Petrogenesis and Metallogenesis of Granitoids in the Yangla Cu-W Polymetallic Deposit, Southwest China: Evidence from Zircon Trace Elements and Hf Isotope

Abstract

Magmatic zircon tends to present characteristic trends in trace element compositions in response to magma petrogenesis and metallogenesis, such that zircon may provide a window into melt evolution not accessible by whole rock chemistry. The Yangla large Cu deposit is located in the central part of the Jinshajiang Suture Zone, southwest China, constrained between the Jinshajiang and Yangla Faults. In this study, the trace elements and hafnium isotopic compositions of zircons from quartz diorite were studied. Previous published relevant data of Yangla granitoid plutons (i.e., dioritic enclave, granodiorite, and quartz monzonite porphyry) also have been systematically cited and discussed. The result shows that the crystallization temperature and two-stage Hf mode ages (t^C_DM; the age of the source rocks for the magmas) gradually increased while the oxygen fugacity (fO₂) and ε_Hf(t) values gradually decreased, corresponding to the diorite enclave (~232 Ma), through granodiorite (~208 Ma) and quartz monzonite porphyry (~202 Ma), and to quartz diorite (~195 Ma). It is suggested that four plutons were from the same three-component mixing of upper crust + lower crust + mantle magmas, while the upper crustal metasediments ratios were gradually increased from the early to late stage. The increasing upper crust inputs resulted in higher melting temperatures and compositions of the initial magma. All melts experienced distinct fractional crystallization of apatite, titanite, and amphibole, and the later granite melts experienced higher assimilation and fractional crystallization degrees than the early ones in the evolution processes four stages of intrusive rocks. These Yangla granitoids are the products of large-scale acid magmatic emplacement activities in the Triassic-early Jurassic and have a good metallogenic potential of the Cu-W polymetallic deposit.

1. Introduction

The Yangla Cu-W polymetallic deposit (YCWPD) (28°51′–28°59′ N, 99°04′–99°07′ E) is located in Deqin County, northwest Yunnan, southwest China, and is one of the representative deposits in the “Sanjiang” Polymetallic Metallogenic Domain (SPMD; Figure 1a, b) [1]. The YCWPD contains ore reserves of 150 Mt Cu @ 1% in the Jinshajiang Suture Zone (JSZ) [1]. Since the discovery of YCWPD in the 1960s, substantial research has been carried out within the Yangla mining district. Fruitful research has been conducted on diagenesis and Cu mineralization, including elucidation of the diagenetic chronology and petro-geochemistry of the Beiwu, Jiangbian, Linong, Lunong, and Tongjige granodiorite plutons [2,3,4,5,6], ore-forming fluid and isotope geochemistry [7,8], isotopic geochronology [8,9], the metallogenic mechanism [8,9,10], and ore genesis [11,12]. The general consensus is that the YCWPD is genetically associated with the magmatic emplacement and other evolution events of the Jinshajiang Ocean Basin (JOB) in the Late Triassic (~230 Ma). The metallogenic materials are mainly related to acidic magma and are partly contaminated by the meteoric water and upper crustal sedimentary materials [1,7,8,9]. The boiling action of hydrothermal fluid accompanied the cooling is the main mechanism of Cu precipitation and enrichment at YCWPD [13]. The YCWPD can be regarded as a composite superimposed genetic deposit, and the mineralization sequence may have undergone the exhalative-sedimentary to skarn-porphyry [12]. Recently, a new (copper-mineralized) quartz diorite was discovered during ore prospecting exploration at the depth (3250 m sections) of the Linong ore block within the YCWPD; the emplacement age, petro-geochemistry and genesis of this new quartz diorite pluton have been well-constrained [13,14], but its magma origin and properties, evolution process, geodynamic setting, and metallogenic potential require further analysis and discussion.

Trace elements (i.e., REE) and the hafnium isotopic system of zircon can be regarded as an important provenance indicator for the origin of granites, which can provide a better constraint on the involvement of magmas [3,15,16,17,18]. Currently, the trace element compositions (i.e., REE, Y, Nb, Ta, Ti, U, and Th) of zircon have been widely used to estimate crystallization temperature and oxygen fugacity, discuss assimilation and fractional crystallization and assess metallogenic potential assessment [19,20,21,22,23,24,25]. Moreover, hafnium isotopes of zircon have been broadly used to trace the magma source, assemblage sequence, petrogenetic types, tectonic setting, and magmatic evolution, combined with the chronology, oxygen isotope, and other whole-rock isotope data (Sr-Nd-Pb) [2,26,27], such as Yangla granodiorite plutons in Sanjiang region [3,5], Pulang composite granite intrusion in Sanjiang region [28], Dazeshan-Tianzhushan granites in Jiaodong peninsula [27], and Erdene granite in south Mongolia [29].

This study mainly presented the zircon trace elements and Hf isotopic composition of the newly discovered quartz diorite in the YCWPD. Moreover, previously published data (i.e., diagenetic chronology data, zircon trace elements, and Hf isotopic compositions) from granodiorite plutons (Beiwu, Jiangbian, Linong, Lunong, and Tongjige), quartz monzonite porphyry and, dioritic enclaves within the granodiorite, are systematically collected and re-interpreted, and the crystallization temperature, petrology, geodynamic setting, and metallogenic potential of acidic magma are discussed in the YCWPD. Combined with previously published data, this study can not only enrich and systematically establish the origin and evolutionary sequence of acid magma in the Yangla region but also analyze the metallogenic potential of the Yangla granitoids plutons.

2. Geological Setting

The Sanjiang region is a key metallogenic zone in SW China (Figure 1a) [14] and mainly consists of several ophiolite suture zones, a magmatic belt, several microcontinental blocks, and regional thrusts (Figure 1b) [2,7]. Studies have shown that four branches of the Paleo-Tethyan Ocean have been recognized, such as the Garzê-Litang Ocean (suture I in Figure 1b), Jinshajiang Ocean (suture II in Figure 1b), Ailaoshan Ocean (suture III in Figure 1b), and the Changning-Menglian Ocean (suture IV in Figure 1b) [30]. Furthermore, numerous igneous rocks (i.e., Baimaxueshan, Jiaren, and Lincang granitoids) and large to super-large Cu-polymetallic deposits (i.e., Yulong, Yangla, Pulang, and Xuejiping) have been formed and discovered since the Paleozoic to Cenozoic [1,12].

The YCWPD is constrained to the east and west by the regional Jinshajiang and Yangla Faults, respectively. This restricts mining to a narrow N-S-trending region with an area of approximately 12.40 km² (Figure 1c). The YCWPD is mainly divided into seven ore blocks (Figure 1c); the Linong is the largest, with proven copper metal reserves of approximately 600,000 tons and average copper content of approximately 1.0% [1,14]. The newly discovered quartz diorite occurs at a depth of the Linong ore block; therefore, this study mainly focuses on the Linong when describing the geological setting.

The Yangla Cu mineralization is closely associated with the spatiotemporal properties and origin of the granitic plutons. The Cu ore bodies are typically produced around acidic intrusions, not only in the surrounding rocks of the outer contact zone but also in the fractured zone and inner contact zone within the plutons [1]. Furthermore, stratigraphic, lithological, and structural controls of the Cu ore body distribution are particularly evident in the YCWPD [31]. The Devonian Jiangbian and Linong formations are important ore-bearing components of the local stratigraphy. Skarn is the main ore-hosting lithology of the YCWPD, followed by altered metamorphic quartz sandstone, sericitic sandy slate, marble, and granodiorite [1,31]. The NNE- and nearly N-S-trending faults and other fracture zones are important ore-controlling or ore-hosting structures [31].

The exposed stratigraphy is mainly dominated by the Silurian (S), Devonian (D), and lower Carboniferous (C₁) units; the Devonian is further divided into a lower sequence (D₁) and a middle-upper sequence (D₂₊₃), which are the main ore-bearing horizons in the Yangla skarn Cu body [11,31]. The Lower Devonian stratigraphy can be further divided into the first (D₁j¹), second (D₁j²), and third (D₁j³) sequence of the Jiangbian Formation, whereas the middle-upper Devonian can be further divided into the first (D₂₊₃l¹), second (D₂₊₃l²), and third (D₂₊₃l³) sequence of the Linong Formation. The third members (D₁j³) of the Jiangbian Formation are the ore-host sequences of JKT1, JKT2, JKT4, and JKT5 ore bodies in the YCWPD. The exposed lithology of the Linong Formation primarily comprises metamorphosed quartz arenite, sericitic sandy-slate, marble, and intruded granodiorite, and its contact zones often exhibit skarnization, silicification, and carbonization, and the main ore body is the KT2 ore-bearing sequence in the YCWPD. The Lower Carboniferous (Beiwu Formation, C₁b) primarily comprises massive basalt, tuff, and sandy slate and outcrops northwest of the YCWPD.

The Yangla mining district has experienced strong tectonic activity and significant fault control of orebodies. The approximately N-S-trending Jinshajiang and Yangla Faults mainly control the regional magmatism and distribution of ore deposits [31,32]. A group of parallel secondary NE-SW-trending normal faults, represented by F4, are also developed in this district, and these faults, together with the Jinshajiang Fault, present a “λ”-shaped structural style in plan-view, which controls the spatial positioning of the pluton and ore bodies. Secondary interlayer faults (mostly dominated by lithologic interfaces) control the morphology of the layered and stratiform-stratoid skarn Cu ore bodies. Fissures in the pluton control the shape of veined Cu orebodies. Late NE-SW- and NW-SE-trending faults, which formed after diagenesis-mineralization cleavage and dislocated the pluton and orebodies, control the spatial positioning and final occurrence of both the pluton and orebodies [31,32].

