petrography and geochemistry of the jajarm karst bauxite...

Petrography and Geochemistry of the Jajarm Karst Bauxite Ore Deposit, NE Iran: Implications for Source

Rock Material and Ore Genesis

DARIUSH ESMAEILY, HOSEIN RAHIMPOUR-BONAB, AMIR ESNA-ASHARI & ALI KANANIAN

Department of Geology, College of Science, University of Tehran, 14155–6455 Tehran, Iran(E-mail: [email protected])

Received 24 June 2008; revised typescript receipt 14 April 2009; accepted 01 July 2009

Abstract: The Jajarm bauxite deposit, northeast Iran, is the largest such deposit in Iran. The deposit is sandwichedbetween the Triassic Elika formation and the Jurassic Shemshak formation, housed within karstic features developedwithin the former unit. The deposit generally shows an internal layering defined by the following four distinct horizons(from bottom to top): (a) a lower argillaceous horizon, approximately 50–80 cm thick, is mainly composed of clayminerals that directly overlies the carbonate footwall (Elika formation); (b) a bauxitic clay layer approximately 2–3 mthick that consists mainly of hematite, kaolinite, anatase, and diaspore; (c) a red bauxite layer (the main high-grade ore),about 5 m thick and composed of diaspore, kaolinite, anatase, and hematite; and (d) an upper kaolinitic layer that is 20–50 cm thick, composed mainly of kaolinite, and overlain by the Shemshak formation. Detailed petrographic studiesreveal diagenetic alteration of the bauxitic protolith. The main observed bauxite textures are microgranular, oolitic,pisolitic, fluidal-collomorphic, and microclastic. Microgranular and microclastic textures associated with the residualfractured and corroded quartz grains, as well as feldspar grains are almost completely replaced by platy diaspore.Geochemical analyses of the red bauxite reveal enrichment of less mobile elements (Nb, Th, Zr, Mo, Ga, and Cr) anddepletion of mobile elements (Rb, K, Na, Sr, La, Mg, and Pb); the opposite result is obtained for the bauxitic clay.Chondrite-normalized REE (Rare Earth Element) patterns for the upper kaolinite layer are similar to those for theunderlying red bauxite, and the patterns obtained for the lower argillaceous layer are similar to those for the overlyingargillaceous bauxite horizon. Ce shows a positive anomaly in the red bauxite and a negative anomaly in the bauxiticclay. The correlation coefficients calculated between REE and other elements demonstrate that the likely REE-bearingminerals are oxides of Ti and Nb, clay minerals, and zircon. In contrast to the present diasporic mineralogicalcomposition of the Jajarm bauxite, the geochemical and mineralogical data indicate an original gibbsitic composition.Finally, the observed mineralogical and textural evidence, combined with the evidence provided by variation diagramsand REE patterns, indicates a mixed origin for the Jajarm bauxite from both basic igneous and sedimentary rocks. Infact, bauxitization was initiated on basic source rocks and continued during reworking and replacement within thekarstic features.

Key Words: Iran, karst bauxite, geochemistry, diaspore, ore deposit

Jajarm (KD İran) Karst Boksit Cevher Yatağının Petrografisi veJeokimyası: Kaynak Kaya Materyali ve Cevher Kökeni İçin Göstergeler

Özet: İran’ın kuzeydoğusunda bulunan Jajarm boksit yatağı İran’daki en büyük yataktır. Yatak, Triyas yaşlı Elikaformasyonu ve Jura yaşlı Shemshak formasyonu arasında yüzeylemektedir. Yatak genel olarak dört farklı düzeyi (alttanüste doğru) takiben tanımlanan bir iç katman yapısı göstermektedir: (a) direkt olarak karbonat (Elika Formasyonu)tabanını üzerleyen, başlıca kil minerallerinden yapılı yaklaşık olarak 50−80 cm kalınlığındaki alt arjilikli seviye, (b)başlıca hematit, kaolinit, anatas ve diyaspor minerallerinden meydana gelen ve yaklaşık olarak 2−3 m kalınlığındaki birboksitik kil seviyesi, (c) diyaspor, kaolinit, anatas ve hematit minerallerinden yapılı olan ve 5 m kalınlığındaki kırmızıboksit seviyesi (yüksek tenörlü ana cevher), (d) başlıca kaolinitten yapılı olan 20−50 cm kalınlığındaki üst kaolinitseviyesi ve bu seviye Shemshak formasyonu tarafından üzerlenmektedir. Detaylı petrografik çalışmalar, boksitik

267

Turkish Journal of Earth Sciences (Turkish J. Earth Sci.), Vol. 19, 2010, pp. 267–284. Copyright ©TÜBİTAKdoi:10.3906/yer-0806-15 First published online 24 July 2009

IntroductionLateritic bauxite deposits developed in tropicalregions principally consist of hydrated aluminiumminerals such as gibbsite Al(OH)3, boehmiteAlO.OH, and diaspore AlO.OH. These mineralscontain variable amounts of iron, silicon andtitanium as atomic substitution for Al, and otherelements in minor and/or trace amounts (e.g., Th, P,Y, Ga, Ge and V), either incorporated in the minerallattice or adsorbed onto its surface (Shaffer 1975;Bárdossy & Aleva 1990; Mordberg 1999; Öztürk et al.2002). Diaspore is the most stable bauxite mineral atthe temperature and pressure conditions of the earthsurface, especially in dry areas, whereas gibbsite, incontrast to diaspore and boehmite, is more stable inhumid climates but it still dissolves in weatheringenvironments (Furian 1994; Dedecker & Stoops1999) and its dissolution is pH dependent (Nahon1991, p. 186; Velde 1992, p. 107).

Bauxite deposits are commonly classified as oneof three genetic types according to mineralogy,chemistry, and host-rock lithology (Bárdossy &Aleva 1990). Of all the known bauxite deposits,about 88% are lateritic type, 11.5% are karst type, andthe remaining 0.5% are Tikhvin type (Bárdossy 1982;Bárdossy & Aleva op. cit.). The specific conditionsthat give rise to different paths of bauxite formationhave been documented previously (e.g., Bárdossy &Aleva op. cit.). Lateritic-type deposits form uponaluminosilicate rocks via in-situ lateritization. Insuch cases, the most important factors in

determining the extent and grade of bauxiteformation are the parent rock composition, climate,topography, drainage, groundwater chemistry andmovement, location of the water table, microbialactivity, and the duration of weathering processes(Grubb 1963; Bárdossy & Aleva 1990; Price et al.1997). Karst-type deposits occur within depressionsupon karstified or eroded surfaces that formed uponcarbonate rocks. Such deposits originate from avariety of different materials, depending upon thesource area (Bárdossy 1982). Finally, Tikhvin-typedeposits are transported or allochthonous depositsthat overlie alumosilicate rocks and that originatefrom pre-existing residual laterite profiles (Bárdossy1982, p. 21; Bárdossy & Aleva 1990, p. 63).

