Carlin-type gold deposits are among the most economically important gold deposits globally, with known resources concentrated mainly in Nevada in the western United States (Muntean et al., 2011) and in the Golden Triangle of South China (Hu et al., 2017). The deposits are typically hosted in carbonates and/or calcareous siliciclastic rocks and are closely associated with extensive hydrothermal alteration dominated by decarbonation, silicification, sulfidation, and argillization (Hofstra and Cline, 2000; Cline et al., 2005). The Middle-Lower Yangtze Metallogenic Belt (MLYB) is one of the most important Cu-Au-polymetallic metallogenic belts in eastern China. The copper, gold, and iron metallogenies of the MLYB occurred in the Mesozoic. They were accompanied by intense magmatism during regional tectonic phases of transition and major adjustments from the early Yanshanian Tethys tectonics to the Paleo-Pacific tectonics for the MLYB, mainly in the compression-extension conversion phase (Zhou et al., 2008; 2011; Mao et al., 2012). The main structure pattern shifted from near EW to NE or NNE (Zhai et al., 1992; Tang et al., 1998; Zhou et al., 2008). The Mesozoic intense metallogeny of the MLYB took place from ∼145 to 120 Ma producing skarn, porphyry, layered copper, iron, gold, and “porphyryite iron” deposits (Nanjing-Wuhu porphyryite iron ore group, 1978; Zhai et al., 1992; Tang et al., 1998; Pan and Dong, 1999; Sun et al., 2017; Xie et al., 2019; Wang Y. et al., 2022; Li et al., 2023; Liang et al., 2023). The Mesozoic large scale mineralization process of the MLYB can generally be divided into three stages: (1) 145–135 Ma: porphyry copper-gold deposits and high-K calc-alkaline series intrusion formed mainly in the lifted block region in a strike-slip compressive tectonic setting (Zhou et al., 2011; Ma et al., 2011; Li et al., 2018; Chen et al., 2024); (2) 135–127 Ma: porphyryite iron and shoshonitic series volcanic rock formation within the basin (e.g., LuZong; Ningwu Project Group, 1978; Zhou et al., 2011; Ma et al., 2011; Chang et al., 2012; Zhang et al., 2023); (3) 127–123 Ma: uranium-gold mineralization and “A” type granite formation in both the lifted block region and basin (Zhou et al., 2011).

Several questions about Mesozoic large scale mineralization in the MLYB remain unresolved. Firstly, What is the metallogenic time and tectonic origin mechanism of the Carlin-type gold deposits in the MLYB? Although Carlin-type gold deposits such as Shewushan and Yangxin have been identified in the western extended part of Edongnan over the last 20 years, their exact formation time and the tectonic processes behind their origin, especially in relation to the large-scale mineralization during the Yanshan period, remain unclear. Secondly, What is the nature and origin of the magmatism and mineralization in the MLYB after 120 Ma? Despite the widespread magmatism and mineralization in southeastern coastal China after 120 Ma (Li, 2000; Li et al., 2023), the MLYB was previously considered a post-metallogenic stage (Chang et al., 2012). However, recent discoveries of magmatism and porphyry copper mineralization in areas like Ningzhen (110–97 Ma) suggest a more complex mineralization history (Zeng et al., 2013; Wang et al., 2014b; Wang et al., 2022 F.; Nie et al., 2016), raising the question of whether this represents a repetition of earlier stages or a distinct magmatic process. Thirdly, What are the origins of the Cu-Au porphyry deposits in the MLYB, and what mechanisms control their formation? The genesis of Cu-Au porphyry deposits in the MLYB remains debated, with proposed mechanisms including intracontinental magmatism (Hou and Yang, 2009; Lv et al., 2014; Zhou et al., 2015) and subduction of the Paleo-Pacific Plate (Ling et al., 2009; 2011; Liu et al., 2010; Li et al., 2021; Sun et al., 2021; Yan et al., 2021; Zhang et al., 2023). Further research is needed to resolve the dominant processes responsible for these deposits.

