mgi 20120202

Majalah Geologi Indonesia, Vol. 27 No. 2 Agustus 2012: 87-107

87Naskah diterima: 11 Oktober 2011, revisi terakhir: 26 Juli 2012, disetujui: 06 Agustus 2012

The Serpentine-Related Nickel Sulfide Occurrences from Latao, SE Sulawesi: a New Frontier of Nickel Exploration in Indonesia

Kehadiran Sulfida Nikel yang berkaitan dengan Serpentin di Latao, Sulawesi Tenggara: suatu Frontier Baru Eksplorasi Nikel di Indonesia

Robby Rafianto, Frans Attong, Alan Matano, and M. Erwin Syam Noor

Mines and Exploration Department PT. INCO Tbk.,Sorowako, South Sulawesi 92984, Indonesia

ABSTRACTAlong the history of nickel exploration and mining in Indonesia, there was no nickel sulfide deposit reported and relatively unknown. All known nickel deposits in Indonesia are lateritic deposits as a weathering product of ultramafic rocks. The major nickel sulfide deposit types in the world are under category of komattites associated, basal and stratabound in mafic-ultramafic intrusions, as-trobleme associated (Sudbury type), and few others reported hosted in black-shale deposits, epigenetic hydrothermal or serpentine-related deposits. The latter type is discussed in this paper that potentially related to nickel sulfide occurrences in PT. INCOs Latao Contract of Work concession. At the begin-ning, the exploration in Latao was taken during 1970 - 1975 and 1980 to confirm the occurrences of podiform chromite deposits. The Exploration Department conducted a modern exploration in 2007 - 2009 in order to get basic information for chromite potentials. The preliminary results showed that the podiform chromite is relatively small potential. However, some findings were surprising during detail geological mapping programs. Some series of silica alteration, rich magnetite rock bodies, altered serpentinized ultramafic rocks related to Ni Co mineralizations (0.5% to 2% Ni; 0.05% - 0.2% Co) were found. During a 2008 drill test program, four holes intersected unusual Ni-Co mineralizations at certain depths and related to the silica alterations. These mineralizations could not be explained by a traditional nickel laterite theory as normally applied in Indonesian geological setting. The visual sulfide minerals are difficult to observe. However, the petrographic and EPMA mineralogical analyses from some surface rock samples showed the occurrences of pentlandite, Co-pentlandite, and millerite as representing the nickel sulfide minerals. The Ni-Co mineralization is interpreted to have relation-ships with serpentine and mainly epigenetic nickel-sulphide type. Due to serpentinization of olivine, the nickel in the olivine was released and nickel bearing sulfides were formed depending on sulfur availability. The 2011 detail IP geophysical survey and further test drilling program are required to validate further this unique mineralization type. The large tonnage-low grade nickel sulfide deposits (+ 200 Mt @ 0.1% Ni) that can produce high grade nickel concentrate like Ronnbacken serpentinite deposit in Sweden is a current major exploration target. This is a potential new frontier of nickel exploration in Indonesia. Keywords: nickel sulfide, serpentine, lateritic, ultramafic, new frontier, nickel exploration, Latao, Sulawesi

SARI

Sepanjang sejarah eksplorasi dan penambangan nikel di Indonesia, laporan mengenai cebakan sulfida nikel tidak pernah ada, sehingga keberadaannya relatif tak diketahui. Semua cebakan nikel yang diketahui di Indonesia berupa cebakan lateritis hasil pelapukan batuan ultramafik. Di dunia, tipe cebakan nikel sulfida utama hadir dalam kategori yang berasosiasi dengan komatit, basal, dan lapisan yang berada dalam intrusi mafik-ultramafik, asosiasi astrobleme (tipe Sudbury), dan beberapa lainnya yang terdapat dalam endapan serpih hitam, hidrotermal epigenetis atau cebakan yang terkait dengan serpentin. Tipe terakhir yang didiskusikan dalam makalah ini adalah yang berpotensi berkaitan


88

INTRODUCTION

The Latao (Contract of Work) CoW located in the western coast of the southeastern arm of Sulawesi Island. Administratively fall under in Batuputih subregency, Kolaka Utara Regency, Southeast Sulawesi Prov-ince. It is approximately 115 km or 3 hours land travel from Sorowako (Figure 1). The 3,148.11 ha areas have altitudes ranging of 10 to 370 m above sea level.

Along the history of nickel exploration and mining in Indonesia, no nickel sulfide de-posit was reported and relatively unknown. All known nickel deposits in Indonesia are lateritic deposits as weathering product of ultramafic rocks. The finding of nickel sul-fide occurrence in Indonesia is an important milestone to the history of nickel exploration in Indonesia.

The nickel sulfide deposits can be defined as a broad group of deposits containing nickel with additional copper, cobalt, and platinum group elements (PGE), occurring as sulfide concentrations and associated with a vari-able mafic and ultramafic rocks.

The major nickel sulfide deposit types in the world are under categorized into komatiites associated, basal and stratabound in mafic- ultramafic intrusions, astrobleme associated (Sudbury type) and few others reported hosted in black-shale deposits, epi-genetic hydrothermal or serpentine related deposits. The latter type is discussed in this paper that potentially related to nickel sul-fides occurrences in PT. INCOs (PTI) Latao Contract of Work concession.

