The Murrin Murrin group of deposits are located between Leonora and Laverton, some 200 km north of Kalgoorlie, with the main processing plant some 50 km east of Leonora, Western Australia. The deposits lie near the centre of the Archaean Norseman-Wiluna greenstone belt and is hosted within a laterite profile developed over sepentinised Archaean peridotite rocks within a sequence predominantly composed of immature feldspathic, clastic and volcaniclastic sediments related to feldspathic volcanic centres.
The sequence also includes mafic volcanic and intrusive rocks and has been intruded by porphyritic granodiorite and felsic to mafic dykes. Metamorphism, which is broadly synchronous with regional deformation, produced an assemblage ranging from prehnite-pumpellyite through greenschist to upper amphibolite. The north-south trending Murrin Murrin ultramafic rich zone is sandwiched between two regional anticlinoria to the east and west respectively. The sequence occurs as a corridor between major NNE trending, westerly dipping, strike faults which are splays off the major NW trending Keith-Kilkenny tectonic zone to the south-west, while the sequence is folded into NNE trending synclinal structure with plunge reversals related to cross-structures and the emplacement of the granitoid intrusives.
Lateritisation of the extensive ultramafic sequences in the Murrin Murrin district has resulted in enrichment of nickel and cobalt within limonite and nontronite clays in at least 10 zones / deposits.
The lateritic profile passes upwards from the unweathered ultra-mafic, through mixed saprolite and ultramafic, to siliceous saprolite, to up to 10 m of saprolite, several metres of ferruginous saprolite, overlain by an around 10 m thick profile of smectite passing up into a narrow transition ferruginous smectite and then to an upper ferruginous zone which may be around 10 m thick. The components of the profile may be summarised as follows:
The ultramafic basement is composed of fresh to slightly weathered, locally silicified ultramafic rock which has a gradational boundary with the overlying saprolite.
The saprolite zone is dominated by serpentine with lesser chlorite and smectite.
The smectite zone is the most prominent feature of the laterite profile. It is dark green to bright apple-green to chocolate to brown in colour, and locally black where manganese oxides are abundant. It consists of 90% smectite with minor chlorite, serpentine and accessory iron oxides and oxyhydroxides (goethite and hematite) and manganese-chromium oxides. The contact with the underlying saprolite is gradational over a few metres, while the upper contact is generally relatively sharp, although it may be represented by a narrow interval of ferruginous smectite clay. The upper transition zone may be host to manganese oxide rich layers and lenses.
The ferruginous zone consists of a zone of goethite and hematite containing minor clay, intermixed with ferruginous and kaolinitic clay towards its base. The upper sections comprise a less consolidated hardcap duricrust of iron oxides, while carbonate, silica and sulphate rich accumulation has locally capped the deposit with calcrete, magnesite, silcrete and gypsum.
The Ni-Co mineralisation, as indicated by a 0.5% Ni cutoff, is primarily confined to the smectite, saprolite and ferruginous smectite zones, and may be up to 25 m thick below the ferruginous zone.
Reserve and resource figures published by Minara Resources (2007) include:
the total measure + indicated + inferred resource at December 31 2006 was 342 Mt @ 1.12% Ni, 0.066% Co, which included
the total proved + probable reserve at the same date of 144 Mt @ 1.09% Ni, 0.087% Co.
The ore is treated by acid pressure leach hydrogen reduction autoclave technology.
The mine and pressure leach facility are currently (2007) producing at a rate of 30 000 tonnes per annum, heading for a target or 40 000 tonnes of nickel and 2500 tonnes of cobalt briquettes.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2007)
