Granites of the Chazangcuo Copper–Lead–Zinc Mining Area in Tibet, China: Magma Source and Tectonic Implications

Abstract

Intermediate-acidic granites occur extensively in the Chazangcuo copper-lead-zinc mining area (hereinafter referred to as the Chazangcuo mining area) in Tibet, China. Exploring their rock types, sources, and tectonic settings is essential for understanding the genesis of granites in the region. This study investigated the petrology of the Chazangcuo granites, as well as the geochemical characteristics of their major elements, trace elements, and rare earth elements (REEs). Results indicate that the Chazangcuo granites are high-K calc-alkaline metaluminous rocks. These granites are enriched in large-ion lithophile elements (LILEs; e.g., Rb and Ba), depleted in high-field-strength elements (HFSEs; e.g., Nb, Ta, Zr, and Hf), with a relative enrichment in light rare earth elements (LREEs), and relatively depleted in heavy rare earth elements (HREEs), exhibiting a V-shaped distribution pattern and weak negative Eu anomalies. The granites are classified as typical I-type granites, displaying characteristics of crust-derived magmas with contributions from mantle sources and exhibiting significant fractional crystallization. The Chazangcuo granites were derived from the partial melting of mafic rocks, with protoliths formed in a moderate temperature environment. Influenced by the subduction of the Neotethys Ocean, the Chazangcuo granites were formed in an arc caused by the collision between the Indian and Eurasian plates (also referred to as the Indo–Eurasian collision) during the Late Triassic. Under the effect of geological activities such as upwelling of the asthenosphere and fluid intrusion and differentiation, metal mineralization was prompted to be distributed in the granite fissures, forming the Cu-Pb-Zn polymetallic deposits of Chazangcou in Tibet, suggesting that the granites are closely associated with mineralization.

1. Introduction

The formation of the Qinghai-Tibet Plateau (QTP) is associated with the Indo-Eurasian collision [1]. Delving into the evolutionary process of this collisional orogeny is significant for understanding geoscience issues such as collisional orogeny, the QTP uplift, and deep magmatism [2,3,4]. The Gangdese belt, situated in the southern QTP, is an area with the most intense magmatic activity since the Meso-Cenozoic [5,6]. The frequent and multi-stage magmatic activity in the belt is a significant response to the evolution of the Neotethys Ocean and the Indo-Eurasian collision [7,8,9,10]. The western portion of the Gangdese belt, the primary distribution area of the Gangdese batholith, preserves considerable geological evolution records on tectonics, magmatism, and mineralization [11], serving as the optimal site for research into the evolution of the Tethys Ocean and the Indo-Eurasian collision. In the study of magmatic rocks in the Gangdese belt, the previous research focuses on the Cenozoic and some Mesozoic magmatism in the southeast. Many achievements have been made, including a 50 Ma large-scale magma eruption, magma mixing, and mantle source contribution, and the lithospheric evolution after the India-Asia collision has been discussed [12,13,14]. In recent years, the spatial and temporal distribution and evolution of Mesozoic magmatic rocks, tectonic-magmatic relationship, mineralization, and mineral resources in the central, western, and northern regions have been paid more and more attention [15,16,17,18]. For example, Zhu Dicheng and other scholars used geological survey data to study the spatial and temporal distribution of magmatism in the Jurassic and Cretaceous, and proposed a two-way scissors subduction model. Through the study of zircon geochronology and Hf isotope geochemistry, the characteristics of Mesozoic magmatism are revealed, and new understandings such as an island arc collage structure are proposed [19,20]. Jiang Xin and other scholars have conducted in-depth research on the granite batholith in the Shiquanhe area in the western Gangdese belt. The granite dating and Hf isotope results reveal that there is an unexposed Paleoproterozoic crustal basement in the Gangdese belt [21]. Granites, an integral part of continental crust, record the historical evolution of the continental crust, and their formation closely relates to continental tectonic movements, metamorphism, and mineralization [22,23,24,25]. The timing of the initial Indo-Eurasian collision, which has attracted significant interest in the geological community, is the key to exploring the origins of granites at different stages, the mechanisms behind the QTP uplift, and the dynamic process of deep magmas. Under this background, the Chazangcuo mining area in the western Gangdese belt, hosting widespread meso-plutonic granites, contains rich information on tectonic evolution and mineralization while serving as an integral part of regional geology.

In recent years, researchers have made notable progress in investigating the Chazangcuo mining area, including its geotectonic setting and regional geologic conditions; the Chazangcuo copper-lead-zinc polymetallic deposit, including its geological characteristics and origin; and Chazangcuo granites, including their petrology, geochemistry, and chronology [26,27,28,29]. Based on the study of fluid inclusions and sulfur and lead isotopes by Gao Shunbao (2015), it is believed that the Chazangcuo deposit has a certain genetic relationship with volcanic magmatic hydrothermal activity. The source is a relatively single source of crustal magma sulfur [27]. Through C-H-O-S-Pb isotope analysis, Jiang Junsheng (2015) found that the formation of the Chazangcuo deposit may be related to the melting of the Nyainqentanglha basement gneiss in the upper crust induced by subduction-collision and the activation of ore-forming materials in the basement. It is considered that the deposit belongs to a typical magmatic hydrothermal vein type deposit [30]. Through the regional geological survey in Shenzha County, Cheng et al. (2014) found that the granites in the Chazangcuo mining area intruded into the Late Paleozoic strata, and some of the rock masses were covered by the Pliocene Wuyu Group. Most of them are distributed along the east-west fault zone and are obviously controlled by the structure. With calc-alkaline characteristics, it is a magmatic S-type granite, which is a Mesozoic product [29]. Liu Wanli (2019) found that alkali feldspar granite in the Chazangcuo mining area is rich in alkali, high in silicon, and poor in magnesium and manganese, and biotite monzonitic granite is rich in alkali, high in silicon, and poor in magnesium, with both belonging to highly differentiated I-type granite. Through geochemical analysis and zircon U-Pb dating, it is found that the biotite monzogranite (203.5 ± 1.4 Ma) and alkali-feldspar granite (204.0±1.3 Ma) in the Chazangcuo mining area were formed in the Late Triassic. It is also judged that its tectonic environment is a syn-collision setting [26]. Therefore, there are some disputes about the genetic types of the Chazangcuo granite. Moreover, there is a lack of in-depth and systematic explorations into the Chazangcuo granites, including their magma source characteristics, their internal relationships with regional tectonic evolution, and their specific contribution to metal mineralization. Hence, this study investigated the mineralogy, petrology, and geochemistry of the granites, aiming to determine their genetic types, provenance, formation temperatures, and tectonic settings, as well as their relationship with mineralization. This study is expected to provide deeper comprehensive insights into the Chazangcuo granites and enhance the scientific understanding of the tectono-magmatic evolutionary process of the Gangdese belt, thus providing fundamental data for further revealing the mechanisms behind the Indo-Eurasian collision.

