Two-dimensional (2D) geophysical survey was done to produce 2–dimensional image of the subsurface in other to define the depth, length and orientation/nature of mineralization within the study. Since all geological structures can be view in 2-D in nature, a fully 2-D survey using a 2-D interpretation model should give the most accurate result imaging of the subsurface (Loke and Barker, 1996). Electrical geophysical method is used
in the survey hence two parameters are measured in other to delineate the veins by differentiate it from the surrounding rock unit. The measured parameters include Chargeability and apparent resistivity. These two parameters are obtained by the use of induced polarization (IP) method and resistivity method respectively.
Induced polarization method
Induced polarization (IP) is a current-stimulated electrical phenomenon observed as a delayed voltage response in earth materials. It has been used extensively in the search for disseminated mineralization in base-metal. The induced polarization (IP) principles is based on the fact that when current injected via electrodes into the subsurface is abruptly interrupted, the voltage across the potential electrodes does not drop to “zero”immediately but decays slowly after an initial large decrease of its steady-state value (Froehlich and Urish, 2002). This decay time could be in seconds or even minutes. Conversely, if the current is switched on again, the potential increases suddenly at first and then gradually reaches the steady- state value. The slow decay and growth of part of the signal are due “Induced Polarization”. Sources of Induced Polarization Effect related to these study is called the Electrode Polarization or overvoltage.
Electrode Polarization occurs when metallic minerals (in these case mineralized vein) are present. The current flow is partly electronic (through groundwater), partly electrolytic (through conductive mineral). The metallic grains conduct charge by electronic conduction, why electrolytic conduction take places around them. However, the flow of electrons through the metal is much faster than the flow of ions in the electrolyte, so the opposite charges accumulate on facing surfaces of a metallic grain that blocks the path of ionic flow through the pore fluid. An overvoltage builds up for some time after the external current is switched on. The magnitude of the effect is related to the metallic concentration. After the current is switched off, the accumulated ions diffuse back to their original positions and the overvoltage decays slowly
Resistivity method
The resistivity method is used in the study of horizontal and vertical discontinuities in the electrical properties of the ground, and also in the detection of two-dimensional bodies of anomalous electrical conductivity. In the resistivity method, artificially-generated electric currents are introduced into the ground and the resulting potential differences are measured at the surface (Kearey and Brooks, 1991). Deviations from the pattern of potential differences expected from homogeneous ground provide information on the form and electrical properties of subsurface inhomogeneities.
The parameters used in defining resistivity
Vertical electrical sounding
In vertical electrical sounding (VES) the goal is to observe the variation of a chosen parameter with depth but when a series of VES is stacked together in the systematic sequence it can be used to study both horizontal and vertical variation of a need parameter or property. The technique is best adapted to determining depth and a need electrical
property of a rock, such as sedimentary beds, the depth to the water table or mineralized veins (Kearey and Brooks, 1991). During the survey the mid-point of the array is kept fixed while the distance between the current electrodes is progressively increased. This causes the current lines to penetrate to ever greater depths, depending on the vertical distribution of conductivity.
Vertical electrical sounding (VES) was performed using the Schlumberger arrangement while Wenner array was used in the subsurface imaging acquisition. The ABEM terrameter SAS 1000 was used for the data acquisition. Also used during the survey were four reels of cables (two for the potential electrodes and two for the current electrodes), four electrodes, global positioning system (GPS), a direct current (DC) source (power source), a data sheet and measuring tape. Vertical Electrical Sounding was carried along a profile format in the suspected mineralized area with each center point mark as station point; space between spread points within the same transverse was 5m.
The data from each station point was then stacked together using kriging interpolation. Two (2) subsurface imaging were produced on average along each profile (chargeability and resistivity imaging). A total of fifteen (15) subsurface imaging were carried out with sixteen (16) VES. The acquired data was subjected to kriging interpolation in other to produce a 2D image of the subsurface.
