Petrogenesis of the granitoids related to skarn-type mineralization in the Nyainqentanglha Metallogenic Belt, Tibet

Abstract

The Nyainqentanglha Metallogenic Belt (NMB) is an economically important lead-zinc ore province located in the Central and Southern Lhasa subterrane, Tibet. The NMB consists mainly of skarn-type lead-zinc polymetallic deposits that form at the contact between Late Cretaceous-Eocene intrusive rocks and carbonatite/volcanic-sedimentary strata. These deposits are generally considered to be related to S-type granites formed by the Indo-Asian continental collision. However, the ε_Hf(t) (ave. −1.6) of zircons from the mineralization-related granitoids indicates that it is crust-mantle mixing products. In addition, the volcanic rocks of the Dianzhong Formation, which have a close spatio-temporal relationship with the skarn-type deposits, typically show mantle-derived features. Therefore, the occurrence of AFC processes in mantle-derived material passing through the thick basement of the Lhasa Terrane may better explain the genesis of the mineralization-related granitoids. In this study, we present results on zircon U–Pb ages, major and trace elements of the granitoids associated with skarn-type mineralization from the Narusongduo district. LA-ICP-MS zircon U–Pb dating shows that the granite formed in the Paleocene (58.6 ± 0.5 Ma). Combined with published regional data, we propose the mineralization age of the skarn-type deposits in the NMB ranges from the Late Cretaceous to Eocene. Detailed petrology and geochemistry of mineralization-related granitoids are evaluated to constrain the magmatic evolution process. The granitoids have high contents of SiO₂ (mean 72.8 %), K₂O (mean 4.0 %), Rb (mean 186.9 ppm), DI (differentiation index) 84.7, A/CNK (mean 1.3), and low contents of MgO, TFe, suggesting that the granitoids have undergone strong differentiation. In addition, the continuous decrease of P₂O₅ with progressive differentiation and the lower average P₂O₅ abundance suggest that the mineralization-related granitoids belong to the I-type granite. The results suggest that the mineralization-related granitoids in the NMB originated from the mantle and is the fractionated I-type granite formed by the process of AFC (Assimilation and Fractional Crystallization). Considering that Pb and Zn often coexist in deposits, we suggest that magmatic differentiation may play an important role in the formation of the granite-related Pb and Zn mineralization.

1. Introduction

During the Paleocene-Eocene, the Lhasa Terrane underwent significant magmatism. The intrusive rocks from this period make up the bulk of the Gangdese batholith and have been extensively studied, accumulating a large amount of chronological and geochemical data [[1], [2], [3], [4], [5]]. In addition, volcanic activity peaked during this period, as evidenced by the Linzizong volcanic rocks [[6], [7], [8], [9]]. The intense magmatic activity also led to the formation of the NMB (Nyainqentanglha Metallogenic Belt) (Fig. 1).

Fig. 1.

(a) Tectonic framework of the Himalayan-Tibetan plateau; (b) Simplified geological map of the Nyainqentanglha Pb–Zn polymetallic metallogenic belt showing the location of Skarn Pb–Zn deposits (modifed after [10]. IYZSZ: Indus-Yarlung Zangbo Suture Zone; LMF: Luobadui-Milashan Fault; GLZCF: Gar-LunggarZhariNam Tso-Comai Fault; SNMZ: Shiquan River-Nam Tso Mélange Zone; BNSZ: Bangong-Nujiang Suture Zone. 1- Yaguila; 2- Dongzhongsongduo; 3- Dongzhongla; 4- Mengya'a; 5- Longmala; 6- Maxionglang; 7- Leqingla; 8- Xingaguo; 9- Dexin; 10-Narusongduo; 11-Sinongduo; 12-Chagele; 13-Longgen; 14-Nuocang.

The NMB mainly consists of skarn-type deposits formed at the contact zone between the Paleocene-Eocene intrusive and carbonate/volcanic-sedimentary strata [[11], [12], [13]]. In recent years, crypto-explosive breccia-hydrothermal vein deposits have been discovered in the Paleocene-Eocene Linzizong volcanic rocks [9,14,15]; Qin et al., 2022) (Fig. 1). Previous studies suggested that the skarn-type deposits in the NMB are related to S-type granites formed by the India-Asia continental collision [11,[16], [17], [18], [19], [20], [21], [22]]. However, the relevant isotopic data show that it is a crust-mantle mixed product (e.g. εHf(t) −13.9 ∼ +8.7) [23]. In addition, the volcanic rocks of the Dianzhong Formation, which have a close spatio-temporal relationship with the skarn-type deposits, typically exhibit mantle-derived features [6,24]. Therefore, the occurrence of (AFC (Assimilation and Fractional crystallization) processes in mantle-derived material as it passes through the thick basement of the Lhasa Terrane may better explain the genesis of the mineralization-related granitoids.

