The first serious suggestion that the Archean atmosphere was reducing was based on the interpretation of round uraninite and pyrite grains in the Witwatersrand Basin in the early 1950s. It was then inferred that these minerals were detrital and that they reflected equilibrium with a reducing Archean atmosphere. Over the past 20 years the understanding of the Witwatersrand Basin has changed dramatically with more integrated studies of the basin and the recognition of widespread alteration in close spatial association with the mineralization in every goldfield. Post-depositional mobility of gold, sulfur, and uranium during alteration is widespread and supports hydrothermal ore genesis, or at least substantial modification of the original mineral assemblage. Pseudomorphic replacements of pre-existing detrital minerals (e.g., pyrite after titano-magnetite), and precipitation and/or chemical rounding to generate round mineral shapes (e.g., uraninite in carbon seams) have all been documented. The recognition that the carbon seams formed by the post-depositional introduction and maturation of migrated hydrocarbons is a dramatic departure from earlier models of coalified algal material deposited with the sediments. The enrichment of both gold and uraninite in carbon seams implies that these minerals are hydrothermal and that their shapes do not reflect detrital processes. Uranium mobility in basinal waters may in fact require a relatively oxidizing atmosphere. None of the existing arguments for the Witwatersrand mineralization unambiguously support a placer or modified placer model for the mineralization. Consequently, round uraninite and pyrite of the Witwatersrand Basin do not provide support for a reducing Archean atmosphere.

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

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