Sulphur isotope composition of Silurian shale-hosted PGE-Ag-Au-Zn-Cu mineralisations of the Prades Mountains (Catalonia, Spain)

ARTICLE

Pura Alfonso Æ Carles Canet Æ Joan C. Melgarejo

Anthony E. Fallick

Sulphur isotope composition of Silurian shale-hosted PGE-Ag-Au-Zn-Cumineralisations of the Prades Mountains (Catalonia, Spain)

Received: 7 December 1999 /Accepted: 12 August 2001 / Published online: 20 October 2001� Springer-Verlag 2001

Abstract Several Silurian metamorphosed shale-hostedsulphide occurrences have been studied in the PradesMountains, southern part of the Coastal CatalonianRanges. Most of the sulphides are found as stratiform orshale-disseminated occurrences. Pyrrhotite is the mostabundant sulphide mineral. Chalcopyrite and arseno-pyrite are less common. Gold, Pd-bearing lollingite(partly replaced by arsenopyrite), sperrylite, hessite,clausthalite, altaite, galena, sphalerite, molybdenite,scheelite, and V-Cr oxides and silicates are minor com-ponents. Sulphur isotopic analyses were made on threeoutcrops (Roca de Ponent, Coma Fosca and SantMiquel) and in the Silurian black shales of the SantBernat series, in order to determine the origin of thesulphur which formed these deposits. The Coma Foscaand Sant Miquel outcrops yield a narrow range of d34Svalues (–11.3 to –4.6&), whereas the Roca de Ponentand Sant Bernat series have a wider range (–19.9 to –7.6& in pyrrhotite, and up to +36& in pyrite from thetop of the Sant Bernat series). In the Roca de Ponentoutcrop the d34S values decrease with increasing strati-graphic height. All the deposits have a mean d34S com-position close to –9&. Annealing of pyrrhotite tookplace during the Hercynian metamorphism. Later, dur-ing the retrograde metamorphism, pyrite replaced pyr-rhotite, and arsenopyrite replaced lollingite. Large-scalecompositional inhomogeneities survived metamorphism(from pyroxene to amphibolite facies). A hydrothermal

source of S has been postulated for the Coma Fosca andSant Miquel outcrops. Sulphur in these deposits is fromleaching of sulphides from the underlying rocks. In theRoca de Ponent outcrop and in the Sant Bernat series,sulphur was derived from a hydrothermal source andfrom biogenic reduction of seawater sulphate. Thecontribution of the latter source increased with time.Biogenic reduction of seawater sulphide in a closed orsemi-closed system for sulphate is considered responsibleof the high d34S values of pyrite from the uppermostpart of the Sant Bernat series. Based on textural andisotopic evidence, we propose an exhalative origin forthe precious metals, cogenetic with the stratiform sulp-hides.

Keywords S isotopes Æ Pyrrhotite Æ Silurian ÆSediment-hosted Æ Platinum-group elements

Introduction

The Silurian series from the southernmost part of theCatalonian Coastal Ranges, NW Spain, host severalstratiform sulphide occurrences (Fig. 1). A sedex originwas proposed by Melgarejo (1992), based on the strati-form character of the mineralisation, their associationwith abnormal sediments assimilated to exhalites, andthe absence of closely associated volcanic rocks. How-ever, alkaline mafic volcanism related to an extensionalgeotectonic setting (Gil Ibarguchi et al. 1990) occurs inthe Silurian basin in other places of the CatalonianCoastal Ranges (Fig. 1).

Among these occurrences, those from the southern-most part of the Catalonian Coastal Ranges are themost important ones due to their mineralogy and vol-ume. These deposits occur at the northern side of thePrades Mountains.

All the Silurian deposits exhibit a very unusual min-eralogy. In addition to base metal sulphides, theassemblage is characterised by the occurrence of Ti, U,REE, V, Cr, and Mo minerals, as well as by high V-Cr

Mineralium Deposita (2002) 37: 198–212DOI 10.1007/s00126-001-0217-8

Editorial responsibility: R. Large

P. Alfonso (&) Æ C. Canet Æ J.C. MelgarejoDepartament de CristalÆlografia,Mineralogia i Diposits Minerals,Universitat de Barcelona,Martı i Franques s/n Barcelona 08028, SpainE-mail: [email protected]: +34-3-4021340

A.E. FallickIsotope Geosciences Unit, SURRC, East Kilbride,Glasgow G75 0QF, UK

contents in garnets, micas, amphiboles and pyroxenes,and the local presence of Ba-rich feldspars. However,the most noticeable mineralogical particularity is theoccurrence of Pt, Pd, Au and Ag minerals in thesemetasediments. The Pd content can reach 0.6 ppm, andPt and Au up to 0.2 ppm each (Jorge et al. 1997). Thepresence of PGE in sediments elsewhere has beenreported by Hulbert et al. (1992), Coveney et al. (1992)and Pasava (1993), and there is some disagreementabout their possible origin. According to several authors(Coveney and Sangster 1996; Pasava et al. 1996; Lottet al. 1999), hydrothermal fluids are the source of metalsin black shales. Other possible origins in discussioninclude extraterrestrial input, direct precipitation fromseawater, microbial processes and precipitation at aredox boundary (Sawlowicz 1993).

The present study reports the results of a sulphurisotope investigation of sulphides from the Siluriansedex deposits of the southwest of the CatalonianCoastal Ranges. The analyses were carried out on

samples in situ with a laser microprobe (Kelley andFallick 1990; Crowe et al. 1990; Crowe and Valley1992; Crowe 1994). This technique has a moderatespatial resolution, allowing analyses on the differenttextures and sulphide minerals at a millimetre scale,otherwise impossible to separate and analyse by con-ventional techniques.

The Prades Mountains occurrences are representativeof a high number of base and precious metal-bearingoccurrences in the Catalonian Coastal Ranges. The Auand PGE contents which we have recorded up to thepresent are too low to consider them as ore deposits forthese elements. However, this discovery opens interest-ing possibilities for mineral exploration in the Silurianseries of southern Europe.

