International Geology Review, Vol. 46, 2004, p. 366–383.Copyright © 2004 by V. H. Winston & Son, Inc. All rights reserved.

Geochemistry of Volcanic Rocks, Albernoa Area,Iberian Pyrite Belt, Portugal

DIOGO R. N. ROSA,1

Instituto Geológico e Mineiro, Estrada da Portela, Alfragide, 2720-866 Amadora, Portugal; Instituto Geológico e Mineiro,Rua Frei Amador Arrais, 39, 7802-491 Beja, Portugal; and CREMINER–Centro de Recursos Minerais,

Mineralogia e Cristalografia, Edifício C2, piso 5, 1749-016 Lisbon, Portugal

CARLOS M. C. INVERNO, Instituto Geológico e Mineiro, Estrada da Portela, Alfragide, 2720-866 Amadora, Portugal and CREMINER–Centro de Recursos

Minerais, Mineralogia e Cristalografia, Edifício C2, piso 5, 1749-016 Lisbon, Portugal

VÍTOR M. J. OLIVEIRA, Instituto Geológico e Mineiro, Rua Frei Amador Arrais, 39, 7802-491 Beja, Portugal; and CREMINER–Centro de Recursos

Minerais, Mineralogia e Cristalografia, Edifício C2, piso 5, 1749-016 Lisbon, Portugal

AND CARLOS J. P. ROSA

Instituto Geológico e Mineiro, Estrada da Portela, Alfragide, 2720-866 Amadora, Portugal; School of Earth Sciences,University of Tasmania, Hobart, TAS 7001, Australia; and CREMINER–Centro de Recursos Minerais,

Mineralogia e Cristalografia, Edifício C2, piso 5, 1749-016 Lisbon, Portugal

Abstract

Volcanic rocks from the Albernoa area essentially consist of calcalkaline quartz-feldspar-phyriccoherent and hyaloclastic rhyodacites, and alkaline and tholeiitic basaltic rocks. Binary plots showthat high-field-strength elements behaved as immobile elements, and allow for the identification oftwo felsic rock suites. Silica and alkali mobility, however, is reflected by compositional scatter onmajor-element diagrams: felsic rocks display rhyolitic to apparent andesitic compositions, and themafic rocks display basaltic to apparent dacitic compositions. Silica and alkali mobility was focusedalong fracture networks and within the matrices of hyaloclastic breccias.

Problematic classification of geotectonic setting for the felsic rocks is a reflection of anomaloushigh-field-strength element systematics; this probably results from a low temperature of crustalfusion, causing decreased solubility of the refractory phases in which these elements reside. Themafic rocks, however, evidently were generated in an extensional setting without involvement ofsubduction; the existence of apparent arc signatures was caused by crustal assimilation. This iscompatible with volcanism in an attenuated continental lithosphere setting, due to strike-slip tecton-ics during oblique continental collision.

Introduction

THE IBERIAN PYRITE BELT (IPB) is an arcuate ter-rane trending from near Seville in Spain to theAtlantic coast in Portugal (Fig. 1). It consists ofDevonian and Carboniferous volcanic and sedimen-tary rocks containing massive sulfide deposits. TheIPB is the largest volcanogenic massive sulfide(VMS) province in the world, and is characterizedby giant deposits such as Aljustrel and Neves-Corvoin Portugal and Rio Tinto and Tharsis in Spain (Fig.1). The IPB is part of the South Portuguese Zone

(SPZ) of the Iberian segment of the Variscan foldbelt (Ribeiro et al., 1979). To the north, the SPZ isbordered by the Ossa-Morena Zone (OMZ), whichcorresponds to the Iberian Autochthon.

Despite the economic importance of VMS miner-alization, and although significant progress has beenachieved regarding the stratigraphy, structure, andmetallogenesis of the IPB, the petrogenesis of theigneous rocks has not been well established. Such amodel would provide important constraints regard-ing the geotectonic setting of the IPB.

A model establishing fractional crystallization ofmafic magma as the origin for the silicic magmas1Corresponding author; email: diogo.rosa@igm.pt

0020-6814/04/731/366-18 $25.00 366

IBERIAN PYRITE BELT 367

yielding the felsic rocks (Routhier et al., 1980) wasdismissed by Munhá (1983) and Thiéblemont et al.(1994, 1998), and crustal melting is presentlywidely considered to be the source for the silicicmagmas. However, no agreement has been reachedconcerning the origin of the mafic magmas, andtherefore on the geotectonic setting of the IPB.Moreover, most previous studies dealing with thegeotectonic setting of the IPB were based on strati-graphic and structural observations, rather than onstudy of the igneous geochemistry. Thus far, pro-posed geotectonic settings for the IPB include: (1)an island or continental arc generated over a sub-duction zone (Schütz et al., 1988; Onézime et al.,2003); (2) a forearc basin within an accretionaryprism associated with N- or NE-dipping subduction,located to the south of the SPZ (Bard et al., 1973;Monteiro and Carvalho, 1987; Leistel et al., 1994;Thiéblemont et al., 1994, 1998); (3) an intraconti-nental backarc basin (Munhá, 1983); and (4) a basin

formed by local extensional tectonics within conti-nental lithosphere, due to transtension caused byoblique continental collision following NE-dippingsubduction of the South Portuguese plate under theOssa-Morena plate (Ribeiro et al., 1990; Silva et al.,1990; Quesada, 1991; Dias and Ribeiro, 1995;Mitjavila et al., 1997).

Studies of igneous petrogenesis have typicallybeen done on a belt-scale, in some cases with a lim-ited number of samples, possibly overlooking rele-v a n t d e t a i l s . A d d i t i o n a l l y, s o m e o f t h einterpretations may have been obtained from olddata sets, with a relatively narrow range of analyzedelements and possibly high detection limits. Thispaper presents new geochemical results of a studyfocused on one specific area of the belt. Hydrother-mal alteration and element mobility are studied indetail, allowing for the classification of the alteredrocks. Finally, the involvement of an arc in thegenesis of the mafic rocks is studied, testing the

FIG. 1. Major structural units and tectono-stratigraphic domains in southwestern Iberia (modified after Quesada,1991). VMS deposits: LS = Lagoa Salgada; A = Aljustrel; NC = Neves-Corvo; SD = São Domingos; T = Tharsis; RT = RioTinto. Study area indicated by square. Inset shows the location of the Iberian Pyrite Belt on the Iberian Peninsula.

