The mining industry faces a challenge due to the scarcity of outcrop or near-surface mineral deposits, necessitating the development of low-cost and efficient prospecting methods to explore through cover. Inductively coupled plasma mass spectrometry has been applied to geological sample analysis, and scientists have gradually improved the method of geogas prospecting. As a result, geogas prospecting has shown promising results in detecting underlying concealed Au, U and Cu–Ni deposits covered by Gobi sand and Quaternary sediments. To verify the effectiveness of this method for exploring underlying concealed mineral deposits developed in igneous covered areas, the Zijinshan high-sulfidation epithermal Cu–Au deposit, a concealed mineral deposit in southeastern China, was selected as the experimental field. Our experiments revealed nanoscale particles composed of Au, Cu–Fe and Cu–other elements in the geogas captured above the ore bodies of the deposit. Furthermore, Cu–nanoscale particles retain the isotopic composition of copper found in both the oxidation zone and deep copper ores. The geogas samples exhibited similar C1-chondrite normalized REE distributions, with REE patterns indicating significant enrichment of LREE relative to HREE and similar (La)N/(Sm)N and (Gd)N/(Yb)N ratios. These characteristics are similar to those of the gold ores, copper ores, altered rock and bedrock near the ore bodies. These findings suggest that deep-penetration geochemical methods using geogas can be a valuable tool for uncovering underlying concealed mineral deposits in igneous covered areas.

With outcrops of mineral deposits becoming increasingly scarce, mineral exploration has turned to loose sedimentary and nascent geological strata overburden, which has made geochemical exploration technology a priority of applied geochemistry research (Wang and Ye 2019). Early techniques, such as ammonium citrate-soluble extraction, were limited to partial element extraction, failing to distinguish mineralization anomalies from other non-mineralization anomalies (Bloom 1955; Bradshaw et al. 1974; Chao 1984). Other advanced techniques, such as enzyme leaching and electrogeochemistry, have contributed to the recent development and success of deep-penetration geochemical methods (Shmakin 1985; Kristiansson and Malmqvist 1987; Clark et al. 1990; Antropova et al. 1992; Clark 1993; Mann et al. 1995; Wang 1998). Further technological advances may bring deeper probing and greater exploration success (Wei et al. 2013; Wang et al. 2021). While these techniques have yielded some success in detecting underlying concealed mineral deposits, like those covered by thick sediment deposits with tens of metres in depth, other harsh geological conditions have impeded their effectiveness (Xie 1998). More recent breakthroughs in deep-penetration geochemical methods, such as gas geochemistry, biogeochemistry, groundwater geochemistry and fine fraction soil extraction, using elemental data from sediments analysed by inductively coupled plasma mass spectrometry (ICP-MS) have provided promising results in finding mineral deposits concealed by Gobi sand and Quaternary sediments, detecting Au, U and Cu–Ni deposits (Lin et al. 2014; Wang et al. 2016; Lu et al. 2019, 2021).

To evaluate the effectiveness of deep-penetration geochemical methods using geogas in detecting underlying concealed mineral deposits, we conducted a study on the Zijinshan high-sulfidation epithermal (HSE) Cu–Au deposit. In this study, we collected and analysed geogas samples in the deposit cover and compared the results with previous studies on the bedrock (Zhang et al. 2001), altered rock (Ruan 2019) and ores (Lu et al. 2016). Our study confirmed the applicability of deep-penetration geochemical methods using geogas in exploring underlying concealed mineral deposits in igneous and Quaternary sediments covered areas. By analysing the nanoscale particles and geogas samples, we have obtained direct geological information about the concealed ore bodies beneath the surface. Our findings indicate that the presence of nanoscale particles and multi-element geochemical anomalies in these samples strongly suggests that these nanoparticles originate from the ore bodies themselves. This discovery highlights the potential of surface sampling techniques, such as geogas prospecting, in providing valuable insights into the geological characteristics of covered areas and facilitating the discovery of mineral deposits.