In recent decades, various studies haveinvestigated the occurrence within bauxite depositsof most of the known chemical elements (e.g.,Bronevoi et al. 1985; Bárdossy & Aleva 1990;Maksimović & Pantó 1991; MacLean et al. 1997;Eliopoulos & Economou-Eliopoulos 2000;Mutakyahwa et al. 2003). In profiles of particulardeposits or bauxitic districts, most of these earlierstudies describe details of the distribution andbehaviour of different elements, as well as theprocess of bauxite genesis.

Bauxites of Upper Triassic to Upper Cretaceousage are widespread throughout Iran, especially inCentral Iran and the Alborz Mountain Range (Figure1a). The layered Jajarm bauxite ore deposit, located18 km from the town of Jajarm (Khorasan province,

KARST BAUXITE ORE DEPOSIT, NE IRAN

268

protolitin diyajenetik alterasyonunu göstermektedir. Çalışılmış örneklerde gözlenen boksit dokuları mikrogranüler,oolitik, pizolitik, kolloform ve mikroklastiktir. Feldspat tanelerinin yanısıra, kırılmış ve aşınmış kalıntı kuvars taneleriile ilişkili mikrogranüler ve mikroklastik dokular, hemen hemen komple levhasal diyaspor tarafından yeri alınmıştır.Kırmızı boksitin jeokimyasal analizleri mobil elementlerin (Rb, K, Na, Sr, La, Mg, and Pb) tüketildiğini, daha az mobilelementlerin (Nb, Th, Zr, Mo, Ga, and Cr) ise zenginleştiğine işaret etmektedir. Zıt bir sonuç, boksitik kil içingözlenmektedir. Üst kaolinit seviyesi için kondrite göre normalize edilmiş NTE (Nadir Toprak Elementleri) örgülerialtlayan kırmızı boksitlerinkilere benzerdir, ve alt arjilikli seviye için gözlenen örgüler, üzerleyen arjilikli boksitseviyesindekine benzerdir. Ce, boksitik kilde bir negatif anomali ve kırmızı boksitte bir pozitif anomali gösterir. NTEve diğer elementler arasında hesaplanmış olan korelasyon katsayıları, NTE-içeren minerallerin Ti ve Nb, kil mineralleri,ve zirkonun oksitleri olduğunu ortaya koymaktadır. Jajarm boksitlerinin var olan diyasporik mineralojik bileşimininaksine, jeokimyasal ve mineralojik veriler, orijinal jipsitik bir bileşime işaret eder. Sonuç olarak, değişim diyagramlarıve NTE örgüleri ile sağlanan kanıt ile gözlenen mineralojik ve dokusal ilişkiler, Jajarm boksiti için hem bazik hem desedimanter kayaçlardan oluşan karışmış bir kökeni işaret etmektedir. Aslında, boksitleşme bazik köken kayaçlarüzerinde başlamış ve karstik özellikler içerisinde yerleşim ve yeniden işlenmesi süresince devam etmiştir.

Anahtar Sözcükler: İran, karst boksit, jeokimya, diyaspor, cevher yatağı

D. ESMAEILY ET AL.

269

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Northeast Iran) and 620 km from Tehran (Figure 1),is more than 8 km long and 20 m thick, making it thelargest bauxite deposit in Iran. The few detailedgeological data for this area mainly compriseexploration, extraction and reconnaissance reports.

The first exploration for bauxite in Jajarm beganin 1970 when the Geological Survey of Iran (GSI)carried out extensive exploration work in this part ofthe country; the exploration for bauxite began in thearea in 1999. A feasibility study on the Jajarm bauxiteore deposit was carried out in 1999 by the Tectono-export Company and an alumina production plantwas constructed near the mine site in 2001. Thebauxite resource at Jajarm is estimated to containmore than 19 Mt ore with an Al2O3/SiO2 ratio of43/14. The bauxite ore is currently mined from anopen pit and processed in the alumina productionplant.

The present paper aims to provide geological,petrographic and geochemical data on the Jajarmbauxite deposit and aspects of its origin arediscussed.

Geological SettingThe Jajarm bauxite deposit is situated in the easternpart of the Alborz structural zone (Figure 1). LowerDevonian sandstone, evaporites, and limestone ofthe Padha formation are the oldest rocks in the area.The Upper Devonian Khosh Yeylagh formationconsists of fossiliferous limestone, dolomite, shale,and sandstone, and is overlain by LowerCarboniferous shale and carbonate of the Mobarakformation (Figure 1). There are no Middle andUpper Carboniferous sediments in the area. Brownindurated claystones and siltstones with small ironconcretions overlie the Mobarak formation. In thesense of Brönnimann et al. (1973) this layer isequivalent of Sorkh Shale formation named byStöcklin et al. (1965) in eastern Central Iran (Tabasarea and Shotori Range). Because of its red colourand rather argillaceous composition it was namedthe Sorkh Shale formation (Sorkh= red) and it is inturn overlain by the Lower Triassic Elika carbonates.

The Elika formation, that hosts the Jajarm karstbauxite, is approximately 215 m thick and is dividedinto two parts: the lower part consists of thinly

bedded dolomite and dolomitic limestone, withlesser marl and yellowish shale (approximately one-third of the total thickness of the formation), whilethe upper part consists of thick light-brown to darkyellow and grey dolomite layers that define thehighlands and mountains of the area.

In many areas throughout the Alborz MountainRange, including some locations close to the studyarea, the Elika formation is overlain by darkcrystalline basic volcanic rocks. Palaeontologicalstudies in the eastern part of the Alborz zoneconfirm the absence of Upper Triassic marinesediments in this area (Stampfli et al. 1976; Seyed-Emami et al. 2005) (Figure 2). The karst features thathost the Jajarm bauxite deposit formed within thickdolomites of the upper Elika formation.Development of the karst topography in the Elikaformation occurred in its upper dolomitic andresistant section (Figures 2 & 3). Alternating shaleand sandstone of the coal-rich Jurassic Shemshakformation (202 m thick) disconformably overlies thebauxite horizon. Thus, the karst bauxite formed inthe upper parts of the Elika formation and issandwiched between the latter unit and theShemshak formation.

During the Triassic and Jurassic, closure of thePalaeotethys Ocean initiated subduction of theoceanic lithosphere of the Neotethys Ocean beneaththe Eurasian Plate (Berberian & Berberian 1981;Berberian & King 1981; Hooper et al. 1994, amongothers). Movement of the Afro-Arabian plate toward


270

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Figure 2. Stratigraphic column of the studied section in theJajarm area. Irregular right boundary shows thedifference of hardness degree for each layer.

Eurasia meant that central Iran and the Alborzstructural zone were located in the tropics at thistime (Berberian 1983). The dominance of shallowwater carbonate environments is one of the mainfeatures of the Tethys Basin, resulting in theformation of thick carbonate units, some hostingbauxite deposits (e.g., the Elika formation). Duringthe late Permian, these carbonate shelves weredistributed around the entire Gondwanaland marginand parts of the Tethys (Marcoux 1993). Following aLower Triassic transgression, the Elika formationwas deposited in the Alborz zone and the Jajarmarea. A progressive sea-level fall during the MiddleTriassic caused development of sabkha environmentsin the area (Stampfli et al. 1976). Finally, during thelate Middle Triassic or early Upper Triassic, a fall insea level led to an epeirogenetic phase and the

subaerial exposure of Triassic dolomites (Elikaformation) in a tropical climate, resulting inkarstification.

