chapter 2 - university of tasmania · chapter 2 - regional geology of the iberian pyrite belt 2-5...

CHAPTER 2

Chapter 2 - Regional geology of the Iberian Pyrite Belt 2-1

CHAPTER 2: REGIONAL GEOLOGY OF THE IBERIAN PYRITE BELT

2.1 Introduction In this chapter, the regional geology of the Iberian Pyrite Belt and its position in the South Portuguese

Zone are discussed. The tectono-stratigraphic evolution of the South Portuguese Zone is also

presented.

2.2 The South Portuguese Zone 2.2.1 Introduction

The Iberian Peninsula is located in western Europe and famous for hosting the largest base metal

massive sulfide province of the world, the Iberian Pyrite Belt. The Iberian Pyrite Belt contains

numerous massive sulfide deposits (Table 2.1), including probably the single largest concentration of

sulfides in the world, the Rio Tinto deposit (~500 Mt). With great historical significance, the Iberian

Pyrite Belt has been exploited at least since the Chalcolithic era but it was the Romans who first

intensified the exploitation of gold from numerous gossan exposures and copper from the supergene

enrichment zones of some ore deposits. The industrial revolution accelerated exploitation of the

deposits in the Iberian Pyrite Belt, namely in the Lousal, Caveira, S. Domingos and Aljustrel mines in

Portugal and Rio Tinto, Tharsis, Aznalcóllar and Sotiel mines in Spain.

Table 2.1: Comparison of the Iberian Pyrite Belt in terms of ore deposits and metal tonnage, with other massive sulfide provinces. Adapted from Leistel et al. (1998b). n = number of deposits.

VHMS Province n Size (Mt)

Cu (Mt)

Pb (Mt)

Zn (Mt)

Ag (t)

Au (t)

Australia (Archaean to Permian) 30 334.9 4.2 4.1 12.8 13447 578

Mount Read Volcanics Tasmania (Cambrian) 6 148.6 1.5 2.5 6.1 7423 156

Canada (Superior Province-Archaean) 87 769.0 11.8 0.6 25.9 28359 712

Abitibi (Archaean) 40 500.3 9.9 0.4 18.2 20919 656

Rouyn-Noranda+Val d’Or (Archaean) 20 394.3 5.8 0.0 6.2 5590 596

Iberian Peninsula (Pyrite Belt) (Devonian to

Carboniferous) 85 1765.0 14.6 13.0 34.9 46188 887

Neves Corvo (Iberian Pyrite Belt,

Portugal) 5 261.5 3.44 0.6 3.74 2977


2.2.2 Geology of the South Portuguese Zone

The South Portuguese Zone is the most external terrain in the southern part of the Iberian Massif of the

Variscan Fold Belt (e.g. Lotze, 1945; Carvalho et al., 1971; Ribeiro et al., 1979, Silva et al., 1990;

Ribeiro and Sanderson, 1996; Quesada, 1998), and correlated with the Devonian-Carboniferous units

of southern Ireland and England, and with the Rheno-Hercynian zone of the Variscan Fold Belt (e.g.

Dias and Ribeiro, 1995; Oliveira and Quesada, 1998) (Fig. 2.1). The South Portuguese Zone is

bordered by the Ossa Morena Zone to the north (Fig. 2.2). The contact between these two distinct

structural and palaeogeographic zones is the Ferreira-Ficalho thrust zone, which exposes a mafic-

ultramafic sequence with ophiolitic affinity (the Beja-Acebuches Complex) along most of its extent

(Munhá et al., 1986b; Silva et al., 1990; Fonseca and Ribeiro, 1993; Quesada et al., 1994; Silva, 1997).

To the south and west, the South Portuguese Zone is concealed by Meso-Cainozoic marine sequences,

and to the northwest and east-southeast, by the Cainozoic basins of the Sado River and the

Guadalquivir River, respectively (Fig. 2.3).