368 ROSA ET AL.

viability of the geotectonic settings proposed in (1),(2) and (3) versus (4) above.

Geological Setting

The lithostratigraphy of the IPB is generallydivided from oldest to youngest into the followingunits: (1) the terrigenous Phyllite-Quartzite Group(PQ), whose top is of Famennian age; (2) the Volca-nic Siliceous Group (VS), hosting the VMS mineral-ization and igneous rocks studied in this paper, oflate Famennian to middle Visean age; and (3) the

turbiditic Flysch Group of late Visean to early West-phalian age (Oliveira, 1990; Oliveira et al., 1997).Metamorphism ranges from zeolite to greenschistfacies (Munhá, 1990), and the Variscan tectonicregime is characterized by a thin-skinned forelandfold-and-thrust belt with south- to southwestwardvergence (Silva et al., 1990).

The Albernoa study area (Fig. 2) consists of aNW-SE–trending exposure of the VS, consideredto be in an autochthonous position, as a windowsurrounded by PQ that was thrusted over it(Oliveira, 1992). The region corresponds to an anti-

FIG. 2. Schematic geological map of the Albernoa study area (modified after Oliveira, 1992). Darkened circles indi-cate the studied drillholes.

IBERIAN PYRITE BELT 369

clinal structure whose axis dips to the southeast,whereas the structure to the northwest is covered byPaleogenic and Neogenic conglomerate and sanddeposits. This study area is part of the IPB’s north-ernmost volcanic lineament, which also includes theSerra Branca and São Domingos areas in Portugal.

As is typical of the IPB, the Albernoa volcanicrocks display a bimodal distribution with abundantfelsic and some mafic rocks, whereas intermediaterocks were not identified. The felsic volcanic rocksconsist essentially of quartz-feldspar-phyric coher-ent rocks and associated hyaloclastites, whereas themafic volcanic rocks comprise subophitic- to inter-granular-texture coherent rocks. The quartz-feld-spar-phyric rocks possibly belong to a domecomplex, enclosed in volcaniclastic sandstones andsiltstones. The mafic rocks occur either as doleritedikes intruding shales or as lava flows.

In addition to diagenetic (syngenetic) hydrother-mal alteration due to spilitization, the volcanic rockswere slightly affected by sub-greenschist faciesmetamorphic reactions. However, no VMS occur-rences have been described in the area, whichmakes its study particularly amenable to identifyingthe original rock chemistry, due to the absence ofsignificant hydrothermal alteration.

Petrography

The felsic volcanic rocks are generally hypocrys-taline with medium- to coarse-grained vitrophyrictextures, and are rarely holocrystaline. Quartz,plagioclase, and K-feldspar phenocrysts are dis-persed throughout a glassy groundmass, displayingcommon perlitic fractures. Alteration mineralsinclude chlorite, possibly replacing biotite, andsericite, calcite, and epidote, replacing plagioclase.These rocks contain fracture networks filled withquartz and largely sericite and epidote. Rare xeno-liths, possibly of trachytic composition, were alsoidentified.

The mafic volcanic rocks, represented by doler-ites and lava flows, are hypocrystaline with fine- tomedium-grained subophitic to intergranular tex-tures. Albitized plagioclase crystals and Fe-richchlorite (replacing mafic minerals) are the mainconstituents; some samples also contain K-feldspar,quartz, and opaque minerals. These rocks are vesic-ular to different extents, with chlorite, epidote, andcalcite filling the vesicles.

The recorded alteration minerals reflect regionalspilitization(Munhá and Kerrich, 1981), resulting

from alkali exchange, carbonatization, hydration,and oxidation induced by seawater, representingdiagenetic (syngenetic) hydrothermal alteration.Rocks were subsequently regionally metamor-phosed to the sub-greenschist facies.

Geochemistry

Analytical methods

Approximately 100 samples were collected fromdrillcore and on traverses along creeks and roads.Petrographic studies of all the samples were subse-quently conducted. Forty-two volcanic rock sampleswere considered to be less altered and amenable forgeochemical studies. These samples were crushedin a tungsten alloy jaw-crusher and subsequentlypulverized in an agate ring mill. These samples wereanalyzed for major elements and Nb, Zr, Sn, Ba, Rb,Sr, and Y by X-ray fluorescence (XRF) on fuseddisks at the IGM laboratory in Porto, Portugal. AtActlabs in Canada, a suite of trace elements wereanalyzed: Au, As, Br, Co, Cr, Cs, Hf, Hg, Ir, Sb, Sc,Se, Ta, Th, U, W, La, Ce, Nd, Sm, Eu, Tb, Yb, and Luby instrumental neutron activation analysis (INAA);and Ag, Cd, Cu, Mo, Ni, Pb, Zn, Al, Be, Bi, V and Sby inductively coupled plasma optical emissionspectroscopy (ICP-OES). Analyses of standardshave shown that the values do not differ fromaccepted values. Actlab’s internal duplicate resultswere shown to be within 10%. Data for the more rel-evant elements are presented in Table 1.

Immobile elements

Binary plots combining high-field-strength ele-ments (HFSE), such as Al, Ti, Zr, Nb, Y, and REE,display covariance (Figs. 3 and 4). Despite somedispersal caused by fractionation trends, these plotsshow alteration lines that converge toward the ori-gin, demonstrating relatively immobile behavior forthese elements, as shown by Barret and MacLean(1999) for equivalent felsic rocks in Aljustrel. Masschanges are estimated to have been approximately20%, based on the variation of the immobile-ele-ment concentration along the alteration lines. Addi-tionally, the Al2O3/TiO2 and TiO2/Zr diagrams allowfor the identification of two quartz-feldspar-phyricrock suites, with distinct Al2O3/TiO2 and Zr/TiO2ratios. Most of the samples have Al2O3/TiO2 ≈ 30and Zr/TiO2 ≈ 300, whereas two samples (IP4 andTE3) have Al2O3/TiO2 ≈ 60 and Zr/TiO2 ≈ 500, rep-resenting a more evolved magma.

