Geological constraints on massive sulphide genesis in the Iberian Pyrite Belt

ORE GEOLOGY REVIEWS ELSEVIER

Ore Geology Reviews 11 (1996)429-451

Geological constraints on massive sulphide genesis in the Iberian Pyrite Belt R. S~iez, G.R. Almod6var 1, E. Pascual Departamento Geologla, Universidad Huelva, 21819 La Rdbida, Huelva, Spain

Received 7 February 1995; accepted 23 August 1996

Abstract

The Iberian Pyrite Belt (IPB), SW Iberian Peninsula, Spain and Portugal, one of the most famous and oldest mining districts in the world, includes several major concentrations of massive sulphides, unique on Earth (e.g., Riotinto, Neves Corvo), as well as a large number of smaller deposits of this same type. All these deposits, in spite of their general similarities, show significant differences in geological setting, age, relations to country rocks, hydrothermal alteration, mineralogy and geochemistry. As a consequence of a review of the available data in the IPB, together with new findings on volcanism, hydrothermal alteration and ore mineralogy, we propose a modified genetic scenario, that can account particularly for the diversity of the geological situations in which sulphide deposits occur, as well as for their mineralogical and petrological diversity. It is concluded that there is no direct genetic relationship between felsic volcanic activity and massive sulphide deposition in the IPB, and that most of the massive sulphide bodies, including all of the giant ones, are closely related to hydrothermal vents, being therefore proximal. The available isotopic data yield additional genetic information: (a) Homogeneous lead isotope values indicate a single (or homogenized) metal source; (b) sea and cormate water are the fluid reservoirs for hydrothermal input, and (c) bacterial reduction of sulphur is the most probable cause of differences in ~34S between stockwork and massive sulphide mineralizations. Finally, current geodynamic models suggested for the IPB are discussed. It is suggested that an intracontinental, ensialic rift or pull-apart environment is the most probable genetic environment for the IPB mineralizations.

1. I n t r o d u c t i o n

The Iberian Pyrite Belt (IPB) is one of the oldest mining districts in the world (Pinedo Vara, 1963). It is characterized by giant and supergiant massive sulphide deposits, including Riotinto, Neves Corvo, Aljustrel, Tharsis, La Zarza, Aznalc611ar, Sotiel, Masa Valverde and other whose total reserves ex-

1 G.R. Almod6var also appears in the references as G. Ruiz de Almod6var.

ceed 1400 million tons. The uniqueness of the region is apparent, in view of the size and abundance of massive sulphide deposits, but especially if the tonnage and number of deposits are compared with the total surface of the district (Leistel et al., 1994). The IPB occupies the southwestern corner of the Iberian Peninsula, extending from SeviUa, in Spain, to the Atlantic Ocean, south of Lisboa, in Portugal, making up a belt of about 230 km in length and 40 km in width (Fig. 1). Massive sulphides are currently mined in Neves Corvo (Cu, Sn) in Portugal and Sotiel-Coronada (S,

0169-1368/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0169-1368(96)00012-1

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R. S{tez et a L / Ore Geology Reviews 11 (1996) 429-451

~

l

o ~::::::::~/~ u

t

o

n

i

PQGroup~._~ ~]

c

~

Chan~ Group

Pulo do Lobo Group ~

'~L ~: L ~ : : : : °

rocks ~ , ~ o °°

CulmGroup VS Complex

~,,,~m

"~ ~

Post-Palaeozoic

I. . . . I cover

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Cu, Pb, Zn) and Aznalc611ar-Los Frailes (S, Cu, Pb, Zn, Ag) in Spain. Au and Ag from oxidized ores are mined in Riotinto and Tharsis-Fil6n Sur. Despite the current decline in mining activity, research interest has remained, since some recent findings (Aguas Tefiidas, Masa Valverde, Lagoa Salgada, Las Cruces) have shown that regional mining potential is still significant. It is difficult to review briefly the enormous volume of geological or mining information published on the IPB. We address the reader to the most thorough and the references therein (Strauss, 1965; Schermerhorn, 1971; Carvalho et al., 1976; Strauss et al., 1977; Routhier et al., 1978; Barriga and Carvalho, 1983; Barriga, 1990; S~ez and Almod6var, 1993; Leistel et al., 1994). Nevertheless, many unsolved- or unprecisely known-questions remain in the region, as it is the case with tectonics and magmatism. This is clearly indicated by the recent interest that both questions have attracted, as well as by the innovative interpretations claimed in these cases and in the origin of massive sulphides (see, for instance, Boulter, 1993a, for a new interpretation of magmatism, or Barriga, 1983 and Barriga and Fyfe, 1988, concerning mineralizations). All these papers question classical geologic and metallogenic views (e.g., L~colle, 1977; Routhier et al., 1978; Carvalho, 1979). To make additional revisions seems therefore necessary, and consequently we present here a comprehensive summary of current ideas about geology

and ore deposits in the IPB. More specifically, we show that the geological and geochemical data now available lead to a modified genetic model for ore deposits at a regional scale.

2. Geologic framework The IPB is a part of the South Portuguese Zone (SPZ) of the Hercynian Iberian Massif (Julivert et al., 1974), which is now interpreted as a tectostratigraphic terrane sutured to the Iberian Massif during the Middle Carboniferous (Quesada, 1991). The sedimentary record of the IPB consists of Devonian and Carboniferous rocks whose most conspicuous features are the intense Dinantian magmatic activity and the abundance of huge massive sulphide deposits. Although numerous local stratigraphic sequences have been proposed (e.g., Van den Boogaard, 1967; Strauss, 1970; Oliveira, 1990), the most useful regional stratigraphic nomenclature was proposed by Schermerhorn and Stanton (1969) and Schermerhorn (1971) and used with minor modifications by many later researchers (Carvalho et al., 1976; Carvalho, 1979; Oliveira, 1983, 1990; Barriga, 1990; S~iez and Almod6var, 1993; Leistel et al., 1994). It consists of three main units: the Phyllite-Quartzite group (PQ), the Volcanic-Siliceous complex (VSC) and the Culm group (Fig. 2). In turn, these units have been used to make a number of subdivisions, especially concern-

R. S6ez et aL/ Ore Geology Reviews 11 (1996) 429-451

431

Shales and litharenites Basal Shaly Sedes

VA3 Flows, epiclastites & conglomerates VA2

Purple shales Basic flows / Shales and epiclestites Fe and Mn jaspers Lavas, breccias & tufts Shales and tuffites Basic flows

'5

Black shales, shales and tuffites

±

VA1

Rhyolitic flows and tufts Basic subvolcanic rocks (sills) Shales, quartzites, conglomerates and litharenites with limestones lenses

Fig. 2. Regional lithostratigraphic sequence of the IPB. Massive sulphides are not included. See Fig. 4 for the stratigraphic position of massive sulphideand manganesedeposits.

