The Origin of Middle Triassic Thick Anhydrite in the Lower Yangtze Region, South China: Implications for the Evolution of Lithium-Rich Brine

Abstract

Lithium­(Li)-rich brines represent an increasingly important strategic resource and are commonly associated with evaporites. Their formation has traditionally been mainly attributed to evaporation-driven concentration under arid paleoclimatic conditions. This study investigates the genetic mechanism of the thick anhydrite deposits and associated lithium-rich brines in the Middle Triassic Zhouchongcun Formation of the Lower Yangtze region. Through integrating petrographic, elemental, and isotopic geochemistry and fluid inclusion analyses, combined with reconstruction of the stratigraphic burial-thermal history, we demonstrate that the formation and evolution of these evaporates are closely linked to multiple tectonic-thermal events. Petrographic characteristics and carbon–oxygen isotopic data indicate that the anhydrite initially formed in a restricted evaporative environment during the Middle Triassic, with δ¹³C ranging from −2.9 to −0.4‰ and δ¹⁸O values from −8.1 to −5.6‰, showing a significant positive correlation. Fluid inclusion data reveals subsequent complex hydrothermal overprinting. Primary inclusions record homogenization temperatures of 181.3–210.2 °C, corresponding to the peak paleo-temperature reached during early deep burial. In contrast, secondary inclusions yield lower homogenization temperatures of 135.8–174.3 °C, consistent with hydrothermal activity associated with the Yanshanian tectonic-thermal event. Burial–thermal history reconstruction indicates significant heating during the Middle Jurassic, with maximum temperature reached between approximately 150–170 Ma, coinciding with regional magmatic-tectonic activity. These results indicate that the anhydrite underwent intense reworking by medium- to high- temperature hydrothermal fluids during the Yanshanian period, superimposed upon its primary evaporative sedimentary origin. Geochemical data further shows that both fluid inclusions within anhydrite and the formation brines belong to a mixed sodium sulfate-sodium chloride hydrochemical type, supporting a common fluid source The lithium content in secondary inclusions is more than 4.8 times higher than that in primary inclusions, which the lithium content in regional intrusive rocks is 9.4 times higher than background levels, indicating a dominant contribution from Yanshanian magmatic-hydrothermal fluids. Micro-Raman spectroscopy combined with brine ion composition analysis jointly provides additional evidence for substantial hydrothermal input of lithium. Based on these observations, a three-stage genetic model is proposed: (1) sedimentary foundation, characterized by anhydrite precipitation and initial brine formation in a closed evaporative basin during the Middle Triassic; (2) lithium addition via hydrothermal reworking, whereby lithium-rich magmatic–hydrothermal fluids ascended during the Yanshanian event (∼150 Ma), modifying pre-existing anhydrite and enriching brines at temperatures of 150 °C; and (3) structural adjustment and accumulation, involving lithium migration and concentration within anhydrite–dolostone anticline traps during post–Late Cretaceous uplift and cooling under extensional tectonic conditions. This model highlights the key controlling role of medium-low temperature hydrothermal events in the formation of evaporite-type lithium resources and provides a new genetic framework and exploration guide for lithium-rich brines in evaporite basins.

1. Introduction

The Triassic marked a pivotal phase in the progressive closure of the Paleo-Tethys Ocean, − during which profound tectonic, climatic, and sedimentary transformations occurred on a global scale. One of the most prominent geological consequences of this evolution was the widespread development of evaporite depositional systems along the circum–Paleo-Tethyan realm. These systems formed a laterally extensive, east–west–trending evaporite belt that stretched from North America through Europe, North Africa, and West Asia to southern China. − The formation of this giant evaporite province was closely associated with large-scale plate reorganization, paleoclimatic differentiation, and basin evolution throughout the Triassic. Since the Middle Triassic, convergence and eventual collision between the Yangtze Plate and the North China Plate resulted in the development of the Sulu orogenic belt, fundamentally reshaping the tectonic architecture and sedimentary environments of eastern China. This tectonic amalgamation drove a regional environmental transition from a relatively stable, warm, shallow-marine system under tropical low-pressure climatic conditions to a restricted tidal flat–lagoon setting governed by a subtropical high-pressure belt and continental arid trade-wind circulation. Such a shift not only created favorable conditions for large-scale evaporite precipitation but also promoted deep-seated hydrothermal circulation linked to crustal thickening and thermal reactivation. The interaction between arid climate conditions and intensified tectono-thermal activity provided both accommodation space and metallogenic materials, facilitating the formation of a variety of evaporite-related mineral systems, , including sulfide mineralization and saline brine resources. Under this combined climatic and tectonic regime, multiple evaporite-bearing basins developed within tectonic depressions of the Lower Yangtze region, laying the foundation for subsequent hydrothermal modification and resource enrichment.

The Wuwei Basin is one of the most significant anhydrite-producing basins in the Lower Yangtze region and represents a typical Mesozoic salt lake basin in southern China. The Middle Triassic evaporite–dolostone succession in this basin attains a thickness exceeding 400 m and is characterized by broad lateral continuity and substantial individual bed thicknesses. Within this confined to semiconfined basin setting, the distribution and evolution of evaporites (e.g., anhydrite and gypsum), hydrocarbon reservoirs, and lithium­(Li)-rich brines are all controlled by complex fluid–rock interactions. − Although previous studies have primarily emphasized the genetic relationship between evaporites and hydrocarbon accumulation, the mechanisms responsible for anomalous lithium enrichment associated with evaporite formation and subsequent modification remain poorly constrained. Consequently, resolving the origin and postdepositional evolution of anhydrite is critical for elucidating the material sources and evolutionary pathways of lithium-rich brines in the basin. Most earlier interpretations attribute the anhydrite in the Wuwei Basin to strong evaporation and concentration of Triassic seawater, with subsidiary contributions from meteoric water circulation. ,, However, widespread occurrences of syngenetic siderite and pyrite suggest that at least part of the anhydrite may represent chemical precipitates linked to cooling hydrothermal mineralizing systems rather than purely evaporative sedimentation. , In addition, deep brine reservoirs hosted within dolostones beneath the anhydrite sequence contain lithium, boron, bromine, and iodine at industrial-grade concentrations, with Li contents commonly exceeding 55.0 mg/L. , Such multielement enrichment patterns are difficult to explain solely by a seawater evaporation model, implying more complex fluid sources and multistage evolutionary processes for both the evaporites and their genetically associated brines.

Carbon and oxygen isotope compositions of carbonate rocks are widely applied in paleoclimatic reconstructions, whereas brine inclusions provide direct and quantitative constraints on paleo-fluid properties, particularly temperature and salinity (i.e., paleotemperature and paleosalinity). − , In particular, the chemical signatures preserved in brine inclusions hosted by anhydrite offer valuable insights into the hydrochemical evolution of deep brines and the material sources of evaporites and related brine systems. − To decode the complex information recorded during anhydrite formation and subsequent brine evolution in the Wuwei Basin, this study integrates microthermometric data (homogenization temperatures and salinities) from brine inclusions in anhydrite with carbon–oxygen isotope compositions of dolostones from the Middle Triassic Zhouchongcun Formation. This multiproxy approach is designed to constrain paleoclimatic conditions, paleo-fluid characteristics, and the depositional environment of the evaporite sequence. The genetic interpretation of the anhydrite deposits and associated lithium-rich brines is further supported by major- and trace-element geochemistry of formation brines, compositional analyses of brine inclusions within anhydrite, and detailed petrographic and mineralogical observations from drill-core samples. − Based on the integration of these data sets, a comprehensive metallogenic model is proposed for the anhydrite deposits and their genetically related lithium-rich brine systems in the Wuwei Basin.

