1 introduction to the geology and metallogeny of sardinia

Geology and Metallogeny of Sardinia

Excursion GuideSEG Student Chapter Geneva

11th - 18th of June 2011

Editors: Johannes Mederer and Cyril Chelle-Michou

Contents1 Introduction to the Geology and Metallogeny of Sardinia 8

1.1 Geology of Sardinia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.1.1 Paleogeographic evolution . . . . . . . . . . . . . . . . . . . . . . . 81.1.2 Geological overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.2 Metallogeny of Sardinia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191.2.1 Metallogenic periods . . . . . . . . . . . . . . . . . . . . . . . . . . 191.2.2 Mining in Sardinia: Historic overview . . . . . . . . . . . . . . . . . 25

2 Pre-Hercynian Stratiform Pb-Zn-Ba Mineralization in the Iglesiente-Sulcis Area 272.1 Description of mineralization and host rocks . . . . . . . . . . . . . . . . . 29

2.1.1 SEDEX type mineralization within the Lower Cambrian Punta Mannaand Santa Barbara formations . . . . . . . . . . . . . . . . . . . . . 29

2.1.2 MVT type mineralization within the lower Cambrian San Giovanniformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

2.1.3 MVT type mineralization at the Ordovician unconformity . . . . . 332.2 Isotopic analyses and their interpretation . . . . . . . . . . . . . . . . . . . 33

3 Supergene Carbonate-Hosted Nonsulfide Zinc Mineralization: The “Calamine”of Southwest Sardinia 373.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 373.2 Supergene nonsulfide zinc deposits . . . . . . . . . . . . . . . . . . . . . . 373.3 Supergene carbonate-hosted nonsulfide zinc deposits in the Iglesiente district

of SW Sardinia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 393.4 “Calamine” occurrences to be visited during our excursion . . . . . . . . . 40

3.4.1 The Nebida mining area. Old exploitation at Canale San Giuseppe 403.4.2 The “Calamine” at Miniera di Monteponi . . . . . . . . . . . . . . . 40

4 Albitite Deposits of Central Sardinia: an Overview 424.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424.2 The albitite deposits of central Sardinia . . . . . . . . . . . . . . . . . . . . 424.3 Magmatic versus metasomatic origin of albitites: a discussion . . . . . . . . 45

5 The epithermal deposits of Osilo and Tresnuraghes and epithermal pro-cesses in generale 475.1 Introduction: epithermal and other hydrothermal deposits of Sardinia . . . 475.2 Epithermal systems in general . . . . . . . . . . . . . . . . . . . . . . . . . 485.3 Kaolinite rich epithermal deposits of Sardinia . . . . . . . . . . . . . . . . 50

6 Bauxite Formation in Sardinia 586.1 General introduction: the formation of bauxite deposits . . . . . . . . . . . 58

6.1.1 Principles of Chemical Weathering . . . . . . . . . . . . . . . . . . 58

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6.1.2 Dissolution and hydration . . . . . . . . . . . . . . . . . . . . . . . 586.1.3 Lateritic Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6.2 The Olmedo Bauxite Deposits . . . . . . . . . . . . . . . . . . . . . . . . . 61

7 High Grade Metamorphic Complex in Sardinia 677.1 Intensity and age of metamorphism . . . . . . . . . . . . . . . . . . . . . . 687.2 Migmatization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 687.3 Field stops during the excursion . . . . . . . . . . . . . . . . . . . . . . . . 69

References 74

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IntroductionMining activity in Sardinia goes back to prehistoric times. Even if most of the small mineswhich were operating in Sardinia (Chapter 1) are now closed, the island still remains ofhigh interest from a metallogenic point of view. During our one-week excursion, we will beable to visit only some of the highlights of geological interest. Table 1 shows our scheduledprogram and Figure 1 an overview map of Sardinia with different field stops. The followingChapters are written by the excursion participants. They give an introduction to the geol-ogy and metallogeny of Sardinia, and cover the different ore deposits and geological pointsof interest we will visit and reflect the choice we necessarily had to make. We start withthe southwestern Iglesiente-Sulcis district where the relationship between Early Paleozoicstratiform base metal mineralization (Chapter 2) and later supergene zinc mineralization(Chapter 3) can be observed. Epithermal and porphyry-type mineralization can be foundon the island and is related to widespread Oligocene-Miocene magmatic activity (Chapter5). We will visit an active mine that nowadays produces high-quality feldspar concentratesfrom albitites with metasomatic origin (Chapter 4). In the north-west of the island, theOlmedo bauxite deposits (still in operation, Chapter 6) are related to extreme weatheringconditions during Middle Cretaceous times. On our way back from the northern point ofthe island to Cagliari, we will visit high-grade metamorphic rocks including migmatitesand eclogite facies rocks of the Hercynian orogenesis (Chapter 7).

We want to thank Thomas Driesner and Pierre Vonlanthen who provided us with fieldguides from former excursions (Matthai 2000 and Vonlanthen et al. 2005) whichwere invaluable for the planning and the organization of this trip. Our thanks also go tothe SEG Regional Vicepresident Europe Maria Boni and to Lluís Fontboté (University ofGeneva), who provided financial support and kept this excursion affordable for us students.Thanks to Honza Catchpole for the english correction of the manuscript.

Finally, special thanks go to Maria Boni, Marcella Palomba and Giacomo Oggiano,who agreed to be our guides in Sardinia. Without them, this excursion would not havebeen possible.

Geneva, June 2011 - Cyril Chelle-Michou and Johannes Mederer

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Figure 1: Road map of Sardinia with some points of interest and overnight stops during the excursion. North-Southextension of the island is about 250 km. Source: http://maps.google.com

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Participants from University of Geneva

Dr. Honza Catchpole UNIGE [email protected] Chelle-Michou UNIGE [email protected] Hémon UNIGE [email protected] M. Tomé UNIGE + IGME Spain [email protected] Mederer UNIGE [email protected] Ortelli UNIGE [email protected]é Pérez UNIGE [email protected]

Field guides from Sardinia and Italy

Prof. Maria Boni University of Napoli [email protected]. Marcella Palomba University of Cagliari [email protected]. Giacomo Oggiano University of Sassari [email protected]

Table 2: Participants and field guides

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1 Introduction to the Geology and Metallogeny ofSardinia

by Cyril Chelle-Michou

1.1 Geology of Sardinia1.1.1 Paleogeographic evolution

Translated and modified from Vonlanthen et al. (2005)

Figure 5 summarizes the major events relevant to the paleogeographic evolution ofSardinia.

Palaeozoic

From Cambrian times until the beginning of the Ordovician, the old Sardinian basementwas split into two terrains located at the northern border of the Gondwana supercontinent(Figure 2). This period is characterized by important accumulation of terrigenous andcarbonaceous sediments at the rim of this passive margin.In the Middle Ordovician, a subduction zone appeared at the northern Gondwana margin,causing the development of an Andean type volcanic cordillera.Slab rollback and back-arc rifting during the Silurian causes the Hun superterrain to sep-arate from the northern Gondwana continental crust, and to drift northwards.During the Devonian, migration of the Hun superterrain led to the closure of the RheicOcean and the opening of Paleotethys. Sardinian terrains are characterized by pelagicsedimentation at this time.Collision of Hun and Laurasia started at the beginning of the Carboniferous marking thebeginning Hercynian orogeny. This event mostly affected Central Europe and led to forma-tion of the Armorican and Saxo-Thuringian massifs. Oblique subduction of the mid-oceanicPaleotethys ridge affected the Hun superterrain with a major transform fault system, lead-ing to the tectonic juxtaposition of the two pieces of Sardinia.A second collision of Gondwana and Hun closed the western Paleotethys. The Precambrianbasement and the volcano-sedimentary rocks that were accumulated during the Paleozoicwere deformed, metamorphosed and thrusted. Two phases are distinguished:

1. an early HP metamorphism episode directly related to deep burying of subductedterrains

2. the main Barrovian episode is characterized by weak to amphibolite facies P-T con-ditions developed during continental collision and orogenesis.

The end of the Hercynian episode is marked by a period of orogenic collapse, andextensional regime. Adiabatic decompression of rock provoked HT-LP metamorphism.

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Associated plutonic rocks were emplaced at this period (until the Permian) and form theHercynian Batholith.

Mesozoic

In the Trias, Sardinia was part of the Laurussian continent and was situated close to Spainand the Balearic Islands but away from Corsica (Fig. 3).In Jurassic time a transform fault system located in the future Pyrenees placed Sardiniaand Corsica next to each other.In the Cretaceous, the Iberian plate (also containing the Corsican-Sardinian bloc) breaksaway from Laurussia. Sardinia constituted the western extension of the BriançonnaisDomain (Fig. 3).Important sedimentation occurred during the Mesozoic; first during the Trias, dominatedby continental evaporite deposition, and later during the Jurassic and Cretaceous, bymostly marine sediments.

Cenozoic

During the Oligocene, the Briançonnais Domain was affected by the alpine orogeny whilethe Corsica-Sardinia bloc broke away from the Iberian plate and rotated counter-clockwiseto reach its present position. This caused the opening of the Liguro-Provencal back-arcbasin and slab-rollback and subduction along the Franco-Spanish margin (Fig. 4). Thissubduction underneath Sardinia was associated with a volcanic episode lasting until Mid-Miocene (28 to 15 Ma).This back-arc extension event was also responsible for a major tectonic restructuring ofthe Mediterranean region leading to the detachment of the Rif-, Betic and Kabyle terrainsas well as the Balearic Islands.By the end of the Oligocene, migration of the Corsica-Sardinia block was stopped bythe Adriatic continental plate. The extensional regime moved eastward from the Liguro-Provencal Basin to what became the Tyrrhenian Basin. Calabria moved to its currentposition at this time.A new volcanic cycle during the Plio-Pleistocene is attributed to this newly establishedextensive regime caused by the opening of the Tyrrhenian Basin. Volcanism associated tothe subduction moved further South to the Aeolian Islands (Etna).Sedimentation during the Cenozoic was mainly coastal to continental, intercalated withshort marine cycles.

1.1.2 Geological overview

Translated from Vonlanthen et al. (2005) and copied and modified from Marcelloet al. (2004)

In Sardinia the sedimentary, magmatic and metamorphic records are well representedwithin three main lithological complexes:

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Figure 2: Paleogeographic reconstruction from Cambrian to Carboniferous times (Stampfli and Borel). The red circlesshow the position of Sardinian terrains

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Figure 3: Paleogeographic reconstruction from Jurassic to Cretaceous times (Stampfli and Borel 2004). Red circles showto position of Sardinian terranes

1. a mainly Paleozoic basement that underwent repeated phases of deformation andmetamorphism during the Caledonian and Hercynian orogenic cycles, and was even-tually intruded extensively by calc-alkaline granitoids

2. a Late Palaeozoic epicontinental sequence and a Mesozoic carbonate platform se-quence, representative of stable shelves, that formed the passive margin of SouthernEurope

3. a Cenozoic to Quaternary volcanic and sedimentary cover consisting of shallow-watermarine carbonates, siliciclastic sediments, continental conglomerates, as well as vol-canic rocks represented by a calc-alkaline suite and alkaline basalts.

Pre-Hercynian Basement The basement of Sardinia is a segment of the South Variscanchain, which after the Cenozoic drifting of the island shows a NW-SE trend and crops outwith good continuity. From south to north its structural framework includes the followingthree different zones:

1. a thrust-and-fold belt foreland consisting of a sedimentary successions ranging in agefrom Upper Vendian to Lower Carboniferous, which crops out in the SW of the island(External Zone)

2. a SW-verging nappe complex that equilibrated under greenschist facies conditionsand occupies the centre of the island. It consists of a Paleozoic metasedimentarysuccession hosting a thick continental arc-related volcanic suite (Nappe Zone)

3. an inner zone that includes a high-grade metamorphic complex juxtaposed to amedium-grade metamorphic complex along a mylonitic belt (Internal Zone), locatedin the north-east of Sardinia

The stratigraphic lowermost sequence in SW Sardinia (Iglesiente- Sulcis region), isprobably of Pre-Cambrian age and consists of mainly terrigenous metasediments, These

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Figure 4: Evolution of the Corsica-Sardinia block during the Cenozoic and the reorganization of the Mediterranean puzzle(Faccenna et al. 2001, Jolivet et al. 2003)

consist of feldspathic metasandstones, quartzites, metaconglomerates, and thin dolomiticintercalations that grade upwards into shales, metasiltites, and metasandstones (BithiaFormation, Fig. 6).The overlying Nebida Formation, the oldest fossiliferous terrain, mostly consists of terrige-nous metasediments with minor oolitic limestones containing Early Cambrian Archaeo-

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Figure 5: Geological evolution synthesis of Sardinia.

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cyathas, Trilobites and algal stromatolites. It is believed to represent a continental shelfenvironment with an eastwards prograding deltaic systems (Matoppa Member) that evolvedinto an oolitic lagoonal environment. This terrigenous formation grades upwards into athick carbonate sequence consisting of dolostones and limestones (the Gonnesa Formation),which represent an arid tidal flat system.

Figure 6: Paleozoic sedimentary succession of the External Zone outcropping at Sulcis (South) and Iglesiente (North).Carmignani (2001)

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The “drowning” of this carbonate platform marks the beginning of the Cabitza Forma-tion, and is recorded by the occurrence of nodular limestones (“Calcescisti” Auctt.), thatis rich in Middle Cambrian trilobites, echinoderms, and brachiopods (Fig. 6).The overlying deeper-environment member consists of a 400-m thick neritic terrigenoussuccession in which the youngest levels contain achritarcs and graptolites of Tremadocianage (“Argilloscisti di Cabitza”). These lagoonal and epicontinental carbonate and terrige-nous deposits correspond to thick siliciclastic sequences in the Nappe Zone (San Vito andSolanas Formations).

