metallogenesis at the neves corvo vhms deposit (portugal): a contribution from the study of fluid...

Ore Geology Reviews 34 (2008) 354–368

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Metallogenesis at the Neves Corvo VHMS deposit (Portugal): A contribution from thestudy of fluid inclusions

António Moura ⁎Departamento de Geologia & Centro de Geologia, Universidade do Porto, Rua do Campo Alegre, 687, 4099-007 Porto, Portugal

⁎ Tel.: +351 220 114 508; fax: +351 222 056 456.E-mail address: [email protected].

0169-1368/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.oregeorev.2008.04.004

A B S T R A C T
A R T I C L E I N F O
Article history:
Neves Corvo, one of Europ Received 27 March 2007Accepted 6 April 2008Available online 8 May 2008
Keywords:Iberian Pyrite BeltPortugalNeves CorvoFluid inclusions

e's most outstanding currently-producing mines was the focus of two recentstudies on fluid inclusions. The present contribution provides new data and is intended to give an overviewof the fluid inclusion research carried out on the deposit. The study focuses on samples from all stratigraphiclevels above the ores, in the copper and tin ores, and in the main thrust. In massive copper ore, pyrite pluschalcopyrite occur along growth zones in quartz, suggesting that quartz and sulfides are coeval in thesesamples. The most frequent fluid is a low-salinity H2O–NaCl fluid, but an aqueous carbonic fluid of lowdensity also occurs, especially in intercrystalline domains. Homogenization temperatures show two distinctmodes, one at 160 to 170 °C and the other between 220 and 280 °C. In one sample a high-salinity H2O–NaClfluid with 30 wt.% NaCl equiv. was observed. This fluid is interpreted to represent the product of phaseseparation at depth, and it could have been derived from either seawater or a magmatic fluid. However, themost likely cause for ore deposition was temperature decrease. It seems, however, that three separate eventsare registered (low, medium and high temperature) in the evolution of the volcanogenic hydrothermalsystem.In all but the massive ore and the main thrust, the dominant fluids are low-salinity aqueous carbonic incharacter, frequently showing liquid CO2 at room temperature. Pressure oscillations frequently found in thesefluids are interpreted as due to variations in the pressure regime from lithostatic to hydrostatic at the samedepth. CO2+CH4 are thought to have resulted from reactions with organic matter dispersed in themetasediments, which are very common at the stockwork and rubané ores and below the orebodies. Thehighest P–T conditions during metamorphism were estimated around 320 MPa and 350 °C. The rubané oresare interpreted as metamorphogenic ores as they were formed during orogeny; also, the fissural copper ore isbetter described as a metamorphosed stockwork.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

TheNevesCorvomine, located in the IberianPyrite Belt (IPB; Fig.1), isthe secondmost important coppermine in Europe. It exploits the richestore deposit ever found in the Belt. Neves Corvo contains more than300 Mt of massive sulfides separated into five orebodies (Corvo, Graça,Neves, Lombador and Zambujal). At present, only the first three are inexploitation for copper and zinc. The two main characteristics of thishuge deposit, and which distinguish it from all other volcanic-hostedmassive sulfide (VHMS) deposits worldwide are: (1) the existence ofhigh grade ores (average 8% Cu, with large volumes exceeding 20% Cu,mainly as chalcopyrite but also as tennantite–tetrahedrite); and (2) theabundance and grades of tin ores, locally with metric-scale blocks ofalmost pure cassiterite (up to 65% Sn). Due to these features, togetherwith the overall size, Neves Corvo is a world-class deposit.

Guilhaumou et al. (1976) began fluid inclusion studies on depositsin the IPB, although they studied only mineralized quartz veins (some

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with sub-economic mineralization) rather than massive sulfides.Wipfler and Sedler (1995) and Germann et al. (2003) also studiedvein type deposits in the IPB. These studies concluded that the veindeposits were probably formed from mineralizing fluids circulatingduring metamorphism. It was only in the last decade that detailedfluid inclusion studies were carried out on IPB massive sulfidedeposits. Among these studies, several concluded that the observedfluids were of primary exhalative origin (Toscano et al., 1997a,b;Almodóvar et al., 1998; Nehlig et al., 1998; Inverno et al., 2000;Sánchez-España et al., 2000, 2003; Jaques and Noronha, 2002). Others,however, found metamorphic-derived fluids with no trace of theoriginal seafloor-related fluids (Moura et al., 1993; Moura, 1994;Moura et al., 1995, 1997, 1999, 2001; Marignac et al., 2003). Moura(2005) showed the existence of volcanogenic-like fluids in themassive sulfide ores of Neves Corvo.

The origin of fluids entrapped in VHMS deposits has been the topicof much controversial debate. Some authors (e.g., Ripley and Ohmoto,1977; Broman, 1987; Solomon and Zaw, 1997) interpret observedfluids as volcanogenic, despite the low- to medium-grade meta-morphism undergone by the deposits. In contrast, other authors who

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355A. Moura / Ore Geology Reviews 34 (2008) 354–368

studied similar deposits (e.g., Hall, 1989; Hall et al., 1991; Giles andMarshall, 1994; Lusk and Krouse, 1997) concluded that the observedfluid inclusions are metamorphic in origin and argued that meta-morphism destroyed all primary fluids. Marshall et al. (2000)discussed this issue, and concluded that most VHMS deposits thatsuffered metamorphim do not contain preserved primary fluids.

Although two articles have already been published about theNeves Corvo paleofluids (Moura, 2005, 2008) the present study is thefirst to present the state-of-the-art regarding the characteristics of thefluids that took part in the genesis of this atypical deposit. It givesdetails on the sample location, presents unpublished data for thehanging-wall lithologies, new and complete data on the microther-mometry and Raman analysis, together with unpublished results onmicroPIXE analysis and geothermometry (vitrinite reflectance andillite crystallinity data). A comparison with fluid inclusion results onVHMS deposits elsewhere and from several modern submarineanalogues is also presented.

Study of the fluid inclusions inside minerals from ores, and fromtheir host lithologies, provides reliable evidence for the P–T–V–X fluidcharacteristics during formation of the Neves Corvo deposit, and thuscontributes to understanding the genesis and the uniqueness of thisdeposit.

2. Regional geology and geodynamics

The IPB is one of the five domains of the South Portuguese Zone(SPZ), which is the outermost Hercynian Zone in Europe. This belt isthe greatest metallogenic province of VHMS deposits in the worldwith respect to ore tonnage, containing around 2257 Mt of ore in 85known massive sulfide deposits (Tornos, 2006). SPZ geology is welldescribed in a series of papers, including Barriga and Carvalho (1983),Munhá (1983, 1990), Oliveira (1983), Ribeiro and Silva (1983), Barriga(1990), Silva et al. (1990) and Silva (1997). Other landmark studiesrelated to metallogenesis in the IPB are those of Schermerhorn (1970),Saez et al. (1996, 1999), Barriga et al. (1997), Tornos et al. (2002, 2005)and Tornos (2006).

