Direct detection of mineralization

The most direct application of gamma ray spectrometry surveys during the 1970’s and 1980’s was the search for U and Th deposits (e.g. Dickson and Scott, 1997). U and Th anomalies may be identified on profile and grid presentations of the data. Ratioing and statistical image processing techniques (Chapters 7 and 8) can enhance subtle anomalies. Anomalies can be

followed up on the ground using portable gamma ray spectrometers. Geochemical analysis and microscopic studies of samples from rock or sediments are needed to fully identify the mineral phases that constitute the radioelements (see for example Charbonneau et al., 1997).

The character of radiometric anomalies associated with subsurface and outcropping U mineralization depends on the forms of U mineralization, the host rock, and the geological setting (Krasnov et al., 1975, Krasnov et al., 1980). Typical features of U mineralization detected at the earth’s surface are:

1. elliptical dispersion halos with dimensions from 70 × 80 m up to 80 × 350 m;

2. anomalous U concentrations in the range 4-20 ppm eU;

3. associated Th anomalies in the range 5-40 ppm eTh;

4. ratios between the radioelement of Th/U<1, U/K>5-10, Th/K in the range 4-5; and 5. increasing gamma radiation with depth.

Due to relatively low penetration of gamma rays through rock and soil, the probability of discovery of uranium mineralization is dependent on the U concentration in the source, its surface dimensions, and the positions of the measured profiles. A small outcrop of high-grade U mineralization is a more difficult target for U exploration than low-grade mineralization with extensive surface outcrop. For example, in a high radioactivity granitic environment, a U anomaly at the 3σ confidence level would be detected by an 8.4 litre NaI(Tl) detector at 80 m height for circular outcropping sources with: 4 m diameter and 4860 ppm eU; 36 m diameter and 53 ppm eU; or 100 m diameter and 9 ppm eU (Matolin, 1980).

Several 4-12 ppm eU anomalies were mapped in airborne survey of the Urt mineral zone in Janchivlan region, 60 km southeast of Ulan Bator, Mongolia. The anomalies are within a coarse-grained porphyric biotite granite of Mesozoic age with U concentration of about 4 ppm. Ground follow-up using a portable spectrometer and radon soil gas measurement verified the presence of U mineralization. K, U and Th concentrations and their ratios mapped the mineralized zone (Figure 9.5). The high ²²²Rn activity concentration illustrates the utility of emanometric methods for the exploration of subsurface U mineralization.

Systematic uranium exploration, based mainly on radiometric methods, was carried out in the Czech Republic between 1946 and 1990. This led to the discovery of 16846 U occurrences and 164 U deposits and U ore objects. Table 9.6 shows the radiometric methods and their efficiency in discovery of these U deposits (Suran, 1998). The most effective methods were radon surveys to a depth of 1 m, radiometric surveying of old mine and exploration sites, car-borne methods, and drilling in sedimentary basins. Aircar-borne gamma ray spectrometry was applied after intensive ground exploration for U at the beginning of the 1960’s. Airborne surveys mapped the regional radioactivity of the country, and led to the discovery of U deposits in the Sokolov Tertiary sedimentary basin, northwest Bohemia.

U ore reserves estimated by gamma ray spectrometry should be corrected for possible disequilibrium between ²³⁸U and ²²⁶Ra. Several methods of directly estimating U were developed and tested for borehole measurements in the 1970’s. These make use of radiation originating directly from U atoms. X-ray fluorescence for the direct determination of U focuses on detection of the U-K_α1 98.428 keV line (Lubecki and Wolf, 1978). Neutron methods are based upon the detection of prompt fission neutrons (Smith, 1977), or delayed fission neutrons (Steinman et al., 1976; Givens et al, 1976), originating from the interaction of neutrons with ²³⁵U atoms.

Dickson and Scott (1997) provide an up-to-date review of the effect of hydrothermal processes, alteration and weathering on radioelement distribution. These processes do not only have implications for the direct detection of U and Th from gamma ray spectrometry

of Sn, W and Mo, porphyry Cu-Au mineralization, gold mineralization and stratabound polymetallic mineralization. The relationships between radioelement distribution and each of these deposit types are varied and complex. A thorough understanding of the effects of silicification, K-alteration, weathering processes and local lithological variations is required to evaluate the mineralization potential associated with radioelement anomalies.

