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Argyle Alluvial Diamonds - Limestone Creek, Smoke Creek, Bow River

Western Australia, WA, Australia

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The Argyle alluvial diamond deposits in Smoke Creek and Upper Limestone Creek and the Bow River alluvial diamond deposit in Lower Limestone Creek,, all of which drain the Argyle AK1 diamond pipe, are located 100 km south of Kununurra in far northern Western Australia.

Alluvial diamonds discovered in Smoke Creek by CRA Exploration Pty Limited (for the Ashton JV) in 1979 led to the discovery of the Argyle diamond deposit which is situated in the headwaters of that creek. Prior to commencement of hard rock open pit operations at the Argyle AK1 Pipe in 1985, Argyle Diamond Mines (then owned by CRA Limited 56.8%; Ashton Mining Limited 38.2%; Western Australian Diamond Trust 5%) mined the upper reaches of both Smoke Creek (Upper Smoke Creek) and Limestone Creek, known together as the ‘Jewelers Box’, where 17.3 million carats of diamonds were recovered from 3.8 Mt of gravel. This alluvial operation was conducted in two campaigns, from 1983 to 1985 and 1989 to 2002, yielding a total of 32.7 million carats of rough diamonds (Boxer, Jaques and Raymond, 2017). During this period, the merger between CRA Limited and Rio Tinto Zinc plc saw CRA's share pass to the merged entity, Rio Tinto Limited. In 2002, Rio Tinto acquired the other partners and assumed 100% control of Argyle Diamond Mines. In 2008, Argyle Diamond Mines surrendered the Prospecting Licences that covered most of the Lower Smoke Creek resources which they had held since 1983. Since then a number of companies have tested and firmed up resources, but not as yet (2019) commenced mining these resources. The Upper Smoke creek mining by Argyle Diamond Mines took place over a 10 km length of the creek. The Lower Smoke Creek alluvial accumulations are distributed over a length of 13 km, ending where Smoke Creek flows into Lake Argyle. A further 12 km of the creek between the two deposits, lies within the Argyle Mining Lease. Historically, the Smoke Creek alluvial diamonds attracted a higher average price than the Argyle AK1 diamonds due to the natural processes that depleted the alluvial deposits of lesser quality diamonds.

In 1982, the Freeport McMoRan Australia (80%) and Gem Exploration and Minerals Ltd (20%) Joint Venture discovered the Bow River deposit, 30 km downstream along Limestone Creek from the Argyle AK1 pipe. Between 1988 and 1995, mining of this alluvial deposit treated 27 Mt of gravel to produce ~7 million carats of rough diamonds at a recoverable grade of 0.27 carats/tonne (Kimberley Diamonds ASX Release 2014). Mining commenced in 1988 under a joint venture between Freeport McMoRan Australia and Poseidon Limited. In 1989, Poseidon took over Freeport McMoRan Australia and in 1991, Poseidon merged with Normandy Resources NL to become Normandy Poseidon Limited and then the sole owner. The mine closed in 1995 and has not reopened (to 2020).

Smoke Creek - flows NNE from the diamondiferous Argyle AK1 olivine lamproite diatreme.   The alluvial deposit has been divided into Upper Smoke Creek, between the AK1 diatreme and a narrow gorge cutting through a prominent ridge of the siliceous Devonian Ragged Range Conglomerate, and the Lower Smoke Creek alluvials below the gorge.   The Upper Smoke Creek alluvials are developed where the creek crosses a broad valley of Cambrian Antrim Plateau Basalts, bounded to the south by the Precambrian Carr Boyd Group quartzites and to the north by the Ragged Range Conglomerate.   The Lower Smoke Creek alluvials formed on a basement of Devonian arkoses and siltstones.

The Upper Smoke Creek alluvials are composed of coarse, poorly sorted, massively bedded and loosely consolidated Modern Flood Plain gravels (as described below) averaging 1.5 m in thickness.   They contain clasts of Carr Boyd Group sediments, weathered Antrim Plateau basalts and occassional AK1 tuffs.   Diamonds occur at a very shallow depth in the creek bed and its incised tributaries.

The Lower Smoke Creek alluvials are below the gorge through the Ragged Range Conglomerate ridge.   Grades drop off downstream with distance from the source. There are four types of deposit, as follows (after Boxer and Deakin, 1990):
• Modern Channel (or D Gravels) - active channel gravels, narrow upstream in Upper Smoke Creek, and up to 400 m wide and 5 m deep downstream near Lake Argyle in Lower Smoke Creek;
• Modern Flood Plains (or C Terrace) - broad, sinuous and partly braided accumulations of unconsolidated channel fill gravels up to 4 m thick, with clasts of Ragged Range Conglomerate, and coarsening with depth. These gravels occur along the current drainages of both Smoke and Limestone Creek with latter containing the largest development. This type is the main host at Upper Smoke Creek, and have been mined extensively by Argyle Diamond Mines from the headwaters of Smoke Creek to the Argyle airport, some 10 km downstream.
• Low Terrace (or B Terrace) - occur as discontinuous remnants of gravel around 3 to 5 m above the present flood plain.   These deposits average 2 m in thickness, exhibit incipient lateratisation and may be covered by 2 m of sand overburden. These gravels have been dated as Pliocene in age (~5 Ma) and occur in both the Smoke Creek and Limestone Creek areas. They are most extensively developed on the southern side of the Argyle pipe, and comprise alluvial fan deposits, alluvial gravel terraces, reworked terraces and areas of remnant gravel lag on the flanks of the higher level eroded terraces. The diamonds in these gravels are interpreted to have been both derived directly from the Argyle pipe, and reworked from the pre-existing High Terrace gravels. They have a mean stone size between those of the High Terrace and the Smoke Creek floodplain gravels.
• High Level Terrace (or A Terrace) - found about 8 to 10 m above the current flood plain and ~5 m above the Low Terrace to form a plateau or mesa to the east of Smoke Creek.   They are around 2 m thick, lateritised and moderately cemented, and have no overburden. They typically form areas of inverted relief due to the 'armouring' effect of the coarse lateritised gravels and the comparatively more rapid erosion of the adjacent bedrock. The diamonds of the High Terrace have the largest average (mean) stone size and have been mined from just south of the Argyle airport to below the Gap on Smoke Creek.