The intrusive bodies, volcanic rocks, and dikes are widely developed within the YCWPD [1]. The intrusive bodies primarily comprise granodiorite and dioritic enclaves within the granodiorite, quartz monzonite porphyry, and quartz diorite in the YCWPD. Granodiorite comprises the largest exposed area and can be divided into five intrusions from north to south (i.e., Beiwu, Jiangbian, Linong, Lunong, and Tongjige) in the YCWPD, all of which were intruded into the Linong Formation and distributed linearly along the western side of Jinshajiang Fault [2,8]. The granodiorite contains plagioclase, K-feldspar, quartz, hornblende, minor amounts of biotite, and accessory zircon [2]. The granodiorite was primarily formed during the Triassic (~230 Ma) and has been confirmed that closely related to Cu skarn mineralization at the Yangla region [2,3,5,8]. The granodiorite samples are dominantly metaluminous and slightly peraluminous (A/CNK < 1.10), and there is a significant negative correlation between SiO₂ and P₂O₅ [2]. Moreover, there is also a significant positive correlation between SiO₂ and Pb [2]. These granodiorite samples are mainly plotted within the I-S-type granite region in the binary diagram of K₂O + Na₂O vs. 10,000 Ga/Al, Y vs. 10,000 Ga/Al, Zr vs. 10,000 Ga/Al, and Nb vs. 10,000 Ga/Al. However, there were no presented muscovite and cordierite in granodiorite samples and corundum lower than 1.0% in CIPW standard mineral calculation [2,3,4,5]. The granodiorite samples were mainly dominated by post-collision granites in the binary-ternary tectonic discrimination diagrams of Nb vs. Y, Ta vs. Yb, Rb vs. Y + Nb, Rb vs. Yb + Ta, and Rb/30-Hf-3 × Ta [2,5]. Therefore, petro-geochemical characteristics show that the granodiorite may belong to the I-type granites and formed in a late- or post-collisional tectonic setting after the closure of JOB [2,3,4,5]. The dioritic enclaves (2–10 cm in diameter) preserves transitional or irregular sharp contact relationship with the host granodiorite and presents a porphyritic texture. The phenocrysts are mainly dominated by plagioclase, biotite, hornblende, and K-feldspar [3]. Zircons U-Pb ages of dioritic enclaves are 232 and 238 Ma [3], which are roughly consistent with the chronology and tectonic setting of granodiorite intrusion.

The phenocrysts of quartz monzonite porphyry are mainly dominated by plagioclase, K-feldspar, minor quartz, biotite, and accessory apatite and zircon; it intrudes the third sequence (D₂₊₃l³) of the Linong Formation [3]. The Rb-Sr chronology shows that the emplacement age of petrogenesis is 202 Ma [33], whereas the zircon U-Pb chronology yields ages of 232 Ma and 234 Ma [3]; this suggests that the quartz monzonite porphyry may have formed in the late Triassic, and has similar petro-geochemical characteristics and tectonic setting to the GR [3]. Furthermore, the quartz monzonite porphyry is considered to be genetically related to the porphyry copper orebody (KT1), and it has been suggested that the porphyry copper orebody exists at a depth of Yangla mining district [33].

The quartz diorite occurs in an underground tunnel in 3250 m sections (levels) of the Linong ore block, where it is emplaced within Linong Formation metamorphosed quartz arenite and sericitic-slate (Figure 2, Figure 3 and Figure 4). The quartz diorite mainly occurred within a NE-SW-trending faults and (or) fractures zone (4~5 m in width and 20~300 m in length), and the contact boundary line show irregular shape between the quartz diorite and Devonian wall-rocks. The petro-geochemical features show that the quartz diorite is relatively enriched in LILEs, depleted in HFSEs, significantly enriched in Pb, and depleted in P [14]. Studies have suggested the quartz diorite may be metaluminous to slightly peraluminous S-type granite and formed in a post-collisional tectonic setting [14]. Zircon U-Pb chronology shows that the crystallization ages of the quartz diorite are 195.3 ± 6.4, 198.40 ± 8.6, and 213 ± 15 Ma (avg. 202 Ma) and indicated it formed in the Late Triassic to Early Jurassic (~195 Ma) [14]. Moreover, plenty of veined, disseminated sulfides (i.e., pyrite, chalcopyrite, and sphalerite) and quartz-calcite veins are developed in the interior fissures of quartz diorite (Figure 3c,d and Figure 4e,f), suggesting that it may be related to Yangla porphyry Cu mineralization.

The light-gray-to-grayish-white quartz diorite samples phenocrysts are quartz (~35%), plagioclase (~30%), and biotite (~10%) (Figure 4c,d), showing subhedral-euhedral grains and corroded platy, and sheet textures. The quartz phenocrysts are mostly hexagonal and quadrangular, with smaller amounts of irregular and euhedral-subhedral crystals with corroded edges. The plagioclase phenocrysts are altered and replaced by the sericite, chlorite, and cryptocrystalline carbonate mineral. The biotite phenocrysts have been altered and replaced by sericite and other secondary minerals (Figure 4g,h). The matrix has a cryptocrystalline-microcrystalline texture and felsic nature, primarily comprising quartz, plagioclase, and biotite.

The volcanic rocks are represented by basalt (primarily composed of plagioclase, pyroxene, and hornblende), which may have formed in the Devonian to Carboniferous (at ~296 and ~362 Ma) enriched mantle source region or the cracking process of the JOB, and are confirmed to have no direct genetic relation with Cu mineralization [34]. Published U-Pb zircon ages of igneous rocks indicate that the JOB is likely to have been generated in the early Carboniferous and underwent westward-directed subduction beneath the Qamdo-Simao terrane during the Early Permian [1]. Subsequently, The JOB gradually closed and was eliminated in the Early Triassic [1]. Zircon U-Pb chronology of late diabase dikes yielded 222 Ma [35] and corresponded to the Triassic. It also has no direct genetic relation with Cu mineralization at Yangla.

3. Sampling and Analytical Methods

Three massive and medium-grained porphyritic quartz diorite samples (No. 3250-Lb41b1, 3250-Lb41b1, and 45-R4) were collected for zircon trace element and Hf isotope analyses (Figure 4a,b). Zircon grains appeared light-gray to charcoal-gray with well-defined crystal morphology, primarily presenting as a long- or short-columnar shape, with a length of 100–300 μm (Figure 5). Owing to the dark color of zircons and the weak cathodoluminescence intensity, clear magmatic oscillation zone textures (growth zones) were observed (Figure 5), indicating that these were typical magmatic zircons [36]. The Th, U, and Pb concentrations were 77–12,967 (avg. 1098 ppm), 228–12,209 (avg. 2495 ppm), and 12–467 ppm (avg.115 ppm), respectively. The Th/U ratios of quartz diorite samples were in the range of 0.15–1.48 (avg. 0.36) [14]. The U vs. Th (Figure 6a) and U vs. Pb (Figure 6b) of zircons present positive correlations and suggest they belong to the magmatic zircons [37].

In situ zircon analyses were conducted at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China, using a GeoLas 2005 Laser Ablation (Agilent, Palo Alto, CA, USA) coupled with an Agilent 7500a ICP-MS. A 50 mJ/pulse (Geolas) 193 nm ArF Excimer (Lambda Physik, Göttingen, Germany) laser was used to ablate the zircons at 10 Hz. The diameter of the laser ablation spot was 32 µm. Helium was used as a carrier gas. The detailed analytical procedures used to follow those described by Yuan et al. (2010) [38]. The detection limits of LA-ICP-MS are generally lower than 0.01 ppm. Based on the zircon-reflected light images and CL images (Figure 5), we avoided inherited cores, cracks, and inclusions as far as possible. Elemental contents were calibrated by using ²⁹Si as the internal standard and NIST SRM 610 as the external standard. Raw data were processed using GLITTER (Version 4.4) [39].

In situ analysis of zircons Lu-Hf isotopic composition was carried out using laser ablation multi-collector inductively coupled plasma mass spectrometry in a laboratory of the Guangzhou Institute of Geochemistry. The ablation-spot diameter, frequency, and time were 50 μm, 8 Hz, and 30 s, respectively. The standard zircon samples (Penglai, Plešovice, and Qinghu) were used as quality control samples during the analytical process. The ¹⁷⁶Hf/¹⁷⁷Hf analytical results of the three standard zircons (i.e., Penglai, Plešovice, and Qinghu) were 0.282915 ± 0.000019, 0.282484 ± 0.000007, and 0.282997 ± 0.000009, respectively, which were highly consistent with the recommended values (¹⁷⁶Hf/¹⁷⁷Hf = 0.282906 ± 0.00001, 0.282482 ± 0.000013, and 0.282996 ± 0.000044, respectively) for these standards [40,41,42]. Standard zircon ¹⁷⁶Hf/¹⁷⁷Hf initial values were calculated based on a decay coefficient of 1.865 × 10⁻⁵ Ma⁻¹ [43]. The relevant parameters for calculating the Hf isotopic composition of zircon are as follows: (a) chondrite (¹⁷⁶Hf/¹⁷⁷Hf = 0.282772, ¹⁷⁶Lu/¹⁷⁷Hf = 0.0332, and f_Lu/Hf = 0.00); (b) depleted mantle (¹⁷⁶Hf/¹⁷⁷Hf = 0.28325,¹⁷⁶Lu/¹⁷⁷Hf = 0.0384, and f_Lu/Hf = 0.16); (c) lower crust (mafic) (¹⁷⁶Lu/¹⁷⁷Hf = 0.022 and f_Lu/Hf = −0.34); (d) upper crust (felsic) (¹⁷⁶Lu/¹⁷⁷Hf = 0.0093 and f_Lu/Hf = −0.72); (e) average crust (¹⁷⁶Lu/¹⁷⁷Hf = 0.015 and f_Lu/Hf = −0.55) [44,45,46].