370 ROSA ET AL.

TABLE 1. Geochemical data for Albernoa samples1

Sample SiO2 TiO2 Al2O3 Fe2O3t MnO MgO CaO Na2O K2O P2O5 LOI Total

Felsic rocksDrill samples

11-1 36.85 67.48 0.50 14.90 4.41 0.07 2.00 2.91 3.49 1.88 0.09 1.86 99.5911-1 37.3 71.33 0.44 13.62 3.10 0.05 0.50 3.83 2.41 2.90 0.18 1.23 99.5911-1 56.1 64.45 0.48 15.47 4.52 0.07 2.26 2.87 2.57 3.13 0.09 3.97 99.8811-1 113.5 58.18 0.59 17.54 5.49 0.08 2.96 6.77 4.62 0.18 0.11 3.09 99.6111-1 122.8 61.69 0.56 15.66 5.37 0.07 3.65 3.47 3.21 2.13 0.11 3.65 99.5711-1 212.5 76.98 0.35 12.27 1.37 0.02 0.27 3.06 2.34 2.06 0.06 1.02 99.8011-1 214.0 74.05 0.32 11.21 2.39 0.03 1.05 6.40 0.86 1.17 0.06 2.25 99.7911-1 292.75 66.10 0.50 15.06 4.63 0.04 3.25 3.36 2.11 1.79 0.10 2.87 99.81TR1 24.3 68.96 0.46 13.07 3.71 0.05 1.85 2.66 2.48 1.90 0.08 4.35 99.57TR1 53.9 54.40 0.69 16.51 6.88 0.09 4.48 5.22 1.65 2.41 0.10 7.42 99.85TR1 64.45 59.27 0.59 15.38 6.83 0.08 4.03 5.21 1.24 1.91 0.08 5.27 99.89TR1 135.3C 60.54 0.58 18.06 6.25 0.09 3.04 1.03 3.38 2.29 0.11 4.22 99.59TR1 135.3M 75.44 0.37 12.15 2.68 0.06 1.50 0.72 1.18 2.81 0.06 2.80 99.77TR1 168.4C 58.49 0.59 19.13 4.72 0.08 2.75 5.69 2.20 2.71 0.10 3.10 99.56TR1 168.4M 62.94 0.58 15.61 6.11 0.09 4.43 2.22 2.36 1.89 0.09 3.27 99.59TR1 250.5 71.22 0.42 13.72 2.65 0.02 1.15 3.16 2.89 2.56 0.07 1.70 99.56TR1 266.1 65.51 0.52 15.54 4.66 0.02 1.80 4.48 3.15 2.05 0.08 2.10 99.91TR1 333.5 64.21 0.60 15.31 5.53 0.07 3.38 3.63 3.45 0.70 0.09 2.64 99.61AB1 64.3 66.39 0.50 14.46 4.58 0.06 2.22 2.75 5.03 1.48 0.11 2.26 99.84AB1 154.6 63.60 0.52 15.11 5.13 0.08 3.29 4.56 2.66 1.91 0.11 2.61 99.58AB1 282.7 73.53 0.37 12.73 1.64 0.10 0.96 1.83 0.34 3.56 0.07 4.54 99.67

Surface samplesCO 1 74.13 0.55 14.23 1.30 <0.02 0.37 0.29 4.24 2.12 0.06 2.21 99.50CO 4 50.94 0.93 19.61 8.01 0.09 5.39 9.00 1.57 0.19 0.11 3.84 99.68CO 5 70.04 0.48 14.96 3.48 0.04 1.95 0.48 6.29 0.22 0.08 1.69 99.71CO 6 65.93 0.51 15.04 4.97 0.07 2.20 4.49 1.83 1.97 0.10 2.50 99.61CO 7 62.36 0.73 16.65 5.33 0.07 3.13 4.14 2.93 1.25 0.12 2.83 99.54TE3 69.59 0.27 15.97 2.53 0.02 2.54 1.97 3.06 1.21 0.06 2.65 99.87TE6 65.41 0.49 14.87 5.67 0.09 3.81 0.77 4.87 0.31 0.11 3.30 99.70TE8 63.46 0.54 16.04 4.29 0.02 3.51 2.97 2.98 2.20 0.10 3.45 99.56TE9C 66.71 0.50 15.15 4.11 0.02 2.27 4.51 2.81 1.22 0.09 2.15 99.54TE9M 64.66 0.55 16.40 4.61 0.02 2.92 2.51 2.89 2.16 0.10 2.71 99.53TE10V 62.49 0.60 18.24 3.88 0.02 2.02 3.22 4.71 1.60 0.10 2.65 99.53TE11 61.35 0.57 16.97 5.86 0.04 3.92 3.00 3.25 1.38 0.10 3.29 99.73TE19 69.78 0.48 14.82 4.05 0.03 0.59 2.48 4.62 1.41 0.09 1.29 99.64IP4 73.46 0.24 14.01 2.26 0.02 0.53 0.24 3.25 4.34 0.05 1.49 99.89MT1 56.83 0.69 19.39 6.74 0.05 3.64 4.13 3.42 1.27 0.07 3.57 99.80MT2C 69.68 0.37 14.59 3.37 0.09 2.53 1.55 2.25 1.91 0.06 3.51 99.91MT3M 59.37 0.48 19.32 4.37 0.08 3.44 2.02 3.26 2.70 0.08 4.63 99.75

Mafic rocksDrill samples

11-1 326.5 59.46 0.70 15.23 5.25 0.13 3.37 3.43 4.56 0.95 0.17 6.35 99.60AB1 341.3 43.75 2.98 19.21 17.37 0.17 4.51 0.95 5.45 0.04 0.41 4.54 99.38

Surface samplesCO 13 49.71 2.23 14.52 12.88 0.17 6.27 3.09 4.10 0.02 0.21 6.47 99.67TE15 42.73 1.57 15.98 10.53 0.60 3.49 10.94 4.54 0.09 0.22 8.99 99.68

IBERIAN PYRITE BELT 371

Rb Sr Ba Y Zr Nb V Hf Sc Ta Th La Ce Nd Sm Eu Tb Yb Lu

Felsic rocksdrill samples

68 163 364 34 144 5 96 5 17.1 1.2 8.9 27.8 62 22 5.4 1.1 0.8 3.9 0.58106 228 698 38 136 5 83 3 14.9 <0.5 7.8 27 54 23 4.9 1.0 0.8 4.3 0.65115 96 384 30 140 5 86 5 16.3 <0.5 8.4 29.7 58 23 5.5 1.2 0.6 3.4 0.52