All over central and SE Sardinia, Middle Cambrian-Early Ordovician metasedimentarysequences are overlain by an Arenigian-Caradocian metavolcanic complex, which includesseveral effusive episodes, with abundant pyroclastic flows and intrusive events. The mag-matic products include a complete sub-alkaline suite ranging in composition from basaltic-andesitic to rhyolitic, with acid terms being more abundant than the intermediate andbasic ones. These features are typical of an orogenic suite involving continental crust.Extensive evidence supports the hypothesis of a magmatic arc connected to a subductionof the oceanic lithosphere under the northern Gondwana margin. The arc-trench gap wasincorporated in the Internal Nappes (Mount Gennargentu, Baronie region).The back-arc basin in the Iglesiente-Sulcis region is devoid of calc-alkaline magmatism andunderwent an Early Hercynian compressional event. This post-Tremadoc and pre-Caradocphase of deformation, which is found in many parts of Europe, is very evident here, espe-cially in the Iglesiente region, where the Cambrian-Lower Ordovician sequences were foldedbefore the Caradocian (“Sardinian Unconformity”). The products of the subsequent ero-sion reach up to several hundreds of meters of thickness (“Puddinga” Auctt.). This angularunconformity has also been reported in south-eastern (Sarrabus-Gerrei region) and centralSardinia, where the Cambrian-Lower Ordovician successions are often separated from theMiddle- to Upper-Ordovician volcanics and sediments by conglomerates that are mainlyderived from the volcanic arc.Both the “Puddinga” continental clastics and the Middle-Ordovician metavolcanics of cen-tral and south-eastern Sardinia are covered by terrigenous continental to littoral sedimentsthat show a large variability in thickness and facies (“Caradocian Transgression”) and areinterbedded with alkalibasaltic metavolcanics. The transgressive Late Ordovician depositsgrade upwards to neritic clay and carbonate deposits (“Ashgillian Limestones”) followedby uniform deposits of Silurian black shales and cherts.The magmatic quiescence, combined with the sedimentary evolution to a pelagic-type de-position indicates that, at least in this time span, the Gondwanan continental edge behavedas a subsiding passive continental margin. On this margin Silurian black shales, which oc-cur everywhere, grade upwards into Lower- and Middle-Devonian pelagic marly shales andnodular Tentaculite-bearing limestones, and then Lower Carboniferous (Tournaisian) thickpelagic limestones, locally replaced, in a few internal areas (Nurra, Baronie) by thick ter-rigenous sequences. In the outermost platforms the carbonate sedimentation was suddenlyinterrupted and changed to Culm-type syn-orogenic deposits.

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Hercynian terrains The above structural framework and the related stratigraphic evo-lution mainly show the effects of the Hercynian orogeny, superimposed upon those of theCaledonian, which occur in the oldest terrains.The main tectonic phases of the Hercynian orogeny occur during the stacking of the Gond-wanan continental margin, and the gravitational collapse of the collisional orogenic wedge.The Hercynian collisional event is well preserved in the Sardinian basement. The over-thrusting continental margin is represented by the “High Grade Metamorphic Complex”(HGMC - Internal Zone) of northern Sardinia and Corsica; the underthrust continentalmargin is represented by the Internal (Nappe Zone) and External (External Zone) NappeComplexes of central and southern Sardinia (Fig. 7). The two domains are separated bythe “Posada-Asinara Line” suture zone.

Figure 7: Simplified geological map of Hercynian terrains (Carosi et al. 2006)

The External Zone Complex, cropping out in the Iglesiente-Sulcis region, is a classicfold-thrust belt characterized by medium- to steeply-dipping thrusts, fold axial planes andcleavages, and very low-grade Hercynian metamorphism. For the late stages of the Hercy-nian collision or early uplift, an age of 344 Ma has been proposed.

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An important extensional event also developed in the Sardinian Variscides as a responseto gravitational re-equilibration within the collisional structure. The extensional evolutionis confined to a time interval extending from the end of the collision to the emplacement ofthe widespread calc-alkaline plutonism (307-275 Ma) of the Sardinian-Corsican batholith,and to the development of the largely coeval Stephanian-Autunian basins.After the end of the Paleozoic and up to the Messinian, the Sardinian-Corsican Massif wasaffected by a number of movements that account both for its present position and for theevolution of the western Mediterranean.During the Late Paleozoic, Sardinia and Corsica were involved not only in the last Hercy-nian collapses, but also in horizontal translations along the north Pyrenean transcurrentfault. A long continental period began, and in Sardinia it was characterized by structuralhighs and lows; a wide peneplanation was reached.

Figure 8: Stratigraphic relationships between Jurassic formations of eastern Sardinia (Dieni and Massari 1985)

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Mesozoic rocks The western margin of the massif (together with its adjacent north-western areas) was bordered by wide platforms and shallow, narrow intracratonic basinseastwards. In these basins Germanic Facies sediments were being deposited (Genna SeloleFormation). The eastern margin of the massif represented the western border of the TriassicAlpine Basin.A transgression started in the Triassic, and during the Jurassic gradually extended towardsthe eastern part of the massif. On this platform a Provençal Facies carbonaceous complexwas deposited (Dorgali, Monte Tului, S’Adde and Monte Bardia Formations, Fig. 8).Confined pelagic episodes also attest to syn-sedimentary extensive tectonics, as an earlysign of the opening of the Ligurian-Piedmontese oceanic basin.The tearing of the sialic crust during the Upper Jurassic separated the Sardinian-CorsicanMassif from the Tuscan one. An oceanic basin opened with simatic crust (ophiolites) andpelagic sediments, which evolved to flysch in the Upper Cretaceous.The Sardinian-Corsican massif has continued to evolve together with the stable side ofthe European plate up to present, while eastward, the Tuscan basement followed the newevolution phenomena of the Apennine continental margin. This is consistent with theCretaceous carbonaceous platform sediments occurring in Sardinia.

Cenozoic rocks By the end of the Cretaceous, the closure of the Ligurian-Piedmontesebasin commenced and the African continent again approached the European. The collisionoccurred during the Middle Eocene. However, clear evidence of compressive phenomenarelated to this event is lacking in Sardinia.Two episode of Cenozoic volcanism are recorded in Sardinia. The Oligocene-Miocene vol-canism (28-15 Ma, Fig. 9), directly linked to subduction of calco-alkaline affinity is com-posed of mostly rhyolitic and rhyodacitic ignimbrites with additional andesites and basalts.The Plio-Pleistocene volcanism (5-0.1 Ma) has no direct link with the subduction and ismostly composed of tholeiitic to alkaline basalts. Rhyolite and rhyodacite are scarce inthis second phase.During the Miocene some Sardinian structural lows (“Sardinian Rift”) were covered byepicontinental seas, others formed lagoonal or lacustrine basins. This sea separated twoemerged areas, south-west and north-east relative to the rift. This transgression is relatedto the opening of the Algerian-Provençal basin, and is characterized by mainly terrigenousdeposition, whose products cover a N-S belt all along the island.In the Upper Miocene almost the whole of Sardinia emerged again, leaving only a few,marginal lagoonal areas. In the Quaternary sea level variations related to glacial cycles areattested by Tyrrhenian fossiliferous sediments (“Panchina”). The filling of the lagoonalbasins continued, and today a few lagoons still exist, mainly at the extremities of theCampidano plain. The alkaline volcanic activity also ceased in the Pleistocene, leavingcinder cones and basalt flows.

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Figure 9: Simplified geological map of the post-Hercynian cover (Carmignani 2001)

1.2 Metallogeny of SardiniaIn spite of a very small surface (24000 km2), Sardinia form a mineral resources perspective,is one of the richest and most varied regions of Italy and probably of Europe. Its resourceshave been exploited since Prehistoric time until now.From an economic point of view Pb-Zn (±Ag±Cd) and epithermal Au deposits are themost important ones in Sardinia. Some epithermal Cu (±Au) is related to Oligocene-Miocene volcanism. Cretaceous bauxite deposits occur in the Mesozoic platform. Barite,fluorite, and magnetite polymetallic skarns are associated with Hercynian granite intru-sions. Kaolin, bentonite, fireclay and albitite deposits have also been exploited. Figure 10syntheses the Sardinian metallogeny as a function of time.

1.2.1 Metallogenic periods

Copied and modified from Marcello et al. (2004)

Sardinian geology is believed to include seven metallogenic periods that occurred dur-ing the geological evolution of the island. At places these periods interacted, so that the

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present shape, grade, and composition of many Sardinian deposits is the result of subse-quent reworking of deposits that were originally not always economic.

The first metallogenic period affected the Cambrian complex of SW Sardinia. Ityielded different ore-mineral accumulations according to the variable paleogeographic con-ditions at times of sedimentation and diagenesis of the Cambrian Middle (i.e. the GonnesaFm; a carbonate platform some hundred meters thick, also called “Metalliferous Lime-stone”), and the transition zones both to the underlying sandy-silty formations and to theoverlying silty-shaly formations. From the lower to the upper section of the sequence, thesedeposits include:

1. residual Fe-oxide; scanty accumulations

2. BaSO4 evaporitic bodies, at places of some economic interest

3. important volcano-sedimentary massive accumulations of FeS2-ZnS with occasionalPbS

4. synsedimentary (possibly volcano-sedimentary) low grade stratabound depositions ofdisseminated ZnS-FeS2 with occasional PbS (the so-called “Blendous” Limestone)

5. deposits of PbS-ZnS with minor amounts of FeS2

The entire Cambrian carbonate member displays a positive geochemical anomaly forBa (local values also exceed 1000 ppm), Pb and Zn (20-100 ppm). The deposis describedin (4) and (5) were often sub-economic, but several important ore-bodies were generatedby mobilizing these protores.

The second metallogenic period is related to the Early Hercynian (Sardinian Phase)folding that took place in the Early Ordovician. Erosion and leaching of the emerged“Metalliferous Limestone” produced local economic deposits of PbS, PbS + ZnS, verypure Fe2O3, BaSO4, andCaF2 as karstic accumulations, and BaSO4 pebbles in the basalconglomerate of the overlying Arenig.

The third metallogenic period took place from the Middle Ordovician to the EarlyDevonian and produced four different types of deposits. Many of these have a syn-sedimentary character; others may be regarded as volcano-sedimentary or decidedly vol-canogenic, and may be grouped as follows:

1. a number of generally small, high-grade, mixed sulfide lenses, are scattered in theSilurian black shales of the entire island; they show evident sedimentary features(load cast, slumping, diagenetic fracturing). These deposits are quite similar to theMeggen and Rammelsberg deposits in central Germany, although of far smaller size(not exceeding a few hundred thousand tons);

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2. stratabound antimonite and scheelite concentrations, often with interesting gold con-tents, are hosted in a volcano-sedimentary sequence outcropping in the SE corner ofthe island (Sarcidano, Gerrei, and Sarrabus Regions). A connection with coeval vol-canics seems much more evident in this case. The ore-bearing formation could beconsidered an extension of the well known Middle Palaeozoic horizon to Sardinia,hosting the stratabound Sb, Hg, W, and As deposits, and running all along theAlpine chain and the southern Appennines (Calabria, Monti Peloritani);

3. gold occurrences, geographically close to those of the above group, are associatedwith metavolcanics. Although gold anomalies have been known for some time, onlyvery recently a veritable gold deposit has been discovered and is currently underexploration;

4. oolitic iron ore accumulations are interbedded in Silurian slates in close connectionwith a mafic laccolith. These deposits occur in the NW corner of the island (theNurra Region).

The fourth metallogenic period is related to metamorphism and magmatism ofthe Hercynian Orogeny. It had strong deformation and remobilization effects on pre-existing ore-mineral concentrations by tectonic and/or post-magmatic fluid circulation,and formation of new ore deposits. The types of deposits formed during this period are asfollow (in decreasing order of economic interest):

1. hydrothermal base metal and industrial-mineral veins, some of which were among themost important mining reserves of Sardinia. A prominent example is Montevecchio-Ingurtosu, a vein system with a total tonnage of about 50-60 Mt of crude ore at10-11 % combined Pb+Zn, 500-1000 ppm Ag in galena Pb, and 1000 ppm of Cd insphalerite. Another very important example is the fluorite-barite-galena deposit ofSilius, whose original reserves included 30 Mt of CaF2, 15 Mt of BaSO4, and 1.5Mt of PbS). One particular vein system is characterized by its richness in Ag, atplaces with recoverable quantities of galena, and minor sphalerite. This type is onlyknown in SE Sardinia (Sarrabus Region), and is assumed to be a reconcentration ofa previous stratabound deposition of the third period

2. skarn deposits, generated by contact metamorphism of previous protores and/ormetasomatic replacement. In such bodies pyrite, pyrrhotite, hematite, and magnetiteare frequent, while other ore minerals (mostly sulfides) were mobilized and drivenaway to various extents. Iron ore deposits, generally depleted in sulfides, may thushave been generated. San Leone (Sulcis) is the most important iron ore example(20-25 Mt at 40- 45 wt.% Fe; now practically exhausted). Scheelite and fluorite alsooccur commonly in these skarns

3. greisen type occurrences; the ore composition includes in decreasing order of abun-dance: molybdenite, chalcopyrite, wolframite, pyrite, cassiterite; recent studies alsoshowed the presence of gold in one of these occurrences

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4. talc-chlorite and albite deposits related to metasomatic-hydrothermal phenomena fol-lowing the emplacement of granite batholiths; the economically feasible occurrences,in central Sardinia, include about 6 Mt of talc-chlorite and at least 25 Mt of albitite

5. pegmatite dykes, pegmatitic granite, and their erosion/alteration products are par-tially exploited for K-feldspar, quartz, and, when kaolinized as raw materials forpottery

6. high-temperature veins carrying W, Mo, As, Ni, Co, and V. Although numerousaround the granites of SW Sardinia, these veins do not reach an overall tonnagesufficient for economic exploitation

7. porphyry type Mo occurrences. Although a few vein-type concentrations were ex-plored and partially exploited in the past (especially during the last World War),no serious studies have ever been carried out on any of these occurrences as such.The correlation between type of deposit and type of granite is not always obvious.A “preference” of most types of deposits for I-type granites, and particularly of hy-drothermal deposits for leucogranites, is commonly accepted

The fifth metallogenic period is related to the post- Hercynian peneplanation. Su-pergene phenomena caused alteration and mobilization of pre-existing ore-mineral concen-trations, and yielded:

1. residual concentrations of sub-economic iron ores, and deposition of kaolin and fire-clays, on the post-Hercynian peneplain. The latter are considerable where preservedby Mesozoic-Cenozoic covers, and the fireclays are currently being exploited

2. karstic concentrations of BaSO4, and oxidized Pb-Zn-Fe ores

3. intensive supergene reworking of the preexisting ore-mineral accumulations; this phe-nomenon is particularly evident in the Pb-Zn-Ag deposits of Iglesiente

4. deposition of high-purity quartz conglomerates, in some instances (Central Sardinia)interdigitated with kaolin-fireclay accumulations outlined in (1).

The post-Hercynian erosion started in the Carboniferous-Permian: in fact, the firstpost-Hercynian discordant sediments are of this age. The first metallogenic effect of thisphase has been recognized in some mineralized karst fillings in the folded Cambrian lime-stone and fossilized by post-Hercynian (probably Triassic) sediments. Upheaval and ero-sion of the Hercynian chain continues up to present, almost incessantly in some areas, as isshown by the fact that some ore-bearing karst systems include both pre-Triassic fossilizeddeposits and columnar bodies whose vertical extent is controlled by the present positionof the water table.

The sixth metallogenic period took place in the Middle Cretaceous and yieldedrather conspicuous bauxite deposits along an emersion surface of the Mesozoic carbonate

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Figure 10: Metallogenic synthesis of Sardinia.