This paper presents new zircon U–Pb ages and whole-rock geochemistry from ore-related granite porphyries at the Narusongduo skarn-type lead-zinc deposit. These new data, combined with previously published data, enable us to re-evaluate the petrogenesis of the ore-related magma.

2. Geological background and sample characteristics

The Tibetan Plateau is made up of several terranes, namely the Songpan-Ganzi Complex, Qiangtang, Lhasa, and the Tethyan Himalaya Terranes, arranged from north to south [25]; Fig. 1a). The Lhasa Terrane, which is a significant crustal segment, is considered to have separated from Gondwana during the Carboniferous-Permian and subsequently drifted northwards. After a long geological period, the Lhasa Terrane collided with the Qiangtang Terrane during the Early Cretaceous and with the Indian continent during the Cenozoic [3]; Fig. 1a and b).

Analysis of zircon Lu–Hf isotope data from igneous rocks, sedimentary strata, and metamorphic basement divides the Lhasa Terrane into three distinct tectonic units: southern, central and northern subterranes. These subterranes are separated by the Shiquanhe-Nam Tso Melange Zone (SNMZ) and the Luobadui-Milashan Fault, as reported by Refs. [3,4]. The Central Lhasa Subterrane consists of extensive Carboniferous-Permian metamorphic strata, Lower Cretaceous volcanic-sedimentary strata, Upper Jurassic-Lower Cretaceous sedimentary strata interbedded with abundant volcanic rocks, and widespread Mesozoic felsic magmatism and Paleocene-Eocene felsic magmatism as described by Ref. [12]. The northern Lhasa Subterrane is dominated by a juvenile crust composed of Jurassic-Cretaceous sedimentary rocks, Early Cretaceous volcanic rocks and associated granitoids. The southern Lhasa Subterrane, on the other hand, consists of juvenile crust with locally preserved Precambrian crystalline basement. The Subterrane mainly consists of Cretaceous Andean arc magmatism, Paleocene-Eocene felsic magmatism and Linzizong volcanic rocks, as reported by Refs. [3,11].

The NMB is located in the central and southern Lhasa subterrane. It contains over 20 polymetallic deposits such as the Yaguila lead-zinc-silver, Narusongduo Lead-zinc and Chagele lead-zinc deposits (Fig. 1b; see Table 1 for details). Skarn is the predominant mineralization type within the NMB, accompanied by minor amounts of breccia pipe, vein, and manto deposits [20]. Evidence of ore-related granites is provided by the presence of skarn alteration at the contact zones between the intrusions and surrounding rocks. The ages of the granites associated with mineralization range from 69 to 51 Ma.

Table 1.

The diagenesis and mineralization ages of Pb–Zn polymetallic deposits in the NMB.

No.	Deposit	Type	Mineralization	Samples	Analytical method	Age (Ma)	Reference
1	Yaguila	Skarn Pb–Zn–Ag	Galena, sphalerite, silver, pyrrhotite, pyrite	Quartz porphyry Molybdenite Garnet	LA-ICP-MS zircon U–Pb Re–Os LA-ICP-MS garnet U–Pb	65.6 ± 1.2 65.0 ± 1.9 65.0 ± 4.7–68.5 ± 3.4	[26] [27] [28]
2	Dongzhongsongduo	Skarn Pb–Zn	Galena, sphalerite, silver, magnetite, pyrrhotite, chalcopyrite, pyrite	Granite porphyry	LA-ICP-MS zircon U–Pb	54.4	[29]
3	Dongzhongla	Skarn Pb–Zn–Ag–Cu	Galena, sphalerite, chalcopyrite, pyrrhotite, pyrite, bornite, chalcocite	Granite porphyry	LA-ICP-MS zircon U–Pb	51.2	[29]
4	Mengya'a	Skarn Pb–Zn–Ag	Galena, sphalerite, pyrrhotite, chalcopyrite, pyrite	Muscovite	Ar–Ar	54.8 ± 0.4	[22]
5	Longmala	Skarn Pb–Zn–Fe–Cu	Galena, sphalerite, magnetite, chalcopyrite, pyrrhotite, pyrite	Biotite monzogranite Molybdenite	LA-ICP-MS zircon U–Pb Re–Os	54.6 ± 0.6 53.3	[30] [31]
6	Maxionglang	Skarn Pb–Zn	Galena, sphalerite, pyrite, and minor chalcopyrite, pyrrhotite	Sericite	Ar–Ar	69.3 ± 1.6	[29]
7	Leqingla	Skarn Pb–Zn–Fe–Cu	Galena, sphalerite, magnetite, chalcopyrite, molybdenite, pyrrhotite	Biotite granite Molybdenite	LA-ICP-MS zircon U–Pb Re–Os	60.8 ± 0.4 59.4 ± 4.5	[32] [33]
8	Xingaguo	Skarn Pb–Zn	Sphalerite, galena, magnetite, pyrite, marcasite, chalcopyrite, pyrrhotite	Biotite granite	LA-ICP-MS zircon U–Pb	56.5 ± 1.3	[32]
9	Dexin	Hydrothermal Pb–Zn	Galena, sphalerite, chalcopyrite, pyrite	Quartz porphyry	LA-ICP-MS zircon U–Pb	56.6 ± 0.8	[34]
10	Narusongduo	Cryptoexplosive breccia-skarn Pb–Zn polymetallic	Sphalerite, galena, pyrite, chalcopyrite, arsenopyrite, rhodochrosite	Granite porphyry Sericite	LA-ICP-MS zircon U–Pb Ar–Ar	58.6 ± 0.5 57.8 ± 0.7	This paper [35]
11	Sinongduo	cryptoexplosive breccia-hydrothermal vein Ag–Pb–Zn	Galena, sphalerite, smithsonite, chalcopyrite, pyrrhotite, silver	Tuff Illite	LA-ICP-MS zircon U–Pb Ar–Ar	61.9 ± 0.4 60.9 ± 0.7–63.1 ± 0.7	[9] [36]
12	Chagele	Skarn Pb–Zn	Sphalerite, galena, pyrite, marcasite, chalcopyrite and pyrrhotite	Granite porphyry	LA-ICP-MS zircon U–Pb	63.3 ± 0.9	[37]
13	Longgen	Skarn Pb–Zn	Sphalerite, galena, chalcopyrite, pyrite, magnetite, pyrrhotite, bornite, molybdenite and arsenopyrite	Granite porphyry Sphalerite	LA-ICP-MS zircon U–Pb Rb–Sr	61.2 ± 0.8 59.1 ± 3.0	[12]
14	Nuocang	Skarn Pb–Zn	Galena, sphalerite, and minor magnetite, pyrite, chalcopyrite, and malachite	Muscovite	Ar–Ar	59.68 ± 0.6	[38]