2. Geological Setting

The Chazangcuo mining area, located in Shenzha County, Nagqu City, Tibet, resides in the east-central part of the Gangdese-Nyainqentanglha plate geotectonically, extending in the EW direction. This area is composed primarily of strata of the Ordovician, along with Meso-Cenozoic intermediate-acidic intrusions and volcanic rocks, exhibiting nearly EW-striking folds and nearly SN-striking tensional fault zones [30] (Figure 1a,b).

Both Paleozoic and Cenozoic strata are exposed regionally. Regarding the Paleozoic strata, only the Carboniferous and Permian strata are sporadically distributed in the central part, with continuous sediments occurring between them. The Upper and Lower Carboniferous strata are well developed, including the Yongzhu and Laga formations. These strata are dominated by sandstones interbedded with quartz sandstones. The Permian Angjie Formation comprises primarily sandstones interbedded with conglomerates. The Cenozoic strata in the region are relatively well developed. They are almost widely exposed throughout the whole region, encompassing the Paleogene, Neogene, and Quaternary strata. The Paleogene strata include the Dianzhong, Nianbo, and Pana formations. The Neogene strata comprise a clastic member and a volcanic member of the Wuyu Group (Figure 1b). Primary structures in the study area include NWW-, NE-, and nearly SN-striking faults. Among them, the Niangrezangbu and Kongjinxiaga faults exhibit large scales, measuring over 140 km in length, with triangular facets of faults, drag folds, and tectonic breccias observed nearby. Granites are commonly exposed near the Kongjinxiaga fault, suggesting that their formation is associated with fault activity. Intermediate-acid intrusions widely occur in the study area. These intrusions are distributed primarily in the nearly EW direction near the Niangrezangbu and Kongjinxiaga faults. Significantly influenced by fault structures, they occur predominantly as stocks, along with a small number of batholiths. Various types of intrusions are found, including granodiorite, biotite granite, two-mica granite, granite porphyry, and granodiorite porphyry. Volcanic rocks are widely distributed in the study area, especially in the Meso-Cenozic Taerma-Xinji volcanic basin in the central part. Significantly dictated by EW-striking structures, these volcanic rocks consist primarily of Cenozoic volcanic rocks, followed by Mesozoic volcanic rocks (Figure 1b).

The exposed strata in the Chazangcuo mining area comprise the Dianzhong Formation of the Paleogene Linzizong Group, the Neogene Wuyu Group, and Quaternary strata. Primary ore-bearing surrounding rocks include andesites, dacites, and rhyolitic brecciated lavas. Structures in the mining area are dominated by NWW-striking faults, serving as ore-controlling, hosting, and transmitting structures. Ore bodies are relatively well developed, distributed as veins in fault structures within volcanic strata. Twelve ore bodies have been identified, exhibiting varying scales and high grades. They are distributed as NW-SE-oriented strips or veins within fault zones (Figure 1c). The estimated ore volume is 1,274,607 tons, including 45,476 tons of copper, 76,385 tons of lead, 60,103 tons of zinc and 29.62 tons of associated silver. The average grade of the deposit is 3.62% copper, 5.99% lead, 4.72% zinc, and 23.24 g/ton of associated silver. The veins are generally characterized by chalcopyrite, galenite, sphalerite, and quartz-calcite veins, and have a sudden contact relationship with the surrounding rock. The types of ore minerals are relatively single, mainly chalcopyrite, galenite, sphalerite, and pyrite, followed by malachite and azurite. Gangue minerals are mainly quartz and calcite, characterized by the development of a large number of dark brown calcites. Additionally, silicification, as well as the pyrite, zoisite, sericite, chlorite, and clay mineral alterations, are observed in the Chazangcuo mining area. Among them, silicification exhibits the closest association with mineralization.

3. Material and Methods

Ten pristine granite samples were collected from the section outcrops, exploratory trenches, and borehole cores in the Chazangcuo mining area in Shenzha County, Tibet. Following hand specimen observation and microscopic identification of polished thin sections, the samples with the weakest alterations were selected for whole-rock geochemical analysis. Their major, trace, and rare earth elements were tested at the State Key Laboratory of Plateau Ecology and Agriculture of Qinghai University. The major elements were assessed using the ME-XRF15c technique in several steps: sample grinding, pre-drying, adding a lithium borate flux with sodium nitrate, thorough mixing, and high-temperature melting. The molten samples were then cast into a flat glass disk for analyses using X-ray fluorescence spectrometry (PANalytical, Almelo, Netherlands), with relative errors in test results of less than 5%. The trace element and REE analyses were conducted using the ME-MS61r method, involving dissolving samples in perchloric, nitric, hydrofluoric, and hydrochloric acids, followed by volume fixing using dilute hydrochloric acids. Then, inductively coupled plasma–optical emission spectroscopy (ICP-ES) (Agilent, Kuala, Malaysia) was employed for analysis. In the case of too-high Bi/Hg/Mo/Ag/W concentrations, dilution was required before an analysis using an inductively coupled plasma mass spectrometer. Spectral interferences between elements were corrected, with relative errors of less than 10%. For REE analysis, lithium borate (LiBO₂/Li₂B₄O₇) was added to samples, followed by thorough mixing and melting in a furnace at 1025 °C. Once cooled, the samples were dissolved in nitric, hydrochloric, and hydrofluoric acids, processed for volume fixing, and then analyzed using a plasma mass spectrometer.

