Geophysics for Magmatic Ni‐Cu‐(PGE) Exploration Bill Peters · 27/10/2017 1 Geophysics for...

27/10/2017

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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


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

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Rejuvenated geophysical exploration for Ni‐Cu in Australia

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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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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


Electromagnetics (EM)

• Surface EM – Detailed, deep penetration

• Airborne EM – Rapid low cost coverage, but less penetration

• Downhole EM ‐ Very successful, Exploration to large depths,

Crone Geophysics

Crone Geophysics

Fugro Airborne

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• Sulphides: Pyrrhotite, chalcopyrite, pentlandite• Pyrrhotite ‐extremely conductive

• 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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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.


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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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


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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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


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


• Forward & inverse plate modelling‐ accurate. Curved surfaces modelling coming.

• Transforms (CDIs, RDIs) ‐ fast

• 1D‐2D Layered Inversions – several options

• 3D Inversions – slow, but improving

Modelling, Transforms, Inversion

AirBEO LEI

GA LEI

EMFlow CDI

TEMPEST AEM

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VpEM3D


Induced Polarisation

• Disseminated sulphides – Highly chargeable (need not be conductive)

• Higher power ‐> Increased depth penetration

• Untethered multiple sensors – new flexibility

• Complex 3D arrays, MT as an adjunct

• 2D &3D inversion with topography for virtually any array including

bore holes. Careful with interpretation of inversions. Use sensitivity

masking

• Time Series – QC, Noise filtering, Analysis, Telluric cancelation

• AMT Data from Time Series?

• High chargeability of disseminated magnetite can be a problem

High power IP transmitter (HPX)

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Zond 2D Inversion

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Magnetics

• Cheap and cost‐effective for mapping, regional targeting and detailed exploration

• Tensor systems – better resolution & modelling

• UAV acquisition – Many systems now

• Miniature Sensors coming – Matchbox size, two watts

• Downhole magnetics – 3D modelling, discrimination of sulphide vs graphite. Collect with DHTEM?

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• Ni‐Cu sulphides: Pyrrhotite –highly magnetic.

• Remanence issues. Hexagonal pyrrhotite –non‐magnetic

• Direct detection rare

• Host UM/M : usually magnetic but depends on metamorphic state (serpentinisation increases, talc alteration decreases)


Modelling, Inversion & Visualisation• Good forward modelling

software

• 3D inversion is now mainstream

• Magnetisation Vector Inversion (MVI) & others ‐ remanence

• Full tensor inversion

• Interpreting it is the important bit!

Geosoft – 3D Magnetic Inversion

Model Vision

Geosoft ‐ MVI

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Gravity

• Ni‐Cu sulphides: Dense @ 4.5 to 4.8g/cc, but direct detection ‐> requires large volume and shallow depth

• Project & Prospect Scale: Mapping favourable dense host rocks & internal structure

• 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


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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Magnetotellurics (MT)

• Natural magnetic and electric fields:• Large depth of exploration (>10kms)• Sensitive to weak resistivity contrasts

• Audio Magnetotellurics (AMT): • high audio frequencies only• shallower depths (0‐2000m)• Faster

• Controlled Source Audio Magnetotellurics (CSAMT): active transmitter

• EM is superior at shallow depths

• Regional scale: Deep sub‐surface structure

• Time Series – Extraction of AMT from other types of surveys?


Magnetotellurics (MT) ‐ ZTEM

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• ZTEM Airborne AMT(30 to 720 Hz) Vertical magnetic field only. Rapid coverage & deep penetration.

• No low frequency or electric field. No absolute conductivity information. Difficult to distinguish between contacts and confined conductors.

• Disko, Greenland: Tubular conductors from ZTEM 3D inversion ‐> deep Ni‐Cu sulphide targets (Noril’sk model)

Disko, Greenland– ZTEM (Blue Jay Mining Plc)

High Power EM survey currently in progress (SGC)


Ni‐Cu‐PGE Deposit Types – Geology and Geophysics

1. Komatiites – Yilgarn, WA

2. Impact Crater ‐ Sudbury

3. Flood basalts – Noril’sk

4. Intrusive – Voisey’s Bay, Thompson, Nova –Bollinger

5. Bushveld style – Bushveld Igneous Complex


Yilgarn (Western Australia) ‐ Komatiite Style Deposits

Yilgarn nickel deposits shown in green

• Along a 600km belt. Paleo‐craton margin?

