion assemblage contains AV and ADCL inclusions with relatively low homogenization temperatures, representing the products of the late fluid evolutionary stage. This inclusion group occurs primarily as secondary inclusions in quartz, fluorite, and late calcite.

(2) Sources of ore-forming fluids

The intimate spatiotemporal paragenesis of carbonatites and alkaline syenites is generally considered significant evidence of liquid immiscibility. Carbonatites exhibit low MgO (below 0.73%) and FeO (below 1.20%) contents, distinctly differing from the primitive Mg-rich carbonatites derived directly from the partial melting of the mantle source [84,85]. Carbonatites share similar REE patterns with alkaline syenites [86,87,88]. However, carbonatites show higher total REE content [86]. Furthermore, carbonatites and syenites show significantly overlapping Sr-Nd isotopic compositions, with similar εSr and εNd values. Specifically, carbonatites exhibit ε_Sr and ε_Nd values of 22.2–50.2 and −3.5–−5.6, respectively, while syenites display ε_Sr and ε_Nd values of 19.1–25.8 and −3.5–−4.6, respectively. This similarity can be attributed to the differentiation of liquid immiscible magmas [89]. The ore-forming fluids have experienced an evolutionary process [90] from melts to melts-fluids to medium-high temperature CO₂-rich supercritical fluids to low-temperature and low-salinity fluids. All the above suggests that carbonatites and alkaline syenites in the REE mineralized zone within the Maoniuping deposit are characterized by liquid immiscibility.

The mineralization of the Maoniuping deposit, occurring within the carbonatite-alkaline complexes, is likely associated with the highly lithified products of CO₂-rich silicic magmas. In addition to daughter minerals CaCO₃ and CaF₂, the inclusions also contain considerable sulfate minerals like BaSO₄, K₂SO₄, and CaSO₄ [91,92,93], suggesting that the fluids are typical of fluids associated with alkaline igneous rocks, particularly carbonatites. Through a Sr-Nd isotopic study on gangue minerals in the Maoniuping deposit, Hou. Reference [70], revealed remarkably similar characteristics with carbonatites (ore-forming parental rocks) and syenites (Figure 16). Combined with the carbon (C) isotopic composition of calcite and the oxygen (O) isotopic composition of fluid inclusions in quartz, it is inferred that the ore-forming fluids originated from carbonate-syenite magmas. Based on the inclusion temperature measurements by Zheng et al., the fluids in the fenitization (SI) stage were dominated by magmatic water at temperatures ranging from 238 to 481 °C [94]. The fluids in the calcite-barite (SII) stage were mixed with meteoric water at temperatures between 260 and 350 °C. In the arfvedsonite-bastnaesite (SIII) stage, the fluids exhibited temperatures ranging from 167 to 240 °C. The extreme development of joints in the near-surface led to the mixing of significant meteoric water, changing the temperature, composition, and pH value of the fluids. These facilitated REE precipitation and mineralization, thus forming a large number of ore bodies.

(3) Sources of ore-forming materials

Previous studies [76,82,94] found that the ore-forming fluids in the Maoniuping deposit were enriched in Na⁺, K⁺, Cl⁻, F⁻ but depleted in SO₄²⁻, suggesting that the ore-forming fluids might originate from the REE-rich carbonate magmas within the lower crust and the lithospheric mantle (Figure 17).

Based on the C and O isotope analysis of the calcite samples from the Maoniuping deposit, the calcite from the deposit exhibited higher δ¹³C_V-PDB (−6.9‰–−3.9‰) and δ¹⁸O_V-SMOW (7.0‰–9.7‰) values compared to the calcite in carbonatites (

Table 4. The C and O isotopic compositions of the gangue mineral calcite in the Maoniuping deposit [70].

Sample No.	Mineral	δ13CV-PDB/‰	δ18OV-PDB/‰	δ18OV-SMOW/‰
MNP-14	Calcite	−5	−20.5	9.7
MNP-15	Calcite	−3.9	−20.9	9.3
MNP-1	Calcite	−6.9	−22.8	7.3
MNP-6	Calcite	−6.9	−22.7	7.4
MO-20	Calcite	−6.3	−22.5	7.7
MO-53	Calcite	−6.5	−23.1	7
MO-72	Calcite	−6.3	−22.4	7.8

). Figure 18 displays a markedly upward δ¹⁸O-δ¹³O trend, which is likely associated with the high-temperature fractionation in the evolutionary process of carbonate magmas [95,96]. Overall, the O and C isotopic compositions of calcite samples from the Maoniuping deposit exhibited characteristics of the primitive mantle-derived carbonatites (Figure 18), suggesting the mantle origin of HCO₃—in calcite [97].

3.2.9. Genesis of the Maoniuping Deposit

The Maoniuping super-large rare earth deposit, located within the Mianning–Dechang rare earth metallogenic belt, formed through the coupled interplay of tectonic, magmatic, and fluid processes. Mineralization involved three critical stages: (1) sourcing of ore-forming materials from carbonate-rich magmas generated by partial melting of the lithospheric mantle at 25–29 Ma; (2) transport along regional first-order faults, which facilitated upward migration of fluids from ~20 km depth to shallow crustal levels (~8 km); and [98]; (3) fault-controlled deposition governed by a hierarchical fracture system. First-order faults enabled deep material ascent, while second-order structures (e.g., the Haha fault) regulated fluid evolution and hydrothermal scale, ultimately determining deposit size. Third-order faults (e.g., F1, F2, and subsidiary fractures) hosted the ore, with vein morphology dictated by fracture geometry-wider apertures produced zoned pegmatitic orebodies, whereas narrow fractures developed veinlet-stockwork mineralization. The exceptional endowment of Maoniuping reflects the unique synergy of the Haha fault system: its dual role as a deep conduit (enhancing fluid flux) and a shallow trap (with dense third-order fractures), combined with prolonged hydrothermal activity, drove large-scale rare earth enrichment [71].

Based on mineralization styles, ore body structures, fluid inclusion data, and the emplacement depths of complexes, Hou et al. established the vein-veinlet-fine stockwork metallogenic patterns (Figure 19, [70,99]). Specifically, at greater depths, the ore-forming fluids are high-temperature, REE-rich, and carbonate-bearing NaCl-KCl-CaCl2 brine differentiated from the carbonatite-syenite magmas. The filling of calcite, fluorite, barite, and bastnaesite into the fractures of intrusive masses along intrusive bodies and their metasomatism on carbonatites constitute the primary mineralization style (Figure 19c).

At medium depths, multistage hydrothermal activity develops within a relatively closed system, with the mixing of meteoric water in the later stage extending the duration of the hydrothermal system. The ore-forming fluids are released and fill the cataclastic zones and fracture networks formed under the transformational-transtensional deformation mechanism, producing a complex vein system characterized by pegmatoidal and coarse- and fine-grained bastnaesite-bearing assemblages. The REE mineralization persists through the evolutionary process of ore-forming fluids from boiling to mixing (Figure 19b).

At shallower depths, the shallow mineralization depths cause intense boiling of ore-forming fluids, producing hydrothermal explosive breccia pipes. The REE mineralization occurs primarily during the fluid boiling stage, forming mineralized breccia pipes and/or hydrothermal breccia ores (Figure 19a). The participation of meteoric water in the hydrothermal system leads to the rapid precipitation of minor fine-grained bastnaesite and quartz under epithermal conditions.