EXPLORATION HISTORY AND METHODOLOGY

Pre 2007

The Latao Block was explored by PT Inco due to its chromite occurrence. Chromite was reported in Sulawesi by Julius and Dickmann during the Dutch colonial era from Warau - Balambano road, Sapiri Island (Latao) and Lapaopao in 1924. A Dutch company called MMC had done a short period of studies on the nickel laterite and chromite potential of Latao area in 1941 (Sumardjo, 1972).

dengan kehadiran sulfida nikel di daerah konsesi Latao, PT.INCO. Eksplorasi awal di Latao dilak-sanakan pada 1970 1975 dan pada 1980 ditemukan keberadaan cebakan khromit podiform. Untuk mendapatkan informasi dasar potensi kromit, pada 2007 2009, Departemen Eksplorasi melak-sanakan sebuah eksplorasi modern. Hasil awal memperlihatkan bahwa kromit podiform potensinya relatif kecil. Namun, selama program pemetaan geologi terperinci ditemukan sejumlah seri alterasi silika, batuan yang kaya akan magnetit, serta batuan ultramafik terserpentinkan dan terubah yang berkaitan dengan mineralisasi Ni-Co (0,5 2 % Ni; 0,05 0,2 Co). Dalam program pengeboran uji 2008, pada kedalaman tertentu, empat lubang menembus mineralisasi Ni-Co yang tidak lazim dan berkaitan dengan alterasi silika. Mineralisasi ini tidak dapat diterangkan dengan teori laterit tradisional seperti yang biasanya diterapkan dalam tataan geologi Indonesia. Secara visual, mineral sulfida ini susah diamati. Namun, analisis petrografi dan mineralogis EPMA terhadap percontoh batuan permukaan menunjukkan keberadaan pentlandit, Co-pentlandit, dan milerit sebagai repre-sentasi kehadiran mineral sulfida nikel. Mineralisasi Ni-Co diduga mempunyai hubungan dengan serpentin dan terutama tipe sulfida nikel epigenetis. Akibat proses serpentinisasi terhadap olivin, nikel dalam olivin terlepaskan dan sulfida pembawa nikel terbentuk bergantung pada ketersediaan sulfur. Survei geofisika IP terperinci 2011 dan program pemboran uji yang lebih lanjut diperlukan untuk memvalidasi lebih jauh tipe mineralisasi yang unik tersebut. Cebakan sulfida nikel yang besar (+ 200 Mt @ 0,1 % Ni) yang dapat menghasilkan konsentrat nikel kualitas tinggi seperti cebakan serpentinit Ronnbacken di Swedia adalah target eksplorasi utama saat ini. Hal ini merupakan suatu eksplorasi nikel new frontier yang potensial di Indonesia.

Kata kunci: : sulfida nikel, serpertin, lateritik, ultramafik, new frontier, eksplorasi nikel, Latao, Sulawesi

The Serpentine-Related Nickel Sulfide Occurrences from Latao, SE Sulawesi: a New Frontier of Nickel Exploration in Indonesia (R. Rafianto et al.)

89

PTI reconnaissance team had done some drilling and pit resampling in 1969 (Nasu-tion, 1973). In 1975, Auger drill program, test pit, and ground magnetic surveys were carried out over the old MMC trenches (Ahmad, 1976). Butler (1980) conducted a geochemical sampling program using backpack augers and field mapping along the Latao shoreline. After 1980, there was no significant exploration activity in the Latao Block.

2007 - Present

In 2007, the exploration program for Latao was designed to confirm its chromite po-tential (Rafianto, 2007). Field activity was carried out by conducting soil sampling using back pack auger combined with a geological mapping. The target was to find anomalous areas of chromite in soil samples, which can be detected from its significant concentration of heavy mineral concentrate

containing chromium in magnetic and non-magnetic fractions. The other program is Ground geophysical survey - IP along the soil sampling anomaly areas to define the vertical distribution of podiform chromite bodies.

The preliminary result showed that the podi-form chromite is relatively small potential. However, some findings were surprising during 2007 - 2008 detail geological map-ping programs. Some series of silica al-teration, rich magnetite rock bodies, altered serpentinized ultramafic rocks related to Ni-Co mineralizations (up to 2% Ni, 0.2% Co) are found. During 2008 drill test pro-gram, four holes intersected unusual Ni-Co mineralisations at certain depths and related to the silica alterations. These mineraliza-tions could not be explained by traditional nickel laterite theory as normally applied in Indonesian geological setting.

o

120 30"0'E o

120 40"0'E o

120 50"0'E o

121 0"0'E o

121 10"0'E o

121 20"0'E o

121 30"0'E o

121 40"0'E o

121 50"0'E o

122 0"0'E

o

120 30"0'E o

120 40"0'E o

120 50"0'E o

121 0"0'E o

121 10"0'E o

121 20"0'E o

121 30"0'E o

121 40"0'E o

121 50"0'E o

122 0"0'E

o

30

"0

'S

o

25

0"0

'S

o

24

0"0

'S

o

23

0"0

'S

o

22

0"0

'S

o

21

0"0

'S

o

20

"0

'S

o

30

"0

'S

o

25

0"0

'S

o

24

0"0

'S

o

23

0"0

'S

o

22

0"0

'S

o

21

0"0

'S

o

20

"0

'S

Jakarta

Makassar

Sorowaka

Legend

PTI Contract of Work

PTI Contract of Work

Roads

Lake

Province

LATAO

PROJECT

Figure 1. Latao Project Geographic Location.


90

In 2008, PTI changed the exploration strat-egy from chromite exploration to exploring for nickel sulfide potential (Rafianto et al., 2008). Ground magnetic survey and core drilling programs were carried out to sharp-ening the 2007 IP/Resistivity results (Su-harto et al., 2009). The EPMA analysis from rock chip samples showed nickel sulfide minerals such as pentlandite and millerite.

Since soil and rock chip samples showed unexplainable broad anomalies across Latao area, in first half of 2011 PTI conducted a 120 km IP/Resistivity survey and directional core rilling program (Attong et al., 2011).

To understand the nickel sulfide mineraliza-tion, we need to differentiate between nickel sulfide and nickel silicate-oxides miner-alisations. A special assaying technique to selectively analyze Ni from NiS only using Bromine Methanol Br(Me)OH has been applied in Latao exploration.

GEOLOGICAL SETTINGS

Regional Tectonics and Geology

A regional tectonic setting description was provided by Fugro (2009) during ground magnetic survey for PTI Latao project. The summary of the tectonic of Sulawesi can be described as below: Sulawesi located at the convergence between the Eurasian (Sundaland), the Pacific-Philippine Sea and the Australian Plates (Hinschberger et al.,2005; Kadarusman et al., 2004, Hall, 1996; Lee and Lawver, 1995). The present config-uration of Sulawesi reflects the interaction between these plates and the amalgama-tion of a number of micro-plates from the Middle Miocene (approximately 15 Ma) to the present.

