ARTICLE

Composition and origin of the Çaldağ oxide nickel laterite,W. Turkey

Robert Thorne & Richard Herrington & Stephen Roberts

Received: 19 February 2009 /Accepted: 4 March 2009 /Published online: 31 March 2009# Springer-Verlag 2009

Abstract The Çaldağ nickel laterite deposit located in theAegean region of W. Turkey contains a reserve of 33 milliontons of Ni ore with an average grade of 1.14% Ni. Thedeposit is developed on an ophiolitic serpentinite bodywhich was obducted onto Triassic dolomites in the LateCretaceous. The deposit weathering profile is both laterallyand vertically variable. A limonite zone, which is the mainore horizon, is located at the base of the profile. A hematitehorizon is located above the limonite, which in the south ofthe deposit is capped by Eocene freshwater limestones andin the north by a siliceous horizon. The deposit is unusualin lacking a significant saprolite zone with little develop-ment of Ni-silicates. The boundary between the limonitezone and serpentinite below is sharp with a markeddecrease in concentrations of MgO from 13 to 1 wt.% overa distance of 2 mm representing the ‘Mg discontinuity’. Niconcentrations within goethite, the main ore mineral, reacha maximum of ~3 wt.% near the base of the limonite zone.Silica concentrations are high throughout most of thelaterite with up to 80 wt.% silica in the upper portion ofsome profiles. The combination of a serpentinite protolithand a high water table at Çaldağ, in association with anaggressive weathering environment in a tropical climate,resulted in the formation of an oxide-dominated deposit.

The precipitation of silica may coincide with a change inclimate with silica precipitation linked to an increase inseasonality. Additional variations within profile morpholo-gy are attributed to transportation during and after lateritedevelopment as a result of faulting, pocket type lateriteformation and slumping, each of which produces acontrasting set of textural and geochemical features.

Keywords Çaldağ . Nickel laterite . Limonite

Introduction

Nickel laterites are formed by the intense weathering ofultramafic rocks exposed in tropical to sub-tropicalclimates. They account for ~40% of annual global nickelproduction and contain ~60% of the world’s total land-based nickel resources (Kuck 2008). During the weatheringprocess, nickel-bearing minerals, olivine and serpentine, aredestroyed in an oxidizing environment producing ironoxide deposits containing residual concentrations of Ni.Secondary silicate deposits are created when Ni leachedfrom the iron oxides reacts with the weathering products ofprimary silicates to form garnierite (Freyssinet et al. 2005).Contemporary nickel laterite formation is confined to lowlatitudes where temperature, rainfall, and consequentlygroundwater through-flow are high. Laterites located athigher latitudes and with cooler climatic regimes (e.g.,Greece and Turkey) are considered to be paleodepositswhich originally formed at lower latitudes during warmerclimes (Freyssinet et al. 2005). Despite the importance ofNi laterite deposits, there is limited data on how and whenNi laterite deposits form. Complex interactions between theprotolith, the water table, topography, climate, and trans-portation mechanisms combine to produce oxide, clay, or

Miner Deposita (2009) 44:581–595DOI 10.1007/s00126-009-0234-6

Editorial handling: B. Lehmann

R. Thorne (*) : S. RobertsSchool of Ocean and Earth Science,National Oceanography Centre, University of Southampton,Southampton SO14 3ZH, UKe-mail: r.thorne@noc.soton.ac.uk

R. HerringtonMineralogy Department, Natural History Museum,Cromwell Road,London SW7 5BD, UK

secondary silicate deposits of varying thicknesses. Thevariable role of these factors in the formation of a specificdeposit is difficult to predict.

The Çaldağ nickel laterite deposit is located within theAegean region of Turkey (Fig. 1) in the province ofManisa, approximately 70 km east of Izmir. The deposit hasa reserve of 33 million tons of ore with a grade of 1.14%nickel and 0.07% cobalt. We describe the results of depositand profile mapping combined with mineralogical inves-tigations of three separate profiles located within the Çaldağdeposit and identify the major formational processes byanalyzing textural, mineralogical and morphological rela-tionships within the deposit.

Geologic setting

The Çaldağ deposit is situated on a fragment of ophiolitelocated on the northern edge of the Menderes Massif,which lies within the Aegean Graben (Fig. 1). Theophiolite complex was tectonically emplaced onto Triassicdolomites in the Late Cretaceous when the Neo-TethysOcean closed obducting 93±2 Ma oceanic crust (Önenand Hall 2000). The deposit is located within the WestAnatolian extensional province which is characterized byN–S extension, caused by the southwestward escape of theAnatolian plate along the North and East Anatolian Faultsas the Arabian, African, and Indian plates collide with theEurasian Plate. The extension results in E–W-trendinghorsts and deep sediment-filled grabens (Yılmaz et al.2000). The Çaldağ deposit is located on a horst block tothe north of the Gediz Graben (Fig. 1), covers an area of9 km2 (Fig. 2), and was developed over a variablyserpentinized ultramafic body. The deposit is partlyoverlain by sandstones, conglomerates, and freshwaterlimestones of Eocene age (Fig. 2). These sedimentsprotected the deposit from erosion, preserving it for thelast ~55 My (Çağatay et al. 1981).

Laterite profiles at Çaldağ

Weathering profiles at Çaldağ can be divided into zonesbased on appearance, texture, mineralogy, and thickness.Investigated profiles across the deposit broadly display thesame characteristics: a limonite zone, which is the main orezone, located below a redder, more hematitic horizon,which in the south of the deposit is capped by carbonatematerial and in the north by a siliceous horizon. Extensionalfaulting is common in the region with a number of profilescut by normal faults with a displacement of up to 10 m.Although the profiles within the deposit are broadly similar,there are marked local variations in texture, morphology,and mineralogical composition (Fig. 2 and 3).