3.3. Xinhua Deposit in Guizhou Province (Sedimentary Rock and Weathered Crust Types)

3.3.1. Rock Units

The Xinhua REE-bearing phosphate deposit (also referred to as the Xinhua deposit) in Zhijin County, Guizhou Province hosts exposed rocks, including the Sinian Dengying Formation (Z₂dy), the Cambrian Gezhongwu (Є₁gz), Niutitang (Є₁n), and Mingxinsi (Є₁m) formations, the Carboniferous Datang (C₂d) and Baizuo (C₂b) formations, the Permian Liangshan (P₂l), Qixia (P₂q), and Maokou (P₂m) formations, the Triassic Yelang (T₁y), and Yongningzhen (T₁yn) formations, and the Quaternary rocks. Field investigations in the deposit revealed no exposed magmatic rocks [100] and the absence of Middle and Upper Cambrian, Ordovician, Silurian, Devonian, and Cretaceous strata. The deposit comprises black carbonaceous shales of the Lower Cambrian Niutitang Formation at the top, and the dolomites of the Sinian Dengying Formation at the base, with the REE-rich dolomitic phosphorites of the Gezhongwu Formation in the middle [101,102,103,104,105].

3.3.2. Structures

The Xinhua deposit is tectonically located at the southwest end of the Qianzhong uplift within the Yangtze stratigraphic region. It principally exhibits an anticline structure known as the Guohua-Gezhongwu anticline in the Zhijin area, a section of the regional Xinhua anticline. The Guohua-Gezhongwu anticline is primarily exposed in northwestern Yinchanggou, characterized by a NE-trending axis and dip angles ranging from 1° to 30° at the core. The exposed rocks on both flanks of the anticline are consistent with the nearby strata at the core, showing gentle occurrence at dip angles ranging from 3° to 10°. Additionally, the syncline shows a significantly varying dip direction and a steeper dip angle near the fold axis and the fault zone. The southeast flank of the Xinhua anticline, predominately disrupted by a W-E-striking fault, exposes locally the Mingxinsi Formation (Э₁m) and primarily the Late Paleozoic and Triassic strata [101,102,104,105] (Figure 20).

Fault structures are well-developed locally in the Xinhua deposit, with faults F1, F4, F5, F8, and F9 (Figure 21) jointly controlling the strata and the phosphorite occurrence morphology in the deposit. They are grouped into the NE- and NW-trending faults, with the former being the most significantly developed. The Guohua-Gezhongwu anticline in the Zhijin area is disrupted by many dominant and derived faults, causing the incompleteness of some ore beds like the Lower Cambrian Gezhongwu Formation.

3.3.3. Characteristics of Ore Bodies and Ore Minerals

(1) Characteristics of ore bodies

The Xinhua deposit consists of stratiform dolomitic phosphorites interbedded with phosphorus-bearing dolomites in the upper layer, laminated-banded arenaceous phosphorites in the lower layer, and phosphorus-bearing dolomites in the middle [104].

The Xinhua deposit holds stratiform ore bodies distributed in the Gezhongwu Formation along the flanks of the Guohua-Gezhongwu anticline. As a giant marine REE-bearing sedimentary phosphorite deposit, it exhibits a strike of approximately 2.8 km, widths ranging from 400 to 800 m, dip directions varying from 280° to 340°, and dip angles ranging from 10° to 30° (generally from 15° to 20°). The ore beds in main ore blocks are typically 9 to 24 m thick. The natural type of ores is dominated by banded dolomitic phosphorites. The ores in the primary zone exhibit a dark gray color and a dense massive structure [102]. The phosphorite ores in the weathered zone exhibit gray and grayish-brown colors and fragmented and silty structures, with ore quality principally being grades I and II. Yang et al. found that the phosphate deposit is associated with 15 REEs, among which yttrium (Y) is the most abundant, with Y-group REEs representing approximately 45% to 50%, slightly lower than cerium (Ce)-group REEs [101].

(2) Characteristics of ore minerals

Phosphorites can be categorized into two types based on mineral composition: siliceous phosphorite and dolomitic bioclastic-bearing phosphorite. In siliceous phosphorites, collophane occurs as aggregates with preferred orientation or disseminated in dolomite, showing alternating quartz-chalcedony banding [101].

The phosphorites exhibit complex textures including: cataclastic, psammitic, fine-crystalline, bioclastic, intraclastic, brecciated, silty, peloidal, and algal debris textures [102,104]. Characteristic structures comprise banded, lenticular, streaky, massive, and dissolution-vuggy types. Banded structures typically alternate organic-rich black-gray/dark brown phosphorite layers with gray/gray-yellow phosphorous dolomite parallel to bedding planes (Figure 21a,c). Lenticular structures manifest as psammitic phosphorite lenses enveloped by siliceous phosphorite (Figure 21b), while dissolution vugs result from carbonate or psammitic phosphorite corrosion (Figure 21d) [101,105].

Petrographic analysis reveals these phosphorites are predominantly composed of collophane and apatite clasts, with subordinate siliceous lithics and quartz grains. Notably, Zhang et al. identified collophane (cryptocrystalline apatite) as the dominant phase in the Xinhua REE-bearing phosphorite deposit, accompanied by accessory apatite, monazite, dolomite, calcite, and clay minerals [106]. Over 97% of total REEs reside in collophane via isomorphous substitution [107].

Collophane particles exhibit gel-like morphology with brown-yellow to dark-brown coloration [102], occurring in two primary forms: as intraclastic constituents (50%–93% abundance), showing well-rounded shapes with good sorting; and as interstitial cement (1%–22% abundance) binding other components. Partial collophane recrystallizes into apatite displaying microgranular, fibrous, or crystalline textures along grain margins [104].

The chief REE carrier is carbonated-fluorapatite [108], existing mainly as amorphous gel aggregates and micritic varieties. The former constitutes the primary REE host in phosphorites, occurring interstitially with bioclastic/silt-sized particles and showing strong correlation with Y₂O₃ [109].

3.3.4. Metallogenic Epochs

The phosphorites in Guizhou Province formed primarily during two key metallogenic episodes: the late Neoproterozoic Doushantuo Stage and early Cambrian Meishucun Stage. The Xinhua REE-bearing phosphorite deposit in Zhijin (NW Guizhou), hosted within the Lower Cambrian Gezhongwu Formation, represents an economically significant occurrence [104]. Multi-method geochronology constrains the mineralization age: Wei et al. obtained a Re-Os age of 522.9 ± 8.6 Ma from basal black shales [110]. Additional constraints include a Pb-Pb age of 531 ± 24 Ma [111], Sm-Nd isochron age of 533 ± 22 Ma [112], SIMS U-Pb age of 535.2 ± 1.7 Ma [113,114], and Re-Os ages of 541.3 ± 16 Ma [115] and 542 ± 11 Ma [116], collectively aligning with the internationally defined Cambrian Period (

Table 5. Geochronological data of ores from the Xinhua REE-bearing phosphate deposit in Zhijin County.

