Geophysics in the Quesnel Belt, BC (Geoscience BC)

Geophysics 101 ½

Garth Kirkham, B.Sc., P.Geoph., P.Geol.Geoscience BC

Vancouver, British ColumbiaSergio Espinosa, Ph.D.

Geophysics, GIS, and DatabasesTerrane Metals CorporationVancouver, British Columbia

© Sergio Espinosa, Terrane Metal Corporation

(3)Source of Energy

X

(4)Measurement of the

Geophysical Response

Surfaceof the Earth

(1) Covered Ore Body with- lithological characteristics, - alteration, and - mineralization.

CoverLithology

Bed and Host Rock Lithology

(2) Body with following geophysical characteristics:

Z: Depth (geometrical value)

V: Volume (geometrical value)

P.P.: Rock Physical Parameter (expressing lithology, alteration, and mineralization)

Conceptual Geological Body

Source of Energy

• Natural

… Techniques

• Man-made

Passive …

Active …

Magnetics

Radiometrics

Electro-Magnetics: Magneto-Tellurics

Seismic

Radiometrics

Electro-Magnetics: FDEM (Dighem, Csamt), TDEM (Protem, Geotem, Vtem)

Gravity

Resistivity / IP

GR = ----------------------------Volume * RPP

Depth (1…2…3)

Measured Geophysical Response(anomaly)

Rock Physical Property

Geometrical Parameter


Geophysical Parameter Relation

TechniqueMass Density GravityMagnetic Susceptibility MagneticsResistivity IPChargeability IPConductivity ElectromagneticsElastic Wave Velocity SeismicElastic Wave Attenuation Seismic Radioactive Isotope Content Radiometrics

Geophysical Techniques

Physical UnitMass Density (kg/m3)

Magnetic Susceptibility (adim)

Resistivity (Ωm)

Chargeability (mV/V)

Conductivity (S/m)

Elastic Wave Velocity (km/s)

Elastic Wave Attenuation (db/m)

Radioactive Isotope Content (%, ppm)

Rock Physical Properties

Geophysical ParameterGravity Earth Gravity Field Magnetics Earth Magnetic InductionInduced Polarization (IP) Ground Voltage Electromagnetics (EM) Induced Field Seismics Ground Particle Velocity Radiometrics Radio-Activity


Physical Unit

Earth Gravity Field (mGal)

Earth Magnetic Induction (nT)

Ground Voltage (volts)

Electromagnetic Induced Field (nT/s)

Ground Particle Velocity (m/s)

Radio-Activity (Curie, CPS)


InstrumentationEarth Gravity Field Gravity-MeterEarth Magnetic Induction MagnetometerGround Voltage Electrodes / VoltmeterElectromagnetic Induced Field EM Receiver Ground Particle Velocity SeismometerRadio-Activity Spectrometer

Geophysical Instrumentation

•Depends on what we are looking for?•Purpose – Regional vs. Property vs. Deposit Scale•Cost $$$•Time and Logistical Considerations

Advantages and Disadvantages

GR = --------------------------Volume * RPP

Depth (1…2…3)


Concept of Gradients

δ/δSδ/δS

Gradients are spatial derivatives ( δ/δS)

Horizontal, vertical, and total derivatives.

Gradient depends on body geometry !!!

Block of Airborne Geophysical VTEM Survey in the Quesnel Terrane

Gravity on Magnetics

Canadian Aeromagnetic Data Base(Natural Resources Canada)

(Belt Scale)


Gravity on Magnetics

Canadian Aeromagnetic Data Base(Natural Resources Canada)

(Belt Scale)


Outline of Quesnel Terrane

Geology of the Quesnel Terrane

Lithology

B.C. Geological Survey(www.mapplace.ca)

(Belt Scale)

Intrusives

Metamorphic Rocks

Sediments

Volcanics

Porphyry Occurrences in the Quesnel Terrane

Gravity on Magnetics(Belt Scale)Canadian Aeromagnetic Data Base(Natural Resources Canada)

Mt Milligan

Chuchi

SwanTakla-Rainbow

Tam

Lorraine

Rondah

Misty

QR

Mt PolleyGibraltar

Boss Mountain

Airborne Geophysical VTEM Survey

in the Quesnel Terrane

Geoscience BC – Quest Project(www.geosciencebc.com)

Data

• EW flight lines; • 4 km line spacing; • gridding with 800 m cell size.

Results

* Very good spatial resolution along profiles or EW flight lines (X);

* Very good depth penetration (Z) and spatial resolution along the EM sounding response;

* Just be careful when extrapolating across flight lines (Y).

x

y

?

Digital Elevation Model




Total Magnetic Field Intensity




Airborne Geophysical VTEM Survey in the Quesnel Terrane


Answer

New geophysical information along profiles.

Question

So, what is the advantage of flying VTEM?

EM responses are associated with conductivity distributions in the sub-surface.

Those conductivity (or resistivity) distributions might be associated with mineralization in the sub-surface or with other geological sources, such as the sedimentary covers.



Amplitude of 682ms channel


Remarks

• A survey with 4km line spacing means gridding with 800m cell size.

• So, by gridding, resolution along profiles is lost, but a regional overview of that

data can be achieved.




Log Amplitude of 682ms channel

Remarks

• A survey with 4km line spacing means gridding with 800m cell size.

• So, by gridding, resolution along profiles is lost, but a regional overview of that data

can be achieved.

Airgrav

Bouguer GravityPreliminary Image

VTEM Response of Mt Milligan


Mt Milligan

spatial windowon a property scale

Mt Milligan Deposit

Mt Milligan Claims

Spatial Window on a Property Scale

Calc-alkaline Volcanics(Upper Triassic)

Sedimentary Rocks

(Triassic Limestones)

Sedimentary Rocks (Triassic Limestones)

Volcano-Sedimentary

Rocks(Triassic)

Monzonite(Early Jurassic)

Paragneiss(Upper Cretaceous-Miocene)

Paragneiss(Upper Cretaceous-

Miocene)

Sedimentary Rocks(Eocene-

Oligocene)

Diorite(Early Jurassic)

Diorite(Early Jurassic)

Granite(Cretaceous)

Volcano-Sedimentary Rocks

(Triassic)

In the Background: Regional Magnetics (source: GSC) with 275m grid cell size

mapping sediments

strong conductor in the metamorphic

very strong conductor

conductor

EM Response

#1315:89

35 km

EM anomaly

mag anomaly in the sediments

conductor with mag anomaly

Magnetic Signal

Mt Milligan orientation: VTEM inversion example

1) Zoom SRTM Topography and Geology with MINFILE Cu Occurrences

2) SRTM Topography with 10x ExaggerationZoom with MINEFILE Cu Occurrences

3) SRTM Topography and GeologyZoom Bouguer Gravity & MINEFILE Cu Occurrences

NEXT CHALLENGES

ProcessingIntegration InterpretationData MiningIdentifying GapsCollecting more geoscience information to fill these gaps

ITERATIVE PROCESS

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Geophysics in the Quesnel Belt, BC (Geoscience BC)