Introduction

The Youjiang Basin, along the south-eastern margin of the Yangtze Block of southern China (Fig. 1a), hosts numerous gold deposits that collectively have resources of > 800 tonnes (> 25 Moz) gold, with Shuiyindong in the Huijiabao trend contributing about 265 tonnes (7 Moz) gold and Jinfeng in the Lannigou trend contributing 167 tonnes (> 5 Moz) gold (Fig. 1b; Hu et al. 2017a). There has been much controversy concerning their classification and genesis in the Youjiang Basin. Most authors have used the term Carlin-like (Deng and Wang 2016; Stephen et al. 2007) or even Carlin-type (Hu et al. 2002; Pi et al. 2017) to describe them, whereas others have suggested they are intermediate between Carlin-type and orogenic gold deposits (Cline et al. 2013; Xie et al. 2017), or Carlin-style orogenic deposits (Tran et al. 2016), or perhaps simply epizonal orogenic gold deposits (Goldfarb et al. 2018), by analogy to those of the Kuskokwim Basin of southwestern Alaska (Goldfarb et al. 2004). Similarly, there have been correspondingly conflicting suggestions concerning the fluid source, with meteoric, aqueous or organic-rich basin, magmatic, and/or metamorphic fluids all suggested by various authors (Hu et al. 2002, 2017a; Cline et al. 2005), as summarized in Goldfarb et al. (2018).

Fig. 1
figure 1

a Simplified geology of the eastern part of the South China Block showing tectonic units discussed in the text. Adapted from Qiao et al. (2015) and Duan et al. (2018); b Geology, gold deposits, and aeromagnetic contours (thin light pink and gray lines) in the Youjiang Basin: adapted from Xiong et al. (2013), Peng et al. (2014), Hu et al. (2017b), and Chen et al. (2015b). The Huijabao Trend includes the world-class Shuiyindong deposit as well as the Zimudang and Taopingdong deposits

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Although there has been some comparison of the Chinese and Nevada deposits, in terms of tectonic setting, thermal history, and district to deposit-scale structural evolution and geometry, the focus was usually on the intra-deposit scale, such as mineralogical, isotopic and fluid inclusion characteristics of the ore bodies and alteration halos.

Since Wyborn et al. (1994) proposed scale-dependant elements for ore formation, several authors have refined this mineral systems approach, which develops a holistic model across the scales from province, through district to deposit scale to recognize synergies between different mineral-deposit classes (Hronsky et al. 2012; Deng and Wang 2016). Such synergies may have important ramifications in mineral exploration (Groves et al. 2016).

We here adopt a mineral system approach to compare the “Carlin-like” gold deposits of the Youjiang Basin and the archetypal Carlin-type gold deposits of Nevada in terms of their key characteristics at different scales. Background information abounds in the published literature and is only summarized here.

Comparisons at the deposit to intra-deposit scale

Deposit geology

Both Nevada Carlin-type and the Youjiang gold deposits comprise a combination of broadly stratabound-replacement and steep brittle fault-controlled mineralization. The Youjiang deposits are hosted both in the platform sequences composed of calcareous and bitumen-bearing rocks and in the basin facies consisting of mudstone and siltstone (Table 1; Gu et al. 2012; Hu et al. 2017a); by contrast, the Nevada deposits are mostly hosted in platform carbonates (Cline 2001; Cline et al. 2013). Both Nevada (Emsbo et al. 2003) and the Youjiang ores (Bao et al. 2005) that are hosted in the platform sequences are associated with bitumen in places.

Table 1 Comparison of gold mineral systems between the Youjiang and Nevada Carlin-type deposits: in order of increasing scale to match discussion in text
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Equivalent alteration, including silicification, sulfidation, argillization, and decarbonation, are recorded between the majority of Youjiang deposits and the Nevada deposits (Su et al. 2009a; Hou et al. 2016), whereas small differences have been identified in several Youjiang deposits, including the presence of ferroan dolomite, only limited decarbonation, and no low-temperature kaolin (Su et al. 2012; Cline et al. 2013; Xie et al. 2017). Moreover, silicification in the Youjiang deposits produced coarser grained quartz than in the Nevada ores where jasperoid is common as a result of decarbonation (Cline 2001).