This karstified carbonate hosted the Jajarmbauxite and was buried by several thousand metresof younger sediments, beginning with the JurassicShemshak formation and other younger units.

Mine GeologyThe Jajarm bauxite deposit is located in an areafolded into an E–W-trending anticline, cut by severalreverse faults, so that its northern extension isoverthrust on to the southern part. Thisoverthrusting has hidden the bauxite depositbeneath Quaternary units. As a result, the bauxitedeposit is only exposed on the northern flank of theanticline, along a length of about 8 km. Exposure ofthe ore body is discontinuous along its length, withthe deposit occurring as isolated blocks that formining purposes are subdivided into eight blocks inthe Golbini area and four in the Zoo area (Figure 1).The variation of Al2O3:SiO2 ratios from 0.87 to 7.52throughout the deposit means that ore grades arelocally heterogeneous.

A complete profile through the Jajarm bauxitedeposit reveals an internal stratigraphy (layering)characterized by the following four distinct horizons(from bottom to top): (a) a lower argillaceous layer,(b) bauxitic clay, (c) red bauxite, and (d) an upperkaolinitic layer (Figure 2). The lower argillaceoushorizon, about 50–80 cm thick, which ismineralogically heterogeneous, directly overlies thecarbonate footwall (Elika formation) of the depositand is mainly composed of clay minerals (inparticular kaolinite and illite) and anatase, withlesser diaspore, hematite, pyrite, and goethite. Thecolour of this layer changes from grey (G 5/5according to Munsel chart) at its base (close to thecarbonate footwall) to pinkish and red (R 4/4) at itstop (close to the bauxitic clay layer).

The bauxitic clay layer is about 2–3 m thick,dominated by clay minerals (mainly kaolinite andillite), hematite, anatase, and diaspore, with rarerutile and quartz, and sharply overlies the lowerargillaceous layer. Layering is locally visible, but theclay layer is not economically viable in terms of

D. ESMAEILY ET AL.

271

Shemshak m.fbauxite horizon

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Figure 3. Outcrops of the Jajarm bauxite with its footwall (Elikaformation) and hanging wall (Shemshak formation).As seen, karstic features show irregular morphologyand hosted bauxite deposit.

alumina production. The clay is friable, and itscolour varies from bright to dark red (R 4/3 to R 5/8).

The red bauxite, which is approximately 5 mthick, is the main high-grade ore extracted for theproduction of alumina. This horizon is physicallyharder than the others, and is mainly red in colour (R5/8), although locally green. The boundary betweenthis horizon and the bauxitic clay is gradual.Minerals identified in this horizon include diaspore,kaolinite, anatase, berthierine, and hematite, alongwith lesser illite, quartz, rutile, and boehmite.Berthierine, which forms under reducing conditions(e.g., Iijima & Matsumoto 1982; Mordberg 1999),gives rise to the locally green colour (G 5/4) of thehorizon.

The upper kaolinite layer is grey (G 5/6), 20–50cm thick, mainly composed of kaolinite associatedwith other minerals such as anatase and hematiteand overlain by the Shemshak formation. The lowerboundary of this layer is very irregular but sharp.

Sampling and Analytical MethodsA total of about 500 rock samples were collectedfrom the four layers described above. Samples werecollected from different chips and along bottom totop of the profiles in different sections. Two hundredthin sections and polished thin sections were studiedby optical microscopy. Thirty-two representativesamples were then selected for whole-rock chemicaland X-ray diffraction (XRD) analysis. 2–3 kgsamples were crushed and powdered. Major andtrace element concentrations were determined byinductively coupled plasma atomic emissionspectrometry (ICP-AES) and inductively coupledplasma-mass spectrometry (ICP-MS) at ActivationLaboratories, Ontario, Canada. The analyticalprocedure is described in Cotten et al. (1995).Relative standard deviations were ≤ ±2% for majorelements and ≤ ±5% for trace elements. The resultsof the analyses are provided in Table 1.

Mineralogical analyses were carried out using aSiemens D4 automatic diffractometer. Samples werescanned with a step size of 0.020 and a step time of 1s, using Cu Kα radiation from a Cu anode X-ray(1.5406 Å).

Mineralogy and TextureDetailed textural and mineralogical analysis basedon the optical microscopy and XRD data was carriedout. It showed that diaspore, the main Al-bearinghydroxide in the Jajarm bauxite, generally appears asa replacement and void-filling cement. In the lattercase, it occurs as coarse-grained crystals that locallyshow features of reworking (Figure 4a). Someintraclast grains consist of various fragments that arewell-cemented by diaspore. The most importantsilicate minerals that accompany diaspore arekaolinite (as reactive silica) and minor quartz (asnon-reactive silica). Hematite is the most importantFe-bearing mineral, producing the red colour of thedeposit. Some samples contain minor berthierineand rare boehmite. Berthierine is an iron-rich,aluminous, l: l-type layer silicate belonging to theserpentine group (Brindley 1981). XRD data showedberthierine to be a minor constituent of some bauxitesamples along with other rock forming minerals.

In some samples reworked, fractured andcorroded quartz grains are embedded in a matrix ofdiaspore, iron oxides, or kaolinite. Many samplesthat contain older fragments of well-roundeddiaspore intraclasts and aggregates are nowcemented by a matrix of fine-grained diaspore andiron oxyhydroxides (Figure 4b). This texturesuggests the transportation and re-deposition ofbauxite, at least locally. The heterogeneousdistribution of iron oxyhydroxides is indicated by thevariable colouring of many samples, ranging fromintense red to light brown.

The matrix-forming minerals are generally 1–20mm in size, although some diasporic minerals are100–200 mm (Figure 4a, b). The wide ranges incrystal size may reflect the old age of the deposit andthe influence of such processes as alteration,recrystallization and diagenesis (Figure 4c). Theeffects of early and burial diagenesis, accompaniedby tectonic stresses, have led to recrystallization andthe growth of large crystals. The grain sizes ofdetrital particles such as intraclasts and diageneticparticles such as ooids and pisoids vary from severalmicrons to several millimetres (Figure 4a, c).Presumably, during bauxitic material formation inthe source area and in the karstic depressions, the


272

D. ESMAEILY ET AL.

273

Tabl

e 1.

Maj

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nd tr

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elem

ent c

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Baux

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lay

Low

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us L

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ple

A036

A006

A010

A013

A015

A017

A019

A022

A026

A029

A031

rang

eav

erag

eA0

01A0

05A0

09A0

21A0

25ra

nge

aver

age

wt%

Min

Max

Min

Max

SiO

229

.430

.526

.117

.827

.728

.325

.024

.431

.424

.435

.317

.835

.327

.38.

