VTEM in Ghana for Gold Exploration

A Case Study of Four VTEM Surveys for
Gold Exploration in Ghana

GEOTECH’S

Versitile
Transient
Elecrtro-
Magnetic

System

VTEM over Bogoso
Sept, 2007

© Condor Consulting 2010

EM Processing Techniques
1 Voltages as profiles

Transient Decays

Magnetic data

3 Model from LEI

Layered-Earth (1D)
Inversion

2 Distance along flight line

1. Data collected as transient decay voltage.
2. Perform LEI on each decay.
3. Display LEIs as model cross-section.
4. Depth slices can be extracted for conductivity
or conductance.

2010

The Four Ghana Surveys

Bogoso
Golden Star Resources Ltd.
Flown in 2007
100 m line spacing

Obuasi
Pelangio Exploration Inc.
Flown in 2007 and 2009
100 m line spacing

Esaase
Keegan Resources Inc.
Flown in 2007
200 m line spacing

Asumura
Flown in 2007
200 m line spacing

The surveys lie within the West
African Craton
The major proterozoic units:
•Birimian: metasediments and
metavolcanics (greenstones)
•Tarkwaian: metasediments – usu. barren


1) Bogoso General EM Character
of the four areas 2) Obuasi

5 km
dB/dt Z Component
Channel 15 (1130 usec)
(a mid-time channel)

4 km

•All flown in northwest direction.
•All show large northeast structures that contain graphite.
•These are thrust faults that have been repeatedly reactivated.
•For each area the gold has a different relationship to the graphite.

4) Asumura
4 km
3) Esaase

2 km


Bogoso/Prestea - Structures
Conductivity from
The resistive Tarkwaian is to the
layered-earth inversion
east, Birimian to the west.
216 m depth slice
Gold occurs along the Main Crush
Zone – conductive.

The wide conductors along the
MCZ are favorable to the where
known gold is located.
Town of
Bogoso
Not all conductors have gold.

Next slide shows cross-section
through the Chujah Pit.

Chujah Pit

Prestea

Line through Chujah Main Pit

Chujah Pit Power line
EM channels

1st VD anomaly
Magnetic data (RTP) Cultural
Not a distinctive TMI anomaly

Ore Power line
LEI section

MCZ

Plan map


Vertical Gradient of Magnetic Data

Bogoso/Prestea area.
Known ore is near Birimian-
Tarkwaian contact

Tarkwaian has larger magnetic
anomalies due to volcanic
content

No apparent magnetic anomaly
with known ore

Magnetic highs/lows are not
good indicators of known ore

Geology from Allibone 2002.


Tilt Angle of Magnetic Data

Can use magnetic data to find
non-conductive structures.

East-West magnetic disruptions
are visible may be relevant to
gold placement.

These are due to D4 and D5
structures that cross the large
D2 faults.

Town of
Bogosoof Bogoso
Town

Prestea


EM dB/dt channel 15
Zoomed to
Bogoso/Prestea area.

There is a strong correlation of
known deposits with conductivity.

Magnetic disruptions appear
related to some conductivity
highs and known gold
occurrences.
Town of Bogoso

Cultural
Contamination

Main Chujah Pit


Conductance for 50 m to 200 m Depth Interval

5 km
55 Target Zones of various rankings are identified
using a variety of known criteria such as: Target Zone 19
Conductivity,
Ground geophysics,
Mapped lithology,
Mapped and interpreted faults, Target Zone 40
Flexures,
Soil geochemistry,
Drilling.

Most targets are along large conductors as seen
with the known gold.

Target Zones 19 & 40 are ranked high priority and
shown on next slide.

Town of Bogoso


Both of these Target Zones are further north Line 9330 B-19 Magnetic
in the survey block, away from the known high is
due to a
gold locations. dyke.
Discrete feature picks are marked by vertical
lines.

B-19:
Very wide conductor on the MCZ.
Right-hand flexure in the fault here.
A north-northwest dyke run through this target.

