Geology and Metallogeny of Sardinia - Excursion Guide SEG Student Chapter Geneva - 11th - 18th of June 2011
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Geology and Metallogeny of Sardinia
Excursion Guide
SEG Student Chapter Geneva
11th - 18th of June 2011
Editors: Johannes Mederer and Cyril Chelle-Michou
Contents
1 Introduction to the Geology and Metallogeny of Sardinia 8
1.1 Geology of Sardinia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.1.1 Paleogeographic evolution . . . . . . . . . . . . . . . . . . . . . . . 8
1.1.2 Geological overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.2 Metallogeny of Sardinia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.2.1 Metallogenic periods . . . . . . . . . . . . . . . . . . . . . . . . . . 19
1.2.2 Mining in Sardinia: Historic overview . . . . . . . . . . . . . . . . . 25
2 Pre-Hercynian Stratiform Pb-Zn-Ba Mineralization in the Iglesiente-
Sulcis Area 27
2.1 Description of mineralization and host rocks . . . . . . . . . . . . . . . . . 29
2.1.1 SEDEX type mineralization within the Lower Cambrian Punta Manna
and Santa Barbara formations . . . . . . . . . . . . . . . . . . . . . 29
2.1.2 MVT type mineralization within the lower Cambrian San Giovanni
formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
2.1.3 MVT type mineralization at the Ordovician unconformity . . . . . 33
2.2 Isotopic analyses and their interpretation . . . . . . . . . . . . . . . . . . . 33
3 Supergene Carbonate-Hosted Nonsulfide Zinc Mineralization: The “Calamine”
of Southwest Sardinia 37
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.2 Supergene nonsulfide zinc deposits . . . . . . . . . . . . . . . . . . . . . . 37
3.3 Supergene carbonate-hosted nonsulfide zinc deposits in the Iglesiente district
of SW Sardinia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.4 “Calamine” occurrences to be visited during our excursion . . . . . . . . . 40
3.4.1 The Nebida mining area. Old exploitation at Canale San Giuseppe 40
3.4.2 The “Calamine” at Miniera di Monteponi . . . . . . . . . . . . . . . 40
4 Albitite Deposits of Central Sardinia: an Overview 42
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
4.2 The albitite deposits of central Sardinia . . . . . . . . . . . . . . . . . . . . 42
4.3 Magmatic versus metasomatic origin of albitites: a discussion . . . . . . . . 45
5 The epithermal deposits of Osilo and Tresnuraghes and epithermal pro-
cesses in generale 47
5.1 Introduction: epithermal and other hydrothermal deposits of Sardinia . . . 47
5.2 Epithermal systems in general . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.3 Kaolinite rich epithermal deposits of Sardinia . . . . . . . . . . . . . . . . 50
6 Bauxite Formation in Sardinia 58
6.1 General introduction: the formation of bauxite deposits . . . . . . . . . . . 58
6.1.1 Principles of Chemical Weathering . . . . . . . . . . . . . . . . . . 58
2
6.1.2 Dissolution and hydration . . . . . . . . . . . . . . . . . . . . . . . 58
6.1.3 Lateritic Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
6.2 The Olmedo Bauxite Deposits . . . . . . . . . . . . . . . . . . . . . . . . . 61
7 High Grade Metamorphic Complex in Sardinia 67
7.1 Intensity and age of metamorphism . . . . . . . . . . . . . . . . . . . . . . 68
7.2 Migmatization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
7.3 Field stops during the excursion . . . . . . . . . . . . . . . . . . . . . . . . 69
References 74
3
Introduction
Mining activity in Sardinia goes back to prehistoric times. Even if most of the small mines
which were operating in Sardinia (Chapter 1) are now closed, the island still remains of
high interest from a metallogenic point of view. During our one-week excursion, we will be
able to visit only some of the highlights of geological interest. Table 1 shows our scheduled
program and Figure 1 an overview map of Sardinia with different field stops. The following
Chapters 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 points
of interest we will visit and reflect the choice we necessarily had to make. We start with
the southwestern Iglesiente-Sulcis district where the relationship between Early Paleozoic
stratiform base metal mineralization (Chapter 2) and later supergene zinc mineralization
(Chapter 3) can be observed. Epithermal and porphyry-type mineralization can be found
on the island and is related to widespread Oligocene-Miocene magmatic activity (Chapter
5). We will visit an active mine that nowadays produces high-quality feldspar concentrates
from albitites with metasomatic origin (Chapter 4). In the north-west of the island, the
Olmedo bauxite deposits (still in operation, Chapter 6) are related to extreme weathering
conditions during Middle Cretaceous times. On our way back from the northern point of
the island to Cagliari, we will visit high-grade metamorphic rocks including migmatites
and eclogite facies rocks of the Hercynian orogenesis (Chapter 7).
