Some Experiences of Resistivity and Induced Polarization Methods on the Exploration of Sulfide: A Review

Sulfide minerals are a group of compounds with the presence of sulfur. This group’s most abundant and economically members are pyrites, pyrrhotite, chalcocite, galena, sphalerite, and the group of copper sulfides minerals. Resistivity and Induced Polarization (IP) methods, which play an essential role in mineral exploration, showed great success in sulfide exploration. This paper started on reviewing sulfide formation by giving details which help to understand their genesis better. To make the reader understand the procedures and appropriate mineral exploration methods, we have briefly covered the theory, the basic principles of resistivity and IP methods, and different investigation techniques using one, two, and three-dimensional surveys. Based on many electrical surveys, we discussed with examples of resistivity and IP methods applied to the exploration of sulfide deposits: the data inversion and interpretation of the geophysical signatures of most of the sulfide deposits in various geological environments were analyzed and end by showing both successful surveys and limitations of the methods.


Introduction
Sulfide minerals are a group of compounds with the presence of sulfur. The most abundant and economically members of this group are pyrites, pyrrhotite, chal- Figure 1 shows the example of sulfides formation at high temperatures.
The older the deposit and the deeper its site of deposition, the more likely it is to contain abundant pyrrhotite, lack barite (Plimer & Finlow-Bates, 1978). Deposits which formed at shallow depth (<500 m) where circulation was probably restricted, pyrite is far more abundant than pyrrhotite (Plimer & Finlow-Bates, 1978).

Biogenic Sulfides Formation
In biogenic sulfides formation, bacteria activities are the primary source of sulfides. Bacterial sulfate reduction is a potential source of high sulfide concentrations in typical marine sediments. However, fixation is limited by a lack of reactive metals and could lead at best to the formation of relatively low-grade ores.
Biogenic sulfide might also contribute to mineralization in hydrothermal situations (Trudinger, 1981). The sulfate reduction process takes place in much lower temperatures (<150˚C) (Trudinger, 1981;McDonald et al., 2018). In hydrothermal systems, thermophilic bacteria biogenically reduce seawater sulfate ( 2 4 SO − ) to sulfide (H 2 S) (McDonald et al., 2018). When these react with 2 Fe + , they can form a very fine particle black precipitates poorly crystalline mackinawite (FeS) or a mixture with greigite (Fe 3 S 4 ). With time, these iron minerals transform into pyrite. Figure 2 indicates three possible pathways of transformations. Table 1 shows examples of sulfide minerals and their host rocks from both abiotically and biogenically. Figure 1. Genesis of volcanogenic massive sulfide deposits (Galley, 1993;Gibson et al., 2007;Piercey, Peter, & Herrington, 2015).   For simplicity, all layers are expected to be horizontal. The electrical resistance, R of a material is related to its physical dimension, cross-sectional area, A and length, L through the resistivity, ρ.

RA L ρ =
(1) The electrical resistance of the cylindrical body R (Ω), is given by: where V is the potential and I is the current.
The conductivity σ (Sm −1 ), is the reciprocal of the resistivity.
In a homogeneous and isotropic half-space, the current density J (A/m 2 ) is: The potential V becomes: The measured voltage with the injected current is converted into apparent resistivity ( a ρ ) by a geometric factor which considers the air-earth interface (Telford et al., 1990a) [ ] where: k is a geometrical factor [m].
, r r  are the distances of the real and mirror effect of the ground surface at potential points (M, N) respectively. V ∆ is the measured difference potential at points M and N; I is the applied electric current.

Theory and Basic Principles of the IP Method
IP has been widely used in geophysical surveys for many decades to provide information about the complex conductivity (chargeability) of the subsurface. IP   are produced by switch-off the transmitted current and measure the potential decay. This is known as the apparent chargeability (M). The most common measurement of the magnitude of the IP effect in the time domain is chargeability M.
The common chargeability expression is: where V p is the primary voltage, V s represents secondary voltage, and V t is the voltage decay with a time interval between t 1 and t 2 . Units of chargeability are millivolts per volt (mV/V) or millisecond (ms).
In the frequency domain, measurements involve measuring the resistivity at varying frequencies one higher than the other, transmitted one after the other.
The IP effect is the percentage frequency effect (PFE) defined as: where 2 ρ and 1 ρ represent apparent resistivity at high and low frequencies respectively. PFE has units of percent.
The IP effect is influenced by the effective resistivity of the host rock and, Metal Factor is defined as the amount of sulfide minerals contained in rocks, in which the amount depends on the value FE. Metal factor is formulated as follows: 1000 is an arbitrary factor chosen to give MF values appropriate values from 10 upwards, and MF has of mhos per foot.
The concept of metal factor arose early in the development of the IP method, because it was felt that low resistivity values would damp the value of PFE more than high resistivity values.