538

.428

.844

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.48.

544

.530

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l 2O3

36.1

29.7

26.5

33.0

38.0

33.2

37.6

42.5

38.6

29.3

29.6

26.5

42.5

34.0

57.7

29.0

47.0

34.6

30.1

29.0

57.7

38.7

Fe2O

316

.723

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.234

.216

.521

.520

.314

.69.

931

.517

.89.

934

.221

.67.

712

.32.

60.

615

.30.

621

.610

.0M

nO0.

050.

020.

020.

030.

010.

020.

010.

010.

040.

010.

020.

010.

050.

020.

000.

000.

000.

000.

000.

000.

020.

00M

gO0.

600.

170.

340.

090.

100.

150.

130.

260.

260.

110.

290.

090.

600.

230.

130.

500.

320.

480.

360.

130.

500.

34C

aO0.

130.

200.

480.

050.

220.

120.

100.

110.

520.

140.

140.

050.

520.

200.

030.

220.

150.

200.

210.

030.

220.

17N

a 2O0.

060.

030.

040.

030.

030.

060.

030.

080.

080.

030.

070.

030.

080.

05nd

0.11

0.07

0.17

0.12

0.05

0.17

0.10

K2O

0.58

0.39

0.22

0.61

0.19

0.49

0.46

1.97

0.89

0.42

1.42

0.19

1.97

0.69

1.38

7.70

4.36

7.59

2.80

0.69

7.70

4.09

TiO

22.

83.

52.

63.

73.

84.

23.

73.

94.

53.

13.

42.

64.

53.

65.

93.

04.

93.

53.

13.

05.

94.

0P 2O

50.

100.

190.

080.

160.

120.

140.

140.

130.

080.

070.

170.

070.

190.

130.

190.

220.

120.

160.

140.

120.

220.

16LO

I12

.311

.010

.29.

212

.311

.111

.410

.712

.69.

710

.79.

212

.611

.017

.37.

110

.47.

310

.27.

117

.310

.5to

tal

98.9

98.7

98.7

98.9

99.0

99.2

98.8

98.7

98.9

98.8

98.9

98.8

98.6

98.7

99.1

99.8

ppm

Ba35

102

4057

5666

6314

832

3217

332

173

7314

177

4614

610

4114

1041

250

Sr27

294

631

154

241

245

367

262

938

330

578

027

294

651

912

0911

5852

375

950

950

912

0977

9Y

2183

8942

2210

857

6022

8365

2110

859

6787

7077

9559

9576

Sc42

4042

5739

5045

4338

3531

3157

4240

3241

3135

3142

37Zr

417

511

417

474

532

570

572

546

607

488

476

417

607

510

838

430

712

530

450

430

838

578

Be3

65

53

65

62

56

26

55

24

45

25

4V

350

411

574

516

485

522

453

396

497

428

423

350

574

460

736

442

496

492

453

442

736

513

Cr

155

108

141

9612

915

613

914

917

620

213

296

202

144

288

112

215

152

119

112

288

172

Co

118

1311

013

822

1623

2419

348

118

3635

77

2016

736

20N

i17

811

214

778

6911

811

915

430

114

134

3017

811

412

161

9211

810

961

121

102

Cund

ndnd

ndnd

42nd

nd33

nd38

3342

3726

6325

46nd

2563

39Zn

63nd

78nd

3912

4nd

6636

nd39

3612

464

nd39

nd39

5239

6448

Ga

5839

2950

3542

4845

4442

3129

5842

6833

5631

4131

6845

Ge

23

43

23

33

33

22

43

31

52

31

53

As

2712

2920

1639

1417

2340

2112

4023

2416

8218

105

1610

545

Rb11

97

144

1110

4317

1034

443

1525

141

7813

867

1514

177

Nb

104

102

9311

713

014

012

611

813

211

399

9314

011

614

895

158

117

9695

158

122

Mo

nd3

83

24

43

25

32

84

nd6

102

42

105

Ag

nd1.

2nd

nd0.

5nd

ndnd

0.6

nd0.

90.

51.

20.

82.

01.

10.

50.

6nd

0.5

2.0

1.0

Innd

ndnd

ndnd

ndnd

ndnd

ndnd

ndnd

ndnd

ndSn

55

56

66

66

65

65

66

45

76

74

76

Sb3

68

84

85

52

94

29

63

22

15

16

3C

snd

1.0

1.4

0.7

nd0.

80.

72.

60.

90.

92.

90.

72.

91.

30.

83.

82.

33.

83.

90.

83.

92.

6La

225

183

8419

123

426

810

612

925

9510

725

268

150

8820

123

414

621

888

234

173

Ce

1080

180

152

443

332

353

215

192

279

215

189

152

1080

330

205

240

198

254

243

198

330

245

Pr44

3424

3626

5827

407

2630

758

3230

7166

4668

3071

52N

d17

397

141

9556

221

112

154

2610

913

926

221

120

136

258

159

180

195

120

258

175

Sm35

1746

167

4523

295

2529

546

2531

4218

2928

1842

29Eu

8.6

4.6

14.5

4.0

1.7

11.9

5.4

7.4

1.3

6.9

7.3

1.3

14.5

6.7

7.1

9.3

3.5

5.8

6.3

3.5

9.3

6.5

Gd

2316

4711

540

1624

423

234

4721

1823

1419

2314

2320

Tb3.

02.

46.

31.

80.

75.

32.

64.

00.

73.

93.

10.

76.

33.

12.

72.

92.

32.

93.

22.

33.

22.

8D

y13

1327

115

2314

203

1914

327

1514

1515

1719

1419

16H

o2.

22.

64.

42.

31.

03.

82.

63.

50.

73.

62.

50.

74.

42.

62.

63.

03.

63.

63.

82.

63.

83.

2Er

78

107

311

810

211

72

118

810

1312

138

1310

Tm1.

01.

11.

31.

00.

51.

51.

31.

6nd

1.6

1.1

0.5

1.6

1.2

1.2

1.5

2.3

1.8

2.1

1.2

2.3

1.7

Yb7

78

73

98

102

107

210

79

1116

1113

716

11Lu

11

11

nd1

12

nd2

11

21

12

22

21

22

Hf

1111

1111

1315

1414

1513

1211

1513

2011

1813

1111

2014

Ta7

66

88

98

89

77

69

76

610

76

610

7w

1810

1829

1415

1528

1120

1610

2918

1611

2011

1111

2014

Tl0.

20.

21.

80.

3nd

0.2

0.1

0.4

0.2

0.2

0.4

0.1

1.8

0.4

0.5

1.3

0.7

1.0

0.6

0.4

1.3

0.8

Pb13

2633

3345

4823

2611

3542

1148

3058

6331

2933

2963

41Bi

107

54

210

610

48

92

107

13

85

141

146

Th20

2019

2023

2522

2625

2121

1926

2231

2131

2222

2131

25U

107

1240

1115

911

1415

107

4014

2525

2010

1210

2518


274

Tabl

e 1.