The island of Sulawesi consists of four dis-tinct lithotectonic belts, bounded by large scale tectonic structures. The four belts

are: (i) West Sulawesi Tertiary Magmatic Arc and Associated Sediments, (ii) Cen-tral Sulawesi Metamorphic Belt, (iii) East Sulawesi Ophiolite (ESO), and (iv) accreted continental fragments of Banggai-Sula Is-lands and the Tukang Besi-Buton Platforms (Figure 2; Kadarusman et al., 2004). The tectonic evolution of the Sulawesi can be chronologically summarized as follows:

Early Paleogene (65 Ma) - Middle Oligo-cene (30 Ma)The formation of Sulawesi began in the Early Paleogene with the opening of the Makassar Basin (Hall, 1996) followed closely by the opening of the Celebes Basins and the rifting of the west and north arms of Sulawesi. The opening of the Celebes Sea was thought to have started by the Middle Eocene. The sea widened to the southeast until the mid-Oligocene. This opening com-bined with the northwards movement of the Indo-Australian Plate, allowed for the ob-duction of the ESO and Pompangeo Schist Belt (PSB) onto the Sundaland margin.

Late Oligocene (25 Ma)By the end of the Oligocene, the ESO had been overthrusted onto the PSB of the west Sulawesi continental margin. This was fol-lowed by a change in plate boundaries and the subduction of the Indo-Australian Plate under the ESO (Hall, 1996). This compres-sional event could be responsible for the NW-SE trending folding of both the East Sulawesi Ophiolite and the Pompangeo Schist Belt.

Early-Middle Miocene (20 - 15 Ma)As a result of the Indo-Australian Plate subducting west under the ESO, arc magma-tism formed the Neogene Volcano Clastics (NVC). Kadarusman et al. (2004) noted that these volcanoclastic rocks have been mistakenly identified in some cases as a ba-saltic layer of the ESO (Monnier et al., 1995; Mubroto et al., 1994; Silver et al., 1983).


91

Middle Miocene - Pleistocene (15 - 5 Ma)The opening of the Ayu Trough in the east, starting in the middle Miocene (15 Ma.) initiated a series of E - W wrench faults ex-tending into Sulawesi (Hall, 1996). Further obduction of the ESO and PSB during the Pliocene was caused by the buoyant subduc-tion of the Banggai-Sula microcontinent (Kadarusman et al., 2004). This period of intense activity was followed by a period of extensional collapse in the Late Pliocene.

The Late Oligocene was a period of thrust-ing, where the ESO was overthrusted onto the PSB. Post or syn-folding was a period NW-SE trending folding which can be at-tributed to compression during the Late Oligocene when the Indo-Australian Plate

was moving approximately northwest, subducting underneath the ESO. The major NW - SE trending sinistral wrench faults (including the Matano and Lewanopo Faults - Figure 3) can be attributed to the opening of the Ayu Trough in the east starting in the Middle Miocene (Hall, 1996) followed by a Pliocene westward extensional transpres-sion and a Quaternary transtensional defor-mation regime.

Closer to Latao area in Southeast Sulawesi, Rusmana et al. (1993) distinguished the re-gion into two geological provinces known as Hialu (oceanic crust - ophiolite) and Ti-nondo (continental terrain). The continental terrain comprises metamorphic basement, and Mesozoic sediments (Triassic Meluhu

Figure 2. Geological setting of the Sulawesi Region (Kadarusman et al., 2004).

West and North Sulawesi

Volcano-Plutonic Arc

Quaternary sediments

Cenozoic volcanics

and plutonic rocks

Tertiary sediments

Central Sulawesi

Metamorphic Belt

Ophiolite Melange

HP Metamorphic Rock

(Pompangeo schists)

East Sulawesi

Ophiolite Belt

Neogene and Quaternary

sediments

Ophiolite

Banggai-Sula & Tukang Besi

Continental Fragments

Continental basement and cover

Continental basement below

sea level

Major thrust

Major strike-slip fault

Active volcano


92

and Tokala Formations). The oceanic crust of Cretaceous Ophiolite rock consists of peridotite, harzburgite, dunite, gabbro, and serpentinite (Figure 4).

RESULT

Rock Types

The Latao survey block is situated in the Southeast Arm of the Sulawesi Region. The geologically the Latao survey block is domi-nated by Cretaceous ophiolitic ultramafic rocks (Figure 5), comprising predominantly moderately to highly serpentinised perido-tite. The ophiolite sequence is believed to be obducted over the Meluhu and Tokala Formations.

In Latao CoW, there are four major types of identified rock types, as follows:

Metamorphic RocksThe metamorphics exposed in northern Latao area comprise phyllite and schist hav-ing a tectonic contact with ultramafic rock. It is doubtful whether it is part of Triassic Meluhu Formation or part of Mekongga Complex.

Petrographically the rocks exhibit hetero-blastic textures, consisting of nematoblastic mineral of opaque (hematite) and tourma-line, granoblastic minerals of quartz, and lepidoblastic minerals of muscovite, fine to medium grain sized (0.2 0.7 mm), anhedral to subhedral in crystals shape. Schistose structure is shown by the orientation of muscovite and opaque minerals (hema-tite). The porphyroblastic texture is present in the some area of thin section where the porphyroblasts (maybe garnet) are usually altered or pseudomorphosed by iron oxide

N

km

Legend

Major fault - inferred

Major fault - thrust

Geological boundary - inferred

0 5 10 20 30 50

Figure 3. Major tectonic and structural geology related to Latao Project (Fugro, 2009).


93

to form brownish red color. This rock is me-dium altered (retrogressive metamorphism). The iron oxide altered or pseudomorphosed the porphyroblasts whilst sericite altered the muscovite.

