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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
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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