2. Geological Background

The Lower Yangtze region is located along the northeastern margin of the Yangtze Block and comprises a series of Meso–Cenozoic sedimentary basins in southern China, bounded by the Tan–Lu and Jiang–Shao fault systems. Covering an area of approximately 225,000 km² (Figure a), the region remained tectonically relatively stable prior to the Early Triassic, during which extensive marine limestone and shale sequences were deposited. , By the Middle Triassic, north–northwestward convergence of the Yangtze Plate toward the North China Plate led to progressive uplift associated with initial frontal collision. This tectonic transition resulted in progressive shallowing of marine environments and the widespread development of evaporites and dolostones across the Lower Yangtze region. , From the Late Triassic to the Early Cretaceous, sedimentation was dominated by clastic successions interlayered with igneous rocks. , Since the Late Cretaceous, basin-controlling fault systems became pervasive, and thick, laterally extensive continental clastic sequences formed the dominant stratigraphic units in the study area (Figure b). The Wuwei Basin, one of the Meso–Cenozoic evaporite-bearing basins in the northeastern Lower Yangtze region, covers an area of approximately 3,100 km². The basin trends predominantly NE–SW and is bounded to the northwest by the Zhaomingshan boundary fault adjoining the Hechao thrust belt, and to the southeast by the Yangtze River fault zone. , Internally, the basin is segmented by the Shijian, Longtang, and Dougou faults into two depressions separated by a central low uplift (Figure c,d). Although most of the basin is concealed beneath Quaternary cover, the northwestern margin exposes Cretaceous to Paleogene clastic strata as well as Neoproterozoic ultrahigh-pressure metamorphic rocks. In contrast, the southeastern and western basin margins are characterized by widespread Yanshanian granitoids, highlighting a potential genetic relationship between magmatic–hydrothermal processes and the formation of lithium-rich brine systems.

1.

(a) Tectonic location and spatial distribution of anhydrite basins in Lower Yangtze region. (b) Geological map of the Wuwei Basin and its surroundings. (c) Structural units of the middle Wuwei Basin and location of sampling drillholes. (d) Seismic line of the Wuwei Basin. The location of the seismic line is shown in panel (c).

The Middle Triassic Zhouchongcun Formation records deposition within a saline lagoon–tidal flat system and is distinguished by the exceptional development of thick anhydrite sequences. Geochemical indicators derived from well data, particularly the Sr/Ba ratio, show that sedimentary salinity reached its maximum during this interval (Figure ). In the study area, individual anhydrite beds attain thicknesses of up to 100 m, and the cumulative thickness of anhydrite exceeds 350 m. , Within the Wuwei Basin, the Zhouchongcun Formation is dominated by anhydrite and dolostone, with subordinate siliceous shale and marl interlayers. These finer-grained lithologies are interpreted to have formed during episodes of seawater reflux and relative water deepening, reflecting short-term fluctuations superimposed on an overall highly evaporative depositional regime (Figure ).

2.

Stratigraphy of the Wuwei Basin based on wells and field outcrops.

3. Samples and Analytical Methods

3.1. Carbon and Oxygen Isotope Analysis of Dolostone

Eight dolostone samples were collected from the core of well B11–1 in the Wuwei Basin for carbon and oxygen isotope analysis (Figure ). Petrographic examination indicates that the dolostones are dominantly fine-crystalline and show no evidence of significant recrystallization, suggesting that primary sedimentary textures are well preserved. , Consequently, their isotopic compositions are considered reliable for reconstructing the paleoenvironmental conditions of the Triassic lacustrine system. For isotopic analysis, samples were initially crushed to 1.0–2.0 mm fragments and subsequently pulverized to 200 mesh. Approximately 1.0–1.5 mg of powdered material was weighed into 12 mL Labco Exetainer vials. Each vial was flushed with high-purity helium for 5 min to eliminate atmospheric contamination, after which eight drops of anhydrous phosphoric acid were injected. The sealed vials were reacted at 70 °C for 45 min in a dry-block thermostat to liberate CO₂ gas. Carbon and oxygen isotope ratios were measured using a GasBench II coupled to a Delta V Advantage stable isotope ratio mass spectrometer at the Sinopec East China Laboratory. Isotopic results are expressed in δ (‰) notation relative to the Vienna Pee Dee Belemnite (VPDB) standard, with analytical precision better than ± 0.1‰.

3.

Columnar sections of anhydrite-dolomite wells and map of sampling locations.

3.2. Fluid Inclusion Analysis of Anhydrite

Fluid inclusion samples were obtained from anhydrite recovered from well B11–1, which penetrates the Middle Triassic Zhouchongcun Formation in the Wuwei Basin. Five representative samples of well-crystallized, gray–white anhydrite were selected for analysis (Figure a,b). Primary anhydrite crystals occur as isolated, elongated columnar or tabular forms (Figure c), whereas secondary anhydrite is characterized by bead-like aggregates arranged along fractures or growth zones (Figure d). These textural features preserve distinct populations of brine inclusions, recording the chemical characteristics of fluids entrapped during both syn-depositional crystallization and subsequent diagenetic or hydrothermal modification. ,

4.

Anhydrite samples and fluid inclusions in Middle Triassic Wuwei Basin. (a) Lumpy fine anhydrite; (b) microbedding anhydrite; (c) anhydrite primary brine inclusion from A02; (d) anhydrite secondary brine inclusion from A04.

3.3. Homogenization Temperature and Salinity Measurements

Raman spectroscopy, a nondestructive analytical technique widely applied in fluid inclusion studies, was used to characterize the composition, structure, and formation conditions of brine inclusions hosted in anhydrite. For anhydrite-hosted brine inclusions, Raman analysis enables identification of dissolved components and provides constraints on formation temperature and salinity. To ensure the integrity of fluid inclusions and obtain high-quality spectral signals, transparent, crack-free anhydrite crystals were selected for analysis. Samples were cut and polished into doubly polished sections with thicknesses of approximately 100–200 μm, allowing optimal exposure of inclusions and effective laser penetration. Brine inclusions were identified under a Zeiss Axiovert 200 polarizing microscope, with preference given to inclusions larger than 5 μm, exhibiting regular morphologies and clear optical characteristics. The locations, sizes, and morphologies of representative inclusions were documented by photomicrography prior to analysis. Microthermometric measurements were performed using a Linkam THM600 heating–freezing stage following standard thermal cycling procedures. − Homogenization temperatures (Th) were measured during heating at a rate of 5 °C/min, whereas final ice-melting temperatures (Tm) were determined during warming at 1 °C/min. Analytical uncertainties are estimated at ± 5 °C for Th and ± 0.1 °C for Tm. All microthermometric analyses were conducted at Yangtze University.