The Shewushan gold deposit, which was initially identified as a red laterite-type gold mine, was discovered in the late 1980s, with an approximately 24 Mt reserve and an average grade of about 1.8 g/t (The Forth team of Hubei Province Geological Prospecting Bureau, 1994; 1999). The Shewushan deposit, a Carlin-type gold deposit located in the Middle-Lower Yangtze Metallogenic Belt (MLYB), represents a newly identified deposit type in the region (Figure 1). The relationship between its mineralization age and the mineralization series of MLYB porphyry and lamprophyre deposits is crucial for understanding regional metallogenic events. This has significant implications for regional exploration and prospecting. The finely disseminated gold hosted within limestone below red laterite is characteristic of a Carlin-type gold deposit. Red laterite-type gold deposits are the oxidation product of Carlin-type gold ore (Liu and Li, 1995; Liu, 1995; Li, 1998). This deposit was confirmed as a typical Carlin-type deposit by Wang et al. (2014b), who studied the fluid inclusions of the mine. Many early Paleozoic fossils, including Brachiopoda coralline, are present within red laterite-type gold bodies and mineralized limestone (Ding, 1992); however, Cretaceous sporopollen is also present in the same stratum (Yu, 1994), and the lack of reliable isotopic dates has led to controversy over the age and genesis of the Shewushan gold deposit (Cai et al., 2016). This study focuses on the geological characteristics of calcite related to mineralization, with the goal of determining the timing of gold mineralization in the Shewushan Carlin-type gold deposit. We measured rare earth element concentrations and the isotopic compositions of Rb-Sr, Sm- Nd in calcite to establish the mineralization age. Additionally, we investigated the coexistence of paleontological fossils from two different ages (Palaeozoic and Cretaceous) within the same stratum. Exploring its potential link to large-scale Mesozoic mineralization in the Middle-Lower Yangtze Metallogenic Belt (MLYB). The relationship between this coexistence and plate subduction dynamics was also examined. These findings contribute to a better understanding of the regional metallogenic processes and provide valuable insights for exploration and prospecting in the region.

The surface of the Shewushan gold deposit is covered by Quaternary, with sporadic outcrops (Figures 2, 3A). The Shewushan gold deposit is located in the Gao Tie duplex anticline (Figures 2, 3A), The Gao Tie duplex anticline axis is cut by an NE–SW high-angle thrust fault, and the Shewushan gold deposit is located in the Southeastern part of the anticline; anticline axis and fault present in east-west directions (Figure 4a), which is composed of early to middle Ordovician bioclastic limestone. Sinorthis typica Wang, Leptella cf. Hubeiensis Zeng, Sinorthis sp., and Tritoechia sp are present within this limestone. A Silurian carbonaceous mudstone containing thin layers of siliceous rock including graptolites, e.g., Orthograptus sp., Climacograptus sp., and Climacograptus cf. Antiquus lineatus Elles and wood, overlies the Ordovician limestone. The near-horizontal, low-angle Brachiopoda shell layer of the Ordovician in the deposit, which exists in the Southeastern part of the anticline in the Shewushan gold deposit, indicates that the attitude of the Ordovician–Silurian strata in this deposit is the lower angle or subhorizontal, rather than near upright as described previously (Li and Liu, 1995; Liu, 1995; Li, 1998).

The two east-west-trending high-angle reverse faults, F2 and F4 cut the country along rock and orebodies, respectively (Figures 4A, B). Deposits are commonly elongated in east-west directions parallel to these two faults (Figure 4A), indicating a close relationship between these two faults and mineralization. F2 is close to the Gao Tie duplex anticline axis; it is composed of fault breccia, fault gouge, gold-bearing siliceous stringers (Figure 5A), and calcite concurrent either with or without As-, Sb-sulfides, such as realgar and orpiment (Figure 5B). This composition implies that there was a multi-stage fault movement. In addition, northeast-southwest-trending faults appear to postdate east-west faults (Figure 4A) and are likely related to the post-metallogenic stage.

