ARTICLE

D. Thie blemont á E. Pascual á G. Stein

Magmatism in the Iberian Pyrite Belt: petrological constraintson a metallogenic model

Received: 3 March 1996 /Accepted: 7 April 1997

Abstract Igneous formations associated with massivesulphide deposits in the Iberian Pyrite Belt (IPB) areessentially composed of basic lavas and dolerites, anddacitic to rhyolitic volcanites; intermediate lavas aresubordinate. The basic rocks show variable geochemicalcharacteristics: lavas and dolerites comparable to recentwithin-plate alkaline basalts seem restricted to thewestern and southern parts of the IPB, whereas basicrocks comparable to continental tholeiites or arc-relatedbasalts occur across the whole belt. The felsic rocks areclassi®ed as calc-alkaline and belong to the ``low-Al2O3

and high-Yb type''. At given SiO2, Al2O3 and TiO2

contents, they show variable Zr, Nb, and HREE con-tents. Heavy-rare-earth element fractionation decreasesfrom the dacites to the rhyolites ([Gd/Yb]N � 1),whereas the negative Eu-anomaly becomes more pro-nounced. The characteristics of the rhyolites are typicalof sulphide-fertile volcanic packages. Trace-elementmodelling suggests that the felsic rocks evolved from adacitic parent magma through fractional crystallizationof hornblende and plagioclase. Partial melting of anamphibolite protolith, which appears as the mostprobable model for the origin of this dacitic magma,requires a high T/P gradient in the crust. The occurrenceof alkaline basalts and continental tholeiites is consistentwith formation of the IPB in a tensional tectonic setting.However, the associated island-arc tholeiites suggest alocation in a domain of plate convergence. Emplace-ment in a fore-arc basin over a recently accreted crustal

segment is envisaged as a possible hypothesis to accountfor the geological and petrological constraints. A highgeothermal gradient and eruption in a submarine ten-sional basin could have been two key ingredients for thedevelopment of massive sulphide deposits within theIPB.

Resumen (translated by E. Pascual) Las formacionesõ gneas asociadas con los depo sitos de sulfuros masivosde la Faja Pirõ tica Ibe rica (IPB) se componen esencial-mente de doleritas y lavas ba sicas y de rocas volca nicasdacõ ticas a riolõ ticas; las lavas intermedias son pocoabundantes. Las rocas ba sicas muestran caracteres ge-oquõÂmicos variables: lavas y doleritas comparables abasaltos alcalinos intraplaca recientes parecen hastaahora restringidos a las partes W y S de la IPB, mientrasque rocas ba sicas de caracteres comparables a las detoleõ tas continentales o basaltos relacionados con arcosaparecen en toda la zona. Las rocas a cidas se clasi®cancomo calcoalcalinas del tipo ``low-Al2O3, high-Yb''.Para un contenido dado en SiO2, Al2O3 y TiO2, muest-ran contenidos variables en Zr, Nb y REE. El contenidoen tierras raras pesadas decrece de dacitas a riolitas([Gd/Yb]N � 1), al tiempo que la anomalõ a de Eu sehace ma s pronunciada. La modelizacio n de elementostrazas sugiere que las rocas evolucionaron a partir de unmagma parental dacõ tico mediante cristalizacio n frac-cionada de hornblenda y plagioclasa. La fusio n parcialde un protolito an®bolõ tico, que parece el modelo ma splausible para el origen del magma dacõ tico, requiere unelevado gradiente T/P en la corteza. La existencia debasaltos alcalinos y de toleõ tas continentales es congru-ente con la formacio n de la IPB en un entorno tecto nicodistensivo. El emplazamiento en una cuenca ``fore-arc'',en un segmento cortical de reciente acrecio n, se conte-mpla como una hipo tesis posible para explicar los car-acteres geolo gicos y petrolo gicos. Un alto gradientete rmico, junto con la erupcio n en una cuenca submarinaextensional, pueden haber sido los dos ingredientes claveen el desarrollo de los depo sitos de sulfuros masivos enla IPB.

Mineralium Deposita (1998) 33: 98±110 Ó Springer-Verlag 1998

Editorial handling: E. Marcoux

D. Thie blemontBRGM, SGN/I2G/GEO, BP 6009, 45060 Orle ans Cedex, Francee-mail: d.thieblemont@brgm.fr

E. PascualDept. Geol. Mineria, University of Huelva, Campus La Ra bida,21819 Palos de la Fontera (Huelva), Spain

G. SteinBRGM, SMN/DEX, BP 6009, 45060 Orle ans Cedex, France

Introduction

The Iberian Pyrite Belt (IPB) is located within the SouthPortuguese Zone in the southern part of the IberianVariscan orogenic belt (Fig. 1). To the south it is bor-dered by the SWPortugueseDomain and to the north it isseparated from the Iberian Autochthon (Ossa MorenaZone) by two units interpreted as two ``oceanic exoticterranes'' (Quesada 1991): the Pulo do Lobo anticline andBeja-Acebuches amphibolites. The IPB comprises threeunits (e.g. Oliveira 1990): (1) the terrigenous Phyllite-Quartzite (PQ) Formation (Schermerhorn 1971), whichincludes an upper carbonaceous horizon of middleFamennian age; (2) a volcano-sedimentary complex (VSFormation) of late Famennian to early Visean age thatcomprises two bimodal and predominantly felsic volcanicunits (V1 and V2) hosting the massive sulphide deposits,and a third felsic unit (V3) (Routhier et al. 1980; MunhaÂ1983); (3) a ¯ysch-like unit (Culm; Routhier et al. 1980) oflate Visean to early Stephanian age.

A large batholith including gabbros, diorites, tona-lites and granites crops out in the northeastern part ofthe IPB (IGME 1972; Soler 1980; SchuÈ tz et al. 1987).These rocks have been interpreted either as the deepequivalents of the volcanic units of the VS Formation(Soler 1980; SchuÈ tz et al. 1987) or as late-stage intrusivesunrelated to the lavas (Simancas 1983). The former in-terpretation has received recent support from a detailed®eld study in the western part of the batholith (Cam-pofrio area; Stein et al. 1996; Fig. 1) but is probably notapplicable to the whole batholith (Simancas 1983; De LaRosa 1992). Evidence for a pretectonic emplacement ofthe western part of the batholith and a close relationshipbetween the volcanic and plutonic rocks includes: (1)the local presence of a N 110° E-striking cleavage in theplutonic rocks, continuous with that observed in thevolcanic rocks; (2) the local thrusting of the plutonicrocks over the VS Formation (GarcõÂ a Palomero et al.1993); (3) the occurrence of hectometre- to kilometre-long and decametre- to hectometre-wide lenses of ®ne-grained granite to microgranite located within the VS or

Fig. 1 Major structural units and tectono-stratigraphic domains inSW Iberia (from Quesada 1991)

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PQ formations o� the outer side of the batholith; (4) thegradual transition from these granitic facies to graniteporphyries and massive rhyolites of the VS Formation.Basic and felsic dykes cutting the plutonic rocks areinterpreted by Stein et al. (1996) as feeders for the V2and/or V3 episodes.