The presence of opaque mineral (hematite), quartz, muscovite, and tourmaline as a major mineralogy of rock suggests to a metamor-

phism process occurring at a relatively low to medium grade and then they have done the retrograde metamorphism, where it is represented by the presence of iron oxide and sericite.

LimestoneThe limestones are present as isolated hills occurring at the boundary between ul-

Figure 4. Regional geology map (Rusmana et al., 1993). Pzm = Palaeozoic Metamorphics (Carboniferous); TRJt = Tokala Formation (Triassic); TRJm= Meluhu Formation (Triassic) Ku = Ophiolites Rocks (Cret-ceous); Tmpp = Pandua Formation (Pliocene).

Figure 5. Latao geologic map.

0 245 490 980 1,470 1,960

1:25000

Latao Project

Legend

Structure

Tidal Zone

Swamp

Basalt

Gabro Cumulate

Metasedimen Limestone

Hydrotermal and Polymict Breccia ?

Tectonic Breccia

Dunite alter

Magnetite

Chromite Zone

Metamorphyc Phyllite

PTI-Cow SK2005

m

Name

Erwin

Remark :

Drawin by

Revised by

Checked by

Approved by

Sign Date

282000 282500 283000 283500 284000 284500 285000 285500 286000 286500 287000 287500 288000 288500 289000 289500 290000

282000 282500 283000 283500 284000 284500 285000 285500 286000 286500 287000 287500 288000 288500 289000 289500 290000

9660000

9660500

9661000

9661500

9662000

9662500

9663000

9663500

9664000

9664500

9665000

9665500

966000

966500

9667000

9667500

9660000

9660500

9661000

9661500

9662000

9662500

9663000

9663500

9664000

9664500

9665000

9665500

966000

966500

9667000

9667500

N


94

tramafic and metamorphic rocks. It is un-clear whether it is part of Triassic Meluhu or Tokala Formations or part of Mekongga Complex. By visual description, the rock can be described as dark to light grey colour, coarse-grained, dominantly consisting of calcite and some samples at Mosiku area contain disseminated pyrites.

The limestone exhibits the clastic and mud-supported texture, poorly sorted and open packed. The rock is dominated by the matrix of carbonate muds with the spotted/patches of opaque (pyrite), and they are usually recrystallized into microcrystalline calcite (micrite). The grains include quartz and fos-sil fragments are present in the some area of thin section. Quartz grains are angular to subrounded. Fossil fragments are dominated by planktonic foraminifera, unbroken in shape, their chamber usually filled by calcite cement. The opaque minerals also present as grain in some areas of thin section, sub-hedral in shape, and up to 0.5 mm in size. Stylolitic structure occures in the some area of thin section.

This sample is fresh but the opaque grains (pyrite) present in the some area of thin sec-tion maybe a product of secondary process. The rock fracture filled by calcite opaque (pyrite) form vein in the some areas within thin section.

Ultramafic RocksThe ultramafic rocks dominate the Latao CoW and have been altered into various ser-pentinization levels (dominantly medium to highly serpentinized) The massive serpenti-nite has N-S orientation, stretches over 6 km in length and 5 km wide. It has a greenish-black color and is composed by serpentine minerals. Most of this ultramafic rocks is completely altered and strongly weathered at the surface. The serpentine and talc pres-ent on the fractures or cleavage the olivine and pyroxene form veinlets of serpentine talc. The veins of silica (quartz) are present to cut the rock in the some areas of thin section (Figure 6a). Magnetite partially re-places chromite on grain peripheries occurs as fracture fillings. The sulfide minerals that occur as fine grains usually are associated with talc and carbonate minerals (Figure 6).

Hydrothermal Associated RocksIn Latao, there are several rocks interpreted to have association with hydrothermal pro-cesses, alterations and mineralization (Fig-ures 7, 8, 9). They are silica ribs, magnetite bodies, and hydrothermal breccias (?). The silica ribs contain chalcedonic silica found in areas of Tanjung Berlian, Lawatuwatu, and Temboe. They have been mistakenly identified as low T lateritic product of silica precipitation by previous explorers. The

Figure 6. Serpentine as alteration product from olivine with mesh structure cross by silica vein (a). Pentland-ite and chromite as opaque minerals in serpentine (b) (ITB, 2009)


95

Figure 7. Latao silica alteration.

Figure 8. Ni Co bearing magnetite outcrop.

a b

Figure 9. Ni Co bearing magnetite outcrop.


96

magnetite bodies are found in Laburino, Roka, and Lawatuwatu. These magnetite bodies unusually containing nickel between 0.5% - 28% andmcobalt between 0.05% - 0.2% Co.

The hydrothermal breccias (?) in Latao are related to magnetite bodies and chromite-silica breccia found at Tanjung Berlian. The major hydrothermal magnetite breccia (?) is present at central area of Latao with linea-tion to NE-SW direction. The breccias have red-brownish colour due to the presence of iron oxides of hematite and magnetite. Millerite and pyrite occur occasionally as coating of fragments and in the matrix of breccia. This breccia has a close correlation with magnetite body areas. The thin section exhibits clastic texture, consisting of perido-tite fragments which are embedded within the mixture of clay matrix, serpentine, silica, iron oxides, and chlorite aggregates. The fragments of peridotite have angular to sub-rounded in shape, grains size of 0.1 - 5.0 cm, locally up to 1.5 cm in size, they exhibited the serpentinization and weather-ing process, consist of olivine, pyroxene and spinel/chromite

The rock sample is strongly altered and weathered, in which the iron oxides + chlo-rite pseudomorphosed the olivine to form the brownish - blackish red colour. The ser-pentine occurson the fractured or cleavage of the olivine and pyroxene to form veinlets of serpentine tremolite talc. Some pyroxenes are pseudomorphosed by talc tremolite. The veins or veinlets of silica (chalcedony) are present to cut the rock.

The sulfur content (S%) were assayed from some samples relatively showing much lower S content (


97

Dumont serpentinite (Canada): NiS deposits related to serpentinization and talc - carbonate alteration. Measured + Indicated resources 1.1 Bt @ 0.27% Ni (Royal Nickel Corp. May, 2011).

