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VTEM in Ghana for Gold Exploration
- 1. 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
- 2. 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.
© Condor Consulting 2006
2010
- 3. 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
Keegan Resources Inc.
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
© Condor Consulting 2010
- 4. 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
© Condor Consulting 2010
- 5. 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
© Condor Consulting 2010
- 6. 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
© Condor Consulting 2010
- 7. 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.
© Condor Consulting 2010
- 8. 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
© Condor Consulting 2010
- 9. 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
© Condor Consulting 2010
- 10. 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
© Condor Consulting 2010
- 11. 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
© Condor Consulting 2010
- 12. 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.
© Condor Consulting 2010
- 13. 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
Keegan Resources Inc.
Flown in 2007
200 m line spacing
© Condor Consulting 2010
- 14. 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
© Condor Consulting 2010
- 15. 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
© Condor Consulting 2010
- 16. 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
© Condor Consulting 2010
- 17. 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
© Condor Consulting 2010
- 18. Primary Targeting Criteria
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
© Condor Consulting 2010
- 19. 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
Keegan Resources Inc.
Flown in 2007
200 m line spacing
© Condor Consulting 2010
- 20. 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
© Condor Consulting 2010
- 21. Esaase Crossing Structures
Magnetic Tilt Angle
Some crossing structures are visible.
As with Bogoso and Obuasi, these seem
relevant to the mineralization.
© Condor Consulting 2010
- 22. 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
© Condor Consulting 2010
- 23. Primary Targeting Criteria
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
1. Cross cutting structures
2. Shallow resistive areas
3. Deeper Conductors
28 Targets Identified
© Condor Consulting 2010
- 24. 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
Keegan Resources Inc.
Flown in 2007
200 m line spacing
© Condor Consulting 2010
- 25. 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
© Condor Consulting 2010
- 26. Asumura magnetic tilt angle with crossing structures
Some disruption around known gold, but not distinctive.
© Condor Consulting 2010
- 27. 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
© Condor Consulting 2010
- 28. Primary Targeting Criteria
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 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
28 Targets Identified
50 Targets Identified
© Condor Consulting 2010
- 29. 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.
© Condor Consulting 2010
- 30. 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.
© Condor Consulting 2010