*Article* **The Improvement and Application of the Geoelectrochemical Exploration Method**

**Ming Kang 1,\*, Huanzhao Guo 1, Wende Zhu 2, Xianrong Luo <sup>3</sup> and Jianwen Yang <sup>4</sup>**


**Abstract:** The anionic and cationic species of elements from deeply buried deposits migrate to the near surface driven by various geological forces. The geoelectrochemical exploration method (GEM), derived from CHIM, consists of the application of an electric field to collect these active ions at the designated electrode. Prospecting effects have been investigated by researchers since the coming up of CHIM. However, the cumbersome technical equipment, complex techniques and low production efficiency have restricted its potential application in field geological survey. This paper presents the newly developed CHIM that is electrified by a low voltage dipole. The improved technique allows both anionic and cationic species of elements to be extracted simultaneously in an anode and in a cathode. Compared with the conventional CHIM method, the innovative techniques called dipole geoelectrochemical method are characterized by simple instrumentation, low cost and easy operation in field, and in particular enables simultaneous extraction of anionic and cationic species of elements, from which more information can be derived with higher extraction efficiency. The dipole geoelectrochemical method was proposed and applied in the experiments of the Yingezhuang gold ore from Zhaoyuan, Shandong Province, the 210 gold ore from Jinwozi, Xinjiang Province, and the Daiyinzhang gold polymetallic deposit from Wutaishan, Shanxi Province. There are clearly anomalies above the gold ore body, indicating the effectiveness and feasibility of the improved dipole geoelectrochemical method in both scientific research and mineral exploration. The results of anode extraction in several mining areas have shown good results, indicating that gold may be mainly negatively charged. In fact, many metal nanoparticles, clay minerals, or complexes of metal ions are negatively charged, so they migrate to the anode electrode and enrich.

**Keywords:** geoelectrochemical method; dipole; low voltage; technique improvement; concealed deposit

## **1. Introduction**

The geoelectrochemical exploration method (GEM), derived from CHIM (Chastichnoe Izvlechennye Metallov), was invented by Leningrad researchers in the late 1960s to early 1970s, and the method refers to partial extraction of metals. The systematic theory and field techniques, together with some practical results, were first published by Ryss and Goldberg (1973) [1]. The laboratory results upon which the method is based, some additional field conditions, equipment parameters, and speed of coverage of the method were described in several other papers [1–3].

In the 1970s and 1980s, the CHIM method was extensively applied in Russia in exploration for base and precious metals, W, U, Be, and oil and gas [4]. In the early 1980s, experimental research was carried out in China [5,6], and in the late 1980s, geoelectrochemical experimentation was started in India [7]. In the 1990s, this method was applied on a trial basis in the USA [8] and Canada [9], and then widely applied to search for concealed ore deposits [10,11]. Since the 2000s, a large amount of research on the halo-forming

**Citation:** Kang, M.; Guo, H.; Zhu, W.; Luo, X.; Yang, J. The Improvement and Application of the Geoelectrochemical Exploration Method. *Appl. Sci.* **2023**, *13*, 2735. https://doi.org/10.3390/app13042735

Academic Editors: Qingjie Gong and Zeming Shi

Received: 2 December 2022 Revised: 13 February 2023 Accepted: 15 February 2023 Published: 20 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

mechanism of this method has been performed, and great progress has been made for its technique improvement [12–15]. Excellent results of searching hidden ore deposits of Cu, Pb, Zn, Au, Ag, Sn, As and Sb have been obtained with this method [16]. Meanwhile, application studies have been carried out for different stages of mineral exploration, with great success [17,18].

The CHIM method has been improved since it was put forward. MDI (Method of Diffusion Extraction) was first proposed for exploration of buried ore bodies by Milkov et al. (1981) [19], "Dipole" CHIM was proposed by Levitski (1993) [20], and NEOCHIM was set up by Leinz and Hoover (1993) [21]. More recently, the adsorption–electric extraction method was developed by Fei (1992) [22], Liu et al. (1997) [14], Luo (1994) [23] and Tan and Cai (2000) [15]. However, there are still many disadvantages as follows:


On the improvement of the CHIM method, the authors have carried out some exploration in China and made some progress [12,13], and other researchers [24] also conducted related research. In order to carry out the research and further improve the technology of this method on a global scale, the authors systematically summarized the previous research results and wrote this paper.

Firstly, the development history of geoelectrochemical exploration method (GEM) was introduced in this paper. Secondly, the halo-forming mechanism was illustrated on mineral exploration. Then, the basic and the improved geoelectrochemical methods including equipment and sampling were described, respectively. Finally, three case studies on concealed gold deposits in China were illustrated to show the applications of the improved geoelectrochemical method developed.

## **2. Halo-Forming Mechanism**

Deeply concealed ore deposits are dissolved in many forms, such as electrochemical dissolution. Metal anionic and cationic species of elements from concealed deposits migrate to near the surface driven by various geological forces, and they are enriched therein [21,25–28]. Ions with a negative charge go to the anode and positive to the cathode under the influence of an artificial electric field. The man-made electric field can activate and change the forms of occurrence of elements in the soil. Firstly, it can bring about decomposition of a great number of complex anions and other stable or sub-stable form of elements; secondly, it can make anions and cations move to the extraction electrodes, and hence accelerate ionic movement [1,17]. The metal ions of electromobile forms are extracted

in either anodes or cathodes under the influence of a man-made electric field, which are called GEM anomalies (Figure 1). There are electrically active fine-grained clay mineral particles in the soil, and charged ions or electrically active ultrafine-grained clay mineral particles can migrate to the designated electrode and be adsorbed in the foam sample under the action of an external electric field [29]. GEM ionic halos are in mobile forms, and they are dynamically related to concealed deposits [7,22,26]. The element composition of the halos is normally correlated with that of the ores, and the halos occur typically directly over the deposits [30]. Therefore, GEM ionic halos can be used to search for concealed deposits.

**Figure 1.** The formation of geoelectrochemical ionic halos.

## **3. Materials and Methods**

The GEM is a prospecting technique that combines geochemical and geophysical exploration. That is, ions or charged complexes of the electromobile forms of elements in soils and rocks are extracted, under the influence of an artificial electric field, into the specially designed element-collectors (ECs). The ECs are analyzed by ICP-MS for indicator elements related to ore deposits.

## *3.1. Basic GEM-CHIM*

The ECs are embedded in the surface sediments along a profile to be explored and are connected to a DC current source as either anodes or cathodes. The collectors comprise cylindrical polyethylene vessels with a semi-permeable diaphragm as a base, filled with specific electrolyte in which a solid electrode is dipped. A common auxiliary electrode is positioned at "infinity" and is represented by a metal or graphite bar(s). The extraction of electromobile forms of occurrence of metals is made with an applied current of 100–200 mA, usually for a duration of 10 or 20 h [7,30–33]. However, the basic CHIM field procedure is complicated and of high cost. As such, some improvements have been made recently, as introduced in this paper.

## *3.2. Improved GEM—"Dipole CHIM" Electrified by a Low Voltage Dipole (Abbreviated as DL-CHIM)*

## 3.2.1. Theoretical Basis

Both cationic and anionic species may bear useful information, and many metals may form anionic complexes, especially in the presence of chlorides, particularly in surface soils. Such complexes include [CuI]−, [CuCl2] −, [Cd(NH3)2 Cl4] <sup>2</sup>−, [HgCl4] <sup>2</sup>−, etc. [34]. Recognizing that anionic as well as cationic species may provide useful information [33], the "Dipole CHIM" technique electrified by a low voltage dipole was then proposed.