Ronnbacken (Sweden); NiS deposits related to serpentinization. Alpine-Type Serpentinite contact with metamorphic phyllite. Measured + Indicated resources 277 Mt @

0.1% NiS and 0.003% Co (SRK consult-ing, April 2011).

From above models, Ronnbacken and Du-mont deposits are the most one considered by PTI as they have large tonnages and possibly occurred in Latao. Both deposits have low average nickel grades but large in tonnages. The geometallurgical model is important to liberate NiS minerals such as pentlandite, heazlewoodite, and millerite, and making projects to be highly econom-ics (discussed in preliminary economic consideration).

Geological Mapping and Soil SamplingGeological mapping with prospecting is a critical part for Latao NiS exploration program. The basic knowledge that we are dealing with an unusual ophiolitic nickel mineralization and unexplainable by tra-ditional nickel laterite theory came from the geological mapping results. Hundreds of rock chip samples were collected and anaylized by Niton portable analyzer and selected samples were assayed for XRF, ICP and FA. The results are shown in Figure 10 and many altered unweathered rock samples are elevated in nikel, cobalt, and iron. The anomalous areas were selected for next geophysical survey prior drilling program.

Some selected surface rock samples were submitted to SGS Canada for mineral-ogical analyses. The nickel sulfide minerals (pentlandite and millerite) are identified by EPMA analysis (Figure 11). These min-

eralizations occur on a chlorite foliation in a Fe-chlorite groundmass altering to Fe-oxides with total Ni in individual mineral more than 20% (Table 1).

Soil sampling program were conducted in 2007 within the framework of a regional sampling program across Latao area. The nickel values from the soil samples could be potentially bias from lateritic mineraliza-tion, however the chemistry associations are very useful to define chromite or anoma-lous hydrothermal zones.

These altered zones are do not following the normal nickel laterite profile pattern and relatively easy to identified.

The soil sampling used hand auger with maximum depth of 10 m and total aver-age is 3.4 m. The soil sampling program designed for ridge and spur sampling and also for regular grid pattern. Since 2007, around 7000 samples have been collected and assayed for XRF analyses.

A special case for chromite prospecting, a simple beneficiation method adopted from Philippines chromite exploration (Kater et al, 1984) with panned concentrate for analyzing heavy mineral concentrate such as chromium and magnetic separation were applied for soil samples in 2007. The chromium soil anomalies were detected but limited in small areas. However, the results were surprising and unexplainable for cobalt values (Figure 12). The high grade cobalt values from panned concentrate of soil samples can reached > 1% up to 2.8% Co. This is an example that high cobalt values from certain areas are not following the conventional laterite theory and interpreted these areas are hydrothermally altered.

Geophysical SurveyThe geophysical surveys of ground mag-netic survey and IP/Resistivity survey were conducted in Latao. Ground magnetic


98

survey was proposed to understand the mag-netic response related to alteration products and general structural geology whereas the resistivity data can guide us to the alteration and mineralization zones which marked by the anomaly values of resistivity and chargeability.

Ground Magnetic SurveyThe ground magnetic survey has initi-ated in mid November 2008 to April 2009 conducted by Fugro (2009). This survey is quite considerable for explore the potential

mineral such us magnetite, chromite, and pyrhotite a total of 120 km of total length in 40 line km within 20 km area at 12.5 m station spacing and 50 m line spacing, than 80 line km within 80 km area at 25 m station spacing and 100 m line spacing has proposed for this survey.

An interpretation was completed using the magnetic and low resolution radar. A number of high-magnetic zones possibly represent more massive zones of magnetite concentrations.

Figure 10. Anomalous nickel values from fresh rock samples.

Figure 11. Nickel sulfide minerals based on EPMA analysis (SGS, 2010).


99

Point MineralWeight

As Ni S Fe Zn Co Cu P Total1 Millerite 0.012 59.50 35.510 2.510 0.008 1.760 0.958 0.000 100.257