4. Results

4.1. Petrography

Field geological surveys, hand specimen observation, and the microscopic analysis of mineral compositions reveal that the granite samples were dominated by granodiorites and biotite granites. This study examined the typical characteristics of copper-lead-zinc ore, granodiorite, and biotite granite samples.

The Chazangcuo copper-lead-zinc ore grade is higher, and primary metallic minerals include chalcopyrite, galena, sphalerite, and pyrite (Figure 2a–c). The chalcopyrite displays hypidiomorphic and xenomorphic granular textures, with grain sizes ranging from approximately 0.01 to 6.00 mm. Some chalcopyrite grains are associated with, and metasomatized by, galena and sphalerite at their edges; some are coated with idiomorphic granular pyrite on their surfaces; and some small ones scatter within gangue minerals (Figure 2e). The galena exhibits hypidiomorphic and xenomorphic granular textures, with black triangular pores occurring. It shows grain sizes ranging from about 0.01 to 4.20 mm and is associated with chalcopyrite at their grain edges (Figure 2d). The sphalerite manifests hypidiomorphic and xenomorphic granular textures, a gray color with a brownish tinge, and grain sizes ranging between 0.01 and 1.10 mm. It is distributed at the edges of chalcopyrite grains locally and exhibits metasomatic replacement of the grains (Figure 2e). The pyrite manifests idiomorphic textures, a bright yellowish-white reflective color, and a grain size of about 0.25 mm, distributed on the grain surfaces of chalcopyrite aggregates (Figure 2f). Gangue minerals are mainly quartz, carbonate minerals, plagioclase, sericite, etc. Under the influence of dynamic action, quartz appears as irregular, angular grains with variable sizes and shapes, containing internal micro-cracks and fissures, sometimes contaminated with impurities, and dispersed irregularly within the rock (Figure 2h). Carbonate minerals are irregular and granular in shape, with different particle sizes and shapes. The interior is polluted by impurities. The rocks are mainly filled and metasomatized by aggregates along the rock fissures, and some of the particles are coarse. Encapsulated quartz is produced (Figure 2g). Plagioclase is in a relatively self-shaped plate shape, and epidotization, sericitization, and clay alteration occur later, which are locally distributed in the gap between garnet or diopside and tremolite (Figure 2i). Sericite is a fine scaly aggregate. The main mixed impurities or carbonate minerals in the rock are locally distributed between the rock cracks, a small amount is aggregated with clay minerals, and the plate-like shape illusion is vaguely seen, which may be formed by plagioclase alteration and locally scattered (Figure 2g). The ore structures are dominated by blocky structures, succeeded by lumpy, disseminated, and directional structures. The ore textures are dominated by hypidiomorphic to xenomorphic granular textures, followed by cataclastic and granoblastic textures.

The weathering surface of granodiorite is light-yellow, and the fresh surface is mostly grayish-white. There are many irregular dendritic gray-black quartz veins in the rock mass, and fine-grained dioritic dark enclaves and dark-banded structures can be seen locally (Figure 3a). It has medium to fine semi-automorphic textures and massive structures. Its primary non-metallic minerals include plagioclase, quartz, potassium feldspar, hornblende, and biotite, which account for approximately 40%, 27%, 25%, 5%, and 3% of the total rock volume, respectively (Figure 3c,e). The plagioclase exhibits a tabular shape, a high idiomorphic degree, colorlessness with a brown tinge, and grain sizes ranging from approximately 0.50 to 2.50 mm; it is disorderly arranged, with stripes and grid twinning observed on its surface. Sericitization, clay alteration, local zoisitization, and muscovite develop (Figure 3e). The quartz displays a xenomorphic granular shape, a smoky-gray color, grain sizes ranging from approximately 0.10 to 0.80 mm, and a mosaic distribution pattern. The potassium feldspar exhibits a nearly semi-automorphic plate-allotriomorphic granular, scattered, or interstitial distribution, grain sizes ranging from 0.30 to 1.25 mm, intense alteration at the center, and turbid surfaces. The hornblende shows a short-prismatic shape, a black color with a glassy luster, and grain sizes ranging from approximately 0.50 to 2.50 mm. The biotite displays a flaky shape, a brown color, and grain sizes ranging from 0.10 to 0.50 mm. Accessory minerals include apatite and zircon.

The biotite granite is light-flesh-red in color. Its prominent feature is that spotted biotite is easy to identify. Generally, it has a medium-fine-grained structure, and some biotite granites contain a small amount of potassium feldspar phenocrysts and a massive structure (Figure 3b). Its non-metallic minerals primarily encompass quartz, potassium feldspar, plagioclase, and biotite, which represent approximately 35%, 35%, 20%, and 10% of the total rock volume, respectively (Figure 3d,f). The quartz is irregular, with fresh grain surfaces and grain sizes ranging from approximately 0.30 to 1.00 mm. The potassium feldspar is tabular in shape and flesh red in color, with grain sizes ranging from approximately 0.50 to 1.55 mm. The plagioclase displays a tabular shape, a high idiomorphic degree, and a light-brown color, with grain sizes ranging from approximately 0.30 to 1.20 mm. The biotite is flaky in shape and brown and green in color, with grain sizes ranging from approximately 0.20 to 1.50 mm. Potassium-feldspar crystals are generally larger than plagioclase and quartz, and are mainly striped feldspar and microplagioclase, with slight kaolinization. Striped feldspar often contains xenoliths of plagioclase and biotite. The twin crystal grain of plagioclase is fine, and the surface often has different degrees of sericitization. The distribution of biotite is uneven, mostly greenish-brown biotite, and there is often chloritization at the edge.

4.2. Major Elements

The analytical results of major elements in the granite samples from the Chazangcuo mining area are presented in

Table 1. Analytical results of major elements in granite samples from the Chazangcuo mining area (%).

