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PROCEEDINGS PIT IAGI LOMBOK 2010
The 39th IAGI Annual Convention and Exhibition
GEOCHEMISTRY CHARACTERISTICS OF NICKEL LATERITE DEPOSIT OF
BAHODOPI AREA, CENTRAL SULAWESI, INDONESIA
Didi Melkybudiantoro, Guntur Suryaning Hadi, Sigit Purnomo
1
1Mine and Exploration Dept. PT International Nickel Indonesia
ABSTRACT
Laterite is a residual soil formation which is rich in iron and alumina, develops by weathering of the
underlying mafic/ultramafic rocks. Ultramafic rocks have widely distribution in Bahodopi, one of
concession area of PT INCO in Central Sulawesi Province. Based on physical and chemical properties,
laterite in Bahodopi could be classified into three zones, un-weathered zone (fresh rock), semi-weathered
zone (saprolite) and high weathered zone (limonite). It is slightly difficult to differentiate them by “naked
eye” in the transition zone, limonite to saprolite and saprolite to fresh rock; therefore chemical properties
of the material have to be assayed. PT INCO assayed data uses XRF analysis which produces 9 elements;
Fe, MgO, Ni, Co, Al, Si, Cr, Ca, Mn. Limonite as high weathered zone generally has high Fe, low MgO,
Ni and SiO2. It is a little bit easier to define limonite since it has homogeny material (fine material) while
saprolite, due to its incomplete weathering, has heterogenic material such as; fine and coarse material
which has high difference chemical properties, especially Fe and MgO. Coarser fraction results in higher
MgO and lower Fe and conversely, finer material results in lower MgO and higher Fe.
Keywords: Geochemistry, nickel laterite, profile weathering, Central Sulawesi.
INTRODUCTION
Laterite comes from Latin word “later”, which
means a brick, introduced by Buchanan in
southern India in 1807. The term laterite has
developed and used to define a residual soil
formation that is rich in iron and alumina and
develops by weathering of the underlying rocks.
Nickel laterite, one of laterite type deposit, is
formed by specific underlying rock such as
mafic/ultramafic rocks. It is required to produce
nickel abundant in the laterite zone.
Bahodopi is one of the Block of PT Inco
concession which lies in Central Sulawesi
Province. It has wide distribution of laterite
deposit and extensively explored by PT Inco’s
exploration team since 2004.
SAMPLING METHODS AND
LIMITATION
Drilling full coring with HQ3 (diameter = 61.10
mm) type and regular drilled pattern was used in
sampling techniques in Bahodopi (Figure 1).
The coring data was prepared and then
controlled by several quality control samples.
The sample is analysed by using XRF
Spectrometer and resulted 9 elements, such as;
Ni, Co, Fe, SiO2, MgO, Cr, Al, Mn, and Ca.
LOI (lost on Ignition) analysis also conducted to
control the reliability of samples.
This paper emphasises to discuss Ni element,
since nickel laterite is the type of deposit
discussed, and major elements (> 10% element
percentage) such as Fe, SiO2 and MgO due to
their sensitivities to layer changes in laterite
zone.
LOCAL GEOLOGY SETTING
Central Sulawesi and parts of the SE arm of
Sulawesi are composed of sheared metamorphic
rocks and in the east a highly tectonised melange
complex is present, together comprising the
Central Sulawesi metamorphic. Similarities
between the pre-Cenozoic rocks and some
potassium–argon dates from the metamorphic
rocks have been used to suggest that these
regions, which include microcontinental
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The 39th IAGI Annual Convention and Exhibition
fragments, had been accreted onto the eastern
margin of Sundaland before the Cenozoic.
However, data from the Central Sulawesi
metamorphic belt, particularly from the SE
arm, is sparse, and the timing of its accretion is
poorly constrained. Throughout the SE arm and
parts of central Sulawesi are rocks that suggest
much of this region are underlain by continental
type crust. Mesozoic rocks are typically deep
water carbonate sequences similar to those of the
Australian margin However, peridotites are the
most common rocks in the east and SE arms and
are interpreted as fragments of a dismembered
ophiolite, known as the East Sulawesi Ophiolite,
interspersed with smaller masses of Mesozoic
and Cenozoic sediments. The ophiolite includes
a full suite of ophiolite lithologies, tectonically
intercalated with Mesozoic pelagic sedimentary
rocks. However it is not clear that these rocks
formed part of a single ophiolite, and it seems
likely that ophiolites in different parts of the
island may have been emplaced at different
times in the Neogene (Figure 2). (Hall and
Wilson, 2000)
LATERITE PROFILE OF BAHODOPI
As residual soil, nickel laterite comes from
continues weathering process which weathered
mafic/ultramafic rock in various degree, depend
on structure geology, morphology, vegetation,
climate, water table and time (Ahmad, 2008).