3.4. Fluid Inclusion Composition Analysis

The chemical composition of brine inclusions in anhydrite was determined using a HORIBA LabRAM Odyssey high-resolution confocal Raman microscope. Excitation wavelengths of 532 or 785 nm were selected depending on fluorescence intensity, and laser power was maintained between 1 and 3 mW to minimize thermal effects on the inclusions. Raman spectra were collected over a spectral range of 100–4000 cm^–1, covering the characteristic vibrational modes of common aqueous species. Individual fluid inclusions were precisely targeted under optical control, with the laser beam focused on the center of the inclusion to maximize signal quality. Raman spectra were acquired from the liquid phase of the inclusions, , and characteristic peaks corresponding to major ionic species, including Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^–, SO₄ ^2–, and HCO₃ ^–, were identified and interpreted. All Raman analyses were performed at Yangtze University.

3.5. Major and Trace Elements of Deep Brines and Yanshanian Granites

Deep brine and granite samples were collected from the Wuwei Basin and its surrounding areas for major and trace elements. Since the brine samples were obtained from gas-rich aquifers, they were immediately filtered through a funnel equipped with filter paper until the solutions became clear. In situ measurements of pH and total dissolved solids (TDS) were conducted using a portable multiparameter water-quality analyzer. Following transport to the laboratory, concentrations of bicarbonate (HCO₃ ^–) and carbonate (CO₃ ^2–) ions were determined by acid titration using phenolphthalein as an indicator. Major cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺) were analyzed using a SPECTRO ARCOS SOP inductively coupled plasma optical emission spectrometer (ICP-OES) at the East China Mineral Resources Testing Center, Ministry of Natural Resources, with analytical precision better than ± 5%. Trace elements, including Li and B, in both deep brine and granite samples were measured by inductively coupled plasma mass spectrometry (ICP-MS), also with analytical precision better than ± 5%. Additional anion analyses were performed as follows: HCO₃ ^– and CO₃ ^2– were quantified by acid titration (AT), whereas other anions were determined by ion chromatography (IC). The analytical precisions for AT and IC were better than ± 3% and ± 5%, respectively. All analyses were conducted at the East China Mineral Resources Testing Center, Ministry of Natural Resources.

4. Results

4.1. Carbon and Oxygen Isotopes of Dolomite

The results of carbon and oxygen isotopes of dolomite are shown in Table . The δ¹³C_PDB values of Middle Triassic Zhouchongcun Formation dolomite range from −2.8 to −0.6‰, with an average value of −1.6‰. The δ¹⁸O_PDB values range from −6.1 to −2.7‰, with an average value of −3.8‰.

1. Analytical Data of Carbon and Oxygen Isotopes of Dolomite Samples in Well B11-1 (‰, PDB).

sample	lithology	13C	18O
D01	dolomite	–2.8	–6.1
D02	dolomite	–1.7	–4.7
D03	dolomite	–2.1	–4.0
D04	dolomite	–1.6	–3.9
D05	dolomite	–1.4	–3.4
D06	dolomite	–1.2	–2.9
D07	dolomite	–1.5	–2.8
D08	dolomite	–0.6	–2.7

4.2. Anhydrite Fluid Inclusions

4.2.1. Types of Fluid Inclusions

Petrographic studies of anhydrite indicate that primary brine inclusions are predominantly isolated and exhibit tabular or elongated shapes, while secondary brine inclusions are arranged in inclined arrays or bead-like trails. The homogenization temperature of individual brine inclusions can reflect changes in the formation water temperature during the deposition and subsequent alteration of anhydrite − This study measured the homogenization temperatures of both primary and secondary fluid inclusions.

4.2.2. Homogenization Temperatures of Fluid Inclusions

Five anhydrite samples containing fluid inclusions were examined petrographically. Primary brine inclusions occur as isolated rectangular to subsquare cavities, whereas secondary brine inclusions are arranged along fractures or growth zones in bead-like trails (Figure c, d). Among the analyzed samples, two (A03 and A04) contain secondary two-phase (liquid–vapor) brine inclusions, one sample (A02) hosts primary two-phase brine inclusions, and two samples (A01 and A05) contain predominantly gaseous inclusions. In total, 21 valid homogenization temperature (Th) measurements were obtained from brine inclusions, and their statistical distribution is shown in Figure . For the sample containing primary brine inclusions, homogenization temperatures range from 181.3 to 210.2 °C, with an average value of 197.6 °C. These data define a unimodal distribution, indicating trapping under relatively stable thermal conditions (Figure a). In contrast, secondary brine inclusions from two samples yield homogenization temperatures between 135.8 and 174.3 °C, with a mean value of 154.2 °C. Their frequency distribution is bimodal (Figure b), suggesting multiple episodes of secondary fluid infiltration or re-equilibration during postdepositional thermal events.

6.

Composition spectra of anhydrite inclusions in well B11–1 obtained by micro-Raman spectroscopy. (a) Primary brine inclusion of sample A02; (b) secondary brine inclusion of sample A04.

5.

Histogram of homogenization temperature plotted against number of anhydrite fluid inclusions. (a) Primary brine inclusions; (b) secondary brine inclusions.

4.3. Chemical Composition of Fluid Inclusions and Deep Brine

High-resolution micro-Raman spectroscopic analyses of brine inclusions hosted in anhydrite from the Wuwei Basin are summarized in Figure and Table . In primary brine inclusions, the characteristic Raman peak area ratios and corresponding relative concentrations of major cations are as follows: Na⁺ (5332; 3006 mg/L), Ca²⁺ (5106; 2872 mg/L), Mg²⁺ (317; 126 mg/L), Li⁺ (206; 104 mg/L), and K⁺ (189; 83 mg/L). The dominant anions in primary inclusions are Cl^– (12,345; 10,472 mg/L), accompanied by minor HSO₄ ^– (462; 133 mg/L) and HCO₃ ^– (532; 142 mg/L). Secondary brine inclusions exhibit distinctly different chemical characteristics. Their major cations display characteristic peak area ratios and relative concentrations of Na⁺ (4250; 6420 mg/L), Ca²⁺ (3900; 5520 mg/L), Mg²⁺ (130; 94 mg/L), Li⁺ (650; 490 mg/L), and K⁺ (96; 58 mg/L). Corresponding anions are dominated by Cl^– (10,800; 15,750 mg/L), with subordinate HSO₄ ^– (132; 95 mg/L) and HCO₃ ^– (122; 78 mg/L). Notably, Li concentrations in secondary inclusions are approximately 4.8 times higher than those in primary inclusions, indicating significant Li enrichment during secondary fluid activity. The bulk chemical composition of deep formation brines determined by ICP-MS is presented in Table . Total dissolved solids (TDS) range from 71.0 to 77.5 g/L, indicating highly saline fluids. Chloride is the dominant anion (31.87–39.44 g/L), followed by SO₄ ^2– (4.46–8.14 g/L) and HCO₃ ^– (408–2768 mg/L). Sodium and calcium are the principal cations, with Na⁺ concentrations of 24.21–29.41 g/L and Ca²⁺ concentrations of 0.80–1.36 g/L. Potassium contents range from 0.37 to 0.54 g/L, whereas Mg²⁺ concentrations vary between 124.1 and 644.6 mg/L. In addition, the brines are enriched in economically significant trace elements, including Li (19.2–56.2 mg/L), B (310–582 mg/L), Br (224–347 mg/L), and I (6.5–16.2 mg/L). The concentrations of these elements meet or exceed the lower thresholds for industrial exploitation of lithium-bearing brine resources, highlighting the substantial resource potential of the Wuwei Basin.

