Nanoparticles in groundwater of the Qujia deposit, eastern China: Prospecting significance for deep-seated ore resources

Journal Pre-proofs Nanoparticles in groundwater of the Qujia deposit, eastern China: Prospecting significance for deep-seated ore resources Xiang Liu, Jianjin Cao, Wanqiang Dang, Zixia Lin, Junwei Qiu PII: DOI: Reference:

S0169-1368(19)30771-1 https://doi.org/10.1016/j.oregeorev.2020.103417 OREGEO 103417

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Ore Geology Reviews

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19 August 2019 11 February 2020 15 February 2020

Please cite this article as: X. Liu, J. Cao, W. Dang, Z. Lin, J. Qiu, Nanoparticles in groundwater of the Qujia deposit, eastern China: Prospecting significance for deep-seated ore resources, Ore Geology Reviews (2020), doi: https:// doi.org/10.1016/j.oregeorev.2020.103417

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Nanoparticles in groundwater of the Qujia deposit, eastern

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China: Prospecting significance for deep-seated ore resources

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Xiang Liua,b,c,d, Jianjin Caoa,b,c,d,*, Wanqiang Danga,b,c,d, Zixia Line, Junwei Qiua,b,c,d

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a School

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b Guangdong

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510275, China

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c Guangdong

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d

Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519000, China

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e

Testing Center, Yangzhou University, Yangzhou 225009, China

of Earth Sciences and Engineering, Sun Yat-sen University, Guangzhou 510275, China Provincial Key Laboratory of Mineral Resources and Geological Processes, Guangzhou

Provincial Key Laboratory of Geodynamics and Geohazards, Guangzhou 510275, China

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*Corresponding

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Guangzhou 510275, China.

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E-mail: [email protected].

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author: J.J. Cao, School of Earth Sciences and Engineering, Sun Yat-sen University,

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Abstract

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The characterization of nanoparticles in groundwater is recognized as an important tool that

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can be applied to mineral exploration. In this paper, we investigate the concealed orebodies of the

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Qujia gold deposit in the Jiaojia orefield of eastern China by analyzing ore-related nanoparticles in

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groundwater using transmission electron microscopy. The nanoparticles contain variable amounts

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of ore-related elements (e.g., Ag, Cu, Zn, La, Ce, Pb, Pt, and S). Some of these elements are rarely

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found in natural settings unrelated to orebodies or mineral extraction, but are strongly linked to

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concealed orebodies in the study area. These nanoparticles, which are diverse in their shapes,

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aggregated features, and degrees of crystallinity, can be grouped in terms of composition as: (i)

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native metal nanoparticles, (ii) metal-based nanoparticles, and (iii) carrier nanoparticles associated

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with trace elements. We propose that these three main types of nanoparticles enable the efficient

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transport of ore-forming elements in groundwater. The results of this study provide insights into

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the migratory behaviour of ore-related nanoparticles in groundwater, and our methods could be

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applied for detecting deep-seated mineral deposits.

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Keywords: Qujia deposit; Deep-seated orebody; Exploration; Nanoparticles; TEM study

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1. Introduction Exploration for deep-seated mineral resources is challenging (Butt and Hough, 2009; Anand

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et al., 2016). Various methods have been employed to explore for

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including primary and secondary geochemical halos, stream sediment geochemistry, mobile metal

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ions, enzyme leaching, leaching of mobile forms of metal in overburden, geogas,

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electrogeochemistry, biogeochemistry, and hydrochemistry (Kristiansson and Malmqvist, 1982;

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Butt and Gole, 1985; Cohen et al., 1987; Clark et al., 1990; Antropova et al., 1992; Mann et al.,

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1998; Luo et al., 1999; Malmqvist et al., 1999; Cameron et al., 2004; de Caritat et al., 2005;

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Kelley, 2006; Anand et al., 2007; Wang et al., 2008; Koplus et al., 2009; Leybourne and Cameron,

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2010; Leslie et al., 2013; Wang, 2015; Yilmaz et al., 2015; Gray et al., 2018; Noble et al., 2018).

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These methods each have specific advantages for the analysis of different near-surface media (e.g.,

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gas, soil, rock, organisms and groundwater).

concealed ore deposits,

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Groundwater is typically of low background value for geochemical anomalies related to

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metallic ore deposits but is capable of facilitating the migration of metals (Giblin, 1994; Taufen,

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1997; Leybourne and Cameron, 2010). As an effective and convenient tool for exploring different

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mineral resources (e.g., Cu, Pb, Zn, Ni, Au, and Ag), groundwater has usually been studied on the

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basis of metal concentrations and isotope analyses (Cameron et al., 2002; Leybourne and Cameron,

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2006, 2010; Noble et al., 2013, 2016; Ilyas et al., 2016; Gray et al., 2018). In some cases, however,

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geochemical patterns can be misdirected by different geological processes that are unrelated to

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concealed mineralization. Therefore, further understanding of the metal transfer mechanisms for

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exploration is very desirable (Robertson et al., 2001; Anand et al., 2016).

