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The Cu Stockwork and Massive Sulfide Ore of the Feitais Volcanic-HostedMassive Sulfide Deposit, Aljustrel, Iberian Pyrite Belt, Portugal:

A Mineralogical, Fluid Inclusion, and Isotopic Investigation

CARLOS M. C. INVERNO,

Servios Geolgicos, Instituto Nacional de Engenharia, Tecnologia e Inovao, Apartado 7586, 2720 Alfragide (Lisboa), Portugal,and Centro de Recursos Minerais, Mineralogia e Cristalografia (CREMINER), Faculdade de Cincias, Universidade de Lisboa,

Edifcio C6, 1749-016 Lisboa, Portugal

MICHAEL SOLOMON,

Centre for Ore Deposit Research (CODES), University of Tasmania, GPO Private Bag 79, Hobart, Tasmania 7001, Australia

MARK D. BARTON,

Department of Geosciences, Center for Mineral Resources, University of Arizona, Tucson, Arizona 85721

AND JOHN FODEN

Department of Geology, University of Adelaide, Adelaide, South Australia 5005, Australia

Abstract

The Variscan Feitais volcanic-hosted massive sulfide deposit in the Aljustrel district of the Iberian Pyrite Beltconsists of 55 million metric tons of Zn-Pb-Cu massive sulfide overlying a Cu-rich stockwork. The massive oreis overlain by up to 30 m of feldspar-phyric, rhyolitic volcaniclastic rock and locally by a jasper and/or chert layerup to 15 m thick. The massive sulfide orebody consists dominantly of pyrite, sphalerite, galena, chalcopyrite,tetrahedrite-tennantite, arsenopyrite, and bournonite, together with minor quartz, chlorite, sericite, carbonate,and barite. The orebody is up to 100 m thick and is underlain by a tabular alteration zone of chlorite-dominated, locally silicified, felsic volcanic rock, the upper 30 to 60 m of which contains chalcopyrite-quartz-chlorite-sericite-carbonatebearing stockwork vein(let)s that prior to deformation were at a shallow angle tothe base of the massive orebody. Chloritized footwall rocks extend up to 20 m below the Cu stockwork zoneand are underlain by up to 50 m of quartz-sericite-pyritealtered rhyolitic rock. The stockwork veins also con-tain pyrite, tetrahedrite-tennantite, sphalerite, and arsenopyrite. Pyrite, both in stockwork and massive ore,locally displays partly recrystallized framboidal, reniform, and cellular textures.

Two generations of quartz, Q1 and Q2, and carbonate in the stockwork veins contain primary (in growthzones) and pseudosecondary fluid inclusions, with homogenization temperatures of 270 to 315C and salini-ties of 2.2 to 8.1 wt percent NaCl equiv. The 34S(CDT)values of massive and stockwork ores range from 15.4to +4.7 (mean, 2.8) and 11.2 to +11.9 (mean, 0.4) per mil, respectively, the lowest values from colloform-textured pyrite. With no evidence of oxidation of sulfide sulfur during mineralization, the most negative valuesindicate an origin by biogenic reduction of seawater sulfate. The 13C(PDB)values for carbonates, 7.5 to 13.7and +9.3 to 14.3 per mil in massive and stockwork ore, respectively, indicate an origin mostly by oxidation ofmethane derived from organic matter in underlying sedimentary rocks and possibly a contribution of magmaticcarbon. There are no significant lateral or vertical variations in S isotope values in sulfides or C-O isotope val-ues in carbonates, either in massive or stockwork ore. The 18O(SMOW)values for quartz in stockwork and mas-sive sulfide are 11.6 to 13.9 and 16.7 to 17.9 per mil, respectively. Coexisting, and texturally contemporaneous,carbonate and quartz in stockwork veins are not in isotopic equilibrium, indicating that the C-O isotope valuesmay have been reset. The 18O values of fluid calculated to be in equilibrium with quartz at fluid inclusionhomogenization temperatures are 4.2 to 5.2 per mil. Barite from the hanging wall and massive ore yields 34S

values (21.927.9) equal to or slightly higher than those of coeval seawater; 87Sr/86Sr ratios (0.7084380.709063) are slightly more radiogenic than those of coeval seawater (0.70800.7085), and much more radi-ogenic than those of coeval volcanic rocks (0.7033040.706642), probably representing mixtures between sea-

water Sr and radiogenic Sr in fluids sourced in the crustal pile.Deposition of the massive sulfide on the sea floor is suggested by its stratiform nature, the stronger alter-

ation of footwall relative to hanging-wall rocks, the stockwork system terminating sharply at the base of themassive sulfide, the presence of sedimentary-like textures in the massive sulfide, the absence of replacementfronts, and the presence of framboidal and other sea-floor depositional textures indicative of fluid quenching.The sheetlike form, lack of rubble mounds and chimneys, scarcity of barite, reduced mineral assemblage, andmetal zoning distinguish Feitais from Kuroko-type deposits. It shares most of the characteristics of thoseIberian Pyrite Belt deposits for which a brine-pool origin has been proposed based on fluid inclusion data,

