U-Pb Ages, O Isotope Compositions, Raman Spectrum, and Geochemistry of Cassiterites from the Xi’ao Copper-Tin Polymetallic Deposit in Gejiu District, Yunnan Province

Abstract

The Xi’ao Cu-Sn polymetallic deposit is located in the inner alteration zone of the Laoka granite. The ore bodies extend to 400 m in the granite rock and primarily occur with fluorite and potassic alterations. Two cassiterite samples of altered rock-type ore and one tourmaline vein-type ore in the Xi’ao Cu-Sn polymetallic deposit yielded U-Pb ages of 83.3 ± 2.1 Ma, 84.9 ± 1.7 Ma, and 84.0 ± 5.6 Ma, respectively. The Raman spectrum peak values of A_1g were shifted to a lower frequency, possibly due to the substitution of Sn with Nb, Ta, Fe, and Mn. Measured δ¹⁸O values of cassiterite samples and calculated δ¹⁸O_H2O values for the ore-forming fluid indicate that the latter was mostly derived from magma. The high Fe and Mn abundances for cassiterite are consistent with those of hydrothermal origin. The Nb, Ta, and Ti contents indicate that cassiterites in the Xi’ao deposit likely formed in a metallogenic environment that was largely affected by granitic magmatism. Therefore, we conclude that the Xi’ao deposit is a magmatic hydrothermal deposit.

1. Introduction

Cassiterite is one of the main tin minerals that is widespread in tin deposits; it can precipitate from hydrothermal ore-forming fluids over a very broad range of P-T-X conditions, and it is resistant to metamorphism, hydrothermal alteration, weathering, and abrasion [1,2]. Hence, it is likely that cassiterite effectively preserves primary information about the trace and rare-earth element geochemistry of the ore-forming fluid, which can provide important information for studies on the genesis of ore deposits [1,2,3,4,5,6,7]. Cassiterite belongs to the rutile group (M⁴⁺O₂). It generally has high U and low Pb contents in its crystal structure and a high Pb closure temperature (560–860 °C) [8]. In recent years, great progress has been made in the use of cassiterite to determine the metallogenic ages of Sn polymetallic deposits [7,9,10,11,12,13,14,15,16,17]. Yuan et al. [18] obtained in situ LA-ICP-MS U-Pb age data (159.9 ± 1.9 Ma) and ID-TIMS U-Pb age data (158.2 ± 0.4 Ma) from cassiterite in the Furong Sn polymetallic deposit. Zhang et al. [16] provided further evidence to constrain the timing of granitic magmatism and hydrothermal mineralization by using LA-MC-ICP-MS U-Pb dating to calculate ages for igneous zircon and hydrothermal cassiterite. Compared with the metallogenic ages obtained from altered minerals, ages obtained directly from cassiterite can more accurately reflect the timing of ore deposit formation [7].

China is extremely rich in tin resources, possessing 32% of the world’s total Sn resources [19]. The Gejiu tin polymetallic district in Yunnan is well known for its large tin reserves containing approximately 335.74 Mt of Sn ores, 357.11 Mt of Cu ores, and 400 Mt of Pb-Zn ores. In the past few decades, this tin district has been extensively studied. Dating is a powerful tool used to determine mineralization characteristics and understand the ore genesis process [9,20,21,22]. Attempts to date the mineralization of the Gejiu district have been primarily performed on hydrothermal minerals (⁴⁰Ar-³⁹Ar ages of micas, Re-Os ages of molybdenites) [23,24,25,26], but in some cases, the ages of these minerals may be inconsistent or inaccurate (the ⁴⁰Ar-³⁹Ar and K-Ar ages range from 43.49 ± 0.87 Ma to 87.5 ± 0.6 Ma) [24,25,27,28]. Thus, ages of ore minerals are still needed to provide precise constraints on the timing of mineralization processes.

In Gejiu district, the Xi’ao ore field, an altered granite-type Cu-Sn polymetallic deposit (containing > 10 Mt Sn + Cu [29]), has been discovered in the inner alteration zone of the western edge of the Laochang–Kafang granite. In this deposit, the granite–marble contact zones rarely contain skarn minerals, and the intensity and scale of the mineralization are substantially constrained by the wall rock alteration and spatial distribution of granite [29,30]. The genesis of this deposit remains unclear, and additional studies are needed. Currently, most studies of cassiterite in the Gejiu district have focused on its mineral typomorphic characteristics [31,32,33,34], and some studies have investigated the geochronology and geochemistry of cassiterites from the Gaosong and Laochang deposits (two ore fields in Gejiu district) [14,35,36]. However, few studies have been performed on cassiterites from Xi’ao deposit, especially for altered rock-type cassiterites.

In this paper, cassiterite samples were collected from altered rock-type and tourmaline vein-type ores in the Xi’ao Cu-Sn polymetallic deposit and studied to constrain the timing of tin mineralization and precipitation environment of cassiterite. Consequently, the mineralogy of cassiterites was investigated by cathodoluminescence (CL) imagery and Raman spectroscopy. The geochemical signatures and oxygen isotopes of these minerals were also analyzed by using electron microprobe analyzer (EPMA) and isotope ratio mass spectrometry (MAT 253). In addition, the U-Pb ages of cassiterites were also measured by LA-ICP-MS. These results provide further insights into ore genesis in the Xi’ao deposit and highlight the potential of using cassiterite as a monitor of hydrothermal processes.

2. Geological Setting

The Gejiu mining district is located in the southeastern region of Yunnan Province. Tectonically, the Gejiu tin polymetallic deposit is located at the junction of the Yangtze Craton and Cathaysia and Indochina blocks (Figure 1). This deposit represents an important part of the southeast Yunnan tin polymetallic belt [37,38]. Sedimentary sequences in Gejiu district comprise Cambrian to Quaternary rocks, but Cretaceous rocks are lacking because of episodic uplift/erosion associated with Indosinian and Yanshanian tectonic events. Most outcrops in the Gejiu area comprise carbonate rocks of the Triassic Gejiu Formation and fine-grained clastic sediments and carbonates of the Falang Formation, the former of which are the main ore-hosting rocks. Numerous faults are present in the area, including the North–Northeast- (NNE) trending Longchahe, Jiaodingshan, and Yangjiatian faults, as well as the Northwest- (NW) trending Baishachong fault and the North–South- (NS) trending Gejiu fault. The Gejiu fault divides the study area into its eastern and western sectors. The eastern area, which contains 90% of the tin reserves in the Gejiu district [38], is dominated by the Wuzhishan anticlinorium, which includes five ore deposit areas; from north to south, these areas are: Malage, Songshujiao, Gaosong, Laochang, and Kafang (Figure 1).

Frequent multistage magmatic activity occurred in the Gejiu area. Gejiu magmatic intrusions (emplacement ages are marked in Figure 1), which show a large variability, including gabbro, nepheline syenite, alkali feldspar granite, alkali granite, porphyritic biotite granite, and granular biotite granite, are widely exposed in the western area of Geiju but are sparsely distributed in the eastern area. The ages of granites in Gejiu range from 53 Ma to 147 Ma according to dating obtained by different methods such as Ar-Ar/K-Ar, Rb-Sr, LA-ICP-MS/TIMS/SHRIMP (Sensitive High-Resolution Ion Microprobe) U–Pb, and Re–Os dating [24,25,41,42,43,44,45]. The latest chronological studies have shown that the magmatism in Gejiu district occurred during the late Cretaceous period (zircon U-Pb ages vary from 76.6 ± 3.6 Ma to 85.0 ± 0.85 Ma,

Table 1. The ages of magmatic intrusions in Gejiu district.