Con

tinue

d.

Red

Baux

iteU

pper

Kao

linite

sam

ple

A003

A007

A011

A014

A016

A018

A020

A023

A027

A030

A032

rang

eav

erag

eA0

04A0

08A0

12A0

24A0

28ra

nge

aver

age

wt%

Min

Max

Min

Max

SiO

211

.29.

829

.06.

94.

36.

328

.217

.519

.341

.85.

84.

341

.816

.439

.336

.431

.934

.135

.731

.939

.335

.5A

l 2O3

49.2

42.4

33.7

56.5

47.1

49.6

39.8

37.4

35.9

36.4

43.5

33.7

56.5

42.9

33.2

31.2

31.8

29.5

30.8

29.5

33.2

31.3

Fe2O

316

.328

.320

.214

.027

.222

.212

.926

.426

.62.

831

.02.

831

.020

.76.

913

.310

.118

.113

.96.

918

.112

.5M

nO0.

110.

040.

000.

010.

270.

120.

080.

040.

020.

010.

310.

000.

310.

090.

000.

310.

000.

230.

220.

000.

310.

15M

gO0.

450.

530.

100.

270.

140.

270.

180.

860.

980.

160.

360.

100.

980.

390.

140.

140.

240.

120.

150.

120.

240.

16C

aO1.

770.

160.

080.

042.

290.

880.

090.

120.

160.

100.

370.

042.

290.

550.

760.

350.

241.

030.

960.

241.

030.

67N

a 2O0.

040.

050.

060.

050.

020.

050.

050.

040.

040.

070.

020.

020.

070.

040.

040.

050.

170.

050.

060.

040.

170.

07K

2O1.

340.

260.

630.

170.

770.

430.

260.

460.

620.

870.

240.

171.

340.

550.

521.

011.

930.

220.

350.

221.

930.

81Ti

O2

5.5

5.5

3.7

7.1

5.3

6.4

4.3

5.8

4.5

4.2

5.3

3.7

7.1

5.2

3.5

4.0

3.6

4.0

3.7

3.5

4.0

3.8

P 2O5

0.05

0.10

0.03

0.08

0.15

0.14

0.02

0.10

0.08

0.01

0.06

0.01

0.15

0.07

0.04

0.02

0.05

0.08

0.03

0.02

0.08

0.04

LOI

13.0

12.0

11.4

13.8

11.7

12.2

13.0

10.6

10.8

12.8

11.7

10.6

13.8

12.1

14.2

12.1

18.7

12.4

12.8

12.1

18.7

14.0

tota

l99

.099

.298

.798

.999

.298

.698

.999

.398

.899

.198

.698

.698

.998

.799

.898

.7pp

mBa

2122

569

333

5711

030

134

8622

2666

926

6652

092

518

462

108

4141

925

264

Sr32

950

586

358

437

1054

8722

932

971

332

7110

5434

718

168

237

165

103

6823

715

1Y

5341

1649

1954

3556

2817

2316

5636

1430

2419

2314

3022

Sc34

5838

4631

5045

4638

3048

3058

4226

3939

3135

2639

34Zr

809

792

527

1020

771

894

643

749

620

609

691

527

1020

739

520

534

608

518

507

507

608

537

Be3

32

52

32

22

21

15

21

31

22

13

2V

501

581

607

705

526

692

549

551

482

448

549

448

705

563

408

403

482

412

402

402

482

421

Cr

256

240

198

287

256

280

210

193

189

182

232

182

287

229

169

174

199

151

151

151

199

169

Co

5347

1355

3226

109

8652

1525

1310

947

449

03

7382

349

013

0N

i71

8742

125

nd35

8194

69nd

nd35

125

7625

101

2437

4424

101

46Cu

ndnd

7523

nd10

58nd

nd24

3nd

1024

382

2066

201

172

264

2026

414

5Zn

8262

nd11

544

114

nd57

60nd

3333

115

71nd

102

nd46

nd46

102

74G

a61

6241

6852

6841

4949

2851

2868

5234

3342

2937

2942

35G

e3

42

33

42

33

12

14

3nd

11

1nd

11

1A

s17

1858

115

1724

2630

4546

017

1558

111

422

826

152

516

567

6752

524

9Rb

266

103

149

59

1216

63

2611

1018

345

75

3415

Nb

155

156

111

181

155

175

134

163

140

109

154

109

181

148

105

126

110

116

119

105

126

115

Mo

85

55

62

63

2nd

92

95

44

46

44

64

Ag

1.4

1.0

0.5

0.7

1.0

0.7

0.6

nd0.

71.

30.

90.

51.

40.

91.

1nd

0.6

ndnd

0.6

1.1

0.8

Innd

0.3

nd0.

30.

20.

4nd

ndnd

nd0.

40.

20.

40.

3nd

ndnd

ndnd

Sn9

86

98

87

77

67

69

75

65

56

56

5Sb

22

82

12

22

26

21

83

44

93

33

94

Cs

1.3

ndnd

0.7

ndnd

ndnd

0.6

0.7

nd0.

61.

30.

81.

00.

82.

6nd

0.5

0.5

2.6

1.2

La13

2623

218

1918

128

264

234

128

2810

652

1114

652

19C

e89

173

108

115

209

219

3628

815

255

183

3628

814

846

3915

710

814

039

157

98Pr

36

44

28

424

61

61

246

22

82

42

84

Nd

1221

1219

837

1884

254

224

8424

711

2111

167

2113

Sm3

52

62

95

157

16

115

62

34

35

25

3Eu

0.9

1.7

0.6

1.9

0.6

2.2

1.4

4.3

1.7

nd1.

60.

64.

31.

70.

41.

21.

00.

91.

50.

41.

51.

0G

d4

52

72

85

157

25

215

62

43

35

25

3Tb

0.8

0.9

nd1.

3nd

1.3

1.0

2.4

1.0

nd0.

80.

82.

41.

20.

30.

80.

60.

60.

70.

30.

80.

6D

y5

52

92

76

135

25

213

62

44

34

24

3H

o1.

01.

10.

51.

80.

51.

41.

12.

30.

9nd

0.9

0.5

2.3

1.1

0.4

0.8

0.8

0.6

0.7

0.4

0.8

0.7

Er3

32

52

43

73

13

17

31

22

22

12

2Tm

ndnd

nd0.

8nd

0.6

nd1.

0nd

ndnd

0.6

1.0

0.8

0.2

0.3

0.4

0.3

0.3

0.2

0.4

0.3

Yb3

32

52

33

62

13

16

31

23

22

13

2Lu

ndnd

ndnd

ndnd

nd1

ndnd

nd1

11

00

00

00

00

Hf

1919

1323

1922

1518

1614

1813

2318

1212

1513

1312

1513

Ta9

107

88

89

119

89

711

97

87

78

78

7w

2231

1864

4536

1639

2713

3713

6432

412

1013

154

1511

Tl0.

