Shi-min Zhn, Wn-bin Jia, Zhi-gao Wang, Zhi-hui Han, Xiao-hang Song, Si-huan Zhao

a Development and Research Centre of China Geological Survey, Beijing 100037, China

b Technical Guidance Center for Mineral Resources Exploration, Ministry of Natural Resources, Beijing 100083, China

c College of Earth Sciences, Jilin University, Changchun 130061, China

d State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China

e Yunnan Geology and Mining International Mining Industry Co., Ltd., Kunming 650051, China

1.Objectives

The Tiechang tin deposit is located in the northern part of the Baoshan Terrane of the Sanjiang Tethys Orogeny in western Yunnan Province of China. Tiechang tin deposit mainly occurs in the migmatite accompany with the structural fracture zone, and only a few of them occur in the internal contact zone between metamorphic rock and migmatite or in the interlayer fracture zone in metamorphic rock (Fig. 1; Lei YA et al., 2013). Different from most tin deposits (such as Lailishan deposit at 47 Ma; Xiaolonghe deposit at 71 Ma;Tieyaoshan deposit at 119 Ma; Chen XC et al., 2014) in western Yunnan, the mineralization of Tiechang tin deposit occured in Oligocene. Therefore, the metallogenic chronology of tin deposits in western Yunnan needs to be further improved. In this paper, the authors use the cassiterite U-Pb LA-ICP-MS method to determine the metallogenic age of the Tiechang tin deposit.

2. Methods

Cassiterite samples were collected from the Tiechang tin deposit for U-Pb dating. All samples were crushed to 40-60 mesh, and the cassiterite grains were separated using heavy liquids and magnetic separation techniques and were then mounted on epoxy disks. Cassiterite U-Pb dating was analyzed by LA-ICP-MS at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry Chinese Academy of Sciences, Guiyang, China, using an Agilent 7700× ICP-MS equipped with a GeoLasPro 193 nm ArF excimer laser, a pulse width of about 4 ns, an adjustable spot size of 60 μm, a continuously adjustable laser pulse frequency of 6 Hz, and an energy density of approximately 4 J/cm2.NIST SRM 610 was analyzed twice and AY-4 was analyzed three times for every fifteen analyses of the tested sample.Details of instrument parameters and conditions are illustrated in Tang YW et al. (2020).

3. Results

The cassiterite grains from the cassiterite-quartz vein ore are light to dark brown in color, generally euhedral with a size of 100-500 μm. The cassiterite shows oscillatory zonation in CL images (Fig. 2). The U-Pb data for the cassiterite samples are summarized in Table 1 and illustrated in Fig. 2. Eighteen spots for sample TC-22 yield a Tera-Wasserburg U-Pb Concordia age of 31.6±1.1 Ma (2σ, MSWD = 2.9). The external standard (AY-4) yielded an isochron age of 155.9±1.7 Ma, which is consistent with the “standard” age of 158.2±0.4 Ma within error (Yuan SD et al., 2011). This indicates that the age of cassiterite can be represented the timing of tin mineralization of the Tiechang deposit.

4. Conclusion

Fig. 1. a-Simplified geological map showing the Tengchong and Baoshan terranes; b-simplified geological map of Tiechang tin deposit (modified from Wang CM et al., 2015).

Fig. 2. 238U/206Pb - 207Pb/206Pb isochron diagram for cassiterite of Tiechang tin deposit. a-Tiechang cassiterite CL images; b-cassiterite isochron diagram.

Previous studies have shown that the main period of tin mineralization in the southern Sanjiang Belt can be divided into three periods: 240 Ma to 210 Ma, 170 Ma to 110 Ma, 90 Ma to 50 Ma (Chen XC et al., 2014). In this paper, the metallogenic age of Tiechang tin deposit is 31.6±1.1 Ma,which indicates the existence of tin mineralization during Oligocene. In the Yunlong tin ore belt of western Yunnan, the tin deposit is closely related to the late differential evolution of fine-grained muscovite granite in terms of time, space and genesis. The tectonic evolution of the Tibetan orogen underwent periodic tectonic-magmatic evolution, including episodes of transpression that lead to adakite-like alkalic magmatism between 33 Ma and 37 Ma diachronously across the belt, such as the Beiya deposit (36 Ma; Fu Y et al., 2015).The Tiechang cassiterite isochron age (31.6 Ma) reported in this study suggests that mineralization in the Tiechang deposit may have been related to the India-Eurasia post-collisional magmatism, and that these mineralizations are affected by the early stage springback of the highly thickened Indian and Asian continental suture. All these metallogenic events are controlled by large-scale thrust and strike-slip fracture systems.

Table 1. LA-MC-ICP-MS cassiterite U-Pb data of Tiechang tin deposit.

CRediT authorship contribution statement

Shi-min Zhen conceived and planed the experiment. Zhigao Wang and Zhi-hui Han carried out the experiment. Wenbin Jia wrote the manuscript with support from Xiao-hang Song and Si-chuan Zhao. Wen-bin Jia helped supervise the project. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Ackowledgement

This study was jointly supported by the National Key Research and Development Program of China(2018YFC0603704) and project of the China Geological Survey (DD20190166, DD20190159).