Therefore, the aim of this sulphur isotope study isto obtain information on the genesis of this unusualtype of sulphide deposits, as well as the genetic rela-tionships between them and the history of Siluriansedimentation, basin formation and subsequent meta-morphism. The interpretation of the sulphur isotopedata obtained allows discussion about the source ofsulphur, the environment of deposition, and the in-fluence of the metamorphism on the above mineralassociations.

Fig. 1 Inset Geological map of the Catalan Coastal Rangesshowing the distribution of Lower-Silurian alkaline volcanic rocksand the stratabound occurrences. Main map Northern PradesMountains. The distribution of the sampled occurrences isindicated

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Geological setting

The Prades Mountains comprise a Hercynian foldedbasement, unconformably covered by Mesozoic andCenozoic sedimentary series (Figs. 1 and 2). ThePaleozoic formations consist of Lower Cambrian (Sanzet al. 2000), Upper Ordovician, Silurian and Devonianseries, unconformably overlain by Carboniferous turbi-ditic sequences (Melgarejo 1992). In addition, rocksfrom the cores of anticline structures are considered asPrecambrian (Melgarejo and Ayora 1990).

These stratigraphically lowermost rocks consist ofmore than 300 m of interbedded greywackes andbrownish shales, which become black shales in the

uppermost part (about 50 m). These black shales passover to a meta-evaporitic series (about 50 m thick) madeup by scapolite-bearing limestones, dolostones and cal-carenites (Melgarejo and Ayora 1990). The transitionalpart between the meta-evaporites and the black shales(about 3 m in thickness) is enriched in disseminatedsulphides, mostly sphalerite, chalcopyrite, pyrrhotite,pyrite and galena.

The Lower Paleozoic sequence outcrops only insmall, isolated areas. Lower-Cambrian quartzites (Sanzet al. 2000) unconformably overlie the underlying rocks.Small outcrops of interbedded quarzites and shales havebeen regionally attributed to the Upper Ordovician(Julivert and Duran 1990).

The Silurian stratigraphic sequence in the PradesMountains can be reconstructed by combining differentsections with dating based on graptolitic fauna (Mel-garejo 1992). The Lower Llandoverian consists of morethan 20 m of metre-thick quartzite beds interbeddedwith black shales. These rocks grade upwards to a sul-phide-rich unit. This unit, which can be more than 30 mthick, consists of interbedded shales, massive sulphides,phosphates and fine-grained feldspar-rich rocks. Allthese metasedimentary rocks contain ore minerals.These series change progressively to black shales withpyrite disseminations (Sant Bernat series), up to 50 mthick (Melgarejo 1992) and belonging to the transitionUpper Llandoverian-Wenlock (Ashauer and Teichmul-ler 1935). The Sant Bernat series is considered asrepresentative of the stratigraphy of these beds, and itwas sampled in several outcrops along the Sant Bernatravine. The lower part of this series exhibits a dark greycolour and contains centimetre-sized pyrrhotite nodules,partly replaced by pyrite. The uppermost part is therichest one in organic matter, and comprises blackshales with graptolites. In these black shales pyrite iscommon as discrete thin beds (up to a few millimetresthick), and as disseminated crystals and nodules (up to20 cm in diameter).

The Devonian series which outcrops in the PradesMountains consists of 50 m of green shales and calc-schists (Emsian?), and 200 m of black shales withquartzite and chert (Eifelian to Famennian; Melgarejo1992). The Carboniferous series begins with a chertyunit (up to 10 m thick), regionally attributed to theTournaisian. This unit unconformably overlies the pre-Carboniferous series. A thick turbiditic series, up to2,000 m thick, unconformably covers all the above unitsand contains several sedimentary exhalative Pb-Zn-Cu-Ag deposits (Melgarejo 1992; Canet and Melgarejo1998).

During the Hercynian orogeny (Fig. 3) the southernpart of the Catalonian Coastal Ranges was affected bytwo main episodes of folding and thrusting. The firstdeformation produced NNW-trending folds and thrustswith fold axial surfaces and thrusts dipping to the NE,whereas the second deformation produced fold axialsurfaces and thrusts dipping to the SW. The regionalmetamorphism produced very low-grade or anchimeta-

Fig. 2 Synthetic stratigraphic profile of the Paleozoic series in thePrades Mountains, showing the location of stratiform andstratabound occurrences

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morphic mineral assemblages. The deformation andmetamorphism affected all the sedimentary series up tothe Westphalian (Colodron et al. 1990).

Undeformed Late-Hercynian granitic stocks anddykes, with a composition ranging from quartzdiorite toleucogranite, intruded the Hercynian sediments(Melgarejo and Ayora 1984). Dykes of porphyriticgranitoids accompany these plutonic intrusives. Thisensemble of rocks has been dated as Permian on thebasis of Rb-Sr geochronology (Enrique and Debon1987). Therefore, the age of the Hercynian deformationcan be attributed to the Upper Carboniferous. A ther-mal metamorphism, ranging from pyroxene to amphi-bole hornfels facies, was developed in the area as aconsequence of these intrusions. Late-Hercynian faults,trending NE-SW and NNW-SSE, have been reactivatedduring the Alpine orogeny, and permitted episodes ofhydrothermal circulation, producing Ba-Pb-Zn-Cu-Ni-Co-Ag veins (Melgarejo and Ayora 1985; Canals andCardellach 1997).

Structure and mineralogy of sulphide occurrences

There are about ten outcrops of Silurian sulphideoccurrences in the Prades Mountains. Among them, theComa Fosca, Sant Miquel and Roca de Ponent outcrops

were sampled. Additional sulphide samples from seriesof the Middle-Silurian rocks (Sant Bernat series) havebeen studied in order to compare the mineralogical andisotopic patterns of these mineralisations with those ofthe Silurian black shales.

