Mineral Chemistry and In Situ LA-ICP-MS Titanite U-Pb Geochronology of the Changba-Lijiagou Giant Pb-Zn Deposit, Western Qinling Orogen: Implications for a Distal Skarn Ore Formation

Abstract

The giant Changba-Lijiagou (Ch-L) Pb-Zn deposit is in the northeast part of the Xicheng ore cluster, Western Qinling Orogen. The ore genesis remains controversial; it could be either a sedimentary exhalative genetic type or an epigenetic hydrothermal genetic type. Here, in situ titanite U-Pb dating for the two kinds of titanite is presented, yielding ages of 212.8 ± 3.0 Ma in the mineralized skarn ore and 214.6 ± 5.1 Ma in the host rock. These ages conform to the previously reported magmatic zircon age (229–211 Ma) based on the in situ zircon U-Pb dating of plutons in this district and the time of large-scale magmatic–hydrothermal activities in Western Qinling Orogen (229–209 Ma). Titanites occurring in mineralized skarn and those that are calcite-hosted are similar to hydrothermal-origin titanites in major element characteristics. The Eu anomalies in the two types of titanite record oxidizing conditions during the mineralization process. A mineral assemblage of garnet, pyroxene, riebeckite, biotite, and potash feldspar, replacing the albite, is well-developed in the deposit. The mineralogical and geochronological characteristics indicate that the Ch-L Pb-Zn deposit is a distal skarn deposit and the result of intensive tectonomagmatic processes in the Xicheng ore cluster during the process of the Late Triassic orogeny.

1. Introduction

The precise age limitations of mineral deposits can reveal the genesis of ores and associated geological processes. Unfortunately, such determinations are usually difficult due to a shortage of proper stages that can be unequivocally associated with mineralization. Titanite (ideally CaTi₂SiO₅) is a common accessory mineral in felsic igneous rocks and hydrothermal ore deposits. Titanite includes sufficient U in its framework for U-Pb geochronology [1] and has the advantage of a high closure temperature (660–700 °C) [2]. Hence, it has significant potential as a U-Pb geochronometer in various geological environments [3,4] and, when related to ore minerals, can be adopted for precisely dating mineralizing events.

In chemical substitutions, titanite includes a number of elements (e.g., Al, Fe³⁺, Nb) at the Ti site, as well as rare earth elements [5]. These elements in titanite are sensitive to temperature, pressure, oxygen fugacity (fO₂), water fugacity, and melt composition and can serve as a significant index of petrogenetic and metallogenic processes.

The Changba-Lijiagou (Ch-L) deposit is the second-largest Pb-Zn deposit in China. So far, a 13 Mt total Pb-Zn reserve has been estimated [6], with a validated 7 Mt Pb + Zn reserve. The genesis of the Ch-L Pb-Zn deposit remains controversial, as it can be interpreted as being either the SEDEX type—generated during the Middle Devonian and the Lower Carboniferous period—or the epigenetic hydrothermal type—formed during the Early Mesozoic [7,8]. The Ch-L deposit is regarded as the genesis of “stratiform” or “strata-bound” sphalerite, as it is not near the granitic intrusion. Discovering the precise age of mineralization provides important information about ore generation conditions and genetic processes. However, obtaining the precise dates using geochronological data of Pb-Zn deposits is difficult. Ma et al. [9] obtained an Rb-Sr isochron age of 389 ± 14 Ma for sphalerite, pyrite, albite, and barite from the Ch-L Pb-Zn deposit. Hu et al. [8] reported the outcomes of the Rb-Sr isotopic dating of sphalerite and pyrite sampled from the major orebody, yielding isochron ages of 222.3 ± 2.2 Ma to 222.0 ± 3.0 Ma. The genesis of the Ch-L Pb-Zn deposit is still uncertain.

In this study, based on detailed research on the metallogenic features of the Ch-L Pb-Zn deposit, mineralogy and chronology analyses on two kinds of titanite were undertaken to obtain precise age data for this deposit. The U-Pb age indicates the last time that mineralization occurred. Furthermore, we present major and trace elemental compositions of titanite in different positions of the orebody to better understand the factors controlling Pb-Zn mineralization. These reveal the ore-forming processes and improve our understanding of regional metallogeny.

2. Geological Setting

2.1. Geology of the Xicheng Ore Cluster

The Qinling Orogenic Belt is formed by the convergence of the North China block and the Yangtze block. The three tectonic units (the North China block, the South Qinling belt, and the north margin of the Yangtze block) are divided by two suture zones, the Shangdan suture and the Mianlue suture from north to south. During the Early Devonian period, the northward subducted Shangdan Ocean closed, resulting in the assemblage of the North China block and the Yangtze block. In the Carboniferous period, the paleo-ocean continued to expand and separate the South Qinling belt from the south margin of the North China block. An independent micro-plate, the South Qinling, was subducted beneath the Mianlue Ocean during the Permian and the Early Carboniferous. Because of the collision between the South Qinling and the Yangtze block, the Mianlue Ocean became extinct during the middle-to-late Triassic period. South Qinling and North Qinling transformed into post-collision collapse in the Early Jurassic period [10].

In terms of the regional aspect, the Xicheng polymetallic ore cluster belongs to the South Qinling belt in the E–W-trending thrust nappe of the Middle Devonian Xihanshui Formation [11]. The bounding of the ore cluster is seen by the E–W trending Huangzhuguan fault to the north and the Rentushan–Jiangluo fault to the south, and the main framework of the ore cluster is the Wujiashan anticlinorium (Figure 1).

With distribution in the Xicheng ore cluster, the lower part of the basement strata is mainly made up of early Mesoproterozoic metamorphic sedimentary layers and marble. Carbonaceous argillaceous clastic rocks, siliceous shale, and minor orthoschist make up the lower Paleozoic strata. Since the late Paleozoic age, the South Qinling belt has been linked to the Yangtze Basin. The Xicheng ore cluster is a normal Yangtze-type steady, shallow oceanic sedimentary generation.

The strata in the Xicheng ore cluster are mostly of Devonian age with several Triassic, Jurassic, and Tertiary strata, as well as Quaternary sediments (Figure 1). The Devonian strata almost strike E–W, including meta-sandstone, quartz schist, and marble of the Lower Devonian Wujiashan Formation (D₁w); bioclastic limestone, meta-sandstone and phyllite of the Middle Devonian Anjiacha Formation (D₂a¹); bioclastic limestone and sericite quartz-schist of the Middle Devonian Xihanshui Formation (D₂a²); and sandstone, limestone, and phyllite of the Upper Devonian Dongshan Generation (D₃d) [12]. The only exposure of Triassic strata is in a small area, with faulted contacts with both Devonian and Jurassic strata. It mostly includes calcareous slate and siltstone breccia (Figure 1). The Rentushan–Jiangluo fault controls the south boundary of the Jurassic strata.

This region has been influenced by stressed Late Triassic magmatism. The plutons include Caoguan quartz diorite in the western part, Huangzhuguan granodiorite and Changba monzogranite in the central part, Shapoli monzogranite–granodiorite in the south-central part, and Mishuling granodiorite in the eastern part. Moreover, dikes of diorite, granodiorite, and granitic aplite form as groups and are allocated across the ore cluster (Figure 1) [8].