South Pit

The unweathered serpentinite protolith which crops out inthe South Pit (Fig. 2) is fine-grained, grey-green, andvariably fractured. Anastomosing calcite and magnesiteveins cut through the serpentinite but do not penetrate intothe limonite above. The overlying limonite zone shows asharp contact with the serpentinite, with small (<2 mm)veins of limonite penetrating up to 15 cm into theserpentinite. The base of the limonite is banded, withvarying amounts of goethite and asbolane, the upperportions are disaggregated and contain limonite blockscemented with calcite. Up section a hematite zone

Black Sea

0 200

Kilometers

N

Northern ophiolite belt

Median (Tauric) ophiolite belt

Southern (peri-Arabic) ophiolite belt

CENTRALMENDERES MASSIF

Küçük MenderesGraben

Büyük Menderes Graben

Plio-Quaternary basins

Neogene sediments

Paleozoic - Cenozoic metasediments

Augen gneiss

Neotethyan ophiolites

AEGEANSEA

25 km

Izmir

Istanbul

Ankara

GedizGraben

Çaldag

Izmir

A

B

38˚00’

38˚30’

27˚00’ 28˚00’

30˚ 35˚ 40˚

40˚

36˚

45˚

Fig. 1 a Location of the Çaldağ deposit in the Aegean region ofWestern Turkey. b Regional geology of the Çaldağ area (Bozkurt andSatir 2000)

582 Miner Deposita (2009) 44:581–595

dominates, containing blocks (<10 cm) of dark greysilicified laterite as well as small (<2 mm) fragments ofcalcite commonly in layers (Fig. 4a). The upper portions ofthis horizon are pervasively veined with calcite, forming areticulate texture (Fig. 3). The hematite zone is unconform-ably overlain by freshwater Eocene micritic limestonewhich contains bands of re-worked hematitic material.

Finally the micritic limestone is overlain by sparitic lime-stone containing rare bi-valves.

North Pit

In the North Pit the protolith is grey-green serpentinitecontaining calcite veins which locally show a reticulateform. The overlying limonite horizon comprises disaggre-gated, predominantly limonitic blocks with calcite or silicacement (Fig. 4b). Up section, the profile takes on a red-brown color suggesting a higher content of hematiticmaterial compared to the limonite zone (Fig. 3). Withinthis zone thick (~20 cm) subvertical continuous veins ofcalcite are present. The uppermost horizon in the North Pitis yellow-brown soil which has a disaggregated appearance.Calcite is found throughout the zone acting as cement.

Hematite Pit

The protolith exposed in the Hematite Pit is light brown-green serpentinite that contains brown goethite veins whichform irregular circular patterns. These veins cross-cut rarewhite magnesite veins and the contact between theserpentinite and limonite, whilst the magnesite veins areconfined to the serpentinite (Fig. 4c). The boundarybetween the limonite zone and the underlying serpentiniteis sharp (Fig. 3) but relic serpentinite can be found withinthe limonite up to 40 cm above the contact. The limonite isgenerally fine-grained, yellow brown and contains irregularcontinuous goethite veins. In places, the limonite is morecompetent and silicified; these areas appear to be in situwith gradational boundaries and are commonly bounded bygoethite veins. At a height of 18 m, the limonite zone takeson a disaggregated appearance and is characterized by alack of goethite veining and a blocky appearance. Someareas at the top of this horizon display a red color similar tothat of the hematite zone within the south pit. Anuppermost silica horizon has a gradational contact withthe limonite below; silica increases in abundance over atotal distance of 20 m until the upper ~15 m of the profile iscomposed of up to 80% silica, giving a distinct whitecoloration to the pit face at this level (Fig. 3).

Differences within the profiles on the north and the southsides of the Hematite Pit are apparent. The profile in thenorth (Fig. 4c) contains goethite veins within both theserpentinite and the limonite zone, and magnesite veinsfound exclusively within the serpentinite. In the south ofthe pit, the goethite veins are no longer present and calciteveins dominate. There is a general dip of all the horizonswithin the laterite in this pit of ~20° to the northwest. Thelog of the profile indicates that disaggregation increasestowards the top of the profile and banded limonitic materialis rare (Fig. 4c).

Blocky silicifiedhematitic material

50 m

40

5860

Blocky silicified area

The South PitN

69

4072

Calcite and quartz veining

Banded hematitic material

Calcite boxwork within hematitic zone

Limonite materialcontaining gypsum

38

60

The North Pit

Thick (~20 cm) calcite veining

Banded limonitic material

Siliceous laterite

Brown blocky zone

Blocky disrupted limonitic material

Quartz veining

54

40

50 m

Scree

N

Serpentinite

Laterite

Dolomite

Marls and siltstone

Key

Siliceouslaterite

Hematite pit

North Pit

South Pit

350m

N

Protolith - Serpentinite

Limonite zone

Hematitic zone

Sparitic limestoneKey

Siliceous material

Fig. 2 Location map of the three excavations in the Çaldağ depositand geological maps of the North and South Pit

Miner Deposita (2009) 44:581–595 583

Summary

The majority of the Çaldağ laterite is composed of limoniticmaterial without original igneous textures, which is in directcontact with the serpentinite beneath. Notably, saprolite is nota significant part of the profiles at Çaldağ and was rarelyobserved. The constituent minerals within the limonite varylittle between profiles, with the exception of carbonate whichis more common in the south of the deposit. Limonite in theform of fine-grained goethite dominates the horizon. Themaindifferences occur within the structure of this zone. Thesouthern profiles display distinct banding and blocky areas,whereas profiles further north display prominent goethiteveins. The hematitic zone is best developed in the south of thedeposit where it displays a banded structure and in some areascontains blocky calcite-rich material. The more northernprofiles do not always have distinct hematitic horizons thoughthey do exhibit the redder colored areas at the top of thelimonite zone associated with hematite. The uppermosthorizons of the South Pit profiles are composed of calcite-rich hematitic material, whilst the uppermost 20 m of theHematite Pit profile is dominated by a siliceous horizon.

Methods

Samples for micromorphological investigations should re-main relatively undisturbed and intact during collection. Insome instances, the Çaldağ samples were sufficiently cohesiveto allow samples to be taken directly from the profile.However, where samples were more friable, a procedure wasadopted whereby samples were preserved within a sample

box, then impregnated with resin allowing for the preparationof thin sections and polished blocks to be made.

