Methods of Prospecting for Radioactive Minerals
A 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.
Five 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 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.
The 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 each method.
For 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
Remote 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.
Radioactivity 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.
Ground Radiometric Surveys:
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
The 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.
Sensitive 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
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
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
Strategy adopted by AMD:
The Atomic Minerals Division (AMD) of the Department of Atomic Energy (DAE) is entrusted with the identification and evaluation of uranium resources required for the first stage of India’s atomic energy programme. AMD’s strategies for uranium exploration are guided by geological criteria and contemporary developments in uranium exploration techniques world over. Exploration for uranium commences with literature survey, followed by study of available satellite images, air photos and geological maps, known radioactivity, geochemical anomalies etc. This is followed by airborne gamma ray spectrometric surveys over favourable areas for narrowing down the target areas. Further detailed radiometric survey is taken up in these target areas using hand held scintillometer. Once uranium anomalies are located, detailed geological and geophysical studies on different scales (1:50000; 1:25000; 1:10000; 1:5000) are then undertaken narrow down the areas for taking up drilling. Simultaneously, the samples generated at various stages of exploration are analysed in Physics, Chemistry, EDXRF, Petrology, XRF, XRD and EPMA laboratories for ore characterization. Bulk samples of the ores are studied in Mineral Technology and Ore Dressing laboratories for optimizing flow sheet characters.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)