We want to thank Thomas Driesner and Pierre Vonlanthen who provided us with field
guides from former excursions (Matthai 2000 and Vonlanthen et al. 2005) which
were invaluable for the planning and the organization of this trip. Our thanks also go to
the SEG Regional Vicepresident Europe Maria Boni and to Lluís Fontboté (University of
Geneva), 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 have
been possible.
Geneva, June 2011 - Cyril Chelle-Michou and Johannes Mederer
4
Figure 1: Road map of Sardinia with some points of interest and overnight stops during the excursion. North-South
extension of the island is about 250 km. Source: http://maps.google.com
5
Date Guide Program Overnight
Saturday Maria Boni Incoming flight from Geneva easyjet 1507, Hotel Artu
2011 06 11 departure 11:00 arrival in Cagliari at 12:30, pickup rental car, Iglesias
drive to Iglesias, field introduction with Maria Boni +39 078122492
Sunday Maria Boni Iglesiente district, stratiform base metal deposits and supergene Zn deposits Hotel Artu
2011 06 12 Iglesias
+39 078122492
Monday Maria Boni Iglesiente district, stratiform base metal deposits and supergene Zn deposits Hotel Artu
2011 06 13 Iglesias
+39 078122492
Tuesday Maria Boni and Iglesiente district, stratiform base metal deposits and supergene Zn deposits Albergo Sas Benas
2011 06 14 Marcella Palomba In the afternoon: drive to Santo Lussurgiu and meet Marcella Palomba Santo Lussurgiu
6
+39 0783550870
Wednesday Marcella Palomba Albitites in the Orani mine, Tresnuraghes kaolin deposit Albergo Sas Benas
2011 06 15 Santo Lussurgiu
+39 0783550870
Thursday Giacomo Oggiano Drive to Sassari and meet with Giacomo Oggiano, Olmedo bauxite deposit Hotel La Funtana
2011 06 16 Depending on time constraints: Osilo epithermal system or a kaolin deposit Santa Teresa Gallura
+39 0789741025
Friday Melissa Ortelli Migmatites and metamorphic core complex in the NE of Sardinia, on the way Hotel Sardegna, Cagliari
2011 06 17 back to Cagliari: close to Tortoli contact between lava flows and carbonates +39 070286245
Saturday Free time in Cagliari
2011 06 18 Outgoing flight from Cagliari at 13:00, arrival in Geneva at 14:35
Table 1: Program for the excursion
Participants from University of Geneva
Dr. Honza Catchpole UNIGE Honza.Catchpole@unige.ch
Cyril Chelle-Michou UNIGE Cyril.Chelle-Michou@unige.ch
Pierre Hémon UNIGE hemon.pierre@gmail.com
Cristina M. Tomé UNIGE + IGME Spain cris.martinez.tome@gmail.com
Johannes Mederer UNIGE jo.mederer@gmail.com
Melissa Ortelli UNIGE melissa.ortelli@unige.ch
José Pérez UNIGE jeplutgardo@yahoo.com
Field guides from Sardinia and Italy
Prof. Maria Boni University of Napoli boni@unina.it
Dr. Marcella Palomba University of Cagliari mpalomba@unica.it
Prof. Giacomo Oggiano University of Sassari giacoggi@uniss.it
Table 2: Participants and field guides
7
Geology and Metallogeny of Sardinia
1 Introduction to the Geology and Metallogeny of
Sardinia
by Cyril Chelle-Michou
1.1 Geology of Sardinia
1.1.1 Paleogeographic evolution
Translated and modified from Vonlanthen et al. (2005)
Figure 5 summarizes the major events relevant to the paleogeographic evolution of
Sardinia.
Palaeozoic
From Cambrian times until the beginning of the Ordovician, the old Sardinian basement
was split into two terrains located at the northern border of the Gondwana supercontinent
(Figure 2). This period is characterized by important accumulation of terrigenous and
carbonaceous 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 Rheic
Ocean and the opening of Paleotethys. Sardinian terrains are characterized by pelagic
sedimentation at this time.
Collision of Hun and Laurasia started at the beginning of the Carboniferous marking the
beginning 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-oceanic
Paleotethys 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 Precambrian
basement and the volcano-sedimentary rocks that were accumulated during the Paleozoic
were deformed, metamorphosed and thrusted. Two phases are distinguished:
1. an early HP metamorphism episode directly related to deep burying of subducted
terrains
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, and
extensional regime. Adiabatic decompression of rock provoked HT-LP metamorphism.
8
Geology and Metallogeny of Sardinia
Associated plutonic rocks were emplaced at this period (until the Permian) and form the
Hercynian Batholith.
Mesozoic
In the Trias, Sardinia was part of the Laurussian continent and was situated close to Spain
and the Balearic Islands but away from Corsica (Fig. 3).
