Prospecting Above Ground

One good way to find hidden minerals may be to study the plants and trees that grow in the neighborhood. In the current Mining and Metallurgy, Finnish Geochemist Kalervo Rankama tells why vegetation is a prospector's friend.

Many kinds of ore cannot be detected by geophysical instruments. Their reactions to electric currents and other physical tests are too much like the "country rock" around them. But all minerals are soluble in water to some extent. Ground water seeping through the rock may pick up so much of a metallic salt that the metal's presence can be detected by ordinary chemical analysis.

In other cases, the exceedingly dilute solution defies the most sensitive tests. Then geochemists look to the neighboring plants, whose roots reach into the soil and draw mineral-laden water to the surface. When the water evaporates through the leaves, the minerals it carried remain in the plant's tissues, eventually falling to the ground and becoming part of the humus on the surface. Geophysicists analyze this "biologically enriched" layer and the leaves of growing plants. Finnish geochemists found a rich copper-nickel deposit by examining the ashes of birch leaves.

Plants occasionally do the analyzing themselves. Many dissolved minerals are poison to some plants and healthy food to others. A certain wild pansy (Viola calaminaria, et zinci) thrives on the waste dumps of zinc mines where little else can survive. The presence of other plants points to copper, lead or petroleum.

Use of plants as mineral "indicators" is still a new science. When it is further developed, a prospector will have to know botany as well as geology. For example, if he finds a plant called Amorpha canescens growing where little else grows, he will have a good hint that the rock beneath the roots is worth investigating for lead ore.

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