Rock units in the South Portuguese Zone range in age from the Late Devonian or older, to the Late

Carboniferous. The South Portuguese Zone is divided into five structural domains, which are, from

northeast to southwest, the Beja-Acebuches Complex, the Pulo do Lobo Antiform, the Iberian Pyrite

Belt, the Baixo Alentejo Flysch Group and the SW Portuguese Domain (Fig. 2.3) (Oliveira, 1990;

Silva et al., 1990; Silva, 1997).

Beja-Acebuches Complex

The Beja-Acebuches Complex has ophiolitic affinities (Munhá et al., 1986b; Silva et al., 1990;

Fonseca and Ribeiro 1993; Quesada et al., 1994) and delineates the contact between the South

Portuguese Zone and Ossa Morena Zone. The Beja-Acebuches Complex extends discontinuously for

~200 km across Portugal and Spain.

Pulo do Lobo Antiform

The Pulo do Lobo Antiform is separated from the Ossa Morena Zone to the north by the Ferreira-

Ficalho thrust zone. The Pulo do Lobo antiform contains the Pulo do Lobo Formation, a phyllite and

quartzite unit interbedded with minor MORB-type basalts (Munhá, 1983) that is considered the oldest

known sedimentary unit of the South Portuguese Zone (e.g. Oliveira, 1990). This formation is overlain

by the Ferreira-Ficalho Group on the northern limb of the antiform, and by the Chança Group on the

southern limb. The Ferreira-Ficalho Group and the Chança Group comprise detrital sedimentary units.

The Ferreira-Ficalho Group is composed of three main units, which are from base to top, the Ribeira

de Limas Formation (phyllite, lithic sandstone, quartzwacke and minor volcanic rocks), the Santa Iria

Formation (shale, siltstone and lithic sandstone) and the Horta da Torre Formation (shale,

100 km

Variscan Front NORTH S

EA

ATLA

NTIC

OCE

AN

MEDITERRANEAN SEA

SPZ IberianPeninsula

Massif Central

RHZRHS

ArmoricanMassif

200 km

WALZ

TBCZBAOCSPZ

OMZ

CIZ

GTO

MZ

CZ

Figure 2.2: Variscan lithostratigraphic units of the Iberian Massif (after Ribeiro and Sanderson, 1996). CZ, Cantabrian Zone; WALZ, West Asturian-Leonese Zone; GTOMZ, Galicia Trás-os-Montes Zone; CIZ, Central Iberian Zone; TBCZ, Tomar-Badajoz-Cordoba Shear Zone; OMZ, Ossa Morena Zone; BAOC, Beja-Acebuches Ophiolitic Complex; SPZ, South Portuguese Zone. Post-Palaeozoic cover in white. Adapted from Onézime et al. (2003).

Figure 2.1: Schematic map of the Variscan Fold Belt in central and western Europe. RHZ, Rheno-Hercynian Zone; SPZ, South Portuguese Zone; SWE, Southwest England; BALG, Beja-Acebuches-Lizard-Giese suture; RHS, Rheno-Hercynian suture. Adapted from Oliveira and Quesada (1998).

IBERIA

N MASSIF

BA

LG

SWE


VAR

ISC

AN

FOLD

BELT

8 00'0 7 00'0

38 00'0

37 00'0

Volcanic Sedimentary Complex(Late Devonian-Early Carboniferous)

Beja-Acebuches Ophiolitic Complex

Ferreira-Ficalho Group (Late Devonian)

Chanca Group (Late Devonian)

Pulo do Lobo Formation(Late Devonian and older)

Phyllite-Quartzite Group (Late Devonian and older)

Undifferentiated(Early to Late Carboniferous)

Massive sulfide deposit

National border

NEVES-CORVO

Overthrust fault

Beja-Acebuches Complex

Pulo do Lobo Antiform

Iberian Pyrite Belt

Baixo Alentejo Flysch Group

Post-Palaeozoic cover

Granitoids (Late Carboniferous)

Monchique Syenitic Massif(Late Cretaceous)

Permian sedimentary Formations(Lower Permian)

Undifferentiated(Late Devonian - Late Carboniferous)

SW Portuguese Domain


PORTUGAL

SPAIN

40 Km

NEVES-CORVO

ALJUSTREL

LAGOA SALGADA

THARSIS

LAS CRUCES

AZNALCÓLLARLOS FRAILES

S DOMINGOS

ATLANTIC

OCEAN

N

Figure 2.3: Geology of the South Portuguese Zone (SPZ), also showing the location of the most important ore deposits and the study areas. Inset shows the location of the South Portuguese Zone in the Iberian Peninsula. Adapted from Thiéblemont et al. (1998b).

