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(i)

Understanding Our Planet Through Chemistry

III. Mapping the Chemistry of the Earth's Surface

IIa. Assessment of Public Lands

Mapping stream sediments for resource exploration

The successes of the old-time and latter-day prospectors have diminished the likelihood for the discovery of additional mineral resources on the surface of our planet. Yet our national and global dependence on mineral resources continues to grow unabatedly, and recycling can only provide a fraction of our needs. By necessity, today's search for the many minerals vital to society is focused on ore deposits that lie beneath the Earth's surface.

Earlier in this Session (back by the illustration of the copper-molybenum porphyry cross section) we discussed the use of models to locate ore deposits. Another way of locating mineral resources is by identifying element-dispersion halos. Dispersion halos are abnormal levels of the metals that develop around deposits. This halo can extend for long distances from the deposit and, once recognized, can be used to trace down the source. The most familiar example of a halo is the dispersion of gold nuggets in drainages downstream from gold mother lodes.

Using today s technology, collected stream-sediment samples may be processed and analyzed for as many as 40 elements, giving an indication of very faint halos at some distance from a variety of deposit types. If elements of economic interest, such as gold, silver, copper, lead, or zinc, are present, they will be revealed in these analyses. This process is repeated for many samples until the entire study area is covered.

Photo of scientists sampling soils in Alaska with helicopter in background.By evaluating our nation's mineral resources, we can determine the appropriate use of Federal lands. Helicopters have little impact on the land and can be used in remote areas, such as Alaska, to efficiently gather samples for geochemical analysis.[30k] [104k]

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