	CZC-1	CZC-2	CZC-3	CZC-4	CZC-5	CZC-6	CZC-7	CZC-8	CZC-9	CZC-10
SiO2	68.05	70.75	69.46	67.32	66.75	69.75	69.25	68.33	67.55	70.95
TiO2	0.42	0.35	0.45	0.55	0.56	0.43	0.35	0.38	0.28	0.22
Al2O3	13.53	13.58	13.38	14.15	13.24	13.06	13.79	14.16	14.21	13.26
Fe2O3	1.75	1.12	1.55	1.21	2.22	1.96	1.82	1.27	1.96	1.35
FeO	1.15	1.02	1.41	1.07	1.02	1.15	1.08	1.12	1.05	1.02
MnO	1.56	0.75	0.45	1.24	2.01	0.53	1.21	1.08	0.95	0.35
MgO	1.47	1.63	1.22	1.65	1.03	1.04	1.15	1.05	1.02	1.21
CaO	3.75	3.35	3.28	3.7	3.93	3.12	3.14	3.75	3.64	2.96
Na2O	4.05	3.44	3.85	4.53	4.15	4.25	3.86	4.32	4.48	3.64
K2O	3.25	3.01	3.7	3.06	4.17	3.76	3.07	3.36	3.65	3.45
P2O5	0.22	0.11	0.14	0.16	0.07	0.14	0.04	0.07	0.05	0.03
LOI	0.83	0.91	1.15	1.32	0.85	0.73	1.32	1.05	1.14	1.62
ALK	7.30	6.45	7.55	7.59	8.32	8.01	6.93	7.68	8.13	7.09
A/NK	1.33	1.52	1.29	1.31	1.17	1.18	1.43	1.32	1.26	1.36
A/CNK	0.80	0.90	0.82	0.81	0.72	0.78	0.90	0.81	0.79	0.88
AR	2.46	2.23	2.66	2.48	2.87	2.96	2.39	2.50	2.67	2.55
δ	2.13	1.50	2.15	2.37	2.91	2.40	1.83	2.33	2.69	1.80
DI	75.57	75.98	78.64	75.26	77.24	81.29	75.90	76.95	77.67	79.13
SI	12.65	15.50	10.42	14.35	8.23	10.08	10.53	9.46	8.43	10.90

Note: LOI is the loss on ignition, δ is the Ritman index, DI is the differentiation index, and SI is the solidification index.

. According to this table, the loss on ignition (LOI) of 10 samples was low, and the average proportion was not more than 1.09% of the total sample. SiO₂ content ranges from 66.75% to 70.95% (average: 68.82%); Al₂O₃ content ranges from 13.06% to 14.21% (average: 13.64%); NaO₂ content ranges from 3.44% to 4.53% (average: 68.82%); K₂O content ranges from 3.01% to 4.17% (average: 3.45%); and MgO content ranges from 1.02% to 1.65% (average: 1.27%). These values suggest high total alkali content (K₂O + Na₂O = 6.45%–8.32%). They manifested differentiation indices (DIs) varying from 75.26 to 81.9 (average: 77.36) and solidification indices (SIs) from 8.23 to 15.5 (average: 11.06), indicating that the rocks experienced high-degree differential evolution. In the total alkali-silica (TAS) diagram, all the samples fell within the junction of the granite, granodiorite, and quartz monzonite zones (Figure 4a). They showed A/CNK ratios ranging from 0.72 to 0.90 and A/NK ratios from 1.17 to 1.52, all falling within the metaluminous zone in the A/NK vs. A/CNK diagram (Figure 4b). The (K₂O+Na₂O−CaO) vs. SiO₂ diagram reveals the calc-alkaline characteristics of the samples (Figure 4c), as corroborated by Ritman indices (δ) varying from 1.5 to 2.91 and alkalinity ratios (AR) from 2.23 and 2.96. The K₂O vs. SiO₂ diagram shows that most samples exhibited high-K calc-alkaline characteristics (Figure 4d). Overall, the intrusions in the Chazangcuo mining area can be categorized as high-K calc-alkaline metaluminous granites.

4.3. Trace and Rare Earth Elements

The analytical results of trace elements and REEs of granite samples from the Chazangcuo mining area are listed in

Table 2. Analytical results of trace and rare earth elements in granite samples from the Chazangcuo mining area (×10⁻⁶).