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platform of Nurra. The recently explored deposit of Olmedo (proven plus inferred reserves:30-35 Mt at 60-62 wt.% Al2O3; total reserves possibly up to 70-80 Mt) is the best exampleof this. It is to be pointed out that bauxite accumulation during the Mesozoic carbonateplatform evolution involved the entire Mediterranean area, both on the European and theAfrican plates.

The seventh and last metallogenic period is related to the Alpidic tectonic andmagmatic activity. Alpidic tectonics are only extensive in Sardinia, and its effects onpre-existing ore occurrences are mainly displacements without important mobilization,as contrary to the Hercynian orogeny, with the exception of those due to local upliftsas discussed above. It is the related Oligocene-Miocene calc-alkaline magmatism thatproduced minerogenetic phenomena, whose importance is just now being recognized. Themain ore occurrence types are the following:

1. porphyry copper (-gold) deposits, linked to subvolcanic bodies at Calabona, close toAlghero, Siliqua in Eastern Iglesiente, and in other areas now under investigation;

2. ochers and/or Mn ores linked to effusive activity (the island of San Pietro, andnorthwestern Sardinia). At places, supergene enrichment played an important rolein the formation of Mn accumulations, yielding thin crusts of highly- pure Mn oxides;

3. precious and base metal occurrences in epithermal systems. These systems have beenfound very recently and are now being actively investigated and exploited. Bothlow- and-high-sulfidation occurrences have been found so far. Their economic worthappears so interesting that the low-sulfidation bodies of Furtei are currently beingmined, and a few tons of gold have already been recovered only in the oxidized partof the explored bodies;

4. kaolins and bentonites after hydrothermal and/or supergenic alteration of the samevolcanics;

5. Cu-Pb-Zn oxidized and sulfide minerals in clastics, at times barite-cemented, beds,at the base of the Miocene sediments, following erosion of the volcanogenic deposits.Their importance has not yet been established. Residual BaSO4 accumulations (ei-ther as pebbles or veinlets) in Quaternary eluvial soils are found close to the outcropsof barite hosted in the underlying Paleozoic of Iglesiente. In one of these occurrencesthat covered karst fillings and was exploited with them, obsidian splinters derivingfrom prehistoric industry are mixed in with the barite clasts.

6. Geothermal systems and thermal waters related to the Tertiary volcanic cycle and stillactive in some districts. Studies on geothermal energy exploitation have not reachedthe operational level yet, while a thermal establishment has long been in operationand others will begin to operate soon; several occurrences zeolitized pyroclastics arescattered all over the areas covered by Tertiary volcanics. Studies on their industrialuses are still in progress.

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For completeness sake, it should be mentioned that two types of fossil coal are alsofound in Sardinia: anthracite (one small occurrence no longer exploited) below Permianterrains, and sub-bituminous coal (a huge deposit) in the Lower Eocene.

1.2.2 Mining in Sardinia: Historic overview

Tranlated and modified from Vonlanthen et al. (2005)

We think the population of the Nuraghe culture (1500-400 BC) started exploitation ofminerals in Sardinia. Many bronze statues show smelting abilities at this time. However,Sardinia is poor in Cu ore, and Cu exploitation is not recorded in prehistoric times, butcopper ingots of Sardinia have been found to come from Cyprus (typology and Pb isotopes).So far, no information is available about the origin of tin as local resource (importation?).Lead tools found in Nuraghic sites probably attest the beginning of exploitation of localdeposits.Phoenicians, coming from the Palestinian coast, reach Sardinia around 800 BC and es-tablish several trading posts in the South of the island. They were probably interested inmetals as they also thought them in Spain. Exploitation in the Iglesiente area probablystarted at this time to produce silver. Nevertheless, artefacts from this exploitation almostdon’t exist. Around 600 BC, Carthaginians, descendants of the Phoenician colons, tookcontrol of Sardinian coasts, while the interior of the country was still dominated by theNuraghe civilization. However, archaeological relicts of mining from this time are veryscarce.Romans conquered Sardinia in 238 BC. Surprisingly ancient Roman authors like Strabonor Pline, don’t mention Sardinian mines. Definite proofs (stamped ingots) from mining inSardinia only appear in the 2nd century. Written testimonies of Pb-Ag and Fe mining aswell as gold panning are dated to the 3rd-5th centuriesThe Vandales, a Germanic tribe, took control of Sardinia after having gone through Galliaand Spain (450-550 AD), and expanded their empire to northern Africa. Then, the islandwas taken over by Constantinople as precautionary measure, while facing the pressure ofthe Arabic expansion (700-800 AD). Although some Arabic authors mention mines in Sar-dinia, it’s difficult to infer the state of the mining industry at this time. Between the 9thand the 11th century, many small autonomous states ruled Sardinia.Around 1000 AD, Genovesi and Pisani repelled the Sarrasins and settled on the island.Pisani later pushed Genoveni out of the island. Some elements show that mines were inoperation at this time. In particular, a mining law probably dating back before 1300 ADdefines exploitation modalities of the Iglesiente mines (taxes, exploration and exploitationrights, status of workers etc.). This statute is very similar to the one existing in Toscanafrom the same time, which is in turn inspired by the first Germanic statute. There is aclear renewal of the mining industry at this period.In 1323, kings of Aragon took control of Sardinia. Some years after, the king, took ad-vantage of a revolt of the Pisani from the Iglesiente area to take control of the mines andreallocate the concessions. During the 14th and 15th centuries, mining industry carried on

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prosperous, and was mainly focused on silver.In 1492 Christopher Colombo discovered the Americas. Earlier, the kingdoms of Aragonand Castilla had merged to form Spain. From 1530, exploitation started in the huge silverdeposits of Mexico and Peru. Sardinian mines carried on being exploited, but without anymajor work.In 1720, the dukes of Savoy took Sardinia and together with their other states (Savoy,Piemont and Nice) found the kingdom of Sardinia. This kingdom is partly dismemberedduring Napoleonian wars (Sardinia keeps its autonomy) but was brought together againin 1815. Between 1848 and 1860, the movement of unification and independence of Italydeveloped and was promoted by the dynasty of Savoy. In 1861, the kingdom of Sardiniamerged with other ones to form the kingdom of Italy. In 1946, the Republic is proclaimedand Sardinia becomes an autonomous region in 1948.From the second half of the 19th century, and during the second phase of the industrial rev-olution, European capitalist wanted to develop new raw material resources to supply theirfactories. New transportation methods and techniques allowed improvement of exploita-tion methods. At this time, public companies, mostly French and British ones, invested alot of money in the mines of the Mediterranean, including the deposits of Iglesiente. Somemines (Monteponi, Montevecchio, and others) were very profitable. Mining was mainlyfocused on supergene Zn at this time.Extraction was intensively carried out during the second half of the 20th century. Theformerly highest grade deposits are now mined out. The Pb-Zn mines closed in the 70’s.Industrial gold mining started in 1997 in the Furtei area and lasted until 2008. Exploita-tion in this area caused an environmental disaster. The accused company operating theFurtei high sulfidation deposit was taken to court.

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Ores in the lower Paleozoic of SW Sardinia

2 Pre-Hercynian Stratiform Pb-Zn-Ba Mineralizationin the Iglesiente-Sulcis Area

by Pierre Hemon

This Chapter is reproduced and modified from Boni et al. (1996).

The Iglesiente-Sulcis area (Fig. 11) is one of the oldest mining districts in the world(production dates back to pre-Roman time), with more than 50 major deposits knownwhich were initially exploited for lead, silver and copper and later for zinc and barium.Two major tectonic trends can be recognized in SW Sardinia. The E-W lineation related tothe Sardic phase and the more recent ∼N-S lineation related to the Hercynian deformation.

Figure 11: Geologicaland structural map of theIglesiente-Sulcis area (fromBoni and Koeppel 1985).1 Post-Hercynian sediments2 Permian porphyry3 Late Hercynian granites4 Silurian and Ordovicianslates, limestones andconglomerates5 Cabitza FM6 Gonnesa FM7 Nebida FMFM Axial planes:8 Sardic deformation9 Hercynian deformation.

The ores exploited can be subdivided into pre-Hercynian stratabound zinc > lead >barium and post-Hercynian lead - barium - silver - copper skarn-, vein- and paleokarst

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deposits. The first ones were deformed together with their host rocks by the Hercyniancompressive tectonics, the latter ones clearly cut the deformed and tilted lithologies.

The pre-Hercynian stratabound deposits have greater economic importance relativeto post-Hercynian deposits. Most of the stratabound ore deposits are hosted by LowerCambrian carbonates (especially within the Gonnesa group: types a to f from Fig. 12)and, to a minor degree by Upper Ordovician metasedimentary rocks (type g and h in Fig.12).

Figure 12: Cambro-Ordovician stratigraphic col-umn showing the stratigraphic position of variousmineralizationA Cambrian-hosted oresB Sardic unconformity-hosted oresa to h refer to mineralization types described inthe text. Figure is taken from Boni et al. (1996).

The pre-Hercynian stratabound ores can be regarded as the result of a combinationof favorable sedimentary environments and Paleozoic tensional tectonics. Two groups ofgenetically distinct ore types are known:

1. SEDEX deposits: the mineralization occurs in the upper part of the Punta Mannaformation as syngenetic and early diagenetic massive sulfides and barite layer (typea and b in Fig. 12).

2. MVT deposits: mineralization occurs in form of void filling, breccia cement andlate diagenetic replacement bodies within the San Giovanni formation (types c to f in

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Fig. 12). MVT-style mineralization also occurs at at the Sardic unconformity (typeg and h in Fig. 12) and is associated with strong hydrothermal silica alteration.

The genesis of stratabound ores within Cambrian sedimentary rocks of SW Sardiniawas apparently an evolving process that produced a series of deposits ranging from SEDEXto MVT.

2.1 Description of mineralization and host rocks2.1.1 SEDEX type mineralization within the Lower Cambrian Punta Manna

and Santa Barbara formations

The Punta Manna and Santa Barbara formations - isolation of the platform:The lower Cambrian sequence of south-eastern Sardinia is divided into the Nebida groupand the overlying Gonnesa group (Fig. 12). The Nebida group consists of terrigeneoussandstones and siltstones, shallow water complexes (Matoppa FM) which grades upwardinto a sequence of alternating beds of detrital and carbonate lithotypes (Punta Manna FM),with a total thickness of 700-800m. The transition from the Matoppa FM to the PuntaManna FM is consistant with a progradation of clastic-carbonates tidal flats (MatoppaTime, Fig. 13) towards a more open marine area (Punta Manna time, Fig. 13).The Punta Manna FM (Nebida group) is followed by the dolomites of the Santa BarbaraFM (Gonnesa group). This transition is consistant with a tectonic tensional phase formingthe Eastern Sulcis Basin (Santa Barbara time, Fig. 13) which separated the platform fromits clastic source.

Figure 13: Tectono-stratigraphic evolution of the Cambrian succession in SW Sardinia, from Matoppa time to Santa Barbaratime (from Bechstädt and Boni 1994).

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Massive sulfide (type b) and barite (type a) layers are hosted within the Punta Mannaformation. Because of their high degree of congruency with depositional structures whichindicates that the paleogeography controlled the distributions of the mineralizations, plussimilar Pb and Sr isotopic composition of the ore and the host rock (see below), the depositsare considered to be of syngenetic to syndiagenetic origin and interpreted as sedimentaryexhalative i.e. SEDEX type deposits (Boni et al. 1996).The ore grade in these massive sulfides was generally high (∼8% Zn, locally exceeding12% Zn+Pb). The deposition of the which occurs at the base of the tidal dolomites ofthe Santa Barbara formation, has been related to the onset of strong tensional tectonicsduring the Early Cambrian (Bechstädt and Boni 1994). In fact, most ores are enrichedalong important tectonic lines, which controlled the distribution of the sedimentary facies(internal platform and slopes) during the Lower Paleozoic.

Barite - type a: Stratiform, irregularly distributed, decimeter-thick layers (max. 2m)of microcrystalline barite occur in the upper carbonate beds in sharp contact with thedolomite horizons of the Santa Barbara FM. This microcrystalline barite displays an al-ternating zebra-like texture within the dolomite. The barite layers are intercalated withinvarve-like laminated dolomites with less microbial laminae and is only locally associatedwith minor pervasive silicification.No evidence of massive replacement could be found, a few detrital barite clasts have beenobserved in the overlying tidal dolomite, indicating a syngenetic to syndiagenetic forma-tion.

Massive sulfides - type b: In some areas, layers of massive sulfide bodies (pyrite >sphalerite > galena) occur in the same carbonates as the barite layer (type a) or withindolomitized host rock. Sulfide layers are generally stratiform, often disrupted or slumped,and are locally characterized by several brecciation events.The ores are typically high in grade (with an average of 8 wt.% Zn), with botryoidal pyriteand microcrystalline dark sphalerite»galena and chalcopyrite. The mineralogy is highlyvariable from place to place, sometimes composed entirely by pyrite. The dolomite host-rocks are often blackened and strongly silicified and were overprinted by the Hercyniandeformation.

Discordant massive sulfide bodies: Some economically important ores are clearlydiscordant and characterized by a strong pervasive silicification of carbonates and sand-stones and/or a soft sediment deformation of sulphides. These ores are concentrated alongtectonic lines of regional importance possibly of Cambrian age.To explain this discordant concentration of ores, Boni et al. (1996) suggest a fluid dis-charge in a shallow, partly euxinic depositional pond or depression, fed by synsedimentaryfaults which were repeatedly reactivated.

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Zonation: The mineralization, interpreted as sedimentary exhalative, shows zonation ona large scale (e.g in the Sulcis District) and is centered in the eastern Iglesias valley wheremassive sulfides (type b) prevail over small amounts of barite (type a). This central partis surrounded by a much larger area where only thin horizons of barite occur.Horizontal zonations are also observed in classical examples of exhalative ores, here locallyfollowed by a vertical gradation from sulfides to barite around hypothesized discharge areasof the mineralizing fluids. Emplacement of the ore bodies is ascribed to several pulses ofhydrothermal fluid circulation, controlled by tectonic movement related to Lower Cambrianrifting.

2.1.2 MVT type mineralization within the lower Cambrian San Giovanni for-mation

The San Giovanni formation - flooding of the isolated platform: The San Gio-vanni FM follows the dolomites of the Santa Barbara FM, consisting of black limestonesat the base, overlain by the “Ceroides” lithofacies (=waxy limestones). The Ceroide con-sists mostly of barren microsparites, representing, at least partially, recrystallized peloidalmudstones and wackestones, commonly heavily bioperturbated. This facies is thought tobe indicative of uniform, deeper, low energy conditions typical of a flooding stage (Fig.14).