4. Results

4.1. Zircon Trace Elements Compositions

The trace element compositions of zircons from samples 45-R4, 3250-42Lb1, and 3250-41Lb2 for the quartz diorite are listed in

Table 1. LA-ICP-MS analytical results of zircon trace elements (ppm), Ti-in-zircon temperatures (T_Ti/℃), and oxygen fugacity parameters (ΔFMQ) in the Yangla quartz diorite.

No.	Ti	Nb	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Y	Hf	Ta	Pb	Th	U	ΣREE	δEu	δCe	TTi/℃	ΔFMQ
Sample No. 45-R4
Min.	1.44	3.11	0.01	5.33	0.01	0.35	1.61	0.37	12.41	4.61	53.80	20.94	104.60	24.12	231.96	48.79	584	7667	1.83	64	210	547	519	0.24	1.12	616	−2.54
Max	19.61	19.49	35.91	99.56	13.13	70.31	52.35	19.91	159.56	38.45	302.07	79.13	310.28	70.22	671.96	146.01	2277	10400	5.95	181	1147	3249	1663	1.52	112.11	853	2.26
Avg.	6.61	6.50	2.43	16.24	1.34	9.53	11.63	5.15	39.73	12.24	131.19	47.10	212.36	48.31	450.90	95.87	1319	8941	3.89	98	620	2144	1084	0.76	12.41	725	−1.42
Sample No. 3250-41Lb1
Min.	1.24	1.44	0.09	5.14	0.07	0.75	1.63	0.31	9.70	4.15	34.39	8.79	27.09	4.39	32.85	5.89	239	6000	0.51	12	77	228	150	0.24	1.01	606	−2.45
Max	26.32	23.39	81.74	176.43	22.61	109.39	38.19	51.54	122.22	33.32	353.63	134.95	604.98	133.76	1174.65	234.10	3593	9834	10.28	322	8398	8358	2898	3.03	20.09	887	3.14
Avg.	10.08	7.21	9.57	38.17	4.14	23.38	16.25	10.54	51.14	16.13	170.57	59.37	257.06	57.40	518.59	107.53	1583	8319	4.34	115	1160	2646	1340	0.99	3.05	761	−0.66
Sample No. 3250-41Lb2
Min.	1.51	2.19	0.01	4.06	0.04	0.50	2.72	0.60	15.34	5.71	70.05	26.34	108.91	25.30	239.91	54.07	690	5058	1.95	46	117	776	617	0.27	1.23	620	−2.49
Max	77.70	65.26	14.94	255.53	11.78	94.32	235.78	66.68	717.38	168.19	1120.49	282.38	1235.61	274.17	2404.16	492.56	7529	9156	13.39	467	12967	12209	6218	2.58	141.31	1036	0.26
Avg.	10.45	9.47	2.43	29.36	2.13	16.31	22.24	11.49	72.85	21.08	205.51	69.05	294.53	65.37	587.53	122.81	1851	8110	4.64	133	1514	2695	1523	0.97	10.00	748	−1.20

Note: The Min, Max, and Avg. values correspond to a single quartz diorite sample. The detailed trace element concentrations are shown in Supplementary Table S1.

and Supplementary Table S1. Moreover, we cited zircons trace element compositions of Yangla dioritic enclaves, granodiorite, and quartz monzonite porphyry plutons, and the details data have been described by reference [3]. Chondrite-normalized REE patterns of most zircons are characterized by left-inclined (depletion of LREE and enrichment of HREE), with positive Ce anomalies (δCe) and negative Eu anomalies (δEu) (Figure 7), which is similar to typical magmatic zircon REE patterns [47]. In addition, we have noticed that a small number of zircons have positive Eu anomalies (δEu > 1.0), which may be that the zircons were affected by crystallization fractionation of mineral phase (i.e., plagioclase, apatite, and titanite) [23], oxidation conditions of the melt [48,49], and (or) later hydrothermal alteration [50]. Zircons in three quartz diorite samples yield a relatively wide range of ∑REE between 150 and 6218 ppm (avg. 1316 ppm), similar δCe values in a range between 1.0 and 141.31 (average = 8.48), and δEu values of 0.24–3.03 (avg. 0.90). The variations of ∑REE may be attributed to (ⅰ) the coeval fractional crystallization of accessory minerals (i.e., apatite) [51] and (ⅱ) the later fluid metasomatism or involved [48,52]. For zircons of two dioritic enclave samples, the ∑REE content ranges from 321 to 917 ppm (avg. 559 ppm), with δCe of 1.06–89.74 (avg. 19.53) and δEu of 0.08–2.31 (avg. 0.53) [3]. For zircons of three GR samples, the ∑REE content ranges from 353 to 1716 ppm (avg. 660 ppm), with δCe of 2.0–139 (avg. 16) and δEu of 0.19–1.70 (avg. 0.51) [3]. For zircons of two quartz monzonite porphyry samples, the ∑REE content ranges from 515 to 1300 ppm (avg. 801 ppm), with δCe of 1.12–49.95 (avg. 8.37) and δEu of 0.12–0.94 (avg. 0.41) [3].

4.2. Hf Isotopic Compositions of Quartz Diorite

Zircon ¹⁷⁶Yb/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios of the quartz diorite ranged from 0.024001 to 0.050774 (avg. 0.037560) and 0.001881 to 0.001446 (avg. 0.001194), respectively. The ratios of ¹⁷⁶Lu/¹⁷⁷Hf were relatively similar, and all ratios were less than 0.002 (

Table 2. Zircon Hf isotopic compositions of quartz diorite, granodiorite, quartz monzonite porphyry, and dioritic enclave in the Yangla Cu-W polymetallic deposit.

Intrusions Type	No.	Age/Ma	176Yb/177Hf	176Lu/177Hf	176Hf/177Hf	2σm	(176Hf/177Hf)t	εHf(0)	εHf(t)	tDM(Ma)	tCDM(Ma)	fLu/Hf	Reference
Quartz diorite (45-R4; N = 11)	1	195	0.047665	0.001408	0.282383	0.000029	0.282378	−13.77	−9.66	1241	1849	−0.96	This paper
	2	195	0.044598	0.001346	0.282420	0.000033	0.282415	−12.46	−8.35	1187	1766	−0.96
	3	195	0.050774	0.001446	0.282403	0.000030	0.282398	−13.03	−8.95	1213	1803	−0.96
	4	195	0.027950	0.000981	0.282404	0.000029	0.282400	−13.03	−8.88	1198	1799	−0.97
	5	195	0.041369	0.001387	0.282400	0.000030	0.282395	−13.17	−9.05	1217	1812	-0.96
	6	195	0.039769	0.001327	0.282421	0.000023	0.282417	−12.40	−8.28	1184	1762	−0.96
	7	195	0.039664	0.001204	0.282385	0.000035	0.282381	−13.67	−9.55	1231	1842	−0.96
	8	195	0.029164	0.001035	0.282410	0.000027	0.282406	−12.81	−8.66	1191	1785	−0.97
	9	195	0.026465	0.000911	0.282473	0.000029	0.282470	−10.58	−6.40	1099	1643	−0.97
	10	195	0.024001	0.000881	0.282446	0.000025	0.282442	−11.54	−7.39	1136	1704	−0.97
	11	195	0.041744	0.001207	0.282337	0.000034	0.282332	−15.39	−11.28	1299	1950	−0.96
	Min.	195	0.024001	0.000881	0.282337	0.000023	0.282332	−15.39	−11.28	1099	1643	−0.97
	Max.	195	0.050774	0.001446	0.282473	0.000035	0.282470	−10.58	−6.40	1299	1950	−0.96
	Avg.	195	0.037560	0.001194	0.282407	0.000029	0.282403	−12.90	−8.77	1200	1792	−0.96
Quartz monzonite porphyry (N = 31)	Min.	188	0.019956	0.000844	0.282401	0.000021	0.282396	−13.12	−8.18	894	1268	−0.97	[3]
	Max.	267	0.051689	0.002051	0.282622	0.000047	0.282617	−5.31	0.37	1207	1785	−0.94
	Avg.	230	0.031792	0.001297	0.282513	0.000027	0.282508	−9.15	−4.29	1054	1536	−0.96
Granodiorite (N = 192)	Min.	213	0.017773	0.000672	0.282391	0.000009	0.282387	−13.47	−8.50	767	1082	−0.98	[2,3,4,5]
	Max.	239	0.092584	0.002346	0.282713	0.000038	0.282708	−2.09	2.86	1217	1805	−0.93
	Avg.	230	0.036373	0.001163	0.282595	0.000021	0.282590	−6.26	−1.43	934	1337	−0.97
Dioritic enclaves (N = 38)	Min.	229	0.016020	0.000735	0.282479	0.000019	0.282475	−10.37	−5.30	806	1147	−0.98	[3]
	Max.	241	0.040573	0.001727	0.282684	0.000027	0.282680	−3.11	1.87	1089	1606	−0.95
	Avg.	235	0.025544	0.001159	0.282587	0.000023	0.282582	−6.53	−1.54	946	1366	−0.97

