Ravensthorpe - Halleys, Hale-Bopp, Shoemaker-Levy
Western Australia, WA, Australia
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The Ravensthorpe Nickel Project, including the Halley's, Hale-Bopp and Shoemaker-Levy deposits, is located 35 km east of the town of Ravensthorpe and 155 kilometres west of the deep-water port of Esperance in the central south coastal region of Western Australia.
(#Location: 33° 38' 28"S, 120° 22' 45"E).
It lies close to the southern margin of the Archaean Yilgarn Craton, within the Ravensthorpe Greenstone Belt which is most likely ~2.9 Ga age. The project is centred on the Halley's deposit, a single 3 x 1 km tabular lateritic nickel body which was mined first, followed by the adjacent Hale-Bopp and Shoemaker-Levy deposits.
The lateritic Ni deposits are developed over an ultramafic suite of the Bandalup Ultramafics, a NNW striking, serpentinised komatiite suite with rare interflow sedimentary units.
The Bandalup Ultramafics, which are up to 1500 m thick, are underlain by the up to 750 m thick Chester Formation (mainly pelites with minor psammite, felsic volcanics and volcaniclastics, and banded iron-formation), and are progressively overlain by the 2000 m thick Maydon Basalt, the 1000 m thick Hatfield formation (metasedimentary clastic rocks, minor dacitic volcanic and volcaniclastic rocks) and the calc-alkaline Annabelle Volcanics.
The Bandalup Ultramafics are represented by thick, irregular intrusions of serpentinised peridotite to dunite, 500 to 3000 m wide, which occur in an arcuate zone some 21 km long, which lies on the northeastern limb of a major syncline and includes inlier bodies in adjacent granitoid rocks of the Manyutup Tonalite (tonalite and quartz diorite).
The ultramafic parent of the lateritic deposits has been described as a serpentinised 'dunite' (34 to 48% MgO and anhydrous) containing close packed pseudomorphs after coarse-grained anhedral olivine and accessory magnetite, which is variably altered to serpentine-talc-magnesite rocks. Units of fibrous tremolite rock (metapyroxenite) occur within the dunite, and the whole intrusion is interpreted as a sill-like body dipping west- southwest at about 50 degrees. The dunite contains 1000 to 3000 ppm Ni, <25 ppm Cu and >20% Mg.
A thick, up to 80 m, lateritic regolith is interpreted to have developed on the Bandalup Ultramafics during the Tertiary Period, and is partially preserved at each of the deposits.
The regolith profile comprises, from the top:
Lateritic residuum - Strongly indurated, ferruginous and siliceous duricrust with a
leached siliceous pedolith, that is largely depleted in Ni, Co and Mg but enriched in silica, Fe and Al.
Ferralite zone - Weakly to strongly indurated, porous, cellular rock, dominantly composed of iron oxyhydroxides (mainly goethite), silica and minor clay. It is highly porous due to the relatively isovolumetric leaching of almost all of the Mg and is generally enriched in Ni, locally enriched in Co and hosts the majority of the Halley's deposit. This zone has been sub-divided into the Upper ferralite - above the Co-Ni-Mn redox horizon, and the Lower ferralite - below the Co-Ni-Mn redox horizon.
Saprolite - Clay-serpentine-goethite-carbonate weathered ultramafic with local silica veining. The top of the saprolite is usually marked by a rapid decrease in Mg and a coincident increase in Fe as the boundary is conformable with the base of limonite and goethite formation. This geochemical boundary, the "magnesium discontinuity", is a redox front marking the contact with the overlying ferralite zone. It ranges from very sharp to gradational over several metres, and tends to rise at the margins of the ultramafic unit (e.g. from less than 1% magnesium to greater than 10% Mg across over intervals of up to several metres, and is conformable with the base of limonite and goethite formation). Moderately weathered saprolite overlies the weathered bedrock, and is the result of partial leaching of mobile cations such as Mg and minor residual enrichment of Ni. Nickel grades increase rapidly above a 0.3% threshold above the base of the saprolite.
