Geophysical Study Of Induced Polarization And Electric Resistivity In Location Of Possible Sulfated Zones For Gold Mineralization

Last update time : 08/09/2019

Author : zhang jie

1. Introduction

On May 28, 29 and 30, 2019, at the request of Mr. Eli Kervahal, Geoprospecto – Solutions in Geophysics, carried out a geophysical study campaign through the geoelectric characterization of the subsurface geological materials, through induced polarization and electro resitivity, which had as objective to locate possible gold mineralization, that occur associated with sulfate zones (primary gold), for the confirmation of the geophysical anomalies.

In the electro resistivity we find possible regions with low resistivity values (less 300 Ohm.m) that may indicate the presence of fractures and faults. While induced polarization, identify high load ability values to pinpoint possible sulfide zones. In this context, due to the history of geophysical studies in Brazil, using induced polarization, the incidence of gold mineralized zones associated with quartz veins, the load anomalies indicate values above 50 mV / V or milliseconds.

2.  Study Area

The study area is located around 35 km from the district of Casa de Tábuas, in the city of Santa Maria das Barreiras, Garimpo Forquilha, State of Pará, with the following coordinates 571509 LESTE and 9039248 North / Datum WGS84. The access to the Fork Fork takes the BR-158 from the Casa de Tábuas District, covering about 20 km of paved road and then following 15 km by unpaved road.

3. Theoretical Introdction

Geophysical prospecting is a branch of applied physics that studies the location and delimitation of subsurface structures due to the contrast of some of its physical properties with respect to the surrounding environment through observations made on the surface of the earth. Among the various methodologies in geophysics, it is not possible to establish relations of superiority, since the effectiveness of these methodologies depends on the area to be applied the method on the proposed problem.

3.1  Eletro Resistivity Method

The method of electro resistance is based on the introduction of electric current in the ground, from an arrangement called an electrode arry. In general, this device consists of four electrodes, according to Figure 1.


Figure 1 – Four-electrode arrangement.l

Electrodes A and B are referred to as current electrodes, by means of which a subsurface electric current is applied. This circuit is connected to an ammeter, by means of electric cables.

The electrodes M and N are conventionalized as potential electrodes, used to measure the potential difference created by the current circuit, whose measurement is carried out by a voltmeter connected between them.

In the case of resistivity measurements in homogeneous media, the obtained value will represent the true resistivity of the medium. However, the geological environment consists of intrinsic heterogeneities and therefore, the resistivity measures represent a “weighted average” of true resistivity, which is calculated product is called apparent resistivity (ρa). The distance between the electrodes, the current intensity (I) supplied by the emitting circuit and the potential difference (ΔV) obtained in the receiver circuit is known, it is possible to calculate the apparent resistivity (ρa), according to the following equation:

Where K is a factor that depends exclusively on the geometric arrangement of the electrodes, which can be obtained as follows:

As for the units used, the apparent resistivity is calculated in ohms. m (ohm meter), the potential difference is measured in mV (millivolts), the current intensity is measured in mA (mA), and the coefficient K is measured in meters. The resistivity parameter depends on the nature and physical state of the analyzed material. The resistivity and the electrical conductivity are related to the propagation mechanisms of the electric current of the materials.

The electrical resistivity in soil or rock is conditioned to the processes of electronic or electrolytic conductivity. The first occurs due to the presence of metallic or conductive minerals. And the second, due to the displacement of dissolved ions in the water contained in pores or fissures.

The electrolytic conductivity process is predominant because groundwater often contains dissolved ions. In contrast, the presence of metallic minerals in a quantity sufficient to raise the conductivity of the medium is rare.

a.METHOD OF INDUCED POLARIZATION

The induced polarization method is based on the measurement of voltage variations as a function of time or frequency, from a current interruption, observing the decay of the load. Rocks act as capacitors, where metal minerals, clays and electrolytes store loads. Thus, it is possible to discriminate the rocks and minerals present in the subsoil (Kearey et al., 2002).

With current injection, negative and positive ions cluster on opposite sides of the material. When the injected current is interrupted, they return to their original positions after a finite period of time, causing a gradual decay of the voltage. This phenomenon is known as membrane polarization, common in the presence of clays (Kearey et al., 2002).

In the case of sulfides, the positive and negative charges are repulsed due to current injection. Negative and positive ions accumulate on either side of the grain, trying to release electrons to the grain or to accept electrons conducted through the grain. Consequently, accumulation of ions occurs (Kearey et al., 2002).

When current is interrupted, the ions slowly disperse back to their source sites and cause a transient voltage decay. Induced polarization (IP) measurements in the time domain involve measuring the voltage decrease after being switched off (Kearey et al., 2002).