Detailed petrographical and geochemical studies ofthe IPB lavas are presented by Routhier et al. (1980),Munha (1981, 1983) and Thie blemont et al. (1994b). Onthe basis of the major-element data, Routhier et al.(1980) conclude that the volcanics represent a singlecalc-alkaline series evolving from basalts to high-Sirhyolites. On the basis of the trace-element data, MunhaÂ(1983) and Thie blemont et al. (1994b) argue for di�erentorigins for the basic and felsic magmas, favouring acrustal origin for the latter. Moreover, Munha (1983)mentions the presence of alkaline basalts and dolerites inthe western part of the belt, but no such rocks wereobserved by Thie blemont et al. (1994b) in its easternpart.

This article re-examines the geochemistry of the IPB,using both new and published analyses, in order to (a)evaluate the geographical extent of the alkaline basicmagmatism, (b) test the possible cogenetic relationshipbetween the plutonic rocks of the Campofrio area andthe VS volcanites, (c) determine the origin of the felsicmagmas and (d) provide constraints on a geodynamicand metallogenic model for the IPB.

Petrographical outline

A petrographical overview of the IPB can be obtained fromRouthier et al. (1980), Soler (1980), Munha (1983), SchuÈ tz et al.(1987) and Thie blemont et al. (1994b), and only a brief summarywill be given here. The V1 and V2 units comprise rhyodacitic torhyolitic pyroclastics and lavas, and the V3 unit includes reworkedtu�s and siliceous shales. Intermediate rocks are generally scarcebut locally abundant, especially in the northern part of the IPB(Routhier et al. 1980). Basic lavas (often with pillow structures) andintrusives (dykes and sills) are intercalated within the felsic lavas.The proportions of felsic and basic rocks are estimated by Routhieret al. (1980) to be 2/3 and 1/3 respectively, and Munha (1983)concludes that they were derived from distinct volcanic centres.Most of the lavas have su�ered recrystallization under prehnite-pumpellyite to lower greenschist metamorphic conditions. In thefelsic lavas, the most common phenocrysts are plagioclase (albite),quartz and biotite (chloritized). Potassic feldspar is rare andprobably of secondary origin (replacing plagioclase). No systematicmineralogical di�erences are observed between the lavas of the V1,V2 and V3 units. The dacites and andesites are mildly to stronglyporphyritic. The former show hornblende phenocrysts and thelatter hornblende and clinopyroxene phenocrysts. Hybrid andesiteshave been observed by Thie blemont et al. (1994b) in the northernpart of the belt (La Joya Mine). These rocks consist of dark patchescomposed of plagioclase, chlorite, clinopyroxene, brown horn-blende and ``rhyolitic'' quartz dispersed in a light matrix ofrhyodacitic composition. The basic rocks are generally stronglyaltered except for rare massive ¯ows and intrusives which showrelict clinopyroxenes. Occurrence of a titaniferrous-salite to sodianferro-salite together with kaersutite and biotite is reported byMunha (1983) in some dolerites and massive lavas from the upperpart of the volcanic sequence. This suggests an alkaline a�nity forthese rocks.

Alteration is generally less intense in the plutonic rocks than inthe lavas. The gabbros are ®ne- to medium-grained and show in-tersertal to ophitic textures. Plagioclase is partially replaced bysericite and/or epidote aggregates and is commonly zoned. Clino-pyroxene occurs as large crystals enclosing plagioclase and as relictswithin hornblende. Amphibole shows a wide range of composition;from brown to green hornblende and actinolite. Opaques are par-tially replaced by titanite. The granites are ®ne- to medium-grained;the most common minerals are plagioclase, perthitic K-feldspar,quartz, biotite and locally amphibole. Granophyric intergrowths ofquartz and feldspars frequently occur in the matrix. Apatite andzircon are the most frequent accessory minerals. Hybrid rocksshowing gabbroic patches within a granitic matrix have been ob-served by Stein et al. (1996) in the Campofrio area. Intermediaterocks of broad quartz-dioritic to tonalitic composition may occurin the transition zone between the basic patches and felsic rocks;they are interpreted as magma mixing (Stein et al. 1996). Otherintermediate rocks appear more homogeneous, tending to agranodioritic composition. They di�er from the granites by ahigher abundance of ferro-magnesian minerals, especially amphi-bole.

Geochemical characteristics

Data base and analytical methods

A total of 323 analyses of volcanic and plutonic rocks from fourdata sets has been used for the present study. Eighty two lavas fromwidespread areas, mainly in the Spanish part of the Belt (Thie b-lemont et al. 1994b) were analyzed at Bureau de Recherche Ge o-logique et MinieÁ re by XRF (major elements), ICP (Sr, Ba, etc.) andICP-MS (REE, Th, Ta, Zr, etc.). They may be obtained by requestto D. Thie blemont. Accuracy of the ICP-MS analyses has beenestimated by Thie blemont et al. (1994a) to be around 10±20% forcontents in the range 0.3±1 ppm and probably less than 5% forcontents >1 ppm. In addition, 44 lavas from the AznalcoÁ llar area(Fig. 1) were analyzed by the Huelva ``team'' by INAA at X-RALLaboratories (Canada). These new data supplement a set of 141lava analyses from both the Spanish and Portuguese parts of theIPB published by Munha (1981); amongst the plutonic rocks, onlythose from the Campofrio area (Fig. 1) have been considered.Thirty analyses, including major and some trace elements (Nb, La,Ce, Zr, etc.) are presented by SchuÈ tz et al. (1987), and 27 additionalanalyses, including ®ve of dolerite and rhyolite dykes cutting theplutonic rocks, were performed at Bureau de Recherche Ge ologi-que et MinieÁ re (XRF, ICP, ICP-MS) following the ®eld study ofStein et al. (1996). They may be obtained by request to D. Thie b-lemont. Representative or average analyses of the di�erent mag-matic units are given in Table 1.

