Discussion on the Genesis of Vein-Type Copper Deposits in the Northern Lanping Basin, Western Yunnan

Abstract

The Sanjiang Tethys orogenic belt in Southwest China is a globally important polymetallic metallogenic domain, hosting numerous world-class Cu-Pb-Zn deposits. Among these, the Lanping Basin is a typical ore concentration area, characterized by complex tectonic evolution and extensive hydrothermal mineralization. Although numerous vein-type Cu deposits occur in the northern and western parts of the basin, research in the north region remains less comprehensive. This study investigates three typical vein-type Cu deposits (Hetaoqing, Hemeigou, and Songpingzi) in the northern Lanping Basin using rare-earth element (REE) analysis, S-Pb-Sr isotope determinations, and tectonic stress inversion. Results show that ²⁰⁶Pb/²⁰⁴Pb ratios range from 18.374 to 18.691, and δ³⁴S_V-CDT values vary from –11.7‰ to +9.4‰, indicating mixed sources of ore-forming materials dominated by deep magmatic sources, particularly related to alkaline rocks around the basin. Sulfur sources are closely associated with thermochemical sulfate reduction (TSR). Additionally, ⁸⁷Sr/⁸⁶Sr ratios range from 0.710949 to 0.711864, ΣREE values range from 85.87 × 10^–6 to 111.86 × 10^–6, Ce/Ce* ratios range from 0.86 to 0.92, and Eu/Eu* ratios range from 1.06 to 2.99. Fluid inclusion microthermometry yields temperatures of 217–252 °C (average 238 °C), indicating that ore-forming fluids experienced water–rock interaction during migration and ultimately exhibited mixed properties. Tectonic stress field inversion reveals that the structures formed by NE–SW compressive stress field before mineralization stage provided ore-hosting spaces and fluid migration pathways, while a late Cenozoic abrupt stress field change promoted the precipitation of ore-forming materials.

1. Introduction

The Sanjiang Tethyan orogen in southwestern China is a globally significant polymetallic metallogenic domain, hosting multiple world-class Cu–Pb–Zn deposits. The Lanping Basin, located on the southeastern margin of this orogen, formed during the Early Cenozoic through the accretion of Gondwana-derived microcontinental blocks and arc terranes along the Paleo-, Meso-, and Neo-Tethyan sutures [1,2,3,4].

Bounded by the Lancang River Paleo-Tethyan suture to the west and the Jinshajiang-Ailaoshan Paleo-Tethyan suture to the east, the basin exhibits complex tectonic evolution and abundant hydrothermal mineralization, making it a typical ore cluster in the region [5].

The basement of the Lanping Basin consists of Proterozoic and Paleozoic metamorphic rocks, including gneiss, amphibolite, sericite schist, and marble [6]. The basin preserves a complete Mesozoic–Cenozoic sedimentary sequence, exposed from Triassic to Oligocene systems, dominated by red clastic rocks and halite-bearing rock series, which record the sedimentary-tectonic evolution of the Tethyan tectonic domain [7,8,9,10].

Multiple phases of thrust-nappe structures and fold deformations occur within the basin, with its boundaries controlled by regional-scale deep faults such as the Lancangjiang and the Jinshajiang-Ailaoshan faults. These structural systems established the fundamental architectural framework of the basin and exerted decisive controls over sedimentation, magmatic activities, and metallogenic processes [11,12,13,14,15].

As an important Cu-Pb-Zn polymetallic metallogenic province in China, the Lanping Basin hosts world-class deposits such as the Jinding super-large Pb-Zn deposit and the Baiyangping ore concentration area, along with structurally controlled hydrothermal vein-type copper deposits along its western margin, including Jinman, Liancheng, Maocaoping, Xiaogela, etc. [16,17,18,19,20,21,22,23] (Figure 1B).

These mineral deposits mainly occur in sedimentary and metamorphic rock series within the oblique collision zone of the India-Asia continent [1,23,24,25]. Their spatial distribution and ore body morphology are clearly controlled by regional-scale thrust faults [23,24,26]. However, the genetic mechanism of these deposits remains controversial, with key viewpoints including “epigenetic deposits” and “hydrothermal sedimentary genesis,” among others [27,28]. The debate centers on the source and evolution of ore-forming fluids [21,23,29,30,31,32], and the dynamic mechanism governing the ore-forming fluid migration remains poorly understood [33,34].

This study focuses on vein-type copper deposits in the northern Lanping Basin. Through systematic geological surveys, analysis of ore-controlling structures, and geochemical analysis, combined with the regional metallogenic background studies, we aim to clarify the characteristics of ore-controlling structures, reveal the source and evolution of ore-forming fluids, establish the metallogenic model, and explain the genetic mechanism. The results provide new evidence for improving the polymetallic metallogenic theory of the Lanping Basin and hold significant guidance for regional mineral exploration.

2. Regional Metallogenetic Background

The Lanping Basin lies within the North Qiangtang-Sanjiang orogenic system of the Sanjiang Tethyan tectonic domain (Figure 1) and belongs to the Changdu-Lanping-Simao block [35,36,37]. Controlled by deep-seated faults such as the Jinshajiang-Ailaoshan fault and the Lancangjiang fault [38,39,40,41], the basin exhibits a unique tectonic-metallogenic frame. The eastern margin is bounded by the Cenozoic Diancangshan-Ailaoshan shear zone, while the western side is bounded by the Cenozoic Chongshan shear zone [20,37]. This tectonic setting has made the basin a superimposed region of multi-stage tectonic-magmatic-metallogenic processes.

The basement comprises Permian to Middle Triassic alkaline to acidic volcanic rocks, tuff, siliceous clastic rocks, and minor carbonate rocks [20,42]. Overlying this is a complete sedimentary sequence recording the transition from marine to continental facies. The Middle Jurassic Bazhulu Formation marks the initiation of the continental graben basin stage [43], while the marine-continental transitional facies of the Huakaizuo Formation represent a significant marine transgression event [44]. Late Jurassic-Early Cretaceous uplift formed the Jingxing Formation fan-delta deposits. Middle Cretaceous basin shrinkage led to Hutousi Formation fluvial deposits, and Late Cretaceous regional uplift caused the widespread absence of Upper Cretaceous strata [45]. This multi-cyclic sedimentary-tectonic evolution facilitated initial enrichment of ore-forming elements and provided dynamic conditions for deep fluid migration.