ing the VSC. Some authors (L6colle, 1977; Routhier et al., 1978) have performed correlations between these local subdivisions. The Phyllite-Quartzite group comprises the oldest rocks known in the IPB. It consist of a monotonous detritic sequence of shales and sandstones, with a number of interbedded limestone lenses towards the top. These latter contain conodonts and other fossils indicating an Upper Fammenian age (Van den Boogaard, 1967; Van den Boogaard and Schermerhom, 1975). The total thickness of the PQ group is currently estimated to exceed 1,000 m (Strauss, 1970). Despite the lack of detailed studies, the available data indicate a shallow depositional environment, probably in an storm-dominated platform (Moreno and Sfiez, 1991). The above quoted uniform features of the PQ group, however, change abruptly towards the top of the unit, closely below the overlying VSC. As a rule, the sand/lutite ratio increases, mostly producing sandstone sequences (Moreno and S~iez, 1990), together with several types of exotic facies, including fan deltas, near-shore bars and mega debris-flows. Deposits related to these latter exotic facies make a mosaic of stratigraphically equivalent sub-units, that

formed in contrasting sedimentary environments, although all them highly energetic (Moreno et al., 1996). This points to the compartimentation of the older Devonian belt in a number of horst/graben-like sub-belts, each of them having a different subsidence rate. In turn, this is an environmental condition that can account for the heterogeneous distribution of volcanics and sediments that dominates the deposition of the overlying VSC. Some of the possible consequences of this geodynamic situation, especially regarding the massive sulphide genesis, are discussed below. The Volcanic-Siliceous complex (VSC) hosts the massive sulphide and manganese deposits. The VSC is Upper Fammenian-Visean in age, and consists of a heterogeneous group of rocks with rapid lateral and vertical facies changes. The thickness of the VSC varies widely, ranging from a few tens to thousands of meters. Rock sequences consist of felsic and mafic volcanics interfingering within a framework of detrital and chemical sediments. Volcanic rocks are mainly felsic pyroclastic and mafic flows. Subvolcanic rocks, both felsic and mafic, are ubiquitous, constituting the bulk of the stratigraphic column in some localities (Boulter,

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R. S{tez et al. / Ore Geology Re~,iews 11 (1996) 429-451

1993a; see also L6colle, 1977 and Routhier et al., 1978). Sedimentary rocks correspond to three main types: volcanic-derived epiclastics which range from fine sandstones to conglomerates; black shales rich in organic matter, commonly associated with massive sulphide deposits; and chemical sedimentary rocks, including massive sulphides, and manganiferous chert and jasper. The VSC is currently known to represent the alternation of felsic (at least three) and mafic (at least two) volcanic episodes interfingered with sequences of sedimentary and volcano-sedimentary affinity (Strauss et al., 1977; L6colle, 1977). As a simplified outline, each felsic package is followed by a mafic package and a sedimentary package, although one or more episodes may be lacking in places. Detrital rocks (slates and epiclastics) are the dominant lithology where volcanics are absent. A typical stratigraphic section is shown in Fig. 2. Alternative local stratigraphic columns are shown in Fig. 4, as well as in Routhier et al. (1978) and Oliveira (1990). Massive sulphide deposits occur at the top of the first and second felsic episodes, either in contact with volcanic rocks or interbedded in sediments in a similar stratigraphic position. A first palaeontological (palynomorph) dating of massive sulphide deposits from Aznalc611ar has been recently performed (Pereira et al., 1996), indicating an Upper Devonian (Strunnian) age. Main manganese deposits are related to the second felsic episode associated with a sequence of jaspers and purple shales. The Culm group (in Portugal, Baixo Alentejo Flysch group, Oliveira, 1983) is a thick and monotonous Upper Carboniferous succession of shales, litharenites and rare conglomerates that overlies the VSC in the IPB. The estimated thickness for this group exceeds several thousand meters. From a sedimentological view, the Culm group represents the infill of a subsident basin by turbidite sediments, whose provenance is within both the IPB and the Ossa Morena Zone (Moreno, 1993).

3. Dinantian magmatism Both mafic and felsic volcanics occurring in the IPB show a wide variety of textures and composi-

tions and, as quoted above, some stratigraphic relations occur between the main types of ore deposits in the region and volcanism. However, in spite of this obvious interest, and in part due to the intense deformation and alteration of the igneous rocks, many uncertainties still remain about physics, chemistry and geodynamic significance of volcanic activity in the IPB. In addition, some recent papers have proposed a completely new scenario for the Palaeozoic magmatism (Boulter, 1993a; Boulter, 1993b), suggesting that ore deposits are not related to volcanism sensu stricto, and claiming instead a sill-sediment interaction to interpret both the volcanic sequences and the related ore deposits. Basic volcanic rocks from the IPB are extrusive submarine (often pillowed flows, in places pillowbreccias) and subvolcanic bodies of spilites and albitic diabases. Basic hydroclastic rocks also occur in several localities (Boulter, 1993a). Subaerial basic flows have been described by L6colle (1977). Although their original chemical features are difficult to assess, basic rocks are mildly alkaline (Munhh, 1983a). Felsic rocks, on the other hand, are lavas and pyroclastics of variable grain size. A new finding is that felsic volcanism changes from (subaerial) pyroclastic to subaqueous shallow subvolcanic with time (Pascual et al., 1994). However, as this evolution is probably related to changes in the local subsidence rate, involving a significant increase of the hydrostatic pressure, it is to be expected that such a change could be less clear, or even lacking, in other areas in the IPB, depending on differential subsidence. Hydroclastic rocks are linked to high-level intrusive sills, injected into water-rich semiconsolidated sediments (Boulter, 1993a). They are abundant in a number of areas in the IPB, such as Riotinto (Boulter, 1993a), and Aznalc611ar (S~iez et al., 1993). All of the felsic rocks mostly range from dacites to rhyolites in composition. Intermediate rocks (andesites) are scarce at a regional scale. However, their local occurrence has been claimed by Munhh (1983a), in particular in the northern segment of the IPB (Pomarao, Albernoa-S. Domingos). A wide variety of felsic pyroclastic rocks have been reported in the IPB, including agglomerates, tufts, ashes, tuffites, ignimbrites and lahars (Strauss and Madel, 1974; Salpeteur, 1976 and Routhier et al., 1978), which strongly support a prevailing explo-

R. S6ez et al. / Ore Geology Reviews 11 (1996) 429-451

sive mechanism for the felsic volcanic activity. However, although volcanic facies have been distinguished and mapped at a regional scale (e.g., Routhier et al., 1978), detailed studies concerning the distribution, relations and temporal evolution of the different felsic volcanic facies are still scarce. Available data suggest fissural-type volcanic lineaments, roughly outlined by grain size distribution around the eruptive centers (Carvalho, 1974; Strauss and Madel, 1974; Munh~, 1983a) with predominance of subaerial or shallow-water volcanic activity (Routhier et al., 1978). On the basis of poorly constrained palaeontological data, Carvalho (1976) has suggested a northward migration of volcanic activity with time. However, rocks overlying the first volcanic episode in Aznalc611ar, within an area included in the central domain by Carvalho (1976), have yielded an Strunian age (Pereira et al., 1996), equivalent to ages obtained in the southern (Cercal-Odemira) domain. Therefore, the northward migration of volcanism seems to be unsupported. Despite the above mentioned changes from pyroclastic to subaqueous shallow subvolcanic character of volcanic rocks with time, no systematic compositional change occurs as a response to these changes. For instance, in the Aznalc611ar area both pyroclastics and overlying subvolcanics are all dacitic in composition, and only late dykes and domes are rhyolitic. In any case, however, both dacitic and rhyolitic types are related by fractional crystallization processes (Pascual et al., 1994). Extensive hydrothermal alteration (hydrothermal metamorphism of Munhh, 1990) makes it difficult to determine a detailed geochemical and petrogenetic model for the IPB volcanics. In spite of this, most of the scientists involved currently interpret them as a bimodal association of basalts and felsic rocks (dacitic to rhyolitic) that evolved separately. The source for felsic magmas could be found in magma chambers developed in the crust by partial melting caused by heat supplied by rising mafic magmas (Munhh, 1983a). Pertinent geodynamic models are further discussed in detail. However, it is to be pointed out here that many of the published proposals on the geodynamic environment of volcanism, especially those relying only on geochemical features, are still subject to caution. For example, recent studies in the Aznalc611ar area (E of the IPB) indi-