Alteration in the Shewushan Gold Mine includes silicification, sulfidization, decarbonization, bartizantion, and fluorite alteration. In the Shewushan gold deposit, gold mineralization is closely associated with decarbonation, silicification, and sulfidation. Decarbonation and silicification of limestone are evidenced by the replacement of limestone lenses within the siliceous rock (Figure 5C), the presence of a chalcedony stickwork in uneconomic low grade mineralized surrounding rock (Figure 5D), and small relict inclusions of calcite in quartz (Figures 5C–E). In addition, the major and trace element and rare earth element (REE) geochemical data show that siliceous rocks emerged through hydrothermal genesis, formed by the transmission of mineralized fluid bearing Si0₂ along faults to the surface, and replacing carbonate rocks to begin deposition (Wang et al., 2014a). The Shewushan gold deposit is associated with widespread hydrothermal alteration, characterized by decarbonation, silicification, sulfidation, and argillization. These features are similar to the Carlin-type gold deposits in Nevada, USA, Guizhou, China, and Osiris (Cline et al., 2005; Su et al., 2009; Steiner and Hickey, 2023).

The Shewushan Gold Mine ore is composed of both oxidized ores (laterite) and primary ores. The gold orebody is a “Z” big lens in the horizontal projection (Figure 4A), with a length of about 3,000 m, and a width of 200–600 m. The orebody output is mainly located in a single layer or two branches at depths of 0–70 m, occasionally 120–140 m below the surface. It is horizontally located in the footwall of a bedding-parallel thrust fault juxtaposing Early Silurian-Late Ordovician grey black poor permeable carbonaceous mudstone splint. It features thin layers of argillaceous slate against Early-Middle Ordovician bioclastic limestone or shell limestone splint with thin layers of carbonaceous siliceous rock. The gold orebody fault contacts wall rock (Figures 4B, C) and is mainly composed of roof horizontal oxidized orebody (laterite) and base horizontal primary orebody. The primary orebody is generally composed of fractured bioclastic limestone, with a thickness of 0–20 m and an Au grade of approximately 0.2 g/t to 3 g/t. The oxidized orebody also has a thickness of 10–20 m, with an Au grade of approximately 2 g/t, and is composed of either laterite interbedded with siliceous rocks (Figure 4B) or porous siliceous rocks splint with clay (Figure 4C) and a little silicified limestone gravel. The oxidized orebody contacts the primary orebody with a leached surface showing unconformity. At the eastern part of the deposit, early Silurian-Late Ordovician grey black carbonaceous mudstone had been denudate and the oxidized orebody had been covered by Quaternary loess.

Minor minerals include native gold, rutile, realgar, pyrite, stibnite, orpiment, cinnabar, anatase, tungsten, galena, white iron ore, epidote, apatite, dolomite, muscovite, garnet, tourmaline, and zircon (Hong and Ye, 1998). Under a microscope, no visible gold minerals were found in oxidized ores. However, electron probe and transmission electron microscopy analyses identified extremely fine (less than 0.03 μm) sub-microscopic gold particles, believed to exist because of physical adsorption or mechanical incorporation. Most gold in goethite is present as lattice gold (Hong and Ye, 1998). Under ×10,000 magnification, a small fraction of goethite particles, showing substantial gold enrichment with a composition of Au 99.82%, Ag 0.18%, and Cu 0.11%, appeared in spherical forms approximately 0.65 μm in diameter, mixed with goethite crystals (Hong and Ye, 1998). This enrichment pattern suggests that arsenian pyrite in goethite may contain independent gold minerals. Primary ore is categorized into fractured mudstone breccia and siltstone clay breccia. Arsenopyrite and realgar in primary ore are primarily distributed in a disseminated manner, with minor vein-like concentrations. In the primary ore, the gangue minerals include kaolinite, dickite, halloysite, and water mica, with minor amounts of quartz, chalcedony, and barite. The gold-bearing minerals are primarily pyrite and limonite, with gold predominantly occurring as microscopic grains within magnetite-rich rims at the margins of pyrite (Cai et al., 2016). Additionally, calcite, orpiment, realgar, cinnabar, and stibnite are also present. Consistent with Hu (2000), arsenopyrite sulfur isotopes show a broad range of variation. Electron probe microanalyzer results indicate that gold exists in arsenopyrite (850 ppm–1,550 ppm Au) and pyrite (470 ppm–1,340 ppm Au) of Shewushan primary ore (Wang et al., 2014b). The elemental composition of gold mineralization bodies displays distinct zoning: the proximal zone is characterized by As, Hg, Sb, Ba, and Ag; the mineralized zone by Au, Mo, Zn, W, F, and Pb; and the distal zone by Ni, Co., Bi, and Cu (Ni and Liu, 1995; Li, 1998). The primary ore bodies at Shewushan occur within clastic rock or bio-clastic limestone breccia zones. Geochemical anomalies of Au-As-Sb-Hg correspond with the widespread presence of realgar, orpiment, and stibnite (Ni and Liu, 1995; Li, 1998). The sulfides in the Shewushan deposit primarily consist of gold-bearing arsenian pyrite and arsenopyrite, with smaller amounts of orpiment, realgar, and stibnite. The gold-bearing arsenian pyrite and arsenopyrite are predominantly disseminated in jasperoid quartz grains or concentrated along the grain boundaries or adjacent areas of the jasperoid quartz (Figures 3E, F; Figure 5E). The sulfur isotopic values of pyrite range from −8.3‰ to +23.1‰, indicating the presence of two or more sulfur sources (Hu, 2000). In the proximal zones of carbonate dissolution, open pores and fractures are typically filled with orpiment and/or realgar veins (Figures 3C, 5F). Moving outward from these zones, calcite veins become more prevalent, often containing orpiment or realgar (Figure 5G).