OSSA MORENA ZONE

Marateca

Seville

RIO TINTOLA ZARZA

Study areas

Albernoa

Neves Corvo

Serra Branca

Paymogo quarryEl Almendro-V. N. CastillejosCerro de Andevalo

Odiel River

La Aulaga

LISBONMADRID

SPZ

a

ba

hgf

e

dcb

dc

fe

hg


siltstone, quartzite and minor red shale and tuffite) (Pfefferkon, 1968; Carvalho et al., 1976; Oliveira,

1990). The Chança Group, on the southern limb of the antiform, is composed of, from older to

younger, the Atalaia Formation (phyllite and sandstone), the Gafo Formation (shale and lithic

sandstone, felsic and mafic igneous rocks) and the Represa Formation (siliceous shale, phyllites,

siltstone and quartzwacke) (Pfefferkon, 1968; Carvalho et al., 1976; Oliveira, 1990). The top of the

Horta da Torre and Represa Formations yield spores and acritarchs from the early-middle Famennian

(Upper Devonian) (Oliveira et al., 1986) and late Famennian (Upper Devonian) (Cunha and Oliveira,

1989; Oliveira, 1990; Oliveira et al., 2005), respectively.

Iberian Pyrite Belt

The Iberian Pyrite Belt occurs to the south of the Pulo do Lobo Antiform, and is described in detail in

the following section (section 2.3).

Baixo Alentejo Flysch Group

Stratigraphically above the Iberian Pyrite Belt lies the Baixo Alentejo Flysch Group (BA Flysch

Group). In Spain, this group is designated the Culm Group. The BA Flysch Group is a marine

sedimentary sequence more than 5000 m thick and dominated by turbidites (Oliveira et al., 1979;

Oliveira, 1983; Silva, 1997). It is interpreted to have been deposited in a syn-orogenic foreland basin

and deposition started in the late Visean (Middle Carboniferous). The BA Flysch Group is composed

of three formations that, from oldest to youngest are: the Mértola Formation, the Mira Formation and

the Brejeira Formation (Oliveira, 1983; Oliveira, 1990).

The Mértola Formation is composed of shale and greywacke with minor conglomerate, and yields

goniatites of the late Visean (Middle Carboniferous) (Oliveira, 1983; Oliveira, 1990). The Mértola

Formation overlies the Volcanic Sedimentary Complex of the Iberian Pyrite Belt. At many locations in

the northern part of the Iberian Pyrite Belt, the Mértola Formation is overthrust by the Volcanic

Sedimentary Complex. At the Cercal-Odemira anticline, the Volcanic Sedimentary Complex is

overlain by the late Visean to early-late Namurian (Middle Carboniferous) Mira Formation (Oliveira

et al., 1979). The Mira Formation consists of shale and sandstone. The Brejeira Formation is

composed of shale, sandstone and greywacke, and contains goniatites of the middle Namurian to early

Westphalian (Middle-Upper Carboniferous) (Oliveira, 1990).

SW Portuguese Domain

The SW Portuguese Domain occurs only in the Portuguese part of the South Portuguese Zone. It

consists of a thick marine sedimentary sequence that is stratigraphically equivalent to, but

lithologically different from, the Iberian Pyrite Belt and the Mértola and Mira Formations. The SW


Portuguese Domain is overlain by the Brejeira Formation of the BA Flysch Group (Oliveira 1990;

Silva, 1997).