Element mobility during hydrothermal alterationand metamorphism

Hydrothermal alteration is commonly intense in thevolcanic rocks, especially in the vicinity of the sulphidedeposits, but is less marked in the plutonic rocks. De-spite careful sampling and petrographic examination,none of the analyzed samples can be considered asperfectly fresh. Consequently, only those elements con-sidered to be immobile or slightly mobile during alter-ation and metamorphism can be used for a petrologicaldescription. Munha (1983) considered Nb, Zr, Y, Ti, Pand the rare-earth elements (REE) to have been rela-tively immobile during moderate alteration in theIPB. Ta, Th, Hf have been added to this list. Relative

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immobility of Nb, Zr, Y and Ti during alteration in thevicinity of massive sulphide deposits have been noted bymany authors (e.g. Lesher et al. 1986; Whitford et al.1988), on the other hand, Eu has been shown by Whit-ford et al. (1988) to be mobile under certain conditions.This point is critical because the existence of a markednegative Eu-anomaly put strong constraints on thepetrological modelling (see petrological discussion). Ona SiO2 vs. Eu/Eu* plot (Fig. 2), a well-de®ned negativecorrelation appears for the plutonic rocks and a similar,though less smooth correlation is observed for the lavasand dolerites of the VS Formation. The average Eu/Eu*

ratios for the basic and the felsic lavas (i.e. SiO2 ³ 68%)are 0.94 and 0.39 respectively; the standard deviationsare 0.16 and 0.2. It may be envisaged that a part of thedispersion observed for the volcanic rocks is of sec-ondary origin, on the other hand, complete redistribu-tion of Eu during alteration or metamorphism is veryunlikely, at least in the analyzed lavas, because the ob-served trends are perfectly consistent with magmaticprocesses (plagioclase fractionation). Thus we believethat the existence of a pronounced negative Eu-anomalyis a primary feature of both the granites of the Cam-pofrio area and felsic lavas of the VS Formation. This

Table 1 Representative analyses of the principal types of magmatic rocks of the IPB. Average compositions are given for the dacite (18analyses) of the AznalcoÁ llar area (see Fig. 1 for location) and the high-Nb type rhyolites (5 analyses, from Munha 1981). The average ofthe medium La/Nb-type of basic rocks (23 analyses) is used as a starting composition in the petrological modelling

Type Basic rock Dacite Rhyolite Campofrio Massif

Low Medium High Average Low MediumHigh Gabbro Granite DykesLa/Nb La/Nb La/Nb Nb Nb Nb

Dolerite Rhyolite

Sample PTR/3 PL1 n = 23 11a DSI25 DSI45 n = 18 DSI44 CH51 n = 5 GS28 GSO7A DS173 GS15

SiO2 (%) 47.00 51.70 49.99 50.57 62.52 64.48 63.87 72.86 73.70 75.14 50.89 72.20 48.31 76.00TiO2 1.80 1.55 1.69 1.30 0.17 0.52 0.49 0.41 0.34 0.11 1.72 0.34 1.84 0.16Al2O3 15.05 15.46 16.17 15.23 17.17 15.69 16.09 13.01 12.37 11.22 14.96 13.95 16.15 12.44Fe2O3t 10.85 9.49 10.61 9.11 5.07 5.68 3.98 2.73 3.21 2.23 10.96 3.15 11.22 2.10MnO 0.11 0.17 0.16 0.20 0.07 0.05 0.05 0.05 0.07 0.05 0.20 0.06 0.18 0.06MgO 3.50 6.58 6.40 7.37 4.71 2.43 2.55 0.89 0.80 0.10 6.70 0.49 7.59 <0.02CaO 8.20 6.25 5.94 3.65 0.35 3.23 2.79 2.33 0.77 0.01 10.05 2.19 11.05 1.52Na2O 4.75 3.97 3.87 3.76 6.63 3.13 3.22 3.68 4.23 0.29 2.84 4.57 2.28 5.43K2O 0.69 0.49 0.46 0.41 0.16 2.05 3.22 2.55 3.12 8.35 0.40 2.44 0.09 0.88P2O5 0.24 0.17 0.21 0.13 <0.05 0.08 0.12 0.07 0.07 <0.05 0.19 0.06 0.13 <0.05LOI 7.75 3.17 4.69 2.47 3.18 3.27 2.85 0.94 0.54 1.39 1.06 0.46 1.65 1.22

Total 99.94 99.00 100.19 99.20 100.03 100.61 99.24 99.52 99.22 98.88 99.97 99.91 100.54 99.81

Rb (ppm) 20 10 9 14 4 54 83 55 77 227 7 70 2 26Ba 341 95 129 141 76 565 696 625 435 467 74 299 34 177Sr 486 144 162 316 215 105 278 84 44 17 125 82 124 129Th 1.1 2.1 2.3 2.2 10.9 9.7 9.5 7.3 11.9 0.8 10.4 n.d. 10.9U 0.6 0.6 1.0 0.4 2.1 1.9 3.9 1.8 2.8 0.2 2.2 0.2 2.9Ta 1.2 ± 0.5 0.2 0.8 0.6 0.9 0.4 ± 0.4 0.6 0.3 0.8Nb 39.8 8.1 8.5 2.5 10.2 8.0 9.9 6.6 15.6 46.0 4.2 6.5 5.2 8.4Hf 2.3 3.2 3.5 2.6 3.2 3.7 4.5 2.8 9.8 2.4 5.8 3.0 5.6Zr 146 156 154 99 101 139 157 116 394 434 129 228 137 183Y 24.4 28.2 32.0 31.1 17.0 18.3 23.3 21.0 64.8 97.3 20.4 41.0 32.2 54.8V 235 218 248 239 54 93 67 79 21 249 35 227 20Co 47 33 39 30 6 12 6 8 8 31 5 39 <5Cr 158 187 181 281 ND 17 61 28 78 1 204 22 195 28Ni 87 34 46 91 15 ND 8 ND 20 8 58 16 92 21