2 Millerite 0.072 41.720 31.510 5.450 0.000 3.060 0.391 0.000 82.203

3 Millerite 0.090 32.940 10.090 5.250 0.000 11.120 0.836 0.000 60.326

4 Millerite 0.027 54.140 33.080 3.210 0.020 4.880 0.279 0.000 95.636

5 Millerite 0.107 41.150 40.250 7.360 0.046 9.570 0.302 0.000 97.785

6 Millerite 0.13 42.860 32.270 8.220 0.021 4.430 0.490 0.000 98.304

7 Millerite 0.00 42.690 26.730 5.980 0.000 5.810 0.421 0.000 81.631

8 Millerite 0.00 56.930 34.570 4.130 0.000 1.410 0.468 0.000 97.526

Average 0.40 46.491 30.501 5.264 0.012 5.130 0.520 0.000 87.959

1 Pentlandite 0.040 34.440 38.860 4.100 0.000 17.260 0.546 0.000 95.246

2 Pentlandite 0.565 43.710 38.570 5.010 0.033 9.550 0.515 0.000 97.444

3 Pentlandite 0.026 21.250 21.450 8.150 0.064 20.180 3.400 0.000 74.520

4 Pentlandite 0.000 36.330 17.220 4.260 0.027 10.470 1.050 0.000 69.357

5 Pentlandite 0.056 27.870 24.950 7.290 0.091 14.820 1.360 0.000 76.437

6 Pentlandite 0.033 22.070 27.820 8.010 0.000 22.020 1.980 0.000 81.933

7 Pentlandite 0.096 25.110 23.100 5.610 0.000 18.360 2.670 0.000 74.946

8 Pentlandite 0.058 23.280 29.410 5.470 0.104 23.810 1.170 0.000 83.301

9 Pentlandite 0.096 24.040 25.270 7.340 0.041 19.650 1.180 0.000 77.617

Average 0.051 28.648 27.406 6.1.39 0.040 17.347 1.541 0.000 91.200

1 Co-Pentlandite 0.016 18.840 26.240 9.670 0.042 21.980 2.840 0.000 79.629

Average 0.016 18.840 26.240 9.670 0.042 21.980 2.840 0.000 79.629

1 Pyrite 0.068 0.016 54.090 47.880 0.042 0.009 0.014 0.000 102.118

Average 0.068 0.016 54.090 47.880 0.042 0.009 0.014 0.000 102.118

1 Chalcopyrite 0.058 10.630 31.570 24.590 0.034 0.414 25.270 0.000 92.566

Average 0.058 10.630 31.570 24.590 0.034 0.414 25.270 0.000 92.566

1 CuZnNiFe 0.015 17.760 0.027 5.990 19.520 0.015 54.760 0.000 98.086

2 CuZnNiFe 0.000 17.100 0.043 4.540 18.040 0.000 58.300 0.000 98.024

Average 0.008 17.430 0.035 5.365 18.780 0.008 56.530 0.000 98.055

Table 1. EPMA Analysis of Latao Nickel sulfides and Alloys (EMD0009916)

IP/Resistivity SurveyThe first IP/Resistivity survey was con-ducted in 2007 at Tanjung Berlian. The IP/Resistivity survey used pole dipole method with 50 m spacing. Total lines are 18 or equal with 16,050 m (Elliot, 2008). The IP survey was following the soil sampling grid lines with east west direction across the silica ribs position.

The second IP/Resistivity was conducted in 2011 and covering larger areas defined as potential anomaly zones by previous geological mapping and soil sampling. The total line of IP survey is 120 km across all the ultramafics and its contact with meta-morphic rocks include hydrothermal associ-ated rocks. This survey proposes to confirm the subsurface of nickel sulfide potential. It


100

is interpreted that the prospect area of nickel sulfide mineralization is related to high resistivity and high chargeability val-ues. The 2011 IP survey shows encouraging result. Some large anomalies (coincident of high resistivity and high chargeability) are detected and open to depth (Figures 13a and 13b; Geoservices, 2011). This survey highly supports for next drilling program and po-tential to achieved large tonnage.

Diamond Drilling ProgramThe first core drilling program in the Latao was conducted in 2008. A total of 2000 m depth of diamond drilling was proposed. The objective of this program was to test the 2007 IP and soil anomalies and to confirm the potential Ni-Co sulfide and Cr mineral-ization. During 2008 drill test program, four holes intersects unusual Ni-Co mineralisa-tions at certain depths (>80 m) and related to the silica alterations. These mineraliza-tions could not be explained by a traditional nickel laterite theory as normally applied in Indonesian geological setting.

Early 2011, PTI started to conduct the 2nd phase diamond drilling program to confirm IP and ground magnetic anomalies related to NiS mineralizations. Total of 10 holes or 1,973.70 m depth were drilled until mid 2011. The 3rd phase of diamond drilling

program is carried out from October 2011 to December 2011.

To understand the occurrences of nickel sul-fide mineralization, PTI selects a special as-saying technique to selectively analyzed NiS by Bromine Methanol Br(Me)OH provided by SGS Canada and Outotec Findland. This method can discriminate the nickel sulfide mineralization with other nickel silicates or oxides minerals (NiS vs Ni total). This is very useful to design the next preliminary flotation test from drill core samples.

Until this paper is written, PTI is still wait-ing the Br(Me)OH assay results from 2008 and 2011 drilling programs with around 4000 samples. However, few random sam-ples were back and preliminary results are confirmed the occurrences of nickel sulfide in the core samples. Table 2 is an example from a preliminary results of Br(Me)OH of nickel sulfide only (%) vs nickel total (%) that composed of nickel silicates, oxides, and sulfides.

The grade of NiS from drill samples is rela-tively similar with Ronnbacken (Sweden) or Dumont (Canada) deposits those related to the serpentinization processes.

The occurrences of NiS mineralizations in drill samples are closely related to high

Figure 12. Unusual very high cobalt values from soil samples (after panned concentrate).


101

resistivity and high chargeability from IP survey. These IP anomalies are good guide for current drilling program and PTI is expecting can achieved potential large ton-nages from current drilling program.

Interpretation of MineralizationLatao deposit is a product of the obduction of peridotitic oceanic crust, intensive hydro-

thermal alteration, and tectonic deformation. It has numerous features in common with other mineral deposits associated with al-tered serpentinites.

From tectonic history, the westward move-ment of the Banggai-Sula microcontinent during the Miocene initiated a new subduc-tion zone, forming Neogene Volcano clastics

Figure 13. Examples of 2011 IP survey anomalies. (a) Section of Latao IP anomalies; (b) 3D model intersec-tions of high resistivity and high chargeability, open to depth.


102

(NVC). Further obduction of the ESO and Pompangeo Complex during the Pliocene was caused by the buoyant subduction of the Banggai-Sula microcontinent (Figure 14, Kadarusman, 2008). Like NVC, it is interpreted that hydrothermal fluids are possibly released through the channeling of major structures during subduction process. Combined with serpentinization processes and depending sulfur availability, the nickel sulfide mineralization in ophiolite such as Latao possibly occured.

The NNE-SSW and NW-SE trends of hy-drothermal associated occurrences in Latao are the logic fault impacts (and as hydro-thermal fluid channels) created by regional WNW-ESE tectonic movements parallel with Lawanopo Fault.

To compare with other normal ultramafic rocks in Sulawesi, Kadarusman (2009) made some geochemical plots. It is con-cluded that Latao bedrocks have different pattern with other peridotite in Sulawesi. Latao ultramafic bed rocks seem to be modified from the original composition by hydrothermal alteration (?). The Latao bedrock more rich in silica, high nickel and low CaO (Figure 15).