Data processing
Surfer®, Microsoft Excel, Res2Dinversion were used to process the subsurface imaging and VES data acquired at the field. The Surfer data was processed using the kriging interpolation to stack the data at their respective depth, Res2Dinversion was also used to plot the pseudosection of the both the resistivity and chargeability data.
2D imaging of the subsurface was carried out at different locations in the study area to determine the depth to mineralized veins. The first survey was carried out on transverse one/Vein one (1), second on transverse two in that order; the results of the surveys are carefully discussed below.
Vein trends and pattern of survey transverses
Fig.10: Subsurface imaging of vein 1-1, Chargeability plot
Fig.11: Subsurface imaging of vein 1-1, Resistivity and geologic model plot
Fig.12: Vertical chargeability drill (one-dimensional sounding) across vein 1-1
Fig.13: Subsurface imaging of vein 2-1, Chargeability plot
Fig.14: Subsurface imaging of vein 2-1, Resistivity and geological model plot
Fig.15: Vertical chargeability drill (one-dimensional sounding) across vein 2-1
Fig.16: Subsurface imaging of vein 2-2, chargeability plot
Fig.17: Subsurface imaging of vein 2-2, resistivity and geologic model plot
Fig.18: Vertical chargeability drill (one-dimensional sounding) across vein 2-2
Fig.19: Subsurface imaging of vein 3-1, chargeability plot
Fig.20: Subsurface imaging of vein 3-1, Resistivity and geological model plot
Fig.21: Subsurface imaging of vein 3-2, Chargeability plot
Fig.22: Subsurface imaging of vein 3-2, resistivity and geological model plot
Fig.23: Vertical chargeability drill (one-dimensional sounding) across vein 3-2
Fig.24: Subsurface imaging of vein 4-1, Chargeability plot
Fig.25: Subsurface imaging of vein 4-1, Resistivity and geological model plot
Fig.26: Vertical chargeability drill (one-dimensional sounding) across vein 4-1
Fig.27: Surface imaging of vein 4-2, Chargeability plot
Fig.28: Subsurface imaging of vein 4-2, Resistivity and modeled plot of vein 4-2
Fig.29: Vertical chargeability drill (one-dimensional sounding) across vein 4-2
Fig.30: Subsurface imaging of vein 5-2, Chargeability Plot
Fig.31: Subsurface imaging of vein 5-2, Resistivity and modeled Plot
Fig.32: Vertical chargeability drill (one-dimensional sounding) across vein 5-1
Fig.33: Vertical chargeability drill (one-dimensional sounding) across vein 5-2
VEIN 1-1 RESULT AND DISCUSSION
One vertical chargeability drill and one subsurface imaging using both electrical induced polarization and electrical resistivity approach were carried out across this identified vein. The chargeability subsurface imaging Fig (10), indicate a high conductive layers with poor differentiation index, the chargeability values range from -31.5 to 10msec. The electrical resistivity subsurface imaging and the geo-electric model of vein 1-1 Fig (11), identified three geo-electric layers inferred as high resistive lateritic zone, baked/un-fractured shale and fractured/mineralized zone. The electrical resistivity values range from 7.96 to 126ohm.m with the low resistivity values indicating conductive/mineralized zone, while as the high resistive zones indicate lateritic and baked shales. The one-dimensional chargeability drill Fig (12) across this point indicates a high chargeability value of 40.23msec at about 30m depth and this provides an approximate depth to the mineralized zone. However, this result is subject to final confirmation using non-porous pot electrode by induced polarization approach on a wet ground.
VEIN 2-1 RESULTS AND DISCUSSION
Vertical chargeability drill and subsurface imaging using both electrical induced polarization and electrical resistivity approach were carried out across this vein. The chargeability subsurface imaging Fig (13), indicates two high conductive points with poor differentiation index, the chargeability values range from -227 to 25.5msec. The electrical resistivity subsurface imaging and the geo-electric model of vein 2-1 Fig (14), identified two geo- electric layers; high resistive lateritic zone and fractured/mineralized zone. The electrical resistivity values range from 3.62 to 240 ohm.m with the low resistivity values indicating conductive/mineralized zones, while as the high resistive zones indicate lateritic/baked shales. The one-dimensional chargeability drill Fig (15) across this point indicates a high chargeability value of 157msec at about 55m depth and this provides an approximate depth to the mineralized zone, this depth is uneconomical base on the prevailing economic
factors on lead/zinc ore mining. However, this result is subject to final confirmation using non-porous pot electrode by induced polarization approach on a wet ground.
