Before and during field evaluation of a favorable area, an effort is made to examine the geology of operating mines and known deposits similar to the targets sought. The geologic characteristics of mineralization in the host rock, alteration, and the geometry of the ore bodies are given special attention. Each observation of ore is examined with the following questions in mind: How did the ore form? Why is it located here? What are the halo characteristics related to ore that are not present in barren ground? When developing a concept of a deposit’s origin to apply to the discovery of new deposits, it is important to keep in mind the idea of multiple working hypotheses. It is also necessary to modify the hypothesis as new facts are accumulated. An example of the chronologic development of genetic theories for sandstone-type uranium deposits in the United States is tabulated by Finch (1967). One should strive to develop, as soon as possible, a concept of origin that is accurate. Discussions of ideas with geologists who have discovered ore deposits are very helpful.
Ore guides are then developed, based on the above studies. These vary among the types of deposits and may range from general to specific. Ore guides common to nearly all uranium deposits are:
1. The presence of a proven host rock.
2. Anomalous uranium mineralization (two times background may be significant in some hosts; 20 times background may be required for other hosts).
3. Position of area with respect to mineralized trends, if any.
Additionally, the habit of uranium deposits is such that they commonly tend to occur in clusters, are elongate, and tend to form distinct trends.
A familiarization with the geologic setting and the experience gained from the above work may be applied to evaluating favorability of the study area. Generally, field examination of a potential area is conducted in stages. The first stage is a brief reconnaissance of the most significant occurrences and outcrops in the area. A hand- held scintillometer (Bailey and Childers, 1977) is commonly used to estimate the grade and extent of mineralization around previously plotted occurrences as well as detecting new ones. These are immediately examined and plotted on work maps. Careful attention to anomalies as low as two times background is prudent. Altered areas in the host rock are plotted, as well as the boundaries of the host.
Airborne scintillation surveys (Bailey and Childers, 1977) are commonly made at this stage, which may detect outcropping deposits, if any, or the surface expression of deposits concealed beneath shallow cover. The geologist directs the airborne work and generally flies as observer to insure the flight lines are as chosen. Flight patterns are selected to parallel and thoroughly cover outcrops of the potential host rock. Grid patterns are generally only used in areas where specific host rocks are unknown. Altitude above ground level should be as low as good safety practices permit, because radiation decreases with the inverse of the distance squared. Flight levels generally range from 15 to 76 m (50 to 250 ft) above the ground. Either an airplane or helicopter is chosen depending upon terrain. Hand-held or airborne scintillation equipment may be purchased or leased. Spectrometers that distinguish between uranium, thorium, or potassium radiation are commonly used, especially in areas of metamorphic or granitic host rocks which often contain thorium. Airborne equipment commonly provides for strip charts, digital printouts, or computer tape storage of data.
It is good practice to calibrate a scintillation counter (Nininger, 1954) before and during use. Some are equipped with an internal calibration setting and an adjusting knob. Others use an external radioactive source and a calibration knob. It is also good practice to obtain hand samples of ore of known grade for a rough estimate of instrument response for various grades of mineralization. It is difficult to estimate grade in a large outcrop of mineralization due to a mass effect. For this purpose, face scanner attachments are available for several of the scintillation and geiger counters (Key et al., 1982).
Geochemical techniques have been used in a number of uranium exploration programs (Dall’ Aglio, 1973; De Voto, 1978; Grimbert, 1972). These techniques include uranium analysis of sediments, stream sediments, stream water and ground water. Results of uranium anomalies in stream and ground waters are generally difficult to interpret. Much of the uranium in water may have been derived from disseminated sources that are uneconomic. Also, composition of the ground water typically determines the amount of uranium that may be held in solution.
Another technique is radon emanometry which consists of measuring radon gas concentration in soil gas generally on a grid pattern over potentially favorable areas. If used, this technique is usually applied to specific drill target areas rather than regional reconnaissance. One method (Miller and Ostle, 1973) uses a one-meter (3 ft) long steel tube driven into the ground. Soil gas is pumped from the tube into an alpha counter with a silver-activated zinc sulfide analyzer. The radioactivity of the sample may be measured in counts per second and the data plotted on a sample map. Background values for the area must be determined several times per day because barometric conditions will influence radon emanations. Another radon technique uses alpha- sensitive film that is placed inside a plastic cup in a shallow hole about 0.6 to 0.9 m (2 to 3 ft) deep (Fleischer et al., 1980). The cup is usually left in place for two to three weeks, then recovered and the film processed. Alpha tracks on the film are then counted. The alpha film method is available from a service company, which supplies the film and cups, processes the film, and furnishes results for a fee. A good summary of radon methods is given by Bailey and Childers (1977a).