2.2. Ore Geology

The Ch-L Pb-Zn deposit, to the south of the Huangzhuguan fault, tectonically ranges from the northern limb of the Ganyulang subsidiary syncline and the southern limb of the Wujiashan subsidiary anticline; both of these are from the northern limb of the E–W-trending Wujiashan anticlinorium. The strata dips steeply south and, in several cases, are overturned. Prevalent faults are divided into two groups: NWW/NW–SEE/SE-trending strike faults and NE–SW-trending cross faults (Figure 1). The F₁ NE–SW-trending cross-fault is formed in the central part. The F₁ fault is produced after the mineralization and classifies the deposit into the Changba section and the Lijiagou section at a high angle (Figure 1). The host rocks in the ore district are the Middle Devonian Anjiacha Formation (D₂a). From bottom to top, these comprise biotite–quartz schist, quartz schist, and mica–quartz schist with marble (D₂a^1-1); dolomitic marble and biotite–calcite–quartz schist (D₂a^1-2); and mica–quartz schist with marble (D₂a²) (Figure 2). The Huangzhuguan and Changba intrusions were placed into the Middle and Lower Devonian strata (Figure 2). Dikes are only found locally in the ore area, with several dikes of granodiorite and lamprophyre occurring with the mica–quartz schist in the south (Figure 2).

The orebodies of the Ch-L Pb-Zn deposit appear in the lower members of the Anjiacha Formation (D₂a¹) (Figure 3), which is formed mainly of marble and calcite–quartz schist. The orebodies conform to the host strata and display layered, layer-like, or lenticular shapes and rare vein types. Changba I, II, and Lijiagou I are hosted in marble; Changba III and Lijiagou II are found in schist. The main orebody (Lijiagou I and II) in marble, which experiences NWW-trending in strike faults, occurred as steeply as SSE-dipping (Figure 4).

The orebodies in marble are more complicated, created as massive sulfides, branches, and reintegration frameworks. The orebodies occurring in schist, however, develop as banded sulfides with laminated, disseminated, and brecciated structures (Figure 5a–f). In conclusion, the main ore types of this deposit are banded, massive, brecciated, laminated, and disseminated (Figure 5a–f).

Skarnization more often occurs in the deep level of a stratum than at a shallow level. Skarnization is rare near the mine surface. At a depth of 900 m, there is weak micasization and tremolitization. In addition, actinolite and diopside are found at a depth of 850 m. The degree of skarnization increased with depth in this deposit. Skarnization was mostly discovered between the hanging wall (orebodies hosted in schist) and the footwall (orebodies hosted in marble). This alteration also occurred in the transitional zone of marble to schist, parallel to bedding. The distances between the skarnization and the deep orebodies ranged from several meters to a dozen meters.

The ore minerals are sphalerite, galena, pyrite, pyrrhotite, and arsenopyrite, with rare chalcopyrite. The major gangue minerals are mainly calcite and quartz with minor biotite, amphibole, potash feldspar, anorthite, and titanite, and the trace gangue minerals are pyroxene, garnet, albite, hyalophane, tourmaline, and zircon. Based on field and petrographic observations, the mineralizing processes at Changba can be divided into three stages (Figure 6).

Stage I is the sphalerite–pyrite arsenopyrite–marcasite-titanite phase (Figure 5a–c,f, Figure 7a–h and Figure 8a–i). The sphalerite is pale yellow, euhedral, and equigranular, with a granularity of 0.1–0.2 mm. The ore is banded, laminar, or brecciated sphalerite with a low Fe bearing. The silicification is related to the granular quartz and generated pyrite with pits (Py-1) (Figure 8c). Calcite, quartz, barite, and titanite (Figure 7c–h,) are the main gangue minerals. Marcasite or arsenopyrite, Py-2, or galena (Figure 8c) replaces Py-1.

Stage II is the black-brown cryptocrystalline sphalerite–galena–pyrite–chalcopyrite stage with characteristics such as massive fine-grained dark-brown sphalerite (n µm, Sp-2) (Figure 5f and Figure 7a,b,g,i,j). The ore is massive with high-Fe sphalerite, and Sp-2 is always intergrown with pyrrhotite (Figure 7j) or galena. The black-brown sphalerite (Sp-2) replaced the early calcite with chalcopyrite exsolutes. Stage II sphalerite (n × 10² µm) is always overgrown with Stage I sphalerite (nµm) (Figure 7g). The pyrite has several growth zonings (Figure 7m). The massive ore of Stage II is in contact with the banded ore of Sp-1 in the form of rhythm strips.

Stage III is the calcite–quartz–sulfide phase. The end of the hydrothermal mineralization has characteristics such as the appearance of the calcite and the calcite vein cutting of the laminated, banded massive orebody (Figure 5e and Figure 7k,l).

The wall rock alteration during the main stage comprised biotitization, garnetization, albitization, potash feldspathization, silicification, and tremolitization (Figure 7m–o and Figure 8a–h).

3. Sampling and Analytical Methods

3.1. Samples

More than 60 samples were gathered from this deposit, from which thin sections for photomicrographic research and in situ analysis (

Table 1. Samples from the Ch-L Pb-Zn deposit for titanite LA-MC-ICP-MS dating.

Sample No.	Location	Mineral	Description
LJG-713	Level 1070, line 94, ore in marble	titanite	Banded ore is rich in sphalerite. The ore band is dark brown aphanitic in the center and reddish brown fine-grained on the edge, interbedded with the grayish white marble. The titanite is euhedral, 30–100 μm, coexisting with the sphalerite.
LJG-714	Level 1070, line 94, ore in marble	titanite	Banded ore is rich in sphalerite, dark brown fine-grained sphalerite, interbedded with grayish-white marble. The titanite is euhedral, 80–150 μm, coexisting with the sphalerite and biotite.
LJG-715	Level 1070, line 94, ore in marble	titanite	Massive dark brown sphalerite with dense disseminated coarse- or medium-grained pyrite. The titanite is euhedral, 60–120 μm, coexisting with the sphalerite, galena, and pyrite.
XKD-722	Level 800, line 57, ore in marble	titanite	Massive dark brown sphalerite with dense disseminated coarse- or medium-grained pyrite. The titanite is euhedral, 80–150 μm, coexisting with the sphalerite, galena, and pyrite.
XKD-727	Level 800, line 57, ore in marble	titanite	Banded ore is rich in sphalerite. The ore band is dark brown aphanitic in the center and reddish-brown and fine-grained on the edge, containing dense disseminated pyrite. The titanite is euhedral, 80–150 μm, coexisting with the sphalerite, galena, and pyrite.
LJG-719	Level 1082, line 83, host rock	titanite	Grayish-green and white banded marble with silicification, containing medium-grained pyrite in white marble. The titanite is euhedral, 300–500 μm, intergrown with chalcopyrite and potash feldspar in the marble.
LJG-720	Level 1082, line 83, host rock	titanite	Skarnization occurred in marble containing a quartz–calcite vein and less sphalerite. The titanite is euhedral, 100–300 μm, intergrown with potash feldspar and biotite in the marble. There is zircon occasionally.
LJG-726	Level 1070, line 94, host rock	titanite	Grayish-green and white banded marble with silicification, containing medium-grained pyrite in white marble. The titanite is euhedral, 50–200 μm, coexisting with the potash feldspar and galena in a quartz vein.

) were produced from the two types of samples (the orebody and the host rock). The samples of ores were collected from Lijiagou II orebodies (94 prospecting baseline) at a depth of 1070 m (Figure 5a). Samples LJG713, LJG714, and XKD727 (type I, banded ore) were composed of sphalerite, pyrite, calcite, quartz, biotite, barite, riebeckite, titanite, and chlorite, as well as minor amounts of garnet (Figure 5b, Figure 7a,f and Figure 8e,f; Table 1). The pyrite in this ore type was usually replaced by marcasite or arsenopyrite (Figure 8c). Samples LJG715 and XKD722 (type Ⅱ, massive ore) were composed of sphalerite, pyrite, galena, calcite, quartz, titanite, potash feldspar, chalcopyrite, and pyrrhotite (Figure 5a, Figure 7a,b,g,i,j and Figure 8h; Table 1). The sphalerite was cryptocrystalline and was intergrown with galena or pyrrhotite. Titanite is widely distributed in these two types of ore, is enclosed in sphalerite, and is closely associated with silicification (Figure 7d and Figure 8f,g).

