This is a presentation I gave at the GSA Cordilleran Meeting in Kelowna, BC, in May 2009. It presents advanced results from geological studies of the Chilcotin Group basalts in south-central BC, and their impact on mineral exploration activities.

1. May 8th 2009 The Chilcotin Basalts and Neogene landscape evolution of the Interior Plateau: implications for mineral exploration Graham Andrews – UBC co-workers: Kelly Russell, Jackie Dohaney, Sarah Brown, Sarah Caven, Randy Enkin, Bob Anderson

4. It obscures basement likely to host epithermal Au and Cu-Au-Mo porphyry 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 Plateaux Coast Mountains adapted from Massey (2006)

8. It obscures basement likely to host epithermal Au and Cu-Au-Mo porphyry 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 Plateaux Coast Mountains adapted from Massey (2006)

12. It obscures basement likely to host epithermal Au and Cu-Au-Mo porphyry 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 Interior Plateaux Coast Mountains adapted from Massey (2006)

15. L. Eocene-Oligocene unconformity Takomkanebatholith Thuyabatholith adapted from Massey (2006)

16. L. Eocene-Oligocene unconformity Chilcotin Ranges – Coast Mountains ≥18 Ma sub-horizontal unconformity – now 5° NE-dipping Cardtable Mtn. – 6400 ft ~18 Ma paleo-valley fill Battleship Peak 6000 ft 5800 ft Mesozoic meta-sedimentary succession Relay Mtn. Andrews et al. (in prep)

17. CG - physical volcanology Andrews and Russell (2007) sub-aerial lavas e.g. Hanceville sub-aqueous pillow delta Eocene rocks 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.

19. coherent,

20. laterally restricted <100 km2,

21. highly vesicular,

22. single lavas <20 m thick,

23. sequences <60 m thick,

24. often emplaced onto / into wet sedimentbasal breccia columnar-jointed lava 0.5 m vesicular top massive interior 3 m basal breccia

26. always restricted to paleo-valleys (e.g. Mio-Plio Chilcotin Valley),

27. typically associated with capping sub-aerial lavas,

28. ≤80 m thick,

29. typically repetitive sequences:

30. repeated pillow-breccia  peperite  sub-aerial lava,

32. sub-aqueous successions transport direction cross-bedded sediment trapped within peperite Gordee et al. (2007)

33. sub-aqueous successions sub-aerial lava hyaloclastite pillow-breccias e.g. Chasm Prov. Park Gordee et al. (2007)

35. areally-restricted (<100 km2),

36. complex vertical and along-valley stratigraphy,

37. ≤150 m thick, 100 – 2000 m wide,

38. 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)

39. paleovalleys – 3 Ma Chilcotin River e.g. Anahim IR

40. paleovalley environment - model “perched” lavas tributary-filling lava – “escaped” valley-fill lithofacies association e.g. Dog Creek

41. paleovalley environment - model 7 1.1 7 2.9 2.9

44. CG locally up to 50 m thick around 100 Mile House (092P) and Miocene (093A)

45. typically <20 m across 092P

46. elsewhere, limited data suggests CG is <20 m thick

47. can also constrain drift thickness and bedrock typeMPBB 40 10 0 50 10 0 Andrews & Russell (2008)

48. schematic cross-section Thuyabatholith adapted from Massey (2006)

49. Implications for exploration Thuyabatholith Extreme thickness variations (0 – 100 m) require many ‘basement windows’ and greatly reduced areal extent.

50. New Distribution Map Dohaney (in prep.)

51. New Distribution Map X X X X X Dohaney (in prep.)

52. Basement Prospects – Windows 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. ARIS Report 25740 - Bonaparte Mb – Chilcotin basalt Tv – Nicola volcanics Mzi - pluton Mb – Chilcotin basalt 1 km

53. Implications for exploration Thuyabatholith 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!!!

54. Mio-Pliocene Fraser River Basin ? inferred paleo-channel flow direction approx. present watershed Stage 3 age constraint 10 Ma 6 Ma Fraser River 8 Ma 6 Ma ? 3 Ma 10 Ma 11 Ma 3 Ma 6 Ma ? 9 Ma 7 Ma 9 Ma

55. Present Fraser River Basin Nechako River reversal approx. present watershed new un-changed Fraser River abandoned 0.2 Ma direct to sea ChilcotinRiver &lt;1 Ma Fraser Canyon Thompson & Clearwater Rivers

57. paleo-landscapes and changes in drainage networks  constraints on geochemical ‘vectoring’.Database of physical properties (e.g., mag. sus; resistivity; density; seismic velocity) tied to CG lithology and lithofacies.

58. Thank you – questions?

60. the ‘middle’ goes down (100s m)X X X X X V – 0 masl

61. Fraser River Basin base-level change 0 m change 25 m change 50 m change &gt;100 m change -ve change 25 &gt;100 0 &gt;100 “warping” recognized by Mathews (1989)

62. Chilcotin Prospects – Placer / U 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. volcanic breccia fluvial gravels

64. synchronous with minor transtensional basin formation (e.g., Hat Creek, Merritt, Tulameen),

65. very few l. Eocene & Oligocene rocks,

66. preserved l. Eocene drainage network in southern BC (Tribe, 2003, 2005),

67. l. Eocene – Present drainage across the ‘high’ Coast Mountains,

68. extensive u/c overlain by sub-horizontal Neogene basaltic lavas (Chilcotin Group & Maitland Frm).“perched” outliers extensive coverage “perched” outliers Neogene Chilcotin Grpvolcs

70. What causes Fraser River reversal after 1 Ma?

71. Balance between post-glacial rebound / base level drop / uplift

72. Are there un-recognized neotectonic landforms across the Interior Plateau (e.g., fault scarps, alluvial fans, truncated valleys)? – YES there are!