A general ore formation model for metasediment-hosted Sb-(Au-W) mineralization of the Woxi and Banxi deposits in South China
Graphical abstract
Introduction
Metasedimentary rock-hosted (= “metasediment-hosted”) Sb-(Au-W) ores occur widely, e.g., in the Tombstone gold belt of Canada (Mair et al., 2006), the La Belliere district of the French Armorican Massif (Cheval-Garabédian et al., 2020), the Western Lachlan Orogen of southeastern Australia (Voisey et al., 2019), Caledonian shear zones in Greenland (Kolb et al., 2016; Steenfelt et al., 2016), West Qinling of North China (Liu et al., 2015; Qiu et al., 2020), the Yellow Pine district of central Idaho (Stewart et al., 2017; Winzter, 2019), and the Jiangnan Orogen of South China (Fig. 1; Gu et al., 2012; Li et al., 2018, Li et al., 2019a). These deposits represent huge Sb, Au, W and Ag resources that have attracted much attention from economic geologists (Neiva et al., 2008; Kontak and Kyser, 2011; MacKenzie et al., 2015; Zhou et al., 2018). However, the metallogenic histories of these metasediment-hosted Sb-(Au-W) deposits are still controversial, with debate centering on their metal and ligand sources (sediment-derived vs. intrusion-originated; Pitcairn et al., 2015; Fusswinkel et al., 2017; Rice et al., 2018; Jia et al., 2019), metallogenic mechanisms (orogenic vs. anorogenic; MacKenzie et al., 2007; Li et al., 2014a; Zhu and Peng, 2015; Lee et al., 2019), fluid compositions and evolution (magmatic fluid, metamorphic water vs. basinal brine; Couto and Roger, 2017; Adomako-Ansah et al., 2018; Krolop et al., 2019; Qiu et al., 2020), and ore emplacement history (single stage vs. multi-stage; Konyshev, 2020; Li et al., 2019a, Li et al., 2020a, Li et al., 2020b, Li et al., 2020c). Thus, a systematic study of such deposits would be helpful in gaining new insights into the origin of metasediment-hosted Sb-(Au-W) ores.
The Xuefeng Mountains in the Jiangnan Orogen (South China) contain >100 Sb-(Au-W) deposits with total proven metal reserves of 1.67 Mt. Sb @ 6%, >100 t Au @ 5–10 g/t, and 0.25 Mt. WO3 @ 0.2–0.8% (Fig. 2; Gu et al., 2007; Li et al., 2018). The main point of controversy concerning the genesis of Sb-(Au-W) deposits in the Xuefeng Mountains is whether they are granite intrusion-related or not (Peng and Frei, 2004; Peng, 2006; Gu et al., 2012; Liang et al., 2015; Zhu and Peng, 2015; Li et al., 2018). Sb-(Au-W) mineralization in the Xuefeng Mountains mainly occurred during three periods: ~420 Ma, ~220 Ma, and ~ 130 Ma (Peng et al., 2003; Wang et al., 2012; Li et al., 2018; Li et al., 2020a, Li et al., 2020b, Li et al., 2020c). Similarly, granitic intrusions were also emplaced in and around the Xuefeng Mountains during three periods: 450–420 Ma, 240–220 Ma, and 160–140 Ma (Fig. 2; Wang et al., 2007b; Xu et al., 2014; Li et al., 2014b; Fu et al., 2015; Kong et al., 2021). However, within the study area, granitic rocks are rarely exposed. Thus, the link between granitic magmatism and Sb-(Au-W) mineralization is still uncertain.
The Upper Silurian to Lower Cretaceous Woxi and Banxi deposits in northwestern Hunan Province (Xuefeng Mountains), South China (Fig. 1) are representative metasediment-hosted Sb-(Au-W) deposits. The Woxi and Banxi deposits share many similarities with regard to orebody setting, host rock lithology, ore mineralogy and geochemistry, and history. These two deposits are hosted by metasedimentary rocks (mainly schist, slate, marble, quartzite, and conglomerate), occur dominantly as quartz vein-type mineralization, contain stibnite, native gold, scheelite, pyrite, arsenopyrite, and quartz as major ore and gangue minerals, and exhibit several stages of mineralization that coincided with local tectonic and magmatic events (Peng et al., 2003; Peng and Frei, 2004; Li et al., 2018; Fu et al., 2020b). These similarities provide an excellent opportunity to study the ore genesis and metallogeny of Sb-(Au-W) deposits on a comparative basis. Though some geochemical and geochronological studies have been carried out for these two deposits, the following issues still need to be resolved: 1) The precise metallogenic age. Previous scheelite Sm-Nd and quartz Ar-Ar dating have constrained the mineralization age at Woxi to 420–402 Ma (Peng et al., 2003), but it is not clear whether this stage of metallogenesis occurred in the Banxi deposit. 2) The ore formation model. Previous studies have proposed several models to explain the genesis of Sb-(Au-W) mineralization for these two deposits, such as syngenetic (SEDEX) (Gu et al., 2007, Gu et al., 2012), Mesozoic granitoid-related (Peng and Frei, 2004; Li et al., 2018), and orogenic scenarios (Zhu and Peng, 2015), but the metal and ligand sources and fluid evolutionary processes are still uncertain, and the mineralization sequence in each deposit is under debate. Thus, a comprehensive study combining new geochronological and in situ beam geochemical analysis is urgently needed.
