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Location: 239 km E from Warburton, Western Australia, Australia
Level 6,190 St Georges TcePerthWestern Australia, Australia6000
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The Wingellina nickel-oxide deposits occur in deeply weathered ultramafic (olivine-rich) members of the Hinckley Range Gabbro, a component of the Giles Complex within the Musgrave Block of central Australia.The Giles Complex is a series of mafic igneous rocks intruded into the gneissic basement rocks of the Musgrave block and consists of numerous separate intrusions of layered mafic and ultramafic lithologies. The Wingellina deposits lie within the Wingellina Hills, a northwest-trending mafic-ultramafic set of ridges and valleys containing pyroxenites, dunites and gabbros.Layering in the intrusions was caused by fractionation and crystal settling within multiple successive magma injections. At Wingellina, this resulted in the formation of a series of ultramafic units overlain by thin pyroxenites and mafic to leucocratic gabbros. Detailed core logging has shown that the rhythmic layering of the differentiated mafic-ultramafic sequence is present on a centimetre to 10 m scale, and compositional variation (both chemical and mineralogical) in the parent rock influences the composition of the weathered product.Steeply southwest-dipping (75–85°) shear zones strike the length of the central part of the Wingellina Hills and affect the ultramafic units and the margins of most of the gabbro unit seen in outcrop. Shearing varies in intensity from strong to mylonitic, and has strongly influenced deep weathering, leaching and limonite formation. East-west sinistral brittle cross-structures are also present across the sequence and are seen to offset the geology in places.WeatheringThe Wingellina nickel-oxide mineralisation is a surficial, tropical laterite style of mineralisation developed over olivine-rich ultramafic stratigraphy. Strong shearing within the ultramafic has promoted deep weathering. The resistant gabbro ridges surrounding the laterites at Wingellina have protected the deposit from subsequent erosion.The weathering profile at Wingellina is consistent with an oxide type of laterite (Elias, 2002). Oxide laterites are comprised largely of iron hydroxides and oxides in the upper part of the profile, with thin layers of altered magnesium silicates overlying fresh bedrock. The Wingellina laterite profile has well-defined regolith boundaries for oxide limonite, and a transitional zone between the iron-rich limonite and the magnesium-rich saprolite at depth.The Wingellina nickel oxide resource consists of two main zones which contain several semi-linear north westerly striking sub-zones of limonitic (iron-rich) and lesser saprolitic (clay-rich) styles of laterite mineralisation.The nickel mineralisation was produced by deep weathering, facilitated by shearing, of olivine-rich ultramafic units in the Wingellina Hills near the northern contact of the Hinckley Range gabbro. Olivine crystals within the ultramafic units originally contained background values of about 0.15% to 0.30% Ni. The almost complete removal of MgO and SiO2 by downward-percolating ground waters during weathering resulted in extreme volume reductions and consequently significant upgrading of Fe2O3, Al2O3 and nickel and cobalt in the weathered profile. The ultramafic units are deeply weathered into asymmetric troughlike shapes that are up to 250 m deep in places. The geological contacts between the completely weathered ultramafic units and the intervening gabbroidal units are transitional.At Wingellina, nickel is largely confined to the ultramafic units, although the boundaries between the ultramafics and mafics are not always sharply defined, partly due to lack of information, but also due to the more transitional effect of a layered intrusion, rather than the sharper boundaries often encountered in other nickel laterites where the protolith is a single ultramafic unit. Cobalt appears to be concentrated largely in the core of the ultramafic units but can also be found in the mafics. There are also areas with high manganese which do not contain cobalt.Cobalt is very mobile and tends to be closely associated with manganese-oxides, often at the water table level or redox boundary.The Wingellina deposits have a strike length of >11 km, a lateral extent of up to 2.5 km and a depth of up to 200 m.