2. Compositions of Anhydrite Fluid Inclusions in the Middle Triassic in Well B11-1.

samples	compositions	Na	Ca	Mg	K	Li	Cl	HCO3 –	HSO4 –
A02	characteristic peak area ratio	5332	5106	317	189	206	12345	462	532
	relative content (mg/L)	3006	2872	126	83	104	10472	133	142
A04	characteristic peak area ratio	4250	3900	130	96	650	10800	132	122
	relative content (mg/L)	6420	5520	94	58	490	15750	95	78

3. Composition of Deep Brines in the Middle Triassic in the Wuwei Basin Obtained by the Laser Denudation ICP-MS Method.

samples	Li (mg/L)	B (mg/L)	Ca (mg/L)	Mg (mg/L)	K (mg/L)	Br (mg/L)	I (mg/L)	Na (mg/L)	Cl (mg/L)	SO4 (mg/L)	HCO3 (mg/L)
WY1	56.2	582.0	812.0	131.0	485.0	258.0	7.4	26680.0	35331.0	7932.0	2768.00
WY2	55.2	570.0	1322.0	130.4	476.3	262.0	6.5	26340.0	34714.1	7867.9	2052.92
WY3	54.8	525.7	795.0	124.1	475.4	224.0	7.7	26750.0	34805.9	8138.0	3132.91
WY4	27.2	328.0	1346.0	514.7	528.3	268.2	15.8	29410.0	36053.0	4863.0	508.90
WY5	27.6	350.0	1360.0	508.8	540.3	271.3	16.2	28260.0	34764.0	4769.0	558.31
WY6	24.4	310.0	1231.0	432.4	508.9	261.7	16.1	26950.0	31865.0	4465.0	574.78
WY7	20.6	472.6	1129.0	642.9	498.2	251.8	15.0	27100.0	36205.0	4619.0	545.13
WY8	20.5	470.1	1203.0	644.6	471.2	257.6	14.6	26920.0	34375.0	4589.0	502.31
WY9	20.0	453.6	1277.0	622.8	500.1	239.7	14.5	26610.0	33003.0	4537.0	601.13
WY10	20.1	454.9	1074.0	633.1	435.8	253.4	14.7	25830.0	37293.0	5049.0	410.09
WY11	20.1	454.0	1156.0	602.7	471.3	251.9	15.1	26030.0	38272.0	5106.0	530.31
WY12	19.5	481.7	1189.0	623.8	454.9	243.2	15.1	26910.0	37237.0	5064.0	449.61
WY13	19.3	479.1	1091.0	625.7	446.7	234.6	15.0	26950.0	34044.0	4615.0	419.97
WY14	19.2	478.7	972.0	620.3	442.9	247.1	14.9	26820.0	36003.0	4994.0	433.14
WY15	19.3	472.3	1108.0	624.9	440.3	261.7	14.8	26840.0	39435.0	5374.0	418.32
WY16	19.7	467.3	1033.0	625.9	440.5	248.3	14.9	26510.0	37585.0	5081.0	419.97
WY17	19.3	462.5	927.0	619.2	435.2	257.7	15.3	26480.0	36698.0	4984.0	419.97
WY18	20.3	409.0	1224.0	571.4	380.1	347.4	13.6	24210.0	36776.3	5104.3	408.22
WY19	19.9	438.2	1179.0	633.5	367.7	276.5	13.8	24920.0	37495.1	5469.9	416.67

5. Discussion

5.1. Structural Characteristics of Dolomite with Anhydrite Veins

Petrographic observations of the WWY1 well core show that anhydrite veins clearly cut across the bedding of the dolostone host rocks, indicating that anhydrite formation postdated primary dolomite sedimentation. , . Based on fracture crosscutting relationships, vein geometry, and filling characteristics, the anhydrite-bearing fractures can be broadly classified into three successive stages (Figure a). The first stage (blue solid lines in Figure a) is characterized by conjugate shear joints completely filled with anhydrite. These structures are interpreted as having formed during an early phase of tectonic uplift and subaerial exposure, corresponding to a fracture–karst development stage. The second stage (white vein-like features in Figure a) consists mainly of tensile joints that are partially to fully filled with anhydrite, representing fracture development under subsidence and burial conditions following the initial uplift. The third stage (red dashed lines in Figure a) comprises high-angle tension–shear joints entirely filled with anhydrite, reflecting fracture formation associated with later deep-seated fluid activity. Mineralogical and structural features further support a hydrothermal origin for the vein-type anhydrite. Contacts between anhydrite and the dolostone host are commonly irregular and serrated (Figure a), displaying well-developed metasomatic textures. In many cases, dolomite is partially replaced by anhydrite (Figure b), indicating that anhydrite formation involved replacement processes rather than simple fracture infilling. The anhydrite crystals are predominantly elongate columnar or fibrous, commonly exhibiting comb textures and growth zoning (Figure b, d). These features contrast sharply with the euhedral to subhedral tabular, short-columnar, or granular morphologies typical of evaporitic anhydrite, and are more consistent with precipitation from hydrothermal fluids within open fractures. Geochemical evidence further corroborates this interpretation. Energy-dispersive spectroscopy (EDS) analyses reveal distinct compositional differences between secondary vein-type anhydrite and primary evaporitic anhydrite, most notably the presence of Sb anomalies in the former (Figure c). Antimony enrichment is widely recognized as a diagnostic indicator of medium- to low-temperature hydrothermal mineralization. Consistent with this, Hou Zengqian (2003) reported that this assemblage of anhydrite is commonly interbedded or closely associated with layered siderite, pyrite, and Pb–Zn ore bodies, with estimated mineralization temperatures ranging from approximately 100 to 250 °C. Such temperatures cannot be achieved by simple evaporative processes, strongly implying that the anhydrite associated with syngenetic mineralization represents a chemical sediment or vein mineral precipitated during the cooling and evolution of a hydrothermal system. Collectively, the fracture architecture, replacement textures, crystal morphologies, and geochemical signatures provide compelling evidence that the anhydrite in the study area underwent significant secondary hydrothermal modification. Although this study proposes preliminary criteria for distinguishing primary evaporitic anhydrite from secondary hydrothermal anhydrite, identification of definitive microscopic diagnostic features (e.g., growth zoning patterns and trace-element distributions) remains limited by sample availability. Future application of high-resolution microanalytical techniques, such as LA–ICP–MS elemental mapping, will be essential for refining genetic discrimination and further constraining the evolution of anhydrite-hosted lithium-rich brine systems.