Deposit/Area	Rock/Mineral	Dated Mineral/Rock	Dating Method	Age (Ma)	Data Source
Ganziping and Sancha	Black shale	—	Re-Os	542 ± 11	Li et al., 1995 [116]
Huangjiawan	Nickel-molybdenum ore	Molybdenite	Re-Os	541.3 ± 16	Mao et al., 2001 [115]
Niutitang Formation	Black shales	Zircon	Pb-Pb	531 ± 24	Chen, 2003 [111]
Zhijin County	Collophanite/small shelly fossil	Collophanite	Sm-Nd	533 ± 22	Shi et al., 2008 [112]
Meishucun Formation	K-bentonite	Zircon	U-Pb	535.2 ± 1.7	Zhu et al., 2009 [113]
Niutitang Formation	Black shale	—	Re-Os	522.9 ± 8.6	Wei et al., 2018 [110]

).

3.3.5. Sources of Ore-Forming Fluids and Materials

The phosphorites from the Gezhongwu Formation REE-bearing deposit in Zhijin exhibit typical hydrothermal-sedimentary characteristics [100,116,117,118,119]. Trace element normalized patterns show flat-topped and hat-shaped distributions with significant negative Ce anomalies (Figure 22), indicating marine hydrothermal sedimentation during their formation. Petrochemical and mineralogical studies by Xing et al. further confirm hydrothermal activity through the development of hydrothermal veins (Figure 23b), occurrence of hydrothermal minerals including barite veins, hydrothermally altered apatite and zircon, nano-sized fluorite, and anatase, as well as REE distribution patterns of siliceous phosphorites characterized by absent negative Ce anomalies but strong positive Eu anomalies (Figure 22). These evidences collectively demonstrate that the deposit underwent intense hydrothermal modification.

A decreased Nd isotopic model age can be recognized as a sensitive tracer for the mixing of mantle-derived materials [120]. Based on this characteristic, Shi et al. analyzed the two-stage Nd model ages of samples from the Xinhua deposit in the Zhijin area (

Table 6. Sm and Nd isotope analysis results of the Xinhua REE-bearing phosphate deposit in the Zhijin area [121].

Sample No.	Sample Name	Sm (10−6)	Nd (10−6)	147Sm/144Nd	143Nd/144Nd	εNd (533 Ma) ①	T2DM ②
Z-1	Small shelly fossil	7.941	59.93	0.080 1	0.512 095 ± 16	−2.63	1 328
Z-5	Collophane	23.47	125.4	0.114 8	0.512 209 ± 15	−2.77	1 338
Z-9	Small shelly fossil	41.85	192.6	0.131 9	0.512 270 ± 12	−2.75	1 336
Z-12	Small shelly fossil	66.49	226.7	0.178 2	0.512 436 ± 14	−2.66	1 330
Z-17	Collophane	13.39	158.4	0.051 3	0.511 998 ± 16	−2.56	1 323
Z-20	Small shelly fossil	37.18	149.2	0.150 3	0.512 350 ± 13	−2.44	1 313

^① (47Sm/144Nd)CHUR = 0.1967; (143Nd/144Nd)CHUR = 0.512638. ^② (47Sm/144Nd)DM = 0.225; (143Nd/144Nd)DM= 0.51315.

), determining the T_2DM ages ranging from 1313 to 1338 Ma, which are significantly lower than the average age of 1.8 Ga for the Chinese continental crust surrounding seawater [121]. The results suggest the presence of substantial new mantle-derived materials in the sediment source area of phosphorites in the Gezhongwu Formation in the Zhijin area, aligning with the characteristic that the normal marine sedimentary process was accompanied by marine hydrothermal sedimentation in the Xinhua deposit.

Figure 22. Linear post-Archean Australian shale (PAAS)-normalized REE patterns (a) and Y/Ho ratios (b) of phosphorites from the Zhijin area in Guizhou Province. (a) Standardized values for Australian shale according to Taylor et al. [122]; (b) Y/Ho ratios for different fluids according to Gadd et al. [123].

Figure 22. Linear post-Archean Australian shale (PAAS)-normalized REE patterns (a) and Y/Ho ratios (b) of phosphorites from the Zhijin area in Guizhou Province. (a) Standardized values for Australian shale according to Taylor et al. [122]; (b) Y/Ho ratios for different fluids according to Gadd et al. [123].

Figure 23. SEM-BSE images of phosphorites (a,b) and transmission electron microscope (TEM) bright field image of nano-apatite (c) from the Zhijin area [124].

Figure 23. SEM-BSE images of phosphorites (a,b) and transmission electron microscope (TEM) bright field image of nano-apatite (c) from the Zhijin area [124].

The geochemical characteristics of the Zhijin phosphorite-hosted REE deposit reveal its unique sedimentary environment and mineralization processes. The phosphorites show δEu values ranging from 0.94 to 1.22, indicating weak negative to slightly positive Eu anomalies, while the overlying carbonaceous shales exhibit δEu values of 0.88–0.99 with weak negative Eu anomalies [100]. This contrast suggests hydrothermal activity during phosphorite deposition versus normal marine sedimentation for the shales. In the ΣREE-La/Yb diagram (Figure 24), phosphorite samples cluster in the granite field, reflecting contributions from deep crustal materials, whereas shale samples plot at the intersection of sedimentary, granitic and basaltic fields, indicating combined marine sedimentation and hydrothermal influences. Diagnostic redox proxies including U/Th ratios (1.52–3.05) and Co/Ni ratios (0.16–0.90) suggest involvement of reducing hydrothermal fluids. The pronounced negative Ce anomalies (δCe = 0.30–0.37), distinct from typical epicontinental/shelf environments [100,125,126], further confirm formation in a unique marginal marine setting (Figure 25). These geochemical signatures collectively demonstrate that the deposit formed through combined marine sedimentation and hydrothermal activity with significant contributions from deep crustal sources.

3.3.6. Genesis of the Xinhua Deposit

The Xinhua deposit in the Zhijin area is a typical marine sedimentary deposit, which primarily comprises dolomites, siliceous dolomites, and dolomitic/siliceous/bioclastic phosphorites. The sedimentary environment, in which the normal marine sedimentary process was accompanied by marine hydrothermal sedimentation, resulted in low phosphorite grades and enriched REEs in the Xinhua deposit. The mineralization in the Xinhua deposit was influenced by various factors, with lithofacies paleogeography and secondary oxidation constituting the primary metallogenic conditions. The Xinhua deposit is located on the eastern margin of the Niushoushan paleocontinent, which controls its sedimentary basement.

