Lake Argyle is a man-made reservoir in the East Kimberley region of Western Australia, formed in 1971 by the damming of the Ord River and ranking, by capacity, among the largest artificial lakes in the southern hemisphere. The lake's principal economic significance, however, lies less in irrigation than in lending its name to the Argyle diamond mine, which between 1983 and November 2020 was the world's largest single producer of diamonds by volume and the dominant source of natural pink, red and violet diamonds. The mine itself is some distance from the lake, in the Smoke Creek area of the East Kimberley, but the geographic association has fixed the name in the gem trade.

Geography and naming

The lake takes its name from Argyle Downs Station, the cattle station established by the Durack family in the late nineteenth century, which was inundated by the formation of the lake in 1971. The lake covers an area that fluctuates between approximately 700 and 1,000 square kilometres depending on the season, and the diamond mine, opened in 1985 after exploration that began in the late 1970s, was sited on the AK1 lamproite pipe in country adjacent to the lake's southern shore. The locality, the lake and the mine are all set within the lands of the Miriuwung-Gajerrong people, and Indigenous Land Use Agreements have governed the post-1990s relationship between the operator (Rio Tinto, from 2001) and the traditional owners.

The Argyle mine

The Argyle mine was developed on a lamproite (not kimberlite) host of Proterozoic age, and it was the first major commercial diamond operation in lamproite. The mine produced approximately 865 million carats over its operational life, predominantly small brown and yellow industrial-quality stones, but with the world's only systematic production of pink, red, violet and purple diamonds. The annual Argyle Pink Diamonds Tender, run from 1984, became the central marketing event for these rare colours, with stones ranked on a proprietary scale (1PP through 9PR for pink, 1BL through 3BL for blue-violet) and sold by closed-bid tender to a small group of qualified buyers. The mine closed in November 2020, marking the end of new Argyle production, and the closure has been followed by a sustained appreciation in the secondary market for documented Argyle pink material.

The geological context

The Argyle lamproite is unusual in world diamond geology in that it formed within a Proterozoic mobile belt rather than on a typical Archean craton, against the conventional expectation set by Clifford's Rule. The host rock is an olivine lamproite of approximately 1.18 billion years age, which intruded as a series of pipes through Proterozoic country rock. The pipe is volcanically distinct from the more familiar kimberlite of southern Africa and Canada, and the Argyle production confirmed that economic diamond can occur in non-kimberlite lamproite under appropriate conditions. The pink colour of the diamonds is generally attributed to plastic deformation rather than to a specific impurity, with the colour centre arising from lattice distortion features.

Lake Argyle as a name in the trade

For the working diamond trade, 'Lake Argyle' is occasionally used as a near-synonym for 'Argyle mine' or as a regional descriptor for the broader area, but the precise designation 'Argyle' refers to the mine and its operator while 'Lake Argyle' refers to the body of water. The two are correlated but not identical. Pink and red diamonds documented as having come from the Argyle production stream are generally certified through the Argyle Pink Diamonds programme (now historic) or, on the secondary market, through laboratory grading reports that reference Argyle origin where chain-of-custody documentation supports it.

Position today

With the closure of the mine in 2020, the Argyle production stream is now finite. The remaining inventory in the hands of authorised partners, in the secondary market, and in collector hands constitutes the entire future supply of documented Argyle pink material, and the trade has accordingly seen the closure as a structural shift in the pink-diamond market. Lake Argyle itself remains a major irrigation and tourism asset for the East Kimberley, and the name continues to resonate in the gem trade well beyond the operational life of the underlying mine.