Figure 14: Tectono-stratigraphic evolution of the Cambrian succession in SW Sardinia, San Giovanni time (from Bechstädtand Boni 1994).

In the westernmost Iglesiente district, Ceroide mudstones locally are characterized byslumps, brecciation, debris-flows and neptunian dikes. The breccias can be matrix-rich,-poor or even -lacking. Due to the enhanced porosity of breccia horizons, they were of-ten susceptible for the later fluid circulation associated with post-Hercynian hydrothermaldolomitization. Therefore, the brecciation phenomenon is generally the most importantore controlling factor in the San Giovanni Formation.The internal breccias (matrix-poor and matrix-lacking) are more enriched in sulfides, whichform the zoned cement between the components, but also fill, together with sparry calcite,the intraclastic porosity. Ores from breccia bodies are late diagenetic to epigenetic, but

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might have been controlled by Cambrian synsedimentary tectonics as well, forming exter-nal and internal breccias in the areas marginal to the platform. The origin of “internally”and “externally” (debris-flows) breccias is still debated.Many different types of stratabound deposits (types c to f in Fig. 12) occur within theSan Giovanni FM, and particularly in its upper part within the “Ceroides”. They consistof several horizons (described below) containing sphalerite, galena, pyrite and more rarelybarite. The sulfides occur as void filling, breccia cement and late-diagenetic replacementbodies in the shallow water limestones of the San Giovanni formation. Some of the orescould also be representing an earlier replacement product. The deposits have been inter-preted as MVT deposits (Boni et al. 1996), possibly related to a widespread fluid-flowevent associated with the Caledonian “Sardic” tectonic phase. Their metal content is inthe range of low-grade MVT ores, averaging 5-7 % combined Zn+Pb. However, thanks tothick and continuous mineable horizons, exploitation was possible until the 1970ies.The iron content of the Sardinian MVT type mineralization is much lower (less pyrite) andthe Pb/Zn ratios significantly higher relative to the SEDEX type mineralization within theSanta Barbara formation. The different metal budget might be related to different sourcesof the metals (i.e. from the pre-Cambrian basement consisting of magmatic/metamorphicrocks, to lower Cambrian siliclastic sedimentary rocks) or to differences in lithology andchemical compositions of the host rocks encountered by the hydrothermal fluids (Boniet al. 1996).

Zn-Pb deposits - type c: At the base of the San Giovanni Formation, small Zn>Pbdeposits occur within the strongly tectonized “black limestone” lithotype. Slumps andslump-breccias are frequent in these successions, involving ore and gangue minerals.The ore-rich sedimentary rocks might have formed as infill of a network of cavities andfractures, as a result of collapse of part of the platform at the beginning of the floodingstage (early San Giovanni time, Fig. 14).

Blendoso - type d: This mineralization type is characteristic for the sphalerite-rich ore,the so-called “Blendoso” limestone. Mineralization occurs as broadly developed horizonsin the mines around the Iglesias valley and today’s western coast of Sardinia.The horizons with 5-6 % Zn consist mainly of vastly diffused stratabound impregnationsof pale yellow (Fe-poor) sphalerite with some pyrite and minor amounts of galena in apeloidal calcareous mudstone facies.The Blendoso ores, hosted by the Mid Ceroide lithofacies, seem to have been emplacedpervasively by diagenetic processes and concentrated by the following Hercynian fluid cir-culation.

Zn-Pb deposits - type e: This mineralization is the economically most important oretype. Zn>Pb deposits occur as cement and/or as matrix of stratabound multigenerationbreccias, which are easy to recognize, but sometimes deformed by post-ore Hercyniantectonic.

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Pb-Zn-Ba deposits - type f: the deposits are characterized by higher Pb/Zn ratios aswell as by the presence of minor barite. They occur throughout the entire Iglesiente-Sulcisregion, immediately below the nodular limestones of the Campo Pisano FM.

2.1.3 MVT type mineralization at the Ordovician unconformity

The Sardic unconformity: The economic significance of the Sardic unconformity de-pends on their depth of erosion in the different areas of SE Sardinia. In most areas theOrdovician erosion cut down into slates of the Middle Cambrian Cabitza FM, where metalconcentrations are low or absent.However, the erosion locally reached the Lower Cambrian San Giovanni and Santa Barbaraformations, where the first sedimentary lithotypes above the unconformity consist of a fewmeters of conglomerates. In those areas most conglomerates and part of the carbonatesbeneath the unconformity show pervasive, but irregular silicification, reaching some tens ofmeters. This impressive quartz horizon contains sub-economic mineralization (type g andh in Fig. 12) consisting mostly of barite with minor galena, plus local traces of sphaleriteand Cu-sulfosalts.

Type g: Mineralization occurs directly beneath the Sardic unconformity as deformedveins and pods which replace the carbonate host rocks.

Type h: The mineralization is hosted by upper Ordovician conglomerates and brecciasand in the slates of the basal M.Orri and Portixeddu formations.

The genesis of type g and h mineralization can be attributed to hydrothermal fluidsmigrating underneath the impermeable cover of Ordovician slates (M.Orri/Portixeddu for-mations) in more porous and leachable conglomeratic lithotypes along the unconformitywith Cambrian carbonates.This mineralizing event is believed to be lower Paleozoic in age, as indicated by the char-acteristic stratigraphic position of the quartzite as a marker horizon and by the contem-poraneous marine sulfur isotopic ratios. Indeed this setting could result in a very effectivemetallogenic trap for metal-bearing fluids that reached the top of the carbonates.

2.2 Isotopic analyses and their interpretationSulfur isotopes

Jensen and Dessau (1966) report sulfur isotopic compositions of galena, sphalerite,pyrite and barite from the Cambrian carbonate sequence (San Giovanni, Campo Pisanoand Acquaresi mines). Based on the significant enrichment of 34S, Jensen and Dessau(1966) suggest that bacterial reduction of sulphates may have been the main source ofreduced sulphur in the stratiform deposits of SW Sardinia.Biogenic sulfides are formed by the reaction of metals with H2S derived from anaerobic

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bacteria during bacterial reduction of sulfate ions. During this reaction, some isotopicexchange can occur:

32SO2−4 +H2

34S 34 SO2−4 +H2

32S

Additional analyses (Fig. 15) compiled by Boni et al. (1996) confirmed this hypoth-esis:

1. S-isotopic data of stratabound barites: Type a and f, hosted in Cambriancarbonates plot into the field of Cambrian, slightly evaporitic seawater (+31 < δ34S <+34h)

2. S-isotopic data of sulfides: Lower Cambrian sulfides (type b to f) show positivevalues of δ34S and are consistent with their formation from bacterial reduction ofCambrian seawater sulfates. Ordovician hosted galena (type g) also show positiveδ34S values indicating again sulfate reduction in a closed system.

Figure 15: a) Sulfur isotopic composition of sulfides and barites from the Cambrian- and Ordovician-hosted deposits b)Sr-isotopic composition of Cambrian and Ordovician carbonates and of Cambrian- and Ordovician-hosted barites. FromBoni et al. (1996); see references therein.

However, accepting sulfur derived from biogenic reduction of sulfate as the precipi-tant, the source of metallic cations for the ore deposits of southern Iglesias still remains aproblem.

Strontium isotopes

Sr-isotopic compositions (Fig. 15b) show two different groups, representing two differentstrontium sources (Boni et al. 1996):

1. Cambrian hosted barite, Cambrian carbonates and Ordovician carbonates show sim-ilar Sr-isotopic compositions which are consistent with a Cambrian (possibly alsoOrdovician) seawater source for the Sr.

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2. The Ordovician-hosted barites (type g and h), show more radiogenic values. Thesource of Sr for the Ordovician-hosted barites has therefore been attributed to anhydrothermal leaching of either Hercynian granites or of the clastic lower Paleozoicsequence.

Lead isotopes

Figure 16: Pb-evolution diagram for the Cambrian- andOrdovician-hosted ores and their host rocks. Data from Boni et al.(1996); see references therein.

The majority of galena from stratiformdeposits hosted by Cambrian carbon-ates forms a tight cluster (Fig. 16).Isotopic compositions also show simi-larly high µ (238U/204Pb) and high W(232Th/204Pb) values in all types of de-posits, which indicate a crustal originfor the lead.Most of the galena lead in the Cam-brian host-rocks is isotopically verysimilar or even identical to lead in theore. Only some galenas occurring inthe Punta Manna FM (type b) containless radiogenic lead with lower µ andWvalues. There is a noticeable tendencyof increasing µ and W values with de-creasing stratigraphic age of the Cam-brian sediments (Boni and Koeppel1985).Galena from deposits located at theSardic unconformity (type g and h) be-long to a second group. Two typesof lead have been recognized togetherin an individual orebody, one with anormal Cambrian composition and one

that is indicative for the Ordovician (or possibly Permo-Triassic) mineralization.The lead isotopic data of these galenas are compatible with the idea that during the Or-dovician transgression, lead from existing deposits (Cambrian-hosted) was remobilized andredeposited together with variable amounts of a more radiogenic component.All the authors, based on lead-isotope analyses in the Iglesiente mining district agree thatlead of the lower Paleozoic ores was derived from a crustal source. Arribas and Tos-dal (1994) envisaged a massive leaching of pre-Cambrian to Paleozoic basement by largeamounts of hydrothermal fluids. This hypothesis explains the genesis of several large de-posits from Cambrian to Miocene in the northwestern Mediterranean region.However, there is much less agreement about the actual source rocks for the lead. Thisdifficulty is mainly due to the lack of outcrops of pre-Cambrian rocks in SW Sardinia,

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which were suggested as possible sources.

Based on the similar µ and W values of all samples, it was suggested that the lead de-rived from the same source or from sources with similar U/Pb and Th/Pb ratios (see Boniet al. 1996 and references therein). On the other hand, Caron et al. (1993) inferredheterogeneity (based on one carbonate sample) in lead isotopic composition between theCambrian host rocks and ore deposits. They suggest a mixed crustal source for the Zn-Pbmineralization.

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Supergene nonsulfide zinc mineralization of SW Sardinia

3 Supergene Carbonate-Hosted Nonsulfide Zinc Min-eralization: The “Calamine” of Southwest Sardinia

by Johannes Mederer

3.1 Introduction

The so called “Calamine” nonsulfide zinc ores were used as the source minerals for brass(an alloy of copper and zinc ± tin) during Roman and medieval times. Brass was manufac-tured until the 18th century by mixing and heating the ground “Calamine” carbonate andsilicate ore with copper in a crucible (Boni and Large 2003). There was no significantexploitation of calamine ores in Europe since the 1970s. Only recently nonsulfide zincdeposits became again commercially attractive targets, as new processing techniques suchas solvent-extraction and electrowinning were developed. Advantageously, calamine oresnormally lack Pb, S, As and other deleterious elements due to metal separating processesduring the supergene formation of the deposits. In 2002, some 11 % of the world’s knownZn reserves were attributed to this kind of ore deposits (Borg 2002).

In this contribution, the different types of supergene nonsulfide zinc deposits are brieflyintroduced together with the geological factors which cause their formation. The specificcharacteristics of the deposits in the Iglesiente district of southwestern Sardinia are pre-sented. A short description of two stops of geological interest which we will visit duringour trip will close this chapter.

3.2 Supergene nonsulfide zinc deposits

Hitzman et al. (2003) distinguishes hypogene and supergene nonsulfide zinc deposits.Figure 17 shows the different kind of deposits which occur within the supergene class.Most of the worldwide occurring supergene nonsulfide zinc deposits are hosted in carbon-ate rocks. This is due to the high reactivity of these rocks with acidic, oxidized, zinc-richfluids derived from the breakdown of sphalerite-rich sulfide ore bodies (Hitzman et al.2003). Whether or not an economic deposit can form is a function of multiple geologicalfactors such as the composition of the primary ore and the possibility to effectively oxidizeit. Influential are uplift or deep weathering, climate, permeability and composition of thehost rock, its structure and the hydrogeological situation, e.g. to avoid dispersion of theZn-charged fluids to an aquifer.

Oxidation of iron-bearing sulfides (particularly pyrite and Fe-rich sphalerite) producesabundant H2SO4, as for example shown for the case of pyrite in Equations 1 and 2. Asa result, zinc goes into solution and can be transported away from the original hypogenesulfide body.

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Figure 17: Models for the formation of supergene types of nonsulfide zinc deposits from Hitzman et al. (2003). a)Direct replacement deposits form by replacement of sulfides (or zinc silicate bodies) by hemimorphite and smithsonite. Leadgenerally remains in situ within the body. b) Wall-rock replacement deposits form where zinc derived from the weatheringof a sulfide body moves out into adjacent carbonate rocks and replaces calcite and dolomite with smithsonite. c) Saproliticsupergene zinc deposits form where original sulfides are weathered causing either a direct-replacement deposit or a wall-rockreplacement deposit. In areas of high rainfall and mass loss in carbonate terranes, consequent development of karst systemsresults in mechanical erosion and concentration of zinc oxide and silicate minerals. Formation of such karst systems alsoforms wall-rock replacement deposits

FeS2 + 3.5O2 +H2O Fe2+ + 2SO2−4 + 2H+ (1)

FeS2 + 14Fe3+ + 8H2O 15Fe2+ + 2SO2−4 + 16H+ (2)

However, oxidation of iron-free sphalerite does not generate acidic fluids (Equation 3).

ZnS + 2O2 Zn2+ + SO2−4 (3)

The pH of the zinc-charged, acidic and oxidized fluids is buffered by the reaction withthe carbonate host rocks under release of CO2 ↑ and the precipitation of nonsulfide zincminerals. The physicochemical characteristics of the fluids change during these reactionsfrom acidic and oxidized to alkaline. The main ore minerals in the deposits are smithsonite(ZnCO3), hydrozincite (Zn5(CO3)2(OH)6), hemimorphite (Zn4Si2O7(OH)2)·H2O) andsauconite (Na0.3Zn3(SiAl)4O10(OH)2·4H2O), a smectite type clay. As seen by Equation4 and in Fig. 18b, it depends on the pH of the fluid, the activity of CO2 or the availabilityof dissolved silica which minerals can precipitate.

5ZnCO3 + 3H2O 2Zn5(CO3)2(OH)6 + 3CO2 (4)

In many deposits, coeval with the decrease of a(CO2) and increasing pH during thevanishing oxidation process of the sulfide body, smithsonite can be replaced by hydrozinciteand late hemimorphite due to solubility differences (Reichert 2009 and Fig. 18b). Thedifferentiation and separation of metals is a function of their different solubilities in thefluid or intensity of surface armoring: e.g. the preservation of galena by precipitationof anglesite (PbSO4) on its surface ensures the supergene formation of very pure zinc orebodies with high grades (up to 35 wt.% Zn). High grades and a rather simple mineralogical

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composition are a characteristic feature of wall-rock replacement deposits, whereas directreplacement deposits often contain higher amounts of other metals such as Pb, Cu, Fe orAg because they were not efficiently mobilized during oxidation.