Samples LJG719, LJG720, and LJG726 (host rock) were collected from the footwall of Lijiagou II orebodies (94 prospecting baseline) at a depth of 1070 m (Figure 8b). The titanite grains explored from five mineralized and three barren samples, which coexist with calcite, quartz, potash feldspar, albite, anorthite, tourmaline, hyalophane, apatite, and a small amount of zircon (Figure 8a–h). Titanite is intergrown with potash feldspar, quartz, and calcite (Figure 8e–f). The dating samples from the host rock were not far from the main orebody in the field observation.

The samples were created as thin sections on which the experiments were performed directly. The morphological characteristics and internal structures of titanite were identified by electron backscatter diffraction (EBSD, MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China). According to the outcomes, the two types of titanite are subhedral and lack any oscillatory zoning, as highlighted in the images (Figure 8i).

3.2. Titanite Major Element Analysis

Proper crystals and points were chosen for the analysis of the main element concentrations. Major element compositions of titanite grains were determined at the MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, with a JEOL JXA-Ihp200F Electron Probe Micro Analyzer equipped with four/five wavelength-dispersive spectrometers. A ca. 20 nm thin conductive carbon film coated the samples before analysis. Yang et al. [14] describe the EPMA approach in detail. During the research, minerals are analyzed with an increasing voltage of 15 kV, a beam current of 20 nA, and a 5 µm spot size. Natural minerals and synthetic oxides were adopted as standards. Data were gathered online, and an altered ZAF (atomic number, absorption, fluorescence) correction process was applied.

3.3. In Situ Titanite U-Pb Dating and Trace Element Analysis

The U-Pb geochronology of titanite was determined by using LA-ICP-MS at Nanjing FocuMS Technology Co., Ltd. (Nanjing, China) An Australian Scientific Instruments RESOlution LR laser-ablation system (Canberra, Australian) and Agilent Technologies 7700x quadrupole ICP-MS (Hachioji, Tokyo, Japan) were integrated for the experiments. Homogenized by a set of beam delivery systems, the 193 nm ArF excimer laser was used to assess titanite surfaces with a fluence of 3.5 J/cm². The ablation protocol adopted a spot diameter of 33 um at a 6 Hz repetition rate for 50 s. Helium was adopted to move aerosol to ICP-MS as a carrier gas.

As an external standard, titanite BRL-1 was adopted to modify instrumental mass discrimination and elemental fractionation during the ablation [15]. Titanite Ontario was adopted as quality control for geochronology [15]. The external calibration of trace elements against NIST SRM 612 and 610 was made by applying a 100% normalization measure without adopting an internal standard by ICPMSDataCal software. Titanites containing abundant Pb were further corrected by the 207Pb-method in the Tera–Wasserburg concordia diagram (Storey et al., 2006) [16].

4. Results

4.1. Chemical Composition of Titanite

The analytical data of titanite collected from the Ch-L deposit are given in

Table 2. Electron microprobe analyses (wt%) of titanite from the Ch-L Pb-Zn deposit.

Sample	Na2O	F	Al2O3	MgO	K2O	FeO	SiO2	Cl	CaO	TiO2	MnO	Total
Hydrothermal titanite in mineralized skarn
LJG713-2-1	0.04	0.86	3.21	0.02	0.04	-	30.57	0.01	28.43	36.92	0.02	99.79
LJG713-2-2	0.08	1.31	3.39	0.02	0.03	-	30.74	0.01	27.87	36.84	0.02	99.76
LJG713-3-1	0.02	0.99	3.12	0.02	-	-	30.67	-	28.36	36.95	0.04	99.76
LJG713-3-3	0.11	1.03	3.78	0.05	0.01	-	30.49	-	28.49	36.47	0.01	100.00
LJG714-4-1	0.09	1.21	3.20	0.02	0.05	-	30.68	-	27.92	36.87	0.05	99.78
LJG714-4-2	-	0.93	3.47	0.02	-	0.02	31.02	-	28.51	36.40	0.01	100.00
LJG714-4-3	0.06	1.16	3.26	0.02	0.02	0.02	30.94	0.01	28.20	36.53	0.06	99.79
LJG714-4-4	0.09	1.10	5.00	0.01	1.86	0.04	33.62	-	25.02	33.37	0.05	100.00
LJG715-1-1	0.01	1.10	3.18	0.02	-	0.01	30.85	-	28.64	36.64	0.02	100.00
LJG715-1-2	0.07	1.09	3.53	-	0.02	-	30.53	0.01	28.31	36.53	0.06	99.77
LJG715-1-3	0.03	1.64	5.59	0.05	0.02	0.01	30.66	0.01	28.93	33.76	-	100.00
XKD722-1-2	0.02	1.11	2.99	0.01	0.03	0.01	29.79	0.01	28.49	37.90	-	99.88
XKD722-1-3	0.05	1.17	3.26	0.01	0.02	-	30.40	0.02	28.54	36.68	-	99.65
XKD722-1-4	0.05	0.76	2.76	-	0.02	0.05	30.55	-	28.49	37.45	0.01	99.82
XKD722-1-5	0.04	1.18	4.30	0.01	0.01	0.05	30.71	-	28.65	35.19	-	99.63
XKD722-3-1	0.05	0.85	3.29	-	0.02	0.01	30.72	-	28.84	36.47	-	100.00
XKD722-3-2	0.03	0.62	2.01	-	0.01	0.01	30.60	-	28.32	38.65	-	100.00
XKD722-3-4	0.14	1.82	5.89	0.07	0.04	0.06	30.72	0.01	28.52	33.49	-	100.00
XKD727-3-1	0.02	1.76	5.95	0.03	0.01	0.04	29.83	-	29.18	33.73	-	99.81
XKD727-3-2	0.02	1.27	3.93	-	-	-	30.61	0.01	28.98	35.51	-	99.80
XKD727-3-3	0.01	1.62	5.59	0.02	-	0.01	31.00	-	28.64	33.79	-	100.00
Calcite-hosted titanite
LJG719-2-1	0.12	1.80	4.58	0.01	0.07	0.01	31.04	-	27.82	34.97	0.01	99.67
LJG719-2-3	0.07	1.93	4.60	0.03	-	0.10	30.67	0.01	28.51	34.68	0.03	99.80
LJG719-2-4	-	1.86	5.17	-	0.01	-	31.31	-	28.69	33.59	0.03	99.87
LJG719-2-5	0.24	1.84	4.87	0.02	0.05	0.04	32.11	0.02	27.70	33.72	0.02	99.85
LJG719-2-7	0.02	1.66	4.90	0.03	0.01	-	30.46	0.01	28.73	34.88	-	100.00
LJG719-3-1	-	1.35	4.76	0.02	0.02	-	30.91	0.01	28.48	34.55	0.02	99.54
LJG719-3-2	0.01	2.04	4.67	0.04	0.01	-	31.27	-	28.49	34.30	0.03	100.00
LJG719-3-3	0.08	1.99	4.84	0.04	0.03	-	31.47	0.01	28.80	33.29	-	99.70
LJG719-3-4	-	1.05	4.76	0.01	0.01	-	31.19	-	28.75	34.39	0.03	99.73
LJG720-1-1	0.07	1.93	4.96	0.05	0.01	-	31.00	-	28.69	33.89	0.05	99.84
LJG720-1-4	0.09	1.04	3.76	0.02	0.02	-	30.99	-	28.50	35.94	0.03	99.95
LJG720-1-5	0.03	1.80	5.52	0.01	-	0.01	30.89	-	28.77	33.44	-	99.73
LJG720-1-6	0.08	1.22	4.53	0.02	0.04	0.03	31.31	-	28.68	34.52	-	99.90
LJG720-2-2	-	2.39	7.29	0.06	-	-	30.86	-	28.94	31.21	0.01	99.76
LJG720-2-3	0.29	1.65	4.75	0.02	0.04	0.04	30.62	-	28.69	34.48	0.01	99.89
LJG720-2-4	0.19	2.09	4.87	0.03	0.04	-	31.37	-	28.36	33.90	0.04	100.00
LJG720-2-5	0.03	1.82	5.44	0.02	-	0.02	30.88	0.01	28.69	33.60	-	99.72
LJG720-2-6	0.07	1.75	4.54	0.03	0.02	0.02	31.17	-	28.47	34.67	0.02	100.00
LJG720-2-7	0.06	1.77	4.43	0.03	0.01	0.02	31.55	0.01	28.56	34.12	-	99.80
LJG720-2-8	0.08	2.67	6.42	0.03	0.06	0.02	30.90	-	28.47	32.50	-	100.00
LJG720-2-9	0.10	2.05	4.69	0.03	0.08	0.08	30.71	0.01	28.24	34.51	0.01	99.63
LJG720-2-10	0.02	1.75	4.38	0.01	-	-	32.11	-	28.57	33.90	-	100.00
LJG720-2-11	0.02	1.75	4.62	0.02	-	-	31.14	-	28.16	34.12	0.03	99.72
LJG720-2-12	0.03	1.28	6.07	0.10	0.01	-	31.22	-	28.74	33.03	0.04	99.97
LJG720-2-13	0.04	1.33	4.60	0.01	-	0.02	31.46	0.01	28.82	34.27	0.01	100.00
LJG720-2-15	-	1.81	4.71	-	-	-	31.41	-	28.76	33.71	0.02	99.67
LJG720-2-16	0.11	1.98	4.50	-	0.06	0.01	29.97	-	28.37	35.84	-	100.00
LJG720-2-18	-	1.29	4.41	0.02	0.02	-	30.03	-	28.79	35.68	-	99.70
LJG720-2-19	0.02	1.34	3.52	0.01	0.02	-	29.79	-	28.77	36.92	0.04	99.85
LJG726-2-1	0.02	1.24	4.91	0.02	0.01	-	30.77	0.01	28.82	34.49	-	99.78