6 115 35 31 146 6 155 5 21 <0.5 8.5 26.5 59 23 5.1 1.0 <0.5 3.2 0.5188 165 246 29 140 5 130 5 21.4 1.2 8.3 26.4 51 23 4.9 1.1 <0.5 3.2 0.5053 122 396 25 121 4 49 4 9.3 1.3 7.5 25.7 55 19 4.7 0.8 0.8 2.6 0.4038 74 144 23 101 3 57 3 9.9 <0.5 6.9 22.3 44 17 4 0.8 0.7 2.3 0.3364 171 198 30 140 4 80 4 13.8 <0.5 7.3 20.4 47 13 4.4 1.0 0.9 2.8 0.4372 82 303 23 121 3 84 4 13.1 <0.5 7 23 48 19 3.8 0.7 <0.5 2.8 0.4792 115 276 30 147 6 125 4 21.7 <0.5 8 24.9 54 16 4.9 1.0 1.0 3.3 0.5274 203 231 25 127 5 128 3 19.8 1.4 6.3 30.2 60 18 5.7 1.3 0.7 3.0 0.4579 48 498 36 146 7 126 4 19.5 <0.5 8.8 29.5 67 28 6.8 1.3 0.7 3.8 0.56

107 34 583 24 92 3 82 3 12.2 <0.5 5.5 18.9 44 20 4.2 0.9 0.6 2.4 0.36118 168 309 32 156 8 73 5 18.2 1.3 9.1 28.7 62 18 5.2 1.2 <0.5 3.3 0.51

82 102 232 34 145 7 105 4 19.2 <0.5 9.1 25.6 59 17 5.4 1.0 0.8 3.9 0.6080 133 304 29 122 4 53 3 13.1 1.2 7 24.3 49 19 4.4 0.9 0.8 2.9 0.4465 148 301 28 124 5 74 3 17.8 <0.5 7.3 23.9 48 16 4.4 1.0 0.8 2.7 0.4123 235 111 30 132 4 98 4 14.5 <0.5 5.8 20.1 43 18 4.4 0.9 0.8 3.4 0.5156 143 227 30 138 4 75 4 13.8 <0.5 7.5 23.4 55 21 4.8 1.0 0.7 3.2 0.4775 237 228 27 138 4 122 4 17.6 <0.5 7.3 23.6 49 18 4.5 1.0 <0.5 3.1 0.46

127 46 404 31 117 3 50 4 8.4 <0.5 7.3 22 49 14 4.4 0.9 0.7 3.3 0.5147 129 122 24 120 3 83 4 12.9 <0.5 6.2 20.8 45 18 4.1 0.8 0.7 2.9 0.44

Surface samples6 477 48 20 139 5 195 3 26.3 <0.5 4.6 15.7 33 10 3.5 1.1 0.5 2.1 0.317 101 82 30 142 4 74 5 13.6 <0.5 8.4 28.2 59 23 5.3 1.0 0.9 3.3 0.50

91 277 332 28 144 6 100 4 16.8 <0.5 7.9 26 57 22 4.9 1.1 0.7 3.1 0.4654 239 321 24 132 4 139 4 18.5 <0.5 6.4 20.9 48 19 4.2 1.0 0.7 2.7 0.4141 149 286 41 142 6 27 5 9.9 <0.5 13 35.6 79 26 6.6 1.1 1.2 4.8 0.72

9 113 119 28 131 4 90 4 16.8 <0.5 6.8 22.7 51 18 4.5 1.0 <0.5 2.8 0.4564 151 227 30 157 4 70 4 15.1 <0.5 8.7 23.7 48 20 4.8 1.2 0.8 3.1 0.4625 193 202 30 142 3 56 4 12.4 1.2 7.9 23.3 48 16 4.2 1.0 0.8 2.8 0.4249 140 340 40 154 5 81 4 17 1.2 9.3 30 66 25 6.1 1.1 0.9 4.7 0.7247 178 237 38 168 6 66 5 15.4 1.7 9.4 23.6 52 20 5 1.1 <0.5 3.2 0.4840 178 340 31 164 6 80 4 16.4 <0.5 9.5 28.6 62 26 5.4 1.1 <0.5 3.3 0.5139 156 237 28 146 5 66 4 13.2 <0.5 8.6 32.3 62 23 6 1.0 0.9 3.3 0.49

119 95 645 40 120 4 29 4 8.3 1.2 11 33.7 71 22 6.3 0.8 1.2 4.4 0.6542 196 184 29 145 6 136 4 25 <0.5 7.4 26.2 57 19 4.8 1.2 0.6 3.5 0.5347 62 261 17 95 3 43 3 9.4 <0.5 7.9 12.9 37 11 2.8 0.5 <0.5 2.1 0.3461 76 313 30 129 5 136 4 13.6 <0.5 11.1 28.4 64 22 5.6 1.1 0.8 3.3 0.51

Mafic rocksDrill samples

<3 236 116 13 97 <3 116 3 12.2 <0.5 1.4 7.9 18 12 2.5 1.0 0.5 1.4 0.244 72 11 33 161 25 379 4 44.9 1.5 2.1 18.3 36 15 5.3 1.5 0.9 3.3 0.50

Surface samples24 129 60 37 142 5 328 4 44.6 <0.5 1.2 11.6 32 16 5.2 1.5 1.1 4.7 0.70<3 300 37 35 107 6 292 2 38.9 <0.5 0.9 9.8 22 10 3.5 1.3 0.6 3.6 0.55

1Prefix explanation: CO-sample along Rib. Cobres; TE-sample along Rib. Terges; IP-Sample from IP2 road exposure; MT-sample near Malhadinha de Torres; 11-1, TR1 and AB1-samples from drillholes near Malhadinha de Torres (locations in Fig. 2).Suffix explanation: C and M correspond to clasts and matrices of hyaloclastic breccias, respectively.

