Hello Joe
The actual term probably has historic references back to the California gold rush but all we are talking about is topsoil and a combination of eroded host rock and kimberlite reduced to gravel and fines.
Generally, because of kimberlite's almost complete lack of silica, preponderance of peridotitic minerals, hence softness, through sun, weather and chemical processes (oxidization), it breaks down or crumbles away very easily.
A point of historic interest in this regard. In the early days of the SA diamond rush, diamonds were found aluvially in a few rivers like the Orange. These deposits were traced back up stream and eventually pipes were discovered under similar overburden like Kimberly's deep hole. Over 1,000 meters of this pipe is believed to have been eroded away in what was a tropical and sub-tropical prehistoric environment in this case over 1 billion years ago. The result was eroded deposits or overburden gravels, river, beach and ocean bench deposits depending on the degree of transport.
The kimberlite the miners originally encountered on and near the surface was quite soft, easily fractured, and yellow in colour, hence the name Yellow Ground was coined for kimberlite ore.
Before long, miners processed the 100 to 150 meters of Yellow Ground and thought they had run out of ore when they encountered a much harder blue/green rock, coined Blue Ground. They quickly discovered that this was in fact the same rock only it had not been weathered.
The economics of the small-scale mining briefly suffered, as the Blue Ground was much harder to mine and process in those days of modest mechanization. However, it was also quickly discovered that if the Blue Ground were blasted out then spread in benches to be exposed to the sun for a week or so, it quickly degenerated and was easily broken up and processed.
That is how quickly an exposed kimberlite can erode into overburden. The process is only limited by the degree of sun exposure and mechanical erosion removing the overburden and the resistivity of the host rock to erosion.
Many SA kimberlites were emplaced including most of those on the Klipspringer property in a fairly soft dolomite, which of course was easily eroded, blown and washed away in the ancient sub-tropical environment. The heavier more resistive minerals such as the garnets, chromites, clyopyroxenes(sp?) and diamonds of course tended to work their way down to bedrock in pay streaks and glory holes hence the tendency for overburden to vary in value according to depth to bedrock, hard pan and/or prehistoric drainage patterns.
Hope this helped answer your question.
Regards |
