Australian Diamond Exploration: Where to next?

Lynton Jaques
Minerals Division
Geoscience Australia

World Diamond Conference,
Perth November 2004

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• Argyle mine is the largest single producer in world
  • 31M carats in 2003
  • 22% of world production (wt)
• Produced >0.5B carats since 1985
• High grade pipe
• Underground 2008?
• Discovered 1979

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• Produced 57,000 cts in 2003
• Resource = 5 Mcts
• >US$200/ct
• Discovered in 1976
• Ellendale Field ~ 80 lamproite bodies
• 3 new diamondiferous lamproites found in 2004

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• Produced >0.5 M carats, incl. 104 ct 'Jungiila Gunajina'
• Average value US$108/ct
• 14 pipes ( < 5 ha), grades 14 - 56 ct / 100t
• Remaining 1.8M carats (Striker Resources)
• Trial mining at Seppelt 2 in the North Kimberley

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Australian Diamond Exploration 1979-2004

Source: ABS

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Australian Diamond Exploration 1979-2004

World Diamond Exploration (Metals Economics Group)

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First generation surveys 1960s-1970s

• Regional stream sediment surveys, airborne magnetics
• Discoveries:
  • Ellendale, Argyle lamproites
  • Kimberlites in North Kimberley, East Kimberley, Gawler, and alkaline ultramafic bodies at Carnarvon Basin and in SE Australia

Second generation surveys 1980s-1990s

• Detailed and regional surveys, HR geophysical surveys, AEM
• Discovered Merlin pipes, Aries, Beta Creek-Ashmore, Forest River, Nabberu, Yilgarn

Current

• Detailed follow-up surveys, HR geophysical surveys, AEM, airborne gravity gradiometry
• Focus: Ellendale, North Kimberley (Seppelt), Merlin, Pilbara, Flinders Ranges

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What can we learn from these discoveries?

• Diamond source regions
• Controls on distribution of diamond pipes (pathways)
• Preservation (erosion levels)
• Other exploration models?

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• >480 kimberlites, lamproites, lamprophyres
• ~30% have traces diamond
• < have 5ct/100 t +

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Red/Brown slower than the world average
Blue faster than the world average.

S. Fishwick & B. Kennett, TSES, ANU, 2004

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Coloured areas have 3D S-waves faster than average mantle at depth slices of 125, 175 and 225 km. Data clipped to coastline (S. Fishwick, RSES).

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• Diamondiferous bodies lie within/at margin of old (>1.6 Ga, basement >2 Ga?), cold lithosphere.
• Crustal thickness = 35-55 km.
• Mostly cratonic palaeogeotherms (limited data).
• Peridotitic indicators indicate moderately refractory mantle source (G9 lherzolite dominant).
• Multiple diamond events
  • Some associated with major extension events e.g. Rodinia (~800 Ma), Gondwanaland (~160-180 Ma)

... but what about province-scale controls?

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Collins et al, 2002

Prism model with "strings" or polylines joining points computed at the maxima of many levels of upward continuation of the gravity response, and plotted at the appropriate upward continuation level.

Coloured by the strength of the horizontal gradient at each point (magenta high, green low).

P. Milligan, Geoscience Australia

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Jaques & Milligan, 8IKC, 2004

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North Kimberley

• >40 kimberlites (800 Ma)
• >50% diamondiferous

East Kimberley

• >20 kimberlites (800 Ma)
• 1200 Ma Argyle lamproite

Central Kimberley

• Aries + satellite pipes

West Kimberley

• >150 lamproites (20 Ma)
• ~2/3 Ellendale pipes diamondiferous (low grade)

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• >20 diamondiferous kimberlites,
• 800 Ma
• NE control on NW gravity high (corridor)

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• Diamondiferous kimberlites
  • Merlin 380 Ma, Roper
  • Timber Creek 180 Ma
• Proterozoic ('Barramundi') basement but inherited Archaean zircons common
  • Archaean nuclei?
• Mesozoic and regolith cover

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Pilbara Craton
1900 Ma kimberlite

Yilgarn Craton
Multiple ages if kimberlite and ultramafic lampropyre

• 850 Ma - lamprophyre
• 13000 Ma - kimberlite, lamprophyre
• 1900 Ma - kimberlite
• 2050 Ma - kimberlite

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Gravity horizontal gradients to 2km

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S-wave seismic tomographic model at 175km (S. Fishwick, RSES, ANU)

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• Significant number of kimberlites, lamproites and ultramafic lamprophyres found
  • new diamondiferous discoveries
• Australian diamond exploration needs a significant new discovery to keep us on the diamond radar screen
• Much of the Australian Shield is prospective but how can we to narrow the search area?
• Exploration difficult because of regolith/cover
  • improved targeting required
  • potential under cover
  • poor survival of indicators, and erratic distribution

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• Role for continental-scale geophysics to define fertile source regions (mantle roots) and identify favourable tectonic zones/corridors
• Many intrusions show structural control
• Structures commonly evident in potential field data (horizontal gradients of upward continued gravity and magnetic data)
  • Multi-scale approach needed (fine & coarse scale structures)
  • Diamond pipes located at boundaries/breaks in lithosphere and crustal domains?
• Increasing use of geophysics - VHR magnetics, AEM, airborne gravity gradiometry

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P. Milligan, M. Nicholas, M. Ratajkoski, GA

Stewart Fishwick, Brian Kennett, RSES, ANU

• De Beers Ltd
• Kimberley Diamond Co
• RioTinto Ltd
• Striker Resources NL