La 26.9 12.1 13.1 6.9 13.5 22.1 24.7 19.8 34.30 45.7 5.5 24.1 6.4 29.9Ce 48.8 26.7 30.3 18.3 36.3 45.4 52.7 41.2 79.4 97.0 13.8 54.9 18.7 68.1Pr 3.0 3.2 3.8 2.5 4.5 5.0 6.0 4.5 9.0 1.9 6.3 2.9 8.4Nd 27.6 16.3 18.8 12.5 21.0 18.3 22.7 16.3 41.3 48.3 9.9 25.6 14.5 32.4Sm 5.8 4.3 4.9 4.2 4.5 3.8 5.0 3.2 10.3 11.8 3.0 6.1 4.5 8.1Eu 1.8 1.5 1.6 1.1 0.5 1.1 1.4 0.8 1.8 0.6 1.1 0.9 1.7 1.1Gd 5.4 4.8 5.5 5.0 4.5 4.0 4.7 3.4 10.9 13.3 3.6 6.9 5.2 8.8Tb 0.8 0.8 1.0 1.0 0.8 0.6 0.7 0.6 1.9 2.7 0.6 1.1 0.9 1.6Dy 3.9 5.5 6.0 6.5 4.2 3.7 4.4 3.9 13.1 4.1 7.2 5.8 10.2Ho 0.8 1.1 1.3 1.4 0.7 0.8 0.8 0.8 2.7 0.8 1.5 1.2 2.1Er 2.2 3.3 3.6 3.9 2.1 2.2 2.5 2.5 8.1 2.4 4.3 3.6 6.6Tm 0.3 0.5 0.5 0.6 0.3 0.3 0.4 0.4 1.3 0.4 0.7 0.5 1.0Yb 1.9 3.2 3.3 3.7 2.0 2.0 2.4 2.6 8.4 12.7 2.3 4.5 3.4 6.3Lu 0.3 0.5 0.5 0.5 0.3 0.3 0.4 0.4 1.2 0.3 0.7 0.5 0.9

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point will be discussed in more detail in the petrologicalsection.

Volcanic rocks

Plots of MgO, TiO2 against SiO2 (Fig. 3A±B) illustratesome general characteristics of the volcanic rocks of theIPB and enable broad comparisons with the plutonicrocks of the Campofrio area. A nearly continuous trendis observed on the SiO2 vs. MgO diagram (Fig. 3A)which suggests that the proportions of basic(SiO2 £ 52%), intermediate (SiO2 52±63%), and acidic(SiO2 ³ 63%) lavas are comparable. As pointed out byRouthier et al. (1980), intermediate lavas are in factsubordinate on a regional scale, and thus their relativeabundance on Fig. 3A re¯ects oversampling. Amongstthe analyzed samples, about a quarter are de®ned as``highly acidic'' (i.e. SiO2 > 74%). As the lavas aregenerally altered, it may be suggested that this highproportion re¯ects a late-stage silici®cation of originallyglassy lavas. However low MgO (i.e. MgO < 1%) andTiO2 (i.e. TiO2 � 0.2%) contents con®rm the highlydi�erentiated nature of these lavas; likewise, high-SiO2

rocks also form a very signi®cant proportion of the

correlative plutonic units of the Campofrio area (Fig. 3).Figure 3B shows that only a minor proportion of thebasic lavas have TiO2 contents in the range 0.9±1.2%.This is to be noted because this range also correspondsto the maximum frequency for recent subduction-relatedbasalt and basaltic andesites (e.g. Miyashiro 1974).Thus, as a whole, the volcanism of the IPB bears littleresemblance to recent arc-magmatism (Munha 1983).

Plots of Zr and Y against SiO2 (Fig. 4A-B) illustratea large overlap between the volcanic and correlativeplutonic rocks. No single trends are observed; at a givenSiO2, the Zr and Y contents are strongly variable, forinstance Zr varies from �50 ppm to more than 200 ppmin the basic range (SiO2 = 45±52%) (Fig. 4A) and from�50 ppm to �450 ppm in the acidic range (SiO2 ³ 63%)(Fig. 4B). Variability for Y is even greater, particularlyin the felsic rocks.

In the following sections we examine more preciselythe trace-element characteristics of the basic and felsiclavas. Because rocks containing between 55% and 63%SiO2 form only a minor part of our own data set, theyare not considered. A detailed description and interpre-tation of these intermediate lavas may be found inMunha (1983).

Basic rocks

As illustrated by the La versus Nb (Fig. 5) and Y vs.Nb (Fig. 6) plots, the basic lavas of the IPB showvariable trace-element characteristics but, as noted byMunha (1983) and Thie blemont et al. (1994b), thesegeochemical variations are independent of stratigraphicposition. Three geochemical ``types'' may be de®ned onthe basis of their La/Nb and Y/Nb ratios; representa-tive compositions are given in Table 1. The ®rst,characterized by low La/Nb (i.e. [La/Nb]N < 1) andY/Nb (i.e. [Y/Nb]N < 1) ratios and fractionated rare-

Fig. 2 SiO2 vs. Eu/Eu* diagram for the lavas and dolerites of the VSFormation and plutonic rocks of the Campofrio area

Fig. 3A, B Plots ofAMgO and B TiO2 against SiO2 for the lavas anddolerites of the VS Formation and correlative plutonic rocks of theCampofrio area

Fig. 4A,B Plots of A Zr and B Y against SiO2 for the lavas anddolerites of the VS Formation and potentially correlative plutonicrocks of the Campofrio area. Same symbols as on Fig. 3

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earth-element (REE) patterns (Fig. 7A), corresponds tothe ``type-B'' basalts and dolerites of Munha (1983).The geochemical characteristics and the occurrence ofTi-rich clinopyroxenes led Munha (1983) to comparethese rocks to recent within-plate alkaline basalts. Thelavas and intrusives of the two other types do not show

any systematic petrographical or mineralogical di�er-ences; both are classi®ed as tholeiitic on the FeOt/MgOvs. SiO2 diagram (Miyashiro 1974, not shown) andgeochemical variations from one to the other are con-tinuous. Thus, this division into two types must beconsidered as purely descriptive. One is characterizedby medium La/Nb (i.e. 1 £ [La/Nb]N £ 2) and Y/Nb(i.e. [Y/Nb]N < 1) ratios, and the other by high La/Nb(i.e. [La/Nb]N > 2) and Y/Nb (i.e. [Y/Nb]N > 1) ra-tios. The former (Fig. 7B) shows primordial mantle-normalized trace-element patterns with moderate neg-ative Nb-anomalies and mild enrichment in highly in-compatible elements, and the latter (Fig. 7C) displayspronounced negative Nb-anomalies. In the Y/Nb ver-sus La/Nb discriminant diagram (modi®ed after Cab-anis and Le colle 1989; Fig. 8), the correspondinganalyses overlap the ®elds of continental tholeiites(medium La/Nb group), back-arc basin and island-arcbasalts (high La/Nb group).