In Jurassic time a transform fault system located in the future Pyrenees placed Sardinia
and Corsica next to each other.
In the Cretaceous, the Iberian plate (also containing the Corsican-Sardinian bloc) breaks
away from Laurussia. Sardinia constituted the western extension of the Briançonnais
Domain (Fig. 3).
Important sedimentation occurred during the Mesozoic; first during the Trias, dominated
by continental evaporite deposition, and later during the Jurassic and Cretaceous, by
mostly marine sediments.
Cenozoic
During the Oligocene, the Briançonnais Domain was affected by the alpine orogeny while
the Corsica-Sardinia bloc broke away from the Iberian plate and rotated counter-clockwise
to reach its present position. This caused the opening of the Liguro-Provencal back-arc
basin and slab-rollback and subduction along the Franco-Spanish margin (Fig. 4). This
subduction 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 of
the Mediterranean region leading to the detachment of the Rif-, Betic and Kabyle terrains
as well as the Balearic Islands.
By the end of the Oligocene, migration of the Corsica-Sardinia block was stopped by
the Adriatic continental plate. The extensional regime moved eastward from the Liguro-
Provencal Basin to what became the Tyrrhenian Basin. Calabria moved to its current
position at this time.
A new volcanic cycle during the Plio-Pleistocene is attributed to this newly established
extensive regime caused by the opening of the Tyrrhenian Basin. Volcanism associated to
the subduction moved further South to the Aeolian Islands (Etna).
Sedimentation during the Cenozoic was mainly coastal to continental, intercalated with
short marine cycles.
1.1.2 Geological overview
Translated from Vonlanthen et al. (2005) and copied and modified from Marcello
et al. (2004)
In Sardinia the sedimentary, magmatic and metamorphic records are well represented
within three main lithological complexes:
9Geology and Metallogeny of Sardinia
Figure 2: Paleogeographic reconstruction from Cambrian to Carboniferous times (Stampfli and Borel). The red circles
show the position of Sardinian terrains
10Geology and Metallogeny of Sardinia
Figure 3: Paleogeographic reconstruction from Jurassic to Cretaceous times (Stampfli and Borel 2004). Red circles show
to position of Sardinian terranes
1. a mainly Paleozoic basement that underwent repeated phases of deformation and
metamorphism 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 Southern
Europe
3. a Cenozoic to Quaternary volcanic and sedimentary cover consisting of shallow-water
marine 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 Variscan
chain, which after the Cenozoic drifting of the island shows a NW-SE trend and crops out
with good continuity. From south to north its structural framework includes the following
three different zones:
1. a thrust-and-fold belt foreland consisting of a sedimentary successions ranging in age
from 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 conditions
and occupies the centre of the island. It consists of a Paleozoic metasedimentary
succession hosting a thick continental arc-related volcanic suite (Nappe Zone)
3. an inner zone that includes a high-grade metamorphic complex juxtaposed to a
medium-grade metamorphic complex along a mylonitic belt (Internal Zone), located
in the north-east of Sardinia
The stratigraphic lowermost sequence in SW Sardinia (Iglesiente- Sulcis region), is
probably of Pre-Cambrian age and consists of mainly terrigenous metasediments, These
11Geology and Metallogeny of Sardinia
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 dolomitic
intercalations that grade upwards into shales, metasiltites, and metasandstones (Bithia
Formation, Fig. 6).
The overlying Nebida Formation, the oldest fossiliferous terrain, mostly consists of terrige-
nous metasediments with minor oolitic limestones containing Early Cambrian Archaeo-
12Geology and Metallogeny of Sardinia
Figure 5: Geological evolution synthesis of Sardinia.
13Geology and Metallogeny of Sardinia
cyathas, Trilobites and algal stromatolites. It is believed to represent a continental shelf
environment with an eastwards prograding deltaic systems (Matoppa Member) that evolved
into an oolitic lagoonal environment. This terrigenous formation grades upwards into a
thick 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)
14Geology and Metallogeny of Sardinia
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.), that
is rich in Middle Cambrian trilobites, echinoderms, and brachiopods (Fig. 6).
The overlying deeper-environment member consists of a 400-m thick neritic terrigenous
succession in which the youngest levels contain achritarcs and graptolites of Tremadocian
age (“Argilloscisti di Cabitza”). These lagoonal and epicontinental carbonate and terrige-
nous deposits correspond to thick siliciclastic sequences in the Nappe Zone (San Vito and
Solanas Formations).
All over central and SE Sardinia, Middle Cambrian-Early Ordovician metasedimentary
sequences are overlain by an Arenigian-Caradocian metavolcanic complex, which includes
several 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 and
basic ones. These features are typical of an orogenic suite involving continental crust.
