During 2019, Rio Tinto completed the drilling of 10 bulk sample holes (trenches) on the Star kimberlite using a trench cutter sampling rig. A significant number of diamonds from the EJF geological units were analyzed, containing a spectrum of diamond sizes from 0.18 carats through all the large stones, up to diamonds of over 15 carats.
The study conducted by Star Diamond confirmed that unusually high proportions of Type IIa diamonds are present in the kimberlite. Of particular note is the exceptionally high proportion of Type IIa diamonds in the larger size fractions of the EJF, of which 53% (8 of the 15) of the largest stones, 4 carats and above, are Type IIa. Nine of the 12 highest-value stones from the EJF trench cutter samples (3 carats and above) are Type IIa. The largest Type IIa diamond identified is a 16.96-carat stone.
“The presence of a significant proportion of Type IIa diamonds recovered in the Star kimberlite by the trench cutter greatly increases the potential for the recovery of large (plus 100 carat), high-value diamonds from a future mine,” George Read, senior technical advisor to Star Diamond, said in a news release.
The analysis also reinforced and expanded on the earlier studies conducted by Star Diamond of Type IIa diamonds at Star (26.5%, 0.32 carats and above), as well as other kimberlites within the Fort à la Corne district. Recent study on the Orion North and Taurus kimberlites also yielded high proportions of Type IIa diamonds.
According to Star Diamond, Type IIa diamonds are very rare and account for less than 2% of all natural rough diamonds mined from kimberlites. Many high-value, top colour, large specials (greater than 10.8 carats) are Type IIa diamonds, which include all 10 of the largest known rough diamonds recovered worldwide.
Only a small number of active diamond mines regularly produce Type IIa diamonds with the most important of these mines being Letseng-la-Terae (Letseng mine) in Lesotho, and more recently Karowe in Botswana, it said.
Star Diamond currently holds a 25% interest in certain Fort à la Corne kimberlites (including the Star – Orion South diamond project).