Deposit formation has been dated, using palynomorphs, atbetween 354 and 354.8 Ma (Late Devonian; Oliveira et al., 2004).The deposits were formed in a volcanic–sedimentary submarineenvironment, possibly linked with an intracontinental rift (Silva et al.,1990) and third order pull-apart basins (Mitjavila et al., 1997), not farfrom the collision zone between South Portuguese and Ossa MorenaZones. About 20 to 30 Ma later, during the Visean, the previoustranstensive tectonics changed to a transpressive regime, forming a

Fig. 1. Location and geodynamic setting of the Ossa Morena Zone (OMZ) and the South PorComplex, PLAP— Pulo do Lobo Accretionary Prism, BAFD— Baixo Alentejo Flysch Domain andCorvo mine. Adapted from Moura (2005).

thin-skinned orogenic belt along the entire SPZ. In the IPB, regionalmetamorphism is mainly of very low grade (prehnite–pumpellyitefacies) but transitions to the greenschist facies (the metamorphicfacies of the Pulo do Lobo Group) are seen in the northern part of thebelt and, occasionally, within shear zones.

Conditions during regional metamorphism are estimated to bearound 310±10 MPa, using fluid inclusion data (Moura, 2003) and 200to 400 °C, from petrologic reactions, vitrinite reflectance, illitecrystallinity, and fluid inclusion studies (Lécolle and Roger, 1976;Bernard and Soler, 1980; Munhá, 1981, 1983, 1990); Caliani et al., 1994;Moura et al., 2001; Moura, 2003; Moura and Rocha, 2003. Priem et al.(1978) used Sr isotopes to date the orogenic metamorphism at 315±7 Ma.

3. Neves Corvo geology

The stratigraphic column at Neves Corvo (Fig. 2) comprises oneallochthonous and one autochthonous sequence. The first is com-posed of the Mértola Formation (Mt1— greywacke and shale) and theUpper Allochthonous Volcano–Sedimentary Complex, composed offour formations, each comprising felsic and intermediate volcanicrocks interbedded with shales (some of them siliceous, black orgraphitic) and occasional carbonate lenses. All these formations areLate Visean A in age. Below this allochthonous sequence there is asection of Early Visean age, composed of gray siliceous shale, blackshale with phosphatic nodules, and felsic volcanic rocks. Unconform-ably underlying these lithologies is the Mértola Formation (Mt2 andMt3) of Late Visean B age. These are turbiditic beds unconformablyoverlying the massive ores or on the jasper and carbonate unit (JC) oreven on the Neves Formation (a pyritic black shale and thin-beddedsiltstone). In places (mainly on the Corvo and Graça orebodies) abovethe massive ores there are banded ores (the so-called rubané). Themassive sulfide ores lie on the top of the Neves Formation or in contactwith felsic volcanics. Both formations are mineralized in certain partsof the mine and are considered in situ stockwork ores. The NevesFormation and the ores were dated Strunian (354.8 to 354 Ma) in ageby palynomorphs (Oliveira et al., 2004). The ores, felsic volcanic rocks,shale and minor quartzite form the Lower Autochthonous Volcano–Sedimentary Complex. The Phyllite–Quartzite Formation (dark shale,quartzite and limestone with conodonts) constitutes the lowerformation, of Late Famennian age (Oliveira et al., 1997, 2004). Someminor mafic volcanics, ranging from Late Famennian to Visean in age,are found on the Neves Corvo section and also widespread in theIberian Pyrite Belt. A stratigraphic study of the Neves Corvo mine area

tuguese Zone (SPZ) including their main domains: BAOC — Beja-Acebuches OphioliticSWP— Southwest Portugal Domain, VP— Variscan plutons, “x” — Location of the Neves

Fig. 2. Tectono-stratigraphic sequence of the Neves Corvo mine, from Oliveira et al.(2004). Explanation: Mt1–3 —Mértola formation beds, r— Brancanes Formation (tuffitesand grey siliceous shales with minor lens of chert, g — Godinho Formation (tuffites andgrey siliceous shales), bv — “borra de vinho” Formation (purple shales), s — GrandaçosFormation (grey siliceous shales and black shale with dispersed carbonate lens andnodules), r′ — Graça Formation (grey silicious shales abd black shale, rich in organicmatter and dispersed siliceous — phosphatic nodules), b1–2 — mafic and intermediatevolcanics, sm1–2 —massive sulfides, n— Neves Formation (black shales), jc— jasper andcarbonate unit, c — Corvo Formation (grey and black shales with intercalations ofcarbonate nodules and locally small clasts of tuffites), ca — limestone lens, v1–3 — felsicvolcanic rocks, PQ — Phyllite–Quartzite Formation (dark shales with intercalations ofthin-bedded silstones and quartzite).

356 A. Moura / Ore Geology Reviews 34 (2008) 354–368

(Oliveira et al., 2004) established a chronostratigraphic chart for theNeves Corvo mine (Fig. 2) and also concluded the existence of threestratigraphic hiatuses corresponding to Early/Middle Strunian, Tour-naisian and Early Visean. Moura (1994, 2003) estimated a thickvolcano–sedimentary cover of ∼11 km during orogenesis.

There are three main types of mineralization and several ore typesat Neves Corvo. Mineralization types are: 1 — massive sulfides [withcupriferous ores (MC), cupro-stanniferous (MS), Zn-rich ores (MZ) andcomplex ores (Cu–Zn–Pb ores, or MCZ)]; 2 — fissural or stockworkmineralization [cupriferous (FC), stanniferous (FT) and zinc ores (FZ)and 3 — rubané ore which is a banded mineralization above themassive ores, interpreted as remobilized parts of the primitivestockwork tectonically transported to the top of the massive sulfides[there are a copper (RC) and a tin rubané ore (RT)]. There are also sub-economic ores related to each of the three mineralization types,namely, massive pyrite (ME), barren stockwork (FE) and barren rubané(RE). Reserves (measured plus indicated) at the beginning of 2007were 19.4 Mt at 5.6% Cu plus 29.4 Mt at 6.2% Zn.

4. Sampling and analytical methodology

Thirty-three drillholes from all five orebodies were sampled inorder to have a representative sample suite for the entire lithologicalcolumn (Fig. 3, Table 1). The following formations were sampled (fromtop to bottom): Mértola Formation (Mt1 and Mt3), AllochthonousVolcano–Sedimentary Complex, jasper and carbonate unit (JC), rubanécopper ore (RC), rubané tin ore (RT), massive cupriferous ore (MC),barren massive sulfides, fissural or stockwork copper ore (FC),Autochthonous Volcano–Sedimentary Complex and Phyllite–Quart-zite Formation. The last two formations did not provide sufficientmaterial for fluid inclusion studies. Six quartz samples from the NevesCorvo main thrust, collected on three orebodies, were also studied.The samples from the RC and RT ores, and many fromMC and FC ores,were collected underground in the followingworking stopes: RTore—C704 2T01, C722 1T01, C722 1T01; RC ore — C730 1B03, C740 5R01,C743 5R 01, C752 5R 01;MC ore— C780 3B01, G844 GW05, N7891B02;FC ore— C700 7B01, C740 7B01, C745 7B01 and C610 7B01. Fig. 4 showsrepresentative samples of the RC and RT ores, JC unit, Mt3 formationand main thrust.