Extensive evidence supports the hypothesis of a magmatic arc connected to a subduction
of the oceanic lithosphere under the northern Gondwana margin. The arc-trench gap was
incorporated in the Internal Nappes (Mount Gennargentu, Baronie region).
The back-arc basin in the Iglesiente-Sulcis region is devoid of calc-alkaline magmatism and
underwent an Early Hercynian compressional event. This post-Tremadoc and pre-Caradoc
phase 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 folded
before the Caradocian (“Sardinian Unconformity”). The products of the subsequent ero-
sion reach up to several hundreds of meters of thickness (“Puddinga” Auctt.). This angular
unconformity has also been reported in south-eastern (Sarrabus-Gerrei region) and central
Sardinia, where the Cambrian-Lower Ordovician successions are often separated from the
Middle- to Upper-Ordovician volcanics and sediments by conglomerates that are mainly
derived 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 sediments
that show a large variability in thickness and facies (“Caradocian Transgression”) and are
interbedded with alkalibasaltic metavolcanics. The transgressive Late Ordovician deposits
grade upwards to neritic clay and carbonate deposits (“Ashgillian Limestones”) followed
by 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 behaved
as 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 and
nodular Tentaculite-bearing limestones, and then Lower Carboniferous (Tournaisian) thick
pelagic limestones, locally replaced, in a few internal areas (Nurra, Baronie) by thick ter-
rigenous sequences. In the outermost platforms the carbonate sedimentation was suddenly
interrupted and changed to Culm-type syn-orogenic deposits.
15Geology and Metallogeny of Sardinia
Hercynian terrains The above structural framework and the related stratigraphic evo-
lution mainly show the effects of the Hercynian orogeny, superimposed upon those of the
Caledonian, 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 continental
margin is represented by the Internal (Nappe Zone) and External (External Zone) Nappe
Complexes of central and southern Sardinia (Fig. 7). The two domains are separated by
the “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 classic
fold-thrust belt characterized by medium- to steeply-dipping thrusts, fold axial planes and
cleavages, 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.
16Geology and Metallogeny of Sardinia
An important extensional event also developed in the Sardinian Variscides as a response
to gravitational re-equilibration within the collisional structure. The extensional evolution
is confined to a time interval extending from the end of the collision to the emplacement of
the 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 was
affected by a number of movements that account both for its present position and for the
evolution 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 transcurrent
fault. A long continental period began, and in Sardinia it was characterized by structural
highs and lows; a wide peneplanation was reached.
Figure 8: Stratigraphic relationships between Jurassic formations of eastern Sardinia (Dieni and Massari 1985)
17Geology and Metallogeny of Sardinia
Mesozoic rocks The western margin of the massif (together with its adjacent north-
western areas) was bordered by wide platforms and shallow, narrow intracratonic basins
eastwards. In these basins Germanic Facies sediments were being deposited (Genna Selole
Formation). The eastern margin of the massif represented the western border of the Triassic
Alpine Basin.
A transgression started in the Triassic, and during the Jurassic gradually extended towards
the eastern part of the massif. On this platform a Provençal Facies carbonaceous complex
was deposited (Dorgali, Monte Tului, S’Adde and Monte Bardia Formations, Fig. 8).
Confined pelagic episodes also attest to syn-sedimentary extensive tectonics, as an early
sign of the opening of the Ligurian-Piedmontese oceanic basin.
The tearing of the sialic crust during the Upper Jurassic separated the Sardinian-Corsican
Massif from the Tuscan one. An oceanic basin opened with simatic crust (ophiolites) and
pelagic sediments, which evolved to flysch in the Upper Cretaceous.
The Sardinian-Corsican massif has continued to evolve together with the stable side of
the European plate up to present, while eastward, the Tuscan basement followed the new
evolution phenomena of the Apennine continental margin. This is consistent with the
Cretaceous carbonaceous platform sediments occurring in Sardinia.
Cenozoic rocks By the end of the Cretaceous, the closure of the Ligurian-Piedmontese
basin commenced and the African continent again approached the European. The collision
occurred during the Middle Eocene. However, clear evidence of compressive phenomena
related 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 is
mostly composed of tholeiitic to alkaline basalts. Rhyolite and rhyodacite are scarce in
this second phase.
During the Miocene some Sardinian structural lows (“Sardinian Rift”) were covered by
epicontinental seas, others formed lagoonal or lacustrine basins. This sea separated two
emerged areas, south-west and north-east relative to the rift. This transgression is related
to the opening of the Algerian-Provençal basin, and is characterized by mainly terrigenous
deposition, 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 are
attested by Tyrrhenian fossiliferous sediments (“Panchina”). The filling of the lagoonal
basins continued, and today a few lagoons still exist, mainly at the extremities of the
Campidano plain. The alkaline volcanic activity also ceased in the Pleistocene, leaving
cinder cones and basalt flows.