), suggesting that there was essentially no accumulation of radioactive Hf after the formation of the zircon crystals, and the initial ¹⁷⁶Lu/¹⁷⁷Hf values can be represented by the ¹⁷⁶Lu/¹⁷⁷Hf ratios of the zircon crystals [5]. The (¹⁷⁶Hf/¹⁷⁷Hf)_t, ε_Hf(0), ε_Hf(t), t_DM, and t^C_DM were 0.28233–0.282470 (avg. 0.282403), −15.39 to −10.58 (avg. −12.90), −11.28 to −6.40 (avg. −8.77), 1099–1299 Ma (avg. 1200 Ma), and 1643–1950 Ma (avg. 1972 Ma), respectively. The f_Lu/Hf was −0.97 to −0.96, with an average of −0.96, significantly lower than that of the mafic lower crust (f_Lu/Hf = −0.34) [54], sialic upper crust (f_Lu/Hf = −0.72) [55], average crust (f_Lu/Hf = −0.55) [46], depleted mantle (f_Lu/Hf = +0.16) [45], and chondrites (f_Lu/Hf = 0.00) [56].

4.3. Hf Isotopic Compositions of Other Granitoid Plutons

The Hf isotopic compositions values of five granodiorites (Beiwu, Jiangbian, Linong, Lunong, and Tongjige), a dioritic enclave, and a quartz monzonite porphyry were systematically obtained to analyze and discuss the origin, evolution, and geodynamic setting of magma in the YCWPD (Table 2). The details data of Yangla dioritic enclaves, GR, and quartz monzonite porphyry plutons have been described by the references [2,3,4,5].

The granodiorite zircons had ¹⁷⁶Yb/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.017773–0.092584 (avg. 0.036373) and 0.000672–0.002346 (avg. 0.001163), respectively. The (¹⁷⁶Hf/¹⁷⁷Hf)_t, ε_Hf(0), ε_Hf(t), t_DM, and t^C_DM were 0.282387–0.282708 (avg. 0.282590), −13.47 to −2.09 (avg. −6.26), −8.50 to 2.86 (avg. −1.43), 767–1217 Ma (avg. 934 Ma), and 1082–1805 Ma (avg. 1337 Ma), respectively. The f_Lu/Hf was −0.98 to −0.93, with an average of −0.97 [2,3,4,5].

Dioritic enclave zircons had ¹⁷⁶Yb/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.016020–0.040573 (avg. 0.025544) and 0.000735–0.001727 (avg. 0.001159), respectively. The (¹⁷⁶Hf/¹⁷⁷Hf)_t, εHf(0), ε_Hf(t), t_DM, and t^C_DM were 0.282475–0.282680 (avg. 0.282582), −10.37 to −3.11 (avg. −6.53), −5.30 to 1.87 (avg. −1.54), 806–1089 Ma (avg. 946 Ma), and 1147–1606 Ma (avg. 1366 Ma), respectively. fLu/Hf was -0.98 to -0.95, with an average of −0.97 [3].

Quartz monzonite porphyry zircons had ¹⁷⁶Yb/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.019956–0.051689 (avg. 0.031792) and 0.000844–0.002051 (avg. 0.001297), respectively. The (¹⁷⁶Hf/¹⁷⁷Hf)_t, ε_Hf(0), ε_Hf(t), t^DM, and t^C_DM were 0.282396–0.282617 (avg. 0.282508), −13.12 to −5.31 (avg. −9.15), −8.18 to 0.37 (avg. −4.29), 894–1207 Ma (avg. 1054 Ma), and 1268–1785 Ma (avg. 1536 Ma), respectively. The fLu/Hf was −0.97 to −0.94, with an average of −0.96 [3].

4.4. Comparison of Yangla Granitoid Plutons

The chronology and Hf isotopic geochemistry of the granodiorite, dioritic enclaves, quartz monzonite porphyry, and quartz diorite were analyzed and compared to facilitate the discussion and constraint of the magma origin, evolution, and geodynamic tectonic setting in the YCWPD.

4.4.1. Geochronology

The emplacement ages of the granodiorite are mainly concentrated between 208 and 246 Ma (N = 28; avg. 230 Ma), suggesting that the Yangla granodiorite pluton was formed in the Triassic (~230 Ma). The crystallization ages of the dioritic enclaves within the granodiorite are 232 and 238 Ma (avg. 235 Ma) [3], also indicating formation in the Triassic (~235 Ma). The whole-rock Rb-Sr age of the quartz monzonite porphyry is 202 Ma [33], whereas the zircon U-Pb crystallization ages are 232 Ma and 234 Ma, giving an overall average of 223 Ma [3], suggesting that the quartz monzonite porphyry also formed in the Triassic, and slightly later than the granodiorite and dioritic enclaves. The emplacement ages of the quartz diorite are 195, 198, and 213 Ma (avg. 202 Ma), indicating that the quartz diorite formed in the Late Triassic to Early Jurassic, slightly later than the above-mentioned Yangla granitoid plutons [14]. Therefore, the emplacement ages of the granitoid plutons show an obvious decrease from dioritic enclaves (~232 Ma), through granodiorite (~208 Ma), and to quartz monzonite porphyry (~202 Ma) and quartz diorite (~195 Ma) in the Yangla mining district (

Table 3. The integration of published geochronological data for various granitic plutons in Yangla Cu-W polymetallic deposit.

Intrusions Type	Minerals/Methods	Age/Ma	References
Quartz diorite	Zircon/U-Pb	195.3 ± 6.4, 198.40 ± 8.6, and 213 ± 15	[14]
Quartz monzonite porphyry	Zircon/U-Pb	232 ± 1.1 and 234.0 ± 1.2.	[3]
	Whole-rock/Rb-Sr	202	[33]
Beiwu granodiorite	Zircon/U-Pb	213.6 ± 6.9 and 233.9 ± 1.4	[2,4]
Jiangbian granodiorite	Zircon/U-Pb	227.9 ± 5.1, 232.0 ± 0.5, 232.0 ± 0.9, 238.0 ± 0.5, 208.09 ± 0.46, 215.7 ± 0.63, 221.28 ± 1.0, 220.3 ± 1.3, and 214.7 ± 0.56.	[3,4,6]
Linong granodiorite	Zircon/U-Pb	239.0 ± 5.7, 229.6 ± 4.4, 233.1 ± 1.4, 234.1 ± 1.2, 235.6 ± 1.2, 232.0 ± 0.9, 233.0 ± 0.9Ma, 224 ± 0.7, 232 ± 1.1, and 234 ± 1.2	[2,3,4,6]
Lunong granodiorite	Zircon/U-Pb	238.1 ± 5.3, 231.0 ± 1.6, 230 ± 1.9, and 234 ± 0.8.	[2,3,4]
Tongjige granodiorite	Zircon/U-Pb	246.1 ± 3.1, 225.6 ± 1.3, and 226.1 ± 3.3	[5,35]
Dioritic enclaves	Zircon/U-Pb	232 ± 0.9 and 238 ± 0.5.	[3]

, Figure 8).

4.4.2. Hf Isotopic Geochemistry

The ¹⁷⁶Yb/¹⁷⁷Hf ratios of the DE, GR, quartz monzonite porphyry and quartz diorite are basically consistent with each other. The average values of ¹⁷⁶Yb/¹⁷⁷Hf moving from the DE to the quartz diorite present a difference of only 0.012016, with a small variation range, and there is a consistent range in the ¹⁷⁶Yb/¹⁷⁷Hf ratio distribution diagram (Table 2; Figure 9a). The ¹⁷⁶Lu/¹⁷⁷Hf ratios of dioritic enclaves, granodiorite, quartz monzonite porphyry, and quartz diorite also generally show a smaller variation. The average values of ¹⁷⁶Lu/¹⁷⁷Hf moving from the dioritic enclaves to the quartz diorite present a difference of only 0.000138, with a small variation range, and there is a consistent range in the ¹⁷⁶Yb/¹⁷⁷Hf ratio distribution diagram (Table 2; Figure 9b). The ¹⁷⁶Hf/¹⁷⁷Hf and (¹⁷⁶Hf/¹⁷⁷Hf)_t ratios of these granitoids generally show that dioritic enclaves are roughly equal to GR, greater than quartz monzonite porphyry, and followed by the quartz diorite (Table 2; Figure 9c,d). The ε_Hf(0) and ε_Hf(t) values of these granitoids generally show that the dioritic enclaves are roughly equal to granodiorite, greater than quartz monzonite porphyry, and followed by the quartz diorite (Table 2; Figure 10a,b). The mean difference of ε_Hf(0) and ε_Hf(t) between quartz diorite and that of granodiorite are 6.64 and 7.34, respectively. The t_DM and t^C_DM values generally show that dioritic enclaves are roughly equal to granodiorite, lower than quartz monzonite porphyry and quartz diorite (Table 2; Figure 10c,d). The mean difference of t_DM and t^C_DM between quartz diorite and that of granodiorite is 266 Ma and 455 Ma, respectively.