Deposit geology

The South China block is bisected into two distinct geological regions by the Jiangshan–Shaoxing–Pingxiang–Yushan fault, namely the Yangtze block in the NW and Cathaysia block in the SE (Yu et al. 2010; Chen and Mao 2022). The Cathaysia Block on the southeastern coastal area of China, owing to the subduction of the palaeo-Pacific plate beneath the Eurasian plate, has experienced extensive Yanshanian magmatism and mineralization, including Cu–Au, Au–Ag, Pb–Zn and W–Sn (Mao et al. 2012). The Zijinshan ore field, situated on the northeastern margin of the Late Mesozoic Shanghang basin (Fig. 1a) at the intersection of the Yunxiao–Shanghang fault and the Xuanhe Anticlinorium within the interior Cathaysia, comprises a typical normal HSE copper–gold deposit and has been considered a significant part of the porphyry Cu system (Jiang et al. 2017; Zhao et al. 2020). With more than 350t Au (at 0.20 g t−1) and 3.50 Mt Cu (at 0.25%) in resources (Chen and Mao 2022), the Zijinshan HSE Cu–Au deposit is renowned for its considerable metal reserves. Gold and copper mineralization are primarily associated with medium-fine-grained granites, porphyritic dacite, crypto-explosive breccias and hypabyssal intrusions, forming a suite of porphyry–epithermal hydrothermal systems with diverse mineralization types within the Zijinshan ore field (Fig. 1b) (Zhang et al. 2001, 2003, 2005; Wang et al. 2009; Hu et al. 2012; Jiang et al. 2013; Liang et al. 2013; Yu et al. 2013; Zhong et al. 2014; Duan et al. 2017). Such mineralization is closely correlated with large-scale tectono-magmatic and hydrothermal events during the Late Mesozoic period (Zhang et al. 2015; Li and Jiang 2017).

The Zijinshan HSE Cu–Au deposit is situated in the central region of the Zijinshan ore field. The prominent lithostratigraphic units observed within the ore field comprise the Neoproterozoic Louziba Group, Late Paleozoic clastic sediments, Early Cretaceous volcanic assemblages and Quaternary alluvial sediments. The Middle Jurassic Zijinshan granitic complex, alongside the Cretaceous cryptoexplosive breccia and dacitic porphyry, serves as the host rocks for the Cu–Au deposits (So et al. 1998; Li and Jiang 2017; Wu et al. 2017; Zhao et al. 2020).The Cu–Au mineralization zone in the Zijinshan deposit has been found to extend over 1400 m vertically, from −400 to 1000 m elevation, with drill holes identifying covellite- and digenite-dominated mineralization at a depth of −400 m elevation (Li and Jiang 2017). Intensive and pervasive alteration of igneous rocks in the Zijinshan granitic complex, comprising the Middle Jurassic Jingmei, Wulongsi and Jinlongqiao granite batholith and Cretaceous cryptoexplosive breccia and dacitic porphyry, shows a typical high-sulfidation alteration zonation (Fig. 1b). The vertical distribution of mineralization in the deposit is controlled by an upper oxidation zone, a leaching zone and a lower primary zone, which contribute to the framework of Au and Cu enrichment (Fig. 1c, d) (Jiang et al. 2017). Weathering of the deposit has led to supergene-enrichment of gold, forming a quartz–limonite–native gold assemblage, with pyrite and limonite serving as the primary gold-bearing minerals. Copper ores occur in veins and veinlets, with chalcocite, enargite, chalcopyrite and bornite being major ore minerals, often accompanied by local azurite occurrences (So et al. 1998; Wu et al. 2017).

Ore features

The Zijinshan HSE Cu–Au deposit is a complex polymetallic deposit consisting of oxide and polymetallic sulfide ores (Wang et al. 2009). The dominant metallic component is pyrite (Fig. 2a, b), which typically occurs at concentrations ranging from 2 to 5%, sometimes exceeding 10%. Additionally, other metallic minerals, such as chalcocite (Fig. 2d), enarite (Fig. 2e) and small amounts of limonite (Fig. 2c), are present in the deposit. The mineral observation results were derived from the analysis of 15 ore samples procured from the Zijinshan deposit open pit and drill cores DZK501, DZK502 and DZK707 (Fig. 1c, d).

The primary mineral found within the gold ores of the Zijinshan deposit is native gold (Fig. 2a, b, d), accompanied by traces of gold-bearing pyrite and limonite. The ore exhibits a compact and nodular structure, with variations in gold content ranging from 1.00 to 10.00 μg kg−1. Notably, the concentration of gold is even higher in the oxidized zone of the deposit.

