27/10/2017 1 www.sgc.com.au Geophysics for Magmatic Ni‐Cu‐(PGE) Exploration Bill Peters Australian Society of Exploration Geophysicists Australian Institute of Geoscientists Perth, October 18 th 2017 www.sgc.com.au This talk discusses: • The current state of the art in geophysical acquisition, power, resolution, sensitivity, inversion, visualisation, and modelling. • The physical properties of Ni‐Cu sulphides, host rocks, and complicating non‐economic geological features. • The key geophysical methods, their application, and global examples. • Regional targeting to identify settings containing prospective mafic ‐ultramafic rocks including the concepts of lithospheric structural control and intracrustal magma chambers. Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters • Radio Hill 1984 – Australia’s first gabbro‐hosted mine – EM discovery • Look at komatiite ‐ hosted & gabbro‐hosted Ni‐Cu‐PGE deposits. • Ni‐Cu sulphides have a strong response to multiple geophysical methods. This is probably why Ni‐Cu exploration has been arguably the most significant factor in the development of minerals geophysical technology. • Outcropping and near‐surface discoveries are now rarer. • Our challenge is to explore covered areas, look deeper, and understand what we are seeing • New technology allowing exploration to depths towards 1000m Introduction Radio Hill TEM 1984 Radio Hill Mine 3 Let’s look at why geophysics works, what new technology we have, some Ni‐Cu deposits, and targeting for new discoveries Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters The Nova‐Bollinger discovery • New gabbro‐hosted Ni‐Cu discovery • Discovered in the Fraser Range of Western Australia in 2012 • An entirely new Ni‐Cu province • Australia previously more focussed on komatiite deposits • New exploration enthusiasm • More about this later 4 Rejuvenated geophysical exploration for Ni‐Cu in Australia
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27/10/2017
1
www.sgc.com.au
Geophysics for Magmatic Ni‐Cu‐(PGE) Exploration
Bill PetersAustralian Society of Exploration Geophysicists
Australian Institute of GeoscientistsPerth, October 18th 2017
www.sgc.com.au
This talk discusses:
• The current state of the art in geophysical acquisition, power, resolution, sensitivity, inversion, visualisation, and modelling.
• The physical properties of Ni‐Cu sulphides, host rocks, and complicating non‐economic geological features.
• The key geophysical methods, their application, and global examples.
• Regional targeting to identify settings containing prospective mafic ‐ultramafic rocks including the concepts of lithospheric structural control and intracrustal magma chambers.
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
• Radio Hill 1984 – Australia’s first gabbro‐hosted mine – EM discovery
• Look at komatiite ‐ hosted & gabbro‐hosted Ni‐Cu‐PGE deposits.
• Ni‐Cu sulphides have a strong response to multiple geophysical
methods. This is probably why Ni‐Cu exploration has been arguably
the most significant factor in the development of minerals
geophysical technology.
• Outcropping and near‐surface discoveries are now rarer.
• Our challenge is to explore covered areas, look deeper, and
understand what we are seeing
• New technology allowing exploration to depths towards 1000m
Introduction Radio Hill TEM 1984
Radio Hill Mine
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Let’s look at why geophysics works, what new technology we have, some Ni‐Cu deposits, and targeting for new discoveries
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
The Nova‐Bollinger discovery
• New gabbro‐hosted Ni‐Cu discovery
• Discovered in the Fraser Range of Western
Australia in 2012
• An entirely new Ni‐Cu province
• Australia previously more focussed on komatiite
deposits
• New exploration enthusiasm
• More about this later
4
Rejuvenated geophysical exploration for Ni‐Cu in Australia
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Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Instrumentation, platforms and software
Now• Off the shelf PCs / CPUs with high
speed AD converters and I/O ports.
• Intelligence incorporated in software
• GPS, Wi‐Fi, Bluetooth
• Self‐powered, untethered sensors
• Large on‐board storage
• Miniaturisation allows use with UAVs
and bore hole probes
ThenInstruments custom designed, hand built ‐ individual components
5 Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Unmanned Aerial Vehicles (UAVs) (Drones)
• Advantages:
Lower cost, safer, fast production, small areas, no ground
access issues
• Limitations:
Small payload, aviation regulations, short range & duration
• Uses:
Magnetics, VLF, spectral, (radiometrics?)