Various degree of weathering process produce
three types of laterite zone (Figure 3), such as;
un-weathered zone (Freshrock/Bedrock), semi-
weathered zone (Saprolite) and high weathered
zone (Limonite).
Bedrock should be the layer which chemical
weathering process is not introduced. It is
possibly attacked by physical weathering
(geology structure) without chemical change.
Bedrock in Bahodopi consists of peridotite in
various serpentinization degrees. High
serpentinized peridotite occurs as dominant
bedrock in core drilling. Olivine, serpentine and
pyroxene constitute primary mineral with
brecciated, vein and fracture structure.
In laterite zone, there will be saprolite above
bedrock. Saprolite has experienced incomplete
weathering process. “Immature saprolite” is
usually found in boulder and called as hard
saprolite or rocky saprolite. Sometimes, it looks
like a bedrock in appearance but different in
chemical composition. In several deposits,
impossible to differentiate them by naked eyes
(Figure 4), except chemical assayed is conducted
to determine chemical composition. Bahodopi is
one of the example deposits which are difficult
to differentiate between rocky saprolite and
bedrock. They look like a bedrock, but
sometimes has high Ni grade (> 1.80%), which
is impossible for fresh peridotite.
“Mature Saprolite” known as soft saprolite
always found in soft material and breakable. It is
finer than hard saprolite. It is difficult to
differentiate them to the layer above, limonite, in
several deposits. Therefore, it needs chemical
analysis to determine their chemical
compositions. Fortunately, it is easy to
differentiate saprolite and limonite by naked
eyes in Bahodopi deposit although the exact
boundary is subtle. Soft saprolite in Bahodopi
has yellow to brown colour with goethite and
serpentine as primary mineral. Total saprolite in
Bahodopi has average thickness 7.00 meters.
Limonite is the ultimate product of weathering
process. It lies above saprolite layer and
constitutes the oldest layer in laterite profile.
Structure and texture of the rock in this layer has
been eradicated. Similar to soft saprolite, they
are always found in soft and fine material and
breakable. They have reddish brown to brownish
yellow colour. Hematite and maghemite occur as
dominant mineral in limonite layer in Bahodopi
with 8.42 meters average thickness.
GEOCHEMISTRY CHARACTERISTICS
OF BAHODOPI LATERITE ZONE
BEDROCK
Bedrock in Bahodopi consists of peridotite in
various serpentinization degrees. Core drilling
usually reach three meters bedrocks to ensure
whole laterite profile (limonite, saprolite and
bedrock) has been gained. It should be done to
reduce the risk of incomplete profile due to
limitation of macroscopic description which can
not difference between bottom saprolite and
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The 39th IAGI Annual Convention and Exhibition
bedrock. The average Ni in bedrock Bahodopi is
0.34%, Fe = 6.05%, SiO2 = 40.20% and MgO =
34.92%.
SAPROLITE
Saprolite has wide range of Ni, MgO, SiO2 and
Fe percentage in Bahodopi saprolite because it
has two different type material such as; rocky
saprolite/hard saprolite and soft saprolite. Rocky
saprolite has material coarser than soft saprolite.
In Bahodopi, hard saprolite usually lies in the
bottom saprolite while soft saprolite in the top,
although due to difference degree of weathering
process, sometimes hard saprolite lies on soft
saprolite. Generally, Ni in rocky saprolite has
percentage lower than soft saprolite, because Ni
element constitutes semi mobile element and
will be more abundant in soft saprolite than hard
saprolite. Average Ni in hard saprolite is 1.44%
while in soft saprolite 1.87%. Behaviour Ni in
Bahodopi saprolite shows higher Ni in the top
and decreasing to the bottom. In several sample
of Bahodopi saprolite, rocky saprolite has higher
Ni than soft saprolite due to in serpentinized
peridotite, serpentine minerals carries
considerable porosity and allows nickeliferrous
waters to penetrate the mineral structure. Some
Mg in the serpentine structure is thus replaced
by Ni giving rises to nickeliferrous serpentine
(Ahmad, 2008).