The basin periphery hosts multi-phase volcano-intrusive rock series, such as the rhyolite-volcanic clastic rocks of the Pantiange Formation in the northeast [46]. The northwestern part exposes a suite composed of andesitic crystal tuff, andesite, basalt, and rhyolite [47,48], and the bimodal volcanic rock series in the south [49,50,51,52]. Middle Triassic bimodal magmatic rocks provided crucial heat sources and ore-forming materials for mineralization. Within the basin, the Ludian granite, Biluoxue Mountain pluton, and Xuelong Mountain metamorphic rocks form a complex source system [53,54,55] (Figure 1B). Regional mineralization exhibits the following distinct “tectonic-magmatic-fluid” characteristics: deep-seated faults control the distribution of ore clusters, multi-stage magmatic activities provide heat sources and partial ore-forming materials, and the gypsum-salt and organic matter layers in the extremely thick sedimentary sequences create essential chemical barriers for the ore-forming fluids [56].

3. Geological Characteristics of Ore Deposits

The exposed strata in the northern Lanping copper deposit primarily belong to the Jurassic and Cretaceous systems of the Mesozoic (Figure 2). On the western side of the mining area, the Jurassic Huakazuo and Bazhulu formations consist of purplish-red clastic rock sequences, with the Huakazuo Formation dominated by siltstone and mudstone. Ore bodies mainly occur within the Cretaceous Jingxing Formation, characterized by grayish-white sandstone interbedded with locally purplish-red sandstone, which has undergone bleaching alteration. The stratum strikes NW-NNW, dips at 204–265°, and has dip angles of 14–70°. The Nanxin Formation comprises lacustrine purplish-red clastic rock deposits. The Hetaoqing copper deposit is primarily hosted in the bleached grayish-white sandstone of the Cretaceous Jingxing Formation, striking NW-NNW and dipping at 14–70° (Figure 3). The Hemeigou and Songpingzi deposits occur in grayish-white to purplish-gray medium-thick bedded fine sandstone interbedded with purple thin- to medium-bedded siltstone of the Jurassic Huakaizuo Formation. The ore body morphologies in all three deposits are controlled by fault zones (Figure 2A,B).

The main structural feature is a monoclinic structure dipping southwest. Influenced by the regional N-S trending Xiayanshan fault and the Sishiliqing fault, secondary faults and fold structures are highly developed, primarily distributed in N-NW and N-S directions (Figure 3). Among these, the N-S trending faults include the regional F₂ (Xiayanshan fault), F₉, and F₁₀ faults. The F₂ reverse fault significantly controls the regional fold structures and stratum distribution.

E-W trending faults are represented by the F₅ and F₈ faults (Figure 3). The F₈ fault, the main ore-hosting structure, strikes nearly E-W (152–199°∠74–87°) with a length of approximately 2300 m, cutting through the Jurassic Huakaizuo and Cretaceous Nanxin Formations. The fault plane exhibits striations, and kinematic analysis indicates it is a dextral reverse fault. Ore bodies are strictly controlled by the F₈ fault fracture zone, occurring as vein-type along the fault strike, demonstrating typical tectonic control over mineralization. The mineralized vein is hosted within tensional fractures (Figure 2C,D).

According to the 2012 resource and reserve verification report, the cumulative verified copper ore resources amount to 3.5 million tons of ore, containing 51,000 tons of copper metal (average grade 1.46%), along with 137 tons of associated silver (average grade 39.3 × 10⁻⁶) [58]. The mineral assemblage includes primary sulfides (chalcopyrite, tetrahedrite, bornite, and chalcocite), secondary oxides (azurite, malachite), and other minerals such as siderite, limonite, and barite. Chalcopyrite, bornite, chalcocite, and tetrahedrite occur as irregular granular, stippled, and dust-like disseminations within siderite and gangue minerals.

4. Sample Testing and Results

4.1. Samples and Methods

As a fundamental technique in ore deposit research, microscopic examination provides crucial evidence for deciphering mineral formation sequences and constraining ore-forming conditions by allowing precise mineral identification and detailed analysis of ore textures and structures. This investigation examined copper ores collected from three deposits in the northern Lanping Basin. Microscopic observations were performed at the Institute of Geomechanics, Chinese Academy of Geological Sciences (located in Beijing, China), using a Leica DM4500 microscope (manufactured by Leica Camera AG, Wetzlar, Germany) under reflected light.

Fluid inclusion thermometry was conducted on quartz from the ore-forming stage of the Hemeigou and Songpingzi copper deposits at the Institute of Geology, Chinese Academy of Geological Sciences (Figure 2E–H). We collected calcite from the Hemeigou copper mine for subsequent rare-earth element (REE) analysis; the samples were obtained from calcite veins contemporaneous with the main mineralization period of the deposit. REE analysis of calcite veins from Hemeigou deposit samples was conducted using inductively coupled plasma mass spectrometry (X-series, Thermo Fisher Scientific, Waltham, MA, USA) at the National Research Center for Geoanalysis, CAGS, calibrated against international standard sample AMH-1 (andesite), with an analytical error < 10%. The above-mentioned calcite from the Hemeigou deposit was crushed to <200 mesh, digested with 1 mL HF and 0.3 mL of 1:1 HNO₃, evaporated to near-dryness, leached with HNO₃, and diluted with 5% HNO₃ for analysis.

Strontium isotope analysis was performed on siderite, dolomite, and calcite from the Hemeigou and Songpingzi copper deposits; all tested samples had their single minerals selected. These minerals are syngenetic and were formed during the main mineralization period. A total of ten samples were collected from different locations within the deposit, using a Thermo Fisher MAT-Triton TI surface ionization mass spectrometer (TMS, Thermo Fisher Scientific, manufactured in Waltham, MA, USA) at the State Key Laboratory of Metallogeny of Endogenous Mineral Deposits, Nanjing University. The Sr isotope standard NIST SRM 987 [59] was used for calibration.

