45 Petrography and Geochemistry of the Jajarm Bauxite Ore Deposit, Northeast Iran 19-2!5!0806-15

8/10/2019 45 Petrography and Geochemistry of the Jajarm Bauxite Ore Deposit, Northeast Iran 19-2!5!0806-15

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Petrography and Geochemistry of the Jajarm KarstBauxite 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, 141556455 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 5080 cm thick, is mainly composed of clay minerals that directly overlies the carbonate footwall (Elika formation); (b) a bauxitic clay layer approximately 23 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 2050 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 bauxitic

clay. 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 ran) Karst Boksit Cevher Yatann Petrografisi veJeokimyas: Kaynak Kaya Materyali ve Cevher Kkeni in Gstergeler

zet: rann kuzeydousunda bulunan Jajarm boksit yata randaki en byk yataktr. Yatak, Triyas yal Elikaformasyonu ve Jura yal Shemshak formasyonu arasnda yzeylemektedir. Yatak genel olarak drt farkl dzeyi (alttanste doru) takiben tanmlanan bir i katman yaps gstermektedir: (a) direkt olarak karbonat (Elika Formasyonu)tabann zerleyen, balca kil minerallerinden yapl yaklak olarak 5080 cm kalnlndaki alt arjilikli seviye, (b)balca hematit, kaolinit, anatas ve diyaspor minerallerinden meydana gelen ve yaklak olarak 23 m kalnlndaki birboksitik kil seviyesi, (c) diyaspor, kaolinit, anatas ve hematit minerallerinden yapl olan ve 5 m kalnlndaki krmzboksit seviyesi (yksek tenrl ana cevher), (d) balca kaolinitten yapl olan 2050 cm kalnlndaki st kaolinitseviyesi ve bu seviye Shemshak formasyonu tarafndan zerlenmektedir. Detayl petrografik almalar, boksitik

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Turkish Journal of Earth Sciences (Turkish J. Earth Sci.), Vol. 19, 2010, pp. 267284. Copyright TBTAKdoi:10.3906/yer-0806-15 First published online 24 July 2009


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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;Brdossy & Aleva 1990; Mordberg 1999; ztrket 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 (Brdossy & 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 (Brdossy 1982;Brdossy & Aleva op. cit.). The specific conditionsthat give rise to different paths of bauxite formationhave been documented previously (e.g., Brdossy & Aleva op. cit.). Lateritic-type deposits form uponaluminosilicate rocks viain-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; Brdossy & Aleva 1990; Priceet al .1997). Karst-type deposits occur within depressionsupon karstified or eroded surfaces that formed uponcarbonate rocks. Such deposits originate from a variety of different materials, depending upon thesource area (Brdossy 1982). Finally, Tikhvin-typedeposits are transported or allochthonous deposits

that overlie alumosilicate rocks and that originatefrom pre-existing residual laterite profiles (Brdossy 1982, p. 21; Brdossy & 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; Brdossy & Aleva 1990;Maksimovi & Pant 1991; MacLeanet al . 1997;Eliopoulos & Economou-Eliopoulos 2000;Mutakyahwaet al . 2003). In profiles of particulardeposits or bauxitic districts, most of these earlier

studies 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

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protolitin diyajenetik alterasyonunu gstermektedir. allm rneklerde gzlenen boksit dokular mikrogranler,oolitik, pizolitik, kolloform ve mikroklastiktir. Feldspat tanelerinin yansra, krlm ve anm kalnt kuvars taneleriile ilikili mikrogranler ve mikroklastik dokular, hemen hemen komple levhasal diyaspor tarafndan yeri alnmtr.Krmz boksitin jeokimyasal analizleri mobil elementlerin (Rb, K, Na, Sr, La, Mg, and Pb) tketildiini, daha az mobilelementlerin (Nb, Th, Zr, Mo, Ga, and Cr) ise zenginletiine iaret etmektedir. Zt bir sonu, boksitik kil iingzlenmektedir. st kaolinit seviyesi iin kondrite gre normalize edilmi NTE (Nadir Toprak Elementleri) rglerialtlayan krmz boksitlerinkilere benzerdir, ve alt arjilikli seviye iin gzlenen rgler, zerleyen arjilikli boksitseviyesindekine benzerdir. Ce, boksitik kilde bir negatif anomali ve krmz boksitte bir pozitif anomali gsterir. NTE ve dier elementler arasnda hesaplanm olan korelasyon katsaylar, NTE-ieren minerallerin Ti ve Nb, kil mineralleri, ve zirkonun oksitleri olduunu ortaya koymaktadr. Jajarm boksitlerinin var olan diyasporik mineralojik bileimininaksine, jeokimyasal ve mineralojik veriler, orijinal jipsitik bir bileime iaret eder. Sonu olarak, deiim diyagramlar ve NTE rgleri ile salanan kant ile gzlenen mineralojik ve dokusal ilikiler, Jajarm boksiti iin hem bazik hem desedimanter kayalardan oluan karm bir kkeni iaret etmektedir. Aslnda, boksitleme bazik kken kayalarzerinde balam ve karstik zellikler ierisinde yerleim ve yeniden ilenmesi sresince devam etmitir.