4.1. Fluid inclusion petrography

Studies of the types and petrography of fluid inclusionswere carriedout on wafers from representative samples of five stratigraphic levels:1—massive sulfide ores, 2— rubané ores, 3— fissural ores, 4— hanging-wall lithologies and 5 — main thrust. The succession of fluids andpossible mineralization events has been studied by interpreting therelationshipsbetweenfluid inclusions and thehostmineral. The texturalrelationship of the quartz and other fluid inclusion-bearing mineralswith the ore minerals was particularly important to the interpretation.

4.2. Microthermometry and laser Raman spectroscopy

Petrography of fluid inclusions has been carried out at thelaboratories of Centro de Geologia da Universidade do Porto on0.3 mm-thick doubly-polished wafers. Microthermometric measure-ments on the fluid inclusions were performed using a Chaixmecaheating-freezing stage (Poty et al., 1976) for cryometry, and a Linkam600 stage (Shepherd, 1981) for the heating studies. The accuracy was±0.1 °C during cryometry and ±1 °C during heating. Molar fractions ofCO2, CH4, N2 and H2S were determined in selected individualinclusions using a Dilor Labram Raman Spectrometer with a He–Nered laser at 20 mW and a radiation of 632.8 nm.

4.3. PIXE microanalysis

The predictable nature of proton trajectories, proton induced X-rayemission (PIXE) yields, X-ray absorption, high spatial resolution andsensitivity make the proton microprobe approach ideally suited to theanalysis of all elements heavier then sulfur. PIXE analysis was done bythe HIAF team at CSIRO (Australia) on previously selected fluidinclusions. The technique, developed at CSIRO, enables individual fluidinclusions as small as 5 µm in diameter, to be analyzed non-destructively. The technique uses a beam of 3 MeV protons and0.5 nA focused to a 1.3 µm spot-size, to penetrate the host mineral andexcite X-rays from elements within the fluid. Using a model of X-rayproduction within each inclusion, the result is a quantitative measureof the composition of the original trapped fluid. The technique is fullyexplained in Ryan et al. (1991, 2002) and can also be found on the website www.nmp.csiro.au.

Prior to analysis, the area of each fluid inclusion was carefullymeasured and the volumewas estimated assuming a proper geometrydepending on the inclusion morphology. The thickness of quartzbetween the wafer surface and the inclusion was measured bycalibrating the screw of the microscope stage.

http://www.nmp.csiro.au


5. Petrographic studies

Petrographic studies were carried out prior to the fluid inclusionstudy. The samples were investigated by hand specimen observation,by standard petrography under binocular microscope and transmittedand reflected light microscopes. Cathodoluminescence observationswere kindly performed by Jens Götze (TU Bergakademie, Freiburg,Germany) in selected thick sections, in order to check previoushypotheses about quartz genesis and deformation. Ore microscopyrevealed the followingmajorminerals: pyrite, chalcopyrite, sphalerite,tennantite–tetrahedrite, galena and cassiterite (this mineral only onthe stanniferous ores). Arsenopyrite, stannite, kësterite, and mawso-nite are accessory minerals. The gangue is composed by phyllosili-cates, quartz, carbonates, and occasional rutile. Pyrite andarsenopyrite were the first minerals to crystallize in the MC, FC andRC ores, and euhedral crystals of cassiterite the first to form in the RTore; coarse grained pyrite in this ore type is interpreted as formedduring metamorphism (Gaspar and Pinto, 1993).

Due to the scarcity of quartz in massive sulfide ores, only foursamples from three orebodies were investigated for fluid inclusions(Fig. 5A). The first sample, collected in the C780 area (Corvo orebody,mine level 780), was a massive copper ore with abundant quartz

Fig. 3. Neves Corvo map showing massive sulfide thick

intergrown with sedimentary bands of massive sulfide (mainly pyriteand chalcopyrite). In this sample it is possible to observe abundantlow-density, aqueous carbonic fluid inclusions in some intercrystal-line boundaries, but the majority lies inside the crystals with anapparent random distribution. In some mm-scale euhedral quartzgrains, it was possible to study aqueous carbonic fluid inclusions ingrowth zones that also contain chalcopyrite. The second sample wascollected in the Neves orebody, from ‘barren’ (b2% Cu) massive sulfidemineralization with marked bedding. The other two studied sampleswere collected from ‘barren’ mineralization within the Lombadororebody; all these samples were from a massive sulfide ore withoutvisible orientation.

The petrography of massive ores with abundant banded quartz(Corvo sample) shows that quartz is coeval with chalcopyritedeposition. In one sample it was possible to study fluid inclusionsalong growth zones of quartz, associated with chalcopyrite. Otherstudied fluid inclusion assemblages consisted of aqueous carbonicintercrystalline inclusions.

The petrographic study of quartz from non-massive sulfide samples(Fig. 5B–D) revealed the presence of three different quartz types (Q1,Q2 and Q3) under the microscope. The first (Q1) is a ‘dirty’ quartz,brown to grey color, due to the presence of numerous fluid inclusions.

ness and sample location (courtesy of SOMINCOR).

Table 1Sample locations (drillhole depth, metres) Bracketed number indicates number ofsamples (if more than one)

Orebody Borehole Hanging-wall

Main thrust JCunit

MC ore FC ore

Corvo SRC 744 272SE 20 A 181 547

Graça GR 859 82–104 (4)GR 877 111GR 840 90–94 (2)GR 885 4.8 116AG 150 48.2 52SJ 3 391SJ 17

Neves SRN 816 110–115 (3)AN 11 39.75 47–126 (4)AN46 105–142 (4)AN53 45SRN 848 60.5SAN 820 50RN 632 100–104 (2)RN 996 13–31 (2)NF 3 218SB 2 193NA 1 213–215 (2)

Lombador NM 34 1446–1504 (4)NF 32 A 1155–1158 (2)NG 28 1152NL 22 700NL 28 830 1226–1283 (4)NK 28 817–825 (2)NI 18 826–877 (5)NN 28 1349–1435 (3)SF 50 901

943Zambujal SM 10 468

SM 14 380SM 18 500–519 (2)SQ 14 213 400


These inclusions, which apparently are two-phase and with a degreeof filling (ratio between the liquid phase and the total volume ofthe inclusion; Flw) of 0.7 to 0.9, cannot be studied due to their smallsize (mostly b3 µm). This quartz is thought to be the first quartzgeneration because it is strongly deformed and is preferentiallyassociated with pyrite (which precedes chalcopyrite in the paragen-esis). This quartz exhibits core–mantle microstructures analogous tothose described by White (1976), strong deformation bands, sub-grain domains (many of them recovered to neo-grains) and intricategrain boundaries.