Cannot carry powerful EM transmitters, but could carry a
sensor used with a ground based loop
Airborne data link for distributed sensors on the ground Medusa ‐‐ Radiometrics
GEM Systems ‐ Magnetics
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Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Modelling, Inversion & 3D Visualisation
• Conventional forward modelling software still being enhanced
• Most significant developments are in inversion ‐ 1D, 2D, 3D, Joint inversions
• Visualisation: It is now expected that geophysical modelling & inversion results will be supplied and incorporated in 3D
• 3D printing of models?
Pitney Bowes
7 Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Electromagnetics (EM)
• Surface EM – Detailed, deep penetration
• Airborne EM – Rapid low cost coverage, but less penetration
• Downhole EM ‐ Very successful, Exploration to large depths,
• Barren pyrrhotite/ graphite – often highly conductive similar to Ni‐Cu sulphides
• Magnetite can be conductive if well connected, but rare
BY FAR THE MOST WIDELY USED & SUCCESSFUL GEOPHYSICAL METHOD FOR NI‐CU DISCOVERY
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Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Advances in EM
HIGHER POWER• Transmitter power: Up from
10 amps to 100‐400 amps
• Much increased depth of exploration and improvement in signal sensitivity and quality
• Downside: Stronger polarisation, SPM and saturation effects
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TIME SERIES(FULL WAVEFORM)
• Analysis of signal and noise.
• Noise rejection, filtering, primary pulse diagnostics, selectable windows
• Extraction of other parameters
AUTONOMOUS SENSORS• Untethered• GPS, Wifi, Bluetooth• Large data storage• Long battery life• Greater flexibility for
survey design and difficult terrain
SIMULTANEOUS TRANSMITTERS/LOOPS• Survey with two or more
loops simultaneously.• Deconvolve from time
series data.
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
High Conductance Targets
• Extreme conductance of massive sulphides: Off‐time dB/dt methods (Coil sensors) can be poor (Derivative of decay is very small).
• Need B‐field or On‐time measurements
• On‐time: Requires accurate calculation of the primary field. OK for Fixed Loop but not practical for In Loop & Slingram surveys.
• B‐field: Increasing use of fluxgate & SQUID sensors
Strong B‐Field response
Weak dBdT response
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Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
High Power Issues (Problems?)
High power (& low sensor height): ‐> Issues
• Superparamagnetism (SPM) from near‐surface ferrimagnetic material results in low amplitude, late time decay anomalies very similar to deep bedrock conductor responses
• Polarisation (IP): Negative decays. Most noticeable in resistive environments
• Saturation: Problem for sensors close to loop e.g. In‐loop surveys
Mutton et al. 2009
Bedrock SPM
IP
11 Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Total Field EM (GAP Geophysics)• Uses a specialized total field magnetometer sensor
• SAMSON: In‐loop & Fixed loop modes. Low frequencies ( 0.125Hz) , long stacking ‐> low noise, deep penetration
• SAMEM: Large fixed loop , walking magnetometer, higher frequency (~3Hz), less stacking ‐> Noisier but higher data density allows spatial filtering
• HeliSAM : Large fixed loop, light helicopter, towed sensor. Rapid acquisition. Downside: Total field only, no late time, large heavy TX loop, null coupling zones. Planned UAV version could fly slower with lower frequencies and flying heights
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Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Airborne EMDevelopments: Higher dipole moments (2.0 M NIA)• Multiple components, B‐field, Time series recording• Fast shut‐off times & improved waveform control ‐> more accurate inversions
Fixed Wing: GEOTEM, TEMPEST, MEGATEM, SPECTREM, etc. ‐> large scale, towed bird, asymmetric. Cheaper than helicopter, but lower resolution.
Helicopter: Focus of most development. Advantages:Omnidirectional coupling, low sensor height, slow flying, short lines
AEM vs Ground EM: Despite rapid coverage and easy logistics, AEM cannot compare to ground surveys using dipole moments over 100 times larger. AEM also has only relatively high transmitter frequencies due to the moving platform
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CGG ‐ MEGATEM
NRG ‐ XCITE
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Down Hole EM• Exceptional for Ni‐Cu exploration at depth & under cover to
depths of 3000m & more
• Surface EM ‐> attenuation of conductor response by depth to conductor, & conductive cover/host rocks.
