economic iron resources, and the paleoplacers include more than one-half of the total known gold resources. No accumulations rivaling in size those banded-iron formations and gold-bearing paleo-placers have been discovered in rocks that have formed since that ancient period.

These cases of mineral formation—distinguished by time of genesis—are relatively straightforward, and there is little likelihood of statistical aberration. An explanation of their formation could be complex, however. For example, it is agreed that the mantle of the younger Earth was hotter than it is today. The very high-temperatures, approaching 1600°C, required to keep komatiites molten could have occurred close enough to the surface to permit eruption of the rock magmas. Formation of the large iron deposits and the gold-uranium-rich placers has been attributed to a different composition of the early atmosphere.

Examples of other time-dependent processes are little more than as yet unexplained correlations. Some well-documented associations between deposit type and geological time may result from a higher probability of exposure and weathering of deposits formed near the surface in continental environments. Erosional surfaces can be recognized, but there is no way of knowing what has been eroded. And because some deposits can be found only in rocks that have formed recently does not prove that the conditions necessary for their formation have occurred only recently. Deposits such as the Mississippi Valley type appear only in relatively young formations, but similar deposits formed in the past may have been subsequently removed from the geological record.

The ores of the Mississippi Valley type, as exemplified in southeast Missouri and the Joplin region of Oklahoma, Kansas, and western Missouri, are prime sources of zinc and lead within the conterminous United States (Figure 4.9). The ores result from the filling of solution cavities in carbonate rocks with metal-rich material at relatively low-temperatures (90° to 125°C). Recent developments that have contributed to understanding this process include the discovery of brines rich with zinc and lead in deep petroleum exploration wells drilled throughout the region, recognition that metal-bearing brines have permeated carbonate rocks extensively through the midcontinent regions, and advanced modeling of fluid flow in compacted sedimentary basins. These scientific developments have produced a refined model of mineral deposition based on the expulsion of metal-bearing brines from a sedimentary basin that concentrates mineralization around the margins of the host basin. The more eastern Mississippi Valley-type mineralization was probably emplaced as a discrete pulse during the collision of an island arc with North America about 450-million-years ago. The larger western episode of mineralization was related to the collision of Gondwana with Laurentia, during the assemblage of Pangea about 300-million-years ago. In both dramatic events, mountains depressed the North American continental margin and drove subsurface fluids toward the interior of the continent.

FIGURE 4.9 Map showing the relationship between Mississippi Valley-type lead-zinc deposits and tectonic belts and basins. After J. E. Oliver, NRC, 1990, The Role of Fluids in Crustal Processes.

Researchers suspect that the cessation of komatiite eruptions resulted from the long-term cooling of the Earth; other changes, such as the closing of ocean basins during continental assemblage, are cyclic. Another example of cyclic change that affects mineral deposition is the global warming that dissipates polar ice caps. Such a warm period occurred 100-million-years ago. Sea level was high; circulation of cold, well-oxygenated polar water toward the equator was diminished; and the deep oceans were warmer than the near-freezing temperatures of today. This caused stagnation in the deep basins; with a continual supply of organic matter from dying organisms near the surface, the deep waters became chemically reduced. This permitted the reduction of manganese oxides on the seafloor to form the relatively soluble divalent manganese. Solution progressed and the manganese content of

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