Both the Youjiang and Nevada deposits have a similar association of mostly invisible lattice-bound gold in texturally and compositionally zoned pyrite and arsenian pyrite with similar ore-element associations of Au, As, Cu, Hg, Sb, and Tl (Cline 2001; Bao et al. 2005; Cline et al. 2013; Hu et al. 2017a). These broad similarities have led to the widespread classification of the Chinese deposits as Carlin-like. The other arsenic-bearing host minerals for both the Nevada and Youjiang deposits are realgar and orpiment in the late ore stage (Su et al. 2009a; Cline et al. 2013; Hu et al. 2017a). In contrast to the Nevada deposits, the commonly coarser-grained pyrite from the Chinese deposits does not have the consistent arsenic- and gold-rich rims, but economic gold is related to later generations of gold-rich pyrite and arsenopyrite (Su et al. 2012; Hou et al. 2016). Free gold is essentially absent in the Nevada Carlin-type ores but does occur rarely in the Chinese deposits such as at Shuiyindong (Su et al. 2008; Hou et al. 2016).

In contrast to the Nevada ores, quartz veinlets are common and some contain breccia fragments of the country rocks. These veinlets are sub-vertical and the nature of breccia fragments (Hou et al. 2016) indicates brittle failure. There are no sub-horizontal veins or crack-seal textures to suggest formation via over-pressured fluids (Sibson 2004; Cox 2005) as for most orogenic gold systems (Goldfarb et al. 2005; Yang et al. 2016). A key question is whether the presence of quartz veins marks a significant difference to the Nevada deposits or is because the classic Nevada Carlin-type deposit has underlying dominantly silica-poor dolomites, limestones, and mudstones whereas the Chinese deposits have significant silica-rich turbidite sequences in underlying, or even hosting, stratigraphic successions.

Ore fluids and stable isotopes

Despite the reports of hydrocarbon fluid inclusions in some bitumen-bearing deposits (Gu et al. 2012), the majority of Youjiang gold deposits formed from near-neutral, low-salinity, CH4-free, and largely aqueous fluids with 6–8 mol% CO2 at temperatures that locally exceeded 300 °C at depths in excess of 1.7 km and up to 4.3 km (Hu et al. 2002; Cline et al. 2013). As such, they are broadly similar to Carlin-type low-salinity, CH4-poor ore fluids with < 4 mol% CO2 at temperatures of 180–240 °C, predominantly at depths between 0.3 and 3.0 km, but possibly as deep as 5 km based on fluid inclusion and geological evidence (Cline and Hofstra 2000). Important differences are the slightly higher CO2 contents, temperatures, and depths of formation of the Chinese deposits. Only epizonal orogenic gold deposits are formed under similar conditions, but normally from fluids with higher CO2 contents and commonly with measurable CH4 (Gebre-Mariam et al. 1995).

Although the δ34S values of the pyrite from wallrock in the Youjiang deposits range largely from −30 to +30 per mil, those for the Au-bearing pyrite and arsenian pyrite concentrate within 1 to 10 per mil (Table 1; Tan et al. 2015; Hou et al. 2016; Yan et al. 2018). This limited range is interpreted to have resulted from mixing between magmatic fluid and sedimentary material (Hou et al. 2016; Yan et al. 2018). The δ34S values for Carlin-type ores have a similar range of −1 to +10 per mil (Kesler et al. 2005). The calculated fluid δ18O values are 1 to 17 per mil for the Chinese deposits (Su et al. 2009a; Peng et al. 2014) and ore-hosting quartz in most Nevada deposits has comparable δ18O values of −5 to 13 per mil (Emsbo et al. 2003; Cline et al. 2005). The oxygen isotopic compositions of the fluids have commonly been interpreted to represent a deep ore fluid, most likely magmatic-hydrothermal, with varying degrees of water-rock reaction and mixing with meteoric water (Hofstra et al. 1991; Lubben et al. 2012; Tan et al., 2015). No negative δ18O values were reported in the Youjiang deposits, suggesting limited mixing with meteoric water, perhaps due to their interpreted greater depth of formation. The Youjiang deposits display a wider range in fluid δ18O values than those of 7 to 13 per mil for Phanerozoic orogenic gold deposits (Goldfarb et al. 2005), possibly due to interaction with contrasting host sequences.

