Yampi Sound, Koolan Island, Cockatoo Island, Irvine Island
Western Australia, WA, Australia
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The Yampi Sound iron ore deposits including Koolan, Cockatoo and Irvine Islands are located in the Kimberley Region of far northern Western Australia, ~130 km north of Derby, 850 km SW of Darwin and 775 km NE of Port Hedland (#Location: 16° 08' 01"S, 123° 44' 27"E).
These deposits are hosted by the Yampi Member of the Pentecost Sandstone within the King Leopold Mobile Belt, on the western margin of the late Palaeoproterozoic Kimberley Basin.
The Kimberley Basin occupies a generally square area of approximately 400 x 400 km with NW-SE and NE-SW oriented margins. It is bounded to the NW and NE by the Timor Sea with no apparent basin margins. To the SE it is interrupted by the Halls Ck Mobile Zone which is occupied by Lower Proterozoic rocks of the Lamboo Complex, basal Whitewater Volcanics and the Archaean or Palaeoproterozoic Halls Ck Group. To the SW, it is limited by the NW-SE trending King Leopold Mobile Belt which is predominantly occupied by the same basement rocks as described for the Halls Ck Mobile Zone. Overall the basin is little deformed with a pattern of gentle interference folding with two axial directions trending NE-SW to N-S and NW-SE to E-W throughout the basin, and dips that are generally shallow in the main part of the basin, seldom exceeding 10°. Dominant fault directions are NW-SE, N-S and NE-SW. Deformation increases as the mobile belts are approached. Near the Halls Ck Mobile zone dips of 15 to 20 and sometimes up to 30° are mapped. The Halls Ck Mobile Zone was probably activated later than the deposition and early folding of the Kimberley Basin sediments, though fairly definitely following a pre existing structural trend.
However, as the King Leopold mobile belt is approached, the folding of the Kimberley Basin sediments becomes more intense and close spaced with dips of 45° to vertical and occasionally overturned. The mobile belt is characterised by asymmetric and overturned folds with a NW axial trend. At the NW and SE extremities of the exposed King Leopold Mobile zone, the Kimberley Basin sediments are well represented across the Mobile Zone and in these areas there are marked facies variations, transgressive relationships and evidence of syn-depositional structural activity. Thrust faults are well developed along this margin of the Kimberley Basin and much of the basal contact of the basinal sediments may be along thrusts. These observations infer that the King Leopold Mobile zone was active during sedimentation of the Kimberley Basin sequence, although it was probably not a basin margin. Current directions throughout most of the succession infer flow directions from the NE and NW.
The sequence within the Kimberley Basin, from the base comprises:
Speewah Group - This group rests unconformably on the basement metamorphics, intrusives and volcanics of the Halls Ck and King Leopold Mobile zones along the south western and south eastern margins of the exposed Kimberley Basin succession. It unconformably overlies the 1855 to 1860 Ma Whitewater Volcanics rhyolites and rhyodacites and comprises a sequence of up to ~1700 m of shallow water feldspathic and quartz sandstone, greywacke, green shale and siltstone.
Kimberley Group - This group conformable overlies the Speewah Group and constitutes the great majority of outcrop within the Kimberley Basin, and comprises the up to 1350 m thick King Leopold Sandstone (blocky to massive, cross-bedded quartz sandstone, grit, conglomerates and minor micaceous siltstones; the 60 to 1150 m thick Carson Volcanics (tholeiitic basalts, commonly amygdaloidal, spilitic lavas (with pillows), agglomerates, tuffs, tuffaceous sandstones, chloritic siltstones and chert); the 60 to 600 m thick Warton Sandstone (medium to coarse grained quartz sandstone, lesser arkosic sandstone and some ferruginous members); and the 40 to 480 m thick Elgee Siltstone (massive red siltstone with scattered interbeds of flaggy, laminated, fine sandstone and minor grey green shale with local thin dolostones).
Pentecost Sandstone - which comprises 420 to 1350 m of quartz and feldspathic sandstone, ranging from fine to coarse grained, with lesser ferruginous siltstone and sandstone and glauconitic sandstone. The Pentecost sandstone has been subdivided into Lower, Middle and Upper members. The Yampi Member, host to the Yampi Sound iron deposits, may be part of the Upper Pentecost Sandstone Member, which rests disconformably onto the Elgee Siltstone and in places onto the Warton Sandstone.
Bastion and Crowhurst Groups - These two groups conformably overlie the Upper Pentecost Sandstone Member, and comprise varying amounts of sandstone, siltstone, shale and dolomite and range from white to pink to green in colour, and are up to 1350 and 160 m thick respectively.