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Since the confirmation of nano-sized materials in geogas (Tong et al., 1998), various geogas-

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based nanoparticles containing Au, Pb, Zn, W, and Hg have been identified and have been linked

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to deep-seated orebodies by composition (Cao et al., 2009a). Cao et al. (2009a) first proposed that

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nanoparticles could be used for mineral prospecting. Many investigations since then have been

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conducted and have shown that ore-related nanoparticles are widely distributed in a diverse range

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of media around concealed orebodies, such as geogas (Cao et al., 2009a, 2010a; Wei et al., 2013;

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Ye et al., 2014; Luo et al., 2015; Zhang et al., 2015; Wang et al., 2016; Li et al., 2017; Jiang et al.,

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2019), fault gouge (Mi et al., 2017), vegetation (Lintern et al., 2013; Hu et al., 2017), and

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invertebrate tissues (Hu et al., 2018). In groundwater, chemically anomalous nanoparticles have

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been recently documented and interpreted as ore-related (Li et al., 2016; Cheng et al., 2018; Hu

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and Cao, 2019; Liu et al., 2019). These nanoparticles reportedly vary in their crystalline features,

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and were grouped as amorphous, single crystals, poly-crystalline, or complex poly-crystalline

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particles to efficiently provide mineralization information (Liu et al., 2019).

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In this paper, the Qujia deposit in eastern China was selected as the study area. Here, deep-

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seated (>800 m below the surface) orebodies have been newly proven via drilling (Shandong

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Jinshi Mining Company Limited, 2013). The area has good groundwater circulation, which is

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amenable to sampling. The specific objectives of this study are to (1) investigate nanoparticles in

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groundwater in a mineralized district using transmission electron microscopy (TEM), (2) elucidate

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the relationship between the nanoparticulate features and the deep-seated mineral resources, and

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(3) document transport behaviour of ore-related nanoparticles in near-surface aquatic systems.

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2. Geological setting

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The Qujia deposit is situated in the western part of the Jiaojia orefield in the Jiaodong

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peninsula, a world-class gold province in eastern China (Fig. 1a and b). Tectonically, the region

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lies to the east of the Tan-Lu Fault, in the southeastern margin of the North China Craton, and the

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northeastern end of the Dabie-Sulu metamorphic belt (Fitches et al., 1991; Mao et al., 2008;

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Goldfarb and Santosh, 2014). In this district, the basement is composed of tonalite-trondhjemite-

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granodiorite (TTG) gneisses, Neoarchean amphibolite, and granulite. The NE- to NNE-trending

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Jiaojia Fault is the major structure (Deng et al., 2015; Yang et al., 2016). Mesozoic igneous rocks

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are mainly Early Cretaceous granitoid and Late Jurassic granitoid, as well as dike rocks that

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include pegmatite, porphyritic granodiorite, and dioritic porphyrite (Shandong Jinshi Mining

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Company Limited, 2013; Song et al., 2015; Yang et al., 2016). The overlying strata is

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predominantly Quaternary sediments, with a thickness of 5 to 10 m (Guo, 2016).

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The Qujia deposit consists of 32 orebodies with ore grades of 1.0–4.4 g/t Au (Guo, 2016).

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Orebody No. I, the most important, is NE-trending (10° to 65°) and dips to the northwest from 2°

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to 48°. It extends between the No. 8 and No. 20 prospecting lines at a level of -800 m to -1600 m

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below the surface (Fig. 2a and b). Orebodies are related to major faults, and commonly occur in

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the zone of quartz–sericite–pyrite altered and cataclastically deformed rocks, and are bedded,

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vein-like, branching-converging, and pinching-swelling in shape (Shandong Jinshi Mining

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Company Limited, 2013; Guo, 2016). Native silver, electrum, native gold, pyrite, pyrrhotite,

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chalcopyrite, galena, sphalerite, and tennantite comprise the metallic mineral assemblages. Non-

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metallic minerals include quartz, sericite, muscovite, feldspar, calcite, and epidote among others.

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Major alteration styles include silicic, beresite, potash feldspar, carbonate, chlorite, and kaolinite

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(Shandong Jinshi Mining Company Limited, 2013; Guo, 2016).

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3. Materials and methods

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3.1. Sampling location and methods

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Groundwater samples were collected in the Qujia district where newly proven but

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unexploited (at the time of sampling) deep-seated orebodies are located. There are no industrial or

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man-made effects in the area. The sampling sites were mainly located between prospecting lines

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No. 8 and No. 20, and the coordinates (latitude/longitude) were recorded and are plotted in Fig. 2a.