## 3.2.2. Method Setting and Field Procedures

The element-collectors comprise a graphite bar wrapped foam electrode pairs connected by a 9-volt alkaline battery (Figure 2). The electrodes were embedded in a rectangular sampling area with a length of 40 cm, a width of 20 cm and a depth of 30 cm, and were placed parallel at the bottom of the sampling area at an interval of 30 cm, and then they were covered with soil while watering and pouring a dilute nitric acid solution of 15% HNO3. Generally, water is in the anode area for one liter, and dilute nitric acid solution is in the cathode area for one liter. These were left for about 24 h, after which time the electrodes were exhumed (depending on the DC power supply and local geological controls). The electrodes need cleaning before using again.

**Figure 2.** Simplified profile of the DL-CHIM. 1—Anion collector; 2—Cation collector; 3—Current flow lines; 4—Disposable DC power supply.

Soil samples were collected at each sampling point to compare the application effects between the DL-CHIM method with the conventional geochemical soil survey (CGS).

## *3.3. Sample Testing*

Acid digestion in pretreating foam samples was made, and the contents of Au were determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), and the analytical work was carried out at Institute of Geophysical and Geochemical Exploration, Langfang, China and Guilin Research Institute of Geology for Mineral Resources, Guilin.

### **4. Results and Discussion**

#### *4.1. Case Study on Yingezhuang Gold Deposit*

The DL-CHIM method was applied at Yingezhuang gold deposit from Shandong, 210 gold deposit from Xinjiang, and Daiyinzhang gold polymetallic deposit from Shanxi, China. The Yingezhuang gold deposit is located in the East of China, whose ore body is situated at a depth of between 100 m and 600 m. The average grade of the ore body is 4.03 g/t with a thickness ranging from 2 m to 10 m [13]. Figure 3a,b show the results obtained using the DL-CHIM method. It can be seen from Figure 3 that the DL-CHIM method identifies distinct anomalies of Au over the gold ore bodies, and that cationic species anomalies of Au have highest values (41.05 ppb) above the buried ore bodies, and anion anomalies of Au have high intensity and good continuity, which corresponds closely to ore body. Anionic species anomalies and cationic species anomalies of Au are much wider and have better continuity (sample numbers from 18 to 54), which can give anomalous responses from deep-seated mineralization. Anionic and cationic species anomalies of Au enable the identification of the position of the deeply buried ore bodies. The maximum anomalous value in cationic species anomalies of Au clearly corresponds to the gold-rich ore body of Yingezhuang gold deposit. In the thick transported covered terrains (sample numbers from 18 to 36), and anion anomalies of Au can show better results for concealed gold deposits in depth because gold may mainly exist as micro–nano particles in surface soils, which are negatively charged and can migrate to the designated electrode and be adsorbed in the foam sample under the action of an artificial electric field. The analytical results of this field work are tabulated in Table 1.

**Figure 3.** Results obtained by employing the DL-CHIM method over Yingezhuang gold deposit from Shandong, China. (**a**) Anion anomalies of Au (Anode extraction); (**b**) Cation anomalies of Au (Cathode extraction); (**c**) geological base map [13].


**Table 1.** Analytical data of the samples obtained using the DL-CHIM at the Yingezhuang gold deposit.

## *4.2. Case Study on 210 Gold Deposit*

The 210 gold deposit from Xinjiang is located in the Northwest of China, where arid residual regolith is about 10 m thick. The ore bodies are hosted in a mylonite belt at depths between 20 m and 60 m, whose grade varies between 3 g/t and 10 g/t with an average of 4.11g/t [13]. Figure 4 shows the results obtained using the DL-CHIM method along line I at the 210 gold deposit. It can be seen from Figure 4 that there are obvious anionic species anomalies of Au above the ore body, and the anomalies of Au have a certain continuity, indicating that gold may mainly exist as micro–nano particles, clay minerals, or complexes of metal ions in surface soils, which is negatively charged. Figure 4 indicates that the cation anomaly of Au only shows a single anomalous point. Thus, it suggests that the DL-CHIM method is a great improvement over the previous monopole extraction. The analytical results of this field work are tabulated in Table 2.

**Figure 4.** Results obtained by employing the DL-CHIM method over 210 gold deposit from Xinjiang, China. 1—Quaternary System Holocene; 2—Quaternary System Pleistocene; 3—Gold ore body; (**a**) Anion anomalies of Au (Anode extraction); (**b**) Cation anomalies of Au (Cathode extraction); (**c**) geological base map [13].


**Table 2.** Analytical results of the samples obtained using DL-CHIM method at the 210 gold deposit.

## *4.3. Case Study on Daiyinzhang Gold Deposit*

The Daiyinzhang gold polymetallic deposit is located in the midwestern section of Wutaishan area from Shanxi, in the middle-high mountainous area. The terrain in the area is high in the east and low in the west, with developed valleys and severe topography, where exposed strata mainly consist of the chlorite schist and sericite schist of the Baizhiyan Formation of the Neoarchean Wutai Group. The alteration phenomena of pyrite mineralization, silicification, tourmaline, sericitization, and carbonation are obvious. The intrusive

rocks are mainly dominated by Wutai plagioclase granite and Lvliang metamorphic diabase [35,36]. The ore body, with strike length 450 m, has a burial depth of 0 m to 558 m and a thickness of 0.53 m to 2.44 m; its Au average grade is 2.97 g/t. It can be seen from Figure 5 that the cation anomalies of Au mainly occur at measurement points from 11 to 18, and the anomalous values are in the range from 6.21 ppb to 11.21 ppb, with an average intensity of 8.83 ppb, which basically corresponds to the ore bodies near the surface; in addition, the extreme values of the anomalies are relatively continuous, which is basically consistent with the distribution of gold ore bodies. The anion anomalies of Au occur at measurement points from 5 to 11, and the anomalous values are in the range from 4.65 ppb to 16.98 ppb, with an average intensity of 9.47 ppb, corresponding to the buried ore bodies. In the geochemical soil survey, the soil anomalies of Au occur at measurement points from 15 to 19, and the anomalous values are in the range from 4.86 ppb to 481.62 ppb, with an average intensity of 121.02 ppb, which has some displacement with the ore bodies near the surface; for buried ore bodies, the soil anomalies of Au are weak, basically showing the background characteristics. In general, in the thick transported covered terrains (measurement points from 1 to 10), the geochemical soil survey shows only background characteristics; however, DL-CHIM method shows obvious anion and cation anomalies of Au at measurement points for 5, 6 and 8. The analytical results of this field work are tabulated in Table 3.

**Table 3.** Analytical results of the samples obtained using DL-CHIM method at the Daiyinzhang gold polymetallic deposit.


**Figure 5.** Results obtained by employing the DL-CHIM method and geochemical soil survey over Daiyinzhang gold deposit from Shanxi, China. 1—Chlorite schist; 2—Sericite schist; 3—Metamorphic diabase; 4—Plagioclase granite; 5—Gold mineralization; 6—Gold ore body; 7—Drill hole; (**a**) Soil geochemical anomalies; (**b**) Anion anomalies of Au (Anode extraction); (**c**) Cation anomalies of Au (Cathode extraction); (**d**) geological base map [35,36].

