The Chilcotin Basalts: implications for mineral exploration

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.




•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.







It’s thickness and distribution are very poorly constrained.

Thuya

batholith


L. Eocene-Oligocene unconformity

Takomkane

batholith

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


sub-aqueous successions

cross-bedded sediment trapped within peperite

transport direction


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)

paleovalleys – 3 Ma Chilcotin River

e.g. Anahim IR

paleovalley environment - model

e.g. Dog Creek

“perched” lavas

valley-fill lithofaciesassociation

tributary-filling lava – “escaped”

7

7

2.92.9

1.1

paleovalley environment - model

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

schematic cross-section

Thuyabatholith


Implications for exploration

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

Thuyabatholith

New Distribution Map

Dohaney (in prep.)

New Distribution Map

X

X

X

X

X

Dohaney (in prep.)

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.

Thank you – questions?

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!

The Chilcotin Basalts: implications for mineral exploration

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