The second quartz type (Q2) is much clear than Q1 and containsabundant workable fluid inclusions. From a microstructural point ofview it has heterogeneous characteristics. Perhaps, the most typical isthe variation in translucency in the same crystal. Zones of clear quartzthat gradually change to darker areas can be observed. These can beinterpreted as recovery or as a syngenetic characteristic due todifferent rates of grain growth. Because recovery destroys almost all ofthe previously existing fluid inclusions, it is suggested that in this casethis is mostly a syngenetic characteristic. Other petrographiccharacteristics include undulose extinction, sub-granulation, defor-mation bands, and recovery in parts of the crystals. Most of theboundaries are straight; some, however, are lobate due to the effect ofrecrystallization. Sometimes, the quartz exhibits a clear band incontact with chalcopyrite. This band that intensely affects only onepart of the crystal is interpreted as a recrystallization feature(elimination of crystalline defects, particularly every visible fluidinclusion). It is not clear whether this recrystallization is due tochalcopyrite deposition, even though it is spatially associatedwith thissulfide, since many quartz–chalcopyrite boundaries do not show thisfeature. In the RT ore it was possible to observe, using cathodolumi-

nescence (Moura et al., 2003), that this quartz occasionally exhibitseuhedral growth zones. All fluid inclusions were studied in this type ofquartz.

The last quartz type (Q3) is a clear quartz generation devoid of fluidinclusions. It is interpreted as the last to form due to the absence ofany visible internal deformation and is also less abundant than theother types. Sometimes it fills pressure shadows around pyritecrystals. Their preferred arrangement, the lack of deformation, theexistence of styloliths in the samples, and their location often atboundaries of opaque phases, suggest genesis by pressure solution.

The petrography of fluid inclusions in ankerite veins from thehanging-wall jasper and carbonate unit was also carried out. Texturalrelationships suggest that the veins are earlier then the last quartzveins. The fluid inclusions are larger than those in quartz. The fact thatmost of them were destroyed (decrepitated) suggests that they are ofprimary origin. Also, fluid inclusions from one carbonate vein of therubané copper ore were analyzed. In general, it was difficult to studythe fluids in the carbonate because there are very few inclusions.

Studies were also conducted on fluid inclusions in cassiterite fromthe rubané tin ore. These inclusions are rare, isolated and identical tothe fluid inclusions found in quartz from the same samples. They wereinterpreted as being of probable primary origin (Moura, 2005).

6. Fluid inclusion study

The minerals used for fluid inclusion studies were quartz,cassiterite and carbonate (ankerite). Due to the scarcity of the fluidinclusions in minerals except quartz, the majority of the analyzedinclusions (N95%) were from quartz. As mentioned earlier, cassiteritewas obtained from the rubané tin ore and ankerite from the jasper andcarbonate unit. A summary of the sampled lithologies and fluidinclusion types is given in Table 2. The observed fluid inclusions couldbe separated in two main types: aqueous and aqueous carbonic.

6.1. Aqueous fluid inclusions

Low-salinity aqueous fluids (Lw) with an average 4 wt.% NaClequiv. are the only fluids found inmost of the hanging-wall lithologies.This observation supports the fact that many of the knownmetamorphic reactions of the area (Munhá, 1981, 1983) are dehydra-tion reactions and also the previous evidence of the predominance ofaqueous fluid inclusions on a regional scale (Guilhaumou et al., 1976;Germann et al., 2003). This suggests that the fluids circulating in theupper crust in this sector were mainly aqueous of low-salinitycharacter.

In quartz from one sample from the fissural Cu ore of Graçaorebody, it was possible to observe fluid inclusions with a Flw around0.90, isolated or associated with aqueous carbonic fluid inclusions,with first melting around −52° to −49 °C, low melting point (∼−25 °C,probably hydrohalite), temperature of melting ice between −5° and−7 °C, low homogenization temperatures (Th), and traces of CO2 andCH4 only detected by Raman spectroscopy. This fluid could becompared to the system H2O–NaCl–CaCl2±CO2±CH4 (with approx.5 wt.% equiv. NaCl plus 5 wt.% equiv. CaCl2) and suggests a genesis byfluid mixing between a low-salinity aqueous carbonic fluid with aH2O–NaCl–CaCl2 fluid. In one carbonate vein from the JC unit, it waspossible to study a H2O–CaCl2–(NaCl)–(MgCl2) fluid, occurring in cm-scale carbonate veins. Hypersaline aqueous fluids with daughterminerals (Fig. 6) were observed only in two samples, one from theCorvo massive copper ores and the other from the Lombadorstockwork copper ores.

6.2. Aqueous carbonic fluid inclusions

In all stratigraphic levels except the massive sulfide ores and in theNeves Corvomain thrust, we can observe aqueous carbonic fluids (H2O–

Fig. 4. Some of the studied samples in hand-specimens (except 4F): A. hanging-wall (Mértola Formation), B. Neves Corvo main thrust at borehole NA1 (213.5 m), C. jasper andcarbonate unit (borehole SAG 150 (52.20 m), D. rubané tin ore from Corvo orebody, E. rubané copper (RC) ore from sector n. 5 at Corvo orebody, F. microscopic view (transmitted light)of the RC ore with quartz and sulfides.


CO2–CH4–NaCl) that form liquid CO2 at room temperature (Fig. 7D).These fluids are termed Lc-w fluids because they are dominantly liquid-rich fluids (L), which form liquid CO2. There are also fluids with less-dense CO2,whichonly formed clathrates upon cooling; these are termedLw-c fluids. The two fluid types were found in all of the ores and

hanging-wall lithologies but never in the main thrust. Lc-w fluids werealso not observed in the massive sulfide. Still less-dense, gas-bearingaqueous fluids were occasionally found associated with the aqueouscarbonic fluids. In this case the gas content is only detected by laserRaman spectroscopy.

Fig. 5.Massive copper ore (MC) and fissural copper (stockwork) ores (FC). A. MC ore from stope C 780 3B 01 (Corvo orebody), B. FC ore from sector 7B at Corvo orebody, C. FC ore fromLombador orebody, D. FC ore from Neves orebody. Scales are cm.


In most cases, the aqueous carbonic fluids are similar incomposition, with the exception of the rubané tin ore, in which CH4

is practically absent. N2 is rarely found in the Neves Corvo fluids and

Table 2Summary of the studied lithologies and fluid types

Studied lithologies Fluid types Fluid type characteristics

1. Hanging-wall Lw, Lw-c, Lc-w Lc-w: H2O–CO2–CH4–(N2)Fluid inclusions that formliquid CO2 on cooling

2. Quartz from the Main Thrust Lw Lw-c: H2O–CO2–CH4–(N2)Fluid inclusions that neverform liquid CO2 on cooling

3. Rubané copper ore Lc-w, Lw-c, Lw,Lw2

Lw: H2O–NaCl low-salinity fluid

4. Rubané tin ore Lc-w, Lw-c, Lw Lw2: H2O–NaCl–CaCl2 fluid5. Massive sulfide copper ore Lw-c, Lw Lw+s: H2O–NaCl high-salinity fluid6. Massive sulfide barrenmineralization

Lw, Lw+s

7. Stockwork copper ore Lc-w, Lw-c, Lw,Lw+s

In bold — most frequent fluid type; italicized — rare fluid type.

