Integrated Exploration Model for Concealed Ore Deposit: A Case Study from Shuitou Fluorite Deposit, Inner Mongolia, North China

Fluorite deposits are widely distributed in the world, such as China, Mexico, Mongolia, South Africa and other countries (Schlegel et al., 2020; Magotra et al., 2017; Gigoux et al., 2016; Sun et al., 2010). Fluorite resources in China are characterized by huge proven reserves, wide distribution, simple genetic type, low ore grade, and close association with other resources (Liu et al., 2020; Zhao et al., 2020; Wang et al., 2018). The fractures-hosted fluorite deposits play the predominant role for industrial exploitation (González-Partida et al., 2019; Öztürk et al., 2019). Furthermore, China is leading the global fluorite market, with 80% of the world's total fluorite production at 2018 (Mineral Commodity Summaries, 2019). However, the fluorite resources at shallow ground were becoming exhausted by overexploitation in recent decades. The concealed ores become the primary target of fluorite exploration in recent years (Castorina et al., 2020; Camprubí et al., 2019). The challenges encountered in prospecting concealed fluorite ore bodies include deep burial, strong interference, low precision and difficulty in understanding the metallogenesis (Meng et al., 2017).

Geochemical anomaly is used as direct ore-finding information. At present, delineations of primary and secondary halo are the primary geochemical methods for fluorite exploration (Bedini, 2019; Thabeng et al., 2019; Lampinen et al., 2017; Zou and Chen, 2005; Cameron et al., 2004). It is effective to use zoning primary halo to prospect outcrop or sub-outcrop of fluorite deposit under eluvium. However, these geochemical methods are powerless for exploration of concealed fluorite bodies beneath cover rocks (Hall et al., 1996; Xie et al., 1996; Erickson, 1993; Lin, 1991). Geophysical anomaly can detect buried ore-forming faults and geological bodies associated with mineralization. For concealed fluorite deposit, the effective techniques mainly include very low frequency electromagnetic and dual-frequency induced polarization methods (Li et al., 2018; Xia et al., 2016). However, the geophysical anomalies are usually non-uniqueness, even have no relationship with fluorite ore body. The results can be easily interfered by various factors, such as geography, and other geological bodies (Penland and Sardeshmukh, 1995). Remote anomaly can detect only shallow evidences related to mineralization. Mathematical ways are powerful tools for extracting and integrated mentioned above ore-finding information. Only ore-finding information from geology, geochemistry, geophysics, and remote surveys can be integrated by mathematical methods. However, there are only few applications of remote sensing and mathematical methods realized by various information integration for fluorite exploration (Sun et al., 2008; Zhu and Bao, 1996). The existence and strength of anomalies revealed by a single geophysical or geochemical technique are difficult to draw reliable conclusions in mineral resource prediction due to the great complexity of geological features. With the increasing difficulty in fluorite exploration, the single technique in the past cannot meet the requirements of concealed fluorite exploration.

The Shuitou fluorite belt in Inner Mongolia, occurring as veins in fracture beneath volcanic sedimentary rocks, presents as an ideal case to evaluate the different prospecting techniques and construct integrated exploration model for concealed fluorite ore bodies. In this study, we present the results of systematic verification of portable geophysical, geochemical and remote sensing methods for exploration of concealed fluorite ore body. These prospecting techniques are used to (ⅰ) select the key prospecting targets; (ⅱ) identify the spatial distribution of structures; (ⅲ) estimate the mineralization potential in structural zones; and (ⅳ) evaluate the economic resource in the deep. The effectiveness of different prospecting techniques is evaluated and their optimum combinations for zonal type and burial type concealed fluorite deposits are proposed.

The Southern Great Xing'an Range (SGXR) metallogenic belt is located at in the easternmost segment of the Central Asian orogenic belt (CAOB, Fig. 1a). The CAOB is considered as the largest accretional orogenic belts on Earth, accompanied with extensive Phanerozoic crustal growth and transformation (Wang S J et al., 2020; Wang J P et al., 2018; Yuan et al., 2018; Zhu et al., 2016). It comprises a series of island arcs, fore-arc/back-arc basins, tectonic mélange zone and micro-blocks (Wang L et al., 2020; Zhu et al., 2019; Pei et al., 2017; Windley et al., 2007; Liu et al., 2005; Xiao et al., 2003). The SGXR is bound by the Xar Moron fault with North China Craton to the south, by the Nenjiang fault with Songliao Basin to the northeast and by the Hegenshan fault with Erguna-Xing'an Block to the northwest (Fig. 1b). As an essential part of the CAOB, the SGXR has experienced complicated geodynamic processes under different tectonic regimes, including: (ⅰ) Paleozoic subduction of the Paleo-Asian oceanic plates and amalgamation of several microcontinental terranes (Wilde, 2015; Wu et al., 2011; Xiao et al., 2003); (ⅱ) Mesozoic subduction of the Mongol-Okhotsk and Paleo-Pacific oceanic plates (Liu et al., 2019; Li et al., 2017; Sha et al., 2008; Metelkin et al., 2007; Kravchinsky et al., 2002).

