films de cul

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0.7%; Au - 0.5 g/t; Ag - 26 g/t; S - 45%; Fe - 40%. Concentrations ranging from tens to hundreds

of ppm of minor metals such as: Sn, Co, Cd, Hg, Bi, Sr, In are commonly present.

About 90 individual massive sulfide deposits are known in the IPB and most of them occur in

clusters. These clusters may represent multiple hydrotherrnal vent fields within the same volcanic

center, or bodies or lenses dismembered and displaced from larger, pre-existing accumulations

due to syn-depositional slumping or to post-depositional slivering by faults and thrusts, or both.

The spectrum of ore lens distribution ranges from S. Domingos (mined out), which is a singular

orebody, to Neves-Corvo, where at least five interconnected bodies have been delineated (see JT

Oliveira et al., this volume); to Lousal (mined out), with 18 separate ore lenses from 100,000 tons

to >3 million tons (Strauss and Madel, 1974). The S. Domingos case is unique, and the reason for

this singular deposit may be due to erosion of other deposits, or because additional lenses remain

to be discovered. An additional, small syn-sedimentary displacement or tectonic deformation on

the big Neves-Corvo sulfide accumulation could have led to the spatial separation of five or more

14

completely separated bodies. Almodovar et al. and Silva et al (this volume) present detailed

accounts of the various deposits at Aznallcóllar, Tio Tinto, Tharsis and Aljustrel.

The bulk of the mineralogy in massive sulfide bodies comprises fine to medium grained pyrite,

with variable amounts of sphalerite, chalcopyrite, galena (and cassiterite at Neves Corvo).

Tetrahedrite-tennantite, arsenopyrite, bournonite and pyrrhotite are common, and many other

minerals have been identified in small and trace amounts. Tin minerals, cassiterite and Cu-Zn-Sn

sulfides (stannite-kesterite, mawsonite and stannoidite) are present in most deposits, and reach

extremely high concentrations in the tin and copper-tin ores at Neves-Corvo. The tin ores at

Neves Corvo are unique among VMS deposits globally in this respect (see JT Oliveira et al., this

volume).

Sedimentary structures such as graded bedding, cross-bedding, sedimentary breccias and slump

structures are common in massive, laminated ores. Primary colloform and framboidal textures are

present in pyrite in most deposits. Gaspar (1991, 1995c) described textures in ores at Aljustrel,

Neves-Corvo and Lagoa-Salgada as similar to those in sulfide chimneys of black smokers from

hydrothermal fields of the East Pacific Rise. In a broad sense, mineralogical and chemical

zonation within IPB deposits is similar to that found in VMS from other areas.

Hundreds of small manganese deposits are known from within the Volcanic-Sedimentary

Complex of the IPB, and have been object of minor mining operations. They occur within various

lithologies, namely tuffites, siliceous slates (often red or purple, hematitic) and cherts and jaspers.

The ensemble of the Mn concentrations and their immediate host lithologies constitute mappable

units, and lie invariably on volcanic rocks, both mafic and felsic, or on their lateral equivalents.

They constitute good field evidence of a late- to post-volcanic environment, but cannot be

considered true "marker horizons" in exploration for massive sulphide deposits, because they

occur associated with both productive and non productive volcanic episodes Jaspers are

particularly significant, and are perhaps the most typical rocks of the IPB (Carvalho, 1979).

The cherts, jaspers and manganese ores related to felsic volcanism are almost exclusively

chemical hydrothermal sea floor precipitates (Barriga and Oliveira, 1986).

A similar, but much more detailed study of the Aljustrel siliceous and metalliferous

sediments (Barriga, 1983; Barriga and Fyfe, 1988) indicates a hydrothermal origin for these

15

15

rocks, and unveils an intimate relationship with the genesis of the associated sulphide ores (see

Silva et al, this volume).

Hydrothermal alteration is a prominent feature of several lithologies of the VS Complex,

notably igneous rocks in general and essentially all host rocks to autochthonous massive sulphide

deposits. Two main types can be considered: regional and ore zone alteration.

Most, if not all volcanic and hypabissal rocks in the IPB have been affected by (sea water)

hydrothermal metamorphism (Munhá and Kerrich, 1981; Barriga, 1983, 1990). The rocks are

spilitized (propylitized), often with nearly total replacement of the igneous minerals.

Alteration of the igneous rocks on a regional scale has importance on metallogenetic studies

and on mineral exploration. In some instances it has been possible to document metal leaching

from volcanic rocks, implying that mineralized solutions may have originated from such

processes (Barriga, 1983; Barriga and Fyfe, 1997).

Ore zone hydrothermal alteration includes stockworks, their peripheral haloes, and in some

instances additional alteration types, particularly hanging wall alteration. The core of stockworks

(Type 1 ore-zone alteration) is invariably constituted by stockwork rock (chlorite-quartz-pyritechalcopyrite).