7.

Structural and mineralogical characteristics of secondary anhydrite-veined dolomite. (a) Hand specimen of well; (b, c) flocculent secondary anhydrite veins with corresponding EDS; (d, e) anhydrite veins with corresponding EDS.

5.2. Paleotemperature-Paleoenvironmental Characteristics Revealed by Carbon and Oxygen Isotopes in Dolomite

Correlation analysis of carbon and oxygen isotope data from dolostones in the study area reveals a pronounced positive relationship between δ¹³C and δ¹⁸O values (R² = 0.7802; Figure a). When considered together with the high homogenization temperatures (181.3–210.2 °C) recorded by primary fluid inclusions in anhydrite, this relationship indicates that the isotopic system was dominantly modified by postdepositional diagenetic to hydrothermal processes rather than reflecting primary depositional conditions. , According to the classical “Algor” model, a strong linear covariance between δ¹³C and δ¹⁸O is diagnostic of high-temperature recrystallization of carbonate minerals (Hudson, 1977; Banner and Hanson, 1990). Under burial or hydrothermal conditions, elevated temperatures (>180 °C) exert a primary control on isotopic fractionation, promoting extensive re-equilibration between carbonate minerals and pore fluids. During this process, oxygen isotopes, which are more temperature-sensitive, typically undergo pronounced negative shifts; accordingly, all analyzed samples exhibit δ¹⁸O values lower than – 2.7‰. In contrast, carbon isotopes, largely inherited from pre-existing marine carbonate sources, display a narrower range of variation and covary with oxygen isotopes within a relatively closed system (Figure b). , The progressive distribution of data points from samples D01 to D08 likely reflects a continuum in the intensity of hydrothermal alteration or thermal exposure, rather than variations in evaporative concentration. Consequently, the observed δ¹³C−δ¹⁸O pattern is best explained by temperature-controlled recrystallization within a semiclosed system, in which any early evaporative isotopic signatures were effectively erased or overprinted. This interpretation is fully consistent with the thermal event recorded by fluid inclusion microthermometry, which documents medium- to high-temperature fluid activity during burial and hydrothermal reworking. Nevertheless, it should be noted that the geochemical proxies applied in this study, such as major and trace element compositions and fluid inclusion chemistry, provide largely indirect constraints on fluid sources and evolutionary pathways. The absence of sulfur and strontium isotope data introduces uncertainty in quantifying the relative contributions of paleoseawater, magmatic–hydrothermal fluids, and host-rock interaction to the mineralizing system. Future studies integrating high-precision sulfur and strontium isotope analyses using multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) on anhydrite and associated minerals will be critical for more rigorously constraining fluid provenance and the genetic evolution of the Lithium-rich brine system.

8.

Relationship between carbon and oxygen isotopes of dolomite from Well B11–1 in the Wuwei Basin and its paleoenvironmental analysis. (a) Relationship between carbon and oxygen isotopes of dolomite; (b) closed vs open paleoenvironment discrimination diagram.

5.3. Geological Significance of Homogenization Temperatures of Brine Inclusions in Anhydrite

Because vapor-rich inclusions can significantly bias homogenization temperature measurements, only liquid-phase brine inclusions were selected for microthermometric analysis. The homogenization temperature distributions of anhydrite brine inclusions from the Middle Triassic Zhouchongcun Formation in the Wuwei Basin are shown in Figure . Primary brine inclusions, characterized by intact, elongated to isolated rectangular morphologies (Figure a), yield homogenization temperatures ranging from 181.3 to 210.2 °C, with a mean value of 197.6 °C. These data define a unimodal distribution, suggesting that the fluids responsible for trapping the primary inclusions were associated with a relatively uniform and thermally stable event or environment. The high temperatures, with a pronounced peak near 200 °C, are interpreted to reflect the maximum paleo-temperature attained by the formation during deep burial diagenesis. In contrast, secondary brine inclusions, which occur as bead-like trails along fractures (Figure b), display homogenization temperatures between 135.8 and 174.3 °C, with an average of 154.2 °C. Their bimodal distribution indicates that these inclusions did not form during a single thermal event but instead record multiple stages of fluid infiltration or re-equilibration. Compared with the primary inclusions, the lower temperatures and bimodal pattern of the secondary inclusions suggest superimposed modification by later, multistage moderate-temperature hydrothermal fluids, most plausibly related to Yanshanian tectonic–thermal activity.

Additional constraints on the thermal evolution of the basin are provided by paleo-temperature modeling. Comparison between the simulated paleo-temperature curve of the Middle Triassic Zhouchongcun Formation and fluid inclusion homogenization temperatures (Figure ) reveals a close correspondence between modeled burial heating and measured inclusion temperatures. Beginning in the Middle Jurassic (∼174 Ma), rapid accumulation of Early Jurassic strata, together with contemporaneous tectonic–thermal activity, led to a regional increase in terrestrial heat flow and the attainment of peak paleo-temperatures. The high homogenization temperatures recorded by primary brine inclusions (mean 197.6 °C) during this interval are therefore interpreted to reflect maximum burial temperatures attained under a normal geothermal gradient, rather than transient heating by short-lived hydrothermal pulses. Following this thermal maximum, the onset of the Yanshanian tectonic regime triggered the most extensive uplift event in the region, resulting in progressive cooling and a decline in paleo-temperature. The lower homogenization temperatures and bimodal distribution of secondary brine inclusions in anhydrite (mean 154.2 °C), which are hosted along fracture systems, are more plausibly attributed to superimposed, multistage hydrothermal fluid activity associated with Yanshanian tectonism. These fluids migrated along newly formed or reactivated fractures, partially re-equilibrating the anhydrite under moderate-temperature conditions. , Although minor increases in paleo-temperature occurred during subsequent deposition of Cretaceous and Paleogene continental strata, two major uplift episodes in the mid–late Cretaceous and during the early Himalayan orogeny, combined with a long-term decrease in regional heat flow, sustained an overall cooling trend. Consequently, the homogenization temperatures of anhydrite-hosted fluid inclusions are interpreted to primarily record peak burial diagenetic temperatures, with localized overprinting by later, predominantly Yanshanian, moderate-temperature hydrothermal events. The formation and modification of anhydrite in the Wuwei Basin thus reflect the combined effects of early burial diagenesis and subsequent magmatic–hydrothermal reworking.

9.

Burial temperatures and anhydrite brine inclusions homogenization temperatures (Th) from the Middle Triassic Zhouchongcun Formation.