Hart Dolerite, dated at 1790±4 Ma, which is up to 3000 m thick, and comprises tholeiitic dolerite, granophyre and lesser gabbro, mapped as an intrusive, but may also be in part extrusive. It occurs in two broad settings, the most extensive being as sills along the margins of the Kimberley Basin, intruding the Speewah and Kimberley Groups, generally below the Carson Volcanics, and secondly as dykes and plugs in the cores of domal structures, occasionally cutting rocks as high as the Pentecost Sandstone.
Wotjulum Porphyry, which is up to 900m thick, and comprises grey to black, massive, quartz feldspar granite porphyry grading to grey green sheared sericitic and biotitic varieties, found only on the Yampi Peninsular and the neighbouring Koolan Island. It is believed to be a sill, but again may in part be extrusive, occurring exclusively within and adjacent to the Elgee Siltstone. It would appear to be similar in age to the Hart Dolerite and the two may well be related. They are mutually exclusive in areal distribution.
The host Yampi Member was deposited between 1880 and 1760 Ma. It comprises a well bedded white, hematite bearing (in matrix) and hematite sandstone (in grains) with minor arkose and numerous thin phyllite and an impersistent basal conglomerate which disconformably overlies the widespread Elgee Siltstone.
The orebodies are restricted to the hematite rich basal conglomeratic sections of the Yampi Member which grade into hard friable hematite. The ore is composed of hard and friable hematite with irregular and impersistent intercalations of hematite sand and hematite conglomerate where the hematite is present as both or either interstitial or clast material. Much of the ore is porous and appears to have undergone de-silicification which has enhanced the iron grade. In places, the siliceous cobbles of the conglomerates have been replaced by hematite.
The host sediments have been folded into overturned synclines and anticlines during a period of regional metamorphism and an axial plane foliation has been observed in the mechanically weaker beds of the sequence on Cockatoo Island. The largest ore bodies occur in the inverted limb of an overturned syncline on Cockatoo and Koolan islands.
On Cockatoo Island the bedding plane of the ore strikes at 120° and dips between 50 and 60°SW. The axial plane of the fold dips at about 25°SW, although the normal limb of the overturned syncline is not exposed on the island. The Main Ore Body on Koolan Island was 2000 m long, up to 30 m thick and was mined from a height of 180 m to sea level, striking at between 100 and 135° and dipping at 45° and 65°SW at the western and eastern ends respectively. The ore has been drilled to 190 m below sea level with un-diminished grades of 66 to 67% Fe. The axial plane of the fold dips at 40 to 45°SW, and the fold axis plunges 8°NW. The normal limb of the fold contains the Acacia orebody, which strikes from 125° to 110° at the eastern and western ends respectively, and dips at 35 to 40°SW (Porath, 1968).
In polished sections, the ore is composed of coarse crystals of hematite (average size: 100 to 500 µm) with sharp edges, which fit together like a mosaic. Hematite crystals often show multiple lamellar twinning. These are signs of an extensive recrystallization of hematite, which probably occurred during the metamorphism of the sedimentary sequence (Porath, 1968).
Mining between 1951 and 1972 amounted to 20 and 13 Mt respectively from Cockatoo and Koolan Islands. Reserves in 1975 were 40 Mt at 64% Fe. It is reported that over 67 Mt has been extracted from these Islands between 1901 and the 1990.
Mount Gibson Iron reported JORC resources and reserves at Koolan Island at 30 July, 2011 as:
Measured + indicated + inferred resources = 71.90 Mt @ 62.80% Fe, 8.61% SiO2, 0.80% Al2O3, 0.01% P, including,
Proven + probable reserves = 32.1 Mt @ 64.00% Fe, 6.76% SiO2, 1.65% Al2O3, 0.01% P.
The resource is located within four deposits, Main Pit, Acacia, Mullet and Barramundi.
The mine was operated by BHP subsidiaries for much of its life, but more recently by Portman Limited and Mount Gibson Iron.
The most recent source geological information used to prepare this summary was dated: 2011.
Record last updated: 10/10/2012
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 to this deposit in the PGC Literature Collection:
B.H.P. Staff, 1975 - Yampi Sound iron ore: in Knight C L, (Ed.), 1975 Economic Geology of Australia & Papua New Guinea The AusIMM, Melbourne, Mono 5 pp 253-255|
Reid I W, 1965 - Iron ore deposits of Yampi Sound: in McAndrew J and Madigan R T (Eds.), 1965 Geology of Australian Ore Deposits Eighth Commonwealth Mining and Metallurgical Congress, Australia and New Zealand, The AusIMM, Melbourne, v1 pp 126-131|
Stocklmayer V 1990 - Yampi Sound Iron Ore: in Hughes FE (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne Mono 14, v1 pp 903-906|
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