5nd

0.2

0.2

0.2

0.1

0.1

0.1

0.1

0.2

nd0.

10.

50.

20.

11.

20.

30.

50.

90.

11.

20.

6Pb

2010

133

2113

2810

1514

229

913

327

2623

9418

1818

9436

Bi2

28

912

37

34

223

222

75

62

57

27

5Th

3635

2539

3134

2628

2623

3023

3930

2226

2619

2219

2623

U22

2222

2922

2516

1816

1214

1229

2019

1128

128

828

16

D. ESMAEILY ET AL.

275

primary materials were colloidal; large particlesformed via secondary processes such asrecrystallization and reworking and erosion ofcomplex oolitic clasts (Figure 4a, b). Spherical grainssuch as ooids and pisoids are important componentsof most samples (Figure 4b, c). Their presence can beattributed to the heterogeneity of the initial colloidsthat originated from alteration of the source rock.Another possibility is the formation of thesespheroidal particles in terrigenous lateriticweathering crusts. Some pore spaces formed bydissolution in the bauxite samples are filled byminerals such as diaspore, goethite, hematite, anddolomite.

Geochemistry32 samples were analyzed for major and traceelement concentrations, including 11 samples eachof bauxitic clay and red bauxite, and 5 samples eachof the lower argillaceous layer and the upperkaolinite (Table 1). The dominant chemicalcomponents of the samples are Al2O3, Fe2O3, SiO2,and H2O (i.e., loss on ignition (LOI), which waschiefly H2O, as the samples are free of othervolatiles). We found considerable chemical variationboth among the four groups of samples and withinindividual groups (Table 1). In the bauxitic clay andred bauxite, Al2O3 contents range between 26.50 and56.47 wt%, Fe2O3 between 2.77 and 34.22 wt%, SiO2between 4.31 and 41.75 wt%, and TiO2 between 2.56and 7.14 wt%. Relative to the average composition ofbauxite deposits (Bronevoi et al. 1985), the Jajarmbauxite ore is enriched in the trace elements Ce (36–1080 ppm), Nb (93–180 ppm), Bi (2–22 ppm), andTa (5.7–10.5 ppm). Relative to crustal averages, theanalyzed samples are enriched in Al, Fe, and Ti, anddepleted in Mg, Ca, Na, and K. Th concentrations arehigher in the red bauxite than in the bauxitic clay,whereas Sc concentrations are similar in the two rocktypes.

When plotted against Al and Ti, theconcentrations of Zr, Nb, and Th produce similarlywell-correlated data arrays for samples from allhorizons (Figure 5). In addition, most red bauxitesamples are enriched in Th, Zr, and Nb. The smalldegree of variation evident in each plot can be

Figure 4. (a) Well-rounded intraclast which is composed ofdiaspore-cemented intraclasts. It shows several phasesof cementation and reworking of bauxitic material,during and after bauxitization (XPL). (b) Oolitictexture with reworking features and fine-graineddiasporic and iron oxyhydroxides matrix from the redbauxite horizon (PPL). (c) Diaspore cemented bauxitewith various particle size resulted from alteration,recrystallization and diagenetic processes (PPL).


276

attributed to minor element mobility, weak source-rock heterogeneity, or the local winnowing oflateritized minerals during subaerial weathering. Thelatter process tends to separate heavy mineralscontaining elements such as Ti, Zr, and Nb from thelighter Al-bearing kaolinite and diaspore.

The average rare earth element (REE)concentrations vary from 0.5 ppm for Lu and Tm inred bauxite up to 225 ppm for Ce in bauxitic clay(Tables 1 & 2). These elements, especially the light

REE (LREE), are concentrated in the bauxitic clayrather than the red bauxite. The chondrite-normalized REE patterns obtained for the upperkaolinite layer are similar to those for the underlyingred bauxite, while the patterns obtained for the lowerargillaceous layer are similar to those for theoverlying bauxitic clay (Figure 6).

Puchelt & Emmerman (1976) explained a strongconnection between REE ionic potential and theirmobility. This is consistent with the distribution

0

300

600

900

1200

R = 0.97

01

23

45

67

0 20 40 60 80

R = 0.85

0

300

600

900

1200

R = 0.89

0

10

20

30

40

R = 0.88

0

50

100

150

0 2 4 6 8

R = 0.92

0

10

20

30

40

R = 0.85

0

50

100

150

R = 0.83

upper kaolinite

red bauxite

bauxitic clay

lower argillaceous layer

Th

Zr

Nb

TiO

2

Al O2 3

TiO2

Nb

Th

Zr

Figure 5. Binary plots of some immobile trace elements (ppm) against Al2O3 and TiO2 (wt%).

D. ESMAEILY ET AL.

277

pattern of REE in the Jajarm bauxite deposit. Ceshows a positive anomaly in the red bauxite andupper kaolinite layer and a weak negative anomaly inthe lower argillaceous clay layer. Ce exists naturallyin two forms: Ce3+ and Ce4+, with the latter having ahigher ionic potential than the other LREE andconsequently the lowest mobility. Accordingly, thepositive Ce anomaly observed in the red bauxite canbe explained by its occurrence as Ce4+ ions. Thelower ionic potential of LREE relative to HREEmeans that they were readily leached from the redbauxite and concentrated in the bauxitic clay (Tables1 & 2). In contrast, the high ionic potential of Ce4+

means that it behaved as a HREE, and wasconcentrated in the bauxitic clay.

Although the ionic potential of HREE increasesfrom Er to Lu, this trend does not appear to affecttheir concentrations in the bauxitic clay. A relativelyhigh correlation coefficient among Er, Tm, Yb, Lu(HREE) and Zr and Al in the red bauxite (Table 2)may indicate that the HREE are more commonlyaccompanied by minerals containing Zr and Al(mostly clay minerals and zircon). Therefore, inaddition to the ionic potential of the REE, their host-mineral chemistry may also have been an importantfactor in determining their degree of leaching. Thestrong correlations between REE (except La), Nb,and TiO2 (Table 2), suggest these REE are probablyhosted by oxides of Ti and Nb (e.g., Gonzáles Lópezet al. 2005). High correlation coefficients obtained

C

0

1

2

3

4

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

A

0

1

2

3


B

0

1

2

3


D

0

1

2

3

4


log

(C/C

)s

am

ple

Ch

on

dri

telo

g(C

/C)

sa

mp

leC

ho

nd

rite

log

(C/C

)s

am

ple

Ch

on

dri

telo

g(C

/C)

sa

mp

leC

ho

nd

rite

Figure 6. Chondrite normalized REE patterns of (A) red bauxite, (B) upper kaolinite, (C) bauxitic clay and (D) lowerargillaceous layer.

Table 2. Average concentrations of REE in red bauxite and bauxitic clay samples from the study area.