From the presently available data (including those ofMunha 1981), a geochemical zoning of the IPB appearsto emerge (Thie blemont et al. 1995). This is illustratedby Fig. 9 where we have plotted the location of thedi�erent geochemical types along the belt. The alkaline-type (type-B) basalts are mostly concentrated in thewestern part of the belt, but also occur sporadically in itssouthern part (AznalcoÁ llar area). Medium-La/Nb andhigh-La/Nb tholeiitic rocks, however, occur across thewhole belt, commonly in close association. This petro-logical zoning is not so apparent where the compositionof the clinopyroxenes is concerned; for instance, Ti-richclinopyroxenes similar to those analyzed by MunhaÂ(1981) in type-B lavas have been observed in basaltsfrom the central part of the belt (Sotiel area; Pascual,unpublished data). Unfortunately, the correspondinglavas have not been analyzed.

Fig. 5 La versus Nb diagram for the most basic lavas (i.e.SiO2 £ 55%) of the IPB. Di�erent key symbols are used for thethree geochemical ``types'': shaded circle, low La/Nb (alkaline) type;®lled circle, medium La/Nb type; empty circle high La/Nb type. Theline labelled PM corresponds to a primordial La/Nb ratio (=1 afterHofmann 1988)

Fig. 6 Y versus Nb diagram for the most basic lavas (i.e. SiO2 £ 55%)of the IPB. See Fig. 5 for explanations

Fig. 7A±C Primordial mantle-normalized diagrams (Wood et al.1979, normalization values from Hofmann 1988 except for P[P2O5 = 0.016%]) for the basic lavas of the IPB. A Low La/Nbratio (alkaline) type; B Medium La/Nb ratio type; C High La/Nbratio type

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Felsic rocks

Felsic rocks in the IPB range from dacite (SiO2 £ 63±68%) to rhyolite (SiO2 ³ 68%) and high-Si rhyolite(SiO2 ³ 74%). On the FeOt/MgO versus SiO2 diagram(Miyashiro 1974, not shown) most samples plot in thecalc-alkaline ®eld, although very low FeOt/MgO ratios(i.e. <2) are common which suggests element mobilityduring hydrothermal alteration (and/or metamorphism).Erratic distributions are also observed for K and Na,

which precludes a discrimination between low-, medium-and high-potassium rocks (Peccerillo and Taylor 1976).According to Arth's (1979) scheme, the rhyolites aregenerally classi®ed as ``low-Al2O3'' and ``high-Yb'' lavas(Fig. 10); the dacites, however, show rather high Ybcontents (i.e. >1.5 ppm) and also high Al2O3 (>14%).A substantial variation is observed for Yb (1.5 to>10 ppm) at a given Al2O3 content, which is indepen-dent of the TiO2 and SiO2 contents. Wide variabilitiesare also observed for Nb, Y, Zr or Hf (Fig. 4, Table 1).Despite these variations, the rhyolites show similar REEpatterns (Fig. 11A); the rocks are LREE-enriched andshow pronounced negative Eu-anomalies and mild or nofractionation between HREE. The dacites di�er fromthe rhyolites by less pronounced negative Eu-anomaliesand signi®cant fractionation between HREE (Fig. 11B).Finally, a particular type of high-Si rhyolite observed byMunha (1981) in the westernmost part of the belt(Cercal area), and referred to by this author as the``cercal type'', shows Yb contents up to 15 ppm andlocal ``LREE-collapsed'' REE-patterns (Munha 1983;Fig. 15).

The highly variable trace element content of the IPBfelsic rocks precludes an unequivocal geotectonic dis-crimination. For instance, the analyses overlap the dis-criminating ®elds of the Nb versus Y diagram (Pearce

Fig. 8 Y/Nb versus La/Nb diagram (modi®ed from Cabanis andLe colle 1989) for the most basic lavas (SiO2 < 55%) of the IPB.Shaded square, basic plutonic rocks of the Campofrio area; shadedtriangle, basic dykes cutting the plutonic rocks. Other symbols as forFig. 5. Representative average compositions of di�erent types ofbasalts are also plotted: N-MORB, N-type MORB from Hofmann(1988); E-MORB, E-type MORB from Wood (1978);WPAB, within-plate alkaline basalt from Baker et al. (1977) (transitional basalt) andBRGM (unpublished data);CFB, continental tholeiite from Lightfootand Hawkesworth (1988) and Cox and Hawkesworth (1985); CAB,subduction-related calc-alkaline basalt from Tormey et al. (1995);IAT, island-arc tholeiite from Dupuy et al. (1982); BAB, back-arcbasin basalt from Boesp¯ug (1990); Prim. Mantle, primitive mantlecomposition from Hofmann (1988)

Fig. 9 Distribution of the dif-ferent basalt-types across theIPB

Fig. 10 Yb versus Al2O3 diagram (Arth 1979) for the dacites (®lledcircles) and rhyolites (empty circles) of the IPB and correlative felsicplutonic rocks (shaded circles)

104

et al. 1984; Fig. 12). It is worth noting that such anambiguity is also observed for the basic rocks (e.g.Fig. 8).

Plutonic rocks

As in the case of the VS Formation, the plutonic massifof the Campofrio area exposes basic, intermediate andfelsic rocks. In most cases, the intermediate rocks appearas hybrid facies showing conspicuous evidence for mix-ing between gabbroic and granitic magmas (Stein et al.1996). The examination of such a process being beyondthe scope of the present study, these rocks are nottreated here. In many places, the plutonic massif is cutby doleritic and rhyolitic dykes which are considered inorder to provide a comparison with the potentiallycorrelative basic and felsic lavas of the V2 episode.