Figure 1.  (a) Schematic map of the Central Asian orogenic belt, modified after Ouyang et al. (2015), showing the location of northeastern China. (b) Geologic map of the Great Xing'an Range and adjacent areas in China, showing the distribution of Late Mesozoic fluorite and polymetallic ore deposits, modified after Zou et al. (2019). Date source from Wang L et al. (2020); Zou et al. (2019); Pei et al. (2018); Sun et al. (2009); Xu (2009).

The geologic terrane exposed in the region include Permian, Jurassic and Cretaceous stratas. The Permian lithology is mainly composed of black slate, siltstone and sandstone, whereas the Jurassic and Cretaceous rocks are dominated by intermediate to felsic volcanoclastic rock, rhyolite, tuff, and ignimbrite. These rocks record plate subduction, collisional orogeny and regional extension during Paleozoic to Mesozoic, which result in the formation of several fault systems with different strike directions (Zhai et al., 2020; Zhang et al., 2019; Shao et al., 2017; Ouyang et al., 2015; Wu et al., 2011). Among them, the early WE striking faults (e.g., Xar Moron fault) plays a leading role in the tectonic framework in the region (Shao et al., 2017). The NE-NNE striking faults, such as Nengjiang fault and Hegenshan fault, occur as abyssal faults in the region. However, their secondary faults striking NE-NNE serve as significant ore-controlling structures (Wang et al., 2006). Multiple stages of intense magmatic activity are recorded in SGXR, especially in the Mesozoic (Ouyang et al., 2015). Previous geochronological data showed two stages of magmatic emplacement, including: (ⅰ) Permian to Triassic intrusions (275–210 Ma) and (ⅱ) Jurassic to Cretaceous intrusions (160–130 Ma; Pei et al., 2017; Ouyang et al., 2014; Zhang et al., 2010). Notably, the most widespread Triassic–Cretaceous granitoids show a younging trend from west to east, which are closely related to mineralization (Zhang et al., 2017; Shu et al., 2015; Wilde, 2015; Zeng et al., 2010).

The Shuitou fluorite belt is located in Chifeng area which hosts 136 discovered fluorite deposits (Chifeng Natural Resources Bureau, 2019). The spatial distributions of fluorite deposits are mainly controlled by NE-striking tectono-magmatic belts. Most fluorite deposits are hosted in NNE-, EW- and NW-striking faults. The wall rocks of fluorite deposits are mainly Jurassic volcanic-pyroclastic rocks and intermediate-acid intrusive rocks.

The Shuitou fluorite belt occurs in the northwestern segment of the Linxi County, approximately 200 km from Chifeng, Inner Mongolia. Dozens of fluorite ore bodies are discovered and they display typical linear distribution and are strictly controlled by faults with different directions. Among them, the ore-controlling faults in a wave-like shape are mainly in NE-NNE directions, followed by SN- and EW-directions. The entire ore-bearing belt extending intermittently for more than ten kilometers can be divided into three parts from north to south (Fig. 2a), including: (ⅰ) the north part is represented by Saiboluogou fluorite deposit (Fig. 2b), which has fluorite veins extending for 2.5 km discontinuously; (ⅱ) the middle part is restricted in Shuitou area, where fluorite veins are developed about 1.5 km in length; and (ⅲ) the south part is located in Hanpaozi-Elimutai area, where the fluorite veins extend about 4 km discontinuously. The major rock types include muddy-sandy slate, volcanic-sedimentary rock and carbonates. The Permian and Late Jurassic granites are exposed in the western and eastern parts of the study area, respectively. A series of lenses-like, pods-like pull-apart spaces in ore-controlling faults provide favorable spaces for precipitation of fluorite (Fig. 3a). The ore bodies strike 345° NW to 30° NE and dip steeply at angles of 65°–85° (Fig. 3b). The thickness of single fluorite vein is generally 0.5–10 m. The fluorite is purple, green, pink, colorless and white in color. The ore texture mainly consists of medium- to coarse-grained, subhedral to idiomorphic granular texture, followed by fine-grained anhedral to subhedral granular texture (Fig. 3c). The fluorite ores display massive, brecciated, banded, and comb structure (Figs. 3d–3i). Based on the feature of ore bodies and mineral assemblages, three stages of primary paragenetic sequences are recognized (Fig. 3i), including: (ⅰ) the early fluorite-quartz stage (stage Ⅰ); (ⅱ) the main quartz-fluorite stage (stage Ⅱ); and (ⅲ) the late carbonate stage (stage Ⅲ). The deposits have a relatively simple mineralogy with mainly fluorite and quartz, which account for more than 95% of the total ore, followed by opal, calcite, kaolinite and barite. The hydrothermal alteration is relatively simple and characterized by silicification, kaolinization, chloritization, sericitization, pyritization and carbonatization (Fig. 3j).