Corresponding author: e-mail, [email protected]

2008 Society of Economic Geologists, Inc.Economic Geology, v. 103, pp. 241267


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Introduction

THE ALJUSTREL DISTRICT, in the southern Portuguese part ofthe Iberian Pyrite Belt (Fig. 1), contains several large vol-canic-hosted massive sulfide (VHMS) deposits, with knownresources of massive ore totalling 190 million metric tons(Mt) grading 45 to 47 wt percent S, 38 to 41 wt percent Fe,3.4 wt percent Zn, 1.2 wt percent Pb, 0.8 wt percent Cu, 36g/t Ag, and 1 g/t Au. An estimated 50 Mt has been mined orleft in mining pillars (Carvalho et al., 1976; Barriga, 1983;Leito, 1997). In addition, stockwork ores also occur in threeof the district orebodies (Feitais, Moinho, and Algares, mostsignificantly in the first two). A recent drilling program, con-ducted by Eurozinc Mining Corp. (now Lundin MiningCorp.), outlined within the 54.5-Mt resources of one of its

orebodies, Feitais, 18.4 Mt of massive zinc ore grading 6.0 wtpercent Zn, 1.8 wt percent Pb, 0.2 wt percent Cu, 67 g/t Ag,and 0. 73 g/t Au (at 4.5 wt % Zn cutoff; Barrett et al., 2008),4.1 Mt of massive copper ore grading 2.1 wt percent Cu, 0.7

wt percent Zn, 0.2 wt percent Pb, 12 g/t Ag, and 0.17 g/t Au (at

1.5 wt % Cu cutoff), and 2.6 Mt of stockwork copper ore grad-ing 2.1 wt percent Cu, 1.0 wt percent Zn, 0.3 wt percent Pb, 14g/t Ag, and 0.37 g/t Au (Dawson et al., 2001a, b; Barrett et al.,2008). In this paper we present the results of a metallogenicstudy, involving geology, mineralogy, fluid inclusion, and stableand radiogenic isotope data, centered on the Feitais copperstockwork and partially extending to the massive ores. Fieldwork included logging of drill cores (Feitais drilling sections160, 240, 320, 520, 560, 600, 640, and 800 ft) and also mapping,at scales of 1:50 and 1:100, of the scarce underground expo-sures of the footwall on levels 140, 190, and 200 m.

Previous Work

The geology of the Aljustrel area and its VHMS depositswas described by Schermerhorn and Stanton (1969), Andradeand Schermerhorn (1971), Carvalho et al. (1976) and Scher-merhorn et al. (1987). Several comprehensive ore microscopy,

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suggesting a similar depositional origin, although the evidence from fluid inclusions in this study is equivocal.The sulfate that underwent biogenic reduction may have been derived from mixing with seawater during earlyfilling of the brine pool; diffusion across the brine-seawater interface; and sulfate reduction in the footwall vol-caniclastic rocks. Stable and radiogenic isotope compositions of sulfates, sulfides, and carbonates suggest in-

volvement of modified seawater and crustal fluids convecting due to magmatic heating, but the calculated highfluid pressures in the stockwork may indicate the additional involvement of magmatic fluids.

Sines

Monchique

Cercal

Gra ndola

Ourique

Castro Verde

Almodo var

FARO

Castro Marim

Pomaro

Mrtola

BEJA

SEVILLA

HUELVA

Lousal

Gavio

ALJUSTREL

Neves-Corvo

S. Domingos

Romanera

San Telmo

Aguas Teidas

Concepcin

Rio Tinto

La Zarza

TharsisMigollas

Valverde

Aznalcollar

Los Frailes

Las Cruces

Salgadinho

Sotiel

Lagoa Salgada

OSSA

MORENAZONE

MADRID

LISBOASPA

IN

FRANCE

PORTUGAL

ATLANTIC

OCEAN

SPAIN

0 20 Km

8 OO' 7 OO'

38 OO'

37 OO'

N

MESOZOIC AND CENOZOIC

NAMURIAN - L. WESTPHALIAN

NAMURIAN

VISEAN

U. DEVONIAN - L. VISEAN

U. DEVONIAN AND OLDER PHYLL ITE- QUARTZITE GROUP; PUL O DO L OBO GROUP ( )

VOLCANIC SILICEOUS COMPLEX

MERTOLA FM.

MIRA FM.

BREJEIRA FM.