The Xinhua deposit in the Zhijin area was mineralized during the Meishucunian of the Early Cambrian. Initially, the environmental influences led to the submersion of the original carbonate platform by water, thereby creating a semi-closed shallow-water basin that served as a habitat for plankton and algal organisms. These organisms absorbed, fixed, and enriched REEs. The intrusion of hydrothermal water (containing REEs brought by Pangaea break-up prior to the Early Cambrian, i.e., terrigenous clasts) and the accumulation of deceased organisms on the seafloor led to significant submarine sedimentary organic matter, which was transformed into inorganic phosphorus through chemical processes [109,118,119]. Under the turbulence of seawater, carbonate and phosphate particles experienced coagulation and gelification, forming a system composed of phosphate rocks, carbonatites, silica nodules, and clay minerals. The carbonate minerals were dissolved and lost due to oxidation. Under the influence of marine currents, waves, and tides, microgranular phosphorites were subjected to fragmentation, transport, and sedimentation or gathered around certain particles to form granular and oolitic phosphorites. Furthermore, phosphate solution might entrain other solid particulate matter to precipitate and form shelly granular phosphorites [101]. Subsequent geological processes led to the formation of gelatinous phosphorites. During the process, ion exchange occurred between REEs and Ca ions in collophane due to their similar radii. As a result, REEs occurred as isomorphs in collophane [107]. Later, due to secondary weathering and leaching, high-solubility carbonate minerals within phosphorites dissolved, accompanied by the loss of some LREEs along with CaO and MgO, thereby leading to relatively enriched collophane [129]. The presence of weathering-altered phosphorites in the deposit [103], the increased mass fractions of P₂O₅, Fe₂O₃, and Al₂O₃, and the decreased mass fractions of MgO and CaO [130] suggest the loss of REEs under the weathering and leaching effects. In summary, the Xinhua deposit in the Zhijin area is genetically a marine biological-chemical sedimentary phosphorite deposit (Figure 26).

4. Regional Mineralization and REE-Rare Metal Mineralization Potential

4.1. Regional Mineralization

Southwest China broadly experienced three mineralization phases: the Precambrian, Paleozoic, and Meso-Cenozoic phases. Among them, the Meso-Cenozoic mineralization phase shows the closest relationship with REEs.

(1) During the Precambrian, the Yangtze region had a relatively thin crust. Ancient mantle plume activity may have led to the formation of the SN-trending Xichang-central Yunnan rift basin. These conditions, accompanied by intense magmatic-volcanic processes, resulted in frequent mantle-crust material exchange, dominant minerals like Cu, Fe, and Au, and magmatic-volcanic-metamorphic mineralization in the region. As a result, the Yangtze region formed distinctive sedimentary-metamorphic reworked copper-iron deposits associated with marine volcanic extrusion, such as the Dahongshan, Lala, and Yi’nachang deposits. Moreover, this mineralization style is also observed in the basement sporadically exposed along the periphery of the Upper Yangtze block. Transitioning into the Meso- to Neoproterozoic and the Early Paleozoic, the ancient Upper Yangtze block took shape, and the geological process shifted to a prolonged period of stable cap rock evolution. During this stage, material exchange primarily occurred within the crust, potentially inheriting early mantle-derived materials. The subsequent intensified sedimentary mineralization led to the formation of iron/manganese/paleo-sandstone-hosted copper deposits closely linked to sedimentation, such as the Laniping-type paleo-conglomerate-hosted copper deposit, the Dongchuan sedimentary-metamorphic reworked copper deposit, and the Manyingou-Baozipu-type weathering-sedimentary iron deposit. During the Late Neoproterozoic, particularly in the Sinian-Cambrian transitional stage, the transport of polymetallic elements in basement rocks by contemporaneous deep faults on continental/platform/basin margins, in relatively enclosed restricted basins or platform-to-basin transition zones, resulted in sedimentation in carbonate formations and the formation of high-abundance layers of polymetallic elements like Pb, Zn, Ag, Hg, and Au. These laid a primary foundation for the final enrichment of the lead-zinc-mercury deposit within the Sinian-Early Paleozoic carbonate formation on the northeastern margin of the Upper Yangtze block and the lead-zinc deposit within the Sinian Dengying Formation-Early Cambrian carbonate formation on the western margin of the Upper Yangtze block. The Mesoproterozoic-Neoproterozoic Qingbaikouan witnessed significant extensive copper mineralization in the Yangtze block, represented by many large and medium-sized copper deposits distributed in the Yinmin and the Luoxue formations of the lower subgroup of the Kunyang Group on the eastern margin of the central Kangdian rift. These copper deposits include the Tangdan, Yinmin, Luoxue, and Xintang deposits in the Dongchuan area and the Tongshi, Shishan, and Sanjiachang Fengshan deposits in the Yimen area. Qiu et al. determined two quartz samples from the Dongchuan-type Luoxue copper deposit, obtaining the 40Ar-39Ar isochron ages ranging from 810 to 770 Ma, possibly suggesting the mineralization age or the enrichment and modification age of the copper deposit from the Jinningian to the Chengjiangian [131]. In the Nujiang-Lancangjiang-Jinshajiang region (also referred to as the Sanjiang region) in Southwest China, the mineralization events associated with the Proterozoic volcanic rocks occurred principally in the Shuangjiang continental margin arc zone in the Lincang area. The mineralization in the region was provided by the metamorphic intermediate-mafic volcanic formation in the Huimin Formation of the Mesoproterozoic Lancang Group. The iron deposit-hosted metamorphic terrane in the Lancang Group is primarily composed of the siliceous and argillaceous formations in the lower part, the volcanic-sedimentary formation in the middle, and the argilloarenaceous flysch formation in the upper part. Among them, the middle metamorphic intermediate-mafic volcanic-sedimentary rock assemblage is essentially a ferrosilicon formation. Its iron-rich mafic volcanic rocks carried a large amount of magnetite along with volcanic materials into the volcanic basin, providing a material source for the formation of iron ore beds. Additionally, elements like Fe, Si, S, P, and CO₂ brought by volcanic activity entered the oceanic basin via gas emission and thermal springs during the intermission of volcanic activity, providing a rich material basis for the sedimentation of siderite and magnetite ore beds. Most ore beds within the deposits formed during the eruption intervals of the mafic eruption cycle. In the middle-late stage of volcanic activity (i.e., the late mafic eruption cycle), volcanic activity peaked, leading to the extrusion of substantial mafic lavas and the subsequent overflow of iron ore molten lava, forming a thick and large iron ore body at the leading edge of mafic lavas. Therefore, the Huimin iron deposit is identified as a typical marine volcanic eruptive-sedimentary iron deposit, with a Mesoproterozoic metallogenic epoch and mineralization ages ranging approximately from 1600 to 1000 Ma.