The discontinuity of the outcrops and the strongtectonisation of the area hinder determination of thestrike length of these occurrences. The Coma Fosca, SantMiquel and Roca de Ponent outcrops are contempora-neous, belonging to the Lower Llandoverian, whereasthe lower levels of the Sant Bernat series correlate withthem, and the higher stratigraphic beds correspond tothe transition Wenlockian-Ludlowian (Ashauer andTeichmuller 1935; Melgarejo 1992). The occurrences arelocated in areas with different metamorphic grades.Their main characteristics are indicated in Table 1.

The mineral occurrences are hosted in organogenousshales. Preliminary analyses of redox sensitive elementsin the host black shales (average 700 ppm V, 11 ppmMo, and 5 ppm U; Canet 2001) indicate that thesesediments formed in an anoxic environment, according

Fig. 3 Geological section along the profile A–B of Fig. 1. TheSilurian series, comprising the stratabound occurrences, acts as adetachment level during Hercynian thrusting. Late-Hercyniangranitoids cut and metamorphose the Paleozoic series and thethrusts

Table 1 Characteristics of the occurrences and sedimentary series in the present study

Area Age Ore mineralsa Metamorphicgrade

Analysedsamples

Numberof analyses

Roca de Ponent Lower Llandoverian Po, Py, Cpy, Sph, Ga, Aspy,Lo, Au, Bi, TeBi, Hes, Sch

Pyroxene hornfels 28 31

Sant Bernat series Lower Llandoverian-Wenlock Po, Py Amphibole hornfels 12 33Coma Fosca Lower Llandoverian Po, Py, Cpy, Sph Pyroxene hornfels 34 37Sant Miquel Lower Llandoverian Po, Py, Cpy, Sph, Ga, Aspy,

Pd-Lo, Spr, Pd, Au, Pet, Hes,Clh, U

Amphibole hornfels 19 37

aPo Pyrrhotite, Py pyrite, Cpy chalcopyrite, Sph sphalerite, Ga galena, Aspy arsenopyrite, Lo lollingite, Pd-Lo Palladian lollingite, Augold, Bi bismuth, TeBi bismuth tellurides, Hes hessite, Sch scheelite, Spr sperrylite, Clh clausthalite, U uraninite

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to the criteria of Thomson et al. (1993), and Quinby-Hunt and Wilde (1994).

Sant Miquel deposit

The Sant Miquel deposit is located 100 m south of LesMasies de Poblet hamlet. A very ancient open pit andsome short galleries were opened in this deposit, whichallow well-controlled sampling. The host sequence isstrongly disturbed by folding and thrusting, but up to5 m of sulphide-rich metasediments can be recognised(Fig. 4). They consist of millimetre-thick interbeddedfeldspar layers (almost totally constituted by anorthite),biotite-muscovite graphitic shales, pyrrhotite-rich shalesand apatite beds (Fig. 5). The sulphide, anorthite andapatite beds are more common at the base of the series.

The sulphide-rich shales can be up to 2 cm thick, andthey consist of muscovite, biotite, quartz and variableamounts of disseminated sulphides (up to 50%). A richvariety of sulphides, tellurides, arsenides, sulphoarse-nides and native elements are scattered throughout allthese sediments. All of them are fine-grained (less than400 lm). They consist of pyrrhotite, lollingite carryingup to 3 wt% Pd (Melgarejo et al. 1994), sperrylite, hes-site, altaite, clausthalite, chalcopyrite, galena, sphalerite,molybdenite, electrum, native silver and native palladi-um. Pyrrhotite is the earliest iron sulphide formed inthese sediments, and displays two types of crystals: (1)euhedral, fine grained (less than 30 lm), and (2) anhe-dral, coarse grained (50–400 lm). There is no evidence ofany precursor mineral for these pyrrhotite grains. Ac-cording to the criteria of Crowe et al. (1990), they seem tobe syn-sedimentary or early diagenetic. Sperrylite occursas scattered grains in the sulphide-rich sediments.Pd-rich lollingite is the most common PGE mineral.

The anorthite beds consist mainly of fine-grained (upto 50 lm) anhedral anorthite crystals (An95–98). Acces-sory minerals in these beds include quartz, muscovite,

ilmenite (with a rim of V-rich titanite), V-Cr spinels,V-rich allanite, goldmanite (garnet group, Ca3V2Si3O12),V- and Cr-rich tremolite, pyrrhotite, chalcopyrite,clausthalite and molybdenite. All of these accessoryminerals are less than 100 lm in size.

The interbedded graphitic shales consist of muscoviteand V-rich biotite, with minor but significant amounts ofxenotime, monazite, uraninite and apatite. The apatitebeds consist of cryptocrystalline apatite grains withsome recrystallised fossil remains, and minor amounts ofuraninite.

Small and irregular sulphide veins (up to 1 mm wide)crosscut all the above sediments. These veins consist ofpyrrhotite, chalcopyrite and sphalerite, and they areinterpreted as remobilisation products of diagenesis orearly metamorphism. This statement is supported by thefact that the above sediments and veins have beenaffected by all the Hercynian phases of deformation atall scales (with development of folding and corre-sponding axial plane cleavage and thrusting). Thermalmetamorphism during the Late-Hercynian episode ofgranite intrusion produced granoblastic textures. How-ever, this outcrop is located in the most external part ofthe contact metamorphic aureola, and these textures arenot very well developed.

A later pyrite generation produced bird’s eye texturesby replacement of all pyrrhotite generations.

Roca de Ponent outcrop

The Roca de Ponent outcrop is similar to the onedescribed above, but the total stratigraphic thickness ofthe mineralised profile can reach 30 m (Fig. 6). Close tothe base it contains several massive sulphide beds (up to

Fig. 4 Cross section of the Sant Miquel occurrence. The sulphide-rich bands are affected by the Hercynian deformation

Fig. 5 Hand specimen of interbedded feldspar layers, pyrrhotite-rich shales and apatite beds

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0.5 m thick) made up by pyrrhotite, sphalerite, pyrite,and chalcopyrite, and minor amounts of galena andscheelite. Pyrite is also found as disseminations in thesulphide-rich metasediments, and it becomes the mainsulphide at the stratigraphic top. In addition to thepyrite generations described in the Sant Miquel deposit,a late pyrite generation occurs as veinlets filling lateundeformed joints.

