Radiometric method - Geophysics for the Mineral Exploration Geoscientist - page 217

loosely bound on grain boundaries and internal surfaces.
The data that are available suggest that, in general, meta-
morphism does not greatly affect radioelement content, so
the concentrations in the rocks mainly re ect the protolith.
For example, Roser and Nathan ( 1997 ) show that there is
no signi cant variation in the concentrations of K, U or Th
in a turbidite sequence whose metamorphic grade varies
from lower greenschist to amphibolite facies. The same
applies to amphibolite-facies gneisses and associated
granulites analysed by Fernandes et al.( 1987 ).
The radioelement distribution of metamorphic rocks
shows a correlation between U and Th ( Fig. 4.13 ), but the
relationship between K and the other two elements is
somewhat less than in igneous rocks, indicating some
element mobility. As would be expected, the data reflect
the radioelement content of the precursors. Felsic gneisses
are among the most radioactive rocks, as are migmatites.
Ma

decreases in radioelement content. Heavy minerals such as
monazite, sphene and zircon are resistant to mechanical
weathering and, if present, will contribute signi cantly to
the observed radioactivity. In heavy mineral sand deposits
these minerals constitute the dominant source of radio-
activity, so these deposits are enriched in U and Th relative
to K compared with other sediment types. Feldspathic and
glauconitic sandstones are clastic sediments that are anom-
alously radioactive, dominantly owing to K.
Pure carbonates have low radioactivity, especially if dolo-
mitic, but when they contain organic matter they may have
relatively high levels of U. The Th content of carbonates is
low since it cannot enter the carbonate lattice easily,
although studies have linked enhanced Th content in
black shales to diagenetic carbonate. Some marine black
shales have U concentrations of hundreds, and locally
thousands of parts per million (Cobb and Kulp, 1961 ) , but
most black shales have radioelement concentrations similar
to other types of argillaceous rock. Phosphatic sediments of
marine origin may have particularly high U content.
Chemical sediments are generally poorly radioactive.
Banded iron formation normally has very low radioele-
ment concentrations, although they appear Th-rich in
radiometric data owing to thorium

c gneisses and amphibolites are much less radioactive.
Metasediments, including schists, pelites, phylites and
slates, plot in similar positions to gneisses, re ecting their
disparate original compositions. Most metacarbonates are
weakly radioactive, but a few may have some U enrichment.

'

saf nity with iron
oxides (see Section 4.6.5 ) . Most evaporites are similarly
poorly radioactive, but become more radioactive when
potassic minerals such as sylvite and carnallite are present.
Figure 4.14b shows that coal has low radioelement con-
tent, especially K content. Occasionally U content can be
quite high, but in general coals are amongst the least
radioactive sediments.

4.6.4 Potassium, uranium and thorium in
sedimentary rocks

The distribution of K, U, and Th in sedimentary rocks is
complex, being in uenced by the composition of the
parent rock, the processes of the sedimentary cycle and
the different geochemical properties of each element. The
radiometric response is determined dominantly by the
feldspar, mica and clay-mineral contents of the rocks,
and supplemented by heavy minerals containing Th and
U. The correlation in the radioelement content of sedi-
mentary rocks is less marked than for igneous rocks, but
still apparent ( Fig. 4.14 ) . The lesser correlation is due to
mobilisation of

4.6.5 Surficial processes and K, U and Th
in the overburden

For the simplest situation of residual cover, the radiometric
response of the cover material is related in a simple way to
the underlying bedrock. Of course, it is unlikely that there
will be any direct relationship where the cover comprises
transported material, so in these cases mapping the bed-
rock with radiometrics is impossible. Even if the cover is
residual, pedogenesis and regolith formation will affect
radioelement content.
Some of the many and complex secondary near-surface
processes that affect the radiometric response from a single
lithotype are summarised in Fig. 4.16 . The most important
processes are those that remove, transport and deposit K,
U and Th. These processes may be sufficient to create

the radioelements in the secondary

environment.
For clastic sediments, key factors affecting radioactivity
are the nature of the source and maturity. A source rich in
radioactive material is, not surprisingly, likely to produce a
radioactive sediment. The lower end of the range corres-
ponds with sediments composed primarily of quartz or
carbonate grains. As the fine-grained fraction increases so
too does radioactivity due to increasing amounts of clay-
minerals. Greywackes have radioelement concentrations
similar to their source, but as sediment maturity increases
quartz becomes increasingly dominant with an associated