SEDIMENTARY ROCKS

FLYSCH GROUP

FERREIRA-FICALHO GROUP (Mainly U. DEVONIAN)

CARRAPATEIRA GROUP (U. DEVONIAN - L. NAMURIAN)

BEJA-ACEBUCHES BASIC-ULTRABASIC COMPLEX

GRANITIC ROCKS

MONCHIQUE MASSIF (76 Ma)

HERCYNIAN

NEW VMS DISCOVERIES (1985-1995)

VMS DEPOSITS

FIG. 1. Regional geology map of the Iberian Pyrite Belt, showing the location of Aljustrel (modified after Carvalho et al.,1999).


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textural, and beneficiation studies have been conducted onthe Aljustrel massive ores, including those of Feitais (e.g.,Gaspar and Conde, 1978; Gaspar, 1984, 1996, 1997). Barrigas(1983) study of Feitais involving transmitted light petrogra-phy, whole-rock geochemistry, and O isotope analysis of thehydrothermally altered rocks and ores, including stockworkand massive ores and electron microprobe mineral analysis,

concluded that the ore-forming fluid was seawater that hadundergone deep convective circulation in older rocks. Thesame isotope data were used by Barriga and Kerrich (1984)and Munh et al. (1986), the latter authors also including newD/H data on stockwork chlorite, to reach the same conclu-sion. These authors calculated from oxygen isotope equilib-rium fractionations, temperatures of 250 to 270 and 200 to215C for stockwork ore and 200 to 220 and 160 to 175Cfor massive ore, respectively.

Barriga and Fyfe (1988, 1991) and Carvalho et al. (1999),following Barriga (1983), proposed that the massive sulfideorebody formed in open space under an older, overlying,jasper and/or chert layer, which acted as a seal during thecooling of the ore-forming fluids. Hutchinson (1991) disputed

this conclusion based on the presence of fragmental orewithin the massive sulfide orebody. Barriga and Fyfe (1998)studied the metasomatism of Aljustrel volcaniclastic rocks notaffected by ore zone hydrothermal alteration, concluding thatthey were altered by seawater that, with increasing depth,changed from a cold, oxidizing, Mg-bearing fluid to a hot, re-ducing, Mg-free, and Si-Fe-Cu-Znrich fluid, with water/rockratios (w/r) in the whole area progressively decreasing fromvery high to low values. The probable amount of Cu and Znleached from volcaniclastic rocks was shown to be compatiblewith the Cu and Zn metal contents in the massive sulfidebody. Barriga (1983) and Carvalho and Barriga (2000) recog-nized a sericitic alteration zone underlying the footwall chlo-ritic alteration zone. Dawson et al. (2001a, b) presented the

main results of a Eurozinc drilling program, with new re-sources for massive Zn ore, massive Cu ore and stockwork Cuore, and proposed that a volumetrically important percentageof the rhyolites in the area are intrusive and not of volcani-clastic nature, as suggested by previous authors. More recentinformation is contained in Barrett et al. (2008) that also pre-sents a lithogeochemical study of the Aljustrel volcanic rocksand proposes that the massive sulfides and the overlying vol-canic rocks and chert formed in a growth fault-controlledbasin.

Regional Geology

The Iberian Pyrite Belt, located in the southwest of theIberian Peninsula (Fig. 1), with an estimated pre-erosional

1.7 billion tons of sulfides, is the largest (in tonnage) VHMSprovince in the world (Carvalho et al., 1999). It is 250 km longand 30 to 60 km wide. The stratigraphic sequence in the belthas been divided into (1) the Late Fammenian (Late Devon-ian) Phyllite-Quartzite Group at the base, (2) the Late Fam-menian to earliest Late Visean intermediate VolcanicSiliceous Complex, consisting of magmatic and sedimentaryrocks that host the massive sulfide deposits, and (3) at the top,the Late Visean to Early Westphalian (Carboniferous) (BaixoAlentejo) Flysch Group (Carvalho et al., 1971, 1976, 1999;Oliveira, 1990; Oliveira et al., 2004).

During the 1970s most of the models for the geotectonicsetting of the Iberian Pyrite Belt involved various plate tec-tonic scenarios (reviewed in Silva et al., 1990, and Carvalho etal., 1999), and in the 1980s the role of oblique, Variscan con-tinent-continent collision and related sinistral transcurrentmovements became important (Badham, 1982; Brun andBurg, 1982). From about 1990 all models envisaged the Iber-

ian Pyrite Belt as part of the South Portuguese zone subduct-ing northward beneath the Iberian massif, part of the Gond-wana continent (Silva et al., 1990; Fonseca and Ribeiro, 1993;Quesada et al., 1994; Dias and Ribeiro, 1995). Fonseca andRibeiro (1993) and Quesada et al. (1994) found evidence ofEarly to Middle Devonian northward obduction of remnantoceanic material (the Beja-Acebuches Ophiolite Complex)during early phases of oblique continent-continent collision,followed by Devonian-Carboniferous sinistral faulting andsouthward thrusting and folding, and associated flysch sedi-mentation (e.g., Quesada, 1998; Onzime et al., 2003). Trans-pression during oblique continental collision resulted in theformation within the South Portuguese zone of extensional,pull-apart basins in which magmatism, massive sulfide miner-

