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Mount
Milligan
BC porphyry deposits: geological interpretation of geophysical data
Dianne Mitchinson Mira Geoscience
Exploration Undercover Workshop April, 2, 2012
KEG Conference 2012
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Outline
• Relating geophysics to geology, and importance of understanding rock properties
• Case studies:
– Mount Milligan
– Lorraine
– Mount Polley
– Mouse Mountain
– Gibraltar
• Comparison of signatures
• Summary
Lorraine
Mount
Milligan
Mouse
Mountain
Gibraltar
Mount
Polley
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Relating geology to geophysics through physical properties
Understanding rock properties will improve our interpretations of geophysics and geophysical models
Magnetic susceptibility model from magnetics inversion
Density model from gravity inversion
X-section
location
Variations in apparently homogeneous rock?
High mag
sus, low
density
Low mag
sus, high
density
Not only that ...geophysical method selection, geophysical survey design, forward modelling, constraining inversions...
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Mafic
minerals Felsic
minerals
Carbonates
Mineralization
Mag
Pyrite
Hem
Pyrr
DENSITY
MA
GN
ET
IC S
US
CE
PT
IBIL
ITY
Modified from Williams (2008)
Relating geology to geophysics through physical properties
• Develop expectations through: – Geological setting
– Deposit model
– Geological processes..
• Think about: – Minerals formed
– Mineral distribution
– Rock textures/porosity..
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Thinking geology while interrogating geophysics
–Mount Milligan
–Lorraine
–Mount Polley
–Mouse Mountain
–Gibraltar
Lorraine
Mount
Milligan
Mouse
Mountain
Gibraltar
Mount
Polley
Case studies useful for
helping to interpret
geophysics data and models
in analogous areas, and to
plan follow-up exploration
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Lorraine
Mount
Milligan
Mouse
Mountain
Gibraltar
Mount
Polley
Mount Milligan
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Mount Milligan local geology
Mount Milligan (Cu-Au) deposit
• Cu-Au porphyry deposit
• Monzonite stock hosted within andesites to
basaltic andesites, and related volcanic
sedimentary units (tuffs, breccias, conglomerates)
• Tilted, and faulted stratigraphy
• Mineralization spatially associated with the
monzonite stock, and hydrothermal breccias
From Jago (2008)
Terrane Metals Corp.
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Magnetics – different features at different scales
800 m line spacing
200 m line spacing
50 m line spacing Survey 1.2 x 1 km
Survey 2.6 x 2.6 km
This map:
~20 km x 15 km
Southern Star
Geotech Ltd. 2008,
airborne magnetics, for GBC
NRCan magnetics
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Physical property variations related to alteration zoning
From Jago (2008)
Resistivity Density
Na-alt’d basalt
Prop (Chl+Ep+Cb) basalt
K-alt’d basalt
K–alt’d monzonite
Na–alt’d monzonite
Magnetic
Susceptibility
Na-alt’d basalt
K-alt’d basalt
K–alt’d monzonite Na–alt’d monzonite
Prop (Chl+Ep+Cb) basalt
Prop o/p K basalt
Na–alt’d
monzonite
Na-alt’d basalt
K-alt’d basalt
K–alt’d monzonite
Prop (Chl+Ep+Cb) basalt
Prop o/p K basalt
Potassic Potassic
Bas.
Monz.
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Magnetic, DC resistivity, and IP inversions
X-section
location
Magnetic susceptiblity
Conductivity
Chargeability
Oldenburg et al. (1997)
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Lorraine
Mount
Milligan
Mouse
Mountain
Gibraltar
Mount
Polley
Lorraine
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Lorraine – local geology and alteration
Metal zoning
Potassic alteration
5 km
Sillitoe 2007 Teck rpt. 2008 Teck Cominco Ltd. assmt. rpt.
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Lorraine magnetics and chargeability
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Contribution to magnetic signature from alteration
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Chargeability and resistivity
Lower Main
Main
Bishop
2Good
IP Line
SW NE
Lower Main
Main
Bishop
NE
SE
NW
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Mount Polley
Lorraine
Mount
Milligan
Mouse
Mountain
Gibraltar
Mount
Polley
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Mount Polley local geology and alteration
Map modified after Logan and Mihalynuk, 2005
From Jackson (2008), redrafted
after Fraser et al. (1995)
C/B
NE
SE
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District scale magnetics – high mag igneous complex and volcanics
Shives et al. (2004)
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Local high mag zones
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Conductivity and chargeability
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Lorraine
Mount
Milligan
Mouse
Mountain
Gibraltar
Mount
Polley
Mouse Mountain
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Mouse Mountain local geology and alteration
Jonnes and Logan (2007)
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Local scale magnetics and chargeability
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Mouse Mountain resistivity and IP inversions
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Susceptibility not related directly to mineralization
DIOR
DIOR DIOR
DIOR
DIOR
DIOR
DIOR
DIOR
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Correlated resistivity and potassium (K) = intrusives?
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Lorraine
Mount
Milligan
Mouse
Mountain
Gibraltar
Mount
Polley
Gibraltar
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Gibraltar local geology
• Hosted in the Mine Phase
Tonalite of the Granite Lake
Batholith
• Other mineralized zones in Mine
Phase and more mafic Border
Phase Diorite
• Formed synchronously with
deformation
• Cu-Mo mineralization hosted
within shear zone stockworks
• Mineralization-proximal
alteration is characterized by
chlorite-sericite-quartz rich
assemblages
After Ash et al. (2004)
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Induced polarization response (related to chargeability)
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IP response (% Frequency Effect) over Granite Lake and Pollyanna
Drummond et al. (1976)
West
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IP/Resistivity in non-mineralized vs mineralized rocks
N S N S
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Summary of ~5 km scale geophysical signatures (relative response)
?
dior
monz
range range
Stage 1
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Summary
• For alkalic porphyries, magnetic (oxidized) rocks are interesting, but magnetite rich intrusives/alteration not as common in calc-alkalic settings
• Later phyllic alteration can destroy primary igneous and secondary hydrothermal magnetite in host rocks in both alkalic and calc-alkalic settings
• Conductivity information can be very useful for detecting structures; Intrusive bodies are usually resistive as a result of coherency
• IP surveys are very effective in detecting peripheral pyrite halos
• Scale important! At 5 km scale, magnetic and resistivity signatures likely relate to rock type and structure, not mineralization; IP most effective
• There is no unifying geophysical model for porphyries - need to understand typical background rocks and alteration, general structural orientations – important for applying outside of BC!
– Host rocks (magnetic/non; brecciated/coherent)
– Depth of formation/erosion (related to alteration mineralogy/halos)
– Metal distribution (connected, disseminated? Zoning?)