MgO in rock saprolite is higher than soft
saprolite due to Mg as high soluble element will
be leached out of laterising environment. The
average of MgO in soft saprolite is 22.65%,
while in hard saprolite 31.98%. Similar to MgO,
SiO2 in rocky saprolite has higher percentage
than soft saprolite due to its solubility. In
laterising environment, high soluble mineral will
has lower percentage than low soluble mineral in
soft saprolite. SiO2 in rocky saprolite has
average 40.33% whereas SiO2 average in soft
saprolite is 36.19%. MgO and SiO2 has similar
behaviour, they have lower percentage in the top
and increasing to the bottom.
Fe has opposite characteristics comparing to
SiO2 and MgO. Fe has higher value in soft
saprolite than rocky saprolite. Ferrous (Fe++
)
iron is quite soluble in ground water while ferric
(Fe+++
) iron is highly insoluble. Originally, Fe
occur in ferrous (Fe++
) state, and due to highly
oxidising environment the ferrous iron in the
ferromagnesian mineral is quickly oxidised to
ferric state in the presence of oxygen present in
pore spaces, particularly above the water table
(Ahmad, 2008). The top of saprolite as soft
saprolite usually lies above water table and
constitute oxidizing environment which means
Fe occur in Fe+++
(highly insoluble) caused
increasing Fe percentage while in the bottom
saprolite that usually lies below water table and
area dominantly rocky saprolite occur, Fe++
is
accommodated caused decreasing Fe percentage
(Figure 5). Fe in soft saprolite has average
14.51% while in rocky saprolite 7.23%.
LIMONITE
Limonite is the term to define layer which has
completely weathered. It is found in the top
layer and also the oldest layer of the laterite
zone. Limonite has homogenous material of soft
and breakable material. The highest percentage
of element in Bahodopi limonite is Fe. Fe has
average 46.06%. Fe in the top of limonite will
gradually decrease to bottom of limonite. Abrupt
change of Fe value indicates a layer change,
from limonite to saprolite. SiO2 and MgO in
limonite has minor occurrence due to their
solubility. Average SiO2 in limonite is 2.63%
and MgO is 1.09%.SiO2 and MgO could be
indicator also of layer change, rapid increasing
of SiO2 and MgO signify the change of layer to
saprolite. Ni has average 0.89% in limonite. The
highest Ni in limonite layer usually lies in
transition zone where Ni as semi mobile element
will be precipitated in bottom limonite to top
saprolite zone (Figure 6).
SUMMARY
Bahodopi laterite shows ideal profile which has
bedrock in the bottom, saprolite lies above
bedrock, and limonite lies in the top of profile.
Ni has low percentage in bedrock. It increases to
the bottom of limonite and decreases to the top
of limonite. Original Fe in peridotite has poorly
percentage; laterisation process caused
increasing Fe to the top of limonite. Conversely,
MgO and SiO2 have highly percentage in
bedrock and continue to decrease until top of
limonite. Rate of solubility of the elements in
PROCEEDINGS PIT IAGI LOMBOK 2010
The 39th IAGI Annual Convention and Exhibition
laterising environment has biggest influence of
element abundances. Immature saprolite,
represented by coarser saprolite, has big
difference to the mature saprolite, represented by
finer saprolite. Ni and Fe in finer saprolite has
higher number than coarser saprolite and
conversely MgO and SiO2 in finer saprolite has
lower number. Abrupt changes of chemistry are
used to define the layer boundary of laterite
zone. Fe, MgO and SiO2 abrupt changes could
be used to define boundary of limonite and
saprolite, while Ni used to define boundary of
saprolite and bedrock.
REFERENCES
Ahmad, W., 2008, Nickel Laterites, PT INCO
Tbk
Hall, R, Wilson, M.E.J, 2000, Neogen Suture In
Estern Indonesia, Journal of Asian Earth
Science. P 781-808.
Melkybudiantoro, Didi, 2009, Technical Report
Myara Hill, PT INCO Tbk
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FIG
URE 1
. Sam
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ahodopi
FIG
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Sum
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. L
ater
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pro
file
of
Bah
odopi
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. E
xam
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of
core
sam
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s B
ahodopi
whic
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sap
rolite
(D
epth
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18)
has
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to b
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Dep
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8 –
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dif
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. M
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elem
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beh
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of
Bah
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eposi
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FIG
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. N
i beh
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of
Bah
odopi D
eposi
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