Prospectors have contributed much to the development of this Nation's mineral resources. Since the time of the earliest settlement, the need for iron for tools and guns, lead for bullets, and copper for utensils has prompted a search for sources of these metals. The lure of gold and silver provided the im- petus for much of the development in the West between 1850 and 1910. Later, prospectors carried out successful ventures to fulfill the country's expanding industrial demands for other metals such as zinc, molybdenum, tungsten, chromium, vanadium, and many others. Even America's uninhabited rugged mountains or barren deserts have been prospected although perhaps only at a reconnaissance scale.
Nearly one billion tons of surface and underground material are mined annually in the United States to recover about one-half billion tons of metallic ores, principally iron and copper(U.S. Bureau of Mines' "Minerals Yearbook, 1986," 1988, v. 1, p. 25). Even greater amounts of ore must be found in the future to meet the Nation's increasing needs and to replace exhausted deposits. The easily found deposits have already been discovered; suc- cess in finding new deposits will de- pend more and more on modern prospecting techniques.
The modern prospector has ad- vantages that to some extent make up for the increased difficulty of finding ore deposits. One advantage is greatly increased knowledge about the geologic factors that localize ore deposition. The search for new deposits has become a complex undertaking, and the pros- pector should be as well-informed as possible. The prospector should acquire the ability to identify not only ore minerals but also common rocks and their minerals, as well as many kinds of geologic structures. This knowledge is best acquired by academic training, but much can be learned from studying reference books such as "Exploration and Mining Geology" (Peters, 1987), "Handbook for Prospectors" (Pearl, 1973), and others listed at the end of this booklet.
The first technique used in a prospecting venture is geological in- ference. The prospector studies re- ports, geologic maps, and cross sections of a region to pinpoint areas where there are structures, rocks, and minerals with which ores are usually associated. These areas warrant further exploration. Topo- graphic maps or aerial photographs of these targeted areas are ob- tained and used in plotting informa- tion, such as locations for sampling. This booklet briefly describes basic prospecting techniques geochem- ical, geophysical, and combination methods that can help lead to the discovery of an orebody. An orebody, as defined by the U.S. Bureau of Mines' "Dictionary of
Mining, Mineral, and Related Terms" (Thrush and others, 1968), is "a mineral deposit that can be worked at a profit under existing economic conditions."
Remote Sensing is a relatively new method of mineral exploration, utilizing imagery of the Earth's sur- face collected by instruments on air- craft and Earth-orbiting satellites. Imagery of radar, color infrared, thermal infrared, and other reflected or radiated electromagnetic energy, can show features such as struc- ture, vegetation, and rock types that may not be discernable by conven- tional ground-based prospecting methods.
Prospecting equipment can be used in many ways, according to the minerals sought and the meth- ods employed in the search. Its effectiveness depends in large measure on the operator's aware- ness of its applications. For exam- ple, radiation counters (the scin- tillometer, for example) not only detect radioactive minerals for the uranium prospector, but can also be useful to placer prospectors. Some placer deposits contain both gold and heavy radioactive minerals
such as monazite; the radiation counter points out gold concentra- tions by detecting the associated monazite. A "black light" (ultraviolet lamp), commonly used in prospec- ting for fluorescent ore minerals such as scheelite (a tungsten ore), is also useful in detecting fluores- cent rock-forming minerals such as calcite, barite, or fluorite, which can be indicators of associated metallic minerals.
The residues left in rocks after ore minerals have been removed by weathering also may be clues to ore deposits. The rusting of iron is a familiar example of the change in materials exposed to weathering. Primary ore minerals (that is, minerals deposited in the rock for- mation by ascending ore-forming solutions) near the surface may be oxidized and carried in solution downward from their original posi- tion. They may be either redepos- ited as secondary minerals in the rocks below, thus enriching lower parts of the mineral deposit; or the minerals may be dissolved and completely removed, resulting in a process of depletion rather than enrichment.
Regardless of what happens to the ore minerals during weathering, some evidence of their former presence remains. Commonly, iron oxides and iron hydroxides form and remain as brown or yellow stains and encrustations on the sur- face rocks. The solution of minerals can result in a sponge-like, iron- stained, porous rock called gossan evidence that primary minerals have been oxidized and at least partly removed. A search for these minerals at depth in such places may be fruitful. Mineral com- pounds containing copper, nickel, cobalt, molybdenum, uranium, and other metals may oxidize to form brightly-colored secondary minerals on the surface rocks. These minerals are found mainly in dry regions because many of them are soluble in water and would be leached away in areas of heavy rainfall.
