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Diamonds from Argyle lamproite pipe

Western Australia's Argyle lamproite pipe is now the world's largest producer of diamond by volume, yields brown, yellowish brown, colourless and red to pink diamonds that have a dominantly eclogitic paragenesis. Fancy coloured Type 1a Argyle diamonds typically have low nitrogen content, mostly in the B-aggregated form. These diamonds are highly strained, colour zoned parallel to {111}, and mostly contain proto/syngenetic inclusions that reveal an eclogitic paragenesis.

The Discovery

Systematic exploration for diamondiferous diatremes in the West Kimberley region of Western Australia commenced in 1969, following the recovery of nine diamonds from the Leonard River by Oilmin N.L. exploration geologists. By 1972 the Kalumburu Joint Venture - consisting of Tanganyika Holdings Ltd, A.O. (Australia) Pty Ltd, Northern Mining Corporation N.L., Jennings Mining Ltd, and Sibeka Societe D'Enterprise et D'Investissements SA - had been formed for the express purpose of exploring the Kimberley Region for diamonds above latitude 19° N.

Early 1976 saw the first successful recovery of kimberlite indicator minerals from routine stream samples taken from the region. This discovery led CRA Exploration to join with the Kalumburu Joint Venturers to form the Ashton Joint Venture (AJV).

In August 1979, the AJV laboratory in Perth reported that two diamonds had been discovered in a sample of gravel collected from Smoke Creek, a small creek that drained north easterly into Lake Argyle. Further progressive sampling upstream led to the subsequent discovery of the Lower and Upper Smoke Creek alluvial deposits, and ultimately, on 2nd October 1979, geologists walked onto and recognised the potentially diamondiferous AK-1 (Argyle Kimberlite No. 1) olivine lamproite pipe. Today this pipe is commonly referred to as the Argyle pipe.

The Argyle Pipe

The Argyle pipe is located at the headwaters of Smoke Creek, in a small valley near the eastern end of the Matsu Range. The pipe is located some 120 km from the nearest town, Kununurra, 25 km upstream from Lake Argyle, and 2,200 km north-east of Perth. When first discovered, the Argyle pipe occupied the whole valley floor and had a surface area of about 45 ha. It had a distinctively linear outcrop with a length of 1,600 m, and a width that varied from 150 to 600 m.

Subsequent petrological investigations revealed that the Argyle pipe contained both tuffaceous and magmatic varieties of lamproite. While the predominantly eclogitic diamonds of the Argyle pipe have been dated at an estimated 1,580 million years old, these diamonds were emplaced hydrovolcanically shortly after their formation between 1,100 and 1,200 million years ago. This relatively short storage time (~ 400 m.y.) and the dominance of B-aggregated nitrogen indicates the diamonds experienced relatively high temperatures during this time.

That the existence of a complex plumbing system below the mobile belt into which the Argyle pipe is emplaced, and a complex annealing history, could account for the dominantly small, highly modified (by dissolution), poor quality B-aggregate rich diamonds that commonly occur in the Argyle pipe. Having initial proven reserves of 61 million tonnes or ore, with an average grade of 6.8 ct/tonne, and further estimated reserves of 14 million tonnes, at a grade of 6.1 ct/ tonne, the Argyle pipe became the world's largest volume producer of diamond7. Exploration of the Argyle pipe and its surrounds was completed when in 1981 a second high-grade deposit of alluvial diamonds was located on Limestone Creek, a non perennial stream that drains the Argyle pipe to the south-east.

Rapid evaluation and development of the Argyle pipe followed; with mining of the diamondiferous Smoke Creek and Limestone Creek gravels commencing in 1983, and open cut mining of the Argyle pipe commencing in December 1985. Ten years later, a drilling program, initiated to assess diamond grade below the planned open pit bottom to about 300 m, revealed a possible diamondiferous resource of up to 100 million tonnes of ore having an average in situ diamond content of 3.7 ct/tonne.

Continuing studies into the future of the Argyle open pit have led to a decision, during June 1998, not to proceed with underground mining. Instead, a decision was made to cut back the west ridge of the mine to access additional ore (64 million tonnes with a diamond content of 2.58 ct/tonne) suitable for open cut mining. Further it has been suggested that underground mining, by sub-level caving, may come on stream later in the first decade of the 21st century.

