Cretaceous intermediate sulfidation ore system in the Dongkeng volcanic Basin, SE China: Constraints from the Dongji and Maluntou gold deposits
Introduction
The Cretaceous epoch, specifically between ~ 110–90 Ma, was a period of intense magmatic-hydrothermal activity in the South China Block, which was associated with the large-scale lithospheric extension and crust-mantle interaction (Li, 2000, Mao et al., 2008). One of the most important economic manifestations of this activity is the formation of an epithermal gold metallogenic belt along the Southeastern China Fold Belt (SCFB, Fig. 1a). Over twenty-two epithermal gold deposits have been currently explored, possessing a total resource of ~480 t Au (Zhong et al., 2017a) and significant amount of Ag, Cu, Pb, and Zn (Jiang et al., 2017, Wang et al., 2017, Zhong et al., 2017b). Ore-related granites and volcanic/subvolcanic rocks have been dated at 97–114 Ma and 149–158 Ma based on U–Pb, Rb–Sr, and Ar–Ar isochron ages (Yu et al., 2013; Zeng et al., 2013; Li, 2016). They generally possess calc-alkaline or high-K calc-alkaline A- or I-type granite affinities and show the characteristics of igneous rocks related to continental arc or arc-back tectonic settings (Li et al., 2011; Li,2 016). By contrast, the origin of these epithermal gold deposits and their possible genetic links to magmatism remain unclear due to the lack of reliable ages and definitive fluid inclusion and geochemistry data, though limited previous studies proposed that most of these deposits are potentially temporal associated with Cretaceous magmatism (Li et al., 2011, Zeng et al., 2013, Jiang et al., 2017, Chen et al., 2020).
Located in the northeastern SCFB, the Dongkeng Volcanic Basin (DVB) with an area of ca. 310 km2 has developed more than seven operational gold deposits, such as the Dongji, Maluntou, Shangshangang, Baoyan, and Shangshan (Fig. 1c; Chen et al., 2020). Orebodies in these deposits are mainly hosted by volcanic and/or subvolcanic rocks and related to quartz vein systems and various styles of mineralized breccias (Wang, 2013, Lu et al., 2017). Previous studies were controversial about whether magmatic water is involved in the ore system, as the published data of microthermometry and H–O isotopes are heterogeneous. Wang and Yan (2019) proposed that hydrothermal fluids are dominated by meteoric water. Instead, Liu (2016) suggested that ore-forming fluids are more likely derived from the mixture of magmatic and meteoric water. Moreover, there is no reported chronological data on mineralization so far. These lead to the poor understanding of the relationship between gold mineralization and Cretaceous large-scale magmatic activity. Apparently, scientific problems related to ore genesis at the DVB is consistent with that of other similar deposits in the region. Therefore, gold deposits in the DVB are representative and are the ideal objects to study mineralization process. Here, we focus on the Dongji and Maluntou deposits, together with fluid inclusions petrography and microthermometry data to understand the characteristics and evolution of the ore-forming fluids. We also discuss trace element compositions of pyrite from different generations to confirm the correlation between gold and pyrite. Pyrite Re–Os and zircon U–Pb geochronological data are presented to confirm the timing of gold mineralization and magmatism in the DVB. Combined the data contribute to the origin of these gold deposits, and are potentially helpful to improve our understanding of other gold deposits within the SCFB that have similar features to the Dongji and Maluntou deposits.
Section snippets
Geological setting
Τhe Dongji and Maluntou deposits are situated in the DVB that is controlled by NE-striking Zhenghe-Dapu fault in the eastern SCFB (Fig. 1). The DVB records multiple volcanic eruptions and subvolcanic events during the Mesozoic period (Guo et al., 2012, Liu et al., 2012). Volcanic rocks in the DVB are divided into an upper and lower series by a regional unconformity (Liu et al., 2016). The lower volcanic series contains the Changlin and Nanyuan formations. The Changlin Formation is mainly
Geology and Mineralization of the Dongji and Maluntou deposits
Gold was first discovered in the Dongkeng district in 1981 and led to the development of commercial mine workings in northern SCFB. Over seven gold deposits and numerous gold occurrences are developed in the DVB, mostly hosted by the upper volcanic series (Fig. 1c). The two largest deposits, i.e., Dongji and Maluntou, that are representative of the geology observed at a large number of artisanal workings were investigated in this study.
LA–ICP–MS zircon U–Pb geochronology
Hydrothermally unaltered / least altered samples, including the rhyolitic ignimbrite of the Huangkeng Formation, the rhyolite of the Zhaixia Formation, the Xiaoshao syenogranite porphyry and the post-mineralization granite porphyry dike, were collected for LA–ICP–MS U–Pb zircon dating. The detailed sample locations are listed in Table A.1 and shown in Fig. 1c and 2a.
The rhyolitic ignimbrite sample (HK-1) is characterized by pyroclastic material (>70%), quartz and feldspar phenocrysts (10–20%),
LA–ICP–MS zircon U–Pb age
All analytical spots were located on pale, euhedral and prismatic zircon grains with clear CL zonation (Fig. 6). The zircon morphological and textural features confirm a magmatic origin. The U–Pb data of the studied volcanic and subvolcanic rocks are listed in Table A.1 and illustrated in Fig. 6.
For the rhyolitic ignimbrite (HK-1) of the Huangkeng Formation, 23 analyses yield a weighted mean 206Pb/238U age of 104 ± 0.7 Ma (MSWD = 1.8; Fig. 6e). This age is in good agreement with previous
Fluid characteristics and evolution
Fluid inclusions hosted in the stage 1 quartz (quartz1) show intermediate-density, homogenization temperatures well above 310 °C, and salinities below 10 wt% NaCl (Fig. 9). These characteristics are comparable to those of fluids in some magmatic-hydrothermal systems (e.g., porphyry, high- and intermediate-sulfidation epithermal, and hydrothermal vein-type; Heinrich et al., 2004, Redmond et al., 2004). The fluids in these magmatic-hydrothermal systems generally possess a magmatic origin (Zhong
Conclusions
Hydrothermal fluids potentially had a magmatic component at high temperature (up to 377 °C; stage 1), and boiled at ca. 329 °C (stage 2), and mixed with infiltrating meteoric water during the late-stage (stage 3).
Visible and invisible gold are both closely associated with pyrite2b. Visible gold forms native gold and electrum and precipitates directly from the fluids during fluid boiling. Invisible gold is in the form of solid solution and related to lattice dislocations caused by rapid
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We express our thanks to reviewers for their critical reviews and comments. The authors also thank Editor-in-Chief Prof. Franco Pirajno and associated Editor-in-chief for their editorial help and constructive suggestions. Thanks are extended to Antonia Hofmann, Chris Ottley, Geoff Nowell, Zeyang Liu and Junjie Liu for analytical support of the Re–Os work, and Chenjin Chen from the Fujian goldmine for assistance during field work. This study was financially supported by the National Natural
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