Samples of pyrite, bornite, chalcocite, and chalcopyrite were collected for lead and sulfur isotope testing at the Beijing Research Institute of Uranium Geology. Sulfur isotope testing followed the DZ/T 0184.14-1997 [60] using a Delta V Plus mass spectrometer (Thermo Fisher Scientific in Waltham, MA, USA). Lead isotope analysis involved sample dissolution, separation, purification, and analysis using a Phoenix thermal ionization mass spectrometer (TIMS)-9444 from the Beijing Research Institute of Uranium Geology, Beijing, China. Measurements included ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁶Pb/²⁰⁴Pb, with errors at 2σ. The testing procedure involved the following steps in compliance with GB/T 17672-1999 [61]: first, the samples were washed with distilled water. Then, they were dissolved using a mixture of double-distilled nitric acid and hydrochloric acid. Next, lead was separated and purified from the solution via hydrobromic acid and anion-exchange resin. After that, phosphoric acid and silica gel were coated onto rhenium tape. Finally, the prepared samples were subjected to mass spectrometry analysis.

4.2. Result

4.2.1. Petrographic Characteristics

Microscopic observations (Figure 4) show that chalcopyrite and tetrahedrite coexist with clear contact boundaries, and tetrahedrite clearly postdates chalcopyrite (Figure 4A–C). As clearly shown in Figure 4A, pre-existing chalcopyrite underwent fragmentation due to later quartz vein infilling, with tetrahedrite growing along the quartz veins and replacing the chalcopyrite. Figure 4B shows residual textures of chalcopyrite altered by tetrahedrite, and Figure 4C shows tetrahedrite crosscutting chalcopyrite as late-stage veinlets. Chalcopyrite occurs as stippled distributions in the quartz matrix (Figure 4D). Based on mineral paragenesis, the deposit formation involved at least two stages of hydrothermal activity: an early high-temperature stage with chalcopyrite precipitation, and a late middle-low temperature stage with tetrahedrite replacing chalcopyrite.

4.2.2. Results of Fluid Inclusion Thermometry

Thermometric results from the quartz formed during the ore-forming stage indicate that the fluid inclusions are predominantly liquid-rich, with occasional three-phase inclusions. The temperature range is 217–252 °C, averaging 238 °C (

Table 1. Fluid inclusion homogenization mode and microthermometric data. For example: “L + L + V → L” denotes the homogenization process of fluid inclusions during heating, transforming from a “liquid–liquid-vapor three-phase coexistence state” to a “single liquid-phase state”.

Sample	Type	Homogenization Temp. (°C)	Homogenization Mode
SPZ-1	Three-Phase	251.6	L + L + V → L
	Three-Phase	240.3	L + L + V → L
	Three-Phase	233.8	L + L + V → L
	Three-Phase	233.8	L + L + V → L
SPZ-3	Liquid-Rich Phase	243.8	L + V → L
	Liquid-Rich Phase	221.9	L + V → L
	Liquid-Rich Phase	217.8	L + V → L
	Liquid-Rich Phase	231.2	L + V → L
	Liquid-Rich Phase	229.2	L + V → L
HMG—8	Liquid-Rich Phase	232.8	L + V → L
	Liquid-Rich Phase	240.5	L + V → L
	Liquid-Rich Phase	251.2	L + V → L
	Liquid-Rich Phase	245.2	L + V → L
	Liquid-Rich Phase	237.8	L + V → L
	Liquid-Rich Phase	252.8	L + V → L
	Liquid-Rich Phase	243.6	L + V → L

).

SPZ-1 has a dispersed range (220–250 °C), with a main peak at 235–245 °C, indicating multiple temperature fluctuations. SPZ-3 has the narrowest range (225–240 °C), densely distributed around 235 °C, indicating a stable hydrothermal environment. HMG-8 has a higher and broader range (230–252 °C), suggesting proximity to the hydrothermal center or deep-heat sources influence (Figure 5).

4.2.3. REE

REE contents in the vein-type copper deposits calcite from the main mineralization period range from 85.87 × 10⁻⁶ to 111.86 × 10⁻⁶ (ΣREE > 50 × 10⁻⁶), with Ce/Ce* = 0.86–0.92 and Eu/Eu* = 1.06–2.99. The calculation methods for Ce/Ce* and Eu/Eu* are, respectively, dividing the normalized Ce value by the average of the normalized La and Pr values, and dividing the normalized Eu value by the average of the normalized Sm and Gd values, where normalization is performed using PAAS as the reference. The deposits are characterized by significant LREE depletion and HREE enrichment. The mass fraction ratio of Y to Ho (i.e., w(Y/Ho)) ranges from 24 to 26.2 (

Table 2. REE composition of calcite in the Hemeigou vein-type copper deposit, northern Lanping Basin. “*” denotes the "rare earth element (REE) normalized value".

	Sample	HMG-3-2	HMG-2	HMG-1-2	HMG2-3	HMG2-5	HMG2-8	HMG-4-2	HMG-7
Element
La (μg/g)		1.49	1.15	0.8	2.48	1.4	0.89	3	1.31
Ce (μg/g)		6.31	5.67	3.98	9.84	6.46	4.26	11.3	5.8
Pr (μg/g)		1.43	1.3	1.03	2.04	1.52	1	2.35	1.28
Nd (μg/g)		9.9	9.01	8.34	13.4	10.6	7.05	14.2	8.55
Sm (μg/g)		6.86	5.44	6.28	7.34	6.3	5.31	7.53	5.56
Eu (μg/g)		5	1.63	1.93	2.33	1.77	1.48	2.56	1.65
Gd (μg/g)		9.5	7.17	7.77	9.41	7.8	8.15	9.03	7.53
Tb (μg/g)		1.63	1.29	1.18	1.53	1.26	1.44	1.5	1.34
Dy (μg/g)		9.95	8	6.69	9.28	7.26	9.13	8.85	8.85
Ho (μg/g)		1.77	1.5	1.18	1.73	1.28	1.73	1.6	1.67
Er (μg/g)		4.96	3.99	3.04	4.59	3.49	4.63	4.42	4.75
Tm (μg/g)		0.67	0.58	0.44	0.65	0.46	0.61	0.6	0.65
Yb (μg/g)		4.11	3.53	2.81	4.14	2.97	3.96	3.81	3.87
Lu (μg/g)		0.56	0.49	0.39	0.54	0.37	0.52	0.5	0.51
Y (μg/g)		45.7	36	29.9	43.1	33.3	44.8	40	42.9
w(Y/Ho)		25.82	24.00	25.34	24.91	26.02	25.90	25.00	25.69
Ce/Ce*		0.88	0.87	0.90	0.92	0.86	0.87	0.84	0.87
Eu/Eu*		1.24	1.23	1.30	1.32	1.19	1.06	1.46	1.20
∑REE		106.97	86.75	75.76	112.40	86.24	94.96	111.25	96.22

).

