joohnno,
That is a great question. Is it valid to try and compare the volume of the material that has been eroded from kimberlite e259 (in that "saucer" shaped depression in my video) to the company's estimates of the volume of alluvial diamond bearing gravels from the terraces along the Cacuilo River?
Well the answer is yes and no and it comes down to how well the company has been able to identify the ages of those gravels and also how well they understand the geochronology (geological timing) of the geological events that have occurred between the emplacement of the kimberlite and the present day erosion of the landscape that is occurring along the active river systems. If you saw my previous video where I point out the slumping of that triangular section of the higher more stable land plain into the "fan/saucer" shaped depression above e259 you will have seen with your own eyes that erosion is present and active at this location today.
Now this is were we need to hunt for the geological facts specific to Angolan and that's what I did this afternoon to answer your question and the same question that was lingering in my mind.
Let's start with the age of emplacement of kimberlites in Angola.
Scientists are more or less agreed that kimberlites (in Angola) were formed around the time of the split which occurred in Gondwana (about 70-150 million years ago), the vast continent that included Africa, Arabia, Latin America, India, Antarctic and Australia. The Atlantic Ocean was taking shape at that same time with the Atlantic plate moving under the continental plate and this movement caused low coherence zones, which made kimberlites escape to the surface within the continental part.
After this kimberlite formation period came to an end a
very important epoch in placer diamond deposit formation in north eastern Angola occurred at the end of the Mezozoic Era(about 66 million years ago).
This was the period in which the kimberlite pipes first started to be destroyed by erosion. The entire sediment pile formed during this period is known as the Calonda Formation and it is recognised by scientists as being the sedimentary collector of diamonds in Angola.
A schematic of this sedimentary formation is shown below.
View attachment 102657
The important diamond bearing layer in this formation (the Calonda gravels) was formed by a mechanical process associated with periods of high rainfall and high energy flows of sediments that accumulated in fault bounded depressions. The sediments overlying the diamond bearing sediments are largely barren of diamonds and characterised by larger periods of dry seasons and the complete disappearance of surface water.
So the very important thing to understand is that the diamond bearing layers in the Calonda Formation are alluvial gravels belonging to an ancient river system (about 66 million years old), NOT a modern one.
The modern river system that has been flowing along the Cacuilo river system is what has eroded and exposed the diamond bearing layers in the Calonda Formation.
So if the diamonds from the kimberlite pipe e259 (that has just been discovered by Lucapa) have contributed to the mineralisation of the diamond bearing layers in the Calonda Formation, those diamonds were eroded from the pipe about 66 million years ago and NOT in the present day.
Well that brings us back to the video I made and my estimate of between 7.5 million to 15 million cubic metres of material that has been eroded from the pipe and surrounding rocks from that fan/saucer shaped depression above the pipe. What is this material then and where did it go? Well if the pipe is mineralised this material is modern alluvium which would also have been deposited along more modern sediment channels associated with the Cacuilo river system (which as mentioned in the video flows in a north westerly direction). I'm not certain of the time span involved with this modern sedimentation but I'm certain it continues today.
Well what has Lucapa been mining? The ancient gravels or the modern gravels and eluvium?
I can't say I've done comprehensive research of Lucapa's past announcements but on the 5th of March 2013 when they announced that they'd applied for the mining lease and gave the market an exploration target for the alluvial gravels they did not once mention in that announcement the Calonda terraces directly in respect of their calculation.
The company framed it this way
"The Mining Lease Application (MLA) will cover an area of ~218km2 and will include the recent alluvial and terrace deposits associated with Cacuilo River..........The recent, lateritic alluvial gravels are associated with the current Cacuilo River and are more widespread and
lower grade than the terraces."
Later in the June quarterly report in 2013 when the company was investigating the large 150ha Se251 kimberlite they reported that they'd exposed 1.6 metres of gravels in a pit near the pipe at block 14 (see photo below).
View attachment 102658
Although described as Calonda gravels in the report the company was actually unsure of their origins describing them as follows; "The gravels exposed in the pitting were up to 1.6m thick and have now been identified over an area of about 31 hectares. At this stage it is believed the gravels are part of the Calonda Formation, but there is the possibility that they are much younger and relate to an old channel of the Cacuilo River."
Then later in presentations given by the company (December 2013) the company introduces and emphasizes the Calonda gravels (upper terraces) as being the more economically important.
"Calonda Formation gravels in the Cacuilo River valley identified as hosting the alluvial diamonds at Lulo....Vast areas of shallow-buried Calonda Formation gravels exist at Lulo"
If you look at the schematic diagram above the Calonda alluvial gravels will be more obvious and easier to identify if the modern river system has exposed them higher in the erosion profile. As you can also see from the schematic those ancient alluvial gravels can be remobilised to form lower river terraces and also have the potential to mix in with younger river sediments closer to the bases of river beds.
So it is pretty clear from this that diamonds eroded from kimberlites on the licence by the modern river system could be mixed with diamonds eroded out of the ancient Calonda alluvial gravels from the same kimberlite but during two different geological epochs separated by approximately 66 million years. The sources of the Calonda gravels would be far harder (if not impossible) to determine due to their more distant in time modes of emplacement.
Eshmun
If you want to learn about the emplacement of kimberlites and diamond deposits in Angola this is a very good source.
http://www.google.com.au/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CBwQFjAAahUKEwi3oODVurXIAhUBKZQKHcnfBvg&url=http://repositorio.lneg.pt/bitstream/10400.9/690/1/Com90_p189.pdf&usg=AFQjCNH8f3yuWAn8l2TJ62mifIIvaQIPYw&sig2=M1VjhTGV20BeX-Tbx2vg3g&bvm=bv.104819420,d.dGo
(20min delay)