Exploration for world-class Ni-Cu-(PGE) deposits in mafic and/or ultramafic igneous rocks has focused on extensional environments where high degrees of mantle melting have occurred in association with mantle plumes. Where continental rifting has been involved, the interaction between large volumes of mafic magma and crustal rocks in either intrusive or extrusive settings may have resulted in contamination that triggered sulfide saturation or melting of sulfides within country rocks. Staging chambers and conduits in the subvolcanic environment and embayments associated with channels in the volcanic environment are localities where immiscible sulfide liquid may accumulate. The large-tonnage, high-grade deposits in conduit and magma chamber environments, such as those at Noril’sk, Siberia, remain high priorities for greenfields exploration, and it is now clear that intrusions with even small footprints may be important exploration targets. Examples of small footprint deposits include the large-tonnage ore systems at Voisey’s Bay in the Nain plutonic suite, Labrador, and the low-tonnage, high-grade mineralization at the Eagle deposit in the Keweenawan of northern Michigan. The high-grade mineralization in small deposits is particularly attractive as incremental feed if smelters are located nearby and transportation routes are available. Low-tonnage, high-grade deposits can also be mined using underground methods, and having lesser environmental impact and remediation is typically more straightforward.
Although convergent margin environments have not been universally viewed as viable target areas for magmatic sulfide-rich Ni-Cu-(PGE) deposits, suprasubduction zone environments have high degrees of mantle melting, and they provide locations for crust-magma interaction and conduit geometries where sulfides may collect. Deposits such as Kalatongke in China, Aquablanca in Spain, and the Turnagain and Duke Island Ural-Alaskan intrusions illustrate that convergent margins should not be dismissed as targets for magmatic Ni-Cu-(PGE) ores.
New advances in hydrometallurgical techniques, particularly pressure leach methods, are making the extraction of Cu, Ni, and PGEs from large-tonnage but low-grade deposits economically promising. The large disseminated sulfide-rich Ni-Cu-(PGE) resources of the Duluth Complex are an example where advances in process technology may permit future development of low-grade occurrences that have traditionally been considered to be of marginal economic value.
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The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries
VOLCANIC-ASSOCIATED and sedimentary-exhalative massive sulfide deposits on land account for more than one-half of the world's total past production and current reserves of zinc and lead, 7 percent of the copper, 18 percent of the silver, and a significant amount of gold and other by-product metals (Singer, 1995). A new source of these metals is now being considered for exploitation from deep-sea massive sulfide deposits. Because the oceans cover more than 70 percent of the Earth's surface, many expect the ocean floor to host a proportionately large number of these deposits. However, there have been few attempts to estimate the global mineral potential. Significant accumulations of metals from hydrothermal vents have been documented at some locations (e.g., 91.7 Mt of 2.06% Zn, 0.46% Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au in the Atlantis II Deep of the Red Sea: Mustafa et al., 1984; Nawab, 1984; Guney et al., 1988). Even more metal is contained in deep-sea manganese nodules. Current estimates in the U.S. Geological Survey (USGS) mineral commodities summaries indicate a global resource of copper in deep-sea nodules of about 700 Mt. In the Pacific "high-grade" area, an estimated 34,000 Mt of nodules contain 7,500 Mt of Mn, 340 Mt of Ni, 265 Mt of Cu, and 78 Mt of Co (Morgan, 2000; Rona, 2003). A number of countries, including China, Japan, Korea, Russia, France, and Germany, are actively exploring this area.