the bauxite of north-western sardinia

The bauxite of North-Western Sardinia

GIACOMO OGGIANO(*), PAOLA MAMELI(*)

Rendiconti Seminario Facoltà Scienze Università Cagliari Supplemento Vol. 71 Fasc. 2 (2001)

(*) University of Sassari, Istituto di Scienze Geologico-Mineralogiche, Corso Angioj 10 – 07100 Sassari.

Abstract. Bauxite outcrops of North-Western Sardinia are described here, taking intoaccount literature data and new SEM-EDS evidences on the development of the mineraltextures in relationship with the genesis. The Nurra bauxite deposits are divided intothree main types related both to the substratum nature and the Mid-Cretaceoustectonics. Textural analyses have highlighted the importance of iron-hydroxides in theformation of accretive structures, such as ooliths and pisoliths, as well as the role ofearly microfractures in the bleaching of the mineral. Latest fractures and faultsconversely favoured the circulation of silica rich solutions which caused the bohemiterisilicization, with consequent production of epigenetic kaolinite. At SEM observations,the neoformed kaolinite appears more crystalline than kaolinite occurring in thebohemitic clays at the base of the bauxite horizon. Two stops were able to illustrate themain features of the Nurra Bauxite: the first one at Olmedo mine where the different roleof fracturation is strikely evident in a bauxite which lies on a mainly marly foot-wall;the second one at Monte Murone where a strongly iron-rich oolitic bauxite developedon a dolomitic foot-wall.

Riassunto. Vengono descritti i giacimenti di bauxite della Sardegna nord-occidentalecompendiando i dati di letteratura esistenti e proponendo nuovi dati analitici al SEM-EDS inerenti lo sviluppo delle tessiture del minerale in funzione della sua genesi. Idepositi bauxitici della Nurra sono stati suddivisi in 3 differenti tipologie in funzione delsubstrato e del controllo che la tettonica – ovvero i differenti tassi di uplift dellapiattaforma carbonatica – ha giocato sul drenaggio e sulla rimobilizzazione deimateriali alteritici al di sopra della paleosuperficie mesocretacica. Le analisi tessituralihanno consentito di mettere in evidenza il ruolo degli idrossidi di ferro nella formazionedi strutture accretive come ooliti e pisoliti e il ruolo della microfratturazione precocenel bleaching del minerale, che in alcune zone si presenta praticamente bianco. Ledeformazioni fragili tardive, invece, favorendo la circolazione di fluidi ricchi in silice,hanno causato la risilicizzazione della bohemite, generando una caolinite epigenetica.Quest’ultima al SEM si manifesta in caratteristici booklets che permettono di distin-guerla morfologicamente dalla caolinite con più bassa cristallinità presente nellealteriti meno evolute che si trovano alla base del profilo bauxitico. Le differentilitofacies e gli effetti della fratturazione che ha controllato la circolazione dei fluidisono state evidenziate negli stops effettuati durante la visita nella miniera di Olmedodove la bauxite si sviluppa su un letto prevalentemente marnoso. In un secondo stop a

60 G. OGGIANO, P. MAMELI

Monte Murone, dove la bauxite poggia su un letto dolomitico, sono stati evidenziate leooliti a prevalente composizione goethitico-ematitica.

1. INTRODUCTION

During the Middle Cretaceous Western Sardinia carbonate platform experienced arelatively long period of emersion, due to an important tectonic instability. Uplift anderosion took place in a hot and wet climate that allowed ferrallitic alteration. In this waybauxite deposits, pertaining to the so called Mediterranean Karstic Type, have beengenerated.

Such deposits are widespread all over North-Western Sardinia (Nurra), where theyoccur showing a discrete number of typologies, among which at least one is peculiar ofthe region (Sardinia Type Deposit [1]).

Among the factors which control the typology of the deposits the Mid-Cretaceoustectonics is the most relevant, whereas their present extension and geometry, as well assome compositional features, are mostly controlled by Cainozoic events.

