Day 13 (Monday Sept. 18th )
Kimberly Mine!
Guides: Dave Bell, Caltech - GPS alumnus now at U. Cape Town
Jock Robey, DeBeers geologist
6:30 AM Argh! We have to get up so we can be ready to go to the diamond mine by
7:45 AM. Freezing in the Kalahari. We are staying on the Dairy Farm, which
is now called DeBeers Geology Division. This used to be a dairy farm owned by
DeBeers, which supplied the miners' camps nearby. For once we arrive on time,
only to discover that there is a shaft inspection under way in the mine we are
to visit. So we have to find something to do until 9:15 AM.
8:15 AM We arrive at the MacGregor Museum, which also turns out to be closed.
Dave Bell comes to the rescue, by talking one of the museum's researchers into
letting us in the back door. One guidebook says the museum is about the Boer
War, another says it is about furniture, and yet another says it is about local
culture. Museum highlights turn out to be displays of San (bushmen) rock art,
a map and explanation of the star constellations (conveniently located in the
bathrooms), and artifacts from the siege of Kimberley.
9:15 AM At last the Bultfontein Diamond Mine is ready to receive us. Everyone
dons blue coveralls and brown shoes. We are now DeBeers property. Before we
enter the depths we are treated to a series of instructional videos on diamond
mining, the history of the Kimberley diamond fields, and the six key
instructions for using our emergency oxygen packs. The oxygen packs are
canteen-sized heavy metal boxes that are strapped onto our belts, which also
have to hold the battery packs for our head lamps that go on our hard hats.
They contain potassium superoxide as the active ingredient! Once we are loaded
with all this equipment, we are compressed sardine-like into a metal elevator
with some mine workers, 34 people total. Down we go, 825 meters below the surface.
The diamonds are found in a type of rock called kimberlite. Kimberlite (named
after Kimberley) is a rock formed by melting the Earth's mantle, around 150 km
below the surface. The kimberlite melt ascends rapidly, carrying along pieces
of the solid rocks it passes through some of these pieces are diamonds. The
rising melt explodes through the surface, creating a volcanic pipe full of
kimberlite and rubble. The kimberlite pipes are roughly the shape of an ice
cream cone 800 m wide at the top. Kimberlite is a very soft rock, so it would
be very dangerous to mine in tunnels through it. Luckily, the rock
surrounding
the kimberlite is Archean gneiss, which is hard and is safer for tunnels. The
mining of kimberlite is like sucking ice cream out of the bottom of the ice
cream cone. Big tunnels through the Archean gneiss lead to the kimberlite.
There are smaller tunnels into the kimberlite. These tunnels represent the
bottom of the ice cream cone. Miners set off controlled explosions in the
ceiling of the tunnels and shovel out the kimberlite that falls in. They will
continue to do this until they run out of diamond-rich material (about 30 more
years for the Bultfontein Mine).
We hike down to the 845 m level, where active mining is taking place, watching
the removal of blasted kimberlite ore. Here, remotely operated sledges feed
ore onto a small underground railway train, which transports the ore to a rough
crusher. After that, a long conveyor belt moves the crushed ore to special
lifts that bring it to the surface. In this mine, about 7 tons of ore are
processed to recover 1 carat of diamonds. Nobody actually sees any diamonds
the whole day, except behind glass cases in the Big Hole Museum (Edwin & Liz).
2:00 PM Lunch at the Big Hole. This is perhaps the most famous museum in
Kimberly, located just at the edge of the original big hole, which was the
first time humans realized where diamonds were found. Jock and Dave took us
first to the two overview locations where we were able to see just how deep the
hole was! The exposure of the sequence was superb, with the Kalahari sands on
top, followed by Permian glacial units and black shales, all on top of the
Archean Ventersdoorp lavas. Next to one of the viewpoints they had located
three ore carts, filled with broken glass, representing the total volume of
diamond that was removed from the hole. It works out to about 0.5 ppm by
volume, or ½ cc (~ 2g) per cubic meter. Although this seemed absurdly small
compared with the Deep Hole in the background, there was general agreement that
it was indeed worth sorting through 1 cubic meter of ore for an average of 10
carats of diamond!
The rest of the museum is a reproduction of what the Main Street of Kimberly
looked like at the turn of the last Century. Jock led a running dialog about
the history of Kimberly, which was the major British outpost in Southern
Africa. In fact, it was the center of the Southern Africa Industrial
Revolution. During the siege of Kimberly in the Bohr War, they were even able
to produce their own cannons.
3:45 PM Dave and Jock then took us to a site about 30 km NE of Kimberly, where
the base of the Karoo Supergroup was exposed. The Permian glaciers which
formed the Dwika Tillite had gouged and carved striations into the Archean
Ventersdoorp lavas, and we were able to put our fingers on the unconformable
contact representing a time gap of nearly 2 billion years. The striated
surface had a good coating of desert varnish, into which the aboriginals had
carved numerous pictographs of African animals (JLK).
4:45 PM Collecting mantle xenoliths at the Boshof Mine Dump. Diamonds aren't
the only interesting minerals that the kimberlites bring up from the mantle.
Jock then led us to a historic dump from the Bultfontein mine, which is being
re-mined by DeBeers using modern techniques. There are many of these giant
man-made mesas of processed kimberlite ore in and around Kimberley. Small
cobbles and boulders of mantle rock which were brought up by the kimberlite
abound at the base of the dump, and we fan out to search for interesting
specimens. The most common mantle rock here is peridotite, which consists of
the minerals olivine and pyroxene with lesser amounts of spinel, garnet, and
mica. Some of the pyroxenes are bright emerald green because of their high
chromium content, and the garnets are lavender-red for the same reason. The
rental vans are now riding low on their springs, which does not bode well for
the return flight on Sunday
6:30 PM The DeBeers Geology gang prepared a proper Kimberly braai (a form of
BBQ with an assortment of various meats) at their headquarters to finish off
the day. When we discover they have a working clothes washer and drier, the
unit is run nearly continuously for the entire evening and night.
Written by Liz and Edwin
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