Figure 2.  Geological sketch map of the Shuitou fluorite belt (a) and Saiboluogou fluorite deposit (b), modified after Zhang et al. (2020).

Figure 3.  Photographs showing the characteristics of ore-controlling structure and representative samples of Shuitou fluorite ores. (a) Characteristics of ore-controlling structure; (b) characteristics of fluorite vein, brecciated fluorite ore and silicified zone; (c) purple, subhedral granular fluorite; (d) impure ore with brecciated structure; (e) white and light blue fluorite ore; (f) pure ore with brecciated structure; (g) mineral assemblage of fluorite and quartz with comb structure; (h)–(i) cutting relationship between different stages of fluorite veins. Note: W. wall rock; Q. quartz; Fl. fluorite; Q (Ⅰ). quartz breccia of stage Ⅰ; Fl (Ⅱ). fluorite breccia of stage Ⅱ; Py (Ⅲ). pyrite veins of stage Ⅲ; Cal (Ⅲ). calcite of stage Ⅲ.

The Saiboluogou fluorite deposit has three main ore veins occurring along the faults in Upper Permian Linxi Formation (Fig. 4a). The outcrop of No. Ⅰ vein is more than 600 m in length and varies from 0.5 to 5 m in width (Fig. 4b). In addition, the richest mineralized zones contain > 50% CaF₂ with up to 8 m thick orebodies (Fig. 4c). The outcrop of No. Ⅱ vein exposes as intense silicified zone (length of 750 m and width of 3–5 m) with quartz, opal and minor fluorite (Figs. 4d–4f). The No. Ⅲ vein is hosted in the contact zone between Permian granodiorite and Linxi Formation. It occurs as a quartz vein or silicified zone with outcrops of brecciated opals and vein quartz which extend 350 m in length and 3–5 m in width (Figs. 4g, 4h).

Figure 4.  Field photographs of the Saibolugou fluorite deposit. (a) Outcrops of No. Ⅰ, No. Ⅱ and No. Ⅲ veins; (b) outcrop of No. Ⅰ vein; (c) purple fluorite ore with banded structure from No. Ⅰ vein; (d) outcrop of No. Ⅱ vein; (e)–(f) silicified zones and quartz veins of No. Ⅱ vein; (g) outcrop of No. Ⅲ vein; (h) silicified rocks of No. Ⅲ vein.

The SGXR hosts widespread Mesozoic volcanic and intrusive rocks, numerous porphyry Mo-(Cu), skarn Fe-(Sn), hydrothermal Ag-Pb-Zn and fluorite deposits (Song, 2019; Shu et al., 2015; Wu et al., 2011). The ages of large-scale magmatic activities and metallogenic events in the SGXR mainly range between Late Jurassic and Early Cretaceous (155–120 Ma), and the peak of metallogeny is slightly younger than the peak of magmatism (Ouyang et al., 2015). The metallogenic ages of fluorite deposits range from 154 to 132 Ma (Wang L et al., 2020; Pei et al., 2018; Cao et al., 2013; Sun et al., 2009; Xu, 2009), which is consistent with large-scale magmatic-hydrothermal events in the SGXR and is closely related to Cretaceous tectonic transition in eastern China. Both fluorite and polymetallic mineralization are considered as a unified mineralization system in extensional tectonic setting (Wang L et al., 2020; Song et al., 2019).