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Groundwater was sampled through water pumps or wells from 20–50 m deep, using 500 ml

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polyethylene bottles. Before samples were collected, groundwater was pumped for several

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minutes. The bottles were pre-rinsed three times with ultrapure water and then dried in pollution-

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free conditions prior to sampling. After sampling, bottles were sealed, labelled, and stored under

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25–27 °C. The pH values of the samples were tested on site and ranged from 7.05–7.95.

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Background water samples were collected at Hailiu village (Longkou, Shandong province,

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approximately 10 km away from industry and mining), which is an area that is unaffected by

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mining and industry. The sampling method was similar to that used in the Qujia district. These

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samples were also stored under 25–27 °C.

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3.2. Analytical methods

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Groundwater samples were processed under clean bench conditions as follows. First, bottles

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were lightly shaken to eliminate the uneven dispersion of nanoparticles in aqueous solution as

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induced by gravitational settling during storage. A carbon-coated nickel TEM grid fixed by

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tweezers was subsequently put into a shaken groundwater sample and slowly moved for 30

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minutes to adsorb nanoparticles. The TEM grids were air dried before being placed into a clean

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sample box. To prevent damage to the TEM grids, tweezers were used to fix the rim of the nickel

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screen.

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Samples were analysed at the Testing Centre of Yangzhou University using a TEM (Tecnai

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G2F30S-Twin, America) equipped with an energy dispersive spectrometer (EDS) for elemental

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analyses. EDS results for C and Ni were not considered due to the use of the carbon-coated nickel

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TEM grids. The focused nanoparticles were morphologically and structurally characterized by

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TEM in combination with scanning transmission electron microscopy (STEM), high-resolution

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transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED). Phase

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analysis was determined by reference to the International Centre for Diffraction Data (ICDD)

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Powder Diffraction File (PDF).

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4. Results

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4.1. Results from the deposit

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Anomalous nanoparticles in the sampled groundwater, as identified by TEM, predominantly

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include a diverse suite of Ag-, Pt-, Cu-, Zn-, W-, Cr-, Fe-, La-, Ce-, and As-bearing nanoparticles

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that vary in elemental content, degree of crystallinity, and morphology. Representative

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nanoparticles (ID1–17) are detailed in this section, and the EDS results are shown in Table 1. The

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analytical information for these nanoparticles is provided in Table S1.

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4.1.1. Native metal nanoparticles

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An irregular nanoparticle of high contrast (ID 1) that only contains Pt is shown in Fig. 3a.

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The SAED pattern (Fig. S1) and the HRTEM image (Fig. 3b) suggest a crystalline feature. This

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nanoparticle resembles the standard diffraction pattern of native Pt sample (JCPDS PDF# 70–

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2057). Another irregular Pt-bearing nanoparticle (Fig. 3c; ID 2) that is 200 × 250 nm in size is

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crystalline (Figs. 3d, S2), and also resembles native Pt with a trace amount of Co.

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4.1.2. Metal-based nanoparticles

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A nearly elliptical Cu-S-O-bearing nanoparticle (ID 3) is shown in Fig. 4a. No diffraction

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spots are visible in the SAED pattern (Fig. S3), which suggests an amorphous feature. The

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nanoparticle might be a Cu sulfate. Figure 4b reveals a nearly round Cr oxide nanoparticle (ID 4)

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that is 60 × 80 nm in size; the amorphous nature is shown in Fig. S4. Figure 4c illustrates a W-O-

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bearing nanoparticle aggregate (ID 5) that consists of two nanoparticles, which are nearly round in

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shape with smooth edges, and share similar chemical components. When combined with the

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SAED pattern (Fig. S5) that shows no diffraction rings, the data suggests that the nanoparticles

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might be an amorphous tungstate.

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A Ag-Cu-bearing nanoparticle (ID 6) is shown using both TEM and STEM imaging (Fig. 5a

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and b). The nanoparticle is nearly round in shape, 25 × 35 nm in size and might be amorphous

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according to the fast Fourier transform pattern (Fig. S6). The EDS spectrum of the nanoparticle is

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shown (Fig. 5g). The elemental mapping analysis (Fig. 5c-f) reveals a concentrated distribution of

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elemental Ag, Cu, and S but not O. Ag is evenly distributed in the nanoparticle, and Cu is

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relatively enriched in the internal part (Fig. 5c and d).

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Figure 6a illustrates an irregular Cu-S-O-bearing nanoparticle (ID 7). This nanoparticle is

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100 × 260 nm in size, and several diffraction spots that are irregularly distributed in the SAED

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pattern (Fig. S7) exhibit a well-defined long-range ordering. In the HRTEM image (Fig. 6b), two

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measured d-spacings resemble those of the standard Cu2O(SO4) sample (JCPDS PDF# 78–0612).

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A nearly round Ag-bearing nanoparticle (ID 8; Fig. 6c) is 30 × 50 nm in size with smooth edges,

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and also has a crystalline feature (Figs. 6d, S8). This nanoparticle might be Ag2O (JCPDS PDF#

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72–2108).