Mineral deposits have also been located by following float (pieces of rock found on the ground surface) uphill to its source. When rocks are being eroded, fragments are carried downhill by gravity and downstream by water. The source of the float can be found by tracing the pieces up slope.
The systematic panning of stream sediment and residual soil for gold or other resistant heavy minerals has long been used to find bedrock lodes. The gold pan, an indispen- sable prospecting tool, is versatile, efficient, inexpensive and portable. From a sample, an experienced panner can recover 80 percent of the minerals that are heavier than quartz (specific gravity 2.65). Even an inexperienced person can obtain a concentrate of heavy minerals from an ore-bearing sample that will contain virtually all of the sample's coarse gold and platinum, and a high percentage of its magnetite (specific gravity 5.2) and heavier minerals such as cassiterite, cerussite, coulumbite-tantalite, scheelite, and silver. Many minerals such as hornblende, biotite, mus- covite, epidote, garnet, pyrite, cin- nabar, and galena are easily recognized during the washing proc- ess, and the skillful panner can recover most of these minerals present in a sample. A sample that is "panned down" to "black sands" generally also contains a number of light-colored heavy minerals such as barite, zircon, sphene, monazite, and scheelite, in addition to the darker minerals such as magnetite, ilmenite, hematite, and sulfides. All these minerals can be clues to valuable deposits in nearby or upstream areas.
Other devices can be used in conjunction with a gold pan to test large samples; dozens of varieties of sluiceboxes, rockers, suction devices, and spiral concentrators are available from prospecting and mining equipment dealers. An inex- pensive portable sluicebox for use in sampling placer deposits can be made from three pieces of 1/8-inch sheet aluminum, each 3 feet long and about 1 1/2 feet wide. The metal is shaped into flat-bottomed troughs about 1 foot wide with sides 3 inches high; the three sections are bolted end-to-end to form a 9-foot sluice. The bottom of the sluice box is fitted with carpet, burlap, wooden cross-slats or riffles, or other material to trap fine heavy mineral particles. Covering the material with a coarse screen prevents pebbles from clogging the trapping material. Water is delivered to the sluicebox by means of a pump driven by a small gasoline engine, or the sluicebox can be placed in the stream to use the natural flow of water. The bottom end of the sluicebox should be part- ly closed to slow down the water flow and to aid in trapping heavier particles. The carpet or burlap con- taining the heavy mineral concen- trate is removed from time to time,placed in a tub, and thoroughly washed. The washings are then fur- ther concentrated by panning. Working diligently, one can wash a ton of gravel in a day and expect to recover 50 percent or more of the black sands present.
Many mineral deposits are not exposed at the Earth's surface. They may be concealed by thick soil cover or may lie buried beneath layers of rock. To find them, more complex techniques based on geochemistry, geophysics, and geobotany can be very helpful. Most of these techniques require specialized training, and, in some instances, expensive equipment. These techniques are described below.
Geochemical prospecting is based
on systematic measurement of
chemical properties of rock, soil,
glacial debris, stream sediment,
water, or plants. The chemical prop-
erty most commonly measured is
the content of a key trace element.
Zones in the soils or rocks of com-
paratively high, or anomalous, con-
centrations of particular elements
may guide the prospector to the
elements in rocks or soils that con-
stitute a geochemical anomaly (dif-
ferent from normal). The actual
amount of the key element in a
sample may be very small and yet
constitute an anomaly if the sam-
ple's concentration is high relative
to the concentration of the surround- ing area. For example, if most samples of soil are found to contain about 0.00001 percent (0.1 parts per million, or ppm) silver, but a few contain as much as 0.0001 percent (1 ppm), the few "high" concentra- tions are geochemical anomalies. Plots of analytical results on a map may indicate zones to be explored further.
Geochemical anomalies are classified as primary or secondary. Primary anomalies result from out- ward dispersion of elements by mineral-forming solutions. High con- centrations of metals surround the deposit, and the dispersion of metals laterally or vertically along fractures or faults may result in a "halo" surrounding the deposit. Halos are especially useful in pros- pecting because they may be hun- dreds of times larger than the deposit they surround and hence are easy to locate.