“-“ means the content is below the detection line.

. Titanite in the orebody shows relatively unified SiO₂ (29.79%–33.62%) and CaO (25.02%–29.18%), high TiO₂ (33.37%–38.65%), and low Al₂O₃ + FeO_T (2.02%–5.99%) and F (0.62%–1.82%) (Table 2; Figure 9). Titanite in the host rock shows relatively unified SiO₂ (29.79%–32.11%) and CaO (27.70%–28.94%), low TiO₂ (31.21%–36.92%), and high Al₂O₃ + FeO_T (3.54%–7.29%) and F (1.04%–2.67%) (Table 2; Figure 9).

Figure 10 and

Table 3. LA-ICP-MS data of hydrothermal titanite trace element data (×10⁻⁶) from the Ch-L Pb-Zn deposit.

Sample No.	La	Ce	Pr	Nd	Sm	Eu	Gd	Tb	Dy	Ho	Er	Tm	Yb	Lu	Y	Zr	Hf	ΣREE	Lu/Hf
Hydrothermal titanite in mineralized skarn
LJG713-2-1	154.2	774.2	147.7	778.1	215.8	31.8	190.4	28.0	159.8	30.9	79.4	11.0	69.3	8.3	785.1	34.2	3.5	3464.0	2.399
LJG713-2-2	118.5	544.4	97.3	480.5	117.0	14.2	98.3	12.9	69.3	13.4	36.1	5.3	35.6	5.2	365.7	46.9	3.1	2014.0	1.704
LJG713-3-1	265.9	1260.0	211.2	1018.0	239.6	44.5	176.5	22.0	103.4	17.0	40.8	5.7	35.9	4.5	433.3	46.4	4.6	3879.0	0.976
LJG713-3-2	259.1	1262.0	223.8	1137.0	311.9	57.3	288.6	44.7	257.7	48.1	120.9	16.2	99.5	11.4	1188.0	52.4	4.6	5327.0	2.463
LJG713-3-3	229.6	1195.0	224.7	1198.0	359.4	50.0	351.5	56.8	337.8	64.0	157.7	20.4	122.9	13.6	1560.0	43.0	4.1	5941.0	3.364
LJG713-4-1	131.7	723.6	151.4	845.0	239.7	30.1	180.0	22.8	115.9	20.9	54.9	7.8	53.7	7.5	548.9	239.9	9.6	3134.0	0.782
LJG714-4-1	256.5	1210.0	195.4	919.5	215.3	27.0	174.5	24.3	136.1	26.3	69.6	10.0	66.0	8.8	712.6	42.3	3.9	4052.0	2.274
LJG714-4-2	172.2	931.3	193.7	1105.0	365.4	46.2	395.2	68.0	429.7	81.4	204.4	26.8	155.8	17.1	1985.0	44.4	4.8	6177.0	3.587
LJG715-1-1	135.2	714.1	141.9	748.5	187.1	30.6	143.0	17.9	94.3	18.7	52.6	8.0	55.0	8.3	514.8	78.0	8.0	2870.0	1.032
LJG715-1-2	186.1	790.5	124.4	556.9	121.6	15.9	88.8	11.1	58.1	12.7	38.1	6.5	51.6	9.3	345.5	43.4	4.1	2417.0	2.284
LJG715-1-3	205.8	1058.0	202.0	1078.0	324.5	59.6	325.1	52.0	312.3	58.3	142.0	18.5	104.8	10.1	1493.0	39.5	4.3	5444.0	2.348
XKD722-1-1	305.3	1511.0	265.8	1372.0	387.1	70.6	356.9	55.3	324.4	60.1	148.6	19.7	118.7	13.4	1456.0	59.5	5.5	6465.0	2.431
XKD722-1-2	210.2	1047.0	195.1	1032.0	306.3	50.7	298.4	48.0	284.9	53.5	133.4	17.7	107.6	12.0	1323.0	41.7	4.1	5120.0	2.891
XKD722-1-3	220.9	1146.0	216.8	1174.0	357.9	57.4	362.6	60.2	365.3	68.4	168.3	22.1	129.1	13.5	1710.0	43.6	4.3	6072.0	3.164
XKD722-3-1	199.8	979.3	172.6	859.1	202.6	31.1	153.8	19.7	101.4	18.5	48.6	7.0	48.1	7.1	488.8	38.4	2.6	3337.0	2.750
XKD722-3-2	171.2	831.7	161.4	868.4	286.4	39.8	324.2	58.8	377.5	72.1	180.6	23.6	134.3	13.6	1715.0	53.7	5.7	5259.0	2.365
XKD722-3-3	301.6	1614.0	291.8	1526.0	381.2	43.4	289.6	36.0	178.8	32.8	87.7	12.8	89.4	13.9	811.0	35.3	3.4	5710.0	4.121
XKD722-3-3	323.3	1835.0	395.5	2423.0	873.5	44.7	698.6	90.9	438.1	73.3	188.3	26.2	180.2	23.1	1787.0	60.9	5.9	9400.0	3.943
Calcite-hosted titanite
LJG719-2-1	154.7	749.9	136.0	655.8	159.8	26.0	139.5	22.5	139.7	26.5	67.4	8.9	51.2	4.8	607.9	32.2	3.2	2951.0	1.527
LJG719-2-2	127.9	626.2	120.9	625.1	179.4	38.2	176.1	27.9	158.6	26.4	58.1	6.9	35.3	2.9	626.7	26.1	2.4	2837.0	1.214
LJG719-2-3	154.6	725.2	132.3	635.1	146.6	33.5	121.6	18.3	104.9	18.5	44.8	5.8	33.1	3.4	454.7	400.1	13.6	2632.0	0.248
LJG719-2-4	152.9	801.6	152.8	789.5	211.4	38.4	191.1	30.3	178.8	31.4	73.7	9.2	50.9	4.8	707.1	23.9	2.5	3424.0	1.932
LJG719-2-5	159.5	773.7	141.5	695.6	168.0	26.8	155.5	24.4	148.3	27.5	66.7	8.7	48.4	4.9	627.3	30.6	3.2	3077.0	1.515