	CZC-1	CZC-2	CZC-3	CZC-4	CZC-5	CZC-6	CZC-7	CZC-8	CZC-9	CZC-10
Cu	6829.13	3820.43	2159.56	272.86	1861.22	156.39	6238.43	15368.67	331.82	639.83
Pb	967.36	1009.18	1723.88	39.50	804.04	111.73	743.01	3270.46	33.91	66.18
Zn	3240.07	596.29	4359.74	333.22	1371.85	54.50	1657.68	1262.02	46.30	98.55
Ba	166.79	230.97	168.30	445.50	98.57	197.29	49.23	53.61	87.08	171.33
Rb	92.90	107.83	89.97	98.93	95.67	106.15	55.08	81.13	58.88	115.32
Nb	12.09	18.74	13.16	11.09	5.20	25.55	6.25	15.68	12.76	25.23
Ta	1.48	1.57	2.01	0.74	0.91	3.08	0.42	2.09	1.21	2.91
La	2.93	1.99	8.75	6.03	5.27	7.32	4.91	2.22	1.48	3.66
Ce	23.82	37.07	29.43	37.94	12.13	75.39	13.55	10.73	10.25	49.09
Sr	18.71	31.06	31.55	57.54	87.22	45.99	22.01	9.39	11.62	13.37
Nd	4.00	2.88	9.45	9.09	5.75	10.24	5.96	3.25	1.36	5.92
Sm	1.35	0.86	2.17	2.35	1.32	2.52	1.35	0.82	0.42	1.79
Zr	154.80	289.27	151.53	172.71	78.71	177.80	81.56	84.27	39.87	143.53
Hf	5.64	10.19	5.47	5.52	2.24	6.78	2.72	2.37	1.67	7.42
Tb	0.56	0.28	0.43	0.44	0.26	0.48	0.25	0.16	0.13	0.42
Y	10.39	6.49	12.51	11.02	7.98	12.64	6.82	4.61	4.75	12.05
Pr	1.21	0.76	2.51	2.14	1.41	2.59	1.49	0.78	0.40	1.46
Eu	0.62	0.33	0.54	0.68	0.55	0.49	0.33	0.21	0.10	0.29
Gd	1.62	1.15	2.20	2.70	1.47	2.92	1.40	0.91	0.53	2.17
Dy	2.21	1.50	2.39	2.57	1.51	3.01	1.37	0.98	0.87	2.92
Ho	0.70	0.41	0.58	0.55	0.32	0.64	0.32	0.22	0.18	0.64
Er	1.66	1.40	1.80	1.62	0.94	2.03	0.93	0.71	0.61	2.08
Tm	0.47	0.27	0.33	0.25	0.14	0.32	0.16	0.10	0.11	0.34
Yb	1.79	1.47	1.99	1.54	0.84	2.11	0.98	0.68	0.80	2.43
Lu	0.49	0.29	0.37	0.25	0.13	0.32	0.18	0.11	0.13	0.37
ΣREE	43.43	50.66	62.94	68.15	32.04	110.38	33.18	21.88	17.37	73.58
ΣLREE	33.93	43.89	52.85	58.23	26.43	98.55	27.59	18.01	14.01	62.21
ΣHREE	9.50	6.77	10.09	9.92	5.61	11.83	5.59	3.87	3.36	11.37
LREE/HREE	3.57	6.48	5.24	5.87	4.71	8.33	4.94	4.65	4.17	5.47
δEu	1.28	1.01	0.75	0.82	1.20	0.55	0.73	0.74	0.65	0.45
δCe	3.04	7.25	1.49	2.54	1.05	4.16	1.20	1.96	3.15	5.11
(La/Sm)N	1.37	1.46	2.54	1.61	2.51	1.83	2.29	1.70	2.22	1.29
(La/Yb)N	1.10	0.91	2.96	2.64	4.23	2.34	3.38	2.20	1.25	1.02
(Sm/Nd)N	1.04	0.92	0.71	0.80	0.71	0.76	0.70	0.78	0.95	0.93
(Gd/Yb)N	0.73	0.63	0.89	1.41	1.41	1.12	1.15	1.08	0.53	0.72

Note: Subscript N denotes chondrite-normalized values.

. The granite samples exhibited total REE content ranging between 17.37 × 10⁻⁶ and 110.38 × 10⁻⁶ (average: 51.36 × 10⁻⁶), LREE/HREE ratios between 3.57 and 8.33 (average: 5.34), and (La/Yb)_N ratios between 0.91 and 4.23 (average: 2.20), suggesting significant LREE and HREE fractionation. The rightward chondrite-normalized REE distribution pattern reveals relative enrichment of LREEs (Figure 5a) and δEu values ranging from 0.45 to 1.28 (average: 0.82), indicating weak negative Eu anomalies. This finding suggests the presence of residual plagioclase or fractional crystallization of plagioclase within the source area. The primitive mantle-normalized trace element spider diagram reveals that the samples were enriched in LILEs (e.g., Rb and Ba) but depleted in HFSEs (e.g., Nb, Ta, Zr, and Hf; Figure 5b). Regarding the contents of ore-forming elements, the granite samples exhibited average Cu, Pb, and Zn contents of 3768 × 10⁻⁶, 877 × 10⁻⁶, and 1302 × 10⁻⁶, respectively, suggesting a close association between granites and mineralization in the Chazangcuo mining area.

5. Discussion

5.1. Petrogenesis

The genetic classification of granites tends to play a fundamental role in granite research [37,38]. Chappell et al. (1974) classified granites into I- and S-types based on their ore-forming sources [39]. Based on previous studies, Loiselle et al. (1979) proposed A-type granites occurring in water-deficient, alkaline, and non-orogenic environments [40]. Pitcher (1982) further introduced mantle-derived M-type granites, whose aluminum saturation indices (A/CNK) generally do not exceed 0.6 when derived from juvenile mantle materials [41]. The Chazangcuo granites exhibit an average A/CNK ratio of 0.83, suggesting they are not M-type granites. A-type granites are typically enriched in alkaline dark minerals, with 10,000 Ga/Al ratios and Zr + Nb + Ce + Y content generally exceeding 2.6 and 350 × 10⁻⁶, respectively [42]. The Chazangcuo granites show lower 10,000 Ga/Al ratios and Zr + Nb + Ce + Y content (2.31–2.62 and 67 × 10⁻⁶–351 × 10⁻⁶, respectively). These values, along with the absence of alkaline dark minerals in the granites, indicate that they are not A-type granites. From the perspective of mineralogy, the accessory minerals of I-type granites include hornblende and biotite rather than muscovite and cordierite. This serves as a typical sign that distinguishes I-type from S-type granites [43]. Furthermore, the A/CNK ratio is frequently used to discriminate I-type from S-type granites. Specifically, A/CNK ratios exceeding 1.1 suggest strongly peraluminous S-type granites, while those below 1.0 suggest metaluminous I-type granites [43]. The granite samples from the Chazangcuo mining area generally exhibited hornblende, biotite, and magnetite, with the absence of muscovite, and an average A/CNK ratio of 0.83 (below 1.1). The aforementioned mineralogy and petrogeochemistry indicate that the granite samples were I-type granites.

Moreover, all the samples fell within the I-type granite zone in the Ga vs. Al₂O₃ diagram (Figure 6a). Studies indicate that during the crystallization differentiation of I-type granites, the solubility of apatite decreases with increasing SiO₂ content, in contrast to the behavior in S-type granites [44,45,46]. The P₂O₅ vs. SiO₂ diagram shows that the P₂O₅ content decreased with an increase in the SiO₂ content in the granite samples and that the projected points of the samples aligned with the evolutionary trend of I-type granites (Figure 6b). Furthermore, all samples fell within the I-type granite zone in the Na₂O vs. K₂O diagram (Figure 6c) and the ACF diagram (Figure 6d). Therefore, the granite samples from the Chazangcuo mining area are categorized as I-type granites.