Characteristics of Different Arrays Configurations Types
Several conventional electrode arrays are used for resistivity and IP surveying. There is no "best" method that can match with all situations encountered in the fields (Daniels, 1977) but the choice depends on the aim of prospecting; the geological aspects of the subsurface, the availability of holes; and economics (Daniels, 1977;Bing & Greenhalgh, 2000). Table 2 describes common arrays configuration depending on the measured potential by assuming that the earth's resistivity is homogeneous and isotropic.
Surface configuration: Surface geophysical methods present a low spatial resolution, which decreases with increasing depth of investigation. Moreover, this can hinder the exposure of small-scale features such as cavities or leachate plumes. The use of electrodes in boreholes and pole-dipole arrays are recommended to extend the adequate depth of investigation (White et al., 2001).
Hole-to-surface measurements are performed by setting a pole or dipole source down a borehole and making surface dipole measurements away from the source hole (Daniels, 1977).
Borehole: Vertical profiles can detect both in-hole and off-hole features, but the enlargement depends on the configuration (Palich & Qian, 2007).
Single-hole arrays (Vertical Resistivity Profiling (VRP)) use a pole or bipole source and potential receiver in the same borehole (Daniels, 1977).
Hole-to-hole measurements (Borehole Resistivity Tomography (BRT)), this is performed by installing a pole or a dipole current source in a borehole and placing a pole or a dipole potential-receiver nearby borehole (Daniels, 1977).
For shallow targets hole-to-surface measurements may locate the target with a minimal field effort (Daniels & Scott, 1981;Daniels, 1983;Tsourlos et al., 2011). In the case of borehole-to-surface, Pole-dipole and dipole-dipole configurations are preferred because other arrays such as Wenner, Wenner-Schlumberger, and Gradient Schlumberger are more prone to artifact distortion (White et al., 2001;Tsourlos et al., 2011). The lateral position of the target can easily set by hole-to-surface or conventional dipole-dipole surface arrays (Daniels & Scott, 1981). Ideally, the horizontal position of the body can be established by a conventional surface survey or a hole-to-surface survey while the vertical position of the body can be determined by hole-to-hole or single-hole measurement (Daniels & Scott, 1981). For the intersection of mineralization and drill holes, mise-á-la-masse surveys appear more promising for imaging the minerals' orientation and strike extent (Tyne, 1980;Guo, Dentith, & Zhao, 2000). However, for near-surface with borehole resistivity surveys, accurate array locations and correct geometry factors are very important to account for bo-  (Guo, Milkreit, & Qian, 2014).
For all, shorter electrode intervals are needed to better define shallow anomalies and mineralized veins (Mammo, 2013).

1D, 2D, and 3D Dimensional Surveys
The Electrical Resistivity and IP methods can provide 1D, 2D or 3D images of their distribution in the subsoil.

1D Dimensional Surveys
Since the early 1920s to the late 1980s, the resistivity method's first commercial use was one-dimensional (1-D) mapping method for the profiling and sounding methods (Ghosh, 1971;Ward, 1990;Meheni et al., 1996;Loke, 2011;Sharma & Verma, 2015). The method is quick and simple in application (Ghosh, 1971;Ogunbo, 2018), it has been widely used to investigate the ground for resource management, such as mineral, petroleum, and groundwater resources (Loke, 2011). With this survey, the subsurface should consist of horizontal layers (Loke, 2011;Loke et al., 2013). In homogeneous, 1-D survey provides good results (Karim, 2015). However, 1-D survey does not provide significant lateral changes, which may bring to gives inaccurate results or misleading results (Loke, 2011;Loke et al., 2013).    which is of great importance for targeting VMS ore (Tavakoli et al., 2016b).

Inversion Process for Sulfide Investigation
To obtain an accurate picture of the subsurface, it is necessary to carry out the pseudo section's inversion (Loke & Barkert, 1995). The inversion is performed by forward modeling operator by calculating theoretical data from input data and thereby calculating derivatives of the data with regard to the parameter (Pelton, Rijo, & Swift, 1978;Madsen et al., 2018). The massive sulfides deposits are indicated by the coincidence of high chargeability and low resistivity values after inversion (Moreira et al., 2012). It is noticeable that a suitable inversion method must simultaneously minimize the effects of data error and model parameter errors (Ja, Oo, & Arce, 2014). Therefore, to produce a better inversion, there should be re-run adjusting default parameters by manual selection of set-  (Robertson & Hart, 2013). The "least-squares method" is the most method used amongst the papers analyzed in this review. The "least-squares method" seems to be the method that can successfully remove the distortions in the apparent resistivity pseudo section because it separates the electrode array geometry's effect on the apparent resistivity values from that which results from the subsurface resistivity (Loke & Barkert, 1995). It also separates overlapping anomalies caused by different bodies (Loke & Barkert, 1995).

