Northern Territory, NT, Australia

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The Merlin kimberlite field is located in the Batten Trough of the southern McArthur Basin, Northern Territory, Australia. It is about 90 km south of Borroloola and 725 km SE of Darwin.

The Batten Trough is located on the eastern side of the North Australian Craton in an area dominated by the Mesoproterozoic McArthur Basin, part of the North Australian Platform. Basement to these sediments include late Palaeoproterozoic volcanics. The Batten Trough is overlain by Cambrian sandstones and flood basalts. The Merlin kimberlites are the youngest volcanic event in the region. The Batten Trough is a structural zone, bounded to the west by the north-south Emu Fault and is also a graben, filled with up to 10 km of McArthur Basin sediments. Two major NW trending faults, the Mallapunyah and Calvert Faults cut the trough in the Merlin district.

The field comprises thirteen single to bifurcating kimberlite pipes and two venting sandstone breccia pipes that occur in five clusters over an area of 10 x 5 kms. They are found on the eastern shoulder of the Batten Trough, 6 km east of the Emu Fault and on the projected trace of the Calvert Fault. The 13 pipes on the Merlin project area are Excalibur, Palomides, Sacramore, Launfal, Kay, Ywain, Gawain, Tristram, Gareth, Ector and Bedevere, as well as the E.Mu1 and E.Mu 2 pipes. The distances between pipes in each of the five clusters is from 100 to 400 m. The largest pipe in the field is E.Mu 1 which covers an area of 4.5 ha and is 250 m in diameter. All of the pipes are steep sided, with generally a cylindrical shape, not diminishing in diameter to depths of more than 100 m.

Numerous intrusive events are indicated by multiple textural phases within each of the kimberlite pipes, typically comprising the following textural groups: i). volcaniclastic kimberlite, ii). micro-breccia volcaniclastic kimberlite, and iii). volcaniclastic kimberlite breccia representing mostly diatreme facies.

Age dating on phlogopite from a number of pipes on the north and south of the field consistently yield emplacement ages of Middle Devonian (370-380 ±2 Ma). The location and attitude of all the kimberlites is controlled by 15° trending structures and intrude basement volcanics and Neoproterozoic shelf sediments, the youngest member of which is the Bukalara Sandstone. All pipes intrude the Bukulara Sandstone. Contact breccias, sandstone breccias, and blind kimberlite apophyses are often associated with each intrusion.

The primary kimberlite mineralogy is often extensively altered and comprises two generations of olivine, now pseudomorphed by predominantly serpentine and carbonate. Other primary phases include phlogopite, chromite, pyrope garnet, apatite, and possibly rare picro-ilmenite. Secondary alteration phases include carbonate, serpentine, chlorite, and kaolinite. The whole rock geochemistry of the Merlin pipes conform to the standard average geochemistry for Group 1 kimberlites.

A localised and restricted Cretaceous marine infill-sequence and a thickened Tertiary weathering profile overlie all of the kimberlites in the field except for the E.Mu 1 & 2 pipes. These infill sediments are almost entirely confined to the pipe depressions in the district, and comprise a basal conglomerate overlain by a sandstone and claystone sequence which commonly 'cork' the kimberlite depressions which are surrounded by the resistent Bukalara Sandstone. The presence of slickenside near the margins of the kimberlites, slumping features, and an increase in jointing within the infill-sediments imply that the kimberlites have undergone post-emplacement subsidence. Smaller diameter pipes have undergone the greatest depth of subsidence, to as much as 42 m. It is suggested the subsidence be related to solution weathering, hydrothermal alteration and/or possibly halmyrolysis.

Diamonds are found in all of the Merlin pipes. Grades vary from trace amounts in E.Mu 1 and 2 to 102 cpht in Ywain based on >1.2 mm diamonds. The value of diamonds (in 1998) from Palomides average USD 60.00 per carat and from Sacramore usd 75.00 per carat.

The Merlin mine, operated for just over three years, closing in 2003 during which time it produced 0.5 million carats of high quality diamonds at an average value of USD 100 per carat. The largest diamond recovered was a 25.9 carat gem quality stone from the Palomides pipe. All pipes, with the exception of Excalibur, produced stones in excess of 10 carats in weight. Initially, production was focussed on the Southern pipes (Palomides, Sacramore, Excalibur and Launfal).

The remaining reserves in 2003 have been declared at: 9 Mt averaging 0.2 carats per hundred tonnes, for 1.8 million carats of diamonds.

Resources quoted by the Northern Territory Geological Survey, 2019 were: 27.8 Mt @ 0.16 carats/t diamonds.

For details consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2004.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Ahmad, M., Dunster, J.N. and Munson, T.J.,  2013 - McArthur Basin: in Ahmad M and Munson TJ (compilers)  Geology and mineral resources of the Northern Territory Northern Territory Geological Survey   Special Publication 5 pp. 15:1-15:72
Lee D C, Milledge H J, Reddicliffe T H and Scott Smith B H  1997 - The Merlin kimberlites, Northern Territory, Australia: in   Proc. 6th International Kimberlite Conference, Volume 1 - Kimberlites, related rocks and mantle xenoliths Russian Geology and Geophysics, Spec. Issue, Alberton Press, New York   v38, no. 1 pp 82-96
Lee D C, Reddicliffe T H, Scott Smith B H, Taylor W R and Ward L M  1998 - Merlin diamondiferous kimberlite pipes: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne   Mono 22 pp 461-464

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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