The Neves-Corvo Deposit, Iberian Pyrite Belt, Portugal: Impacts and Future, 25 Years after the Discovery
Jorge M.R.S. Relvas, Fernando J.A.S. Barriga, Álvaro Pinto, Alfredo Ferreira, Nelson Pacheco, Paulo Noiva, Gonçalo Barriga, Rui Baptista, Delfim de Carvalho, Víctor Oliveira, José Munhá, W. Hutchinson Richard, 2002. "The Neves-Corvo Deposit, Iberian Pyrite Belt, Portugal: Impacts and Future, 25 Years after the Discovery", Integrated Methods for Discovery: Global Exploration in the Twenty-First Century, Richard J. Goldfarb, Richard L. Nielsen
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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.
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Integrated Methods for Discovery: Global Exploration in the Twenty-First Century
The dominant forces that affect the mining industry in today's economy are globalization, entrepreneurship, and unprecedented minerals availability. The current malaise and diminished economic importance of the base and precious metal mining industry in the world economy are the result of ongoing, longterm trends. Shifting market forces within a changed geopolitical landscape have resulted in excess supplies and reduced demands for high-unit-value mineral products and for geologists and engineers in the exploration and mining industries. Although these changes are still evolving, professionals and the mining companies that employ them must realize that these changes are irreversible.
Mining has five dominant characteristics: (1) it is essential to society, (2) it is effective in keeping society supplied with abundant, low-cost mineral products, (3) historically, it has been wealth destructive for investors, (4) it has been subsidized, and (5) it is shaped by social and political forces. What is different now is that instead of being subsidized, the industry is being handicapped by governments and abandoned by investors. The industry has reacted to a marketing problem with an inappropriate strategy of cost reduction and increased production. The increased supply has resulted in decreased prices. Mining companies are earning low or negative rates of return.
Remedies for the larger mining companies and the high-unit-value metals businesses include: Exemplary behavior to regain the trust of the public, governments and investors; Continued consolidation to become sector leaders; Profit through value-added vertical integration and direct marketing to consumers Development of innovative consumer-based financing mechanisms, especially for exploration ventures Individuals and small groups that have been displaced from the mainstream must become entrepreneurs. They must engineer their own survival by shifting careers or by finding, developing, and capitalizing on exploration and production opportunities that are unrecognized by, or are too small for, the major companies.
The mining industry is here to stay. It is too necessary to society to be abandoned; however, the future size and shape of the industry will probably be unrecognizable to most of us. It will be leaner and more opportunistic. It will be characterized by entrepreneurial corporations of all sizes that dominate niches and sectors and that anticipate and profit from the changing needs of society