Electrical Properties of Sulfide Host Rocks
The  Table 3.
Sulfide in a sedimentary host may be derived directly from the mantle or by reduction of sulfate either by a chemical or biological mechanism (Trudinger, 1981). Resistivity and chargeability of sulfide mineralization in sedimentary host Table 3. Electrical resistivities range sulfide minerals and host rocks. Chalchopyrite (Ward, 1988;Telford, Geldart, & Sheriff, 1990b;Cardimona, 2002;Idornigie, Olorunfemi, & Omitogun, 2006). expects to much contrast and detectable (Meju, 2002;Airo, 2015), because of their resistivity variations: Shale and clays present low values followed by sandstones with intermediate values, while coal and limestone beds have high values (Lau, 2000). Generally, sulfide in sedimentary hosts indicates low resistivity values with a strong correlation between the low resistivity and high chargeability areas for most of sedimentary host (Côrtes et al., 2016;Evrard et al., 2018). Massive sulfide surveyed around the submarine area usually shows low resistivity values lower than the seawater value (Goto et al., 2013;Komori et al., 2017;Ishizu et al., 2019). But, the sulfide hosted by quartz and carbonates leads to act as good electrical insulators with higher resistivity values (Katsube et al., 2003;Moreira et al., 2014;Côrtes et al., 2016;Han et al., 2016;Ishizu et al., 2019).
Seafloor massive sulfide (SMS) deposits are considered a high potential source of economic minerals s such as zinc, copper, tin, gold, and silver (Emsbo, 2007;Haroon et al., 2018). The electrical resistivity methods have revealed that SMS deposits exhibit a strong IP effect and lower resistivity than the surrounding host rock (Iijima & Sayanagi, 2013;Hördt et al., 2016;Ishizu et al., 2019 (Lau, 2000;Airo, 2015). In the Iberian Pyrite belt, in Bathurst districts in Canada and Kuroko province of Japan, sulfide observed as volcano-sedimentary stratigraphic layers for being closely associated with volcanic lavas and pyroclastic rocks. They have formed a different relative participation of sedimentary/bacterio-genic and hydrothermal processes (Boulter, 1996;Barrie & Hannington, 1999;Sáez et al., 1999;Lentz & McCutcheon, 2006;Lentz, Thorne, & Beal, 2009;Walker, 2010;Piercey, Peter, & Herrington, 2015;Almod et al., 2019). VMS deposits (Sulfide in mafic or felsic hosted or occur as intercalation of basalts) are characterized by high electrical conductivity, high chargeability bordering zones, and sediments (such as graphitic shales) are more conductive than the associated volcanic host (Tavakoli et al., 2016b). However, the geochemical processes of weathering such as humidity, porosity, and hydrothermal alteration form clay can affect the target's resistivity. Also, major faults, fracture zones, and related structural control features in VMS prospects may be saturated with saline fluids, making them electrically conductive (Meju, 2002;Moreira et al., 2014;Tavakoli et al., 2016b). Though, when mineralized, they will be recognized by their elevated chargeability (Hawke & Brooker, 2001).