433

cate that local chemical variation found in felsic volcanics is similar to the regional variation of the same rocks throughout the IPB. Therefore, it cannot be solely attributed to a source inheritance, but to differentiation processes in shallow magma reservoirs (Pascual et al., 1994).

4. Structure and metamorphism The rocks of the IPB were deformed and regionally metamorphosed in the Asturian phase of the Hercynian orogeny during the Upper-Visean to Westfalian-D times. Three stages of deformation have been recognized in the IPB. D~ phase generated regional structures and low-grade regional metamorphism, whereas D 2 and D 3 only slightly modified D 1 structures. Both deformation and metamorphism seem to increase in intensity from SW to NE (L6colle, 1977; Routhier et al., 1978; Ribeiro and Silva, 1983; Munhh, 1979; Munhh, 1983b; Munhh, 1990). However, regarding the regional metamorphism, and therefore the P - T evolution of the IPB during Variscan times, some aspects need further discussion. In particular, metamorphic zoning (e.g., Munhh, 1990) is generally sketched on the basis of large-scale comparisons, including geological units located outside the IPB itself (for instance, Pulo do Lobo group), which have unclear relations with the IPB (Giese et al., 1994). In other cases, relationships are considered with the Culm group, that evolved at a shallower structural level. Additional mineralogical precisions are also needed. In fact, most of the mineral assemblages described as related to regional metamorphism of basic rocks (L6colle, 1977; Munhh, 1979; Munhh, 1983b; Munhh, 1990) are hard to distinguish from those related to hydrothermal metamorphism, to be described below. On the other hand, illite crystallinity measurements performed on IPB sediments have yielded uncertain results, in places suggesting a temperature-dominated control, as well as an EW - not a NS --variation. Therefore, we suggest that the possibility of a regional metamorphism focused on shear zones should be considered as an alternative to metamorphic zoning, as previously pointed out (Fern(mdez-Caliani and Gal~in, 1991; Fern~ndezCaliani and Galen, 1992).

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The structure of the IPB has been defined as a thin-skinned foreland thrust and fold belt (Ribeiro and Silva, 1983; Silva et al., 1990; Quesada, 1991). Deformation causes asymmetric folds verging to SW, which often show transposed bedding on the short limb, mimicking the structural features of a thrust belt. Folding is accompanied by development of a penetrative foliation that shows sinistral transection of the axial planes. These are related to a left-lateral shear component during folding (Silva, 1983), in a general tectonic context of a transpressive orogen (Badham, 1982; Andrews, 1983; Silva et al., 1990). During Late- and Post-Hercynian times, a strikeslip tectonic regime prevailed in the European Hercynian Belt (Arthaud and Matte, 1977). In the SPZ, this setting involved the formation of E - W shear and fault zones and major tectonic activity along the boundary between the SPZ and the Ossa-Morena Zone (OMZ) (Simancas, 1983; Crespo and Orozco, 1988). Local transtension related to this tectonic environment favoured the intrusion of bimodal magmatic rocks (ganitoid to gabbroic) outcropping specially in the NE of the IPB (Simancas, 1983; De la Rosa, 1992).

5. Metailogenesis The metallogenesis of the IPB includes the massive sulphide and manganese deposits associated with Dinantian volcanism, and vein-type hydrothermal mineralization associated with Late-Hercynian brittle deformation and magmatic processes during Upper Carboniferous times. Manganese deposits are represented by hundreds of small-size rhodocrosite, rhodonite and Mn-oxide deposits associated with shales and Fe-jasper within the VSC of the IPB. The economic significance of this kind of deposits is small, and all the mines are today out of production. The Mn-associated lithologies constitute mappable units in a stratigraphic position near to the massive sulphide horizons. In general, there is no temporal equivalence between massive sulphide and Mn horizons. Possible indirect relations are discussed in Routhier et al. (1978), Barriga (1983, 1990) and Leistel et al. (1995). Late- and Post-Hercynian hydrothermal mineralization is mainly represented by vein-type ores, al-

though they also occur as replacement bodies within suitable lithologies (Sfiez et al., 1988; Sfiez and Ruiz de Almod6var, 1991). Element associations include: F-(Pb, Zn); Sb-(As, Cu); Pb-Zn-(Ag, Ba); AsCu-Bi-Co-Ni and Sn-W-As. Recent lead isotope determinations (Marcoux and S~ez, 1994) indicate the relation of these mineralizations with the Lateand Post-Hercynian evolution of the IPB. There is no mining activity today of this mineralization style, possibly because their economic potential has been masked by the magnitude of the massive sulphide bodies, so that they have attracted little exploration interest. In spite of this minor economic significance, Late- and Post-Hercynian hydrothermal mineralization is important in understanding the Upper Carboniferous evolution of the IPB.

6. Massive sulphide deposits Mining in the IPB began in pre-Roman times and has continued to the present day with the exception of a few idle periods. The elements of interest have not changed through time. Early mining activities were mainly looking for gold, silver and copper from oxidized ores which are still being mined today. In the twentieth century, pyrite (for sulphuric acid), copper, base metals, gold and silver are the main mining targets in the IPB. About eighty mines have been operative during the last hundred years (Fig. 3), with a total production of about 300 million tons of polymetallic ores, although in most of the cases only sulphur and copper have been processed. The recovery of other elements (including Pb and Zn) is often hindered by the fine grain size of the ores and the complex mineral intergrowths. During the last twenty years, gold and silver from gossans and slags have contributed the most of the metal production in Riotinto and Tharsis (Fil6n Sur). In addition, tin is an important metal resource in Neves Corvo. Reserves of massive sulphide ore in the IPB, computing only major mining districts, are summarized in Table I. As previously suggested (Rambaud, 1978; Routhier et al., 1978), massive sulphides (and Mn) deposits may be palaeogeographically controlled. Late-Devonian tectonic movements have been claimed to be responsible for this palaeogeographic

R. S6ez et al. / Ore Geology Reviews 11 (1996) 429-451

435

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R. Sdez et al. / Ore Geology Reviews l I (1996) 429-451

Table 1 Ore grade and reserves in giant deposits in the IPB. Data from literature Reserves (Mt)

Aljustrel a

Cu (%)

Zn (%)

Pb (%)

Sn (%)