The parameter measured in the induced polarization is the chargeability (M), defined as the area (A) below the decay curve, in a time interval Delta t, normalized by Delta Vc continuous, since the value of M depends on the initial V, (Kearey et al., 2002), according to the formula below:


b.DATA ACQUISITION TECHNIQUE

In the method of electro resistance there are several techniques of field surveys, basically divided into electrical probing and electric walking, within which there is a great variety of possible configurations of electrodes that gives the method great versatility.

The devices for resistivity measurements consist of a four-electrode system, two of which are used to send an electric current (I) to the ground (electrodes A and B), and the other two (electrodes M and N) used to measure the difference potential between them.

The Electric  technique is based on the realization of measures of apparent resistivity along a line, with the objective of investigating variations in one or more levels in depth.

Among the several types of field arrangements available for the electric  array technique, the present work uses the dipole-dipole arrangement, due to the arrangement of the electrodes in symmetrical arrangement, operational ease during acquisition, low signal-to-noise ratio, and a lateral resolution appropriate to the objectives of the work.

Figure 2 – Dipole-Dipole arrangement – CE

The dipole-dipole arrangement is characterized by using equal spacing between MN and AB, with displacement of the center of both dipoles along the line. The spacing between the dipoles AB and MN can be varied from the simultaneous use of several MN dipoles along the line.

The depth of investigation increases with the spacing between the centers of AB and MN, which in theory corresponds to R / 2 or to the crossing of two 45-degree angles, that is, a line from the center of AB and another which leaves from the center of MN. In this way, the readings are performed from the pair of potential electrodes M1N1 corresponding to the theoretical depth n1.

The geometric factor K is calculated by the formula:

4.Works Executed

Six electrical resistivity survey lines were performed with the Dipole-Dipole array (F). Each survey line has electrode spread of 230 m length with electro spacing of 5 m and 10 m. 19 levels of investigation are performed. To conduct the geo-referencing of the CE stations, Garmin Etrex 10 GPS was used. The geographic coordinates were located at Universal Transversal of Mercator – UTM, WGS 84 / Zonal 22 South. The survey lines were setup in place with the help of field assistants. The geophysics survey aimed at profiling the subsurface and to facilitate the positioning of the potential locations with promising subsurface characteristics for future investigations.

For the geophysical survey of IP / Resistivity, model GD-10 from the GD SERIES D.C. GEO-ELECTRICAL SYSTEM was used in the electrical resistivity profiling. GD-10 is manufactured by the Geomative Inc. (Figure 4). Twenty-four stainless steel electrodes with 12.5 mm diameter and 50 cm length, and 24 non-polarizable lead sulphate electrodes were used (Figure 5). Multi-core electrode cabling was used to connect the electrodes to the measurement equipment, in order to perform automated measurements.

IP acquisition is performed in time domain, with paste-paste windows and current injection intervals (5A) of 8s and 16s. This is achieved through the constant voltage injection of 1000 V from an external source (7 kVA generator).

Figure 3 – Map of the location of the 6 IP Lines / Resistivity in Forklift (Source: Google Earth, 2019).

Figure 4 – Equipment GD SERIES D.C. GEO-ELECTRICAL SYSTEM, model GD-10, manufactured by Geomative

Figure 5 – Stainless steel electrode and non-polarizable lead sulphate electrode.e

5.  Deta Survey

5.1 Electric Walk (EC)

For the electrical resistivity measurements, the Dipole-Dipole array was used. The great advantage of this array type is that the high lateral sensitivity to the electrical properties in the subsurface with several levels of depth. In this type of array, the spacing between AB and between MN is equal, the centers of the dipoles move along the line of the survey so that several depth levels are investigated.

In this field work, our team fixed the AB electrodes and moves the MN electrodes along the entire profile.Upon the completion of acquisition, the AB electrode pair is moved to the next location and the MN electrodes are moved along the entire profile again, until AB has traversed its last location. For example: we first put AB electrodes at locations 1 and 2 respectively, while the MN electrodes are placed at location 3 and 4 respectively. The measurement is conducted at the line intersection that passes with a 45 ° angle to the centers of the dipoles used.  MN is then moved to stations 4 and 5 respectively. This is repeated with the same manner until the complete profiling is measured.

6.  Inversion Of The EC Date

Since the electrical geophysics exploration is executed through 2-D imaging, its measurement data was interpreted through inversion method. The inversion methods essentially attempt to find a resistivity model with responses that are in accordance with the measured data. For the least squares method used in the RES2DINV software, the parameters to be fitted are the resistivity values ​​of the model blocks, while the measurement data are the measured apparent resistivity values. It is well known that for a given set of data there is a wide variety of models which may fit well according to the measured data to a certain extend and with some ambiguities.  In addition to the attempt of minimizing the differences between the calculated and the measured values, inversion methods also seek to reduce other parameters that produce certain desired characteristics in the resulting model. The RES2DINV program uses smoothness-constrain method which, starting from an initial model, tries to find an improved model whose apparent resistivity values ​​are closer to the measured values. The analysis software used was the version 3.4, 2D Resistivity and IP Inversion, Copyright (1995-2002) Geotomo Software, Malaysia, and is operated within the Microsoft Windows operating system.