2.3 Regional geology of the Iberian Pyrite Belt The Iberian Pyrite Belt is by far the most important palaeogeographic domain of the South Portuguese

Zone. It defines an arcuate belt parallel to the Variscan suture (Ferreira–Ficalho thrust zone), and is

characterised by E-W structures in Spain and NW-SE structures in its western part, in Portugal. The

Iberian Pyrite Belt ranges from 25 km to 70 km wide, and extends for approximately 250 km from

Seville (Spain) at its eastern end, to near Marateca (on the Atlantic coast of Portugal) at the western

end (Fig. 2.3). The Iberian Pyrite Belt is divided into northern and southern branches (Oliveira, 1990;

Silva et al., 1990), based mainly on differences in the deformation style. The northern branch is

characterised by isoclinally folded thrust nappes, whereas the southern branch is characterised by

gently folded thrust nappes that define open antiforms (Silva et al., 1990; Silva, 1997).

The Iberian Pyrite Belt contains a great amount of sulfide resources, more than 1700 Mt (Leistel et al.,

1998b; Tornos, 2006). That total includes 14.6 Mt of Cu, 13.0 Mt of Pb, 34.9 Mt of Zn, 46188 t of Ag

and 887 t of Au (Leistel et al., 1998b), distributed in more than 85 massive sulfide deposits, of which

at least seven are larger than 100 Mt (Leistel et al., 1998b, Carvalho et al. 1999, Relvas et al., 2002;

Tornos, 2006) (Table 2.2). The average Cu grade ranges from 0.5% to 1.5%, but at the Neves Corvo

ore deposit, this grade is higher, reaching 14.4% in the massive Cu-Sn ores (e.g. Relvas, 2000 unpub.).

The most impressive deposit of the Iberian Pyrite Belt is the Neves Corvo massive sulfide deposit

(Fig. 2.3) that contains more than 300 Mt of sulfides, of which 100 Mt contain 3.46% Cu and 3.54%

Zn and 50 Mt contain ~6% Cu and Zn. These high grades combined with 300 000 t of tin, mainly in

the form of cassiterite and stannite, make Neves Corvo a unique case in the belt.

The Iberian Pyrite Belt is composed of two major lithostratigraphic units: the Phyllite-Quartzite Group

(PQ Group) and the Volcanic Sedimentary Complex (VS Complex). The PQ Group consists of a

terrigenous sedimentary sequence, mainly composed of phyllite intercalated with quartzite, and

conglomerate and limestone units at its top. The limestone beds are interpreted to have formed in a

shallow marine continental platform environment (Oliveira, 1990; Silva, 1997) and contain a conodont

fauna from the middle-late Famennian (Upper Devonian) (Boogaard and Schermerhorn, 1975;

Fantinet et al., 1976). Recent biostratigraphic studies based on palynomorph associations identified in

units of the PQ Group also indicate that its top is from the middle-late Famennian (Oliveira et al.,

2004, 2005). The PQ Group is more than 200 m thick, and its base is not exposed (Oliveira, 1990).


Table 2.2: Tonnage and metal grades for some of the most important Iberian Pyrite Belt massive sulfide deposits. Modified after Relvas et al. (2002).

DEPOSITS Tonnage(Mt)

Cu (%)

Zn (%)

Pb (%)

Sn (%)

Neves Corvo Total sulfides

Original reserves

>300 100

1.603.46

1.403.54

0.280.80

0.10 0.25

Rio Tinto Total sulfides

Original reserves

>500 335

0.300.39

0.540.34

0.160.12

Aljustrel >270 1.01 3.40 1.16

Tharsis 110 0.50 2.70 0.60

La Zarza 100 0.70 1.50 0.60

Massa Valverde 92 0.44 1.92

Aznalcóllar 90 0.51 1.80 0.85

Sotiel 75 0.56 3.60 1.34

Los Frailes 70 0.35 3.87 2.21

Migollas 58 0.88 2.23 1.12

Concepcion 56 0.57 0.48 0.19

Las Cruces 45 2.92 2.54 1.49

Aguas Teñidas 41 1.30 3.10 0.90

La Romanera 34 0.42 2.30 1.18

São Domingos 30 1.20 3.00

Lousal >20 0.70 1.40 0.80

Above the PQ Group is the VS Complex which hosts the massive sulfide deposits in the Iberian Pyrite

Belt. The VS Complex ranges in age from the late Famennian (Upper Devonian) to early-late Visean