Pluton Name	Lithology	Research Object	Dating Method	Age	Petrogenesis	Reference
Longchahe	Porphyritic biotite granite	biotite	K-Ar	100~115 Ma	A-type; peraluminous, alkaline granite	[51]
		whole rock	Rb-Sr	147 ± 3 Ma
		zircon	U-Pb	82.0 ± 0.3 Ma~83.2 ± 1.4 Ma		[43]
Masong	Porphyritic biotite granite	biotite	K-Ar	100~102 Ma	A-type and S-type; metaluminous—peraluminous, calc-alkaline granite	[51]
		K-feldspar	K-Ar	91.5~116 Ma
		whole rock	Rb-Sr	90.4 ± 6.3Ma
		zircon	U-Pb	82.8 ± 1.7 Ma		[43]
Shenxianshui	Equigranular granite	biotite	K-Ar	72~87 Ma	A-type; peraluminous, alkaline granite	[51]
		whole rock	Rb-Sr	84.4 ± 1.1 Ma
		zircon	U-Pb	81.0 ± 0.52 Ma ~81.4 ± 0.4 Ma		[43]
Baishachong	Equigranular granite	biotite	K-Ar	53 Ma	Peraluminous, calc-alkaline granite	[52]
		whole rock	Rb-Sr	81.0 ± 2 Ma
		zircon	U-Pb	77.4 ± 2.5 Ma		[43]
Laoka	Equigranular granite	biotite	K-Ar	64 Ma~80 Ma	S-type; peraluminous, calc-alkaline	[52]
		whole rock	Rb-Sr	81.0 ± 4.9 Ma
		zircon	U-Pb	85.0 ± 0.85 Ma		[41]
	Porphyritic granite	zircon	U-Pb	83.3 ± 1.6 Ma		[43]
Baiyunshan	Alkali feldspar granite	biotite	K-Ar	59.5 Ma~62 Ma	/	[52]
		whole rock	Rb-Sr	94.3 ± 2.4 Ma
		zircon	U-Pb	76.6 ± 3.6 Ma		[53]
Jiasha	Gabbro	zircon	U-Pb	84.0 ± 0.6 Ma	/	[54]
Lamprophyre		zircon	U-Pb	77.2 ± 2.4 Ma	/	[53]

). Trace elements and Sr-Nd-Hf isotopes demonstrate that the Gejiu granites were mainly derived from melting of continental crust in an intraplate environment [43]. The predominant granitoids and the synchronous mafic and ultramafic rocks in Gejiu thus constitute a bimodal magmatic association controlled by lithospheric extension and asthenosphere upwelling within the western Cathaysia block in Late Cretaceous [46,47,48,49], indicating a late Yanshanian intracontinental extensional tectonic context [40,43,50,51]. Previous studies have also shown that the granites emplaced in Yanshanian are closely temporally and spatially related to the Gejiu tin polymetallic deposit [23,40,43].

3. Ore Geology of the Xi’ao Deposit

The Xi’ao Cu-Sn polymetallic deposit is located in the western part of the Laochang ore field (Figure 1), and the mining area is approximately 25 km² [55]. The outcrop of the area mainly comprises Triassic Gejiu Formation carbonate rocks, which are the main ore-hosting rocks. The NW-trending Huangmaoshan and East–West- (EW) trending Wanzijie anticlines, which are the subsidiary anticlines of the Wuzhishan anticlinorium, are the main fold structures. The faults comprise three groups—NW-, NE-, and NS-trending—all of which are closely associated with mineralizations.

The Laoka equigranular biotite granite intruded into the Gejiu Formation in the Late Cretaceous (85.0 ± 0.85 Ma, [43]) at depths of 200–1800 m beneath the surface. It is mainly distributed in the Laochang and Kafang ore fields. Rock-forming minerals in equigranular biotite granite include K-feldspar (38%), plagioclase (25.2%), quartz (32.8%), and biotite (4%). The accessory minerals are zircon, apatite, titanite, allanite, monazite, tourmaline, and fluorite. Petrographic and geochemical features suggested that the hidden Laoka biotite granite has affinities of calc-alkaline S-type granite with higher contents of Sn (33.3 ppm), Cu (12.8 ppm), W (5.3 ppm), and mineralization-associated elements (F: 2500 ppm and Cl: 250 ppm) than normal granite [56,57]. It is peraluminous and enriched in silica and potassium (SiO₂ content is high, SiO₂ > 74%; K and Na rich, K₂O + NaO = 7.19%, K₂O/Na₂O = 6.8; aluminum saturation, Al/(K₂O + Na₂O) = 1.83; low oxidation rate, Ox = FeO/(FeO + Fe₂O₃) = 0.46 and highly fractionated, DI = 85~95) [29,38]. In addition, this granite is highly evolved and fractionated and possibly formed during the late evolution stage of the Gejiu granite [43,51].

The ore bodies are located in the inner alteration zone along the western edge of the Laoka granite (Figure 2) approximately 1000 m below the surface [29]. These ore bodies are mainly controlled by the EW- trending faults, occur as veins and lenticular bodies, and extend to 400 m into the granite. The grades of tin and copper range from 0.2% to 1.34% and from 0.3% to 3.0%, with metal contents of 127.6 and 69.8 thousand tons, respectively [50]. The alteration of rocks is intensive, and skarn is poorly developed in the ore field. The country rock alterations are potassic alteration, tourmalinization, fluoritization, pyritization, silicification, chloritization, and carbonatization. The alteration zone boundary is vague. The alteration zones are classified into two major types—potassic and epidote-chlorite. The former is closely related to mineralization [56,58,59]. The mineral assemblages are listed in

Table 2. The mineral assemblage characteristics of altered-rock type ores in Xi’ao deposit.

Mineral Assemblage Type	Minerals	Ore Structure	Mineralization Significance
Potassium-fluoride-sulfide	K-feldspar, pyrite, chalcopyrite, and less fluorite, quartz, tourmaline, mica, etc.	Block, crumb, veinlet and disseminated	Big
Epidote and chlorite-pyritization-fluoride	Quartz, plagioclase, k-feldspar, biotite, epidote, chlorite, pyrite, fluorite, tourmaline, etc.	Veinlet and disseminated.	Small

.

Based on the occurrence, locality, and paragenesis of tin ore, two mineralization types were collected from the mining area, i.e., altered rock- and tourmaline vein-type ore (Figure 3).

Altered rock-type Sn polymetallic ore (Figure 3a,b) (due to the paucity of studies, we name it provisionally here) represents a newly discovered mineralization type, providing an interesting direction for deep prospecting in the future. The distribution of orebodies is controlled by EW-trending fractures, which are located in the inner alteration zone of the Laoka granite edge (approximately 10 m to 400 m within the rock mass). The main alteration types associated with mineralization are potassic alteration and chloritization. In the inner granite alteration zone, ore bodies are found only in K-altered granite. Orebodies mostly occur as veins, veinlets, or banded bodies such as parallel arrangements. When intersecting fractures, lenticular ores are also present. The average grade of Sn is 0.51 wt %, and it can reach up to 10 wt %. In potassic alteration zone, the original plagioclase, quartz, and other minerals are overprinted and essentially replaced by secondary K-feldspar, perthite, and microcline. The main metallic minerals are chalcopyrite, pyrrhotite, sphalerite, cassiterite, pyrite, scheelite, and wolframite (Figure 3d,g). These minerals are mainly disseminated (rich), and stringers fill the structures (barren) of altered granite (Figure 3d,e). The gangue minerals mainly contain K-feldspar, fluorite, and tourmaline, and less mica, quartz, epidote, chlorite, and apatite. Ore textures are mainly metasomatic relict, poikilitic, and skeletal (Figure 3d,e).

Tourmaline vein-type ores (Figure 3c) occur as veins, veinlets, and stockwork and are predominantly controlled by fractures. The vein ores are present at a wide range of depths, from proximal to the granite to near the ground surface. The ore minerals, which are mainly disseminated, mainly contain chalcopyrite, pyrite, cassiterite, and arsenopyrite with lesser sphalerite and pyrrhotite (Figure 3h). The gangue minerals mainly include tourmaline and fluorite, with little quartz (Figure 3i).

4. Samples and Analytical Methods

Samples XA-3 (Figure 3a) and XA-5 (Figure 3b) were obtained from altered rock-type ore and tourmaline vein-type ore in the Xi’ao 1800 m adit, respectively. XA-4 (Figure 3c) was obtained from altered rock-type ore in granite at Fengliushan. Prior to analysis, samples were crushed, and cassiterite grains were separated using heavy liquid and magnetic separation techniques. They were then handpicked under a binocular microscope, mounted in an epoxy resin disc, and polished.

Cassiterite samples were examined carefully under the binocular microscope and scanning electron microscope to observe mineral and fluid inclusions and cracks. We chose the clean surface of cassiterites when experimenting in order to avoid fractures and inclusions, as common Pb is mostly associated with fluid inclusions [10].