18Geology and Metallogeny of Sardinia
Figure 9: Simplified geological map of the post-Hercynian cover (Carmignani 2001)
1.2 Metallogeny of Sardinia
In 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 resources
have been exploited since Prehistoric time until now.
From an economic point of view Pb-Zn (±Ag±Cd) and epithermal Au deposits are the
most 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 10
syntheses 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
19Geology and Metallogeny of Sardinia
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. It
yielded different ore-mineral accumulations according to the variable paleogeographic con-
ditions at times of sedimentation and diagenesis of the Cambrian Middle (i.e. the Gonnesa
Fm; 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 the
overlying silty-shaly formations. From the lower to the upper section of the sequence, these
deposits 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 occasional
PbS
4. synsedimentary (possibly volcano-sedimentary) low grade stratabound depositions of
disseminated 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 for
Ba (local values also exceed 1000 ppm), Pb and Zn (20-100 ppm). The deposis described
in (4) and (5) were often sub-economic, but several important ore-bodies were generated
by 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, very
pure F e2 O3 , BaSO4 , andCaF2 as karstic accumulations, and BaSO4 pebbles in the basal
conglomerate of the overlying Arenig.
The third metallogenic period took place from the Middle Ordovician to the Early
Devonian 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 the
Silurian black shales of the entire island; they show evident sedimentary features
(load cast, slumping, diagenetic fracturing). These deposits are quite similar to the
Meggen and Rammelsberg deposits in central Germany, although of far smaller size
(not exceeding a few hundred thousand tons);
20Geology and Metallogeny of Sardinia
2. stratabound antimonite and scheelite concentrations, often with interesting gold con-
tents, are hosted in a volcano-sedimentary sequence outcropping in the SE corner of
the island (Sarcidano, Gerrei, and Sarrabus Regions). A connection with coeval vol-
canics seems much more evident in this case. The ore-bearing formation could be
considered an extension of the well known Middle Palaeozoic horizon to Sardinia,
hosting the stratabound Sb, Hg, W, and As deposits, and running all along the
Alpine chain and the southern Appennines (Calabria, Monti Peloritani);
3. gold occurrences, geographically close to those of the above group, are associated
with metavolcanics. Although gold anomalies have been known for some time, only
very recently a veritable gold deposit has been discovered and is currently under
exploration;
4. oolitic iron ore accumulations are interbedded in Silurian slates in close connection
with a mafic laccolith. These deposits occur in the NW corner of the island (the
Nurra Region).
The fourth metallogenic period is related to metamorphism and magmatism of
the 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 as
follow (in decreasing order of economic interest):
1. hydrothermal base metal and industrial-mineral veins, some of which were among the
most 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 at
10-11 % combined Pb+Zn, 500-1000 ppm Ag in galena Pb, and 1000 ppm of Cd in
sphalerite. Another very important example is the fluorite-barite-galena deposit of
Silius, whose original reserves included 30 Mt of CaF2 , 15 Mt of BaSO4 , and 1.5
Mt of PbS). One particular vein system is characterized by its richness in Ag, at
places with recoverable quantities of galena, and minor sphalerite. This type is only
known in SE Sardinia (Sarrabus Region), and is assumed to be a reconcentration of
a previous stratabound deposition of the third period
2. skarn deposits, generated by contact metamorphism of previous protores and/or
metasomatic replacement. In such bodies pyrite, pyrrhotite, hematite, and magnetite
are frequent, while other ore minerals (mostly sulfides) were mobilized and driven
away to various extents. Iron ore deposits, generally depleted in sulfides, may thus
have 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 also
occur 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 also
showed the presence of gold in one of these occurrences
21Geology and Metallogeny of Sardinia
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 for
pottery
6. high-temperature veins carrying W, Mo, As, Ni, Co, and V. Although numerous
around the granites of SW Sardinia, these veins do not reach an overall tonnage
sufficient 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 preserved
by 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 first
post-Hercynian discordant sediments are of this age. The first metallogenic effect of this
phase 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 is
shown by the fact that some ore-bearing karst systems include both pre-Triassic fossilized
deposits and columnar bodies whose vertical extent is controlled by the present position
of the water table.
The sixth metallogenic period took place in the Middle Cretaceous and yielded
rather conspicuous bauxite deposits along an emersion surface of the Mesozoic carbonate
22Geology and Metallogeny of Sardinia
Figure 10: Metallogenic synthesis of Sardinia.
23Geology and Metallogeny of Sardinia
platform of Nurra. The recently explored deposit of Olmedo (proven plus inferred reserves:
30-35 Mt at 60-62 wt.% Al2 O3 ; total reserves possibly up to 70-80 Mt) is the best example
of this. It is to be pointed out that bauxite accumulation during the Mesozoic carbonate
platform evolution involved the entire Mediterranean area, both on the European and the
African plates.