(Middle Carboniferous) (Boogaard, 1963; Strauss, 1970; Schermerhorn, 1970; Carvalho, 1976;

Carvalho et al., 1976; Boogaard and Schermerhorn, 1980; Oliveira, 1983, 1990; Oliveira et al., 2004,

2005). The VS Complex is exposed in elongate, discontinuous, separate areas that are aligned parallel

to the major Variscan structures. It consists of interbedded volcanic and sedimentary rocks, and ranges

in thickness from approximately 600 to 1300 m (Silva, 1997; Relvas, 2000 unpub.; Tornos, 2006). The

volcanic rocks are subordinate, amounting only to ~25 % of the lithostratigraphic record (Tornos,

2006), occupying an area of approximately 2500 km2. The remainder consists of volcanogenic

sedimentary units and abundant, thick, non-volcanic sedimentary units, mainly mudstone that occurs

below, interbedded with and above the volcanic units. The volcanic component of the VS Complex is

bimodal in composition but strongly dominated by felsic units (~70 %; Tornos, 2006). Abundant

marine fossils such as radiolarian, conodonts, brachiopods, ammonoids and dasycladales (Boogaard

1963, 1967 unpub.; Boogaard and Schermerhorn, 1975, 1980; Fantinet et al., 1976; Oliveira, 1983;


Oliveira, 1990; Moreno et al., 1996; Korn, 1997; Soriano and Marti, 1999; Oliveira et al., 2004)

indicate the depositional environment for the VS Complex was submarine. The Pomarão area, at the

western end of the Puebla de Guzman anticline, is the type area of the VS Complex, where three felsic

volcanic cycles were identified and firstly described by Boogaard (1967 unpub.).

The mafic volcanic rocks occur at the base of the VS Complex in Spain, and at the top in Portugal

(Strauss and Madel, 1974; Barriga et al., 1997). Many mafic units in the Spanish part of the Iberian

Pyrite Belt are interpreted as sills (e.g. Boulter, 1993a, 1993b, 1996, 2002; Pascual et al., 1999;

Soriano and Marti, 1999; Valenzuela et al., 2001, 2002, 2003; Boulter et al., 2001, 2004). At some

localities, both in Portugal and Spain, pillow lavas have been described (Strauss et al., 1977; Garcia-

Palomero, 1980 unpub.; Munhá, 1983; Almodóvar et al., 1998). In Portugal, the mafic units are

tholeiitic or alkaline and came from different mantle sources (Munhá, 1983).

Andesitic units are minor in volume compared to the felsic and mafic units. Andesites occur mainly in

the northern branch of the Iberian Pyrite Belt and may have been derived from the tholeiitic basaltic

magmas by fractional crystallization (Munhá, 1983).

The felsic volcanic rocks consist of calc-alkaline dacites, rhyolites and high-silica rhyolites (Munhá,

1983; Mitjavila et al., 1997; Thiéblemont et al., 1998). The felsic units are considered to result from

melting of continental crust, probably induced by ponding of hotter mafic magmas in the crust

(Munhá, 1983; Thiéblemont et al., 1994, 1998). Some dacites may have been derived from the

andesitic magmas by fractional crystallization (Munhá, 1983). The massive sulfide deposits occur

close to the felsic volcanic rocks or black mudstone.

Previous descriptions and interpretations of the VS Complex have concentrated on petrogenetic,

stratigraphic or metallogenetic aspects (e.g. Boogaard, 1967 unpub.; Strauss, 1970; Strauss and Madel,

1974; Carvalho, 1976; Carvalho et al., 1976; Lecólle, 1977 unpub.; Routhier et al., 1980; Munhá,

1983; Barriga and Kerrich, 1984; Oliveira, 1990; Munhá et al., 1997; Barriga et al., 1997; Mitjavila et

al., 1997; Sáez and Moreno, 1997, 1999; Barriga and Fyfe, 1998; Carvalho et al., 1999; Tornos,

