Airborne EM (VTEM-ZTEM) Applications for Gold

Transcription

1 Airborne EM (VTEM-ZTEM) Applications for Gold with Recent examples over the Pebble cu-mo-gold Porphyry and Lalor Lake VMS cu-zn-gold deposits By Jean M. Legault *, Vlad Kaminski, Harish Kumar, Marta Orta and Alex Prikhodko Geotech Ltd., Aurora, ON All That Glitters: Geophysical Exploration for Gold KEGS-PDAC SYMPOSIUM 2010 Presented at KEGS-PDAC Symposium, 6-March-2010, Toronto, ON.

2 Introduction Outline VTEM-ZTEM Description & Features VTEM Gold examples Northern Empire (ON), Tusker (TZ), Nkran (GN) VTEM-ZTEM Silver-Gold example Cinco de Mayo (MX) ZTEM Gold examples Pebble (AK), Lalor Lake (MB) Conclusion KEGS-PDAC SYMPOSIUM 2010

3 Part 1: VTEM Versatile Time Domain electromagnetics Award Winning and Industry Leading Airborne EM Technology

4 Specifications Parameter VTEM 26 Base Frequency Waveform 25 Hz/30Hz Trapezoid Current Maximum: 310 A (4ms pulse) 200 A (7.5ms) Peak dipole moment Maximum: NIA (4ms) Typical: NIA (7.5ms) Tx loop diameter (area) 26 m (540 m 2 ) Tx number of turns 4 Survey speed Tx/Rx Clearance Tx turn-off time Tx Pulse On Time Rx Time gates Rx coil diameter, m Rx number of turns Rx Effective Area, m 2 Magnetometer Mag Clearance & sensitivity 80 km/h m 1.1 ms Programmable 4.6 to 7 ms 28 (0.083 ms 7.89 ms) 1.2 (Z) & 0.32 (X) 100 (Z) & 245 (X) (Z) & 19.7 (Z) Optically pumped caesium vapour 60m & 0.02nT (0.001nT base) The VTEM System Outlining VTEM System Specifications and Key Elements

5 VTEM Technical Highlights 25/30 Hz base frequency (permits long decay measurements), up to 50 channels with latest system 26m Transmitter coil (the largest diameter loop available on any airborne geophysical platform), 35m for VTEM 35 Large Dipole moment (425,000 Am 2 for VTEM 26 / 950,000 Am 2 for VTEM 35 ) with Extremely Low System Noise Deep Penetration (arguably best of HTEM systems), m for shallow dipping targets, >750m in Athabasca Basin Focused footprint to also discriminate smaller targets (i.e., kimberlites) Superior Repair or Replace Time (few hours) Concentric Transmitter Receiver geometry ensures accurate anomaly location, Z & X (+/- Y) sensors VTEM is widely considered the best HTEM massive sulphide detection & imaging tool, with fly-to-drill capability.

6 Part 2: ZTEM Z-axis Tipper electromagnetics Introducing Airborne AFMAG (audio-frequency magnetic) Technology

7 AFMAG - Background AFMAG stands for Audio Frequency (electro) Magnetics or Magneto-variational). ZTEM ( Z-axis Tipper Electromagnetics) is a variant of AFMAG only Hz receiver is mobile, Hx-Hy sensors at fixed base-station. Passive Electromagnetic (EM) technique in same family as Magnetotellurics (MT), but measuring magnetic fields only - similar to VLF EM but does not use a man-made transmitter, lower frequency, 3D measurement. Frequency range - audio range Geotech s AFMAG can operate from 22 to 2800 Hz (based on digitizing rate), but in practice we operate from 30 to 360Hz (based on signal strength). The EM source is the natural field of Earth caused by lightning strikes at a distance mainly equatorial thunderstorms and other electrical storms 1000 s of km away. The Earth and the Ionosphere, both conductive, act as a wave guide to transmit these energies to great distance. Like MT, AFMAG fields are characterized by deep penetration, ranging from 100 s to 1000 s of metres (skin depth depends on host rock resistivity).

