Gamma-ray Response of Weathered Materials

Gamma-rays emitted from the Earth's surface will relate to the mineralogy and geochemistry of the bedrock and weathered materials (e.g., soils, saprolite, alluvial and colluvial sediments). Weathering typically modifies the distribution and concentration of radioelements from the initial bedrock concentrations. Understanding bedrock and regolith responses has proven invaluable for not only mapping regolith materials but also understanding geomorphic processes (Dickson et al. 1996, Wilford et al. 1997; Figure 20). Fortunately, from a regolith perspective K, Th and U behave quite differently from one another during bedrock weathering and pedogenesis. As a general rule, K concentration decreases with increasing weathering. This is because K is soluble under most weathering environments and tends to be leached from a regolith profile. Exceptions to this are where K is incorporated into potassic clays such as illite, where it is absorbed onto clays such as montmorillonite and kaolinite or where K is associated with either large K-feldspar phenocrysts or mica that take time to weather (Dickson 1997). In contrast, Th and U are associated with more stable weathering products in soil profiles. Thorium and U released during weathering are readily adsorbed onto clay minerals, Fe- and Al-oxyhydroxides and organic matter in soils (Figure 21). In addition, Th and U also reside in resistate minerals that persist for a long time in the soil. It is therefore not uncommon for Th and U concentrations to increase in the regolith as other more soluble minerals are lost in solution (Table 2). These relationships are summarised in the diagram below:

Sequence diagram

Diagram showing element weathering and gamma-ray response (Wilford et al. 1997).


The behaviour of these radioelements during weathering will depend on the initial bedrock composition or chemistry. For example, weathered felsic rocks usually show a loss of K whereas weathered basic or ultramafic rocks that are initially low in all three radioelements can show little change in their radioelement distributions. Different bedrock types and weathering profiles in combination can produce a range of radioelement patterns and in many cases similar radioelement responses can relate to completely different surface materials. For example, highly weathered siliceous soils developed on granite can have the same radioelement signature as unweathered ultramafic bedrock. For this reason it is best to interpret the gamma-ray response within major lithological-geochemical groups (e.g., sandstone, granite, basalt). In addition, dividing the landscape into major geomorphic units (e.g., erosional and depositional landscapes) can add further clarity in understanding the distribution of radioelement patterns and their relation to specific materials (regolith and bedrock).

Figure 20

Figure 20 (above). Conceptual diagram showing factors that can influence the distribution of radioelements over the same rock or sediment derived from the same rock.


Figure 21a Figure 21b

Figure 21c Figure 21d

Figure 21 (above). Regolith materials and their characteristic gamma-ray responses. A (upper left) - bauxite, usually high in Th and U which are associated with Fe oxides, resistate minerals (e.g., zircon) and clays. B (upper right) - slightly weathered granitic saprolite. Responses will vary according to the geochemistry of the granite, but usually very high in K associated with K-feldspars. C (lower left) - Quartz sands or sandstone, usually low in all three radioelements due to the abundance of quartz which is radiometrically barren. D (lower right) - Ferricrete, typically high in Th due to scavenging (adsorption) by Fe-oxyhydroxides.