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Crystalline nanoparticles containing rare earth elements (REE) are also detected. Figure 7a

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shows a nearly round nanoparticle (ID 9), which is crystalline (Figs. 7b, S9). Figure 7c reveals an

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irregular REE-bearing nanoparticle (ID 10). The sections with different contrasts (ID 10I and 10II)

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share similar chemical compositions, whereas elemental Zn is only detected in the section of

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relatively lower contrast (ID 10II). The SAED pattern (Fig. S10) and HRTEM image (Fig. 7d)

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indicate a crystalline nature. These nanoparticles could be La-Ce-Zn-Fe oxides.

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4.1.3. Carrier nanoparticles associated with trace elements

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Figure 8a shows an ellipsoidal Si-O-Cu-bearing nanoparticle (ID 11) that is 200 × 300 nm in

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size. The amorphous nature of this particle is illustrated by Fig. S11. In contrast to the TEM

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results for the internal section of the nanoparticle (ID 11I), Cu contents (wt%) are relatively higher

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on the edge (ID 11II). This nanoparticle could therefore be a silicate associated with trace

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elements. A Ca-O-Zn-Co-Cu-bearing nanoparticle (ID 12; Fig. 8b) is poorly crystalline according

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to the SAED pattern (Fig. S12). The nanoparticle is inferred to be a CaCO3 associated with

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additional ore-related elements. Different Fe oxide/hydroxide nanoparticles with varying

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crystalline features are recorded. These include an ellipsoidal poorly crystalline nanoparticle (ID

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13; Figs. 8c, S13) and a nearly round amorphous nanoparticle (ID 14; Figs. 8d, S14).

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Figure 9a illustrates an Fe-Mn-Si-O-bearing nanoparticle aggregate (ID 15), which is made

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up of different crystalline nanoparticles with d-spacings clearly shown in the HRTEM images (Fig.

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9b and c). These nanoparticles, i.e., Fe-Mn oxide (ID 15I), Fe oxide (ID 15II), and Fe-Si oxide (ID

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15III), are associated with Zn. As shown in Fig. 9d, one nanoparticle aggregate (ID 16) consists of

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two Ca-P-O-bearing nanoparticles of different contrasts. These might be Ca phosphates. The

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nanoparticle of low contrast (ID 16I; Fig. 9d) is amorphous (Fig. S15a) and associated with Zn,

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Cu, and S, while the nanoparticle of high contrast (ID 16II; Fig. 9d) is also amorphous (Fig. S15b)

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but contains small amounts of Pb and Cu.

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A 150 × 200 nm nanoparticle (ID 17; Fig. 10a and b) contains several tiny particles, some of

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which are further illustrated in the HRTEM image (Fig. 10c). This nanoparticle is crystalline (Fig.

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S16), and is mainly composed of Si and O with small amounts of Ag, Fe, and Mg among others.

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Elemental mapping (Fig. 10d-g) differentiates the tiny particles, and those shown in Fig. 10c

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correspond to one of the Ag enrichments (Fig. 10h). This nanoparticle might be a silicate with

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several Ag- and Fe-bearing tiny particles included within.

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4.2. Results from the background area

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Nanoparticles detected in the background water samples mainly include Ca-, Fe-, Si-, Al-,

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Mg-, O-, and Ti-bearing nanoparticles. Representative nanoparticles (ID 18–20) are shown in Fig.

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11, and the EDS results are listed in Table 1. The analytical information of these nanoparticles is

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provided in Table S1. Figure 11a shows a Si-Ca-Fe-O-bearing nanoparticle (ID 18), which

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contains significant O. As revealed by the SAED pattern (Fig. S17), the nanoparticle is poorly

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crystalline. Figure 11b shows an irregular Si-O-bearing polycrystalline nanoparticle (ID 19; Fig.

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S18), which contains Si and significant O. Figure 11c shows an aggregate, which is made up of

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two Ti-O-bearing polycrystalline nanoparticles (ID 20; Fig. S19). The aggregate is inferred to be a

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TiO2.

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5. Discussion

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5.1. Different types of nanoparticles containing ore-forming elements

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Various anomalous nanoparticles are presented in this study, which occur as individual

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particles and/or particle aggregates. The contents (wt%) of ore-forming elements in the

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nanoparticles are evident to varying degrees (i.e., high and low contents), and these nanoparticles

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could be grouped into (i) native metal nanoparticles, (ii) metal-based nanoparticles, or (iii) carrier

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nanoparticles associated with trace elements. Native metal nanoparticles composed almost

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exclusively of one metallic element were rarely found in groundwater samples. When EDS results

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are combined with SAED patterns and HRTEM images, only a few native Pt nanoparticles (Fig.

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3a and c) are identified. This is similar to Au nanoparticles in an ore system reported by Hough et

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al. (2011), where the native metal nanoparticles were composed of one noble metal of an

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extremely high fineness of Au. In contrast, the metal-based nanoparticles dominate our samples.