Chalcocite and enarite (Fig. 2d, e) are the predominant components of the copper ore in Zijinshan, along with minor quantities of bornite (Fig. 2e), chalcopyrite (Fig. 2f) and sporadic occurrences of azurite (Fig. 2e). The copper content in these ores varies between 5.00 and 221.70 g kg−1. Significantly, the co-occurrence of gold within the copper ores is noteworthy, with an average gold content of approximately 1.95 μg kg−1 in chalcopyrite and 2.85 μg kg−1 in enarite. Consequently, the copper minerals play a crucial role in hosting gold in this deposit.

Sampling

To assess the potential for geogas exploration technology to detect underlying concealed mineral deposits in the Zijinshan HSE Cu–Au deposit, two geogas survey lines were arranged along NE and NW directions, intersecting above the ore body, with an average sample point distance of 25 m, half the distance above the ore body. A total of 75 samples were collected along the NE line, and an additional 46 samples were collected along the NW line (Fig. 1c). Also, 37 background geogas samples were collected far from the deposit as control samples (Fig. 1b).

In this study, we optimized a new and rapid method for the dynamic collection of geogas (Wang et al. 1995), using millipore-filter (MF) membranes (nitrocellulose, d ≤ 0.5 μm) to capture nanoscale particles and polyurethane foam to capture metal in the geogas collection process. The MF membranes were pre-checked to ensure they did not contain any target elements before being used. Polyurethane foam was washed with 10% concentrated aqua regia and ultrapure water (ρ ≥ 18 MΩ cm, 25°C), and then immersed in 5% aqua regia for further cleaning (Ye et al. 2014). Three holes were drilled using a steel chisel to 0.8–1.0 m depth at every sampling point, with the spacing of these holes more than 1 m apart. A spiral sampler was screwed into the holes and connected to a particulate filter, a capture device and an air extractor by silicone tubes with ultra-filter (UF) membranes (nitrocellulose, d ≤ 200 μm) (Fig. 3). Each individual hole undergoes two successive gas extraction cycles, resulting in a cumulative volume of 3 l, with the gas flow rate controlled at 1.5 l min−1. Each geogas sample was collected from these three holes, with a total volume of 9 l of gas. The foams used for blank samples were prepared using the identical washing method employed for the sampling foams. Following preparation, the blank foams were individually stored in sample bags until their transportation to the laboratory.

Transmission electron microscopy analysis

Nanoscale particles captured on the MF membrane were observed and analysed using a Hitachi H9000NAR transmission electron microscope equipped with an X-ray energy dispersive spectroscope (EDS) capable of detecting 88 elements ranging from boron (B) to uranium (U). The instrument had a point resolution, lattice resolution and minimal spot radius of 0.18, 0.1 and 0.8 nm, respectively, with an accelerating voltage of 300 kV and a beam spot diameter of less than 0.2 μm (Wei et al. 2013; Ye et al. 2014; Lu et al. 2017). To determine the composition of the particles, we employed the EDS without reference material in this study, enabling us to obtain their elemental compositions, albeit without precise mass percentages.

Chemical analysis

Trace element concentrations in the polyurethane-foam-captured metal from the geogas samples were measured using high-resolution ICP-MS after being ashed and dissolved in 20% concentrated aqua regia and adjusted to 10 ml with ultrapure water (Ye et al. 2014; Lu et al. 2019). Accuracy evaluation of the foam-capture medium was challenging due to the lack of standard material. To ensure precision control, five laboratory replications were carried out for each batch of 50 samples in this study. The blank foams also underwent the same analysis process as the geogas samples.

Copper isotope ratios

Copper isotope ratios were determined using a Thermo Fisher Scientific Neptune Plus MC-ICP-MS instrument, with NIST 976 Cu serving as the reference standard. The samples were dissolved in Savillex beakers using concentrated double-distilled solutions of HF + HNO3, HCl + HNO3 and HCl. Subsequently, the sample solution was loaded onto AG MP-1M (Bio-Rad) strong anion resin to separate the copper (Maréchal et al. 1999; Larson et al. 2003). Mass bias correction was performed using the sample standard bracketing method. The copper isotope ratios are reported in the standard d-notation per mil relative to NIST 976:
δ65Cu=((δ65Cu/δ63Cu)sample/(δ65Cu/δ63Cu)standard1)×1000
To minimize analytical errors, we employed the standard–sample–standard bracketing approach, comparing all copper ratios to the NIST 976 Cu standard. The 2σ error for the standard variation over two analytical sessions was determined to be ±0.12‰.