At Lagoa Salgada, the paragenesis includes pyrophyllite (Relvas et al., 1994). A

peripheral halo of sericite + quartz + pyrite ± sphalerite is frequently found (Type 2 ore-zone

alteration). In some cases (e.g. Aljustrel and Gavião), a third type of ultraperipheral alteration has

been found (Type 3), characterized, among other features, by weak, Na-sericite bearing alteration

(Relvas et al., 1990; Relvas, 1991). Minor amounts of carbonate are often present. Locally, these

may become abundant.

Stockwork zonation is present at two scales: megascopically it is clear that stockwork rock

(chloritic) predominates largely in the core of the alteration pipe, and is enveloped by a relatively

well defined halo of outer stockwork rocks (sericitic) before feldspar-bearing rocks are found

(Type 3, or no ore-zone alteration). At the scale of individual sulfide-rich veins chlorite-quartz

selvedges are surrounded by sericite-quartz rock which sometimes grades into rock unaffected by

16

ore zone alteration. When this happens, regional alteration types are found, clearly as relics in the

stockwork, implying that regional alteration predates stockwork formation. From the periphery to

the core of the stockwork, original volcanic textures are progressively obliterated by corrosion of

the original minerals and comcomitant precipitation of alteration phases. Stockwork alteration

extends laterally beyond the limits of the sulfide orebodies, and thus can be used as an ore guide

in exploration.

In some instances, hanging wall alteration has been described, such as above the Salgadinho

deposit (Plimer and Carvalho, 1982), and above Feitais (Barriga, 1983; Barriga & Fyfe, 1988; see

Silva et al., this volume). The presence of this type of alteration indicates that hydrothermal

activity persisted after deposition of the immediate hanging wall rocks. These may include not

only volcanic rocks but also shales, phyllites, tuffites, cherts and jaspers. Hanging wall alteration

may be mineralogically similar to stockwork alteration (especially in volcanic rocks), but can also

consist of manganese enrichment (oxides or carbonates).

Many isotopic studies have been carried out in relation with the IPB mineral deposits.

Sulphur isotopic studies (e.g. Arnold et al., 1977; Munhá and Kerrich, 1981; Mitsuno et al., 1988;

Kase et al., 1990; Yamamoto et al., 1993) have been recently summarized by Carvalho et al.

(1997), who have showed that these data fit the trend of sulfur isotope ratios in massive sulfide

deposits and coeval marine sulfate deposits noted on a global scale by Sangster (1968).

Furthermore, the data also suggest variable degrees if involvement of bacteriogenic sulfur in the

generation of the sulfides.

Oxygen and hydrogen isotopic studies have been undertaken in several areas and deposits in

Portugal, and in the Cerro Colorado stockwork in Spain (Barriga and Kerrich, 1984; Munhá et al.,

1986). These data are interpreted as meaning that mineralizing fluids in the IPB were mostly

derived from sea water, perhaps with small contributions of fluids derived from other reservoirs

(magmatic, metamorphic), although these are not required by the isotopic data. Massive sulfide

formation temperatures may have spanned 200-270ºC.

Lead isotope composition studies on massive sulfides have been reported by Marcoux et al.

(1992), Leistel et al. (1994) and Marcoux (1997). These authors concluded that the isotopic

compositions of massive and stockwork ores, excluding Neves Corvo, are fairly similar and very

17

17

homogeneous along the belt, indicating that most of the ore lead was derived from the same

continental crust environment as the accompanying volcanic rocks. Neves Corvo is the only IPB

orebody that has a heterogeneous isotopic composition, including highly radiogenic stanniferous

The sea water convective model for massive sulfide generation is firmly established in the

IPB. In some deposits containing exceptional ores, especially Neves Corvo, a significant

magmatic fluid contribution may be indicated. Several lines of evidence also suggest that variable

proportions of the massive sulfides may have precipitated slightly below the paleo sea floor,

protected from direct contact with basinal, unmodified sea water. Carvalho et al. (1997) have

recently summarized these and other genetic aspects of the IPB ores. Some of the models are

controversial, especially in detail, but there can be no question that the IPB is a province where

the variables conducive to VMS generation were matched with very unusual efficiency. As

proposed by those authors, the IPB deserves recognition as the type-reference VMS province for

deposits hosted in bimodal siliciclastic settings.

Acknowledgments

We wish to thank all the authors who participated in the Guidebook for their efforts to

comply with the requests of the reviewers and/or editors. The reviewers are also deeply thanked

for their hard work with the manuscripts. Very special thanks to Prof. António Mateus

(GeoFCUL), whose technical assistance and nearly unlimited availability were essential for the

preparation of most of the figures, including redrafting many of them, as well as for assembling

the text in its final version.

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