## **5. Conclusions**


**Author Contributions:** M.K., conceptualization, methodology, data curation, writing—original draft, investigation, project administration; H.G., investigation, data curation; W.Z., investigation, funding acquisition; X.L., supervision; J.Y., writing—review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the National Natural Science Foundation of China (Grant No. 40743018).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**


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**Bing Zhou 1,2,\*, Zhixue Zhang 3, Zeming Shi 1, Hao Song <sup>1</sup> and Linsong Yu 1,4,\***


**Abstract:** Triassic granitoids are abundant on the northwestern margin of the Tibetan Plateau. The Dahongliutan pluton, located in the eastern Western Kunlun orogen, formed in the Late Triassic.Previous field studies have identified potential mixing of crustal and mantle magmas. In this study, we used zircon U–Pb ages and major and trace elemental analyses to investigate the tectonic evolution of the pluton, and to determine whether any exchange of mantle-derived material occurred between the pluton and the source area. We found that the pluton has relatively high SiO2 contents, and the aluminum saturation index is consistent with peraluminous high-K calc-alkaline granite. The pluton is enriched in light rare earth elements; both light and heavy rare earth elements are highly fractionated. The magma that formed the pluton was predominantly derived from the crust; however, a small amount of upper mantle material was involved in the early stages of magma formation. The pluton underwent composite emplacement as a result of tectonic extension and magmatic emplacement, which may have occurred in the late Triassic post-collisional orogenic stage. Late Triassic magmatism provided heat and ore-forming material for Pb–Zn, Cu, Fe, and rare metal mineralization, which is of considerable importance for geological prospecting.

**Keywords:** zircon U–Pb dating; geochemistry; late Triassic; Paleo-Tethys Ocean; syn-collisional; Dahongliutan pluton; western Kunlun orogen

## **1. Introduction**

The Western Kunlun orogenic belt is located in the northwestern Qinghai–Tibet Plateau at the junction of the Paleo-Asian and Tethys tectonic domains. Magmatic activity has occurred frequently throughout this region, producing granitoid rocks. The Western Kunlun orogen was accompanied by evolution of the Paleo-Tethys Ocean during the Indosinian period. As collisional compressive conditions changed to a collisional extension setting, strong intermediate-acid magmatism occurred along the Mazha–Kangxiwa suture zone [1]. Determining the characteristics, genesis, and evolution of the emplaced granitoids is crucial for investigating the evolution of continental orogenic belts.

The Dahongliutan pluton is one of the main plutons of the Sanshiliyingfang Qitaidaban granite belt [2]; it is located in the northern Tianshuihai–Karakoram terrane, on the southern side of the Mazha–Kangxiwa suture zone, and is bounded by the Dahongliutan Fault to the north. Based on geochronological data, the Bureau of Geology and Mineral Resources of Xinjiang suggested that the Dahongliutan pluton formed during the late Cretaceous. Based on petrography, petrochemistry, and chronology, Qiao, et al. [3] suggested that the Dahongliutan monzogranite is a Late Triassic, highly differentiated S-type granite that experienced crustal source contamination in a collisional setting. Wei, et al. [4] suggested that the Dahongliutan pluton is a compound pluton produced during a single period of tectonomagmatic activity that included partial melting of crustal material in a collisional setting after the closure of the Paleo-Tethys Ocean. Zhang, et al. [5] measured

29

**Citation:** Zhou, B.; Zhang, Z.; Shi, Z.; Song, H.; Yu, L. Geochemistry, Geochronology, and Prospecting Potential of the Dahongliutan Pluton, Western Kunlun Orogen. *Appl. Sci.* **2022**, *12*, 11591. https://doi.org/ 10.3390/app122211591

Academic Editors: Nikolaos Koukouzas and Andrea L. Rizzo

Received: 7 October 2022 Accepted: 14 November 2022 Published: 15 November 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

<sup>1</sup> College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China

an emplacement age of ~217.5 Ma for the Dahongliutan pluton and determined that the pluton was the product of partial melting of crustal material in a post-collisional setting. Ding, et al. [6] measured the emplacement age of the Dahongliutan biotite monzogranite to be 214 ± 1.8 Ma, which indicates that the pluton was produced by late Indosinian magmatism; they postulated that the magma that formed the Dahongliutan pluton contained mantle and Mesoproterozoic crustal components [6,7]. The Dahongliutan rare metal pegmatite deposit is located south of the Mazha-Kangxiwa Fault in the eastern part of the Western Kunlun orogen and is a large Li-Be-Nb-Ta deposit [8]. More than 400,000 tons of Li2O resources have been explored, and many of the pegmatite veins have not yet been evaluated; thus, this deposit has great metal prospecting potential [9].

Exploration data indicate that the Dahongliutan belt has large- and super large-scale rare metal prospecting potential. The Dahongliutan rare metal pegmatite deposit, a large Li–Be–Nb–Ta deposit, is located south of the Mazha–Kangxiwa Fault in the eastern Western Kunlun orogen [8]. More than 400,000 t of Li2O resources have been explored; however, many of the pegmatite veins have not yet been evaluated and this deposit has great metal prospecting potential [9]. The pegmatite dikes that surround Triassic rocks in this area suggest ideal conditions for rare metal mineralization [10]. However, it remains unclear whether the emplacement ages of the different lithofacies in the Dahongliutan pluton are consistent. Moreover, it is unclear whether the pluton formed over multiple magmatic stages. Previous research has suggested that the magmatic source of the Dahongliutan pluton was crustal; however, this does not account for dioritic enclaves developed in the samples. The Muztagata pluton (located to the north), Shangqimugan pluton, and Dahongliutan pluton all formed under similar regional geologic settings and exhibit similar emplacement ages. They contain dark inclusions, which previous field studies attributed to mixing of crustal and mantle magmas. Thus, it is important to determine whether any exchange of mantle-derived material occurred between the Dahongliutan pluton and the source area.

In this study, we used data obtained by a 1:50,000 mineral prospective survey and 1:50,000 regional geological survey, along with previous research results, to investigate the petrology, geochemistry, genesis, and tectonic setting of the Dahongliutan pluton and its geodynamic setting in the Late Triassic. The findings of this study provide important data for systematic studies on tectonomagmatic events related to the Western Kunlun orogeny.

## **2. Geologic Setting**

The Western Kunlun orogen, which is located at the junction of the northwestern margin of the Qinghai–Tibet Plateau and the Tarim Block, comprises a composite orogenic belt formed by multiple arc-continent collisions (Figure 1a). The evolution of the Western Kunlun orogen is complex and includes three tectonic units: the northwestern Kunlun, southwestern Kunlun, and Tianshuihai–Karakoram terranes [7,11].

The Dahongliutan pluton is located ~150 km southwest of Hotan County, Xinjiang Uygur Autonomous Region; the Xinjiang–Tibet highway passes through the north bank of the Karakash River on the northern side of the study area. The tectonic setting is the northern Tianshuihai–Karakoram of the Western Kunlun orogen, which lies to the south of the Mazha–Kangxiwa suture zone. To the north, it is adjacent to the Dahongliutan Fault. The rocks surrounding the Dahongliutan granite are from the Triassic Bayanhar and Paleoproterozoic Kangxiwa groups. The pluton, which forms a linear/oval feature (Figure 1) with its long axis trending NW–SE, was emplaced parallel to the Dahongliutan fault zone [3]. According to the intrusive relationships, rock types, intrusion sequence, and genetic types, the intrusive rocks can be divided into seven different types: muscovite monzogranites (first intrusion, Figure 2a), mica monzogranites (second intrusion, Figure 2b), biotite granodiorites (third intrusion, Figure 2c), biotite monzogranites (fourth intrusion), biotite quartz diorites (fifth intrusion, Figure 2d), tonalites (sixth intrusion), and mica granites (seventh intrusion). These intrusions underwent homologous magmatic and spatial evolutions and belong to a relatively complete magmatic evolution sequence. All

the rock types contain gray-black deep-sourced diorite inclusions that are predominantly elliptical; however, the long axes of inclusions near the edges of the pluton are roughly parallel to the contact boundary with the surrounding rock. The pluton has experienced regional or dynamic metamorphism to different degrees, and local sections exhibit flaky and gneissic structures. The rocks generally contain feldspar phenocrysts, which have evolved from neutral to acidic (early to late) and form a relatively complete homologous magmatic sequence. There is a pulsating contact with rocks adjacent to each intrusion in the sequence, with the presence of condensation edges.