Cathodoluminescence (CL) images of three cassiterite samples were obtained at Wuhan SampleSolution Analytical Technology Co., Ltd., Wuhan, China, using an analytical scanning electron microscope (JSM-IT100) (Japan Electron Optics Laboratory Co., Ltd., Tokyo, Japan) connected to a GATAN MINICL system (Gatan, Inc., Pleasanton, CA, USA). The operating conditions included an accelerating potential of 10 kV, a temperature of 20 °C, and an image acquisition time of 35 s/sheet. To compare the CL images of cassiterite grains, specific analytical conditions were adopted during the experiment such that the instrument automatically adjusted the parameters to obtain an image with the highest resolution after the scale was changed.

Raman spectra of three cassiterite samples were determined at the State Key Laboratory of Geological Processes and Mineral Resources (GPMA), China University of Geosciences (Wuhan), using a Renishaw RW-1000 Raman microspectrometer (Renishaw, London, UK). An argon ion laser with a wavelength of 514.5 nm was used for detection at 25 °C. The laser beam was focused on a small area measuring 2.0 μm. Working conditions: the laser power was 0.2 W, the collected time of Raman spectrum was 60 s, and the spectral resolution was ±1 nm⁻¹. The spectrograms were processed using OriginPro 8.0 (OriginLab, MA, Northampton, USA).

Mineral compositions were determined at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan) using a JEOL JXA-8100 Electron Probe Micro Analyzer equipped with four wavelength-dispersive spectrometers (WDS) (Japan Electron Optics Laboratory Co., Ltd., Tokyo, Japan). The samples were first coated with a thin conductive carbon film prior to their analysis. The precautions suggested by Zhang and Yang [60] were used to minimize differences in the thicknesses of carbon films between samples and to obtain a uniform coating of approximately 20 nm. During the analysis of these minerals, an accelerating voltage of 15 kV, a beam current of 20 nA, and a spot size of 2 µm were used. Data were corrected online using a modified ZAF (atomic number, absorption, fluorescence) correction procedure. The peak counting time was 10 s for Sn, W, Ta, Si, Fe, and Nb, and 20 s for In, Ti, and Mn. The background counting time was one-half of the peak counting time on the high- and low-energy background positions. The following standards were used: cassiterite (Sn), tungsten (W), tantalum (Ta), olivine (Si), indium phosphide (In), pyrope garnet (Fe), niobium (Nb), rhodonite (Mn), and rutile (Ti). Analytical uncertainty was estimated at <1 wt % error for major elements and up to 5 wt % for trace elements.

Pure cassiterite separates were prepared for oxygen isotope analysis. The O isotope compositions were analyzed following the BrF₅ method [61]. The δ¹⁸O values of these separates were determined on a Finnigan MAT 253 ratio mass spectrometer (Thermo Fisher Scientific, USA). The O isotope results are reported relative to V-SMOW (Vienna standard mean ocean water), and the analytical precision is ±0.2‰ for δ¹⁸O. The degree of isotopic fractionation between cassiterite and water was calculated using Equation (1) [62] at the minimum trapping temperature, which was defined based on the average homogenization temperatures of the fluid inclusions in each quartz and calcite sample, as reported by Liao et al. [50]:

δ¹⁸O_water-SMOW = δ¹⁸O_V-SMOW − 10³lnα_{cassiterite-water} = δ¹⁸O_V-SMOW − (10.13 × 10⁶/T² –26.09 × 10³/T + 12.58)

(1)

In situ U-Pb cassiterite dating was performed using a Neptune ICP-MS (Thermo Fisher Scientific ICAP Q) (Thermo Fisher Scientific, USA) coupled with an ESI 193 nm COHERENT Compex Pro 102F Excimer laser ablation system (Cohernet Coherent Inc., Santa Clara, CA, USA) at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan). The Squid smoothing device was used to reduce statistical error induced by laser ablation pulses and to improve the data quality [63,64]. Helium gas carrying the aerosol of the ablated sample was mixed with argon carrier gas and nitrogen as an additional di-atomic gas to enhance sensitivity before finally flowing into the ICP. Typical gas flow settings for the Ar cooling gas, Ar auxiliary gas, and He carrier gas during the course of this study were 15 L/min, 0.75 L/min, and 0.86 L/min, respectively. The samples were analyzed using an energy density of 5 J/cm², a spot size of 32 μm, and a laser pulse frequency of 10 Hz. NIST SRM 610 (National Institute of Standards and Technology, Standard Reference Material 610) and an in-lab cassiterite standard AY-4 were used as external calibration standards. AY-4 was collected from the skarn orebody in the Anyuan tin deposit of the Furong orefield in the middle Nanling Range. This cassiterite sample has been well studied using ID-TI0MS, and it has a U-Pb age of 158.2 ± 0.4 Ma [18]. Details can be found in Yuan et al. [13,18]. NIST SRM 610 was analyzed once after every ten analyses; AY-4 was analyzed twice after every five analyses. Each spot analysis incorporated approximately 20 s of background acquisition followed by 40 s of sample data acquisition. Isotopes were measured in time-resolved mode. For U-Pb dating, dwell times for each mass scan were 15 ms for ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁸Pb, ²³⁸U, and ²³⁵U, and 25 ms for ²⁰⁷Pb. Data errors of single spot were 1σ. Raw data reduction was performed off-line using ICPMSDataCal software (Liu Yongsheng, China University of Geosciences, China) [65,66]. The uncertainty of single populations, ratio uncertainty of the AY-4 reference material, and decay constant uncertainties were propagated to the ultimate results of the samples during the process of data reduction by ICPMSDataCal 10.1 [66]. Tera-Wasserburg concordia lower intercept age calculations were processed using Isoplot 3.0 (Kenneth R. Ludwig, United States Geological Survey, USA) [67].

5. Results

5.1. Color and CL Images

The typomorphic characteristics of the color of cassiterite have been well studied by researchers due to its easily recognized and distinguishable features [4,12,31,32,33,34,68].

As shown in Figure 4, the colors of the three cassiterite samples varied widely (e.g., black, dark brown, brown, red brown, light brown, gray, colorless). The colors of altered rock-type cassiterites varied from colorless to dark, while those of tourmaline vein-type cassiterites were mainly dark. In addition, the grain sizes of cassiterites that formed in the same ore-forming setting were uniform, while they varied widely among cassiterites of different genetic types, ranging from n × 10 μm to n × 100 μm. The particle sizes of cassiterites from XA-3 (350 × 200 μm to 550 × 380 μm) and XA-4 (280 × 240 μm to 490 × 330 μm) were larger than those from XA-5 (130 × 110 μm to 270 × 180 μm).

Pronounced differences in the luminescence and internal structures of CL images were observed among cassiterites that formed in different crystallization environments (Figure 5). Based on the luminescence and complexity of the internal structures observed in CL images, the cassiterites could be divided into two zones: (1) the H zone (homogeneous zone), which is characterized by black luminescence and homogeneous internal structures (Figure 5a,b,d), and (2) the O zone (oscillatory zone), which exhibits gray, off-white luminescence with some hourglass structures and oscillatory zoning (Figure 5a,b) and others homogeneous (Figure 5c,e). XA-3 cassiterites had both H and O units, and oscillatory zoning was common in O units. XA-4 cassiterites were predominantly composed of O units with no H units. XA-5 cassiterites were primarily composed of O units with oscillatory zoning and few H units.

5.2. Raman Spectrum

Cassiterite has a rutile structure with space group ${\mathrm{D}}_{4\mathrm{h}}^{14}$-P4/mnm (Z = 2). The tin atom is located in the octahedral coordination of oxygen. The mechanical representation of the normal vibration modes at the center of the Brillouin zone is given as Γ = A_1g + A_2g + B_1g + B_2g + E_g + 2B_1u + A_2u + 3E_u [69,70], among which A_1g, B_1g, B_2g and E_g were assigned to Raman active while A_2g + 2B_1u was assigned to Raman inactive.

The Raman spectra of Xi’ao cassiterite samples are shown in Figure 6. The most intense peak can be attributed to A_1g mode, which was found to shift to 633.693 cm⁻¹ in XA-3, 632.295 cm⁻¹ in XA-4 and 633.693 cm⁻¹ in XA-4, while those exhibited at 473.355 cm⁻¹ to 531.539 cm⁻¹ and 774.156 cm⁻¹ to 776.176 cm⁻¹ may have been due to vibrational modes E_g and B_2g, respectively. B_1g mode was found to shift to 88.731 cm⁻¹ in XA-3, 87.346 cm⁻¹ in XA-4 and 85.058 cm⁻¹ in XA-5. The Raman spectra peaks of cassiterites collected from Xi’ao are in good agreement with the pure SnO₂ Raman spectra peaks of A_1g = 646 cm⁻¹, B_2g = 752 cm⁻¹, E_g = 441 cm⁻¹ and B_1g = 100 cm⁻¹ reported by Katiyar et al. [71].