Electron microprobe analyses were completed at theNatural History Museum on a Cameca SX-50 electronmicroprobe equipped with a wavelength dispersive system.Analyses were conducted at 15 kV and 20 nA and countingtimes ranged from 10 to 50 s for spot analysis. Thedetection limits in weight percent for NiO, SiO2, Al2O3,FeO, MgO are 0.03, 0.02, 0.01, 0.03, and 0.01, respective-ly. All 24 samples were subjected to major element XRFanalysis at the University of Southampton using a PhillipsPW1400 X-ray spectrometer with a 3-kW rhodium anodeX-ray tube. Moreover, 0.5 g of homogenized powderedsample was fused with di-lithium tetraborate flux toproduce a dilution of 5:1. Scanning electron microscopy(SEM) was carried out on a Jeol JSM 6400 SEM located inthe National Oceanography Centre, University of South-ampton. Images were acquired in backscattered electronimaging mode. Micro XRF element scans were completedon an EDAX Eagle 3 Micro X-ray fluorescence spectrom-eter with a Rhodium anode X-ray tube. The sample wasimpregnated with resin and cut to produce a flat surface.The sample was analyzed in the X-ray fluorescencelaboratory of the University of Southampton with thefollowing operating conditions: 40 kV, 100 mA with anirradiation spot of 277 μm and a count time of 15 s.

Micromorphological analysis of the weathering profile

Microprobe analyses, transmitted light microscopy, microXRF scans, and SEM images enable the changes in

7 m

NE

In-situLimonite

Hematite PitSiliceous

Allochthonous

Serpentinite

W

5mLimonite

Hematite

Soil zone

NorthPit

Serpentinite

ECalcite veins

Carbonate

South Pit

5 m

Serpentinite

LimoniteHematite

Fig. 3 Profiles from the North,South, and Hematite Pit, show-ing the main zones within thelaterite

584 Miner Deposita (2009) 44:581–595

mineralogy and texture within the laterite profiles to beassessed. The observed variations are related to the weath-ering and transportation history of the deposit.

South Pit

Within the protolith of the South Pit, original igneous texturesin the peridotite are evident and where serpentinization is notcomplete relict olivine and orthopyroxene with unalteredcores are present (Fig. 5a). The degree of serpentinization isheterogeneous and can vary at the scale of a thin section withalteration of olivine to serpentine along fractures and ascoatings on the edges of olivine grains. Olivine andorthopyroxene become increasingly altered towards the

boundary with the overlying limonite zone and as theproportion of iron oxide increases. In particular, olivineshows thin coatings of dark, fine-grained iron oxide-oxyhydroxide along grain boundaries and fractures. Thisalteration is particularly apparent adjacent to calcite veinswhich cross-cut the peridotite. In areas of more intensealteration, olivine is completely replaced by iron oxides; thismore intense alteration occurs in patches affecting five or sixindividual crystals, or it can focus on one crystal withoutaffecting those nearby. Voids are common in areas wherecomplete replacement of olivine by iron oxide has occurred(Fig. 5a). Within 30 cm of the limonite boundary, no originaligneous textures are evident and the profile is dominated byiron oxides.

60

40

20

0

(M)

Red

White

Red

White

Fractured serpentinite withsilica and calcite veins.

Black metallic sometimesbotryoidal layers withinlimonite.

Layered fine grainedlimonite and blocky unitscomposed of dark greycarbonate with rare brokenwhite calcite veins andsilicified limonitic material.

Thin (30 cm) layer of lightgrey low density silicifiedmaterial.

Silicified hematitic materialCalcite boxwork surroundingfine grained hematiticsediment.

Se

rpe

ntin

iteL

imo

niti

czo

ne

Hem

atiti

czo

neH

emat

itic

zone

calc

itebo

xwor

kC

arb

on

ate

Hem

atiti

czo

neS

parit

iclim

esto

ne

Varying amounts ofhematitic materialwithin fine grainedcarbonate.

Bench one, South Pit

0

20

40

60

Magnesite and ironveined serpentinite.

Iron veins cross cutprotolith - limonite boundary.

Relic serpentinite presentwithin 1 m of contact.

Limonitic zone, pervasivelyiron veined, rare silicifiedareas.

Red patchesincrease

Brown in colour,silicified blocks arepresent.

Discontinuousiron veining.

Silica blocksdominate, with rarebanded limoniticmaterial.

Ser

pent

inite

Iron

vein

edlim

oniti

czo

neS

ilici

fied

zone

limon

itezo

neS

ilici

fied

limon

itezo

ne

(M)

North wall of the Hematite Pit

Cl0

20

60

Cl Si Peb

Blocky calcite rich material.Rare banded fine grainedlimonite zones.

Dark calcite, blockscommon.

Thick (<20 cm) calciteveins.

Brown colour,soil -like composition.

Rare bands of finegrained layered limonite.

Blocks becomepredominantlysiliceous.

Red siliceous blockyunit.

(M)

Lim

oniti

czo

neB

row

nso

ilzo

neR

edsi

liceo

uszo

ne

Eastern wall of North Pit

40

Si PebCl Si Peb

A) B) C)

Fig. 4 Lithostratigraphic logs of the laterite profiles in the South, North, and Hematite Pit, the three principal excavations within the Çaldağdeposit

Miner Deposita (2009) 44:581–595 585

There are no primary igneous minerals present withinthe limonite horizon which comprises orange and yellow-brown layers of limonite (Fig. 5b). The banding in thelimonite zone is most prominent toward the base of thehorizon but decreases in regularity and definition upwardsand is no longer apparent 6 m above the serpentinite–limonite boundary. Samples from the hematite zone aremuch darker in appearance (Fig. 5c); quartz veins arecommon at the base of this horizon and are often cross-cutand offset by later calcite veins. In a gradational process,

the quartz veins are replaced by fine-grained calcite whichbecomes coarser grained with height in the profile(Fig 5d). Re-crystallized bivalve shells are present withinsparitic limestone overlying the laterite. The presence ofcalcite as both veins and disaggregated pieces in thelaterite indicates that lateral migration of HCO3

−-bearingfluids must have occurred after or during laterite forma-tion. The source of the carbonate is unknown but the mostobvious source is the sparitic limestone cover of thesouthern laterite.