Ag (ppm)

Au (ppm) 0.8

250

0.8

3

1

38

43 30 70

0.4 0.58 0.4

3.3 0.4 3.8

1.8 2.2

67 10 63

164

1.2

2.5

1.1

50

0.7

1.4

0.8

Masa Valverde

120

0.5

1.3

0.6

Neves Corvo Cu-rich ore Sn-rich ore Zn-rich ore

220 28 3 50

1.1 6.8 11.7 0.5

1.5 1.2 1.7 5.9

0.3

1 0.15

2 0.15

1

59 58

0.6 0.9

4.9 2.2

1.9 1.1

110

0.5

2.7

0.6

Aznalc611arGroup Aznalc611armassive Aznalc611arstockwork Los Frailes La Zarza Lousal

Riotinto massive Riotinto stockwork Sotiel Group Sotiel Migollas Tharsis

250 2000

0.01

47

1.8

38

0.8

0.05 0.3 2.9

1.2 30 7 0.02 22

0.7

a Aljustrel data about reserves are for the whole Aljustrel district whereas ore grades are only for the Moinho deposit according to Leit'~o (1993).

control by Moreno et al. (1996). Rambaud (1978) suggests that volcanic foci lineaments were crucial to ore deposition, whereas Routhier et al. (1978) proposed a control by the vicinity of volcanic foci and by Late-Devonian epirogenic movements resulting in different sedimentary and volcanic facies associations. Finally, Moreno et al. (1996), have described the development of Late-Devonian horsts and grabens, related to transtensional movements, since Late-Devonian series show large differences in subsidence rates, yielding in places sediments that range from continental to deep marine. In this way, the IPB sediment distribution, including the Culm group, would be palaeogeographically controlled. In particular, the regional distribution of black shales, that are especially interesting with regard to massive sulphide deposits, is also palaeogeographicaily controlled. However, the intense Hercynian deformation occurring at a regional scale, including in places large

overthrusts and nappes, obliterate the original facies distributions. As a final result, many (or most) of the above criteria cannot be used in a simple way to map palaeogeographic zones. Previous attempts to depict such zones are to be treated with caution, although some examples tend to confirm that structural controis did operate (Solomon et al., 1980). Also as shown by Carvalho and Ferreira (Carvalho and Ferreira (1993), Fig. 7) the limit between Cu- and Zn-rich mineralizations in the Masa Corvo is marked by a N E - S W trend, roughly coincident with the Messejana fault. The general distribution of massive sulphide lenses in Neves Corvo seems also to be related to this trend.

7. L o c a t i o n o f t h e m i n e r a l i z a t i o n

Massive sulphide deposits in the IPB are associated to sedimentary horizons (mainly black shales

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and tuffites), located close to the top of the felsic episodes Vhl and Vh2 (see Fig. 4), as previously reported by many authors (e.g., Routhier et al., 1978; Barriga, 1990). Therefore, we interpret them to represent waning stages of the volcanic activity (Fig. 4). In areas where more than one volcanic episode occurred, massive sulphides may be found in different stratigraphic positions, close to the end of each of these volcanic episodes (Fig. 4). An essential exploration tool in the IPB is to recognize productive stratigraphic horizons in the target areas. Considering the Spanish part of the IPB, in the

so-called Southern Zones (Routhier et al., 1978) major ore bodies commonly occur at the top of the first felsic volcanic episode (e.g., Tharsis group, Sotiel, Torerera, Campanario-Cibeles group). Aznalc611ar, which is considered by Routhier et al. (1978) as related to the transition zone, should be better included in the Southern Zone, related to the Vhl episode (Pons et al., 1993). In contrast, massive sulphide deposits in the Northern and Transition zones (Routhier et al., 1978) are mainly related to the second felsic episode (Riotinto, La Zarza, Lomero-Poyatos, La Joya, San Telmo, San Miguel,

THARSIS - HERRERIAS

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~ 1 4

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Los Silos I, E Cura, Chart,=. 2: Gibraltar, Los Silos II.

C A B E Z A S RUBIAS

- EL

PATRAS

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1: Buitron, Tinto-S'.Rola, Campanado 2: Sotlel, Mlgollas, Torerera, Valvetde AZNALCOLLAR

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Monte Romero, Agmm Te~iidM, ~' Platbn 2: Cueva de la Mora, Angellta, Coneepci6n

1: Rlotlnto, San M~guel 2: La Zarza

1

1: Aznalcdllar, Los Fraik~, Tintillo, Zarcita, Caridad

Fig. 4. Local stratigraphic sequences and the position of some massive sulphide deposits from the spanish IPB with regard to volcanic episodes. (1) Shales and sandstones (PQ); (2) mafic subvolcanics (sills); (3) felsic tufts, locally lavas (VAI); (4) tuffites and organic matter-rich black shales; (5) mafic flows; (6) dacitic sills and hydroclastic breccias, locally tufts (VA2); (7) felsic debris flow deposits; (8) purple-green tufts and tuftites; (9) massive sulphide deposits; (10) purple slates; (11) Mn-Fe jaspers; (12) siliceous tuffites and epiclastites; (13) felsic epiclastites and minor felsic volcanics (VA3); (14) shales and sandstones (Culm group). Adapted from Ruiz de Almod6var and S~ez (1992).

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Monte Romero). In the Paymogo region, mineralization is associated with the second felsic episode, but in two different positions: at the top of dacitic breccias and tufts (Gibraltar, Los Silos), or at the top of volcanoclastic debris flows (Sierrecilla, Romanera, E1 Carmen). Correlations between the Spanish and Portuguese parts of the IPB are not easy, due to the use of a different stratigraphic nomenclature. However, it seems clear that Neves Corvo, Lousal, Caveira and S~o Domingos have an equivalent stratigraphic position, close to the top of the regional VA1 episode, whereas the Aljustrel deposits occur in a stratigraphic environment very similar to La Zarza and Riotinto (see Freire D'Andrade and Schermerhorn, 1971; Carvalho et al., 1976; Strauss and Madel, 1974; Garcla Palomero, 1980), and therefore it should be considered to be related to the top of the VA2. The same position is to be assumed for the Chan~a group deposits.