Some millimetre-thick calc-silicate beds, formed bygrossular garnet and Ni-, Cr-rich diopside, are scatteredbetween the series. Anorthite beds are up to 0.5 m thick.Here, anorthite crystals develop a coarse granoblastictexture.

Lollingite crystals have been partly replaced byeuhedral poikiloblastic arsenopyrite crystals. These arelarge enough to be recognised with the naked eye, andform discrete layers. Unfortunately, they do not containPGE elements, and mineral inclusions in the arsenideassemblage consist of some small grains of native bis-muth, bismuthinite, joseite and electrum. This occur-rence is in the vicinity of a granite intrusion. The gradeof the contact metamorphism is high, as indicated by thecoarse granoblastic texture and the occurrence of diop-side in the calc-silicate assemblages and andalusite-cor-dierite in the pelitic assemblages.

Coma Fosca occurrence

It is about 15 m thick and has a low content of feldsparand calc-silicate beds at the bottom. In this occurrenceneither PGE- nor Au-, Ag-bearing minerals have beenfound. The metamorphic grade is medium, as indicatedby the presence of cordierite-andalusite in the host shales.

Sulphide minerals in the Sant Bernat series

The Sant Bernat series has been affected by extensivethrusting, but its reconstruction allows to interpret that

it consists roughly of more than 50 m of shales. Thelower part of the series is rich in pyrrhotite dissemina-tions, and correlates with the top of the Roca de Ponentoccurrence. Pyrrhotite is fine grained (less than 100 lmin size).

The intermediate part of the series represents thetransition between the previously mentioned unit andthe black shales of the top. Pyrrhotite was formed early,and it can occur as centimetre-sized nodules which arereplaced at the borders by pyrite.

The uppermost part of the series is constituted byampelitic black shales. Pyrite is the only major sulphidemineral, forming euhedral cubic-faced crystals rangingin size from 1 to 5 mm, and rounded nodules up to20 cm in diameter. Some centimetre-thick beds can beenriched in pyrite (up to 50 modal %). Other sulphidesare extremely rare, and include chalcopyrite and smallpyrrhotite remains.

In synthesis, at least three stages of metal minerali-sation can be recognised in these occurrences: (1) aprimary pyrrhotite-rich mineralisation, affected by allphases of deformation and metamorphism; (2) mobili-sation during folding and regional metamorphism, thefirst sulphidation process, probably related to contactmetamorphism, which produced replacement of lolling-ite by arsenopyrite; and (3) a second, post-tectonic, verylate sulphidation process leading to pyrrhotite replace-ment by pyrite.

Late-Hercynian porphyritic granitic dykes cut all thementioned occurrences. The phenocrysts are quartz,plagioclase, orthoclase and biotite. Close to the contactwith the host rock, the dykes contain disseminations ofsulphides as euhedral crystals in the groundmass(arsenopyrite and pyrite). In addition, anhedral aggre-gates of different sulphides (pyrite, chalcopyrite, pyr-rhotite, sphalerite), in association with chlorite andsericite, occur as pseudomorphs of plagioclase and bio-tite phenocrysts.

A synthetic sequence of crystallisation of the ensem-ble of these occurrences is represented in Fig. 7.

Fig. 6 Geological section ofthe Roca de Ponent occur-rence

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Analytical methods and sampling

The sulphur isotopic composition was analysed in themost representative Silurian occurrences (Coma Fosca,Sant Miquel, Roca de Ponent) as well as in the sulphide-rich black shales of the Silurian series of Sant Bernat. InRoca de Ponent, pyrrhotite (n=28), pyrite (n=1) andarsenopyrite (n=1) were measured, in Coma Foscapyrrhotite (n=32), pyrite (n=3) and chalcopyrite (n=2),in Sant Miquel pyrrhotite (n=19) and pyrite (n=1), andin Sant Bernat pyrrhotite (n=3) and pyrite (n=6).

Samples were analysed as powder or directly in thickpolished sections. In the first case, mineral separationswere made by hand-picking and magnetic separation.Purity of the samples was tested by examination with abinocular microscope and X-ray diffraction.

Analyses were carried out at the Scottish UniversitiesResearch and Reactor Centre (SURRC). In-situ lasercombustions were carried out with a Nd-YAG lasersystem following the method of Fallick et al. (1992) andHall et al. (1994). Powder combustions were performedsimilarly, and purified SO2 isotopically analysed on aVG-SIRA II mass spectrometer.

The results are given as d34S & values relative to theCDT standard. The lateral resolution of the laser beamis about 250 lm. The analytical precision is within±0.2& at 1r. The data are presented in Tables 2 and 3,and plotted as histograms in Fig. 8.

Results

We consider the distribution of S isotope values in thesulphide-rich black shales of the Silurian series,represented by the Sant Bernat series. The lower partof this series contains disseminations of pyrrhotite, andthis part can be correlated to the sulphide occurrences.The d34S values of this pyrrhotite range from –13.5 to–9.1&. By contrast, pyrite from the middle part of theSilurian series yields d34S values between –3.9 and+36&. Pyrite from the hanging wall of the series hasthe widest range of d34S values for the whole area,between –21.0 and 36.7&. These crystals of pyrite aresurrounded by a later generation of pyrite with d34S of+1.2&. On the other hand, as indicated above, it ispossible to distinguish at least three different stages of

Fig. 7 Sequence of crystalli-sation of the sulphide ores inthe Silurian deposits of thePrades Mountains

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mineralisation, according to textural evidences in theLower Llandoverian occurrences. The first stage cor-responds to the formation of primary ores disseminatedin sediments or forming stratiform massive sulphideoccurrences; the two later ones correspond to remo-bilisation processes.