V

Pro

tolit

hLi

mon

iteH

emat

iteB

oxw

ork

South Pit

A

B

C

D

Lim

Pro

tolit

hLi

mon

iteS

ilice

ous

FeO

Lim

E

F

G

H

Hematite Pit

CalQtz

Qtz

Qtz

Qtz

Ol

Sp

Qtz

S

Sp

Sp

Sp

Qtz

Qtz

Qtz

Lim

Qtz

Qtz

I

J

K

L

Spl

Qtz

Lim

Spl

Lim

Qtz

QtzQtzSp

Sp

Sp

Ol

V

Photomicrographs SEM imagesPhotomicrographs

V

5 mm5 mm 0.75 mm

Fig. 5 Photomicrographs (XPL)of protolith and laterite samplesfrom Çaldağ. South Pit. a Orig-inal igneous textures at the baseof the profile. Olivine (Ol) al-tered to both amorphous ironoxides and serpentinite (Sp) withcross-cutting calcite veins. Closeto the contact with the limoniteolivine has been completely al-tered to serpentine and ironoxide. b No igneous textures;the rock is highly porous anddominated by iron oxides whichdisplay banding in the base ofthe limonite (Lim). c Brokenquartz (Qtz) veins surrounded byvery dark hematite rich material.d Calcite (Cal) dominates withrare fine-grained quartz.Hematite Pit. e Serpentinite withmicrocrystalline quartz, stainedorange around iron veins.f Protolith–limonite boundary.The protolith is composeddominantly of serpentine; within1 mm of the boundary quartz iscommon. g The base of thelimonite zone contains a meshtexture similar to that observedin the serpentinite. h Theamount of quartz increases andiron oxide decreases towards thetop of the profile. The upper-most horizon is composed al-most entirely of microcrystallinesilica. i Serpentine surroundingolivine and iron oxide. Dissolu-tion of olivine leaving voids (V)within serpentine. j Serpentine–limonite transition zone; serpen-tine and quartz are replaced byiron oxide veinlets over a dis-tance of less than 2 mm. kHomogeneous goethite, no evi-dence of movement. Brokenclastic textures provide evidenceof movement within the profile.l Silica-rich laterite. Field ofview in all photomicrographs=10 mm. Field of view in allSEM images=2.5 mm

586 Miner Deposita (2009) 44:581–595

Hematite Pit

The protolith in the Hematite Pit also comprises serpenti-nized peridotite exhibiting a characteristic mesh texture(Fig. 5e, i). Olivine is present within the least alteredprotolith samples (Fig. 5i), surrounded by a serpentinelattice. As the protolith weathers, olivine is the firstmineral to be destroyed leaving behind voids (Fig. 5i);these voids are subsequently filled with Fe oxides andmore rarely with microcrystalline quartz (Figs. 5e and 6).Micro XRF element maps demonstrate that, with a fewexceptions, both Ni and Si are distributed relatively evenlythrough the serpentinite (Fig. 7). Areas where Si isconcentrated can be attributed to the precipitation of silicaproduced during the weathering of primary silicates.Goethite veining can penetrate up to 2 m into theserpentinite from the weathering front. The veins followfractures and often show a gradational contact with theserpentinite. Iron-oxide staining spreading from the goe-thite veins leads to large areas of the protolith beingdistinctly orange (Fig. 5e). Cr-spinel grains are presentthroughout the protolith and are commonly fractured andcut by serpentine veins. Cr-spinel is preserved during theweathering process and is therefore common within thelimonite and overlying zones (Fig. 7).

The boundary between the serpentinite and the limoniteis sharp with geochemical and structural changes occurringover a distance of less than 2 mm. The serpentinite incontact with the limonite zone contains quartz distributedin a band parallel to the weathering front. The limonite indirect contact with the serpentinite appears structurelessunder transmitted and reflected light, but SEM andmicroprobe map images reveal numerous iron oxideveinlets penetrating into the serpentinite (Figs. 5j and 6).The iron oxide veins will exploit microfractures in theprotolith and precipitate as Mg- and Si-bearing serpentiniteis destroyed (Fig. 6). The leaching process that destroys theserpentine and removes the Mg and Si is so efficient thatrelic material is rare within the limonite.

The limonite within 1 m of the weathering frontexhibits a millimeter-scale banded texture with alternat-ing bands of structureless iron oxide between bandsdisplaying a texture reminiscent of serpentinite. Themesh texture within the limonite is thought to bepseudomorphic after serpentinite. Similar textures havebeen observed in other Ni-laterite deposits; when theserpentinite is dissolved, a small amount of nickel-richgoethite organized as a replica of the serpentinite latticeremains. This lattice is preserved until all the serpentinitehas been destroyed (Trescases 1997). Asbolane and

Fe

SiNi

MgLim

Sp

Qtz

Goethite veins

Weathering front

400 mµ

1

2

3

4

5

Fig. 6 Electron microprobe ele-ment map of the boundary(weathering front) between thelimonite (Lim) and serpentinite(Sp). The more intense yellowindicates a higher element con-centration. 1 Mg contentdecreases as leaching removesthe most soluble element fromthe profile. 2 Goethite veinspenetrate into the protolith; thisboundary represents the base ofthe limonite zone. 3 Relic Siwithin serpentine is presentwithin the base of the limonitezone. 4 The less altered protolithdisplays a relatively homoge-nous distribution of Si aroundquartz (Qtz) crystals. 5 Thehighest Ni concentrations areassociated with relic silica

Miner Deposita (2009) 44:581–595 587

quartz observed as layers of Mn and Si in Fig. 7 also occurwithin the base of the limonite zone. At the base of thelimonite zone, Ni concentrations are highest whereresidual Si remains (excepting quartz) suggesting Ni isheld within secondary silicates (e.g., Ni-rich serpentine)(Fig. 6). As Si is rapidly removed from the profile duringweathering, Ni within silicates represents a very minorproportion of the total reserve. The majority of the Ni isassociated with goethite which at the base of the limonitezone (above the 1-m horizon with the mesh texture) isstructureless and contains small voids and rare Cr-spinelgrains (Fig. 5k). Siliceous areas in the limonite zone areconfined to blocks (<10 cm) which contain silica veinletsand as a result are more competent than the surroundinglimonite. The texture of the limonite changes up section,with banded iron oxide fragments and silica veins with anangular disaggregated texture (Fig. 5k) present at a heightof 20 m within the profile. Above this point, limonitetextures remain brecciated and the amount of silicaincreases with height. The upper portion of the profile in

the west of the Hematite Pit is dominated by a ~15-m-thick silica zone (Fig. 5h, l). This horizon comprises whiteto dark grey silicified material which displays a variationin morphology across the horizon from a white incompe-tent powder to more indurated silicified blocks and veinnetworks which in some places forms a reticulate patternsurrounding limonite material.

