Radiometric method - Geophysics for the Mineral Exploration Geoscientist - page 218

γ -rays

Highly
weathered
palaeosurface

Residual soil/
regolith

Bedrock exposed

Accummulation
of resistate
minerals
& oxides

Bioturbation

Active
erosion

Lateral movement
of Fe-oxides
& clays

Old
alluvium

Soil
formation

Groundwater
discharge

Recent
alluvium

Figure 4.16 Schematic illustration of near-surface
processes that affect radiometric responses.
Redrawn, with permission, from Wilford et al.
( 1997 ).

Groundwater
flow

Eluviation/
illuviation

Capillary
rise

Leaching of soluble bases (Na, Ca, Mg, K)

ore-grade U concentrations in sur cial materials. Also
important are changes in density/porosity that affect the
ability of the

initial radioelement concentrations, such as limestones and
clean sandstones, generally produce soils with similarly low
elemental concentrations. Recent organic sediments may
be rich in uranium.
The relative and absolute abundances of the three
radioelements for rocks, and the overburdens derived
from them, from a deeply weathered Australian terrain
and a glaciated Canadian terrain are shown in Fig. 4.17 .
Although these are two geologically different terrains, simi-
lar behaviour is observed in both regions. The relative
concentrations of the radioelements are largely unaltered,
but in virtually every case the cover has signi

γ

-rays to escape from the source rock, and
also facilitate the loss of mobile isotopes, promoting
disequilibrium.
Weathering decomposes the main K-hosting minerals in
the order biotite, alkali feldspar, muscovite. The K so liber-
ated is taken up in minerals such as illite, and to a lesser
extent other clay minerals. Of the main U- and Th-bearing
minerals, only zircon and monazite are stable during
weathering. Uranium liberated by weathering tends to
occur in authigenic iron oxides and clay minerals, or may
go into solution as a complex ion. Thorium liberated by
weathering may be found in oxides and hydroxides of iron
or titanium oxides, and also clays. Similar to U, Th can be
transported and absorbed on colloidal clays and iron oxides.
The relationship between the radiometric response of
the bedrock and its weathered equivalents depends on
factors such as rainfall, temperature, and pH and salinity
of water, all of which affect the weathering process and the
behaviour of the radioelements. Depending on the local
conditions, opposite effects may result. K-feldspar is rela-
tively resistant to weathering, resulting in a relative
increase in K content early in the weathering process.
However, under lateritic weathering conditions, kaolinisa-
tion may cause substantial loss of K. Uranium is not easily
liberated by weathering when it is trapped within heavy
minerals such as zircon, but when it is present in less
resistant minerals, or located along grain boundaries, it
may be lost relatively easily. Weathering of shales tends
to result in the loss of K, but less so U and Th, especially
during lateritic weathering. On the other hand, silici

cantly lower
concentrations of the radioelements than the bedrock. This
reduction may in fact be even greater than those suggested
by the data (Dickson and Scott, 1997 ) . In other places the
concentrations in the soils may exceed that of the protolith.
This is common for soils formed above ma candultrama c
rocks, and iron formations, with U and Th exceeding those
of the source. This is thought to be due to the concentration
of U and Th in iron oxides during weathering.
The distribution of the three main radioactive elements
is not constant in the soil/regolith profile ( Fig. 4.18 ). The
fact that cover materials are more prone to erosion than
lithified materials means that the vertical distribution of
elements in the sequence determines the radiometric
response, because dissection may expose different parts of
the soil/regolith profiles. In the case of regolith, the pres-
ence of clays or iron oxides at different depths can lead to
anomalous concentrations of U and Th. Concentration of
Th in iron-rich materials, such as ferricrete and pisoliths, is
also a complicating factor.
Other near-surface phenomena that may be of signifi- -
cance are related to the hydrological setting. In areas of

ca-
tion tends to preserve the K content. Sediments with low