7. Interpretation Of Results

7.1 Line 01

In Line 01 (230 m in length and 54 m in depth) , two IP anomalies detected were considered strong, due to the fact that they have a value around 270 milliseconds. For this reason, it is recommend to confirm these anomalies through borehole drill between pickets 9 and 14 (depths between 17 and 54 meters) and between pickets 17 and 18 (depths between 9 and 27 meters),. They are named Zone 1 and Zone 2 respectively, with a slope of 60º, to confirm the possible sulphide zone. Due to the high chargeability value, there is a great possibility of success in the detection of gold accumulation (Figure 6).

Figure 6 – Line 01.

7.2 Line 02

In Line 02 (115 m in length and 37.7 m in depth), two IP anomalies detected were considered strong, due to the fact that they have a value around 88 milliseconds. For this reason, it is recommend to confirm these anomalies through borehole drill between pickets 10 and 12 (depths between 19 and 38 meters) and between pickets 14 and 15 (depths between 6 and 38 meters),. They are named Zone 1 and Zone 2 respectively, with a slope of 60º, to confirm the possible sulphide zone. Due to the high chargeability value, there is a great possibility of success in the detection of gold accumulation (Figure 7).

Figure 7 – Line 02.

7.3 Line 03

In Line 03 (230m in length and 54 m in depth) , only one IP anomaly detected were considered strong, due to the fact that it has a value around 280 milliseconds. For this reason, it is recommend to confirm these anomalies through borehole drill between pickets 10 and 16 (depths between 19 and 54 meters), named Zone 1 respectively, with a slope of 60º, to confirm the possible sulphide zone. Due to the high chargeability value, there is a great possibility of success in the detection of gold accumulation (Figure 8).

Figure 8 – Line 03.o

7.4 Line 04

In Line 04 (230m in length and 54 m in depth) , only one IP anomaly detected were considered strong, due to the fact that it has a value around 358 milliseconds. For this reason, it is recommend to confirm these anomalies through borehole drill between pickets 9 and 14 (depths between 16 and 54 meters), named Zone 1 respectively, with a slope of 60º, to confirm the possible sulphide zone. Due to the high chargeability value, there is a great possibility of success in the detection of gold accumulation (Figure 9).

Figure 9 – Line 04.

7.5 Line 05

In Line 05 (230 m in length and 46.2 m in depth) , two IP anomalies detected were considered strong, due to the fact that they have a value around 576 milliseconds. For this reason, it is recommend to confirm these anomalies through borehole drill between pickets 8 and 10 (depths between 27 and 46 meters) and between pickets 12 and 16 (depths between 26 and 46 meters),. They are named Zone 1 and Zone 2 respectively, with a slope of 60º, to confirm the possible sulphide zone. Due to the high chargeability value, there is a great possibility of success in the detection of gold accumulation (Figure 10).

Figure 10 – Line 05.

7.6 Line 06

In Line 06 (230m in length and 46 m in depth) , only one IP anomaly detected were considered strong, due to the fact that it has a value around 182 milliseconds. For this reason, it is recommend to confirm these anomalies through borehole drill between pickets 8 and 14 (depths between 13 and 54 meters), named Zone 1 respectively, with a slope of 60º, to confirm the possible sulphide zone. Due to the high chargeability value, there is a great possibility of success in the detection of gold accumulation (Figure 11).


Figure 11 – Line 06.
Figure 12 – Map of location of anomalies. The lines in yellow refer to possible mineralized zones.

8.  Final Considerations

The results obtained with the 6 lines of induced polarization and resistivity allowed to identify possible sulphide zones that probably can accumulate gold in the area of study in the GarimpoForquilha. For this, the following premises must be observed:

  1. A total of 9 IP anomalies were identified, all of them considered strong due to the high chargeability value (greater than 80 milliseconds) and consequently a great possibility of success for gold accumulation;
  2. Lines 01, 05 and 06 have anomalies in the same direction (basically N-S), as well as related lines 03 and 04 (E-W);
  3. It is suggested to make more lines in the area in question, parallel to line 02 on the west side to verify the lateral tendency of the possible mineralized zone. As well as lines above line 06 (north) to below line 01 (south);
  4. It is worth mentioning that geophysical anomalies are obtained indirectly, which can lead to ambiguous interpretations.
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Last update time 08/09/2019

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