alization, and marine sedimentation occurred (Munh, 1983;Dias and Ribeiro, 1995; Mitjavila et al., 1997; Oliveira andQuesada, 1998; Fonseca et al., 1999; Rosa et al., 2004).Solomon and Quesada (2003) suggested that steep transcur-rent faults were conduits for crust- and mantle-derived mag-mas and that their exsolved fluids were important compo-nents of the fluids responsible for mineralization.

In the Aljustrel area, Phyllite-Quartzite Group rocks areabsent but crop out in Corte Vicente Anes, 3 km northeast ofAljustrel. The lower part of the Volcanic Siliceous Complexsequence (Fig. 2), including that at Feitais, consists of sub-marine crystal and lapilli rhyolitic volcaniclastic rocks, and afew massive or autobrecciated lavas, with a total thickness ofover 300 m in the central parts and less than 80 m laterally

(Andrade and Schermerhorn, 1971; Carvalho et al., 1976;Barriga, 1983; Dawson et al., 2001b). Six orebodies (Algares,S. Joo, Gavio, Moinho, Feitais, and Estao; Fig. 2) lie nearthe top of this sequence. A U-Pb zircon age of 352.9 1.9 Mawas obtained for the hydrothermally altered footwall rhyoliteof the Algares orebody by Barrie et al. (2002). This age is veryclose to that of the hanging-wall rock at Las Cruces, which ison the same horizon as Aznalcllar, dated by palinology of thehost shales as Late Strunian (Pereira et al., 1996; Conde et al.,2003); a Late Strunian age has been assumed for Feitais indiscyssing isotope studies. Sedimentary rocks, mainly black orsiliceous shales, are interbedded with the volcaniclastic rocksin places (Andrade and Schermerhorn, 1971; Leito, 1997).T.J. Barrett and W.H. MacLean (unpub. report for Eurozinc

Mining Corp., 1999, 65 p.), Dawson et al. (2001b), and Bar-rett et al. (2008) indicated the existence of intrusive rhyolitesin this lower part of the Aljustrel Volcanic Siliceous Complexsequence, but our interpretation is that the field evidence re-mains inconclusive.

The ore horizon at Feitais is overlain by up to 30 m offeldspar-phyric, rhyolitic volcaniclastic rocks (Barrett et al.,2008) that are themselves overlain by a 0- to 15-m-thick,hematitic to manganiferous jasper and/or chert layer thatmarks the base of the Paraso Formation (Fig. 3; Barriga andKerrich, 1984). This layer directly caps the massive sulfide in

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Gavio

Estao

Feitais

Algares

S. Joo

Roldana

Moinho

Sto. Anto

ALJUSTREL

0 1000 m

N

Clastic Sedimentary Cover Rocks

Dolerite Dike

Flysch Group - Mrtola FmUPPER VISEAN

PLEISTOCENE

MID-JURASSIC

MIO-PLIOCENE

TOURNAISIAN--LOWER VISEAN SILICEOUS COMPLEX

VOLCANIC

SS

SS

VMS deposits (not in production)

Siliceous Phyllite, Volcanic Sandstone and Shale

Rhyolitic Volcaniclastic Rock (hosting Ore on top);Lateral Sedimentary Rock ( )

Jasper

SS

*Road Fault Thrust fault

8 12'o o8 10' o8 08'

37 52'o

o37 53'

FIG. 2. Geology map of Aljustrel (modified after Andrade and Schermerhorn, 1971; Sto. Anto area after Leito, 1992).

Jasper / chert

Feldspar-phyric rhyolitic volcaniclastic

Massive sulfide orebody

Paraso Fm: siliceous shale, phyllite,

Mrtola Fm: graywacke and shale

Late Visean

Tournaisian to early Visean

Rhyolitic volcaniclastic rock (footwall)

Stockwork

FLYSCH GROUP

VOLCANIC SILICEOUS COMPLEX

Fault

Drill-hole trace

rock ( hanging wall )

volcanic sandstone and shale

0 100 m

0 level

100 level

200 level

300 level

400 level

500 level

600 level

SEA

198,4

00E

100,6

00N

198,7

00E

100,9

00N

LEVEL

SW NEA A'

LC13 (124 m)

F063 (50 m)

L064 (132 m)

F003 (52 m)

F014 (59 m)

FS9803 (365 m)

F015 (99 m)

F016 (126 m)

FS9944 (389 m)

FS9942 (422 m)

FS11 (506 m)

FS9802 (492 m)FS13 (520 m)

FS22 (642 m)

FS21 (574 m)

F065A (177 m)

F003 (52 m)

FIG. 3. Feitais drilling section 560 ft Northwest. Position of profile A-A' is shown in Figure 4 (modified after Sousa, 1999;Eurozinc 1:1,000 scale section, 2000).