Stage 1

The occurrence of pyrrhotite and lollingite is indicativeof low ƒO2 and ƒS2 in this early episode. By contrast,

primary pyrite is the iron sulphide disseminated in theregional black shales of the hanging wall of the UpperLlandoverian-Lower Ludlowian Sant Bernat series. Thesulphide samples from the Coma Fosca and Sant Mi-quel outcrops, dominantly pyrrhotite, have a very ho-mogeneous isotopic composition of sulphur (Table 2,Fig. 8). Both occurrences have an unimodal distribu-tion of d34S close to –9&. The Coma Fosca outcropyields d34S values for pyrrhotite ranging from –11.8 to–7.3&, with chalcopyrite ranging from –11.9 to –10.7&. Pyrrhotite from Sant Miquel is isotopicallyheavier, ranging from –10.1 to –5.8&. The Roca de

Table 2 d34S values of Roca de Ponent (MT, Mal), Coma Fosca (Ag) and Sant Miquel (M, Es) occurrences

Sample Morphology Lithology Poa Py Ga Aspy Sample Morphology Lithology Po Py Cpy

MT 1 Disseminated An –7.6 M 3 Disseminated Schists –5.8MT 2 Disseminated An –8.4 M 5 Disseminated Schists –6.9MT 3 Disseminated Qz+An –10.1 M 6 Disseminated Schists –10.1MT 5 Disseminated Qz+An –9.6 M 7 Disseminated Schists –9.3MT 7 Disseminated Schists –14.2 M 8 Disseminated Schists –7.6MT 8 Disseminated Schists+An –11.0 M 9 Disseminated Schists –6.3MT 9 Disseminated Calc-silicate –13.3 M 11 Disseminated Schists –8.1MT 11 Disseminated An –9.0 Ag 1 Disseminated Schists –8.5MT 12 Disseminated Schists –14.9 Ag 2 Veins Black schists –7.0MT 13 Disseminated An+Ap –8.6 Ag 4 Disseminated Calc-silicate –8.2MT 14 Disseminated An –10.2 Ag 8 Disseminated Anorthite beds –9.7MT 15 Disseminated Schists –13.7 Ag 9 Disseminated Quartzite –8.9MT 16 Disseminated Schists –16.2 Ag 10 Disseminated An-rich schists –9.0MT 17 Disseminated Schists –16.3 Ag 11 Disseminated An-rich schists –9.4MT 18 Disseminated An –12.6 Ag 11 b Disseminated An-rich schists –11.4 –10.7MT 19 Disseminated Schists –14.5 Ag 11 c Veins An-rich schists –11.9MT 20 Disseminated Schists+Ap –14.7 Ag 12 Disseminated Schists –9.5MT 23 Disseminated Schists+Ap –19.9 Ag 13 Disseminated Schists+Ap

beds–8.9

MT 28 Disseminated Schists+Ap –11.0 Ag 14 Disseminated Schists+Apbeds

–9.5

MT 29 Disseminated Schists+Ap –17.1 Ag 15 Disseminated Schists –8.5MT 30 Disseminated An –8.0 Ag 16 Disseminated Schists

+An+Qz–8.8

MT 31 Disseminated An+calc-silicate –13.2 Ag 17 Disseminated Qz+An –9.1MT 32 Disseminated Schists –12.0 Ag 17 b Veins Qz+An –11.2 –10.7MT 33 Disseminated Schists –11.3 Ag 18 Disseminated Schists –11.3MT 34 Disseminated Schists –9.3 Ag 19 Disseminated Schists+An –9.0MT 35 Disseminated Schists –8.0 Ag 20 Disseminated Schists –9.4MT 36 Disseminated Schists –8.6 Ag 21 Disseminated Schists –11.9MT 37 Disseminated An –8.0 Ag 23 Disseminated Schists+Ap

beds–8.4

Mal 17 Disseminated An –9.2 –10.3 Ag 23 b Disseminated Schists+Apbeds

–7.8 –7.6

Mal g Massive Massivesulphide

–13.1 –11.9 Ag 23 c Disseminated Schists+ Apbeds

–7.4

M 15 Disseminated Schist –8.5 Ag 24 Disseminated Schists+Apbeds

–8.5

M 16 Disseminated An-rich schists –7.7 Ag 25 Disseminated Schists+An –8.2M 17 Disseminated Schists –8.6 Ag 26 Disseminated Schists+An –8.0M 18 Disseminated Ap, schists –8.4 Ag 27 Disseminated Schists –8.6M 19 Disseminated Ap, schists –9.5 Ag 28 Disseminated Schists –7.4M 20 Disseminated Schists –8.3 Ag 28 b Disseminated Schists –8.0M 21 Veins Schists –9.4 Ag 29 Disseminated Schists

+An+Ap–8.9

M 22 Disseminated Schists –9.4 Ag 30 Disseminated Schists –9.0M 23 Disseminated Schists –8.8 Ag 30 b Disseminated Schists –9.3M 24 Veins Schists –8.3 Ag 31 b Disseminated Qz+An –9.4Es-15 a Veins Schists –8.5 –8.7 Ag 41 Disseminated Qz+An –8.9Es-15 b Massive Schists –8.6

aPo Pyrrhotite, Py pyrite, Ga galena, Aspy arsenopyrite, Qz quartz, An anorthite, Ap apatite, Pet petzite

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Ponent outcrop exhibits a wide range of d34S values,with pyrrhotite ranging between –19.9 and –7.6&. Thedistribution of these values is bimodal (at –8 and–14&).

The distribution of the d34S values in the Roca dePonent outcrop is related to the stratigraphic position.

In the upper part of the outcrop there is a trend towardsmore negative isotopic compositions of sulphur (Fig. 9).

In all the occurrences, the d34S values in disseminatedpyrrhotite are very similar to those in pyrrhotite fromthin veins, about –9&, suggesting the same origin forboth.