TiO2 Al2O3 Zr Cr Sn Nb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu REE

TiO2 1.00 0.99 0.26 0.58 0.51 0.50 0.64 0.77 0.71 0.68 0.77 0.79 0.79 0.80 0.80 0.78 0.60

Al2O3 1.00 -0.26 0.25 0.00 0.05 0.20 0.34 0.31 0.28 0.42 0.46 0.46 0.51 0.51 0.49

Zr 1.00 -0.01 0.37 0.24 0.24 0.39 0.55 0.50 0.46 0.61 0.65 0.65 0.67 0.67 0.64

Cr 1.00 -0.17 0.21 0.03 0.03 0.15 0.28 0.25 0.16 0.34 0.43 0.43 0.46 0.46 0.44

Sn 1.00 -0.07 0.32 0.12 0.13 0.28 0.42 0.37 0.30 0.46 0.54 0.54 0.56 0.56 0.53

Nb 1.00 0.32 0.60 0.55 0.53 0.65 0.78 0.72 0.66 0.76 0.80 0.80 0.81 0.81 0.79


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for Ti–Nb (0.99) and Ti–∑REE (0.6) demonstratethat Nb- and Ti-bearing minerals such as anatasedetermined the distribution of some of the REE.Other titanium and niobium oxides such as lopariteand euxenite (Mariano 1989) may also haveinfluenced the distribution of some of the REE inthese rocks, as REE and Y can be replaced by Ca inthe mineral structure.

Discussion and ConclusionDiagenetic and Epigenetic Processes Different diagenetic processes in the Jajarm bauxitedeposit formed ooids and pisoids, as well as thereplacement of boehmite by diaspore. Also, thesespheroidal particles could have been formed byterrigenous lateritic weathering crusts. Like many

other diasporic deposits (Nikitina 1986; Bárdossy &Aleva 1990) the original mineralogical compositionof the Jajarm bauxite appears to have been gibbsitic,which altered to boehmite then to diaspore duringdiagenesis.

Epigenetic processes that began after dehydrationand crystallization of the primary gel were enhancedby burial or tectonic pressures, resulting in chemicaland physical deformation (e.g., pressure solution andstylolites; Figure 7a). These burial pressures may wellhave led to alteration of boehmite to diaspore. Thetextures observed in the Jajarm bauxite indicate thatthe initiation of bauxite formation occurred underreducing conditions, followed later by oxidizingconditions (hematite formation). Accordingly, localreducing conditions were dominated by thedeposition of the OM-rich Shemshak formation on

50 µm 64 µm

50 µm 50 µm

a b

c d

Figure 7. (a) Pressure-solution fabric in the reworked diasporic clasts, formed during burial (XPL). (b) Grain-boundedepigenetic fractures in a pisolite grain from red bauxite horizon (XPL). (c) Epigenetic micro-fractures that cross-cutthe matrix of red bauxite sample (XPL). (d) An intraclast in a fine-grained matrix formed by deferrification.

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the bauxitic horizons. This coal-rich formation wasdeposited in a series of paralic marshes covering thebauxitic horizon. Then, due to oxidation of organicmatter, Fe was stabilized as soluble ferrous ions(Biber et al. 1994).

As mentioned, leaching of mobile elements fromthe upper horizon (red bauxite) and depositionwithin the lower (bauxitic clay) is another epigeneticprocess (e.g., Valeton et al. 1987). Another importantepigenetic process was deferrification, and theleaching of iron, which occurs at all scales from themacro to the micro; with the conversion of hematiteinto limonite, is one of the results of this process.Deferrification provides evidence of microbial andbiological activity that produced acidic and reducingconditions (Augustithis 1982). Generally, biologicaland herbal activities decrease the pH of sedimentaryenvironments: this reduces Fe3+ to Fe2+ and finallycauses iron to be leached from iron-bearing mineralsas organometallic complexes. In the Jajarm bauxitethe degree of deferrification is variable and traceableeither in different layers of a single ooid and pisoid orin different beds with variable amounts ofdeferrification (Figure 7d).

Fractures and joints are the other importantepigenetic features of the deposit, and these aredivided into two main groups. The first group isconfined to within individual grains (e.g., withinpisoids), terminating at grain boundaries (Figure7b). These fractures originated from the dehydrationand compaction of the original material. The secondgroup of fractures cuts through the matrix, andindividual fractures are of varying origin. Someformed following the dehydration of the originalbauxite-forming material, while others are oftectonic origin and transect both grains and thesurrounding matrix (Figure 7c). Various fractures,joints, and veinlets are filled with materials such asdiasporic and boehmitic cement that in somesamples occur as coarse crystals; other infill mineralsare hematite, goethite, pyrite, Mn oxides, ankerite,etc.

Source Rock MaterialMany bauxite deposits can be directly related, viatexture and chemistry, to the underlying bedrock;

although in sedimentary limestone sequences we arefaced with a wider array of choices, ranging fromargillite components of the underlying limestone tofluvially transported debris sourced from basementrocks (e.g., Bárdossy 1982, 1984) and deposits ofvolcanic ash (Bárdossy 1984; Lyew-Ayee 1986). In allthese models, Al is concentrated in situ as an inert(immobile) residue of lateritization.

In studies of other deposits, Pye (1988) andBrimhall et al. (1988) proposed windborne transportas the re-concentration process that formed localhigh-grade bauxites. Given the nature of karsterosion, in which both chemical and mechanicalweathering is active, it is generally difficult todetermine the relative contributions of differentsources of argillaceous debris during bauxiteformation.

The concentrations of immobile elements can beused to trace source materials, as they showcontrasting distributions in different sources. In thisstudy, variation diagrams for Zr, Nb, and Th versusAl2O3 and TiO2 (Figure 5) demonstrate a singlestrongly correlated trend for all four main horizonswithin the bauxite deposit. Based on the findings ofMacLean (1990), the four horizons within the Jajarmbauxite appear to have originated from a singlehomogeneous source. In the Ni-Cr diagram (Figure8), the Jajarm bauxite samples plot close to thekarstic bauxite field with a basaltic source material(see Schroll & Sauer 1968). According to the Zr-Cr(Ni)-Ga ternary diagram of Balasubramaniam et al.(1987), our samples generally show a mixed origin ofbasic igneous and sedimentary rocks (Figure 9).

In considering the concentrations or proportionof selected trace elements, Mordberg (1993)proposed several variation diagrams that can be usedto distinguish the mineralogical composition ofbauxite deposits of different ages. Contrary to thediasporic mineralogical composition of the Jajarmbauxite, Mordberg's plot indicates an originalgibbsitic mineralogical composition (Figure 10).This observation suggests that an original gibbsiticcomposition converted to diaspore over time.

REE patterns can also be used to identify sourcematerials (e.g., González López et al. 2005).Chondrite-normalized REE patterns obtained for theJajarm bauxite resemble those of the composition ofupper continental crust (UCC) (Figure 11) and shale


280

0.1 101 100 10001

10

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1000

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basic

rock

skarstbauxite

basalt

low ironlateriticbauxite

high ironlateriticbauxite

sandstonegranite

carbonate rocks

syenite

Ni (ppm)

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(pp

m)

s shale, late

red bauxitebauxitic clay

red bauxite bauxitic clay

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acidic andmetamorphic

rocks

sedimentaryrocks

basic rocks

ultrabasic

20

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Pb

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2 4 6 8 10

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C r/Ni

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40

60

80

100

20 40 60 80 100

Ga

Pb

G B D

PZ

MZ

CZ

Figure 8. Binary correlation diagram of Cr/Ni in karst andlateritic bauxites in different parent rocks (afterSchroll & Sauer 1968).