B-40
Sharp conductor with high time constant on the
MCZ.
May have some right-hand flexure in the fault. Target
Zone B-19
Line 6870

5 km

Target
B-40 Zone B-40


Primary Targeting Criteria

Bogoso/Prestea

1. Large conductors in 50 to 200 m DI.
2. Conductors identified through high
ranking discrete feature picks.
3. Cross cutting structures

58 Targets Identified.


Four Ghana Surveys

Bogoso
Golden Star Resources Ltd.
Flown in 2007
100 m line spacing

Obuasi
Pelangio Exploration Inc.
Flown in 2007 and 2009
100 m line spacing

Esaase
Flown in 2007
200 m line spacing

Asumura
Flown in 2007
200 m line spacing


Obuasi – Correlation of Known Gold with Conductivity
•Good correlation of gold and conductivity, particularly south in Obuasi Mine area.
•However, plenty of gold showings without significant conductivity.
•Not all lie on a major fault.

Conductance from Layered Earth Inversion
50 m to 200 m Depth Interval

Granodiorite

Gold
Mineralization

VTEM time channels
P.L.
Mines

LEI conductivity

1 km Cross-section
Obuasi Color pallet is linear, 0 to 5000 millisiemens


Obuasi Cross-Cutting Structures
Northeast to east-west structures visible in magnetic data.
Some correlation of gold to magnetic highs.

Magnetic Tilt Angle
Granodiorite

Gold
Mineralization

Mines

Obuasi


Tikal Section
•Two holes drilled on flank of EM conductor.
•Encountered Sulphidation and Silicification.
•Clearly some mineralization is resistive.

N S

300 m Silicification

100 m


Obuasi Target Zones
Line 1720
High conductivity and large structures not as
important as at Bogoso.
Most target zones are conductors, such as TZ-12.
TZ-12
Some target zones are resistors.

50 m to 200 m conductance
Color pallet is linear, 0 to 5000 millisiemens

Northern end of Obuasi Mine



Bogoso/Prestea Obuasi

1. Large conductors in 50 to 200 m DI. 1. Conductors in 50 to 200 m DI
2. Conductors identified through high 2. Cross cutting structures
ranking discrete feature picks. 3. Resistive areas
3. Cross cutting structures 4. Magnetic highs

58 Targets Identified. 51 Targets Identified


Esaase EM Data
Multi-million oz. resource area is not conductive.
Gold is in quartz veins along major fault.
Gold is in close proximity to conductors.

EM Channel 15

Magnetic Dikes

Resource Area


Esaase Crossing Structures

Magnetic Tilt Angle

Some crossing structures are visible.
As with Bogoso and Obuasi, these seem
relevant to the mineralization.


Esaase EM profiles
Line 3300
Many target zones are resistors.
Conductors still targeted, just in case there is another
mineralization style present.

Big west dipping structure

Line 3610
TZ-25

TZ-20

30 m conductivity depth slice Mag
RTP-TMI
showing shallow resistors. Mag total grad
vert grad

Power line monitor

Resource
Time Constant

Only mildly conductive

Tarkwaian






Esaase

1. Cross cutting structures
2. Shallow resistive areas
3. Deeper Conductors

28 Targets Identified


Asumura Conductivity
No significant gold mineralization found associated with the graphitic faults.
Gold is found along the NW Fault - which does not contain graphite.

92 m conductivity
depth slice


Asumura magnetic tilt angle with crossing structures

Some disruption around known gold, but not distinctive.


Asumura EM profiles for Target Zone TZ-36
Weak surface conductor at known gold. Line 1710
Resistors at 92 m provided some targets.

Conductivity at 92 m depth TZ-36

Line 1450






Esaase Asumura

1. Cross cutting structures 1. Near large faults
2. Shallow resistive areas 2. Resistive areas
3. Deeper Conductors 3. Crossing structures
4. Possible deep conductors


Conclusions

•Known gold is not always indicated by conductors:
•Bogoso - Most known gold is coincident with conductors
•Obuasi - Some known gold is associated with conductors
•Esaase - Known gold is near conductors
•Asumura - Little indication that gold is associated with conductors

•Resistors are important for gold exploration.

•Structures are important with all four survey areas.

•VTEM could be used to assist in gold exploration for all four survey areas.


Acknowledgements

Condor Consulting thanks the following companies for
allowing their data to be used in this study:

•Golden Star Resources Ltd.

•Pelangio Exploration Inc.

•Keegan Resources Inc.


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