Argyle Diamonds

Over the first fifteen years of the Argyle pipe's exploitation as an open cut mine, annual production has increased substantially and to such an extent that annual production peaked in 1996 at 42 million carats of which almost 2.7 million carats were derived from associated alluvial deposits. Despite a small drop in production to 40.8 million carats in 1998, this rate of production has assured that Australia continues to be the world's leading producer of diamonds, by volume, but not by value. The reasons for this apparent contradiction are quite simple. First, the average mix of diamonds in the Argyle pipe consists of 55 per cent industrial quality diamond, 45 per cent near gem quality diamond, and only 5 per cent gem quality diamond11. Second, of the small percentage of the gem quality diamonds recovered from the Argyle mine about 95 per cent are brown and brownish yellow hued, some 4 per cent are either colourless or grey, and much less than 1 per cent have the very desired pink to red hues. While Argyle diamond sales has little problem marketing its popular pinks and reds, persistent and innovative marketing has resulted in ever increasing acceptance for champagne and cognac browns, as well as markets for small sized diamonds that are cut from near gem rough by labour intensive Indian manufacturers.

Gemmological Characteristics

After more than a decade of intensive marketing, and following more than a decade of intensive research, it is indeed surprising that so little has been published in the gemmological literature on the characteristics of Argyle's brown to yellow, pink and colourless diamonds.

It is clear that a significant feature of diamonds from the Argyle pipe is that the majority of them have suffered deformation of their crystal lattice. In instances where diamonds are plastically deformed, and their nitrogen content is low, a brown or pink colour is produced. The nitrogen content varies from 500 to 1,000 ppm in colourless diamond, from 100 to 500 ppm in brown diamond and 10 to 100 ppm in pink-red-mauve diamond.

The majority of diamonds are brown (~ 80%), then yellow (15%), while colourless (2%), grey (2%), pink and green (<1%) are less than 1/20 of an average parcel. Coloured stones show planar colour zoning associated with one or rarely more (111) slip planes. When examined between crossed polars displays a 'tatami' pattern of strain birefringence.

Most Argyle diamonds are heavily frosted, have prominent etched channels, and have external surfaces patterned with hexagonal etched pits.

Diamonds mined from Argyle present mainly irregular shapes (<60%), then macles (~ 25%), crystal aggregates (~ 10%), strongly resorbed dodecahedra and octahedra-dodecahedra (~ 5%). Cubes are rare.

Inclusions are 75 % eclogitic, 10 % peridotitic, 10 % indeterminable sulphides.

Eclogitic proto/syngenetic inclusions were orange garnet (57%), garnet + clinopyroxene (16%), omphacitic pyroxene (6%), kyanite (3%), rutile (2%), coesite (1%), mixtures of rutile-garnet, garnet-sulphide, garnet-clinopyroxene-sulphide, garnet-kyanite, kyanite-sulphide (15%). Peridotitic proto/syngenetic inclusions were olivine (45%), pyrope garnet (9%), enstatite (9%), mixtures of olivine-diopside, olivine-garnet, olivine-garnet-enstatite, enstatite-garnet (37%). Epigenetic graphite lining cleavages and fractures is the commonest inclusion in Argyle diamond.

Fluorescence and phosphorescence
UV Fluorescence: Blue (LWUV>SWUV) due to N3 centre for colourless, dull green (LWUV>SWUV) for browns and blue (LWUV>SWUV) for pink to mauve diamonds.
UV Phosphorescence: Yellow for colourless, dull yellow-inert for browns and yellow for pink to mauve diamonds.
X-ray Fluorescence: Blue-white for colourless, browns and pink to mauve diamonds.
X-ray Phosphorescence: Yellow for colourless, browns and pink to mauve diamonds.

Absorption
Spectral- Infrared: B-aggregate absorptions (8.4m >7.8 m ) for colourless diamonds, mixed B- and A-aggregate absorptions (8.4m " 7.8m ) for brown diamonds and A-aggregate absorptions (7.8m >8.4m ) for pink to mauves.
Visible: N3 absorptions for colourless, increasing absorption towards the blue with superimposed N3 (415.2 nm), N2 (478 nm), and H3 (503.2 nm) absorptions for browns and a broad absorption centred at 550 nm, N3 absorptions for pink to mauve.
Ultraviolet: Absorption edge at ~ >230 nm for colourless, absorption edge between 230 and 320 nm for browns and absorption edge at ~ <320 nm for pink to mauve.