The document discusses the distribution and thickness of volcanic and glacial cover on the Interior Plateaus of south-central British Columbia. The Chilcotin Group volcanic rocks cover over 35,500 km2 but their thickness and distribution are poorly constrained. Recent research indicates the volcanic rocks have extreme thickness variations from 0-100 meters, forming paleo-drainage channels and leaving many "basement windows" exposed. This challenges the previous assumption of a homogeneous thick sheet and greatly reduces the mapped areal extent of the volcanic rocks. Identifying these paleo-drainage systems could have implications for regional mineral exploration methods.

1. March 22nd 2010 – GAC Cordillera / TGI-3 workshop, Vancouver Distribution and Thickness of Volcanic and Glacial Cover on the Interior Plateaus Graham Andrews – GSC Vancouver Graham.Andrews@NRCan-RNCan.gc.ca Kelly Russell – UBC Vancouver krussell@eos.ubc.ca

4. It obscures basement likely to host Cu-Au-Mo porphyryand epithermal Au deposits.

5. The CG is wholly within the MPBB infestation zone.

6. It is extensively covered by ?? m of drift.Nicola Arc Cariboo Mountains Cariboo Mountains Interior Plateaus Coast Mountains adapted from Massey (2006)

8. It obscures basement likely to host Cu-Au-Mo porphyryand epithermal Au deposits.

9. The CG is wholly within the MPBB infestation zone.

10. It is extensively covered by ?? m of drift.It’s thickness and distribution are very poorly constrained. Nicola Arc Cariboo Mountains Cariboo Mountains Interior Plateaus Coast Mountains adapted from Massey (2006)

12. It obscures basement likely to host Cu-Au-Mo porphyryand epithermal Au deposits.

13. The CG is wholly within the MPBB infestation zone.

14. It is extensively covered by ?? m of drift.It’s thickness and distribution are very poorly constrained. Nicola Arc Cariboo Mountains Cariboo Mountains STUDY AREA Coast Mountains adapted from Massey (2006)

15. Regional Geology - MINFILE WL 100

16. Regional Geology - MINFILE Mt Polley Gibraltar ??? WL ??? ??? Takomkane ??? 100 Thuya ??? Blackdome Prosperity

17. Chilcotin Group – typical exposure Andrews and Russell (2007) sub-aerial lavas e.g. Hanceville sub-aqueous pillow delta subaerial lavas Eocene rocks subaqueous breccias e.g. Chilcotin River / HWY 20 Extensive sub-aerial / sub-aqueous successions along the tributaries of the Fraser River. Complex horizontal stratigraphic transitions between sub-aerial and sub-aqueous lithofaciesat the margins of paleovalleys.

18. e.g. Chilcotin River / HWY 20 e.g. Chasm Provincial Park

19. Chilcotin Group – typical exposure e.g. Chasm Provincial Park Farrell et al (2010) – GSC OF6230

20. 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. ??? Chilcotin Group - lavas Basement ??? what’s observed what’s inferred Andrews & Russell (2007)

21. e.g. Nazko River Valley evidence for river valleys till - overburden ~6.3 Ma columnar-jointed lava 4 m sediment diapir soft-sed deformation ≤3 m bedded lacustrine sediments

22. evidence for river valleys e.g. Vedan Lake Valley lava Basalt-buried Miocene channels are known locations of basal U deposits (e.g., Blizzard site, near Kelowna, BC). Basalt-buried channels are knownhosts for placer Au deposits most Cariboo placers are Miocene or Pliocene hydro-volcanic breccia river gravels

23. mapping valley-fill successions valley margin ~2 km valley axis e.g. Bull Canyon Prov. Park Gordee et al. (2007)

24. mapping valley-fill successions valley margin subaerial lava characteristic orange pillow-breccias e.g. Bull Canyon Prov. Park Gordee et al. (2007)

25. exposed paleovalleys – 3 Ma Chilcotin River e.g. Chilcotin River / HWY20

26. a new facies model 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. hydro-volcanic breccia subaerial lava Andrews & Russell (2007)

27. implications: areal extent Dohaney MSc (2009)

28. implications: areal extent Dohaney MSc (2009)

29. implications: areal extent Up to 50% reduction in mapped Chilcotin Group extent ? ? ? ? Dohaney MSc (2009)

30. implications: areal extent Dohaney et al (2010) – GSC OF6344 & OF6284

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

33. typically <20 m elsewhere

34. can also constrain drift thickness and bedrock typeVanderhoof Andrews et al - CJES paper and GSC OF in 2010

35. implications: buried paleochannels Prince George 100 Mile House Andrews et al (subjudice) Andrews et al - CJES paper and GSC OF in 2010

36. summary Extreme thickness variations (0 – 100 m) require many ‘basement windows’ and greatly reduced areal extent. Andrews et al - CJES (subjudice)

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

38. Thank you – questions?