372 ROSA ET AL.

Series discriminationAs documented by the AFM diagram (Fig. 5) of

Irvine and Baragar (1971), the quartz-feldspar-phyric rocks belong to the calc-alkaline series,whereas mafic rocks seem to belong to the alkaline(Y/Nb ≈ 1, sample AB1-341.3) or the tholeiitic

series (Y/Nb > 2, remaining three samples). SampleAB1-341.3 is akin to Upper Mafic Lavas/Type Bdolerites, whereas the other three mafic samples arecomparable to the Lower Mafic Lavas/Type A doler-ites described in Munhá’s (1983) belt-scale study.

FIG. 3. Al2O3/TiO2 plot, displaying two quartz-feldspar-phyric suites, fractionation trend (continuous), and alterationlines (dashed).

FIG. 4. Zr/TiO2 plot, displaying two quartz-feldspar-phyric suites, fractionation trend (continuous), and alterationlines (dashed).

IBERIAN PYRITE BELT 373

FIG. 5. AFM diagram (Irvine and Baragar, 1971). Quartz-feldspar-phyric rocks represented by squares, mafic rocksby circles.

FIG. 6. C1-Chondrite–normalized REE profiles (normalizing values after McDonough and Sun, 1995). Quartz-feld-spar-phyric rocks represented by open squares, tholeiitic dolerites by filled circles, and alkaline dolerite by open circles.Gd calculated by linear interpolation between chondrite-normalized Sm and Tb. In a few cases, Tb was below the detec-tion limit, in which case it was plotted at the detection limit value.

374 ROSA ET AL.

On a chondrite-normalized REE diagram (Fig.6), the alkaline mafic sample has a more pro-nounced LREE-HREE fractionation than the tholei-itic mafic samples, which commonly reflects a lowerdegree of partial melting at greater depths. The calc-alkaline quartz-feldspar-phyric rocks display LREEenrichment (LaN/YbN ≈ 4–7), relatively flat HREEat 15 to 30 × chondrite, and negative Eu anomalies(Eu/Eu* ≈ 0.5–0.8).

Rock ClassificationQuartz-feldspar-phyric rocks

When plotted on the TAS diagram of Le Maitre etal. (1989), analyzed quartz-feldspar-phyric samplesrange from andesitic to rhyolitic compositions, aver-aging dacitic (Fig. 7). The compositional scatterlikely reflects silica and, to a certain degree, alkalimobility, resulting from syngenetic hydrothermalalteration. Silica alteration occurred as silica addi-tion along fracture networks and/or pervasive silicaaddition or removal, preferentially affecting thehyaloclastite matrix. In some cases, the addition ofcalcium, causing silica dilution, can also contributeto misclassification of the original rock. This occursin situations where calcite fills amygdales withincoherent domains (hyaloclastite clasts) that may

have not been completely removed during samplepreparation.

The analyzed coherent samples that seem to nothave fracture networks filled with quartz have, in allbut one case, dacitic compositions. Considering thatcoherent samples would have been relatively unal-tered, this suggests that the precursor was probablya dacite. To confirm this, and in order to readthrough the silica alteration, matrices and clastswere analyzed separately in four hyaloclastite sam-ples in which fracture networks filled with quartzwere not identified. Due to the higher permeabilityof the matrices, it is presumed that silica alterationwas stronger in them than within the hyaloclastiteclasts and, therefore, that the latter would corre-spond to the least-altered compositions. Except forone case (TR1-168.4) in which the clasts were sig-nificantly carbonatized as indicated by the high Cacontent, therefore yielding an andesitic composi-tion, clasts show a dacitic composition whereas thematrices are more mafic or silicic. It is, therefore,concluded that these rocks were originally dacites.Using the de la Roche et al. (1980) classification,and the results of norm calculations for the Streck-eisen (1978) classification (Figs. 8 and 9, respec-tively), the least-altered samples can be morenarrowly or precisely classified as rhyodacites.

FIG. 7. TAS diagram, with quartz-feldspar-phyric rocks represented by open squares, and mafic rocks by filled cir-cles. Larger symbols represent clasts of hyaloclastic breccias that were analyzed separately.

IBERIAN PYRITE BELT 375

FIG. 9. Streckeisen (1978) classification of the quartz-feldspar-phyric rocks, based on normative compositions.Larger symbols represent clast of hyaloclastic breccias analyzed separately.

FIG. 8. de la Roche (1980) diagram, with the classification of the quartz-feldspar-phyric rocks. Larger symbols cor-respond to clasts of hyaloclastites analyzed separately.

376 ROSA ET AL.

The chondrite-normalized REE diagram forclasts and matrices analyzed separately (Fig. 10)show similar patterns for each matching clast/matrixpair. However, the matrices of hyaloclastic samplesdisplay higher values than the corresponding clasts,due to residual concentration of REE in cases wheresilica leaching (samples MT and TE9) or carbonateleaching (TR1-168.4) occurred; or, they may displaylower values than their clasts, due to dilution ofREE, in cases where there was silica addition (sam-ple TR1-135.5). For sample TR1-168.4, alterna-tively to carbonate leaching of the hyaloclastitematrix, one can consider that carbonate was addedat a later stage, by filling the amygdules presentwithin the coherent domain (clasts) and thus dilut-ing the REE.

Mafic rocksThe mafic rocks have compositions that range

from trachybasalt to andesite and even dacite (Fig.7). Considering their petrography and the silicamobility described for the quartz-feldspar-phyricrocks, these rocks were probably originally basaltsor trachybasalts, but silica (and alkali?) mobilitycaused significant compositional scatter.

Geotectonic Setting

Distribution of volcanismRecently, the bimodal distribution of the IPB vol-

canism (Munhá, 1983; Mitjavilla et al., 1997) hasbeen disputed by Onézime et al. (2003), who consid-ered that significant amounts of intermediate rockexist. As shown above, most intermediate composi-tions more likely reflect silica mobility and subse-quent dispersal of plotted results on the TASdiagram; hence a traditional bimodal nature of vol-canism is favored, at least in the Albernoa area. Thisis significant for the geotectonic classification of thevolcanism, as arc settings are typically dominatedby intermediate rocks (andesites), whereas volcan-ism generated in extensional settings is generallybimodal.