5.4. Geological Implications of Anhydrite brine Inclusion Compositions and Lithium-Rich Brine Components

The Piper diagram summarizes the hydrochemical characteristics and evolutionary trends of brine inclusions and lithium-rich formation brines (Figure ). All samples plot within the upper-right sector of the diamond field, defining a mixed sodium sulfate–sodium chloride (Na₂SO₄–NaCl) water type. This compositional affinity indicates that the brines experienced strong evaporative concentration, accompanied by substantial water–rock interaction during their evolution. Rather than clustering tightly, the sample points define a systematic evolutionary trend from a typical Na–Cl composition toward higher Ca²⁺ contents (red arrow in Figure ). This trend is interpreted to reflect progressive cation-exchange processes, whereby Na⁺ in the brine is partially replaced by Ca²⁺ derived from interaction with carbonate or sulfate-bearing host rocks. In addition, the displacement of several samples toward the Ca–Mg–Cl compositional field suggests mixing between the original evaporitic brine and Ca-rich deep formation waters or hydrothermal fluids. These hydrochemical features collectively indicate that the brine system in the study area underwent complex multistage evolution involving intense evaporation, prolonged water–rock interaction, and episodic input of hydrothermal fluids.

10.

Hydrochemical type classification using Piper diagram (samples 1–2: anhydrite brine inclusions; samples 3–19: lithium-rich brine).

Comparison between the chemical composition of the deep brines and the standard seawater evaporation trajectory reveals pronounced and systematic deviations (Figure ), indicating that simple evaporative concentration cannot fully account for the observed brine chemistry. Bromide (Br^–), which behaves conservatively during evaporation and water–rock interaction, is therefore used as a reference to assess evaporative evolution. − In the Ca²⁺–Br^– and SO₄ ^2––Br^– diagrams, the deep brine samples (star symbols) plot distinctly above the paleo-seawater evaporation curve, showing anomalously high Ca²⁺ concentrations with increasing Br^–, while SO₄ ^2– and Mg²⁺ contents are comparatively depleted. This pattern is characteristic of evaporite diagenetic modification. During burial diagenesis, dolomitization reactions release Ca²⁺ into the brine while simultaneously consuming Mg²⁺, resulting in elevated Ca²⁺/Mg²⁺ ratios. As evaporation proceeds further, the precipitation of gypsum and anhydrite (gray shaded interval in Figure ) progressively removes SO₄ ^2– from the brine and consumes part of the dissolved Ca²⁺. The inflection observed along the Ca²⁺ trend marks the onset of sulfate mineral precipitation, recording a key transition in brine evolution. In the K⁺–Br^– diagram, all samples plot below the standard seawater evaporation path, indicating systematic K⁺ depletion. This feature suggests that evaporative concentration did not reach the sylvite (KCl) precipitation stage, consistent with the absence of potash evaporites in the regional stratigraphic record and implying truncation of the classical evaporation sequence. By contrast, the Li–Br^– relationship displays a markedly different behavior: lithium concentrations in the deep brines are far higher than those predicted by the seawater evaporation curve. Such enrichment cannot be explained by evaporation alone and instead requires an additional lithium source. This excess lithium is most plausibly attributed to the input of nonevaporative fluids, such as Li-enriched deep formation waters or magmatic–hydrothermal fluids associated with Yanshanian tectono-thermal activity, as independently suggested by burial and thermal history modeling. Taken together, the systematic deviations of Ca²⁺, SO₄ ^2–, K⁺, Mg²⁺, and especially Li from the standard seawater evaporation path demonstrate that the lithium-rich brines in the study area formed through a multistage evolutionary process. , This process involved initial marine evaporation, intense late-stage burial diagenetic modification of evaporites and carbonates, and subsequent mixing with deep, lithium-bearing fluids, rather than derivation from simple evaporative concentration of seawater alone.

11.

Curves of seawater evaporation trajectory.

The Gibbs diagram is employed to identify the dominant processes controlling the hydrochemical evolution of the brines (Figure ). All samples plot consistently within the evaporation–crystallization dominance field, indicating that evaporative processes exerted the primary control on brine chemistry. This distribution provides robust evidence for an evaporative origin of the brines and effectively excludes atmospheric precipitation or dilution by meteoric water as the principal controlling mechanism. The uniformly high total dissolved solids (TDS) values, together with the alignment of sample points along the evaporation–crystallization trend, further demonstrate that progressive evaporative concentration was the fundamental driver of salinity enrichment in the study area. At the same time, the extremely high Na⁺/(Na⁺+Ca²⁺) ratios (approaching 1.0) observed in all samples indicate substantial modification of the primary evaporative brine through water–rock interaction. This feature is consistent with the Piper diagram results and reflects cation exchange and mineral–fluid reactions during burial and diagenesis. − Consequently, the chemical characteristics of the brines record a coupled evolutionary process involving intense evaporative concentration followed by significant water–rock interaction, with evaporation constituting the dominant mechanism and diagenetic reactions playing a secondary but essential role in modifying ionic compositions.

12.

Gibbs diagram of brine samples from the well WWY1 in the Wuwei basin.

5.5. Coupled Metallogenic Model of Lithium-Rich Brines and Anhydrite Deposits in the Wuwei Basin

The Wuwei Basin represents a typical Mesozoic–Cenozoic salt-bearing rift lacustrine basin characterized by graben–horst structural architecture controlled by deep-seated basement faults. Since the Middle Triassic collision and subsequent orogeny between the Yangtze and North China blocks, the basin has experienced long-term subsidence and maintained a relatively closed and well-sealed hydrogeological system. Within this tectonic framework, multiple sources, including hydrothermal fluids released from the subducting slab, Yanshanian granitic magmatism, and recurrent geothermal (hot spring) activity, have continuously supplied heat and trace elements (e.g., Li and B) to deep brines confined within anhydrite–dolomite successions. , Under the combined influence of an arid paleoclimate, a closed basin configuration, and the availability of volcanic- and basement-derived elements, the formation of thick anhydrite deposits and lithium-rich brines in the Wuwei Basin can be divided into three major evolutionary stages.

Stage 1: Triassic – Preparation Period for Deep Brine and Anhydrite Material Foundation

Beginning in the Middle Triassic, the Wuwei Basin developed under persistently hot and arid climatic conditions, promoting intense evaporation of paleo-saline lake waters. Progressive evaporative concentration led to the formation of dense brines, while the depositional environment evolved into a hypersaline lagoon system (Figure ), favoring the precipitation of anhydrite and anhydrite-bearing dolomite. , Owing to the closed hydrological conditions and sustained evaporation, anhydrite layers continuously thickened, forming regionally extensive evaporite sequences (Figure a). By the late Middle Triassic, mineralized materials buried beneath the subduction-related accretionary wedge experienced increasing compaction and tectonic extrusion, resulting in the expulsion of substantial volumes of deep-seated hydrothermal fluids. These fluids were enriched in ore-forming elements such as Li, B, and Rb, which were released into basin-bottom hydrothermal systems and preferentially transported along the subduction direction toward major fault zones. However, fluid migration during this stage was spatially restricted, and large-scale fluid circulation within the basin had not yet been established. Consequently, although trace elements were introduced into the system, no significant lithium enrichment occurred within the basin depressions at this time (Figure ).

13.

Lithofacies Paleogeographic map of the Middle Triassic Zhouchongcun Formation in the Wuwei Basin.

14.

Coupling mineralization model of deep lithium-rich brine and anhydrite deposits in the Wuwei Basin.