FIG. 9.5. K, U, Th concentrations and ²²²Rn activity concentration oven U mineralization in the Urt mineral zone, Mongolia (Matolin, 1994).

TABLE 9.6. EFFECTIVENESS OF RADIOMETRIC METHODS IN U EXPLORATION IN THE CZECH REPUBLIC IN THE PERIOD 1946 - 1990 (Suran, 1998)

Method Number of U ore occurrences discovered

Percentage of discovered U occurrences (%) Radiometric revision of mining activities 37 23

Airborne gamma survey 4 3

Car-borne gamma survey 14 9

Radon survey to the depth 1 m 66 40 Radon (and gamma) survey to the depth 2 m 5 3

Surficial gamma survey 12 7

Gamma survey in shallow holes 2 1 Gamma survey to the depth 15 m 7 4 Combination of radiometric methods 2 1

Gamma survey in boreholes 15 9

The detection of K-alteration by gamma ray spectrometry surveys has received particular attention, as it has resulted in several mineral discoveries (Dickson and Scott, 1997; Shives et al., 1997). K alteration halos can be repeatedly distinguished from normal K variations by their characteristic low Th/K ratios (Shives et al., 1997).

A high-resolution magnetic/radiometric survey, undertaken by the Geological Survey of Canada in 1994, delineated a large hydrothermal system in the Lou Lake area, Northwest Territories, Canada (Shives et al., 1997). Figure 9.6 shows that the hydrothermal system is characterized by enrichment of potassium and magnetite within a volcano-plutonic magmatic zone. The potassium enrichment is evident as a potassium high, and confirmed by a low in the ratio of Th/K (i.e. preferential enrichment of potassium relative to thorium). The magnetite enrichment produces a significant magnetic anomaly. The Sue-Dianne polymetallic (Au-Co-Cu-Bi-W-As) deposit is located 20 km north of this hydrothermal system. It was originally discovered by exploring for the source of a uranium and U/Th ratio anomaly measured during a regional survey in 1974. A uranium halo around the edges of the hydrothermal system (high values in the ratio of U/Th) correlates with several veins of pitchblende. The geophysical responses to the alteration and uranium mineralization are clearly seen in profile form (Figure 9.7). Mapping the alteration system leads directly to the polymetallic deposits (Figure 9.8). Ground truth studies show that the effects of hydrothermal potassium enrichment within each lithologic unit are easily measured using gamma ray spectrometry (Figure 9.9).

FIG. 9.6. Radiometric responses, magnetic response and the geology of the Lou Lake area, Northwest Territories, Canada. Polymetallic mineralization associated with potassium and magnetite-enriched hydrothermal system. Uranium enrichment occurs on the margins of the hydrothermal system (after

FIG. 9.7. Radiometric responses and magnetic response for flight line 16 (see FIG. 9.6) of the Lou Lake area, Northwest Territories, Canada. The hydrothermal system is evident in the high potassium and magnetic responses, and the coincident low in eTh/K. The high responses in eU/eTh reflect uranium mineralisation (after Shives et al., 1997).

FIG. 9.8. Potassium versus thorium concentrations determined by in-situ gamma ray spectrometry, from the Lou Lake area, Northwest Territories, Canada. Increasing potassium within a lithologic unit reflects the hydrothermal potassium enrichment (after Shives et al., 1997).

FIG. 9.9. Geological cross-section for line A-B (see FIG. 9.6) of the Lou Lake area, Northwest Territories, Canada, showing the mineralization relative to the halo of potassium and magnetite hydrothermal enrichment (after Shives et al., 1997).

FIG. 9.10. Potassium versus thorium concentrations determined by in-situ gamma ray spectrometry, over the Mount Milligan (a) and Mount Polley (b) porphyry copper-gold deposits, British Columbia, Canada. Increasing potassium within a lithologic unit reflects the potassium enrichment associated with the mineralization (after Shives et al., 1997).

FIG. 9.11. A-B-C: Potassium response, magnetic response and geology over the Mount Milligan porphyry copper-gold deposit, British Columbia, Canada. Mount Milligan intrusive complex shows high magnetic and potassium responses, whereas the mineralised zones show more discrete, lower amplitude potassium anomalies. D-E-F: Radiometric responses over the Casino porphyry gold-copper-molybdenum deposit, Yukon Territory, Canada. Deposit distinguished by potassium enrichment (potassium high and coincident eTh/K low) (after Shives et al., 1997).