• Within channels at base of ultramafic flows, no significant copper

• Surface EM Discoveries: Cosmos, Waterloo, Amorac, Sinclair, Emily Ann, Maggie Hays North

• DHEM: Deeper discoveries ‐ Flying Fox, Cosmos Deeps

• 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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Emily Ann (Western Australia) ‐Komatiite Style Geophysical

Discovery • Typical komatiite deposit discovered with Surface In‐Loop EM

• Depth ‐ 200m. Shallow dip, resistive felsic host.

• Structurally remobilized several hundred metres from ultramafic contact.

• Magnetic anomalies from ultramaficsand BIF. No anomaly over mineralisation.

• Most EM responses ‐> sulphidicsediments

Peters et al. 2000

Discovery In‐Loop EM profile and CDI section

Deposit (red) over Magnetics

Cross Section


Camelwood (Mt Fisher East) (Western Australia) ‐ Komatiite Style Deposit • Airborne EM Discovery

from a VTEM survey in 2011 designed primarily for gold exploration.

• Surface and down hole EM surveys lead to discovery of the additional deposits Musket and Cannonball

• No significant magnetic anomaly over deposits

Deposits (red) over Aeromagnetics

Huizi et al. 2013

Camelwood

Cannonball

Musket

Interpreted Geology

Rox Resources Ltd

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Rox Resources Ltd


Camelwood (Mt Fisher East): EM Geophysics

• The VTEM shows a very clear diagnostic twin peaked anomaly indicating an east dip

• Fixed Loop and In‐Loop data are easily modelled and used for accurate drillhole targeting

• Borehole EM very effective for deeper delineation

• Most other conductors are barren sulphides

Huizi et al. 2013

VTEM Profile

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VTEM Late Time B Field Image


Sudbury (Canada) ‐ Impact Style Deposit

• Sudbury Intrusive Complex (SIC): Meteor impact structure

• Southeast margin of Superior Craton.

• Large structure: 60kms x 30km by 15kms deep

• Mined since late 1800’s

• One of the world’s largest deposits (1.6Bt @ 1.2% Ni & 1.0% Cu )

• Multiple deposits along the norite /gabbro footwall contact

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• A vast amount of geophysical data has been collected over seventy years

• It has been a testing ground and inspiration for much geophysical development since the 1940’s.

Bouger Gravity

Total Magnetic Intensity

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• Basin architecture has been derived from seismics and 3D inversion and 2D modelling of gravity and magnetic data

•• Magnetics defines basin well and shows layering & internal structure

• Gravity shows deeper information, particularly on southern margin

Sudbury – Magnetics and Gravity

Allan King, 2007

1947 EM system


Sudbury: EM & IP • Surface EM: Diminishing now because of near

complete coverage and increasing depths involved

• DHEM: Now most dominant method in holes to depths of up to 3000m.

• Modelling evolving from simple plate modelling to curved facets and 3D voxel inversion.

• Conductor discrimination is on the basis of conductance

• Cross‐hole radar imaging (RIM): Signal loss in conductive sulphides

DHEM to 2300m showing well defined anomalies

Lamontagne 2007 Cross‐hole Radar Imaging

A. King 2007

Conventional Plate DHEM Modelling DHEM Modelling with curved facets

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Sudbury: Reflection Seismics• Reflection seismics commenced in the 1990’s

• Detected footwall contact and some internal layering

• Detected massive sulphides at 1800m via scattering response?

Seismic level slice showing sulphide signature

Mapping of footwall contact

Footwall contact and mineralisation responseMilkereit et al

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• Requires high frequencies, large offsets, high stacking/ fold

• Used with magnetics, and gravity to build 3D basin architecture


Noril’sk‐Talnakh (Russia): Flood Basalt Style Deposits• Known since the

1600’s

• NW margin of Siberian Craton.

• World’s largest Ni‐Cu deposits.

• 1.3Gt @ 1.8% Ni & 3.6% Cu

• Mineralisation is at base of flat‐lying lava conduits (dolerite‐gabbro sills).