Figure 18: a) Simplified cross-section of a calamine-type deposit (Monteponi) in southwest Sardinia. CD = Cambrianlimestone and dolomite, CS = Calcschists, SC = Cabitza schists; 1 = primary lead and zinc sulfides, 2 = smithsonite andminor cerussite, 3 = hemimorphite and cerussite (from Boni and Large 2003) b) Stability of zinc carbonates in the chemicalsystem Zn-O-H-C in relationship to PCO2(g) and pH. The activity of zinc is a(Zn) = 10−5mol/l (after McPhail et al. 2003)

3.3 Supergene carbonate-hosted nonsulfide zinc deposits in theIglesiente district of SW Sardinia

The following descriptions of the Sardinian deposits are compiled from Boni and Large(2003), Boni et al. (2003) and Boni and Bechstädt (2009).

The mining history in the Iglesiente district goes back to pre-Roman times, where ini-tially Ag-Pb-Cu and later Zn, fluorite and barite deposits were exploited. The Sardiniannonsulfide zinc deposits are supergene and most of them belong to the class of wall-rockreplacement deposits even though many times combinations of direct and wall-rock re-placement deposits can be found. Fig. 19 shows the occurring supergene nonsulfide zincdeposits in the Iglesiente district, partially derived from SEDEX- and MVT type hypogenesulfide mineralization. Residual and karst-fill deposits can also be found on the island;despite being characterized by high zinc grades, they are usually small and irregular insize.Nonsulfide zinc mineralization in the Iglesiente district was formed by the oxidation ofhypogene stratiform carbonate-hosted MVT and SEDEX type lead-zinc sulfide mineraldeposits which are described in Chapter 2 on Page 27. Together with their host rocksthese deposits were tilted nearly vertical during the Variscan orogeny (Fig. 18a), and up-lifted and altered during the Alpine deformation. Primary sulfides were oxidized, metalswere remobilized and redeposited into vugs and karst cavities which were related to the pa-leowatertable. The supergene oxidation of sulfides commonly extends to depths of several

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hundred meters in different tectonic blocks and is not related to the present water tablebut to fossil oxidation phenomena. The relationship between supergene mineralization atshallow levels and the transition towards the hypogene sulfide mineralization at depth isillustrated in Fig. 18a.

The most important ore minerals in the deposits are smithsonite, hydrozincite andhemimorphite with minor occurrences of cerrusite and anglesite. The main gangue phasesare calcite, quartz, iron-oxy-hydroxides and barite. Stable oxygen isotope analysis byBoni et al. (2003) show formation temperatures for the zinc carbonates between 20 and35◦C, whereas temperatures dropped to 5-10◦C during the precipitation of late ganguecarbonates. 13C depleted organic carbon and 13C enriched marine carbonate carbon derivedfrom paleozoic limestones were identified as two discrete carbon sources by carbon stableisotope analysis.

The “Calamine” formation in Sardinia is controlled by the favorable composition andstructure of the sulfide-hosting carbonate rocks, which allowed a deep infiltration andcirculation of the oxidized fluids. The timing of supergene mineralization is difficult todetermine due to multiple oxidation events. According to Boni et al. (2003), middleEocene (emersion phase of Sardinia) to Plio-Pleistocene (tensional phase with differentiateduplift) seems to be the most reliable time span in which tectonic and climatic conditionswere favorable for deep oxidation of hypogene sulfides.

3.4 “Calamine” occurrences to be visited during our excursionThe following two stops are described in more detail in Boni and Bechstädt (2009).

3.4.1 The Nebida mining area. Old exploitation at Canale San Giuseppe

Very rich “Calamine” ores can be found north of the village of Nebida, where a small pathleads to the old mining area of the Canale San Giuseppe and several small open pits andadits can be found. Remnants of the original hypogene MVT-type sulfide mineralization(sphalerite>galena) can still be observed near one of the adits. Patchy hydrothermaldolomite (“Dolomica Geodica”) as well as thin veins of post-Variscan barite can be foundhere.In the San Giacomo valley, the relics of the Santa Margherita Calamine orebody are visible.In the upper levels which are situated well above the water table, the nonsulfide zinc ore(almost absent of sulfides) was mined by sub-level stoping. Descending to lower levels,the transition towards zinc-lead sulfides is visible. The heavily dolomitized carbonate hostrock is karstified and various zinc-carbonates, zinc-silicates, iron oxides- and hydroxides aswell as stalactitic, cadmium-rich yellow concretions were abundant in this area.

3.4.2 The “Calamine” at Miniera di Monteponi

In Monteponi the deformation and the nearly vertical tilting of the former sulfide miner-alization together with the Paleozoic carbonate host rocks can be observed (see Fig. 18a).

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Figure 19: Geologic sketch map of southwest Sardinia with location of the “Calamine” orebodies, taken from Boni et al.(2003). The calamine at Candiazzus, San Benedetto, Seddas Moddizzis, and Campo Pisano are derived from SEDEXsulfides, whereas in Buggerru, Planu Sartu, Masua, Monteponi, San Giovanni, Mount Agruxiau, Marganai, Sa Duchessa,Mount Scorra, Montecani, Acquaresi, and San Giorgio the primary ores were Mississippi Valley-type.

Oxidation extends several hundred meters below the surface. In this district, the threedifferent styles of calamine mineralization occur: partial replacement of the host carbon-ates, partial replacement of the hypogene sulfide mineralization and irregular karst filldeposits. Grades in this area were generally higher than 20 wt.% zinc with a dominanceof smithsonite and hydrozincite.

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Albitites and their Origin

4 Albitite Deposits of Central Sardinia: an Overview

by Cristina M. Tomé

4.1 Introduction

The albitites (also called albititic rocks, albite rocks, albite granites and plagiogranites)are uncommon rocks mainly composed by albite plagioclase (NaAlSi3O8) and quartz withsome accessory minerals such as biotite, zircon and titanite. Despite its simple mineralogyand appearance, these rocks are a subject of discussion in the literature, as they are foundin different geological settings from magmatic suits to purely hydrothermal environments.Here, we describe the metasomatic albitite occurrences from central Sardinia, summarizedfrom the following references: Carcangiu et al. (1997), Palomba (2001), Biddauet al. (2002) and Castorina et al. (2006). A second part deals with different typesof albitite deposits in the world and the current knowledge of their origin (i.e. magmaticversus metasomatic).

4.2 The albitite deposits of central Sardinia

The largest albitite occurrences in Europe are located at the Ottana - Sarule - Orani -Oniferi sector in central Sardinia (Fig. 20). The outcrops cover an area of 90 km2 andare best preserved at the southern margin of the Tirso river rift valley. They are hostedin Variscan granodiorites that were emplaced into Paleozoic metasediments. The albititesfrom this area are a valuable resource and are mined for feldspar which is mainly used inthe ceramic industry.

The albitite deposits are principally composed of albite-plagioclase and quartz withsome accessories, such as allanite, epidote, K-feldspar, chlorite and titanite. They occur aselongated kilometer long lenses or as vein-shaped bodies along joints and fractures in thegranodiorite. They also show a characteristic “patchy zone” structure with albitite coresgrading outward to granitoids or the reverse, granitoid relics within the albitite bodies.The mineralogical composition of the rocks changes gradually from the parental granodi-orite to the albitite, with few meters of a transitional zone called “albitized granitoid” :

Unaltered granitoid

The Variscan granitoids are considered to be the parental rocks of the albitites. Com-positions range from granodiorite to biotitic granodiorite with medium grained textures.The main mineralogy consists of plagioclase, quartz and biotite with K feldspar, allanite,apatite, amphibole, zircon, titanite, magnetite and epidote as accessories. Plagioclase isoften zoned with andesine core and albite rim.

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Figure 20: Geological map of the Otana - Sarule - Orani - Oniferi area and location of the albitite oucrops. Modified afterPalomba (2001) and Castorina et al. (2006)

Transitional albitized granitoid

This transitional stage between the most altered albitite and the unaltered granodioritecan sharply vary (in less than 1m) from very well preserved zones to sectors where the iden-tification is difficult. Magmatic textures of the parental granodiorite are still preserved andthe rock can be recognized by its intense alteration and its lighter whitish color comparedto the fresh granitoid. Some minerals are starting to be replaced such as biotite by chloriteand K-feldspar by albite, the latter preserving the original shape and twin planes of themicrocline. The main mineralogy varies from quartz, oligoclase, allanite, zircon, titanite,magnetite and K feldspar to oligoclase, albite, chloritized biotite and a second generationof quartz filling open spaces. Plagioclase is usually zoned with oligoclase core and albiterim.

Albitite

Up to the 70 % of the albitite consist of albite plagioclase. No igneous textures are preservedand the color has changed to whitish-greenish or yellowish depending on the chlorite or the

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epidote content. The mineralogical association varies as follows : albite, quartz, oligoclase,zircon, allanite, titanite and K feldspar into albite, quartz, chlorite, K feldspar, allanite,epidote, LREE rich epidote, titanite and oligoclase. This latter mineral association can befurthermore altered into muscovite, kaolinite and smectite. Depending on the temperatureand chemistry of the fluids, two types of albitite can be distinguished in the area:

1. Chlorite-bearing albitite: near the metasedimentary basement or within thebiotite-rich granitoids, where it overprints or partially replaces biotite.

2. Epidote-rich albitite: within the albitite bodies far from the basement, and closelyrelated to the most intensely metasomatized and fractured zones.

In summary, these compositional variations result from the replacement of Ca-plagioclase,K-feldspar and biotite into newly formed minerals such as albite, chlorite, epidote andquartz as well as a second generation of quartz (Figure 21). This is possible thanks to thefluid-rock interaction that is influenced by the permeability and porosity of the granitoidalong which aqueous, weakly saline and oxidizing fluids of low temperature are able tocirculate.

Figure 21: Flow chart metasomatic events and mineral paragenesis. The formulae of the main minerals are shown for betterclarity. Modified after Castorina et al. (2006)

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Regarding field evidence, petrography and mineral chemistry, there is a common con-sensus about the metasomatic origin of the albitites from central Sardinia. Na-,Mg- andCl-rich fluids circulated through the granitoid and the metasediments during the late stagesof the Variscan orogeny replacing the primary mineral associations. Fluids are interpretedto be of non-magmatic origin, as indicated by the higher Sr isotope ratios of albitites withrespect to the initial ratio of unaltered granitoids. They probably acquired radiogenic Srduring circulation through the Paleozoic sedimentary sequence. Nd isotopes suggest thatalbitization occurred close to the emplacement age of the Variscan granitoids at 310 - 290Ma.

A particular characteristic of the albitites from central Sardinia is the mobilization ofREE (Rare Earth Elements) during the metasomatism and their redistribution in newlyformed minerals. LREE elements (Sc - Gd) are thus partitioned into epidotes while HREE(Tb - Lu) fractionate into the titanites. This distribution of REE depends on the natureof the original REE-bearing minerals (allanite in the granodiorite), the composition of thefluid (e.g. F, Cl and CO2 ligands), and the structure of the new minerals that will hostthem (e.g. cationic coordination in epidote). This considerations support the hypothesisof metasomatic alteration involving Cl-bearing fluids.

4.3 Magmatic versus metasomatic origin of albitites: a discus-sion

The albitites are uncommon rocks that are present in very different geological settings.They occur as part of ophiolite suits (e.g. Bay of Islands, USA, Elthon 1991), in miner-alized U and U-REE deposits (e.g. Orani, central Sardinia, Palomba 2001; Asir region,southern Saudi Arabia, Sherbini and Qhadi 2004), Fe-oxide deposits (e.g. Cloncurrydistrict, Australia, Mark 1998, 2000), as altered granitic bodies (e.g. Rockford Granite,USA, Drummond et al. 1986) and in metamorphic environments (e.g. AridondackMountains, USA, Mclelland et al. 2002).

The most common and accepted origin for their formation is related to the actionof metasomatic fluids (Kovalenko and Kovalenko 1978; Cheilletz and Giuliani1982; Chauris 1985; Cathelineau 1988; Castorina et al. 2006; Bachiller et al.1996) replacing primary minerals when they are hosted by granitic rocks. Magmatic ori-gin for the albitites is less well documented and only found in ophiolites and some pla-giogranites (Clark and Lyons 1986; Elthon 1991; Kovalenko and Kovalenko1984; Schwartz 1992; Sherbini and Qhadi 2004). The latter studies suggest thatmagmatic albitites were formed by Na, Li and F-rich magmas but the evidence is not wellconstrained, and their ultimate origin is still enigmatic.

Discrimination of magmatic and metasomatic albitites has been based on petrologicalcriteria and field evidence (Schwartz 1992). For example, the typical replacement of

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K-feldspar by albite (Fig. 22a and 22b), often preserving the original textures, has beenused as indication of metasomatic alteration. Some clear transitional zones between theparental rocks and the albitites are also unambiguously suggesting a metasomatic origin(Palomba 2001). In other cases, however, these characteristics are not that evident andlead to misunderstandings about the ultimate origin of the albitites.

This confusion is in part due to the fact that regardless of their origin, the albitites arecompositionally similar, as the preliminary study of geochemical data suggest (Fig. 22c;Tomé, unpublished data).

Figure 22: Photomicrographs of incipient perthite K-feldspar replacing into albite (a) and the complete alteration stage,where checkerboard albite has replaced all but a small remnant of perthite (b, arrow). Pictures are taken from Mclellandet al. 2002. (c) Sample/chondrite diagram where the trace elements of six albitite deposits of different origin are plotted.Note the similar geochemical pattern for all of them.

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Epithermal deposits of Sardinia

5 The epithermal deposits of Osilo and Tresnuraghesand epithermal processes in generale

Edina Szappanosné-Vágó

5.1 Introduction: epithermal and other hydrothermal depositsof Sardinia

Figure 23: Geological map of Cenozoic volcanic rocks of Sardinia. Some ofthe most important Oligocene and Miocene porphyry and kaolinite rich ep-ithermal deposits of Sardinia are also shown (after Palomba et al. 2006).

There is a wide spectra ofthe magmatic-hydrothermal min-eral deposits in Sardinia. Amongthese are porphyry, epithermal,and W-Mo intrusion related veins.During the seventh metallogenicepisode of Sardinia, which is re-lated to the Alpine geodynamicevolution, especially during theOligocene and Miocene, severalporphyry copper deposits (Cal-abona, Siliqua) and related ep-ithermal systems (Tresnuraghes,Osilo, Romana, Monti Ferro,Furtei) were formed (Figure 23).