This paper presents original in-situ major- and trace-element concentration, S-Pb isotope, and Re-Os geochronological data for stibnite, pyrite, and arsenopyrite from the Woxi and Banxi Sb-(Au-W) deposits, South China. The main research objectives of this work are 1) determining similarities and differences between the Banxi and Woxi deposits, 2) reconstructing their detailed metallogenetic histories, and 3) developing a general ore formation model for metasediment-hosted Sb-(Au-W) deposits.
Section snippets
Regional geology
The Woxi (28°27′45″-28°34′00”N, 110°48′30″-110°57′45″E) and Banxi (28°21′03″-28°22′14”N, 110°54′22″-111°56′19″E) Sb-(Au-W) deposits are located in northwestern Hunan Province, South China, within the Xuefeng Mountains (Fig. 1). These mountains are part of the larger Jiangnan Orogen, which marks the boundary between the Yangtze Block to the northwest and the Cathaysia Block to the southeast, recording the Early Neoproterozoic continent-arc-continent collision that formed the South China Craton (
Ore deposit geology
The Woxi Sb-Au-W deposit is located in the central part of the Xuefeng Mountains (Fig. 2). It contains significant Sb (220,000 t), Au (>50 t), and WO3 (25,000 t) reserves, with average grades of 2.84%, 9.8 g/t, and 0.3% for Sb, Au, and W, respectively (Zhu and Peng, 2015). The Banxi Sb deposit lies on the southeastern margin of the Jiangnan Orogenic Belt, a transitional region between the Xuefeng Mountains and the Xiangzhong Basin (Fig. 2). It possesses a total Sb metal reserve of up to
Sampling and methods
Representative ore samples of the major mineralization stages of the Woxi and Banxi deposits were collected in underground mine tunnels at several elevations (see Table 1 for detailed sample information). A part of each sample was prepared as a polished section, and the remaining part was used for mineral separation (i.e., stibnite, pyrite, and arsenopyrite).
Back-scattered electron (BSE) images were taken at State Key Laboratory of Geological Processes and Mineral Resources, China University of
BSE imaging
The BSE images of pyrite and stibnite from the Woxi deposit are shown in Fig. 11. It can be seen that both Py1 and Py2 possess core-rim textures: the core of Py1 is characterized by porous textures that are filled by quartz and feldspar (Fig. 11a, b), whereas the core of Py2 possesses an oscillatory zoning texture (Fig. 11d-e). The rims of both Py1 and Py2 are relatively clear and homogeneous in textures on the BSE images (Fig. 11a-f). Thus, the cores of Py1 and Py2 are noted as Py1a and Py2a,
Sources of ore metals and ligands
The concentrations and ratios of trace elements (such as Co and Ni) in pyrite have been used to determine the source of ore-forming fluids (Thomas et al., 2011; Large et al., 2013; Franchini et al., 2015; Adam et al., 2020). The Co/Ni ratio is widely used to reveal the genesis of pyrite and to reveal the source of ore metals. Pyrite with Co/Ni < 1 is considered to be of sedimentary origin, whereas pyrite with Co/Ni > 1 (especially between 1 and 5) is considered to be of magmatic-hydrothermal
Conclusions
Early-stage Au mineralization at Woxi may have been orogeny-related, and ore metals were derived mainly from local epimetamorphic sedimentary rocks with limited inputs from magmatic fluids. In contrast, late-stage Sb mineralization at Banxi may have received more ore metals and ligands from deep granitic magmatism.
The shift in mineralization from gold to stibnite at Woxi can be described as: Py1 + native gold + fluid (Au) (aq) → Py2 + Stb + Gn + Ccp + fluid (As3+, Sb3+, Pb2+, Cu2+) (aq). The Sb
Declaration of Competing Interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
Acknowledgments
This research benefited from the financial support provided by the National Key Research and Development Plan (Grant No. 2018YFC0603902) and the Central South University (Grant No. 218059). Prof. Balz Kamber is thanked for efficient editorial handling. Two anonymous reviewers are also acknowledged for their careful reviews that resulted in an improved manuscript.
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