Anahtar Szckler:ran, karst boksit, jeokimya, diyaspor, cevher yata


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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 of the 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 of 43/14. 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 Setting The Jajarm bauxite deposit is situated in the easternpart of the Alborz structural zone (Figure 1). LowerDevonian sandstone, evaporites, and limestone of the 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 Mobarak formation (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 Brnnimannet al . (1973) this layer isequivalent of Sorkh Shale formation named by

Stcklinet 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 dark yellow 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 study area, the Elika formation is overlain by dark crystalline basic volcanic rocks. Palaeontologicalstudies in the eastern part of the Alborz zoneconfirm the absence of Upper Triassic marinesediments in this area (Stampfliet al . 1976; Seyed-Emamiet al . 2005) (Figure 2). The karst features thathost the Jajarm bauxite deposit formed within thick dolomites 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 Shemshak formation (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; Hooperet al . 1994, amongothers). Movement of the Afro-Arabian plate toward


270

U p p e r

T r i a s s i c

M i d d l e T r i a s s i c

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J u r a s s i c

carbonate basement(Elika formation)

lower argillaceous layer

bauxitic clay

red bauxite

upper kaolinite

shale and sandstone withcoal beds (Shemshak formation)

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


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Eurasia meant that central Iran and the Alborzstructural zone were located in the tropics at thistime (Berberian 1983). The dominance of shallow water 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 (Stampfliet 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 metres

of younger sediments, beginning with the JurassicShemshak formation and other younger units.

Mine Geology The Jajarm bauxite deposit is located in an areafolded into an EW-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 of the 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 5080 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 the

carbonate footwall) to pinkish and red (R 4/4) at itstop (close to the bauxitic clay layer).The bauxitic clay layer is about 23 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

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Shemshak m. f b a u x i te h o r iz o n

El ika m.f

Elika m.f

Shemshak m. f

k a r s t

i c f e a

t u r e s

b a u x i t e h o r i z o n e

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


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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 physically

harder than the others, and is mainly red in colour (R 5/8), 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), 2050

cm 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. 23 kgsamples were crushed and powdered. Major andtrace element concentrations were determined by inductively coupled plasma atomic emissionspectrometry (ICP-AES) and inductively coupledplasma-mass spectrometry (ICP-MS) at ActivationLaboratories, Ontario, Canada. The analyticalprocedure is described in Cottenet al . (1995).Relative standard deviations were 2% for major

elements and 5% for trace elements. The resultsof the analyses are provided in Table 1.Mineralogical analyses were carried out using a

Siemens D4 automatic diffractometer. Samples werescanned with a step size of 0.020 and a step time of 1s, using Cu K radiation from a Cu anode X-ray (1.5406 ).

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 as

a replacement and void-filling cement. In the lattercase, it occurs as coarse-grained crystals that locally show 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 of diaspore, iron oxides, or kaolinite. Many samplesthat contain older fragments of well-roundeddiaspore intraclasts and aggregates are now cemented by a matrix of fine-grained diaspore andiron oxyhydroxides (Figure 4b). This texturesuggests the transportation and re-deposition of bauxite, at least locally. The heterogeneousdistribution of iron oxyhydroxides is indicated by the variable colouring of many samples, ranging fromintense red to light brown.

The matrix-forming minerals are generally 120mm in size, although some diasporic minerals are100200 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 of detrital 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


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primary materials were colloidal; large particlesformed via secondary processes such asrecrystallization and reworking and erosion of complex 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 by dissolution in the bauxite samples are filled by minerals such as diaspore, goethite, hematite, anddolomite.

Geochemistry

32 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 other volatiles). We found considerable chemical variationboth among the four groups of samples and withinindividual groups (Table 1). In the bauxitic clay and

red 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 of bauxite deposits (Bronevoiet al . 1985), the Jajarmbauxite ore is enriched in the trace elements Ce (361080 ppm), Nb (93180 ppm), Bi (222 ppm), andTa (5.710.5 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 rock types.