Additional peaks were observed at 438.109 in XA-3 and XA-5 and 233.004 in XA-4, which may have been due to the nano-inclusions, possibly requiring further study.

5.3. Cassiterite Composition

Trace element contents of cassiterite are listed in

Table 3. Electron probe micro-analyzer (EPMA) data on cassiterites from Xi’ao Cu-Sn polymetallic deposit.

XA-3 Altered Rock-Type Ore
	XA-3-1	XA-3-2	XA-3-4	XA-5-6	XA-3-7	XA-3-8	XA-3-9	XA-3-10	XA-3-11	XA-3-12	XA-3-13	XA-3-15	XA-3-17	XA-3-18		XA-3-20
	H	H	O	O	O	O	O	O	O	O	O	O	O	H		H
SnO2	100.416	98.882	99.667	99.445	99.716	98.754	99.966	99.775	96.631	98.772	98.278	98.957	99.815	100.087		98.392
WO3	0.000	0.667	0.047	0.487	0.387	0.017	0.161	0.080	0.082	0.031	0.030	0.029	0.000	0.000		0.037
Nb2O5	0.000	0.058	0.192	0.000	0.024	0.038	0.010	0.039	0.558	0.019	0.005	0.063	0.039	0.029		0.592
Ta2O5	0.000	0.014	0.255	0.000	0.000	0.277	0.000	0.213	0.000	0.000	0.000	0.047	0.000	0.000		0.110
FeO	0.625	0.782	0.626	0.744	0.678	0.501	0.498	0.530	0.712	1.168	0.952	0.721	0.876	0.653		0.621
MnO	0.023	0.032	0.015	0.024	0.000	0.016	0.000	0.006	0.002	0.005	0.000	0.000	0.000	0.002		0.000
TiO2	0.000	0.018	0.211	0.150	0.240	0.000	0.000	0.145	0.810	0.209	1.052	0.772	0.235	0.219		0.638
SiO2	0.012	0.005	0.080	0.024	0.011	0.041	0.004	0.047	0.054	0.000	0.000	0.026	0.000	0.031		0.000
Total	101.076	100.458	101.093	100.874	101.056	99.644	100.639	100.835	98.849	100.204	100.317	100.615	100.965	101.021		100.390
Cation Formula Based on Four Atoms of Oxygen
Sn4+	1.9795	1.9593	1.9580	1.9603	1.9615	1.9798	1.9849	1.9737	1.9327	1.9639	1.9383	1.9504	1.9700	1.9745		1.9428
W6+	0.0000	0.0086	0.0006	0.0062	0.0049	0.0002	0.0021	0.0010	0.0011	0.0004	0.0004	0.0004	0.0000	0.0000		0.0005
Nb5+	0.0000	0.0013	0.0043	0.0000	0.0005	0.0009	0.0002	0.0009	0.0126	0.0004	0.0001	0.0014	0.0009	0.0006		0.0133
Ta5+	0.0000	0.0002	0.0034	0.0000	0.0000	0.0038	0.0000	0.0029	0.0000	0.0000	0.0000	0.0006	0.0000	0.0000		0.0015
Fe2+	0.0388	0.0488	0.0387	0.0462	0.0420	0.0351	0.0346	0.0367	0.0498	0.0812	0.0657	0.0497	0.0605	0.0451		0.0429
Mn2+	0.0010	0.0013	0.0006	0.0010	0.0000	0.0007	0.0000	0.0003	0.0001	0.0002	0.0000	0.0000	0.0000	0.0001		0.0000
Ti4+	0.0000	0.0007	0.0078	0.0056	0.0089	0.0000	0.0000	0.0054	0.0306	0.0078	0.0391	0.0287	0.0087	0.0081		0.0238
Si4+	0.0006	0.0003	0.0039	0.0012	0.0005	0.0020	0.0002	0.0023	0.0027	0.0000	0.0000	0.0013	0.0000	0.0015		0.0000
Total	2.0199	2.0204	2.0175	2.0205	2.0184	2.0225	2.0219	2.0232	2.0295	2.0540	2.0436	2.0324	2.0401	2.0299		2.0247
Nb + Ta	0.0000	0.0015	0.0077	0.0000	0.0005	0.0047	0.0002	0.0037	0.0126	0.0004	0.0001	0.0020	0.0009	0.0006		0.0147
Fe + Mn	0.0398	0.0501	0.0393	0.0472	0.0420	0.0358	0.0346	0.0370	0.0499	0.0814	0.0657	0.0497	0.0605	0.0452		0.0429
XA-4 Altered Rock-Type Ore
	XA-4-1	XA-4-2	XA-4-3	XA-4-4	XA-4-6	XA-4-7	XA-4-8	XA-4-9	XA-4-5	XA-4-10	XA-4-11	XA-4-12	XA-4-13	XA-4-14	XA-4-15	XA-4-16	XA-4-17
	O	O	O	H	H	H	H	H							H
SnO2	98.483	99.626	99.369	99.397	99.949	99.423	98.811	99.598	96.516	98.760	99.027	98.777	99.634	99.005	98.994	98.236	100.022
WO3	0.062	0.065	0.000	0.052	0.011	0.000	0.037	0.000	0.087	0.143	0.000	0.000	0.042	0.000	0.000	0.547	0.012
Nb2O5	0.029	0.000	0.039	0.000	0.106	0.067	0.000	0.082	0.513	0.250	0.005	0.000	0.000	0.086	0.000	0.077	0.038
Ta2O5	0.047	0.077	0.000	0.000	0.000	0.019	0.000	0.049	0.339	0.184	0.000	0.025	0.000	0.000	0.118	0.093	0.000
FeO	0.614	0.680	0.965	0.597	0.521	0.800	0.795	0.553	0.959	0.588	0.587	0.751	0.656	0.606	0.802	0.606	0.590
MnO	0.028	0.000	0.028	0.000	0.007	0.000	0.000	0.000	0.000	0.019	0.011	0.000	0.015	0.000	0.014	0.000	0.000
TiO2	0.400	0.381	0.308	0.066	0.282	0.014	0.036	0.000	1.448	0.225	0.754	0.392	0.179	0.628	0.632	0.849	0.201
SiO2	0.048	0.019	0.032	0.033	0.030	0.057	0.064	0.003	0.064	0.028	0.021	0.024	0.043	0.021	0.000	0.016	0.048
Total	99.711	100.848	100.741	100.145	100.906	100.380	99.743	100.285	99.926	100.197	100.405	99.969	100.569	100.346	100.560	100.424	100.911
Cation Formula Based on Four Atoms of Oxygen
Sn4+	1.9648	1.9667	1.9631	1.9810	1.9727	1.9759	1.9762	1.9838	1.8733	1.9632	1.9570	1.9662	1.9749	1.9591	1.9557	1.9394	1.9753
W6+	0.0008	0.0008	0.0000	0.0007	0.0001	0.0000	0.0005	0.0000	0.0011	0.0018	0.0000	0.0000	0.0005	0.0000	0.0000	0.0070	0.0002
Nb5+	0.0007	0.0000	0.0009	0.0000	0.0024	0.0015	0.0000	0.0018	0.0121	0.0056	0.0001	0.0000	0.0000	0.0019	0.0000	0.0017	0.0009
Ta5+	0.0006	0.0010	0.0000	0.0000	0.0000	0.0003	0.0000	0.0007	0.0045	0.0025	0.0000	0.0003	0.0000	0.0000	0.0016	0.0013	0.0000
Fe2+	0.0428	0.0470	0.0667	0.0416	0.0360	0.0556	0.0556	0.0386	0.0651	0.0409	0.0405	0.0523	0.0455	0.0419	0.0554	0.0418	0.0408
Mn2+	0.0012	0.0000	0.0012	0.0000	0.0003	0.0000	0.0000	0.0000	0.0000	0.0008	0.0005	0.0000	0.0006	0.0000	0.0006	0.0000	0.0000
Ti4+	0.0151	0.0142	0.0115	0.0025	0.0105	0.0005	0.0013	0.0000	0.0530	0.0084	0.0281	0.0147	0.0067	0.0234	0.0235	0.0316	0.0075
Si4+	0.0024	0.0009	0.0016	0.0017	0.0015	0.0028	0.0032	0.0002	0.0031	0.0014	0.0010	0.0012	0.0021	0.0010	0.0000	0.0008	0.0024
Total	2.0284	2.0306	2.0448	2.0274	2.0234	2.0366	2.0368	2.0251	2.0122	2.0247	2.0272	2.0348	2.0303	2.0274	2.0368	2.0236	2.0269
Nb + Ta	0.0013	0.0010	0.0009	0.0000	0.0024	0.0018	0.0000	0.0025	0.0166	0.0081	0.0001	0.0003	0.0000	0.0019	0.0016	0.0030	0.0009
Fe + Mn	0.0440	0.0470	0.0678	0.0416	0.0362	0.0556	0.0556	0.0386	0.0651	0.0417	0.0410	0.0523	0.0461	0.0419	0.0560	0.0418	0.0408
XA-5 Tourmaline Vein-Type Ore
	XA-5-6	XA-5-8	XA-5-9	XA-5-10	XA-5-11	XA-5-12	XA-5-13	XA-5-15	XA-5-16	XA-5-17	XA-5-18	XA-5-19	XA-5-20	XA-5-21	XA-5-22		XA-5-23
	O	O	O	O	H	O	O	O	O	O	O	O	O	O	O		O
SnO2	99.885	99.022	99.149	99.757	99.161	99.584	99.124	98.823	99.568	98.975	99.168	99.867	99.452	98.699	99.362		99.816
WO3	0.000	0.031	0.128	0.000	0.033	0.000	0.016	0.066	0.000	0.070	0.062	0.000	0.046	0.000	0.035		0.000
Nb2O5	0.082	0.000	0.067	0.048	0.048	0.043	0.149	0.000	0.000	0.000	0.034	0.000	0.111	0.000	0.115		0.024
Ta2O5	0.000	0.000	0.066	0.000	0.376	0.003	0.334	0.123	0.060	0.189	0.104	0.000	0.000	0.044	0.305		0.071
FeO	0.655	0.666	0.723	0.543	1.081	0.645	0.643	1.040	0.705	0.991	1.082	1.181	0.588	1.065	0.857		0.649
MnO	0.034	0.000	0.011	0.000	0.000	0.009	0.000	0.000	0.009	0.012	0.000	0.006	0.035	0.017	0.000		0.010
TiO2	0.000	0.216	0.070	0.098	0.135	0.154	0.237	0.188	0.177	0.005	0.021	0.000	0.095	0.085	0.202		0.154
SiO2	0.028	0.000	0.040	0.000	0.012	0.041	0.037	0.030	0.077	0.068	0.033	0.070	0.053	0.025	0.025		0.011
Total	100.684	99.935	100.254	100.446	100.846	100.479	100.540	100.270	100.596	100.310	100.504	101.124	100.380	99.935	100.901		100.735
Cation Formula Based on Four Atoms of Oxygen
Sn4+	1.9752	1.9704	1.9671	1.9780	1.9526	1.9707	1.9592	1.9558	1.9665	1.9608	1.9600	1.9584	1.9707	1.9612	1.9557		1.9715
W6+	0.0000	0.0004	0.0016	0.0000	0.0004	0.0000	0.0002	0.0008	0.0000	0.0009	0.0008	0.0000	0.0006	0.0000	0.0004		0.0000
Nb5+	0.0018	0.0000	0.0015	0.0011	0.0011	0.0010	0.0033	0.0000	0.0000	0.0000	0.0008	0.0000	0.0025	0.0000	0.0026		0.0005
Ta5+	0.0000	0.0000	0.0009	0.0000	0.0050	0.0000	0.0045	0.0017	0.0008	0.0026	0.0014	0.0000	0.0000	0.0006	0.0041		0.0010
Fe2+	0.0408	0.0417	0.0452	0.0339	0.0670	0.0402	0.0401	0.0648	0.0439	0.0618	0.0673	0.0751	0.0367	0.0666	0.0531		0.0403
Mn2+	0.0014	0.0000	0.0005	0.0000	0.0000	0.0004	0.0000	0.0000	0.0004	0.0005	0.0000	0.0003	0.0015	0.0007	0.0000		0.0004
Ti4+	0.0000	0.0081	0.0026	0.0037	0.0050	0.0057	0.0088	0.0070	0.0066	0.0002	0.0008	0.0000	0.0036	0.0032	0.0075		0.0057
Si4+	0.0014	0.0000	0.0020	0.0000	0.0006	0.0021	0.0018	0.0015	0.0038	0.0034	0.0017	0.0037	0.0026	0.0013	0.0013		0.0006
Total	2.0207	2.0206	2.0214	2.0167	2.0318	2.0200	2.0180	2.0315	2.0219	2.0301	2.0327	2.0374	2.0182	2.0335	2.0246		2.0200
Nb + Ta	0.0018	0.0000	0.0024	0.0011	0.0061	0.0010	0.0078	0.0017	0.0008	0.0026	0.0022	0.0000	0.0025	0.0006	0.0067		0.0015
Fe + Mn	0.0422	0.0417	0.0456	0.0339	0.0670	0.0406	0.0401	0.0648	0.0442	0.0623	0.0673	0.0754	0.0382	0.0673	0.0531		0.0407

