Geological mapping

Gamma ray spectrometric data have been applied with variable degrees of success to the mapping of lithological units. The degree to which bedrock units can be delineated depends on many factors. The most important factors are:

1. the contrasts in radioelement content between lithological assemblages;

2. the extent of bedrock exposure and soil cover;

3. the relative distribution of transported and in-situ soils;

4. the nature and type of weathering;

5. the soil moisture content; and 6. the vegetation cover.

Unlike U, the average K and Th content of soils reflect the average K and Th content of the rocks from which they are derived. But the differences in soil radioelement concentrations are relatively small (Table 9.4; Dickson and Scott, 1997).

In general, a useful strategy for geological mapping is to first outline the major lithological units and than enhance the radioelement patterns within the individual units. A regional geological mapping example from the Pilbara, Australia, with outlines of major batholiths and greenstone belts, is shown in Figure 9.2. The ternary maps presented in Chapter 7 (Figures 7.14 and 7.15) show detailed views of the Pilbara and illustrate the richness in additional lithological details that can be extracted from these broad assemblages.

Enhanced products of gamma ray spectrometry data have often assisted in detailed mapping or further subdivision of lithological units. In some cases units with distinct radioelement signatures (mostly volcanic in origin) were identified that could be used as lithological markers in unravelling the geological map pattern in complex areas (Jaques et al., 1997).

Integrated interpretation with aerial photograph, satellite imagery and other airborne geophysical data sets allows exploiting the complementary geological information and enables the radioelement distributions to be studied in a structural geologic and geomorphologic context.

FIG. 9.2. Geological mapping example from the Pilbara, Australia. Outline indicates the area imaged in the ternary radioelement maps shown in Figures 7.14 and 7.15.

Gamma ray spectrometric data have shown to be uniquely applicable for mapping subtle compositional variations within igneous suites, particularly granitoid plutons and batholiths (Broome et al., 1987; Charbonneau, 1991; Goossens, 1992; Jaques et al., 1997; Wellman, 1998; Schetselaar, 2002). Zonation patterns in granites have been commonly recognized in gamma ray spectrometry surveys, many of which were not recognized by conventional field mapping (Broome et al., 1987; Schetselaar, 2002). This is because the bulk of the radioelements in igneous rocks occur within accessory minerals, such as monazite, xenotime, zircon, allanite, sphene and apatite. Subtle (but diagnostic) variations in the concentrations of these accessory phases are difficult to recognize in bedrock exposures. Also, late-stage magmatic and hydrothermal processes may control regional radioelement distributions in granitoids, particularly U.

Normal zoning in granitoid plutons formed by fractional crystallization, show a gradual increase in K and Th (and Th/U) from the margin to the centre. Along this path the SiO₂ content gradually increases and the mafic index decreases with the lithological composition ranging from quartz diorite through granodiorite to granite.

The positive correlation between SiO₂ and the radioelements breaks down in the last stages of magmatic differentiation when highly felsic pegmatites and aplites (with potentially high U concentrations) are emplaced. However, these late magmatic phases may be too small (aerially) to be detected by gamma ray surveys (Dickson and Scott, 1997).

Zoned granitoid plutons formed as a result of magmatic fractionation within a single magmatic pulse are relatively rare in nature. Many of the compositional variations in granitoid plutons are actually composed of distinct intrusive phases with relatively discrete boundaries.

Such composite plutons were formed in multiple magmatic pulses that can both resemble normal or inverse zoning (Pitcher, 1993).

Thin section studies are useful to establish the mode of occurrence and textural associations of K, Th and U-bearing minerals, which in combination with trace element analyses shed light on the petrogenetic associations of the radioelements. Charbonneau (1991) and Charbonneau et al. (1997), for example, observed in the study of several peraluminous granites of the Canadian Shield that monazite and zircon are commonly spatially associated to biotite and Ti-Fe oxides (Figure 9.3). This spatial association between radioactive accessory minerals and early crystallising phases, particularly biotite, has important implications for the interpretation of gamma ray spectrometry surveys over granitoids. This association may be interpreted as restite (in which clusters of biotite with radioactive minerals, such as zircon and monazite are solid state residues) or to result from the nucleation of accessory minerals by the local saturation of slowly diffusing rare earth elements, (including Th, U) rejected from the hosting crystal/liquid interface of phenocrysts (Bacon, 1989). Variations in the distribution in U and Th over granitoid plutons may therefore, in addition to magmatic differentiation and hydrothermal processes, also reflect differentiation related to the separation of restite from the source rocks.

FIG. 9.3. Photo-micrograph of biotite crystals with inclusions of monazite and zircon in peraluminous granite. Pleochroic halos around monazite indicate damage of the crystal lattice of biotite by alpha radiation from Th and/or U decay.

Quantitative approaches to the estimation of the radioelement concentration within accessory minerals are based on radiometric methods, whole rock trace element analysis, and microprobe and fission track analysis. Barritt (1983), for example, used mass-balance equations to compute the contributions of accessory phases to whole rock Th and U concentrations for estimating radiogenic heat production of granite plutons in the Scottish Highlands.

Gamma ray spectrometry surveying also offers a promising tool for lithofacies mapping of sedimentary basins as evaporites, carbonates, sandstones and shales can usually be differentiated on their radioelement content and because clastic sediments often reflect the