The seventh and last metallogenic period is related to the Alpidic tectonic and
magmatic activity. Alpidic tectonics are only extensive in Sardinia, and its effects on
pre-existing ore occurrences are mainly displacements without important mobilization,
as contrary to the Hercynian orogeny, with the exception of those due to local uplifts
as discussed above. It is the related Oligocene-Miocene calc-alkaline magmatism that
produced minerogenetic phenomena, whose importance is just now being recognized. The
main ore occurrence types are the following:
1. porphyry copper (-gold) deposits, linked to subvolcanic bodies at Calabona, close to
Alghero, 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, and
northwestern Sardinia). At places, supergene enrichment played an important role
in 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 been
found very recently and are now being actively investigated and exploited. Both
low- and-high-sulfidation occurrences have been found so far. Their economic worth
appears so interesting that the low-sulfidation bodies of Furtei are currently being
mined, and a few tons of gold have already been recovered only in the oxidized part
of the explored bodies;
4. kaolins and bentonites after hydrothermal and/or supergenic alteration of the same
volcanics;
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 outcrops
of barite hosted in the underlying Paleozoic of Iglesiente. In one of these occurrences
that covered karst fillings and was exploited with them, obsidian splinters deriving
from prehistoric industry are mixed in with the barite clasts.
6. Geothermal systems and thermal waters related to the Tertiary volcanic cycle and still
active in some districts. Studies on geothermal energy exploitation have not reached
the operational level yet, while a thermal establishment has long been in operation
and others will begin to operate soon; several occurrences zeolitized pyroclastics are
scattered all over the areas covered by Tertiary volcanics. Studies on their industrial
uses are still in progress.
24Geology and Metallogeny of Sardinia
For completeness sake, it should be mentioned that two types of fossil coal are also
found in Sardinia: anthracite (one small occurrence no longer exploited) below Permian
terrains, 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 of
minerals 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, but
copper 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 local
deposits.
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 in
metals as they also thought them in Spain. Exploitation in the Iglesiente area probably
started at this time to produce silver. Nevertheless, artefacts from this exploitation almost
don’t exist. Around 600 BC, Carthaginians, descendants of the Phoenician colons, took
control of Sardinian coasts, while the interior of the country was still dominated by the
Nuraghe civilization. However, archaeological relicts of mining from this time are very
scarce.
Romans conquered Sardinia in 238 BC. Surprisingly ancient Roman authors like Strabon
or Pline, don’t mention Sardinian mines. Definite proofs (stamped ingots) from mining in
Sardinia only appear in the 2nd century. Written testimonies of Pb-Ag and Fe mining as
well as gold panning are dated to the 3rd-5th centuries
The Vandales, a Germanic tribe, took control of Sardinia after having gone through Gallia
and Spain (450-550 AD), and expanded their empire to northern Africa. Then, the island
was taken over by Constantinople as precautionary measure, while facing the pressure of
the 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 9th
and 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 in
operation at this time. In particular, a mining law probably dating back before 1300 AD
defines exploitation modalities of the Iglesiente mines (taxes, exploration and exploitation
rights, status of workers etc.). This statute is very similar to the one existing in Toscana
from the same time, which is in turn inspired by the first Germanic statute. There is a
clear 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 and
reallocate the concessions. During the 14th and 15th centuries, mining industry carried on
25Geology and Metallogeny of Sardinia
prosperous, and was mainly focused on silver.
In 1492 Christopher Colombo discovered the Americas. Earlier, the kingdoms of Aragon
and Castilla had merged to form Spain. From 1530, exploitation started in the huge silver
deposits of Mexico and Peru. Sardinian mines carried on being exploited, but without any
major 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 dismembered
during Napoleonian wars (Sardinia keeps its autonomy) but was brought together again
in 1815. Between 1848 and 1860, the movement of unification and independence of Italy
developed and was promoted by the dynasty of Savoy. In 1861, the kingdom of Sardinia
merged with other ones to form the kingdom of Italy. In 1946, the Republic is proclaimed
and 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 their
factories. New transportation methods and techniques allowed improvement of exploita-
tion methods. At this time, public companies, mostly French and British ones, invested a
lot of money in the mines of the Mediterranean, including the deposits of Iglesiente. Some
mines (Monteponi, Montevecchio, and others) were very profitable. Mining was mainly
focused on supergene Zn at this time.
Extraction was intensively carried out during the second half of the 20th century. The
formerly 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 the
Furtei high sulfidation deposit was taken to court.
26Ores in the lower Paleozoic of SW Sardinia
2 Pre-Hercynian Stratiform Pb-Zn-Ba Mineralization
in 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 known
which 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 to
the Sardic phase and the more recent ∼N-S lineation related to the Hercynian deformation.
Figure 11: Geological
and structural map of the
Iglesiente-Sulcis area (from
Boni and Koeppel 1985).