This study focuses on the mineralization-related granites found in five representative deposits. The petrographic descriptions provided in this study are based on previous research conducted by Refs. [12,13,32]. Among these deposits, the Narusongduo deposit mainly includes two types of mineralization: cryptoexplosive-breccia and skarn. The skarn type orebody is developed at the interface between the granite porphyry and limestone from the Xiala Formation (Fig. 2). This type of mineralization is characterized by the presence of skarn-type mineral assemblages, including garnet and actinolite, and locally high-grade lead-zinc ores (Fig. 3a and b). The Narusongduo granite porphyries, represented by samples TD019-1∼5 and located at coordinates 29°58′15″, 88°46′57″, exhibit a porphyritic texture. They are primarily composed of quartz, alkali feldspar, and biotite. The groundmass consists of smaller quartz, alkali feldspar, and sericite (Fig. 3c and d). In terms of accessory minerals, apatite and zircon are present in these samples. Additionally, slight alteration has been observed in the samples.

Fig. 2.

Geological sketch map of Narusongduo Pb–Zn deposit (Modified from [14].

Fig. 3.

Petrographical and microphotograph photos from the Narusongduo Pb–Zn deposit. (a) and (b) the garnet skarn the massive sulfide skarn ores; (c) and (d) Granitoids associated with Skarn-type mineralization.

3. Analytical methods

3.1. Major and trace elements analysis

To conduct comprehensive rock geochemical analyses, the initial step involved crushing the four samples to 200 meshes using an agate mill, followed by thorough cleaning in distilled water using ultrasonic waves. Subsequently, major and trace element analyses were performed at China's National Geological Experimental Test Center. Major element analyses were carried out using XRF (X-ray fluorescence spectrometry), with an analytical uncertainties of less than 0.5 %. Trace elements were analyzed using an X-series 2 ICP-MS instrument, with an analytical uncertainties of less than 5 %. The results of the element analyses for the granitic porphyry samples are showed in Table 2.

Table 2.

Major and trace element compositions of the ore-related granite porphyry in the Narusongduo Pb–Zn deposit.