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some areas, but for the most part does not, being separatedfrom it by up to 30 m of hydrothermally altered (chlorite,sericite, pyrite) rhyolitic, volcaniclastic rock (Fig. 3). In thevicinity of, and above, the orebody, red hematitic jasper gradesinto a gray, black, or dark brownish manganiferous chert, withdisseminated carbonates, magnetite, and pyrite; local Mnoxide-rhodocrosite-rhodonite accumulations have been ex-

ploited in small open pits (Barriga and Fyfe, 1988). Chlorite isat least locally abundant in the layer, parts of which also con-tain intercalated altered feldspar-phyric, rhyolitic volcaniclas-tic rock; the layer is crosscut by abundant veins filled withquartz and later carbonate (-barite). Leistel et al. (1998)thought that the Iberian cherts in general were emplacedbelow the sea floor through chemical precipitation or replace-ment of a preexisting rock, but Barriga and Fyfe (1988) con-sidered the jaspers and associated rocks in the Aljustrel area tobe hydrothermal precipitates deposited on the sea floor.

The jasper and/or chert layer at Feitais is locally overlain bybasaltic lavas and volcaniclastic rock, and volcanic sandstonesand shales (Leito, 1997). The remainder of the Paraiso For-mation, the top of the Volcanic Siliceous Complex sequence,

consists of 50 m of siliceous shales, volcanic sandstones andshales, and minor, thin, rhyolitic volcaniclastic rock layers inthe lowermost part (Carvalho et al., 1976; Barrett et al.,2008). Members of the Paraso Formation above the jasperand/or chert layer have been hydrothermally altered, with sig-nificant development of chlorite, sericite, carbonate, and sul-fides (Schermerhorn, 1978; Barriga, 1983). Carbonate-quartz(barite) veins occur in the feldspar-phyric, rhyolitic volcani-clastic rocks, the jasper and/or chert layer and at least the low-ermost 20 m of the Paraso Formation rocks. Barriga (1983)noted the presence of cymrite, a hydrated barium silicate, ina barite-free vein cutting the jasper and/or chert layer. Thereare barite-rich layers, up to over a decimeter thick, in thefeldspar-phyric, rhyolitic volcaniclastic hanging-wall rocks.

The Tournaisian to early Visean Volcanic Siliceous Complexsequence (Schermerhorn et al., 1987; Barrie et al., 2002) isconformably overlain by and in gradational contact with theMrtola Formation of the Flysch Group, consisting of severalhundred meters of a turbiditic sequence of graywackes andshales, of late Visean age, but the Volcanic Siliceous Complexrocks have been thrust over the flysch along much of the con-tact (Andrade and Schermerhorn, 1971; Leito, 1997).

The whole sequence was deformed during the Hercynianorogeny, with a pre-late Westphalian main deformation phase(Carvalho et al., 1999), involving development of northwest-southeast folds with steep regional slaty cleavage dipping tothe northeast, as well as overthrusts. The Aljustrel orebodieslie in the steep limbs of four asymmetrical anticlines that dis-

tribute the orebodies into three lineaments (Fig. 2), fromnortheast to southwest, Feitais-Estao, Algares-Moinho-S.Joo-Gavio (Schermerhorn et al., 1987), and Gavio SW(Conde and Leito, 1984; Conde et al., 1986). S. Joo andGavio, and Feitais and Estao, are displaced relative toeach other by an offset of 3 and 0.5 km, respectively, alongnortheast-southweststriking (Messejana) and north-north-east-south-southweststriking (Represa) faults, respectively.Each pair of sulfide lenses was initially a single orebody,reaching 1.7 km along strike in the case of Feitais-Estao(Andrade and Schermerhorn, 1971; Dawson et al., 2001b).

Dawson et al. (2001a, b) and Barrett et al. (2008) suggestedthat the Feitais orebody formed in a restricted basin within amajor basin, adjacent to a growth fault that focused the fluidsresponsible for alteration and veining in the footwall.