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Noril’sk: Geophysics • Information (English) sparse

• Magnetic, gravity and seismic surveys for deep structure and signatures for origin of mineralisation

• Filtering of overlying volcanic sill responses to show deeper magnetic bodies ‐> intermediate magmatic chambers?

• Modern large loop EM, AMT & IP are used. But overlying conductive sediments are a problem.

• DHEM: To depths in excess of 1800m

Filtered TMI showing postulatedmagma chamber at depthTMI showing mainly host volcanics

Rempel 1994Rempel 1994


Voisey’s Bay (Canada) ‐ Intrusive Style Deposit• Labrador ‐ unexpected discovery

in outcrop 1994.

• New province ‐ large significant deposit.

• 140 Mt @ 1.6% Ni

• Hosted by troctolite dike(s) thought to be feeders to intrusion

• Extensive geophysical surveys

• Good publications by Steve Balch & Alan King

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Voisey’s Bay ‐ EM , IP, AMT

Ovoid Traverse

UTEM

GEOTEM

HEM

• Extreme conductance of massive sulphides diminishes the response of conventional dBdt EM

• Associated semi‐massive and disseminated sulphides are detected by dB/dT EM, IP, AMT

Balch et al, 1998

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• MaxMin HLEM traced the mineralisation east under cover and lead to the discovery of the Ovoid Zone

• Airborne EM, Surface EM and DHEM.

• Mineralisation seen to 1000m Original MaxMin HLEM


• Ground and airborne magnetic surveys.

• No strong correlation of magnetics with mineralisation

• Sulphides are mainly non‐magnetichexagonal pyrrhotite

• Host troctolites & gabbro only weakly magnetic – magnetic lows

• Most magnetic responses due to gneisses Balch 1998

Voisey’s Bay – Total Magnetic Intensity

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Voisey’s Bay ‐ Magnetics

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• Ground and airborne gravity

• Ovoid Deposit & main mafic intrusives show up well

• The gravity response is dominated by host troctolites

• Ovoid: Four mgalanomaly. 3D inversion works well

UBC‐GIF

UBC‐GIF

A. King 2007

Ovoid Deposit Gravity

Ovoid Deposit 3D Inversion

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Ovoid

Voisey’s Bay ‐ Gravity


Thompson (Canada) ‐ Intrusive Style• Discovered in 1956

• Margin of Trans‐Hudson Orogen, and Archaean Superior province in Manitoba

• 150 Mt @ 2.32% Ni

• Multiple deposits along 150km of strike length

• Host: Peridote lenses within sulphidic metapelites

• Virtually no outcrop ‐> Exploration is largely driven by geophysics.

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Thompson ‐ EM & AMT

• Discovered with very early airborne EM as a combined EM and magnetic anomaly due to massive pyrrhotite content

• Exploration at depth is now based on AMT, FLEM, DHEM

• AMT 2D and 3D inversions used to map conductive metapelitestratigraphy in 3D to at least 2000m depth.

• Favourable targets from the AMT are correlated with magnetics and geology to prioritize

• At “shallow depths” (<700m) targets are tested with large loop UTEM

• Deeper targets are tested with DHEM

J. Dowsett, 1970

AMT 2D Sections & 3D Magnetic Inversion

Discovery AEM & Magnetic Profiles

A. King 2007


• Reflection seismic combined with AMT is used to map geology in 3D.

• Cross‐hole seismic has been used to map low velocity sulphides between boreholes.

Cross‐hole seismic low velocity zones coinciding with sulphide intersections shown as warm colours.

A. King 2007

A. King 2007

White et al. 2000

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Combined Seismic – AMT section

Thompson ‐ Seismics

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Nova‐Bollinger (Western Australia) ‐ Intrusive Style Deposit

• 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


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


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


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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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


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


Bushveld Igneous Complex: Mineralisation & Geophysics

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Geoff Campbell 2011

• 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.