The Nurra Bauxites are essentially bohemitic in composition. When compared withother Mediterranean bauxites, they exhibit higher alumina and relatively lower ironcontent. The silica content is strictly related to the occurrence of kaolinite, and showssome vertical and horizontal variability, depending both on the foot-wall rock type andon late brittle failures, which affect the deposits exerting a strong control on silica richfluids circulation.

2. GEOLOGY OF THE NURRA BAUXITE

The Nurra represents a tertiary structural high, where older rock sequences areprogressively exposed westward. The Variscan metamorphic basement is well exposedin the westernmost sector near the coast. Onto the basement, grey Autunian arenites andsiltites, unconformably capped by Upper Permian-Triassic continental red beds andinterlayered alkaline volcanics, are found. The first transgressive deposits consist ofdolostones, fossiliferous limestones and evaporites of Middle Trias age with typicalGermanic facies.

Since this time, shallow marine sedimentation, in a carbonate platform environment,was almost continuous until Aptian-Albian time, before the emersion that gave rise to thebauxitic gap.

During Albian-Aptian time an important tectonic phase took place in Nurra (AustrianPhase [2]).

During the Coniacian all the Nurra bauxitic paleosurface was submerged, due to a newtransgression which led to carbonate-terrigenous sedimentation and lasted up to theMaastrichtian. The post Maastrichtian emersion is supposedly related to a new tectonicphase (Laramic phase [3]).

THE BAUXITE OF NORTH-WESTERN SARDINIA 61

Since the Paleocene, all the region experienced weathering, erosion, widespreadcalcalkaline volcanism and two important deformative events linked both to the Pirenaic[2] and to the North Apennine [4] orogeneses. These deformations generate minorthrusts and mild NE trending folds, which control the present geometry of the ore-bodies(fig.1).

Figure 1. Geological sketch map of Nurra (Ceccarini et alii [11] modified)


2.1. Stratigraphy

The nature of the pre-gap succession plays an important role for the bauxite genesis,as it contains some marly-clayey beds that can supply aluminium-rich material whichcould have acted as the mother rock of the ore [5].

Due to a mild angular unconformity, the bauxite deposits in Nurra rest on a foot-wallrepresented by rocks with age spanning from the Oxfordian up to the Lower Aptian,depending on the structural arrangement that took place during the Meso-Cretaceousemersion. The following main lithologic units can be roughly distinguished:

• a mainly dolomitic succession, consisting of coarse grained dolostones and whiteoolitic limestones, Kimmeridgian-Oxfordian;

• a calcareous-dolomitic succession, consisting of fine grained dolostones and whitemicritic limestones, Portlandian;

• a mainly marly succession, consisting of typical greenish marls with «Purbeckian»facies, Berriasian-Valanginian;

• dolomitic limestones, consisting of iron rich dolostones, and yellowish biospariticmore or less dolomitic limestones, Barremian;

• limestones consisting of a thick, roughly bedded, succession of white biomicriteswith «Urgonian» facies, Barremian-Lower Aptian;

• bauxitic horizon, ? Turonian;• calcareous conglomerate bearing reworked bauxite pebbles, Coniacian;• mainly biomicritic limestone bearing hippurites fragments and miliolidae,

Coniacian;• a thick succession consisting of yellowish sandy-marly, glauconitic calcarenites,

glauconitic breccias and grey marls, Coniacian;• nodular, white hippurite-bearing limestone, Santonian;• greenish marls and fine grained arenites, Campanian-Maastrichtian.Upon the Upper Cretaceous rocks, the following volcanic and sedimentary complexes

rest:• reddish, highly welded pyroclastic flows alternating mainly with ash-pumice fall

deposits, often altered into bentonite, Lower Miocene;• transgressive, bioclastic limestones and marls, Burdigalian-Serravallian.

2.2. Tectonics

The tectonic instability started in Middle-Cretaceous time with transtensive dynamic(Bedoulian movements) followed by transpressive regime [3]; it caused the bauxitic gapand an angular unconformity between the Lower Aptian and the Upper Turonian and/orLower Coniacian [6, 7]. These structures are probably connected to the divergent-convergent transverse motion between the European and Iberian plates in the pyreneandomain.