Secondary anomalies result from dispersion of elements by weather- ing. Some primary minerals, such as gold or cassiterite, are resistant to chemical weathering and are
transported by the streams as fragmental material. Other primary minerals may be dissolved and the metals may be either redeposited locally or carried away in solution in ground and surface waters. Some metals in solution are taken up by plants and trees and can be con- centrated in their tissues. Many studies have been made of the metal content of residual soils over sulfide deposits, and in general the distribution of anomalous amounts of metal in the soil has been found to correspond closely with the greatest concentration of metals in the underlying rock.
Most products of weathering in a drainage basin enter the streams and rivers that flow across it. The weathered products occur as chem- icals in solution in the streams' water and in their sediments. Either
or both can be sampled and tested, and composition of the samples will reflect the chemical nature of the rocks in the drainage basin. The presence of ore may be determined by sampling water and sediment from each successive tributary and by analyzing the samples for anom- alous amounts of metals. This pro- cedure narrows the search for ore deposits to the most favorable areas.
Contamination of surficial material by human activity is an ever-present hazard in geochemical surveys. The most common sources of contam- ination are materials derived from mine workings. Similarly, smelter fumes, wind-blown flue dust, and metallic objects may also con- taminate the soils and rocks. Such materials may oxidize and go into solution, contaminating the soil, stream sediment, and water nearby, thus masking natural anomalies.
Analytical methods used in geochemical prospecting must be sensitive enough to determine minute amounts of key elements, accurate enough to show small dif- ferences in concentration, fast enough to permit large numbers of samples to be analyzed in a day, and inexpensive. Wet chemical techniques are usually confined to rapid colorimetric procedures that require a minimum of equipment and materials. Instrument tech- niques, such as emission spec- trographic and X-ray fluorescence, require expensive equipment and trained personnel, but usually yield a lower cost per determination if thousands of samples must be analyzed.
Wet Chemical Methods
Analytical techniques for many elements have been devised for use in geochemical prospecting. These range from very simple procedures that can be accomplished in the field, through less simple pro- cedures that can be carried out in an improvised laboratory at a camp- site, to complex procedures that re- quire a well-equipped laboratory.
Simple procedures to test for heavy metals, as well as campsite tests mostly requiring heating and leaching, are described in U.S. Geological Survey Bulletin 1152 (Ward and others, 1963). Their precision is adequate for prospec- ting, and the costs are not high. Commercial kits for some of these tests are available starting at reasonable cost; they are advertised in most popular mining journals.
U.S. Geological Survey Circular 948 (O'Leary and Meier, 1986) describes methods for determination of gold, calcium, indium, lithium, magnesium, potassium, sodium,
tellurium, thallium, tin, tungsten, and uranium; U.S. Geological Survey Bulletin 1408 (Ward, F.W., ed., 1975) discusses methods for testing for antimony, arsenic, bismuth, cadmium, cobalt, copper, fluorine, lead, mercury, molyb- denum, nickel, selenium, silver, and zinc.
More sophisticated methods of analysis, particularly those employ- ing hazardous chemical or com- plicated procedures, are best done in an established laboratory. Labor- atory methods usually permit about the same productivity as the camp- site methods but require a trained chemist to perform them correctly.
Instrument techniques. The types of instruments used mostly for large-scale prospecting are emis- sion spectrographs, atomic absorp- tion spectrophotometers, and X-ray spectrographs because they permit quick identification of most elements. Instrument techniques are described in detail in U.S. Geological Survey Bulletin 1770A-K (Baedecker, 1987) and in U.S. Geological Survey Circular 948 (O'Leary and Meier, 1986).
Emission spectrographic methods have been widely used and have the distinct advantage of giving results for 40 to 60 elements or more in each sample. To ac- complish the analysis, heat of an electric arc or spark vaporizes a sample, which excites the atoms of the elements in the sample so that they emit light. A prism or diffrac- tion grating disperses this light into a spectrum containing lines of definite wavelengths that are characteristic for various elements.
From the intensity of these lines, as recorded photographically or elec- tronically, the concentrations of sought-after elements in the sample may be determined. Many commer- cial laboratories offering spec- trographic analyses are advertised in mining journals.
