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Distribution and Thickness of Volcanic and Glacial Cover on
the Interior Plateaus
Graham Andrews – GSC [email protected]
Kelly Russell – UBC [email protected]
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?