(2) The Paleozoic witnessed the formation and evolution of the Yangtze block, which exhibited relative internal stability, intense marginal extension, basalt eruption, and the emplacement of mafic-ultramafic rocks. During the Chengjiangian, the Yangtze block took shape, accompanied by large-scale intrusion of granitic magmas on its western margin and extensional rifting in the southeastern Guizhou region, leading to the formation of Mn-Fe-bearing sediments in the marginal sea basin and its margins. During the Caledonian, the Yangtze block developed an epicontinental sea. The Late Sinian-Early Cambrian neritic carbonate formations hosted lead-zinc/phosphate/gold deposits, whereas the Early-Middle Cambrian carbonate formations in the southeastern Yunnan region hosted copper-lead-zinc-tungsten-tin-silver polymetallic deposits. Moreover, the Caledonian structural modification contributed to the enrichment and mineralization of the tectonic-hydrothermal vein antimony deposit and the altered-rock gold polymetallic deposit hosted in the Meso- to Neoproterozoic epimetamorphic rock series in the southeastern Guizhou region. The Hercynian manifested weak magmatic activity in the early stage and basalt extrusion and the emplacement of ultramafic-mafic magmas in the late stage, generating vanadium-titanium-magnetite/copper-nickel-platinum-palladium/lead-zinc deposits. Due to the rise of the mantle plume during the Hercynian, extremely thick Devonian and Carboniferous sediments occurred in the Liupanshui rift trough, which formed due to crustal thinning, extension, and subsidence in the initial stage. Moreover, the possible circulation of Pb-Zn-rich hydrothermal fluids in deep faults along the rift trough boundary might have led to the sedimentation in the carbonate formation and the formation of a high-abundance layer of polymetallic elements like Pb, Zn, and Ag, providing an initial foundation for the final enrichment of the Huize-type lead-zinc deposit in the Late Paleozoic carbonate formation. The lead-zinc deposits might have formed concurrently with the sedimentation and diagenesis of Devonian carbonate formations. During the Late Hercynian, the activity of the Emeishan mantle plume reached its peak, marking another significant mineralization event in Southwest China, forming Panzhihua-type magmatic fluids vanadium-titanium-magnetite deposits, volcanic rock-hosted copper deposits, and lead-zinc deposits. In the early Late Permian, Southwest China exhibited tropical rainforests with lush vegetation, contributing significantly to coal formation. Bauxite deposits within the Middle Permian Liangshan Formation and the Upper Triassic Xuanwei Formation were associated with the littoral-swamp facies. From the Neoproterozoic to the Early Paleozoic, the marginal active zone on the east side of the Sichuan-Yunnan rift zone along the western margin of the ancient Yangtze block evolved into a significant lead-zinc metallogenic belt. These deposits are distributed within dolomites with algal layers in the second member of the Sinian Dengying Formation, frequently associated with the anhydrite layers. Based on the attitudes of ore bodies, the deposits can be categorized into stratiform and vein deposits, with the latter type being larger-scale, exemplified by the Daliangzi, Tianbaoshan, and Tuanbaoshan deposits [132]. The Early Paleozoic mineralization in the Sanjiang region in Southwest China occurred primarily on the passive margins of the Zhongza and the Baoshan blocks, which are located on the east and west sides of the Qamdo-Pu’er block, respectively. It was predominately characterized by lead-zinc mineralization, thus forming sedimentary exhalative lead-zinc deposits. Such lead-zinc deposits are hosted by the Early Paleozoic marine carbonate rocks and exemplified by the Najiaoxi lead-zinc deposit in the Zhongza block in Batang County and the Mengxing lead-zinc deposit in the Baoshan block. Specifically, the Najiaoxi lead-zinc deposit is situated within the Paleozoic carbonate platform, with a model age of lead-zinc ores at 552 ± 73 Ma, which is comparable to the age (615 to 520 Ma) of the Cambrian and the ore-hosting surrounding rocks. Therefore, the Cambrian units can be considered the synsedimentary ore source layers, with their mineralization closely associated with hydrothermal sedimentation. Based on the information above, the Paleotethys Ocean can be understood as an archipelagic ocean that developed from the continental margin system of the Prototethys Ocean. It was an oceanic system characterized by alternative distributions of multiple blocks, oceanic basins, and island arcs, along with a complex continental margin. Zhong et al. regarded the Paleotethys Ocean as an archipelagic ocean system due to its archipelagic ocean pattern [133]. However, Pan et al. highlighted the continental margin system, terming the Paleotethys Ocean as a multi-island arc basin tectonic system [134]. Zhong et al. revealed that the Changning-Menglian (Lancang River) between Gondwana- and Yangtze-affiliated blocks served as the main oceanic basin, whereas the Jinsha River—Ailao Mountains, Ganzi-Litang, and the South Kunlun Mountains—Aemye Ma-chhen Range were considered as subsidiary oceanic basins [133]. Pan et al. systematically analyzed five ophiolite melange belts and four arc-basin systems in the Sanjiang region and compared them with the arc-basin tectonic system in Southeast Asia [134]. They proposed that the prototype basins represented by the Paleozoic ophiolite melange belts in the Sanjiang region were predominantly back-arc oceanic basins, interarc basins, and marginal sea basins. During the Paleotethys Ocean evolutionary stage, the coexisting arc chains (frontal/island/volcanic arcs), the back-arc, interarc, and marginal sea basins in an alternative distribution, and microblocks constituted a complex continental margin tectonic system. Driven by the subduction of back-arc basins and their oceanic crusts, the archipelagic orogenesis involving arc-arc and arc-continent collisions resulted in the Southeast Asia-type orogenic process during the Mesozoic. The Jinshajiang arc-basin system is characterized by the Qiangtang-Jitang-Chongshan-Lancang remnant arc as its frontal arc along the western margin of the Qamdo-Simao Block. Simultaneously, the Lancangjiang back-arc basin expanded northeastward of this remnant arc. The Yidun arc-basin system resulted from the westward subduction of the Ganzi-Litang oceanic basin, which formed through back-arc spreading [135]. The Qamdo-Simao Basin, nestled between the Jinshajiang suture zone and the northern Lancangjiang fault zone, is recognized as a composite back-arc foreland basin formed on the Proterozoic-Lower Paleozoic basement from the Late Paleozoic to the Mesozoic. Based on the above, during the Late Paleozoic in the Sanjiang region in Southwest China, the successive closure of paleo-oceanic basins that opened from the Carboniferous to the Early Permian and the subduction-induced orogenic processes resulted in an archipelagic arc-basin system. This system produced the Tongchangjie copper deposit (typical of Cyprus-type deposits) associated with the Permian ocean basalt series and the Laochang lead-zinc-silver polymetallic deposit (typical of volcanogenic massive sulfide (VMS) deposits) associated with the slightly alkaline intermediate-mafic volcanic series. The Late Paleozoic mineralization events were characterized by submarine hydrothermal exhalative-sedimentary mineralization, forming various VMS deposits in three significant metallogenic environments. Specifically, the Changning-Menglian oceanic basin environment developed the Tongchangjie copper deposit (typical of Cyprus-type deposits) associated with the Permian ocean-bridge basalt series and the Laochang lead-zinc-silver polymetallic deposit (typical of VMS deposits) associated with the slightly alkaline intermediate-mafic volcanic series [136,137]. The Carboniferous-Permian volcanic arc (Tenasserim) environment, created by the eastward subduction of the Changning-Menglian oceanic basin, produced the Dapingzhang-type VMS deposit in the Carboniferous marine quartz keratophyre series (360 Ma; [138,139]) and the Sandashan-type massive sulfide copper deposit in the Late Permian marine intermediate-felsic volcanic series [140]. The intra-oceanic arc environment, caused by the westward subduction of the Jinshajiang oceanic basin, developed massive sulfide deposits associated with the Permian arc volcanic series, exemplified by the Yangla copper deposit [141]. Overall, the Late Paleozoic mineralization phase was dominated by copper, lead, and zinc mineralizations, succeeded by iron mineralization, predominantly forming large-scale VMS deposits, showing promising exploration potential. The Changning-Menglian rift oceanic basin exhibited different mineralization types in various evolutionary stages. In the early rift development stage, volcanic activity resulted in highly alkaline rift basalts, which, combined with hydrothermal fluid mineralization, led to the VMS deposits represented by the Laochang deposit. In the late oceanic basin stage, volcanic activity reduced the alkalinity of volcanic rocks, resulting in ocean-ridge basalts. The ocean-ridge basalts under volcanic-sedimentary mineralization formed the CVHMS deposits represented by the Tongchangjie deposit. Although the volcanic activity in the rift basin was generally controlled by the rift valley—oceanic basin system, the resulting deposits were significantly dictated by volcanic edifices and fault structures. The early hydrothermal fluid mineralization was dominated by metallic elements including Ag, Pb, and Zn, associated with S and Cu. Individual ore bodies featured upper black ores (massive silver-lead-zinc ore body) and lower yellow ores (Cu-bearing pyrite ore body). The copper ore body exhibited a veinlet-disseminated structure at the lower part, with a concealed porphyry mass in the deep part. The late volcanic-sedimentary mineralization showed primary metallic elements including Cu and Zn. The Carboniferous-Permian volcanic arc environment, produced by the eastern subduction of the Lancangjiang oceanic basin, developed the massive sulfide copper deposits (e.g., the Sandashan copper deposit) associated with the Carboniferous-Permian marine intermediate-felsic volcanic series and the volcanic-sedimentary iron deposits (e.g., the Manyang iron deposit) associated with the marine mafic volcanic rocks. Although the mineralization in the Yunxian-Jinghong rift basin was generally controlled by the rift zone, the deposits and orefields were primarily distributed in volcanic edifices and depressions along fault zones, with the deposit types and chemical structures varying with rift segments. For instance, the volcanic-sedimentary mineralization in the southern segment formed the Sandashan volcanic-hosted massive sulfide (VHMS) deposit with a dominant metallic element of Cu, while the mafic volcanic rocks in the rift basin formed the volcanic-sedimentary Manyang iron deposit. The Late Triassic mineralization events occurred principally in the magmatic arc environment and partially in the superimposed basin environment above magmatic arcs, resulting in various VMS deposits and porphyry copper deposits. In the Yidun island arc belt, the slab tearing and differential subduction of the Ganzi-Litang oceanic basin caused the arc segmentation. Specifically, the northern segment developed the Changtai extensional arc due to the steep subduction of the oceanic crust slab, hosting interarc rift basins. The southern segment developed the Zhongdian compressional arc due to the gentle subduction of the oceanic crust slab, developing significant intermediate-acid porphyry systems [142]. The interarc rift basins in the Changtai extensional arc developed the VMS zinc-lead-copper deposits like the super-large Gacun deposit [143]. The bimodal volcanic series associated with VMS mineralization showed ages ranging from 220 to 218 Ma [135]. The Re-Os ages of the sulfide ores ranged from 218 to 217 Ma [142]. The porphyry magmatic system within the Zhongdian compressional arc developed porphyry and skarn deposits, exemplified by the large Pulang porphyry copper deposit and the Hongshan skarn copper polymetallic deposit. For the Pulang porphyry copper deposit, the copper-bearing porphyry showed ages ranging from 216 to 213 Ma [144], and the molybdenite yielded a Re-Os age of 213 ± 3.8 Ma [145]. In the southern segment of the Jiangda-Weixi continental margin arc, the Late Triassic extension led to the formation of a volcanic-rift extensional basin, which was superimposed upon the Permian continental margin arc terrane [146]. The basin developed massive sulfide deposits associated with the Late Triassic bimodal rock assemblages. Furthermore, the deep-water marine felsic volcanic series (Rb-Sr age: 230 Ma) produced the Luchun-type copper polymetallic deposit, while the shallow-water intermediate-acid volcanic series produced the Chuzhage-type iron-silver polymetallic deposit. During the contraction and extinction stage of the volcanic rift basin, hydrothermal-sedimentary mineralization generated large barite and gypsum deposits [146,147]. The superimposed basin located in the northern segment of the Jiangda-Weixi continental margin arc holds massive sulfide deposits associated with the Late Triassic intermediate-acid volcanic series in shallow-water volcanic environments, such as the Zhaokalong and the Dingqingnong iron-silver polymetallic deposits. Moreover, the superimposed basin hosts submarine hydrothermal-sedimentary deposits associated with the Late Triassic basalt series and extensional basins, for example, the Zuna silver-lead-zinc deposit in the Shengda Basin [147]. In summary, the Late Paleozoic mineralization in the Sanjiang region in Southwest China is principally associated with the archipelagic arc-basin systems. The hydrothermal exhalative sedimentation and arc volcanism in rift valleys/basins primarily led to copper, lead, and zinc mineralizations, followed by iron mineralization. The resulting deposits are dominated by VMS deposits, succeeded by hydrothermal deposits, showing overall large scales.