Atomic absorption is the opposite
of emission spectrography, in that
atom vapors in an unexcited state
will absorb the light that they
characteristically give off in an ex-
cited state. This phenomenon is
used in mercury detectors. The in-
terest in mercury is twofold. Not on-
ly a valuable metal in itself, mercury
also occurs in small quantities with
many different ores, such as those
of silver, gold, lead, zinc, and cop-
per. The presence of mercury,
therefore, may indicate the
presence of these other metals. Further, mercury is a volatile ele- ment and is transported as a gas that easily diffuses through small fractures and porous rock. Thus, a mercury halo presents a larger target for the prospector than halos produced by many other elements. In X-ray spectrography (some-times called X-ray fluorescence spectrography), bombardment with X-rays excites the atoms in solid samples that then release their acquired energy in a radiation spec- trum characteristic of each element. An X-ray spectrograph is an instru- ment designed to use this property for determining the concentrations of elements in a sample. It requires high voltages and adequate radia- tion shielding to protect the oper- ator. X-ray analyses can be obtained from many commercial laboratories.
Plants, humus, and bacteria have been successfully used as aids in mineral prospecting, and under certain con- ditions they may assist the prospec- tor in locating buried mineral deposits. So many factors are in- volved, however, that it is not possi- ble to predict conditions under which biological prospecting will be helpful. However, biological pros- pecting can be a valuable adjunct to conventional prospecting methods.
Many plants, by means of their extensive root systems and the ab- sorptive ability of their roots, effec- tively sample many of the elements that are within reach and transfer these elements to the branches, stems, and leaves, which can be chemically analyzed. Thus, under ideal conditions, the plant has sam- pled the underlying soil or rock in its root zone to depths of as much as 50 feet. The advantages to the prospector of being able to sample plants and thus to obtain informa- tion about the metals that occur at considerable depth are obvious, although problems in interpreting this information may render this method of prospecting impractical under many field conditions. For instance, some plants, because of their genetic makeup, selectively concentrate elements in their roots, stems, or leaves in higher concen- trations than are found in the underlying soil and rocks. When- ever possible, soil and rock samples should also be analyzed before concluding that a geobotanical anomaly indicates the presence of certain minerals in an area.
Forest humus also has been suc- cessfully used to locate mineralized rock, especially where it is hidden by soil. Elements are immobilized and concentrated in the humus layer as twigs, leaves, and other parts of the forest vegetation fall to the ground and decay. Studies in the United States, Canada, Scan-
dinavia, and the Soviet Union have shown that chemical analysis of forest humus yields results which delineate zones of gold and other metals much more accurately than results from the underlying soil (Curtin and others, 1971.)
For a review of the use of geo- botany and biogeochemistry in mineral exploration, see "Biological Methods of Prospecting for Minerals" (Brooks, 1983), "The Use of Plants in Prospecting for Precious Metals, Principally Gold
A Selected Reference List and Topic Index" (Erdman and Olson, 1985), and "Mineral Exploration- Biological Systems and Organic Matter" (Carlisle and others, 1986).
Geophysical prospecting com- bines the sciences of physics and geology to assist the prospector in exploring for both mineral and energy fuel deposits. Familiar ex- amples include the use of scintilla- tion counters for detecting radioac- tive uranium deposits and magnetic surveys for locating iron deposits.
Five major geophysical methods- magnetic, gravimetric, geoelectric, radiometric, and seismic are routinely used in mineral explora- tion. Application of some of these methods and techniques requires complex and costly instruments and sophisticated methods of processing and interpreting the data, but others are relatively simple and inexpen- sive. Among the latter are the magnetic and radiometric methods and some of the geoelectric tech- niques, which are outlined here.
prospecting is based on the natural magnetic properties of some min- erals such as magnetite. When held near a magnetite-rich rock, the needle in a compass behaves erratically because the Earth's magnetic field is distorted by the magnetic field of the rock. Rocks containing minerals such as mag- netite (iron oxide), and pyrrhotite (iron sulfide) are usually magnetic enough to be recorded by sensitive magnetic instruments.
The common unit of measure for the strength of a magnetic field is the gamma. Where not disturbed by highly magnetic rocks, the strength of the Earth's magnetic field in the conterminous United States ranges
from a low of about 48,000 gammas in Texas and Florida to a high of about 60,000 gammas in Minnesota.
Instruments called magnetometers are used for direct detection of magnetic anomalies (that is, the distortion of the Earth's magnetic field by magnetic minerals in crustal rocks). The magnetic readings over weakly magnetic rocks may depart from local average (background) values by 10 to 500 gammas, but over magnetic iron formation the readings may depart from back- ground by 100 to 100,000 gammas. The magnetometer can be used to trace concealed rock formations
that have magnetic properties differ- ing from those of adjacent forma- tions. It can also be used indirectly in the search for ore minerals. For example, the "black sand" of
Placer deposits commonly contain grains of magnetite that affect the magnetometer. Thus, it can be used in the search for gold or other heavy minerals present in the black sand.
