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 and Prospecting Techniques:

The 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.

 

Geological Mapping:

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 prepared.

 

Remote Sensing:

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.

 

Airborne Gamma-ray Spectrometry:

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:

Radiometric 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 available.

 

Radon Measurement:

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.

 

Geochemical Techniques:

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 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 Techniques:

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.

 Exploratory drilling:

Once 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.

Notes & Handouts

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Department of Geology

Aligarh Muslim University, Aligarh - 202 002 (India)

Phone: 9719421011

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