The radiometric signatures of several porphyry systems in the Canadian Cordillera have been studied (Figures 9.10 and 9.11). They demonstrate the need to process the data, to differentiate between lithologic and alteration signatures (e.g. preferential enrichment of potassium versus thorium).

More than thirty Canadian examples of geological mapping and mineral deposit characterization are provided in Shives et al. (1995), using airborne gamma ray spectrometry and systematic ground truthing. The deposit types covered include:

1. volcanic-hosted massive sulphides with associated potassium enrichment;

2. volcanic-hosted epithermal base and precious metals with associated potassium enrichment;

3. granite-hosted gold associated with illite alteration (potassium enrichment);

4. metavolcanic/metasediment-hosted gold with hydrothermal alteration (potassium enrichment);

5. metasediment-hosted skarn mineralization (Au-U-W-Mo-Co);

6. porphyry-hosted gold-uranium and copper–gold (molybdenum) with hydrothermal alteration (potassium enrichment);

7. metasediment-hosted polymetallic (Bi-Cu-Co-Au-As) with hydrothermal alteration (potassium enrichment);

8. carbonatite-hosted rare earth elements;

9. syenite-hosted rare metal/rare earth elements associated uranium enrichment;

10. dolomite-hosted epigenetic uranium-copper;

11. limestone-hosted uranium;

12. uraniferous pegmatites; and 13. metasediment-hosted pitchblende.

The Ashanti Belt in Ghana is a prolific producer of gold, and a high-resolution magnetic/radiometric survey has been flown over the area. The mineralization is hosted in two settings: Birimian metavolcanics and Tarkwaian conglomerate. Lo and Pitcher (1996) shows that the Birimian deposits are associated with sericite alteration, with a potassium enrichment signature. The Tarkwaian conglomerate can be mapped through its association with a potassium-rich phyllite marker horizon.

The physical properties of diamondiferous kimberlite sills in the Guaniamo area of Venezuela were studied and compared to neighbouring igneous rocks, which include granite, gabbro and volcanics (Versteeg and Paterson, 1997). The magnetic susceptibilities range widely and there is a significant overlap between rock types. The degree and depth of weathering makes geological mapping difficult and minimizes the effectiveness of electromagnetic surveys.

Measurement of the radioelement concentrations from fresh rock samples and drill core clearly showed that the kimberlite could be distinguished from most other igneous rocks (altered and foliated granites, granodiorites, gabbros, rhyolites, lamprophyre dykes, altered mafic dykes) and metasediments through anomalously high levels of K, U and Th. The unaltered granites show similar levels of these radioelements, but the kimberlite is differentiated through higher Th/K and lower U/K ratios.

Gold mineralization within the La Libertad region, Nicaragua, is dispersed in epithermal quartz veins within Tertiary basaltic lavas. There are five alteration zones (I-V) characterized by secondary mineral assemblages (Darce, 1990). A regional, 30 km long, portable gamma ray spectrometer profile of the region showed an increase in K concentration and of the Th/K and KU/Th ratios (Table 9.7) toward the Au mineralized veins (Matolin, 1991).

TABLE 9.7. MEAN K, U AND Th CONCENTRATION DETERMINED BY GROUND GAMMA RAY SPECTROMETRY IN VOLCANIC ROCKS ON THE REGIONAL PROFILE JUIGALPA - LA LIBERTAD, AND AT GOLD BEARING QUARTZ VEIN STA ELENA, LA LIBERTAD, NICARAGUA

An airborne gamma ray spectrometry potassium anomaly (5% K in a background of 1.5-2%) with an associated Th/K ratio anomaly (Th/K=1 in a 4-6 background) led to the discovery of Zn-sulphidic mineralization in a belt of Devonian and Lower Carboniferous volcanites and sediments in Northern Moravia, Czech Republic. The distribution of natural radionuclides in an alteration zone was used to site drill holes that intersected the Zn mineralization (Gnojek and Prichystal, 1985).

Jayawardhana and Sheard (1997) presented a study of radiometric data over the Mount Isa Inlier, Australia. The area is host to more than eighty base and precious metal deposits. Over 700,000 line-km of airborne gamma ray spectrometric data were acquired in the province. The data provided critical contributions to mapping both lithology and regolith. GIS techniques were used to localize zones of radioelement enrichment or depletion, and correlated well with various types of base and precious metal deposits. The radiometric signatures in these areas were used to explore for similar deposits in the region.