Epithermal systems in Sar-dinia typically have extendedkaolinite alteration zones that areeconomic for exploitation. Thesekaolinite deposits have only re-cently been recognized as poten-tial gold prospects and they arecurrently under exploration by themining company Sardinia Gold(Simeone et al. 2005).Before we look at the Tres-nuraghes and the Osilo depositslet us revisit the characteristics ofporphyry copper and epithermalsystems.

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5.2 Epithermal systems in generalEconomic importance and average concentrations Porphyry Cu systems presentlysupply nearly three-quarters of the world’s Cu, half the Mo, perhaps one-fifth of the Au,most of the Re, and minor amounts of other metals (Ag, Pd, Te, Se, Bi, Zn, and Pb)(Sillitoe 2010). Porphyry copper systems beside the Cu can be huge reserves of Mo andAu. Average concentrations that are typically found in porphyry deposits are 0.5-1.5 wt.%for Cu, 0.01-0.04 wt.% for Mo and approximately 1.5 g/t Au.

Figure 24: Telescoped porphyry Cu system of Sillitoe (2010). Relationships of a multiphase porphyry stock with its hostrock developping various deposit facies. Proximal and distal skarn, chimney and manto type carbonate replacement bodies,sediment hosted deposits and overlying HS and IS epithermal deposits. Note: temporal sequence of rock types, porphyrystock → lithocap development and phreatic brecciation → maar-diatreme emplacement.

Size and complexity of the whole system Epithermal systems are believed to begenetically related to the circulating hydrothermal fluid system of deeper seated porphyryintrusions. These porphyry-epithermal (HS, IS, LS) systems produce large volumes (10 to

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100 km3) of hydrothermally altered rock centred on the porphyry copper stock, together(or not) with related skarn, carbonate replacement bodies, sediment hosted deposits, andbase- or precious metal mineralization (Figure 24).

Geological setting Volcanic arcs with large bodies of calc-alkaline batholiths aboveactive subduction zones of convergent plate boundaries preferably host such kind of depositassociations.

Deposit genesis Porphyry deposits are associated with the generation of oxidized I-typegranite magmas adjacent to subducted oceanic crust. After rising up to higher crustal levelsvapor saturation occurs, and due to first boiling the vapor phase will scavenge copper andgold from the silicate melt.

Wall rock alteration Typical of porphyry and especially the related epithermal systemsare the widely extended, depth dependent and centrally zoned alteration mineral assem-blages (see Figure 25). This is so characteristic that it’s one of the most important andevident exploration tools for field geologists. For additional information on deposit form,ore textures, fluid chemical characteristics and typical ore and gangue mineral assemblagesin HS and LS epithermal systems see the Table below.

Figure 25: Generalized alteration-mineralization zoning pattern for porphyry-epithermal systems. Sericitic alteration ismore abundant in porphyry Cu-Mo and chlorite-sericitic is more abundant in porphyry Cu-Au deposits (Sillitoe 2010)

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HIGH-SULFIDATION LOW-SULFIDATIONGeological setting volcanic terrane, often in caldera- spatially related to intrusive centre;

filling volcaniclastic rocks veins in major faultsHost rock silicic to intermediate (andesite) intermediate to silicicDeposit form Disseminated ore dominant, replacement Open-space veins dominant, stockwork

ore common; Stockwork ore minor, veins ore common; Disseminated andcommonly subordinate replacement ore minor

Texture wallrock replacement, breccias, veins veins, cavity filling, brecciasOre mineralogy pyrite, enargite, chalcopyrite, tennantite, pyrite, electrum, gold, sphalerite,

covellite, gold, electrum, tellurides galena (arsenopyrite)Gangue mineralogy quartz, alunite, barite, kaolinite, pyrophyllite quartz, chalcedony, calcite, adularia,

illite, carbonatesC-H-S isotopes magmatic fluids indicated magmatic water (H2O) may be obscured

alunite: δOH2O =∼ +7; δSM−Halu =∼ 0 by mixing; surface waters dominate;C,S typically indicate a magmatic source

5.3 Kaolinite rich epithermal deposits of SardiniaThis section is compiled from Palomba et al. (2006)

Nine major kaolin deposits have been identified in three main districts: Romana, Tres-nuraghes and Serrenti-Furtei. All are related to the Oligocene-Miocene calc-alkaline (OMC)volcanic sequence.The Romana and the Tresnuraghes kaolin deposits are hosted by rhyodacitic and rhyoliticignimbrites. The Furtei deposit in the south is hosted in dacitic to rhyodacitic pyroclastitesand ash-flow tuffs and contains gold mineralization which was exploited from 1997 to 2003by the Sardinia Gold Mining Company.The Furtei epithermal gold deposit is highly enriched in tellurides. The hessite, stützite,sylvanite, petzite, coloradoite, altaite association with native tellurium indicates directmagmatic inputs to the mineralizing solutions. Sulfidation states of telluride bearing orefluids fluctuated between IS and HS conditions and telluride minerals are related to bothIS and HS assemblages. In 1998, reserves of 5 Mt with an average grade of 3 ppm Au havebeen estimated in the Furtei area.

Tresnuraghes epithermal deposit

Geological setting The Tresnuraghes Miocene hydrothermal system is hosted in silicicpyroclastic rocks and associated with silicic domes. Ore forming fluid flow was controlledby steeply dipping faults. The mineralogical and geochemical features (Au, Hg, As) of thearea indicate a typically shallow level epithermal environment.The volcanic sequence consists of rhyodacitic-rhyolitic ignimbrites, in which two main unitscan be recognized. The lower unit that hosts the largest deposits consists of ignimbritescharacterized by a fluidal-like texture with plagioclase (andesine) and cristobalite, minorquartz, potassium feldspar (sanidine), and rare biotite in a cryptocrystalline matrix. Theupper ignimbrites, have a similar mineralogical composition, but typically have sphericalstructures ranging in diameter up to 5 to 6 mm, and formed by overcooling of the ign-imbritic unit during its emplacement (concentric shells around a core are generally formedby a plagioclase crystal). Sometimes, a radial growth of cristobalite and kaolinite fibersare recognizable from core to rim. Miocene marine sediments (Langhian-Serravalian) and

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basalt flows overlie the volcanic sequence. The emplacement of both Tertiary and Qua-ternary volcanic rocks is controlled by NE-SW and NNW-SSE striking rift-related faults.Late EW striking faults displace the altered units.

Figure 26: a) Geologic and alteration map of the Tresnuraghes epithermal deposit b) Schematic section through theTresnuraghes epithermal district showing alteration zonation (Simeone et al. 2005)

Deposit characteristics The surface exposures at Tresnuraghes generally lack sulfides,and there is no drill information, so the sulfidation state of the hydrothermal fluids cannotbe directly determined. However, we can estimate other characteristics of the hydrothermalsystems based on the silicate and sulfate mineralogy. In the Tresnuraghes area silicifiedbodies include fault-filling quartz-chalcedony-barite veins within volcanic rock altered toK-feldspar-quartz-illite and shallow chalcedonic sinter. The quality of the kaolin in thesedeposits is low and precludes commercial extraction.

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Alteration pattern The kaolin bodies are generally hosted by dacitic to rhyolitic py-roclastic units. The most intense alteration occurs at fault intersections, frequently withsilicification (silica caps, sinters, silica veins). The kaolin bodies are lens-shaped and typ-ically 20 to 30 m thick. They are generally white or pale pink in colour. The transitionfrom fresh to mineralized rock is gradual over a few meters. In several deposits, opal andsmectite veinlets (a few up to 10 cm wide) occur, cross-cutting the bodies.

Figure 27: Model of theTresnuraghes epithermal deposit(Simeone et al. 2005). Thedeposit is related to ascendingweakly acidic to neutral pH (lowsulfidation) fluids that producedlocal sinter on the paleosurfaceand extensive steam-heated zonesnear the paleosurface at <120◦C.

Kaolinite occurs in two types: poorly ordered (pseudomonoclinic) and well-ordered (tri-clinic). A total of six alteration types can be differentiated based on the spatial zoning andmineral association. The hydrothermal alteration is zoned laterally from kaolinite-alunite-opal, close to vertical fractures that occur in the central part of the quarries, outward tokaolinite-montmorillonite-cristobalite, and finally to fresh rock. The vertical zonation inthe quarries is characterized by opal at the top, kaolinite-alunite-opal in the central part,and montmorillonite in the lower part accompanied by a decreasing alteration intensitydownward. This zonation suggests a major structural control of the fluid flow.Kaolinite bodies show an enrichment of As and Hg in the siliceous feeders with gener-ally low amounts of chloride-complexed metals such as Pb and Zn. Because Hg is readilytransported in the vapor phase at epithermal temperatures and As is not (Barnes andSeward 1997), the Hg-As association suggests fluctuation from aqueous transport of As inthe hydrothermal fluid to vapor transport of Hg in the steam-heated vadose environment.Possibly, this is the result of a late steam-heated overprint upon an earlier and underlyingAs-rich rock produced by condensed fluids.

Stable Isotopes Two dickite samples from the blanket-like kaolinite-opal-alunite zonehave isotopic compositions in equilibrium with meteoric water with -20 per mil δD at tem-peratures of 40◦ to 50◦C. The dickite might have originally formed at >120◦C from meteoricwater with a small component of magmatic water, and later isotopically reequilibrated at25◦ to 50◦C in the steam-heated environment. An alunite sample from the kaolinite-alunite-opal assemblage has a δ34S value of 12.0 per mil. Barite from central siliceous feeders aTresnuraghes plots on the δ34S versus δ18O diagram in the field of magmatic-hydrothermal

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alunite, and sulfate produced by 200◦C fluids with H2S/SO4 ratios of about unity.The occurrence of kaolinite associated with alunite indicates an acid environment witha pH between 2 and 4. By comparison with active geothermal systems, the kaolinite-opal association suggests temperatures up to 120◦C. Based on the S-O-H isotopes and thealteration zoning at Tresnuraghes the kaolinite-opal ± alunite deposits likely formed ina steam-heated environment where hot spring-derived, H2S-rich steam condensed in thenear-surface vadose zone, mixed with local ground water, cooled, oxidized, acidified, anddescended. The Sardinian kaolin minerals in general formed from ascending magmaticfluid mixture with Miocene meteoric water.The fluids that formed the central and deeper siliceous feeders were hotter and geochem-ically different from the fluids that formed the overlying and distal kaolin deposits. Theoccurrence of K-feldspar-quartz-illite alteration proximal to the vein systems with quartz-chalcedony and local barite suggests circulation of weakly acidic to near-neutral pH fluidand temperature higher than 220◦C.Thus, the fluid-dominated hydrothermal system of Tresnuraghes most likely represents anupflow zone of neutral alkali-chloride fluid that is overlain by a steam-heated, acid-sulfateenvironment containing kaolinite deposits. The Tresnuraghes system can be classified asadularia-sericite type and, due to the lack of sulfides, is inferred to have formed underlow-sulfidation conditions based on common sulfide-silicate associations (Hayba et al.1985).

Osilo epithermal deposit

Geological setting The Osilo epithermal area is located in northern Sardinia in theLogudoro district and covers ∼30 km2. The area consists of Tertiary volcanic rocks, whichwere generated following the detachment of the Sardinia-Corsica microplate from the Eu-ropean continental margin during the lower Oligocene. The calc-alkaline volcanic cycle isrelated to the subduction zone which has been activated due to the opening of the BalearicBasin.Three eruptive episodes characterize the volcanic activity between 24 and 13 Ma. Typi-cal bimodal volcanism is represented by rock masses of basalt, basaltic andesite, andesite,dacite and rhyolite. At Osilo, the most extended rock type is andesite which shows typicalpotassic, propylitic and argillic alteration. Lower to middle-Miocene marine and lacustrinesediments and recent travertines overlies the volcanic especially on the south.

Mineralization Mineralization is closely related to the Tertiary calc-alkaline volcanism.ESE-WNW and ENE-WSW trending quartz veins cross the pervasively altered host rockwith 0.3 to 1 m width. Veins are brecciated and recemented by comb and saccharoidalquartz. Up to several ppm Au are present. Mineral assemblages consist of py, aspy, stibnite,tetrahedrite, electrum, and subordinate galena, sphalerite and chalcopyrite.

Hydrogeochemistry Hydrogeochemical prospecting of Au deposits is not a promisingmethod in general, considering the low concentrations of Au in natural waters (in dissolved

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forms or in suspensions). However, Cidu and co-workers have done systematic water sam-pling from springs and surface waters monthly over a year and found nice positive correla-tions between the location of known auriferous veins and elevated gold concentrations inwater draining these vein systems (Figure 28).

Figure 28: Geological map of Osilo district (numbered points are the sample positions for hydrogeochemical sampling) fromCidu et al. (1995)

How to find outcrops near Osilo:The following description is copied and modified from Matthai (2000):

The village of Osilo (township of Sassari) lies in the center of an Oligocene-Mioceneandesite-basalt and rhyodacite complex of the monte S. Antonio (24-13 Ma) at the north-

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eastern margin of the “Logudorro” graben (the eastern extension of the Campidano graben.Geophysical studies indicate that this volcanic complex extends to a sub-surface depth ofabout 9 km.

Probably, while it was still cooling, fluid circulation along faults, fractures and withinpermeable pyroclastic and epiclastic units, altered the volcanics which erupted into a sub-aerial to shallow marine environment. The present-day volcanic layering is close to horizon-tal such that the topographic relief provides an opportunity to examine the manifestationsof the hydrothermal activity at different sub-surface levels. Ongoing gold exploration bythe company SGM (Sardinia Gold Mining S.p.a), has identified gold mineralization inseveral exploration drill holes intersecting subparallel approximately E-W striking quartzveins east of Abba Ruia (about 5 km SW of Osilo). The vein mineralization formed in awell-preserved epithermal system, perhaps comparable to the active systems on the northisland of New Zealand or the northern Flinders ranges in Australia. The hydrothermalalteration ranges from propylitic to argillic. Laumontite and other zeolites are common,espeically in fragmental lavas exposed in the valley around Osilo.

Quarry 1 (SS127, 300 m east of junction described below) From the main roadSS127, coming from the east of Sassari (in the guide Osilo is written, but I think they meantto write Sassari, because the road SS127 is east of Sassari and very far from Osilo) onerecognizes in the quarry face a subvertical E-W trending zone of alteration marked by a setof iron-stained anastomosing fissures with a bleached, up to several meter wide halo. Thesefissures cross-cut miarolitic, and locally fragmental, andesites, and rhyodacitic ignimbritesthat alternate with massive basaltic flows. This zone is exposed on the south-eastern endof the quarry. The iron staining is due to sulfides weathering in the periphery of cm-widecalcite-mineralized fissures. The basalt is altered into clay minerals, and mm-sized augitephenocrysts are replaced by white aphanitic alteration products.On the north-western end of the quarry, near an old shed, the andesite is agglomeratic andcontains small N-S trending clefts filled with carbonate. In the interstices of the lava zeo-lites occur (probably laumontite or stilbite). Next to the contact with a massive basalt flowthere are clefts which contain euhedral calcite crystals. Disseminated and partly weatheredsulfides give an iron-stained and spotted appearance.