In summary, the quartz diorite, granodiorite, dioritic enclaves, and quartz monzonite porphyry have similar characteristics in terms of their ¹⁷⁶Yb/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios but show evident differences in their ¹⁷⁶Hf/¹⁷⁷Hf, ε_Hf(t), t_DM, and t^C_DM values. This may be related to the differences in emplacement age, magma source, and evolutionary processes of each granitoid pluton. The reasons for the differences in the Lu-Hf isotopic system of the Yangla granitoids will be discussed in the following Section 5.

4.5. Crystallization Temperature

Zircon is regarded as a preferred accessory mineral for estimating magma formation temperature due to its high stability and early crystallization in granitic magmatic systems [57,58,59]. Ferry and Watson (2007) [60] have suggested that the ZrSiO₄, ZrTiO₄, and TiSiO₄ are the independent variation phases, and the Ti mainly replaces the Si, and the replacement equation is ZrSiO₄ + TiO₂ ↔ ZrTiO₄ + SiO₂ or TiO₂ + SiO₂ ↔ TiSiO₄ in zircons. They have revised and proposed a new calculation equation of zircons crystallization temperature as follows:

lg(Ti)zircon = (5.711 ± 0.072) − (4800 ± 86)/T(k) + lg(αTiO₂) − lg(αSiO₂)

(1)

The lg(Ti)zircon represents the concentrations of Ti in zircons (ppm). It is generally assumed that αSiO₂ and αTiO₂ are equal to 1 if quartz and rutile are present in the magmatic system, respectively. In particular, if the magma system does not contain rutile but contains other Ti-containing minerals (i.e., titanite and ilmenite), the αTiO₂ generally varies between 0.5 and 1.0 [61]. The Yangla granodiorite intrusions are SiO₂ saturated, and coexisting Ti-bearing phases (ilmenite) indicate αTiO₂ is not low [3]. The αTiO₂ has been assumed to be 0.70 in the Yangla granodiorite, and this article will continue to cite this hypothetical result [3]. The calculation reveals that the zircons in three quartz diorites have a model temperature range of 606–1036 °C, with average crystallization temperatures of approximately 745 °C (Table 1 and Figure 11a), which are notable that the crystallization temperatures of quartz diorite are slightly higher than those of most granodiorite (avg. 661 °C, n = 72), dioritic enclaves (avg. 634 °C, n = 31), and quartz monzonite porphyry (avg. 651 °C, n = 24) zircons samples (Table 1 and Figure 11a) [3]. Therefore, we preliminarily believe that the magma crystallization temperature gradually increased from early to late stages, corresponding to the dioritic enclaves → granodiorite → quartz monzonite porphyry → quartz diorite in the Yangla mining district.

4.6. Oxygen Fugacity

Zircon rare earth elements (REEs) oxy-barometers have been widely used to calculate magma oxygen fugacity, and there are mainly seven types of commonly used zircon REEs oxy-barometer [62], such as (Ce⁴⁺/Ce³⁺)_Zircon [19], (Ce/Ce*)_D, (Eu/Eu*)_D, (Ce/Ce*)_CHUR [63], (Ce/Ce*)_C [23,25], Ce/Nd ratios [64], and $(X_{C{e}^{4+}}^{melt}/X_{C{e}^{3+}}^{melt})$_Zircon [65]. The (Ce⁴⁺/Ce³⁺)_Zircon oxy-barometers will be largely affected by an obvious deviation to the lattice strain model, and only when δK < 3.0 can (Ce⁴⁺/Ce³⁺)_Zircon yield a robust semi-quantitative oxygen fugacity [62]. The (Ce/Ce*)_D, (Ce/Ce*)_CHUR, and Ce/Nd oxy-barometers largely depend on the accurate La concentrations in zircons [57,62]. However, when zircons with La ≤ 0.1 ppm (clean zircons) are considered to truly reflect the REE composition of zircon, free from excess nonlattice bound REEs contributed by inclusions, and accurate oxygen fugacity can be obtained [62]. The La concentrations of zircons in Yangla quartz diorite, granodiorite, dioritic enclaves, and quartz monzonite porphyry are generally greater than 0.1 ppm, indicating that the above-mentioned oxy-barometers may not reflect accurate oxygen fugacity. The $(X_{C{e}^{4+}}^{melt}/X_{C{e}^{3+}}^{melt})$_Zircon oxy-barometer does not rely on the accurate measurement of La content in zircons or limited deviation from the lattice strain model. However, the $(X_{C{e}^{4+}}^{melt}/X_{C{e}^{3+}}^{melt})$_Zircon oxy-barometer still requires accurate measurement of H₂Owt%, temperature (T), and $D_{C{e}^{4+}}^{zircon/whole rock}$ in the melt [62]. The estimates of H₂Owt% values have a greater impact on final oxygen fugacity, and it is generally difficult to obtain an accurate water content of the magma at the time of zircon crystallization [62]. The (Ce/Ce*)_C calculation formula is as follows:

(Ce/Ce*)_C = Ce_N/[(Nd_N)²/Sm_N]

(2)

This formulation is much more robust than the conventional method as it does not require the accurate determination of either La or Pr [23]. However, when combined with Equation (3) to calculate the oxygen fugacity and the calculated results are dominated by negative values and significantly lower than those of most ore-bearing intrusions (i.e., Relin, Tongchanggou, Disug, and Pulang in Sanjiang polymetallic metallogenic belt of Southwest China, and Dexing in Jiangnan orogen belt of southern China) [66,67].

This calculation formula is not suitable for the estimation of oxygen fugacity in Yangla granitoids plutons.

ln(Ce/Ce*) = (0.1156 ± 0.0050) × lnfo₂ + (13.860 ± 708)/T − (6.125 ± 0.48)

(3)

Therefore, this study mainly adopts the zircon oxy-barometer independent of the melt composition proposed by Loucks et al. (2020) [68], which can well avoid the main problems involved in the above-mentioned oxy-barometers. The calculation formula is as follows:

$$\log f{\mathrm{O}}_{2\left(\mathrm{sample}\right)}-\log f{\mathrm{O}}_{2\left(\mathrm{FMQ}\right)}=\mathsf{\Delta }\mathrm{FMQ}={3.99}_8(\pm {0.12}_4)\times \log \left[\frac{\mathrm{Ce}}{\sqrt{{\mathrm{U}}_{\mathrm{i}}\times \mathrm{Ti}}}\right]+{2.28}_4(\pm {0.10}_1)$$

(4)

$${\mathrm{U}}_{\mathrm{i}}={\mathrm{U}}_{\mathrm{zircon}}\times e^{1.98173\times {10}^{-4}\times t}$$

(5)

The Ce and Ti represent the concentrations in zircons. The t represents the age of zircon crystallization (Ma). The calculation reveals that the zircons in three quartz diorites have a ΔFMQ range of −2.54–+3.14, with an average of −1.09. Correspondingly, logfO_{2(quartz diorites)} ranges from FMQ – 2.54 to FMQ +3.14, with an average of FMQ −1.09. The oxygen fugacity of granodiorite, dioritic enclaves, and quartz monzonite porphyry zircons samples mainly concentrated in FMQ −3.76 to FMQ + 2.96, FMQ −1.60 to FMQ +2.31, FMQ −2.27 to FMQ +1.43, respectively. The oxygen fugacity of quartz diorite is slightly lower than those of most granodiorite (ΔFMQ = −3.76 to 2.96, avg. −0.80, n = 72), dioritic enclaves (ΔFMQ = −1.60 to 2.31, avg. −0.52, n = 31), and quartz monzonite porphyry (ΔFMQ = −2.27 to 1.43, avg. −1.10, n = 24) samples (Table 1 and Figure 11b), indicating that the oxygen fugacity of granitic magma gradually decreased during the magma evolution from early to a late stage in the Yangla mining district. This evolving pattern can be further supported by the varied Ce/Nd ratios of Yangla granitoid zircons. The Ce/Nd ratios of zircons show a slight decrease from dioritic enclave and granodiorite, through quartz monzonite porphyry, and to quartz diorite (Figure 12a), which indicates that they are relatively reduced compared to the dioritic enclave and granodiorite in the Yangla mining district.

5. Discussion

Zircon (ZrSiO₄) is an important accessory mineral in granitoids, and absorbs various trace elements during its crystallization process (such as REE, U, Th, and Ti) [26,62,69]. Laser-ablation inductively-coupled-plasma-mass-spectrometry (LA-ICP-MS) has made it possible to analyze trace element concentrations in zircons, along with U–Pb dating [51]. Previously published research has suggested the geochemistry of trace elements in zircons, in spite of the wide range of contents, can produce a very significant set of information useful for metallogenetic, petrogenetic, geochronological, and provenance studies [3,19,20,21,22,23,26,51,70,71]. Currently, the trace elements in zircons have been widely used to estimate the crystallization temperature and oxygen fugacity, determine magma source and crustal sediments assimilation, constrain the separation and crystallization of minerals and analyze the metallogenic potential of intrusions [62,63,64,65,66,67,68,69,71,72,73].

