Gamma-rays can penetrate several hundred metres of air, but only 30-35 cm of rock or soil. Therefore, most gamma-rays measured above the earth's surface originate in the top 35 cm of the Earth's surface. The interpretation of gamma-ray data requires an understanding of surface processes such as weathering and the surficial transport of regolith material. Interpreters also need to be aware that the radiation measured by an airborne spectrometer at 60-100 m above the ground represents the average radioactivity over a large area (Figure 3). Also, be aware that disequilibrium is common in the U decay series. High eU estimates may well represent Ra concentrations rather than that of 238U.

The radioactivity of rocks, soils and vegetation

Rocks

Potassium has an average crustal abundance of 2.3 wt. % and is much more common than Th and U, with estimated crustal averages of 3 ppm and 12 ppm respectively. Fractionation of K, Th and U during rock formation generally increases in accordance with increasing silica content (Figure 16). This is reflected in a general trend of increasing K, U and Th from mafic (silica-poor) to acidic (silica-rich) igneous lithologies. Potassium is common in orthoclase and microcline feldspars, muscovite, alunite and sylvite (Table 1). High K abundance is typically associated with acid igneous rocks including, for example, granite, rhyolite and pegmatite. Potassium is absent or at very low concentrations in mafic minerals and associated mafic to ultramafic rocks such as basalt, dunite, serpentine and peridotite. Uranium occurs as two main valency states: U+4; and, U+6. The oxidised U+6 forms complexes with oxygen as a uranyl ion (UO2+2). Uranyl ions are mobile and typically form chemical complexes with carbonate, sulphate and chloride ions. The mobility of U+6 is modified by adsorption to hydrous iron oxides, clay minerals and colloids (Dickson and Scott 1997). Under reducing conditions the more reduced U+4 form is contained in insoluble minerals. Uranium is associated with pegmatite, syenite, radioactive granite and some black shales. Thorium occurs in a single valency state (+4) and therefore its mobility does not alter under changing redox conditions. Thorium solubility is generally low, although it can be soluble in acid solutions or at neutral pHs when it is associated with organic complexes. Thorium is found in minerals such as thorianite and thorite. In rocks it is associated with granite, pegmatite and gneiss. Uranium and Th are found in accessory and resistate minerals such as zircon, titanite (sphene), apatite, allanite, xenotime, monazite and epidote.

Figure 16

Figure 16 (above). Fractionation of K, Th and U during rock formation generally increases in accordance with increasing silica content (Dickson and Scott 1997).



Table 1. Radioactive minerals (from Telford et al. 1976).

Potassium
Mineral
  1. Orthoclase and microcline feldspars [KAlSi3O8]
  2. Muscovite [H2KAl(SiO4)3]
  3. Alunite [K2Al6(OH)12SiO4]
  4. Sylvite, carnallite [KCl, MgCl2, 6H2O]
Occurrence
  1. Main constituents in acid igneous rocks and pegmatites
  2. Same
  3. Alteration in acid volcanics
  4. Saline deposits in sediments
Thorium
Mineral
  1. Monazite [ThO2 + Rare earth phosphate]
  2. Thorianite [(Th, U)O2]
  3. Thorite, uranothorite [ThSiO4 + U]
Occurrence
  1. Granites, pegmatites, gneiss
  2. Granites, pegmatites, placers
  3. Granites, pegmatites, placers
Uranium
Mineral
  1. Uraninite [Oxide of U, Pb, Ra + Th, Rare earths]
  2. Carnotite [K2O.2UO3.V2O5.2H2O]
  3. Gummite [Uraninite alteration]
Occurrence
  1. Granites, pegmatites and with vein deposits of Ag, Pb, Cu, etc.
  2. Sandstones
  3. Associated with Uraninite