. The average SnO₂ contents of XA-3, XA-4, and XA-5 in the study area were 99.170 wt % (96.631 wt % to 100.416 wt %), 99.037 wt % (96.516 wt % to 100.022 wt %), and 99.338 wt % (98.699 wt % to 99.885 wt %), respectively. The main trace elements in these different types of cassiterite comprised Fe, Nb, Ta, and Ti, and minor trace elements included W, Mn, and Si (Table 3). A summary of the EPMA data is shown in

Table 4. Statistical characteristics of EPMA results.

Sample	SnO2	WO3	Nb2O5	Ta2O5	FeO	MnO	TiO2	SiO2
XA-3	99.170 ± 0.95 a	0.137 ± 0.21	0.111 ± 0.19	0.061 ± 0.1	0.713 ± 0.18	0.008 ± 0.01	0.313 ± 0.34	0.022 ± 0.02
	96.631~100.416 b	0–0.667	0–0.592	0–0.277	0.498–1.168	0–0.032	0–1.052	0–0.080
XA-4	99.037 ± 0.82	0.062 ± 0.13	0.076 ± 0.13	0.056 ± 0.09	0.686 ± 0.14	0.007 ± 0.01	0.400 ± 0.37	0.032 ± 0.02
	96.516–100.022	0–0.547	0–0.513	0–0.339	0.521–0.964	0–0.028	0–1.448	0–0.064
XA-5	99.338 ± 0.38	0.030 ± 0.04	0.045 ± 0.05	0.105 ± 0.13	0.820 ± 0.22	0.009 ± 0.01	0.115 ± 0.08	0.034 ± 0.02
	98.699–99.885	0–0.128	0–0.149	0–0.376	0.543–1.181	0–0.035	0–0.237	0–0.077

^a: mean ± sd, ^b: range of concentration.

. Iron was the most abundant minor element in cassiterites (0.498 wt % to 1.181 wt %). In XA-3 and XA-4, Nb abundance was higher than Ta, while in XA-5, Ta abundance was higher. Ti was an important element in cassiterite and contents ranged from 0 to 1.448 wt %.

5.4. Oxygen Isotopes

The results of oxygen isotope determination of the cassiterite samples are shown in

Table 5. The δ¹⁸O values of cassiterite samples in Xi’ao Cu-Sn polymetallic deposit.

Samples	Minerals	δ18OV-PDB (‰)	δ18OV-SMOW (‰)	Temperature (°C) a	δ18OH2O-SMOW (‰)
XA-3	Cassiterite	−25.40	4.7	374.10	8.25
XA-4		−25.70	4.4	374.10	7.95
XA-5		−26.20	3.9	353.82	7.16

^a, temperatures were taken from [50].

. The cassiterite samples showed δ¹⁸O values ranging from 3.9‰ to 4.7‰ (Table 5), exhibiting a gradual decrease from XA-3 to XA-4 to XA-5. The δ¹⁸O values of XA-3, XA-4 and XA-5 were 4.7‰, 4.4‰ and 3.9‰, respectively. Calculating the precise δ¹⁸O_H2O values of fluids that were in equilibrium with cassiterite was difficult due to the large variation in temperatures obtained from fluid inclusions [50]. Here, we used the average homogenization temperatures of these fluid inclusions to calculate their δ¹⁸O_H2O values. The calculated δ¹⁸O_H2O values of these fluids were similar and within a limited range (7.16‰ to 8.25‰). The δ¹⁸O_H2O values of XA-3, XA-4, and XA-5 were 8.25‰, 7.95‰, and 7.16‰, respectively.