Stage 2: Yanshanian – Period of Lithium-Rich Granite Intrusion and Metasomatism

From the Late Jurassic to the Early Cretaceous, volcanic and magmatic activity became widespread in and around the Wuwei Basin. Progressive increases in temperature and precipitation activated extensive thermal spring systems along basin-margin faults (Figure b), enhancing fluid circulation between deep magmatic sources and sedimentary strata. Previous studies have demonstrated that intermediate–acidic, silica-rich, strongly peraluminous to alkali-rich granites are particularly favorable for lithium enrichment. , Consistent with this framework, Yanshanian intrusions surrounding the Wuwei Basin are dominated by quartz diorite, quartz monzonite, granodiorite, monzogranite, and granite porphyry, all of which exhibit variable but generally advanced degrees of magmatic differentiation (Figure ; Table ). Geochemically, these granites display SiO₂ contents ranging from 60.04 to 77.91 wt.% (average 68.75 wt.%), and CIPW normative quartz (Q) values of 2.28–37.91, indicating SiO₂-saturated to oversaturated compositions. Total alkali contents (K₂O + Na₂O) range from 7.45 to 9.50 wt.%, reflecting alkali-rich characteristics. The rock series index (δ = 3.07–4.02, average <4) classifies the intrusions as calc-alkaline, while their positions on the SiO₂–K₂O diagram indicate affinity with the high-K calc-alkaline to shoshonitic series (Figure ). Aluminum saturation indices (ANKC = 0.84–1.08; average 0.95) suggest metaluminous to weakly peraluminous compositions. In addition, solidification indices (SI = 12.38–1.00) and high differentiation indices (DI = 74.06–94.60) point to extensive fractional crystallization and/or assimilation processes, conditions that are particularly conducive to lithium enrichment in granitic systems (Table ). Independent petrographic and microthermometric evidence further confirms intense hydrothermal activity during this stage. Secondary anhydrite veins with irregular, serrated contacts and elongated columnar to fibrous crystal habits are widely developed within dolomite host rocks (Figure b–e), reflecting hydrothermal replacement rather than simple fracture infilling. Homogenization temperatures of primary fluid inclusions (181–210 °C) record a medium- to high-temperature hydrothermal regime, consistent with magmatic heat input. Notably, lithium concentrations in secondary inclusions are approximately 4.8 times higher than those in primary inclusions (Table ), directly quantifying the lithium enrichment effect associated with Yanshanian hydrothermal fluid injection. Regional volcanic rock data provide further constraints on the lithium source. Lithium concentrations in Yanshanian intrusive rocks range from 22.3 to 86.9 ppm (average 48.6 ppm), whereas Neo-Proterozoic volcanic rocks contain only 3.1–7.4 ppm Li (average 5.2 ppm). Thus, Yanshanian magmatic rocks exhibit lithium contents more than 9.4 times higher than older volcanic units (Figure ), supplying abundant lithium-, boron-, and volatile-rich minerals to the basin system. Concurrently, plastic flow and differential compaction of thick evaporite sequences led to the development of several second-order anhydrite anticlinal structures within the Wuwei Basin (Figure b). The anhydrite–dolomite assemblages of the Zhouchongcun Formation underwent further fracture development during burial compaction, enhancing both permeability and sealing capacity. At this stage, lithium, boron, and other elements were preferentially emplaced near the contact zones between Yanshanian intrusions and evaporite strata, but had not yet undergone large-scale convergence or economic concentration. Nevertheless, this stage established both the material basis and structural–hydrogeological pathways required for subsequent large-scale fluid migration and elemental focusing along extensional faults and fracture networks (Figure b).

15.

K₂O-SiO₂ relation diagram of granites­(The intrusive rocks are shown in Figure ).

4. Main Elements of Yanshanian Intrusive Rocks in the Wuwei Basin.

sampling sites	sample ID	lithology	SiO2	TiO2	Al2O3	Fe2O3	FeO	MnO	MgO	CaO	Na2O	K2O	Σ	ALK
intrusive rock①	1–1	quartz monzonite	77.91	0.15	12.33	0.62	0.58	0.02	0.17	0.07	4.76	3.80	99.87	6.56
	1–2	quartz monzonite	77.71	0.06	12.19	0.42	0.30	0.02	0.10	0.46	3.73	3.72	99.56	7.45
	1–3	quartz monzonite	76.39	0.16	12.49	0.75	0.64	0.04	0.22	0.55	3.73	4.82	100.38	8.55
	1–4	quartz monzonite	76.69	0.12	12.05	0.67	0.55	0.06	0.18	0.55	3.61	4.75	99.62	8.36
	1–5	quartz monzonite	76.19	0.04	12.61	0.41	0.49	0.03	0.22	0.49	3.45	4.74	99.49	8.19
	1–6	quartz monzonite	76.07	0.12	12.46	0.52	0.82	0.08	0.04	0.64	3.67	4.75	99.73	8.42
intrusive rock②	2–1	syenogranite	73.61	0.26	13.14	1.80	0.60	0.10	0.23	0.17	3.45	5.40	99.90	8.85
	2–2	syenogranite	73.55	0.27	13.42	1.58	0.90	0.06	0.28	0.24	3.55	5.40	100.14	8.95
	2–3	syenogranite	73.10	0.29	13.81	1.50	0.23	0.05	0.22	0.27	4.20	5.42	99.88	9.62
	2–4	syenogranite	73.28	0.27	12.91	2.36	0.91	0.10	0.33	0.36	4.20	5.40	100.56	9.60
	2–5	syenogranite	72.24	0.28	13.51	1.36	1.09	0.14	0.12	0.68	4.05	5.42	99.42	9.47
intrusive rock③	3–1	monzonitic granite	73.68	0.17	14.46	1.18	0.65	0.05	0.11	0.64	6.10	3.66	100.14	6.76
	3–2	monzonitic granite	71.87	0.38	14.22	2.10	0.15	0.03	0.15	0.11	0.23	5.57	99.71	8.80
	3–3	monzonitic granite	71.25	0.49	14.79	2.65	0.07	0.10	0.22	0.24	1.91	3.97	98.76	7.88
intrusive rock④	4–1	granodiorite	67.53	0.77	17.29	4.57	0.19	0.02	0.31	0.74	0.05	2.95	106.76	1.00
intrusive rock⑤	5–1	granite porphyry	62.72	0.54	16.10	2.11	1.93	0.24	1.04	2.55	3.90	5.15	97.90	9.05
	5–2	granite porphyry	65.44	0.55	16.09	1.91	1.94	0.15	1.26	1.78	4.15	5.35	99.79	9.40
	5–3	granite porphyry	63.86	0.62	16.36	2.65	1.76	0.01	1.38	3.22	3.70	3.90	98.77	7.70
	5–4	granite porphyry	60.45	0.60	16.12	2.24	0.95	0.03	0.54	4.92	2.88	4.88	96.78	9.76
	5–5	granite porphyry	61.17	0.52	17.27	2.45	1.74	0.29	1.08	2.78	4.48	4.90	98.23	9.38
	5–6	granite porphyry	61.74	0.63	16.40	1.76	2.45	0.24	1.31	3.18	4.05	5.00	98.71	9.05
intrusive rock⑥	6–1	quartz monzonite	63.98	0.55	16.22	2.23	1.85	0.09	1.73	5.18	4.10	3.75	100.97	7.85
	6–2	quartz monzonite	63.21	0.52	14.92	1.13	2.71	0.08	1.64	3.09	3.70	3.38	100.63	7.18
intrusive rock⑦	7–1	quartz diorite	65.18	1.33	18.63	1.64	0.68	0.05	0.21	1.16	10.35	2.50	101.06	10.45
	7–2	quartz diorite	70.41	0.25	16.57	0.99	0.82	0.05	0.09	0.61	8.40	2.46	101.72	9.86
	7–3	quartz diorite	62.14	0.61	17.50	1.25	1.73	0.12	1.47	5.68	5.28	3.46	101.55	8.74
	7–4	quartz diorite	60.04	0.63	15.75	2.97	1.90	0.15	0.98	5.33	4.22	3.23	102.25	7.45
	7–5	quartz diorite	60.70	0.94	16.45	4.52	1.38	0.06	2.52	1.76	7.68	2.70	101.66	10.38
	7–6	quartz diorite	61.78	0.70	15.65	5.42	0.99	0.20	1.39	4.14	5.67	1.88	99.23	7.55
	AVG		68.75	0.44	14.92	1.92	1.07	0.09	0.67	1.78	4.25	4.10		8.35