8 Specifications Parameter Transmitter Sampling Frequency Receiver Survey speed Rx Clearance ZTEM None required (Passive EM method) A/D = 2000 Hz A/D (0.0005sec) Output = 2.5Hz (0.4sec ~ 10m/sample) Bird = Hz (Vertical Dipole), Base = Hx-Hy (Horizontal Dipole) 80 km/h 50 m (nominal) Rx coil diameter Mobile = 7.2m Base = 3.5m Rx Frequencies 32, 45, 90, 180, 360 Hz (+/- 720Hz) Rx Derived Measurements Rx Transfer Functions Nominal Noise floor Skin Depth Penetration Hz/Hx and Hy/Hz Tippers (Tensor FFT) In-Phase and Quadrature <1% IP & QD m for 1k Ω-m avg. Host m for 100 Ω-m avg. Host m for 10k Ω-m avg. Host The ZTEM System Base Station Receiver Airborne Receiver Helicopter Magnetometer In ZTEM only Vertical component (Hz) of AFMAG field is measured in receiver coil. The horizontal (Hx-Hy) primary fields are measured at the base-station. This is a distinct advantage in terms of data quality. 90m EM Receiver

9 ZTEM - Features Excellent sensitivity to lateral resistivity contrasts, for example fault-fracture zones, clay-alteration, silicification, water, etc. but also sensitive to absolute conductivity, such as nickeliferous and VMS massive sulphide orebodies Hz frequency Bandwidth provides for deep penetration, makes ZTEM a depth- sounding and profiling tool, mid-low Hz permits near-4 season survey capability. Superior Exploration Depth over 2000 metres from MT skin depth (δ S =503* (ρ/f)), numerical simulations, field tests. Relative insensitivity to flight-height variations (due to relatively larger primary field penetration depths, small 1/R 2 fall-off rate) Most importantly, the uniform, plane-wave nature of the natural EM fields permits fast 2D-3D forward & inversion possible on PC, makes ZTEM unique Geologic Mapping Tool.

10 DEPTH (METRES) 360 Hz 30 Hz ZTEM Features: Depth Penetration Simplest Case: 1D Skin Depth Rule 80m 290m Earth Surface 270 m 1000 m 920m 840 m ZTEM AFMAG Minimum Penetration Depth 2000 m 3000 m 4000 m δ s ~ 503 (ρ/f) [metres] ZTEM AFMAG Maximum Penetration Depth 2900 m 10 Ohm*m 100 Ohm*m 1000 Ohm*m 30 Hz 2700 m 10,000 Ohm*m 360 Hz 9200 m EARTH RESISTIVITY (OHM-METRES) MT PLANE WAVE SKIN DEPTHS in 1D HALF-SPACE *(Ref. Vozoff, 1972)

11 ZTEM Phase-Rotated Z/X In-Phase Results ZTEM Features: Deep Penetration Example at Shea Creek, NW Sask. 3D Pole-Pole DC Inversion Shallower Penetration ZTEM In-Line IP & QD Results Deeper Penetration 2D ZTEM Resistivity Inversion SHEA CREEK HORIZON (modified after Bingham et al., 2006) Note Reverse colour scale (Blue=Low Resistivity) relative to all other figures DEPTH TO BASEMENT=700M Apparent Resistivity Low in Athabasca Sandstone above SK-Lake Graphite consistent with NE-offset alteration observed in Pole-Pole Resistivity Data (Courtesy AREVA Resources (Canada) and UEX Corporation) Showing ZTEM Results over Kianna Zone, at Shea Creek NW Saskatchewan Canada

12 Gold Deposit Types 1 HEM Detectability South African Paleoplacer (47% of World Au)» Only occur in South Africa Porphyry (20%)» Mostly copper deposits, with gold associated» Also includes Skarns, Mantos, etc. Greenstone (13%)» Most are Archean» Types 1) Vein deposits, 2) Disseminated-Vein-Stockworks Epithermal (10%)» Types 1) High Sulphidation, 2) Low Sulphidation, 3) Exhalative Sediment-hosted (9%)» Types 1) Carlin-type, 2) Clastic-sediment hosted? Usually Difficult for HEM Extremely Difficult for HEM Sometimes Difficult for HEM Sometimes Difficult for HEM 1 From Hodgson, PDAC 2010

13 Northern Empire Mine ( ) 149,053 ounces of gold produced VTEM for Gold: Northern Empire Mine, Beardmore, ON (Vein hosted Au associated with py-po sulphides) Geologic Section Contact Zone 25,000 oz Au VTEM B-field (mid-channel) From Dubé, and Gosselin, 2007 Power Zone (mined out) VTEM 2.5D Plate Model Contact Zone Conductor 63Siemens Dipping Conductor (courtesy Kodiak Exploration Ltd., 2010)

14 400m 1-2 million ounces Au grading above 3.0 g/t VTEM for Gold: Nkran Mine, Ghana 05 (Au associated with py-as sulphides) Conductivity Depth Image derived from VTEM db/dt data Waste dump Mine pit (Courtesy PMI Gold, 2008) Potential new targets

15 VTEM for Gold: Tusker Mine, Tanzania (Sediment-hosted Au associated with py-po sulphides) Mt 1.15g/t Au Gold Intercepts in Drill holes Drill hole NYZRCDD032 VTEM data over greyscale 1VD of RTP airborne magnetic data (courtesy Barrick Gold, 2009)