**Figure 1.** (**a**) Geologic map of the Kangxiwa–Dahongliutan area (base map according to [10]), and (**b**) a simplified geologic map of the study area (1. Modern snow cover; 2. Holocene alluvium; 3. Pleistocene

alluvial diluvium; 4. Upper formation of the Bayankala Mountain Group; 5. Middle formation of the Bayankala Mountain Group; 6. Lower formation of the Bayankala Mountain Group; 7. Upper formation of the Huangyangling Group; 8. Middle formation of the Huangyangling Group; 9. Lower formation of the Huangyangling Group; 10. Second member of the upper formation of the Tianshuihai Group; 11. First member of the upper formation of the Tianshuihai group; 12. Second member of the lower formation of the Tianshuihai group; 13. First member of the lower formation of the Tianshuihai Group; 14. Second member of formation B of the Saitula Group; 15. First member of formation B of the Saitula Group; 16. Second member of formation B of the Kangxiwa Group; 17. First member of formation B of the Kangxiwa Group; 18. Second member of formation A of the Kangxiwa Group; 19. First member of formation A of the Kangxiwa Group; 20. Late Triassic seventh intrusive secondary biotite monzogranite; 21. Late Triassic sixth intrusive secondary tonalite; 22. Late Triassic fifth intrusive secondary biotite quartz diorite; 23. Late Triassic fourth intrusive secondary biotite monzogranite; 24. Late Triassic third intrusive secondary biotite granodiorite; 25. Late Triassic second intrusive secondary biotite monzogranite; 26. Late Triassic first intrusive secondary muscovite monzogranite; 27. Late Ordovician quartz diorite; 28. Middle Cambrian second intrusive secondary biotite granite; 29. Middle Cambrian first intrusive secondary granite porphyry; 30. Fault; 31. Inferred fault; 32. Remote sensing interpreted fault; 33. Geological boundary; 34. Inferred geological boundary; 35. Pulsating contact boundary; 36. Surging contact boundary; 37. Hornization; 38. Marble; 39. Sample locations and numbers).

**Figure 2.** (**a**) Rock surrounding a xenolith in muscovite monzogranite. (**b**) Joints developed inmica monzogranite. (**c**) Dark inclusions developed in biotite granodiorite. (**d**) Tonalite in pulsating contact with biotite quartz diorite.

## **3. Analytical Methods**

## *3.1. Major and Trace Elemental Analyses*

We analyzed the major and trace elemental contents of 23 rock samples from the study area. Sample analyses were performed at the Urumqi Supervision and Inspection Center of Mineral Resources, Ministry of Land and Resources. Major elements were analyzed using Xray fluorescence (model # Axios, manufacturer:Panaco Netherlands, country of origin: The Netherlands); the analytical accuracy was better than 1–5%. Trace elements and rare earth elements (REEs) were analyzed using inductively coupled plasma mass spectrometry (ICP– MS; model # NexION 300x ICP-MS iCAP6300 Simultaneous spectrometer, manufacturer: PerkinElmer, country of origin: the United States). Detailed descriptions of the analytical methods and analyses can be found in Liu, et al. [12].

## *3.2. Zircon U–Pb Dating*

Zircon U–Pb dating was performed using laser ablation ICP–MS (LA–ICP–MS) on six samples (P22-39TW01, P20-25TW01, P25-08TW01, P25-12, P21-32TW01, and P7-65TW01). Zircon sorting and analyses were performed at the Shared Collaboration (Beijing) Analysis and Testing Center. The rock samples were first mechanically crushed, then washed and sorted using conventional flotation and electromagnetic separation methods. Zircon grains were carefully chosen using a binocular microscope to ensure grains with good crystal form and transparency, and without inclusions or cracks. The number of zircon grains sorted from each sample ranged from 500 to >1000; these grains were then fixed in a colorless and transparent epoxy resin. Transmitted light, reflected light, and cathodoluminescence (CL) photography were performed to select analytical spots on zircon grains. The selected analytical points avoided the locations of internal cracks and inclusions in order to obtain accurate age values.

Zircon age dating was performed using LA–ICP–MS (Neptune, Finnigan) and a Newwave UP 213 laser ablation system. The spot beam diameter used for laser ablation was 25 μm, the frequency was 10 Hz, and the energy density was ~2.5 J/cm2; He was used as the carrier gas. The 206Pb, 207Pb, 204Pb (+204Hg), and 202Hg signals were received by ion counters, while 208Pb, 232Th, and 238U signals were received by the Faraday cup, allowing for the simultaneous reception of all target isotopic signals. The analytical accuracy of 207Pb/206Pb, 206Pb/238U, and 207Pb/235U was ~2%. Denudation of the sample was measured using the point denudation method, and GJ-1 was used to debug the instrument before analysis. Zircons CJ-1 and M12 were used as external standards. Zircon GJ-1 was analyzed twice before and after every 10 samples for calibration, and one plesovice zircon standard was measured simultaneously.

Data processing of isotope ratios and elemental contents were performed using ICPMS-DataCal 8.0. The age calculations and harmonic maps were completed using lsoplot ver. 3.0. Samples with 206Pb/204Pb ages of ≥1000 Ma were excluded from the calculation because they contained a large amount of radiogenic Pb. For samples with ages of <1000 Ma, the more reliable 206Pb/238U surface age was used. Detailed descriptions of the experimental procedures can be found in Hou, et al. [13].

## **4. Results**

## *4.1. Petrographic Characteristics*

We collected 23 samples from the northern, central, southern, southwestern, and southeastern parts of the pluton. The muscovite monzogranites (samples P22-55 GS, P22-71 GS, P20-15 GS, and D3801GS01) are gray-white with fine-grained hypidiomorphic-granular and massive structures. Euhedral plagioclase (30–35%) crystals have dimensions of 0.1 × 0.25–1 × 2.3 mm; some are replaced by sericite. Interstitial microcline (30%; 0.1–2 × 4.5 mm) is associated with biotite, plagioclase, and quartz. Fine albite stripes are distributed within coarse microcline grains. Quartz grains (25–30%; 0.05–1 × 0.5 mm) are xenomorphic-granular and exhibit strong wavy extinction. Muscovite (5%; 0.05 × 0.2–0.4 × 1.1 mm) is flaky. Flaky biotite (1–5%) is sometimes replaced by chlorite. Small amounts of apatite and white titanium were also observed.

The mica monzogranites (samples P22-39XT01, P26-88 GS, P20-19 GS, P20-03 GS, and P11-15GS01) are gray-white, with fine-grained hypidiomorphic-granular and massive structures. Mineral grain sizes are 0.05–1.7 × 3.5 mm; however, samples are predominantly composed of fine-grained minerals. Plagioclase (30–35%) is euhedral and ranges in size; potassium feldspar (20–25%) exhibits irregular shapes; inclusions of microcline and striped feldspar often replaced interstitial plagioclase. Quartz (25%) is allotriomorphic-granular and exhibits strong wavy extinction. Muscovite (10–15%) is flaky, variable in size, and has an uneven distribution. Biotite (5%) is flaky and commonly replaced by chlorite, with residual visible crystals. A small amount of apatite is also present, along with sericite exhibiting strong wavy extinction. Reticular cracks are filled a small amount of calcite.

