A Case History of the Neves-Corvo Massive Sulfide Deposit, Portugal, and Implications for Future Discoveries

Abstract

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 km2 to over 8,000 km2.