(3) During the Mesozoic Indosinian (260–205 Ma), the subduction, collision, and rifting of block margins resulted in Carlin-type gold deposits. Frequent and intense magmatic activity enhanced metamorphic clastic-rock gold-iron-manganese deposits in the Barkam region on the Western Sichuan Plateau. Due to the subduction of the Pacific plate beneath the Yangtze block, the Cathaysia block moved further towards the Yangtze block, enriching the fine disseminated gold deposits through structural modification. In the folding and uplifting stage of continental blocks during the Mesozoic Yanshanian (205–80 Ma), the subduction of the Pacific plate toward the Chinese continent caused regional folding and uplifting, shifting the structural line from EW to NE-NNE directions. This led to the modification and enrichment of the initial lead-zinc protore beds, forming lead-zinc vein deposits. Furthermore, the Yanshanian magmatic activity in the southeastern Yunnan region played a significant role in the modification, enrichment, and localization of the copper-lead-zinc-tungsten-tin-silver polymetallic deposits, and sedimentary (sandstone-hosted) copper-iron deposits formed in the Mesozoic red-bed basins. During the Triassic, the structural framework characterized by alternating continental-margin island arcs and basins emerged. The Barkam-Xiangcheng and Yushu-Zhongdian areas in the north inherited the active basin environment that had formed since the Late Paleozoic. Flysch basins and island arc volcanic-sedimentary basins formed in the east and the west, respectively. In the early stage of the Late Triassic, typical bimodal volcanic formations were extensively developed in the Yidun island arc belt, and syn-collisional arc granite and diorite porphyries were developed in the northwestern Yunnan region, along with large-scale submarine volcanic exhalation and porphyry mineralization. During the Jurassic, seawater receded to the Tibet and western Yunnan regions due to the impacts of the Indosinian movement on the Chinese paleogeographic environment, surviving the Tethys-type sea area. The Riwoqe-Lhorong area in the northwestern Sanjiang region still developed metastable marine basins and flysch formations. The Shiqu-Xinlong-Muli area also developed similar sedimentary rocks in unconformable contact with the underlying Late Triassic volcanic rocks, suggesting that the Tethyan marine sedimentation had extended to the Ganzi area in the western Sichuan region. During the transition from the plate collisional orogeny in the Indosinian cycle to the intracontinental orogeny in the Yanshanian-Himalayan cycle, the Songpan-Ganzi orogenic belt transitioned into a relatively stable postcollisional stage. At the end of magmatic activity, large-scale intrusion of intermediate-felsic magmas occurred in the relatively stable and closed environment, laying the foundation for the post-magmatic pegmatite rare metal mineralization. During the Cretaceous, the Sanjiang region seldom received sediments, except for the Qamdo-Lanping-Simao basin and the southern side of the Bangonghu-Nujiang suture zone. During the Late Cretaceous, subjected to the subduction of the Bangonghu-Nujiang Ocean and the collisional orogeny, the granites formed in a back-arc thrusting setting in the Tengchong area were significantly associated with tungsten-tin mineralization. In the Baoshan area, the mountain belt subjected to intracontinental deformation developed post-orogenic A2-type granites, which played a vital role in the localization of lead-zinc polymetallic deposits. In addition, constrained by the large Longmenshan-Daxueshan-Jinpingshan nappe structure and magmatism, post-collisional granites/granite porphyries formed in the Songpan-Ganzi and Yidun-Zhongdian areas, dominating the Yanshanian copper polymetallic mineralization.