H2S was never detected. A summary of the microthermometric data isgiven in Tables 3 and 4. Raman data and bulk fluid composition isgiven in Tables 5 and 6.

Fig. 6. High-dense fluid inclusion with halite daugther mineral from quartz withinmassive sulfides from drill hole NL 28 (Lombador orebody).

Fig. 7. Aqueous carbonic fluid inclusions associated with the fissural copper ore. Cpy — chalcopyrite. Image A — low magnification and cross nicols; B and C — medium and highermagnification; D — two aqueous carbonic fluid inclusions with liquid CO2 at room temperature.


6.3. Fluids from the massive sulfide ores

It is possible to observe three fluid inclusion types in the massivesulfide ores: (1) aqueous low-salinity fluids in groups, isolated or intransgranular trails; (2) aqueous carbonic fluidswith a lowgas content[up to 5mol% CO2 and 6.5mol% CH4 (Fig. 8), and very small amounts ofN2 and H2S in a few inclusions]; and (3) hypersaline aqueous fluids(located in randomly oriented fluid inclusions found in only onesample). The relative abundance of these three inclusion types is ca.90, b10 and 1%, respectively. Some aqueous carbonic fluid inclusionsare found along quartz growth zones with chalcopyrite. They aretexturally intergrown and can be observed under the reflected andtransmitted light suggesting that the minerals are coeval.

Homogenization temperatures (Th; Fig. 9) lie in the interval 140 to370 °C, with two modes, one around 160 to 170 °C and the other

Table 3Summary of the microthermometric characteristics for the studied fluid inclusions (hangin

FI type

Hanging-wall lithologies Mértola Formation and Volcano–Sedimentary Complex Lc-w

Lw-cLw

Jasper and carbonate unit Lc-w

Lw-cLw

FI in carbonate Lw2Neves Corvo main thrust Lw

FI — fluid inclusion; Flw — ratio between the liquid phase and the total volume of the inclusphase; TmI — melting of the last ice crystal; TmC — melting of clathrates; ThCO2 — homhomogenization in vapour (V), liquid (L) or critical (F).

between 220 and 280 °C. The salinities determined from the meltingpoint of the last ice crystal are confined to a narrow interval between 3and 6 wt.% NaCl equiv., except for inclusions from one sample, whichcontained a cubic daughter-mineral, most probably halite. For theseinclusions the salinity values obtained with the equation of Sterner etal. (1988) is 30 wt.% NaCl equiv.

6.4. Fluid inclusions from the rubané and fissural ores

Themost important characteristic of thesefluid inclusion types is theircomposition: they are H2O–CO2–CH4–NaCl fluids that frequently formthree-phase H2O (liquid)+CO2 (liquid)+CO2 (gaseous) fluid inclusions atroom temperature. These inclusions, which have a fillingwater ratio of ca.0.8 are often located a few µm away from chalcopyrite (Fig. 7). They arenormally distributed in small clusters and are sometimes isolated.

g-wall lithologies and main thrust)

n Typical Flw TmCO2 TmI TmC ThCO2 Th

13 0.80 −57.1/−57.8 8.7/10.2 26.0/29.5 V 220/2503 0.80 22.5/27.5 L11 0.80 −4.2/−0.9 8.4/9.0 227/24978 0.85 −4.1/−0.3 120/31038 0.80 −58.2/−57.4 8.0/11.1 26.0/29.4 V 240/27010 0.80 26.2/29.0 L4 0.80 23.2/23.4 F6 0.80 9.0/10.2 260/294

21 0.90 −2.9/−0.3 170/1845 0.90 −24.8/−24.5 102/108

144 0.90 −4.7/−0.9 142/238

ion. The following temperatures (°C) were recorded; TmCO2 — melting of the CO2-richogenization of the gas phase and Th — final homogenization. L, V, F — mode of CO2

Table 4Summary of the microthermometric characteristics for the studied fluid inclusions (ores and massive sulfide barren mineralization)

FI type n Typical Flw TmCO2 TmI TmC ThCO2 Th

Rubané copper ore (RC) FI in carbonate Lc-w 29 0.80 −58.2/−57.3 8.5/9.4 22.5/29.4 V 228/330Lw-c 15 0.80 −4.6/−0.5 6.4/9.2 244/265Lw 12 0.90 170/200Lw2 8 0.90 −17.0/−15.0 100/120

Rubané tin ore (RT) FI in cassiterite Lc-w 13 0.80 −57.7/−57.2 9.1/9.8 27.5/30.5 L 244/250Lc-w 22 0.80 −58.1/−56.6 7.6/8.7 26.7/29.3 L 215/245Lw-c 11 0.80 −4.7/−3.0 8.0/8.2 217/248Lw 24 0.90 −4.9/−1.2 158/190

Fissural copper ore (FC) Lc-w 349 0.80 −64.0/−56.9 8.0/14.4 −10.5/26.5 L 210/359Lc-w 58 0.80 −62.8/−57.6 8.0/12.4 2.4/27.4 V 225/347Vc-w 5 0.40 360/390Lw-c 16 0.75 −4.5/−4.4 8.0/12.8 266/336Lw 195 0.90 −4.7/−1.3 154/3581Lw+s 5 0.90 145/1522Lw2 5 0.90 −7.0/−5.0 175

Massive sulfide copper ore Lw-c 8 0.90 −3.8/−1.8 5.8/12.5 239/247Lw 104 0.90 −4.8/−1.7 155/374

Barren mineralization Lw 58 0.90 −3.2/−1.4 140/3103Lw+s 6 0.90 165/168Number of fluid inclusions: 1276

Flw— ratio between the liquid phase and the total volume of the inclusion. The following temperatures (°C) were recorded: TmCO2—melting of the CO2-rich phase; TmI—melting ofthe last ice crystal; TmC—melting of clathrates; ThCO2— homogenization of the gas phase and Th— final homogenization. L, V, F—mode of CO2 homogenization in vapour (V), liquid(L) or critical (F). 1Lw+s— group of five three phase (L+V+halite) FI found on one sample from borehole NG 28 (Lombador orebody). On heating first disappears the vapor phase (atT=103/122 °C) followed by liquid homogenization between 148 and 152 °C. 2Lw2 — small group of FI observed in one quartz crystal from the stockwork of Graça orebody; Thcorrespond to onemeasurement. 3Lw+s— small group of three phase FI found on one sample from drill hole NL 28 (Lombador orebody). The vapor phase disappear between 95° and104 °C.


Another type of inclusions (Lw-c type) comprises a less-dense aqueouscarbonic fluid, which only form clathrates on freezing. Fluid inclusionsthat resembleH2O (gas-free)fluids are occasionally found associatedwiththe aqueous carbonic fluids; in these inclusions; CO2 is only detectedduring Raman spectroscopy. A population of low-salinity fluid inclusionsdistributedalong transgranular trails, and therefore interpretedof beingofsecondary origin, is normally present in all samples.

