“In some cases, such as at the DeBeer's Orapa mine in Botswana, the grade of the volcaniclastic rocks can be many times higher than the grade of the associated Orapa pipe material.”
I have spent some time trying to confirm this. Here is an internet location that reviews diamond production at that mine and others. When you get there, go to the bottom - see “complete report”.
layfieldresources.com
Some quotes:
“Mining of the crater facies, eg. at the Orapa Mine, is a critical part of the Botswana diamond industry which generates an annual revenue of close to US$2 billion from three kimberlite mines. The latter includes the Jwaneng Mine, located to the east of, and possibly along the same crustal fracture zone as Molopo and which is the richest diamond mine in the world.”
“Orapa Kimberlite Field
The Orapa AKI pipe is the largest of about 30 pipes in this field and was discovered by De Beers in 1967. It and two others, DK1 and DK2, are economically diamondiferous. The latter two were brought into production in 1977 as the Letlhakane Mine.
In Hawthorne's model of the Orapa AKi kimberlite pipe, the present land surface across the pipe is shown as being very close to the level of the original land surface at the time of kimberlite emplacement. As a result, it was originally thought that all 30 pipes would be at a similar erosional level. However, only the AKI, AK2 and AK3 pipes contain crater facies type kimberlite, although it has been reported that it is difficult to distinguish highly altered and weathered tuffisitic varieties of the diatreme facies from the pyroclastic varieties of the crater facies in drill core.
At Orapa, five types of crater facies sedimentary kimberlite are reported to have been recognised. The coarsest-grained are steeply dipping debris flow deposits around the margin of the crater. Gritty mud flows are located within this perimeter of coarse clastics, as are a small volume of sedimentary rocks derived by fluvial reworking within the crater. A centrally-disposed group of playa lake facies sedimentary rocks comprises alternation between fine grits and shales representing less viscous debris flows, and sediment settled from suspension. The crater facies sediments contain a diverse assemblage of fossil insects and plant remains which have been dated as Middle to Upper Cretaceous.
In 1993, the level of excavation at Orapa exposed an 80 metre section of epiclastic (sedimentary) kimberlite overlying pyroclastic and diatreine facies kimberlites. In addition, a hypabyssal facies plug was mapped in the southwest portion of the pipe, and assumed to represent a later magmatic event post-dating the crater development.
The remaining pipes in the Orapa field all appear to be either diatreme or hypabyssal facies kimberlites. The DK1 and DK2 pipes at Letlhakane are hypabyssal facies. All the Orapa field kimberlites appear to be Group I kimberlites and mineral inclusions in diamond at Orapa indicate a predominant eclogitic paragenesis.”
Perhaps Jesse would be willing to give this the benefit of a review for comment and pass it on to Richard Boulay.
Larry Meneely |