LJG719-2-6	135.3	632.9	115.6	574.3	150.9	29.5	142.9	22.8	136.0	24.0	58.2	7.0	40.3	3.7	548.2	34.3	3.1	2622.0	1.188
LJG719-2-7	166.2	806.5	146.1	701.3	163.7	27.1	140.0	21.8	136.7	25.4	64.7	8.6	49.0	4.9	573.8	37.0	3.6	3036.0	1.362
LJG719-3-1	167.0	815.2	155.0	810.0	234.4	47.9	213.1	34.5	204.1	33.1	72.0	8.4	42.7	3.6	716.3	23.4	2.3	3557.0	1.523
LJG719-3-2	188.7	916.6	167.4	833.7	207.0	33.9	176.2	27.4	160.9	27.7	66.0	8.3	45.9	4.4	636.4	40.3	3.6	3500.0	1.201
LJG719-3-3	152.4	774.4	151.4	798.4	230.6	49.7	201.3	31.3	176.6	27.7	59.1	6.7	32.5	2.6	666.8	24.1	2.4	3361.0	1.064
LJG719-3-4	135.3	643.7	117.1	576.0	142.4	26.5	132.1	20.7	129.1	23.8	58.3	7.5	43.0	4.2	543.8	33.1	3.1	2603.0	1.351
LJG719-3-5	219.2	1286.0	269.7	1527.0	511.2	52.1	425.9	68.3	369.8	54.6	113.0	12.5	62.4	4.8	1174.0	18.8	2.5	6151.0	1.953
LJG720-1-1	134.8	660.9	127.3	647.6	187.1	37.4	172.9	27.0	155.3	25.7	56.9	6.8	35.4	3.0	593.8	30.3	3.1	2872.0	0.965
LJG720-1-2	169.3	809.3	145.7	696.7	165.1	25.7	141.8	22.6	139.5	25.3	62.7	8.1	45.8	4.4	572.5	33.7	3.5	3034.0	1.249
LJG720-1-3	139.1	675.1	124.6	619.3	158.8	27.9	156.3	24.9	152.1	27.6	65.9	8.3	46.8	4.4	615.8	28.2	2.8	2847.0	1.532
LJG720-1-4	107.2	503.5	94.7	489.7	138.9	29.1	131.4	20.2	118.3	19.9	44.2	5.2	28.6	2.5	444.6	32.5	3.2	2178.0	0.793
LJG720-2-1	118.6	752.1	156.1	836.5	210.9	62.8	157.1	20.2	111.0	19.1	47.9	6.3	39.1	4.7	510.6	42.4	3.6	3053.0	1.328
LJG720-2-2	107.9	828.7	199.7	1190.0	345.4	67.4	246.7	31.4	161.9	27.3	64.9	8.3	51.5	6.5	681.0	30.9	3.5	4019.0	1.841
LJG720-2-3	160.5	901.7	173.9	874.8	195.6	42.0	130.5	16.5	91.0	18.1	52.3	7.9	59.4	9.1	443.2	39.3	3.6	3176.0	2.499
LJG720-2-4	105.4	751.6	182.7	1125.0	387.5	55.9	316.1	45.0	229.6	34.3	71.2	7.9	39.3	3.4	853.0	64.1	4.8	4208.0	0.705
LJG720-2-5	166.5	918.6	157.9	755.4	165.7	46.6	121.9	15.0	80.4	14.8	39.1	5.6	38.9	5.8	406.6	38.5	3.7	2939.0	1.554
LJG720-2-6	160.5	870.0	156.3	764.5	170.2	45.8	132.9	17.3	100.0	19.9	53.6	7.8	53.1	7.3	532.0	32.6	3.4	3091.0	2.125
LJG720-2-7	39.0	258.3	58.3	330.8	86.1	25.9	51.5	5.1	19.5	2.2	3.3	0.3	1.2	0.1	53.6	66.0	6.5	935.0	0.010
LJG720-2-8	142.1	813.1	167.6	898.0	249.1	53.3	205.6	29.2	165.6	31.0	77.8	10.1	60.2	6.5	778.3	37.9	3.5	3687.0	1.845
LJG720-2-9	168.7	943.3	182.4	971.7	257.3	47.4	188.8	24.9	136.1	23.5	58.2	8.0	49.4	6.0	615.5	37.5	3.7	3681.0	1.596
LJG720-2-10	137.8	966.8	215.8	1179.0	295.8	66.8	200.4	25.0	127.2	21.2	49.9	6.6	39.3	4.5	564.6	43.7	4.3	3901.0	1.042
LJG720-2-11	338.9	1325.0	206.0	982.1	242.6	61.4	191.7	27.9	152.0	25.4	60.0	7.5	42.5	4.3	603.5	79.7	2.3	4271.0	1.848
LJG720-2-12	161.9	984.0	214.6	1201.0	362.0	92.5	230.0	25.1	97.3	10.0	14.7	1.1	4.1	0.2	280.5	57.6	5.7	3679.0	0.042
LJG720-2-13	66.0	470.7	106.8	589.6	148.2	40.9	83.45	8.0	28.3	2.8	3.8	0.3	0.9	0.1	69.8	179.9	10.4	1620.0	0.006
LJG720-2-14	106.7	490.4	94.9	504.6	151.7	39.9	110.8	13.7	63.5	7.8	14.0	1.4	6.3	0.4	185.8	65.2	6.2	1792.0	0.065
LJG720-2-15	124.4	791.1	169.0	941.8	257.5	42.4	204.7	27.8	153.1	27.7	70.5	9.4	59.5	7.2	707.6	34.9	3.4	3594.0	2.131
LJG726-2-1	31.5	162.9	36.6	222.2	79.4	24.0	55.8	6.3	27.2	3.5	5.6	0.6	2.6	0.2	79.4	61.4	6.3	738.0	0.026
LJG726-2-2	133.4	991.8	239.9	1471.0	482.6	65.6	379.8	53.3	273.2	41.4	85.7	9.6	50.1	4.5	1014.0	37.3	3.3	5295.0	1.390
LJG726-2-3	252.3	1466.0	289.2	1581.0	401.7	54.9	281.5	36.6	194.7	33.2	77.6	9.8	56.4	6.2	860.0	55.0	5.4	5601.0	1.142

,

Table 4. In situ U-Pb isotope data for the titanite in mineralized skarn.