Discussion

The Youjiang and Nevada deposits generally share similar alterations including silicification, argillization, sulfidation, and decarbonation. Sulfidation shows the most consistent relationship to gold mineralization (Cline 2001; Kesler et al. 2003; Ye et al. 2003). The dissolution of ferroan carbonate minerals in the host rocks to provide Fe for sulfidation reactions has been suggested as the most likely precipitation mechanism of gold either as “invisible” or visible gold in arsenian pyrite (Kesler et al. 2003; Su et al. 2008). Given natural intra-group variations at the deposit to intra-deposit scale, the Youjiang deposits have more similarities than contrasts to the Nevada deposits than the orogenic gold deposit (Groves 1993; Goldfarb et al. 2005; Deng et al. 2015; Yang et al. 2016), reinforcing the Carlin-type classification. The main difference is the higher maximum temperature of ~ 60 °C at which the Chinese deposits are interpreted to have formed. This equates to a 1–2 km greater depth of formation, consistent with contrasts in estimated depth ranges. This can potentially explain the coarser-grained auriferous pyrite in the Chinese ores and lesser participation of meteoric water in mixed ore fluids. The approximately 160 Mys difference in mineralization age between the Upper Triassic age of the Chinese deposits (robust Re-Os and Ar-Ar ages listed by Hu et al. (2017a), as discussed further below) and the Eocene age of the Nevada deposits (Cline et al. 2005; Muntean et al. 2011) is also compatible with a different erosional level for the two groups of deposits.

The key question is whether an approximately 60 °C and 1–2 km depth difference mitigates against the classification of the Chinese deposits as Carlin-type, particularly since other mineral deposit classes, such as orogenic gold (Groves 1993), IOCG (Groves et al. 2010), and porphyry copper-gold deposits (Sillitoe and Perelló 2005), can each form a continuum with much greater P-T extremes. This is further examined below at greater scales.

District-scale structural comparisons

Structural histories and geometries

As summarized by Cline et al. (2005) for Nevada deposits and Goldfarb et al. (2018) for the Youjiang deposits, the host terranes for the gold deposits experienced a classic orogenic cycle from compression and transpression to post-orogenic collapse and extension. Early-orogenic thrust and fold structures were both re-activated or overprinted by later extensional structures (Rhys et al. 2015). Although a few authors (Chen et al. 2011, 2015a) suggest that the Chinese deposits formed with a similar timing to the majority of orogenic gold deposits (Goldfarb et al. 2005), at the transition from compression to transpression, the time gap between precise mineralization ages at ca. 210 Ma and initiation of the regional extension at ca. 220 Ma (Wang et al. 2007) support formation of the Youjiang deposits at the onset of extension in the host terranes.

A specific comparison of the structural timing and controls of the gold deposits from Nevada (Rhys et al. 2015) and those of the Youjiang Basin is summarized concisely in Rhys (2017). For Nevada Carlin-type, extension was focused locally on structural highs formed in the earlier compressional deformation phase, with reactivation of existing reverse faults and the development of new extensional faults controlling auriferous fluid ingress to depositional sites (Rhys et al. 2015). Cross-sections of the Nevada deposits commonly illustrate a structural geometry of a combination of stratabound replacement-style orebodies, in gentle monoclines to anticlinal half-horsts, connected to steeply-dipping fault-controlled orebodies (Cline et al. 2005; Rhys et al. 2015; Fig. 2a).

Fig. 2
figure 2

Cross-sections of characteristics of large gold deposits in the Nevada district and Napanjiang Basin. a Screamer-Betze-Deep Post deposits in the Nevada district, from Kesler et al. (2003); b Taipingdong deposit in the Huijiabao Trend, from Hou et al. (2016); c Shuiyindong deposit in the Huijiabao Trend, from Peng et al. (2014); d Lannigou deposit, from Chen et al. 2011

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There are similar controls of the Chinese deposits through extensional reactivation of thrusts and folds and generation of an extensional fault system (Figs. 1b and 2b–d; Hu et al. 2017a; Rhys 2017). In places this reactivation was preferentially focused on paleogeographic highs, represented by isolated carbonate platforms that developed from the Early Paleozoic (Su et al. 2009a; Chen et al. 2015b): for example at the Lannigou, Jinya and other deposits (Fig. 1b). The margins of these platforms developed into fault surfaces for selective ore fluid infiltration due to lithological contrasts between platform and surrounding basinal sequences in the extensional setting.