5.5. U-Pb Ages

The cassiterite grains used for U-Pb isotope analysis had a few cracks and fluid inclusions. Care was taken when selecting laser positions to avoid cracks and fluid inclusions to reduce the influence of common Pb in inclusions and to improve the accuracy of analyses. Both O and H zones were used for U-Pb dating. The LA-ICP-MS results are summarized in

Table 6. LA-ICP-MS U-Pb dating results of cassiterite from the Xi’ao Cu-Sn polymetallic deposit.

XA-3 Altered Rock-Type Ore
Spots	Th	U	Isotopic Ratios									Age (Ma)
			238U/206Pb	1σ	207Pb/206Pb	1σ	206Pb/238U	1σ	207Pb/235U	1σ	207Pb/206Pb	1σ	206Pb/238U	1σ	207Pb/235U	1σ
XA16-3-3	0	20,627	75.1314801	3.217501	0.03543	0.00497	0.01331	0.00057	0.06532	0.00877	0.1	0	85.3	3.62	64.2	8.36
XA16-3-4	8	64,216	77.5193798	2.223424	0.02693	0.00255	0.0129	0.00037	0.04806	0.00438	0.1	0	82.6	2.33	47.7	4.24
XA16-3-5	0	38,135	77.4593338	2.699977	0.03421	0.00376	0.01291	0.00045	0.06101	0.00639	0.1	0	82.7	2.86	60.1	6.11
XA16-3-6	0	32,544	79.491256	3.033053	0.031	0.00423	0.01258	0.00048	0.05385	0.00708	0.1	0	80.6	3.07	53.3	6.82
XA16-3-8	0	23,724	79.4281176	3.532942	0.06637	0.00797	0.01259	0.00056	0.11525	0.01289	818.2	232.61	80.7	3.55	110.8	11.74
XA16-3-11	0	46,501	75.8150114	2.759	0.0378	0.00616	0.01319	0.00048	0.06922	0.01094	0.1	0	84.5	3.08	68	10.39
XA16-3-12	0	51,406	80.1282051	6.869966	0.05307	0.01635	0.01248	0.00107	0.09228	0.02726	331.9	579.52	80	6.84	89.6	25.34
XA16-3-13	0	64,041	75.5857899	2.742342	0.05508	0.00849	0.01323	0.00048	0.102	0.01516	415.3	312.01	84.7	3.08	98.6	13.97
XA16-3-14	0	23,742	75.5287009	3.650934	0.07457	0.01498	0.01324	0.00064	0.13904	0.02702	1056.5	359.19	84.8	4.08	132.2	24.09
XA16-3-16	0	32,855	75.8725341	3.511551	0.07766	0.01786	0.01318	0.00061	0.14698	0.0334	1138.3	400.23	84.4	3.86	139.2	29.57
XA-4 Altered Rock-Type Ore
Spots	Th	U	Isotopic Ratios									Ages (Ma)
			238U/206Pb	1σ	207Pb/206Pb	1σ	206Pb/238U	1σ	207Pb/235U	1σ	207Pb/206Pb	1σ	206Pb/238U	1σ	207Pb/235U	1σ
XA-4-1	0	38,164	70.52186	2.5364	0.07462	0.00723	0.01418	0.00051	0.14623	0.0133	1058	183.86	90.8	3.26	138.6	11.78
XA-4-2	1	36,083	69.39625	3.178454	0.09839	0.01033	0.01441	0.00066	0.19591	0.01873	1593.9	184.1	92.2	4.18	181.7	15.9
XA-4-3	3	91,084	74.46016	1.940511	0.04016	0.00318	0.01343	0.00035	0.07453	0.00565	0.1	0	86	2.26	73	5.34
XA-4-4	0	31,407	72.83321	2.811479	0.0469	0.00583	0.01373	0.00053	0.08894	0.01058	44	273.39	87.9	3.4	86.5	9.86
XA-4-5	0	58,372	85.6898	3.597944	−0.00366	0.00568	0.01167	0.00049	−0.0059	0.00916	0.1	0	74.8	3.13	-6	9.35
XA-4-6	0	72,882	74.51565	2.276559	0.04511	0.00408	0.01342	0.00041	0.08358	0.00721	0.1	157.16	85.9	2.62	81.5	6.75
XA-4-7	0	18,842	74.46016	3.492919	0.04477	0.00769	0.01343	0.00063	0.08302	0.01377	0.1	305.34	86	3.99	81	12.91
XA-4-8	0	6665	63.77551	4.392701	0.08595	0.01659	0.01568	0.00108	0.18605	0.03366	1336.9	333.6	100.3	6.88	173.3	28.81
XA-4-9	0	11,272	63.85696	3.466055	0.12024	0.01501	0.01566	0.00085	0.25995	0.02943	1959.8	207.41	100.2	5.42	234.6	23.72
XA-4-10	0	7160	58.34306	3.676225	0.12216	0.01768	0.01714	0.00108	0.28893	0.03792	1988	237.23	109.6	6.82	257.7	29.88
XA-4-11	0	126,487	80.84074	2.156625	0.03191	0.00289	0.01237	0.00033	0.05443	0.00475	0.1	0	79.2	2.1	53.8	4.58
XA-4-12	0	5181	74.23905	6.448381	0.12237	0.02421	0.01347	0.00117	0.22732	0.04059	1991.1	315.34	86.2	7.43	208	33.58
XA-4-13	0	58,639	78.125	2.563477	0.05157	0.00471	0.0128	0.00042	0.09105	0.00785	266.4	196.22	82	2.65	88.5	7.31
XA-4-14	0	1704	68.3527	9.1573	0.11805	0.04805	0.01463	0.00196	0.23816	0.09165	1926.9	591.79	93.6	12.46	216.9	75.16
XA-4-15	0	9525	80.97166	6.294153	0.05115	0.01484	0.01235	0.00096	0.08714	0.02439	247.4	558.56	79.2	6.11	84.8	22.78
XA-4-16	0	6612	74.34944	5.970067	0.09207	0.01935	0.01345	0.00108	0.17076	0.03324	1468.7	354.03	86.1	6.9	160.1	28.83
XA-4-17	0	97,931	75.18797	2.20476	0.04487	0.00372	0.0133	0.00039	0.08227	0.00645	0.1	128.04	85.2	2.48	80.3	6.06
XA-4-18	0	26,046	77.27975	3.404131	0.03189	0.0063	0.01294	0.00057	0.0569	0.01097	0.1	0	82.9	3.64	56.2	10.54
XA-5 Tourmaline Vein-Type Ore
Spots	Th	U	Isotopic Ratios									Age (Ma)
			238U/206Pb	1σ	207Pb/206Pb	1σ	206Pb/238U	1σ	207Pb/235U	1σ	207Pb/206Pb	1σ	206Pb/238U	1σ	207Pb/235U	1σ
XA-5-1	57	33,209	10.9769484	0.277135	0.70382	0.02819	0.0911	0.0023	8.88783	0.30736	4738.5	56.34	562	13.59	2326.6	31.56
XA-5-2	1	20,863	70.2247191	2.958907	0.07856	0.00878	0.01424	0.0006	0.155	0.01621	1161	206.92	91.1	3.8	146.3	14.25
XA-5-4	35	30,390	30.0390508	0.776016	0.46305	0.01974	0.03329	0.00086	2.13586	0.0787	4127.7	61.84	211.1	5.33	1160.5	25.48
XA-5-5	8	25,787	49.4071146	4.027754	0.23846	0.03313	0.02024	0.00165	0.66883	0.07615	3109.7	205.5	129.2	10.44	520	46.33
XA-5-6	133	143,761	59.2417062	1.228353	0.19086	0.00774	0.01688	0.00035	0.44634	0.01674	2749.6	65.1	107.9	2.2	374.7	11.75
XA-5-7	60	27,856	17.6584849	0.763964	0.53733	0.03368	0.05663	0.00245	4.21513	0.20401	4347	88.87	355.1	14.93	1677	39.72
XA-5-8	0	125,483	75.4716981	1.822713	0.04799	0.00317	0.01325	0.00032	0.0881	0.00555	97.6	150.41	84.9	2.04	85.7	5.18
XA-5-9	23	8254	66.0938533	4.106294	0.15994	0.01956	0.01513	0.00094	0.33533	0.03576	2455.1	193.27	96.8	5.94	293.6	27.19
XA-5-10	74	112,650	78.1860829	3.239924	0.11092	0.00998	0.01279	0.00053	0.19659	0.01594	1814.6	155.02	81.9	3.38	182.2	13.52
XA-5-11	44	47,919	47.3484848	1.546897	0.25907	0.01524	0.02112	0.00069	0.75849	0.03861	3241	89.79	134.8	4.35	573.1	22.29
XA-5-12	55	32,069	52.6315789	1.717452	0.3831	0.02155	0.019	0.00062	1.00882	0.04815	3844.1	82.39	121.3	3.95	708.3	24.34
XA-5-13	40	6225	2.12417954	0.049588	0.67373	0.03003	0.47077	0.01099	43.97262	1.81795	4675.7	62.73	2486.9	48.17	3864.6	41.05
XA-5-14	31	5892	16.2839928	0.654966	0.5602	0.035	0.06141	0.00247	4.77057	0.24063	4408	88.3	384.2	15.03	1779.7	42.34
XA-5-15	0	75,871	70.5716302	1.942338	0.04468	0.00343	0.01417	0.00039	0.08782	0.00641	0.1	104.97	90.7	2.51	85.5	5.98
XA-5-16	0	151,920	76.5110941	1.873263	0.03887	0.00258	0.01307	0.00032	0.0705	0.00446	0.1	0	83.7	2.04	69.2	4.23