16.

Superimposed map of rock mass distribution and Middle Triassic anhydrite-dolomite traps in the Wuwei Basin. (a) Plan map; (b) section map.

Stage 3: Late Cretaceous – Period of Fluid Migration, Convergence, and Accumulation along Extensional Faults

From the Late Cretaceous to the Paleogene, basin-controlling normal faults in the Wuwei Basin underwent strong extensional reactivation, leading to the development of multiple graben–horst structural assemblages. Reactivation of the Tan–Lu Fault and the Yangtze River Fault significantly enhanced regional crustal permeability, thereby accelerating the upward discharge of lithium- and boron-bearing fluids associated with thermal mountain spring systems (Figure c). At the same time, lithium and boron initially stored in deep formation brines were effectively remobilized under the combined influence of tectonic extension and hydrothermal circulation. These solutes were subsequently channeled by synchronously active basement faults, such as the Shijian and Dougou faults (Figure c), and migrated laterally and vertically toward structural culminations within the basin. Driven by fault-controlled fluid flow and density-driven brine migration, lithium-rich brines progressively converged into the crests of second-order anticlinal traps developed within the anhydrite–dolomite sequences (Figure c). Notably, all currently identified high-lithium brine occurrences (e.g., Well WWY1, Li > 50 mg/L) are spatially confined to these anticlinal highs, providing direct evidence for structurally focused enrichment. Ultimately, economically significant lithium-rich brine deposits formed within the anhydrite–dolomite anticlinal traps, where favorable structural geometry, effective evaporite sealing, and sustained hydrothermal input acted synergistically − (Figure ). The present-day distribution of lithium-rich brines exhibits a strong correspondence with both the planar arrangement of fault-bounded brine traps (Figure a) and their subsurface structural configuration revealed in cross-sectional profiles (Figure b), underscoring the dominant role of late-stage extensional tectonics in controlling lithium brine accumulation and preservation.

6. Conclusions

One The anhydrite of the Zhouchongcun Formation in the Lower Yangtze region is not the product of a single evaporative process. Its formation commenced with evaporative deposition in a closed saline lake under the arid climatic conditions of the Middle Triassic, resulting in the development of thick primary anhydrite layers. The critical transformation occurred during the Yanshanian period, when intense tectono-magmatic activity introduced medium- to high-temperature, lithium-rich hydrothermal fluids, as recorded by fluid inclusion homogenization temperatures of 150–210 °C. These hydrothermal fluids extensively dissolved, recrystallized, and metasomatized the early anhydrite, producing secondary anhydrite veins. This genetic process, characterized by early sedimentation as the foundation and later hydrothermal activity as the finalizing agent, accounts for the complex anhydrite fabrics and the presence of anomalously high-temperature fluid inclusions.

Two The formation of lithium-rich brine reflects a dynamic coupling of source, migration, and accumulation within a specific burial–thermal history. Lithium was primarily sourced from Yanshanian acidic, lithium-rich magmatic–hydrothermal fluids. The lithium concentration in secondary fluid inclusions is more than 4.8 times higher than that in primary inclusions, providing direct evidence for hydrothermal lithium input. Evaporation of paleo-seawater contributed to the initial salinity framework and part of the ionic composition of the brine. Lithium activation and migration are closely linked to two key thermal events. Burial history reconstruction indicates that the strata reached temperatures of ∼ 150 °C during the early deep-burial stage, as recorded by primary inclusions, initiating early material activation. The Yanshanian (∼150 Ma) tectono-thermal event, recorded by secondary inclusions showing hydrothermal overprinting at ∼ 130 °C, acted as the principal driver of lithium mineralization by providing sustained heat and effective fluid pathways for large-scale lithium extraction and migration. Subsequent tectonic uplift and cooling since the Late Cretaceous led to the gradual waning of the hydrothermal system. Within an extensional tectonic regime, the activated lithium ions migrated with the brine along basin-controlling normal faults, converging and ultimately being preserved within anhydrite–dolomite anticlinal traps.

Three This study proposes a three-stage metallogenic model explicitly coupled with burial–thermal evolution. Stage I (Sedimentary Foundation Period, T₃): Under an arid climate, evaporation in a closed basin led to the formation of primary anhydrite and initial saline brine, establishing the material foundation and potential trapping architecture for mineralization. Stage II (Hydrothermal Modification and Lithium Enrichment Period, J₃–K₁): Corresponding to the temperature peak on the burial history curve, Yanshanian tectono-magmatic activity supplied heat, hydrothermal fluids, and lithium. Medium- to high-temperature hydrothermal fluids (150–210 °C), during circulation, leached surrounding strata, introduced lithium into the basin brine system, and simultaneously modified the pre-existing anhydrite. This stage represents the core lithium enrichment phase. Stage III (Tectonic Adjustment and Accumulation Period, K₂–present): Corresponding to the cooling and uplift stage of the burial history, regional tectonic extension reactivated basin-controlling faults and formed graben–horst structures. Under decreasing temperatures and weakening hydrothermal activity, lithium stabilized within the brine and migrated toward structural highs (horsts and anticlines), where it accumulated and was effectively sealed within lithologically favorable anhydrite–dolomite sequences, forming industrially valuable lithium-rich brine deposits.

Four The proposed time–temperature–mineralization event coupling framework indicates that prospective lithium-rich brine systems require not only early evaporite deposition but, more importantly, a subsequent tectono-hydrothermal event operating within a medium- to low-temperature window (100–200 °C). This framework provides a new theoretical basis and predictive criterion for lithium resource exploration in the Lower Yangtze region and in other tectonically comparable evaporite basins worldwide.

The authors declare no competing financial interest.