Fig. 8A–D d34S values ofthe studied sulphides. A SantMiquel; B Coma Fosca;C Roca de Ponent; D SantBernat series

Table 3 d34S values of the SantBernat series and a Late-Her-cynian porphyritic granite dyke(N). The letters in sample N1refer to different crystals

Sample Morphology Stage Lithology Po Py

N 1 Bed Black shales 21.1N1 a rim Bed 1 Black shales 29.9N1 a core Bed 1 Black shales 36.7N1 b rim Bed 1 Black shales 34.0N1 c rim Bed 1 Black shales 30.1N1 c core Bed 1 Black shales 29.9N1 d Bed 2 Black shales 8.7N1 e Bed 2 Black shales 12.7N1 f Bed 3 Black shales 1.7N1 g Bed 3 Black shales –3.5N 2.1 Veins Black shales –11.2N 2.2 Nodule Black shales 18.4N 2.3 Bed+veinlets Black shales 4.8N 2.4 Veins Black shales –15.0N 3 Nodule Black shales 18.1N 4 Nodule Black shales –5.8N 5 Dissemination Porphyritic

granite dyke–11.2 –7.7

N 6 Veinlets Black shales –10.0N 7 Nodule Black shales 19.8N 9 Nodule Black shales –13.5 –6.3N 11 Dissemination Black shales –9.1

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Stage 2

The second stage is characterised by the sulphidation ofthe above-formed sulphides, and produces pyrite afterpyrrhotite and arsenopyrite after lollingite. d34S valuesof pyrite, carefully obtained by in-situ laser extractionon polished sections, are slightly higher than those ofpyrrhotite. Arsenopyrite has d34S values of the sameorder of magnitude as the contiguous sulphide minerals.In the Roca de Ponent outcrop pyrite has a d34S value of–9.2&, and arsenopyrite –10.3&.

Stage 3

There is a third stage in which a late generation of fine-grained pyrite or marcasite replaced pyrrhotite. Pyriteand marcasite form bird’s eye texture in the Lower-Llandoverian occurrences, or overgrow the primary

pyrite in the Ludlowian sediments. In the first case thed34S values are lower than those of pyrrhotite and,therefore, there is a lack of isotopic equilibrium betweenthis generation of pyrite and the sulphides formedearlier. In the Coma Fosca outcrop the d34S values forpyrite range from –10.7 to –7.4&. In the Sant Miqueldeposit the only sample analysed has a d34S value of–8.7&.

The late generation of pyrite in Ludlowian sedimentshas lower d34S values than the pyrite formed earlier (upto +36&). However, it was difficult to extract sulphurfrom the second generation of pyrite due to the inade-quate resolution of the laser beam and, therefore, thepossibility of contamination from the pre-existing pyritemust be taken into consideration.

Influence of late processes

As indicated above, the mineralogy and textures of thehost metasediments and sulphide occurrences indicatethat metamorphic recrystallisation took place. Thesulphides have been affected by the Hercynian defor-mation, and this fact can be easily observed in theoccurrences influenced by Late-Hercynian lower-gradecontact metamorphism. This is the Sant Miquel case,where the textures of the Hercynian regional meta-morphism are well preserved. The contact metamor-phism produced neoformation of biotite in the hostrocks. By contrast, some outcrops such as Roca dePonent and Coma Fosca were affected by contactmetamorphism of higher grade, as indicated byextensive development of cordierite-andalusite cornu-bianites or diopside hornfels. Sulphides from theseoutcrops exhibit granoblastic textures. Moreover, theseries and some outcrops are crosscut by porphyriticgranite dykes, which are enriched in sulphides similarto those in the Silurian metasedimentary rocks andsulphide occurrences (pyrrhotite, pyrite, chalcopyrite,arsenopyrite, with minor amounts of galena andsphalerite). Therefore, before any consideration aboutthe source of sulphur can be made, it is necessary todecipher the possible effects of this contact metamor-phism on the isotopic composition of the sulphideoccurrences. Note, however, that the Roca de Ponentoutcrop is located in the highest metamorphic facies,that the lower part of the outcrop is very close to thetop of a granite intrusion, and that nevertheless thereis a systematic variation of the d34S with stratigraphicheight (Fig. 9). This fact, as well as the broad range ofd34S values, indicates that in this area a resettingof sulphur isotopic compositions at field scale did nottake place during metamorphism.

The sulphur isotopic fractionation between pyriteand pyrrhotite (Dpy–po) is as expected under equilibriumconditions. Because the laser-based combustion methodwe used is inherently less precise and accurate thanconventional combustion, and because there is a cor-rection factor involved (with an additional inherent

Fig. 9 Distribution of d34S values according to the stratigraphicposition in the Roca de Ponent deposit

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error), we suggest that the estimation of temperaturesfrom small mineral–mineral d34S differences is unwiseand does not allow us to establish the effects of meta-morphism on the d34S of sulphides in the PradesMountains occurrences.

Several authors have also found a localised re-equil-ibration in sediment-hosted sulphide mineralisations(Seccombe et al. 1985; Skauli et al. 1992; Kribek et al.1996; Whelan et al. 1984, in metamorphosed sulphidesof the Balmat-Edwards deposit). These authors, aftermeasuring several tens of mineral pairs, found thatmetamorphic re-equilibration depends on several factorsincluding grain size, and does not affect all sulphides tothe same degree.

In addition, some samples were collected at the con-tact of the Late-Hercynian porphyritic granitic dykes, inorder to check the possibility of magmatic sulphur input.However, their textural pattern and mineral/isotopiccomposition are similar to those of the rest of the min-eralisation. We conclude that the intrusion of thesedykes did not produce significant changes in the min-eralisation. There is no evidence for a significant input ofmagmatic sulphur associated with the intrusion of theLate-Hercynian porphyritic dykes, as indicated by thestrongly negative values in all cases (close to or far fromthe contact).

The third generation of pyrite has lower d34S valuesthan pyrrhotite, which indicates a lack of isotopicequilibrium between the sulphides.

Source of sulphur in the Silurian sedimentary series

Pyrrhotite is the sulphide which forms earliest in thesedimentary series, and it exhibits a wide range of neg-ative d34S values. These values are typically produced bybacterial reduction of seawater sulphate in a closedsystem.