5.2. Magma Sources and Crystallization Temperatures

I-type granites are magmatic products from the melting of unweathered igneous rocks [50], exhibiting two genetic models: the crystallization differentiation of intermediate-mafic magmas [51] and the partial melting of crustal rocks with the contribution from mantle-derived materials [52]. The granite samples from the Chazangcuo mining area were relatively enriched in LILEs and LREEs but depleted in HFSEs and HREEs. The geochemical element trends indicate consistent evolutionary patterns, suggesting derivation from cognate magma evolution [53]. The weak negative Eu anomalies of these samples suggest the fractional crystallization of plagioclase in the magma sources [54]. The characteristic ratios of some trace elements can also effectively reflect the characteristics of rock sources [55]. The granite samples exhibited Nb/Ta ratios ranging from 5.71 to 14.99, averaging 9.72, which is close to that of the crust (11) [56]. Their Zr/Hf ratios ranged from 19.34 to 35.56, with an average of 28.49, which is slightly lower than that of the crust (33) [57,58]. These results further indicate a crustal origin. The samples exhibited Sm/Nd ratios ranging from 0.23 to 0.33 (average: 0.27), consistent with those of the continental crust (0.17 to 0.25) [59]. They manifested Rb/Sr ratios ranging from 1.10 to 8.64, averaging 4.12, which is higher than that of the crust (0.35) and much higher than that of the mantle (0.034) [60,61]. These findings indicate the crustal origin characteristics. The (La/Yb)_N vs. δEu diagram illustrates that the granite samples fell within the crustal source zone (Figure 7a), suggesting that their materials were derived from the melting of crustal sediments. The (La/Sm) vs. La diagram shows that the samples were distributed along the partial melting curve (Figure 7b), implying that their materials were sourced from the partial melting of the crust. In the A/MF vs. C/MF diagram of granites, all samples fell within the partial melting zone of mafic rocks (Figure 7c). It can be comprehensively inferred that the intrusions in the Chazangcuo mining area primarily originated from the partial melting of crustal mafic rocks.

Given that zircon emerges as one of the minerals in granitic magmas that crystallize the earliest, zircon saturation temperature (T_zr) can be used to constrain the crystallization temperature of granitic magmas using Equation (1) [65,66,67]. According to the Zr content of granite samples, the calculation results of a zircon saturation thermometer indicate that the granite samples from the Chazangcuo mining area exhibited T_zr values ranging between 643 and 819 °C (average: 730 °C), indicating that their source rocks were formed under moderate temperatures. This result aligns with the crystallization temperatures of I-type granites [68]. Additionally, a Ti-in-zircon thermometer can also be used to calculate the zircon crystallization temperatures [69,70,71], according to the Ti content in TiO₂ of granite samples and Equation (2), yielding the crystallization temperatures of the granite samples ranging between 568 and 636 °C (average: 609 °C). The Q-Ab-Or isothermal–isobaric contour map of granitoids shows that the granite samples all fell within the zone with temperatures ranging from 700 to 750 °C (Figure 8). Accordingly, it can be inferred that the Chazangcuo granites primarily originated from the melting of crustal materials at moderate temperatures, mixed with mantle-derived materials and the product of fractional crystallization.

T_zr = 12,900/[2.95 + 0.85 × M + ln(496,000/Zr)_melt] − 273.15

(1)

where (496,000/Zr)_melt is the ratio of Zr content in zircon (496,000 × 10⁻⁶) to Zr concentration in Zr-saturated melts and M = (Na + K + 2 × Ca)/(Al × Si).

T_Ti = 4800/[5.711 − logα_SiO2 + logα_TiO2 − logTi_zircon] − 273.15

(2)

The formula is calibrated to a certain extent according to a certain lithology. According to the previous summary of the activity of SiO₂ and TiO₂, the following estimates can be made: (1) if there is zircon in the system, α_TiO2 ≥ 0.5; if there is ilmenite, α_TiO2 ≥ 0.6; if there is titanite and magnetite, α_TiO2 ≥ 0.7; with rutile, α_TiO2 = 1; (2) For siliceous melts, α_SiO2 ≥ 0.3; if there is quartz, α_SiO2 = 1.

5.3. Tectonic Setting

The magmatic activity in the Gangdese belt has a complex history. Some researchers divide the magmatic activity into four stages: ca. 205–152 Ma, 109–80 Ma, 65–41 Ma, and 33–13 Ma [14,17,73,74]. The granite samples from the Chazangcuo mining area exhibited zircon U-Pb ages of 203.5 ± 1.4 Ma and 204.0 ± 1.3 Ma, indicating that they were formed by Mesozoic Triassic collisions [26]. The Mesozoic magmatic activity was governed by the northward subduction of the Neotethys Ocean slab. The granites formed during this period are generally enriched in LILEs such as K, Rb, Sr, and Ba, and LREEs like La and Ce, but relatively depleted in HFSEs including Nb, Ta, and Ti, exhibiting the properties of arc magmatic rocks [75]. Arc magmatic rocks were first discovered in the circum-Pacific subduction zone. The melting process of a subduction zone is generally accompanied by the metasomatic replacement of the overlying mantle wedges with the subduction products of the oceanic crust and the sediments and other fluids carried by it. Furthermore, the melting process is accompanied by partial melting. Consequently, calc-alkaline magmatic rocks in island arcs and active continental margins are formed. Since the Neotethys Ocean slab and fluids in the subduction products are enriched in LILEs and LREEs but relatively depleted in HFSEs, the trace element composition typical of arc magmatic rocks was formed [75,76]. Figure 5 shows that all the granite samples from the Chazangcuo mining area were enriched in LILEs and LREEs but relatively depleted in HFSEs, that is, the samples possessed the characteristics of arc magmatic rocks. Therefore, the tectonic setting of the Late Triassic Chazangcuo granites reflects the subduction environment of the Neotethys Ocean slab.