The biotite granodiorite (samples P25-08XT01, P25-1-05 GS, and D259GS) is graywhite, with medium to fine-grained hypidiomorphic and massive structures. The main mineral components are plagioclase (45%), microcline (20–25%), quartz (25–30%), biotite (5%), and amphibole (1%). Mineral grains are 0.1–4.5 × 5 mm in size; medium-grained minerals (>2 mm) account for ~45% of the total. Some euhedral plagioclase are replaced by sericite, which has a random distribution. Microcline is irregular and interstitial, and is associated with plagioclase and biotite. Quartz is irregular, exhibits strong wavy extinction, and is associated with plagioclase and biotite. Biotite is flaky, and some grains are replaced by chlorite. Other minerals include small amounts of euhedral amphibole, epidote, sphene, and apatite. Samples are jointed and the surface is locally affected by spherical weathering.

The biotite monzogranite (sample P25-12GS) is gray-white with medium–fine-grained hypidiomorphic-granular and massive structures. The main mineral components are plagioclase (30–35%), microcline (25%), quartz (30–35%), biotite (10–15%), and amphibole (1%). Plagioclase crystals are elongate and exhibit ring structures; some grains are replaced by sericite and zoisite. Microcline is interstitial to plagioclase and biotite. Quartz is irregular and exhibits strong wavy extinction. Biotite is flaky, and some grains are replaced by chlorite. Some amphibole grains are associated with biotite. Epidote, apatite, sphene, and a small number of other minerals are also present. Samples are jointed and the surface is locally affected by alteration.

The biotite quartz diorite (samples P21-20GS and P23-01GS01) is gray with medium– fine-grained hypidiomorphic-granular and massive structures. The mineral grains are 0.1–2.1 × 3.1 mm in size; medium-grained minerals (>2 mm) account for 20–25% of the total. Plagioclase (50%) is tabular with variable grain sizes; some grains are replaced by sericite (1–5%). Hornblende (15%) ranges from euhedral to irregular in shape, and is associated with biotite. Biotite (15–20%) is flaky and randomly distributed. Quartz (15–20%) is interstitial, exhibits strong wavy extinction, and has an uneven distribution. Epidote, sphene, apatite, tourmaline, and small amounts of other minerals are also present.

The tonalite (samples P21-32XT01 and P21-08GS) is light gray-white with fine-grained hypidiomorphic-granular and massive structures. The main mineral components are quartz (25%), plagioclase (40–45%), biotite (20–25%), and amphibole (10%). The mineral grains are predominantly 0.1–2.6 mm in size. Plagioclase exhibits a hypidiomorphic plate strip shape, and some sericite replacement is observed. Biotite is flaky and exhibits weak alteration.

The mica granite (samples P7-65XT03, P9-32GS, and P10-01GS) is gray-white, medium grained, and exhibits hypidiomorphic-granular and massive structures. The main mineral components are plagioclase, potassium feldspar, biotite, muscovite, and quartz. Plagioclase is semi-idiomorphic and columnar; crystals are cloudy and some are extremely undeveloped bicrystals. Potassium feldspar is xenomorphic-granular, and the crystals are clean. Quartz is allomorphic-granular, and the crystals are smaller than those of feldspar. Biotite and muscovite are hypidiomorphic shaped wafers with low degrees of hypidiomorphism. Alteration is less pronounced than in other rock types, with only plagioclase kaolinization and sericitization; however, samples are weathered and the surfaces are broken.

## *4.2. Major Element Contents*

Major elemental analyses (Table 1) indicate that the Dahongliutan pluton is silica-rich (SiO2 =53.91–73.95 wt.%; average = 68.33 wt.%). The Al2O3 content is 12.81–17.41 wt.% (average = 15.08 wt.%), and the aluminum saturation index (A/CNK) ranges from 1.01 to 1.43, indicating peraluminous rocks (Figure 3a). The Na2O content is 2.06–3.39 wt.% (average = 2.72). The K2O content is 1.76–6.60 wt.% (average = 3.98 wt.%) and the K2O/Na2O ratio is 0.53–2.73 (average = 1.49). The total alkali content (K2O + Na2O) ranges from 4.24 wt.% to 9.02 wt.% (average = 6.7). The alkali aluminum index (AI) is 0.24–0.63 (average = 0.45) and the Rittman Portfolio Index (σ) ranges from 0.96 to 3.18, with all values < 3.3 (average = 1.82), thereby exhibiting typical calc-alkaline characteristics. On an SiO2-K2O diagram (Figure 3b), the samples fall in the high-potassium calc-alkaline series region. The CaO and TiO2 contents are low (0.82–6.47 wt.% and 0.13–0.87 wt.%, respectively). The MgO content is 0.21–3.75 wt.% (average = 1.22 wt.%). The total iron (TFeO) content is 1.28–9.49 wt.% (average = 4.07 wt.%), and the TfeO/MgO ratio ranges from 2.53 to 8.19 (average = 4.12), indicating the pluton is Fe-rich and Mg-poor. According to the calculated CIPW norms, corundum (0.16–4.58 wt.%) is present, indicating peraluminous characteristics. The differentiation index (DI) ranges from 40.3 to 90.51 (average = 75.69), indicating a high degree of magmatic differentiation. The consolidation index (SI) ranges from 1.97 to18.21 (average = 7.96), indicating that the magma underwent a high degree of fractional crystallization and was highly acidic. On a total alkali vs. silica (TAS) diagram (Figure 4), 17 samples plot in the granite region. In summary, the Dahongliutan pluton is therefore characterized by high silica, alkali, and potassium contents, but low MgO, TiO2, MnO, and P2O5 contents. These characteristics indicate that the pluton underwent strong fractional crystallization and is a peraluminous granite of the potassium-rich calc-alkaline series.



**Table 1.** *Cont.*


Note: 1–7, Damourite Adamellite; 8–12, Two-mica Adamellite; 13–15, Biotite Granodiorite; 16, Biotite adamellite; 17–18, Biotite Quartz Diorite; 19–20, Tonalite; 21–23, Two-Mica Granite; Qz, quartz; An, anorthite; Ab, albite; Or, orthoclase; C, corundum; Hy, hypersthene; Ilm, ilmenite; Mt, magnetite; Ap, apatite; Zr, zircon; Chr, chromite. (1) Mg# = 100 × Mg2+/(Mg2+ + Fe2+), in which Mg2+ and Fe2+ are molar fractions; (2) A/NK = Al2O3/(Na2O+K2O) (mol); (3) A/CNK = Al2O3/ (CaO + Na2O+K2O) (mol); (4) AI = (Na2O+K2O)/Al2O3 (mol); (5)<sup>σ</sup> = (Na2O+K2O)2/(SiO2 − 43) (wt.%); (6) DI = Qz + Or + Ab + Ne + Le + Kp.

**Figure 3.** (**a**) A/NK vs. A/CNK (according to Maniar, et al. [14]) and (**b**) K2O wt.% vs. SiO2 wt.% plots for the Dahongliutan pluton (according to Peccerillo, et al. [15]).

**Figure 4.** Total alkali vs. silica (TAS) plot of the Dahongliutan pluton samples (according to Middlemost, et al. [16]).

## *4.3. Trace Element Contents*

Table 2 lists the trace element compositions of the samples. Among the large ion lithophile elements (LILEs), Rb is enriched and Sr is depleted (Figure 5a). Among the high field strength elements (HFSEs) Th is enriched, while Ta and Nb are depleted. The observed Sr depletion may be related to the fractional crystallization of plagioclase. HFSE depletion indicates that the magma source was predominantly crustal material.

**Table 2.** Trace (×10<sup>−</sup>6) and rare earth element (×10<sup>−</sup>6) contents of Dahongliutan granite samples.



**Table 2.** *Cont.*

Note: 1–7, Damourite Adamellite; 8–12, Two-mica Adamellite; 13–15, Biotite Granodiorite; 16, Biotite adamellite; 17–18, Biotite Quartz Diorite; 19–20, Tonalite; 21–23, Two-Mica Granite. The calculation method for zircon saturation temperature (TZr) was based on [17,18]. Zircon LA-ICP-MS U-Pb dating (Table 3) was performed on six samples: P22-39TW01, P20-25TW01, P25-08TW01, P25-12, P21-32TW01, and P7-65TW01.