6.5. Fluids from the hanging-wall lithologies

The majority of the fluid inclusions belong to the Lw type. Their Thvalues lie in the interval between 120 °C and 310 °C with strong modebetween 170 and 190 °C. The salinity of these fluids is between 0.6 and6.6 wt.% NaCl equiv., with mode between 2.6 and 3.2 wt.% NaCl equiv.It was possible to find aqueous carbonic fluid inclusions in only two ofthe eleven samples studied from the Mértola Formation and theVolcano–Sedimentary Complex away from the ores. These fluids are

Table 5Raman data and bulk compositions for selected fluid inclusions from Neves Corvo (hanging

Fluid type FI reference Raman

CO2

Hanging-wall lithologiesMértola Formation Lc-w 820-1-4 97.50

Lc-w 820-1B 97.00Jasper and carbonate unit Lc-w 150-5-3 98.5

Rubané copper oreLc-w 1rc1-1 94.76Lc-w 1rc1-2 94.25Lc-w 3rc2-2 100.00Lc-w 3rc4-1 96.94

Rubané tin oreLc-w 3rt2-1 100.00Lc-w 3rt2-2 98.47Lc-w cs1 100.00Lc-w cs2 100.00

nd — not detected. (⁎) values determined using the software of Bakker (2003).

similar to the fluids found in the rubané and fissural copper ores.Compositional ranges from six representative fluid inclusions are: 90to 92 mol% H2O, 5 to 9 mol% CO2, 0.17 to 0.05 mol% CH4: and 0.01 to1.60 mol% for both Na+ and Cl−.

Fluids from the jasper and carbonate units were studied in samplesfrom three drill holes. Here, the quartz exhibits abundant aqueouscarbonic fluids and also aqueous low-salinity fluids (the latterdominantly in trails). The aqueous carbonic fluids have compositionsvery similar to those on the other hanging-wall lithologies. MeasuredThwas between 220 and 290 °C. The salinities of the aqueousfluids arelow (between 0.5 and 4.8 wt.% NaCl equiv.). It was possible to studyfluid inclusions in one carbonate sample. The fluid inclusions are largerthan those entrapped in quartz, show last ice melting temperaturesbetween −24.5 and −24.8 °C and Th between 102 and 108 °C. Thesefluids probablycarried bivalent ions (Ca2+,Mg2+ and Fe2+). Ifwe assumethis fluid as belonging to the H2O–CaCl2 system, then the compositionwould be 87 mol% H2O, 8.7 mol% Cl− and 4.3 mol% Ca2+.

-wall lithologies and rubané ores)

data CH4 Bulk composition (⁎)

CH4 N2 H2O CO2 CH4 N2 NaCl

2.50 nd 89.52 9.01 1.07 nd 0.643.00 nd 92.60 7.35 0.04 nd 0.001.50 nd 94.05 5.86 0.05 nd 0.02

5.23 nd 94.23 4.37 0.20 nd 0.605.75 nd 92.98 4.88 0.14 nd 1.00nd nd 92.28 5.81 nd nd 0.951.14 1.92 92.06 5.86 0.04 0.06 0.99

nd nd 88.23 9.19 nd nd 1.291.53 nd 93.19 4.83 0.03 nd 0.01nd nd 90.69 9.29 nd nd 0.00nd nd 90.54 8.73 nd nd 0.37

Table 6Raman data and bulk compositions for selected fluid inclusions from Neves Corvo (massive sulfide Cu ore and fissural Cu ore)

Fluid type FI reference Raman data Bulk composition

CO2 CH4 N2 H2O CO2 CH4 N2 NaCl

Massive sulfide copper ore Lw-c R1 86.60 13.40 nd 93.91 2.98 0.13 nd 1.49Lw-c R3 96.40 3.50 nd 93.44 3.72 0.04 nd 1.40Lw-c R5 37.10 62.90 nd 91.14 3.31 3.52 nd 1.01Lw-c mc-cz 80.00 20.00 nd 90.40 6.89 1.19 nd 0.76Lw-c 24–71 21.60 66.50 11.81 91.68 2.00 3.59 0.63 1.05Lw-c 24–73 32.80 67.20 nd 93.63 1.96 1.67 nd 1.37

Fissural copper ore Lw-c 1fc1-1 59.59 40.41 nd 93.26 3.16 0.86 nd 1.36Lc-w 1fc31 68.94 29.61 1.45 93.21 4.19 0.57 0.03 1.00Lc-w 4fc2-1 93.36 6.64 nd 89.19 8.47 0.42 nd 0.96Lc-w G1 97.21 2.78 nd 87.14 11.07 0.25 nd 0.77Lc-w G2 74.73 25.26 nd 78.52 14.48 4.42 nd 1.34Lc-w G5 63.27 36.72 nd 78.42 12.20 6.22 nd 1.58Lc-w N1 78.09 21.90 nd 92.63 5.32 0.87 nd 0.59Lc-w N2 89.19 10.81 nd 85.73 10.30 1.01 nd 1.43Lc-w L1 91.09 8.90 nd 90.98 7.76 0.52 nd 0.37Lc-w L3 90.05 9.94 nd 88.00 10.38 0.92 nd 0.35Lc-w Z1 82.67 17.33 nd 88.19 8.59 1.34 nd 0.94Lc-w Z2 86.29 13.70 nd 83.88 11.78 1.56 nd 1.39

nd — not detected. (⁎) values determined using the software of Bakker (2003).


6.6. Fluids from the Neves Corvo main thrust

Six samples were also studied from the Neves Corvo main thrust(Fig. 4B). Here, only aqueous low-salinity fluids were found. Theirsalinity values are between 2.6 and 7.4 wt.% NaCl equiv. (mode from2.6 to 3.4 °C). Th values for these fluids are between 140 and 230 °C,with a strong mode around 170 °C. Fluid densities are between 0.85and 0.96 g cm−3.

6.7. Complementary studies for fluid characterization

In order to constrain the temperature of geological events relatedwith orogenesis, studies on chlorite geochemistry, illite crystallinityand vitrinite reflectance were carried out. Chlorite geothermometricdata are not reliable because of the local diversity in lithology (Zaneand Sassi, 1998). Both illite and chlorite crystallinity measurementswere performed on 25 samples from the hanging-wall, footwall, mainthrust, stockwork and rubané copper ores. Illite crystallinity data areexpressed by the Kubler index (referred to the Bragg angle, Δ° 2Θ)which is obtainedmeasuring the full width at half maximum intensityof the first, 10 Å, X-ray powder-diffraction peak of dioctahedral illite–muscovite, as measured on the b2 μm size-fraction of air-driedsamples using Cu-Kα radiation. The illite crystallinity data range from0.19 to 0.35 Δ° 2Θ that correlates with temperatures from b250 to360 °C (Moura et al., 2001; Moura, 2003; Moura and Rocha, 2003).However, as some samples contain paragonite or illite/paragonitemixed minerals, the data are to be used with caution.

