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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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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

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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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