1 Post-Hercynian sediments
2 Permian porphyry
3 Late Hercynian granites
4 Silurian and Ordovician
slates, limestones and
conglomerates
5 Cabitza FM
6 Gonnesa FM
7 Nebida FM
FM Axial planes:
8 Sardic deformation
9 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
27Ores in the lower Paleozoic of SW Sardinia
deposits. The first ones were deformed together with their host rocks by the Hercynian
compressive tectonics, the latter ones clearly cut the deformed and tilted lithologies.
The pre-Hercynian stratabound deposits have greater economic importance relative
to post-Hercynian deposits. Most of the stratabound ore deposits are hosted by Lower
Cambrian 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 various
mineralization
A Cambrian-hosted ores
B Sardic unconformity-hosted ores
a to h refer to mineralization types described in
the text. Figure is taken from Boni et al. (1996).
The pre-Hercynian stratabound ores can be regarded as the result of a combination
of favorable sedimentary environments and Paleozoic tensional tectonics. Two groups of
genetically distinct ore types are known:
1. SEDEX deposits: the mineralization occurs in the upper part of the Punta Manna
formation as syngenetic and early diagenetic massive sulfides and barite layer (type
a and b in Fig. 12).
2. MVT deposits: mineralization occurs in form of void filling, breccia cement and
late diagenetic replacement bodies within the San Giovanni formation (types c to f in
28Ores in the lower Paleozoic of SW Sardinia
Fig. 12). MVT-style mineralization also occurs at at the Sardic unconformity (type
g and h in Fig. 12) and is associated with strong hydrothermal silica alteration.
The genesis of stratabound ores within Cambrian sedimentary rocks of SW Sardinia
was apparently an evolving process that produced a series of deposits ranging from SEDEX
to MVT.
2.1 Description of mineralization and host rocks
2.1.1 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 group
and the overlying Gonnesa group (Fig. 12). The Nebida group consists of terrigeneous
sandstones and siltstones, shallow water complexes (Matoppa FM) which grades upward
into 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 Punta
Manna FM is consistant with a progradation of clastic-carbonates tidal flats (Matoppa
Time, 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 Barbara
FM (Gonnesa group). This transition is consistant with a tectonic tensional phase forming
the Eastern Sulcis Basin (Santa Barbara time, Fig. 13) which separated the platform from
its clastic source.
Figure 13: Tectono-stratigraphic evolution of the Cambrian succession in SW Sardinia, from Matoppa time to Santa Barbara
time (from Bechstädt and Boni 1994).
29Ores in the lower Paleozoic of SW Sardinia
Massive sulfide (type b) and barite (type a) layers are hosted within the Punta Manna
formation. Because of their high degree of congruency with depositional structures which
indicates that the paleogeography controlled the distributions of the mineralizations, plus
similar Pb and Sr isotopic composition of the ore and the host rock (see below), the deposits
are considered to be of syngenetic to syndiagenetic origin and interpreted as sedimentary
exhalative i.e. SEDEX type deposits (Boni et al. 1996).
The ore grade in these massive sulfides was generally high (∼8% Zn, locally exceeding
12% Zn+Pb). The deposition of the which occurs at the base of the tidal dolomites of
the Santa Barbara formation, has been related to the onset of strong tensional tectonics
during the Early Cambrian (Bechstädt and Boni 1994). In fact, most ores are enriched
along 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 the
dolomite horizons of the Santa Barbara FM. This microcrystalline barite displays an al-
ternating zebra-like texture within the dolomite. The barite layers are intercalated within
varve-like laminated dolomites with less microbial laminae and is only locally associated
with minor pervasive silicification.
No evidence of massive replacement could be found, a few detrital barite clasts have been
observed 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 within
dolomitized 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 pyrite
and microcrystalline dark sphalerite»galena and chalcopyrite. The mineralogy is highly
variable from place to place, sometimes composed entirely by pyrite. The dolomite host-
rocks are often blackened and strongly silicified and were overprinted by the Hercynian
deformation.
Discordant massive sulfide bodies: Some economically important ores are clearly
discordant and characterized by a strong pervasive silicification of carbonates and sand-
stones and/or a soft sediment deformation of sulphides. These ores are concentrated along
tectonic 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 synsedimentary
faults which were repeatedly reactivated.
30Ores in the lower Paleozoic of SW Sardinia
Zonation: The mineralization, interpreted as sedimentary exhalative, shows zonation on
a large scale (e.g in the Sulcis District) and is centered in the eastern Iglesias valley where
massive sulfides (type b) prevail over small amounts of barite (type a). This central part
is 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 locally
followed by a vertical gradation from sulfides to barite around hypothesized discharge areas
of the mineralizing fluids. Emplacement of the ore bodies is ascribed to several pulses of
hydrothermal fluid circulation, controlled by tectonic movement related to Lower Cambrian
rifting.