Two commonly used magnetom-
eters are the fluxgate and the
proton. A fluxgate is an electronic
device that measures the strength
of the field in a particular direction.
The proton magnetometer's sensing
element is a container filled with a
proton-rich liquid such as water or
kerosene surrounded by a coil of
wire; protons are subatomic par-
ticles that spin about rotational
axes. The frequency with which the spin axes of the protons wobble, or "precess," after being aligned by a strong current passed through the coil, is directly related to the strength of the Earth's field. This frequency is measured and con- verted into readings in units of gam- mas. The proton magnetometer measures the total intensity of the Earth's field rather than the intensi- ty in a vertical or horizontal direction.
Magnetic surveys may be con- ducted either along a series of lines or in a grid pattern. The size of the area being prospected and the type of deposit being sought determine the spacing of stations. Stations spaced 10 to 20 feet (approximately 3 to 6 meters) apart may be re- quired to locate small magmoderately magnetic rocks, but sta- tions spaced 100 feet (approximate- ly 30 meters) or more apart may suffice if the presence of highly magnetic rocks is suspected in a large area. Powerlines, rails, auto- mobiles, and other large metallic objects should be avoided in any type of magnetomer survey be- cause they create strong local magnetic fields that mask the anomalies inherent in the rocks.
Today most magnetic surveys are airborne or marine and use total- field detecting systems and vertical gradient systems. Instruments called vertical gradiometers measure the vertical magnetic gradient; this helps to locate the edges of mag- netic zones, rock units, and other geologic features. These surveys provide comprehensive reliable data about regional magnetic trends. Ground magnetometer surveys are still used to locate anomalies from small subsurface structures.
Geoelectric methods. Most elec- trical prospecting is based on the fact that various minerals and rocks offer differing degrees of resistance to the flow of electric current. Elec- trical resistivity of rocks, measured in ohm-meters, can vary from sev- eral thousand ohm-meters for some igneous and metamorphic rocks to a few ohm-meters for shales and clays. Some orebodies have such low resistivity that geophysicists refer to them as conductors (con- ductivity is the inverse of resistivity). For example, the resistivity of most common sulfide minerals, such as chalcopyrite (copper-iron sulfide) and galena (lead sulfide), but not in- cluding sphalerite (zinc sulfide), is very low a fraction of an ohm- meter. If the individual grains in a sulfide orebody are in good elec- trical contact with each other, the entire orebody may offer a very low resistance to the flow of electricity compared to the surrounding rocks, and hence be called a conductor. Other bodies, such as clay pockets, graphite schists or sediments saturated with brine can also be good conductors.
Many electrical techniques are used in searching for conductors that may be orebodies. Some re- quire very expensive and com- plicated instruments and large field crews to make the measurements, and mathematical computations must be applied to the data before interpretation. Other techniques, however, use only moderately ex- pensive equipment that is easily operated by one or two persons and require little or no use of math- ematics. For example, the VLF method employs very low frequency (VLF) radio signals, electromagnetic fields transmitted from a number of powerful stations around the world
that continuously broadcast in the range of 15-25 kHz. The primary fields from these stations penetrate the Earth to depths of 30-300 feet (approximately 10-100 meters) and cause electrical currents of the same frequency to flow in the Earth. This current creates secon- dary magnetic fields that can be detected at the Earth's surface. In areas where the electrical resistivity
of the Earth is uniform, the primary and secondary fields at the surface are also uniform and are oriented in the horizontal direction. Where the resistivity of the Earth is not uni- form, however, the currents tend to concentrate along low resistance paths such as may be provided by orebodies. This causes disturbances
in the secondary fields at the sur- face that can be measured and used to predict the presence and location of orebodies.
The basic measuring instruments in the VLF method consist of one or more induction coils, which are used to sense the VLF magnetic field, a VLF radio receiver, and a readout device. In some instruments the tilt of the VLF magnetic field from the horizontal plane is mea- sured; in other instruments its amplitiude in the horizontal direction is measured. Measurements are made along lines or a grid in the
same manner that magnetic surveys are conducted. Ordinarily a station spacing of 25 to 50 feet (approx- imately 5 to 15 meters) is adequate. Powerlines, pipelines, metal fences and other large metallic objects, even if they are not steel, should be avoided because they act as con- ductors and cause anomalous fields not related to orebodies.