The Shuitou deposit was formed at 132±11 Ma based on fluorite Sm-Nd isotopic age (Pei et al., 2018). Notably, the emplacement age of Herimu Tai granite from the southern part of the Shuitou fluorite belt is 136.9±1.7 Ma (Pei et al., 2018). Pei et al. (2018) proposed that the fluorite mineralization and granitoid magmatism are formed in the same tectonic setting within a post-collisional extension setting. The ore-forming fluid of fluorite deposits was considered as mixed fluid from meteoric water and formation water (Wang L et al., 2018; Pei et al., 2018; Zhang et al., 2014). The F came from the deep F-rich geologic bodies, and Ca was sourced from wall rocks through leaching of hydrothermal fluid. Water-rock reaction was considered as the main mechanism for fluorite mineralization along faults and other fractures (Wang L et al., 2020; Cao et al., 2014; Zeng, 2013).

Prospecting techniques including WorldView-2 high resolution multispectral remote sensing (WorldView-2), VLF electromagnetic (VLF-EM), portable X-ray fluorescence analysis (PXF), ground gamma-ray spectrometry (GGS), partial extraction geochemistry (PEG), high-accuracy ground magnetic survey (HGM), near infrared spectroscopy (NIR) and Stratagem^TM EH4 (EH4) were carried out to study the feasibility of multi-techniques for prospecting concealed fluorite ore bodies. The prospecting techniques were applied in the three ore veins and alteration zones of the Saiboluogou deposit in the northern segment of the Shuitou fluorite belt (Figs. 2b, 5a). The details of prospecting techniques and results are described below.

Figure 5.  (a) Geological sketch map of Saiboluogou fluorite deposit, modified after Pei et al. (2018); and (b) WorldView-2 high resolution multispectral remote-sensing image of Saiboluogou fluorite deposit.

The ore-bearing geological bodies in shallow ground can be identified in the WorldView-2 remote sensing images as line, ring, color, band and block under certain conditions (Bedini, 2019; Thabeng et al., 2019; Fang, 2014). Mineralized and hydrothermal altered zone show obvious linear distribution in remote sensing images. Therefore, the following two signs in WorldView-2 remote sensing images are reasonable to identify the occurrence of veins and hydrothermal altered zone in Saiboluogou fluorite deposit, including (ⅰ) the mineralized and hydrothermal altered zones show obvious linear structures in Fig. 5b; and (ⅱ) the extracted ridges with potholed-shape or grainy-shape represent the exposure of fluorite veins, silicified zone or quartz veins, which have strong resistance to weathering (Fig. 5). The discontinuous ridge terrains can be easily formed in coverage areas, which are manifested as potholed-shape or grainy-shape in remote sensing images. Among them, the No. Ⅰ vein shows obvious grainy feature, while the No. Ⅱ and No. Ⅲ silicified zone and quartz veins show clear potholed feature (Fig. 5).

The VLF-EM is widely used in metallic mineral exploration since 1970s. The intensity, scale and continuity of mineralization in cross section can be defined by VLF-EM (Chen et al., 2017; Zhang et al., 2008; Eze et al., 2004). The fluorite veins in fractures can be effectively demonstrated as low-resistivity zone by using VLF-EM, and the extension of these anomaly zones to the surface can be further evaluated.

The No. Ⅰ fluorite vein corresponds to a consecutive low-resistivity zone with undulation feature (Fig. 6). The No. Ⅱ silicified zone corresponds to a medium-high resistivity zone. The result suggests that the fault zone is filled with quartz veins with relatively high resistance, whereas the wall rocks on both sides of the fault zone are silicified within a certain depth and width. There is a trend of superposition in the southern part of Saiboluogou fluorite deposit between No. Ⅰ and No. Ⅱ veins, indicating the two belts merge in the deep. In Fig. 7, fluorite and quartz veins show obvious low-resistivity feature, and the anomalies corresponding to fluorite ore body are much clear than those of quartz veins and silicified zones. The result suggests that the silicified degree of quartz vein is significantly higher than fluorite ore body, resulting in the different behaviors during the subsequent fragmentation.

Figure 6.  Planar distribution of dip-angle anomaly by Fraser filtering in Saiboluogou fluorite deposit, modified after Fang et al. (2014).

Figure 7.  Integrated profiles including geological sections and VLF-EM profile of line 13 in Saiboluogou fluorite deposit; (a) geological section; (b) density diagram of equivalent current; (c) profile of Magnetic dip (D) and Fraser filter (F).