and plotted in Figure 7. Figure 7 indicates that cassiterites from the XA-3, XA-4, and XA-5 yielded U-Pb Tera-Wasserburg concordia lower intercept ages of 83.3 ± 2.1 Ma (1σ, MSWD = 0.29) (MSWD: mean squares weighted deviates), 84.9 ± 1.7 Ma (1σ, MSWD = 0.78), and 84.0 ± 5.6 Ma (1σ, MSWD = 4.1), respectively. These data represent the metallogenic age of the Xi’ao Cu-Sn polymetallic deposit.

6. Discussion

6.1. Timing of Sn Mineralization

Cassiterite is the main ore mineral in tin polymetallic deposits; thus, directly dating cassiterite can provide precise constraints on the timing of mineralization processes. With recent improvements in analytical techniques, the LA-ICP-MS U-Pb dating of cassiterite (SnO₂) has proven to be a powerful tool for dating tin polymetallic deposits [7,12,13,15,16,17,18,72]. Li et al. [10] discussed the reliability of using LA-ICP-MS to determine the U-Pb age of cassiterite and proposed the relative merits and reliability of isochron ages, concordia ages, and Tera-Wasserburg lower intercept ages.

In this study, we applied the LA-ICP-MS technique to obtain the U-Pb chronology of cassiterites from the two types of ores (altered rock- and tourmaline vein-type ores) in the Xi’ao Cu-Sn polymetallic deposit. The ²⁰⁷Pb/²⁰⁶Pb-²³⁸U/²⁰⁶Pb Tera-Wasserburg concordia lower intercept ages of 83.3 ± 2.1 Ma to 84.9 ± 1.7 Ma (Figure 7) obtained from our data agree well with those of the Kafang (one of the five deposits in Gejiu district) cassiterite (84.4 ± 2.0 Ma) obtained by Guo et al. [35], and those of the Gaosong (one of the five deposits in Gejiu district) cassiterites (83.5 ± 2.1 Ma to 85.1 ± 1.0 Ma) obtained by Guo et al. [14]. The U-Pb system in cassiterite has a high closure temperature, and cassiterite has stable chemical properties and is resistant to hydrothermal alteration. Therefore, we conclude that the timing of Sn mineralization in the Xi’ao deposit occurred at 83.3 ± 2.1 Ma to 84.9 ± 1.7 Ma.

Numerous geochronological studies have been performed on the Laoka granite. The reported U-Pb age for the Laoka equigranular biotite granite is 85.0 ± 0.85 Ma [41,43]. Furthermore, the U-Pb ages (83.3 to 84.9 Ma) of the cassiterites obtained in our study are consistent with the previously mentioned ages, which indicates that the altered rock- and tourmaline vein-type ores in the Xi’ao Cu-Sn polymetallic deposit have a close temporal relationship with the Late Cretaceous granitic magmatism.

6.2. Color, CL Images and Raman Spectrum

The color variation of cassiterite has been attributed to elemental isomorphous substitution in the mineral lattice [7,73,74], or Fe content [75,76] or Fe²⁺/Fe³⁺ ratio [77]. However, in our cassiterites collected from Xi’ao deposit, there were no obvious relationships between Fe + Nb + Ta + Ti, Fe, or W contents and the color variation (Figure 8). Moreover, previous studies also show W and U contents are the main factors controlling the color variation of Gaosong cassiterites [14,78,79]. Both of the results illustrate other mechanisms may exist, which still need further study.

Some studies show Fe, Ti, and W contents are related to the luminescence intensity [80,81,82,83,84]. Figure 5 shows that the O units had elevated Ti content, and the H units usually had no or little Ti. The W content had no distinct relation with luminescence intensity. Therefore, the Ti content played an important role in the luminescence intensity of CL images of the Xi’ao cassiterites. In addition, A_1g was one of the characteristic peaks of cassiterite, whose Raman frequency could be significantly affected by impurities [69,70,71,85]. In Figure 6, low-frequency drift of A_1g may have been caused by the substitution of Sn by Nb, Ta, Fe, and Mn [70].

6.3. Metallogenic Conditions

The δ¹⁸O values of cassiterite samples were indistinguishable and exhibited a narrow range from 3.9‰ to 4.7‰. The δ¹⁸O_H2O values were calculated using the formula based on the δ¹⁸O values of cassiterite that may represent the δ¹⁸O values of ore-forming fluid, which were in the range from 7.16‰ to 8.25‰. The oxygen isotope data are consistent with the values of magmatic water (δ¹⁸O_H2O = 5.5‰ to 9.5‰) proposed by Ohmoto [86] and Sheppard [87]. Additionally, previous studies of rocks and ore minerals from the Gejiu district have reported oxygen isotopic compositions (

Table 7. The δ¹⁸O of other geological samples in Xi’ao Cu-Sn polymetallic deposit.

Samples	Minerals	δ18Omineral (‰)	Reference
Laoka granite	Whole rock (4)	11.85	[88]
	Biotite (2)	8.58
	Quartz	12.40	[57]
Ore-bearing quartz vein	Quartz	12.60	[88]
Gejiu Formation limestone	Limestone	27.31	[89]
Gejiu Formation marble	Marble (2)	22.25
Gejiu Formation dolomite	Dolomite	18.70
Basalt amygdala	calcite	19.29

): the whole-rock δ¹⁸O of Laoka granite was 11.85‰ [88], and the calculated δ¹⁸O_H2O of Laoka granite was 9.3‰ [57], the δ¹⁸O_biotite of Laoka granite was 8.58‰ [88]; the δ¹⁸O values of Gejiu Formation carbonate rock limestone, marble, and dolomite were 27.31‰, 22.25‰, and 18.70‰, respectively [89]. The δ¹⁸O_H2O values of cassiterite samples are consistent with the calculated δ¹⁸O_H2O of Laoka granite rather than those of carbonate, and the H-O isotopic data of quartz in different mineralization stages indicate that ore-forming fluids were mostly derived from magma as well as the late-stage addition of meteoric water [50,88]. Therefore, we conclude that the ore-forming fluids were derived from magmatic hydrothermal system.

Numerous studies have analyzed the geochemistry of cassiterites from different metallogenic environments and have observed systematic variations in their chemical compositions [3,90,91]. Cassiterites from the Xi’ao deposit had high Fe and Mn contents, which plot in the field of hydrothermal cassiterites (Figure 9). The Fe and Mn contents of cassiterites from the Xi’ao deposit were obviously higher than those of cassiterites from the Gaosong deposit. The (Ta + Nb)/(Fe + Mn) atomic ratio of cassiterite samples varied greatly, and all data fell above the line of (Ta + Nb)/(Fe + Mn) = 1 (Figure 9). This result could be attributed to the excess Fe present in most cassiterite samples (Table 3). The (Ta + Nb)/(Fe + Mn) atomic ratios and their correlations demonstrate that various charge compensation mechanisms occur when Sn⁴⁺ is substituted [4,90]. The cations could be incorporated into the cassiterite structure according to the equation 3Sn⁴⁺ = (Fe, Mn)²⁺ + 2(Nb, Ta)⁵⁺ and 2Sn⁴⁺ = Fe³⁺ + (Nb, Ta)⁵⁺ [74,90]. The remaining Fe would then be incorporated into the crystal by the substitutions Sn⁴⁺ = Fe³⁺ + H⁺, Sn⁴⁺ + O²⁻ = Fe³⁺ + OH⁻ [90], and Ti would be incorporated by the mechanism of Sn⁴⁺ = Ti⁴⁺.