It was initially believed that the subduction of the Neotethys Ocean began in the Cretaceous [77]. However, Early Jurassic arc granites (188.1 Ma) have been discovered in recent years [78], and Late Triassic granites (205.3 Ma) have been identified in the northern part of Dazhuka most recently [75]. The period from the Late Triassic to the Early Jurassic witnessed continuous magmatic events, with I-type granites exhibiting arc magmatic properties. Therefore, the subduction of the Neotethys Ocean possibly occurred before 205 Ma during the Late Triassic. In recent years, siliceous rocks containing Middle-Late Triassic radiolarian faunas have been found in the Yarlung Zangbo suture zone, indicating the complete opening of the Neotethys Ocean and the presence of a deep-water environment during the Middle Triassic [79,80]. Furthermore, the discovery of Late Triassic ophiolites in the suture zone [81] proves a possibility for the Late Triassic subduction of the Neotethys Ocean.

The Rb-Hf-Ta ternary diagram shows that the granite samples from the Chazangcuo mining area fell within the volcanic arc zone and the collisional setting zone (Figure 9a). Additionally, the Late Triassic granites in the Menba area within the Gangdese belt were formed in an island arc environment due to the early subduction of the Neotethys Ocean [82]. These granites might have experienced a similar diagenetic environment to those in the Chazangcuo mining area, thus being the product of the same tectonic evolution stage [83]. The tectonic setting discriminant diagram shows that the granite samples fell within the transition zone between pre-collision and post-collision zones (Figure 9b), indicating that Chazangcuo granites were possibly formed in a tectonic setting prior to the Indo-Eurasian collision during the Late Triassic [84,85]. The hinterland of the Gangdese belt, lying on the north side of the Yarlung Zangbo suture zone, had undergone uplift and magmatic activity by the Late Triassic, with some subduction-type granites being emplaced due to the subduction of the oceanic crust of the Yarlung Zangbo River [73,86]. The earliest Mesozoic magmatic rock found in Lhasa block is Late Triassic granite. These Late Triassic granites are distributed in the southern margin of the Gangdese arc-back fault-uplift zone, 100–150 km from the Yarlung Zangbo River suture zone, and 200–250 km from the Bangong Lake-Nujiang River suture zone in the north. Considering the influence of crustal shortening in the Mesozoic and Cenozoic, it is farther from the northern suture zone and is obviously more affected by the northward subduction of the Neo-Tethys oceanic plate [73,75,78,87]. Moreover, the Early and Middle Jurassic magmatic rocks are mainly developed in the southern and central parts of the Lhasa block, while the northern margin of the Lhasa block is rarely found. Most of the magmatic activities developed in the northern margin are in the Middle-Late Jurassic and Early Cretaceous. In summary, with the spatial and temporal distribution of magmatic rocks from south to north, the magmatic activity of the Lhasa block shows a trend from old to new. This is consistent with the new trend of active continental margin magmatic activity from the subduction zone to intracontinental magmatic activity [75,88]. Therefore, the temporal and spatial distribution characteristics of Mesozoic magmatic rocks indicate that the Mesozoic magmatic activity of the Gangdese batholith is the product of the northward subduction of the Neo-Tethys oceanic slab.

Eu/Eu* values are frequently employed to reflect magma source depths of granites derived from crustal melting [91,92,93]. The Chazangcuo granites exhibit Eu/Eu* values ranging from 0.05 to 0.31. Based on the positive correlation between the zircon Eu/Eu* values and the crustal thickness, the formation depths of granite magmas in the Chazangcuo mining area can be calculated using Equation (3). The calculation results indicate that average depths of the granite magmas range from approximately 28.71 to 50.60 km, roughly aligning with the crustal source depths of rock masses like Shesuo and Zhulang on the western margin of the Gangdese belt [27].

H = (84.2 ± 9.2) × Eu/Eu* + (24.5 ± 3.3)

(3)

Overall, the spatial and temporal distributions of Mesozoic magmatic rocks reveal that magmatic activity in the Gangdese belt during this period was significantly influenced by the northward subduction of the Neotethys Ocean slab. This influence is particularly evident in the western Gangdese belt during the Late Triassic, where intense magmatism and crustal thickening are believed to have resulted from asthenospheric upwelling due to the break-off of the Neotethys Ocean slab within the context of the Indo-Eurasian collision. At 205–152 Ma, the continuous convergence and subduction of the Indian and Eurasian continents after their collision further increased the resistance against the subducted slab. As a result, the subducted Neotethys oceanic crust might have undergone slab rotation and break-off, which caused asthenospheric upwelling. Accordingly, the crust in the Gangdese magmatic arc experienced intense tectonic compression and thickening, during which dehydration led to the partial melting of the upper mantle. Consequently, basaltic magmas were formed and then upwelled and underplated the lower crust. The considerable thermal energy produced melted the lower-crustal materials, leading to the formation of felsic magmas. After the extensive mixing of the basaltic and felsic magmas, the rapid magma emplacement during the Early Eocene led to the formation of extensively distributed granites in the Gangdese belt [94,95].

5.4. Relationship Between Rock Masses and Mineralization

By clarifying the role of granite in the metallogenic process, we can further explore the geological significance of granite and reveal the metallogenic mechanism [96,97,98]. Disseminated and veinlet-disseminated limonite mineralization, pyritization, malachitization, and other mineralization are often developed. Quartz veins are developed in some rock fissures, which are favorable parts for the development of ore bodies [99,100,101]. There is a clear and obvious boundary between the disseminated ore bodies and the host rocks in the Chazangcuo mining area, showing a rapid transition relationship, which may indicate that the mineralization and diagenesis were carried out at the same time, that is, the formation process of the magmatic deposit and the condensation crystallization process of the parent rock are generally consistent in time. Geochemical data show that the granite samples are enriched in large ion lithophile elements and light rare earth elements, and depleted in high-field-strength elements (Figure 5). The contents of the main ore-related elements such as Cu, Pb and Zn are high (Cu: 3768 × 10⁻⁶, Pb: 877 × 10⁻⁶, and Zn: 1302 × 10⁻⁶), indicating that they are rich in ore-forming elements. The source of ore-forming materials is partial melting of the upper crust, with the participation of mantle-derived materials, and has the characteristics of arc magma. This not only enriches the source of ore-forming materials, but also enriches the ore-forming elements through the differentiation in the process of magmatic evolution [102,103]. Overall, the intrusions in the Chazangcuo mining area exhibit the same characteristics as the rock masses related to the mineralization of typical hydrothermal deposits.