Sample No.	TD019-1	TD019-2	TD019-3	TD019-4	Sample No.	TD019-1	TD019-2	TD019-3	TD019-4
SiO2(%)	72.67	77.28	79.74	75.46	Nb(μg/g)	10.7	11	8.05	9.75
TiO2(%)	0.25	0.23	0.18	0.2	Ta(μg/g)	1.08	1.03	0.8	0.85
Al2O3(%)	13.2	13.2	10.72	11.46	Zr(μg/g)	152	141	105	140
Fe2O3(%)	0.52	0.39	0.43	5.12	Hf(μg/g)	4.38	4.18	3.18	3.8
FeO(%)	1.2	1.06	0.88	1.13	La(μg/g)	31.7	21.1	30.2	63.8
MnO(%)	0.14	0.16	0.1	0.05	Ce(μg/g)	58.1	36.5	52.1	108
MgO(%)	0.34	0.33	0.27	0.38	Pr(μg/g)	5.71	4.18	5.91	11.4
CaO(%)	0.17	0.19	0.16	0.1	Nd(μg/g)	18.1	13.8	22.8	40.7
Na2O(%)	0.76	0.01	0.16	0.01	Sm(μg/g)	2.74	2.21	2.99	5.77
K2O(%)	3.93	3.88	3.19	3.46	Eu(μg/g)	0.17	0.18	0.18	0.34
P2O5(%)	0.07	0.08	0.06	0.04	Gd(μg/g)	2.08	1.75	2.02	4.69
DI	87.35	86.78	89.38	81.13	Tb(μg/g)	0.28	0.24	0.25	0.78
A/CNK	2.34	3.02	2.77	2.98	Dy(μg/g)	1.34	1.23	1.09	4.56
CIPW(corundum)	8.1	9.12	7.15	8.18	Ho(μg/g)	0.24	0.28	0.21	0.97
Sr(μg/g)	6.33	6.57	6.12	5.73	Er(μg/g)	0.66	0.83	0.59	2.66
Ba(μg/g)	191	339	243	134	Tm(μg/g)	0.11	0.16	0.11	0.42
Rb(μg/g)	252	246	198	225	Yb(μg/g)	0.72	1.07	0.74	2.57
Th(μg/g)	30.8	32.9	26.3	27.5	Lu(μg/g)	0.12	0.19	0.14	0.37
U(μg/g)	3.27	2.99	2.88	3.17	Y(μg/g)	7.46	8.3	5.74	31.7

3.2. LA-ICP-MS zircon U–Pb dating

To investigate the internal structures of zircon and determine suitable locations for zircon isotope analyses, CL images were captured at the Zirconium Year Technology Company in Beijing. LA-ICP-MS was utilized to acquire U–Pb isotope and trace element data at the GPMR (State Key Laboratory of Geological Processes and Mineral Resources) in Wuhan, China. During the LA-ICP-MS analysis, we calibrated U–Th–Pb concentrations using the NIST SRM 610 external standard and the ²⁹Si internal standard. We utilized ICPMSDataCal for offline selection and integration of background and analyzed signals, time drift correction, and quantitative calibration. Finally, we employed Isoplot to generate Concordia plots, age spectra, and age calculations. The mean age of ²⁰⁶Pb/²³⁸U results is reported with an uncertainty of 2-sigma. Detailed analytical results of zircon U–Pb dating can be found in Table 3.

Table 3.

Zircon LA–ICP-MS U–Pb data of the ore-related granite porphyry in the Narusongduo Pb–Zn deposit.

spot	Th(ppm)	U(ppm)	Th/U	207Pb/206Pb	1σ	207Pb/235U	1σ	206Pb/238U	1σ	207Pb/235U	1σ	206Pb/238U	1σ
TD019-5-1	1842.8	2058.1	0.9	0.0509	0.0019	0.0655	0.0025	0.0093	0.0001	64.4	2.4	59.7	0.7
TD019-5-2	1745.2	1556.3	1.12	0.0524	0.0016	0.0665	0.002	0.0092	0.0001	65.3	1.9	59.3	0.5
TD019-5-3	1449	1757.5	0.82	0.049	0.0013	0.0607	0.0017	0.009	0.0001	59.8	1.6	57.6	0.5
TD019-5-4	673.5	452.2	1.49	0.0498	0.006	0.062	0.007	0.0092	0.0003	61.1	6.7	59.1	1.8
TD019-5-6	561.5	268.2	2.09	0.0539	0.0118	0.0641	0.0131	0.0089	0.0004	63.1	12.5	57.4	2.3
TD019-5-7	957.8	465.7	2.06	0.0476	0.0027	0.0591	0.0033	0.0091	0.0001	58.3	3.1	58.1	0.7
TD019-5-8	489.7	192.2	2.55	0.0541	0.0117	0.064	0.0141	0.0093	0.0004	63	13.4	59.5	2.6
TD019-5-9	1090.9	1030.1	1.06	0.0475	0.0021	0.0594	0.0025	0.0091	0.0001	58.6	2.4	58.6	0.6
TD019-5-11	286	245.5	1.16	0.0524	0.0067	0.0658	0.0074	0.0095	0.0002	64.7	7	60.7	1.6
TD019-5-13	604.9	346.7	1.74	0.0477	0.003	0.0583	0.0035	0.0089	0.0001	57.5	3.3	57.4	0.7
TD019-5-14	598.2	317.3	1.89	0.0517	0.0033	0.0635	0.004	0.0091	0.0001	62.5	3.8	58.6	0.9
TD019-5-15	1743.6	2073.7	0.84	0.0475	0.0012	0.0597	0.0014	0.0092	0.0001	58.9	1.4	58.8	0.5
TD019-5-16	738.3	340.7	2.17	0.0527	0.0116	0.0657	0.0134	0.0091	0.0004	64.6	12.8	58.7	2.3
TD019-5-17	569.1	425.6	1.34	0.0518	0.0053	0.0676	0.0069	0.0095	0.0002	66.4	6.5	61.2	1.5
TD019-5-19	393.7	345.4	1.14	0.0502	0.0045	0.062	0.0061	0.0088	0.0002	61.1	5.9	56.4	1.4
TD019-5-20	484.1	276.5	1.75	0.0564	0.0095	0.0644	0.009	0.0091	0.0003	63.4	8.6	58.2	1.9