The effects of such tectonics result in some uplifted blocks bounded by normal faults,


as well as in mild folds within a sinistral wrench shear belt running between Uri and CapoCaccia.

Hence during the planation of the bauxitic surface, due to the erosive elision of, atleast, 600 meters of Mesozoic sequence, the older rocks were exposed north-westward(bauxite lying on Oxfordian dolostones at La Campana) whereas the younger ones wereexposed to ENE and to WSW (Lower Aptian at Uri and Capo Caccia respectively). Foldswith different length-waves are also detectable near Olmedo, where the bauxitic horizonand the Upper Cretaceous beds seal a wide NNW trending syncline bearing Valanginianmarls in the core.

The earlier evidences of tectonic movements subsequent to the bauxite horizon andits Coniacian cover consist of syn-tectonic breccias and olistostromes within UpperCretaceous sediments, close to an important fault. This caused the uplift of the Mesozoicplatform south of a line joining Uri and Alghero (Mamuntanas-Su Zumbaru Fault, [3]).This tectonic was tentatively ascribed to the Laramic phase. Other faults and folds, withNE axial strike, involve the whole Mesozoic sequence, but not the Lower Miocene ones;they could be referred to the Pyrenaic phase [2] as well as to the apenninic collision [4].Starting from Upper Burdigalian till Pliocene, an extensional regime took place, givingrise to normal faults with different directions. The structural depression where UpperCretaceous sediments are preserved – i.e. where the occurrence of bauxite is highlyprobable – are linked to the Cainozoic movements; particularly, a wide ENE trendingsyncline hosts the most important productive deposit (fig. 2).

3. ORE-BODIES: TYPOLOGY AND TECTONIC CONTROL

COMBES et alii [5] have distinguished three main types of deposits which encompass

Figure 2. Schematic view of the Olmedo deposit.


the types previously distinguished by OGGIANO et alii [3]:– Type 1 – Stratiform, autochthonous ore (Sardinia type sensu Combes [1]). It consists

in a deposit with wide lateral continuity characterised by an almost constant thickness (2-3 m). The bauxite derives from in situ ferrallitization at the expense of illitic Berriasian-Valanginian marls (Purbeckian facies), which led to a kaolinite-bohemite bauxite, withlow content of hematite and goethite. These iron phases are confined within irregularreddish patches; at places totally bleached ore is common. Kaolinite is confined in thelower part, and progressively diminishes toward the core of the horizon. A new increaseof kaolinite content is recorded toward the hanging-wall in the detrital bauxite, which lieson the erosional surface that caused the upper part truncation of the horizon. Arepresentative vertical distribution of silica content is shown in fig. 3.

– Type 2 – Deposits on irregularly connected karst pockets, developed on calcareousor dolomitic footwall of pre-Berriasian age. This type has been explained as a Type 1bauxite profile which sunk into the Purbeckian facies during alteration of marls, and wassuperimposed on Portlandian substratum through karst pockets, within which theweathering and evolution of the whole alterite continued. The thicknesses, revealed insome boreholes, range between 5 and 10 m. The genesis is consistent with parallochtony.

– Type 3 – These deposits are not common. They rest on post-Berriasian foot-wall i.e.on the Urgonian (Barremian-Lower Aptian) shelf limestone. The shape is generallyregular; the thickness ranges between 0 and 5 m. In consideration of the relatively youngerage of the foot-wall rocks, they must have developed on depressed areas. Such aconsideration, joined to the occurrence of bedded detrital bauxite, point to a genesis fromthe erosion of Type 1 and Type 2, followed by resedimentation of mature or sub-maturebauxitic material, which could have (parauthocthony) or not (allochthony) undergoneextension of weathering.

On the more or less dolomitic-limestones lying at the transition between the Purbeckianand Urgonian facies, the deposits show intermediate characteristics between Type 1 andType 3: thickness is more variable due to the karst morphology of the foot-wall, and thebauxite profile shows a strong maturity due to a better drainage. Clayey bauxite is almostabsent, and an evolute bauxite directly rests on the bedrock (fig. 4).

