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Rocklea

Western Australia, WA, Australia

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The Rocklea channel iron deposit (CID) is located 10 to 33 km SW of Tom Price, 50 km NW of Paraburdoo and 260 km SSW of the port of Dampier in the Pilbara region of north-western Western Australia.

The channel has an intermittently preserved cumulative length of ~45 km and extends across the Rocklea Dome from a point to the SE of its core, to the downstream stretches across the northwestern limb.

The Rocklea Dome is a shallowly double-plunging anticline with a hinge zone trending approximately WNW-ESE (Thorne and Tyler, 1996). The 30 x 15 km core of the Rocklea Dome comprises a metamorphosed Archaean (>2750 Ma) monzogranite pluton, schists and cherts, with minor crosscutting mafic and ultramafic intrusions that belong to the Pilbara Craton basement, underlying the Hamersley basin.

This core is surrounded by Archaean to Proterozoic metasedimentary and volcanic rocks that were deposited within the Hamersley Basin, subdivided into the Fortescue, Hamersley, Turee Creek and Wyloo Groups (Thorne and Tyler, 1996).

The 2765 and 2687 Ma (zircon U-Pb; Arndt et al., 1991) Fortescue Group, which comprises basalt, dolerite, volcaniclastic rocks, sandstone, siltstone, mudstone, conglomerate and chert, rests unconformably on the granite-greenstone basement (Thorne and Tyler, 1996). The Fortescue Group contains significant massive mafic and layered sills that are doleritic to gabbroic, and range from only a few metres thick to laterally persistent intrusions with a thickness of several hundred metres (Thorne and Tyler, 1996).

The Hamersley Group, which conformably overlies the Fortescue Group, contains the major bedded iron deposits of the Hamersley Basin. The lower portion of the Hamersley Group was deposited during the late Archaean (Compston et al., 1981) and consists of shale, chert, bedded iron formation (BIF), dolomite and volcaniclastic rocks in the Rocklea Dome area (Thorne and Tyler, 1996). The upper parts of the Hamersley Group were deposited during the early Palaeoproterozoic (Compston et al., 1981) and comprise jaspilitic iron-formation with shale and chert, often intruded by dolerite sills, massive medium-grained rhyolite and BIF (Thorne and Tyler, 1996).

See the Hamersley Basin Iron Province record for the regional setting.

The Tertiary palaeochannel crosscutting the Rocklea Dome is filled with 15 to 45 m of pisolitic limonite, with calcrete and detritals at surface (Thorne and Tyler, 1996). The pisolitic limonite at Rocklea contains ochreous goethite, hematite, small amounts of ferrigenous detritus, and scattered fragments of fossil wood. Parts of the channel now form elevated mesas (Thorne and Tyler, 1996) except for the Rocklea channel iron deposit, which is undercover.

The channel iron deposits of the Pilbara form part of the detrital infill of sinuous Tertiary river channels that were part of mature river systems that incised the Hamersley surface (Morris et al., 1993), an elevated and dissected peneplain surface of probable late Mesozoic to Early Tertiary age (Campana et al., 1964; Twidale et al., 1985). These palaeochannels were incised into granitoids, volcanic rocks, metasedimentary rocks and BIF (Morris and Ramanaidou, 2007).

Channel iron deposits originate where a palaeochannel lies within or close to the laterised palaeo-surface exposing BIF on a slope, and persist for several km downstream above metabasalts and clastic metasediments (Morris et al., 1993). The similarity in trace element composition of the CID and its direct basement indicates a strong influence by the varying geologic background of the different sections of the channel drainage system (Morris and Ramanaidou, 2007).

The Rocklea CID was filled with iron (oxyhydr-)oxide-rich sediments duing the middle Eocene to the middle Miocene (Morris et al., 1993; Macphail and Stone, 2004; Heim et al., 2006; Morris and Ramanaidou, 2007). These palaeochannels typically contain a mixture of iron (oxyhydr-)oxide pelletoids and ferruginised wood fragments 1 to 10 mm in size. The pelletoids comprise both hematite and goethite nuclei with goethitic cortices as well as nonstructured iron oxyhydroxide granules.

The iron-rich pelletoids are interpreted to have been formed on the slopes of deeply weathered outcropping banded iron formations and deposited along the palaeochannels, where they were subsequently cemented in a goethite matrix, almost devoid of quartz grains, clays and country-rock lithic fragments (Morris et al., 1993). Late-stage iron oxyhydroxide solution-reprecipitation processes have formed different types of goethite (i.e., vitreous, ochreous and brown), which influence the iron ore type/quality. The base of the CID comprises a clay horizon of variable composition, and the deposits are capped in places by calcrete and silcrete (Morris and Ramanaidou, 2007). Parts of the Rocklea test site are covered with Quaternary alluvium (Thorne and Tyler, 1996).