The geometries of ore-controlling structures in the Youjiang Basin are very similar to those in the Nevada district (Table 1). A structural map of the Huijiabao trend, that includes the world-class Shuiyindong deposit, demonstrates well this geometry of a combination of an early anticline with axial-surface parallel reverse (thrust) faults cut by oblique normal faults (Fig. 2b; Hou et al. 2016). Cross-sections of the Taipingdong (Fig. 2b) and Shuiyindong (Fig. 2c) deposits in the Huijiabao trend, and the Lannigou deposit (Fig. 2d), show stratabound gold mineralization sited in gentle anticlines or monoclines, almost identical, if more simplistic, in geometry to cross-sections of the Nevada Carlin-type deposits. Other deposits, such as Yata, consist of steeply dipping orebodies in extensional faults (Zhang et al. 2003).

In the Nevada district, a critical structural component controlling the spatial location of ore deposits the Roberts Mountain Thrust. The thrust juxtaposed siliciclastic and basaltic rocks with the carbonate shelf sequences, the main host of Carlin-type gold ores (Emsbo et al. 2006). Similarly in the Youjiang basin, the carbonate-dominated units were juxtaposed against siliciclastic rock sequences (Hou et al. 2016; Figs. 1b and 3). The ore deposits in both the Youjiang and Nevada districts are preferentially developed adjacent to the thrust, and the majority of the gold resource, including the world-class Shuiyindong and Jinfeng deposits in the Youjiang Basin, are hosted in the dominantly permeable carbonate units.

Fig. 3
figure 3

Schematic maps providing a province-scale comparison between the setting of gold deposits in the Nevada district a and in the Youjiang Basin b. Features to note include location at near-orthogonal margins of a fragment of continental crust, with gold deposit trends subparallel to these margins, and close spatial association of the gold deposits with a major thrust juxtaposing two parts with contrast lithology. Nevada data derived from, Grauch et al. (2003), Cline et al. (2005), Morrow and Sandberg (2008), Muntean et al. (2011), Watts et al. (2016), and references therein. Youjiang data derived from Stephen et al. (2007), Peng et al. (2014), Chen et al. (2015b), Deng and Wang (2016) and Hou et al. (2016), and references therein

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Discussion

There are obvious similarities in structural timing, structural regime, and structural geometry between the Youjiang deposits and the Nevada deposits, as also concluded by Rhys et al. (2015), Hu et al. (2017b), and Rhys (2017). These structural geometries are in marked contrast to those of orogenic gold deposits where fold-related deposits are normally sited in tight, reclined, “locked-up” anticlines, and associated thrusts and are typified by sub-horizontal extensional quartz veins (summarized by Groves et al. (2018) and references therein). It must be concluded that the Chinese Carlin-like deposits of the Youjiang Basin meet the criteria for classification as Carlin-type deposits at the district to deposit scale.

Province-scale comparisons

Tectonic setting and geometry of continental crustal margins

The precise tectonic settings of the Nevada province and the Youjiang Basin of China are different. Whereas the Nevada district lies on the fractured margin of the North American Craton which faced the Pacific Ocean throughout its subduction-related compressional to extensional history, the Youjiang Basin is generally considered a foreland basin related to closure of eastern branches of the Paleo-Tethys and subsequent collision between continental blocks (Lehrmann et al. 2007; Deng et al. 2017; Duan et al. 2018). Support for the foreland model is provided by southward thickening of Triassic strata in the Youjiang Basin and the northeast and northwest vergence of folds and thrust faults in northeast Vietnam (Lepvrier et al. 2011) and on the southeastern margin of the Youjiang Basin (Qiao et al. 2015), respectively.

Despite this, the province-scale geometries of the two regions are very similar as are their order-of-magnitude scales. The Nevada district is sited adjacent to a major junction between the regional, broadly NW-SE trending craton margin and a broadly NE-SW trending indentation (Fig. 3a). These almost orthogonal margins, defining the continental crustal block enclosing the majority of the deposits, are defined by Sr and Pb isotope initial ratios and geophysical interpretations (Crafford and Grauch 2002; Grauch et al. 2003). The goldfield trends reflect the near-orthogonal geometry of the continental block margins. The Carlin Trend (approx. 75 km long) and Battle Mountain-Eureka Trend (approx. 125 km long) are subparallel to the regional NW-SE trending craton margin, whereas the Getchell and Jerritt Canyon Trends (approx. 35–40 km long) are subparallel to the NE-SW trending indentation in the craton margin (Fig. 3a).