Feitais Orebody and Footwall

The Feitais massive sulfide orebody, in the northeast flank

of the Feitais anticline, is 750 to 1,000 m long (remains open),500 m wide, and 60 m thick (max 100 m; Figs. 3, 4). It has aCu-(Bi-Sb)enriched base of medium- to coarse-grainedhomogeneous to banded massive ore, an intermediate, low-grade, pyritic zone (fine-grained massive ore), and an upperZn-Pbrich fine-grained banded ore zone (Conde and Leito,1984; Gaspar, 1996, 1997; Dawson et al., 2001a; Barrett et al.,2008). The massive ore consists predominantly of pyrite,sphalerite, galena, chalcopyrite, tetrahedrite-tennantite, ar-senopyrite, and bournonite, with minor pyrrhotite, jamesonite,cassiterite, stannite, meneghinite, marcasite, cobaltite, nativebismuth, gudmundite, kobellite, magnetite, and hematite asmetallic minerals (Gaspar and Conde, 1978; Gaspar, 1996).Where the stratigraphic base of the massive ore is underlain

by a stockwork zone, sphalerite contains abundant chalcopy-rite inclusions (chalcopyrite disease). The highest Bi contents,both in Feitais and Moinho, are in the base of the massive sul-fide orebody, where native bismuth occurs locally in the latter(Conde and Leito, 1984; Gaspar, 1997). The massive oregangue (a few percent in the Moinho orebody: Gaspar, 1997)comprises quartz, chlorite, sericite, carbonate, and minorbarite (Barriga, 1983; Gaspar, 1996). Barite is more commonnear the hanging wall and in the periphery of the orebody(Barriga, 1983; Mitsuno et al., 1988) but overall is scarcewithin massive ore. It also occurs in carbonate-quartz-bariteveins that cut the orebody.

The massive ore has been recrystallized, consisting mostlyof an aggregate of idiomorphic, fine-grained (rarely medium-

grained) pyrite (Barriga, 1983). The massive ore in placesdisplays mineral layering consisting of sphalerite-rich layersin pyrite and also bands of different grain size or texture inpyrite; in places, the bands are parallel to slaty cleavage andmay be due to deformation, but Barriga (1983) and Gaspar(1996) believed the layering to be in most cases primary.Structures such as graded bedding, convolute lamination,and small slumps are not abundant in massive ore accordingto Barriga (1983) but are common in its stratigraphic lowerportion according to Gaspar (1996, 1997). Framboidal, aswell as cellular and zoned botryoidal (colloform) pyrite tex-tures observed in the present study are shown in Figure 5A-D. The massive ore, in places, also grades locally into frag-mental, microconglomeratic ore, commonly with pyrite (or

rarely quartz) fragments up to 1 cm wide in a pyrite-rich, orlocally quartz- or carbonate-rich matrix. Domains of alteredrhyolitic volcaniclastic rock up to 2 m long are found locallywithin massive sulfide ore immediately adjacent to the strati-graphic footwall, e.g., in the 190-m level (depth relative to areference level 0 m that is 150 m above sea level; surface is065 m above level 0 m). The position and attitude of thestope faces containing those domains suggests that such facesmay represent the intersection with irregular terminations ofthe massive sulfide orebody, and are not evidence for re-placement, as claimed by Barriga (1983), although, in a few


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other cases reported in that study, there is evidence of minor,local replacement.

Most of the massive sulfide orebody overlies a broadly tab-ular zone of altered volcaniclastic rock, the upper part ofwhich, 30 m to locally 60 m thick (Fig. 3), contains chalcopy-rite-bearing stockwork quartz veins and veinlets. In placesthis stockwork extends for some tens of meters beyond thelimits of the massive sulfide orebody, but particularly at itssouthwest margin and in the northwest and southeast ends(Fig. 4), the orebody is not underlain by a stockwork zone; inthese portions, up to 220 m in extent, the orebody commonly

has neither a Cu-rich stratigraphic base nor chalcopyrite dis-ease in sphalerite.The stockwork veins lie mostly within strongly chloritized

rhyolitic volcaniclastic rock, and this altered rock extendsdownward up to an additional 20 m without veins, then gradesinto a dominantly sericitized, pyrite-bearing, rhyolitic volcani-clastic rock, which continues for at least 50 m below the chlo-rite-altered zone. Carvalho and Barriga (2000) recognizedboth the sericitic- and chloritic-altered zones in the Feitaisorebody footwall. A paragonite-quartz-sulfide assemblage(Relvas et al., 1990; Massano et al., 1991) in the outermost

stockwork rocks of the Gavio orebody footwall was not rec-ognized in Feitais by those authors or in our study. In placesthe chloritized volcaniclastic rock within the Feitais stock-work grades into a medium- to pale-gray, chloritized, partlysilicified, or even completely silicified, whitish rhyolitic rock.In the uppermost stockwork, local silicification also produceda white to buff silicified and sericitized rhyolitic volcaniclasticrock, with millimeter- to centimeter-scale bands and inter-vening anastomosing stringers of pyrite (chalcopyrite; Fig.5E, F). Patches of this rock are also locally present within theunderlying chloritized stockwork. Dominant sericitic alter-

ation, and minor chloritic alteration (more intense in associa-tion with the jasper and/or chert layer) occurred in thefeldspar-phyric, rhyolitic volcaniclastic hanging-wall rocks upto the lowermost 10 to 20 m of the Paraso Formation shales,with enrichment in Fe, Mn, and Ba (Barriga, 1983) and de-velopment of quartz-carbonate (calcite, Fe dolomite) veins.

