The magnetic method: A frontline geophysical approach for gold exploration

Transcription

1 The magnetic method: A frontline geophysical approach for gold exploration Marc Boivin MBGeosolutions, Quebec QC Presented to the KEGS-PDAC 2010 Symposium All that Glitters: Geophysics for Precious Metals Toronto, ON, March,

2 OUTLINE Introduction Gold exploration and MAG surveys Structural interpretation procedures Example Conclusions 2

3 INTRODUCTION Gold and precious metals geophysical exploration A challenge for a geophysicist Very weak concentration (10g/t Au = 10ppm) Direct detection. Not possible! Indirect detection. YES! Gold exploration is a real challenge for geophysicists. Economic gold occurrences represent a weak concentration. No direct geophysical detection is possible. Geophysics can be use for indirect detection. 3

4 INTRODUCTION Gold and precious metals geophysical exploration A challenge for a geophysicist INDIRECT DETECTION Host rocks (Quartz,..) Traps (veins, shear zones ) Accessory minerals -Sulphides -alteration The indirect detection can target the host rock, the ore traps or path finder minerals 4

5 INTRODUCTION MAGNETIC METHOD Direct footprint of the geology Uniform sampling and complete coverage Strong potential for a structural interpretation - Magnetic image is a direct footprint of the GEOLOGY -Because of the low cost, the method can offer a uniform sampling and a larger coverage -The MAG results have a strong potential for structural interpretation. 5

6 GOLD exploration and MAG surveys Geological structures Major role in gold deposit Geological structures Larger target Geological structures Can be possibly seen on MAG Why interpreting geological structures using Geophysics in gold exploration? Because structures play a major role in several gold deposits Because structures represent larger targets compare to the gold deposits Because structures can be possibly seen geophysical survey and more particularly on MAG surveys 6

7 GOLD exploration and MAG surveys Understand gold deposit models Understand structural geology Recognize structures on MAG data To ensure a high quality structural interpretation of MAG data, the interpreter has to: 7

8 STRUCTURAL INTERPRETATION FOR GOLD Understanding structural geology This cartoon illustrates the development of shears by faulting. 8

9 STRUCTURAL INTERPRETATION FOR GOLD Recognize geological structures on MAG An appropriated MAG image can show similar structural features (faults and shears) 9

10 STRUCTURAL INTERPRETATION FOR GOLD Understanding structural geology This cartoon illustrates the development of a structural «pressure shadow» produces by faulting 10

11 STRUCTURAL INTERPRETATION FOR GOLD Recognize geological structures on MAG An appropriated MAG image can show similar features directly associated with a pressure shadow 11

12 STRUCTURAL INTERPRETATION FOR GOLD Which type of magnetic survey is suitable? Ground or Airborne? 12

13 STRUCTURAL INTERPRETATION FOR GOLD Ground MAG survey Ground magnetic surveys offer a better spatial resolution. 13

14 STRUCTURAL INTERPRETATION FOR GOLD High resolution helicopter MAG survey On the same property, an airborne high resolution MAG survey (helicopter-borne survey) shows an optimized signal/noise ratio for structural interpretation and data processing. 14

15 STRUCTURAL INTERPRETATION PROCEDURES 1) Define the appropriate data 2) Define the interpretation criteria 3) Deliver a usable product A useable and trustable structural interpretation of the magnetic data is only possible with well defined procedures 15

16 STRUCTURAL INTERPRETATION PROCEDURES Define the appropriate data Shaded relief of 1 st vertical derivative Appropriated sun azimuth 1D and/or 2D vertical derivative processing -The ideal magnetic data to interpret structural lineament is the shaded relief of the 1st vertical derivative - The sun azimuth will be chosen according the known structural model 16

17 STRUCTURAL INTERPRETATION PROCEDURES Define the interpretation criteria and procedures Defining the magnetic ridge 17

18 STRUCTURAL INTERPRETATION PROCEDURES Magnetic ridge A magnetic ridge is defined as the line joining continuous magnetic highs. A magnetic ridge is not a structural lineament. 18

19 STRUCTURAL INTERPRETATION PROCEDURES Define the interpretation criteria and procedures magnetic ridge Defining the structural lineament - Truncation of magnetic ridges - Border between magnetic patterns Using the magnetic ridges, the interpreter can define structural lineaments using some criteria 19

20 STRUCTURAL INTERPRETATION PROCEDURES Structural lineament -TRUNCATION Here an example of defining a structural lineament defines by truncations. In highly deformed geology, the angular relation between the truncated magnetic ridges and the structural lineament can be very subtle. 20

21 STRUCTURAL INTERPRETATION PROCEDURES Structural lineament BORDER (BORDER between magnetic domains) The border between two magnetic domains can be used to interpret a structural lineament. Lithostratigraphic contact is the host of major structure in highly deformed geology. 21

22 EXAMPLE OF A STRUCTURAL INTERPRETATION This example come from a structural magnetic interpretation of the kinojevis property owned by Cartier. This gold and VMS project is located in the Abitibi greenstone belt. 22

23 EXAMPLE Fixed-wings survey Helicopter-borne survey As preciously mentioned, choosing the best dataset to enhanced sub-surface features is one of the first step 23

24 EXAMPLE Fixed-wings survey Helicopter-borne survey A comparison of the shaded relief image of the 1st vertical derivative for both surveys 24

25 EXAMPLE Shaded relied (2D FFT 1 st Vertical Derivative) Choosing the best filtering to minimize levelling artefacts. The 2D FFT calculation of the 1st vertical derivative shows some levelling problems 25

26 EXAMPLE Shaded relied (1D FFT 1 st Vertical Derivative) The calculation of the 1D FFT of the 1st vertical derivative tends to minimize levelling problems. 26

27 EXAMPLE MAGNETIC RIDGES LAYER Completion of the magnetic ridges layer over the Kinojevis survey 27

28 EXAMPLE MAGNETIC RIDGES LAYER (enlarged view) Enlarged view 28

29 EXAMPLE STRUCTURAL LINEAMENT INTERPRETATION 29

30 EXAMPLE STRUCTURAL LINEAMENT INTERPRETATION (enlarged view) 30

31 EXAMPLE STRUCTURES CLASSIFICATION (1 st order & 2 nd order) Possibility to define several classes of structures 31

32 EXAMPLE STRUCTURES CLASSIFICATION (deformation( corridor) Possibility to define deformation corridor 32

33 EXAMPLE VALIDATION Validation of the interpretation by comparing public domain geological data. 33

34 EXAMPLE PROPERTY SCALE TARGETTING FOR GOLD EXPLORATION According the gold exploration model, it is possible to generate conceptual gold targets at the property scale. 34

35 CONCLUSIONS Complex geological structures associated with gold occurrences can be interpreted on MAG surveys High resolution MAG survey is recommanded structural interpretation 35

36 CONCLUSIONS Appropriate data processing can enhance the structural pattern on the MAG image Well defined interpretation procedures will make the results more trustable and progressive 36

37 CONCLUSIONS The magnetic method A frontline geophysical approach for gold exploration 37

38 THANKS CARTIER RESOURCES INC VIRGINIA MINES INC SOQUEM INC I would like to thanks these companies for their support 38