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.
Techniques WorldView-2 VLF-EM PXF GGS HGM PEG NIR EH4 Major purposes Identification of ore-controlling structure Prospecting location of low-resistivity zone Evaluation of mineralized potential of fault zone Location of ore-controlling structure Location of ore-controlling structure Evaluation of mineralization potential Identification of alteration zone Identification of deep ore-controlling structural Predict Objects Remote sensing image features of different geological bodies The resistivity of different geological bodies Multi-element contents in soil and rock samples The radioactivity (U, Th, K) of various geological bodies The magnetism of various geological bodies Adsorbed ions in soil Altered minerals The resistivity of different geological bodies Anomalous characters Linear distribution of structures Low-resistivity abnormal zones High value anomaly of Ca Low value anomaly zone of TC, K U and Th or anomaly transition zone Low magnetic anomaly zone High value anomaly of F High value anomaly of alteration minerals Low resistivity anomaly Advantages Wide detection range, no terrain restriction; Portable, economic and efficient Low cost, light, sensitive, accurate, and reliable Low cost, portable, and sensitive Low cost Lost cost, portable and sensitive Low cost, portable, and sensitive High depth of detection Disadvantages Easily affected by the Quaternary cover and Easily affected by external things Easily effected by foreign substances Shallow detection depth and easily affected by external things Easily effected by external things Multiple solutions of anomaly Shallow detection depth and easily affected by external things Multiple solutions of anomaly
Table 1. The effectiveness of different prospecting techniques
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).
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).
Types Techniques Major purposes Anomalous characters Necessity Application Zonal type Worldview-2 Identification, tracking and delineation of mineralized alteration zone Obvious linear structure in remote sensing image Necessary Areal detection VLE-EM Location prospecting and horizontal extension of mineralized low-resistivity zone Low-resistivity anomalous zone Necessary Areal detection, around 50 m effective depth HGM Location of mineralization and alteration in structure Fluorite, quartz veins, and tectonic alteration zone show low-value anomaly, but wall rocks show high-value anomaly Necessary Areal detection PXY Evaluate mineralization potential of fault zone High-value anomaly of Ca Necessary Areal detection or key section dissection PEG Evaluate mineralization potential of fault zone High-value anomaly of F Necessary Areal detection or key section dissection NIR Recognition of mineralized alteration zone Alteration minerals like chlorite, sericite, kaolinite Auxiliary key section dissection EH4 Recognition of deep ore-controlling structure Low-resistivity anomalous zone or high - low resistivity gradient zone Significant Key section dissection, around 1 000 m effective depth Burial type Worldview-2 Identification, tracking and delineation of mineralized alteration zone Obvious linear structure in remote sensing image Auxiliary Areal detection VLE-EM Location prospecting and horizontal extension of mineralized low-resistivity zone Low-resistivity anomalous zone Necessary Areal detection, around 50m effective depth PXF Evaluate mineralization potential of fault zone High-value anomaly of Ca Necessary Areal detection or key section dissection PEG Evaluate mineralization potential of fault zone High-value anomaly of F Necessary Areal detection or key section dissection GGS Recognition and location of mineralized alteration zone Low-value anomaly of TC, K, U, Th Auxiliary Key section dissection EH4 Recognition of deep ore-controlling structural feature Low-resistivity anomalous zone or high - low resistivity gradient zone Significant Key section dissection, around 50m effective depth
Table 2. Optimum combination of prospecting techniques for the two types of fluorite deposits
(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.