These hydrothermal alteration assemblages overprinted alower temperature regional alteration event that convertedbasalts into spilites and rhyolites into quartz keratophyres.This alteration is thought to result from the interaction be-tween seawater and the volcanic rocks (Munh and Kerrich,

246 INVERNO ET AL.

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N

Surface projection of massive

Surface drill holes

sulfide orebody at 200 level

mentioned in text

A

A'

LC3

FS9975

FS17

FS9801

FS22

FS21

FS13

FS11

FS9802

FS9803

FS9942 / FS99444

FS9963

FS9932 / FS9933

FS9934

FS9939

FS9822

FS9949 / FS9953

FS9947 / FS9950

0Ft

4080

120

160

200

240

280

320

360

400

440

480

520

560

600

6406

8072

076

080

084

088

092

096

010

00Ft

101,000

100,500

100,000 198,0

00

198,5

00

199,0

00

Fault

0 200 m

Represa

fault

Fault (approx. position)

FS9951 / FS9956 /

FIG. 4. Plan of Feitais deposit, showing the drill sections and surface projection of the massive sulfide body outlined onthe 200-m level. Line A-A' denotes position of profile shown in Figure 3 (after Eurozinc maps, 1999).


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1980) and was followed by prehnite-pumpellyite (to lowergreenschist) facies regional metamorphism during the Her-cynian orogeny (Munh et al., 1986; Munh, 1990).

Chlorite and white mica compositions

Hydrothermal choritization altered the pale gray por-phyritic (albite phenocrysts) rhyolitic volcaniclastic rock (An-

drade and Schermerhorn, 1971) to a medium to dark gray-green fine-grained (


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A

FE

DC

B

p y

c p y

FIG. 5. Photomicrographs of polished thin sections from Feitais massive sulfide and stockwork ore samples. Samplelocations are indicated in tables. A. Microconglomeratic massive sulfide ore, with fragment of pyrite with colloform texturein quartz (black) matrix. Sample F57, reflected light (RL). B. Microconglomeratic massive sulfide ore, with fragment ofbotryoidal pyrite, with an outer more recrystallized pyrite shell. Tetrahedrite-tennantite (tt) and chalcopyrite (cpy) are alsopresent in quartz (black) matrix. Sample F57 (RL). C. Massive sulfide ore with zoned, colloform texture; shells of tetra-hedrite-tennantite (tt), chalcopyrite (cpy), chlorite (chl), and quartz (qtz), in pyrite. Sample FXIA (RL). D. Silicified andsericitized peripheral stockwork rock, 1 m below MS body footwall, 140-m level. Partly recrystallized pyrite (py) framboidsoccur within quartz-sericite (black) and also rimming sphalerite I (sp). Sample FXXII (RL). E. Silicified and sericitized stock-

work containing anastomozed pyrite (py)-minor chalcopyrite (cpy) stringers in quartz-sericite rock (dark gray); the pyritegrains have abundant chalcopyrite inclusions and microveinlets. Sample F32a, transmitted light (TL + RL). F. Silicified and

sericitized peripheral stockwork, 190-m level, with pyrite II (py)-chalcopyrite (cpy)-quartz (qtz)-sericite microveinlet.Quartz-sericite host rock in the upper left corner. Sample FVIB (TL + RL), crossed polars. G. Quartz (white)-chlorite hostrock to stockwork veins, with ghosts of phenocrysts, now fully replaced by quartz and chlorite. Sample F33 (TL). H. Q2 (qtz)-chlorite II (dchl)-chalcopyrite (cpy)-pyrite (py) stockwork vein. Sample F60 (TL + RL). I. Stockwork vein with chalcopyrite(cpy) replacing pyrite II (py). The arrowed sphalerite I (sp) microveinlet is later than the narrow chalcopyrite microveinletextending from the upper right to the lower middle of the photomicrograph. Sample F47 (RL). J. Colloform texture in stock-

work, with stringers of chlorite II-pyrite-chalcopyrite (all black) within very fine grained chlorite (gray). Sample F38 (TL).K. Stockwork with chlorite II (black) with arsenopyrite grains (apy), mostly coarser than pyrite (py), both corroded by chal-copyrite (cpy). Sample F51 (RL). (L) Stockwork quartz vein containing colloform pyrite-chalcopyrite (py-cpy) band, partlyrecrystallized. Sample F58 (RL). M. Detail of left end of band in L. Zones, within quartz (black), of very fine grained inter-grown pyrite-chalcopyrite and intercalated more recrystallized zones (whiter) of both minerals. Sample F58 (RL). N. Stock-

work containing pyrite with cellular, colloform texture, with inner, finer grained zone and outer, more (solid) inclusion-freerecrystallized zone. Surrounded by finer pyrite, chlorite II (black), and a quartz (qtz) grain. Sample F22 (RL).