Published results on geothermometry based on vitrinite reflec-tance (Fernandes et al., 1997; McCormack, 1998) gave a temperaturefor Neves Corvo samples between 289 and 346 °C, which is inagreement with fluid inclusion data.

Thirty-five fluid inclusions in quartz were selected for analysis byproton induced X-ray emission (PIXE) in order to quantify their metalcontent. The inclusions were selected from the following stratigraphiclevels: hanging-wall (n=5), main thrust (6), rubané tin ore (5), rubanécopper ore (3), massive sulfide ore (6), stockwork copper ore (10). Onlyten inclusions (three from the hanging-wall lithologies, two from theRC ore, three from the MC ore, and two from the stockwork ore)presented conditions that allowed the determination of elementconcentrations (K, Ca, Cr, Mn, Fe, Cu, Zn, As and Sn). The main resultswere as follows: all the inclusions contain K, Ca, Fe and Cu; the valuesfor K (between 1225±181 and 9155±378 ppm) and Ca (802±367 to3723±515 ppm) are consistent with those expected for low-salinity

fluids. The Cu content of the fluid inclusions from the mineralizedzones gave values (average of 827±151 ppm), much higher than thosefrom samples of the non-mineralized areas (two analyses are belowthe detection limit and one sample gives a value of 263±80 ppm).Minimum detection limits (ppm) lies in the ranges 285 to 1735 for K,184 to 1066 for Ca, 143 to 534 for Cr, 77 to 498 for Mn, 69 to 398 for Fe,88 to 159 ppm for Cu, 83 to 502 for Zn, 80 to 404 for As and 713 to4,489 for Br. Errors (ppm) lie between 181 and 1204 for K, 163 and 790for Ca, 88 and 275 for Cr, 39 and 265 for Mn, 37 and 411 for Fe, 35 and261 for Cu, 33 and 244 for Zn, 30 and 204 for As and 199 to 1390 for Br.

7. Discussion and conclusions

7.1. Fluids coeval with massive sulfide formation

The abundance of H2O-rich gas-free fluid inclusions suggests thatthis was the most important fluid responsible for ore deposition.Based on the position of these inclusions as clusters in the cores of thecrystals and as pseudosecondary arrays, they are probably of primaryorigin (Moura, 2005). Their salinity is similar to that of seawater,concordant with the hypothesis that these fluids were essentiallymodified seawater that convectively percolated the basement. Theparticipation of low-density aqueous carbonic fluids is thought to be alocal variation of the fluid chemistry, mainly due to exchange betweencirculating seawater and organic-rich matter in metasedimentaryrocks underlying the deposit. Since CO2 favors quartz precipitation,the incorporation of this gas in the percolating fluid could beenvisaged as a local mechanism favoring deposition of phases fromthe hydrothermal system (Shettel, 1974). The origin of CO2 and CH4

from a supposed magmatic chamber underneath the deposit is lessprobable considering the abundance of black shales in the footwalland also the similar proportion of both gases in the fluid inclusions. Infact, CO2 is much more abundant than CH4 in magmatic fluids(Charlou et al., 2000) and both gases are generated in similar amountsby reaction within organic matter-rich metasedimentary rocks (Lilleyet al., 1993).

The pressure of fluid entrapment is constrained by the isochores ofthe fluid inclusions, as the Th corresponds to the lowest possibletemperature of its entrapment. The pressure corresponding to thehighest Th on this near-seafloor environment, gives a lower boundaryaround a seawater depth of 2000 m. However, since these fluids havesome gas content, it is expected that the water column could havebeen higher, even up to 4000 m. The study of more aqueous carbonic

Fig. 8. Raman spectrum for selected fluid inclusions from the massive copper ore. Scales are 10 µm.


fluid inclusions from the massive sulfides will allow the depth ofmassive sulfide formation to be constrainedwith greater accuracy. Thefluid with halite is interpreted as brine resulting from seawater phaseseparation in the supracritical domain; alternatively, it could haveresulted from immiscibility of a parent magmatic fluid liberated indepth. However, due to the rare presence of this fluid, it is likely that itrepresents a local phenomenon in the Neves Corvo hydrothermalsystem.

PIXE measurements of element concentration inside individualfluid inclusions gave values for Cu identical to those obtained byVanko et al. (2001) in fluid inclusions from the Mathematician Ridgeand the Oceanographer Transformer. However in the present case theCa and K values are one order of magnitude lower which denotes amuch higher Cu:Ca and Cu:K ratio in the fluid.

The syngenetic fluids are analogous to fluids found in otherorebodies from the Iberian Pyrite Belt, e.g., Aznalcollar and Los Frailesorebodies (Toscano et al., 1997a; Almodovar et al., 1998), Salgadinhoorebody (Inverno et al., 2000), Lagoa Salgada orebody (Jaques and

Noronha, 2002), Corta Atalaya, RioTinto orebodies (Nehlig et al.,1998),Masa Valverde oredody (Toscano et al., 1997b; Sánchez-España et al.,2000) and in VHMS deposits from other parts of the world (Ripley andOhmoto, 1977; Spooner, 1981; Costa et al., 1983; Pisutha-Arnond andOhmoto, 1983; Broman, 1987; Bajwah, 1997; Zaw et al., 2003). Thesefluids are also similar to most modern analogues presently forming inthe seafloor (LeBel and Oudin, 1982; Delaney et al., 1987, Peter andScott, 1988; Ramboz et al., 1988; Halbach et al., 1989; Vanko et al.,1991; Peter et al., 1994; Petersen et al., 1998; Tivey et al., 1998; Lécuyeret al., 1999; Luders and Banks, 2001 and Luders et al., 2001). Thesecomparisons can be clearly seen in Figs. 10 and 11.

7.2. Compositions, temperatures, pressures and depth of fluidentrapment during orogeny

About 20 million years after the deposition of the 300 Mt ofmassive sulfides (Fig. 12), the Neves Corvo deposit area underwentorogenic metamorphism. A thick pile of metasedimentary and

Fig. 9. Homogenization temperatures histograms for the studied fluid inclusions.

Fig. 10. Salinity and temperature data for fluid inclusions found in VMS depositspresently forming on the Sea Floor, and a comparisonwith the fluids found in the NevesCorvo massive ores. EPR — East Pacific Rise; TAG — Trans-Atlantic Geotraverse; KFZ —

Kane Fracture Zone. Data taken from LeBel and Oudin, 1982; Delaney et al., 1987; Peterand Scott, 1988; Ramboz et al., 1988; Halbach et al., 1989; Vanko et al., 1991; Peter et al.,1994; Petersen et al., 1998; Tivey et al., 1998; Lécuyer et al., 1999; Luders and Banks,2001 and Luders et al., 2001. ⁎ One sample with fluid with 30 wt.% NaCl equiv.


metavolcanic rocks buried the ores to a depth of ca. 11 km (Moura,1994). This depth was estimated from the isochores and temperatureof homogenization of the aqueous carbonic fluids in the rangebetween 210 and 390 °C, with 95% of the values located in the 210to 330 °C range. Pressures corresponding to the highest Th arebetween 280 and 320 MPa, indicating a depth of entrapment ofbetween 10 and 13 km, assuming lithostatic pressure. The tempera-tures obtained from illite crystallinity measurements and the vitrinitereflectances of dispersed organic matter agree very well with the fluidinclusion data.