**Figure 5.** (**a**) Primitive mantle-normalized multi-elemental plots (normalizing values from [17]) and(**b**) chondrite-normalized rare earth element (REE) patterns for the Dahongliutan pluton samples (normalizing values from [18]).






**Table 3.** *Cont.*

## *4.4. REE Contents*

The total REE contents (Table 2) range from 65.5 × <sup>10</sup>−<sup>6</sup> to 293.09 × <sup>10</sup>−<sup>6</sup> and the pluton is enriched in light REEs (LREEs) and depleted in heavy REEs (HREEs), indicating pronounced fractionation between the two groups. There is no pronounced Ce anomaly. The δEu is >1 for three samples (P26-88 GS, P21-32XT01, and P21-08 GS), and from 0.34 to 0.94 for the other 20 samples, with relatively pronounced negative Eu anomalies. This indicates that the fractional crystallization of plagioclase occurred during magmatic differentiation. The chondrite standardized REE distribution patterns are all right-inclined (Figure 5b). Eu is depleted, and the distribution curves all exhibit pronounced Eu depletion, indicating that all samples shared the same source material and formation mechanism.

## *4.5. Zircon U–Pb Ages*

Table 3 lists the zircon U–Pb isotopic results. Among the light gray-white, fresh, unaltered granites and diorites (P22-39TW01, P20-25TW01, P25-08TW01, P25-12TW, P21- 32TW01, and P7-65TW01), zircons are predominantly elongate (100–300 μm in length with aspect ratios concentrated between 2:1 and 3:1) and transparent to light yellow in color. The crystal surfaces are bright and clear, with straight edges and pronounced rhythmic bands, consistent with the characteristics of magmatic zircons.

The muscovite monzogranite (P22-39TW01) has 232Th/238U ratios of 0.0070–0.4316 (average = 0.0741; <0.1), with an abnormal Th/U ratio, indicating that these magmatic zircons crystallized in a magma with a special composition [19–21]. The weighted average age obtained from 14 points is 214.0 ± 1.2 Ma (MSWD = 0.79, n = 14; Figure 6a).

The mica monzogranite (P20-25TW01) has 232Th/238U ratios of 0.0042–1.0226 (average = 0.4486; >0.4). A strong positive correlation is present between Th and U, indicating that they are typical magmatic zircons [19–21]. The weighted average age obtained from nine points is 216.6 ± 1.6 Ma (MSWD = 0.47, n = 9; Figure 6b).

The biotite granodiorite (P25-08TW01) has 232Th/238U ratios of 0.2413–0.5179 (average = 0.2913), indicating that the Th/U ratios are related to the Th and U contents of the magma and the distribution coefficient between the zircon and magma [19–21]. CL images show clearly developed oscillating bands (Figure 7a), which are characteristic of magmatic zircons. The weighted average age obtained from 19 points is 212.71 ± 0.89 Ma (MSWD = 0.93, n = 19; Figure 6c).

The biotite monzogranite (P25-12TW) has 232Th/238U ratios of 0.1930–0.6078 (average = 0.3095), indicating that the Th/U ratios are related to the Th and U contents of the magma and the distribution coefficient between the zircon and magma [19–21]. CL images show oscillating bands (Figure 7b) typical of magmatic zircons. The weighted average age obtained from 19 points is 214.7 ± 1.0 Ma (MSWD = 1.10, n = 19; Figure 6d).

The tonalite (P21-32TW01) has 232Th/238U ratios of 0.4416–0.8648 (average = 0.5938; >0.4), with a strong positive correlation between Th and U, indicating that they reflect magmatic crystallization [19–21]. The weighted average age obtained from eight points is 209.3 ± 2.8 Ma (MSWD = 0.45, n = 8; Figure 6e).

The mica granite (P7-65TW01) has 232Th/238U ratios of 0.0043–0.1461 (average = 0.0897; <0.1), with an abnormal Th/U ratio, indicating that these zircons crystallized in a magma with a special composition [19–21]. The weighted average ages obtained from 19 points is 205.99 ± 0.79 Ma (MSWD = 0.36, n = 19; Figure 6f).

**Figure 6.** Concordia diagrams for granite samples from the Dahongliutan pluton. (**a**) P22-39TW01, (**b**) P20-25TW01, (**c**) P25-08TW01, (**d**) P25-12TW, (**e**) P21-32TW01, and (**f**) P7-65TW01.


**Figure 7.** Cathodoluminescence images of selected (**a**) biotite granodiorite (sample P25-08TW01) and (**b**) biotite monzogranite zircons (sample P25-12TW). Solid circles indicate analytical spots used for U–Pb dating.

## **5. Discussion**

#### *5.1. Chronological Significance*

Granitoids occur throughout the Western Kunlun region and existing emplacement age data are predominantly concentrated along the China–Pakistan and Xinjiang–Tibet highways [22–28]. The main body of the Dahongliutan pluton is located above the snow line at an altitude of 6000 m. The terrain is relatively steep and the natural environment is harsh, inhibiting systematic field studies. In recent years, chronological studies of granitoids have indicated that the Western Kunlun orogen is a multi-stage intrusive composite batholith [3]. Although early Paleozoic–Mesozoic granites are exposed the region, the late Variscan Indosinian and early Yanshanian periods contained the peak magmatic activity [29]. The Mesozoic Indosinian granitic belt is the largest tectonomagmatic belt in the region [30]. According to the emplacement ages of different rock bodies in the Western Kunlun region (Table 4), the 258–200 Ma granitic bodies form a large-scale magmatic belt that reflects different periods of magmatism under a compressive extensional setting. Chronological studies of Indosinian granitoids provide an important basis for understanding the spatiotemporal distribution, alteration, and mineralization of Indosinian granites in the Western Kunlun orogen.

**Table 4.** Isotopic ages of Indosinian intrusive rocks in the Western Kunlun region.


The Dahongliutan pluton is located in the eastern Western Kunlun orogen. Previous studies [3–6] have obtained isotopic ages of 220 ± 2.2–209.6 ± 1.5 Ma. This is consistent with the zircon isotopic ages obtained for the six intrusive units in this study (214.0 ± 1.2, 216.6 ± 1.6, 212.71 ± 0.89, 214.7 ± 1.0, 209.3 ± 2.8, and 205.99 ± 0.79 Ma). Therefore, the emplacement age of the Dahongliutan pluton is 220 ± 2.2 to 205.99 ± 0.79 Ma, indicating that the pluton was likely the product of Late Triassic magmatism.

