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Cawse |
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Western Australia, WA, Australia |
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Ni Co
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Cawse project area is located some 55 km to the north of Kalgoorlie in Western Australia (#Location: 30° 23' 11"S, 121° 9' 13"E).
It lies within the Archaean Norseman-Wiluna greenstone belt of the Archaean Yilgarn Craton. The Cawse deposit is on the SW side of the Coongarries-Mount Pleasant anticline, a regional fold with a width of about 20 km, with a core of granitoid, overlain on the limbs by a greenstone sequence. The mafic-ultramafic sequence of the greenstone succession on the SW side of the anticline is 10 km thick and can be broadly correlated with the sequence in the Kalgoorlie-Kambalda region to the south, and comprises from the base:
Pole Group - not present near Cawse, mainly basalts;
Linger and Die Group, divided into,
Walter Williams Formation - coarse-grained olivine adcumulate dunite composed of fresh olivine and minor chromite; with a thin orthocumulate present at the base and top of the formation, with the upper orthocumulate separated from the underlying adcumulate by a thin harrisitic, olivine layer. The contact with the overlying Siberia Komatiite is a complex zone of peridotite, pyroxenite, gabbro and high Mg basalts.
Siberia Komatiite - a 2600 m thick sequence of thin spinifex-textured kamatiite flow units, and includes minor high Mg basalt and gabbro.
These are overlain by a sequence of epiclastic sedimentary rocks.
The lateritised protoliths of the Cawse deposit are serpentinised olivine mesocumulates and adcumulates of the Walter Williams Formation. In this area the unit 300 m thick and strikes NNW. Orthocumulates are present at the upper and lower margins of the unit and locally intertongued within the dunitic host. The protolith is cokposed of forsterite, antigorite, lizardite, minor magnetite, brucite and hydroxy-carbonates with minor chromite, magnesite, dolomite, talc, chlorite and silica. The un-enriched protolith contains 2500 ppm Ni and is enriched in olivine-compatible elements (mean wt%, 42.35% MgO, 36.6% SiO2, 8.33% Fe2O3, 100 ppm Co, 820 ppm Mn, 9 ppb Ir, 12 ppb Os, 7 ppm Ru); and is depleted in olivine compatible elements (Al2O3, TiO2, Cu, Zr, Pt, Pd, and Rh).
The deposit comprises an up to 500 m wide corridor that extends for around 25 to 30 km containing lateritic nickel and cobalt mineralisation. This corridor reflects a fault structure related surface "channel" cutting the Siberia Komatiite within the Archaean Norseman-Wiluna greenstone belt.
Around 80% of the mineralisation in the main Cawse Central tenements consists of a shallow, flat lying zone associated with limonitic clays in the upper 40 m of the weathering profile in this structure, overlying a barren saprolite.
The lateritic profile comprises, from the base:
i). Immediately above the saprolite a 10 to 30 m thick smectite layer is developed, composed of lime green. mottled green and brown nontronitic clays with variable overprinting silica, while talcose clays occur in subvertical shear zones;
ii). Limonite zone, which is 10 to 30 m thick, 1.7% Ni, 0.09% Co, composed of residual clay with finely divided irnoxides particles adding a orange to yellow tinge. A distinctive 1 to 54 m thick interval of indurated silica and manganese oxide with pisolitic, crustiform or botryoidal texture is consistently developed at the top of the limonite zone containing high grade silica-cobalt with 1.6% Ni, 0.73% Co and 5% Mn. A narrow zone of manganiferous cobalt ore occurs at the transition from the limonite to the underlying siliceous ores and carries similar grades of Ni and Co, but higher Mn;
iii). Lateritic duricrust - 0 to 10 m thick, comprising a pisolitic ironstone, cemented in part near the surface to form a duricrust;;
iv). Transported soil, alluvial silt and clay from 0 to 3 m thick.
There are four zones of Ni-Co laterite enrichment:
i). Nontronitic ore, comprising green to chocolate brown smectitic clays; ii). shear controlled high grade Ni in talcose zones; iii). a 1 to 10 m thick siliceous Mn-Ni-Co horizon at the top of the limonite zone; and iv). a sub-horizontal blanket of limonitic clay extending over the full length of the deposit containing Ni and Co.
Around 90% of the resource at Cawse has <0.5% Mn, although the Co rich zones may contain up to 20% Mn. The deeper nontronitic clay and talc zone is found on the margins of bedrock structures.
The oxide (limonite) ore at Cawse is the result of a second phase of weathering that oxidises the first stage nontronite formed from the protolith, which at Cawse was a dunite, not a peridotite.
In 1997 the total resource amounted to 213 Mt @ 0.7% Ni, 0.04% Co, over the 25 km interval.
Proven + probable reserves were of 30.3 Mt @ 1% Ni, 0.11% Co. The ore deposit is an oxide laterite.
The operation was originally developed by Centaur Mining & Exploration Ltd, but has since (2001) been sold and is now owned by OMG Cawse Pty Ltd, a member of the US based OM Group, and has subsequently been purchased by Norilsk Nickel.
The most recent source geological information used to prepare this decription 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.
Cawse
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Brand N W, Butt C R M, Hellsten K J 1996 - Structural and Lithological Controls in the Formation of the Cawse Nickel Laterite Deposits, Western Australia -: in Grimsey E J, Neuss I, (Eds), Nickel 96: Mineral to Market, Kalgoorlie, Western Australia, 17-29 November, 1996 AusIMM Publication Series 6/96 pp 185-190
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Hellsten K, Lewis C R, Denn S 1998 - Cawse Nickel-Cobalt Deposit: in Berkman D A, Mackenzie D H (Eds), Geology of Australian and Papua New Guinean Mineral Deposits The AusIMM, Melbourne pp 335-338
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