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Mt Weld
Western Australia, WA, Australia
Main commodities:
P REE Ta
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Deposit Description
The Mt Weld carbonatite intrudes greenstones 30 km SSE of Laverton, in the north-south trending Laverton tectonic zone which lies in the eastern part of the Laverton greenstone belt, part of the greater Norseman-Wiluna greenstone belt within the Archaean Yilgarn Craton of Western Australia.
The Laverton greenstone belt comprises three major cycles of ultramafic to mafic volcanics separated by thin heterogeneous units of one or more of banded iron formation, chert and carbonaceous shale. The third cycle, which surrounds the Mt Weld carbonatite, is overlain by conglomerate along the western margin of the Laverton tectonic lineament. These greenstones are bounded by a variety of granitoids with intrusive contacts. The greenstone belt has been divided into a series of sectors of open folding and minimal metamorphism separated by linear to sinuous tectonic zones chacaterised by periodic shearing, faulting and peralkaline igneous activity and subsequent sediment filled grabens.
The Mt Weld carbonatite has been dated at 2064 ±40 Ma, and comprises a pipe of primary carbonatite approximately 2.5 km in diameter, with an outer 500 m wide annulus of glimmerite which has a gradational outer margin with the intruded greenstones. The glimmerite is composed of strongly brecciated ferriphlogopite. The carbonatite is dominantlty sovite with beforsite and dolomitic sovite, while biotite and apatite may locally predomiate. A weathering/regolith layer over the carbonatite varies from 20 to 100 m in thickness.
Apatite occurs as a residual layer near the base of the regolith, varying from 6 to 30 m thick, at depths of 50 to 90 m below surface. The best beneficiable type is a lightly cemented fine sand of apatite, magnetite and vermiculite. The primary carbonatite contains around 3.5% apatite. Niobium and tantalum bearing pyrochlore, ilmenite and niobium rutile from the primary carbonatite are concentrated in the apatite and magnetite rich residual zone. Yttrium and lanthanides are distribued throughout the carbonatite regolith and are derived from low grade occurrence in the primary carbonatite of monazite, apatite and trace synchysite.
The most important rare earth resource, the 'Central Lanthanide Deposit' is in the regolith above the central part of the carbonatite, while the higher grade Nb and Ta zones are closer to the outer margins of the pipe. The majority of the REOs are contained within secondary, low Th phosphate minerals with low levels of deleterious elements (eg F & Ca)
The regolith over the carbonatite was estimated (1989) to contain an indicated resource of:
250 Mt @ 18% P2O5,
270 Mt @ 0.9% Nb2O5,
145 Mt @ 0.034% Ta2O5,
15.2 Mt @ 11.2% lanthanide + yttrium oxides
Resource estimates in 2002 were quoted as (Lynas Corp 2002 Annual Report):
Measured - 1.2 Mt @ 15.6% REO,
Indicated - 5.0 Mt @ 11.7% REO,
Inferred - 1.5 Mt @ 9.8% REO,
Total - 7.7 Mt @ 11.9% REO
The Nb and Ta deposits contain > 400 000 t Nb and 29 000 t Ta metal.
For detail consult the reference(s) listed below.
Description based on information available at, or was up to date when written in 2003.
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.
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| References to this deposit in the PGC Literature Service Collection: |  |
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Duncan R K, Willett G C 1990 - Mount Weld Carbonatite: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v1 pp 591-597
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