Under the action of the subduction of the Neo-Tethys Ocean, the Chazangcuo mining area is affected by the NW-SE tectonic stress extrusion and the NE-SW extension, accompanied by deep granitic magmatism [28]. The extension caused the host rocks to rupture and gradually developed into a tensile normal fault. With the continuous extension, a series of parallel normal faults is gradually developed at a certain interval behind the formed faults. These normal faults are gradually connected at the bottom to form a large-scale low-angle normal fault. In the part where the extension occurs, the granitic magma uplifts and continues to invade upward, so that the surrounding rock is continuously arched. In the late stage of magmatic activity, the magmatic intrusion begins to cool from the top, the volume shrinks, and the stress direction changes, so that the expanded surrounding rock also shrinks and cracks are generated [2,28,88]. Therefore, there is a large number of folds and faults in the Chazangcuo mining area. The tectonic activity of the northwest-southeast fault provides ore-bearing space for magmatic emplacement. As the ore fluid migrates and condenses in different levels of fault zones and fracture zones, various minerals are filled and continuously mineralized.

The Chazangcuo granite is mainly a product formed before the collision of the Indo-Asian continent in the Late Triassic [26]. In the late Yanshan period, the Indian plate began to subduct under the Eurasian plate, making the Gangdese tectonic magmatic subzone an active continental margin [12]. First, the Neo-Tethys oceanic crust and the terrigenous materials deposited on the oceanic crust subducted beneath the Eurasian plate. The Neo-Tethys began to subduct northward and formed the Gangdese magmatic island arc along the active continental margin. Then, it subducted beneath the Gangdese intermediate continental block, and the remelting magma intruded upward along the intersection of faults in different directions. The magma in the melting channel continued to melt and undergo differentiation and evolution. Under the action of stress, it was strongly in place in the form of diapir, forming a concentric ring distribution pattern of intrusive rocks in this period [2,28,104,105,106]. In the early stage of subduction, the speed was slow, the friction was small, and the influence range of subduction was not large, mainly based on the intrusion of basic magma. Subsequently, the subduction rate of the oceanic crust continued to accelerate, and the friction generated along the subduction zone continued to increase. Due to the water intervention and the participation of some active components, the remelting rate accelerated, resulting in the melting of deep crustal materials and the production of a large number of intermediate-basic and intermediate-acid magmas. In the strong compression tectonic environment, granite formed [107,108,109]. The Gangdese-Nyainqentanglha plate was developed from the early oceanic island arc to the late continental margin arc stage. The magmatic arc gradually moved inland, resulting in internal arc magmatic activity, forming a granitic magmatic arc in the Gangdese belt. Chazangcuo granite is one of its components.

The magmatic activity associated with mineralization in the Chazangcuo mining area occurred during the Mesozoic (205–152 Ma) [26], when the Neotethys Ocean underwent subduction [110,111]. The plate subduction caused the mantle-derived magmas to upwell and underplate the mafic, thickened juvenile lower crust. This process created considerable heat sources and released substantial ore-forming hydrothermal fluids containing Cu and rich in S, causing the thickened lower crust to melt under water-rich conditions and granites with high Cu content (273 × 10⁻⁶ to 15,368 × 10⁻⁶, average: 3768 × 10⁻⁶) to form. During syn-collisional crustal extension, the upwelling and shallow emplacement of magmas, along with the rapid fractional condensation of ore-bearing hydrothermal fluids, led to pronounced rock silicification and, accordingly, the formation of magmatic rocks that bear Cu and are rich in Si and K. Previous studies argued that the residual melts and hydrothermal fluids formed by granitic magmas in the late-stage crystallization differentiation were rich in ore-forming metal elements [112,113]. For the Chazangcuo granites, the hydrothermal fluids derived from the fractional crystallization of magmas were rich in elements like Mo, Pb, Zn, Au, Cu, and Ag. Since the behavior of ore-forming elements was affected by the degree of magmatic differentiation, acidity, and alkalinity, the Cu-Mo and Pb-Zn minerogenetic series were formed. The continuous subduction of the Neotethys Ocean slab led to the formation of magmas with a crust-mantle mixing source. Pipes formed during magma upwelling and fractures formed in the late stage provided pathways for the ascent, enrichment, and precipitation of ore-bearing hydrothermal fluids, as well as spaces for ore storage [114]. The energy and solution released during the magma intrusion and condensation could activate the ore-forming materials in surrounding rocks, creating mineral-rich fluids. As a result, the mineralization of metals like Cu, Mo, Pb, and Zn occurred largely within granites, contributing to the formation of the Chazangcuo deposit in Tibet (Figure 10).

6. Conclusions

(1) The Chazangcuo granites are high-K calc-alkaline metaluminous rocks, relatively enriched in LILEs like Rb and Ba, but depleted in HFSEs like Nb, Ta, Zr, and Hf. Moreover, they are relatively enriched in LREEs but depleted in HREEs, exhibiting a V-shaped distribution pattern and weak negative Eu anomalies.

(2) The Chazangcuo granites are typical I-type granites. They are derived from the partial melting of mafic rocks, dominated by crustal sources and contaminated by mantle. Their protoliths were formed in a moderate temperature environment. It is an arc granite in the Late Triassic Indo-Asian collision.

(3) Influenced by the subduction of the Neotethys Ocean, the asthenosphere moved up to the lithospheric mantle and released Cu-rich and S-rich fluid. The fluid intruded upward, and differentiated and enriched metals such as copper, molybdenum, lead, and zinc. The granite fissures provided channels and spaces for the migration and enrichment of ore-bearing hydrothermal fluids, which promoted the distribution of metal mineralization in the rock mass and formed the Chazangcuo Cu-Pb-Zn polymetallic deposit in Tibet.