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Location: 40 km SE from Ravensthorpe, Western Australia, Australia
Lot 1269 South Coast HighwayLocked Bag 100RavensthorpeWestern Australia, Australia6346
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The RNO deposits are characteristic of nickel laterite deposits formed in tropical conditions (high rainfall and warm temperatures) of weathered and serpentinised olivine rich ultramafic rocks over long periods of time (greater than 1 million years - Golichtly, 1981). The high rain fall and warm temperatures increase the kinetics of the weathering process. The primary Ni content of these rocks is typically between 0.2 and 0.4%.The Bandalup Ultramafics dominate the RNO area with a strike length of approximately 45 km and are typically between 500 m to 3000 m wide. The host rocks are comprised of a serpentinised (greenschist facies metamorphism) komatiite complex with rare interflow sedimentary units; the primary ultramafic rock was dunitic in composition. The serpentinite typically exhibits pseudomorphs of mesocumulate to accumulate olivine textures and secondary magnetite. The sequence has been altered in areas with overprinting carbonate rich rocks, mostly in the saprolite. The serpentinite sequence dips at about 50 degrees to the west. The Bandalup sequence is in turn bound by:- Metabasalt and metadolerite members of the Maydon basalt; and- Gneissic granitoids of monzogranodiorite to granodiorite composition.The Shoemaker-Levy and Hale-Bopp Archean strata have been intruded by a series of stacked typically northeast trending sub-vertical Proterozoic dolerite dykes. The dykes are often associated with faulting. Late stage cross cutting dykes are evident at some of the other deposits but not to the same extent. The Hale-Bopp and to a lesser extent Shoemaker-Levy deposits are also cut in places by narrow discrete talc dominated shear zones typically associated with faulting.Ravensthorpe Nickel Laterite MineralisationThe RNO Nickel laterite deposits are residual products formed by the pervasive weathering of Bandalup serpentinite rocks during the Cretaceous period (wet humid climate). The weathering/leaching process has resulted in horizontally defined deposits with four typical layers being overburden, limonite and saprolite developed over altered/weathered saprolitic rock (saprock) grading to bedrock. The degree of layer development and the extent of any transition zones are dependent on the local conditions and may be influenced by geochemical characteristics of the protolith. At RNO the laterisation process stopped as the climate started to become increasingly arid in the Tertiary period after which the laterite became eroded and covered in transported sediments. This is why the overburden layer is largely barren in terms of Ni content.The weathering results in a volume reduction of the rock mass as Mg, Si and other soluble components are removed (primary minerals are replaced by more stable secondary phases). The limonite layer is composed predominantly of iron oxides that are the residual product of the laterisation of ultramafic/serpentinite rocks. Nickel is usually leached from olivine or its metamorphosed derivative, serpentine and is concentrated in the form of nickel silicates and or in iron and manganese oxides. Some Ni is hosted in green smectite clays however this style of mineralisation makes up only a few percent of the total number of drilled metres at RNO. The background concentration of Ni in the RNO serpentinite typically ranges from about 0.1 to 0.4%. The Mg content of unweathered serpentinite is typically in excess of about 16%. The low level of aluminiumbearing minerals (particularly pyroxene) in the primary dunite has resulted in a laterite that is generally low in aluminium at depth. Shear zones, dykes and the surface caprock layer however all contain relative higher levels of Al. Nickel and cobalt mineralisationThe majority of the nickel mineralisation at RNO is hosted in the Limonite and the upper portions of the saprolite layer. Nickel and cobalt react differently to the laterisation process and as such are often not coincident. During the weathering process Ni will tend to migrate down the profile while cobalt will often tend to precipitate out due to changes to the oxidation state (redox boundary’s) with depth often in association with accumulations of manganese in minerals like asbolane.During the weathering process the cumulate texture of the original olivine minerals is well preserved. The presence of this cumulate texture in the Bandalup ultramafics and its partial preservation in the regolith is very important to the beneficiation success of the deposits at RNO. As serpentine has broken down in the weathering process, silica released has precipitated pseudomorphing the cumulate olivine boundaries provided thus preserving the cumulate texture. This silica lattice is generally low in nickel, while the fine minerals within the relict olivine crystal boundaries (mostly Fe oxides) contain the bulk of the nickel mineralisation. The physical difference between fine, nickel-bearing minerals and coarse, hard nickel-poor silica rich minerals means the ore can be upgraded by a process of washing and screening known as beneficiation.
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