Quarry 2 The second quarry represents the topmost portion of the first quarry and isreached on a small road running from the village center to the south, opposite of the roadto Castel Sardo. Instead of entering the quarry directly, one walks up a small footpath tothe right into the upper part of the quarry. This is an area where mineral collectors areactively mining amethyst specimens from fissures and miarolitic cavities (1-15 cm) withagate rims within partly welded pyroclastics (trachytic because of large sanidine crystals?).Chalcedony and quartz precipitated first and are partly covered with later calcite crystals.The best mineralized cavities appear to occur in the pyroclastics directly below a massive

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basaltic flow. This suggests that the heat from the basalt drove fluid circulation in themore permeable unit below. The quartz crystals appear to have formed rapidly out ofa supersaturated fluid since they often show skeletal growth. Fifty meters further to theeast, on the northern end of the quarry, there is a subvertical cm-wide cleft with smallamethyst sceptor quartz crystals inside.

SGM Exploration Lease at Abba Ruia 8.3 km after the junction to Osilo at thesouthern end of Sassari, there is a turnoff from the SS127 onto a small asphalt-coveredroad on which one reaches the core shed of SGM (a joint venture between the Italianmining company Progemisa and senior people from the former Newcrest mining companyof Australia) after 4 km. The exploration lease consists of a total of 37 separate areasnone of them bigger than 900 hectares on which exploration drilling is carried out. Theenvironmental sensitivities are great, especially on the south of the prospect since there isa valley with the water wells of Sassari. Importantly, the leases are handed out by the stategovernment of Sardegna based in Cagliari such that there is a pre-programmed conflict ofinterest between the state government and the government of the province (Nurra) withthe capital Sassari.The hydrothermal manifestations on the lease comprise ESE-WNW striking epithermalquartz-chalcedony-carbonate veins hosted by a set of faults with a small component ofdextral shear and a potentially conjugate set of NW-SE striking faults. The highest goldgrades of about 40 ppm occur over a width of several meters in pipe-shaped ore bodiesdefined by the intersection of the two fault sets. Concentric ring faults delineate thevolcanic complex and are roughly coeval with the mineralized fault zones. In the south ofthe prospect, sinters on the paleo-surface are exposed topping several Logudorru-grabenparallel normal faults. Individual veins with a length of up to 3 km, form en echelonarrays. The whole system has an east-west extent of greater than 8 km. The epithermalactivity finished before late rhyolitic and trachytic explosive base-surge deposits formed inthe Pliocene.The epithermal veins are named after exposure localities in the rolling hills south of Osilo.The most important veins are Bunnari, Sa Pala, Perda Bianca and Perda Edra. Theirmineralogy varies slightly and all show boiling fluid inclusion assemblages. Thus, the levelbelow boiling has not been reached in any of the exploration drill holes. The best surfaceexposures exist on a hilltop defined by the Bunnari vein and are recommended for theexcursion.The Perda Edra vein is discontinuously exposed over several km distances crossing theSS127 about 1.5km east of Osilo where the 3 m-thick vein crops out on the west shoulderof the road.In the exploration drill holes a transition occurs from fresh to altered andesite enrichedin potassium and bleached in the immediate vicinity of the veins. In the bleached zones,plagioclase phenocrysts are leached out and this secondary porosity is partly infilled by clayminerals, celadonite and sub-mm-sized tarnished pyrite crystals. This alteration envelope

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is typically less than 5 m wide. A breakdown of amphibole phenocrysts in the andesitemay be observed up to larger distances from the veins.The gangue minerals range from banded-laminated chalcedony with agate like vugs tofinely crystalline vuggy silica. Several cm-wide open clefts may contain euhedral skeletalquartz crystals some of which are amethysts. Other vein textures include host rock brecciasand sparry calcite which has been dissolved but is indicated by negative crystal shapes inmilky quartz precipitated later. With hand lense, a finely dispersed sulfides and free goldin the veins are visible. The veins contain several ppm gold (5-40 ppm) and up to 130ppm silver. Some quartz crystals show surface etching by acid steam. In a few localitiesadularia has been observed as alteration mineral. Preliminary fluid inclusion work identifieshomogenization temperatures of 160◦ to 230◦C and fluid salinities ranging from 0.5 to 8.5wt.% NaCl equivalent. In many holes the fluid salinities define a narrow range from 1 to2.5 wt.% NaCl equivalent. The paleo sub-surface depth of the examined vein intersectionsis interpreted as around 500 meters. The vein system is well suited for research since itdid not experience any post-mineralization overprint.

Accessibility Osilo is reached on the SS127 from Sassari. From the edge of the town ittakes 15 minutes to drive to Osilo center where there is a junction to the road to CastelSardo. The Perda Edra vein system is located on the mining lease of SGM. A permission isneeded to enter the lease. The exploration office of SGM is located on the SS127 betweenOsilo and Sassari. The chief geologist is Giovanni Funaiol. The company has an excellentcollection of remote sensing data processed by Earth Vision, a Perth-based geophysics firm.The team also explores other parts of Sardinia. Most other outcrops of Osilo veins andsinters are also inside of the exploration lease of SGM. The core shed of SGM is locatedto the east of the SS127, in a garden after a sharp corner on the descend from a limestoneplateau into a valley from which the view opens on Osilo.

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The Olmedo Bauxite deposits

6 Bauxite Formation in Sardiniaby José Enrique Pérez Lutgardo

6.1 General introduction: the formation of bauxite depositsOnce metals have been concentrated in the Earth’s crust and then exposed at its surface,they are commonly subjected to further concentration by chemical weathering. The rela-tionship between weathering and ore formation is often a key ingredient that leads to thecreation of a viable deposit and many ores would not be mineable was it not for the factthat grade enhancement commonly occurs in the superficial environment. There are sev-eral deposits where the final enrichment stage is related to supergene weathering processes.Bauxite deposits of economic importance form by these supergene processes.

In this section we present general processes relevant to the bauxitization that followexplanations by Robb (2005). We also present compiled information from MacLeanet al. (1997), Oggiano and Mameli (2001) and Mameli et al. (2007), addressingthe general geology of the Sardinian bauxites and results of studies with emphasis on theOlmedo bauxite mine.

6.1.1 Principles of Chemical Weathering

From a metallogenic point of view, chemical weathering can be subdivided into threeprocesses:

1. Dissolution of rock material and the transport/removal of soluble ions and moleculesby aqueous solutions

2. Production of new minerals, in particular clays, oxides and hydroxides, and carbon-ates

3. Accumulation of unaltered (low solubility) residual material such as silica, aluminaand gold

6.1.2 Dissolution and hydration

The relative solubilities of different elements in surface waters depends on a variety offactors, but can be qualitatively predicted (Fig. 29) in terms of their ionic potential (orthe ratio of ionic charge to ionic radius). Cations with low ionic potentials (<3) areeasily hydrated and are mobile under a range of conditions, although they will precipitateunder alkaline conditions and are readily adsorbed by clay particles. Similarly, anions withhigh ionic potentials (>10) form soluble complexes and dissolve easily, but will precipitatetogether with alkali elements. Ions with intermediate values (ionic potentials between 3and 10) tend to be relatively insoluble and precipitate readily as hydroxides. Over thepH range at which most groundwaters exist (5-9), silica is more soluble than aluminum

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(Fig. 30) and consequently chemical weathering will tend to leach Si, leaving behind aresidual concentration of immobile Al and ferric oxides/hydroxides. This is typical of soilformation processes in tropical, high rainfall areas and yields lateritic soil profiles, whichcan also contain concentrations of bauxite (Aluminum ore) and Ni. Lateritic soils will not,however, form under acidic conditions (pH<5) as Al is more soluble than Si (Fig. 30) andthe resultant soils (podzols) are silica-enriched and typically depleted in Al and Fe.

Figure 29: Simplified scheme on the basis of ionic potential (ionic charge/ionic radius) showing the relative mobility ofselected ions in aqueous solutions in the superficial environment (Robb 2005).

6.1.3 Lateritic Deposits

Laterite Formation Laterite is defined as the product of intense weathering in humid,warm, intertropical regions of the world, and is typically rich in kaolinitic clay as well asFe- and Al-oxides/oxy-hydroxides.Laterites form on stable continental land masses, over long periods of time. Laterites canbe subdivided into ferruginous (ferricretes) and aluminous (alcrete or bauxite) varieties.

Bauxite ore formation Bauxitic ore, in the form of the minerals gibbsite (Al(OH)3)/boehmite (γ-AlO(OH)) and diaspore (α-AlO(OH)), is the principal source of aluminiummetal. The demand for this metal has increased dramatically in the second half of thetwentieth century. In the upper zone of the lateritic profile the accumulation of an alumina-rich residuum, as opposed to one enriched in iron, is a function of higher rainfall, but alsolower average temperatures (around 22◦C rather than 28 ◦C for ferricretes) and higherhumidity. Actual alumina enrichment in the upper part of the laterite profile is mainly

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due to relatively high Si mobility compared to Al, and probably reflects near neutral pHconditions (Fig. 31).

Figure 30: a) The solubility of Si and Al as function of pH. b) The solubility of amorphous silica at 25 ◦C (from Robb2005).

Figure 31: Eh-pH diagram showing conditions relevantto the formation of laterites and bauxite ore. The sol-ubility contours are in mol/l and assume equilibrium ofthe solution with gibbsite and hematite/goethite (fromRobb 2005).

Seasonal climatic variations are also consid-ered important to the formation of bauxite oresas the alternation of wet and dry spells pro-motes fluctuations in groundwater levels and,hence, dissolution and mass transfer. Varia-tions in bauxitic profiles, as well as transfor-mation from hydrated gibbsite to the relativelydehydrated version, boehmite, or to diaspore(AlO(OH)), result from such fluctuations. Theredistribution of iron, and the segregation of Aland Fe, is a necessary process in bauxite forma-tion because ferruginous minerals tend to con-taminate the ore. High quality bauxite ores re-quire that both Fe and Si be removed, but notalumina, whereas ferricretes and conventionallaterites are characterized by different combina-tions of element leaching. The interplay of Ehand pH is critical to the formation of high qual-ity bauxite ores (Fig. (31).

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In terms of Eh and pH, Si is leachable by hydrolysis (Si4+ + 4H2O H4SiO44 + 4H+)and is increasingly soluble at higher pH, whereas Al is most soluble at either very low or highpH (Fig. 30a). Fe is most mobile as ferrous iron at low Eh and pH. In lateritic environmentswhere concentrations of both Fe and Al occur, it is the rather special conditions where thesetwo metals are segregated that provide the means for high quality bauxite formation.

6.2 The Olmedo Bauxite Deposits

Introduction

Figure 32: Geological sketch of northeast Sardinia showingthe position of bauxite occurrences (MacLean et al. 1997).

Bauxite deposits can be classified intotwo main categories, depending on thebedrock lithology: bauxite deposits over-lying (A) alumosilicate rocks and (B) car-bonate rocks. Bauxite overlying alumosili-cate rocks can be further subdivided into:(1) laterite bauxites that consist of residualdeposits and/or deposits that experiencedonly local redeposition; and (2) Tikhvin-type bauxites, represented by transported(allochthonous) deposits without any re-lationship with the original residual pro-file. Bauxite lying on carbonate rocks canbe identified as the ’karstic’ category, re-gardless of whether the bedrock surface iskarstified or not, or the degree of kars-tification. These karst bauxites repre-sent ∼14% of world resources and are welldeveloped around the Mediterranean area(e.g. Pyrenees-Provence, Sardinia, Greece).They are related to a period of extremeweathering conditions in the Middle Cre-taceous.

Location

The Olmedo bauxite deposit occurs in theNurra district of northwest Sardinia (Fig.32). The bauxites deposits are in sequencesof limestones and marls that have a com-plex history of deposition, uplift, subaerialerosion and karst weathering.

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Olmedo Geology

The Olmedo bauxite forms a stratiform unit within sequences of Cretaceous marine carbon-ates deposited up to mid-Cretaceous. Footwall rocks are predominantly lagoonal-lacustrinemarls and intraclastic carbonates typical of internal marine platforms.

In mid-Cretaceous the area was uplifted and a karst erosional surface formed underclimatic conditions favorable to bauxite formation. The Casa Galante - Su Zumbaru tec-tonic lineament was developed at this time with subsidiary renewed movement along LateHercynian basement faults (N60E and N150E, Fig. 32).

In the late Cretaceous, the region subsided and marine limestones and marls weredeposited on the bauxitic horizon. Overlying units of the stratigraphy are a thin biospariticlimestone (platform deposition) or a thick bioclastic limestone (deposit in the interiorparts of the platform), a tick and widespread succession of yellow to grey-black marls andcalcarenites, rare occurrences of green marl, covered by lenses of tuff, ignimbrite, ash andaltered volcanic products.

Uplift in Oligocene-Miocene time and ensuing erosion exposed the bauxite horizon in itspresent state. The Casa Galante - Su Zumbaru tectonic line marks the southern boundaryof bauxite occurrences in the Nurra. The bauxite horizon crops out continuously over astrike length of about 4 km, and has a fairly uniform thickness averaging 2.6 m (max. 5m). It trends ENE, and dips about 20◦ to the SSW.

Figure 33: Geological map and cross section of the mined area of Olmedo. The different types of bauxites deposits areevidenced both in the map and in the cross-section (Mameli et al. 2007)

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Deposit types

The Nurra deposits can be divided into four different types (Fig. 33 and 34):

Type 1: regular, stratiform, residual deposits (autochthonous) In this type ofdeposit the bauxite was derived from in situ lateralization of illitic marls (Purbeckianfacies). The profile consists of clayey bauxite near the footwall, which grades upwardsto lithic boehmite bauxite, with low hematite and goethite contents. Thickness isusually from 2 to 5 m and Al2O3 grade can exceed 70% in the central part.

Type 2: karst pockets. The bauxite and bauxitic clay infill karsts developed on cal-careous or dolomitic beds underlying the Purbeckian marls. They are derived fromthe Type-1 bauxite profile collapsed into karst pockets hosted in Late Jurassic rocks.

Type 3: detrital deposits. These deposits are uncommon and rest on post-Berriasianbedrock, i.e. on Urgonian (Barremian-Lower Aptian) shelf limestone with an averagethickness of 5 m on average. In places, the bauxite is missing and is replaced byconglomerates. They were formed by transportation and deposited bauxite detritusin topographic lows.