The vein-type copper deposits exhibit HREE enrichment. However, previous studies have suggested that REE in sedimentary rocks is expected to exhibit HREE depletion characteristics [62]. However, the calcite in the veins exhibits the opposite result (Figure 6). Based on previous studies of deposits in southwestern China, the following consistent pattern has been observed: if calcites in hydrothermal deposits of this region exhibit characteristics during the mineralization stage similar to those shown in Figure 4 [63], it confirms that the calcites collected in this study are indeed from the mineralization stage.

4.2.4. Sr Isotopes

The ⁸⁷Sr/⁸⁶Sr ratio serves as a reliable indicator for determining the source of ore-forming materials. In ore deposit geology, it has been widely used to trace fluid origins, magmatic water, and crust-mantle contamination involving deep-sourced fluids [64]. The hydrothermal minerals used in this study were all formed by fluids from the main mineralization stage and should therefore be representative of the characteristics of the ore-forming fluids during that period.

The ⁸⁷Sr/⁸⁶Sr values range from 0.710949 to 0.711864 (difference ≈ 0.0009), showing a relatively tight distribution. The lowest value is HMG-6 (siderite, 0.710949), and the highest is HMG-7 (dolomite, 0.711864). Ten of eleven samples have values >0.711. Dolomite samples cluster within 0.711164–0.711864 (mean ≈ 0.71156), while calcite samples overlap with the high-value interval (

Table 3. Sr isotopic compositions.

Serial Number	Sample	Mineral	87Sr/86Sr
1	HMG-6	Siderite	0.710949
2	HMG-1-2	Dolomite	0.711341
3	HMG-2	Dolomite	0.711707
4	HMG-2-3	Dolomite	0.711494
5	HMG-2-5	Dolomite	0.711667
6	HMG-2-8	Dolomite	0.711164
7	HMG-3-2	Dolomite	0.711651
8	HMG-4-2	Dolomite	0.711741
9	HMG-7	Dolomite	0.711864
10	SPZ-5	Calcite	0.711743

).

4.2.5. S and Pb Isotopes

The δ³⁴S_V-CDT values range from −11.7 to +9.4 (

Table 4. Sulfur and lead isotope data of sulfide minerals (“-” indicates missing data).

Sample	Mineral	δ34SV-CDT (‰)	206Pb/204Pb	207Pb/204Pb	208Pb/204Pb
SPZ-1	Chalcopyrite	−6.9	18.663	15.614	38.755
SPZ-1	Chalcocite	−5.7	18.515	15.561	38.648
SPZ-3	Chalcopyrite	−3	18.67	15.606	38.825
SPZ-4	Chalcopyrite	−0.6	18.665	15.637	38.833
SPZ-5	Bornite	−4.1	18.437	15.475	38.361
SPZ-5	Chalcopyrite	−1.3	18.405	15.453	38.35
SPZ-6	Chalcocite	2.8	18.374	15.414	38.244
SPZ-6	Chalcopyrite	9.4	-	-	-
HMG-1-2	Chalcopyrite	−11.5	18.691	15.689	39.025
HMG2	Chalcopyrite	−6.8	18.641	15.675	38.931
HMG-3-2	Chalcopyrite	-	18.656	15.629	38.822
HMG-3-2	Pyrite	0.1	-	-	-
HMG-4-1	Chalcopyrite	−1.6	-	-	-
HMG-4-2	Chalcopyrite	−11.2	18.605	15.627	38.836
HMG-7	Chalcopyrite	−8.7	18.602	15.602	38.787
HMG-8	Chalcopyrite	-	18.435	15.525	38.459
HMG-8	Pyrite	0.9	-	-	-
HMG2-3	Chalcopyrite	−9.1	18.484	15.519	38.569
HMG2-5	Chalcopyrite	−11.7	18.558	15.588	38.68
HMG2-5	Pyrite	−11.2	18.59	15.608	38.762
HMG2-8	Chalcopyrite	−5.3	18.455	15.529	38.551
LP14006-5	Chalcopyrite	1.1	18.587	15.635	38.815
LP14006-6	Chalcopyrite	−1.4	18.637	15.664	38.947
LP14006-7	Chalcopyrite	−1.5	18.622	15.653	38.912
LP14006-9	Chalcopyrite	−2.5	18.661	15.672	38.99

; Figure 7). Twenty-one data points (87.5%) are negative, with only three positive values as follows: SPZ-6 chalcopyrite (+9.4‰), SPZ-6 chalcocite (+2.8‰), and LP14006-5 chalcopyrite (+1.1‰). Three data points have values ≤−11.0‰: HMG2-5 chalcopyrite (−11.7‰), HMG-4-2 chalcopyrite (−11.2‰), and HMG-1-2 chalcopyrite (−11.5‰).

The ²⁰⁶Pb/²⁰⁴Pb ranges from 18.374 to 18.691, ²⁰⁷Pb/²⁰⁴Pb from 15.414 to 15.689, and ²⁰⁸Pb/²⁰⁴Pb from 38.244 to 39.025 (Table 4). The minimum values are from Songpingzi SPZ-6 chalcocite, and the maximum from Hemeigou HMG-1-2 chalcopyrite.

Sulfur isotope composition can effectively trace the source of ore-forming materials. In hydrothermal deposits, the sulfur sources include mantle-derived sulfur (δ³⁴S ≈ 0, small variation), crustal sulfur (large variation), and mixed sulfur (magma assimilation). Mixed sulfur exhibits distinct sulfur isotope characteristics [65].

Figure 7. δ³⁴S_V-CDT (‰) distribution histogram of vein-type copper deposits in northern Lanping Basin (data from Hemeigou and Songpingzi are from this study, while part of the data from Hetaoqing are from previous studies [66,67,68,69]).

