4.1_arid zone conference talk - j robey
DESCRIPTION
science, workTRANSCRIPT
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Geology of Kimberley Area
• It is relatively simple
• Alluvial gravels (Vaal River) and Hutton Sands <1my• Kimberlite pipes ~90my• Karoo dolerite sills ~180my• Basal Karoo glacial sediments – Dwyka group ~300my• Allanridge Fm andesite lavas and quartzite VSG ~2600my• Basement granitoids, amphibolites and schists ~3200my
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Karoo dolerite sill
(180my) intrusive
into Dwyka Shales
Karoo Dwyka
Glacial Shales
~300my
Ventersdorp
Allanridge Fm
andesite lavas
2600my
Karoo Dwyka Glacial Shales
Kimberlite
Kimberley Mine
– the Big Hole
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Nooitgedacht
Glacial Pavement
cut on
Ventersdorp
andesite lava
showing glacial
striations and San
rock art
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Detail of glacial striations and San rock art
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Unconformity between
Karoo Dwyka Tillite
(300my)
And Ventersdorp lava
(2600my) at Nooitgedacht
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Agates formed in gas bubbles in the Ventersdorp lavas make up
a significant portion of young diamondiferous gravels
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Dolerites are the feeders to the massive outpouring of Stormberg basalts
forming the Drakensberg - Maluti mountains
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Diamonds are the natural high pressure form of the element carbon
and its classic crystal growth form is an octahedron
DIAMONDS, the MANTLE and KIMBERLITES
presentation by jock robey (de beers)
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The ultimate dyke, from Bellsbank: ~30% olivine xenocrysts, minor garnet, flow zoned and multiple
intrusions
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Column - vertical
Wind - sorting
Surface – maar crater + pyroclastic fall deposits
What do kimberlite eruptions look like at surface? don’t know – no modern activity
Ukinrek volcano
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Kimberley
reconstruction at ~90my
ago; we believe there has
been ~850m erosion at
Kimberley from ~90my
to present; release of
diamonds into fluvial
(Vaal and Orange rivers)
and marine (West
Coast) systems
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crater formed by violent
kimberlite eruption,
surrounded by low tuff ring
and possible internal tuff
cone?; reconstruction of a
typical Orapa and probably
Kimberley Grp 1 style
kimberlite volcano at
~90my
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Old Archaean crust
Convecting asthenosphere
Subducting slab
Cold lithosphere
model for South Africa – Kaapvaal SCLM
SW NE
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Deep (100 – 200km) mantle rocks (peridotites) brought to the surface by kimberlites
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diamond inclusion in a purple peridotite garnet – photo JJG form Menzies PhD at UCT; in one
garnet saw max 32 d’s - Newlands
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Mantle peridotites are beautiful rocks