Pyrite from the hanging wall of the Sant Bernatseries has anomalously heavy d34S values, up to+36.6&. These values could be due to the lateststages of reduction of seawater sulphate by bacterialactivity, in a closed system for sulphate which wasopen or partially open to H2S (Ohmoto 1986), by aRayleigh distillation process. Extremely heavy d34Svalues of pyrite have also been described in otheroccurrences. Examples are the pyrite from the Lower-Paleozoic strata below the Navan Zn+Pb deposit,Ireland (Anderson et al. 1989), pyrite in theKanmantoo group, South Australia (Seccombe et al.1985), and the diagenetic pyrite from Paradise Valley,western succession of Mount Isa, Australia (Davidsonand Dixon 1992).

Source of sulphur in the primary mineralisation

Sulphide sulphur in sediment-hosted sulphide depositsmay be derived from bacterial reduction of coeval

seawater sulphate, from hydrothermal solutions, orfrom a mixture of both. In the second case, it has beenoriginated via the inorganic reduction of seawater sul-phate, or is derived from the surrounding sediments orfrom a deep source. It is necessary to take into accountthe following considerations to discuss the source ofsulphur in the occurrences of the present study: (1)modal d34S values, (2) the range of d34S values, (3) theisotope composition of the Silurian seawater, and (4) thedependence of d34S values on the stratigraphic positionin the series.

The similarities in modal d34S values for the sulphidesfrom all three outcrops (about –9&) suggest a commonsource of sulphur for all of them.

The Roca de Ponent outcrop exhibits a wide range ofd34S values, with a trend towards more negative isotopicvalues in the uppermost parts of the outcrop (Fig. 9).Nevertheless, the Coma Fosca and Sant Miquel occur-rences have a narrow range of d34S values.

The seawater sulphate in the Llandoverian wasabout +28& (Claypool et al. 1980). Thus, the frac-tionation between the d34S values of the seawater sul-phate and the sulphides in these occurrences is about37&. This value is compatible with that which wouldbe expected in sulphide obtained by bacterial reductionof seawater sulphate. According to Ohmoto and Rye(1979), the bacterial reduction of seawater sulphateproduces fractionations of 45±20& (see also Ohmotoet al. 1990).

Bacterial reduction of sulphate seawater can occur ina system open or closed to sulphate. Bacterial reductionin an open system produces a narrow range of values,which could explain the isotopic composition of theComa Fosca and Sant Miquel outcrops. However, thisopen system does not explain the spread of d34S valuesin the Roca de Ponent outcrop. In the second case,although a closed system produces a wide range of d34Svalues (Hoefs 1997), they exhibit an unimodal distribu-tion with the mode in the lowest d34S values, and anincrease in d34S with time (Schwarcz and Burnie 1973;Goodfellow 1987; Goodfellow et al. 1993). Therefore,this possibility has to be ruled out as source of sulphurfor this occurrence.

Sulphur from many sedex and Mississippi Valley-type deposits found in the literature has been consid-ered as formed by bacterial reduction of seawatersulphate. Examples are the Kapunda deposit (Lambertet al. 1980) and the Mount Isa deposit (Smith et al.1978), both in Australia. Murowchick et al. (1994)suggest a bacteriogenic origin for sulphur in combi-nation with a hydrothermal fluid as main source ofmetals in Cambrian stratabound Ni-Mo-(PGE-Au)ores of southern China. However, in other deposits, asthe Aberfeldy barite-zinc-lead deposit in the Late-Proterozoic Dalradian terrain, Scotland, the inorgani-cally reduced seawater sulphate is the dominant sourceof sulphur (Willan and Coleman 1983; Hall et al.1991). These possibilities must be rejected in our case,because the d34S values exhibit an inverse depth trend

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compared to the cited examples. A decrease in thed34S values of the sulphides with stratigraphic height isa common phenomenon in VMS deposits (Ohmotoand Rye 1979). A negative trend in d34S is alsoobserved in the Late-Proterozoic copper mineralisationof Copper Claim, South Australia (Lambert et al.1984). This observation has been explained in theseVMS deposits by influence of a second, isotopicallymore negative source of sulphur, by a change ofconditions favouring open-system bacterial fractiona-tion of sulphur isotopes, or by an increase in theoxidation state of the fluid. However, as discussedabove, there are several arguments against a simplybacterial origin of the sulphur for these occurrences.Otherwise, the hypothesis of changes in the redoxconditions of the fluids are not supported in our caseby the mineralogical evidence, because there are nobarite beds, or manganese or iron oxide caps at thetop of the sulphide occurrences.

Therefore, a hydrothermal origin of sulphur must beconsidered. A first possibility is to consider the thermalreduction of seawater sulphate in a deep circulatingconvective system. This process occurs at temperatureshigher than 250 �C (Shanks et al. 1981). At thesetemperatures, this process produces a lower degree offractionation than that necessary for the sulphidesfrom Prades Mountains, according to the equationspresented by Ohmoto and Rye (1979). Therefore,although the thermal reduction of seawater sulphatecannot be ruled out as contributing a small component,it cannot be a quantitatively important source of sul-phur.

A second possible source of hydrothermal sulphur isthe leaching of the underlying rocks, as suggested byRussell (1983, 1986). These rocks are those metasedi-ments attributed to the Precambrian which do notoutcrop in the Prades Mountains area, but showextensive outcrops in the southernmost part of theCatalonian Coastal Ranges. These rocks carry sulp-hides with an average d34S value of –9& and scapolite,which has been interpreted as the product of meta-morphism of evaporite sequences (Melgarejo andAyora 1990). Thus, a possible reservoir of sulphurexists in the underlying rocks. Lange et al. (1980) alsosuggest a hydrothermal source of sulphur in the sulp-hides at the Red Dog sediment-hosted zinc-lead-silverdeposit, Alaska, which formed proximal to the sourcevent.