The hydrothermal alteration corresponded with the serpentinization processes while the ophiolites were emplaced during a pe-riod of crustal collision resulting in obduc-tion. The hydrothermal mineral assemblage is a product of temperature, the presence of CO2, and oxygen and sulfur fugacity. The

BHID From To Samp ID NiS Ni Total NiS Average Ni Total Average

LTD0C31 38.00 39.00 CXL0004052 0.15 0.21

0.11 0.22

LTD0C31 39.00 39.72 CXL0004053 0.13 0.22

LTD0C31 39.72 40.00 CXL0004054 0.12 0.12

LTD0C31 40.00 41.00 CXL0004055 0.11 0.11

LTD0C31 41.00 41.72 CXL0004056 0.10 0.10

LTD0C31 41.72 42.00 CXL0004057 0.10 0.10

LTD0C31 42.00 43.00 CXL0004058 0.09 0.09

LTD0C31 43.00 43.72 CXL0004059 0.10 0.10

Table 2. Example of Br(Me)OH assay for Nickel sulfide (%) occurrences.

Figure 14.1nterpreted regional tectonic and Latao mineralization (Kadarusman, 2008).


103

silica was subsequently remobilized and redeposited forming a stockwork of quartz veins and veinlets. The silica (occasionally with carbonate) alteration within this en-vironment is called listwaenite to birbirite alteration. Listwaenites are described as silicified and carbonated rocks from ser-pentinized ultramafic rocks in ophiolite complexes (Buisson - Leblanc, 1985), while birbirite alteration has more silica content >85% (Esteban et al., 2011). The chromite breccia occurrence in Latao is originated from podiform chromite which is related to upper mantle cumulate associations and possibly affected by hydrothermal injection fluid.

The genesis of Latao magnetite-hematite bodies and in hydrothermal breccia (?) is still unknown. However referring to the Divrigi iron mineralization in Turkey, the magnetite bodies were formed during a hydrothermal alteration of serpentinites. In Turkey, the iron is enriched by serpentini-zation processes and further concentrated by hydrothermal convective cells caused by intrusion of granite pluton (Unlu et al., 1995). Some parts of this concept perhaps

can be adopted but required further studies.

NiS mineralization in Latao is hosted by serpentine and hydrothermal breccia (?). Nickel was originally located mainly in the olivine lattice within the ultramafic rocks, such as dunites and peridotites. Due to ser-pentinization of the olivine, nickel in the olivine was released and nickel bearing sul-phides were formed depending on sulphur availability. Serpentinization of ultramafic rocks and the olivine occurs through the sup-ply of water, S, and CO2. The reaction can be summarised as follows (SRK, April, 2011):

Olivine (Ni) + H2O + S

2 + CO

2 Ser-

pentinite + Brucite + Carbonates + Fe3O4 + NixSy* + H2* Ni-rich sulphides

By experimental studies (Filippidis, 1982), synthetic nickel bearing olivine has been serpentinized at 350C under a pressure of 2 kbar by adding sulphur and water. Olivine was transformed to serpentinite, brucite, and magnetite, and nickel rich sulphides were formed such as millerite (NiS), pentlandite ((Fe, Ni)9 S8), heazlewoodite (Ni3S2), and bravoite ((Fe, Ni, Co) S2). Low sulfur fugac-

Figure 15. Silica rich Latao ultramafic bedrock plot vs normal ultramafic rocks in Sulawesi (Kadarusman, 2009).


104

ity favours the formation of heazlewoodite and nickel rich magnetite. At a higher sulphur fugacity, lower nickel values are found in magnetite, including iron sulphides as pyrite. Dehydration experiments (at 500C) have shown that, based on the pres-ence of brucite and serpentine, it is possible to get reformation of new olivine which is more magnesium rich than the original ol-ivine. This is interpreted as magnetite and pentlandite being stable during the created metamorphic conditions.

Preliminary Economic and Environmen-tal Considerations

The large tonnage low grade nickel sulfide deposits are the main target for the Latao NiS exploration program. The rationale be-hind this target is referring to Ronnbacken (Sweden) or Dumont (Canada) projects. Both deposist have large tonnage (>200 Mt) but low nickel grade ($250 million (Royal Nickel Corp, April 2011 and Nickel Moun-tain, 2011). The summary of Ronnbacken and Dumont projects can be summarized in Table 3.

Nickel rich heazlewoodite and millerite + no iron sulfides = high grade nickel concentrate (IGE Nordic, November, 2010).

The 2011 IP survey across the Latao area shows encouraging result. Some large areas

Figure 16. Fe-Ni-S phase diagram (Misra and Fleet, 1973).


105

with combination of high resistivity and chargeability are shown for next drilling tar-gets. PTI has a great expectation to achieved large tonnage of NiS deposits in Latao.

Environmental ConsiderationFor environmental issue, the serpentine-related NiS deposits have a good position since the mine environmental costs are in-creasing. Very low sulfide content (Nickel Mountain, 2011) gives very low Acid Mine Drainage (


106

a preliminary metallurgical flotation test from drill core samples sometimes in 2012.

Experiences learned from the Latao, there is a new frontier of nickel exploration in Indo-nesia. The geology and tectonic settings of Indonesian ophiolite complexes in Sulawesi, Papua, Kalimantan, and Halmahera are needed to be re-assessed.

ACKNOWLEDGMENTS

The paper has been presented in Seminar MGEI-IAGI of the Sulawesi Mineral Resources 2011 in Manado, and compiled within the proceedings. The committe is acknowledged. To publish the article in the MGI-IAGI some improvements of the content have been carried out.

REFERENCES

Ahmad, W., 1976. Latao Chromite Exploration - A Preliminary Report, PT. Inco Internal Memorandum.

Ahmed, A., Arai, S., and Ikenne, M., 2009. Mineral-ogy and Paragenesis of the Co-Ni Arsenide Ores of Bou Azzer, Anti-Atlas Morroco. Society of Economic Geologists Inc.

Attong, F., Matano, A., and Erwin, M., 2011. Latao Exploration 2011, PT. Inco Internal Presentation.

Buisson, G. and Leblanc, M., 1985. Gold-bearing listwaenites (carbonatized ultramafic rocks) from ophiolite complexes. In: Gallagher J.M., Ixer,R.A., Neary, C. R., (eds), Metallogeny of Basic and Ultra-basic Rocks . Transactions - Institution of Mining and Metallurgy: London; p.121 - 132.