The PXF method is mainly used for rapid analysis of mineralization-related elements in rock or soil. It has been widely used in mineral exploration due to the advantages of portability, inexpensive and high efficiency (Li et al., 2018; Wang et al., 2012; Piorek, 1997). The positive anomaly of Ca is the best indicator for fluorite ore body.

In the PXF profile, the Ca anomalies are consistent with the location of concealed fluorite ore body. As shown in Fig. 8, a series of positive Ca anomalies exist at the east side of No. Ⅰ vein, which is caused by the migration of element in topography. The positive Ca anomalies also occur above the vein and on both sides of the No. Ⅱ vein, which are similar to the anomaly of VLF-EM. The result suggests that the outcrops of the No. Ⅱ vein represent the silicified cap of concealed fluorite ore body.

Figure 8.  Ca content distribution map of PXF in Saiboluogou fluorite deposit, modified after Fang et al. (2014). L-Ca: lower limit of calcium anomaly, refer to Fang et al. (2014).

The concealed ore bodies are deduced by GGS through analyzing the total contents and general trends of U, Th and K in various geological bodies (Lampinen et al., 2017; Xin et al., 2008). The radioactive elements (U, Th and K) and their parameter ratios could reveal the location of ore body in shallow coverage areas. The high-value anomalies of U, Th and K correspond to the wall rock alteration, whereas the low-value anomalies correspond to the quartz vein and fluorite ore body.

Figure 9 shows the curve trend of radioactive elements from GGS at line 13 in Saiboluogou fluorite deposit. The Th, U and K curves are consistent, and their low-value anomalies correspond to fluorite ore bodies with the anomalies of later fluorite vein are more prominent than those of the former veins (Figs. 9b, 9c). In addition, the high-value anomalies of Th, U and K occur in the roof of No. Ⅰ fluorite ore body. The results could be caused by the vertical dissipation of radioactive elements in the upward migration process along fracture zone, resulting in the sharp decrease of radioactive elements in the foot-wall of fluorite ore body in the fault.

Figure 9.  Integrated profiles including geological section (a) and GGS composite profiles (b)–(c) of line 13 in Saiboluogou fluorite deposit.

The HGM is a geophysical technique for ore prospecting by detecting the magnetic characteristics of geological bodies (Yang et al., 2013). Fluorite is non-magnetic and corresponds to negative magnetic anomaly. Quartz veins and ore-bearing veins have weak magnetism and correspond to low magnetic anomalies. The magnetic susceptibility of wall rocks is significantly different with fluorite and quartz, showing relatively strong magnetic property and positive magnetic anomaly (Yu et al., 2019). Hence, the distribution of alternation zone can be basically recognized by HGM based on different susceptibility of fluorite, quartz, altered and unaltered wall rocks.

The result of HGM magnetic anomaly in Saiboluogou fluorite deposit is displayed in Fig. 10. From blue to red, the magnetic anomalies increase gradually. The locations of fluorite ore body and quartz veins show relatively low-magnetic anomalies or are in anomaly transition zones. The magnetic survey of fluorite, quartz and altered zone shows low-value anomalies; on the contrary, the magnetic survey of wall rocks displays high-value anomaly. It can be concluded that the magnetic survey of geological bodies related to fluorite mineralization shows low magnetic anomalies by HGM.

Figure 10.  Planar (a) and sectional (b) distribution of magnetic anomaly by GGS in Saiboluogou fluorite deposit.

The PEG technique is widely used to extract alteration information, unravel the state of elements and strengthen the geochemical anomaly. It is also a useful tool to increase the prospecting depth of concealed ore (Kelley et al., 2003; Wang et al., 1998; Clark, 1993). The soil samples with the distance of 10 m and the depth of 50 cm were dealed with ultra-pure hydrofluoric acid for further analysis by ICP-MS. Detailed analytical methods were described by Lu (2019). The content of soil-adsorbed F is an effective indicator for fluorite mineralization, showing positive anomaly above the fluorite ore body. Other elements like Sr, Y, Pb, Zn and Ag, also show anomalies.