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Composed of different elements, the metal-based nanoparticles contain relatively high contents

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(wt%) of ore-forming elements, and thus, a variety of elemental combinations are presented: Ag-O,

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Ag-Cu-S-O, W-O, Cr-O, Cu-S-O, and La-Ce-Zn-Fe-O among others. These nanoparticles

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commonly occur as oxides and sulfates (e.g., Ag oxide, Cr oxide, and Cu sulfate), and show a

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diversity of shapes (e.g., nearly round, nearly elliptical, and irregular) and crystalline features,

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including amorphous and crystalline structures. This diversity of nanoparticles provides general

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mineralization information.

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Nanoparticles composed mainly of common elements (e.g., Ca, Si, O, and Fe) should not be

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ignored, even though they are widely distributed in nature. The nanoparticles and nanoparticle

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aggregates that were detected in sampled groundwater over the deposit are usually associated with

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various trace elements (e.g., Cu, Co, As, Zn, and Pb) at low contents (wt%), such as the silicate

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(Fig. 8a), Ca carbonate/phosphate (Figs. 8b, 9d) and Fe oxide/hydroxide (Fig. 8c and d). Serving

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as carriers in groundwater, these nanoparticles facilitate the transport of different kinds of ore-

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forming elements, which are roughly similar to those contained in the native metal nanoparticles

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and the metal-based nanoparticles. Nanoparticles containing ore-bearing tiny particles are another

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viable transport form. For instance, several Ag- and Fe-bearing tiny particles included within a

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silicate nanoparticle (Fig. 10a) suggest the potential for the co-transport of different particles.

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Thus, common nanoparticles from the studied ore deposit are capable of providing mineralization

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information and deserve more attention.

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5.2. Relationship between the collected nanoparticles and deep-seated orebodies

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The nanoparticles in the background samples only contained common elements (e.g., O, Na,

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Ti, Si, Cl, Ca, Mg, and Al). On the contrary, several Ag-, Pt-, Cu-, La-, Ce-, Zn-, Pb-, S-, and As-

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bearing nanoparticles, although uncommon, were detected and are of significance in the

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groundwater samples from the Qujia deposit. Since no anthropogenic contamination (e.g., mining,

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factory, and landfill) affects the study area, these anomalous nanoparticles might be linked to

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concealed orebodies, which are made up of native silver, native gold, electrum, pyrite, pyrrhotite,

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chalcopyrite, galena, sphalerite, and tennantite among others (Shandong Jinshi Mining Company

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Limited, 2013; Guo, 2016). In addition, the primitive ores contain La and Ce (Shandong Jinshi

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Mining Company Limited, 2013). The potential for Pt mineralization can also be expected, since

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Wang et al. (1999) reported that Pt has been detected in the concealed orebodies from the

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Xincheng Au deposit, which is near our study area. Therefore, the occurrence of anomalous

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nanoparticles in groundwater, to some extent, appears to be due to the enrichment of ore-forming

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elements (e.g., Ag, Zn, Cu, La, Ce, Pb, S, As, and Pt) located beneath the study area. Moreover,

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we postulate that nanoparticles with different contents (wt%) of ore-forming elements might

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originate from different parts of the concealed mineral resources. Hence, the native metal

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nanoparticles and metal-based nanoparticles that contain relatively high contents (wt%) of ore-

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forming elements might be linked to high grade orebodies, whereas the carrier nanoparticles with

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minor ore-related elements are possibly derived from low grade ores.

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Additionally, as a precious metal, Ag is important for economic development. The main

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differences (e.g., size, element composition, and crystalline feature) of Ag-bearing nanoparticles

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include i) the amorphous Ag-Cu-S-O-bearing nanoparticle that indicates the complexity of

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elemental combinations (Fig. 5a), ii) the Ag2O nanoparticle that exhibits a well-defined long-range

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ordering (Fig. 6c), and iii) several Ag-bearing tiny particles within a silicate that provide

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considerable mineral information (Fig. 10a). Since there are few reports of Ag in natural

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conditions other than orebodies, the concealed orebodies in the study area may be the source for

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Ag anomalies in the groundwater samples, because it is already known that the grade of Ag in

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orebody No. I ranges from 0.4–25.70 g/t (mean 6.69 g/t ) (Shandong Jinshi Mining Company

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Limited, 2013). The average content of Ag in the earth crust is 0.1 ppm (Etris, 1997; Purcell and

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Peters, 1998). A mineralogical study from the Qujia deposit has indicated that Ag-bearing

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minerals in irregular particulate forms are commonly included in the crystals of pyrite and quartz,

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and also occur in the cracks of other minerals (Shandong Jinshi Mining Company Limited, 2013).

