Geology and geochronology of the Jinmuguo Mo polymetallic deposit: Implications for the metallogeny of the Bangongco- Nujiang belt of Tibet

Abstract

The genesis of the Bangongco-Nujiang metallogenic belt (BNMB), located in the central portion of the Qinghai-Tibet Plateau, is closely related to the tectonic evolution of the Bangongco-Nujiang Ocean. Although the metallogeny of the western and central BNMB is well understood, the eastern part of this belt has not received much attention to date. The Jinmuguo deposit, the subject of this study, is a recently discovered porphyry Mo polymetallic deposit in the eastern BNMB. Here we present the geology, geochronology and isotopic characteristics of this deposit. Zircon U-Pb dating of the ore-related granite porphyry yielded an age of 118.6 ± 0.7 Ma (n = 20) whilst Re-Os analyses of molybdenite constrain the timing of mineralization as 117.9 ± 2.4 Ma (n = 4). These new age data suggest that magmatism and mineralization were coeval during early Cretaceous. In addition, the S isotope values of sulfide minerals from the Jinmuguo deposit show a narrow range of 2‰ to 4‰, suggesting a magmatic sulfur source. The Pb isotopic character of sulfides indicates a heterogenous lead source. Whilster the H-O isotopic composition (δD values −84.8‰ to −69.1‰, δ¹⁸O_H2O values 4.2‰ to 4.7‰) is consistent with a dominantly magmatic hydrothermal fluid. Overall, our study of the mineralization and alteration characteristics suggests that the Jinmuguo deposit is a porphyry Mo polymetallic deposit related to early Cretaceous felsic magmatism. A comparison of the latter with the other deposits of the BNMB suggests that the entire belt has potential to host a diverse range of magmatic arc-related mineral deposit types.

Introduction

The Bangongco-Nujiang metallogenic belt (BNMB) is one of the most important metallogenic belts in the Qinghai-Tibet Plateau (Deng et al., 2020a, Lin et al., 2017, Song et al., 2014, Tang et al., 2016, Wang et al., 2020a, Yang and Cooke, 2019, Wang et al., 2020b). The belt extends from Bangongco in the west, to Gêrzê, Nyima, and Basu in the east, and further southeast along the Nujiang River into western Yunnan (Fig. 1a). It hosts multiple ore deposits associated with magmatic suites that occur along the Bangongco-Nujiang suture zone (Song et al., 2014, Tang, 2019.

Intense exploration at the beginning of the century identified several very large ore deposits in the western BNMB, including the Duolong Cu (Au) ore district (Li et al., 2011, Li et al., 2013b, Li et al., 2015a, Li et al., 2015b, Lin et al., 2016, Lin et al., 2019b, Tang et al., 2014), the Ga’erqiong-Galale Cu (Au) ore district (Wang et al., 2019a, Zhang et al., 2015), and the Fuye and Caima Fe deposits (Feng et al., 2006, Feng et al., 2007). Small and moderately-sized Au or Cu deposits have been discovered in the central belt, for example, the Shangxu, Shesuo, Xiongmei, Kuga, and Sangri deposits (Fang et al., 2020, Lin et al., 2018, Lin et al., 2020, Wang et al., 2019b). The eastern BNMB contains several Cu (Mo) deposits, including the Baomai, Yulong, Mangzong, and Malasongduo deposits (Fig. 1b).

The genesis of deposits in the central and western BNMB has been linked to the subduction of the Bangongco-Nujiang Ocean, and the collision of the Qiangtang and Lhasa terranes (Lin et al., 2017, Wang et al., 2019b), as well as Mesozoic magmatism (Fig. 1b) (Lin et al., 2018). However, eastern magmatism and mineralization is thought to have resulted from the Eocene collision of the Indian and Eurasian plates (Hou et al., 2003). It is also unclear whether there is Mesozoic mineralization in the eastern BNMB (Changdu-Mangkang).

The recently discovered Jinmuguo deposit in the eastern BNMB is entirely hosted in hornfels with no evidence of a causative intrusive body, either at the surface or from drilling. The deposit contains base and precious metal (Pb-Zn, Au, Ag) orebodies, which are controlled by large faults. Borehole drilling in the deposit has also revealed molybdenum orebodies, as well as chalcopyrite veinlets within hornfels and skarns. However, further exploration has been hindered by a lack of detailed characterization of the ore minerals, timing of ore formation, and genesis of the Jinmuguo deposit. Therefore, in this study we investigate the geological, geochronological, and isotope geochemical characteristics of the Jinmuguo molybdenum polymetallic deposit to better understand its genesis. The findings presented here provide new insights into the nature and origin of mineralization in the eastern BNMB and provide a better understanding of the mineralization potential of the eastern BNMB and metallogeny in the BNMB as a whole.