A total of eleven elements (F, Ca, Ba, Pb, Zn, Hg, Ag, Sr, Y, Cu and Rb) were measured by PEG (Fig. 11). The curve of F is relatively flat with an anomaly only above the fluorite ore body, and no anomaly on the quartz veins (Fig. 11). The curve of Ca shows many anomalies, so the results of the adsorbed Ca contents cannot reflect the location of the concealed fluorite ore body. The curves of Pb, Zn, Ag and Hg are similar, showing low-value anomalies corresponding to fluorite ore body and quartz veins. The results indicate low mineralization potential for metal resources in the deeper level of the Saiboluogou fluorite deposit. The curve of Ba shows a zigzag shape without obvious correspondence with fluorite ore body or quartz veins. Strontium, Y and Rb are relatively enriched in fluorite deposit due to their similar ionic radius with Ca (Öztürk et al., 2019). There is an obvious anomaly of Sr above the No. Ⅰ fluorite ore body and weak anomaly above the quartz veins. However, Y and Rb curves have no obvious peaks above the three veins.

Figure 11.  Integrated profiles including geological section and elements measured by PEG in Saiboluogou deposit.

In recent years, NIR has been widely used for mineral exploration, especially for epithermal deposit (Tang et al., 2015; Xian, 2014; Yang et al., 2012). The altered minerals associated with fluorite mineralization can be identified by NIR, according to the characteristics of intensity, symmetry, shift, half-width, intensity ratio and reflectivity of NIR for alteration minerals (Fig. 12). Notably, the intensity, symmetry and half-width of NIR vary with the distance from the detected mineral. The closer detection distance always yields higher intensity. On the contrary, the shift, intensity ratio and reflectivity of NIR for alternation minerals decrease gradually with the distance from the ore body.

Figure 12.  Near infrared spectroscopy (NIR) results of chloritization (a), kaolinization (b) and sericitization (c).

The hydrothermal alternation is relatively simple in Saiboluogou fluorite deposit, including silicification, kaolinization, chloritization, sericitization, pyritization and carbonatization. In this study, the hydrothermal alterations of sericitization, chloritization and kaolinization are well identified by NIR (Fig. 12).

EH4 is a new and powerful technique for concealed ore exploration, which can reflect the shape and scale of mineralized zone in profile directly (Fu et al., 2015; Li et al., 2009). For hydrothermal vein type fluorite deposit, the fault and mineralized belt are manifested as low-resistivity zone or gradient sharp changing zone.

The EH4 profile of line 13 in Saiboluogou fluorite deposit is displayed in Fig. 13. According to the geological characteristics, the points of Z1329, Z1339, and Z1350 correspond to the No. Ⅲ, No. Ⅱ and No. Ⅰ veins, respectively. All three veins overlap with the gradient changing zone between high- and low-resistivity anomaly zones (Fig. 13). Hence, the electrical differential interface of EH4 is an important evidence for location prediction of deep structure, fluorite mineralization or altered zone in coverage areas. Furthermore, the occurrence of high-, low-resistivity zones and gradient changing zone reveal the west dipping occurrence of deep structure, which is consistent with the field observation.

Figure 13.  EH4 conductivity profile of line 13 in Saiboluogou fluorite deposit.

The prospecting techniques have different characteristics, application restrictions and corresponding conditions. In order to discuss the advantages and disadvantages of each technique and synthesize the best combination of techniques for prospecting concealed fluorite ore bodies, the effectiveness of different techniques is summarized in Table 1.

The crucial issues for prospecting concealed fluorite ore body mainly include: (ⅰ) locate the ore-controlling structure, and (ⅱ) evaluate the mineralization potential in structure. The techniques including WorldView-2, VLF-EM, GGS, HGM and EH4 aim at locating the ore-controlling structure of concealed fluorite ore bodies. WorldView-2 is suitable for bedrock exposed areas with few Quaternary coverage and undeveloped vegetation. In WorldView-2 image, the spatial distribution characteristics of regional linear structural pattern are clear, especially for Zonal type concealed fluorite deposits. However, the results can be easily affected by the thickness of Quaternary sediment and vegetation coverage. The VLF-EM and GGS methods are effective to locate the ore-controlling structure. Compared to GGS, the VLF-EM can reveal greater detective depth (~50 m) and the results are less influenced by cover rocks. The HGM and EH4 methods can reveal the feature of ore-controlling fault in deep. Compared to HGM, the EH4 has greater detective depth (~1 000 m) and the results are less influenced by topography. Hence, EH4 is an effective tool for exploration of concealed ore bodies.