Butler, H., 1980. Latao Chromite. PT. Inco Internal Memorandum.

Elliot, P.J., 2008. Report on a Dipole-Dipole IP Survey Over the Latao Chromite Prospect, Sulawesi Indonesia. Elliot Geophysics International Pty. Ltd.

Esteban, J., Cuevas, J., Tubia, J., Vegas, N., Velasco, F., and Birbirite, 2011. Occurence in the Ronda Peridotites, (Betic Cordilleras, Southern Spain). Geophysical Research Abstracts, 13.

Filippidis, A., 1982. Experimental study of the ser-pentinization of Mg-Fe-Ni Olivine in the Presence of Sulfur. Canadian Mineralogist, 20, p.567 - 574.

Fugro, 2009. PT. Inco tbk. Integrated Interpretation of Ground Magnetic and Remote Sensed Data, Latao Area, Sulawesi. Fugro Airborne Surveys Pty Ltd. Geoservices, 2011. PT. Inco tbk - Latao IP Resistivity Data, PT. Geoservices Exploration Division.

Hall, R., 1996. Reconstructing Cenozoic SE Asia, In: Hall, R. and Blundell, D. (eds) Tectonic Evolution of Southeast Asia, Geological Society of London Special Publication 106, p. 153 - 184.

Hinschberger, F., Malod, J-A., Rehault, J-P., Ville-neuve, M., Royer, J-Y., and Burhanuddin, S., 2005. Late Cenozoic geodynamic evolution of eastern Indonesia, Tectonophysics. 404, p. 91 - 118.

IGE Nordic, 2010. Ronnbacken - Developing a major nickel sulphide project in Sweden, Raw Materials. Group 7th Annual Exploration & Mining Investment Conference 2010 Stockholm, 11 November 2010.

ITB, 2009. Latao Petrographic Description, Report for PT. Inco tbk.

Kadarusman, A., Miyashita, S., Maruyama, S., Parkinson, C.D., and Ishikawa, A., 2004. Petrology, geochemistry and paleogeographic reconstruction of the East Sulawesi Ophiolite, Indonesia. Tectono-physics, 392, p.55 83,.

Kadarusman, A., 2008. Interpretation of Latao tec-tonic setting, PT. Inco internal presentation.

Kadarusman, A. and Noor, M. E. S., 2009. Ultra-mafic Geochemistry of Sulawesi, PT. Inco internal presentation.

Kater, G., Wallner, P., and Friedrich, G., 1984. A Method of Exploration for Lateritic and Bed-Rock Chromite Mineralization Used in the Philippines. Economic Geology, 79, p. 372 - 381.

Kontinen, A. and Peltonen, P., 2004. Modelling of the Geology and Genesis of Outokumpu-type Massive-Semimassive Sulphide Deposits in Eastern Findland - Summary of the Results and Conclusions of the Geomex Modelling - Subproject, Geological Survey of Findland.

Lee, T-Y. and Lawver, L.A., 1995. Cenozoic plate reconstruction of Southeast Asia, Tectonophysics. 251, p. 85 - 138, 1995.


107

Misra, K.C. and Fleet, M.E., 1973. The Chemical compositions of synthetic and natural pentlandite assemblages., Economic Geology, 68, p. 51 - 530.

Monnier, C., Girardeau, J., Maury, R.C., and Cotton, J., 1995. Back arc basin origin for the East Sulawesi Ophiolite, Eastern Indonesia, Geology, v. 23, p. 851 - 854.

Mubroto, B., Briden, J.C., McClelland, E., and Hall, R., 1994. Paleomagnetism of the Balantak ophiolite, Sulawesi. Earth and Planetary Science Letters, 125, p. 193 - 209, 1994.

Nasution, A.D., 1973. Preliminary Report on Chro-mite Deposits, Sulawesi. PT. Inco Internal Memo-randum.

Nickel Mountain A. B., 2011. Ronnbacken - A major, long-life nickel sulphide project in Sweden, Corpo-rate Presentation.

Rafianto, R., 2007, Latao Chromite 2007. Objectives and opportunities, PT. Inco Internal Memorandum.

Rafianto, R., Pamungkas, A., Sutrisno, A., Erwin, M., and Parawangsa, A., 2008. Latao 1st Phase Report and Ni - Co Sulphides Potential. PT. INCO Internal Memorandum.

Royal Nickel Corporation, 2011. The elements of value creation. Corporate Presentation.

Rusmana, E., Sukido, Sukarna, D., Haryono, E., and Simandjuntak, T.O.,1993, Geological Maps of the Lasusua-Kendari quadrangles, Sulawesi, Geological Research and Development Centre, Bandung.

SGS Canada Inc., 2010. An investigation into min-eralogical characteristics of twelve samples from the Sorowako deposit. Prepared for PT. Inco Indonesia, Project 12214-002 MI5031-APR10.

Silver, E.A., McCaffery, R., Joyodiwiryo, Y., and Ste-vens, S., 1983. Ophiolite emplacement and collision between the Sula Platform and the Sulawesi island arc, Indonesia. Journal of Geophysical Research, v. 88 p. 9419 - 9435.

SRK Consulting, 2011. Preliminary Economic Assessment for the Ronnbacken Nickel Project, Sweden. Report prepared for Nickel Mountain Re-sources AB.

Suharto, Erwin, M., Sutrisno, A., Usman, A., and Abriadi, 2009. Status Report on 2009 Exploration of Latao Project. PT. Inco Internal Memorandum.

Sumardjo, H., 1972. Chromite Deposit Latao, PT. Inco Internal Memorandum.

Unlu, T., Stendal, H., Makovicky E., and Sayili, 1995. Genesis of Divrigi Iron Ore Deposit, Sivas, Central Anatolia, Turkey - an ore microscopy study. Mineral Research and Exploration Bulletin, 117, p. 17-28.

mgi 20120202

Documents