VEIN 2-2 RESULTS AND DISCUSSION
The subsurface chargeability imaging of this transverse Fig (16) has a chargeability range of -4.04 to 45.6msec with two point of chargeability high within the transverse line. The resistivity imaging Fig (17) has resistivity values range from 3.32 to 160ohm.m, this also indicates two major conductive zones within the profile line. The geo-electric model indicates three layers inferred as high resistive lateritic top, baked/un-fractured shale and fractured/clayey/mineralized zone. The one-dimensional vertical chargeability drill Fig (18) indicates high chargeability value of 82.61msec at about 25m depth and this provides an approximate depth to the mineralized zone. However, this result is subject to final confirmation using non-porous pot electrode by induced polarization approach on a wet ground.
VEIN 3-1 RESULTS AND DISCUSSION
The chargeability subsurface imaging Fig (19) of this transverse shows unitary display of data in three (3) iso-chargeability zones of non-differentiated index with chargeability range of -1.23 to 2.0msec. However, the subsurface resistivity imaging Fig (20) provides a clear plot of the subsurface image of this location with resistivity values range between 7.51 to 618ohm.m. The geo-electric model Fig (20) indicates three (3) layers which are inferred as high resistive lateritic top, baked/un-fractured shale and fractured/clayey/mineralized zone. The one-dimensional vertical chargeability plot of this location shows masked data. However, re-drill on wet ground using non-porous electrode is recommended.
VEIN 3-2 RESULTS AND DISCUSSION
Vertical chargeability drill and subsurface imaging using both electrical induced polarization
and electrical resistivity approach were carried out across this identified vein. The chargeability subsurface imaging Fig (21), indicates a high conductive layers with poor differentiation index, the chargeability values range from 21.6 to 106msec. The electrical resistivity subsurface imaging and the geo-electric model of vein 3-2 Fig (22), identified three geo-electric layers inferred as high resistive lateritic zone, baked/un-fractured shale and fractured/mineralized zone. The electrical resistivity values range from 2.67 to 364ohm.m with the low resistivity values indicating conductive/mineralized zone, while as the high resistive zones indicate lateritic and baked shales. The one-dimensional chargeability drill Fig (23) across this point indicates a high chargeability value of 60.47msec at about 28m depth and this provides an approximate depth to the mineralized zone. However, this result is subject to final confirmation using non-porous pot electrode by induced polarization approach on a wet ground.
VEIN 4-1 RESULTS AND DISCUSSION
Vertical chargeability drill and subsurface imaging using both electrical induced polarization and electrical resistivity approach were carried out across this vein. The chargeability subsurface imaging Fig (24), the chargeability values range from -19.6 to 184msec, the electrical resistivity subsurface imaging and the geo-electric model of vein 4-1 Fig (25), identified three geo-electric layers; high resistive lateritic zone, baked/un-fractured shale and fractured/mineralized zone. The electrical resistivity values range from 3.84 to 290 ohm.m with the low resistivity values indicating conductive/mineralized zones, while as the high resistive zones indicate lateritic/baked shales. The one-dimensional chargeability drill Fig (26) across this point indicates a high chargeability value of 120msec at about 25m depth and this provides an approximate depth to the mineralized zone. However, this result is subject to final confirmation using non-porous pot electrode by induced polarization approach on a wet ground.
