RETIRED & LEISURED GROUP APRIL MEETING REPORT
Our April meeting was a talk by Tony Ahmat on Australia’s yellow diamonds, specifically the Ellendale diamond mine. Diamonds are the hardest naturally occurring substance and alluvial diamonds were known in Australia from 1851 and discovered in commercial quantities in 1895.
The Ellendale Diamond Field is one of only three hard-rock diamond mine locations in Australia and holds a unique and special place in Australia’s and the world’s diamond history. In 1937 Professor Rex Prider, Professor of Geology at UWA, predicted that diamonds might be found in lamproitic rocks in the West Kimberley region and this proved to be the case when the Ashton Joint Venture discovered diamonds in olivine lamproite at Big Springs and later in November 1976 at Ellendale. Ellendale was the first hard rock deposit to be found and it stimulated later discoveries at Argyle in 1979 and a further discovery in 1993. Until the discovery at Ellendale, kimberlitic structures had been seen as the only source of diamonds, but now the new host-rock, olivine lamproite, offered a new possibility.
The 1960s produced a more favourable situation for diamond mining and modern diamond exploration began in 1971. Mining commenced at Ellendale in 1976 and by the end of 1980, 46 pipes had been discovered, but test grading them proved the mine to be uneconomic and it was put on care and maintenance. In 1994 the Kimberley Diamond Company became involved with the Ellendale Mining Lease and commenced mining at Ellendale 9 pipe in 2002. Since then over 2.1 million carats of diamonds have been mined from Ellendale 4 and 9 pipes.
Ellendale is a source of high-value yellow diamonds, which comprise 14 per cent of Ellendale 9’s production. The yellow colour is due to trace amounts of nitrogen. Ellendale 4 has now been closed as the resource dwindled—and Ellendale 9’s future is not certain.
Tony showed us pictures of the laboratory processing of diamonds, which go through shaking tables to concentrate the heavy minerals. Then the materials are examined microscopically by mineral observers, followed by various other processes.
Indicator minerals have been important in the discovery of Ellendale 4; aeromagnetic data has been the most successful exploration tool for new discoveries because most of the pipes are more magnetic than their sandstone hosts.
Whilst mining is in process, the soils being removed are banked up on the sides of the mine and when mining work finishes, this soil is returned by the company involved, in order to restore the site. Some sites pose a problem, where companies have abandoned their sites without carrying out restoration.
This talk proved to be most interesting and was supported by excellent display material and specimens of rock. Tony was a very warm and friendly guest speaker.
Margot Bentley