8. Morphology and size of the deposits The sulphide mineralization of the 1PB occurs as concordant tabular bodies or lenses, commonly underlain by crosscutting stockworks in which sulphides occur in veins and as pervasive disseminations. As a rule, lenses and tabular bodies of massive sulphides are stratabound, concordant and syngenetic with the host rocks, although replacement relationship with host rocks are also noticeable. The most common host rocks are black shales rich in organic matter, and occasionally felsic pyroclastics and epiclastics. Although in some exceptional cases stockwork mineralizations without related massive sulphides have been reported, (e.g., Salgadinho, see Plimer and Carvalho, 1982), both mineralization types are associated as a rule. Where the massive sulphides are located on the top of a well developed stockwork in felsic volcanics, they commonly show a lenticular shape with a considerable thickness at the center of the body (e.g., Riotinto, San Miguel, S~o Domingos). Ore bodies associated with more porous rocks (for instance, pyroclastics) and thick black shale horizons exhibit tabular form (e.g., Aznalc611ar, Lomero-Poyatos, Monte Romero). In some

cases, massive sulphides hosted by shales and tuffites have been described as consisting of several superposed lenses (e.g., Lousal, see Strauss (1970), or Sotiel, el. Santos et al. (1993)). However, in other areas, as in Aznalc611ar, up to five mineral ore lenses are tectonically superposed, including the overthrust of a stockwork zone, actually emplaced above a massive sulphide body (Pons et al., 1993). Therefore, and considering tectonic complexities, we suggest that prior descriptions of occurrences of several ore lenses in different stratigraphic positions, although plausible, should be reviewed in more detail. Stockwork mineralization may be developed in different rock types and represent the feeder zone of hydrothermal fluids (Rambaud, 1969; Williams et al., 1975; Garc~a Palomero, 1980). This kind of mineralization is commonly conical in shape, with diameters ranging from 100 to 600 m and with a vertical extent of 20 to 250 m. An exception to be quoted is Riotinto, where the stockwork zone has a vertical extent of more than 700 m, with a lateral extent of several square kilometers (Garcla Palomero et al., 1993). Massive sulphide bodies in the IPB are very diverse in size. Fem~mdez Alvarez (1974) stated four categories of tonnage of massive sulphide bodies: (a) Very large deposits bigger than 20 million metric tons (Mt). This category represents more than 75% of total ore reserves in the IPB; (b) large deposits with tonnage between 5-20 Mt representing about 17% of ore reserves; (c) medium size deposits from ! to 5 Mt and 7% of total reserves; and (d) small deposits with less than one Mt and 1% of total reserves. Seven known mining districts in the IPB include individual lenses of more than 50 Mt: Riotinto (where the sum of massive sulphide bodies exceeds 500 Mt), Neves Corvo (225 Mt), Aljustrel (250 Mt), Tharsis (130 Mt), Aznalc611ar-Los Fralles ( = 125 Mt), Sotiel-Migollas (-- 100 Mt), La Zarza (100 Mt), All of them could be classified as super-giant massive sulphide deposits according to the classification of Sangster (1972). Riotinto is the largest known massive sulphide concentration in the IPB, and probably in the world (Barfiga, 1990). The average composition estimates of known mineralizations have been summarized in Table 1.

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9. Relations with country rocks As previously quoted, massive sulphide deposits in the IPB are associated with detrital and volcaniclastic horizons, located near the top of the VA1 and VA2 episodes, very often related to black shales. These latter may occur as thin, discontinuous levels, as in Riotinto or San Miguel, or as much wider levels up to 100 m thick, as in Tharsis, Sotiel and Aznalc611ar. Given the stratigraphic significance that is commonly attributed to black shales (Bitterli, 1963; Einsele, 1992; Arthur and Sageman, 1994), it can be deduced that in many cases a significant time span elapsed between volcanic activity and massive sulphide deposition in the IPB. Therefore, relationships between massive sulphides and volcanics could have been palaeogeographic. Relationships between massive sulphides and siliceous rocks (mainly cherts and jaspers) have also been considered to be of interest in previous works on the IPB (Strauss, 1965; l_,6colle, 1972; L6colle and Roger, 1973; Routhier et al., 1978; Garcla Palomero, 1980; Barriga, 1983; Lydon, 1984). These rocks may be located either laterally or at the hanging-wall of the massive sulphides, their hydrothermal origin having been shown (Barfiga and Oliveira, 1986). However, genetic relationships between siliceous rocks and massive sulphides remains unclear (e.g., Leistel et al., 1995).

10. Hydrothermal alteration Mineral transformations of volcanic rocks in the IPB, prior to regional metamorphism, occur in two different contexts: (1) On a regional scale, most of the volcanic piles are affected by so-called 'hydrothermal metamorphism' (Munhh and Kerrich, 1980; Munhh, 1990) or 'regional alteration' (Barriga and Carvalho, 1983; Barriga, 1990); (2) on a local scale, strong hydrothermal alteration is usually linked to stockwork zones extending below the massive sulphide lenses. Regional hydrothermal alteration has been reported as a process of spilitization that modifies the original mafic (basaltic) and felsic composition of volcanic and subvolcanic rocks of the IPB (Munhh and Kerrich, 1980). In mafic rocks, spilitization in-

439

volves the growth of mineral assemblages characterized by chlorite, carbonates, epidote, albite and actinolite. Munhh (1990) describes hydrothermal metamorphism as an open-system process, involving extensive N a - K exchange, hydration, oxidation and carbonatization. The regional extent and the immense rock volume involved suggest the participation of a large amount of fluids from a reservoir external to volcanic rocks. Isotopic x80 and D values of altered basic rocks, close to those in Carboniferous sea water, indicate that hydrothermal transformations proceeded under conditions of high water:rock ratios (Munhh and Kerrich, 1981; Munhh et al., 1986). A shift in isotopic ratios suggest that the system progressively evolved to lower water:rock ratios (Barfiga and Kerrich, 1981; BmTiga and Kerrich, 1984). It has been also suggested that the regional hydrothermal pattern in the IPB is analogous to those found in ophiolites (Munh~, 1990). Felsic volcanics, including rhyolites, dacites and related rocks, are also affected by regional hydrothermal alteration, and common related metasomatic processes are albitization, sericitization, chloritization, silicification and exceptionally adularitization. The resulting keratophyric rocks have also been called 'felsic spilites' by Munhh and Kerrich (1980). Three questions are to be remarked regarding chemical modifications of volcanic rocks during regional hydrothermal alteration: (1) The designation of the IPB volcanic rocks as 'primary' spilites, quartz-keratophyres and keratophyres (Rambaud, 1969; Schermerhorn, 1970; Soler, 1973) is inappropriate, considering the Na - - and other element - metasomatism that regional alteration involves; (2) in view of the open-system and non-isochemical character of the hydrothermal metamorphism, any chemical comparison with fresh igneous rocks must be made with caution. In particular, the correlation of these volcanics with plutonic rocks located in the northern IPB (Schlitz et al., 1987; Sawkins, 1990) has insufficient support, and (3) this regional hydrothermal alteration plays an important role in leaching metals and in the ore genesis in the IPB (Munhh and Kerrich, 1980; Barriga, 1983; Barriga, 1990). Hydrothermal alteration focused in the ore zones of the IPB represents the interaction between orebearing fluids, country rocks and sea water near the

