bc porphyry deposits: geological interpretation of ... · pdf filemount milligan bc porphyry...

Mount

Milligan

BC porphyry deposits: geological interpretation of geophysical data

Dianne Mitchinson Mira Geoscience

Exploration Undercover Workshop April, 2, 2012

KEG Conference 2012

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

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...

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..

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

Lorraine

Mount

Milligan

Mouse

Mountain

Gibraltar

Mount

Polley

Mount Milligan

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.

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

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.

Magnetic, DC resistivity, and IP inversions

X-section

location

Magnetic susceptiblity

Conductivity

Chargeability

Oldenburg et al. (1997)

Lorraine

Mount

Milligan

Mouse

Mountain

Gibraltar

Mount

Polley

Lorraine

Lorraine – local geology and alteration

Metal zoning

Potassic alteration

5 km

Sillitoe 2007 Teck rpt. 2008 Teck Cominco Ltd. assmt. rpt.

Lorraine magnetics and chargeability

Contribution to magnetic signature from alteration

Chargeability and resistivity

Lower Main

Main

Bishop

2Good

IP Line

SW NE

Lower Main

Main

Bishop

NE

SE

NW

Mount Polley

Lorraine

Mount

Milligan

Mouse

Mountain

Gibraltar

Mount

Polley

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

District scale magnetics – high mag igneous complex and volcanics

Shives et al. (2004)

Local high mag zones

Conductivity and chargeability

Lorraine

Mount

Milligan

Mouse

Mountain

Gibraltar

Mount

Polley

Mouse Mountain

Mouse Mountain local geology and alteration

Jonnes and Logan (2007)

Local scale magnetics and chargeability

Mouse Mountain resistivity and IP inversions

Susceptibility not related directly to mineralization

DIOR

DIOR DIOR

DIOR

DIOR

DIOR

DIOR

DIOR

Correlated resistivity and potassium (K) = intrusives?

Lorraine

Mount

Milligan

Mouse

Mountain

Gibraltar

Mount

Polley

Gibraltar

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)

Induced polarization response (related to chargeability)

IP response (% Frequency Effect) over Granite Lake and Pollyanna

Drummond et al. (1976)

West

IP/Resistivity in non-mineralized vs mineralized rocks

N S N S

Summary of ~5 km scale geophysical signatures (relative response)

?

dior

monz

range range

Stage 1

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?)