Figure 10. Trace element ratios in bauxites of different ages and mineral forms of Al (G –gibbsite; B – boehmite; D – diaspore; PZ – Palaeozoic; MZ – Mesozoic; CZ –Cenozoic) (Mordberg 1993). Jajarm bauxite samples are shown by stars.

Figure 9. Zr-Cr(Ni)-Ga ternary diagram for bauxites derivedfrom different parent rocks (Balasubramaniam et al.1987).

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(Figure 12). HREE patterns for the Jajarm bauxite areclosely related to those for diabase, although withsome enrichment (Figures 11 & 12). The fact thatREE patterns obtained for the bauxite are similar tothose for UCC and shale can be explained by themixed origin of the source material: bauxitizationwas initiated on the basic source rock, but continuedduring reworking and replacement within the karsticfeatures. A contribution from sedimentary materialduring the latter process explains the observed REEpatterns.

The palaeogeography of Iran in the Late Permianand Early Triassic indicates that the early LateTriassic compressional phase (Early CimmerianEvent) was followed by extensional movements inNorth and Central Iran. The initiation of thisextensional phase is locally indicated by continentalalkali-rift basaltic lava flows and vesicular maficrocks (ranging from a few metres up to 300 m thick)(Berberian & King 1981; Vollmer 1987). In theAlborz mountains (Figure 1), the Late Triassicandesitic to basaltic volcanic rocks cover an eroded

and often karstified surface of Middle Triassiccarbonates (Early Cimmerian palaeorelief), at thebase of the Shemshak formation (for exampleFiroozkouh and Shahmirzad in Figure 1) (Annelleset al. 1975; Nabavi & Seyed-Emami 1977; Nabavi1987). The volcanic activity occasionally continuedinto the lower part of the Shemshak formation. Sothe proposed basic source material of the Jajarmbauxite is widespread within the Elika formation incertain parts of Alborz Mountain Range. Thisinterpretation of a basic source material is supportedby petrographic evidence, such as the replacement ofigneous feldspar by platy diaspore and the well-rounded nature of reworked oolitic and pisoliticgrains.

Origin of Bauxite LayeringGeochemical results (Table 1) demonstrate theenrichment of less mobile elements (such as Nb, Th,Zr, Mo, Ga, and Cr) and the depletion of mobileelements (such as Rb, K, Na, Sr, La, Mg, and Pb) inthe red bauxite relative to the bauxitic clay. This

A0

1

2

3


granite diabase red bauxite UCC

B0

1

2

3


red bauxite UCC

log

(C/C

)sam

ple

Ch

on

dri

telo

g(C

/C)

sam

ple

Ch

on

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te

A0

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granite diabase red bauxite shale

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log

(C/C

)s

am

ple

Ch

on

dri

telo

g(C

/C)

sa

mp

leC

ho

nd

rite

Figure 11. Comparison of chondrite normalized patterns of redbauxite and Upper Continental Crust (UCC) (Taylor& McLennan 1981), as well as granitic and diabasicrocks (Klein & Hurlbut 1999). The chondriticcomposition is after Boynton (1984). Diagram A isfor comparison of chondrite normalized patterns ofred bauxite with granitic, diabasic and UCCcomposition. The red bauxite and UCC arecompared in diagram B.

Figure 12. Comparison of chondrite normalized patterns of redbauxite with average composition of European shale(Haskin & Haskin 1966), granitic and diabasic rocks(Klein & Hurlbut 1999). The chondritic compositionis after Boynton (1984). Diagram A is forcomparison of chondrite normalized patterns of redbauxite with granitic, diabasic and averagecomposition of European shales. The red bauxiteand European shale are compared in diagram B.


282

pattern could reflect the leaching of mobile elementsfrom the upper horizon (red bauxite) and depositionwithin the lower (bauxitic clay). Such a trend is alsodescribed from other karst-hosted bauxite deposits(e.g., Valeton et al. 1987; Maksimović & Pantó 1991;MacLean et al. 1997). In addition, the carbonate hostacted as a geochemical barrier to further migrationof the mobilized elements, leading to theirconcentration in the lowermost horizon. A lack ofactive drainage in the bauxitic clay and the nature ofits interface with carbonate rocks are the mainreasons for the dominance of basic pH values. Thisexplains the weak bauxitization observed in thelower horizon and the evolution of the bauxitic clay.In comparison, the occurrence of active drainage andacidic pH in the red bauxite acted to enhance thebauxitization process.

Chondrite-normalized REE values are similar forboth the bauxitic clay and the lower argillaceoushorizon, while the values for the red bauxite aresimilar to those for the upper kaolinite horizon(Figure 6). These findings indicate a similar originfor the lower argillaceous horizon and the bauxiticclay on one hand, and a similar origin for the upperkaolinite horizon and red bauxite on the other. Thedirect interface of the lower argillaceous layer withthe carbonate footwall, and relatively poor drainagecondition in the lowermost part of the depositcaused separation of the bauxitic clay and the lowerargillaceous layer as both have differentmineralogical and geochemical properties.

In contrast to the lower argillaceous horizon, theupper kaolinite horizon is relatively homogeneous in

terms of chemical and mineralogical composition,with kaolinite being the most abundant mineral. Theupper 1–4 m of the red bauxite is completelyreplaced by secondary kaolinite, and the lowerboundary of this horizon with the red bauxite isirregular but sharp. Resilification could be the originof the Si for secondary kaolinite. According toBárdossy & Aleva (1990, p. 181–182) in somedeposits, secondary kaolinite completely replaces theupper part of the bauxite horizon and dissolved silicais introduced into the bauxite horizon by the groundwater, either laterally or from the overburden. Theoptimum pH for kaolinitization is close to 4 (Yariv &Cross 1979, p. 321–324), and it generally involves ahigh degree of crystallization (Bárdossy & Aleva1990). This high acidity resulted from thedevelopment of swamps upon the red bauxite duringEarly Jurassic deposition of the Shemshak formation(Figure 2). Dominance of swamps and reducingconditions is evident from the presence of theoverlying Shemshak formation (in a large area innorth and central of Iran), containing a huge amountof organic materials such as coal and kerogen,formed mainly in a deltaic environment.

AcknowledgmentsThis work was supported by grant no 84/124 fromthe National Research Council of I.R. Iran (NRCI),received by Dr. H. Rahimpour-Bonab. TheUniversity of Tehran provided some facilities for thisresearch, which we are grateful. Turkish translationof abstract is made by Selman Akdoğan. The Englishof the final text is edited by John A. Winchester.

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