Calcite veins are divided into three groups: The first group (G1) primarily fills fractures within the host rock. Veins are generally <2 cm wide, white, relatively short, have poor crystal transparency, The G1 calcite veins, which pre-mineralization, do not contain any gold or sulfides. G2 calcite veins are most likely the product of decarbonation during the main gold mineral stage. In summary, G2 is concurrent with As- and Sb-sulfide mineralizations, and features siliceous rocks fluid inclusions. Especially the spatial correlation of vein minerals to alteration zoning is assumed to have formed where acidic ore fluids were neutralized near gold mineralization, and are contemporaneous with main gold mineralization (Su et al., 2008). In this study, this spatial correlation has been used to constrain the age of gold deposition. G3 calcite veins are mostly concurrent without As- and Sb-sulfide mineralization and crosscut chalcedony veins that had formed in the main mineral stage (Figure 5E). This indicates that G3 precipitated after the main ore-stage alteration.

The second group of calcite veins (G2) commonly fill fractures or open cavities on the periphery of decarbonated rocks (Figure 3D), especially in proximity to reverse faults F2 (Figure 5B). These appear concurrent with As- and Sb-sulfides, such as realgar and orpiment (Figure 5G), occasionally stibnite and Au. G2 veins presented in large crystal clusters (Figure 5B), with weak weathering and higher transparency than G1. Two types of fluid inclusions exist in G2, namely, aqueous fluid inclusions and CO₂-rich fluid inclusions (Wang et al., 2014b). The homogenization temperatures determined from aqueous fluid inclusions in Shewushan gold deposit range from 140°C to 220°C (Wang et al., 2014b). Salinity levels range from 2.2 to 9.34 wt% NaCl eq (Liu and Li, 1995), which is consistent with Carlin-type or Carlin like-type gold deposits with an ore fluid of ∼250°C and <8 wt% NaCl eq from Navada, USA (e.g., Kuehn, 1989; Hofstra et al., 1991; Lamb and Cline, 1997; Cline et al., 2005; Muntean et al., 2011). This indicates that the Shewushan gold deposit is a classical Carlin-type deposit, formed at a depth of 1.five to two km (Liu and Li, 1995; Liu, 1995; Hu, 2000; Wang et al., 2014b; Cai et al., 2016), or finely disseminated gold deposit (Li, 1998).

The main samples of hydrothermal calcite veins concurrent with. As-, Sb-sulfides and Au,for Sm and Nd isotope dating samples were mainly obtained from along the F2 (G2: B201, HS39, BD13, BD-31,BD-13–2,F39-16),secondary sample (G2: JK230320-2,JK270320-32,JK310320-4) from the drill ZK6303. Samples from the calcite veins, collected after the main ore-stage of mineralization (G3: LS-1, G1: D112, BD102, H39-16, BD12-2, BD23, BD-28) was selected for comparison. Syn-mineralization calcite veins contain relatively higher concentrations of REEs and medium rare earth elements (MREEs) than pre-ore calcite veins, The rare earth element characteristics are similar to those observed in the Shuiyingdong and Zimudang Carlin-type gold deposits in Guizhou (Su et al., 2009; Tan et al., 2019; Wang et al., 2021); they also present variable Sm/Nd ratios suitable for isotopic dating (Bell et al., 1989; Halliday et al., 1990; Chesley et al., 1991; Darbyshire et al., 1997; Peng et al., 2003a; 2003b). The Sm-Nd isochron ages of hydrothermal calcite veins in the Shuiyindong gold deposit were successfully determined by Su et al. (2009). In their study, nine samples of G2 were selected to test REE, Rb-Sr, Sm-Nd, and five sample of G1 and one sample of G3 (LS-1) were selected to test REE for a comparison with G2 samples.

