Distribution and Thickness of Volcanic and Glacial Cover on

the Interior Plateaus

Graham Andrews – GSC VancouverGraham.Andrews@NRCan-RNCan.gc.ca

Kelly Russell – UBC Vancouverkrussell@eos.ubc.ca

March 22nd 2010 – GAC Cordillera / TGI-3 workshop, Vancouver

Mineral Deposits in south-central BC

Cariboo

Mountains

Coast M

ountains

Nicola A

rc

Cariboo

Mou

ntain

s

Interior

Plateaus

adapted from Massey (2006)

•The Chilcotin Group (~22 – 0.1 Ma) covers >35,500 km2 of the Interior Plateaus.

Mineral Deposits in south-central BC

Cariboo

Mountains

Coast M

ountains

Nicola A

rc

Cariboo

Mou

ntain

s

Interior

Plateaus

adapted from Massey (2006)

•The Chilcotin Group (~22 – 0.1 Ma) covers >35,500 km2 of the Interior Plateaus.

•It obscures basement likely to host Cu-Au-Mo porphyry and epithermal Au deposits.

•The CG is wholly within the MPBB infestation zone.

•It is extensively covered by ?? m of drift.

Mineral Deposits in south-central BC

Cariboo

Mountains

Coast M

ountains

Nicola A

rc

Cariboo

Mou

ntain

s

Interior

Plateaus

adapted from Massey (2006)

•The Chilcotin Group (~22 – 0.1 Ma) covers >35,500 km2 of the Interior Plateaus.

•It obscures basement likely to host Cu-Au-Mo porphyry and epithermal Au deposits.

•The CG is wholly within the MPBB infestation zone.

•It is extensively covered by ?? m of drift.

It’s thickness and distribution are very poorly constrained.

Mineral Deposits in south-central BC

Cariboo

Mountains

Coast M

ountains

Nicola A

rc

Cariboo

Mou

ntain

s

adapted from Massey (2006)

•The Chilcotin Group (~22 – 0.1 Ma) covers >35,500 km2 of the Interior Plateaus.

•It obscures basement likely to host Cu-Au-Mo porphyry and epithermal Au deposits.

•The CG is wholly within the MPBB infestation zone.

•It is extensively covered by ?? m of drift.

It’s thickness and distribution are very poorly constrained.

STUDY

AREA

Regional Geology - MINFILE

WL

100

Regional Geology - MINFILE

Thuya???

???

???

???

???

Gibraltar

Mt Polley

Takomkane

Prosperity

Blackdome

WL

100

Chilcotin Group – typical exposure

sub-aqueous pillow delta

sub-aerial lavas

Andrews and Russell (2007)

Eocene rocks

e.g. Hanceville

Extensive sub-aerial / sub-aqueous successions along the tributaries of the Fraser River.

Complex horizontal stratigraphic transitions between sub-aerial and sub-aqueous lithofacies at the margins of paleovalleys.

subaerial lavas

subaqueous breccias

e.g. Chilcotin River / HWY 20

e.g. Chilcotin River / HWY 20

e.g. Chasm Provincial Park

Chilcotin Group – typical exposure

Farrell et al (2010) – GSC OF6230

e.g. Chasm Provincial Park

Common Wisdom?

Assumed to be an extensive and thick (100 – 200 m), homogeneous sheet.

This is the least favorable geological model for successful, low-risk exploration.

Basement

Chilcotin Group - lavas

what’s observed

what’s inferred

???

???

Andrews & Russell (2007)

till - overburden

bedded lacustrine sediments

~6.3 Ma columnar-

jointed lavasediment diapir

soft-sed deformation

evidence for river valleys

e.g. Nazko River Valley

≤3 m

4 m

hydro-volcani

c brecci

a

river gravels

lava Basalt-buried Miocene

channels are known locations of basal U deposits (e.g., Blizzard site, near Kelowna, BC).

Basalt-buried channels are known hosts for placer Au depositsmost Cariboo placers are

Miocene or Pliocene

evidence for river valleyse.g. Vedan Lake Valley

Gordee et al. (2007)

mapping valley-fill successions

e.g. Bull Canyon Prov. Park

valley margin

valley axis

~2 km

Gordee et al. (2007)

subaerial lava

characteristic orange

pillow-breccias

mapping valley-fill successions valley margin

e.g. Bull Canyon Prov. Park

exposed paleovalleys – 3 Ma Chilcotin River

e.g. Chilcotin River / HWY20

A valid geological model for the Chilcotin basalts must include:

Significant paleo-relief, probably similar to today;Thick sections in paleo-drainages, thin sections over paleo-highs;

Varied lithofacies – related to paleo-environment;Basement windows “poke” through the basaltic

cover.

a new facies model

subaerial lava

hydro-volcanic breccia

Andrews & Russell (2007)

implications: areal extent

Dohaney MSc (2009)

implications: areal extent

Dohaney MSc (2009)

implications: areal extent

Dohaney MSc (2009)

??

? ?

Up to 50% reduction in

mapped Chilcotin Group

extent

implications: areal extent

Dohaney et al (2010) – GSC OF6344 & OF6284

Andrews et al - CJES paper and GSC OF in 2010

• BC Prov. Govt. water-well log data

• CG locally up to 50 m thick around 100 Mile House and Vanderhoof

• typically <20 m elsewhere

• can also constrain drift thickness and bedrock type

implications: buried paleochannels

Vanderhoof

Andrews et al (subjudice)

100 Mile House

implications: buried paleochannels

Prince George

Andrews et al - CJES paper and GSC OF in 2010

summary

1. Extreme thickness variations (0 – 100 m) require many ‘basement windows’ and greatly reduced areal extent.

Andrews et al - CJES (subjudice)

1. Extreme thickness variations (0 – 100 m) require many ‘basement windows’ and greatly reduced areal extent.

2. Identifying paleo-drainages (esp. Pliocene / Pleistocene) reveals major drainage direction changes what are the implications for regional-scale detrital mineral / till and geochemical sampling? --- POTENTIALY A BIG PROBLEM!!!

summary

Thank you – questions?