Another technique, the Slingram electromagnetic method, uses a local transmitter consisting of a bat- tery powered source of alternating current and an air- or metal-cored induction coil that serves as the , antenna. The operating frequencies range from about 200 Hz to 4,000 Hz. The separation between the transmitter and the receiver varies from about 100 to 800 feet (approx- imately 30 to 240 meters). The receiver for this technique mea-
sures the in-phase and the out-of-
phase portion of the received
signal, that is, the amount of secon- dary field that aligns with the broad- cast field and the amount that is perfectly misaligned with it. The presence of a low-resistivity orebody distorts the fields that are observed at the surface. In general this technique has a greater depthrange than the VLF method and often provides data that are easier to interpret than data from the VLF method. Disadvantages are that two persons are needed to make measurements and that survey lines must be cleared and measured in advance to work in wooded terrain.
Another group of electromagnetic devices, metal detectors, are small portable instruments consisting of a wire loop suspended above the ground along which an alternating current flows, inducing currents underground. The secondary mag- netic fields thus created can be measured by audible signals. Metal objects below the ground surface (at depths of a few feet) distort these fields, creating a change in the frequency of the audible signal.
Electromagnetic techniques re- spond to the presence of rocks bearing significant quantities of sulfides, graphite, or clays, to water- filled shear zones, and to overbur- den, particularly when clays or saline waters are present. Where geologic evidence indicates the possible presence of an orebody, magnetometer measurements may help discern anomalies likely to represent valuable mineral deposits.
Electrochemical methods are used as a follow-up to the above- mentioned methods. An orebody that is actively being oxidized can act as a natural battery, causing the surface of the Earth above it to have an electrical potential that is different from the surrounding area.
In the self-potential method, two
nonpolarizing electrodes placed on
the surface of the Earth are con-
nected to a sensitive millivoltmeter,
and the difference in potential is
measured. Ideally, one electrode is
left in a fixed position, and the
other electrode is moved along lines or a grid to measure the variations in self potential. No calculations are necessary unless more than one location is used for the fixed elec- trode. Sources other than mineral deposits also have variations in self potential. Thus, results from this method should be correlated with other geologic evidence to indicate the presence of an orebody.
Radiometric methods. Naturally occurring radioactive elements such as potassium, uranium, and thorium decay to other elements or isotopes by emission of subatomic particles. Gamma rays (similar to X-rays, but higher in frequency), alpha particles (nuclei of helium atoms), and beta particles (electrons) are most com- monly emitted during this process.
Radiation counters (Geiger coun- ters, scintillometers, and gamma ray spectrometers) detect differences in intensities of radioactivity and are used in finding deposits of radioac- tive minerals. The Geiger counter is a tube filled with a gas such as helium, argon, or krypton. A high- voltage wire extends into the central part of the tube. When gamma radiation or beta particles pass into the tube from a radioactive source, some of the rays collide with gas molecules and produce electrically charged particles that are then attracted to the central wire and produce electrical pulses. The elec-
trical pulses can be translated into dial readings of counts per minute. Scintillometers use crystals of cer- tain compounds, such as sodium iodide, which emit flashes of light when struck by radiation. A photoelectric cell "sees" the flash of light or scintillation and elec- tronically counts the numbers of flashes per unit of time. This can be transmitted to a dial reading in counts per minute. Scintillometers are more sensitive than Geiger counters; Geiger counters are no longer widely used, as they lack the necessary sensitivity to find lower- grade ("lean") deposits. While sen- sitive to very small differences in amounts of radioactive elements in rocks, Geiger counters and scin- tillometers do not show what ele- ment produces the radioactivity; such distinctions are made by chemical analysis of the radioactive rock. The gamma ray spectrometer will give ppm levels of thorium, and uranium,or the percent of potassium.
For ground surveys, the prospec- tor commonly walks while listening to the counts on earphones or
watching the dial of the counter. Radioactive deposits may produce readings that are 10 to 100 times as great as "background" readings. If the deposits are covered by even a few tens of inches of overburden, however, the radiation cannot be detected. When a portable counter is used, the information should be interpreted with caution until it is verified by adequate sampling and chemical analysis.
