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Soroako

Sulawesi, Indonesia

Main commodities: Ni
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The Soroako lateritic nickel deposit of PT International Nickel Indonesia (61% owned by CVRD Inco, 20% by Sumitomo Metal Mining) is located in central Sulawesi Island of Indonesia, some 300 km NNE of Ujung Pandang.

After an exploration program commencing in the late 1960's PT Inco had outlined sufficient reserves to commence mining in 1978. The operation has been progressively expanded to its present capacity of 68 000 tpa of Ni in matte.

Lateritic nickel deposits are found in the central to southern sections of eastern Sulawesi. The best developed are located at the southern end of the south-east peninsular, where a stable surface preserves mature laterites (such as the Pomalaa deposits of PT Aneka Tambang), while areas of greater uplift and erosion are obvious in the north. The Soroako deposits are in between, in the intermediate setting. The formation of commercial laterites on the peninsular has taken place over the last 10 Ma, ie. from the commencement of the Miocene uplift to the present. The weathering profiles have been developed on both the un-serpentinised harzburgite and strongly serpentinised lherzolites of an extensive Cretaceous serpentinised peridotite.

The district is flanked to the west by a west dipping thrust across which are limestones. Along to this fault the ultramafics are strongly serpentinised and mylonitised. Immediately to the east the rocks are un-serpentinised harzburgite with minor dunite and lherzolite, bounded to the west by more serpentinised lherzolite, which again become less serpentinised further to the east. The Ni of the laterites is derived from both direct weathering of olivines, and indirectly by weathering the serpentine that replaces olivine. Virtually all of the ultramafics are overlain by residual nickel laterites.

The weathering profile varies from 5 to 30 m in thickness. The Soroako deposit occurs as spurs, ridges and on hill flanks where there are remnants of a number of extensive erosional surfaces at different topographic levels. In general, the thickest laterite deposits occur on the lower levels. Although topography is an important control of nickel enrichment, so also is the degree of fracturing of the protolith.

Laterite deposits are in two groups, namely i). those over un-serpentinised bedrock; and ii). those over bedrock containing 20 to 90% serpentinised olivine. In both there is an upper limonite and a lower saprolite zone.

The profile for both groups, generally comprises a thin discontinuous iron cap less than a metre thick overlying limonite overburden with <1.3% Ni which may be 5 to 10 m or more thick. This is followed by a <5 m thick medium grade limonite with around 1.3 to 1.5% Ni, 44% Fe, 7 to 10% SiO2, and 3% MgO for both groups. The limonite tends to be yellow-orange over un-serpentinised bedrock and brown to yellow-brown over serpentinised sections. It is principally very fine goethite, with gibbsite or kaolinite from the Al2O3 rich peridotite, plus hematite, and variable amounts of quartz, magnetite and chromite. In both types there is commonly a lower interval in the limonite zone containing higher concentrations of asbolite or ferri-manganese wad with high Ni, Co, Y, Zn & Cu. Over the un-serpentinised peridotite, the limonite is separated from the saprolite layer by a thin limonite ore band with around 1.6% Ni, 42% Fe, 10% SiO2, and 3% MgO.

The saprolite zone shows more variability between the two bedrock types. Saprolite from un-serpentinised rocks contains cores of unweathered harzburgite with yellow to orange saprolite rims and fracture fill of garnierite, quartz and MnO. This ore can be substantially and cheaply upgraded by screening out the un-weathered rock. Saprolite over serpentinised peridotite comprises lesser boulders in a soft, brown-green matrix of serpentine and goethite with minor chromite, magnetite, MnO and talc. Bulk saprolite ore, for both groups before screening is generally 5 to 10 m thick with 1.7 to 1.9% Ni, 16 to 18% Fe, 35 to 38% SiO
2, and 14 to 24% MgO.

Reserves in 1998 were in excess of 100 Mt @ 1.8% Ni and the mine claimed to be one of the most efficient and low cost in the industry.

In 2005 reserves and resources were (Inco, 2006):
    Proven reserves - 59 Mt @ 1.80% Ni
    Probable reserves - 88 Mt @ 1.81% Ni.
    Measured resources - 0.4 Mt @ 1.85% Ni,
    Indicated resources - 28 Mt @ 1.67% Ni
    Inferred resources - 322 Mt @ 1.7% Ni.

Production in 2005 was 58 000 t Ni.

The most recent source geological information used to prepare this summary was dated: 2000.    
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.


  References & Additional Information
   Selected References:
Golightly J P  1979 - Geology of Soroako Nickeliferous Laterite Deposits: in Evans D J I, Shoemaker R S, Veltman H (Eds), 1979 International Laterite Symposium, New Orleans, Louisiana, Feb, 19 to 21 Soc Mining Engineers, of the AIMM&PE, New York    pp 39-56
van Leeuwen, T.M. and Pieters, P.E.,  2011 - Mineral Deposits of Sulawesi: in   The Sulawesi Mineral Resources 2011 Seminar, 28‐29 November 2011, Manado, North Sulawesi, Indonesia, Masyarakat Geologi Ekonomi Indonesia - Indonesian Geologists Association (MGEI‐IAGI), Jakarta    Proceedings, 130p.


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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