Quartz-feldspar-phyric rocksInasmuch as the HFSE appear to have been rel-

atively immobile during alteration, they can be usedas petrogenetic indicators. Using the empiricallyderived tectonic discrimination diagrams of Pearceet al. (1984), the rhyodacites display volcanicarc characteristics, with low Rb content that is

FIG. 10. C1-Chondrite–normalized REE profiles (normalizing values after McDonough and Sun, 1995) for constitu-ents of hyaloclastic breccias. Matching clast (C = filled symbol, continuous line) and matrix (M = open symbol, dashedline) pairs. Gd was calculated by linear interpolation between chondrite-normalized Sm and Tb. In two cases, Tb wasbelow detection limit, in which case it was plotted at the detection limit value.

IBERIAN PYRITE BELT 377

incompatible with a syncollisional setting (Figs. 11and 12).

Assuming the above classification of least-altered rocks to be correct, the quartz-feldspar-phy-ric rocks are misclassified on the Nb/Y-Zr/TiO2 dia-gram of Winchester and Floyd (1977). Thisindicates that Zr, and probably other HFS elementsused in the tectonic discrimination diagrams ofPearce et al. (1984), appear to have anomalously lowconcentrations (Fig. 13). This may reflect a low sol-ubility of refractory phases in which these elementsreside during low temperature fusion. This sameprocess has been proposed by Piercey et al. (2001)

and Lentz (1999) for felsic rocks from the FinlaysonLake District, Yukon, and Bathurst, New Bruns-wick, respectively. If the HFSE concentrations areanomalously low, geotectonic settings indicated bydiagrams that use these elements, such as the tec-tonic diagrams of Pearce et al. (1984), should beconsidered invalid. In fact, considering that Ta, Nb,Yb, and Y, similarly to Zr, would have been two orthree times more concentrated if fusion hadoccurred under normal temperature conditions, thestudied rocks would plot within or close to the WPGfield. This is more compatible with models invokingpartial melting of crustal rocks as a source of silicic

FIG. 11. Y/Nb diagram (Pearce et al., 1984), for the quartz-feldspar-phyric rocks. WPG = within-plate granites; ORG =oceanic-ridge granites; VAG = volcanic-arc granites; SynCOLG = syncollisional granites.

FIG. 12. Y+Nb/Rb diagram (Pearce et al., 1984), for the quartz-feldspar-phyric rocks. The same acronyms are usedas in Figure 11.

378 ROSA ET AL.

magmas, proposed by Munhá (1983), Mitjavila et al.(1997), and Thiéblemont et al. (1998).

Mafic rocksIn the Nb-Zr-Y (Meschede, 1986) and Th-Ta-Hf

(Wood, 1980) diagrams, the three tholeiitic samplesplot in or very close to the VAB field, whereas thealkaline sample (AB1 341.3) plots as WPB (Figs. 14

and 15). Similarly, in the Zr-Y-Ti (Pearce and Cann,1973) diagram, in which only tholeiitic rocks can beplotted, the three tholeiitic samples plot on or veryclose to the VAB field (Fig. 16). However, thesethree tholeiitic samples plot within the MORB + FBfield and not on the island-arc field of the Shervais(1982) Ti-V diagram (Fig. 17), considered to be lesssensitive to crustal interaction (Pearce, 1996). This

FIG. 13. Nb/Y-Zr/TiO2 diagram (Winchester and Floyd, 1977), with the quartz-feldspar-phyric rocks as squares andthe mafic rocks as circles.

FIG. 14. Meschede (1986) diagram, with geotectonic classification of the mafic rocks. Filled circles = tholeiitic maficrocks; open circles = alkaline mafic rock; WPAlk = within-plate alkaline basalt; WPT = within-plate tholeiite; VAB =volcanic-arc basalt; MORB = mid-ocean ridge basalt.

IBERIAN PYRITE BELT 379

is an indication that Th enrichment and coupled Nbdepletion, evident in the Nb-Zr-Y and Th-Ta-Hf dia-grams (Figs. 14 and 15), likely result from theassimilation of continental crust rather than from atrue subduction component (Pearce, 1996). Simil-

arly, Ti and Y depletions in the Zr-Y-Ti diagram(Fig. 16) probably are due to the magma interactionwith relatively Ti- and Y-poor, Zr-rich continentalcrust. It is not completely clear what the originalmagma type would have been before assimilation of

FIG. 15. Wood (1980) diagram, with geotectonic classification of the mafic rocks. The same symbols and acronymsare used as in Figure 14.

FIG. 16. Pearce and Cann (1973) diagram, with geotectonic classification of the tholeiitic mafic rocks. The samesymbols and acronyms are used as in Figure 14.

380 ROSA ET AL.

such continental crust and the described acquisitionof an apparent arc signature. Yet the original magmamay have had a MORB source, as Mitjavila et al.(1997) argued, a WPB source, in which case it couldbe represented by the alkaline sample, or may havehad a transitional source between the two. Thedeveloped bimodal volcanism, with possible WPB/MORB transitions and superimposed crustal assim-

ilation is characteristic of magmatism in attenuatedcontinental lithosphere settings, such as the magma-tism developed in continental crust by local exten-sional tectonics.

The influence of continental crust on the trace-element signature of the mafic rocks studied can beseen on the spider diagram of Figure 18. The alka-line sample (AB1 341.3) has a typical WPB pattern,

FIG. 17. Shervais (1982) diagram, with geotectonic classification of the mafic rocks. Abbreviations: IAT = island-arctholeiites; BAB = backarc basalts; CFB = continental flood basalt. The same symbols are used as in Figure 14.

FIG. 18. NMORB-normalized profiles for the tholeiitic mafic rocks (filled circles), alkaline mafic rocks (open circles),and continental crust (crosses). NMORB values are from Sun and McDonough (1989) and continental crust values arefrom Rudnick and Fountain (1995).

IBERIAN PYRITE BELT 381

with no Nb anomaly and with relative enrichment ofTi over Y (Pearce, 1996). The tholeiitic samples dis-play patterns that are intermediate between a WPBor possibly an E-MORB pattern and average conti-nental crust values. The tholeiitic samples displaynegative Nb anomalies that make them resembleVAB. However, these anomalies probably do notreflect a true subduction component, but ratherresult from assimilation of continental crust.

