Abstract Pre and Post Meeting trips (Figures 1.1 and 1.2) The aim of this visit is to observe and discuss stratigraphic, petrologic and structural features of selected outcrops of the Volcano Sedimentary Complex in the Neves Corvo mine vicinity. Emphasis is put on the felsic and mafic volcanic sequences exposed at the Neves Corvo-Rosario and Castro Verde Anticlines (Figure 1.3). The trip departs from the Neves Corvo mine. This stop is behind the main ventilation raise (CPV2) of the Neves Corvo mine. The exposed shales and thinly bedded siltstones belong to the Brancanes Formation, the uppermost unit of the allochthonous suite of the Volcano Sedimentary Complex. Although weathered to yellow or gray colors (the fresh rocks are dark) it is possible to observe the bedding planes and the shale/siltstone alternations. The siltstones display graded bedding, suggesting deposition by weak turbidity currents. The shales are rich in organic matter. Fossils of Posidonia becheri and goniatites are common, indicating a Late Visean age. The cleavage trends N135, 70 NE and its attitude relative to bedding shows that the outcrop is the normal limb of a large anticline. Intersection lineation L1 plunges 101 to SE. It is worth noting that the overlying Mértola Formation turbidites of the Baixo Alentejo Flysch Group provided the same faunal association and Late Visean spores. This suggests a gradual lithological transition between the Brancanes and the Mértola Formation. The cut to the north of the road between the mine main transformer station and the main entrance exhibits the siliceous

Abstract Neves-Corvo, the jewel in the crown of the Iberian pyrite belt, was the most important massive sulfide discovery of the last three decades, both in gross value and significance to exploration for these ores. The deposit consists of five orebodies, which lie beneath a 230-to 800-m-thick succession of rocks of Upper Devonian and Carboniferous age. The massive sulfide orebodies were dated at ca. 350 Ma (Rb-Sr) and are hosted by rhyolitic to rhyodacitic volcanic and volcaniclastic rocks, which are part of a relatively thin Volcanic-Siliceous Complex (≤600 m). The latter is overlain by a thick cover of younger flysch and rests on older, Upper Devonian metasedimentary rocks. Low-angle thrusting has complicated the stratigraphic relationships between these major units. The deposit contains more than 300 million metric tons (Mt) of massive sulfides with about 100 Mt of mineable grade, the balance being highly pyritic. Like most other Iberian pyrite belt deposits, the Neves-Corvo deposit combines many distinctive characteristics of volcanic-hosted massive sulfide (VHMS) deposits worldwide, including very fine grained and metallurgically complex Zn-Pb-(Cu) ores, prominent metal zoning, and hydrothermal alteration of its footwall rocks; with some features akin to those of sedimentary exhalative (SEDEX) deposits. The uncommonly low ratio of volcanic versus sedimentary rocks in the overall footwall succession of the Iberian pyrite belt, the extremely homogeneous lead isotope signatures of the ores throughout the whole district, and the gigantism of many deposits are among the reasons that account to envisaging these deposits as hybrids linking those two major types of massive sulfide mineralization. In addition to these Iberian pyrite belt-like features, the Neves-Corvo deposit possesses some unusual characteristics as well. Among these stand out its uncommonly high copper-to-zinc ratio, which more than doubles that of typical Iberian pyrite belt deposits; its extremely copper rich ores that reflect zone refining within the deeper parts of the orebodies and late enrichment by remobilization phenomena; and its remarkably high tin content. The Neves-Corvo deposit is unique among massive sulfide deposits, as it encloses primary, extremely high grade, stringer and massive cassiterite ores, which may reach 60 wt percent Sn in small lenses of almost pure cassiterite. Additionally, about 200,000 t of tin metal occurs as lower grade cassiterite disseminations in the copper-rich parts of the deposit (avg 0.25 wt % Sn). These exceptional features of the Neves-Corvo ores call for unique ore-forming processes, which probably included a history of metal supply involving multiple sources. Its abnormal ore geochemistry, together with both stable and radiogenic isotope data, suggests that its generative sea-floor hydrothermal system may have included metal and fluid contributions from a deep-seated magmatic or metamorphogenic source, as well as mainly modified seawater, which leached the footwall rocks. The ongoing deposit-scale research in Neves-Corvo has put in evidence critical geologic and genetic factors that determined both the size and the extraordinary ore geochemistry of this deposit and, thus, ultimately, its high value. Relevant insights to integrated exploration have emerged from the identification and characterization of these factors. Also, these insights significantly widen the constraints that must be considered and applied in metallogenic modeling, thereby broadening the number and range of geologic relationships for consideration and application in future exploration projects. The collective consideration of all these understandings is especially significant in defining areas of remaining high exploration potential in long-studied districts like the Iberian pyrite belt and others worldwide.