Basic rocks

No gabbros comparable to the ``type-B'' basalts anddolerites have been observed in the Campofrio area. Allthe analyzed samples show moderate Nb contents(<10 ppm), indicative of a subalkaline a�nity. They areclassi®ed as tholeiitic in the FeOt/MgO versus SiO2 di-agram (Miyashiro 1974, not shown) and show moderateTiO2 contents (1.3±1.85%). As illustrated by the paral-lelism between primordial mantle-normalized trace-ele-ment patterns (Fig. 13), the gabbros show geochemicalcharacteristics very similar to those of some lavas anddolerites of the ``medium-La/Nb type''. Their similarityis also evident in the La/Nb vs. Y/Nb diagram (Fig. 8);this makes the gabbros comparable to recent continentaltholeiites. The geochemically similar volcanic units anddolerites are from widespread areas (Rio Tinto to Vil-lanueva) and belong either to the V1 or V2 volcanicepisode.

Analyses of dolerite dykes cutting the plutonic massifhave been compared with those of the lavas. Again, theprimordial mantle-normalized trace-element patterns(Fig. 13) and Y/Nb vs. La/Nb plot (Fig. 8) show anobvious similarity between the two rock types.

Felsic rocks

The felsic plutonic rocks of the Campofrio area are de-®ned as granodiorite to granite (Stein et al. 1996); thegradual transition between these two facies is marked byan increase in SiO2 content from 68 to 80% (Fig. 4). TheK2O content is highly variable, ranging from 0.8 to 5%,but this variability is probably a consequence of altera-tion. A moderate Th content (7±16 ppm) is consistentwith an originally medium- to high-K a�nity (Gill1981). Like the felsic lavas, the granites and granodi-orites are of ``low-Al2O3'' and ``high-Yb type'' (Fig. 10)and overlap the three discriminant ®elds of the Nb vs. Ydiagram (Fig. 12). Likewise, as illustrated by Fig. 14,their chondrite-normalized REE patterns are very simi-lar, i.e. slightly LREE-enriched, unfractionated forHREE, and with marked negative Eu-anomalies.

Fig. 11A,B Chondrite-normalized rare-earth element patterns for thefelsic lavas of the IPB (normalization values from Anders andGre vesse 1989). A Lower, upper and average patterns for therhyolites, B representative patterns for the dacites, the upper patterncorresponds to the average dacite given by Munha (1983)

Fig. 12 Nb versus Y diagram of Pearce et al. (1984) for the felsicrocks of the IPB. Filled circle, dacite; empty circle, rhyolite; shadedcircle, plutonic rocks of the Campofrio area. VAG + Syn-COLG,®eld for volcanic-arc and syn-collision granites;WPG, ®eld for within-plate granites; ORG, ®eld for ocean-ridge granites

Fig. 13 Primordial mantle-normalized trace element patterns for thebasic plutonic rocks (®ne lines) and dolerite dyke cutting the plutonicrocks (heavy line) and comparison with the patterns of some basicvolcanic rocks (broken lines)

105

The REE-patterns of two rhyolitic dykes cutting theplutonic rocks are plotted on Fig. 14. Again, these pat-terns are very similar to those of the felsic lavas andplutonic rocks; in fact, no systematic di�erences areobserved between the three rock types (Table 1).

Petrological discussion

From the geochemical data discussed, especially thoseprovided by Munha (1981) on the basic rocks of thePortuguese part of the IPB, an apparent geochemicalzoning can be deduced with alkaline basalts located inthe westernmost and southernmost parts of the belt(Fig. 9). Amongst our own samples, only one comesfrom Portugal (Ponte da Rotorta area, NW of NevesCorvo) and it also belongs to the alkaline group. How-ever, of the more than 30 basaltic samples that we col-lected in Spain, only two are alkaline (Fig. 9). Thus, wecannot envisage the observed zoning to be purely for-tuitous. In Portugal, Munha (1983) notes that the al-kaline basaltic lavas are restricted to the upper part ofthe volcanic sequence.

Another important result concerns the plutonismversus volcanism relationship. Striking similarities be-tween the basic and felsic plutonic rocks and lavas ofcomparable SiO2 content are perfectly consistent withthe conclusions of Soler (1980), SchuÈ tz et al. (1987) andStein et al. (1996) who correlate the lavas of the VSFormation and the plutonic facies of the western part ofthe batholith. Likewise, the strong similarities observedbetween the basic and felsic dykes cutting the plutonicrocks and the VS lavas are consistent with the interpre-tation of Stein et al. (1996) that these dykes could befeeders for the V2 volcanic episode. Conversely, the re-sults of Simancas (1983) and De La Rosa (1992) suggestthat a large (eastern ?) part of the batholith was em-placed after the main deformation phase a�ecting the VSFormation and thus cannot be correlative of those lavas.The conclusions of Soler (1980), SchuÈ tz et al. (1987) andStein et al. (1996) are therefore only valid for a restrictedpart of the batholith, which leads us to consider it as acomposite batholith.

A third important point concerns the origin of theacidic magma. As the sulphide deposits are preferen-tially hosted by the felsic lavas, this point is relevant tometallogenic models. In recent investigations, two pet-rological models have been envisaged for the origin ofthe IPB felsic rocks: fractional crystallization of a basicmagma (Routhier et al. 1980) and crustal melting (Mu-nha 1983; Thie blemont et al. 1994b). We have testedboth hypotheses from the trace-element aspect using(a) the average of the medium La/Nb basic lavas as theparent melt composition (Table 1), (b) the mineral/liq-uid partition coe�cients compiled by Gill (1981), Hen-derson (1984) and Cocherie (1984) for titanite, and(c) the simplest equations of fractional crystallizationand batch partial melting (Shaw, 1970), inasmuch asthese equations are accurate as far as mass balanceconsiderations are concerned.

