Methods of Prospecting for Radioactive Minerals
good understanding of the geochemical behavior of nuclear fuel metals, their
mineralogy and geological processes favorable for and responsible for their
formation have facilitated sustained efforts in the exploration for these.
Significant from the exploration point of viewis the time- bound characteristic
of uranium deposits. Major uranium deposits are found in rocks of Lower to
Middle Proterozoic, Permo-Carboniferous, Mesozoic, and Tertiary ages.
major orogenies at 2500,1800, 1000, 350 and 30 million years have helped in
recycling of U by magmatic-anatectic reworking of the pre-existing rocks and
through the action of groundwater, leading to a variety of deposit types.
Recognition and characterization of these geological environments is the
first stage in the exploration program.
Exploration and Prospecting Techniques:
exploration techniques are principally based on physico-chemical properties of U
and associated elements. U and it's
daughter elements are undergoing constant disintegration and releasing alpha and
beta particles and gamma radiation characteristic of daughter elements Bi214,
Ra226 of the U238 series indicates the presence of U.
They are a number of U-exploration techniques, direct and indirect,
having advantages and disadvantages. The
combination of techniques is used such that the limitations of one are
supplemented by the advantages of the other.
results of indicted methods like remote sensing, geophysics and geochemistry
must as the making the related to the bedrock geology to become meaningful.
The exploration geologist should know the advantages and limitations of
exploration purposes, published maps of the area may be used or if none is
available, mapping may be required. This
may be done using remote sensing techniques.
A suitable map scale for such an exercise is 1: 50,000. Maps of this scale may help in locating the most probably and
potential geologic environments of U mineralization. In the initial stages of the exploration program, geological
maps help in the interpretation of geophysical and geochemical anomalies.
In the advanced stages of exploration of more detailed map, perhaps on
scales of 1: 2000 or 1:1000 may need to be prepared.
sensing refers to the gathering of information about the earth without actually
coming in contact with it. Aerial
photography, satellite imageries, gamma-ray spectrometry are all remote sensing
techniques using different wavelengths of the electromagnetic radiation.
Instruments used are scintillometers, photography cameras and sensors
(scanners). Aerial photographs and
imageries are used for making thematic maps.
Drainage patterns, erosional features, rock types, surficial deposits,
igneous extrusives, and intrusives, attitudes of beds, fold axes, faults and
joint planes can be mapped and relationships studied so that sufficient
information is gathered.
emanates mainly from U, Th and K. 99
percent of gamma radiation are attenuated by 30 cm of rock or 45 cm of soil,
therefore the success of this method depends upon rock exposure.
Radiation detectors used in gamma-ray spectrometry are NaI scintillation
counters which are mounted on aircraft flying at about 120 m.
Flight lines are spaced 1-5 km apart for reconnaissance surveys and 250 -
5000 m for detailed surveys. The
amount of radiation detacted reflects the size, shape and source of intensity. Contour maps of both the grounds are prepared which reflect
the lithology and other characteristics of the host rock. These anomalies I later used for ground follow-up surveys.
The superposition of gamma-ray contour on geological map makes a
meaningful contribution to the exploration program.
surveys measure the distribution of three radioactive elements (uranium, thorium
and potassium) in the Earth's crust, by recording the gamma-radiation emitted
during the decay of these elements. Approximately 90% of measured gamma rays are
received from the top 30cm of the ground. These measurements enable the
interpretation of rock and soil types. Once
an anomaly is identified using airborne methods, a hand-held or jeep mounted
scintillometer or GM counter is used for detail surveys.
This is the most common and cheapest method of uranium exploration.
Ground radiometric surveys are most effective where rock exposures are
decay series of uranium produces radon (Rn222) which has a half life of 3.82
days. By emitting an alpha particle
it decays to Bi218. The detection of radon serves as a useful Pathfinder for
uranium, and is possible through a variety of alpha particle detectors.
Faults and fractures often serve as conduits through which radon issues,
hence these surveys are capable of delineating these structures in soil covered
terrains. Though sensitive, this
method is subject to diurnal changes.
methods of geochemical analysis can detect uranium concentrations in ppm or ppb
in water, rock, or soil samples. This
makes it possible for for analytical methods to be applied at both the
reconnaissance and detailed stages of exploration. Stream/spring sediments and plants may also be analyzed for
their uranium content. Geochemical surveys help in identifying the primary and
secondary halos around uranium deposits. Hydro
geochemical surveys are suitable in areas of scanty outcrop or thick overburden.
Stream sediments samples help in zeroing in on the target.
Usually second and third order streams are sampled.
The method is specially useful in forested and mountainous regions.
Geophysical methods are used in conjunction with airborne gamma-ray spectrometry for both reconnaissance and detailed surveys. Geophysical techniques are helpful indirectly since they help in the identification of major rock types. A clue to be possible concentrations of uranium can be obtained by the presence of rocks which are either hosts to uranium deposits or are sources of uranium. Gravity, magnetic, electrical or seismic methods may be used to bring out the contrast in lithology and help in locating faults and dislocations, sulfide and altered zones and unconformities.
an anomaly has been deciphered with a fair level of confidence, it is explored
by drilling. The boreholes may be
of coring or non coring type. Drilling
helps in the precise location and sampling of the ore zone.
The boreholes are logged in respect of lithology, resistivity, IP,
magnetic susceptibility, neutron and gamma-rays to get the maximum information
about the prospect. The borehole samples are studied in the lab to determine the
ore mineralogy and gangue mineralogy.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)