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hydrothermal vents. As a consequence, chemical composition of footwall rocks is strongly modified in an open system, in which most of elements are mobilized, including those commonly classified as 'immobile' during hydrothermal processes (e.g., Zr, Hf, REE and Y, see Almodtvar et al., 1995). Footwall rock alteration is characterized by a roughly concentric zonation, with an inner chloritic and a sericitic peripheral zone. Differences in alteration intensity are often shown by changes in phyllosilicate compositions, or by the extent of alteration zones and variations in the chlorite/sericite ratios. In Masa Valverde, for instance, the ore-related sericitic zone can be distinguished from outer zones (having suffered metamorphism but not hydrothermal alteration) by high values of the (Ba + K ) / N a ratio in sericite. High-Ba sericites are found in zones of maximum alteration in this area (Toscano et al., 1994; see also Barriga, 1986 and Leistel et al., 1994). Changes in chlorite composition may be also indicators of the hydrothermal alteration intensity. In some cases, as in Aznalctllar and Masa Valverde, hydrothermal zoning can be traced by a progressive decrease of the M g / ( M g + Fe) ratio in chlorites, which is minimum in the inner chloritic zones (Ruiz de Almod6var et al., 1994), whereas the opposite zoning has been described in other areas (e.g., Riotinto), where maximum M g / ( M g + Fe) ratios are found in chlorites from inner alteration zones (Leistel et al., 1994). Some comparisons between the IPB and other areas containing massive sulphides are pertinent. In some cases, as for Ba content in sericites, most (although not all) of the available data are in agreement with those obtained in massive sulphides elsewhere (McLeod, 1987; Poupon et al., 1988), suggesting so a similar geochemical alteration mechanism. However, variations of M g / ( M g + Fe) ratio in chlorites indicate that some alteration haloes in the IPB show features similar to those in Kuroko and other areas (Ohmoto and Skinner, 1983; Large, 1977; Costa et al., 1983), whereas other show the opposite. These discrepancies point to a significant difference between the IPB and other massive sulphide provinces, that could have been caused by local, relative changes in fo2, fs (Bryndzia and Scott, 1987), temperature and other variables, related to water:rock ratios (Caritat et al., 1993). This suggests that local envi-

ronmental changes would have had a stronger influence in the IPB, with regard to other massive sulphide provinces. Apart from whole-rock alteration indices based on major elements, many chemical variations have been described as related with progressive, ore-related hydrothermal alteration in the IPB (Barriga and Relvas, 1993; Leistel et al., 1994). In addition, other chemical parameters have been used to characterize alterations. For instance, a relatively high C o / N i ratio was found to be characteristic of footwall rocks in La Zarza (Strauss et al., 1981), the cobalt content reaching values of about 1% in the Tharsis stockwork. The usefulness of this parameter has been more recently confirmed by a detailed study on the Aznalc611ar-Los Frailes area (Almodtvar et al., 1995). Finally, it is to be borne in mind that low-grade Hercynian metamorphism had little influence on the whole-rock composition: chemical features produced during ore-related hydrothermal alteration seem to have not been disturbed by this regional event. Alteration processes other than chloritization and sericitization are usually present in the ore zones, the most common being silicification, pyritization and carbonatization (Garcfa Palomero, 1980; Barriga, 1983; Toscano et al., 1993; Ruiz de Almodtvar et al., 1994). The presence and significance of these other alteration processes varies locally, and it is controlled by physicochemical parameters related to composition of footwall rocks, temperature, water/rock ratios and composition of hydrothermal solutions.

II. Mineralogy, textures and zonation Table 2 represents the most significant features of the sulphide mineralizations of the IPB. Pyrite is the most abundant metallic mineral species in the massive sulphide deposits of the IPB. Sphalerite, galena, chalcopyrite, arsenopyrite and pyrrhotite are also common. Prevalent minor mineral components are: B i - S b - P b - A s sulphosalts, tetrahedrite, stannite, cassiterite, magnetite and hematite (Pinedo Vara, 1963; Strauss, 1970; Routhier et al., 1978; Mitsuno et al., 1988). The most conspicuous mineral feature of the IPB is the ubiquitous presence (Mitsuno et al.,

R. S6ez et al. // Ore Geology Reoiews 11 (1996) 429-451

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R. Srez et al. / Ore Geology Re~,iews 11 (1996) 429-451

1988) of subordinate bismuth minerals (bismutinite, native bismuth and Bi-sulphosalts), as well as tin minerals (cassiterite, stannite, kesterite). Cassiterite, although known in a number of massive sulphide bodies in the IPB (Strauss, 1970; Aye, 1974; Aye and Picot, 1976; Routhier et al., 1978; Garcia de Miguel, 1990), is mined only at Neves Corvo, where it makes part of massive sulphides and stockwork mineralizations with contents up to 1% Sn (Gaspar and Pinto, 1994). In addition, cobalt sulphides characterize some stockwork mineralizations (Strauss et al., 1981; Marcoux and Moelo, 1993; Ruiz de Almodrvar et al., 1994; Marcoux et al., 1996). Non metallic minerals include chlorite, sericite, quartz, carbonates and barite. The fabrics of massive mineralizations are characterized by fine grain size and framboidal and colloform textures. Three main textural types can be recognized: (1) B r e c c i a t e d to m a s s i v e ores, mainly cupriferous or barren pyritic, characterizing mineralization deposited directly over, or very near, hydrothermal feeder zones; (2) d e t r i t a l ores, characterized by abundant sedimentary structures (graded bedding, parallel and cross lamination, slumping, etc.). It is inferred that they were formed through redeposition of pyritic muds (Schermerhorn, 1970; Strauss et al., 1981; Lrcolle, 1977; Routhier et al, 1978), in zones lateral to massive sulphide bodies; (3) b a n d e d ores. This type shows alternating bands of different sulphide composition. It is thought that banded ores may be related to chemical precipitation from hydrothermal brines far from the vents (Garcia Palomero, 1980). Some replacement, diagenetic textures also occur. The zonal distribution of minerals within massive sulphide mineralization is still insufficiently known in many localities in the IPB. In addition, and whatever this original pattern would have been, intense deformation, including overthrusting and faulting, is superposed to zoning in many points. The most obvious zonation pattern consists of copper-rich ores near the central and lower parts of the sulphide lenses; zones of barren pyrite; and Zn + Pb concentrations towards the upper and lateral zones of the bodies (Fern~ndez Alvarez, 1974; Lrcolle, 1977; Routhier et al., 1978; Hofstetter, 1980). This model is in agreement with ore distribution found in Riotinto (Garcfa Palomero, 1980), Monte Romero

(Fernandez Alvarez, 1974) and other areas; however, discrepancies are found with ore distributions in Los Frailes (Pons et al., 1993), Neves Corvo (Carvalho and Ferreira, 1993), Migollas (Santos et al., 1993) and La Zarza (Strauss et al., 1981).

12. Genetic model There is a general consensus that the massive sulphide deposits of the IPB were formed by exhalative-sedimentary processes (Apps, 1961; Williams, 1962; Rambaud, 1969; Routhier et al., 1978; Carvalho, 1979; Barriga, 1990), in which the stockwork mineralizations were the hydrothermal feeders. This general outline has common features with many other models proposed for massive sulphides throughout the world, including recent seafloor deposits, which in turn can form in a wide variety of geologic environments (Solomon, 1976; Sato, 1972; Francheteau et al., 1979; Finlow-Bates, 1980; Franklin et al., 1981; Lydon, 1988; Large, 1992; Rona and Scott, 1993). But apart from these general similarities, massive sulphides in the IPB differ from those in other metallogenic provinces in the world, both recent and ancient, in a number of features, that make this province unique. First of all, the IPB represents the greater concentration of giant and supergiant massive sulphide deposits on Earth; IPB is also characterized by the occurrence of a large number of minor deposits; mineralizations were formed on a continental crust; volcanics are bimodal, dominantly felsic, and finally both large and small deposits exhibit individual features that make some single deposits different from any other in the province, or in the world. The strikingly high tin and copper contents in Neves Corvo can be taken as an extreme example of these unique features. Consequently, we will summarize first the interpretations given to date to account for these peculiarities. However it should be stressed that, despite these unique features, interpretations of the IPB deposits as 'normal' volcanic hosted massive sulphides (VHMS), or as deposits of a given reference type (for instance, as Kuroko type mineralization) are still common in recent times (e.g., Sawkins, 1990). Schermerhorn (1970) and Carvalho (1979) distinguished two main types of deposits in the IPB,