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 limestones
at the base, overlain by the “Ceroides” lithofacies (=waxy limestones). The Ceroide con-
sists mostly of barren microsparites, representing, at least partially, recrystallized peloidal
mudstones and wackestones, commonly heavily bioperturbated. This facies is thought to
be 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 Bechstädt
and Boni 1994).
In the westernmost Iglesiente district, Ceroide mudstones locally are characterized by
slumps, 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 hydrothermal
dolomitization. Therefore, the brecciation phenomenon is generally the most important
ore controlling factor in the San Giovanni Formation.
The internal breccias (matrix-poor and matrix-lacking) are more enriched in sulfides, which
form 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
31Ores in the lower Paleozoic of SW Sardinia
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 the
San Giovanni FM, and particularly in its upper part within the “Ceroides”. They consist
of several horizons (described below) containing sphalerite, galena, pyrite and more rarely
barite. The sulfides occur as void filling, breccia cement and late-diagenetic replacement
bodies in the shallow water limestones of the San Giovanni formation. Some of the ores
could 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-flow
event associated with the Caledonian “Sardic” tectonic phase. Their metal content is in
the range of low-grade MVT ores, averaging 5-7 % combined Zn+Pb. However, thanks to
thick and continuous mineable horizons, exploitation was possible until the 1970ies.
The iron content of the Sardinian MVT type mineralization is much lower (less pyrite) and
the Pb/Zn ratios significantly higher relative to the SEDEX type mineralization within the
Santa Barbara formation. The different metal budget might be related to different sources
of the metals (i.e. from the pre-Cambrian basement consisting of magmatic/metamorphic
rocks, to lower Cambrian siliclastic sedimentary rocks) or to differences in lithology and
chemical compositions of the host rocks encountered by the hydrothermal fluids (Boni
et al. 1996).
Zn-Pb deposits - type c: At the base of the San Giovanni Formation, small Zn>Pb
deposits occur within the strongly tectonized “black limestone” lithotype. Slumps and
slump-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 and
fractures, as a result of collapse of part of the platform at the beginning of the flooding
stage (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 horizons
in 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 impregnations
of pale yellow (Fe-poor) sphalerite with some pyrite and minor amounts of galena in a
peloidal calcareous mudstone facies.
The Blendoso ores, hosted by the Mid Ceroide lithofacies, seem to have been emplaced
pervasively by diagenetic processes and concentrated by the following Hercynian fluid cir-
culation.
Zn-Pb deposits - type e: This mineralization is the economically most important ore
type. Zn>Pb deposits occur as cement and/or as matrix of stratabound multigeneration
breccias, which are easy to recognize, but sometimes deformed by post-ore Hercynian
tectonic.
32Ores in the lower Paleozoic of SW Sardinia
Pb-Zn-Ba deposits - type f: the deposits are characterized by higher Pb/Zn ratios as
well as by the presence of minor barite. They occur throughout the entire Iglesiente-Sulcis
region, 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 the
Ordovician erosion cut down into slates of the Middle Cambrian Cabitza FM, where metal
concentrations are low or absent.
However, the erosion locally reached the Lower Cambrian San Giovanni and Santa Barbara
formations, where the first sedimentary lithotypes above the unconformity consist of a few
meters of conglomerates. In those areas most conglomerates and part of the carbonates
beneath the unconformity show pervasive, but irregular silicification, reaching some tens of
meters. This impressive quartz horizon contains sub-economic mineralization (type g and
h in Fig. 12) consisting mostly of barite with minor galena, plus local traces of sphalerite
and Cu-sulfosalts.
Type g: Mineralization occurs directly beneath the Sardic unconformity as deformed
veins and pods which replace the carbonate host rocks.
Type h: The mineralization is hosted by upper Ordovician conglomerates and breccias
and in the slates of the basal M.Orri and Portixeddu formations.
The genesis of type g and h mineralization can be attributed to hydrothermal fluids
migrating underneath the impermeable cover of Ordovician slates (M.Orri/Portixeddu for-
mations) in more porous and leachable conglomeratic lithotypes along the unconformity
with 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 effective
metallogenic trap for metal-bearing fluids that reached the top of the carbonates.
2.2 Isotopic analyses and their interpretation
Sulfur isotopes
Jensen and Dessau (1966) report sulfur isotopic compositions of galena, sphalerite,
pyrite and barite from the Cambrian carbonate sequence (San Giovanni, Campo Pisano
and Acquaresi mines). Based on the significant enrichment of 34 S, Jensen and Dessau
(1966) suggest that bacterial reduction of sulphates may have been the main source of
reduced sulphur in the stratiform deposits of SW Sardinia.
Biogenic sulfides are formed by the reaction of metals with H2 S derived from anaerobic
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