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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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


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


Regional Targeting & Geophysics• Key regional geophysical methods: magnetics, gravity, seismic, and MT

• Magnetics/ gravity ‐> Key data sets & commonly available

• Seismic (Active & Passive) / MT data ‐> rarer, costly and usually sparse

• At very large scale (A): Locate major cratonic margins and intra‐cratonic sutures

• At intermediate Scale (B): At mid‐crustal level, find evidence for magma chambers and feeder systems

• At local Scale (C): Near surface – Find plug, dyke, or sill intrusions into dilational settings

Hronsky 2007

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(B) Magma Chamber

(A) Mantle plume/ suture

(C) Magma discharge (intrusion)


Fraser Range: A Regional Targeting Success

• Following the Voisey’s Bay discovery (1996), Prospector Mark Creasy decided to look for similar settings in WA.

• A study identified Fraser Range as a major crustal suture/margin, with prominent gravity anomalies & known ultramafic rocks

• 1996: Gravity and magnetic data were compiled, followed by a GEOTEM AEM survey 30km SW of Nova

• No significant Ni‐Cu sulphides found from the AEM but many graphite conductors

• The key tenements were joint ventured to Sirius Resources who discovered Nova‐Bollinger in 2012

Kirkland et al. 2011

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Fraser Range Belt ‐ Yilgarn Craton margin

100 km

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Sirius Resources and GSWA

Regional Government Magnetics Regional Government Gravity

Sirius Targeting

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100 km 100 km

Raglan

Thompson

• Sirius saw similarities to the Canadian Thompson & Raglan settings.

• Major suture/ craton boundary – seen in both magnetic and gravity data

• Sirius selected specific targets from government geochemical & aeromagnetic data


Intrusion targets can be interpreted from magnetic data

In this case they are non‐magnetic zones within surrounding magnetic gneissic rocks

Favourable dilational settings – see fault jog in right hand image

Identifying Intrusions from Magnetics

Mt Ridley Mines LtdSirius Resources

TMI image ‐ Nova “Eye” target TMI image ‐ Intrusive target MT‐02

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• Southeast of Nova

• Subtle diffuse gravity anomaly indicating a deep dense source

• Poor correlation with shallower magnetic rocks

• Possible deep magma chamber?

Magma Chamber?

Nova‐Bollinger Nova‐Bollinger

Deep magma chamber?Deep magma chamber?

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Gravity Magnetics

25 km 25 km


• GA regional seismics is interpreted to show mainly east dipping structures

• The east dips are consistent with the “Magma Chamber” being a potential source for Nova?

• Regional MT ‐> not so clear but looks convincing to 30 km depth or so

Seismics & MT ‐ Fraser Range GSWA‐GA Seismic Traverse across Fraser Range

GSWA‐GA MT Traverse across Fraser Range

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<‐ 50 km ‐>

<‐ 50 km ‐>

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Conclusions ‐ 1• Geophysics is ideal for Ni‐Cu sulphides – Very successful & dominates discoveries

• Use rock property measurements for planning, modelling ‐ Some unexpected scenarios.

• Instrumentation: Faster, multiple independent sensors, complex arrays, data storage ‐ > 3D, increased spatial density, better logistics.

• Time series recording ‐> advanced signal processing

• Software: Better & faster inversion (2D, 3D, joint), time series analysis, handling magnetic remanence, 3D visualisation & integration with geology

• Increased power ‐ EM: Increased depth penetration & sensitivity, but some issues (SPM, Polarisation, Saturation)


Conclusions ‐ 2• EM is dominant & the first choice.

• Ground EM is superior to AEM

• Magnetics & gravity fundamental for mapping & structure, but rarely useful for direct detection.

• Seismic & MT at prospect scale: Increasing use for deeper depths, especially for lithological/ structural mapping. Maybe direct detection.

• Borehole magnetics is underutilised. Include with DHTEM surveys?

• Time series recording: Can we extract EM, IP, AMT, TMI all from one measurement?

• Regional targeting: Magnetics, gravity, seismic, MT: To identify lithospheric margins, staging chambers, favourable near‐surface structure

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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.


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

Geophysics for Magmatic Ni‐Cu‐(PGE) Exploration Bill Peters · 27/10/2017 1 Geophysics for...

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Transcript of Geophysics for Magmatic Ni‐Cu‐(PGE) Exploration Bill Peters · 27/10/2017 1 Geophysics for...