R. S6ez et al. / Ore Geology Reviews 11 (1996) 429-451

'autochtonous' and 'allochtonous'. The former type are consistent with the general genetic model, whereas the 'allochtonous' type involve the redeposition by gravity flows of previously deposited sulphide muds, permitting the accumulation of greater sulphide masses. The above authors proposed that Tharsis (Fil6n Norte) and Lousal were examples of this 'allochtonous' type. As pointed out above, Barriga (1983) and Barriga and Fyfe (1988) have suggested an alternative explanation to account for the formation of huge massive sulphide bodies. According to their model, a siliceous, colloidal cap could have protected the sulphide masses from erosion or alteration processes. This capping is now represented by chert horizons overlying the sulphide masses. According to these authors, this model is appropriate in particular to the Aljustrel area. Finally, Boulter (Boulter, 1993a; Boulter, 1993b) has proposed that massive sulphide deposition at Riotinto is related to sill-sediment interaction, relying on a comparison between the IPB and the Guaymas Basin. Apart from the explanation of all of the igneous rocks in terms of hydroclastic brecciation and alteration of lavas, the massive sulphide deposits in the Riotinto area are related to hydrothermal activity triggered by sill emplacement. It is note worthy that (a) Boulter does not admit the existence of true pyroclastic rocks in the areas of the IPB he has studied, and (b) he does not intend a complete generalization of his model outside these specific areas. However, no one of these three alternatives to the general model seem to be sufficient. First, and despite the local existence of textures indicating transport and redeposition of sulphides, it is known that most of the sulphide masses in the IPB exhibit an underlying stringer zone, including masses claimed to be alloctonous (sensu Schermerhorn, 1970), like Tharsis (Strauss and Beck, 1990; Kase et al., 1990) or Lousal (Mitsuno et al., 1988). On the other hand, a siliceous cap, as suggested by Barriga and Fyfe (1988), could account for the genesis of the Aljustrel deposits, but it does not seem to be of general application to the whole IPB, because an overlying siliceous level is not a general feature of the sulphide masses. Finally, the suggestion by Boulter (Boulter, 1993a; Boulter, 1993b) is doubtful even in the Riot-

443

into area, as the stockwork in this area cuts the subvolcanic rocks that this author suggests to triggered the hydrothermal activity. Consequently, we suggest a different, alternative genetic approach, which can account both for the size of deposits and for a number of facts we consider relevant to the sulphide genesis. First, the problem posed by the enormous size of many of the IPB deposits relies upon the nature and amount of the hydrothermal fluids involved, as well as the environment in the depositional basin. On the other hand, significant variations in the physico-chemical conditions during massive sulphide genesis, involving regional redox changes, are to be deduced from the contrasting mineral assemblages occurring in points throughout the IPB, since oxides and sulfates are major components of massive sulphide ores in a number of places (San Telmo, Concepci6n, Cueva de la Mora). Finally, the available isotopic data indicate two facts: (a) The metal source of the IPB massive sulphides must have been homogeneous at a regional scale, in view of the tight cluster of lead isotope values (Marcoux et al., 1992), and (b) a significant participation of bacterial activity during some stages of the massive sulphide deposition is to be deduced from their slightly positive 834S values in a number of places, which contrast with values close to zero found in stockwork zones (Arnold et al., 1977; Routhier et al., 1978; Eastoe et al., 1986; Mitsuno et al., 1986; Mitsuno et al., 1988; Yamamoto et al., 1994). Hydrogen and oxygen isotope data indicate that hydrothermal fluids could have consisted of seawater modified by water-rock interaction, possibly with a minor participation ( < 15%) of magmatic fluids, or connate fluids (Munh~t and Kerrich, 1981; Barriga and Kerrich, 1984; Munh~ et al., 1986). In these conditions, the enormous sulphide amount found in larger deposits cannot be explained in terms of single convective cell models, as both heat and metal supply should have been insufficient. More probably, a larger fluid reservoir, as well as a multi-stage heat source, are needed. On the other hand, if the potential metal source is enlarged, involving heterogeneous rock types, some homogenization is required, prior to the sulphide deposition, to account for the even lead isotopic values (Marcoux et al., 1992). Finally, bacterial activity, as well as the above quoted

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R. S6ez et al. / Ore Geology Reviews 11 (1996) 429-451

inherent to our model. These piles would have included the Lower VSC and the upper part of the underlying PQ group. Among the various possibilities for heating, we find that a single heating system related to felsic dome emplacement, as proposed for japanese massive sulphide deposits, cannot account for the IPB. Alternatively, we suggest that isotherms were abnormally raised during the whole VSC deposition, due to the repeated stages of volcanic activity and to more general geodynamic causes, as quoted below. The final concentration of sulphides would occurred later, during a further isotherm rise, which we suggest to link to the ascent of basic magmas. The existence of the corresponding basic rocks, emplaced

relation of massive sulphides and black shales or other strongly reduced horizons, would indicate that sulphides were mainly deposited in the euxinoid zones of a compartmented basin. This basin compartimentation is also strongly supported by the above quoted spatial variations in redox conditions. The genetic model proposed is shown in Fig. 5. It is first apparent that sulphide masses (especially the largest) should have been related to fracture zones, which would have caused both the compartimentation of the basin and a focusing of fluids along them, as seen in Neves Corvo (Carvalho and Ferreira, 1993). Nevertheless, it is to be stressed that some pre-storage of fluids in an aquifer-like reservoir, involving leaching of large piles of materials, is

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R. Stez et al. / Ore Geology Reviews 11 (1996) 429-451

as sills underlying mineralizations, but closely postdating the sulphide deposition, is apparent in a number of points in the IPB (Sotiel-Coronada, Neves Corvo, Tharsis, La Joya, Cueva de la Mora). A similar role of mafic magmas has been found at Riotinto by Boulter (1996), however an important difference remain between the Riotinto case and all the above quoted. At Riotinto, mafic sills are crosscut by the stockwork system whereas that other ones lacks hydrothermal alteration related to massive sulphide genesis. With regard to current models, the proposed variations permit a better explanation for a number of other features. For instance, it is clear that these conditions do n o t include a direct relation between massive sulphides and felsic volcanic activity. In addition, the relation between massive sulphides and major fault lineaments, as well as between sulphides and black shales, is due to a tectonic control on both genesis of sulphides and sedimentary process, not only during the VSC deposition, but also from the Late-Devonian, as evidenced by recent studies on PQ group stratigraphy (Moreno et al., 1996). It is equally easy to explain how a definitely submarine sulphide deposit may follow closely in time the deposit of subaerial pyroclastics, provided that a tectonic collapse had occurred within a segment of the sedimentary basin. 34S values, on the other hand, would indicate some degree of mixing between ascending fluids, having positive or near-zero 834S, with bacterial-reduced sulphur, having much lower b'34S values, these latter being formed in conditions similar to those in SEDEX. Nevertheless, these 34S variations are not inherent to the model proposed. Regional alteration shown by the VSC, that can also be better explained according to the proposed model, deserves special mention. In general, this alteration is too intense, even if compared to other ancient volcanic provinces, and affects any volcanic rock, disregarding the distance to hydrothermal feeders. These latter show a more intense alteration, but it is of another type, that in general can be easily distinguished from the regional one, as discussed above. On the other hand, regional alteration does not only affect the submarine volcanics, but also the pyroclastic, subaerial acid volcanics. Given that alterations products are similar in all the cases considered, we suggest that the proposed first stage of

