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Graham Andrews – UBCco-workers: Kelly Russell, Jackie Dohaney, Sarah Brown, Sarah Caven, Randy Enkin, Bob Anderson
May 8th 2009
Mineral Deposits in south-central BC
adapted from Massey (2006)
•The Chilcotin Group (~22 – 0.1 Ma) covers >35,500 km2 of the Interior Plateaux.
Mineral Deposits in south-central BC
adapted from Massey (2006)
•The Chilcotin Group (~22 – 0.1 Ma) covers >35,500 km2 of the Interior Plateaux.
•It obscures basement likely to host epithermal Au and Cu-Au-Mo porphyry deposits.
•The CG is wholly within the MPBB infestation zone.
•It is extensively covered by ?? m of drift.
Mineral Deposits in south-central BC
adapted from Massey (2006)
•The Chilcotin Group (~22 – 0.1 Ma) covers >35,500 km2 of the Interior Plateaux.
•It obscures basement likely to host epithermal Au and Cu-Au-Mo porphyry 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
adapted from Massey (2006)
•The Chilcotin Group (~22 – 0.1 Ma) covers >35,500 km2 of the Interior Plateaux.
•It obscures basement likely to host epithermal Au and Cu-Au-Mo porphyry 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.
Andrews et al. (in prep)
Cardtable Mtn. – 6400 ft
Battleship Peak
Chilcotin Ranges – Coast Mountains
Mesozoic meta-sedimentary succession
~18 Ma paleo-
valley fill
≥18 Ma sub-horizontal unconformity – now 5°
NE-dipping
Relay Mtn.
6000 ft
5800 ft
L. Eocene-Oligocene unconformity
CG - physical volcanology
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.
massive interior
columnar-jointed lava
vesicular top
1. sub-aerial lavas
0.5 m
sub-aerial lavas are typically:
• columnar-jointed,
• coherent,
• laterally restricted <100 km2,
• highly vesicular,
• single lavas <20 m thick,
• sequences <60 m thick,
• often emplaced onto / into wet sedimentbasal
breccia
3 m
basal breccia
massive interior
e.g. Dog Creek
2. sub-aqueous successionssub-aqueous successions are:
• exposed in Present-day drainages
• always restricted to paleo-valleys (e.g. Mio-Plio Chilcotin Valley),
• typically associated with capping sub-aerial lavas,
• ≤80 m thick,
• typically repetitive sequences:
• repeated pillow-breccia peperite sub-aerial lava,
• indicates gradual damming, infilling, and breaching of valleys / lakes with short-lived highstands (sub-aerial lavas).
Gordee et al. (2007)
SW NE
sub-aqueous successions
basement
sub-aerial lavas
pillow brecciasbasement
basement
peperites
e.g. 6 Ma Chilcotin River Valley
Gordee et al. (2007)
sub-aqueous successions
cross-bedded sediment trapped within peperite
transport direction
Gordee et al. (2007)
sub-aqueous successionssub-aerial lava
hyaloclastite pillow-breccias
e.g. Chasm Prov. Park
paleovalleyscharacterized by:
• coincidence of sub-aqueous and sub-aerial lithofacies,
• areally-restricted (<100 km2),
• complex vertical and along-valley stratigraphy,
• ≤150 m thick, 100 – 2000 m wide,
• rapidly filled (10s – 100s years) multiple, single eruptions.
They are typically sub-parallel to modern river valleys (e.g. the Chilcotin, Nazko, Fraser, and Taseko Rivers).
Many seem to follow Eocene paleo-valleys (e.g., Selina Tribe)
paleovalley environment - model
e.g. Dog Creek
“perched” lavas
valley-fill lithofaciesassociation
tributary-filling lava – “escaped”
duration of valley-filling eruptions
clustered, non-polar paleomagnetic mean directions
rapid (<< 10 kyr), un-interrupted in-filling single, short-duration, proximal eruptions
• probably mono-genetic many vents, cones, in a single field
Enkin et al. (Round-Up 2008)
Dog Creek – 3.1 Ma
Bull Canyon – 6.2 Ma
Chilcotin Group sources
Nazko area
Wells-Grey VF
inferred Chilcotin
Group sources
Holocene volcanic field
there are probably 100s of ‘missing’ vents – 1
vent for each lava
10
0
• BC Prov. Govt. water-well log data
• CG locally up to 50 m thick around 100 Mile House (092P) and Miocene (093A)
• typically <20 m across 092P
• elsewhere, limited data suggests CG is <20 m thick
• can also constrain drift thickness and bedrock type Andrews & Russell (2008)