• DHEM ‐> sensor close to target ‐> attenuation minimised
• Step response or B‐field measurements for high conductance targets.
• Advances include power & data storage in the probe, optical cables
• Down dip drilling of prospective contacts14
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
• Regional Scale: Margins, sutures, feeder zones, magma chambers
• Airborne gravity ‐ low spatial resolution: Best suited to large scale regional work
• Airborne gravity gradiometry: Higher spatial resolution. New systems appearing ‐> NEOS / Lockheed ‐> Full Tensor Gradiometry (20 times sensitivity and 10 times greater bandwidth?), VK1
• Down hole gravity ‐> identify dense sulphides close to drill hole, discriminate graphitic EM conductors from sulphide conductors. Slow & expensive
19 Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Seismics
• Prospect Scale: Use accelerating, 3D surveys for mapping lithology, structure, and possibly detection of sulphides
• Core measurements & refraction tomography to get accurate velocity models.
• Amplitude preservation to detect sulphides.
• Data is often patchy. Interpretation not easy
• Cross‐hole seismic ‐> low velocity sulphides
• Regional Scale: Mapping deep structure, crustal thickness, prospective mantle tapping zones
• Passive seismics – Rapid advances. Lower resolution but cheaper
Kambalda: 3D seismic showing sulphide targets
HiSeis‐ ConsMin
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• Massive sulphides low velocity but high density• Acoustic Impedance ~ velocity x density• Acoustic impedance contrast ‐> seismic reflector
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Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Magnetotellurics (MT)
• Natural magnetic and electric fields:• Large depth of exploration (>10kms)• Sensitive to weak resistivity contrasts
• Can have very small/ negligible EM footprints/responses:• Cosmos: High grade mine ‐> Very small EM footprint• Silver Swan: Not detectable with EM due to vertical
pipe‐like geometry & conductive cover
• Magnetics: delineates host rocks, but rarely a directmineralisation response
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Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Emily Ann (Western Australia) ‐Komatiite Style Geophysical
Discovery • Typical komatiite deposit discovered with Surface In‐Loop EM
• Major gabbro‐hosted Ni‐Cu discovery in 2012, Fraser Range
• In a new Ni‐Cu province as for the Voisey’sBay discovery
• 14.3Mt @ 2.3% nickel, 0.9% copper.
• Hosted in a highly metamorphosed gabbro sill intruded into an extensional sedimentary basin
Sirius Resources
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Nova – Bollinger Section: Sulphides shown in red
200 m
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Nova‐Bollinger: Geochemistry & Magnetics
• Initial exploration based on a single anomalous Ni‐Cu soil sample (GSWA) on the flank of an eye‐like magnetic feature.
• Initial thoughts was that the “Eye” was a gabbro intrusive plug. Now thought to be a double‐fold structure in the host gneisses.
• The host gabbro sill is only weakly magnetic at best
Sirius Resources
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Magnetics image showing “Eye” structure and Sirius tenement
5 km
Magnetics image showing single anomalous nickel soil sample (271ppm)
2.5 km
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Nova‐Bollinger ‐ EM• Surface In‐loop EM detected three
conductors on the margins of the “Eye”.
• Drilling of the main anomaly lead to the discovery of Nova.
• Highly saline groundwater in paleo‐channels hinders EM
• Samson Total Field EM and AMT surveys have been done
• Other conductors have been mainly barren sulphides or graphite
Sirius Resources
In Loop Ch32 Image
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1000 m
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Nova‐Bollinger: GravityNova is located on a prominent regional gravity high
Detailed gravity showed a dense body NE & down dip from Nova.
Drilling of this lead to the discovery of Bollinger Sirius ResourcesGSWA ‐ GA
Residual Gravity ImageRegional Gravity Image
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100 km
1000m
Nova
BollingerNova ‐ Bollinger
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Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Bushveld Style Deposits (crossover with intrusive style)The Bushveld style refers to Ni‐Cu‐PGE mineralisation found in large igneous layered complexes.
