Giant metallic deposits鈥擜 century of progress
详细信息   
摘要
Giant (and super-giant) metallic deposits are defined as those that store the trace metal (and some major metal like Fe, Al) equivalent in1011 (1012) tons of continental crust in Clarke (mean crust content) concentration. Deposits of metallic ores that have very contrasting Clarke values (like Fe, Cu and Au) can be compared on geochemical basis, with political–economic and technologic factors minimized. Under these terms, there are now 1171 giant and 137 supergiant accumulations of 37 metals worldwide, contained in 915 deposits, as several deposits have two or more giant metal accumulations (Olympic Dam has 5). These deposits store and supply between 60 and 95% of global metallic resources on land, with several individual deposits monopolizing the supply. It is predicted these exceptional deposits will remain the principal metal source for the industry at least through this century. The “ore giants” are dominated by gold and copper (278 and 268 entries), followed by Mo (166), Ag (119) and Pb (90). This metal selection is more the consequence of demand and price than geological availability, proving that when there was a demand and the “right” price, the resources industry has been able to discover and develop new deposits to satisfy this demand. This may change in the future so unconventional metal sources are reviewed and compared with the classical, high concentration factor ores.I have not been able to find a single case where a giant deposit would be a unique, one-of-a-kind product. In all instances, the ore giants are magnitude end members of a population of lesser deposits of the same, or similar, type so they are only quantitatively distinct from the rest. For that reason discovery of the ore giants is statistical and cannot be exclusively targeted, despite the fact that in some cases the “giants” were among the first deposits discovered. Although the ore giants are not qualitatively different from the lesser members of the same type, they are the product of the best optimized mineral system. A number of metal accumulation magnifiers have been suggested in the literature and they are briefly reviewed here.More than 70% of the ore giants have been discovered in the past sixty years; the discovery rate grew steadily since the Industrial Revolution in the 19th Century when a number of new chemical elements had been discovered, to accelerate in the 1890s, then again in the 1950s. The 1965–1970 and 1990–1995 intervals had the greatest number of giant deposits discovered (38 and 43, respectively) and this not only replenished the consumed metals, but provided a significant surplus. These periods may have been discovery peaks followed by diminishing ore finding rate, suggesting future metal supply scarcity. With continuing depletion of ore discoveries exposed at the surface the number of deposits found under cover has grown almost exponentially; by 2010, close to 150 partially to completely buried ore giants have been found, some in a depth approaching 3000 m. This has only been possible by continuously increasing complexity and cost of ore finding, from visual on-foot discoveries prevalent before 1950 to the instrument-assisted discoveries by corporate teams. The prevalent technique of ore discovery (followed by proving) is drilling, positioned by geochemical and geophysical anomalies, in turn the product of practical geological models and creative human reasoning. As metal mining is an economic activity made possible by geological availability of resources, it is believed that the production costs (combined with environmental and political considerations) will govern the future transition from the “classical” (high concentration factor) ores to the lower-grade and more difficult to extract sources of tomorrow like oceanic resources, with increased role of recycling.