Comments on: A True Ghost Story, Part 6: But first, since we’re talking geology …

By: Greg Laden

Greg Laden — Sun, 13 Nov 2011 15:44:46 +0000

CherryBombSim, that is partly true, but quite a few of the pipes stick up out of the surface. The big hole itself is one of them; It started out as a hill.

Placer deposits are the best localities to look, because as you say diamonds, being rocks and relatively heavy, can concentrate in gravel deposits. It can take a lot of work to find deposits that actually have diamonds in them. I’ve spent a fair amount of time poking around in the trenches dug by diamond prospectors (who use backhoes) and did a major geological/archaeological project on a place known as “Diamond Kojpe” which was obviously misnamed because there were none.

As the “Orange River” passes over and among the remains of volcanic plugs that have diamonds in them, the diamonds become part of the “lag” (gravel moving along the base of the river) but so does a lot of other stuff. There are plenty of other metamorphic rocks in the area that are hard and dense as well.

The river moving through a diamond deposit is much more likely to spread out diamonds than to concentrate them. But then, at a few key spots along a river you can get the reverse, where gravel more or less stays in place while softer bits are eroded away, lighter rocks drawn off, etc. and then there can be concentration. Those are the places to look for diamonds.

Locally people believe there may be concentrations of diamonds at the base of Augrabies Falls, but if anyone had gone in there to find out, they have not returned!

By: CherryBombSim

CherryBombSim — Sun, 13 Nov 2011 03:00:16 +0000

Oh, ya. There are placer-type deposits where the diamonds have washed out of the pipe. The diamonds concentrate much like gold, as they are heavier than almost all of the other minerals. The pipes they originally come from are not really so rare, but they are hard to find as they are only a couple of hundred meters across and they just look like normal dirt when you walk across them. A little yellowish, maybe, but nothing sticking up like a volcano or any obvious signs of it from the surrounding geology. People mined the placer deposits for centuries without ever realizing where the diamonds came from.

By: Greg Laden

Greg Laden — Mon, 07 Nov 2011 16:19:02 +0000

Alan, Kimberley South Africa is the boom-town from the diamond days. The Australian region is named after Kimberley, South Africa.

By: Alan

Alan — Mon, 07 Nov 2011 14:19:26 +0000

Interesting, I didn’t know the diamond area in S. Africa was called Kimberly. The diamonds in Australia are also mined in a place called “The Kimberly”.

By: Greg Laden

Greg Laden — Thu, 03 Nov 2011 00:00:13 +0000

These would have been in situ deposits.

By: Jon Tomas

Jon Tomas — Wed, 02 Nov 2011 22:32:55 +0000

The “yellow rocks” are, perhaps, weathered “blue rocks”, and may have experienced physical displacement to some degree, and comprise placer type deposits, which would also suggest (again, perhaps) preferential sorting. The trick, then, when hunting diamonds in the yellow substrate, would be where you look within that substrate, as opposed to the blue substrate, wherein the diamonds are more isotropic, with regard to spatial distribution. The total diamond content, per volume, doesn’t change, merely the distribution within the equivalent volume.

By: Greg Laden

Greg Laden — Wed, 02 Nov 2011 20:40:29 +0000

OK, you may be right, but that is in conflict with geological reports contemporary with the mining of the Big Hole in Kimberly, which was pretty much mined out before you were born. It is quite possible that we are talking about two different phenomena here.

Next time you are in South Africa stop in at the museum (at the Big Hole) in Kimberly and ask around!

By: CherryBombSim

CherryBombSim — Wed, 02 Nov 2011 20:03:14 +0000

I may be full of it on other things (and feel free to say so!), but trust me on this thing. The yellow rock has just as many diamonds as the blue it came from. It is very crumbly, though, so you can basically mine it with a shovel and a sieve.

By: Greg Laden

Greg Laden — Tue, 01 Nov 2011 02:03:38 +0000

The yellow rock did not have diamonds in it (to the same degree) so I’m not sure that we are talking about the same thing here. Perhaps there is a reason you did not find any diamonds!

By: CherryBombSim

CherryBombSim — Tue, 01 Nov 2011 00:17:18 +0000

The yellow rock is just weathered kimberlite. Kimberlite is not very hard to start with, and the yellow stuff will crumble in your hand. The volcanic plugs you mentioned earlier are probably from more typical volcanoes; the kimberlite pipes eroded down into depressions called “pans” where the first diamonds were found. I was diamond mining myself a couple of months ago. 🙂 Didn’t find any. 🙁