During the Triassic, the Yajiang remnant basin constituted the main body of the Songpan-Ganzi fold belt in the Sanjiang region, with the east and west sides of the fold belt bounded by the Luhuo-Daofu and the Ganzi-Litang ophiolite melange zones, respectively. At the end of the Triassic, the closure of the Ganzi-Litang oceanic basin and the arc-continental collision orogeny caused the Sanjiang region to transition into the folding stage, developing granites in the Late Triassic-Jurassic collisional—post-collisional orogenic environment (238–179 Ma and 137–97 Ma). In this stage, the mineralization was characterized by the enrichment of rare metal deposits, which were primarily distributed in the central and eastern metallogenic belt. The identified rare metal deposits include the super-large Jiajika pegmatite lithium-beryllium deposit, the large Zhawulong pegmatite lithium-beryllium deposit in Shiqu County, the medium Rongxuka pegmatite lithium-beryllium deposit in Daofu County, the medium Daqianggou pegmatite lithium-beryllium deposit in Jiulong County, and the small Murong pegmatite lithium-beryllium deposit in Yajiang County. Among them, the most significant is the Jiajika pegmatite lithium-beryllium deposit, which represents the largest pegmatite lithium polymetallic deposit in China. The Jiajika pegmatite lithium-beryllium deposit is located within the dome-shaped short-axis anticline, along which the Late Triassic-Jurassic Li-bearing two-mica granite stocks intruded into the flysch primarily composed of extremely thick clastic rocks in the Triassic Xikang Group. Many granitic pegmatite veins were formed around the inner and outer contact zones of granites, exhibiting vein, irregular vein, and lenticular patterns. Their minerals include spodumene, beryl, columbite-tantalite, and cassiterite, with spodumene primarily occurring in the fine- to medium-grained quartz-albite-spodumene metasomatic zone. Wang et al. determined the Ar-Ar plateau ages of 195.7 ± 0.1 Ma and 198.9 ± 0.4 Ma and the isochron ages of 195.4 ± 2.2 Ma and 199.4 ± 2.3 Ma for the ore veins [148]. Moreover, Li et al. determined the Ar-Ar plateau ages of 176.25 ± 0.14 Ma and 152.43 ± 0.60 Ma for the pegmatite veins in the Ke’eryin rare metal deposit [149]. These results suggest that the mineralization of the ore veins occurred primarily during the Early-Middle Jurassic in a post-collisional (magmatic rock subfacies) crustal thickening setting.

4.2. Mineralization Potential

Rare metal and REE deposits have been discovered in many provinces across Southwest China. Specifically, 32 REE deposits/occurrences have been identified, including five large, four medium, and nine small deposits and 14 mineralized spots. They are categorized into seven genetic types. However, most mineral localities have seen limited exploration and have low proven reserves, with industrial deposits primarily containing LREEs. The Mianning-Dechang area in Sichuan holds proven REE resources of approximately 250 × 104 t, serving as the most promising area for bastnäsite exploration, with predicted prospective resources exceeding 500 × 10⁴ t [14]. The Maoniuping REE deposit in the area is identified as a world-class large deposit, characterized by shallow ore burial depths, high ore grades, ease of mining and beneficiation, and high quality. Weathering crust REE deposits are distributed in Longchuan and Jinping areas in western and southern Yunnan Province, respectively. Some phosphorite-hosted REE deposits with REEs as by-products are distributed in northwestern Guizhou Province [150]. Additionally, the palaeo-weathered crust-sedimentary-clay-type REE polymetallic deposits have been discovered in the eastern Yunnan-western Guizhou region. Their unique REE enrichment characteristics have significantly increased the potential for REE exploration in Southwest China. The three-rare elements (e.g., REEs, niobium, scandium, gallium, and zirconium) are enriched in the lower claystone sedimentary layers of the Xuanwei Formation. Moreover, the TREOs in many areas exhibited grades exceeding 0.1%, with the highest grade up to 0.42%, and an average grade of about 0.2%. The ore bodies manifest exposed thicknesses ranging from 2 to 6 m, and prospective resources of over 100 × 104 t, which surpass those of large/super-large REE deposits currently being exploited and utilized in China, such as the Mianning carbonatite deposit in Sichuan Province, the Bayan Obo carbonatite deposit, the Weishan carbonatite deposit in Shandong Province, and some iREE deposits in South China. Compared to iREE/paleoplacer REE deposits, the sedimentary deposits in Southwest China show significant advantages in ore grades, resource scales, concentration degrees, exploitation methods, and environmental impacts. Moreover, they are characterized by thick ore beds, high ore grades, considerable resource potential, and high proportions of critical REEs (CREEs). With the advancement of beneficiation and smelting, the leaching rate of REEs from kaolinite approaches or even exceeds 90%, while those of major impurities including aluminum, iron, titanium, and silicon are below 5%, further enhancing the exploitation potential of sedimentary/clay REE deposits. Upon the completion and strengthening of the assessment system for sedimentary REE polymetallic deposits, such deposits in Southwest China may constitute China’s significant strategic reserve base for three-rare mineral resources [15,16].