Fresh ultramafic - Komatiitic olivine cumulate rocks underlie the profile, but are rarely intersected by evaluation drilling, with most holes terminating in weakly weathered serpentinite, typically carrying >20% Mg and <0.3% Ni.
Studies of the geochemical, mineralogical and textural characteristics of the profile reflect
weathering processes that include: i). leaching of Mg, ii). residual enrichment of Ni, Fe, Al and Cr, iii). dissolution and reprecipitation of silica, iv). residual and supergene enrichment of Co and Mn.
The zone of nickel enrichment and ore occurs as a single gently undulating slab. Around two thirds of the zone lies within the ferralite horizon while the remainder is within the upper saprolite. Higher Ni grades tend to occur at one or two horizons within the enrichment zone, with the most continuous being towards the top of the ferralite zone, associated with the main zone of Co enrichment, although peak Co grades are generally found 2 to 4 m higher in the profile than the Ni peak. The Co mineralisation has a strong association with Mn accumulation and is interpreted to be controlled by a redox front. The second, but deeper, higher grade Ni interval often accompanies the Mg discontinuity.
The Halley's deposit is being development as an open pit mine, with the ore to be treated by pressure acid leach (PAL). The three principal elements of interest are Ni, Co and Mg. Ni is the main ore element, while Co is an economically significant by-product. Mg is the most important gangue component in the ore as it is the principal consumer of sulphuric acid in the PAL circuit. As such there is a direct relationship between Mg grade and operating cost.
The three Ravensthorpe orebodies are distinctive in that they have a high silica content enabling the limonite ore to be upgraded to almost twice the mined grade through beneficiation, using a simple scrubbing and screening process to remove the barren, hard silica. The saprolite ore can also be upgraded but not to the same extent.
Testing of the Halley's deposit has involved reverse circulation drill holes on a regular 50 x 40 m grid. All were vertical and were sampled at 2 m intervals.
In 2003, the three orebodies of Halley's, Hale-Bopp and Shoemaker-Levy had published reserves anad resources (Comet Resources):
Proved reserves of 125.3 Mt @ 0.73% Ni and 0.032 % Co and
Probable reserves of 137.9 Mt @ 0.57% Ni and 0.026% Co, for a
Total proved and probable reserves of 263.3 Mt @ 0.65 % Ni and 0.029 % Co.
Published JORC compliant ore reserves and mineral resources at December 31, 2012 (First Quantum Resources, 2013) were:
Measured + indicated resources
Halley's - 57.9 Mt @ 0.76% Ni, 0.03% Co, 10.7% Fe, 1.2% Al, 8.3% Mg, 1.0% Ca, 11.8% CO3, 18 ppm Cu, 68 ppm Zn
Shoemaker-Levy - 176.9 Mt @ 0.59% Ni, 0.03% Co, 12.2% Fe, 1.6% Al, 5.1% Mg, 1.5% Ca, 12.2% CO3, 34 ppm Cu, 87 ppm Zn
Hale-Bopp - 25.9 Mt @ 0.62% Ni, 0.03% Co, 10.8% Fe, 1.7% Al, 8.1% Mg, 0.8% Ca, 14.4% CO3, 24 ppm Cu, 61 ppm Zn
TOTAL - 260.7 Mt @ 0.63% Ni, 0.03% Co, 11.7% Fe, 1.5% Al, 6.1% Mg, 1.3% Ca, 12.3% CO3, 30 ppm Cu, 80 ppm Zn
TOTAL - 114.1 Mt @ 0.53% Ni, 0.02% Co, 10.1% Fe, 1.8% Al, 8.6% Mg, 0.7% Ca, 8.5% CO3, 39 ppm Cu, 74 ppm Zn
Measured + indicated reserve
TOTAL - 238.0 Mt @ 0.62% Ni, 0.03% Co, 11.7% Fe, 1.5% Al, 5.8% Mg, 1.3% Ca, 8.2% CO3, 24 ppm Cu, 77 ppm Zn.
The most recent source geological information used to prepare this summary was dated: 2004.
This description is a summary from published sources, the chief of which are listed below.
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