Chilcotin Group – buried channels
100
50
40
Implications for exploration
1. Extreme thickness variations (0 – 100 m) require many ‘basement windows’ and greatly reduced areal extent.
Thuyabatholith
Several deposits identified within windows or adjacent to CG margins: How are mineral trends /
geochemical anomalies traced below the basalt?
How are geophysical signals modified?
How many similar deposits lie hidden under thin basalts elsewhere?
Windows can be delineated by careful mapping, supported by regional- and property-scale geophysical surveys.
Basement Prospects – Windows
ARIS Report 25740 - Bonaparte
Mb – Chilcotin basalt
Mb – Chilcotin basalt
1 km
Implications for exploration
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!!!
Thuyabatholith
Mio-Pliocene Fraser River Basin
Stage 3approx. present watershed
3 Ma
7 Ma
9 Ma
11 Ma
10 Ma
inferred paleo-channel flow direction
10 Ma age constraint
8 Ma6 Ma
6 Ma
6 Ma
9 Ma
?
?
?
3 Ma
Present Fraser River Basin
direct to sea
approx. present watershed
Nechako River
ChilcotinRiver
Thompson & Clearwater
Rivers
reversal
un-changed
new
abandoned
0.2 Ma
<1 Ma
Aid for Interior explorationists1. New distribution and thickness maps of the Chilcotin Group
tested against regional-scale geophysical data-sets.
2. Identification / estimation of basement affinities in the new ‘windows’.
3. Use of the Chilcotin Group as a “clock” to quantify:
• Neogene uplift and erosion Barkerville placer gold release and exhumation of Eocene epithermal systems,
• paleo-landscapes and changes in drainage networks constraints on geochemical ‘vectoring’.
4. Database of physical properties (e.g., mag. sus; resistivity; density; seismic velocity) tied to CG lithology and lithofacies.
Neogene uplift
V – 0 masl
The ‘high’ Coast Mountains have significant (>2 km) Mio-Pliocene uplift
This is linked to Cordilleran-scale uplift across the Interior Plateau and into the Omineca Mountains
The Interior Plateau is warped:• the ‘corners’ goes up (100s –1000s m)• the ‘middle’ goes down (100s m)
Fraser River Basin base-level change
0
>100
25
0 m change
-ve change
25 m change
50 m change
>100 m change
“warping” recognized by Mathews (1989)
volcanic breccia
fluvial gravels
lava
Buried Miocene channels are known locations of basal U deposits (e.g., Blizzard site, Kelowna).
Buried Miocene channels are probable hosts for placer deposits (Au, PGE) most Barkerville / Fraser placers
are Miocene in age
Mapping the courses of buried channels is a first step towards further exploitation.
Chilcotin Prospects – Placer / U
l. Eocene peneplainThe Cordillera has a widespread late-Eocene – Oligocene unconformity:• post major transtensional regional extension (core complexes, etc),• synchronous with minor transtensional basin formation (e.g., Hat Creek, Merritt, Tulameen),• very few l. Eocene & Oligocene rocks,• preserved l. Eocene drainage network in southern BC (Tribe, 2003, 2005),• l. Eocene – Present drainage across the ‘high’ Coast Mountains,• extensive u/c overlain by sub-horizontal Neogene basaltic lavas (Chilcotin Group & Maitland Frm).
Neogene Chilcotin Grp
volcs
Outstanding questions• How does Neogene vertical tectonics link to plate-margin processes?
• What causes Fraser River reversal after 1 Ma?
• Balance between post-glacial rebound / base level drop / uplift
• Are there un-recognized neotectonic landforms across the Interior Plateau (e.g., fault scarps, alluvial fans, truncated valleys)? – YES there are!