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Ultramafic rocks and their 

geological environment

Geology of Sulawesi

 Ade Kadarusman PhD

Senior Geologist Specialist

Exploration and Mine Development Department

PT INCO Tbk, Sorowako, South Sulawesi

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COMPOSITION OF

THE UPPER MANTLE

• The upper mantle is aperidotitic metamorphiccomplex dominated byolivine with increasinglylesser amount of 

orthopyroxene,clinopyroxene and analuminous phase(plagioclase, spinel or garnet depending on

pressure). Eclogite andpyroxenite are omnipresent.

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Ultramafic Rock the presence of essentially mafic

minerals without considering the silica content

Ultramafic rocks

classification

Rock classification for peridotites and pyroxenitesClassification according to Aluminuous phase content :

-plagioclase peridotite, spinel peridotite or garnet peridotite

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Three principle sources of upper mantle

rock exposed in the Earth’s surface 

• Orogenic peridotite massifspackages of ultramafic rocks (m to km in scale) that were

tectonically emplaced onto continental crust or island arc during

continental collision

- Layered intrusion (Mostly Pre-Cambrian Environment)- Alpine type of Peridotites

• Oceanic peridotite

uplifted onto continental margin or island arc in ophiolite sequences

• Peridotitic xenolithscm to m scale samples carried in rapidly erupted host volcanics

such as alkali basalts and kimberlites

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 The upper mantle beneath pre-Cambrian cratonperidotite as mantle xenoliths

Menzies &

Chazot, 1995

• Peridotitic rocks were sampled by kimberlite-like rock fromdepth of 40 km minimum as mantle xenoliths

Th tl k i ti t l lli i

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The upper mantle rock in continental collisions

-Orogenic Peridotite-Deep subducted crustal rocks (to depth of at least 40 km) in continental

collision zone can uplift peridotitic rocks to the surface

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The Origin of Ophiolites

SSZ SSZOPMOR

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Origin of Ophiolites (oceanic crust on the land):• Mid Oceanic Ridge (MOR)

• Supra Subduction Zone (SSZ)

• Oceanic Plateau / Seamounts (OP)

Worldwide Database showed that SSZ ophiolite is

common for:

-Ultramafic hosted chromite deposit

-Chromite and nickel sulphides hosted PGM-Volcanic Massive Sulphides Deposit

i iti tl

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primitive mantle =

basalt/gabbro + peridotite

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Key for laterites development : ultramafic rock on

the surface and equatorial locations

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Ultramafic rocks

of oceanic origin

emplacement

onto continental

margins or 

island arcs :

- Obduction

- Accretion

Obduction

Accretion

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Ultramafic rocks

(Upper Mantle Origin)

Expose on the Earth’s Surface 

(oceanic or orogenic peridotite)

Laterite Development Sulphides Mineralization

Equatorial Location Magmatism

Nickel LateriteNickel Sulphides

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Geology of Sulawesi

See the animation

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Eastern Indonesia :The World’s largest of ultramafic rock outcrops,

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East Sulawesi Potential :the Batui

Thrust Development

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Lakes area and Matano fault

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Coastal Area and Mekongga Block

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Conclusions

• Unique geology and tectonic of EasternIndonesia exposed large distribution of 

ultramafic rocks on the land

• Exposing ultramafic rocks in the surface,means potential economic values not only

for Nickel laterite, but also for Nickel

sulphides, diamond, rare metal/PGE

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SSZ

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Supra Subduction