The link between the typology and the foot-wall lithology (chiefly the Purbeckianmarls) of the deposits can easily be established. Less obvious is the role that differentuplift rates played on their spatial distribution and on their genesis. The different upliftrates controlled: i) drainage, and therefore the starting of alteration; ii) motion anddistribution of the alteritic material over the palaeosurface.

The tectonic model of COMBES et alii [5] relates the Type 1 and Type 2 with highly tomoderately uplifted areas (regardless of the type of structure: thrust, horst, anticline),which delivered aluminous material to those occupying low structural sites (fig. 5).

In conclusion, the type and genesis of bauxite deposits are connected to the evolutionof a morphostructural paleosurface in a still tectonically unstable setting, which enhancedthe alteritic cover to move under strong weathering conditions and evolve to pure bauxites.


4. TEXTURE AND COMPOSITION

The Nurra Bauxites are essentially bohemitic; gibbsite and diaspore have beendetected as traces in a couple of cases. The silica is confined in kaolinite, even if illite andrare montmorillonite can occur at the base of the bauxitic horizon in the Type 1 deposits.Hematite and goethite show strongly variable contents. Generally the former is more

Figure 3. Silica profile on marly Purbeckian foot-wall (Ceccarini et alii [11], modified).

Figure 4. Silica profile on Barremian calcareous-dolomitic foot-wall (Ceccarini et alii [11],modified).

mm


abundant than the latter, which is often replaced by alumo-goethite.Titanium can reach 4-5%; it mainly occurs as neoformed anatase and minor detrital

rutile. Zircon is also common within detrital material; quartz and muscovite are practicallyabsent.

From a textural point of view, the Nurra bauxites show a high compaction degree dueto strong diagenesis; bauxite with a good pedogenetic evolution and weak diagenesis canoccur in Type 2 deposits. As a rule, the porosity does not exceed 10% and, on the basisof pore dimensions (voids dimension < 0.05 mm), it can be defined as a microporosity.The uncommon porous bauxite must be regarded as an epigenetic product.

Particularly in the Type 1 deposits, where the reworked bauxite is absent and only thetop of the in situ deposits have been removed by erosion, a lateritic profile that in turnevolved toward an almost complete bauxitization can be observed. From the hanging-wall to the foot-wall the following zones can be distinguished:

• a zone of strong eluviation dominated by bohemite, anatase and less iron oxides;

Figure 5. Tectonic control on parent rock alteration (after COMBES et alii [5], modified).


high field strength elements such as Ga, Cr, Hf, Nb etc. concentrate in this portion.Texture is oolitic, in an afanitic, generally bleached, matrix;

• an illuviation zone, where iron concentrates relatively to Al and Ti, and HFSE arestill present in high concentration. The textural organization is strongly oolitic-pisolitic,the inter-oolitic spaces are filled by pelitomorphic bohemitic material.

These two zones are characterized by good bauxite with low silica (i.e. low kaolinitecontent), high alumina, and variable iron contents. Normally Al

2O

3 ranges between 60

and 75%, SiO2 between 1 and 8%, iron oxides between 2 and 16%; TiO

2 normally is

around 4%. Bohemite is in the range of 75-90%, kaolinite spans between 2-8%,hematite+goethite 3-20%, rutile+anatase 4-5%.

• a zone of clayey bauxite where the alteritic processes clearly proceed at the expenseof the marly bedrock. The upper part of this zone grades into the previous illuvial one, itshows hematite and bohemite ooids in a still kaolinite rich matrix (fig. 6). Iron oxides areabundant, HFSE content is lowered. The lower part of this zone consists of an alteritic claywith dominant kaolinite and variable amounts of illite. The good cationic exchangecapacity of these minerals led to a concentration of LFSE such as Pb, Ba, Li, Zn, Ce aswell as some REE.

Chemical and mineralogical representative data of these upper and lower parts are

Figure 6. A: BSE image of ooliticbauxite evolved on Purbeckian marls(lower part of the horizon). B: Silicamap showing a silica (kaolinite) richmatrix supporting bohemite-hematiteoolites.