The iREE deposits in Yunnan Province are primarily distributed in southeastern Yunnan, where the Lincang rock mass has been significantly explored and received extensive attention in recent years. Compared to the REE deposits in the Nanling region, the iREE deposits in Yunnan occur at elevations above 1000 m, belonging to the weathered crust type formed in plateau-low mountain-hilly environments. Their ore-forming lithologies, distributed along the Sanjiang fault zone, comprise primarily monzogranites and K-feldspar granites featuring medium- to coarse-grained textures. Their zircon U-Pb ages are concentrated in two age intervals: 208–240 Ma and 52–80 Ma. Based on the distributions of LREEs and HREEs, the bedrocks can be classified into the LREE and the HREE types. The LREE-type bedrocks are principally enriched in LREEs, whereas the HREE-type bedrocks are relatively enriched in HREEs. REE-bearing minerals like sphene, allanite, and fluorocarbonate that are susceptible to breakage and dissociation under weathering serve as the primary source of REE³⁺ (Trivalent rare earth element ions) in the deposits. The ore-bearing weathered crusts are composed of detrital, clay, and accessory minerals, with clay minerals being the major REE³⁺ carrier. A complete granite weathered crust consists of a topsoil layer, a completely weathered layer (the primary ore-bearing layer), a semi-weathered layer, and a slightly weathered layer. Based on the distributions of REEs in weathered crusts, REEs exhibit arched, trumpet-shaped, and wavy patterns. Furthermore, they display concealed and exposed patterns due to the varying thickness of the topsoil layers. The migration and enrichment of REE³⁺ in weathered crusts are dictated by factors, such as bedrock, geomorphology, and environment. Moreover, the enrichment and mineralization of REE³⁺ are significantly influenced by continuous and repeated weathering processes [151]. More than 95% of iREE deposits in Yunnan Province are distributed in igneous rock areas along the Tengchong-Yingjiang-Longchuan and Lincang-Menghai regions south of 26° N, with ore-forming parental rocks dominated by Yanshanian and Himalayan monzogranites. The hot and humid climate, abundant rainfall, and lush vegetation in Yunnan Province lead to a rich source of organic acids and intense chemical weathering. Consequently, minerals like sphene, allanite, and apatite are prone to break and decompose under supergene conditions, forming extremely thick weathered crusts. Thereupon, activated REEs are absorbed by clay minerals and transported and enriched for mineralization, showing promising mineralization potential for iREE deposits. Notably, the granite weathered crust zone formed from the Mesozoic Cretaceous granite parental rocks exhibits significant prospects for exploring large-scale and high-grade LREE deposits. Additionally, the claystone distribution zone within the Xuanwei Formation in northeastern Yunnan Province shares similar metallogenic conditions to the sedimentary REE polymetallic deposits in the Hezhang-Weining area in Guizhou Province, displaying a significant prospecting potential for REE deposits. Sedimentary REE polymetallic deposits are expected to be a new source of three-rare mineral resources in China [16].

Based on the distributions of 14 REEs in South China, Cheng et al. revealed the exceptional geochemical anomalies of REEs centering on the Xuanwei area at the junction of Guizhou, Yunnan, and Guangxi provinces [152]. By comparing weathered crusts near the Middle and Upper Permian boundaries in western Yunnan and Guizhou provinces, Zhao et al. [153] found that basalt weathered crusts are enriched in elements like Cu, REEs, and Al, tending to form bauxite and REE deposits. A typical example is the Lufang REE deposit in the Weining area, Guizhou Province. Through detailed studies of profile sedimentary sequences and REE geochemistry, researchers proposed that REEs released from the weathered Emeishan basalts were transported by aqueous media, absorbed by the unconsolidated kaolinite mineral grains, and subjected to diagenesis, finally forming hard kaolinitic claystone-hosted REE deposits. REE mineralized occurrences have been identified in Weining, Hezhang, and Bijie areas in Guizhou Province. Previous field surveys and geochemical analyses suggested that the weathered crust of the Emeishan basalts provides ore-forming materials for these mineralized localities and that the top weathered crust layer of the Emeishan basalts serves as a significant layer for searching for REE deposits. Within the Yulong niobium deposit in the Weining area, a REE enrichment layer has been discovered in the aluminous claystones at the base of the coal-bearing rock series of the Upper Permian Xuanwei Formation. This REE enrichment layer exhibits slightly higher REE content compared to the basalt weathered crust REE deposits in western Guizhou Province. Moreover, its REE content shows an upward trend from massive basalts to tuffaceous and bauxitic claystones, combined with the increasingly strong negative δEu anomalies, indicating that the weathering of basalts led to the enrichment of REEs.

Overall, based on the metallogenic conditions, ore-controlling factors, and REE production potential of various REE deposits in Southwest China, six prospect areas of REE deposits can be identified in Southwest China: the Mianning-Dechang, Weining-Zhijin, Long’an, Simao-Adebo, and Shuiqiao prospect areas of REE deposits, as well as the prospecting area of palaeo-weathered crust-sedimentary-clay-type REE deposits in the Xuanwei Formation in Muchuan (Figure 27).

5. Conclusions

(1) The southwestern region of China is tectonically situated within the Tethyan domain, demarcated by the Longmenshan-Ailaoshan fault zone that separates the eastern Upper Yangtze Block from the western orogenic belt. As the principal component of the Tibetan Plateau, the western orogenic belt formed through subduction of the Paleo-Tethys oceanic basin and subsequent arc-continent collisions. Its present structural complexity (including the Hengduan Mountains), resulted from later modifications due to the Indian Plate collision. The region’s lithostratigraphy comprises six major terranes: the Qin-Qi-Kun, Qiangtang-Changdu-Sima, Bangong Lake-Shuanghu-Nujiang, Gangdese-Himalaya, Indian, and Yangtze terranes, each characterized by distinct lithological assemblages. Metallogenically, the area contains two first-class metallogenic domains—the Tethyan-Himalayan and Pacific Rim domains—which host abundant mineral resources dominated by copper, zinc, bauxite, and rare earth elements.

(2) Comparative analysis of regional tectonic-metallogenic settings reveals that the distribution of REE deposits in southwestern China is fundamentally controlled by the evolution of the Tethyan tectonic domain. The eastern region is predominantly characterized by sedimentary-weathering type deposits, while the western area hosts deposits genetically associated with orogenic magmatic activities. This systematic comparison not only elucidates the metallogenic patterns of REE mineralization, but also establishes a robust scientific foundation for cross-regional resource assessment and optimization of exploration strategies. The findings provide critical insights for understanding the spatial distribution and genetic types of REE deposits in relation to the regional tectonic framework.

(3) A total of 32 REE deposits and occurrences have been identified in southwestern China, predominantly enriched in light REEs. These deposits can be genetically classified into three main types: (1) endogenic deposits, including alkaline rock-carbonatite type and granite-related type; (2) exogenic deposits comprising weathering crust type, ion-adsorption type, and sedimentary type; and (3) metamorphic deposits. Cretaceous granite weathering crusts are particularly noteworthy for their significant potential to host ion-adsorption type REE deposits. Representative examples include the Mianning-Dechang prospect areas in Sichuan Province with estimated resources of 2.5 million tonnes REO, the Longchuan deposit in western Yunnan, and the phosphorite-associated REE occurrences in northwestern Guizhou. The recent discovery of paleo-weathering crust-sedimentary type REE deposits in the eastern Yunnan-western Guizhou region has substantially expanded the exploration potential and opened new avenues for REE prospecting in this area.

(4) The metallogenic processes in southwestern China can be chronologically divided into three major epochs: the Precambrian, Paleozoic, and Mesozoic-Cenozoic. The Mesozoic-Cenozoic epoch shows the most significant genetic relationship with REE mineralization. Based on comprehensive metallogenic studies, six prospective exploration zones have been delineated: the Mianning-Dechang, Weining-Zhijin, Long’an, Simao Adebo, Shuiqiao prospect areas, and the Muchuan Xuanwei Formation paleo-weathering crust-sedimentary-clay type REE deposit exploration area. These prospective zones exhibit significant potential for REE mineralization, particularly the Muchuan Xuanwei Formation area, which represents a newly identified target characterized by its unique paleo-weathering crust-sedimentary-clay type REE mineralization. The delineation of these zones provides a scientific foundation for future exploration and resource assessment in southwestern China.