## *5.2. Genetic Types and Source Material*

The LILE contents of the Dahongliutan pluton indicate Rb enrichment and Sr depletion. The HFSE contents indicate Th enrichment and strongly indicate Ta and Nb depletions. The total REE contents are 65.5 × <sup>10</sup>−<sup>6</sup> to 293.09 × <sup>10</sup>−<sup>6</sup> and zircons have inherited cores that share similarities with Himalayan leucogranites [40]. Leucogranites are metasedimentary rocks formed by the dehydration and melting of hydrous minerals [41,42] and are regarded as representative S-type and syn-collisional granites [40]. The Zr contents of the Dahongliutan pluton are relatively low (65.51 × <sup>10</sup>−<sup>6</sup> to 354 × <sup>10</sup><sup>−</sup>6). Even at low temperatures, zircon crystallization also requires Zr contents of ~100 × <sup>10</sup>−<sup>6</sup> [26]. Therefore, the pluton has experienced zircon fractional crystallization. Only four samples (D3801GS01, P6-06XT01, P22-39XT01, and P7-65XT03) have Zr/Hf ratios of >40; the other 19 samples have low Zr/Hf ratios (17.85–39.85), which supports this conclusion, as zircons have high Zr/Hf ratios (>40). The fractional crystallization of zircon leads to lower Zr/Hf ratios in the residual magma. The presence of inherited cores indicates that the initial magma was Zr-saturated during the formation of the pluton. According to Zr-saturation thermometer calculations, the zircon saturation temperature of the Dahongliutan pluton was 724–872 ◦C [43,44]. This is similar to the emplacement temperature of I-type granites (750–850 ◦C). Among the sample, two (P22-71 GS and P21-08 GS) had Zr-saturation temperatures of >800 ◦C; the remaining 21 samples had Zr-saturation temperatures that were substantially lower than the emplacement temperature of A-type granite (>800 ◦C), which is the upper limit for the initial magma temperature [45]. However, given that zircon fractional crystallization requires higher temperatures and lower viscosity, the actual temperature of the magma may have been higher [40].

On an S- and I-type granite discrimination diagram (Figure 8) most of the Dahongliutan pluton samples plot in the S-type granite region, and some plot in the I-type granite region. The A/CNK of the pluton is 1.01–1.43 (average = 1.21), which is greater than 1.1 and belongs to the peraluminous granite region. The presence of biotite and Al-rich minerals such as muscovite and garnet is consistent with S-type granites. Dark elliptical or irregular diorite xenoliths of different sizes are common, but are unevenly distributed. The boundaries between the xenoliths and the surrounding rock are clear. Some boundaries have clear condensation edges, indicating that the xenoliths had a magmatic origin. Such boundaries are the result of mixing between relatively more basic and acidic magmas. In summary, the magma that formed the Dahongliutan pluton mainly originated from the crust and was an S-type granite. However, a small amount of upper mantle material was involved in the early stages of magma formation, as reflected by the I-type granite characteristics.

The Dahongliutan pluton samples are enriched in LREEs and depleted in HREEs (La/YbN = 7.05–49.9), except for three samples (P26-88 GS, P21-32XT01, and P21-08 GS). One sample has an δEu value of >1, with a pronounced negative Eu anomaly, low Y (~34.4 × <sup>10</sup><sup>−</sup>6) and Yb (~3.83 × <sup>10</sup>−6) contents, and a high Sr/Y ratio (~38.97). This is similar to adakite, and indicates that garnet was present during magma formation. The other 20 samples have δEu values of 0.34–0.94, relatively pronounced negative Eu anomalies, and insignificant Sr losses, which indicates that no plagioclase residue was present in the source. Given the lower pressure limit of rutile stability (1.5 GPa), the formation pressure of the Dahongliutan pluton was likely 1–1.5 GPa. Biotite and polysilicon muscovite have stronger affinities for Nb than Ta [46] in granitic magmas. In a low pressure environment (<2 GPa), the melting temperature of polysilicon muscovite is lower than that of biotite. At 1–1.5 GPa, the melting temperature of polysilicon automica is 760 ◦C, while that of biotite is 820 ◦C [47]. The Nb/Ta ratios of the Dahongliutan samples are high, which also suggests that dehydration and melting of polysilicon muscovite and biotite may have

occurred during magma formation. Thus, the Dahongliutan pluton may have formed via the partial melting of crustal rocks at 1–1.5 GPa.

**Figure 8.** S- and I-type granite discrimination diagram for the Dahongliutan pluton samples (according to Nakada, et al. [48]). A = n (Al2O3 − Na2O − K2O); C = n (CaO − 3.33 P2O5); F = n (FeO + MgO + MnO).

The relative enrichment of LREEs, flat HREE contents, and weak Eu depletion of the Late Triassic granitoids suggests that the magma may have had mixed crustal and mantle sources [49]. The granitoids also have pronounced negative Nb, Ta, and Sr anomalies, which also indicates that their source was predominantly crustal material, and that contamination by mantle-derived magma may have occurred. The Rb/Sr value of a mantle-derived magma is <0.05, that of a mixed crustal and mantle source is 0.05–0.5, and that of a crustal source is >0.5. Four samples have Rb/Sr ratios of 0.21–0.46, indicating a mixed crustal and mantle source. The other samples have Rb/Sr ratios of 0.62–4.38, indicating magma with a dominant crustal source. The Nd/Th values of the samples are 0.84–3.71 (average value = 1.88), which is close to the value of a crustal source (~3) [50], but inconsistent with a mantle source (>15) [51]. The Nb/Ta values of the samples are 2.89–14.40 (average = 8.05), which is close to the crustal Nb/Ta value (11) [52]. The Zr/Hf values range from 17.85 to 50.76 (average = 32.32), which is close to the crustal value (33) [52]. The La/Nb values range from 1.17 to 3.30 (average = 1.99), which is close to the average crustal value (2.58) [52]. Thus, the Dahongliutan pluton predominantly formed as the result of the partial melting of crustal material, but included some mantle-derived material.

#### *5.3. Tectonic Setting and Geological Significance*

Many Middle and late Triassic granites [3,24,25,53] in the Western Kunlun orogen are predominantly distributed along both sides of the Mazha–Kangxiwa Fault. The Late Triassic granite belt of the Western Kunlun area is related to the closure of the Paleo-Tethys Ocean. An LA–ICP–MS zircon U–Pb age of 231.4 Ma was previously obtained for the Mushitage pluton [25], while the SHRIMP age was 230.3 Ma [53]. This pluton was the product of a transition from a mainly collisional compressive tectonic setting to a back arc extensional setting [40], and of the detachment of the subducted slab in a post-collisional tectonic setting [53]. Isotopic ages ranging from 213 to 225 Ma have been obtained for the Akaraz Shan granite [22–24,26], which has been postulated to have formed in postcollisional, subduction, and syn-collisional settings [23,24,26]. Studies of the Middle and Late Triassic Yuqikapa, Taer, Mazha, Shengliqiao, and Qitaidaban plutons indicate that they are related to the subduction and closure of the Paleo-Tethys Ocean and subsequent continental collisional orogeny [22,27,28,53].

Zircon U–Pb dating of metamorphic rocks in the Kangxiwa Fault zone and fission track dating of detrital zircons in areas adjacent to the Western Kunlun region indicate that strong compressive uplift occurred between 235 and 267 Ma in the Western Kunlun region. This uplift was closely related to the subduction and collision of the Paleo-Tethys Ocean [1,7,54]. Kang, et al. [55] also found that the Western Kunlun region lacks sedimentary records from the late Permian to the Middle Triassic, which is likely related to the orogenic event that occurred during this period. Before the late Permian, predominantly marine sedimentation occurred; after the Middle Triassic, sediment was composed of continental material, which may indicate the end of the collisional event [55]. Thus, as the Paleo-Tethys Ocean retreated northward until it closed in the Late Triassic [30,56], continental collision occurred along both sides of the Kangxiwa Fault zone. This collision resulted in crustal thickening and large-scale re-melting under a compressive setting, which led to strong magmatism that formed the Dahongliutan pluton on the southern edge of the Kangxiwa Fault zone.

The Dahongliutan pluton is located in the Hoh Xil–Songpan foreland basin, and its long axis trends NW–SE. The pluton is controlled by fault structures consistent with the regional tectonic direction. The steep surrounding rocks were affected by the lateral compression of the pluton, indicating that magma actively expanded and pushed the surrounding rock outward during emplacement. The pluton then expanded further through structural expansion and magmatic encroachment. During the Late Triassic, the study area underwent large-scale crustal uplift, and heat flow values increased accordingly. The melting layer moved upwards and Al-rich granitic magma was produced through the partial melting of the crust. The pluton was emplaced and along a structurally weak zone in the uplift. As the magma ascended through the crust, the viscosity increased, owing to crystallization, and the speed of ascent slowed. When ascent stopped, the magma expanded laterally through tectonic expansion (i.e., along the fault zone). Magma encroachment occurred into the surrounding rock and produced xenoliths in the pluton. The Dahongliutan pluton therefore had a composite emplacement mechanism that was dominated by active expansion supplemented by tectonic expansion and magma intrusion mechanisms.

