Remote Sensing

No ore deposit has yet been found directly by the highly publicized "remote sensing" techniques of exploration from spacecraft. Most of the methods used are adaptations of techniques well known and evaluated in a variety of laboratory, ground station, surface vehicle, and aircraft installations. A tremendous amount of basic scientific data is being collected which cannot fail to be of major value in mineral exploration if properly coordinated with basic geologic concepts and evaluated by personnel experienced in ore search.

Several applications of activation analysis techniques show considerable promise in mineral exploration, and improved versions of instrumentation are becoming available for field use. An intense radioactive source is mounted within lead or paraffin shielding. When the shielding is raised so that the surf ace area to be sampled is subjected to radiation, some elements respond be giving off a radiation that is measured by a counter within the apparatus. The method might be compared to an interrogation-reply mechanism.

A typical portable instrument can be used only for one element, and the equipment is cumbersome, expensive, and must be operated by trained personnel under Energy Research and Development Agency (formerly Atomic Energy Commission) license. In spite of these shortcomings, limited use thus far has been spectacular in such applications as the search for beryllium ores. Several major deposits of beryllium have been found in old mining districts generally considered to have been thoroughly explored.

Most ore deposits in the western United States do not respond well to any kind of geophysics or are too small and irregular to produce an anomaly sufficiently distinctive to interpret and explore. On the Canadian Shield, the typical ores possess good geophysical response characteristics. Outcrops were scoured clean of weathering effects by ice Age glaciation, so that weathering does not interfere. The thin layer of glacial drift over much of the region made traditional prospecting methods ineffective, and many ore deposits have been discovered in recent years by geophysics. Canadian explorationists are therefore much more likely to be enthusiastic about geophysical exploration than their colleagues in western North America, who are more accustomed to the complicated, unresponsive, weathered ores of the deserts and mountains.

The geophysical method that might be useful in one area may prove wholly inappropriate in another. For exam pie, airborne scintillation counters were used very effectively in radiometric reconnaissance for bedded uranium ores of the Colorado Plateau in Colorado, Utah, Arizona, and New Mexico. The same technique, when applied to exploration for uranium in Canada, was a total failure. It was found that the Colorado deposits were relatively high grade, and enclosed in a sequence of virtually non-radioactive sediments. The Canadian exploration was conducted in a terrain of granite and metamorphic wall rocks that themselves were radioactive, resulting in such a hash of background signal and false anomalies that the airborne surveys failed to delineate useful target areas within the static.

In general, the most discouraging aspect of geophysical exploration is the spurious result frequently obtained. For example, the ore deposits that furnish the best electromagnetic responses are massive sulfides, which are found in rocks containing variable amounts of pyrite and graphite. The pyrite and graphite, which are worthless and commonly show no meaningful distribution pattern in relation to the ores, yield a geophysical response that cannot be distinguished from that of the ore itself.

Many exploration holes are drilled into electromagnetic anomalies, only to encounter barren pyrite or graphite. Conversely, negative geophysical results by no means rule out the presence of an important ore deposit. For example, the most careful magnetic survey over an "invisible" gold deposit of the kind being found in northern Nevada could not be expected to delineate ore, because these deposits contain no minerals capable of measurably distorting magnetic patterns.

It is obvious that the application of geophysics involves more than the simple ability to make the equipment work. To be successful, the geophysicist must be thoroughly grounded in fundamental ore deposit theory, or must work closely with an exploration geologist in planning and interpreting the work.