The Limestone Creek - deposits are located immediately east of the AK1 diatreme and are a composite of an alluvial fan, terraces and current channel and flood plain accumulations.   The fan deposits are similar in age to the Low Terrace at Smoke Creek and have been reworked into Modern Channel deposits and flood plains.   The fan deposits are pisolitic, around 3.5 m thick and merge into low terrace deposits downstream.   Neither type has any overburden.   The current flood plain deposits are partly braided with up to 4 m of gravel below nearly 3 m of overburden.

In these two creeks, each type of deposit has a characteristic diamond size and distribution, with the higher level and older deposits having the coarsest, but with highest grades near the change in slope where the gradient drops off.   Diamonds occur throughout the gravel profile.   The product is around 10% gem and 90% industrial being frosted, irregular shaped and with numerous inclusions.   The largest diamond recoverered was 34 metric carats

Bow River,   Lower Limestone Creek - is ~30 km downstream from the Argyle AK1 pipe and the Upper Limestone Creek deposits. The deposit lies along Limestone Creek, which flows into the Bow River near its junction with the Ord River.   They comprise five types, as follows (after Fazakerley, 1990):
• T5 - Modern channel deposits - gravels within the bed of the current creek.   Like the provious type these are only low grade.
• T4 - Flood plain deposits - found both to the north and south of the current creek, well bedded and unweathered;
• T3 - Channel deposits cutting the previous types, at a level of around 2 m above the current flood plain.   These are the youngest gravels in the main channel and are similar in compsotion and character to the 2 preceeding types;
• T2 - Ferruginous terraces approximately 4 m above the current flood plain, forming a layer some 6 km long, 1 km wide and covered by a 2 m thickness of black soil and clay. The deposits range from a few cms to 5 m thick. Diamond grades tend to increase downwards;
• T1 - Old High Level lateritised terrace, approximately 8 m above the current flood plain, made up of moderately well sorted, deeply oxidised, subrounded pebbles, cobbles and boulders with diamonds throughout the profile.

The highest grade resources were in the T1 and T2 terraces where the resources listed below were contained. The run of mine at Bow River averages 18 to 25% gem quality, 65 to 72% industrial and 8 to 10% boart.   They range from brown to yellow to clear white, and include a few pink diamonds.

The production for the Argyle alluvials, mined in the first campaign from January 1983 to December 1985, was 8.6 million metric carats (Boxer and Deakin, 1990), comprising:
    Upper Smoke Creek - 0.580 Mt @ 4.6 Metric Carats/tonne;
    Upper Limestone Creek - 1.7 Mt @ 3.5 Metric Carats/tonne. Subsequent mining from 1989 to 2002, yielded a further 24.1 million carats of rough diamonds (Boxer, Jaques and Raymond, 2017)

The pre-mining reserve/resource at the Bow River deposit in 1988 (Fazakerley, 1990) was:
    Proven+probable reserve - 10.75 Mt @ 0.43 metric carats/tonne at a 0.3 metric carat/tonne cut-off;
    Possible resource - 2.19 Mt @ 0.39 metric carats/tonne.
The economic sections of these reserves and resources plus subsequently discovered resources were mined out between 1988 and 1995 with 27 Mt of gravel yielding ~7 million carats at a recoverable grade of 0.27 carats/tonne.
The size distribution and grades at Bow River were as follows (Marx, 1996):
  T1 gravel - 0.571 carats/tonne, 0.300 carats/stone mean size, and 1.840 stones/tonne;
  T2 gravel - 0.420 carats/tonne, 0.250 carats/stone mean size, and 1.680 stones/tonne;
  T3 gravel - 0.168 carats/tonne, 0.209 carats/stone mean size, and 1.804 stones/tonne.

The Lower Smoke Creek resource, which is un-mined (as of 2019), is 22 km NE and downstream from the Argyle AK1 pipe. It contains lower grade JORC compliant inferred resources, of 21.5 Mt @ 0.28 carats/tonne for 6 million carats (cut-off 0.1 carats/tonne), as reported by Kimberley Diamonds Limited (ASX announcement, 2014). The same report listed a global resource of 48.5 Mt @ 0.15 carats/tonne at a lower cut-off, which it said was consistent with the resource estimate in the CRA Exploration relinquishment report.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2018.     Record last updated: 3/9/2020
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:
Boxer G L, Deakin A S  1990 - Argyle alluvial Diamond deposits: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v2 pp 1655-1658
Fazakerley V W  1990 - Bow River alluvial Diamond deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v2 pp 1659-1664


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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