On a R1–R2 diagram (Figure 9), the data are predominantly in Zone 6 during the same collision period. On an Rb − Y + Nb diagram (Figure 10a), the data fall into the volcanic arc granite and syn-collisional granite areas, but are closer to the syn-collisional area. On an Rb − Yb + Ta diagram (Figure 10b), most data fall into the syn-collisional granite area. Thus, the Dahongliutan pluton may have formed in a syn-collisional tectonic setting. The Late Triassic tectonic setting was the extensional post-collisional orogenic stage of the Paleo-Tethys Ocean [57]. Peak magmatism occurred during the late Hercynian–early Yanshanian period in the Western Kunlun region. However, there is no chronological evidence of magmatic activity from 230 to 250 Ma [56], which may be because the strong compressive stress was not conducive to magmatism [58]. When the tectonic stress changes to post-collisional extension, decompression melting and magmatic upwelling are more likely to occur [58], leading to intense magmatism. With the development of extension and collapse of the orogenic belt, a substantial amount of heat was generated, thereby causing the partial melting of crustal material that ascended and was emplaced. With continuous extension and lithospheric thinning, large-scale magma upwelling formed the Late Triassic intrusive rocks in the Western Kunlun region.

**Figure 9.** R1–R2 diagram for the Dahongliutan pluton (according to Batchelor, et al. [59]). -1 R1: 4Si-11(Na + K) − 2(Fe + Ti); R2: 6Ca + 2Mg + Al; Mantle anorthosite granite; -2 Destructive active plate edge (before plate collision) granite; -<sup>3</sup> Granite from the uplift stage after plate collision; -4 Late tectonic granite; -<sup>5</sup> Non-orogenic A-type granite; -<sup>6</sup> Syn-collisional (S-type) granite; -7 Post-orogenic A-type granite.

**Figure 10.** (**a**) Y + Nb vs. Rb and (**b**) Yb + Ta vs. Rb diagrams for the Dahongliutan pluton (according to Pearce, et al. [17]) (WPG: within-plate granite; VAG: volcanic arc granite; syn-COLG: syn-collisional granite, ORG: ocean ridge granite).

## *5.4. Implications for Geologic Prospecting*

Magmatism in the study area not only provided metallogenic hydrothermal fluids, but also promoted the activation, migration, and enrichment of metallogenic elements in the surrounding rock, which is often accompanied by mineralization. Minerals often form in or around plutons. Spatially, most mineralized areas are concentrated in the internal and external contact zones between the Late Triassic pluton and the surrounding rock. Compared with their average crustal abundances, the elements enriched in the Late Triassic pluton include Pb, Zn, W, Sn, Mo, Ag, and Bi. Elements with variation coefficients of >1 include Pb, Zn, As, Sb, Ni, and Ag. The Pb and Zn contents are 1.24–24.37 and 1.03–5.28 times the average value of the Western Kunlun region, respectively. The enrichment of other elements is less pronounced. The Kangxiwanan Ag–Pb, Kalakashihenan Pb–Ag, Ahelangan Pb–Ag, Fulugouxiayoubei Pb–Zn, Fulugou Pb–Zn, and Dahongliutanxi Pb ore deposits are all located in the outer contact zones of Late Triassic rocks. These findings are consistent with geochemical and remote sensing anomalies observed in the study area.

Rare metal mineralization is closely related to the Late Triassic intrusive rocks. The granite pegmatites in the study area were derived from the differentiation of these rocks and provided rare metals, a heat source, and ore-forming materials. The Dahongliutan and Ahelangan Li–Be ore deposits occur in granite pegmatites. Mineralization in the study area is predominantly hydrothermal, and was controlled by NW–SE structures. Mineralization is also mainly distributed in the high strain zones of the regional structures. The orebodies are controlled by a group of joints that are consistent with the overall strike, and exhibit contemporaneous mineralization characteristics. In Triassic strata, Li and Be deposits formed as a result of the intrusion of Indosinian magmatic hydrothermal fluids. The metallogenic mode of these deposits was predominantly crystalline metasomatism. The host rock was granite pegmatite and the surrounding rock was anorthosite biotite quartz schist. The strikes of the orebodies are consistent with those of the regional tectonic trend. The orebodies exhibit discontinuous, lenticular, and vein distributions that are often accompanied by strong albitization. The orebodies are controlled by the pegmatite veins, which have ore bearing properties that are related to the distance between plutons. Poor ore-bearing properties occur near the pluton, while good ore-bearing properties occur further from the pluton [10].

#### **6. Conclusions**

Based on analyses of the isotopic ages, mineralogy, and geochemistry of the Dahongliutan pluton in the Western Kunlun orogen, the following conclusions were drawn:

(1) Combined with the zircon CL images and U and Th data, zircon U–Pb dating indicates that the emplacement age of the Dahongliutan pluton is 220 ± 2.2 to 205.99 ± 0.79 Ma, and was the result of Late Triassic magmatism.

(2) The Dahongliutan pluton is enriched in silica, alkali elements, and potassium. The A/CNK ranges from 1.01 to 1.43, indicating a strongly peraluminous rock type. LREEs and HREEs are highly fractionated. The negative Eu anomalies of 20 samples are relatively pronounced. The pluton is enriched in HFSEs (e.g., Th) and LILEs (e.g., Rb); however, it is depleted in Sr, Ta, and Nb. Thus, the pluton belongs to the high potassium calc-alkaline peraluminous rock series.

(3) The Dahongliutan pluton is an S-type granite; however, a small amount of upper mantle material was involved during early magma formation. The pluton also exhibits I-type granite characteristics. The negative Eu anomalies, low Y and Yb contents, high Sr/Y ratios, low Nb, Ta, and Sr contents, and high Nb/Ta ratios (2.89–14.40) indicate that garnet and rutile residue were present in the source. Thus, the Dahongliutan pluton may have formed via the partial melting of crustal rocks at 1–1.5 GPa, with the contribution of some mantle-derived material.

(4) The U–Pb ages and regional geology indicate that the Dahongliutan pluton had a composite emplacement mechanism that was dominated by tectonic expansion and magma intrusion, which may have occurred in the same collisional tectonic setting. During the Late Triassic, the Western Kunlun region entered the post-collisional stage of the orogeny and intrusive rocks formed in an extensional setting.

(5) Late Triassic magmatism provided important heat and ore-forming materials for the mineralization of Pb–Zn, rare metals, Cu, Fe, and other minerals in the study area. Mineralization in this region is closely related to magmatism, which is important for geologic prospecting.

**Author Contributions:** Conceptualization, B.Z., Z.Z. and Z.S.; methodology, B.Z., Z.Z., Z.S. and H.S.; writing—original draft preparation, B.Z., and Z.Z.; writing—review and editing, B.Z., Z.Z. and H.S.; visualization, B.Z. and Z.Z.; supervision, Z.S., H.S. and L.Y.; funding acquisition, B.Z. and Z.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the China Geological Survey Project (Grant No. 1212011220655) and the Xinjiang Uygur Autonomous Region Geological Exploration Fund Project Management Center Project (Grant No. K15-1-LQ04).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** We would like to thank the editor and reviewers for valuable comments and suggestions, which have greatly improved this manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

## **References**