4. Analysis results

4.1. Major and trace elements

In the paper, we tested four samples of granite porphyry from the Narusongduo skarn deposit for whole-rock geochemistry. We supplemented the data published in the literature to make the data statistically significant (see Table S1; [12,13,32].

The SiO₂ content is relatively high, ranging from 65.5 % to 79.7 %, with an average of 72.8 %. Al₂O₃ varies from 10.7 % to 17.9 %, with an average of 13.5 %. CaO, MgO and TFe contents are low. K₂O has a higher content, ranging from 1.7 to 7.0 %, with an average of 4.0 %. The Harker diagram shows that the major element all vary nearly linearly with the SiO₂ (Fig. 4).

Fig. 4.

Diagrams of major elements plotted versus SiO₂ showing the fractionation trends for the granitoids associated with Skarn-type mineralization in the NMB.

The granite porphyry from the Narusongduo deposit displays a steep pattern for light rare earth elements (LREE) and a flat pattern for heavy rare earth elements (HREE) (see Fig. 5a). The (La/Yb)_N ratios range from 14.1 to 31.6. A 'V'-shaped REE distribution pattern is created by negative Eu depletions (δEu = 0.19–0.27). In the primitive mantle-normalized spider diagram, the samples exhibit strong enrichment of Rb, Th, U, and La, and depletion of Nb, Ta, Ba, Eu, and Sr (see Fig. 5b).

Fig. 5.

Chondrite-normalized REE pattern (a) and primitive mantle-normalized spidergram (b) for the granitoids associated with Skarn-type mineralization in the Narusongduo Pb–Zn deposit.

4.2. Zircon U–Pb age

The zircon grains in the sample (TD019-5) are transparent and contain only a few inclusions. Their diameters range from 70 μm to 120 μm, and their length to width ratios range from 3:1 to 1:1. Oscillatory zoning is present in all the grains, and their Th/U ratios range from 0.6 to 2.5, indicating a magmatic origin (Table 3, [39]. The 16 spots have ²⁰⁶Pb/²³⁸U ages ranging from 56.4 ± 1.4 to 61.2 ± 1.5 Ma, giving a weighted mean ²⁰⁶Pb/²³⁸U age of 58.6 ± 0.5 Ma (Fig. 6).

Fig. 6.

Zircon U–Pb concordia diagram for the granitoids associated with Skarn-type mineralization in the Narusongduo Pb–Zn deposit.

5. Discussion

Magmatic-hydrothermal lead-zinc polymetallic deposits are widespread throughout China, such as the Erdaohe skarn lead-zinc deposit in the Great Hinggan Range [40], the Caixiashan Zn–Pb deposit in the Eastern Tianshan [12], the Huangshaping lead-zinc deposit and the Tongshanling deposit in the Nanling Range [28,41,42]. This paper will specifically focus on the lead-zinc deposit in the NMB.

5.1. The important metallogenic events between Late Cretaceous to Eocene

The NMB is located in the central and southern Lhasa subduction zone, between the Shiquan River-Nam Tso melange zone and the Indus-Yarlung Zangbo suture zone. It has experienced three tectonic-magmatic events, including the Mesozoic subduction of Neotethyan oceanic crust, Palaeogene India-Asia continental collision and Miocene post-collision [4,16,25,43,44]. The NMB contains subduction associated porphyry Copper deposits (e.g. Xiongcun; [45], collision associated lead-zinc deposits (e.g. Longmala; [46], post-collisional associated porphyry copper deposits (e.g. Qulong [47]; and skarn copper deposits (e.g. Jiama; Zheng et al., 2016). It is one of the most important metallogenic belts in the Himalayan-Tibetan continental orogenic system. Skarn is the dominant type of mineralization within the NMB, accompanied by a small amount of breccia pipe, vein and manto deposits. The metallogenic belt hosts over 20 polymetallic deposits, including four significant ones: Yaguila, Mengyaa, Leqingla and Narusongduo.