The micromorphological analysis of the South andHematite Pit profiles shows the laterite is mineralogicallysimilar in both pits, with the majority of the lateritecomposed of fine-grained goethite in the form of limonite.Both profiles display distinct changes in morphology; theSouth Pit laterite is banded near to the contact with theprotolith and becomes more disaggregated and structurelessover a distance of 2 m up section. The Hematite Pit lateritedisplays continuous goethite veins within the base of theprofile which again becomes more disaggregated withheight, losing any continuous features. The observedchanges in morphology may be attributed to transportationmechanisms during syn- and post-laterite formation.

CrFe

Si Ni Mn

CrFeSample

1cm

Sp

Lim

Fig. 7 Micro XRF scan acrossthe weathering front, displayingvariations in Si, Mn, Fe, Cr, andNi. The sample photograph dis-plays the area scanned and theboundary between the serpen-tinite (Sp) and the limonite(Lim), representing the weather-ing front. Discrete spinel grainsare apparent within the Cr map

588 Miner Deposita (2009) 44:581–595

Electron microprobe analysis

An electron microprobe profile across the protolith–limoniteboundary, which represents the weathering front (Fig. 8),demonstrates a dramatic increase in FeO from 10 wt.% in theprotolith to 46 wt.% in the limonite and decreases in SiO2

from 20 to 7 wt.% and MgO from 13 to 1 wt.%. The changein MgO concentrations over a distance of 2 mm marks the“magnesium discontinuity” above which the leaching pro-cess has removed Mg by dissolution of silicate minerals.Nickel percentages stay approximately constant (~2.2 wt.%)across the boundary but are much more variable within theserpentinite. On average, peridotite contains 0.3 wt.% Ni(Lelong et al. 1976) with the majority of the Ni presentwithin fosteritic olivine which contains Ni concentrationsbetween 0.2 and 0.4 wt.% (Gleeson et al. 2003). The Niwithin the protolith in the Çaldağ deposit is present withinNi-rich serpentine as all olivines have been destroyed at thisheight within the profile. The variation in Ni concentrationsreflects the heterogeneity within the serpentinite as variousminerals found across the profile line are analyzed. Alumi-num shows the opposite trend to Ni concentrations withmore variation within the limonitic zone with concentrationsaveraging 0.02 wt.%. The increased variability of aluminumconcentrations within the limonite zone probably representsdiffering degrees of aluminum substitution for iron withingoethite. The changes in element concentrations can beattributed to the destruction of primary minerals andprecipitation of goethite. This process has occurred over adistance of less than 2 mm (Fig. 8) and has produced a sharpphysical and chemical contact between the protolith andlimonite.

Whole-Rock geochemistry

The weathering of an ultrabasic rock begins with thebreakdown of olivine which results in the removal of Mgand silica, leaving behind an insoluble residue whichincludes iron and nickel which precipitated as ferrichydroxides and minor amorphous silicates, forming anoxide deposit. This weathering sequence results in a lateriteprofile with a specific geochemical signature and anydeviation from this signature will indicate that the weath-ering process has been interrupted or altered. We completeddown-hole profiles of the Hematite and South Pit illustrat-ing variations in element concentration with depth.

Hematite Pit

The lithogeochemical variation of the elements within theweathering sequence of the Hematite Pit (Fig. 9a) showthat the SiO2 and Fe2O3 concentrations vary antitheticallythrough the profile. MgO decreases dramatically from22 wt.% in the protolith to <1 wt.% in the limonite zoneand remains low throughout the rest of the sequence.Nickel concentrations are relatively high within the top ofthe serpentinite but reach a maximum of 3 wt.% within thelimonite zone, above which concentrations graduallydecrease. Chromium concentrations increase graduallythrough the profile to a maximum of 1.5 wt.%, followinga similar pattern to Al2O3; both of these elements (alongwith iron) are considered to be relatively insoluble(Oliveira et al. 1992) and the increase in values towardsthe top of the profile can be attributed to residualconcentration processes.

0 4 8 12 16

0.00

0.05

0.10

0.15

0.201.0 2.0 4.0 5.0

0

20

40

60

80

An

aly

sis

nu

mb

er

MgO %Al O %2 3

NiO %

3.0

100

Sp

Lim

Lineof analysis

0 5 10 15 20 25

SiO %2

0 10 20 30 40 50 60

FeO %

Limonite

Serpentinite

Fig. 8 Electron microprobe profile across the serpentinite–limonite boundary. One hundred points were analyzed over a distance of 2 mm; profileline is shown on the backscattered electron image

Miner Deposita (2009) 44:581–595 589

South Pit

The distribution of the major elements within the South Pitprofile displays a different pattern (Fig. 9b) to that of theHematite Pit. Fe2O3 increases from 9 wt.% in the protolithto a maximum of 45 wt.% in the limonite zone decreasingto 16 wt.% in the hematite zone above. SiO2 concentrationsare relatively high within the serpentinite (40 wt.%) andshow major variations which vary antithetically with Fe2O3

concentrations. MgO concentrations decrease graduallyfrom 6 wt.% in the protolith to 0.3 wt.% in the limonite,above which there is a dramatic increase to 6 wt.% in thehematite zone. Nickel values are relatively low andunexpectedly show a general trend to increasing valuestowards the top of the profile, reaching a maximum of0.5 wt.% within the hematite zone. Low nickel valuescorrespond with higher CaO and SiO2 concentrations.Overall, Ni values are relatively low possibly due totransportation and associated leaching of the Ni. Dilutionof Ni grades by the precipitation of calcite and silica willalso decrease Ni concentrations. CaO values are compara-tively high within the South Pit and increase towards thetop of the weathering sequence where values reach amaximum of 50 wt.%. Chromium values show a general

decrease through the lateritic profile, with the highestconcentrations within the limonite zone.