5.1. Magma Source

Zircons have good stability and are an important tool for exploring crustal evolution and tracing rock source areas. Moreover, zircons usually have higher Hf concentrations (0.5–2.0%) and lower Lu concentrations, resulting in low Lu/Hf ratios. Therefore, there is no obvious accumulation of radioactive Hf in zircon after its formation, and the Hf isotopic values of the initial system of zircon formation can be accurately obtained [15,46]. Furthermore, the Hf isotopic composition of the zircon parent melt is difficult to reset after the formation system is closed and is not affected by another geological process [74]; thus, the heterogeneity of zircon Hf isotopic values is related to the magma source and records useful origin information [2]. In particular, the t^C_DM values can better record and reflect source materials region information, such as the time of source materials being extracted from the depleted mantle or remaining in the crust, because of relatively lower zircon f_Lu/Hf values of Yangla granitoids plutons [75].

The quartz diorite samples have (¹⁷⁶Hf/¹⁷⁷Hf)_t values of 0.282332–0.282470 (avg. 0.282403), and the ε_Hf(t) are −11.28 to −6.40 (avg. −8.77). The quartz diorite samples primarily plot near the evolution line of the lower crust in (¹⁷⁶Hf/¹⁷⁷Hf)_t and diagenetic chronology diagrams (Figure 13a,b). The quartz diorite samples also primarily plot near the evolution line of the lower crust in diagrams of ε_Hf(t) and diagenetic chronology (Figure 13c,d), indicating that the source region may be the negative ε_Hf(t) values component of the lower crust. Therefore, Figure 13 indicates that the parent magma of the quartz diorite may have mainly derived from the melting of ancient lower crustal materials [15,75,76]. Furthermore, the corresponding T^C_DM of quartz diorite is 1.6–2.0 Ga, with an average of 1.8 Ga, which also suggests that the magma may originate from ancient Paleoproterozoic lower crustal material.

The quartz monzonite porphyry samples have (¹⁷⁶Hf/¹⁷⁷Hf)_t values of 0.282396–0.282617 (avg. 0.282508), and the ε_Hf(t) are −8.18 to 0.37, with only one sample being positive, and the others being negative, with an average of -4.29. The quartz monzonite porphyry samples primarily plot within the region between lower crustal materials and chondrite evolution lines in the (¹⁷⁶Hf/¹⁷⁷Hf)_t and chronology diagrams (Figure 13a,b). These samples also primarily plot within the region between lower crustal materials and the chondrite evolution lines in the ε_Hf(t) and chronology diagrams (Figure 13c,d), and the corresponding t^C_DM are concentrated between 1.2 and 1.8 Ga, with an average of 1.5 Ga, suggesting that the magma may originate from Paleo-Mesoproterozoic components of the lower crust. Granodiorite and dioritic enclaves have similar Lu-Hf isotopic composition values, including similar ε_Hf(t) and t^C_DM values. The ε_Hf(t) values are concentrated between −8.6 and +2.8, dominated by negative values (only a few are positive), and the t^C_DM values are concentrated between 1.1 and 1.8 Ga, indicating that they may have consistent magma source regions. Granodiorite and dioritic enclave samples primarily plot in the region between the lower crust and chondrite evolution lines, and a small number of samples plot in the upper regions of the chondrite evolution line in the (¹⁷⁶Hf/¹⁷⁷Hf)_t and chronology diagrams (Figure 13a,b). The granodiorite and dioritic enclave samples also primarily plot in regions between the lower crust and the chondrite evolution lines, and a small number of samples (ε_Hf(t) > 0) plot in the upper regions of the chondrite evolution line in the ε_Hf(t) and chronology diagrams (Figure 13c,d); this indicates a potentially heterogeneous magma source, where ancient Mesoproterozoic negative ε_Hf(t) value components may have been mixed with newborn positive ε_Hf(t) value components [3]. The negative and positive ε_Hf(t) values usually show the characteristics of crustal melting and mantle-derived magma or new crustal material components, respectively [2,77]. However, previous studies have pointed out that most of the negative ε_Hf(t) values of the granodiorite in the Yangla mining district mainly originate from Paleo-Mesoproterozoic continental crustal materials and a small number of positive ε_Hf(t) values represent a certain mixing proportion of mantle-derived materials, resulting in an inhomogeneous isotopic composition of the magma source region, which is a mixed magma formed by crust-mantle interaction [2,3,5]. Moreover, a large number of mafic microgranular enclaves (MME) and Sr-Nd-Pb-O isotopic features of the granodiorite plutons further confirm that mantle-derived components contribute to the magma system without contamination by new crustal materials at Yangla [2]. Therefore, the magma of the granodiorite and dioritic enclaves may have originated from lower crust Paleo-Mesoproterozoic material, combined with contaminations of mantle-derived material components in the deep lower crust, and crust-mantle interactions that occurred to form a mixed magma [2,33]. This has similar magma origin characteristics to the Yidun island arc belt granitoids in the southwest Sanjiang region to the east, which also originated from the remelting of Paleo-Mesoproterozoic material components in the lower crust, accompanied by mantle-derived components added to form a mixed magma [78,79,80].

Regionally, the JOB was formed by the separation of the Qamdo-Simao Block (QSB) from the Yangtze Block (YB), and the basement materials of the JSZ and QSB should be consistent with those of the YB [2,81]. Greentree et al. (2006) [82] and Zhao et al. (2012) [83] have confirmed that the western YB basement mainly underwent numerous tectono-magmatic and metamorphic events at 2700–2600, 2500–2400, 2000–1900, 1100–1000, and 910–720 Ma. However, the t^C_DM of the quartz diorite, quartz monzonite porphyry, and granodiorite−dioritic enclaves are 1500–2000, 1200–1800, and 1100–1800 Ma, respectively, and none of them correspond to obvious igneous and metamorphic events in the YB. In addition, Neoproterozoic basement mafic rocks at the western margin of the YB usually have positive ε_Hf(t) values (~12.0), suggesting that Neoproterozoic materials were added to the crust at that time [84]. The Lunong and Jiangbian granodiorite intrusions of Yangla and the Neoproterozoic mafic rocks at the western margin of the YB have a partially similar distribution range of Hf isotopic composition, indicating the existence of Neoproterozoic crustal materials that were added to the Triassic acidic magmatic system and participated in the diagenesis process of granitoids in Yangla [3]. Therefore, the magma may primarily originate from Paleo-Meso-Neoproterozoic materials in the lower crust, coupled with a small amount of mantle-derived components contamination in the YCWPD.

Studies have suggested that the Yangla granodiorite is an I-type granite [2], whereas the quartz diorite is an S-type granite [14]. I- and S-type granites primarily originated from the partial melting process of the pre-existing lower crustal igneous rocks and upper crustal sediments, respectively [77,85]. However, previous studies on zircon Hf-O isotopic values have suggested that I- and S-type granites can also be formed by the mixing of magma with different proportions of mantle-derived components and remelted crustal sedimentary material [77,86]. Given that the quartz diorite, quartz monzonite porphyry, granodiorite, and dioritic enclave were mainly derived from two source regions (mantle and lower crustal materials), the Hf-O-Pb isotopic compositions of GR further indicate that the evolution and diagenesis of the magmatic system involved mixing with upper crustal sediments; hence, these magmas may be involved three-component upper crust + lower crust + mantle mixing materials [2]. Moreover, studies have confirmed that the granitoids plutons (i.e., quartz diorite, quartz monzonite porphyry, granodiorite, and dioritic enclave) are the products of the same magmatic diagenesis system at Yangla region [2,3,5,13] and the differences in diagenetic chronology can be attributed to multiphase emplacement [3]. Therefore, the Yangla granitoids plutons should have the same magma source region, which is mainly derived from three-component mixing magmas, these components being upper crustal sediments + lower crustal Paleo-Meso-Neoproterozoic materials + mantle-derived components.