The underlying metasedimentary rocks could there-fore be the main source of sulphur in the formation ofthe Coma Fosca and Sant Miquel outcrops. To explainthe wide range and distribution of d34S values in theRoca de Ponent outcrop, it is necessary to consider anadditional sulphur source. Bacterial reduction ofseawater sulphate could provide an important contri-bution to the occurrence of Roca de Ponent, and couldbe responsible for the most negative values of d34S inthis occurrence. The bacteriogenic contribution ofsulphur would be more important with time, producing

the decrease in d34S values in relation to the strati-graphic height observed at Roca de Ponent (see Fig. 9).Sulphur from a dual source has been considered asresponsible for the formation of many other sediment-hosted massive sulphide deposits, such as Tynagh(Boast et al. 1981) and Navan (Anderson et al. 1998) inIreland, and Rammelsberg in Germany (Eldridge et al.1988).

Genesis of the occurrences

The sulphide occurrences in the Silurian marine blackshales of the southern Catalonian Coastal Ranges arestratiform and stratabound, and are regionallyrestricted to the Lower-Llandoverian sediments.Therefore, the age of these occurrences is assumed tobe Lower Llandoverian. The environment for its for-mation would be an anoxic basin. In addition to theabundance of organic matter in the black shales, thereis evidence of a low activity of oxygen in this stage(cf. low SO4/H2S ratio, as demonstrated by the for-mation of pyrrhotite instead of pyrite or magnetite atrelatively low temperatures; Ohmoto and Lasaga 1982).This fact also explains the absence of barite in theseoccurrences, despite the existence of other Ba minerals.The physicochemical conditions in the Silurian basinchanged with time, as is recorded in the sulphurisotopes from the Sant Bernat series. The SO4/H2Sratio increased, at the same time as the hydrothermalactivity decreased, so that primary pyrite was formed atthe highest stratigraphic levels.

As proposed above, a dual source of sulphur isconsidered in these ore deposits: hydrothermal sulphurleached from the basement and sulphur of bacterio-genic origin. In the studied area, this leached sulphurwould have a d34S not heavier than –9&, i.e. an ulti-mately bacteriogenic origin. The fluids would behomogenised at depth, where the sulphur wouldacquire a uniform isotopic composition. This processcan be accomplished by means of convective cells.During the Lower Llandoverian these hydrothermalfluids would upwell toward the seafloor through faultswhich would act as feeder channels. Decreasing tem-perature, together with probable decompression, wouldproduce precipitation of sulphides on the seafloor orwithin the underlying permeable sediments, forming theSant Miquel and Coma Fosca outcrops. Some H2Swould migrate to more distal areas where it would mixwith sulphur produced by bacterial reduction ofseawater sulphate, giving place to the Roca de Ponentoutcrop. Here, the relative contribution from bacterialactivity would increase progressively with time, as thehydrothermal activity decreased.

Finally, a later episode of mineralisation took place.Here, pyrite with bird’s eye texture replaced the sulp-hides formed earlier. The similarity between the sul-phur isotopic composition of this late pyrite and thatof the hydrothermal sulphide formed in the previous

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stages indicates a common source of sulphur. Aremobilisation of the earlier sulphides and/or a leachingprocess in the sedimentary host rocks took place.A mixed hydrothermal-bacteriogenic process was alsoinvoked to explain PGE enrichment in sulphide-richblack shales in Proterozoic deposits in the CzechRepublic, Russia and Finland (Pasava et al. 1996).

The source of the precious metals is more prob-lematic. The regional setting could suggest an epige-netic, post-sedimentary source. However, sperrylitedisplays textural evidences of a pre-metamorphic ori-gin. The occurrence of Pd-lollingite is consistent withthe low values of sulphur and oxygen activity (primarypyrrhotite, black shales) which were predominantduring the deposition of the stratiform sulphides.Lollingite is affected by sulphidation, and this couldsuggest the introduction of a very late fluid unrelatedto the exhalative processes. However, the association ofcomplex textures of replacement between lollingite andarsenopyrite has been described in some present-daydeposits (Koski et al. 1994). Furthermore, the isotopicd34S composition of arsenopyrite is similar to that ofthe other sulphides from the three occurrences. Theseaspects suggest a common origin for the hydrothermalsulphur and the precious metals.

Conclusions

Isotopic data presented in this study strongly suggestthat the sulphur source for the Coma Fosca and SantMiquel occurrences is located in the underlying me-tasediments. Hydrothermal fluids leached the sulphurfrom sulphides of those metasediments, and pyrrhotitewas formed with a mean d34S of about –9&. Bacterialreduction of seawater sulphate, in a closed orsemi-closed system, at the time of mineralisation is anadditional source of sulphur for the Roca de Ponentoccurrence. This latest source is responsible for the mostnegative d34S values. Bacterial activity in a closed orsemi-closed basin was responsible for the isotopicallyheavy pyrite in the hanging wall of the Sant BernatSilurian series (d34S up to 36&).

During high-grade metamorphism (pyroxene toamphibole facies) sulphur isotopic re-equilibration didnot take place at a large scale.

The Pd-rich lollingite was replaced by arsenopyritewhich has d34S values similar to the rest of the sulphides.This fact and textural evidences suggest that the exha-lative processes which formed the stratiform minerali-sation also led to precious metal enrichment.

Acknowledgements The SEM-EDS analyses and BSE images wereobtained at the SEM unit at the Serveis Cientıfico-Tecnics (SCT) dela Universitat de Barcelona (R. Fontarnau). X-ray diffractionanalyses were performed at the X-ray unit at the SCT (X. Alcover).This research has been sponsored by the CICYT Spanish researchproject AMB94-0953-CO2-01, and by a postdoctoral grant of theSpanish Ministerio de Educacion y Cultura to P. Alfonso. A. Taitis thanked for his assistance in the isotope analyses. R. Willan and

V. Pascual are thanked for reviewing the manuscript, andB. Lehman for the editing and for improving the English. We alsoacknowledge the accurate reviewing by M. Solomon andP. Seccombe.

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