1999a, 1999b, 2006; Relvas, 2000 unpub.; Oliveira et al., 2005). Recently, the physical volcanology

and facies architecture have been addressed (e.g. Boulter, 1993a, 1993b, 1996, 2002; Soriano and

Marti, 1999; Pascual et al., 1999; Allen, 2001; Valenzuela et al., 2001, 2002, 2003; Boulter et al.,

2001, 2004; Donaire et al., 2002; C. Rosa et al., 2004a, 2004b, 2005). The felsic units have been

interpreted as lavas, domes, cryptodomes and sills, and pyroclastic units. The volcaniclastic units are

thought to include pyroclastic, autoclastic and epiclastic deposits. The felsic volcanic rocks are

considered to have been emplaced in a submarine setting, although some units have been interpreted

as ignimbrites emplaced in a subaerial setting.


The massive sulfide ore deposits of the Iberian Pyrite Belt are mainly composed of pyrite and variable

amounts of sphalerite, chalcopyrite and galena. Relatively small amounts of tetrahedrite-tennantite,

arsenopyrite and pyrrhotite, among many other minerals are also present (Sáez et al., 1999). The ore

typically occurs in: (1) stratiform, massive sulfide lenses, (2) disseminated in the host volcanic or

sedimentary rocks, or (3) as stockworks below the massive sulfide lenses (Strauss et al., 1977; 1997;

Carvalho et al., 1999). Strongly silicified and chlorite-altered host rocks are commonly associated with

the stockwork ore (Barriga and Carvalho, 1983). The massive sulfide deposits are considered to have

formed on the sea floor, although textures indicating replacement of the host rocks, particularly at the

base of the deposits, have been widely described (Sáez et al., 1999; Relvas, 2000 unpub.; Tornos,

2006). The metals in the sulfides are considered to have been mostly supplied from the PQ Group

(Sáez et al., 1999; Relvas, 2000 unpub.; Tornos, 2006) and transported by hydrothermal fluids

(Barriga and Carvalho, 1983; Barriga, 1990; Barriga and Fyfe, 1998; Leistel et al., 1998b; Carvalho et

al., 1999) but some deposits may also contain metals sourced from magmatic fluids (Relvas, 2000

unpub.; Relvas et al., 2001; Sánchez Espana et al., 2003; Solomon et al., 2002; Tornos et al., 2005).

Manganese deposits, associated with jasper, chert or purple mudstone, are abundant throughout the

Iberian Pyrite Belt (Barriga et al., 1997; Jorge et al., 2005).

Jasper and chert units are commonly intercalated within the VS Complex at several stratigraphic

levels. They occur throughout the Iberian Pyrite Belt, although no single unit extends across the entire

belt (Barriga and Oliveira, 1986; Leistel et al., 1998a; Jorge, 2000 unpub.). The jasper and chert are

dominantly of hydrothermal origin (Barriga and Oliveira, 1986; Leistel et al., 1998a; Jorge, 2000

unpub.). The jasper and chert are interpreted to have formed at the early, or late weak stages of

hydrothermal activity that produced the massive sulfide deposits, or from low-temperature diffuse

hydrothermal activity unrelated to the massive sulfide deposits (Barriga and Oliveira, 1986; Leistel et

al., 1998a).

The Freixial Formation is a transitional formation between the VS Complex and the overlying BA

Flysch Group. It is typical of the northern branch of the Iberian Pyrite Belt, and consists of a turbidite-

mudstone succession up to 200 m thick (Silva, 1989 unpub.; Cunha and Oliveira, 1989; Oliveira,

1990). Recent biostratigraphic ages determined using palynomorphs indicate the Freixial Formation at

Serra Branca, São Domingos mine and Albernoa (all in Portugal), has middle-late Visean age (Middle

Carboniferous) (Oliveira et al., 2005).

Ages of the felsic volcanic rocks (U/Pb isotopes in zircon, Barrie et al., 2002), massive sulfide ore

deposits (Pb/Pb, Re/Os, Rb/Sr and Re/Os isotopes, Marcoux, 1998; Mathur et al., 1999; Relvas et al.,

2001; Munhá et al., 2005), or mudstone (palynomorphs, Pereira et al., 1996; Rodriguez et al., 2002;


González et al., 2002; Oliveira et al., 2004, 2005), are scattered from the late Famennian (Upper

Devonian) to late Visean (Middle Carboniferous).

