Opening the jewel box
Clear objectives were set for the search: firstly, to find a diamond larger than a quarter of a carat; secondly, to find a diamond-bearing pipe; and, thirdly, to establish that the pipe was of a diamond grade high enough to justify a mine.
The first objective was reached in July 1976 when a diamond of more than a quarter of a carat was found in the King George River in the north Kimberley, although the second objective had already been achieved with the discovery of a diamond-bearing pipe – Big Spring No 1 – at the remote site of Mt North Creek.
Bulk testing of this pipe and four others found nearby in 1977 disappointingly revealed diamonds of only very small size. However, the sampling did turn up some important new geological information: in contrast to the South African diamond finds, chromite was revealed as an indicator mineral for diamonds in Australia and lamproite was indeed the host rock for the diamonds. Rex Prider’s theory had finally been proven.
While samples from the west Kimberley eventually led to the discovery of 49 lamproite pipes, and a central treatment plant was built to recover some 92,000 diamonds, it was determined that the grade of these deposits was not quite high enough to warrant establishing a mine.
Happily, further exploration in the eastern part of the Kimberley was to deliver on the search’s third objective. Sampling of Smoke Creek – running some 35 kilometres from the Matus Range to Lake Argyle – revealed not just the usual indicator minerals for diamonds, but whole diamonds. Indeed, members of the venture team were astonished to find up to 15 diamonds in their pan when they sieved gravel from certain parts of the creek and fittingly dubbed it “the jewel box”. Initial fears that the diamonds might just be alluvial were allayed when a sizeable lamproitic host pipe, later to be named AK1, was discovered.
Over the next three years, deep drilling was carried out to fully evaluate the pipe and confirm that it was indeed commercially viable. Open pit mining commenced in December 1985 and while the diamonds unearthed were of much lower value than world average, the volumes were immense and the grade was of a richness that would rewrite geological textbooks. Argyle would become one of the world’s largest producers of diamonds and its largest supplier of natural coloured diamonds, including white, champagne, cognac, blue and the highly-coveted rare pink diamonds.