English: Argyle Lamproite - olivine lamproite lapilli-ash metatuff from the Mesoproterozoic of northwestern Australia (field of view ~3.7 cm across).

Kimberlites and lamproites have tremendous economic importance because they are host rocks for gem-grade and industrial-grade diamonds. Kimberlites & lamproites are unusual igneous bodies having overall pipe-shaped geometries. Their mode of formation is only moderately understood because they have not been observed forming. Kimberlites & lamproites are known from scattered localities throughout the world - only some are significantly diamondiferous. Classic localities for diamonds are India and Brazil. Africa was also discovered to have many kimberlites and is world-famous for producing large numbers of diamonds. Other notable diamondiferous kimberlite-lamproite occurrences include Russia, China, northwestern Australia, and northwestern Canada.

Kimberlites are named for the town of Kimberley, South Africa. Several kimberlite pipes occur in the Kimberley area. Kimberlites have a gently tapering-downward, pipe-shaped cross-section. Lamproites have a cross-section more closely resembling that of a martini glass.

Northwestern Australia’s diamondiferous Argyle Lamproite was discovered in October 1979 using the classic prospecting technique of identifying indicator minerals in stream sediments. It is a unique diamondiferous unit due to its relative abundance of pink diamonds. Elsewhere on Earth, pink diamonds are exceedingly rare (two famous examples of subtly pink diamonds were found at Golconda, India and Tanzania, Africa). Pink diamonds from the Argyle Lamproite can be intensely pink, unlike those from outside Australia.

Several different colors of diamonds are processed at the Argyle Mine, including clear (“white”), pink, brown, yellowish-brown, and a few unimpressive blue diamonds. About 5% of Argyle diamonds are gem-quality.

Two major facies are present in the Argyle Lamproite (a.k.a. Argyle AK1 Lamproite, Argyle Diatreme, Argyle Lamproite Diatreme, Argyle AK1 Pipe): 1) olivine lamproite lapilli-ash metatuff; and 2) sandy lamproite tuff. The rock shown above is from the first facies - olivine lamproite lapilli-ash metatuff. The original olivines are gone, and have been replaced by the metamorphic minerals serpentine, talc, and chlorite (the serpentine & chlorite give the rock its greenish coloration). Mineral studies indicate that Argyle Lamproite material also contains sericite, K-feldspar (replacing original leucite), some carbonate, chromite, phlogopite, and several accessory minerals.

The Argyle Lamproite Pipe erupted during the mid-Mesoproterozoic, at about 1.178 billion years ago. The included diamonds are much older (~1.6 billion years). The Argyle Pipe is fortuitously preserved. Nearby pipes have eroded down to their roots. A quartzite ridge adjacent to the Argyle has protected it from erosion.

The pink coloration in some Argyle diamonds is apparently not the result of chemical impurity, which gives most diamonds their color variation. The pink color appears to be related to metamorphism-induced microfaulting or slippage of carbon atoms within the diamond crystals.

Geologic context: lamproite pipe emplaced along a splay of the Glenhill Fault, near-eastern margin of the Halls Creek Mobile Zone, eastern edge of the Kimberley Craton.

Location: Argyle Mine, near the headwaters of Smoke Creek, west of Lake Argyle, northwestern Western Australia (~16° 14’ South, ~128° 23’ East).

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Info. mostly synthesized from:

Karen Rice (pers. comm.)

Boxer et al. (1989) & Jaques et al. (1989a, 1989b) & Pidgeon et al. (1989) - the latter four references are published in Kimberlites and related rocks, Volume 1, their composition, occurrence, origin and emplacement. Geological Society of Australia Special Publication 14.