Figure 7. δ³⁴S_V-CDT (‰) distribution histogram of vein-type copper deposits in northern Lanping Basin (data from Hemeigou and Songpingzi are from this study, while part of the data from Hetaoqing are from previous studies [66,67,68,69]).

Lead pattern tracing uses the lead structural model diagram (Figure 8) [70]. On the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb diagram, data form a straight line with strong correlation, distributed between the orogenic belt and upper crust, suggesting a mixed source. On the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁸Pb/²⁰⁴Pb diagram, all data indicate orogenic Pb as the dominant source. Samples from copper or copper-bearing polymetallic deposits within the basin show good clustering, implying similar Pb sources and a single evolutionary history (Figure 8).

5. Discussion

5.1. Properties and Sources of Ore-Forming Fluids

The identification of ore-forming fluids can be achieved through trace elements, REE, Sr isotopes, and fluid inclusions. No primary minerals indicative of an oxidizing environment have been identified, and the Ce negative anomaly in calcite is minor, likely originating from primary anomalies in the fluid. Mantle-derived fluids can carry Ce negative anomalies, but mantle REE anomalies are typically distinct [82]. Thus, the mantle is unlikely to be the direct source of rare-earth elements (REE) in the hydrothermal fluids. The wall rocks are primarily clastic rocks, where Ce occurs as Ce³⁺ [83], making it unlikely for the fluid to retain significant Ce⁴⁺. The Ce negative anomaly in the fluid is likely from the water–rock interaction, leading to Ce loss. Eu exhibits a distinct positive anomaly, typically indicating the high-temperature (>200–250 °C) water–rock interaction before low temperatures precipitating [83]. Eu²⁺ is more easily released into the fluid, resulting in positive Eu anomalies. Fluid inclusion temperatures (217–252 °C) support the water–rock interaction. Y and Ho exhibit similar geochemical behavior, with w(Y)/w(Ho) generally constant. Most clastic sediments and magmatic rocks have w(Y)/w(Ho) ratios of approximately 28, while eastern China continental crust ranges from 20 to 35 and the mantle from 25 to 30 [84]. The w(Y)/w(Ho) data for vein-type copper deposits in the northern Lanping Basin (Table 2) indicate crust-mantle mixing characteristics of the fluid. Various rare-earth element (REE) geochemical indicators, particularly the Ce and Eu anomalies, support that the ore-forming fluid underwent water–rock interaction.

The Sr isotopes (⁸⁷Sr/⁸⁶Sr = 0.710949–0.711743) are higher than the mantle value (0.7035) but lower than the continental crust average (0.7190), indicating a mixed fluid source (Figure 9). Sr shares geochemical behavior with Ca and Mg, while Rb differs [85]. In Ca- or Mg-bearing minerals (e.g., calcite, dolomite), Sr²⁺ can substitute for Ca²⁺, but Rb⁺ cannot. This results in high Sr and low Rb content [86]. Selective substitution results in the characteristic that calcium- and magnesium-bearing minerals typically exhibit relatively high Sr contents and extremely low Rb contents. It is worth emphasizing that the process by which rubidium (Rb) forms the stable isotope ⁸⁷strontium (Sr) via β decay—given Rb’s extremely low initial abundance and minimal incorporation into minerals during their formation—causes a negligible disturbance to the initial Sr isotopic composition of the system.

Therefore, calcium- and magnesium-bearing minerals can effectively preserve the initial Sr isotopic composition information of ore-forming fluids. Siderite contains a certain amount of Mg, which is attributable to the similar geochemical behaviors of Mg²⁺ and Fe²⁺. Given that the chemical formula of siderite is FeCO₃, it can thus serve as a test mineral for indicating initial fluids.

A comparison with other deposits in the basin, sedimentary clastic rocks, and igneous rocks reveals that the Sr isotope ratios of vein copper deposits fall between the Sr isotope ratios of igneous rocks and those of clastic sedimentary rocks. The deposit Sr isotope ratios are significantly lower than those of the wall rocks (Nanxing and Jingxing Formation sandstones), suggesting at least one high-ratio endmember and one low-ratio endmember, that is, the high-ratio endmember represented by clastic sedimentary rocks and the low-ratio endmember represented by alkaline rocks (Figure 6). This may result from the fluid mixing after acquiring Sr from igneous rocks and flowing through wall rocks. Similarities with the Hexi gypsum deposit indicate contributions from gypsum rock layers (Figure 9). Constraints from the above indicate that the fluid underwent water–rock interaction during migration, leading to REE fractionation, Ce negative anomalies, and Eu positive anomalies. Continuous mixing of multiple components formed the current fluid geochemical characteristics.

5.2. Ore-Forming Material

The δ³⁴S_V-CDT values of the Songpingzi and Hetaoqing fluctuate around zero, indicating a mantle-derived relationship, while Hemeigou data concentrate in the negative range, suggesting non-mantle sources (Figure 10). The Jinding lead-zinc mining area contains 1% to 25% bitumen in the Triassic Sanhedong Formation, and bitumen/crude oil in the Tertiary Yunlong Formation [95,96], indicating abundant organic matter. The basin hosts substantial sedimentary clastic rocks that could have served as a potential provenance for the ore-forming substances. δ³⁴S values of the gypsum-salt layers in the clastic sedimentary rocks of the basin range from 9.6‰ to 17.99‰ [88,97,98].

Thermochemical sulfate reduction (TSR) and bacterial sulfate reduction (BSR) are primary mechanisms of sulfate reduction. Organic matter thermal decomposition (> 100 °C) can lead to sulfides δ³⁴S values of 0‰ ± 5‰. TSR requires a temperature of 100–140 °C (up to 160–180 °C), while BSR occurs at 0 to 60–80 °C [104]. Fluid inclusion temperature supports TSR. Thus, another significant sulfur source is the gypsum-salt layers derived from organic matter thermal decomposition.

Sulfur isotope diagrams indicate a strong correlation with volcanic or igneous rocks. The δ³⁴S distributions of the northern vein-type copper deposits are nearly identical to those of Jinman, Liancheng, and Kedengjian deposits (Figure 10), similar to granite and island arc basalt + andesite ranges. Overall, the sulfur sources of copper deposits in the northern Lanping Basin and the entire western basin result from the mixing of deep magmatic hydrothermal sulfur and the organic matter-reduced gypsum-salt sulfur (i.e., TSR).

