4.1_arid zone conference talk - j robey

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

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

Nooitgedacht

Glacial Pavement

cut on

Ventersdorp

andesite lava

showing glacial

striations and San

rock art

Detail of glacial striations and San rock art

Unconformity between

Karoo Dwyka Tillite

(300my)

And Ventersdorp lava

(2600my) at Nooitgedacht

Agates formed in gas bubbles in the Ventersdorp lavas make up

a significant portion of young diamondiferous gravels

Dolerites are the feeders to the massive outpouring of Stormberg basalts

forming the Drakensberg - Maluti mountains

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)

The ultimate dyke, from Bellsbank: ~30% olivine xenocrysts, minor garnet, flow zoned and multiple

intrusions

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

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

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

Old Archaean crust

Convecting asthenosphere

Subducting slab

Cold lithosphere

model for South Africa – Kaapvaal SCLM

SW NE

Deep (100 – 200km) mantle rocks (peridotites) brought to the surface by kimberlites

diamond inclusion in a purple peridotite garnet – photo JJG form Menzies PhD at UCT; in one

garnet saw max 32 d’s - Newlands

Mantle peridotites are beautiful rocks