vtem in ghana for gold exploration

© Condor Consulting 2010

VTEM over BogosoSept, 2007

GEOTECH’S

VersitileTransientElecrtro-Magnetic

System

A Case Study of Four VTEM Surveys for

Gold Exploration in Ghana


EM Processing Techniques

Layered-Earth (1D)

Inversion

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.

Transient Decays

Voltages as profiles


1

2

3 Model from LEI

Magnetic data

Distance along flight line


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


General EM Character

of the four areas

5 km

4 km

4 km

2 km

dB/dt Z Component

Channel 15 (1130 usec)

(a mid-time channel)

1) Bogoso

2) Obuasi

4) Asumura

3) Esaase

•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.

Next slide shows cross-section through the Chujah Pit.

Not all conductors have gold.

The wide conductors along the MCZ are favorable to the where known gold is located.

Gold occurs along the Main Crush Zone – conductive.

The resistive Tarkwaian is to the east, Birimian to the west.


Bogoso/Prestea - Structures

Conductivity from

layered-earth inversion

216 m depth slice

Town of Bogoso

Prestea

Chujah Pit


Power line

Ore Power line

Chujah Pit

Line through Chujah Main Pit

EM channels

Magnetic data (RTP)

LEI section

Plan map

Not a distinctive TMI anomaly

MCZ

Cultural1st VD anomaly


Vertical Gradient of Magnetic Data

Geology from Allibone 2002.

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


Town of Bogoso

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 Bogoso

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

55 Target Zones of various rankings are identified using a variety of known criteria such as:

Conductivity,

Ground geophysics,

Mapped lithology,

Mapped and interpreted faults,

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

Target Zone 40

5 km

Target Zone 19

Magnetic high is

due to a dyke.

Line 9330 B-19

B-19:

Very wide conductor on the MCZ.

Right-hand flexure in the fault here.

A north-northwest dyke run through this target.


Line 6870

B-40

B-40

Sharp conductor with high time constant on the MCZ.

May have some right-hand flexure in the fault.

Target Zone B-40

5 km

Target Zone B-19

Both of these Target Zones are further north in the survey block, away from the known gold locations.

Discrete feature picks are marked by vertical lines.


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


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.



Conductance from Layered Earth Inversion

50 m to 200 m Depth Interval

Color pallet is linear, 0 to 5000 millisiemens

Mines

Gold Mineralization

Granodiorite

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.

Obuasi

Cross-section

P.L.

1 km

LEI conductivity

VTEM time channels


Obuasi Cross-Cutting StructuresNortheast to east-west structures visible in magnetic data.

Some correlation of gold to magnetic highs.

Magnetic Tilt Angle

Mines

Gold Mineralization

Granodiorite

Obuasi

Tikal Section

TK03

TK01

100 m

300 m

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


N S

Silicification


Obuasi Target Zones

TZ-12

Line 1720

50 m to 200 m conductanceColor pallet is linear, 0 to 5000 millisiemens

Northern end of Obuasi Mine

High conductivity and large structures not as important as at Bogoso.

Most target zones are conductors, such as TZ-12.

Some target zones are resistors.



Bogoso/Prestea





Obuasi

1. Conductors in 50 to 200 m DI


3. Resistive areas

4. Magnetic highs

51 Targets Identified


Four Ghana Surveys


Obuasi Pelangio Exploration Inc.

Flown in 2007 and 2009 100 m line spacing



Asumura Keegan Resources Inc.



EM Channel 15

Esaase EM Data

Resource Area

Magnetic Dikes

Multi-million oz. resource area is not conductive.

Gold is in quartz veins along major fault.

Gold is in close proximity to conductors.


Esaase Crossing Structures

Magnetic Tilt Angle

Some crossing structures are visible.

As with Bogoso and Obuasi, these seem relevant to the mineralization.


TZ-25

Line 3300

Esaase EM profiles

Line 3610

RTP-TMI

Power line monitor

Time Constant

Mag

vert gradMag total grad

30 m conductivity depth slice

showing shallow resistors.

Big west dipping structure

Only mildly conductive

Tarkwaian

Many target zones are resistors.

Conductors still targeted, just in case there is another mineralization style present.

TZ-20

Resource



Bogoso/Prestea





Esaase


2. Shallow resistive areas

3. Deeper Conductors


Obuasi



3. Resistive areas

4. Magnetic highs



Four Ghana Surveys


Obuasi Pelangio Exploration Inc.

Flown in 2007 and 2009 100 m line spacing



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


Asumura Conductivity

92 m conductivity

depth slice

No significant gold mineralization found associated with the graphitic faults.

Gold is found along the NW Fault - which does not contain graphite.


Asumura magnetic tilt angle with crossing structures

Some disruption around known gold, but not distinctive.

Line 1450


Asumura EM profiles for Target Zone TZ-36

Line 1710

TZ-36

Weak surface conductor at known gold.

Resistors at 92 m provided some targets.

Conductivity at 92 m depth



Bogoso/Prestea





Asumura

1. Near large faults

2. Resistive areas

3. Crossing structures

4. Possible deep conductors


Obuasi



3. Resistive areas

4. Magnetic highs


Esaase


2. Shallow resistive areas

3. Deeper Conductors



Conclusions


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.

vtem in ghana for gold exploration

Technology

barren condor consulting

flight line

line spacingthe surveys

gold exploration

known goldoccurrences

tilt angle of magnetic

whereknown gold

em dbdt channel