445

aquiferous-like extraction and concentration of water from sediments could have had marked chemical effects on subaerial and shallow-depth volcanics and sediments, equivalent to those of the so-called hydrothermal metamorphism (Munhh, 1990) on submarine flows, in part resulting in a converging mineral assemblage. Therefore, the aquiferous-like generalized metal and fluid extraction could have produced a peculiar type of alteration, starting at each point in the IPB when the basin collapsed, not necessarily during the volcanic activity.

13. Devonian/Carboniferous geodynamic environment in the IPB Another condition favoring the proposed model would have been a tectonic environment dominated by crustal thinning, that would have favoured additionally the isotherm rise. This rift, or pull-apart tectonic evolution would have started near the Devonian-Carboniferous boundary (Moreno et al., 1996), and provides the idoneous palaeogeographic environment for the deposit of massive sulphides, with small, separate and differentially subsident basins. This could account for the great variety of sulphide mineralization styles, even in the same district (e.g., Neves Corvo), the rapid change from subaerial to submarine conditions of volcanism, and the basin instability, recorded in the detfital features of some massive sulphides and in the volcano-detritic sequences. However these conditions, that in our opinion seem to be the most probable during volcanism and massive sulphide development, are not universally agreed, so that a number of geotectonic models have been also proposed for the IPB or, more generally, for the southern Iberian Massif. We will discuss this matter as a concluding remark. It is apparent that sulphide deposits in the IPB could not form in an oceanic environment, as shown by the platform character of the underlying PQ (Moreno et al., 1996), as well as by the subsequent volcanism, containing large volumes of acid rocks. Terrigenous sediments associated with VSC also support the same conclusion. Accordingly, most of previous geodynamic models dealing with the IPB have focused on comparisons with arc or intracontinental basins, mostly arguing the character of the volcanism.

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A destructive plate margin environment in the IPB has been claimed by a number of authors (e.g., Carvalho, 1972, 1979; Vegas and Mufioz, 1976; Soler, 1980; Schtitz et al., 1987). In addition, some other (e.g., Sawkins, 1990) favor an arc-related rift environment, mostly on the basis of the previous work by Munhh (1983a). Considering first strictly an arc environment, this interpretation, in its more updated version, is based on three facts, all of which are to be considered as insufficiently supported according to the above review: (a) The migration in the age of volcanism, suggested by Carvalho (1976) and not confirmed by recent palaeontological data (Pereira et al., 1996); (b) an increasing metamorphic grade towards the northern IPB, which is also in discussion; and (c) the geochemical character of the IPB volcanic rocks, and in particular the equivalence between the IPB volcanics and the plutonic rocks cropping out in the northern part (Schiitz et al., 1987; Sawkins, 1990). This latter argument deserves further discussion. First, several of the chemical arguments used by Schiitz et al. (1987) are questionable, as the sodic (trondjhemitic) character of plutonic rocks cannot be compared with that in the intensely altered volcanics in the IPB, in which a high sodium mobility is characteristic (Barriga, 1990). But beyond this, the point is that the oldest of the plutonic rocks of the IPB, the Gil Mftrquez pluton, has yielded an Upper Visean age (Giese et al., 1993), which is younger than the youngest IPB volcanics. In a more general way, it seems not possible to correlate the above referred plutonic rocks with the IPB volcanism, as also done in other more local interpretations (Thi6blemont et al., 1995), because of the clear differences in the time of emplacement (Syn- to Post-Hercynian and Pre-Hercynian, respectively) and chemical character. Regarding this latter, differences between the two rock populations referred to are apparent from previous works by Simancas (1983) and Munhh (1983a), and have been stressed more recently by Quesada et al. (1994). We tend to agree with this latter work, also in view of recent studies on plutonic rocks by De la Rosa (1992). Extensional models for the IPB are supported by the bimodal character of volcanism, as it has been clearly shown by Simancas (1983) and Munhh (1983a). This latter work, in addition, has pointed out an independent genesis of mafic and felsic mag-

mas, especially conspicuous for the upper part of the volcanic sequence. A back-arc, ensialic environment is commonly favoured (Munhh, 1983a; Sawkins, 1990), although intracontinental rifting cannot be discarded. In fact, a major reason by which a back-arc spreading model is suggested for the IPB is that an arc/collisional environment is commonly invoked for the whole of the southern Iberian Massif at a larger scale (Carvalho, 1972; Vegas and Mufioz, 1976; Bernard and Soler, 1974). All these models, however, are more or less questionable, due to the scarcity of ophiolites and arc-related volcanics (i.e., true andesitic or calc-alkaline volcanic series; see Munhh, 1983a; Quesada et al., 1994). Moreover, some recent models suggesting such an arc/collisional environment in the southernmost part of the Iberian Massif during Variscan times recognize the differences between the IPB volcanics and arc-related volcanic and plutonic rocks, concluding that the former are intracontinental, within-plate suites (Quesada et al., 1994). In view of the current opinions and uncertainties, we can only conclude that both the IPB volcanics and the associated massive sulphide deposits were generated in an ensialic environment, perhaps arc-related, but most probably within an intracontinental rift or pull-apart zone. This is a sufficient constraint to set the genetic environment of the ore deposits, whereas much additional work is still needed to understand the geodynamic situation of the whole southern Iberian Massif at this time. If we consider that SEDEX massive sulphides, in which large deposits are much more common, are often associated with a rift intracontinental environment, involving small, compartmented basins, we suggest that most of the peculiarities of the IPB ore deposits could be due to a genesis that would be in some aspects intermediate between SEDEX and VHMS. Recently, Boulter (1996) has reached independently this same conclusion on the intermediate character for the Riotinto massive sulphides. However, his model departs from a purely 'non-volcanic' view in which massive sulphide ores would have formed either predating any volcanic activity or simultaneously to late, shallow intrusive magmatism. In contrast, and as stated above, we support that the most probably scenario for the IPB includes the massive sulphide genesis during prolonged waning stages of magmatism, occurring after

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a volcanic cycle and followed by late raising basic magmas. This two events were significantly separated in time, as evidenced by tectonic and palaeogeographic changes, involving deposition of black shales and related sediments.

Acknowledgements This work has been financed by the Spanish government project AMB94-0243, as well as by the regional government of Andalucfa (PAI Group 4067). Helpful suggestions by the referees M. L6colle, R. Large and H. F~irster are acknowledged.

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