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.
Scintillometer
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
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.
Biological prospecting
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
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.
Magnetic methods.
Magnetic
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
Magnetometer
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-
Geiger counter
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.
Reprinted: USGS