The northwestern, highly mineralized part of the Youjiang Basin is sited in a continental crustal block bounded by the broadly NE-trending Shizong Mile Fault to the northwest and the broadly NW-SE trending Ziyun Yadu Fault to the northeast (Figs. 1a, 3b). These faults separate the basement rocks of the Yangtze Block from the metasedimentary sequences of the Youjiang Basin, and must have been two of the fundamental crustal to lithospheric faults that controlled both the evolution and gold distribution of the basin (Hu et al. 2017a and references therein). As in the Nevada district, the goldfields define roughly orthogonal trends that subparallel the fault boundaries (Figs. 3a, b). The Huijiabao Trend (approx. 40 km long), that includes the world-class Shuiyindong deposit, as well as the Zimudang and Taopingdong deposits, trend broadly WNW-ESE subparallel to the Ziyun Yadu Fault, whereas the Getang Trend (approx. 30 km long) and Lannigou Trend (approx. 30 km long), that includes the world-class Jinfeng deposit, as well as the Panxian trend along the northewestern boundary of Youjiang Basin are subparallel to the NE-SW trending Shizong Mile Fault. Other gold trends are less well-defined and are shown as less-robust trends in Fig. 3b based on the trends of sporadic individual deposits. The definition of such trends has obvious exploration implications for the distribution of as-yet-undiscovered deposits.

Thermal drivers for gold mineralization

Although the precise tectonic processes for generation of a regional-scale thermal anomaly responsible for gold mineralization over tens of thousands of square kilometers at both the Nevada and Youjiang districts are debated, the thermal drivers appear broadly similar.

In the Nevada district, possibly through the removal of the Fallon subduction slab (Cline et al. 2005), onset of extension and uprise of asthenosphere resulted in melting of previously metasomatized lithosphere to produce basic to ultrabasic melts that ponded at the MOHO and melted continental crust. This led to intrusion of a series of hybrid high-K granitic melts which are considered to be the heat and probable ultimate fluid sources for the Nevada deposits, as summarized by Cline et al. (2005) and Muntean et al. (2011). Despite the recognition of a few Eocene syn-gold intrusions exposed in the Nevada district (Maroun et al. 2017), their spatial relationship to the Nevada gold deposits is largely inferred from geophysical data (Ressel and Henry 2006). The evolution from the inferred high-T magmatic hydrothermal ore fluids to the low-T fluids that deposited the gold and the depositional mechanisms themselves are still controversial (Cline et al. 2005; Muntean et al. 2011). Continued extension resulted in the development of a series of metamorphic core complexes extending for over 1000 km along the eastern margin of the Nevada province, characterized by the Basin-and-Range terrain (Konstantinou et al. 2013), with the slightly off-trend Ruby Mountains metamorphic complex being the most proximal to the Carlin Trend (Howard 2003).

The thermal driver for the evolution of the Youjiang goldfields has been unclear as there are no recorded granitic intrusions coeval with the gold mineralization in the basin (Zhu et al. 2017), one of the major factors in the controversy concerning the classification of the Chinese deposits. Any heat or fluid source is constrained by the age of gold mineralization. The recent robust ages include ca. 223–185 Ma from Ar/Ar of sericite and Re-Os of arsenopyrite for Jinfeng (Chen et al. 2015a); ca. 218–209 Ma for Zhesang from Ar/Ar in sericite and U-Pb in rutile (Pi et al. 2016, 2017); ca. 218–209 Ma for Laozhaiwan from U-Pb in monazite (Hu et al. 2017a); and 268 (240)-202 Ma for Shuiyindong (Chen et al. 2015a). Collectively, these data suggest that gold mineralization took place in the Upper Triassic at ca. 210 Ma. Samarium-Nd isochron ages of 134 to 136 Ma reported for hydrothermal calcite from the Shuiyingdong deposit (Su et al. 2009b), probably represent a later limited hydrothermal overprint.