5.2. Assimilation and Fractional Crystallization

5.2.1. Assimilation

Many magmas assimilate wall rock during their residence in the crust, and crustal assimilation of magmas deriving from various degrees of crustal assimilation could lead to distinct zircon trace element trends with magma compositional evolution [87]. The chemical effects of crustal assimilation are most obvious in S-type granites, which assimilate large amounts of metasediment and consequently show higher aluminosity and more reduced chemistry [87]. The Yangla quartz diorite and granodiorite were mainly plotted within the slightly metaluminous to peraluminous and metaluminous to slightly peraluminous in the A/NK-A/CNK binary discrimination diagrams, respectively [2,14], which suggested the crustal assimilation may be responsible for the above-mentioned plutons geochemistry. Furthermore, if the magmatic system is contaminated by crustal sediments, which have distinctive high δ¹⁸O values [88], reduced chemistry based on the Ce anomaly [89], and elevated concentrations of P [90] in zircon grains. Bell et al. (2019) [91] and Bell and Kirkpatrick (2021) [87] further confirmed that the sediment assimilation is supported by the higher δ¹⁸O, lower oxygen fugacity (fO₂), and higher P contents within zircons. The zircon oxygen δ¹⁸O values of Yangla granodiorite mainly range from 7 to 9‰ (Avg. 8.0‰; Figure 14a) and are significantly higher than the transition-eastern and western zone plutons and relatively lower than high-Sr/Y plutons in Peninsular Ranges Batholith PRB of Baja California and Southern California, which indicated that there is obvious assimilation of crustal metasediments [87]. Zircon P concentrations of Yangla quartz diorite are significantly higher than that of plutons in PRB (Figure 14b), which suggests that the Yangla granitoids plutons probably contain a considerable assimilated metasedimentary component. The oxygen fugacity (fO₂) of Yangla granitoids plutons is roughly consistent with that of plutons in PRB (Figure 14c), which also indicates that Yangla granitoids plutons assimilate large amounts of metasediments. The above-mentioned variation pattern is also supported by the zircons (i.e., elevated LREE contents and increasing average Nd/Dy ratios) and whole rock geochemistry (i.e., more radiogenic ⁸⁷Sr/⁸⁶Sr, less radiogenic ε_Nd, and higher Pb isotope ratios) in PRB of southern California and Baja California [87,92]. The LREE concentrations and Nd/Dy ratios show slightly increase from dioritic enclave and granodiorite, through quartz monzonite porphyry, and to quartz diorite (Figure 15), which indicates that there are obvious sediments assimilation and the degree of sediments contamination gradually increased in the Yangla magmatic system. Zhu et al. (2011) [2] have revealed whole rock Sr, Nd, and Pb isotopic compositions of Beiwu, Linong, and Lunong granodiorite and (⁸⁷Sr/⁸⁶Sr)_i, ε_Nd(t), ²⁰⁶Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb mainly range from 0.7078 to 0.7148 (avg. 0.7095), and −6.70 to –5.10 (avg. −6.0), 18.213 to 18.598 (avg. 18.407), and 38.323 to 38.791 (avg. 38.601), respectively. These whole rock Sr, Nd, and Pb isotopic compositions of Yangla granodiorite are roughly consistent with the PRB plutons (i.e., avg. ⁸⁷Sr/⁸⁶Sr = 0.704, ε_Nd=-5.0, ²⁰⁶Pb/²⁰⁴Pb =18.787, and²⁰⁸Pb/²⁰⁴Pb = 38.445; [93]), suggesting that Yangla plutons have assimilated continental materials (i.e., sediments and/or metasediments). Moreover, we also referred to whole rock chemistry of the “hybridization index” (FeO^T + MgO + TiO₂ + MnO) for comparison with K₂O, Na₂O, U, Th, Ba, Rb, Nb, and Ta contents of Yangla quartz diorite and granodiorite [2,14]. There are identifiable correlations between the hybridization index and K₂O, Na₂O, U, Th, Ba, Rb, Nb, and Ta [2,14], and indicate contamination of granitic magma by metasedimentary rock caused by chemical interaction with the granitoids [24].

Consequently, we preliminary believe that Yangla granitoid plutons have assimilated crustal metasediments, and the degree of sediment assimilation shows a gradual increase from dioritic enclave and granodiorite through quartz monzonite porphyry and to quartz diorite.

5.2.2. Fractional Crystallization

The variations of zircons trace element compositions can well reflect the degree of magma differentiation and the crystallization of accessory minerals during the crystallization and differentiation process [24,69,94]. Studies suggest the contents of incompatible elements (i.e., Th, U, Y, and REE) in residual magma will gradually increase, coupled with the increased degree of magmatic crystallization fractionation, and these characteristics will be recorded by the zircons from the magmatic system [69,73,95]. The contents of ΣREE, Th, U, and Y in the four stages of magmatic zircons gradually increased in the Yangla deposit (Figure 16a–c), showing that the fractionation of the Yangla granitoids plutons gradually increased from the early to late stage. Furthermore, if there were separated and crystallized MREE-rich minerals (i.e., apatite, titanite, and hornblende) in the early stage of magmatic evolution, the residual melt would relatively lose REE (i.e., Sm and Gd) and then the Ce/Sm and Yb/Gd ratios will simultaneous increased [96]. Figure 16d shows that the four stages of magmatic rocks in the Yangla deposit have undergone significant crystallization differentiation of apatite, titanite, and hornblende during the diagenesis process. In particular, the positive correlation between P and Y in quartz diorite zircons further confirms the existence of apatite separation and crystallization in Figure 16e [69]. Moreover, the Ta and Nb of dioritic enclaves, quartz diorite, and quartz monzonite porphyry zircons show a significant positive correlation in Figure 16 f, indicating that the source region may undergo obvious rutile crystallization and differentiation process [69]. There is no clear correlation between Ta and Nb in granodiorite zircons, indicating that there may be no separation and crystallization of rutile that formed the granodiorite magma.

In summary, the trace elements variations of zircons indicated that the Yangla granitoids plutons (dioritic enclaves, quartz monzonite porphyry, and quartz diorite) mainly experienced the separation and crystallization of apatite, titanite, hornblende, and rutile. The Yangla granodiorite plutons mainly experienced the separation and crystallization of apatite, titanite, and hornblende. These results are consistent with the mineralogical characteristics of apatite, titanite, and rutile as the accessory minerals and hornblende as the main minerals of the Yangla granitoid plutons [2,3,4,5,14]. The degree of magma differentiation gradually increased from dioritic enclaves and granodiorite through quartz monzonite porphyry and to quartz diorite, corresponding to the early to late stages of magmatic evolution in the Yangla mining district.

5.3. Magmatic Evolutionary Sequence

The geochronology of the Yangla granitoids shows an obvious decrease from dioritic enclave (~232 Ma) through granodiorite (~208 Ma) and quartz monzonite porphyry (~202 Ma) and to quartz diorite (~195 Ma). Combined with the diachronous collision-closure process and tectonic setting of the JOB, indicating that the corresponding magmatic activity in the YCWPD may have lasted up to 33 Ma [35], and various granitoids formed via “multiphase emplacement” of the same magmatic system [3]. Furthermore, Yangla granitoids are the products of different evolution stages of the same magmatic system formed by the partial melting and (or) fractional crystallization action [2,14]. Studies have verified that the ¹⁷⁶Hf/¹⁷⁷Hf ratios gradually decrease owing to the continuous addition of continental crustal materials during the evolution of the magmatic system [77], which usually corresponds to the evolution of the magmatic system from an early to late stage. Given that the ¹⁷⁶Hf/¹⁷⁷Hf ratios of the Yangla granitoids show the various features of quartz diorite < quartz monzonite porphyry < granodiorite and dioritic enclaves, this further clarifies the notion that the granitoids are the products of different evolutionary stages in the Yangla mining district (Table 2; Figure 9c), which also supported by the evidence of petrography about the above-mentioned intrusions. The variation in ¹⁷⁶Hf/¹⁷⁷Hf ratios indicates that the mid-to-late Triassic granodiorite and dioritic enclaves were followed by the quartz monzonite porphyry, which was in turn followed by the Late Triassic to Early Jurassic quartz diorite, and the crustal materials mixed into the magmatic system gradually increased. This is also consistent with the ε_Hf(t) values of the quartz diorite are all negative; the quartz monzonite porphyry has only one positive value, and the granodiorite and dioritic enclaves are primarily negative, with a few positive values. Moreover, Meng et al. (2016) [3] have proposed the magma which resulted in the formation of quartz monzonite porphyry is a later and more evolved melt stage compared to the granodiorite, which, based on a good correlation (i.e., both ratios decline with progressive magmatic evolution) between Th/U and Zr/Hf ratios in zircons. This evolving pattern can be further supported by the elevated LREE and HREE contents of the studied zircons from quartz diorite, quartz monzonite porphyry, granodiorite, and dioritic enclaves in the Yangla deposit. As seen in the binary diagrams for Ce/Nd vs. (Yb/Sm)_N (Figure 12a), the HREE relative to LREE enrichment decreases systematically from granodiorite and dioritic enclaves through quartz monzonite porphyry and to quartz diorite (although overlaps in Figure 15a), showing an evolutionary process from early to later magmatic stage. Furthermore, the ε_Hf(t) values of the zircons Yangla granitoids have a positive correlation with Th/U ratios (although they overlap in Figure 12b), with the Th/U ratios being proxies for the degree of differentiation [77]. This positive correlation between ε_Hf(t) and Th/U ratios revealed the progressive evolution from dioritic enclaves and granodiorite through quartz monzonite porphyry and to quartz diorite, which indicated the magmatic system increasingly assimilated crustal components throughout its evolution [3]. In addition, Meng et al. (2016) [3] have confirmed the rare xenocrystic zircons observed in the quartz monzonite porphyry, which further supported the assimilated crust materials within the magmatic system. Therefore, our preliminary interpretation is that during the the evolution of magma with a continuation of assimilation and fractional crystallization, the contamination amount of upper crustal metasediments gradually increased, and dioritic enclaves, granodiorite, quartz monzonite porphyry, and quartz diorite were formed in sequence, corresponding to the early to late stages of Yangla magmatic system evolution.