2.4 Metamorphism Regional metamorphism has affected the South Portuguese Zone. The metamorphic grade has been

determined using the metamorphic mineral assemblages in the mafic units and illite-crystallinity data

(e.g. Munhá, 1990). The metamorphic zones lie parallel to the Variscan suture zone (Ferreira-Ficalho

thrust zone), and range from zeolitic facies in the SW part of the South Portuguese Zone, through

prehnite-pumpellyite and lower greenschist facies in the Iberian Pyrite Belt, to greenschist facies in the

Pulo do Lobo Formation in the NE part of the South Portuguese Zone (Munhá, 1983, 1990). More

recent work suggests the metamorphic grade throughout the South Portuguese Zone is low, and

increases to lower greenschist facies in proximity to important shear zones (Sánchez Espana, 2000

unpub.).

Pre-orogenic alteration has been recognised in the volcanic units of the Iberian Pyrite Belt and

attributed to syn-depositional interaction with seawater (e.g. Munhá and Kerrich, 1981; Barriga, 1983

unpub., Barriga and Kerrich, 1984; Munhá et al., 1986a; Munhá, 1990) and/or hydrothermal systems

associated with the massive sulfide deposits (Strauss et al., 1977; Barriga and Carvalho, 1983). The

sea-floor alteration was responsible for extensive alkali exchange (Na and to a lesser extent K),

hydration, oxidation and carbonate alteration of the lithologies (e.g. Munhá and Kerrich, 1981).

2.5 Geotectonic setting of the South Portuguese Zone The geotectonic setting of the South Portuguese Zone is contentious. The most widely accepted model

proposed for the evolution of the South Portuguese Zone involves northeast (actual coordinates)

directed subduction starting in the Famennian (Upper Devonian), and located at the boundary between

the South Portuguese Zone and the Ossa Morena Zone. The subduction evolved into sinistral oblique

continental collision during the Visean to middle Westphalian (Middle-Upper Carboniferous), forming

intracontinental “en echelon” extensional basins that hosted the magmatism of the Iberian Pyrite Belt

and the massive sulfide deposits (Silva, 1989 unpub., 1997; Silva et al., 1990; Ribeiro et al., 1990;

Quesada, 1991, 1998; Dias and Ribeiro, 1995; Mitjavila et al., 1997; Tornos et al., 2005).

In this model, the PQ Group was being deposited on a passive continental margin platform during the

Middle and Upper Devonian (Silva et al., 1990; Dias and Ribeiro, 1995). During the late Famennian

(Upper Devonian) to the late Visean (Middle Carboniferous), a transtensional regime favoured local

crustal extension forming “en echelon” extensional intracontinental basins parallel to the orogen front

and volcanic activity (Iberian Pyrite Belt) (Silva, 1989 unpub., 1997; Silva et al., 1990). During the

late Visean (Middle Carboniferous), there was a change to a SW verging (actual coordinates), sinistral


transpressional regime that led to the end of volcanism, and initiated the deposition of a syn-orogenic

sedimentary sequence (BA Flysch Group) in a foreland basin environment (Silva, 1989 unpub., 1997;

Silva et al., 1990; Oliveira, 1990; Oliveira et al., 2004). The thrust faults of the Iberian Pyrite Belt

were possibly former extensional structures inverted to low-angle thrusts, and conferred a “thin-

skinned” tectonic style to the orogen (first deformation phase, D1a; Silva, 1989 unpub.; Silva et al.,

1990; Quesada, 1998). During the middle Westphalian (Upper Carboniferous), a second pulse of the

same deformational episode (second deformation phase, D1b; Silva, 1989 unpub.; Silva et al., 1990)

folded the previously formed structures into NW-SE to E-W trending, SW-S verging (actual

coordinates) folds and generated the regional cleavage.

chapter 2 - university of tasmania · chapter 2 - regional geology of the iberian pyrite belt 2-5...

Documents