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As reported for other deposits, Ag at the nanoscale can also be included in hypogene or supergene

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ores, for example, digenite, tennantite, and arsenian pyrite (Reich et al., 2010; Deditius et al., 2011;

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Wu et al., 2016). Hence, different types of Ag-bearing nanoparticles found in this work might be

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strongly related to the Ag-bearing minerals of the concealed orebodies.

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5.3. Prospecting significance of anomalous nanoparticles

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Various geological processes are responsible for the formation of natural nanoparticles

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(Hochella et al., 2008, 2019), including mineralization (Palenik et al., 2004; Reich et al., 2005;

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Deditius et al., 2011; Ciobanu et al., 2012), weathering (Hough et al., 2008), and fault activities

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(Wilson et al., 2005; Dor et al., 2006). With respect to mineral exploration, ore-related

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nanoparticles are regarded to be generated at depth through oxidation and fault activity (Cao et al.,

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2010b, 2015). In this paper, Ag-, Pt-, Cu-, La-, Ce-, Zn-, Pb-, S-, and As-bearing nanoparticles are

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collected in groundwater, and they vary in their shapes, aggregated features, chemical

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compositions, and degrees of crystallization. Pt and Ag as precious metals commonly have low

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dissolved concentrations in aquatic condition (Barriada et al. 2007; Soyol-Erdene and Huh, 2012),

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and the formation of their dissolved phase is strictly dependent on the climatic conditions and the

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groundwater chemistry (Freyssinet et al. 2005; Likhoidov et al., 2008). In many cases, the

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materials precipitated out of solutions quickly surpass the nano-size phase and form larger

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particles (Wigginton et al., 2007; Hochella et al., 2008). However, many studies revealed that ore-

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related nanoparticles can be directly carried by geogas (Cao et al., 2009a; Luo et al., 2015; Jiang et

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al., 2019). Wang et al. (2016) reported that diverse Zn-, Cu-, Mo-, and Pb-bearing nanoparticles

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have been found in geogas and soil samples from the Xiaohulishan molybdenum polymetallic

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deposit, which is located in the arid area of northern China. Mi et al. (2017) found anomalous

288

nanoparticles with crystal structure deformations or jagged edges in deep fault gouges, and those

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nanoparticles might be directly linked to the action of fracture. Therefore, based on characteristics

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of the anomalous nanoparticles in this study, these nanoparticles might be derived from ore

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minerals. In the Qujia deposit, the deep-seated orebodies are controlled by the major fault (Fig.

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2b). On one hand, fissures that have been induced by faults might provide transport channels for

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oxygen-rich water and free oxygen to deep-seated orebodies, thus resulting in oxide and sulfate

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nanoparticles (i.e., oxidation products). These nanoparticles contain significant O, and they are

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identified in this study such as Ag oxide (Fig. 6c), La-Ce-Zn-Fe oxides (Fig. 7a and c), and Cu

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sulfates (Figs. 4a, 6a). On the other hand, fault activities can mechanically grind bulk minerals

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into nanoparticles, and may be predominantly responsible for the formation of the native metal

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nanoparticles. In this case, a resultant ore-bearing nanoparticle might be transported when

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groundwater flows through the orebody.

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Metals are capable of migration upwards and laterally through transported cover from

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concealed orebodies to produce various geochemical anomalies, and groundwater transport driven

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by dilatancy pumping, convection, bubbles, etc. might be one of the critical mechanisms of

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migration (Cameron et al., 2004; Kelley, 2006; Anand et al., 2016). Likewise, ore-bearing

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nanoparticles in groundwater have been regarded as important for providing mineralization

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information, because their small sizes allow for high efficiencies and long-distance migrations in

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phreatic processes (Li et al., 2016; Cheng et al., 2018; Liu et al., 2019). In contrast, we conclude

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that ore-related nanoparticles detected in groundwater in our study differ depending on the type of

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corresponding deposit in terms of chemical composition and/or elemental combinations. Abundant

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Cu-bearing nanoparticles have been recorded in groundwater from the Bofang copper deposit,

310

south China (Cheng et al., 2018), and diverse Cu-, Pb-, Zn-, Fe-, and Mn-bearing nanoparticles

311

have been detected in groundwater from polymetallic deposits (Li et al., 2016; Liu et al., 2019).

312

However, we find several REE-, Pt-, and Ag-bearing nanoparticles that have rarely been reported

313

previously. Based on previous geochemical exploration works conducted in Qujia, Au, As, Sb, Ba,

314

Ag, Cu, Pb, Zn, Bi, Mo, Co, Sr, W, Cr, and Sn are indicative elements of the deposit (Guo, 2016).

315

Moreover, the detection of the W-bearing nanoparticles potentially corresponds to the results of

316

previous drilling in the area surrounding our study area, which used structures superimposed with

317

halos of elemental W in order to assist with locating concealed orebodies (Liu et al., 2014; Pan et

318

al., 2017).