Electrical Resistivity and Chargeability of Sulfide Ores
Common sulfides except sphalerite, have high electrical conductivities (Wells, 1914;Ross, 1957;Meju, 2002;Pearce, Pattrick, & Vaughan, 2006;Aradis, Annigan, & Ewing, 2007;Airo, 2015) Resistivity and IP of Sulfide deposits depend very strongly on their quantity and the degree of connection between the various mineral grains or veins in the relevant host rock. When sulfide grains are isolated, they are not good electrical conductors because conductivity increases with the concentration of metallic particles (Revil, Florsch, & Mao, 2015). The ore bodies constituting more than 80% sulfide are considered as having "massive" texture; it is clear they are good electrical conductors (Langore et al., 1989).
20% up to 80% of the total volume is considered as "veinlet" texture. Due to electrical connections between the veinlets, such sulfide are also good electrical conductors (Langore et al., 1989). Sulfide content in the disseminated ore is 10% -20% (Langore et al., 1989), because of the disseminated texture, sulfide is not good electrical conductors. It is also noted that the chargeability increase depending on the volumetric content of metallic particles (Scott & West, 1969;Revil, Florsch, & Mao, 2015).
The Pyrite and galena are frequently available as reasonably large single crystals. Simultaneously, chalcopyrite, pyrrhotite, and arsenopyrite occur in a compact, microcrystalline forms (Parasnis, 1956) massive sulfide vary in thickness from 2 to 10 cm, massive chalcopyrite layers vary in thickness from 2 to 5 mm (Tivey et al., 1995). Pyrrhotite laths are 20 to 30 μm long, and 3 μm wide and occur in interstices and intergrown with outer edges of chalcopyrite grain. Forty to 100 μm clusters of sphalerite are intergrown with minor amounts of Pyrite (<10 μm) (Tivey et al., 1995).
Pyrrhotite is highly conducive and can significantly influence a sulfide body's general electrical conductivity signature (Roach & Fitzpatrick, 2003). Pyrite and chalcopyrite also give potentials higher polarized (too conductive) state than galena (Wells, 1914;Roach & Fitzpatrick, 2003), but sphalerite is generally insultator with less conductivity and lower IP signatures than other sulfide minerals (Komori et al., 2017;Evrard et al., 2018). It can be conducive when associated with other conducive/chargeable sulfide minerals such as pyrite and pyrrhotite, even in small quantities (Roach & Fitzpatrick, 2003;Evrard et al., 2018). Even though (Hawke & Brooker, 2001)'s research showed that sphalerite is the primary mineral source of the induced polarisation effect observed in their investigated samples (Hawke & Brooker, 2001). It is also noted that Cu-bearing ores are likely to be more conductive than sphalerite-rich Zn ores (Airo, 2015).
Chalcopyrite is highly conductive but usually constitutes only a small part of the total sulfide volume in many deposits. Table 4 clearly describes the conductivities and chargeability of major sulfide minerals.
As mentioned in Table 4, the geophysical responses of massive sulfide usually dominated by low resistivity and high chargeability. It is not easy to make any conclusive deduction about exact resistivity and IP values for each type of sulfide.   (Parasnis, 1956;Telford, Geldart, & Sheriff, 1990b;Pearce, Pattrick, & Vaughan, 2006;Airo, 2015;Evrard et al., 2018).
Different geological settings of the areas, make identical minerals to have different geophysical signatures or different minerals to produce the identical geophysical signature (Langore, Alikaj, & Gjovreku, 1989;Moreira et al., 2014;Côrtes et al., 2016). Some of the reviewed papers did not found differences between various sulfides, i.e., pyrite and chalcopyrite, in the spectral IP parameters with laboratory analysis (Langore, Alikaj, & Gjovreku, 1989). Also, mineralization may be of a chargeable nature because of the presence of other minerals which strongly associated with them like pyrite/marcasite (Côrtes et al., 2016;Evrard et al., 2018). Thus, minerals with high metallic conductivity even for disseminated sulfide ores, can have strong IP effects (Wang & Strangwayt, 1981).
Though, the presence of graphite-bearing limestone, IP surveys is less useful for mapping sulfide mineralization (Guo, Dentith, & Zhao, 2000). But, graphite rarely occurs in massive sulfide. Table 5 presents examples of resistivity and IP methods in the exploration of sulfide deposits. It describes the types of surveys used, types of minerals and types of host rocks encountered, and shows their resistivity and IP responses.

Factors Affecting Exploration of Sulfides
Here are some factors that affect exploration of sulfide:  The exploration of sulfides is potential sources of pollution, of the air and of surface waters and soils (Vaughan & Corkhill, 2017).  The 1-D survey does not provide significant lateral changes, which may bring to gives inaccurate results or misleading results.  When using 2D, the geological structures do not change in the direction perpendicular to the survey line, creating errors in interpretation.  To delete negative IP data may cause the loss of essential information (Dahlin & Loke, 2015).  Using large electrode spacing can mask data of thin layers (Mammo, 2013;Tavakoli et al., 2016b).   Interference from grounded structures such as pipelines, power lines, and fences can distort the signals (Sternberg, 2002).  Sometimes pyrite in fine-grained sediments such as mudstones and shales may produce false anomalies (Sternberg, 2002).  In the interpretation of resistivity data, the author should emphasize the fact that there are other causes of low resistivity other than sulfides and explain how to constrain the interpretation to be unique to sulfides.

Conclusion
It is much known about resistivity and IP surveys in mineral exploration. These methods are simple, fast, cheap, and effective for sulfide mineral exploration.
The reviewed papers showed that these methods have been successfully used in sulfide exploration whereby many results showed that low resistivity correlates with high chargeability and vice versa. This review is the wealth of resistivity and IP properties exhibited by sulfide minerals and their host. This review covered different distinct techniques of investigation to make the reader understand the procedures and appropriate methods that can be applied in mining exploration by showing the success and problems affecting exploration. This research could be useful as a guide for the future geophysical studies such as geophysics, mineralogy or in mineral exploration. However, many authors did not give the values C. Dusabemariya et al. Journal of Geoscience and Environment Protection attributed to each type of minerals but the range value. Some failed to include the area covered during their survey. It could be suggested therefore to present the area covered during the geophysical survey or give the number of traverse and line with their space interval from which interested readers can get the idea of the area covered for future research.