DESCRIPTIVE MODEL OF LATERITIC Ni

MODEL 38a

By Donald A. Singer

DESCRIPTION Nickel-rich, in situ lateritic weathering products developed from dunites and peridotites. Ni-rich iron oxides are most common. Some deposits are predominantly Ni silicates.

GENERAL REFERENCE Evans and others (1979).

GEOLOGICAL ENVIRONMENT

Rock Types Ultramafic rocks, particularily peridotite, dunite, and serpentinized peridotite.

Age Range Precambrian to Tertiary source rocks, typically Cenozoic weathering.

Depositional Environment Relatively high rates of chemical weathering (warm-humid climates) and relatively low rates of physical erosion.

Tectonic Setting(s) Convergent margins where ophiolites have been emplaced. Uplift is required to expose ultramafics to weathering.

Associated Deposit Types Podiform chromite, PGE placers, serpentine-hosted asbestos.

DEPOSIT DESCRIPTION

Mineralogy Garnierite, poorly defined hydrous silicates, quartz, and goethite. Goethite commonly contains much Ni.

Texture/Structure Red-brown pisolitic soils, silica-rich boxworks.

Alteration Zoned--from top: (1) Red, yellow, and brown limonitic soils; (2) saprolites--continuous transition from soft saprolite below limonite zone, hard saprolite and saprolitized peridotite, to fresh peridotite. Boxwork of chalcedony and garnierite occurs near bedrock-weathered rock.

Ore Controls Upper limonite zone containing 0.5-2 percent Ni in iron-oxides; lower saprolite and boxwork zone typically contains 2-4 percent Ni in hydrous silicates. The oxide and silicate ores are end members and most mineralization contains some of both.

Weathering The profile from red-brown pisolitic soil down to saprolite represents the products of chemically weathered ultramafic rocks.

Geochemical Signature Enriched in Ni, Co, Cr; depleted in MgO relative to fresh peridotite (less than 40 percent MgO).

EXAMPLES

Poro, NCAL (Troly and others, 1979)

Cerro Matoso, CLBA (Gomez and others, 1979)

Nickel Mountain, USOR (Chace and others, 1969)

Greenvale, AUQL (Burger, 1979)