After analyzing a significant amount of chronological data, we have confirmed that the mineralization period of NMB during the Late Cretaceous-Eocene is second only to the mineralization explosion period of Miocene. These deposits are mainly concentrated in the central and eastern part of the belt. Recently, some deposits have also been discovered in the western region, including Chagele skarn lead-zinc polymetallic deposit, Longgen skarn lead-zinc deposit, and Nuocang skarn lead-zinc polymetallic deposit [10]. The Chagele deposit was dated using the Re–Os method on molybdenite, and an isochron age of 61.49 ± 0.6 Ma was obtained [37]. Notably, some deposits have been found in the Linzizong volcanic sequence in the middle section of the belt, such as the Sinongduo deposit, which is a cryptoexplosive breccia-hydrothermal vein silver-lead-zinc deposit [48], the Narusongduo deposit, which is a cryptoexplosive breccia lead-zinc-silver deposit [35], and the Dexin hydrothermal lead-zinc deposit. The sericite in the quartz sulphide veins from the Sinongduo silver-lead-zinc deposit was dated using ⁴⁰Ar-³⁹Ar analysis, yielding ages ranging from 60.9 ± 0.7 to 63.1 ± 0.7 Ma [36]. Furthermore, the ⁴⁰Ar-³⁹Ar age of sericite in the Narusongduo lead-zinc deposit is 57.8 ± 0.7 Ma [35].

Numerous skarn deposits are found in the middle part of the metallogenic belt, primarily occurring at the interface of granitoids and the Takena Formation [49]. By in-situ LA-ICP-MS U–Pb zircon dating, the intrusion age of ore-forming biotite granite in the Leqingla deposit is determined to be 60.8 ± 0.4 Ma, which can be inferred as the mineralization age [32]. Using the molybdenite Re–Os dating method, the isochron age of the Yaguila deposit is determined to be 65.0 ± 1.9 Ma, which is consistent with the zircon U–Pb age (65.6 ± 1.2 Ma) of the ore-forming porphyry within the error range [26,27]. LA-ICP-MS analysis was conducted on zircon U–Pb dating of the biotite monzogranite associated with mineralization at the Longmala deposit, resulting in a weighted mean age of 54.6 ± 0.6 Ma [30]. The age is consistent with the molybdenite Re–Os dating which yielded a weighted mean age of 53.3 Ma [31]. In addition, ⁴⁰Ar-³⁹Ar dating of muscovite from the Mengya'a deposit yielded a plateau age of 54.8 ± 0.4 Ma [22]. Zircon U–Pb analysis was used to date the Dongzhongla and Dongzhongsongduo deposits, yielding ages of 51.2 Ma and 54.4 Ma, respectively [29]. Furthermore, the Maxionglang sericite was dated using ⁴⁰Ar-³⁹Ar analysis, yielding an age of 69.3 ± 1.6 Ma [29] (Table 1). Based on the published data, we propose the mineralization age of the skarn-type deposits in the NMB ranges from the Late Cretaceous to Eocene.

5.2. Petrogenesis of the granite related to skarn-type deposits

It is widely believed that the granitoids related to lead-zinc mineralization in the NMB are linked to the formation of S-type granitoids during the collision of the Indo-Asian continents [11,[16], [17], [18], [19], [20], [21], [22]]. The main evidence for this theory is that the granitoids are mostly peraluminous (as shown in Fig. 7a) and their metallogenic ages are mostly less than 65 Ma, which occurred during the India-Asia collision stage (Yin & Harrison, 2000; [50]. Therefore, it is generally accepted that the granitoids associated with lead-zinc mineralization are co-collision S-type granite. However, this paper challenges the evidence provided by previous researchers. Firstly, it cannot be determined that the granitoids are S-type granite because the S-type and I-type granite classification schemes are independent of the tectonic background. Secondly, peraluminous characteristics are not a robust indicator for judging S-type granite because I-type granite also exhibits peraluminous characteristics (as shown in Fig. 7b) [51,52,53].

Fig. 7.

(a) A/CNK vs SiO₂ diagram for the granitoids associated with Skarn-type mineralization in the NMB (symbols are as in Fig. 4.); and (b) A/CNK diagram of I-type and S-type granite for Lachlan Fold Belt [51].

5.2.1. Genetic types of the granite related to skarn-type deposits

The zircon U–Pb geochronology results indicate that the granite porphyry in the Narusongduo area is 58.6 ± 0.5 Ma, which is in agreement with the granitoids associated with lead-zinc mineralization in the NMB [11,12].