The Rocklea CID is developed over weathered basalt/metadolerite and metasedimentary rocks (black shale - weathered to a clay layer in the footwall of the deposit) on the flank of the Rocklea Dome. The deposit has an internal channel stratigraphy from bottom to top, comprising:
i). Well-ordered (in situ), 20 to 60 cm thick white kaolinite and a partially denaturated channel iron deposit. This layer is largely developed above a Fortescue Group black shale and forms the base of the overlying CID resource.
ii). Poorly ordered (transported) kaolinite, commencing with a brown goethitic clay, locally with irregular vitreous goethite nodules with variable diameters of between 0.1 and 1.5 mm, consisting of both massive and cell-like goethite crystals, the latter sometimes replacing wood stems. The edges of the vitreous goethite nodules have pores filled with hematite and/or silica. Ochreous, finely crystalline goethite with quartz clusters, crosscut the vitreous goethite. Iron (oxyhydr-)oxide nodules and other 1 to 10 mm diameter fragments are increasingly developed toward the top of the interval. The nodules and fragments include: (1) ooids with a goethite nucleus and an ochreous or vitreous goethite shell; (2) very rare broken ooids with a hematite nucleus; (3) ferruginised wood; (4) irregular nodules of massive Fe oxyhydroxides without any internal structure; and (5) large irregular masses comprising goethite nodules/fragments, cemented by vitreous goethite.
iii). ochreous goethite with scarce ooidal textures, no clays, and late-stage vitreous goethite/silica replacement, representing a transition from a clay cemented conglomerate of Fe (oxyhydr-)oxide ooids at the bottom to a typically yellow to light-brown clay horizon at the top, with some development of poorly ordered kaolinite. This layer is generally thin and pinches out to the NE.
iv). Overall, a poorly textured ochreous goethite with little or no clay, and occasional late stage vitreous goethite and silica infill (of dissolution-formed vugs) as well as overgrowths. However, some nodules and fragments characteristic of level ii persist a short way up into this level, as well as ferruginised wood fragments. A later stage of hematite replaces (dehydroxylates) both ochreous and vitreous goethite, and the vitreous goethite and associated silica are crosscut by quartz veins.
v). Mostly poorly ordered kaolinite, occurring as a 5 to 15 m thick layer of dominantly kaolinite, but also with aluminium smectite, especially over the centre of the CID. No ferruginised wood fragments or Fe (oxyhydr-)oxide ooids are observed at this level, which is not considered to be part of the CID ore profile, although spherical concretions with a vitreous goethite framework and silica cavity fill are developed.
vi). A flat-lying calcrete, which is variably developed from massive to discrete in different part of the deposit area. The massive calcrete contains abundant mixed rock fragments, including chert, BIF and vitreous goethite ooids. Unconsolidated, dark red-brown alluvial gravels cover the calcrete.

The main channel at Rocklea is typically 800 m to 1.25 km wide in the main section, narrowing on the extremities to be 200 to 600 m in width. In general the CID is 5 to ~50 m thick and has been delineated over a strike length of >10 km. The extensions of this palaeochannel host additional iron resources of the Beasley River CID, which commences some 20 km "downstream".

Published JORC compliant indicated + inferred mineral resources (Dragon Resources, 2012) include:
    at 50% Fe cut-off - 182.6 Mt @ 52.7% Fe, 8.3% SiO2, 3.4% Al2O3, 0.031%P, 11.5% LOI,
    at 53% Fe cut-off - 72.6 Mt @ 54.4% Fe, 7.2% SiO
2, 2.7% Al2O3, 0.031%P, 11.2% LOI,

The most recent source geological information used to prepare this summary was dated: 2012.    
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
 References to this deposit in the PGC Literature Collection:
Haest M, Cudahy T, Laukamp C and Gregory S,  2012 - Quantitative Mineralogy from Infrared Spectroscopic Data. II. Three-Dimensional Mineralogical Characterization of the Rocklea Channel Iron Deposit, Western Australia : in    Econ. Geol.   v.107 pp. 229-249
Haest M, Cudahy T, Laukamp C and Gregory S,  2012 - Quantitative Mineralogy from Infrared Spectroscopic Data. I. Validation of Mineral Abundance and Composition Scripts at the Rocklea Channel Iron Deposit in Western Australia: in    Econ. Geol.   v.107 pp. 209-228


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