The Δβ–Δγ diagram (Figure 11) shows that most samples fall into the subduction zone lead region (magmatic, mixture of upper crust and mantle), near the orogenic lead field. Some samples plot within the orogenic lead field, indicating a significant magmatic lead source.

Comparison with igneous rocks and high-grade metamorphic rocks from the basin margin confirms the magmatic origin of lead. Most igneous rocks or metamorphic rocks differ from Hetaoqing, while alkaline rocks from Jianchuan, Beiya, and Yongan are similar (Figure 12). The Jianchuan Basin alkaline rocks may share the same origin as the copper deposits. Alkali-rich plutons around the Lanping Basin have ages of 50–25 Ma [105].

In summary, the primary source of ore-forming materials for the northern Lanping Basin vein-type copper deposits is the orogenic belt, with the basement rocks playing a dominant role. Igneous and metamorphic rocks from the basin margin also contribute, with Cenozoic Jianchuan alkaline rocks being the most significant.

5.3. Ore-Controlling Structures

Structural analysis of the F₈ fault system was conducted using FaultKin 6 and Stereonet 2020 software [110,111]; detailed occurrence data are provided in Appendix A,

Table A1. Attitudes of the main ore-hosting fault and conjugate joints near it; Hetaoqing copper deposit, northern Lanping Basin.

Conjugate Joints								Fault				Fault Striations
HTQ2307-1		HTQ2307		HTQ2309		HTQ2315		HTQ2311		HTQ2318		HTQ2311		HTQ2318
Dip Direction	Dip Angle	Dip Direction	Dip Angle	Dip Direction	Dip Angle	Dip Direction	Dip Angle	Dip Direction	Dip Angle	Dip Direction	Dip Angle	Trend	Plunge	Trend	Plunge
321	44	267	60	83	60	330	69	193	74	177	76	119	32	252	40
305	38	249	51	94	70	329	71	194	79	167	81	115	37	252	37
307	41	270	64	110	62	318	59	180	67	177	81	118	33	263	13
308	45	261	68	160	60	317	67	192	81	163	75	125	39	248	34
312	50	265	80	166	56	316	68	196	84	179	76	121	38	247	30
310	55	266	72	177	74	320	66	195	87	133	64	120	33	254	32
312	56	268	67	163	85	342	74	199	75	158	71	127	35	254	23
322	60	255	65	175	79	334	67	198	78	152	73	120	38	242	34
309	45	257	67			339	70
311	56	271	66			319	55
304	51	269	67			313	55
313	52	272	69			315	61
310	55	273	66			102	62
312	54	280	77			98	51
9	57	50	84			95	64
11	56	52	89			116	60
7	58	47	85			115	60
8	60	210	88			118	63
23	64	24	66			111	67
19	63	22	63			105	60
11	66	18	74			311	25
12	67	36	67
5	65	41	65
32	61	38	66

. Stress inversion of fault striations at HTQ2309 and HTQ2318 (Figure 13) indicates that a principal compressive stress axis (P-axis) orientation of NE-SW is 58°∠13° and a principal tensile stress axis (T-axis) of NW-SE is 160°∠42°. The stress field shows a typical thrust fault mechanism, with the P-axis nearly perpendicular to the fault dip direction.

Stress inversion of conjugate joints (

Table 5. Principal stress results of conjugate joints in Hetaoqing.

Point	Joint Attitude				σ1 (°)		σ2 (°)		σ3 (°)
	Dip Direction	Dip Angle	Dip Direction	Dip Angle	Dip Direction	Dip Angle	Dip Direction	Dip Angle	Dip Direction	Dip Angle
HTQ2307	308	45	7	58	58	13	315	45	160	42
HTQ2307-1	268	67	41	65	153	48	336	42	245	1
HTQ2309	95	64	168	71	220	6	119	62	313	27
HTQ2315	324	65	107	61	213	58	38	32	306	2

, Figure 14) indicates that the three sets of joints (HTQ2307, HTQ2309, and HTQ2315) reflect a NE-SW compressive stress field, consistent with the formation mechanism of the F₈ fault, representing coeval tectonic events. These pre-ore-forming structures provided space and dynamics for fluid migration and ore body emplacement.

Two conjugate joint systems at HTQ2307 (Table 5, Figure 14) show an early NE-SW compressive stress field and a late SE-NW compressive stress field, differing by nearly 90°. The pre-ore-forming NE-SW compressive stress field formed the F₈ reverse fault system, generating a fracture zone and joint network. During the ore-forming stage, a stress field change reactivated the fault system (Table 5; Figure 14), allowing ore-bearing hydrothermal fluids to migrate along structural weak zones. Post-ore tectonic activity locally remodeled the ore bodies.

5.4. Genesis of Mineral Deposit

The northern Lanping Basin vein-type copper deposits are hosted in purple-red and grayish-white clastic sedimentary rocks, occurring as veins and lenses. Their inferred ore-forming age is inconsistent with the wall rock age, precluding classification as stratiform copper deposits (SSCs). However, it still has some similarities with SSC-type minerals. Iran is an important enrichment area for sediment-hosted strata-bound copper deposits (sediment-hosted strata-bound copper, SSC-type) in sedimentary rocks [112], where multiple phases and types of copper deposits are developed (e.g., Dehmadan, Khongah [112,113]; Markasheh [114]). Globally, there are also other copper deposits of this type, such as Timna Valley in Israel [115], Corocoro in Bolivia [116], Lisbon Valley in the United States [117], and Aragón (NE Spain) [118]. The SSC (sediment-hosted stratiform copper) deposits in Iran and Spain indicate that the initial sulfur source originated from evaporite layers within ancient sedimentary basins [114,118,119], consistent with the Cu mineralization in the northern Lanping Basin of China. This suggests that in low-to-medium temperature hydrothermal Cu deposits, evaporite layers formed under arid paleoclimate conditions in ancient sedimentary basins act as massive sulfur reservoirs. Sulfur is incorporated into copper sulfides through either bacterial sulfate reduction (BSR) or thermochemical sulfate reduction (TSR) processes, ultimately forming Cu sulfide minerals and completing the mineralization process.