319

Although the ore-forming elements differ in contents within the orebodies, the nanoparticles

320

in the groundwater samples serve as indicators of the ore source, and efficiently transport

321

mineralization information in diverse forms. Both high and low contents of ore-forming elements

322

could be detected in nanoparticles, and as a result, diverse mineralization characteristics are

323

evidenced for the groundwater in the study area. Different nanoparticles are detected from

324

different sampling sites, and a sketch map is shown (Fig. 12). A schematic genetic model of the

325

ore-related nanoparticles from the Qujia deposit shown in Fig. 13 illustrates the three main types

326

of nanoparticles: native metal nanoparticles, metal-based nanoparticles, and the carrier

327

nanoparticles that are associated with trace elements. Given the convenience of groundwater

328

sampling, the simple pre-treatment procedure, and the low limit of detection in TEM in our study,

329

identification of these three types of nanoparticles in groundwater can be easily conducted for ore

330

exploration, not only for determining metallic mineral assemblages, but also for narrowing the

331

areas of interest. Beyond that, ore-related nanoparticles may be distributed in other media, for

332

example, surface vegetation and invertebrate tissues (Lintern et al., 2013; Hu et al., 2018), which

333

may also provide insights to the translocation of geochemical anomalies and further mineralization

334

information.

335

6. Conclusions

336

This paper presents a method to investigate deep-seated orebodies by studying nanoparticles

337

in a groundwater system. Anomalous nanoparticles containing various degrees of ore-related

338

elements (Ag, Cu, Zn, La, Ce, Pb, Pt, S, etc.) were collected from groundwater samples and found

339

to be strongly related to the concealed orebodies. With high contents of ore-forming elements,

340

several nanoparticles in the sampled groundwater commonly include native metal nanoparticles

341

and metal-based nanoparticles. Nanoparticles that are predominantly composed of common

342

elements are also of significance and are capable of carrying a few ore-related elements/tiny

343

particles in groundwater. Mineralization information was efficiently transported in groundwater in

344

diverse forms of nanoparticles. The roles of ore-related nanoparticles are further demonstrated by

345

this study and might be helpful for delineating anomalous areas and locating deep-seated

346

orebodies.

347

Acknowledgments

348

This research was supported by the National Key R&D Program of China (Grant No.

349

2016YFC0600602). We wish to thank Deping Yang and Yuxin Xiong of the Shandong Institute of

350

Geological Sciences for fieldwork assistance.

351

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Tables

78

Table 1. EDS analytical data for nanoparticles in this study (ID 1-20) Particle ID wt% at% wt% 2 at% wt% 3 at% wt% 4 at% wt% I at% 5 wt% II at% wt% 6 at% wt% 7 at% wt% 8 at% wt% 9 at% wt% I at% 10 wt% II at% wt% I at% 11 wt% II at% wt% 12 at% wt% 13 at% wt% 14 at% wt% I at% wt% 15 II at% wt% III at% wt% I at% 16 wt% II at% wt% 17 at% wt% 18 at% wt% 19 at% wt% 20 at%

O

Na

Mg

Al

Si

S

Cl

Ca

Ti

P

Mn

Elements Fe Co

Cu

Zn

Pb

As

Cr

Ag

La

Ce

1

1.15 3.72 52.03 77.66 46.84 73.23 27.24 75.59 16.77 62.13 8.80 33.65 4.03 11.83 7.93 33.90 21.05 55.02 28.14 69.01 21.44 62.36 63.86 77.09 60.79 74.76 33.32 60.08 38.98 68.81 51.53 77.74 43.04 70.77 44.21 71.65 46.99 70.65 57.92 78.99 53.72 75.95 54.52 72.32 84.32 92.90 62.57 76.71 37.82 64.13

1.31 1.11 1.31 1.17

2.95 6.43

4.31 7.83 1.00 1.71 1.87 3.78

0.86 2.42 2.11 3.64 0.11 0.18 0.33 0.63 0.68 0.56

1.11 2.70 1.45 2.17 3.65 5.10 3.57 2.55 4.17 3.04

0.72 0.64

2.36 2.18

1.15 1.07 3.04 2.66

2.96 2.33

0.81 0.69

2.88 2.32

3.07 2.23

25.47 17.52 26.07 18.27 1.60 1.64 0.87 0.87 0.46 0.40 4.96 4.65 5.42 5.01 15.31 13.11 0.35 0.27 0.59 0.47 18.92 14.29 2.35 1.48 19.61 13.69 1.62 1.57