Discussion

Weathering profile formation and evolution

The Çaldağ deposit is dominated by oxide ore. Manylaterites, e.g., Cerro Matoso in Columbia and laterites ofNew Caledonia (Gleeson et al. 2003; Freyssinet et al.2005), possess secondary silicate zones, below the limonitezone which contains garnierites, sepiolite, and chlorite. Theabsence of secondary silicate minerals at Çaldağ isindicative of specific environmental and geological con-ditions. The lack of a hydrous silicate zone suggests that theleaching and transportation of nickel required to formsecondary silicates has not occurred or has been interrupted.This could be taken to indicate that the weathering period atÇaldağ was relatively short; however, in situ profilethicknesses close to ~20 m suggest that weathering in theregion was sustained over an extensive time period.Assuming on average laterite weathering rate of 20 mmper 1,000 years (Nahon and Tardy 1992) the Çaldağ deposit

0 25

Hei

ghti

npr

ofile

(m)

MgO % SiO2 %0 100 0 3.01.5

Cr %0 3.01.5

Ni %0 3.01.5

Al O2 3 %

0

10

20

30

40

50

60

0 40 80Fe O2 3 %

0 60300 10050SiO2 %

0 2 4 6Cr %

0 7 14Al O2 3 %

0

10

20

30

40

50

Hei

ght i

npr

ofile

(m)

0 2010MgO %

0 20 40 60Fe O2 3 %

0 10.5Ni %CaO %

Siliceous zone

Hematite zone

Limonite zone

Serpentinite

50

A

B

Fig. 9 Lithogeochemical variation diagrams and idealized logs from aHematite Pit, b South Pit. The Hematite Pit profile displays a typicalin situ element variation, with decreases in MgO and increases in Ni

concentrations from the protolith into the laterite. The South Pit profiledisplays an increase in MgO towards the top of the profile and Niconcentrations are relatively low throughout

590 Miner Deposita (2009) 44:581–595

took at least 1 million years to form. Alternatively, a highwater table during laterite formation would preventleached nickel from interacting with actively weatheringprimary silicates, resulting in little or no formation ofnickel-rich secondary silicates (Freyssinet et al. 2005).The prevailing climate during laterite formation also playsan important role in the type of laterite deposit developed.Most laterites forming today are formed in humidsavannah type environments (e.g., Cerro Matoso, Colom-bia and Vermelho, Brazil), which have a pronouncedseasonality with a dry season lasting up to 3 months. Theseasonality promotes the formation of secondary silicates.As humidity and rainfall increases, the difference in theweathering rate of the framework minerals containingnickel (olivine and serpentine) decreases, leading to theformation of an oxide-dominated deposit (Lelong et al.1976; Trescases 1997). This suggests that Çaldağ mayhave formed in a climate with little seasonality. Thedegree of serpentinization of the protolith also affectsthe relative development of different horizons within thelaterite. Protolith dominated by serpentinite will normallyweather directly into goethite. In contrast, the delaycaused by the different weathering characteristics ofolivine and serpentine together allows for the formationof a silicate layer at the base of the profile (Trescases1997). At Çaldağ, the Hematite Pit protolith is composedalmost entirely of serpentinite and the profile abovecontains significant amounts of silica. A combination ofa serpentinized protolith, a high water table, and a climatewith little seasonality can account for the thickness of theoxide zone and the lack of secondary silicates.

Downward progression of the weathering front

Textural analysis of the weathering front on the north sideof the Hematite Pit shows that there is no gradation fromserpentinite to limonite; instead, there is a very sharpboundary where intensely veined iron oxides are found indirect contact with serpentinite. Above this contact, thelimonite shows evidence of a mesh structure suggestingrelict serpentinite texture is preserved. The banded nature(on a millimeter scale) of the base of the limonite indicatesthat the weathering front does not move downwards in asimple linear fashion. The progression of the limonite zoneis preceded by intense iron veining observed today withinthe protolith. This veining provides a pathway for fluidsand the alteration of the protolith spreads from these veins.The stages in the progression of the weathering front areshown in Fig. 10 and are all observed within the HematitePit. Initially, alteration is confined to the iron veins, asweathering proceeds alteration of the serpentinite becomesmore intense adjacent to the iron veins and iron oxidesbegin to dominate the area between the veins. Serpentinite

is often left isolated, forming relics that can be found up to2 m from the base of the limonite zone. Further, weatheringresults in the destruction of relics and the downwardprogression of the weathering front.

Transportation mechanisms

The identification of transportation mechanisms which mayhave modified the Çaldağ laterite during formation iscrucial to the understanding of the present-day depositmorphology. Transportation identified by disaggregationwithin laterites has most often been described as profilecollapse, where the mechanical strength of the protolith hasbeen reduced by weathering (Golightly 1981). This type ofmovement results in a broken and blocky profile appear-ance but the laterite can still be described as in situ.Transportation mechanisms that produce allochthonouslaterites include erosion, slumping, and fault-related move-ment. The Çaldağ laterite contains evidence for a number ofdifferent transportation events.

Hematite Pit

The base of northern Hematite Pit profile is the only part ofÇaldağ that can definitively be described as in situ.Goethite veining within the limonite is continuous andcrosses the contact between the protolith and limonite,indicating no lateral movement has occurred since theformation of these veins. The magnesium content of theprofile exhibits a dramatic decrease from the protolith intothe laterite and there is a general decrease in the amount ofnickel above the limonite zone; these compositionalchanges are indicative of leaching within an in situ laterite.The contact between the limonite and serpentinite isirregular and often displays interdigitation between thetwo lithologies (Fig. 11), a pattern unlikely to survive ifmovement had taken place. Goethite veining is no longerpresent ~20 m above the limonite–protolith contact and theprofile becomes more siliceous and blocky indicating somedegree of movement resulting in the disaggregation of theprofile. In contrast, the northwest side of the Hematite Pit,which due to the dip of the laterite represents the upperportions of the profile, shows distinct zones with anintensely disaggregated appearance suggesting that trans-portation has occurred over a short distance or in a numberof different stages.

The transportation history observed in the Hematite Pitsuggests the laterite may have originally formed as a pocketin a topographic low (Fig. 12a). Laterites initially formuniformly over uneven topography, but over time theprofile forming in the highs is eroded and transported intothe lows producing profile morphology similar to thatwithin the Hematite Pit.