A

B


respectively in table 1 and 2.Around Olmedo mine, a large sector (35%) of the deposit is rather bleached due to iron

leaching. At places leaching can give rise to a completely bleached white bauxite with arenewed, weak oolitic, homogeneous texture. In this strongly bleached ore, iron oxides(mostly alumo-goethite) span in the range of 1-3% and Al

2O

3 is higher than 75%. Such

a strong bleaching might have occurred before the hardening and compacting of the orein order to allow the flow of the leaching solutions.

5. GENESIS AND EVOLUTION OF THE ORES

As regards the genesis of karst bauxite, according to BARDOSSY [8], on the bases ofactualistic considerations concerning the West Pacific Islands, the dominant climaticconditions favourable to the bauxite formation must have been similar to those ofmonsonic type with mean annual temperature of 23°C and rainfalls close to 2,000 mm.Beside a favourable climate, good drainage and appropriate geochemical and physico-chemical conditions are needed.

Field and analytical evidences point to an origin of Nurra Bauxite from clayey, mainlykaolinitic, aluminous material. pH conditions and carbon dioxide partial pressurefavourable to kaolinite dissolution must have occurred in the groundwater.

In a CaCO3 dominated environment, with CO

2 free water, relatively high alkalinity

can be achieved which leads to aluminium hydroxides and amorphous silica formationat the expense of kaolinite [8].

Conversely, weathering in an acidic buffered environment cannot lower the Si/Al

Table 1. Clayey bauxite: chemical and mineralogical composition of the upper part.

Al2O

3SiO

2Fe

2O

3TiO

2CaO L.O.I.

38% 20% 26% 2% 3% 11%

Kaolinite Bohemite Hematite-Goethite Anatase+Rutile Calcite

43% 25% 25% 2% 5%

Table 2. Clayey bauxite: chemical and mineralogical composition of the lower part.

Al2O

3SiO

2Fe

2O

3TiO

2K

2O CaO L.O.I.

33% 40% 11% 2% 2% 1% 11%

Kaolinite Illite Hematite Goethite Bohemite Anatase+Rutile Calcite

65% 18% 5% 6% 2% 2% 2%


Figure 7. A: Oolith in clayeybauxite. B: Iron map. C:Aluminium map. BSEimages.

A

B

C


ratio, as in such conditions alumina undergoes stronger leaching compared with silicawhich can precipitate from the groundwater.

Also oxidizing conditions are favourable for bauxitization of clayey material, even ifsome grey pyrite bearing bauxite crop near the Olmedo deposit. This bauxite isaccompanied by thin coal layers and probably evolved in a vegetation rich, swampyenvironment. The effect of humic acids (buffering) on pH caused the stability of highamount of kaolinite which in this bauxite generally exceeds bohemite.

The main steps through which the ore with the present compositional featuresdeveloped can be summarized as follow:

• accumulation of alterites, mainly from marly rocks, mobilization of iron that leadsto the appearance of first nodules in a mainly kaolinitic matrix;

• leaching of silica, followed by remobilization of iron which starts to organize intoooids and/or pisooids mainly composed by hydroxides. Depending on the drainage rate

Figure 8. A: Microfracturesin high alumina bauxite. B:Iron map. BSE images.

A

B


also hydroxides of aluminium that coat iron nodules (fig. 7) can form in this stage. Theporosity of such alterite is high. The coherence is scarce, particularly if large amount ofooliths are present;

• general mobilization of iron and aluminium. The eluviation of the alteritic profile,according to LAVILLE [9] due to both chemical and mechanical processes, tends to erasethe coated textures in the upper part, whereas the middle and lower parts are characterizedby illuvial features such as inter-oolitic voids infilled by micro-ooliths and very fineparticles. The process results in porosity diminishing and hardening of bauxite, so that,due to brittle behaviour, microfractures can develop and enhance the iron evacuation(fig. 8);

• burial and further hardening of bauxite accompanied by definitive conversion ofthe eventual gibbsite into bohemite. Due to the Cainozoic deformations the depositsexperienced severe jointing and faulting;

• during the Early Miocene, in concomitance with the calcalkaline volcanic activity,low thermal silica rich solutions caused the silicification of bohemite and neoformationof kaolinite close to the fractures that enhanced their circulation (fig. 9).

