Diamonds

Diamond-bearing kimberlite pipes and dykes have been discovered in geological basins across the northern portion of the NT. Opencut mining between 1999 and 2003 at the Merlin Diamond Field produced in excess of 500 000 ct, valued at US$108 per carat. The high value of diamonds from the Merlin pipes is related to the significant recovery (30% by weight) of gem quality stones and relatively high (55%) proportion of white gems.

The Merlin Diamond field contains 13 kimberlite intrusives distributed in five discrete clusters. Emu 1 is the largest pipe, some 250 m in diameter. Mining was restricted to 9 pipes that ranged from 40 to 110 m in diameter. The largest diamond in Australia (105 carat and valued at US$525 000) was recovered from the Gareth pipe. A 44 carat diamond (valued at US$110 000) was also recovered from the Palomides pipe. Striker Resources NL are aiming to increase the current resource under and between the Palomides, Sacramore and Launfal kimberlite pipes and recommence mining in the future. Additional untested kimberlite targets (eg HUM 7 Anomaly) exist near the mine site.

The Devonian-age (367 Ma) pipes in the Merlin Diamond field intrude sedimentary rocks of the southern McArthur Basin and northern Georgina Basin. Other significant diamond occurrences in the area include the Coanjula litharenites and Abner Range diatreme. Diamond-bearing kimberlite dykes have intruded sediments of the McArthur Basin at the Packsaddle and Blackjack prospects, northeast of Mataranka.

Jurassic-age (179 Ma) diamond-bearing kimberlite pipes and dykes intrude Victoria Basin sediments near Timber Creek. Bulk sampling of the main pipe (TC-01) has recovered several hundred gem and industrial quality diamonds. Approximately 40% appear as gem quality, consisting of white, yellow, brown and pink varieties. Some 20% are near gem and 40% are industrial. Tawana Resources NL estimate an average diamond value of US$40 per carat. Ore grade for TC-01 is estimated at 1 ct/t. Potentially diamond-bearing alluvial material has been identified immediately downstream from the Timber Creek Kimberlite cluster.

The Northern Territory is presently receiving intense interest from both major global diamond players and several active juniors. Together, these diamond explorers account for a significant proportion of Exploration Licence (EL) applications and grants in the Territory.

As a result of increasing diamond exploration activity the Northern Territory Geological Survey has developed a digital database of sample points with associated diamond indicator mineral results which is linked to a diamond mineral chemistry database (Diamond Indicator Mineral and Diamond Mineral Chemistry Database).

Diamond exploration surveys over the past 30 years have identified a great swathe of microdiamonds that extends across the heart of the cratonic Northern Territory. This swathe envelopes known kimberlite occurrences and extends well beyond them. There are literally thousands of microdiamond occurrences within this region. Individual grain shapes are mainly octahedra and dodecahedra, but there are some twins and irregular grains, and some curious frosted spongy cubes. There has been some discussion within the industry on the significance of this swathe, including suggestions that microdiamonds were recycled through the Cretaceous and have been redistributed more widely by fluvial and aeolian processes.

Some important exploration insights from the Merlin discoveries include:

• there is a correlation between microdiamond and macrodiamond tenor
• kimberlite intrusives occur within tableland geomorphology with ill-defined drainage
• intrusives are small at the surface, but occur in clusters that merge at depth
• resistant Cambrian sedimentary host rocks act to preserve the crater facies
• Cretaceous sediments cap the kimberlite pipes, even when they are largely exhumed and this inhibits the ability of the kimberlites to shed indicators
• kimberlite intrusives have no anomalous magnetic or gravity signature (but do have an EM expression)
• intrusives within kimberlite fields are controlled by fractures in a cratonic environment (not mobile belts).

These features have drawn today's diamond explorers away from mobile belts (setting for the Argyle diamond deposit in Western Australia) and into the heart of the North Australian Craton (NAC), with its challenges of ill-defined drainages and lateritic weathering. In the vanguard are the junior explorers. This new wave of explorers is chasing subtle indicator minerals including the microdiamond swathe, but is also employing geologically-based area-selection concepts.

The recent availability of high-resolution aeromagnetic, gravity, crustal temperature and teleseismic geophysical datasets allows favourable tectonic areas to be identified over much of the North Australian Craton. Deep lithospheric roots below ?old and cold? crustal nuclei may extend into the elevated diamond stability field, which may be tapped by much younger kimberlite and related diatremes. There is geophysical and geochronological evidence that much of the NAC is underlain by Archaean basement, or by basement reworked prior to the Barramundi Orogeny. Cratonic nuclei may be quite prevalent in the craton, but are obscured by the effusive Antrim Basalt.

Elkedra Diamonds NL has identified and pursued numerous diamond targets in the southern Georgina Basin overlying the Altjawarra cratonic nucleus within the NAC. This basement feature may be a classic Archaean nucleus that contains a deep cold sub-lithospheric root, and as such, is considered to be highly prospective for diamond-bearing kimberlite. Previous sampling by CRAE and Stockdale recovered many microdiamonds and at least one macrodiamond from this area.

The prognosis for the Northern Territory must be considered high by world standards, because:

• the microdiamond swathe must have prospectivity significance
• Archaean basement under the NAC must be extensive
• the NAC deep lithosphere is demonstrably fertile for diamonds
• Palaeozoic cover offers protection and preservation for kimberlite intrusives
• Cr-spinels and microdiamonds are robust in the weathering environment
• there is extensive coverage of high-resolution airborne geophysical surveys that depict internal and marginal structural features of Precambrian basement blocks.