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G Hp y

I J

L

M N

K

p y- c p y

p y

FIG. 5. (Cont.)


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chalcopyrite-chlorite II association is also present in localized,rosary-like stringers of fine- to medium-grained chlorite II,pyrite and, locally, abundant chalcopyrite, within massive,very fine grained chlorite II, in a partially recrystallized earlytexture (Fig. 5J).

Two varieties of sphalerite are recognized, both commonlyfine grained. Sphalerite I is medium brown (medium green inplane-polarized light under the microscope) with abundant in-clusions of chalcopyrite (chalcopyrite disease) and yellow inter-

nal reflections. Sphalerite I typically precedes, but in placespostdates, chalcopyrite (Figs. 5I, 7). Sphalerite II is darker in

color (red in plane-polarized light under the microscope), withfew or no chalcopyrite inclusions and enriched in Fe, as in-ferred from its red internal reflections. It is generally parage-netically later and less common than sphalerite I (Fig. 7). Bar-riga (1983) reported 1.4 to 2.5 wt percent Fe in sphalerite fromthe stockwork, and this likely corresponds to our sphalerite I.

Tetrahedrite-tennantite is a distinctive and relatively abun-dant mineral in the Feitais stockwork. It is commonly an-hedral, forming aggregates up to 2 mm long, and, like spha-

lerite, is nearly synchronous with chalcopyrite (Fig. 7). Fine-to medium-grained euhedral to anhedral arsenopyrite isabundant in the pyrite-chalcopyrite stockwork bands and in-tervening stringers mentioned above in silicified and/or seric-itized volcaniclastic rock, as well as, locally, with chlorite II instockwork quartz veins and predominantly in their chloritizedwalls (Fig. 5K). Galena, bournonite, and boulangerite areminor minerals. Pyrrhotite was identified in three chloritizedrhyolitic volcaniclastic rock samples, where it occurs asminute grains within chalcopyrite and in one case is partlyconverted to marcasite and pyrite. Native bismuth is presentlocally in stockwork veins (Mitsuno et al., 1988).

Several generations of vein quartz are present (Fig. 7),postdating the anhedral, fine-grained quartz of the hydro-

thermally altered host rock (Quartz H). Quartz 1 (Q1), com-monly anhedral but locally displaying a comb structure, is upto 6 mm long and contains abundant fluid inclusions thatimpart a milky white appearance. In these same veins and vein-lets, in proximity to chalcopyrite, Q1 is cut by a more transpar-ent, anhedral to euhedral quartz 2 (Q2). Q2 is typically morefine grained (


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formed as quartz fibers aligned in the pressure shadows ofcoarse pyrite grains (e.g., cubes). Q2 is also accompanied byup to 2-mm carbonate crystals in proximity to chalcopyrite(Fig. 7); both the quartz and carbonate locally form growth-zoned, idiomorphic crystals with terminations pointing intothe vein center, characteristic of open-space growth.

At the periphery of the stockwork, centimeter-long bands

in chloritized host rock to the stockwork (Fig. 5L) consist ofvery fine grained intergrown pyrite and chalcopyrite (both


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TABLE1.Elec

tronMicroprobeAnalysesofChloritefromFeitais

Rocktype

Stockworkore

Sampleno.

F31

F9a

F14

F21

F33

F42(R)

Drillhole

section,(m)or

level/exposure

FS9963-540,452.6

FS9803-560,357.2

FS9944-560,365.1

FS9947-600,412.3

FS9953-640,330.2

FS9949-640,387.7

Chloritetype

ChloriteII

ChloriteI

ChloriteII

ChloriteI

Ch

loriteII

ChloriteI

ChloriteII

Ch

loriteII

ChloriteII

ChloriteI

Ch

loriteII

ChloriteII

andhost

Stwveins

Stw

hostrock

Stwveins

Stwhostrock

St

wveins

Stwhostrock

Stwveins

Stw

hostrock

Stwveins

Stwhostrock

St

wveins

Stwhostrock

No.ofanalyses

3

3

3

3

3

3

3

2

5

1

4

3

Wt%

SiO2

24.65

24.85

24.35

24.12

24.39

23.56

23.83

23.33

23.98

26.78

25.96

26.92

Al2O3

25.64

24.66

23.80

23.81

25.91

25.02

20.21

20.27

22.31

25.36

23.08

23.44

TiO2

0.01

0.03

0.04

0.07

0.04

0.03

0.03

Inverno08 FeitaisVMS EG

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