Abstract The Neves-Corvo massive sulfide deposits in the Iberian pyrite belt of southern Portugal consist of several clustered deposits that have a unique stratigraphy and much higher grades of copper and tin than other massive sulfide deposits in this and other metallogenic provinces in the world. These deposits include the Neves, Corvo, Graca, and Zambujal orebodies, which have more than 150 million metric tons of polymetallic massive sulfides, of which 31 million metric tons average 8 percent copper. Grades of more than 5 percent tin, mainly from cas-siterite, are locally associated with the copper ore. About 3 million metric tons average 2.5 percent tin. The Iberian pyrite belt forms the main part of the South Portuguese zone, which is a geotectonic unit of the Iberian segment of the Hercynian fold belt. Rocks of upper Paleozoic age in the pyrite belt are divided into three major lithostratigraphic units, which are, from oldest to youngest: the Phyllite-Quartzite group, the Volcanic Siliceous complex, and the Flysch group. The Volcanic Siliceous unit, which consists of volcanic and sedimentary rocks, is host to massive sulfide deposits throughout the Iberian pyrite belt. The main volcanism represented by the unit took place during Upper Devonian and lower Carboniferous time. The Neves-Corvo area lies in a southern subbelt that trends northwesterly and is defined by the Estação de Ourique-Neves volcanic lineament. The Neves-Corvo deposits were explored and developed by an association formed in 1972 by a Portuguese company (Sociédade Mineira de Santiago) and two French mining companies (Société Minière et Métallurgique Peñarroya Portuguesa and Société d’Etudes et Recherche Minières). Neves-Corvo was chosen as a target area on the basis of an earlier discovery by the Serviço de Fomento Mineiro of the Direcção Geral de Minas e Serviços Geológicos (Portuguese Geological and Mining Service) of a 0.5-mGal Bouguer anomaly over geologically favorable terrane. Detailed surface studies by the association included geologic, geophysical, and geo-chemical investigations in the Neves-Corvo area as well as in other parts of the Iberian pyrite belt in Portugal. The initial drill hole, drilled in 1973 and located over the Neves gravity anomaly, passed from the Volcanic Siliceous unit into monotonous flysch-type sediments without encountering sulfide mineralization. As a result, the area was assigned a low priority and was temporarily abandoned. A second drill hole in 1977, four years later, resulted in the discovery of the Neves deposit. Since then, reserves in the four deposits have been established by over 140 km of drilling, and the mine, operated by SOMINCOR, went into production in October 1988. The discovery of the Neves-Corvo deposits suggests the possibility of other similar high-grade deposits along the southeast projection of the Estação de Ourique-Neves trend. It also demonstrates the feasibility of exploring for massive sulfide targets at depths greater than 300 m, which in the past was considered economically prohibitive; consequently, it has enlarged the favorable prospecting terrane in the Portuguese part of the Iberian pyrite belt from little more than 3,000 km 2 to over 8,000 km 2 .

