Darling Range Bauxite - Jarrahdale, Dwellingup, Huntley, Willowdale, Mt Saddleback

Western Australia, WA, Australia

Main commodities: Al
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The Darling Range bauxite deposits, including those at Jarrahdale, Dwellingup, Huntly, Willowdale and Mt Saddleback, fall within a 50 km wide, north-south trending corridor on the western margin of the continental plateau between 31° and 34°S, bounded to the west by the north-south Darling Fault. Bauxite deposits are distributed over a north-south interval of almost 240 km immediately to the east of Perth in Western Australia. The main deposits listed are located over the southern half of this interval, from just to the south-east of Perth to just to the north-east of Bunbury.

Deposits are generally found at between 250 and 340 m asl, but locally may occur as high as 550 m asl.   Mean hill slopes are around 5° to a maximum of around 9°   Bauxite is more widespread within the region, although in this corridor it is more continuous and of higher grade.   The bauxite appears to have been developed as residual soil material on at least one erosional surface.   The bedrock on which the bauxite and laterite has been developed is predominantly Archaean in age.   In the northern part of the corridor these rocks are mainly acid biotite or hornblende gneisses with areas of meta-sediments, mainly quartzites, micas schists and minor jaspilite.   Further south, in the area in which Jarrahdale, Dwellingup and Saddleback are located medium grained granite dominates, while further south there are acid gneisses and ultramafic rocks.

At Jarrahdale, as elsewhere in the district bauxite is best developed on ridge slopes in contrast to the crests and valleys, occuring as scattered sheet like bodies covering areas of up to 80 ha.   These orebodies are the more aluminous zones within the overall lateritic developments which have a much greater areal extent.

The bauxites are generally overlain by 0.3 to 0.6 m of loose sandy gravel overburden concealing a hardcap up to 1.5 m thick which is more ferruginous than the underlying earthy zone that ranges from massive to fragmental, pisolitic and occassionally tubular or layered varieties.   The ore below this hardcap is paler, more earthy and often porous and friable with pisolites and nodules in the upper transitional zone.   Together the hardcap and earthy zone form the orebody, which at Jarrahdale averages 4 m in thickness, but locally exceeds 12 m.   The friable zone grades downward into mottled kaolinitic saprolite/bedrock forming the lower boundary to ore.   The thickness of the ore varies rapidly over short distances as do variations in the alumina content, both laterally and vertically.   In addition the boundaries between different mineral zones may be either sharp or gradational.   The grade however, generally decreases downwards, with in some profiles showing evidence of multiple episodes of enrichment.

The mineral assemblage in the ore zone includes gibbsite, boehmite, kaolinite, goethite, maghemite, metahalloysite, illite, hematite, amorphous gel material, quartz, diaspore and corundum.   Gibbsite is the dominant ore mineral with only minor boehmite.

At Mount Saddleback (Worsley) bauxite is predominantly developed over gabbroic basement, although sections of the ore is also found over granitoids.   The laterite profile, which includes the bauxite, is up to 25 m thick and comprises from the top:
i). Surface iron-rich hard cap - up to 6 m in thickness with 40 to 50% Fe2O3 and abundant pisolites over th gabbros, while over granitoids it seldom exceeds 2 m and includes significant quartz sand.
ii). Bauxite zone - an Al enriched interval with significant Fe and pisolites at the top grading down to a clay rich base, over a thickness averaging 5.5 m, but which may be up to 20 m thick.
iii). Clay zone - a transition zone to the bedrock, comprising clay contaminated bauxite and kaolinitic weathered bedrock, with a white clay band near the base.
iv). Weathered bedrock - commonly with a sharp contact to the overlying bauxite, with the clay zone often being poorly developed to absent.

The orebodies at Mt Saddleback are irregular and lenticular with the largest being 20 Mt over an area of 600 ha, while the smaller deposits are down to 0.35 Mt.   Overall the bauxite comprises around 25 to 30% of the total laterite.

Reserves at Mt Saddleback in 1975 totalled around 200 Mt @ 32% Al
2O3, and in 2004 were 400 Mt of bauxite.
Grades at individual orebodies at Jarrahdale range from 25 to 45% available alumina, with mean values of 30 to 35%, while reactive silica is between 1 and 2%.
To its closure in 1998, Jarrahdale produced 168 Mt of bauxite.
Grades at Huntley and Willowdale in 2002 averaged 27.5% Al
2O3. Resources in 2004 were 1000 Mt of bauxite at each mine. Indicated reserves at Huntley in 2006 (MBendi website) were 700 Mt of bauxite.

Total district resources in 2004 were 1.25 Gt of contained economically extractable Al
2O3 (WA DOIR, 2004).

For detail 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:
Baker G F U  1975 - Darling Range bauxite deposits, WA: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 5 pp 980-986
Owen H B, Hargreaves M R  1975 - Mount Saddleback bauxite area, WA: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 5 pp 987-991

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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