Miner Deposita (2009) 44:581–595 591

North Pit

On the evidence of this study, there is no part of the profilein North Pit which is in situ as all structures or bands withinthe profile are discontinuous and rare. However, thereremains a distinct hematite zone observed over the limonitehorizon, although this is covered by a browner soil-richhorizon. The profile increases in thickness down dip withthe top of the section composed of a 1.5-m-thick limonite

zone overlain by a relatively unbroken 2-m-thick band ofsiliceous material. The thickening of the profile to 30 m inheight down dip and its heterogeneity suggests erosion andtransportation of lateritic material from previously formedlaterites (Fig. 12b). This process was initiated along faultsin the region which created lows where transported lateritewas deposited. Post movement silicification has producedsiliceous horizons which are generally unbroken and inplaces silica cements blocks of lateritic material together.

Limonite

Protolith

Limonitezone

Weatheringfront

Protolith

Serpentiniterelics

Serpentinite

Iron veining

Rare relics

Fig. 10 Stages in the progres-sion of the weathering front inthe Hematite Pit. The progres-sion of the limonite zone ispreceded by intense iron vein-ing. This provides a pathway forfluids, leading to the alterationof the protolith progressing out-wards from these veins

592 Miner Deposita (2009) 44:581–595

South Pit

The profile in the east of the South Pit dips at ~60° to thesouth and is zoned with a distinct limonite and hematitezone. Small veins of limonitic material are found penetratinginto the serpentinite from the limonite above. The profileshows distinct continuous bands, one of which is composedof grey, low-density siliceous material which is traceableover a distance of 60 m. However, the top of the hematitezone, comprising fine-grained red hematitic material withsiliceous blocks randomly distributed throughout the unitappears to be re-worked and entirely allochthonous. Thelateritic material grades into micritic limestone which alsocontains bands of hematitic laterite suggesting re-workingoccurred at the top of the weathering sequence. The increasein MgO concentrations towards the top of the profile mustrepresent an input of MgO from another source. As thiselement is the first to be leached from the serpentinite at thebeginning of the weathering process, it should not be presentin significant quantities within in situ laterite. The increase invalues observed in the hematite zone indicates transportationand movement after profile formation has taken place.

The morphology and geochemistry of the South Pitprofile suggests tilting of the laterite has resulted in faultingand slumping within the serpentine and laterite (Fig. 12c).This process has left some continuous horizons at the baseof the profile and resulted in increasing disaggregationtowards the top of the weathering sequence.

Silica precipitation and formation of the Hematite zone

A significant amount of silica is present within the Çaldağdeposit, especially within the Hematite Pit where it is foundthroughout the laterite, reaching a maximum in the upperparts of the weathering profile. The precipitation of thesilica has effectively replaced the goethite, diluting nickelconcentrations in the top of the profile. The antitheticvariation in FeO and SiO2 observed in the weatheringsequence is unusual though a similar pattern has beenobserved over a distance of 8 m in the base of the limonitezone from the Sipolou Plateau laterite, in western IvoryCoast (Nahon et al. 1982). The SiO2 concentrations inÇaldağ are very high compared to other oxide depositsespecially when considering that thick siliceous horizonsnormally form over a dunite protolith where the lack of Alpromotes the formation of silica instead of clays (Freyssinetet al 2005). Siliceous zones are, however, reported in anumber of different laterites around the world, includingMt. Keith, Western Australia (Butt and Nickel 1981),Vermelho, Brazil and Tagaung Taung, Burma (Schellmann1989). The 25% increase in silica content within theuppermost horizon at Tagaung Taung has been attributedto the addition of material from other rock types (e.g.,granites) in the area. The increase in silica corresponds to achange in lithology observed at the top of the profile andcoincides with the addition of TiO2 and zircon which arenot found within the protolith (Schellmann 1989). As nochange in lithology is observed in the Hematite Pit andlimonitic material is present above the silica zone, it seemsunlikely that the silica has been transported into the region,especially as it can compose 80% of the profile in someareas. The presence of silica horizons in the Çaldağ deposit

Ophiolite

DolomiteThrust fault

Laterite

Pocket

Transportation

Pocket Type Formation

3 m

Serpentinite

Limonite zoneHematite

zone

Serpentinite

In- situ laterite12 m

Fault Related Transport Slumping

200 m

A) B) C)

In- situ laterite Erosion and transportation

Allochthonouslaterite

Fig. 12 Representations of the possible transportation mechanisms which formed the Çaldağ deposit. a Pocket type formation. b Fault-relatedlaterite transportation and deposition. c Slumping related to faulting and tilting of the deposit

E

Serpentinite

Limonite

2m

Fig. 11 Interdigitation between serpentinite and limonite at the baseof the in situ laterite profile located on the east side of the Hematite Pit

Miner Deposita (2009) 44:581–595 593

morphology, at least in terms of silica, can be in partexplained by precipitation from silicic acid formed duringthe destruction of serpentine. This process takes place bythe progressive replacement of Mg by hydrogen ions:

Mg3Si2O5 OHð Þ4þ2Hþ ¼ Mg2H2Si2O5 OHð Þ4þMg2þ

Serpentine

Mg2H2Si205 OHð Þ4þ4Hþ ¼ 2H4SiO4 þ H2Oþ 2Mg2þ

Silicic acid

The development of the siliceous zone at theMt. Keith deposithas been attributed to the process of silicic acid formationfrom the weathering of serpentinite (Butt and Nickel 1981).The precipitation of silica from silicic acid does notnecessarily happen immediately and the fact that silica isfound throughout the Çaldağ deposit, including the serpen-tinite, suggests that the silica is highly mobile and that pre-cipitation occurred after a significant portion of the lateritehad formed. The precipitation of silica can effectively insu-late the profile from further weathering and in some in-stances can completely halt the weathering process. Theprocess of silicification at Mt. Keith is thought to correspondto the change in climate from tropical to a less humid envi-ronment in the Mid-Miocene (Butt and Nickel 1981). Thisled to a reduction in the through-flow of water, consequentlyresulting in the evapotranspirative concentration and precip-itation of silica. This mainly occurs in topographic lowswhere silica-rich fluids become concentrated and the watertable is high. The formation of the silica horizon provides anindurated cap that protects the underlying laterite fromerosion.

