Diamond-bearing kimberlite pipes and dykes have been discovered 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 comprises 14 kimberlite pipes in five separate clusters. In 2014, the combined Probable Ore Reserve for all diamond pipes at Merlin was 4.04 Mt at 0.15 carats per tonne (ct/t) for a total of 0.61 Mct, and the Indicated and Inferred Mineral Resource is 27.8 Mt at 0.16 ct/t for a total of 4.35 Mct. 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.
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.
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 (NTGS DIP 011, available on DVD from the InfoCentre).