Nanoscale particles

Nanoscale particles composed of Au, Cu, Fe and other elements were observed in the geogas above the Zijinshan ore body using transmission electron microscopy, as depicted in Figure 4. The particles exhibited a diverse array of shapes, including single grains and small nanoparticles forming aggregates, with spherical, granular, oval and polygonal shapes being typical. The metal particles’ radii ranged from several tens to several hundred nanometres. Based on their elemental composition, the particles could be classified into four types: (1) Au nanoscale particles, which represent the primary component particle (Zhang et al. 2015) (Fig. 4a); (2) Cu–Fe nanoscale particles with Al and other elements (Fig. 4b, c; the Cu–Fe nanoscale particle in Fig. 4b exhibits an ordered crystal structure); (3) Fe–Ti nanoscale particles containing Au (Fig. 4d); and (4) other complex nanoscale particles predominantly composed of Fe or Cu (Fig. 4e, f).

The particles captured from the background area (Fig. 4g–i) contained Al, Si, K and O, but no metal elements associated with highly siderophile element mineralization such as Au, Cu or Fe were detected.

Copper isotope ratios

Table 1 summarizes the data obtained from various samples, including nanoscale particles, pyrite, chalcocite, bornite, enarite, azurite and chalcopyrite. Duplicate analyses were performed on copper ore samples to assess the degree of copper isotope heterogeneity within each sample. The results reveal a robust correlation between the observed variability in δ65Cu values for nanoscale particles (−1.35 to 0.62‰) and copper ore samples (−2.33 to 0.57‰).

The δ65Cu values of nanoscale particles exhibit a variation range of 1.97‰, spanning from −1.35 to 0.62‰, which is two orders of magnitude greater than the analytical uncertainty (±0.06‰; 2SD). Bornite and enarite samples display a more depleted copper isotope composition compared to chalcocite samples, Bornite exhibit δ65Cu values ranging from −0.52 to −0.04‰, with an average of −0.26‰ (n = 6), enarite demonstrate a wider δ65Cu range of −2.33 to −0.13‰, with an average of −1.06‰ (n = 5). The δ65Cu values of the four pyrite samples range from −0.83 to 0.57‰. The data for single azurite sample and chalcopyrite sample are sourced from a previous study (Wu et al. 2017).

Geogas samples

Upon subtracting the average value of the blank samples (Wang et al. 2008), the average value of the background area samples demonstrates a close resemblance to the 85th percentile value of the cumulative frequency distribution for the ore area samples. Following the approach outlined by Sinclair (1974), where the 85th percentile value of the cumulative frequency distribution for the ore area samples serves as the lower threshold for anomalies, we identified a total of 22 geogas samples exhibiting geochemical anomalies. The trace element concentrations of these anomalous samples are presented in Table 2.

Cu and Au are the primary metallogenic elements in the Zijinshan HSE Cu–Au deposit. As shown in Figure 5, geochemical anomalies featuring Cu and Au were detected above the ore bodies, particularly the highly contrasting Cu anomaly exceeding the threshold. This implies that Cu is a high-abundance component in the deep-seated mineralization system of the deposit and that Cu nanoscale particles exhibit superior penetration (Ye et al. 2012). Additionally, Pb, Zn, Cd and other indicator element anomalies occur at the same locations as the Cu and Au anomalies when the ore bodies are projected horizontally. Furthermore, the boundaries of these anomalies are closely related to the alteration zone's location (Fig. 5).

Provenance of nanoscale particles

The inheritance relationship between the nanoscale particles collected from the Zijinshan HSE Cu–Au deposit and the underlying concealed ore bodies is a pivotal finding in this study. This relationship is supported by the close resemblance of the particles’ chemical and isotopic compositions to those of the concealed ore, as well as their similar morphology. Specifically:

(1) Different types of metal minerals exhibit different metallogenic element assemblages, with Au nanoscale particles solely present in large-scale Au deposits (Zhang and Wang 2018).