The La versus Nb plot (Fig. 15) shows that the La/Nb ratio is generally higher in the felsic lavas than in thebasic ones. Nevertheless, the two rock types showbroadly the same ranges of Nb contents. Thus a strongfractionation of Nb relative to La is required to explainthe felsic rocks as derivatives of the basic ones by frac-tional crystallization. This cannot be achieved by pre-cipitation of plagioclase, olivine or pyroxene because thepartition coe�cients for Nb and La in these phases arevery low and very similar. The only minerals capable offractionating Nb and La in a basaltic to andesitic liquidare hornblende (Hb) and oxides (magnetite-ilmenite)(Mt). The latter phase cannot have a signi®cant e�ect,both because oxides represent only a minor part of thecrystallizing solid and because their partition coe�cientsfor Nb are too low (=1 according to Gill 1981). Asillustrated by Fig. 15, fractionation of Nb relative to Laby hornblende crystallization requires the proportion of

Fig. 14 Chondrite-normalized rare-earth element patterns for thefelsic plutonic rocks of the Campofrio area (®ne lines) and discordantrhyolitic dykes (heavy lines) and comparison with the average REEpatterns of the felsic lavas (broken line)

Fig. 15 Modelled fractional crystallization plotted on a La versus Nbdiagram. The vectors indexed Pl, Hb and Mt correspond to thecrystallization of plagioclase, hornblende and magnetite respectively.The FC vector corresponds to the crystallization of 50% Hb, 48% Pland 2% Mt from a liquid having the average composition of themedium La/Nb type of basic lavas (Table 1). At the right end of theFC line the fraction of residual liquid is 0.2

106

this phase in the crystallizing solid to be at least 0.5,which is clearly unrealistic. Thus, as pointed out byMunha (1983) and Thie blemont et al. (1994b), deriva-tion of the felsic lavas by fractional crystallization ofma®c rocks is not consistent with the geochemical data.

In his alternative partial melting model, MunhaÂ(1983) envisages a crustal protolith of felsic to interme-diate composition (given by the average upper conti-nental crust). Such an hypothesis appears ratherunlikely. For instance, in sharp contrast with the rhyo-lites and granites derived from ``evolved'' crustal reser-voirs that are peraluminous and contain cordierite and/or muscovite (e.g. Chappell and White 1974; Pichavant1993), biotite and hornblende are the only ferromagne-sian phases in the felsic rocks of the IPB. In addition, asnoted by Thie blemont et al. (1994b), the trace-elementcharacteristics of the IPB lavas di�er strongly from thoseof the crustally derived peraluminous granites andrhyolites; this includes the low Nb contents (<10 ppm)and Nb/Zr ratios (i.e. [Nb/Zr]N < 1.5) of many of therhyolites and granites. Such characteristics also precludederivation from an evolved crustal reservoir (Taylor andMacLennan 1985).

As pointed out earlier, in Arth's (1979) scheme therhyolites of the IPB are classi®ed as low-Al2O3 and high-Yb felsic rocks (Fig. 10). Both Arth (1979) and Barker(1979) envisage two models for the origin of such rocks:(1) fractional crystallization of a basaltic magma but, asnoted already, this hypothesis may be discarded in thecase of the IPB felsic lavas; (2) partial melting of anamphibolite protolith at medium to low pressure, in thefollowing section we test this model from the trace-ele-ment aspect using the average of the medium-La/Nbbasic lavas as a protolith composition (Table 1).

Two solutions are plotted on the La versus Nb(Fig. 16A) diagrams; one (m1) assumes an amphiboliteprotolith with (45% Hb ± 55% Pl ± 0.025% Tit ±0.032% Zircon) and the other (m2) a garnet-amphibo-lite protolith with (48% Hb ± 49.2% Pl ± 1.8% Gt ±0.95% Tit ± 0.05% Zircon). The amount of possiblegarnet is tightly constrained by the low [Gd/Yb]N ratioof the felsic lavas (Fig. 11B). More than 2% Gt wouldlead to [Gd/Yb]N ratios of more than 2, which is outsidethe observed range (0.5±2.0; Fig. 16B). Figure 16Ashows that both solutions broadly reproduce the rangeof La/Nb ratios observed in the felsic lavas. Theoreticalcompositions obtained from these two solutions, as-suming 15% melting of the amphibolite protolith, areplotted as primordial mantle-normalized trace-elementpatterns (Fig. 17) and compared with di�erent IPB da-cites (from Table 1). Bearing in mind the uncertainty inthe protolith composition, the agreement between thecalculated and observed patterns may be considered assatisfactory. Thus, as far as the dacites are concerned,the partial melting model can be considered as adequateto account for the trace-element characteristics.

In order to test this hypothesis further, the calculatedpartial melting trends have been plotted on a (Gd/Yb)Nvs. Eu/Eu* diagram (Fig. 16B). Clearly this model does

not account for the strong negative Eu-anomalies (i.e.Eu/Eu* < 0.7) observed in the rhyolites. This let us toconsider a two-step model involving ®rst, the partialmelting of an amphibolite protolith to give a daciticmagma and then, the fractional crystallization of feld-

Fig. 16A,B Modelled partial melting of an amphibolite protolith andfurther fractional crystallization of the derivative liquid plotted on ALa versus Nb and B (Gd/Yb)N versus Eu/Eu* diagrams. Samesymbols as for Fig. 15. Two trends are shown: one (m1) assumes aprotolith with (45%Hb ± 55% Pl ± 0.025% Tit ± 0.032% Zircon) andthe other (m2) a protolith with (48% Hb ± 49.2% Pl ± 1.8% Gt ±0.95% Tit ± 0.05% Zircon). These trends (heavy lines) have beencalculated up to a degree of partial melting of 15%. Fractionalcrystallization (®ne lines) was then modelled assuming crystallizationof (90% Pl ± 9% Hb ± 1% Tit ± 0.05% Zircon)

Fig. 17 Comparison between calculated and observed dacite compo-sitions represented as primordial mantle-normalized trace elementpatterns. Details on the calculation are given in the caption of Fig. 16.The theoretical dacite compositions have been calculated assuming adegree of partial melting of 15%. They have been compared to (a) theaverage dacite of Munha (1983), (b) a fresh dacite of La Joya mine, (c)the average of 18 analyses of a porphyritic dacite from the AznalcoÁ llarmine. The pattern of the protolith used for the calculation is alsoplotted

107

spar (+hornblende � titanite and zircon) from thisanatectic magma to give the rhyolites. As illustrated byFig. 16B, this two-step model can reproduce the Gd/Yband Eu/Eu* ratios observed in the felsic lavas of the IPB.It is also consistent with the data on other trace elements(e.g. La and Nb; Fig. 16A).