Figure 9. A: Open fracture oncompact bauxite. B: Si map;notice the Si concentration closeto the fracture. BSE images.

A

B


Figure 10. A: Large, thickstacks of kaolinite crystalgrown on the walls fractureof fig. 9. B: Tiny crystalsand aggregates of kaolinitewithin clayey bauxite. SEimages.

Kaolinite from bohemite can easily form when the latter phase reacts with watercontaining silica higher than 2.5 x 10–5 mole/litre [10].

Kaolinite generates spontaneously according to the following reaction

Al2O

3 · H

2O + 2H

4SiO

4 = H

4Al

2Si

2O

9 + 3H

2O

The neoformed kaolinite differs from the primary one because of its higher crystallinity.The Hinckley index of the kaolinite of the bohemitic clays at the base of the bauxitehorizon falls around 0.8, whereas neoformed kaolinite, which infills fractures or pores inepigenetically altered bauxite, ranges between 1.15 and 1.25. A striking evidence of thedifferent crystallinity results from the SEM images (fig. 10).

Acknowledgement

Thanks are due to Mr. Aldo Orrù and to the Property of the Sardabauxiti for their


technical assistance during sampling at Grascioleddu mine.SEM and EDS analyses were carried out at CME of University of Sassari.

REFERENCES

[1] P.J. COMBES, Typologie, cadre geodynamiques et genese des bauxites francaises. GeodinamicaActa 4, 2 : 91 (1990).

[2] A. CHERCHI & P. TREMOLIERES, Nouvelles donnèes sur l’evolution structurale au Mesozoiqueet Cènozoique de la Sardaigne et leurs implications géodynamiques dans le cadremediterranéen. C.R. Acad. Sc. Paris 289, 889 (1984).

[3] G. OGGIANO, G. SANNA & I. TEMUSSI, Caractères geologiques, gitologiques et geochemiquesde la bauxite de la Nurra. Groupe Francais du Crétacé. Livret-Guide exc. Mai 1987 enSardaigne, 72 (1987).

[4] L. CARMIGNANI, F.A. DECANDIA, L. DISPERATI, P. FANTOZZI, D. LIOTTA, A. LAZZAROTTO & G.OGGIANO, Relationship between the Sardinia-Corsica-Provencal domain and the NorthernAppennines. Terra Nova 7, 128 (1995).

[5] P.J. COMBES, G. OGGIANO & I. TEMUSSI, Geodynamique des bauxites sardes, typologie, geneseet controle paleotectonique. C.R. Acad. Sc. Paris 316, Série II: 403 (1993).

[6] J. PHILIP & J. ALLEMANN, Comparaison entre les plates-formes du Crétacé Supérieuer deProvence et de Sardaigne. Cret. Res. 3, 35 (1982).

[7] R. FILIGHEDDU & G. OGGIANO, Contributo alla stratigrafia delle bauxiti del Cretaceo dellaNurra mediante lo studio di un livello pollinico. Atti Soc. Tosc. Sc. Nat. 91, Serie A: 1 (1984).

[8] G. BARDOSSY, Karst Bauxites (Bauxite deposits on carbonate rocks). Elsevier, Amsterdam(1982).

[9] P. LAVILLE, Etude de textures de la formation bauxitique du gisement d’Olmedo. Sardaigne.Rapport confidentiel du B.R.G.M. 83 SGN 401 GEO (1983).

[10] R.M. GARRELS & C.L. CHRIST, Solutions, minerals and equilibria. Ed. Harper & Row, NewYork (1965).

[11] C. CECCARINI, G. OGGIANO & I. SALVADORI, Nurra (Sardinia, Italy) bauxite deposit. Bauxite,Ed. Leonard Jacob Jr., 525 (1984).

the bauxite of north-western sardinia

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