The prospecting techniques including PXF, PEG and NIR were applied to evaluate mineralization potential in structure. The PXF can analyze the Ca content effectively, which is the direct evidence for mineralization potential in fractures. It is noteworthy that a comprehensive analysis with the combination of geological investigations and other techniques is required in high-Ca background areas. Unfortunately, the accuracy and reliability are always influenced by adventitious alluvial sediment or drift load. The PEG can obtain the F content directly, which make up for the shortage of VLF-EM in some high-Ca background areas. The NIR is an effective technique to analyze the distribution of alteration minerals (such as chlorite, kaolinite and sericite). In addition, the results of GGS are closely related to the minerals containing radioactive components in measured rock or soil.

Based on geological features and different manifestations of prospecting techniques, 10 anomalies are presented below.

(1) Geological features: fluorite mineralization, hydrothermal alteration, quartz veins and silicified breccia zone along faults.

(2) Geomorphic features: spine-like positive landform and paternoster negative landform.

(3) Remote-sensing features: linear distribution of structures.

(4) VLF-EM features: low-resistivity anomaly zones.

(5) GGS features: low value anomaly of TC, K, U, Th and anomaly transition zone of low value anomaly (ore body) to high value anomaly (alteration).

(6) HGM features: low magnetic anomaly zone.

(7) PXF features: high value anomaly of Ca.

(8) PEG features: high value anomaly of F.

(9) NIR features: alteration minerals such as chlorite, kaolinite and sericite show high value anomaly of intensity, symmetry and half-width, and low value anomaly of peak shift and peak to strong ratio.

(10) EH4 features: low-resistivity zone or gradient sharp changing zone between low-resistivity and high-resistivity.

The factors controlling the dispersion halo of fluorite veins at Shuitou include: (ⅰ) type and thickness of coverage rocks, and (ⅱ) the existence of silicified zone in the apical part. Based on these factors, the fluorite deposits are divided into two types: (ⅰ) Zonal type concealed fluorite deposit, and (ⅱ) Burial type concealed fluorite deposit (Fig. 14).

Figure 14.  Schematic diagram for the zonal type (a) and burial type (b) concealed fluorite deposit.

Zonal type concealed fluorite deposit (Fig. 14a): The rock outcrops, ore-controlling fracture zone and hydrothermal alteration zone are well developed in shallow surface. From the surface to deep, silicified zone occurs at near-surface and fluorite ore bodies are well preserved in the deep with the main fluorite ore buried in depth ranging from a few dozen of meters to several hundred meters.

Burial type concealed fluorite deposit (Fig. 14b): the concealed fluorite ore body was once exposed at the surface because of weathering, tectonic movement or crustal uplift. Then, it was covered by autochthonous weathering residues or adventitious alluvial sediments. The thickness of the Quaternary sediments is generally tens of centimeters to several meters, which can be up to more than ten meters in the low-lying terrain, and even up to tens of meters in valleys.

According to the geological features, effectiveness, similarity in detection efficacy and complementarity of different methods, the optimum combinations of techniques prospecting for the Zonal and Burial types fluorite deposits are summarized below (Table 2).

(1) Optimum combination of prospecting techniques for Zonal type concealed fluorite deposit includes WorldView-2 (Necessary), VLF-EM (Necessary), HGM (Necessary), PXF (Necessary), PEG (Necessary), NIR (Auxiliary) and EH4 (Significant).

(2) Optimum combination of prospecting techniques for Burial type concealed fluorite deposit includes WorldView-2 (Auxiliary), VLF-EM (Necessary), PXF (Necessary), PEG (Necessary), GGS (Auxiliary) and EH4 (Significant).

We propose an integrated exploration model for prospecting concealed fluorite ore body as follows. The technical process is shown in Table 3. Firstly, select key prospecting targets based on the combination of regional metallogenic background, geochemical anomaly of F and Ca, and high resolution multispectral remote sensing image. Secondly, identify the spatial distribution of ore-controlling structure on the basis of key profile dissection and rapid aerial reconnaissance through VLF-EM, unraveling the spatial distribution of low-resistivity anomaly zones. Thirdly, evaluate mineralization potential of structure zone with low resistance features, and identify Ca and F anomalies in key section by PXF and PEG. In cases, disclose the exposure of alternation zone and apply NIR spectrum analysis for alternation minerals. Fourthly, evaluate the potential of deep resources according to the comprehensive analysis of geophysical and geochemical anomalies. The spatial distribution of concealed ore body and extension of ore-controlling structure (~1 000 m below the surface) are further clarified by applying EH4 in some key sections. The evaluation of resource potential in deep and the deployment of further prospecting projects are guided on the basis of geological characteristics and metallogenic regularities of fluorite.