Analysis #	Pb	U	Th	Th/U	Measured Isotopic Ratios								207Pb-Corrected Ages (Ma)
	ppm	ppm	ppm		Pb207/Pb206	1σ	Pb207/U235	1σ	Pb206/U238	1σ	Pb208/Th232	1σ	Pb206/U238	1σ
LJG715-1	122.43	274	93.56	2.92	0.57788	0.00447	11.23120	0.20497	0.03548	0.00237	1.83974	0.12118	224.7	13.4
LJG715-2	8.52	217	79.39	2.73	0.05055	0.00234	0.23712	0.01155	0.03410	0.00054	782.39588	51.91849	216.2	3.4
LJG715-3	20.32	274	115.17	2.38	0.19988	0.00251	1.14270	0.01587	0.03281	0.00045	28.59561	0.78140	208.1	2.8
LJG715-4	36.03	209	70.64	2.97	0.39263	0.00434	3.71615	0.07686	0.03489	0.00094	34.71689	2.44654	221.1	5.7
LJG715-5	118.42	93	59.91	1.55	0.68410	0.01039	32.84093	0.72439	0.03491	0.00622	0.60505	2.40627	221.2	5.4
XKD722-1	6.73	118	85.82	1.37	0.11947	0.00403	0.61789	0.02493	0.03336	0.00067	0.05435	1.60290	211.5	4.2
XKD722-2	73.42	80	92.07	0.87	0.50103	0.01718	11.48482	0.33606	0.05121	0.00385	0.00472	1.64284	321.9	6.8
XKD722-3	7.05	75	94.18	0.79	0.18763	0.00773	1.31677	0.07050	0.03491	0.00096	0.00426	1.14413	221.2	5.9
XKD722-4	21.60	262	103.27	2.53	0.22682	0.00556	1.44592	0.06409	0.03332	0.00099	0.00096	0.97121	211.3	6.1
XKD722-5	20.34	225	83.27	2.70	0.25208	0.00462	1.62151	0.03881	0.03298	0.00067	0.00263	0.96331	209.2	4.2
XKD722-6	38.78	261	78.00	3.35	0.37282	0.00571	3.07559	0.06533	0.03227	0.00087	0.00041	1.12898	204.8	4.7
XKD722-7	87.32	190	121.05	1.57	0.58485	0.00529	11.18409	0.16940	0.03360	0.00184	0.00496	2.85096	213.0	10.4
XKD722-8	12.97	171	156.65	1.09	0.09519	0.00664	0.47187	0.02625	0.03313	0.00081	0.00073	0.83414	210.1	5.0
XKD722-9	3.98	36	38.32	0.93	0.27721	0.00795	1.88355	0.05760	0.03408	0.00080	0.00043	1.24961	216.0	4.9
XKD722-10	6.17	88	81.70	1.08	0.16387	0.00424	0.91095	0.02376	0.03432	0.00055	0.00798	1.05586	217.5	3.4
XKD727-1	121.90	432	158.74	2.72	0.49943	0.00506	6.59159	0.11512	0.03475	0.00146	0.06745	0.00292	220.2	8.6
XKD727-2	4.86	40	45.38	0.88	0.30330	0.00824	2.24480	0.05386	0.03465	0.00088	0.08037	0.00177	219.6	5.4
XKD727-3	12.51	186	156.00	1.19	0.15926	0.00295	0.89381	0.02132	0.03426	0.00056	0.19115	0.00583	217.2	3.5
XKD727-4	39.64	100	98.38	1.01	0.55270	0.01097	10.38180	0.44312	0.03725	0.00432	15.67269	1.51140	235.8	3.8

# means the number of the sample.

and

Table 5. In situ U-Pb isotope data for the titanite in host rock.

Analysis #	Pb	U	Th	Th/U	Measured Isotopic Ratios								207Pb-Corrected Ages (Ma)
	ppm	ppm	ppm		Pb207/Pb206	1σ	Pb207/U235	1σ	Pb206/U238	1σ	Pb208/Th232	1σ	Pb206/U238	1σ
LJG719-6	71.59	48	76.32	1.60	0.38185	0.00781	3.47351	0.09698	0.03342	0.00121	0.04710	0.00093	211.9	13.8
LJG719-7	54.37	97	115.50	1.19	0.49817	0.01042	8.00059	0.18619	0.03548	0.00216	0.15582	0.00459	224.7	20.5
LJG720-1	17.51	30	69.75	2.33	0.34076	0.02085	4.67447	0.37937	0.03403	0.00383	0.04799	0.00327	215.7	5.6
LJG720-2	17.53	78	106.14	1.35	0.62662	0.00798	21.23110	0.51117	0.03521	0.00508	0.28176	0.00423	223.1	4.6
LJG726-1	25.55	138	53.50	0.39	0.44994	0.00601	5.47519	0.13014	0.03467	0.00131	0.26554	0.00620	219.7	6.2
LJG726-3	20.54	122	99.88	0.82	0.51735	0.00590	8.45942	0.22496	0.03572	0.00239	0.19709	0.00468	226.3	4.3
LJG726-4	63.24	134	87.48	0.65	0.59518	0.00416	13.43099	0.23589	0.03561	0.00249	0.38189	0.00558	225.5	15.5
LJG726-6	10.83	97	75.34	0.77	0.39953	0.00568	3.79732	0.06509	0.03478	0.00093	0.09434	0.00142	220.4	4.5
LJG726-7	31.08	119	83.59	0.70	0.33932	0.00525	2.85320	0.05708	0.03489	0.00076	0.07910	0.00153	221.1	10.2
LJG726-9	10.09	128	110.12	0.86	0.62664	0.00546	22.46797	0.67098	0.03179	0.00703	0.46245	0.00857	201.7	2.5
LJG726-10	26.62	136	84.08	0.62	0.34079	0.00541	2.94257	0.04849	0.03334	0.00072	0.09587	0.00150	211.4	6.6
LJG726-11	27.86	105	87.11	0.83	0.60059	0.00423	14.97072	0.26411	0.03279	0.00284	0.33964	0.00554	208.0	5.2
LJG726-13	27.10	119	88.43	0.74	0.23516	0.00464	1.55384	0.04203	0.03346	0.00073	0.04144	0.00085	212.2	7.8
LJG726-15	8.51	159	128.78	0.81	0.22353	0.00712	1.62665	0.06709	0.03408	0.00120	0.06512	0.00398	216.0	5.0
LJG726-17	8.22	169	94.16	0.56	0.37762	0.00555	3.36434	0.08782	0.03368	0.00109	0.11631	0.00281	213.6	7.4
LJG726-19	66.93	157	84.00	0.54	0.55894	0.00585	10.25499	0.25063	0.03354	0.00254	0.36408	0.00864	212.6	14.5
LJG726-21	42.61	138	67.28	0.49	0.50962	0.00450	7.20173	0.10820	0.03385	0.00129	0.27894	0.00388	214.6	7.6
LJG726-22	7.75	149	51.28	0.34	0.10731	0.00235	0.55687	0.01228	0.03493	0.00046	0.03040	0.00054	221.3	2.9
LJG726-23	7.17	154	53.22	0.35	0.08542	0.00250	0.41866	0.01118	0.03436	0.00039	0.02271	0.00212	217.8	2.4
LJG726-24	9.35	150	49.37	0.33	0.16655	0.00396	0.88225	0.02331	0.03201	0.00055	0.04925	0.00108	203.1	3.4
LJG726-25	10.34	141	45.52	0.32	0.21766	0.00639	1.34647	0.05254	0.03291	0.00075	0.07140	0.00283	208.7	4.6

# means the number of the sample.

list the trace element concentrations of the two kinds of titanite grains. The titanites from the mineralized skarn had slightly higher concentrations of U (36–432 ppm; average: 132) and Th (38.32–158.74 ppm, average 90.18), with Th/U ratios ranging from 0.30 to 1.79 (Table 4). The concentrations of Hf and Lu were 2.6–9.6 and 4.6–23.1, respectively, with Lu/Hf proportions of 0.782–4.121 (average: 2.49) (Table 3). Moreover, the titanite grains from the mineralized skarn showed relatively higher ΣREE concentrations (2014.0–9400.0 ppm; average: 4782.0 ppm) and were slightly depleted in HREE, with LREE/HREE proportions varying from 0.814 to 3.623 (average: 1.850) (Table 3). On chondrite-normalized REE diagrams, these titanites showed moderate negative Eu anomalies (Eu * = 0.84–0.99) (Figure 10).

The titanites from calcite had slightly lower concentrations of U (30–169 ppm; average: 122) and Th (45.52–128.78; average: 81.47), with Th/U proportions varying from 0.32 to 2.33, which are not very different from those recorded for titanite in mineralized skarn (Table 5). The concentrations of Hf and Lu were 0.1–9.1 ppm and 2.3–13.6 ppm, respectively, with a Lu/Hf proportion of 0.006–2.499 (average: 1.23) (Table 3). On the other hand, calcite-hosted titanite grains had relatively lower LREE concentrations than titanite in mineralized skarn, ranging from 31.5 to 338.9 ppm (average: 146.7) (Table 3). On chondrite-normalized REE diagrams (Figure 10), these titanites showed slight fractionation between LREE and HREE (LREE/HREE = 1.535–7.200; average: 2.432). The titanites showed moderate negative Eu anomalies (Eu^* = 0.3–1.1) (Table 3, Figure 10).