Proposed Petrogenetic Model

The tectonic classification of the mafic rocksproposed above can be accounted for by consideringa model in which basaltic magmas of alkaline andtholeiitic affinities, depending on the degree of par-tial melting in the mantle, formed within an attenu-ated continental lithosphere setting. The degree ofmelting would have been controlled by the gradualstretching of the lithosphere, favoring progressivelyshallower melting. The alkaline magmas wouldhave had a WPB signature, whereas the tholeiiticmagmas would possibly have had a WPB/MORBsignature.

Although alkaline magmas preserved theirwithin-plate signatures, the tholeiitic magmas pro-gressively assimilated continental crust, yieldingapparent volcanic arc signatures. These basalticmagmas could have locally promoted crustal melt-ing, yielding a silicic magma. As suggested by thelow HFSE contents of the felsic rocks, this crustalmelting must have occurred under relatively low

temperatures. The resulting silicic magma subse-quently underwent a small degree of plagioclasefractionation, as indicated by the increase of the Euanomaly with decrease of P or Ti (Fig. 19), andgenerated the two suites of rhyodacitic rocks, onerepresented by the samples IP4 and TE3, and theother represented by all other analyzed quartz-feld-spar-phyric rocks (Table 1).

The fact that no arc rocks were identified, withapparent arc signatures dismissed by the identifica-tion of crustal assimilation processes for the maficrocks and low-temperature crustal melting for thefelsic rocks, supports a model in which volcanismoccurred in an attenuated continental lithospheresetting. This is compatible with the model proposedby Silva et al. (1990), and Quesada (1991), amongothers. In this model, an oblique collision betweenthe South Portuguese and the Ossa Morena platesgenerated strike-slip tectonics within the continen-tal crust of the South Portuguese plate. The strike-slip tectonics would have favored the opening ofpull-apart basins where volcanism, with the charac-teristics described above, would have occurred. Asimilar conclusion was drawn by Mitjavila et al.(1997), who in addition to general geochemical data,also presented isotope data. However, consideringthe limitations imposed by a limited number of ana-lyzed samples from only one area of the IberianPyrite Belt, models partially invoking subduction,such as proposed for this belt by Munhá (1983) andThiéblemont et al. (1998), should not be rejected atthis stage.

FIG. 19. Increase of the Eu anomaly with fractionation of the quartz-feldspar-phyric rocks, providing an indication ofplagioclase fractionation.

382 ROSA ET AL.

Acknowledgments

This work was sponsored by the POCTI programof the Fundação para a Ciência e Tecnologia (Portu-gal). In addition to the laboratory assistance referredto in the text, further laboratory support was pro-vided by the CREMINER–Centro de Recursos Min-erais, Mineralogia e Cristalografia, at the Faculdadede Ciências, Universidade de Lisboa.

REFERENCES

Bard, J. P., Capdevilla, R., Matte, P., and Ribeiro, A.,1973, Geotectonic model for the Iberian Variscan oro-gen: Nature, v. 241, p. 50–52.

Barret, T. J., and MacLean, W. H., 1999, Geological rela-tions at the Feitais VMS deposit and in some outcropareas at Aljustrel, Portugal: Vancouver, BC, EurozincMining Corporation, internal report, 65 p.

de la Roche, H., Leterrier, J., Grande Claude, P., and Mar-chal, M., 1980, A classification of volcanic and plu-tonic rocks using R1-R2 diagrams and major elementsanalysis—its relationships and current nomenclature:Chemical Geology, v. 29, p. 183–210.

Dias, R., and Ribeiro, A., 1995, The Ibero-Armorican arc:A collision effect against an irregular continent: Tec-tonophysics, v. 246, p. 113–128.

Irvine, T. N., and Baragar, W. R. A., 1971, A guide tochemical classification of the common volcanic rocks:Canadian Journal of Earth Sciences, v. 8, p. 523–548.

Le Maitre, R. W., Bateman, P., Dudek, A., Keller, J., Lam-eyre, J., Le Bas, M. J., Sabine, P. A., Schmid, R.,Sørensen, H., Streckeisen, A., Woolley, A. R., andZanettin, B., 1989, A classification of igneous rocksand glossary of terms: Oxford, UK, Blackwell.

Leistel, J. M., Bonijoly, D., Braux, C., Fressynet, P. H.,Kosakevitch, A., Leca, X., Lescuyer, J. L., Marcoux,E., Milesi, J. P., Piantone, P., Sobol, F., Tegyey, M.,Thiéblemont, D., and Viallefond, L., 1994, The mas-sive sulphide deposits of the South Iberian PyriteProvince: Geological setting and exploration criteria:Orléans, France, Bureau de Recherches Geólogiqueset Minières, document no. 234, 236 p.

Lentz, D. R., 1999, Petrology, geochemistry, and oxygenisotope interpretation of felsic volcanic and relatedrocks hosting the Brunswick 6 and 12 massive sulfidedeposits (Brunswick Belt), Bathurst Mining camp,New Brunswick, Canada: Economic Geology, v. 94, p.57–86.

McDonough, W. F., and Sun, S. S., 1995, The compositionof the Earth: Chemical Geology, v. 120, p. 223–253.

Meschede, M., 1986, A method of discrimination betweendifferent types of mid-ocean ridge basalts and conti-nental tholeiites with the Nb-Zr-Y diagram: ChemicalGeology, v. 56, p. 207–218.

Mitjavila, J., Martí, J., and Soriano, C., 1997, Magmaticevolution and tectonic setting of the Iberian Pyrite Beltvolcanism: Journal of Petrology, v. 38, p. 727–755.

Monteiro, J. H., and Carvalho, D., 1987, Sea floor volcan-ism and polymetallic sulfide deposits in ancient activemargins: The case of the Iberian Pyrite Belt, in Teleki,P. G., Dobsdon, M. R., and Stackelberg, V., eds.,Marine minerals: NATO ASI Ser. C, Mathematics andPhysical Sciences, v. 194, p. 375–387.

Munhá, J., 1983, Hercynian magmatism in the IberianPyrite Belt, in Sousa, M. J. L., and Oliveira, J. T., eds.,The Carboniferous of Portugal: Memórias ServiçosGeológicos de Portugal, v. 29, p. 39–81.

______, 1990, Metamorphic evolution of the South Portu-guese/Pulo do Lobo Zone, in Dallmeyer, R. D., andMartinez Garcia, E., eds., Pre-Mesozoic geology ofIberia: Berlin-Heidelberg, Germany/New York, NY,Springer-Verlag, p. 363–368.