It is thus instructive to examine what thermal events were occurring in the vicinity of the Youjiang Basin during the mineralization period. In the Upper Triassic, there was also a post-collisional extensional event along the Ailaoshan-Songma Paleotethyan Suture on the western margin of the Yangtze Block. This is manifested by the development of extensional basins and basic-felsic magmatism (Yang et al. 2018; Fig. 1a). South of the Youjiang Basin, this critical period was also characterized by extension, after Early to Middle Triassic intraplate crustal shortening with folding and associated thrusting in North Vietnam and the Shiwandashan basin (Lepvrier et al. 2011; Wang et al. 2007). On the north-western margin of the Yangtze Block, a > 1000-km long Mesozoic domal belt associated with synchronous granite intrusion (Fig. 1a) has been interpreted to be related to asthenosphere upwelling and consequent crustal extension (Roger et al. 2004; Sigoyer et al. 2014). However, as discussed above, there is no direct evidence of magmatism in the Youjiang Basin at this time.

Discussion

Although their tectonic settings are different, both the Nevada and Youjiang gold districts formed during post-orogenic extension adjacent to crustal- to lithosphere-scale faults bounding the margins of continental crustal blocks. They are both sited in anomalous geometrical configurations on these fragment margins where first-order faults are locally orthogonal (Fig. 3). The linear trends of the goldfields and gold deposits in both cases are subparallel to the bounding faults, with individual trends almost orthogonal to each other, although the Nevada trends are more extensive, commensurate with their far greater gold endowment.

Formation of the Nevada Carlin-type deposits was driven by asthenosphere uprise that acted as a thermal anomaly (Muntean et al. 2011) to instigate the uprise of metamorphic core complexes and melt both metasomatized lithosphere and crust to produce hybrid magmas that were intruded as granitic plutons into the sedimentary rocks of the continental margin (Cline et al. 2005). The Carlin-type ores are interpreted to be deposited from a highly modified magmatic-hydrothermal fluid that was exsolved from these plutons.

There are no recorded granitic intrusions within the Youjiang Basin despite the clear evidence for extensional tectonics at the time of gold mineralization. This does not necessarily mean that no granitic plutons were intruded as the magmatic association in the Nevada district was only recognized initially from high-quality company-generated aeromagnetic surveys (Ressel and Henry 2006), and only later confirmed from rare outcrops of gold-synchronous granites (Muntean et al., 2011). In the Youjiang Basin, there are positive aeromagnetic anomalies spatially related to the ore districts (Xiong et al. 2013; Fig. 1b), but it is currently unclear if these relate to concealed intrusions of appropriate age to be gold-related. The specific trending of the Youjiang gold deposits parallel to the margins of a continental block and the extensional timing of gold mineralization in that block are in direct contrast to the tectonic settings and transpressional timing of the majority of orogenic gold deposits.

An important similarity between the Nevada and Youjiang gold districts is the presence of metasomatized lithosphere beneath the continental crust (Muntean et al. 2011; Zhao et al. 2011). The south-eastern margin of the Yangtze Block has witnessed an episode of oceanic subduction in the Neoproterozoic resulting in such metasomatism (Zhao et al. 2011), and the world-class Huijiabao and Lannigou deposits are above similarly modified lithosphere along the south-eastern boundary of the Yangtze Block (Fig. 1a). However, unless it can be shown that melting of such metasomatized lithosphere produced intrusions coeval with gold mineralization, the possibility that the Youjiang gold deposits had a more distal thermal energy and fluid source than the Nevada deposits must be considered. Such a model has the potential to explain their order-of-magnitude lower gold endowment.

Conclusions

An across-scales comparison between the Triassic, largely carbonate-hosted, gold deposits of the Triassic Youjiang Basin and the Tertiary Nevada deposits is instructive. At the deposit to intra-deposit scale, there are numerous similarities to the Tertiary Nevada deposits, with differences that can be explained by formation of the older Youjiang deposits at up to 60 °C higher temperatures and up to 2 km greater depths commensurate with deeper erosion levels: a difference well within the limits of variation of other coherent gold-deposit classes. At the district to deposit scale, the Nevada and Youjiang deposits are both characteristically sited in gentle anticlines, monoclines, and/or half-horsts or along extensional faults in previously-deformed sedimentary sequences. At the province scale, both the Nevada and Youjiang gold deposits form sets of almost orthogonal trends that are subparallel to the margins of continental crustal blocks underlain by metasomatized lithosphere. An apparent more proximal source of thermal energy and deep fluid in the Nevada district can best explain its order-of-magnitude greater gold resource in the current absence of definitive evidence for intrusion of syn-gold hybrid magmas into the Youjiang Basin. It can be concluded that the Chinese deposits are comparable with the Nevada deposits within a mineral system context and should be classified as Carlin-type deposits.