4.2. Titanite U-Pb Aging

The U-Th-Pb isotopic data for titanite grains explored herein are summarized in Table 4 and Table 5. The titanite grains were euhedral to subhedral in shape, 80–120 µm long, and 40–100 µm wide (Figure 8i), and had characteristics such as being homogeneous and lacking any oscillation according to EBSD imaging (Figure 8i).

The U-Pb age data acquired herein are presented in Table 4 and Table 5 and shown graphically in Figure 11. Most zircons and several titanites included negligible common Pb; therefore, the Stacey and Kramers [25] correction model for Pb isotopic data were suitable. However, for titanite with a higher common Pb content, the application of the Pb isotopic composition correction model may not be appropriate [1]. In this study, our analyzed titanite grains had high contents of common Pb, indicating that the selection of initial Pb isotopic composition can greatly affect the calculated ages [5]. This problem was solved by following [1], which involved plotting the uncorrected isotope data on a Tera–Wasserburg total Pb plot (²³⁸U/²⁰⁶Pb-²⁰⁷Pb/²⁰⁶Pb; Figure 11) and then fitting a regression line. The lower intercept age (²³⁸U/²⁰⁶Pb-²⁰⁷Pb/²⁰⁶Pb; Figure 11) represents the real age of the titanite. In the case of an unknown, the initial Pb isotope composition, this is a helpful method. The calculation of initial ²⁰⁷Pb/²⁰⁶Pb as the Y-intercept is made on the plot. Later, the weighted average age can be found by calculating the individual ²⁰⁷Pb-corrected ²⁰⁶Pb/²³⁸U ages. Recent studies using the common Pb correction approach have been effective in the U-Pb dating of hydrothermal titanite.

A total of 23 analyses were performed on 23 titanite grains from mineralized skarn samples LJG-713, LJG-714, LJG-715, XKD-722, and XKD-727. Table 4 lists the U-Pb isotope products of this type of titanite. On the Tera–Wasserburg diagram, a linear array is defined by uncorrected common Pb data, providing a lower-intercept age of 212.8 ± 3.0 Ma (MSWD = 1.7) (Figure 11a,b). With this common Pb composition, a common Pb correction was performed using the ²⁰⁷Pb-based approach. Fifteen analyses produced a weighted average ²⁰⁶Pb/²³⁸U age of 214.2 ± 2.3 Ma (MSWD = 1.05) (Figure 11a,b), conforming to the lower intercept age within the error range.

Thirty-three analyses were performed on thirty-three titanite grains from barren mineralized samples LJG-719, LJG-720, and LJG-726. Table 5 lists the U-Pb isotope results of this type of titanite. On the Tera–Wasserburg diagram, the common Pb-uncorrected data define a linear array, providing a lower intercept age of 214.6 ± 5.1 Ma (MSWD = 4.8) (Figure 11c,d). With this common Pb composition, the common Pb correction was performed, applying the ²⁰⁷Pb-based approach. Twenty-one analyses produced a weighted average ²⁰⁶Pb/²³⁸U age of 214.0 ± 3.7 Ma (MSWD = 3.1) (Figure 11c,d), conforming to the lower intercept age within error.

5. Discussion

5.1. Features and Origin of Titanite

Titanite (CaTiSiO₄O) is a frequent accessory mineral in igneous rocks, metamorphic rocks, and hydrothermal deposits. Their texture, paragenetic mineral assemblages, and geochemistry can distinguish between the various sources of titanite. The titanite grains in Ch-L-mineralized skarn are mainly subhedral and homogeneous, as shown in the EBSD images (Figure 8i), differing from the oscillatory zoning in magmatic titanite. Most grains coexist with retrograde potash feldspar and biotite, indicating a hydrothermal origin. The titanite grains in the host rock share features with those in the mineralized skarn and have the same origin. Titanite geochemistry corroborates this conclusion.

Figure 9a shows a strong negative association between TiO₂ and FeO_T + Al₂O₃; Figure 9b shows a positive correlation between F and FeO_T + Al₂O₃. Titanite consists of chains of corner-sharing Ti octahedral sites and Si tetrahedral sites, enclosing a large seven-fold Ca site. The mineral includes large ion elements, such as Al, Fe, and F, in its crystal lattice [5]. Later, it appeared likely that the main substitution system in our data were (Al, Fe)³⁺ + (F, OH)⁻ = Ti⁴⁺ + O²⁻, conforming to previous research [18,26].

As shown in Figure 7a,b, the titanite grains from the Ch-L orebody and host rock share similar compositions to hydrothermal titanite. Titanites from the orebody fall on the left-hand part of the diagram, with higher Ti and lower Al, Fe, and F contents than the titanite from the host rock, indicating that the ore-forming fluid has a composition close to that of the magmatic fluid. The source of titanite can be determined by the Th/U proportion in titanite. Generally, hydrothermal titanite has a lower Th/U (mostly < 1) than magmatic titanite. The mineralized skarn titanite produced low Th/U (0.30–1.79; average: 0.92; mostly < 1; Table 4). The Th/U ratio of the host rock titanite mostly ranged from 0.32 to 1.73, with an average of 0.84 (eliminate extremum of 2.33). The Th/U ratio indicates that Ch-L is a typical hydrothermal deposit comparable to the hydrothermal titanite from other skarn deposits.

The titanite grains in mineralized skarn have moderate negative Eu anomalies (14.22–70.58 ppm; average: 41.38; Figure 10), with similar features to the titanite grains in calcite. The Eu-negative anomalies, implying the formation of REE in fluids, were controlled by a complex (OH⁻, F⁻, CO₂) instead of absorption [27]. According to past research, the redox conditions of titanite production can be monitored by the Eu anomalies in titanite. Under oxidizing circumstances, Ca²⁺ is usually replaced by REE. As Eu³⁺ has similar geochemical characteristics to Ca²⁺, Eu³⁺ will be included with Ca²⁺ in titanite, which records positive Eu anomalies [28]. When Eu exists as Eu²⁺, the radius (0.117 nm) is much larger than that of Ca²⁺; thus, it is difficult to replace in titanite. The titanite grains from the Pb-Zn deposit showed negative Eu anomalies (Figure 10), suggesting that they formed under oxidizing conditions.

5.2. Age of Pb-Zn Mineralization

The in situ titanite U-Pb dating of the orebody and host rock near the orebody show that the ²⁰⁷Pb-corrected weighted average ²⁰⁶Pb/²³⁸U ages (Figure 11b,d) and lower intercept ages in the Tera–Wasserburg Concordia diagrams (Figure 11a,c) of these titanite grains are about 214 Ma. The age error of the host rock is somewhat large, which may be due to the large range of Pb concentrations (7.17–71.59 ppm; average: 25.67 ppm, Table 5) and the high common Pb concentration of the host rock titanite grains, which influenced the accuracy of the age. The U-Pb ages of the two kinds of titanite acquired herein conform to the rock-forming ages of the ore-associated granitic rocks (229–211 Ma) [29]. The agreement of these ages shows that titanite U-Pb ages are reliable. The mineralization age of hydrothermal deposits can be acquired through titanite U-Pb dating.

Titanite U-Pb dating has been adopted for magmatic and metamorphic rocks but less often for hydrothermal mineralization. Dating hydrothermal minerals places direct age limitations on hydrothermal activities and the related metallogeny, which is a key aspect in ore deposit research. Therefore, increasing attention has been paid to the U-Pb dating of hydrothermal titanite.