16 VTEM & ZTEM over Carbonate Replacement Deposits Cinco de Mayo, Chihuahua MX (Mo-Gold & Silver-Pb Discoveries) VTEM db/dt Tau ZTEM 360Hz In-Phase DT Jose Manto Ag-Pb-Zn Zone Pozo Seco Au-Mo Zone Pozo Seco Au-Mo Zone VTEM db/dt Profiles & CDI Section CM (courtesy Mag Silver Corp., 2010) 4:1 Vertical Exaggeration Jose Manto Ag-Pb-Zn Zone

17 ZTEM over Porphyry Cu-Gold Deposit: Pebble, Alaska (1 st Largest Gold and 4 TH Largest Copper Deposit) 360Hz IN-PHASE TIPPER TOTAL PHASE ROTATED (Shallower Penetration) 30Hz IN-PHASE TIPPER TOTAL PHASE ROTATED (Deeper Penetration) ZTEM is the ONLY Airborne technique designed and proven to map Porphyry deposits to great depth Pebble Deposit appears nearly invisible in highest frequency data due to limited penetration (<500m). Mainly nearsurface geologic contacts & structures are likely 0 1.5km being highlighted. Pebble Deposit is best defined in low frequency image (>1.5km). More resistive rocks in center coincide with K-altered core; lower resistivities outside the deposit are 0 1.5km 2D ZTEM Resistivity consistent with surrounding Inversion clay-altered pyrite halo. 600m 1500m 5.1BT 0.43% Cu, 0.35 g/t Au and 256 ppm Mo (courtesy Pebble Partnership, 2010) Showing ZTEM Results for Structural & Lithologic Mapping over a known Porphyry

18 Graphite-> ZTEM over VMS Cu-Gold Deposit: Lalor Lake, Manitoba (12.3 MT buried at > m Depth) ZTEM Phase-Rotated Z/X In-Phase Results OUTLINE OF LALOR LAKE DEPOSIT Powerline Powerline TOWN of SNOW LAKE Snow Lake OUTLINE OF LALOR LAKE DEPOSIT Powerline Powerline TOWN of SNOW LAKE Snow Lake PHOTO LAKE Graphite-> PHOTO LAKE CHISEL NORTH CHISEL NORTH L1020-> CHISEL LAKE CHISEL OPEN PIT 360Hz XIP PHASE-ROTATED ZTEM Tipper AFMAG Survey 0 1km L1020-> Lalor Lake Outline 900m NS x 700m EW CHISEL LAKE CHISEL OPEN PIT 2D ZTEM Resistivity Inversion 30Hz XIP PHASE-ROTATED ZTEM Tipper AFMAG Survey 0 1km Lalor Lake Deposit drill holes and mineralized zone outlines in a) long section (left) and b) plan ZTEM is the ONLY Airborne technique that has detected the Lalor Lake VMS deposit (12.3MT 1.6 g/t Au, 24.2 g/t Ag, 0.66% Cu, 8.70% Zn) 550m 1200m (courtesy Hudbay Minerals Corp, 2010) Showing ZTEM Results over Lalor Lake VMS Deposit, Snow Lake, Manitoba Canada

19 KEGS-PDAC SYMPOSIUM 2010 Conclusions 4 Common types of Gold Deposits (Hodgson, PDAC-2010):» 1) Porphyry, 2) Greenstone, 3) Epithermal, 4) Sediment-hosted. Many gold deposit types (1-2), due to disseminated nature of sulphides, and non-conductive host-rocks, pose difficult but not always insurmountable challenges to helicopter time-domain EM. For example, VTEM was successful over Northern Empire Archean vein-hosted deposit (Type 2), but similar examples more rare. Other examples (not shown) might include Epithermal (Type 3), high sulphidation targets (i.e., LaRonde, QC) and Porphyries (Type 1) with supergene enrichment (i.e., Collahuasi, Chile). VTEM has been successful for Sedimentary hosted (Type 4) gold deposits in Africa (Ghana, Tanzania). ZTEM & VTEM were successful for carbonate replacement (Type 1) silver +/- gold deposits. ZTEM is best suited for most porphyries (Type 1) and deep Lalor-Lake type (Epithermal Type 3?) targets, beyond reach of VTEM penetration (>500m); potentially also useful structural mapping tool for other types of gold deposits.

20 KEGS-PDAC SYMPOSIUM 2010 Thank You! Kodiak Exploration Barrick Gold PMI Gold Condor Consulting Mag Silver Pebble Partnership. Hudbay Minerals