This paper and literature data indicate that the granitoids associated with lead-zinc mineralization in the NMB are peraluminous and strongly peraluminous (A/CNK>1.0). Moreover, corundum is present to some extent in CIPW standard mineral calculation (Table 2). These features are often observed in S-type granite [51], and researchers have frequently linked mineralization-related granitoids with S-type granite [11,[16], [17], [18], [19],22]. However, this contradicts two facts. Firstly, the granitoids exhibit Sr-Nd-Hf isotopic characteristics that tend to be mantle-derived (Fig. 8, Fig. 9). In Fig. 8, the granitoids associated with mineralization display a wide range of ε_Hf(t) values (−13.9 to 8.7), which fall within the range of crust-derived and mantle-derived rocks in the NMB. This suggests that the granitoids have a composite origin. Fig. 9 shows that the mineralization-related granitoids are distributed between the mantle-derived Linzizong volcanic rocks and the Middle-Upper Crust, Amdo orthogneiss, indicating that the granitoids are a mixture of mantle-derived magma and crustal components. The ⁸⁷Sr/⁸⁶Sr data exhibit distinct characteristics. The degree of ⁸⁷Sr isotope enrichment is lower than that of Amdo orthogneiss, but closer to the mantle-derived Linzizong volcanic rocks, indicating that the granitoids were not formed by remelting of ancient sedimentary rocks, i.e., they are not S-type granite, and should originate from the mantle together with the Linzizong volcanic rocks. Additionally, the ⁸⁷Sr/⁸⁶Sr value of the granitoids is higher than that of the Linzizong volcanic rocks. This is because Rb is a large ion lithophile element that tends to aggregate in the magma during the latest stages of magma evolution, i.e., in highly fractionated granites. ⁸⁷Rb decays into ⁸⁷Sr, leading to an increase in ⁸⁷Sr/⁸⁶Sr in fractionated granites. Therefore, mineralization-related granitoids of the same age will exhibit a higher ⁸⁷Sr/⁸⁶Sr value compared to the Linzizong volcanic rocks. Secondly, the Linzizong volcanic rocks are mainly mantle-derived rocks [6,8], which are spatially consistent with the mineralization-associated granitoids. It is generally believed that volcanic rocks are more representative of the source components, which are formed by the direct rise and overflow from the magma chamber. The mantle source characteristics of the Linzizong volcanic rocks indicate that the mineralization-related granitoids with relatively consistent temporal and spatial distributions should also originate from the mantle.

Fig. 8.

Histogram of ε_Hf(t) of zircons from the granitoids associated with Skarn-type mineralization in the NMB (Data from Ref. [12] and therein).

Fig. 9.

Sr–Nd isotopic compositions of the granitoids associated with Skarn-type mineralization in the NMB. Data sources: the Yarlung MORB [54]; the Lower crust [54]; the Middle-Upper crust [11]; Amdo orthogneiss [55]. Linzizong volcanics [24]; Yaguila quartz porphyry [26]; Longgen granite porphyry [12]; Chagele granite porphyry [18]; and Narusongduo granite porphyry [56].

Based on the summary and analysis of this paper and the literature data, it is believed that the mineralization-related granitoids in the NMB are fractionated I-type granites formed by the AFC process when mantle-derived magma intrudes into the thick continental crust (the crystalline basement of the Lhasa terrane). Firstly, we used CIPW to calculate the mineralization-related granitoids, and the DI (differentiation index) value ranges from 75.2 to 94.5, with an average value of 84.7 (Table 2), suggesting strong differentiation. Secondly, the granitoids have high SiO₂ (mean 72.8 %), K₂O (mean 4.0 %), Rb (mean 186.9 ppm), A/CNK (1.0–3.0, average 1.3), and low contents of MgO, TFe, consistent with the characteristics of fractionated I-type granites. Thirdly, the mineralization-related granitoids are dominated by quartz and potassium feldspar, with low calcareous plagioclase content, which is a mineralogy sign of fractionated I-type granite. However, these features are also often displayed by the S-type granite. This paper denies the petrogenesis of S-type granite because, in addition to the two piece of robust evidence listed above, the evidence for determination S-type granite is ambiguous. Firstly, the main evidence for judging S-type granite includes strong peraluminous (A/CNK>1.1) characteristics, and corundum in CIPW standard minerals. However, these characteristics are ubiquitous in fractionated I-type granite [57]. Furthermore, the high A/CNK value is due to the separation of plagioclase, which leads to the gradual decrease of Ca and Na content (Fig. 10). S-type granite often contains peraluminous minerals such as cordierite, muscovite or garnet, but these minerals are not found in the mineralized granitoids in the NMB. Secondly, the behavior of phosphorus (P) is a crucial indicator for distinguishing between I- and S-type granites in the case of fractionated granites [58]. have shown that P₂O₅ exhibits opposite trends during fractional crystallization, decreasing in I-type granites and increasing in S-type granites. The continuous decrease of P₂O₅ with progressive differentiation (as shown in Fig. 4c) and the lower average P₂O₅ abundance (0.067) in the mineralization-related granitoids suggest that they belong to the I-type granite category.

Fig. 10.