Massive ores in the Ch-L deposit are related to retrograde mineral assemblages, such as titanite, which tends to coexist with potash feldspar. The traditional chemical and in situ U-Pb dating of titanite grains gathered from distal skarn deposits suggests that the mineral is a credible geochronometer. Hydrothermal titanite grains coexist with potash feldspar (Figure 8g) and provide a ²⁰⁶Pb/²³⁸U age of 212.8 ± 3.0 Ma (five samples) with in situ titanite U-Pb dating (Figure 8). Repeated research on three samples from host rock yielded a ²⁰⁶Pb/²³⁸U age of 214.6 ± 5.1 Ma (Figure 11), which supports the idea that the mineralization should be no later than the age indicated by the titanite U-Pb age. Considering the content of common Pb, we present the titanite age as the Tera–Wasserburg age. In this study, we used this initial Pb ratio as the titanite’s upper intercept, anchored by the upper intercept, which produces a more reasonable age. The common Pb component of the method adopted by Simonetti et al. [30] was determined by the change in the U/total Pb ratio of the uncorrected data. The upper intercept is the initial ²⁰⁷/²⁰⁶Pb proportion, which can also be obtained from non-U-bearing minerals such as plagioclase and pyrite Pb isotopes [31,32].

The best proof of a hydrothermal source of titanite comes from its texture, especially the mineral inclusions of Kf and galena in Figure 8i. The galena inclusion not only indicates the hydrothermal origin but also supports the notion that mineralization should be no later than the age constrained by the titanite U-Pb age.

Several previous studies have considered the metallogenic age of the Ch-L Pb-Zn deposit. Wang et al. [11] studied the Pb isotopes of sulfide in ore deposits, yielding an anomalous lead age of 232 Ma. Ma [9] studied the banded albitite and breccia albitite in the No. 2 orebody, obtaining an Rb-Sr isochronous age of 389.42 ± 13.95 Ma. Previous research on Rb-Sr isotopes of sulfide minerals from the deposit produced an isochron age of 222.0 ± 3.0 Ma for combined sphalerite and pyrite data [8]. However, albitite did not coexist with the ore minerals in our field observations, and the potassium alteration is more closely related to mineralization (Figure 8g).

The Erlihe Pb-Zn deposit, in the Fengtai ore cluster to the east of the Xicheng ore cluster, produced a sphalerite Rb-Sr isochron age of 220.7 ± 7.3 Ma and a pyrite Rb-Sr isochron age of 226 ± 17 Ma. The age of the hydrothermal mineralization of the Ch-L deposit is identical to that of the Pb-Zn in the Fengtai ore clusters.

5.3. Genesis of the Ch-L Pb-Zn Deposit

The metasomatism of Na, K, and Ba is widely developed in the Ch-L Pb-Zn deposit. The barium–potassium feldspar or hyalophane series minerals are distributed in the footwall (Figure 8f), and the separation of potassium and sodium during hydrothermal metasomatism is widespread in this deposit.

There are rare earth elements in minerals, barium feldspar, potash feldspar, and albite in different stages of mineralization. We can infer that the REE, Na, K, and Ba-rich hydrothermal fluid, separated from the alkaline magma through fractional crystallization, replaced the dolomite of the Anjiacha group and formed riebeckite skarn. With the rise in residual hydrothermal fluids, Na, Ba, and K metasomatism occurred within the schist in the roof wall, forming potassium metasomatic rock and biotite-type ore (Figure 8e,f). The material composition and structure of the potassium metasomatism rocks, especially the chemical composition and microstructure of barium feldspar series minerals, indicated that the order of metasomatism of the roof wall schist by hydrothermal solution was Na→Ba→K [33].

Both field geological and microscopic observations (Figure 7a–l and Figure 8c–h) suggest that the deposit is the result of epigenetic mineralization, not syngenetic sedimentation. Skarnization more often occurred deep in the stratum than at a shallow level. At a depth of 900 m, there was weak micasization and tremolitization. Moreover, actinolite and diopside were found at a depth of 850 m. The skarnization was mostly discovered between the hanging wall (orebodies hosted in schist) and the footwall (orebodies hosted in marble). This alteration also occurred in the transitional zone of marble to schist, parallel to bedding. The distances between the skarnization and the deep orebodies ranged from several meters to a dozen meters.

Permian complex plutons (Changba monzonitic granite and Huangzhuguan granodiorite) are located 300–400 m from the orebodies. Due to the cooling of the intrusions, the ages of the middle parts are older than those of the marginal parts. The plutons do not come into contact with the orebodies in exposed or deep areas. Skarnization and silicification develop in marble near intrusions. The mineral assemblages near the intrusions are high-temperature minerals: garnet, diopside, tremolite, and quartz; mineral assemblages near the orebodies are low-temperature minerals: albite, potash feldspar, biotite, and epidote.

There have been several findings about mineralization related to magmation in the Ch-L deposit recently. There is a metasomatic relict texture caused by hydrothermal metasomatism in pyrite, arsenopyrite, and marcasite (Figure 5a). At the same stage of hydrothermal metasomatism, the content of Fe in sphalerite increases from 1.28%–3.64% to 5.36%–8.44% (unpublished data). The growth ring in pyrite (Figure 5b) is another piece of evidence to prove that hydrothermal fluid is involved in the mineralization progress. Chalcopyrite and pyrite were found in diopside skarn located in the north of the Huangzhuguan granodiorite. Sphalerite, galena, and molybdenite were found in or near the Changba monzonitic granite in the south of the ore deposit [11]. We estimated the process of mineralization as follows: hydrothermal fluid ore derived from the magma traveled a long distance and precipitated when it encountered rich oxygen and sulfur in a low-temperature fluid, such as the basin brine.

The plutons exposed in the Xicheng ore cluster and adjacent area were placed in the Late Triassic period, such as the Mishuling intrusion (214 Ma) [34], Changba monzonitic granite (219–211 Ma), and Huangzhuguan granodiorite (229–216 Ma) [29]. At the southern margin of the Changba monzonite intrusion, in contact with the Lower Devonian Wujiashan formation, quartz vein-type molybdenite developed. Re-Os isotopic chronology showed that molybdenum mineralization occurred at 209 ± 15 Ma [35].

In summary, the ages of magmatic–hydrothermal activities in the Xicheng ore cluster and adjacent area were found to be concentrated in the period 229–209 Ma, corresponding to the post-collision orogenesis of the Qinling Orogenic Belt in the Late Indosinian period [36]. The main stage of collisional orogeny in the Qinling region happened during the Middle-to-Late Triassic period and then changed to an intracontinental tectonic phase. Large-scale fluid mineralization was triggered by continental collision at the end of the Indosinian period in Western Qinling. Metallogenic hydrothermal in the Xicheng and Fengtai ore clusters were generated during the Triassic continental collision and post-collision periods [37]. The agreement between the regional tectonic evolution, magmatic activity, and hydrothermal mineralization suggests that the Xicheng ore cluster in Western Qinling experienced wide tectonomagmatic–mineralization processes.

6. Conclusions

The titanites developed within the orebody and host rock of the Ch-L Pb-Zn deposit are products of epigenetic hydrothermal activity. In situ titanite U-Pb dating yielded ages of 212.8 ± 3.0 Ma and 214.6 ± 5.1 Ma, respectively.

Based on major and trace elements, the titanites of Ch-L were distinguished as hydrothermal titanites, and the negative Eu anomalies suggested that they formed under oxidizing conditions.

Integrated with deposit geology, mineral assemblage, ore fabric, and magmatic activity, the data we obtained in this study indicate that the Changba-Lijiagou Pb-Zn deposit is an epigenetic hydrothermal-type deposit, but not a SEDEX type deposit. This is also the result of intensive tectonomagmatic procedures within the Xicheng ore cluster during the Late Triassic orogeny.