Pure calcite separates were hand-picked under a binocular microscope and were crushed to 200 mesh in an agate mortar. Prior to isotopic analysis, concentrations of REEs in subsamples of separated calcites were determined by inductively coupled plasma optical emission spectrometry (plasma direct-reading spectrometer JY38 S and fluorescence spectrophotometer Magix_pro2440) after acid digestion of samples in Teflon bombs at the Comprehensive rock and mineral testing Center of Wuhan, China. Sm and Nd contents and isotopic composition analyses were conducted according to procedures described by Peng et al. (2003b) and Su et al. (2009). Samples were dissolved in Teflon vessels with a mixture of HF and HClO₄ at a ratio of 10:1 at 150°C for at least 12 h, until decomposition was complete. Two separated sample aliquots, each weighing 150 mg, were dissolved, one for determining present-day ⁸⁷Sr/⁸⁶Sr and ^l43Nd/¹⁴⁴Nd ratios and the other for spiking and determining Sm and Nd concentrations. Separation of Sm and Nd was achieved by reverse-phase extraction with a HDEHP coated in Teflon powder. Sm and Nd concentrations were determined by isotope dilution using the ¹⁴⁹Sm-¹⁴⁶Nd spike solutions. Isotopic ratio measurements were conducted on a thermal ionization mass spectrometer at the Tianjin Institute of Geology and Mineral Resources, China. Nd isotopic ratios were normalized to a¹⁴⁶Nd/¹⁴⁴Nd ratio of 0.7219 and Sr isotopic ratios were normalized to an ⁸⁶Sr/⁸⁸Sr ratio of 0.1194. The reproducibility of isotopic ratios was about 0.005% at the two-sigma level, and the precision of the Nd and Sm concentrations at the two-sigma level was about ±0.5% of the quoted values. Concentrations of BCR-2 determined in this study were 6.61 ppm Sm and 28.13 ppm Nd. An ^l43Nd/¹⁴⁴Nd ratio of 0.512,633 ± 10 (2σ, n = 10), consistent with the reference value (Weis et al., 2006), was also determined. The SRM987 standard yielded an ⁸⁷Sr/⁸⁶Sr value of 0.710,245 ± 0.000025 (2σ, n = 10); and the JMC Nd standard provided a ^l43Nd/¹⁴⁴Nd value of 0.511,132 ± 0.000020 (2σ, n = 10). Over the course of this study, blanks were 30 pg for Sm, 54 pg for Nd, and 0.56 ng for Sr The Sm-Nd isochron age was calculated using the ISOPLOT 3.0 plotting program (Ludwig, 2003). In this study, the decay constant, λ, used for ¹⁴⁷Sm was 6.54 × 10⁻¹² a⁻¹, and the ¹⁴⁷Sm/¹⁴⁴Nd and ^l43Nd/¹⁴⁴Nd values from the chondritic uniform reservoir used for the εNd(t) calculations were 0.1967 and 0.512,638, respectively.

(1) Hydrothermal calcite veins, concurrent with As- and Sb-sulfides, are relatively enriched in both MREEs and HREEs. ∑REEs contents of hydrothermal calcite veins were found to be higher than calcite veins prior to mineralization. Sm-Nd isochron ages (99.1 ± 2.4 Ma) for hydrothermal calcite veins, with initial εNd values ranging from −12.2 to −12.6, represent the age of decarbonation and gold deposition in the Shewushan gold deposit. This study provided the metallogenic age constraint of the Carlin-type gold deposit in the MLYB for the first time.