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The Chilcotin Basalts: implications for mineral exploration
 

The Chilcotin Basalts: implications for mineral exploration

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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 ...

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.

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    The Chilcotin Basalts: implications for mineral exploration The Chilcotin Basalts: implications for mineral exploration Presentation Transcript

    • 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
    • Mineral Deposits in south-central BC
      • The Chilcotin Group (~22 – 0.1 Ma) covers >35,500 km2of the Interior Plateaux.
      Nicola Arc
      Cariboo Mountains
      Cariboo Mountains
      Interior Plateaux
      Coast Mountains
      adapted from Massey (2006)
    • Mineral Deposits in south-central BC
      • The Chilcotin Group (~22 – 0.1 Ma) covers >35,500 km2of 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.
      Nicola Arc
      Cariboo Mountains
      Cariboo Mountains
      Interior Plateaux
      Coast Mountains
      adapted from Massey (2006)
    • Mineral Deposits in south-central BC
      • The Chilcotin Group (~22 – 0.1 Ma) covers >35,500 km2of 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.
      Nicola Arc
      Cariboo Mountains
      Cariboo Mountains
      Interior Plateaux
      Coast Mountains
      adapted from Massey (2006)
    • Mineral Deposits in south-central BC
      • The Chilcotin Group (~22 – 0.1 Ma) covers >35,500 km2of 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.
      Nicola Arc
      Cariboo Mountains
      Cariboo Mountains
      Interior Plateaux
      Coast Mountains
      adapted from Massey (2006)
    • L. Eocene-Oligocene unconformity
      Takomkanebatholith
      Thuyabatholith
      adapted from Massey (2006)
    • 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)
    • 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.
    • e.g. Dog Creek
      1. sub-aerial lavas
      massive interior
      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 sediment
      basal breccia
      columnar-jointed lava
      0.5 m
      vesicular top
      massive interior
      3 m
      basal breccia
    • 2. sub-aqueous successions
      sub-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).
    • sub-aqueous successions
      SW
      NE
      sub-aerial lavas
      peperites
      pillow breccias
      basement
      basement
      basement
      e.g. 6 Ma Chilcotin River Valley
      Gordee et al. (2007)
    • sub-aqueous successions
      transport direction
      cross-bedded sediment trapped within peperite
      Gordee et al. (2007)
    • sub-aqueous successions
      sub-aerial lava
      hyaloclastite pillow-breccias
      e.g. Chasm Prov. Park
      Gordee et al. (2007)
    • paleovalleys
      characterized 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
      “perched” lavas
      tributary-filling lava – “escaped”
      valley-fill lithofacies association
      e.g. Dog Creek
    • paleovalley environment - model
      7
      1.1
      7
      2.9
      2.9
    • duration of valley-filling eruptions
      Bull Canyon – 6.2 Ma
      Dog Creek – 3.1 Ma
      Enkin et al. (Round-Up 2008)
      clustered, non-polar paleomagnetic mean directions
      rapid (&lt;&lt; 10 kyr), un-interrupted in-filling  single, short-duration, proximal eruptions
      • probably mono-genetic  many vents, cones, in a single field
    • Chilcotin Group sources
      inferred Chilcotin Group sources
      Holocene volcanic field
      Nazko area
      Wells-Grey VF
      there are probably 100s of ‘missing’ vents – 1 vent for each lava
    • Chilcotin Group – buried channels
      • 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
      MPBB
      40
      10
      0
      50
      10
      0
      Andrews & Russell (2008)
    • schematic cross-section
      Thuyabatholith
      adapted from Massey (2006)
    • Implications for exploration
      Thuyabatholith
      Extreme thickness variations (0 – 100 m) require many ‘basement windows’ and greatly reduced areal extent.
    • New Distribution Map
      Dohaney (in prep.)
    • New Distribution Map
      X
      X
      X
      X
      X
      Dohaney (in prep.)
    • 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
    • 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!!!
    • 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
    • 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
    • Aid for Interior explorationists
      New distribution and thickness maps of the Chilcotin Group tested against regional-scale geophysical data-sets.
      Identification / estimation of basement affinities in the new ‘windows’.
      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’.
      Database of physical properties (e.g., mag. sus; resistivity; density; seismic velocity) tied to CG lithology and lithofacies.
    • Thank you – questions?
    • Neogene uplift
      X
      The ‘high’ Coast Mountains have significant (&gt;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)
      X
      X
      X
      X
      X
      V – 0 masl
    • 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)
    • 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
    • l. Eocene peneplain
      The 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).
      “perched” outliers
      extensive coverage
      “perched” outliers
      Neogene Chilcotin Grpvolcs
    • 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!