Geodynamic and metallogenic implications

The Iberian Variscan Belt is interpreted as the result ofthe accretion of di�erent terranes on the margin of anIberian Autochton Terrane which presently constitutesthe central part of Iberia (Quesada 1991). In thesouthern part of the belt, two oceanic exotic terranes arerecognized: the Beja-Acebuches ophiolite and the Pulodo Lobo accretionary prism (Fig. 1). To the north theseterranes are overthrust by the Iberian Autochton (OssaMorena Zone), to the south, they are in tectonic contactwith the IPB (South Portuguese Zone) (Fig. 1). At theend of the Middle Devonian, the oceanic terranes werea�ected by a major compressional phase (e.g. Silva et al.1990); following this phase, the IPB was emplaced dur-ing the Upper Devonian. As pointed out by MunhaÂ(1983), the occurrence of basalts and dolerites of alkalinea�nity suggests emplacement of the IPB in a tensionaltectonic setting. Sedimentological data (Oliveira 1990),as well as the occurrence of basalts similar to continentaltholeiites (this study), lead to the same conclusion.Conversely, the existence of basalts with arc-type char-acteristics suggests a relationship between the IPB and asubduction process. To account for this, Munha (1983)envisages an emplacement in an ensialic back-arc basin.However, no trace of the corresponding arc and oceanicdomain seems to exist in the Iberian Variscan Belt. Silvaet al. (1990) envisage that the Pyrite Belt was emplacedon a continental block clearly distinct from the Pulo doLobo and Beja-Acebuches terranes and was subse-quently accreted against them by strike-slip movements.This model is not consistent with the occurrence of ba-salts with arc-type characteristics in the IPB. Consider-ing that there is no major suture zone between theoceanic terranes and the Pyrite Belt and that the earlycompressive phase in the formers occurred just beforethe deposit of the latter, Thie blemont et al. (1994b, 1995)consider the deposition of the IPB to have been in a fore-arc basin (palaeo-accretionary prism) opened immedi-ately above the recently deformed northern terranes(Pulo do Lobo, Beja Acebuches amphibolites; Fig. 1).The latter model tries to account for the widespreadcrustal melting by invoking the combined e�ects of thethermal relaxation of the recently deformed crustalsegment and heat transfer during extension and intru-sion of basic magmas. According to this model, a con-tinuous north-directed oceanic subduction should haveoccurred during the Devonian and lower Carboniferous.Evidence for this is given by igneous rocks of EarlyCarboniferous age which occur in the Ossa-Morena-Zone, 50 to 100 km north of the IPB, and show geo-

chemical characteristics similar to those of recent sub-duction-related volcanic and plutonic rocks (Quesada1991).

Examination of other ancient massive-sulphideprovinces, show that massive-sulphide deposits may beassociated with diverse types of volcanic rocks. For in-stance, the volcanic rocks associated with the massive-sulphide deposits in the Boliden-LaÊ ngdal area (Skelleftedistrict, Sweden) display petrological and geochemicalfeatures similar to subduction-related volcanic arcs; theyare considered to have formed in a rifted arc (Vivallo1987). Massive-sulphide deposits at Que River (Tasma-nia) are hosted by a high-K andesite-dacite-rhyodacitecalc-alkaline series similar in many ways to moderncontinental margin andesitic rocks (Whitford et al.1989). High-TiO2 basic rocks suggesting an alkaline in-traplate or rift association have been observed in theMount-Windsor massive-sulphide sub-province (Aus-tralia), but they represent the earliest phase of ma®cvolcanism; deposition in a back-arc basin is envisagedfor these volcanics (Stolz 1995). These variable geo-chemical characteristics suggest that massive-sulphidedeposits may formed in a variety of tectonic settings.

Geochemical modelling provides arguments for theexistence of a high geothermal gradient within the IPBbasement at the time of emplacement. As noted earlier,the slight HREE fractionation in the felsic lavas suggeststhat crustal melting would have occurred close to theedge of the garnet stability ®eld. According to the ex-perimental results of PatinÄ o Douce and Beard (1995),this implies a maximum pressure of 10±12 kbar, whichalso corresponds to a crustal thickness of around 30 km.Such a value is in fact a maximum considering thesubmarine emplacement of the IPB lavas. For pressuresaround 10 kbar, dehydration melting of an amphiboliteprotolith will give a dacitic liquid at a temperature of950 °C (PatinÄ o Douce and Beard 1995). Therefore, atheoretical geothermal gradient of 30±35 °C/km may bededuced for the IPB, which is consistent with the generallow-P/high-T conditions of the Hercynian metamor-phism in the South-Portuguese Zone (Munha 1990).

As noted previously, the massive sulphide deposits ofthe IPB are mainly hosted by felsic lavas. This is also thecase in many major provinces such as the SuperiorProvince (Lesher et al. 1986), the Kuroko Province(Urabe 1987) and the northeastern Massif Central (Le-scuyer et al. this volume). Systematic geochemical in-vestigations on such felsic rocks are presented byCampbell et al. (1982) and Lesher et al. (1986). Theseauthors show that the REE-patterns of the rhyolites andhigh-Si rhyolites associated with massive sulphidedeposits are characterized by slight LREE-enrichment,unfractionated HREE and negative Eu-anomalies.Clearly, the felsic lavas (and some granitoids) of the IPBmeet these criteria (Fig. 11A). Lesher et al. (1986) showthat, in addition to these criteria, felsic lavas hostingimportant base-metal-sulphide deposits have low Zr/Yratios (i.e. Zr/Y £ 5±7). More than 90% of the rhyolitesin our data base have Zr/Y ratios in the range 7±1, and

108

no clear relationship appears between this ratio and theirgeographical distribution. Thus, in the case of the IPB,this criterion cannot be used for discriminating therhyolites associated with the richest Cu-Pb-Zn massivesulphides inasmuch as most of the IPB deposits lackeconomically signi®cant base-metal concentrations(Leistel et al. this volume).

Our results suggest that melting of a basic protolith(amphibolite) at relatively low pressure, therefore a highgeothermal gradient, was probably the main control onthe REE characteristics of the IPB felsic rocks. Thismodel is similar to that proposed by Barrie (1995) forthe rhyolites associated with massive-sulphide depositsin the Archean Abitibi sub-province (Canada). As sug-gested by this author, the association of many massivesulphides world-wide with a particular type of felsic lavacould re¯ect the preferred location of such deposits inzones of high-geothermal gradient rather than their re-lationship to a particular tectonic setting.

Acknowledgements This work was funded by the European Com-munity (DGXII contracts MA2M-CT90-0029 Raw Materials andRecycling Programme, and BRE2-CT92-0299 Brite Euram pro-gramme) and by the BRGM (French National Research Centreand Geological Survey). Constructive reviews by D.J. Whitfordand A.J. Stolz were greatly appreciated. We thank our colleaguesJ.M. Leistel for his assistance in the ®eld study, and M. Te gyey forpetrographical descriptions.

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