8.94 6.66 0.99 0.77

0.43 0.83 23.78 34.83 0.51 1.10

0.72 0.49 1.20 0.85

1.59 3.06

1.80 1.12 3.23 3.58 3.41 5.05 0.37 0.57

1.84 0.84

2.61 1.26 1.71 0.84 25.13 18.08

1.47 1.00

5.28 3.18

0.90 0.57

18.75 8.98 6.55 3.09 12.09 5.29 1.07 0.72 1.06 0.70

1.19 0.71

15.92 8.66 19.42 10.96 2.12 1.12 11.36 4.99 3.30 1.61

7.10 5.00 9.38 6.85 0.23 0.10

60.54 34.28

0.75 0.92 16.45 12.31 12.19 8.56 13.24 11.03 0.49 0.17 0.32 0.11 14.86 7.67 52.53 26.57 36.01 15.56 32.21 15.17 42.42 19.69 24.18 10.41 6.08 2.37 4.47 1.81 4.80 1.82 1.95 0.61 7.73 2.71

W

33.76 12.68 42.50 20.44 62.22 15.02 70.20 22.62 22.66 21.80 72.18 53.33

0.55 0.94

0.84 0.46 0.77 0.43 2.46 2.00

3.20 1.37 3.47 1.55 7.30 5.80 9.60 10.18

Pt 100.00 100.00 98.84 96.27

64.75 36.69 86.69 54.95 7.86 5.03

9.98 3.00 19.80 5.59 23.20 7.77

3.03 2.16

8.49 4.15

3.16 0.96 4.57 1.42 9.85 4.47 3.24 1.44

4.26 1.88 3.49 1.50 4.50 1.45

10.39 3.56 6.58 2.34

1.02 0.41 1.37 0.54 1.40 0.51 1.13 0.37 4.65 0.50 8.73 1.71

36.74 10.96 35.08 9.82 36.86 12.24

580

Figure captions

581 582

Fig. 1. (a) Location of the study area in China. (b) Sketch map of the Jiaojia orefield (modified from Song et al., 2014; Yang

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et al., 2016; Cai et al., 2018).

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Fig. 2. (a) Sketch map of the Qujia deposit (modified from the unpublished map provided by

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Shandong Institute of Geological Survey). (b) Profile of No. 16 exploration line (modified from the unpublished map

587

Shandong Jinshi Mining Company Limited).

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Fig. 3. Different polycrystalline native Pt nanoparticles: (a) TEM image of the nanoparticle of high contrast; (b) HRTEM

590

image obtained from the rectangle in a; (c) TEM image of the nanoparticle of low contrast; (d) HRTEM image obtained

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from the rectangle in c.

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Fig. 4. TEM images of different metal-based nanoparticles: (a) a Cu-S-O-bearing nanoparticle; (b) a Cr-O-bearing

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nanoparticle; (c) a W-O-bearing nanoparticle aggregate.

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Fig. 5. A Ag-Cu-S-O-bearing nanoparticle: (a) TEM image; (b) STEM image; (c-f) elemental mapping images; (g) EDS

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spectrum.

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Fig. 6. A Cu-S-O-bearing nanoparticle: (a) TEM image; (b) HRTEM image obtained from the rectangle in a. A Ag-O-

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bearing nanoparticle: (c) TEM image; (d) HRTEM image obtained from the rectangle in c.

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Fig. 7. Different La-Ce-Zn-Fe-O-bearing nanoparticles: (a) TEM image of the nearly round nanoparticle; (b) HRTEM image

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obtained from the rectangle in a; (c) TEM image of the irregular nanoparticle; (d) HRTEM image obtained from the

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rectangle in c.

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Fig. 8. TEM images of different carrier nanoparticles: (a) a silicate nanoparticle associated with Cu; (b) a CaCO3

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nanoparticle associated with Co, Cu, and Zn; (c) an Fe oxide/hydroxide nanoparticle associated with Zn and Cu; (d) an Fe

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oxide/hydroxide nanoparticle associated with As.

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Fig. 9. An Fe-Mn-Si-O nanoparticle aggregate associated with Zn: (a) TEM image; (b) HRTEM image obtained from the

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black rectangle in a; (c) HRTEM image obtained from the white rectangle in a. (d) TEM image of a Ca-P-O nanoparticle

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aggregate associated with Pb, Zn, S, and Cu.

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Fig. 10. A silicate nanoparticle containing several tiny particles: (a) TEM image; (b) STEM image; (c) HRTEM image

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obtained from the rectangle in a; (d-g) elemental mapping images; (h) EDS spectrum of tiny particles in c.

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Fig. 11. TEM image of different nanoparticles from the background area: (a) a Si-Ca-Fe-O-bearing nanoparticle; (b) a Si-O-

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bearing bearing nanoparticle; (c) a TiO2 nanoparticle aggregate.

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Fig. 12. Sketch map showing anomalous nanoparticles distribution for different sampling sites.

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Fig. 13. Schematic diagram of the ore-related nanoparticles formation and transport at Qujia.

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Highlights:

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• Nanoparticles in groundwater were investigated using TEM.

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• Nanoparticles with different contents of ore-forming elements can be grouped into three main types.

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• A relationship between metal-bearing nanoparticles and concealed orebodies was demonstrated.

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• Classification of ore-related nanoparticles was important for the detection of mineralization information.

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