As stated above, fluids from the hanging-wall lithologies areseparable into two main types: aqueous low-salinity fluids andaqueous carbonic fluids. Occasionally, fluids of higher salinity thatcould contain bivalent ions could be observed. These fluids are notrare in orogenic environments at very low to lowmetamorphic grades.For example, Guilhaumou et al. (1976) observed fluids with salinitiesup to 32 wt.% NaCl equiv., which they interpreted as resulting fromimmiscibility due to a pressure drop. In addition, Wipfler and Sedler

(1995) refer the existence of hypersaline fluids (also up to 32wt.% NaClequiv.) in association with vein copper mineralization. These fluidscould have been formed by reactions that involved Ca minerals, suchas either:

3chloriteþ 12quartzþ 10calcite ¼ 3actinolite þ 2epidote þ 10CO2 þ 8H2O

5prehniteþ chloriteþ 4ferricepidote þ 2quartz¼ 8epidoteþ actinolite þ 6H2O

potassicfeldspar þ oligoclaseþ 1=2H2O þ 3; 5Hþ

¼ epidoteþ muscovite þ 11quartz þ 4Naþ

Pressure variation is a common phenomena promoting mineralprecipitation, due to decrease of solubility of the metal species insolution. The observation of variations on the pressure at the Thstrongly suggests that there must have been pressure oscillationsduring precipitation of quartz and oreminerals. These variations couldhave been promoted by the seismic activity of the subduction zone(bounding the South Portuguese Zone) that must have existed few kmto the north.

One possible explanation for the absence of CO2 and other gases inthe fluids from the Neves Corvo main thrust is that rapid percolation(and entrapment) of the fluids prevents wall rock exchange and,consequently, the assimilation of CO2 and CH4 even from the locallyabundant black shales.

7.3. Causes for deposition of the rubané and fissural ores

The rubané copper and rubané tin ores contain abundant aqueouscarbonic fluids in both quartz and cassiterite. These ores, which arelocated above the massive ores, display extensive evidence of strongdeformation (e.g., isoclinal folds and cataclastic fluxes). All evidencesupports the hypothesis that they were formed during orogenesisresulting from strong remobilization of themetallic elements from theunderlying ores. By analogy with examples found by others (Gilliganand Marshal, 1987; Pohl, 1992; Marshall and Gilligan, 1993; Marshallet al., 1999, 2000; Cartwright and Oliver, 2000) these ores have been

Fig. 11. Salinity and temperature data for fluid inclusions found on other IPB deposits, and a comparisonwith the fluids found on the Neves Corvomassive ores. A.T.E.— Aguas TenidasEast orebody. Data from Toscano et al., 1997a,b; Almodovar et al., 1998; Nehlig et al., 1998; Inverno et al., 2000; Sánchez-España et al., 2000 and Jaques and Noronha, 2002. ⁎ Onesample with fluid with 30 wt.% NaCl equiv.


recently considered metamorphogenic ores (Moura, 2008). Thefissural copper ore, which is a stockwork ore, has also been affectedby metamorphism. The copper content of this ore type was notdisplaced a great distance. This ore remained below the massivesulfides; however, chalcopyrite was remobilized inside the ore. Thisexplains why the sulfide content of this ore is in most cases almostpure chalcopyritewith only veryminor pyrite and other sulfides. Thus,the FC ore could be considered a metamorphosed ore.

The deposition of quartz and ore minerals was probably aconsequence of: (1) pressure drop; (2) mixing of CO2-rich with H2O-rich fluids; and (3) decreased temperature of the hydrothermalsystem. As the orogenic event evolved, the pressure gradient musthave changed from near-lithostatic to hydrostatic. Low-salinity water-rich fluids of probable meteoric provenance are found widespread intransgranular trails in all samples. Their Th is around 170 to 180 °C andthey were presumably entrapped at a relatively shallow depth duringuplift and exhumation.

Although syn-metamorphic remobilization of copper ore fromhuge Cu-rich VHMS deposits has been documented by others (e.g.,Lincoln, 1981; Swager, 1985; De Roo, 1989; Mauk et al., 1992; Alonso-Azcarate et al., 1999), this study is the first to document the presenceof both types of fluids in the same VHMS deposit from the IPB.

Fig. 12. Proposed P–T–t path for the Neves Corvo deposit. t1 — massive sulfideformation (Oliveira et al., 2004); t2 — beginning of burial (Visean-342 to 325 Ma); t3 —

burial, diagenesis, tectogenesis and beginning of metamorphism; t4 — progrademetamorphism and beginning of exhumation (age data from Priem et al., 1978); t5—retrograde metamorphim, cooling, last fluid circulation and exhumation; t6 —

Exhumation, cooling and cratonization.

The Neves Corvo deposit was formed on or below the ocean floor atabout 354 Ma, by the deposition of a huge amount of sulfidesprecipitated from a hydrothermal fluid that must have percolatedthrough the surrounding rocks, possibly in convective cells heated bya probable source at depth. The question about the ultimate origin ofthe ore-forming fluid remains controversial. Hennigh (1996) whostudied the tin ores concluded that “a granitic source is unlikely”.Inverno et al. (2007), however, in a recent study of the Lombadororebody proposed that a major component of the ore-forming fluidwas derived from a granitic pluton.

About 20 to 30 million years after formation, the deposit wastectonized and percolated by meta-hydrothermal fluids generatedduring the orogenic metamorphism. This promoted the formation ofnew ores above the massive sulfides; the stockwork was affected to alesser extent. Remobilization inside the massive sulphide massescould have occurred, but the evidence is ambiguous due to theprimary hydrothermal reworking. The fluids that formed the massivecopper ores are considered to be volcanogenic fluids, while those thatformed the rubané ores are metamorphic fluids.

Acknowledgements

I wish to thank SOMINCOR for permission to sampling and theNeves Corvo geologist for many interesting discussions about thegeology of the deposit. Jens Götze (Department of Mineralogy, TUBergakademie, Freiberg, Germany) is thanked for cathodolumines-cence observations on the Neves Corvo samples. Prof. Fernando Rocha(Aveiro University, Portugal) is thanked for X-ray diffraction measure-ments on selected samples. M.C. Boiron (Nancy University, France),Alexandra Guedes and Armanda Dória (both from CGUP, Portugal) arethanked for helping with the Raman spectroscopy on selected fluidinclusions. Helpful reviews by Professor Frederico Sodré Borges(CGUP, Portugal), David Vanko (Tucson University, USA) and CarlosInverno (INETI, Portugal) are also acknowledged.

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