Geoelectric Structure of the Western Superior Province

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Electrical and magnetic properties of the Duport
gold deposit, western Ontario, Canada
I. J. Ferguson, A. Krakowka, B. Cook, and J.
Young
University of Manitoba, Manitoba, Canada
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1. INTRODUCTION
Deposit location Cameron Island, Shoal Lake,
Northwest Ontario
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Deposit history

• Deposit discovered in 1896 and mined for gold
  several times
• Drilling indicates along-strike extent of gt1000 m
  and depth gt500 m
• Estimated reserves 1.8 Mt (proven to inferred)
  grading 12 g/t Au
• In-depth geological study by P.M. Smith (1987)
• 2005 airborne geophysical survey by Halo
  Resources

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Objectives of this study

• Define small-scale ground magnetic and ground EM
  responses on Cameron Island
• Use ground responses to relate airborne
  geophysics to smaller-scale geological features

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2. GEOLOGICAL BACKGROUND
? Western Wabigoon spr. ? Lake of the Woods
Greenstone Belt
Modified from Percival (2000)
Modified from Ayer et al. (1991)
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Snowshoe Lake Batholith
Stevens Island Diorite
Modified from Smith (1986)
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Gold emplacement

• Mineralization was syn- to late-tectonic.
• Prograde amphibolite facies metamorphism in
  aureole of Snowshoe Lake batholith and subsequent
  retograde metamorphism caused by large volumes of
  high temperature fluid enriched in CO2 and H2O in
  the deformation zone.
• Gold mineralization associated with sulphidation,
  silicification, biotization, and carbonatization.
  
• Precipitation of gold from bisulphide complexes
  was possibly related to iron content in host
  rocks.

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3. GEOPHYSICAL SURVEYS
(a) Airborne geophysical survey
1. DIGHEM 2743 km 2. Azimuth 123o 3.
Line-spacing 50 m 4. Sampling 10 Hz (3.3 m) 5.
Sensor clearance 30 m
Modified from Garrie (2005)
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(b) Ground geophysical surveys
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(c) Core susceptibility measurements
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4. MODELLING AND INTEGRATION
(a) Modelling magnetic responses
? Blocks 1,2 induced-dominant (k0.1, 0.7
SI) ? Blocks 3,5 remanent-dominant (J23, 7
A/m, reversed) ? Blocks 4,6 either
Modelling using POTENT
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EM31 Modelling
Modelling using EMIGMA
1. Conductive responses dominantly in
quadrature 2. Magnetic responses dominantly in
in-phase 3. HCP, VCP magnetic in-phase responses
have opp. sign 4. HCP magnetic in-phase response
positive (zltltr)
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(b) Magnetic signature in southeast of Cameron I.
? Surface samples k0.065 SI ? Magnetic
modelling Modelling 0.1 SI ? EM31 in-phase
anomalies HCP ve, VCP ve, k0.3 SI ?
HEM 900 Hz in-phase HCP ve k0.04 SI
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(c) Magnetic signature in northwest of Cameron I.
? Susceptibility of core samples Schistose
basalt k0.2 SI Brecc. basalt klt0.05-0.1 SI ?
Magnetic modelling Schistose basalt
k0.7 Brecc. basalt J10 A/m ? EM
anomalies Negligible EM in-phase
anomalies
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(d) Electrical conductivity
? EM31 Quadrature s20-100 mS.m-1
In-phase HCP ve, VCP ve s 300-400 mS.m-1 2.
TEM For 100x20m plate t0.11 ms?400
mS.m-1 3. HEM Integrated response
All frequencies In-phase and
quadrature
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5. GEOLOGICAL INTERPRETATION
Mafic intrusive rock (Stevens Island diorite)
? Induction-dominant magnetization due to
magnetite ? Petrographic analysis up to 5 mt on
grain boundaries and disseminated in
clinopyroxene pseudomorphs
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Schistose basalt
? Induction-dominant magnetization due to
magnetite ? Petrographic analysis up to 5-15
mt, typically fine- grained, and evenly
distributed
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Brecciated and sulphidized basalt
? Remanent-dominant magnetization due to
pyrrhotite ? Petrographic analysis lt3 mt, up to
10 sulphides
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Magnetic susceptibility
Geophysical responses
Petrographic analyses
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Koenigsberger ratio (qualitative)
Modified from Clark Emerson (1991)
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Electrical conductivity
Geophysical responses
Petrographic analyses
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6. GEOPHYSICAL MODEL (CONCLUSIONS)
Regional controls on mineralization
Airborne magnetics ? Location of Snowshoe Lake
batholith ? Zones of enhanced (secondary?)
magnetite ? Defines deformation zones including
Duport Def. Zone ? With filtering identifies
some narrower geological units
Airborne EM ? Location and integrated
conductance of zones containing significant
sulphidization ? Broader zones of induced
magnetization
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Local controls on mineralization Determination
of physical properties Ground
magnetics ? Location and width of lithological
units and alteration facies ? Discrimination of
remanent and induced magnetization and
estimates of k and J Ground EM (EM31
and TEM) ? Location, width, and conductivity of
sulphidized zones ? Relationship between
conductive and magnetic zones ? Integrated
conductance of these zones (TEM) ? Zones of
induced magnetization, estimates of k Core
Susceptibilty ? Relationship of alteration
facies and lithology ? Definitive estimates of
k ? Link between geology and geophysical
responses
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ACKNOWLEDGMENTS
? HALO RESOURCES ? Manitoba Geological Survey ?
Petros Eikon and Geophysical Software Solutions ?
KEGS Foundation