Abstract This Guidebook contains information to support the three SEG field trips included in the SEG Neves Corvo Field Conference 1997 (Lisbon, May 11–14, 1997). Collectively, these field trips cover the whole Iberian Pyrite Belt and beyond. Given that the Conference is aimed primarily at participants unfamiliar with the geology of the Belt, it was considered appropriate to introduce the subject prior to the field guides. Many studies have presented the overall characteristics of the geology and mineral deposits of the IPB (e.g. Carvalho et al., 1976; Strauss et al., 1977; Carvalho, 1979; Routhier et al., 1980; Barriga, 1990). Very recently, Carvalho et al. (1997) have summarized rather thoroughly the present state of the art concerning the IPB geology and metallogenesis. The present introduction is drawn largely from this. The Iberian Pyrite Belt (IPB) corresponds to an area of Devonian-Carboniferous volcanic and sedimentary rocks containing massive polymetallic sulfide deposits. This area forms an arcuate belt, about 250 km long and up to 60 km wide, trending westwards from near Seville in Spain to west-northwest in South Portugal. Both the eastward and westward extents of the belt are covered by Tertiary sedimentary rocks (Figure 1). The IPB is arguably the largest and most important volcanogenic massive sulfide (VMS) metallogenic province in the world. Some of its mineral deposits have been known and mined since the Chalcolithic era such as the Rio Tinto deposit, renowned for its historical role in the study of ore deposits. Only after the discovery of the large and rich copper-tin ore body of Neves Corvo (Southern Portugal) in 1977 has the true importance and potential of the IPB become fully appreciated. The original, pre-erosional amount of sulfides concentrated in about 90 known deposits are estimated at more than 1. 7 billion tons. Of this amount, about 20 percent has been mined, and 10-15% lost to erosion. This impressive amount of metals, in concentrations that range from small lenses with thousands of tons to giant bodies with hundreds of million tonnes, in such a relatively small area, represents an outstanding global geochemical anomaly of S, Fe, Zn, Cu, Pb, Sn and several other metals. The Iberian Pyrite Belt is located in the Southwest of the Iberian Peninsula, and comprises a large part of the Setubal and Beja districts in Portugal, and Huelva and Seville provinces in Spain (Figure 2). The region is an eroded peneplain, plunging gently to the

Abstract The Kidd Creek mine is an Archean volcanogenic Cu-Zn deposit with total past production and current reserves of more than 138.5 Mt at 2.4 percent Cu, 6.5 percent Zn, 0.23 percent Pb, 90 g/t Ag, and up to 0.15 percent Sn. The massive sulfides occur at the top of a locally thickened felsic volcanic pile, within and overlying a succession of massive rhyolite flows, volcaniclastic rocks, and coarse epiclastic units. The felsic volcanics occupy the core of an anomalous, S-shaped fold structure and attain a maximum thickness of approximately 300 m beneath the deposit. Massive autobrecciated rhyolite occurs at the base of the mine sequence and is interpreted to be a proximal vent facies. The local volcanic basement comprises mainly ultramafic flows, intercalated with minor rhyolite. The ultramafic rocks are interpreted to be early extrusive lavas associated with the development of an extensional rift. Basaltic pillow lavas and breccias occur in the hanging wall of the mine and are extensively intruded by gabbroic sills. South of the mine, this stratigraphy is truncated along the contact with younger, regional metasedi-mentary rocks. Kidd Creek is typical of a class of large volcanogenic massive sulfide deposits that occur within thick successions of permeable felsic volcaniclastic rocks and are dominated by large, stratiform, Zn-rich lenses with laterally extensive zones of ore-grade Cu mineralization. The deposit consists of three main ore-bodies (the North, Central, and South orebodies) that are distributed along an inferred boundary fault of a linear, grabenlike depression. The present deposits have a restored strike length of at least 2 km, indicating remarkable continuity of the hydrothermal system along the length of the graben. The main ore lenses formed by infilling and strata-bound replacement of volcaniclastic rocks, coarse volcanic breccias, and finer grained tuffs that filled the graben. Abundant relics of silicified rhyolite within the massive sulfides, gradational contacts between the massive sulfides and unmineralized fragmental rocks at the margins of the ore zones, and extensive replacement within the hanging-wall breccias confirm that a large part of the deposit formed below the sea floor. Burial of the deposits by mass flows was coincident with mineralization, and subsea-floor deposition