Type 4: irregular stratiform deposits. Located on dolomitic-limestones at the transi-tion between the Purbeckian and Urgonian facies, they are characterized by featurestypical for Type 1 and 3 deposits. The have a variable thickness and high-gradebauxite rest directly on carbonate substratum.

Figure 34: Different ore deposits typologies (Mameli et al. 2007)

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Type 1 and type 4 bauxite deposits are the most economic ones as they reach highgrades and volumes. Exploitation of type 2 bauxite deposits is rendered difficult due tothe complex geometry of karst pockets and mixing with different clays at various levels.

Mineralogy and texture

The bauxite deposit profile shows a basal limestone with overlying desiccated marl andargillite, passing upwards through bauxitic argillite, argillaceous bauxite and compact redand white bauxite (Fig. 35 a). Calcite and minor dolomite constitutes up to 25% of thealtered marl, and the remainder of this unit and the overlying argillite are visibly composedlargely of illite, kaolinite, montmorillonite, quartz, hematite and minor goethite.X-Ray diffraction (XRD) studies on the Nurra bauxites indicate that the porous matrix iscomposed of kaolinite (Al2Si2O5(OH)4); boehmite(γ−AlO(OH)); goethite(α−FeO(OH))and anatase with traces of its polymorph rutile (TiO2). The ooids are composed of con-centric and irregular shells of alternating boehmite and goethite (and/or hematite) (Fig.35 b).

Figure 35: a) Profile showing the mineralogy and texture of the bauxite deposit b) Compilation profile of normativeminerals plotted against total Al2O3 in wt%. Cc, calcite; Chl, chlorite; Qz, quartz; Ser, sericite; Hem, hematite; Kaol,kaolinite; Bhm, boehmite (MacLean et al. 1997)

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Element mobility during the bauxitization process

Studies in the Nurra district suggest that bauxites are formed by weathering of clay-richdebris accumulated onto bedrock. The bauxitization proceeded from the surface down-wards, with the build-up of residual ’immobile’ elements (Al, Ti, and HFSE), involvinglarge-scale mobility and ultimate loss of SiO2 and Fe2O3 with increasing Al2O3.Although mostly immobile, Al was removed in small amount (∼ 1.2 wt% of Al2O3 avail-able) mainly form the white bauxite (Fig. 35 b) and added to the argillite altered marl.Al is least soluble in neutral pH groundwater; hence its downward migration in solutionmay have been enhanced by seasonal fluctuations in the acidity of saline groundwater.The same factors can be invoked for the formation of Fe oxy-hydroxide throughout theprofiles. Fe-rich horizons concentrate REE (mostly LREE), Y, U and transition elements(Ni, Co), probably because of the scavenging action exerted by goethite. However, highREE contents not correlated with Fe2O3 are due to the occurrence of REE minerals (bast-nasite group). Internal complexities in Type-1 and Type-4 bauxites have been identifiedbut are not described here. Immobile elements ratios and distributions show that the al-tered marl, argillite and bauxite were derived from the underlying argillaceous limestoneand marl. The immobile elements are also used to quantify the losses of mobile material(SiO2,MgO,K2O, etc.).

.

Figure 36: Eu-anomaly vs. TiO2/Al2O3(a) and vs. Ti/Cr (b) diagrams. Eu/Eu*and T iO2/Al2O3 depart from the aver-age composition of upper crustal rocks andpoint towards a more mafic composition.A similar conclusion is reached using theTi/Cr ratio. In general, the Nurra baux-ites have Ti/Cr values decisively lower thanthose of the UCC, further supporting amafic source for the argillaceous sedimentsfrom which the bauxite is formed. UCC:Upper Continental Crust (Mameli et al.2007)

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Parental affinity

Conservative indices, including TiO2/Al2O3 and Ti/Cr ratios and the Eu anomaly, suggestthat beds from which the bauxites formed are derived from mafic rocks of the Variscanbasement of western Sardinia (Fig. 36). This implies that the Variscan basement wasexhumed and eroded during the middle Cretaceous.

The Olmedo Mine

Until the 1980s, Sardinian bauxite was poorly known and unexploited. In the 1980s and1990s, a joint venture aimed to assessed the economic potential of the main deposits,triggering new research and exploration and also leading to the opening of a mine close toOlmedo village. Between 2002 and 2007, 800,000 tons of bauxite were mined, with Al2O3grades as high as 62.5 % and SiO2 about 9% on average. Most of the production derivedfrom underground exploitation. The geometry of the bauxite bed allows mining along thedip of the bed using room and pillar excavation. The probable reserves cover an area of6.5 km2 and have been estimated at 30 Mt; proven reserves within the area of the mineare 3.8 Mt.The Olmedo mine is characterized by extensive type 1 deposits developed on Purbeckianmarls. Most of the current production comes from type 1 deposits, the remaining fromtype 4 deposits. The Type-1 deposits have a average thickness of 2 m, and a kaolinite-illitesubstratum that grades into Fe-rich, oolitic, kaolinitic bauxite.

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High grade metamorphic basement

7 High Grade Metamorphic Complex in Sardiniaby Melissa Ortelli

Several sections of this chapter are translated from the Sardinia excursion field guide ofVonlanthen et al. (2005).

The High Grade Metamorphic Complex (HGMC) is mainly built of more or less migma-tized paragneiss and orthogneiss. Metabasic lenses in the HGMC outcrop at the “GolfoAranci” or the “Punta de li Tulchi” (Stop 1 and 2, Fig. 37). They partly have eclogiticand amphibolitic facies metamorphic grade. The paragneiss is believed to originate fromPrecambrian sedimentary sequences, and the protolithes of orthogneiss and metabasite areintrusives of an Ordovician magmatic event (Chapter 1).

Figure 37: Simplified geological map of the Hercynian terrains in the northern part of Sardinia (see Chapter 1, after Carosiand Palmeri 2002)

During the Hercynian orogenesis, the HGMC underwent a metamorphic history thatcan be divided into two stages:

1. Pre-collisional subduction of the Paleotethys resulted in the burial of the lithosphereto a certain depth of around 50 to 60 km, producing eclogite and amphibolite fa-cies metamorphism. Contrary to their Corsican equivalents, the HGMC ortho- andparagneiss from Sardinia show no trace of this HP phase. This absence indicateseither a shallower burial depth for the gneiss compared to the metabasite followed bytheir late juxtaposition or a complete mineralogical reequilibration during exhuma-tion. This question is not yet solved to date. During the Barrovian phase, the HGMCmetabasite remained at depth, while the overlying sections (today eroded) formed athrust nappe which spread over the nappe’s zone toward the South-West.

2. The post-collisional relaxation of the compressive tectonic event during the end ofthe orogenesis led to the exhumation of the HP rocks with erosion and denudationof the chain. During the uplift, the complex suffered an adiabatic decompressionthat caused a partial anatexis, and paragneiss migmatization. This event of HT/LPinspired the name of the complex : High Grade Metamorphic Complex.

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7.1 Intensity and age of metamorphismThe visible mineralogical assemblage in the ortho- and the paragneiss (K-Feldspars, quartz,aluminosilicate, garnet, biotite) and their migmatization witness P-T conditions of up-per amphibolite to granulite facies (700 ◦C / 1 GPa). Basic rocks are partly metamor-phosed to the eclogitic facies (750 ◦C / 1.3-1.7 GPa; presence of garnet and clinopyroxene:Franceschelli et al. 2002; Giacomini et al. 2005). Some of them have later beenreequilibrated during the following amphibolites facies metamorphism (Fig. 38). Manydating studies have been conducted on the Golfo Aranci and Punta de li Tulchi rocks. Zir-con U-Pb dating of eclogite yield 460±5 Ma, which is considered as the formation age ofthe magmatic protolith (gabbro) during the Ordovician magmatism. The age of HP meta-morphism has not yet been precisely determined. However it is related to the subduction ofthe paleotethys lithosphere just before the Hercynian collision sensus stricto. Amphiboliteand paragneiss yield an age of 352±3 Ma and 344±7 Ma respectively, corresponding to theHercynian orogenesis.

7.2 MigmatizationThis section is compiled from Winter (2001)

Migmatites are partially molten metamorphic rocks, which contain mesosomes andleucosome components. The main mechanism responsible for their formation is the partialmelting, or anatexis, of a rock with pelitic or granitic composition. The first portions ofthe original rock to melt are the ones which are rich in white micas (muscovite). Thedestabilization of these minerals locally allows the lowering of the rock liquidus by waterrelease. The general anatexis process can be simplified by the next reactions:

1. Release of water:

KAl2AlSi3O10(OH)2 + SiO2 KAlSi3O8 + Al2SiO5 +H2O (5)

2. Lowering of liquidus and anatexis:

KAl3Si3O10(OH)2 + (Ca,Na)Al(Al, Si)3O8 + SiO2 +H2O Al2SiO5 +melt (6)

The temperature at which these reactions occur depends on the partial pressure ofH2O. In reaction 5 for example, the destabilization of muscovite will be increased bylowering of the partial pressure of H2O. Aluminosilicate formed by reaction 6 can bedissolved in the melt, which becomes peraluminous (Al2O3 > CaO + Na2O + K2O).The melt later recrystallizes to form leucosomes, and the refractory minerals remain asmesosomes. This segregation is supported by either (1) a differential pressure within therocks allowing a concentration of liquid in cracks or (2) by the lowering of the surfaceenergy at the interface solid-liquid. For example, the contact between a siliceous magmaand feldspar grains is energetically more favorable than the contact between the same meltwith biotite. This explains why the melt will preferably concentrate along feldspar-rich

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Figure 38: P-T-t paths of a) basic rocks and b) ortho- and paragneisses. (E) eclogite, (GR) granulite, (HT-amph) hightemperature equilibrated amphibolites, (MT-amph) medium temperature equilibrated amphibolites, (Pg out 1) Disappear-ance curve of paragonite for a H2O partial pressure at 0.1, (Pg out 2) same for H2O partial pressure at 0.9, eclogite-granuliteboundary (Ringwood and Green 1966), (Ms out 1) partial melting by dehydratation of muscovite (Spear et al. 1999),(Ms out 2) same (Patiño Douce and Harris 1998), (Opx in) appearance of orthopyroxene (Spear et al. 1999)

zones. It is only after reaching 30-40 % volume melt fraction that a transport over greaterdistance becomes possible. Different textures can develop according to the partial meltrate (Fig. 39). However, similar textures like the ones in migmatites can be formed bydifferent mechanisms, such as preferential dissolution of quartz by metasomatic fluids andreprecipitation.

7.3 Field stops during the excursionStop 1 Punta Falcone / (41◦15’25”N; 9◦13’31”E)

The tafoni (Fig. 39) are granitic blocks with typical erosional features found in semi-arid(around the Mediterranean Sea for example) or arid (desert) climates. They show rounded,decimetric to metric cavities that are often cut into the blocks from their bottom to their

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Figure 39: Tafoni example from the KangarooIsland, Australia (Photo: A. Hennig http://www.mineralienatlas.de/lexikon/index.php/Tafoni) Precipitation from element present in thewater: superficial hardening and formationof a protecting crust. Upwelling of aqueoussolution by capillarity allowing salt crystallizationaccelerating the erosion.

upper part. These cavities are formed by differential erosion, as a function of lithology,texture, and structure of the rock (Conca and Rossman 1985), and also of externalfactors like the velocity of wind and water streaming, ambient moisture, soil composition(Dragovich 1969). Secondary and superficial precipitation of mineral on granite surfacecan form where high evaporation condition occur (sun-exposed surfaces), and resist erosion.

The Punta Falcone is the northernmost point of Sardinia (apart from the Razzoli andLa Maddalena Islands). Tardi-and post-hercynian magmatic rocks with shreds of HGMCgneiss outcrop along the coast. Good examples of magmatic textures can be seen in thebasic rocks: fluidal textures, magma mingling and/or mixing.

The following outcrops (Golfo Aranci and Punta de li Tulchi, Fig. 37) belong to theHGMC of the axial Zone. They regroup mainly orthogneiss, migmatized pelitic paragneissand basic lenses (amphibolites and eclogites) of variable dimension (a few meters to 2kilometers).

Stop 2 Golfo Aranci (Migmatites), 41◦00’18”N; 9◦37’26”E

compiled from Giacomini et al. (2005)

The Golfo Aranci (Fig. 41)is an outcrop of the HGMC with good examples migmati-zation and deformation processes in rocks (Fig. 40). Giacomini et al. (2005) summarize:

“The migmatitic paragneisses are usually coarse-grained pelites with stromatic fabricsand well-developed foliation defined by alternating lensoidal leucosomes and mesosomes.Nebulitic agmatic textures are less abundant Felsic orthogneisses bodies (mainly two micas,K-feldspar gneisses with augen texture) occur within the paragneisses. Foliation strikescommonly N180-140◦ and is often subvertical or steeply E dipping.”“Banded amphibolites with minor ultramafic layers and amphibolitized ecolgites crop outin the region as large boudins within the migmatites and their setting is concordant with

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http://www.mineralienatlas.de/lexikon/index.php/Tafoni

http://www.mineralienatlas.de/lexikon/index.php/Tafoni


Figure 40: Some examples of migmatite tex-tures (Mehnert 1968): (a) brecciated : ag-matites (b) reticulate with possible injectionof external leucosome : arterites (c) raft-like(d) veiniform with leucosome derived from thehost rocks : veinites (e) stromatic or banded (f)boudinate (dilatation) (g) Schlieren, (h) nebu-litic.

the main regional foliation. Amphibolites are foliated and layered rocks: the layers aredefined by varying relative modal amounts of amphibole (± garnet) and plagioclase.”“The amphibolitized eclogites are clinopyroxene, garnet and amphiboles-rich rocks andare easily distinguishable from the amphibolites by the large modal amount of garnetporphyroblasts.”

Stop 3 Punta de li Tulchi (Migmatites, basic lenses), 40◦44’39”N; 9◦42’54”E

The following explanations are taken from Cruciani et al. (2001): Along the coastbetween Porto Ottiolu and la Punta de li Tulchi (Fig. 42) migmatites are exposed, whosestyle and intensity of migmatization are variable. A good example of a metabasic lensecrops out at la Punta de li Tulchi. It is oriented N80◦ NE60◦, and contains relics of aneclogite facies assemblage. The eclogitic rocks exhibit a brownish to greenish compositional

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Figure 41: Geological map of the Golfo Aranci (Giacomini et al. 2005)

layering, and are cross-cut by several quartz-rich veins and by a quartz-feldspat vein. Theyare characterized by two main compositional layers: the garnet + pyroxene layer and theplagioclase + amphibole layer. Both types of layers range in thickness from 20 to 50 cm.

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Figure 42: Geological sketch map between Porto Ottiolu and Punta de li Tulchi. (Cruciani et al. 2001)

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1 introduction to the geology and metallogeny of sardinia

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