Munhá, J., and Kerrich, R., 1981, Sea water–basalt inter-action in spilites from the Iberian Pyrite Belt: Contri-butions to Mineralogy and Petrology, v. 75, p. 15–19.

Oliveira, J. T., 1990, South Portuguese Zone, in Dallm-eyer, R. D., and Martinez Garcia, E., eds., Pre-Meso-zoic geology of Iberia: Berlin-Heidelberg, Germany/New York, NY, Springer-Verlag, p. 333–346.

______, 1992, Carta geológica de Portugal (1/200.000),Notícia explicativa da folha 8: Lisbon, Portugal,Serviços Geológicos de Portugal, 91 p.

Oliveira, J. T., Carvalho, P., Pereira, Z., Pacheco, N.,Fernandes, J. P., and Korn, D., 1997, Stratigraphy ofthe Neves-Corvo mine region [abs.] , in Barriga, F. J. A.S., ed., SEG Neves Corvo Field Conference 1997,Abstracts and Program, p. 86–87.

Onézime, J., Charvet, J., Faure, M., Bourdier, J. L., andChauvet, A., 2003, A new geodynamic interpretationfor the South Portuguese Zone (SW Iberia) and the Ibe-rian Pyrite Belt genesis: Tectonics, v. 22, no. 4, p. 1–17.

Pearce, J.A., 1996, A user’s guide to basalt discriminationdiagrams, in Wyman, D. A., ed., Trace elementgeochemistry of volcanic rocks: Applications for mas-sive sulphide exploration: Geological Association ofCanada Short Course Notes, v. 12, p. 79–113.

Pearce, J. A., and Cann, J. R., 1973, Tectonic setting ofbasic volcanic rocks determined using trace elementanalysis: Earth and Planetary Science Letters, v. 19, p.290–300.

Pearce, J. A., Harris, N. B. W., and Tindle, A. G., 1984,Trace element discrimination diagrams for the tectonicinterpretation of granitic rocks: Journal of Petrology, v.25, p. 956–983.

Piercey, S. J., Pardais, S., Murphy, D. C., and Mortensen,J. K., 2001, Geochemistry and paleotectonic setting offelsic volcanic rocks in the Finlayson Lake volcanic-hosted massive sulfide district, Yukon, Canada: Eco-nomic Geology, v. 96, p. 1877–1906.

IBERIAN PYRITE BELT 383

Quesada, C., 1991, Geological constraints on the Paleo-zoic tectonic evolution of tectonostratigraphic terranesin the Iberian massif: Tectonophysics, v. 185, p. 225–245.

Ribeiro, A., Antunes, M. T., Ferreira, M. P., Rocha, R. B.,Soares, A. F., Zbyszewsky, G., Almeida, P. M., Car-valho, D., and Monteiro, J. H., 1979, Introduction à lagéologie général du Portugal, in 26-ème CongrèsGéologique International, Colloque G14: Orléans,France, Bureau de Recherches Géologiques etMiniéres, 141 p.

Ribeiro, A., Quesada, C., and Dallmeyer, R. D., 1990,Geodynamic evolution of the Iberian Massif, in Dallm-eyer, R. D., and Martinez Garcia, E., eds., Pre-Meso-zoic geology of Iberia: Berlin-Heidelberg, Germany/New York, NY, Springer-Verlag, p. 397–410.

Routhier, P., Aye, F., Boyer, C., Lecolle, M., Picot, P., andRoger, G., 1980, La ceinture sud-iberique à amas sul-furés dans sa partie espagnole mediane, tableaugeologique et metalogenique, synthèse sur le typeamas sulfurés volcano-sedimentaires: MémoiresBureau de Recherches Geólogiques et Minières, v. 94,265 p.

Rudnick, R. L., and Fountain, D. M., 1995, Nature andcomposition of the continental crust: A lower crustalperspective: Reviews in Geophysics, v. 33, p. 267–309.

Schütz, W., Ebneth, J., and Meyer, K. D., 1988,Trondhjemites, tonalites and diorites in the South Por-tuguese Zone and their relations to the volcanites andmineral deposits of the Iberian Pyrite Belt: Geolo-gische Rundschau, v. 76, p. 201–212.

Silva, J. B., Oliveira, J. T., and Ribeiro, A., 1990, SouthPortuguese Zone. Structural outline, in Dallmeyer, R.D., and Martinez Garcia, E., eds., Pre-Mesozoic geol-ogy of Iberia: Berlin-Heidelberg, Germany/New York,NY, Springer-Verlag, p. 348–362.

Shervais, J. W., 1982, Ti-V plots and the petrogenesis ofmodern and ophiolitic lavas: Earth and Planetary Sci-ence Letters, v. 59, p. 101–118.

Streckeisen, A., 1978, Classification and nomenclature ofvolcanic rocks, lamprophyres, carbonatites and melili-tic rocks: Neues Jahrbuch für Mineralogie Abhandlun-gen, v. 134, p. 1–14.

Sun, S. S., and McDonough, W. F., 1989, Chemical andisotopic systematics of oceanic basalts: Implicationsfor mantle composition and mantle evolution: Earthand Planetary Science Letters, v. 35, p. 429–448.

Thiéblemont, D., Marcoux, E., Tegyey, M., and Leistel,J. M., 1994, Génèse de la province pyriteuse sud-ibe-rique dans un paleo-prisme d’accretion?: BulletinSocieté Géologique de France, v. 165, p. 407–423.

Thiéblemont, D., Pascual, E., and Stein, G., 1998, Mag-matism in the Iberian Pyrite Belt: Petrological con-straints on a metallogenetic model: MineraliumDeposita, v. 33, p. 98–110.

Winchester, J. A., and Floyd, P. A., 1977, Geochemicaldiscrimination of different magma series and their dif-ferentiation products using immobile elements: Chem-ical Geology, v. 20, p. 325–343.

Wood, D. A., 1980, The application of a Th-Hf-Ta diagramto problems of tectonomagmatic classification and toestablishing the nature of crustal contamination ofbasaltic lavas of the British Tertiary volcanic province:Earth and Planetary Science Letters, v. 50, p. 11–30.