When plotted against Al and Ti, theconcentrations of Zr, Nb, and Th produce similarly well-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 of diaspore-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).


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attributed to minor element mobility, weak source-rock heterogeneity, or the local winnowing of

lateritized 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 clay rather 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

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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 naturally in 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 relatively high 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 commonly accompanied 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 probably hosted by oxides of Ti and Nb (e.g., Gonzles Lpezet 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


l o g ( C

/ C

)

s a m p l e

C h o n d r i t e

l o g ( C

/ C

)

s a m p l e

C h o n d r i t e

l o g ( C

/ C

)

s a m p l e

C h o n d r i t e

l o g ( C

/ C

)

s a m p l e

C h o n d r i t e

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 TiNb (0.99) and TiREE (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 ProcessesDifferent 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 by terrigenous lateritic weathering crusts. Like many

other diasporic deposits (Nikitina 1986; Brdossy & 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 m 64 m

50 m 50 m

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(Biberet al . 1994).

As mentioned, leaching of mobile elements fromthe upper horizon (red bauxite) and depositionwithin the lower (bauxitic clay) is another epigeneticprocess (e.g., Valetonet 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, biological

and herbal activities decrease the pH of sedimentary environments: this reduces Fe3+ to Fe2+ and finally causes 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 of deferrification (Figure 7d).

Fractures and joints are the other importantepigenetic features of the deposit, and these aredivided into two main groups. The first group is

confined 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 of tectonic 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 Material Many 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., Brdossy 1982, 1984) and deposits of volcanic ash (Brdossy 1984; Lyew-Ayee 1986). In allthese models, Al is concentratedin situ as an inert(immobile) residue of lateritization.

In studies of other deposits, Pye (1988) andBrimhallet 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 show contrasting 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 of MacLean (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 the

karstic bauxite field with a basaltic source material(see Schroll & Sauer 1968). According to the Zr-Cr(Ni)-Ga ternary diagram of Balasubramaniamet al .(1987), our samples generally show a mixed origin of basic 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 of bauxite 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., Gonzlez Lpezet al . 2005).Chondrite-normalized REE patterns obtained for theJajarm bauxite resemble those of the composition of upper continental crust (UCC) (Figure 11) and shale


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0.1 101 100 10001

10

100

1000 u l t r a

b a s i

c

r o c k

skarstbauxite

basalt

low ironlateriticbauxite

high ironlateriticbauxite

sandstonegranite

carbonate rocks

syenite

Ni (ppm)

C r

( p p m )

s shale, late

red bauxitebauxitic clay

red bauxite bauxitic clay

Zr

GaCr(Ni)

acidic andmetamorphic

rocks

sedimentaryrocks

basic rocks

ultrabasic

20

4 0

60

80

10 0

20 4 0 6 0 80 100

P b

Y

5

10

15

20

2 4 6 8 10

Z r /

P b

C r /Ni

20

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 and

lateritic bauxites in different parent rocks (afterSchroll & Sauer 1968).

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

Figure 9. Zr-Cr(Ni)-Ga ternary diagram for bauxites derived

from different parent rocks (Balasubramaniamet 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 this

extensional 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 of igneous feldspar by platy diaspore and the well-rounded nature of reworked oolitic and pisoliticgrains.

Origin of Bauxite Layering Geochemical 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

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3

La Ce Pr Nd Sm Eu G d Tb Dy Ho Er T m Yb Lu

granite diabase red bauxite UCC

B0

1

2

3


red bauxi te UCC

l o g

( C

/ C

)

s a m p l e

C h o n d r i t e

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( C

/ C

)

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C h o n d r i t e

A0

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La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er T m Y b Lu

granite diabase red bauxite shale

B0

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red bauxi te shale

l o g ( C

/ C

)

s a m p l e

C h o n d r i t e

l o g

( C

/ C

)

s a m p l e

C h o n d r i t e

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 of red 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.


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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., Valetonet al . 1987; Maksimovi & Pant 1991;MacLeanet 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 of active drainage in the bauxitic clay and the nature of its 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 14 m of the red bauxite is completely replaced 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 toBrdossy & Aleva (1990, p. 181182) 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. 321324), and it generally involves ahigh degree of crystallization (Brdossy & Aleva1990). This high acidity resulted from thedevelopment of swamps upon the red bauxite during

Early 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 Akdoan. The Englishof the final text is edited by John A. Winchester.

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45 Petrography and Geochemistry of the Jajarm Bauxite Ore Deposit, Northeast Iran 19-2!5!0806-15

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