Examples of these are:• Bushveld Igneous Complex (South Africa)• Duluth Igneous Complex (Minnesota)• Stillwater (Montana)• Munni Munni (Western Australia)• Lac Des Iles (Canada)• Great Dyke (Zimbabwe)
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Two main mineralisation types:• Large low grade Ni (PGE dominant) in extensive sub‐horizontal disseminated layers• Smaller basal and margin massive sulphide deposits
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Bushveld Igneous Complex (South Africa)
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• Very large at 65000 sq kms and 350km across.
• Primary structural mapping tools are regional gravity, magnetics & seismics to delineate structure and layering
Geoff Campbell 2011
Geology Magnetics Gravity
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
• PlatReef, UG2 and Merensky Reef Units are the most important mineralised horizons
• Merensky Reef and UG2 ‐ geophysically transparent being too thin, and with low sulphide and magnetite content, but conformable horizons often allow indirect mapping with aeromagnetics, gravity and reflection seismics.
• PlatReef• Can be geophysically detected with IP due to its thickness and high disseminated
sulphide content.• Inversion modelling of Falcon gravity and magnetic data incorporating remanence
has been used to predict deep extensions (Williams & De Wet 2016)
• Bushveld satellite bodies – small massive and disseminated Cu‐Ni sulphide deposits amenable to EM, IP and CSAMT.
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Bushveld Igneous Complex – Reflection Seismics
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Geoff Campbell 2011
UG2 Section showing fault displacements
Regional Section
UG2 Section
The Bushveld Complex has almost certainly been the basis of the most extensive research and use of reflection seismics in hard rock mineral exploration
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Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Uitkomst Igneous Complex – (Bushveld Satellite)
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Nyoni & Bishop, 2000
• One of several satellite intrusive complexes associated with the Bushveld Complex
• 12kms long x 850m thick
• Ni‐Cu‐Co‐PGE disseminated and massive sulphide mineralisation in basal pyroxenites and gabbros, as well as in the footwall
Idealised Cross SectionAt Nkomati CSAMT, DHMMR, DHEM have all been successful to a varying degree
Geology Plan
CSAMT Resistivity section showing conductive zone due to sulphides
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Displacement of Craton margin from lithospheric suture/ plume
Regional Targeting – The Big Picture Theory• Deposit locations are related to the architecture of the subcontinental lithospheric mantle.
• At crustal level they are close to craton and paleo‐craton margins – China & Superior Cratons examples
• First ‐ major lithospheric sutures/ faults provide dilational settings for magma intrusion into the crust. (mid‐crustal staging magma chambers)
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Beggs et al. 2010
• Next ‐ intrusion of magma from the staging chambers into weaknesses in the upper crust (dilational faults, fold noses, etc.). Structurally active mobile belts are favourable.
• There is commonly a cluster of intrusions (plugs/sills/dykes) forming a Province/Camp
• Intrusions can be a considerable distance displaced from the underlying magma chamber and/or lithosphere sutures and occurring within unlikely host rocks
China Cratons – Deposits in red
Superior Craton – Deposits in red
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
Conclusions ‐ Final
• At the end of the day, all of this fancy technology and software still depends on the skills of the user, and interpretation can be the weakest link
• We usually only ever have very incomplete information, and there is always ambiguity and uncertainty, but looking for new discoveries is certainly going to depend greatly on geophysics, and geophysicists
• A thorough understanding of geology, mineralising systems, and the physical properties of the target and the host is essential
• Finally, the implications of the large increase in depth capability of new EM technology is that vast areas surveyed over the past forty years can be considered prospective for new exploration.
59 Geophysics for magmatic Ni-Cu-(PGE) exploration | Bill Peters
ACKNOWLEDGEMENTS
I have benefited from useful discussions and input from:
• SGC Colleagues• Ken Witherly• Kim Frankcombe• Theo Aravanis• Greg Turner
Some of the technical content in this presentation has been derived from publications and information publically available, in particular by:
• Steve Balch• Graeme Begg• Steve Beresford• Jon Hronsky• Alan King• Peter Lightfoot• Tony Naldrett• Sirius Resources