Worldwide, another important type of mesothermal-epithermal copper deposits is the SEDEX(Sedimentary Exhalative)-type deposits, including the Sullivan Deposit in Canada, the Jason Deposit in Canada, the Mount Isa Copper Deposit in Australia, the Century Deposit in Australia, the Rammelsberg Mine in Germany, the Silvermines Deposit in Ireland, and the Howards Pass Deposit in Canada [120,121,122,123,124,125,126,127,128,129,130,131,132]. The copper mineralization of this type shares certain commonalities in terms of structural control. Most deposits are developed in Proterozoic or Paleozoic rift basins; the mineralization occurred in an extensional tectonic background, such as the Selwyn Basin (Howards Pass, Jason [133,134]) in Canada, the Belt-Purcell Basin (Sullivan [135]) in Canada, the Mount Isa Basin (Dugald River, Century [126,136]) in Australia, and the Irish Midlands (Lisheen, Silvermines) in Ireland [137,138,139]. This indicates an important prerequisite for the formation of SEDEX deposits, namely, development in an extensional setting. This prerequisite is the same as the metallogenic environment of the vein-type copper deposits in the northern Lanping Basin, as both are mineralized in an extensional setting.

Previous studies have classified vein-shaped copper deposits in the northern Lanping Basin as a variant of SEDEX deposits [140,141,142]. The Hetaoqing copper deposit is near transpressional faults, but the main ore-hosting fault underwent a later tectonic inversion to an extensional event, consistent with the SEDEX setting.

However, a critical distinction lies in the timing of mineralization relative to the host rock and the morphology of the ore bodies. A defining characteristic of SEDEX deposits is that the mineralization is essentially syngenetic with diagenesis, and the ores are predominantly stratiform, stratoid, or lenticular [143]. In contrast, the vein-type copper deposits in the northern Lanping Basin, which are the focus of this study, are different. They occur as discordant vein-type mineralization strictly controlled by faults. The clear cross-cutting relationships between the faults/veins and the host strata demonstrate that the mineralization age is significantly younger than that of the country rocks. Consequently, it is not tenable to arbitrarily classify them as SEDEX-type or other similar deposit types. They can only be definitively categorized as sediment-hosted vein-type copper deposits.

Previous geochemical studies suggested ore-forming materials and fluids are primarily basin-derived [30,144,145,146,147,148], while others propose mantle or magmatic origins via deep structures [23,149,150]. Fluid inclusion studies on western Lanping vein-type copper deposits [17,23,34,99,142,148,149,151,152,153,154,155,156,157,158,159,160] show low salinity and temperatures below 300 °C (mostly <250–150 °C), similar to the northern deposits, indicating a low-temperature hydrothermal type. Under such conditions, copper migrates as hydrogen sulfide complexes [161].

Ore-forming fluids, driven by NE-SW compression and gravity, migrated toward the sedimentary basin, incorporating signals from evaporite layers and sedimentary rocks. During migration, fluids continuously leached ore-forming elements, resulting in mixed metal provenances. Ore-forming fluids and elements migrated along deep faults (formed by NE-SW compression), and an abrupt stress perturbation reactivated pre-existing fractures, enabling the fluid to ascend and ore body precipitation (Figure 15).

The Jinman copper deposit is controlled by interlayer fracture zones and fractures near the core of the Jinman-Liancheng compound overturned anticline, occurring as vein-like, lenticular, and sub-stratiform bodies [162]. Pb and S isotope characteristics of the Jinman are similar to those of the northern Lanping Basin vein-type copper deposits [101,102]. Both are hosted in thrust fault structures and share consistent sources of ore-forming elements, suggesting similar ore-forming epochs (Table 6). The ore-forming material and ore-forming fluid of the vein-shaped copper deposit in the northern part of Lanping should have been in place for the first time between 59 and 48 Ma, and this stage is the mineralization stage of high-temperature chalcopyrite (Figure 4), consistent with the development of the western basin thrust belt and the main collision period of the Tibetan Plateau (65–41 Ma) [5]. Additionally, its main mineralization period should be the late collision stage of the Qinghai–Tibet Plateau collision (40–26 Ma); this stage is also the later stage of the metasomatism of chalcopyrite by tetrahedrite (Figure 4), during which a large number of alkaline rocks around the basin were developed [5].

Worldwide, non-conventionally classified mesothermal-epithermal vein-type copper deposits hosted in sedimentary rocks are not numerous, but there are still examples such as the Frontier Copper Deposit (Congo/Zambia Copperbelt) [163]. Frontier Mine is a unique structurally controlled vein-type copper deposit in the Zambian Copperbelt, distinct from the typical sediment-hosted stratiform copper deposits in the region; traditionally, most deposits in the Zambian Copperbelt are of the SSC type [164], but Frontier Mine exhibits vein-type mineralization with significant structural control [165,166]. It is located in the Lufilian Arc (Pan-African rogeny) [167], a segment that records the rifting-collision process between the Kalahari and Congo Cratons approximately 880–500 Ma [165,168]. Mineralization was synchronous with the Lufilian orogeny, lasting about 300 Myr and spanning from the syndepositional stage to the post-orogenic stage [165,169,170,171] as follows: during the early mineralization stage, bedding/schistosity-parallel veins formed, embrittling the country rock; in the middle stage, high-grade saddle reef veins formed at fold hinge zones due to extensional spaces and brittle fractures, constituting the core of the orebody; in the late stage, continuous compression led to brittle faults, forming brecciated cross-cutting veins [165]. This mineralization process is similar to that of vein-type copper deposits in the northern Lanping Basin, as both are located near orogenic arcs, associated with post-orogenic activities, and experienced tectonic transition processes and major mineralization caused by late-stage tectonic activities.

This means that a crucial prerequisite for this type of copper deposit is that it formed in the vicinity of an “arc” and experienced multiple repeated tectonic transformations during the mineralization process. This is because tectonic transitions can form vein-type ore-hosting spaces, and ore-forming fluids carrying ore-forming materials can accelerate their precipitation under drastic changes in the external environment (e.g., [172,173,174,175]).

