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Cerro Matoso
Main commodities: Ni

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The Cerro Matoso lateritic nickel deposit in Colombia is located some 20 km to the south-west of the town of Montelibano in the Department of Cordoba on the western side of the Andes, and approximately 400 km north of the capital Bogota (#Location: 7° 54' 5"N, 75° 33' 10"W).

The ore deposit is developed over pre-Late Cretaceous ultramafic rocks, principally comprising lightly serpentinised harzburgite. The peridotite is flanked by ferruginous sandy sediments with interbedded coal beds of the early Oligocene to early Miocene Cienaga de Oro Formation and Recent alluvial gravels and sands. The harzburgite is predominantly composed of olivine with lesser orthopyroxene and secondary serpentine. Any more intense serpentinisation is primarily confined to areas of faulting and brecciation, particularly along the western and eastern boundaries of the peridotite body. The nickel laterite profile was best developed in weakly serpentinised peridotite, both vertically and in degree of enrichment of nickel-rich secondary products, rather than in strongly serpentinised peridotite.

The Cerro Matoso peridotite has been subjected to two major tectonic events. The first of these involved compression associated with the late phases of the middle Eocene to late Eocene Pre-Andean orogeny, which brought the peridotite to the surface. It also generated a major NE trending fault system within the peridotite body and local serpentinisation, particularly along fault zones. Lateritisation of the harzburgite is regarded to probably have begun in the late Eocene to early Oligocene, with chemical weathering and erosion related to a tropical humid and rainy climate with probable alternating wet to relatively dry seasons which continued throughout the Oligocene. A major NW trending fault system was developed within the peridotite body during the subsequent late Miocene to Pliocene Andean orogeny, with the southwestern part of the weathered peridotite body being uplifted relative to the northeastern part. This uplift is interpreted to have been sufficient for intense erosion to remove most of the laterite profile from the uplifted block, while the northeastern block was only slightly modified.

The ore deposit takes the form of a layered blanket on a 2500 x 1700 m oval shaped hill that rises some 200 m above the surrounding countryside. The thickness of the ore varies from a few up to more than 100 m. The lateritic profile is as folllows, from the surface to the bedrock:

i). Canga - up to more than 20 m thick, comprising a hard ferruginous crust of lateritic soil cemented by hydrated iron oxide covering a large aprt of the deposit, and having a low Ni content;
ii). Laterite - comprising an amorphous, reddish-brown, soft soil made up of hydrated iron and aluminium oxides, with generally <1.5% Ni;
iii). Saprolite - partially weathered clays that retain the texture and appearance of the peridotite bedrock. Two types of saprolite are recognised, namely: a). an upper green variety which has a higher Ni content and ferrous:ferric iron ratio, found below the canga capping; and b). a brown coloured variety. Overall the saprolite averages >3% Ni;
iv). Saprolitised peridotite - composed of fractured bedrock with soft, enriched weathered margins to the fractures and joint planes, sometimes containing garnierite;
v). Silica boxwork - occuring a veins of chalcedony and quartz on fractures within the peridotite bedrock - essentially barren at surface, but reaching ore grade in the lower sections of the deposit.

The original peridotite had a high magnesia and silica content, with the balance being predominantly iron and 0.28 to 0.36% nickel. Magnesia, silica and nickelwere dissolved during weathering and re-deposited lower in the weathering profile, while iron and aluminium remained near the surface as relatively insoluble hydrated oxides. Nickel maxima in the profile correspondes to a zone of 15 to 25% MgO. The ore comprises a zone of saprolite and saprolitised peridotite. The footwall is un-weathered peridotite.

Based on a 1.5% Ni cut-off, ore occurs as massive and fracture filling types. The former constitutes the economically most important part of the deposit. Smectite and serpentine are the dominant Ni-bearing minerals in the massive ore and are particularly abundant in the upper saprolite zone. Pimelite, nimite and Ni-bearing sepiolite fill fractures in both the upper and lower Saprolite zones. The massive ore averages 3% Ni, but ranges to as much as 7% locally. Nickel grades in fracture fillings range to as much as 30%. Variations in pH, and to a lesser extent Eh, of the weathering solutions are interpreted to have influenced the selective differential element accumulation and mineral formation throughout the laterite profile. Oxidising and slightly acidic conditions apparently influenced the Laterite (limonitic) and Canga zones and led to precipitation of Fe as goethite. A slight increase in pH in the Laterite zone, in contrast to the Canga zone favored Mn precipitation in the lower part of the Limonite zone as Mn oxide. Ni was highly mobile in the Canga and Laterite zones and alkaline conditions present in the saprolite zones is interpreted to have promoted deposition in garnierite. Co was less mobile than Ni and the zone of maximum Co concentration formed near the base of the Laterite zone. Moderate mobility of silica occurred in the upper profile levels, but mobility decreased rapidly in the Upper Saprolite zone apparently due in part to the presence of higher amounts of dissolved Mg in the weathering solutions. Cr mobility was very low in this weathering environment. Alumina behaved as a strongly immobile component and concentrated primarily in the near surface part of the Canga zone (the information in this paragraph is drawn from Lopez-Rendon, 1986).

In 1999 proven + probable reserves were more than 35 Mt @ 2.3% Ni using a 1.5% Ni cut-off,
      and up to 60 Mt at a 1% Ni cutoff.
In 2006 the proven + probable reserves were 43 Mt @ 2.3% Ni.
At 30 June, 2012, proven + probable reserves (including stckpiled ore) were: 91 Mt @ 1.2% Ni,
  included within
measure+indicated+inferred laterite resources of: 319 Mt @ 0.9% Ni, + stockpiles of 43 Mt @ 1.2% Ni.

Mineral Resources and Ore Reserves, as at 30 June, 2022 (South32 Annual Report, 2022) were:
  Measured + Indicated + Inferred Laterite Mineral Resource - 270 Mt @ 0.9% Ni (includes 17 Mt @ 0.8% Ni Inferred resource);
  Measured + Indicated Mineral Resource in stockpile - 52 Mt @ 0.9% Ni
  Proved + Probable Laterite Ore Reserve - 15 Mt @ 1.3% Ni;
  Proved + Probable Ore Reserve in stockpile - 12 Mt @ 1.1% Ni.

It has historically been one of the highest grade lateritic nickel deposits in the world, as illustrated by the excerpts from Lopez-Rendon (1986) above.

The mine and ferro-nickel plant are operated by Cerro Matoso S.A., a subsidiary of BHP Billiton between 1980 and 2015, before being passed to the demerged South32.

The most recent source geological information used to prepare this decription 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.

Cerro Matoso

  References & Additional Information
   Selected References:
Gleeson SA, Herrington RJ, Durango J, Velasquez CA and Koll G  2004 - The Mineralogy and Geochemistry of the Cerro Matoso S.A. Ni Laterite Deposit, Montelíbano, Colombia: in    Econ. Geol.   v99 pp 1197-1213
Gomez R, Ogryzlo C T, Dor A A,  1979 - The Cerro Matoso Nickel Project: in Evans D J I, Shoemaker R S, Veltman H (Eds), 1979 International Laterite Symposium, New Orleans, Louisiana, Feb, 19 to 21, 1979 Soc Mining Engineers, of the AIMM&PE, New York    pp 412-458
Lopez-Rendon, J.,  1986 - Geology, Mineralogy and Geochemistry of the Cerro Matoso Nicheliferous Laterite, Córdoba, Colombia.: in    Master Thesis, Colorado State University,    410p.
Robertson R R,  1985 - Colombias Cerro Matoso: in    E&MJ May 1985   v186, No. 5 pp 18-22

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, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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