(2) The prevalence of Cu in the identified nanoscale particles is consistent with the observations of other researchers. The abundance of Cu in Cu deposits provides a bountiful source for the integration of Cu into the nanoscale particles, thus strengthening the extensive distribution of Cu–other elements nanoscale particles across the examined samples. The significant detection of these Cu–other elements nanoscale particles underscores the pivotal role of Cu abundance in Cu deposits in the genesis of nanoscale particles (Ye et al. 2012).

(3) The formation of well-ordered crystalline structures necessitates prolonged timeframes and specific conditions of temperature and pressure. The Cu–Fe nanoscale particles display ordered crystalline structures. This observation implies nanoscale particles could have originated from profound geological processes. The existence of such ordered crystalline structures within these Cu–Fe nanoscale particles holds the potential to provide valuable elucidation regarding the mechanisms underlying nanoscale particle formation and the processes involved in deep-seated mineralization (Wang et al. 2012).

(4) The δ65Cu ranges observed in the nanoscale particle samples (−1.35 to 0.62‰) and copper minerals (−2.33 to 0.23‰) are similar to or significantly larger than those reported in previous studies, which reported ranges of 0.05 to 0.29‰, −2.97 to 0.25‰, and −2.76 to 0.48‰ (Duan et al. 2016; Wu et al. 2017; Zhao et al. 2022), thereby suggesting that the sampled nanoscale particles originate from underlying concealed ore bodies, rather than being formed elsewhere.

Characteristic of copper isotopes

Copper is a highly mobile element that undergoes significant changes in its isotope ratios during hydrothermal processes. The original copper isotope compositions in primary minerals are prone to alteration through low-temperature and secondary processes (Ikehata and Hirata 2012), in particular the weathering and leaching of copper-bearing rocks can lead to the remobilization, migration and re-enrichment of supergene copper isotopes (Mathur et al. 2012, 2013). Heavy copper isotope is more enriched in leaching fluids than the primary Cu-rich minerals and can be precipitated during migration with those fluids (Zhu et al. 2002; Ehrlich et al. 2004; Duan et al. 2016). The copper isotope composition of copper-bearing minerals can serve as a sensitive geochemical tracer to indicate the source of ore-forming materials, the processes of mineralization and the formation of ore deposits (Asael et al. 2012; Li et al. 2015; Liu et al. 2016, 2021; Jansen 2018; Masbou et al. 2020; Sherman and Little 2020; Dekov et al. 2021; Luo et al. 2023; Sarjoughian et al. 2024).

The Zijinshan Cu–Au mineralization was formed under acidic conditions and associated with highly valence sulfates such as alunite and dickite, indicating that it was formed under highly oxidizing conditions. Primary copper sulfides formed at the early ore-stage are susceptible to modification by acidic and oxidizing fluids. This process involves oxidization of Cu+ in the sulfides and release of Cu2+ into the fluid. Many leaching experimental studies demonstrate that the leaching fluids are more 65Cu-enriched than the residual phase (Ehrlich et al. 2004; Jiang et al. 2013). As the fluids migrate away from the leach cap, and the temperature drops, the fluid 65Cu would be precipitated and enriched via chemical reduction in the deeper zone, resulting in altered Cu isotope compositions (Zhao et al. 2022).

In the Zijinshan HSE Cu–Au deposit, chalcocite and enarite play a dual role as both primary copper minerals and significant carriers of gold (Wang et al. 2009; Hu et al. 2012; Yu et al. 2013; Zhong et al. 2018; Zhao et al. 2022). Furthermore, the nanoscale particles entrapped in the overlying atmosphere exhibit a predominant composition of copper elements. Figure 6 shows that the Cu isotopes of nanoscale particles and copper minerals have distinct horizontal and vertical zoning, and the δ65Cu values for nanoscale particles fall between the values of copper minerals. The above phenomenon is consistent with the fact that copper isotopes can be fractionated significantly in low-temperature aqueous reactions involving Cu oxidation, which formed different isotopic reservoirs (Ehrlich et al. 2004). According to the Cu isotope composition of nanoscale particles and copper minerals, and their relationship with elevation, this isotopic pattern reflects the leaching of 65Cu from the upper zone and its transfer to the lower zone, resulting in spatial variations of copper isotopes. The nanoparticles not only inherit the copper isotope characteristics of copper ore in the oxidation weathering zone, but also show a close provenance relationship with deep copper ore.