Table 6. Characteristics comparison of vein-type copper deposits on the western side of Lanping Basin (“\” indicates missing data).

Deposit	S Source	Pb Source	Metallogenic Epoch	Source
Northern Vein-type Copper Deposit in Lanping	Mixed source of deep magmatic hydrothermal sulfur generated during basin tectonic activities and gypsum rock sulfur reduced by organic matter (i.e., thermochemical sulfate reduction, TSR).	Mainly from orogenic-belt lead, with certain contributions from magmatic sources on both sides of the basin and metamorphic rocks.	This paper	This paper
Jinman Copper Deposit in Lanping	Deep metamorphic rock series may also have contributions from surrounding rocks, sedimentary rocks, biogenic sulfur, and bacterial reduction in sulfates to the sulfur source.	Mainly from sedimentary rocks of the upper crust in the Lanping Basin; may have some addition of deep lead.	56–54 Ma	[102,162,176,177]
Liancheng Copper Deposit in Lanping	Mainly from the deep part; in the late stage, sulfur from the strata of surrounding rocks in the basin is added.	Mainly from sedimentary rocks of the upper crust in the Lanping Basin and mixed with the mantle, leading to varying degrees.	51–48 Ma	[102,155,162,178]
Qingyangchang Copper Deposit in Lanping	From evaporite (gypsum-salt) formations in basin strata, being the reduction product of sulfates.	\	\	[99]
Shuixie Copper Deposit in Lanping	Mainly from evaporite strata in the basin.	Mainly from sedimentary strata in the basin and may have some addition of minerals from the basement rock series.	59.2 ± 0.8 Ma	[177,179]
Kedengjian Copper Deposit in Lanping	The main sulfur source is marine sulfate.	Lead mainly comes from upper crustal sedimentary rocks with the involvement of the deep crust.	~100 Ma	[19,100,101]

Table 6. Characteristics comparison of vein-type copper deposits on the western side of Lanping Basin (“\” indicates missing data).

Deposit	S Source	Pb Source	Metallogenic Epoch	Source
Northern Vein-type Copper Deposit in Lanping	Mixed source of deep magmatic hydrothermal sulfur generated during basin tectonic activities and gypsum rock sulfur reduced by organic matter (i.e., thermochemical sulfate reduction, TSR).	Mainly from orogenic-belt lead, with certain contributions from magmatic sources on both sides of the basin and metamorphic rocks.	This paper	This paper
Jinman Copper Deposit in Lanping	Deep metamorphic rock series may also have contributions from surrounding rocks, sedimentary rocks, biogenic sulfur, and bacterial reduction in sulfates to the sulfur source.	Mainly from sedimentary rocks of the upper crust in the Lanping Basin; may have some addition of deep lead.	56–54 Ma	[102,162,176,177]
Liancheng Copper Deposit in Lanping	Mainly from the deep part; in the late stage, sulfur from the strata of surrounding rocks in the basin is added.	Mainly from sedimentary rocks of the upper crust in the Lanping Basin and mixed with the mantle, leading to varying degrees.	51–48 Ma	[102,155,162,178]
Qingyangchang Copper Deposit in Lanping	From evaporite (gypsum-salt) formations in basin strata, being the reduction product of sulfates.	\	\	[99]
Shuixie Copper Deposit in Lanping	Mainly from evaporite strata in the basin.	Mainly from sedimentary strata in the basin and may have some addition of minerals from the basement rock series.	59.2 ± 0.8 Ma	[177,179]
Kedengjian Copper Deposit in Lanping	The main sulfur source is marine sulfate.	Lead mainly comes from upper crustal sedimentary rocks with the involvement of the deep crust.	~100 Ma	[19,100,101]

Fluids associated with western basin vein-type copper deposits have a higher Cu mass transfer rate than Co, Pb, and Zn, indicating preferential Cu transport [180]. Cu-rich ore bodies may form from the mixing of metal-rich fluids with H₂S-rich thermochemical fluids within the basin. Ore bodies are hosted in purplish-red sandstone layers affected by bleaching metamorphism, where H₂S reduces hematite (Fe₂O₃) to magnetite (Fe₃O₄) or other reduced iron minerals, enhancing Cu enrichment under reducing conditions [181,182,183].

6. Conclusions

• Integrated geological, geochemical, and structural analysis of the vein-type copper deposits in the northern Lanping Basin leads to the following conclusions: the ore-forming materials primarily originated from deep magmatic hydrothermal activities. The lead isotope compositions of the northern copper deposits are consistent with other copper deposits within the basin and exhibit a close affinity to the Cenozoic alkaline rocks around the basin. The sulfur source is closely linked to TSR.
• Strontium isotope signatures indicate that the ore-forming fluids were derived from, or at least interacted extensively with, the gypsum-salt strata within the basin, indicating the migration path of the fluid. Data from rare-earth elements (REEs), including LREE depletion, negative Ce anomalies, and positive Eu anomalies, collectively reveal that the ore-forming fluids underwent significant water–rock interaction during their migration.
• The mineralization was structurally prepared and triggered by specific tectonic events. Pre-ore NE–SW compression created the primary fault architectures (e.g., the F₈ fault system) that provided conduits and spaces for fluid migration and ore emplacement. The actual precipitation of ore-forming materials was likely facilitated by a late Cenozoic abrupt change in the stress field, which caused fluid pressure release and mixing.

The northern Lanping vein-type copper deposits are classified as structurally controlled, low–medium temperature hydrothermal deposits. Their formation involved metal sourcing from deep magmatic rocks and basin-margin alkaline units, fluid–rock interaction, and ore deposition controlled by a specific tectonic sequence of early compression and subsequent stress field perturbation.

This model provides new insights into three aspects. Temporally, mineralization postdates the host rocks, indicating an epigenetic origin related to late-stage tectonic-thermal events, which distinguishes it from SEDEX or SSC-type deposits. Geochemically, isotopes reveal a hybrid metal source from deep magmatic fluids and the sedimentary basin. Mechanically, stress field inversion dynamically controls fluid migration, focusing, and precipitation.

The geochemical similarities among the Lanping Basin vein-type copper deposits point to a common genesis and the model’s applicability, yet they call for resolving precise mineralization dating and pinpointing ore-forming fluid sources in the future.