Sr versus Ba diagram (a) and Sr versus Rb diagram (b) of the granitoids associated with Skarn-type mineralization in the NMB. Labeled vectors correspond to up to 50 % fractionation crystallization of the main rock-forming minerals (after [52]. Symbols are as in Fig. 4.

5.2.2. AFC process

AFC (Assimilation and Fractional Crystalization) process are typically linked to the formation of granitoids. This is due to the high large viscosity of granitic magma, long emplacement time, and its location several kilometers underground, which slows down the dissipation of magma heat and enables ample matter and energy exchange with the surrounding rocks. Additionally, the fractional crystallization of the magma can also occur slowly and completely.

In the Harker diagram, there is a noticeable linear decrease in Al₂O₃, CaO and Na₂O as SiO₂ content increases (Fig. 4a, b and c). This is believed to be due to the fractional crystallization of feldspar, which is rich in Sr, Ba, and Eu. The significant loss of Sr, Ba, and Eu (Fig. 5a and b) confirms the separation of feldspar. Potassium feldspar, which is abundant in K and Rb, is a common mineral in the final stage of magma evolution. The high enrichment of K and Rb suggests that granitoids related to mineralization have reached the final stage of magma evolution. The lower content of major elements Fe, Mg, and Ti is often associated with the crystallization differentiation of hornblende, biotite, and iron-titanium minerals, indicating that these minerals have separated during magma evolution. This indicates that potassium feldspar and plagioclase are the main minerals that separate during magma evolution, with a small amount of biotite, muscovite, and hornblende also occurring (Fig. 10a and b).

Granitic magma has a high viscosity and migrates slowly, which leads to assimilation and contamination with surrounding rocks. As a result, it often exhibits isotopic characteristics that are neither enriched nor depleted, and can undergo a wide range of changes. The ε_Hf(t) values of granitoids related to mineralization range from −13.9 to +8.7, with an average value of −1.6 (Fig. 8) [12]. This suggests that the formation of mineralization-related granitoids is linked to interactions between the crust and mantle.

Partitioning processes can significantly enrich incompatible elements, such as Pb, W, Sn, and rare metals [10,[59], [60], [61], [62]]. During magmatic differentiation, Pb is continuously enriched as an incompatible element. This process can lead to a significant enrichment of incompatible elements, up to 30 times higher than the original basaltic parent [56,63,64]. However, unlike highly incompatible elements, Zn behaves as a compatible to moderately incompatible element and is sensitive to changes in mineralogy or major element composition [65]. Given that Pb and Zn often coexist in deposits, we suggest that magmatic differentiation may be one of the causes of Pb and Zn enrichment. However, this does not exclude the possibility that Pb and Zn enrichment in the source area plays an important role in Pb and Zn mineralization.

6. Conclusion

The NMB is a crucial metallogenic belt within the Himalaya-Tibetan continental orogenic system. Skarn is the primary form of mineralization found within the NMB, accompanied by smaller quantities of breccia pipe, vein-type, and manto deposits.

• (1)LA-ICP-MS zircon U–Pb dating shows that the granites associated with skarn-type mineralization from the Narusongduo district formed in the Paleocene (58.6 ± 0.5 Ma). Combined with the regional published data, we propose the mineralization age of the skarn-type deposits in the NMB ranges from Late Cretaceous to Eocene.
• (2)The granitoids associated with mineralization in the NMB have high SiO₂ (mean 72.8 %), K₂O (mean 4.0 %), Rb (mean 186.9 ppm), DI (differentiation index) 84.7, A/CNK (mean 1.3), and low contents of MgO, TFe, suggesting that the granitoids have undergone strong differentiation.
• (3)The continuous decrease of P₂O₅ with progressive differentiation and the lower average P₂O₅ abundances suggest that the mineralization-related granitoids belong to the I-type granite. Based on the published Sr-Nd-Hf isotopic data, we suggest that the granitoids originated from the mantle and underwent the AFC process. The formation of lead-zinc polymetallic deposits may be related to the highly fractional crystallization of the magma.

Funding statement

This paper was supported by Central Finance Research Project Surplus Funds Project of the Institute of Geophysical and Geochemical Exploration (JY202109); Science & Technology Fundamental Resources Investigation Program (2022FY101800); National Nonprofit Institute Research Grant of IGGE (AS2022J11); National Natural Science Foundation of China (41504063, 42202106).

CRediT authorship contribution statement

Fu Yangang: Writing – review & editing, Writing – original draft, Project administration, Investigation, Funding acquisition, Conceptualization. Duan Zhuang: Methodology, Funding acquisition, Formal analysis, Conceptualization. Gao Jianweng: Visualization, Formal analysis, Data curation. Hao Zejiang: Visualization, Data curation. Yang Jianzhou: Visualization, Data curation. Zhao Keqiang: Formal analysis, Data curation. Wang Zhenliang: Writing – review & editing, Writing – original draft, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following is the Supplementary data to this article.