VEIN 4-2 RESULTS AND DISCUSSION
Vertical chargeability drill and subsurface imaging using both electrical induced polarization and electrical resistivity approach were carried out across this vein. The chargeability values Fig (27) range from 1.28 to 42.05msec, with two points of high chargeability along the survey profile line (at 40meters and 60meters). The electrical resistivity subsurface imaging and the geo-electric model of vein 4-2 Fig (28), identified three geo-electric layers; high resistive lateritic zone, baked/un-fractured shale and fractured/mineralized zone. The electrical resistivity values range from 13.9 to 332 ohm.m with the low resistivity values indicating conductive/mineralized zones, while as the high resistive zones indicate lateritic/baked shales. The one-dimensional chargeability drill Fig (29) across this point indicates a high chargeability value of 280msec at about 40m depth and this provides an approximate depth to the mineralized zone. However, this result is subject to final confirmation using non-porous pot electrode by induced polarization approach on a wet ground.
VEIN 5-2 RESULTS AND DISCUSSION
The subsurface chargeability imaging of this transverse has a chargeability range of 0.57 to 20.4msec Fig (30) with one point of chargeability high within the transverse line (at 40meters). The resistivity imaging has resistivity values range from 7.08 to 134ohm.m Fig (31). The geo-electric model indicates three layers inferred as high resistive lateritic top, baked/un-fractured shale and fractured/clayey/mineralized zone. The one-dimensional vertical chargeability drill Fig (33) indicates high chargeability value of 110msec at about 37m depth and 260msec at about 47m this provides an approximate depth to the mineralized zone. However, this result is subject to final confirmation using non-porous pot electrode by induced polarization approach on a wet ground.
VERTICAL ELECTRICAL DRILL OF VEIN 5-1
The one dimensional drill of vein 5-1 shows a high chargeability value of 120msec at about
27m Fig (32), further drill is recommended for this transverse for confirmation using non- porous pot electrode by induced polarization approach on a wet ground.
We considered this survey to be very consistent with the general expected lead-zinc anomaly within the study area. The dissemination of lead/zinc deposit in the area is judge on average to occur in a minable quantity. Therefore, a propose target testing via drilling or pitting/trenching is recommended to be carried out as most of the lineaments from the area have been confirmed to be deep seated to near subsurface within the mineralized veins. Based on the observed chargeability and resistivity anomalies, most of the prospective bodies occur within 30m depth on average; therefore, target drilling should be down to 30m depth.
Vein 1-1 shows mineralization viability at a depth of 30meters on both the subsurface imaging and vertical electrical drill, Fig (11) and Fig (12) respectively, this is however, recommended for confirmation using induced polarization of non-porous pot electrodes on a wet ground.
Vein 2-1 indicates a mineralization potential at a depth between 22meters to 28meters on the pseudosection plot Fig ( 14), the one-dimensional electrical drill Fig (15) indicates a depth of 58meters, this point is recommended for re-evaluation and final confirmation using non-porous pot electrodes on a wet ground.
Vein 2-2 shows a mineralization depth at about 25meters Fig (17) also the one-dimensional
drill Fig (18) confirm the mineralization depth of 25meters with a close anomaly at about 40meters.
Vein 3-1 this transverse point Fig (20) shows two points of inferred mineralization at a depth range of 25meters to 30meters, reconfirmation is also recommended before drill.
Vein 3-2 indicates a great mineralization potential at a depth of about 28meters to 40meters Fig (22), however re-evaluation is also recommended on this profile point.
Vein 4-1 shows high mineralization potential at the depth of about 25meters with good clarity at about 40meters distance along the profile line Fig (25).
Vein 4-2 indicates zones of mineralization at a greater depth of about 40meters Fig (27) although evidence/signs of mineralization are absent at about 30meters with two clear distinctive points.
Vein 5-2 shows high mineralization viability at about 25meters on the subsurface imaging plot, the vertical electrical drill Fig (33) shows a deeper mineralized vein from about 35meters to 50meters. Re-evaluation and confirmation is necessary on this point using non
-porous electrodes on a wet ground. The vertical electrical drill of vein 5-1 Fig (32) indicates high mineralization potential with mineralization of 25meters, this also need confirmation on a wet ground
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