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