The search for diamonds in a remote and underdeveloped region of Western Australia in the early 1970s was a bold and challenging undertaking that would deliver a number of surprises and effectively rewrite geological record books.
When Ewen Tyler was a geology student at The University of Western Australia in the 1940s, kimberlite was then the only known host rock for diamonds. However, Tyler was intrigued by the observations of his geology professor, Rex Prider, that lamproitic minerals found in the Kimberley region of Western Australia bore a strong resemblance to those accompanying diamond discoveries in South Africa.
Returning to Australia in the late 1960s after a decade working in mining exploration in Africa and Europe, Tyler was keen to test Prider’s theory that lamproite might also be a host rock for diamonds. He formed the Kalamburu Joint Venture to begin commercial exploration for diamonds in the Kimberley.
The search began in 1972, with the team’s geologists charged with the enormous task of sampling sediments from all the creeks and rivers in the Kimberley – a vast and isolated region roughly the size of the British Isles. The hope was that if there was an exposed diamond pipe, it would have been crossed by creeks or rivers during the region’s pronounced wet season and at least some speck of indicator minerals would have been washed downstream as it eroded.
By early 1974, indicator minerals and diamonds had been found in samples from the north, east and west Kimberley and the joint venture group decided it needed more resources to intensify its exploration. A timely meeting with John Collier, who was head of exploration for CRA Ltd (later to become Rio Tinto Ltd) helped to secure CRA’s participation in the project under the new name of the Ashton Joint Venture. Collier, who had had great success in opening up CRA’s giant Hamersley iron ore project in Western Australia, was an important champion of the diamond project, ensuring that the best equipment and resources were made available to continue the search in earnest.
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 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.
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 the largest supplier of natural coloured diamonds, including white, champagne, cognac, blue and the highly coveted rare pink diamonds.