The Çaldağ deposit may have been modified by achanging climate in a similar manner to the Mt. Keithlaterite. Presently, the west of Turkey has a temperateclimate with an average 645 mm of rainfall per year. Thisis significantly less than the 900 to 1,800 mm of rainfallnormally experienced in areas of laterite formation(Freyssinet et al. 2005). Temperatures are also on averagesignificantly lower today. From the Eocene until present theδ18O signature of benthic foraminifera records a decrease indeep oceans temperatures of ~6°C (Zachos et al. 2008). Thestudy of micro- and mega-floral changes through Europeindicates that changes in European continental temperaturescoincide with the decrease in temperatures observed in themarine record (Mosbrugger et al. 2005). Further to this, themicro- and mega-floral temperature records suggest thatthere has been an increase in seasonality in the yearlyaverage temperatures with increasingly cold winters overthe last 45 million years. At Çaldağ, this change in climatemay have resulted in less water through-flow and morevariability in the height of the water table, promoting the

concentration and precipitation of silica (Butt and Nickel1981; Golightly 1981). Subsequently, the protection thesilica offers, combined with faulting, has protected theHematite Pit laterite from erosion.

Where silicification is not as extensive or absent, there isoften a marked hematite zone at the top of the profile.Hematite is thought to form during seasonal saturation anddehydration (Freyssinet et al. 2005) as a replacement productof goethite. The change in climate associated with the pre-cipitation of silica may also result in the formation of fine-grained hematite as seasonal fluctuations in the level of thewater table increase as the climate becomes drier and moreseasonal.

Conclusions

The morphological and mineralogical composition of theÇaldağ oxide laterite reflects its formational history.Weathering in a tropical climate with relatively littleseasonality combined with a serpentinite protolith and ahigh water table has formed an oxide laterite with a highconcentration of silica. Silica precipitation may have beeninfluenced by a change in climate, with an increase inseasonality resulting in evapotranspirative concentrationand precipitation near the end of the lateritization process.The laterite has been modified by deformation and surfaceprocesses during and after its formation. Three differenttypes of transportation mechanisms have been identifiedfrom the analysis of the deposit. These have affected profilemorphology, thickness, and the nickel grade. Limonitewithin the Hematite Pit contains the highest Ni concen-trations within the Çaldağ deposit. Ni grades decrease withheight within the profile as silica comes to dominate,effectively diluting Ni concentrations.

Acknowledgements This study was funded by a National Envi-ronment Research Council PhD studentship to R. Thorne, NER/S/A/2006/14212. Field work at Çaldağ was made possible by the supportof European Nickel Plc. The authors also express gratitude to J.Spratt for his assistance with microprobe analysis at the NaturalHistory Museum, R. Pearce for his aid with SEM imaging, andI. Croudace for his assistance with XRF analysis. This paperhas also benefited from comments and suggestions by D. Teagle,B. Lehmann, and G. Paterson.

References

Bozkurt E, Satir M (2000) The southern Menderes Massif (westernTurkey): geochronology and exhumation history. Geol J 35:285–296

Butt CRM, Nickel EH (1981) Mineralogy and geochemistry of theweathering of the disseminated nickel sulfide deposit at Mt.Keith, Western Australia. Econ Geol 76:1736–1751

594 Miner Deposita (2009) 44:581–595

Çağatay A, Altun Y, Arman B (1981) The mineralogy of the Çaldağlateritic iron, nickel–cobalt deposits. MTA Maden Etut DairesiRep 1709, Ankara

Freyssinet P, Butt C R M, Morris R C, Piantone P (2005) Ore-formingprocesses related to lateritic weathering. Econ Geol 100thAnniversary Volume, pp 681–722

Gleeson SA, Butt CRM, Elias M (2003) Nickel laterites: a review.SEG Newsl 54:10–16

Golightly JP (1981) Nickeliferous laterite deposits. Econ Geol 75thAnniversary Volume, pp 710–735

Kuck PH (2008) Nickel. In: U.S. Geological survey, mineral commoditysummaries January 2008 USGS http://minerals.usgs.gov/minerals/pubs/commodity/nickel/#pubs. Accessed 8 Nov 2008

Lelong F, Tardy Y, Grandin G, Trescase JJ, Boulange B (1976)Pedogenesis, chemical weathering and processes of formation ofsome supergene ore deposits. In: Wolf KH (ed) Handbook ofstrata-bound and stratiform ore deposits. Elsevier, Amsterdam, pp92–173

Mosbrugger V, Utescher T, Dilcher D (2005) Cenozoic continentalclimatic evolution of Central Europe. Proc Nat Acad Sci102:14964–14969

Nahon D, Tardy Y (1992) The ferruginous laterites. In: Butt CRM,Zeegers H (eds) Regolith exploration geochemistry in tropical

and sub-tropical terrains. Handbook of exploration geochemistry4. Amsterdam, Elsevier, pp 41–55

Nahon D, Paquet H, Delvigne J (1982) Lateritic weathering ofultramafic rocks and the concentration of nickel in the WesternIvory Coast. Econ Geol 77:1159–1175

Oliveira BSM, Trescases JJ, Melfi JA (1992) Lateritic nickel depositsof Brazil. Miner Deposita 27:137–146

Önen AP, Hall R (2000) Sub-ophiolite metamorphic rocks from NWAnatolia, Turkey. J Metamorph Geol 18:483–495

Schellmann W (1989) Composition and origin of lateritic nickel ore atTagaung Taung, Burma. Miner Deposita 24:161–168

Trescases JJ (1997) The lateritic nickel–ore deposits. In: Paquet H,Clauer N (eds) Soils and sediments, mineralogy and geochem-istry. Springer, Berlin, pp 125–138

Yılmaz Y, Genç SC, Gürer OF, Bozcu M, Yılmaz K, Karacık Z,Altunkaynak Ş, Elmas A (2000) When did the western Anatoliangrabens begin to develop. In: Bozkurt E, Winchester JA, PiperJDA (eds) Tectonics and magmatism in Turkey and thesurrounding area, Geological Society Special Publication 173.Geological Society, London, pp 353–384

Zachos JC, Dickens GR, Zeebe RE (2008) An early Cenozoicperspective on greenhouse warming and carbon-cycle dynamics.Nature 451:279–283

Miner Deposita (2009) 44:581–595 595