5.4. Metallogenic Potential

Currently, zircon has become an effective indicator mineral to judge the mineralization potential of granitoids intrusions, which is based on the continuous improvement of trace element analytic technology (i.e., LA-ICP-MS) [66,73,97,98]. Ballard et al. (2002) [19] suggested that Cu mineralization is directly associated only with intrusions with zircon δ Eu > 0.40 (Figure 17), which is based on the barren and fertile calc-alkaline intrusions from Chuquicamata El Abra porphyry copper belt of northern Chile. The zircon δ Eu values of Yangla quartz diorite are mainly concentrated between 0.5 and 1.2, with an average of 0.90, and significantly higher than 0.4 and indicates the quartz diorite has good Cu metallogenic potential. The zircon δ Eu values of quartz monzonite porphyry, granodiorite, and dioritic enclaves mainly range from 0.2 to 0.50 (avg. 0.4), 0.2 to 0.4 (avg. 04), and 0.2 to 0.40 (avg. 0.50) (Figure 17), respectively. These zircons δ Eu values of Yangla granitoids roughly similar to the intrusions in the Zhongdian ore concentrated area of SW China (i.e., Relin, Tongchanggou, Disuga, and Pulang; [67]), Jiangnan orogen belt of China (Dexing; [66]), Hualgayoc mining district of northern Peru [49], and Chuquicamata El Abra porphyry copper belt of northern Chile [19] (Figure 17), suggesting that Yangla granitoids have a good potential for Cu mineralization. In particular, the Xiuwacu deposit was dominated by the tungsten and shows slightly lower δ Eu values in the Zhongdian ore concentrated area of SW China [67], and generally similar to the Yangla granitoids (i.e., quartz monzonite porphyry, granodiorite, and dioritic enclaves) [3]. It further indicates that Yangla granitoids also have tungsten metallogenic potential.

Lu et al. (2016) [22] further research suggested that the fertile magmatic suites have collectively higher δEu > 0.30, 10,000*(Eu/Eu*)/Y (>1.0), (Ce/Nd)/Y (>0.01), and lower Dy/Yb (<0.3) ratios than infertile suites which based on the zircon compositions as a pathfinder for porphyry Cu ± Mo ± Au deposits. In particular, zircon (Eu/Eu*)/Y ratios are positively correlated with (Ce/Nd)/Y ratios in the fertile suites, but this correlation is lacking in the infertile suites. The zircon δEu values of most Yangla quartz diorites are greater than 0.30. Only one sample spot of Yanla quartz diorite has a 10,000*(Eu/Eu*)/Y ratio lower than 1.0, and the others have greater than 1.0. Most quartz diorite zircons also show (Ce/Nd)/Y and Dy/Yb ratios greater and lower than 0.01 and 0.30, respectively. Furthermore, there is a slightly positive correlation between (Eu/Eu*)/Y and (Ce/Nd)/Y ratios in quartz diorite. Therefore, we preliminarily believe that Yangla quartz diorite has Cu mineralization potential. Moreover, the other Yangla granitoids (i.e., quartz monzonite porphyry, granodiorite, and dioritic enclaves) have similar features with the quartz diorites in zircon δ Eu, 10,000*(Eu/Eu*)/Y, (Ce/Nd)/Y, and Dy/Yb values [3], which also indicate these granitoids have a good Cu metallogenic potential in the Yangla mining district.

Geng et al. (2019) [98] have proposed the discriminant diagram between ore-forming and barren granitoids intrusions based on zircons REE compositions of big data mining, such as the binary Eu/Dy × 100 vs. Ce/Nd × 0.1 (Figure 18a), Er/Lu vs. Ce/Sm (Figure 18b), Er/Lu vs. Dy/Yb × 10 (Figure 18c), Er/Lu vs. Dy/Lu (Figure 18d), Er/Yb × 10 vs. Gd/Dy × 10 (Figure 18e), Er/Lu vs. Tb/Tm × 10 (Figure 18f), and the triangular Eu/Gd × 100-Eu/Dy × 100-Gd/Yb × 100 (Figure 18g) and Eu/Gd × 100-Dy/Yb × 10-Er/Yb × 10 (Figure 18h). These binary and ternary discriminant diagrams can effectively distinguish the mineralization types of granitoids, such as Cu and W. Most Yangla granitoids (i.e., granodiorite, dioritic enclaves, quartz monzonite porphyry, and quartz diorite) mainly plotted within the Cu-related intrusions in Figure 18, suggesting that these granitoids have great potential for Cu mineralization, which is consistent with the geological fact that the Yangla deposit is dominated by Cu [1]. Moreover, a few Yangla granitoids samples are plotted within the W-related intrusions regions, indicating that these granitoids also have a certain W metallogenic potential, which is consistent with the geological facts of the newly discovered tungsten orebodies in the depth of the Yangla deposit (Figure 19) [99]. Furthermore, the Cu and W contents of the quartz diorite mainly range from 2019 to 12,170 ppm (avg. 6466 pm) and 79 to 1103 ppm (avg. 492 ppm) (

Table 4. The metallogenic elements (Cu and W) concentrations of quartz diorite in the Yangla Cu-W polymetallic deposit.

Sample No.	Cu/ppm	W/ppm	Reference
3250-41-3	5209	1103	[32]
3250-41-4	12,170	295
3250-41-5	2019	79

; [32]), respectively, which also indicated that the quartz diorite has a certain Cu-W metallogenic potential.

Previous studies have shown that the lower Dy/Yb ratios (<1) of zircons in the ore-forming plutons usually show the H₂O-rich features of the magma system, and the Dy/Yb ratios of zircons in the W ore-forming pluton are less than 0.4, which are lower than Cu ore-forming plutons [73]. Experimental petrology shows that the Y/Ho ratios in crystallized zircons from granitic magma are positively correlated with the F concentration in the magma [100]. Tungsten (W) is often enriched in F-rich and highly fractionated magma as a typical high-field strength element [101]. Ye et al. (2022) [73] further proposed two binary discrimination diagrams for ore-bearing intrusion based on zircon variations of Dy/Yb and Y/Ho ratios, such as Dy/Yb vs. Y (Figure 20a) and Y vs. Y/Ho (Figure 20b). The Dy/Yb ratios of all granodiorite, dioritic enclaves, and most quartz monzonite porphyry and quartz diorite samples from the Yangla deposit are less than 0.4 in Figure 20a, indicating that these granitoids have the potential to form tungsten deposit. The Y/Ho ratios of all dioritic enclaves and most granodiorite, quartz monzonite porphyry, and a small number of quartz diorite samples are mainly plotted within the region of Y/Ho ratios greater than 28 (enriched F system) in the Figure 20b. Most quartz diorite samples are mainly plotted within the region of the Y/Ho ratios are less than 28 (enriched Cl system) in Figure 20b. Therefore, zircons Dy/Yb and Y/Ho ratios features further confirm that Yangla granitoids have the potential to form not only Cu but also W deposit.

Mao et al. (2020) [102] suggested the tungsten deposits mainly related to highly differentiated reductive S-type granite, followed by oxidized I-type granodiorite in the Jiangnan tungsten belt, south China. The Yangla granodiorite and quartz diorite pluton belong to the I- and S-type granite, respectively [2,14], and dioritic enclaves, granodiorite, quartz monzonite porphyry, and quartz diorite experienced strong mineral crystallization differentiation and the corresponding oxygen fugacity values gradually decreased, which were highly similar to the W-related granites in the Jiangnan tungsten belt. Therefore, we preliminary believe that Yangla granitoids have certain W metallogenic potential. Li et al. (2022) [67] suggested that the oxygen fugacity of the magma forming the Cu deposit was greater than that of the W deposit; Cu and W are mainly related to mantle-derived components and ancient crustal materials, respectively. Oxygen fugacity is a key factor in Cu mineralization because magma with high oxygen fugacity is more conducive to the transport of metal elements and sulfur from the mantle to the epithermal setting of the crust [103]. However, tungsten mainly tends to be enriched in the crust, and the various states of oxygen fugacity cannot change the valence of tungsten (W⁶⁺), but the feldspar-sericitization in the magmatic-hydrothermal period and the release of Ca²⁺ from calcium-bearing wall rocks are conducive to W enrichment and formation W deposit [67]. The oxygen fugacity of the Yangla granitoids gradually decreased (Figure 11b), and the upper crustal components gradually increased from early to late stages (Figure 10b), corresponding to the dioritic enclaves → granodiorite → quartz monzonite porphyry → quartz diorite. Moreover, combined with the Yangla deposit, geological facts of the intrusions usually coupled with sericitization, and the marble in Devonian Linong Formation were the main host rocks. This further shows that the Yangla granitoids plutons have W metallogenic potential. To sum up, Yangla granitoids plutons have a good potential for Cu-W polymetallic mineralization.

6. Conclusions

• The crystallization temperature and oxygen fugacity of the magma show a gradually increased and decreased from dioritic enclaves and granodiorite through quartz monzonite porphyry and to quartz diorite in the Yangla mining district, respectively.
• Yangla granodiorite, dioritic enclaves, quartz monzonite porphyry, and quartz diorite have consistent magma sources, all of which originate from three-component upper crust + lower crust + mantle mixed magmas; the provenance is primarily Proterozoic basement components and a small quantity of mantle-derived materials.
• The degree of crustal metasediments assimilation and differentiation have gradually increased from dioritic enclaves and granodiorite through quartz monzonite porphyry and to quartz diorite. These granitoids plutons mainly experienced the separation and crystallization of apatite, titanite, and hornblende.
• Yangla granodiorite and dioritic enclaves, quartz monzonite porphyry, and quartz diorite were formed in the early, mid, and late stages of magmatic evolution, respectively.
• Yangla granitoids plutons have a better potential for Cu-W polymetallic deposits.