Table 3.  Integrated exploration model for concealed fluorite deposit

The Saiboluogou area is covered by widespread Quaternary sediments with sporadical outcrops of silicified cap and weak fluorite mineralization on the surface. The integrated results of geological, remote sensing, geophysical and geochemical techniques indicate that the concealed fluorite bodies are probably developed in the deeper level of Saiboluogou area based on the following lines of evidence, including: (ⅰ) the Saiboluogou area belongs to the northern part of the Shuitou fluorite belt which has good potential for fluorite mineralization; (ⅱ) the parallel distribution characteristics of the three veins striking NNE-SN are well revealed by WorldView-2 remote sensing images. The No. Ⅱ vein shows converging trend to the north and branching off to the south, thus the No. Ⅲ vein is probably the branch of No. Ⅱ vein; (ⅲ) the VLF-EM results delineate a group of nearly SN-trending structures, which are mainly composed of NNE-trending, SN-trending and NNW-trending faults (Fig. 6). The distribution of low-resistivity zone is consistent with No. Ⅰ fluorite vein; (ⅳ) the Ca and K anomalies in PXF profiles are consistent with the location of the three veins (Fig. 8); (v) the Th, U, K and total gamma rays display the low-value anomalies over fluorite bodies, and the distribution of anomaly zone is in good agreement with the No. Ⅰ fluorite vein (Fig. 9); and (vi) all three veins overlap with the gradient changing zone between high- and low-resistivity anomaly zones in EH4 profile (Fig. 13).

Xu and Zhang (2013) proposed that the vertical zonation of fluorite orebody can be divided into four parts from top to bottom, including silicified cap, head ore, main ore and tail ore. The outcrops of the three veins occur as white quartz stockworks and silicified rocks along with weak fluorite mineralization which are considered as typical silicified cap. Drill cores disclose the fluorite ore body at 80 m beneath the No. Ⅱ vein (Fig. 15a). The mineralized zone controlled by structure is 4.5 m wide with 3.7 m of quartz vein and 0.8 m of fluorite vein (Figs. 15b–15f). Fluorite veins mainly occur in the roof and floor of the ore-controlling structures. However, the fluorite ore is still sub-economic with low quality, and belongs to head ore showing higher fluorite content than those in silicified cap. The main ore has yet to be unrevealed and could exist at least 250 m below surface based on the vertical zonation pattern of fluorite ore and the EH4 profile (Fig. 13). Hence, it is reasonable to propose the considerable fluorite resources potential at Saiboluogou.

Figure 15.  Field photographs of fluorite ore in drilling cores. (a) Ore-bearing structure; (b) white fluorite ores with brecciated structure; (c) white and light blue fluorite ore; (d)–(e) fluorite-quartz vein with brecciated wallrock; (f) banded ore with purple fluorite and white quartz veins. Q. Quartz; Fl. fluorite.

(1) On the basis of geological study and validity analysis of different prospecting techniques, we identify the following anomalies for prospecting concealed fluorite ore bodies: (ⅰ) geological features: fluorite mineralization, hydrothermal alteration, quartz veins and silicified breccia zone along faults; spine-like positive landform and paternoster negative landform; (ⅱ) remote-sensing features: linear distribution of structures; (ⅲ) geophysical features: low-value anomaly zones or anomaly transition zone; and (ⅳ) geochemical features: high value anomaly of Ca and F, and low value anomaly of TC, K, U, and Th.

(2) The concealed fluorite deposits are divided into Zonal and Burial types. The optimum combination of WorldView-2 (Necessary), VLF-EM (Necessary), PXF (Necessary) and PEG (Necessary) is proposed for Zonal type concealed fluorite deposit, and the combination of VLF-EM (Necessary), PXF (Necessary) and PEG (Necessary) is suggested for Burial type concealed fluorite deposit.

(3) A comprehensive exploration model for concealed fluorite deposit in coverage areas is established, including: (ⅰ) select key prospecting target by WorldView-2 and geological investigation; (ⅱ) recognize the low resistance anomaly zone through VLF-EM rapid scanning; (ⅲ) evaluate mineralization potential of fault zone by PXF and PEG; and (ⅳ) evaluate the deep fluorite resource potential through geophysical techniques, such as EH4.