Distribution of trace elements and REEs

During the metallogenic process of the Zijinshan HSE Cu–Au deposit, pyrite was the most prevalent metallic mineral, constituting 15–20% of the deposit (Zhang et al. 2014). Its cubic structure enabled the incorporation of considerable amounts of trace elements (Cu, Pb, Zn, As, Se, Co and Ni), leading to Fe and S deficiencies within the deposit (Lu et al. 2016). Furthermore, trace element analyses of pyrite provide valuable insights into ore paragenesis, metal source and the origin of various ore deposits. Trace elements can also serve as indicators of hydrothermal ore deposits (Large et al. 2007; Gregory et al. 2015; Large and Mukherjee 2017; Wang et al. 2021). Moreover, high temperatures favour Co and Ni isomorphism replacing Fe, with Co and Fe being easier to form complete isomorphism (Yan et al. 2012; Zhang et al. 2014). In this study, the ores’ Co/Ni values ranged from 0.09 to 0.14, which is similar to geogas samples (0.02 to 0.14). Bivariate plots of related trace element concentrations commonly exhibited parallel compositional profiles (Fig. 7).

Table 3 displays the concentrations and associated geochemical parameters of REEs for geogas samples (mean ZJg, n = 22), gold core, copper ore, altered rocks and bedrock. As indicated in Table 2, the average ΣREE concentration in geogas samples was 0.06 mg kg−1, significantly lower than the concentrations observed in C1 (2.53 mg kg−1), gold ore (1.76 to 1.88 mg kg−1), copper ore (4.53 mg kg−1), altered rocks (0.78 to 6.18 mg kg−1) and bedrock (5.92 to 22.98 mg kg−1).

Figure 8 exhibits the C1-normalized REE distribution patterns for the aforementioned samples. The REE distribution patterns of geogas samples, gold ore, copper ore, altered rock and bedrock formations reveal considerable enrichment in LREEs and right-inclined REE patterns, implying that these samples are predominantly enriched in LREE and relatively depleted in HREEs.

All samples, except for the geogas samples (δEu = 20.09), exhibited negative δEu anomalies, accompanied by slightly negative δCe anomalies with similar (La)N/(Sm)N and (Gd)N/(Yb)N ratios (Fig. 8, Table 3). The remarkable positive δEu anomalies observed in the geogas samples are attributed to the variable valence nature of Eu, which is prone to fractionation from other REEs during fluid–rock interactions, especially under favourable conditions of temperature, fo2 and pH (Shannon 1976). The Zijinshan HSE Cu–Au deposit is classified as a high sulfidation epithermal (HSE) mineral deposit (Zhao et al. 2020), where the abundant sulfur combines with oxygen to form SO2 and SO42–. Under lower oxygen fugacity, a fraction of Eu3+ is reduced to Eu2+ (Bau 1991). During fluid–rock interactions, Eu2+ tends to enter the liquid phase and migrate to the surface via the geogas. Given the extremely low concentrations of ΣREE in the geogas samples (Table 3), the addition of a trace amount of Eu2+ would result in the notable positive δEu anomalies observed in these samples.

Migration model of multielement geochemical anomalies in geogas

Simulation experiments have confirmed that metal elements in ore-bearing supercritical fluids, under extreme conditions and during pressure reduction, can be found in both the liquid and the CO2-rich vapour phases, even under specific conditions (22 MPa, 250–270°C). For instance, the gold content in some vapour phase samples was higher than that in the liquid phase samples (Zhang and Hu 2002). Nanoscale metal particles are a natural occurrence in deep-seated mineralization processes (Zheng and Zheng 2002). In high reducing environments rich in hydrogen and CO, nanoscale metal particles can conglomerate at lower melting points, forming volatile and migratory carbonyl and hydride compounds with CO and H2. Upon transport to an oxidizing environment, these special complexes can decompose and oxidize into nanoscale metal particles (Zheng et al. 2007). Additionally, supergene transformation of ores can result in the formation of secondarily mobile nanoscale particles through weathering and biological actions (Fairbrother et al. 2012; Frank et al. 2012).