Exploration for uranium has
changed markedly over the past
half century. The simple ionization chambers and Geiger counters of the 1940's have been superseded by sophisticated spectrometers ot great reliability and sensitivity that are capable of discrimination among uranium, thorium, and potassium.
Research on the movements of radon, helium, and other daughter products of uranium has produced new or improved tools and methods to detect concealed uranium depos- its. Radon gas, for example, can be detected in soils by use of portable radon counters or by radon cups containing radiation-sensitive film.
Claim to Mineral Discovery and Exploration
Any U.S. citizen or any person who has declared an intention to become a citizen may locate a min- ing claim on public lands, which are mainly in the Western States. Al- though minerals are classified for purposes of mineral laws as locata- ble, leasable, or salable, only locatable mineral deposits can be staked and claimed under the General Mining Law of 1872. Locatable materials include metallic minerals (gold, silver, lead, and others) and nonmetallic minerals (fluorspar, asbestos, mica, and others).
All minerals on certain public lands, such as acquired lands (lands in Federal ownership ob- tained by the Federal Government by purchase, condemnation, gift, or exchange) and areas offshore, are subject to special leasing laws and regulations. Further, the location of mining claims is prohibited on some public lands. Regulations governing operations on mining claims apply to most public-domain lands in the national forests and the land ad- ministered by the Bureau of Land Management. The regulations ap- pear in Parts 9 and 228 of Title 36, and Part 3809 of Title 43, of the Code of Federal Regulations.
A mining claim can be validly located and held only after a valuable mineral deposit has been discovered. The Department has established and the courts have followed the "prudent man" test to determine what constitutes discov- ery of a valuable mineral, (Chrisman v. Miller, 197 US 313(1905).) The Secretary, in Castle v. Womble, 19 Land Decisions 455,457 (1894), defined the test as, "Where miner- als have been found and the evi- dence is of such a character that a person of ordinary prudence would be justified in further expenditure of his labor and means, with a reason- able prospect of success in devel- oping a valuable mine then the re- quirements of the law have been met." Environmental factors and economic costs are important con- siderations in applying this test. In 1968, the U.S. Supreme Court in United Sates v. Coleman, 390 U.S. 599 (1968), approved the market- ability test (that one must mine and market a mineral at a profit). In 1983, the Department of the Interior adopted the position that only a reasonable prospect of success in marketing, not guaranteed profit- ability, was the proper marketability standard (Interior Regulations Pacific Coast Molybdenum, 90 ID 352(1983).
Although the number of claims that can be held is unlimited, an ac- tual physical discovery on each and every mining claim on public land must be made. Traces, minor in- dications, geological inference, or hope of a future discovery are not sufficient to satisfy the "prudent man" test. Making minor im- provements, posting a notice, or performing annual assessment work will not create or perpetuate a right or interest in the land if there are no valuable minerals within the claim. (See Bureau of Land Management leaflet, "Staking a mining claim on Federal lands.")
Federal mining regulations per- taining to the acquisition of mineral rights on public land are adminis- tered by the Bureau of Land Management. For answers to ques- tions on how and where prospecting is allowed on public lands, and to secure copies of their publications listed in the reference section at the end of this booklet, write to:
Office of Information
Bureau of Land Management (130) Washington, D.C. 20240
That office can also provide ad- dresses of their regional offices in the Western States.
On privately owned land, the mineral rights must be obtained from the owner, generally through
purchase or lease. State geologists and officials at county courthouses are other sources of information pertaining to the acquisition of mineral rights on public or privately owned land.
The prospector who succeeds in making a mineral discovery must consider how to explore the deposit in order to estimate its size and grade. Evaluating the economic potential of a deposit is sometimes difficult and may require the pros- pector to hire an experienced min- ing engineer. Estimates of the length, width, and depth of a deposit are needed to determine the tonnage of mineralized material present, and samples must be ob- tained for analysis to determine the grade of the deposit. Such samples must be representative of all the material that might be mined as ore, not just selected parts. For a review of sampling methods, see "Exploration and Mining Geology" (second edition) by Peters (1987), chapter 16.
Services Available to Prospectors
The U.S. Geological Survey does not identify, analyze, or assay sam- ples of rocks, minerals, or ores at the request of individuals or cor- porations. Some State geological surveys will identify the minerals in ore samples submitted by residents of their State. These State agencies can also furnish helpful information on State mining laws and the geol- ogy of specific areas within the State.