The trace element concentrations of cassiterites can provide information about the ore-forming fluid. For example, the high field strength elements (HFSEs, Nb, Ta, and Ti) contents of the cassiterites related to magmatic hydrothermal fluids are elevated when compared to those of cassiterites associated with sedimentary or metamorphic hydrothermal fluids [2,6,7,92,93]; for example, studies have shown that in the Furong tin deposit, HFSE concentrations of cassiterite in granite are high (Sc: 2.81~22.9 ppm; Ti: 81.1~1952 ppm; Nb: 8.94~1046 ppm; and Ta: 0.063~74.0 ppm). In contrast, cassiterite in carbonate rock, which is greatly influenced by country rock, has less Sc (0.181~1.80 ppm), Ti (3.44~75.2 ppm), Nb (0.002~0.129 ppm), and Ta (0.001~0.022 ppm) [7]. Cassiterite collected from a magmatic hydrothermal tin deposit related to granite in northern Portugal contains high contents of Nb₂O₅ (0.05 wt %~1.12 wt %), Ta₂O₅ (0.23 wt %~2.37 wt %), and TiO₂ (0.24 wt %~0.45 wt %) [6]. Cassiterite collected from the Yunlong metamorphic hydrothermal tin deposit contains low contents of HFSEs, e.g., Nb (0.17 ppm ~0.31 ppm), Ta (0.07 ppm~0.09 ppm), Sc (2.34 ppm ~2.48 ppm), and Ti (115~128 ppm) [2]. All of our studied samples contained relatively higher contents of Nb₂O₅ (0~0.592 wt %), Ta₂O₅ (0~0.376 wt %), and TiO₂ (0~1.448 wt %) (Table 3), indicating that the sources of the ore-forming fluid are enriched in HFSEs. The contents of HFSEs were high in the Laoka granite (Nb: 53.86 ppm, Ta: 19.4 ppm; Ti: 179.85 ppm [94]) but very low in the Gejiu Formation carbonate rocks, both of which are related to the Laochang tin polymetallic deposit [57,94]. Hence, the high contents of HFSEs in cassiterites may be derived from the Laoka granite, and these cassiterites likely formed in a metallogenic environment that was largely affected by granitic magma. In addition, as shown in

Table 8. Composition of sulfur isotope in the Xi’ao Cu-Sn polymetallic deposit.

Minerals	δ34SV-CDT (‰)	Reference
Laoka granite	−3.7~0.1	[23]
Carbonate	7.4~11.1	[57]
Pyrrhotite	−0.2	[94]
Chalcopyrite	1.9
Pyrrhotite	0.2	[38]
Arsenopyrite	2.2
Chalcopyrite	2.9

, the S isotopic compositions of different sulfide minerals (pyrrhotite, arsenopyrite, chalcopyrite) indicate that their δ³⁴S values (−0.2‰~2.9‰, [38,94]) fall within the range of the δ³⁴S values for granite (−3.7‰ to 0.1‰, [23]), which is obviously lower than that of the Gejiu Formation carbonate rock (7.14‰ to 11.1‰, [57]), and interpreted to be of magmatic origin. All of the above data demonstrate that the Laoka granite provided the source material for tin mineralization.

As important carriers of Sn, W, and other metal elements, F and Cl play a key role in the separation, migration, and enrichment of Sn, W, and HFSEs (e.g., Nb, Ta, Ti) from melt. Studies have shown that the solubilities of Nb and Ta significantly improve in fluoride solutions [95,96], as does that of Ti; these solubilities are 20–200 times higher in fluorine solutions than in pure water [97]. Within a reducing environment, the solubility of Sn in fluid can obviously increase due to the fact that Sn²⁺ and Cl⁻ can form more stable complexes [98,99]. The Laoka granite is peraluminous, highly evolved, and fractionated, and has a high fluorine content of 2500 ppm and a high F/Cl value (8.26) [41,42,43,56]. The abundance of fluorite associated with these ores also supports the interpretation that the ore-forming fluid contained a considerable amount of fluorine. Hence, fluorite mineralization is attributed to low pH and copious availability of Ca²⁺ derived from dissolution of the host limestone [100]. Under these conditions, it can be inferred that F⁻ beside Cl⁻ played possible roles as relevant complexing agents of Sn. Previous studies have shown that, during magmatic differentiation, fluorine preferentially enters the melt until it becomes saturated in a liquid phase due to its highly incompatible behavior [50,101]. Meanwhile, fluorine may also decrease the viscosity, density, and solidus temperature of magma, thus extending the length of fractional crystallization and facilitating the extraction of tin, copper, and other ore-forming elements in the residual liquid phase [101,102,103]. The value of D_F fluid/melt increases exponentially with increasing F content in the melt [104]; thus, the F content of the exsolved fluid from the F-rich melt should be high. During the late stage evolution of a F-rich magma, the ore-forming fluid formed, which was enriched in metallogenic elements (e.g., Sn, W) and HFSEs (e.g., Nb, Ta, Ti). As the ore-forming fluid migrated upward, it interacted with the Gejiu Formation carbonate rocks and was accompanied by the addition of meteoric water during the late metallogenic stage—this resulted in the formation of cassiterites with high contents of HFSEs in different regions.

6.4. Ore Genesis of the Xi’ao Deposit

At present, most large and superlarge tin deposits are of magmatic hydrothermal origin and are closely temporally and spatially associated with granites [105,106,107]. However, tin deposits that record other geneses, such as the Portugal Neves Corvo VMS Cu-Sn deposit [108,109] and the China Yunnan Yunlong metamorphic hydrothermal tin deposit have also been found [2]. To the best of our knowledge, deposits of types similar to the Xi’ao tin deposits described here have not been reported previously. This deposit allows us to study cassiterite mineralization more comprehensively. In this study, the colors, Raman spectra, elemental compositions, oxygen isotope results, and U-Pb geochronology indicate that the cassiterites from the Xi’ao deposit formed in the Late Cretaceous and are genetically related to Laoka biotite granite.

The ore bodies of the Xi’ao Cu-Sn polymetallic deposit are hosted within the granite, which extends to 400 m. In addition, the fluorite and the potassic alterations in the ore bodies suggest that the fluid system is rich in potassium and fluorine. The sulfur and oxygen isotopes characteristic of the sulfides and cassiterite indicate a magmatic origin [38,57,88,94]. Thus, the Xi’ao deposit may be classified as a magmatic hydrothermal deposit. Moreover, the younger U-Pb ages for the cassiterites reported here compared to the age of the Laoka granite corroborates this interpretation. The occurrence and alteration of the ore bodies provide useful information for exploring the altered rock-type Cu-Sn polymetallic deposit in Xi’ao.

7. Conclusions

The U-Pb Tera-Wasserburg concordia lower intercept ages of two samples of altered rock-type ore and one sample of tourmaline vein-type ore in the Xi’ao Cu-Sn polymetallic deposit are 83.3 ± 2.1 Ma, 84.9 ± 1.7 Ma, and 84.0 ± 5.6 Ma, respectively. These ages are highly consistent with the U-Pb age of Laoka granite, which indicates that mineralization has a close temporal relationship with the Late Cretaceous granitic magmatism. The peak values of A_1g were shifted to a lower frequency, possibly due to the substitution of Sn by Nb, Ta, Fe, and Mn. The δ¹⁸O values of cassiterite samples and the δ¹⁸O_H2O values of ore-forming fluid indicate that ore-forming fluids were mostly derived from magma. The high Fe and Mn contents show that the cassiterites belong to hydrothermal cassiterites. The Nb, Ta, and Ti contents indicate that cassiterites in the Xi’ao deposit likely formed in a metallogenic environment that was largely affected by granitic magma. The later-stage hydrothermal activity dominated by Cl- and F-rich fluids was responsible for cassiterite deposition. Thus, we conclude that the Xi’ao deposit is a magmatic hydrothermal deposit.