The Mysteries of Eastern Australian Diamonds

by Dr Julian D. Hollis B.Sc (Hons) Ph.D (London)

Are you a fellow diamond enthusiast? If so I would like to share some of my aesthetic pleasure derived from this beautiful gemstone. My approach is that of a geo-scientist working as a consultant to the gemstone industry, especially the Diamond Exchange.

My in-line to diamonds is via their crystallography – beautiful natural shapes reflecting their formation, provenance and tortuous history in volcanic systems.

Like snowflakes, no two crystals are ever the same. – Unique expressions of natural artistry.

In Eastern Australia we have a great geological mystery, shared by Alaska, California, Kamchatka, Burma, Thailand and the Urals: Theoretically diamonds should not be concentrated in the “blind placers” of such places. Most diamonds apparently come from stable ancient crust areas – cratons, beneath which the rocks are relatively cool, within the stability field for diamond. Being meta-stable, diamond readily converts to graphite or just burns to carbon dioxide unless stringent protection can be afforded. High temperatures cause destruction of diamond if upward transport is too slow, releasing pressure without appropriate cooling rates.

Eastern Australia presents a puzzling situation where certain local areas yield abundant diamonds, at least 500,000 carats from the Copeton and Bingara Fields of Northern New South Wales alone. Prior to production from Argyle , Western Australia this was the only commercial production in Australia. But ; one big problem: The NSW diamonds occur over 1000 km from the nearest craton, straddling a geothermally hot “mobile belt” left over from a major mountain – building episode (orogeny) from some 400 to 200 million years ago. This does not appear to be a suitable place to find a primary diamond-bearing volcanic system! But; how does one explain local concentrations and peculiarities of form characterising each find?

Long distance alluvial travel or ice carting dilute and scatter diamonds to uneconomic grades.

At various points along the Eastern Australian mobile belt there are strange relics of sub-volcanic structures, some directly associated with diamonds.

These range from narrow fissures to large basin like features, filled with ground up:”Tuff sites”. Tuff sites are gas-dissociated rocks that have been churned and intruded.

Associated diamonds show minimal abrasion and strong rounding, also a high degree of polishing due to surface melting. They are unlikely to have travelled far. But, back to the crystals:

An Australian company exploring for diamonds recently recovered some 8500 diamonds from a test program in the area. These included an unusually fine suite of crystals.

Curiously the larger diamonds seldom show clear crystal form. It is distinctly unusual this way around, even more so as there are no sub-1mm micro diamonds.

Crystal types are also abnormal; being dominated by highly rounded forms resulting from magmatic resorption. These are frequently sculptured to oddly shaped 12 sided Dodecahedra or 24 sided Dido decahedral forms.

The normal Octahedra are extremely rare, in sharp contrast to their dominance in Kimberlitic systems. However these too are present and show the usual progressive resorption from crystal face edges (coigns).

Many crystals are twinned, with planes forming ultra –hard zones (naatz). Other naatz reflect planes of stress formed under intense shearing in upper mantle, over 120 km (75miles) down.

Diamond crystal forms appear to be dictated by depth (pressure) and temperature conditions during formation. Those from shallow levels are mostly cube forms which are extremely rare in Eastern Australia.

During rapid transport to the surface, propelled by volatile rich melts, diamonds experienced extensive attack, with size reduction due to gas and magmatic etching.

This hot, chemical buffeting was responsible for a wide range of diamond surface features, such as micro-ovals, pits ,laminations and geometric ornamentations (trigons,etc) adding to the great range of crystal variety.

So back to our Eastern Australian diamonds. Where do they come from?

During the Palaeozoic, between 400 and 300 million years ago, it appears that the Pacific ocean crust was spasmodically forced beneath the eastern margin of Australia successive slabs descended into the mantle where they remained sufficiently cool (perhaps near 300 – 500º C as opposed to over 1000ºC in the surrounding mantle) for long enough to host diamond formation.

Diamonds probably grew from carbon released from the dissociation of primitive, deep mantle methane, out gassing into favourable sites in the slab material. This may have included biogenic carbon from the slab itself.

The potential volatile content held in the altered slab material at over 120 km must have been phenomenal. This may have been triggered some 220 million years ago by a deep hot plume and a head of volatiles and molten carbonate rich material ascended rapidly to violently burst out at the surface. This carried the released diamonds largely intact and in great numbers.

Not a normal craton hosted Kimberlite ( or Lamproite) system but... Any more ideas?

We would like to hear from you.

Please contact: Dr. Julian Hollis at jhollis@diamondexchange.com.au