of sulfides progressed laterally into the volcaniclastic rocks adjacent to the ore lenses. Metalliferous sediments or exhalative horizons are notably absent, and there is little evidence that widespread venting of high-temperature fluids occurred at the sea floor. Deposition of sulfides within the thick sequence of basin fill ensured that ore-forming fluids were confined to the graben and relatively little metal was lost to high-temperature discharge. The development of the three main orebodies is best explained by a long-lived, low-temperature hydrothermal system punctuated by several higher temperature pulses of Cu-rich fluid. Focusing of the fluids was caused by intense silicification of the rhyolite above and adjacent to the main upflow zone. Extensive lateral flow occurred within the bedded volcaniclastic rocks, and the highest temperature fluids appear to have occupied a number of high-level aquifers beneath the deposits. These are marked by conformable lenses of chlorite alteration, semimassive chalcopyrite, and strata-bound chalcopyrite stringer mineralization. The larger alteration envelope is broadly conformable to the ore lenses and consists of quartz and sericite, together with chlorite, Fe-rich carbonate, and minor tourmaline. Two main ore suites occur at Kidd Creek: a low-temperature, polymetallic suite enriched in Zn, Ag, Pb, Cd, Sn, Sb, As, Hg, ±Tl, ± W, and a higher temperature suite of Cu, Co, Bi, Se, In, ± Ni. The massive ores consist mainly of pyrite, pyrrhotite, sphalerite, and chalcopyrite, together with galena, tetrahedrite, ar-senopyrite, and cassiterite, in a quartz and siderite gangue. However, more than 60 different ore minerals and ore-related gangue minerals are present, including complex assemblages of Co-As sulfides, Cu-Sn sulfides, Ag minerals, and selenides. Tin is present as cassiterite in the upper part of the massive sphalerite lenses and as stannite in the underlying chalcopyrite-rich ores. Despite the high Ag content of the deposit, Kidd Creek is remarkably Au poor. The ores exhibit a close chemical affinity with their immediate felsic host rocks, including strong coenrichments of Ag, Pb, As, Sn, W, and F However, the complex metal assemblage suggests that a more primitive mafic suite may also have played a role in metal supply. The extensive metagraywackes to the south of the mine are younger than Kidd Creek and therefore could not have been a source for metals. An abundance of pyrrhotite, arsenopyrite, high Fe sphalerite, and Fe-rich chlorite indicates predominantly low fO 2 –fS2 conditions, and the abundant siderite in the ore indicates that the hydrothermal fluids were highly enriched in CO 2 . Sulfur isotope compositions range from -2.4 to +3.3 per mil, with the bulk of the massive sulfides having S 34 S values close to 0 per mil. The mineralogy and bulk composition of the Kidd Creek ores bear a closer resemblance to those of many Phanerozoic Zn-Cu-Pb deposits (e.g., Bathurst, Neves Corvo) than to other Archean Cu-Zn deposits. The predominance of Zn-rich ores (ca. 70–80 Mt) implies that most of the deposit formed at low temperatures (ca. 250°C). Solubility modeling indicates that a large hydrothermal system at relatively low temperatures would have been sufficient to account for about 75 percent of the metals. The significant enrichments in Ag, Pb, and Sn reflect not only the abundance of felsic volcanic rocks in the mine sequence but also the sustained, low-temperature venting history of the deposit. In contrast, the Cu-rich ores appear to have been introduced during relatively short-lived, hydrothermal pulses at much higher temperatures. The higher temperatures most likely coincided with discrete felsic magmatic events that occurred at several intervals during the ∼3.5 m.y. history of the volcanic complex. The late-stage introduction of Cu may indicate that the Cu-rich fluids evolved separately from the lower temperature, con-vective part of the hydrothermal system. This model is supported by the presence of a high-grade bornite zone in the South orebody, which represents a massive influx of Cu metal at peak hydrothermal temperatures late in the development of the Cu stringer zone. Kidd Creek resembles sulfide deposits that are currently forming in young, intraoceanic back-arc rifts, such as the Lau basin, and this may be an appropriate modern analogue for the Kidd Creek setting. The combination of voluminous mafic-ultramafic flows in the footwall of the deposit, punctuated by anomalous felsic volcanism, and the extensive deposits of coarse epiclastic rocks and volcaniclastic sediments suggest that Kidd Creek formed within a subsiding rift basin. The importance of a plumelike source for the ultramafic melts and the longevity of the hydrothermal system may indicate that rifting occurred above a stationary hot spot.