Nanoscale metal particles exhibit high surface energy and can adsorb onto the surface of geogas bubbles such as CO2 and CH4 (Fig. 9). This mineral element transformation is beneficial for their migration and enrichment, as the particles can preserve their fundamental geochemical stability during long-distance or long-term transport (Wang et al. 2012). As geogas ascends, nanoscale particles can penetrate the thick covering layers and migrate to the surface (Fig. 9). Furthermore, owing to their high diffusivity, they can also vertically migrate in the gas phase directly to the surface. Upon reaching the surface, some nanoscale particles remain in the geogas, while others are captured by soil geochemical traps such as clays, colloids and oxides, resulting in multi-element geochemical anomalies in geogas (Wang et al. 2007, 2012).

Evaporation is one of the methods capable of producing nanoscale particles. Conditions in geological processes such as volcanic eruption, magmatism and hydrothermalism resemble this process and may generate abundant nanomaterials during diagenesis and mineralization. At the Zijinshan HSE Cu–Au deposit, nanoscale particles were observed in geogases above the ore body, but no Au-, Cu- or Fe-bearing counterparts were found in samples from the background area. The particles in geogas were comparable to those in ores, exhibiting similar features regarding size, shape, assemblage and component. In areas far from the Zijinshan deposit, background geogas samples (n = 37) displayed gold (Au) values ranging from 0 to 0.25 μg kg−1, with an average of 0.15 μg kg−1, and copper (Cu) values ranging from 0.07 to 0.98 g kg−1, with an average of 0.72 g kg−1. Out of the total of 121 samples collected over the mineralized areas, 22 geogas samples exhibited multi-element geochemical anomalies above the ore body. These anomalous samples showed Au values ranging from 0.50 to 1.75 μg kg−1, with an average of 0.73 μg kg−1, and Cu values ranging from 0.18 to 29.16 g kg−1, with an average of 1.65 g kg−1, which were twice the concentrations observed in the background samples. Furthermore, other metallogenic elements and associated elements displayed concentrations 1.5 to 5 times higher than those found in the background samples. Based on the types of anomalous elements and their spatial distribution on the surface (Fig. 5), the anomalous elements were closely linked to metallogenic elements, with the anomaly locations related to those of the ore bodies and alteration zones. Copper isotope, trace elements, REE concentrations and associated geochemical parameters were alike in nanoscale particles, geogas samples, gold ore, copper ore, altered rock and bedrock. Hence, it can be inferred that nanoscale particles in geogas and ores have the same genesis, with geogas derived from ores inheriting some of their relevant characteristics related to endogenic mineralization, and with the phase and features of nanoscale particles in geogas being stable.

At the Zijinshan HSE Cu–Au deposit, these nanoscale particles can penetrate volcanic coverings and migrate to the surface by the geogas streaming and multi-agent mechanism (Wang 2005; Wang et al. 2007), including mantle degassing, barometric pumping, gas release from ore minerals, evaporation, capillary action and plant uptake. These particles can be adsorbed onto bubbles and ascend along with the geogas stream to the surface, due to their considerably large surficial area. Alternatively, based on their characteristics similar to gases, particles can ascend through regolith cover independently. While some particles remain in the geogas, others become ensnared by soil geochemical traps. These particles alongside multi-element geochemical anomalies constitute direct geological evidence of the underlying concealed mineral deposit (Fig. 9).

Here, the utilization of geogas prospecting is effective in identifying locations of ore mineralization buried under thick igneous coverages. More research is required for evaluating the exact mechanism of nanoparticles release from mineral deposits and determining the applicable use of this technology for regional-scale exploration.

Constructive comments by anonymous reviewers were very helpful and deeply appreciated. The authors express their special appreciation to ‘Deep-Penetration Geochemical Detection Technology Project' (201011055-1, 201011055-3) supported by the Ministry of Land and Resources, China; such support is much appreciated.

QL: investigation (lead), methodology (lead), validation (lead), visualization (lead), writing – original draft (lead); RY: conceptualization (lead), data curation (lead), writing – review & editing (equal); HD: software (equal), supervision (equal); KX: investigation (equal), methodology (equal); SS: software (equal), validation (equal); YT: validation (equal), visualization (equal).

This work was funded by the Deep-Penetration Geochemical Detection Technology Project (201011055-1, 201011055-3).

The authors assert that no financial support or relationships that could have influenced the content of the manuscript have been acknowledged.

All data included in this study are available upon request by contact with the corresponding author.

This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/)