Radiometric method - Geophysics for the Mineral Exploration Geoscientist - page 212

Table 4.3 Radioelement contents of selected minerals in which
the radioelements substitute for other elements

in uences. Data that plot outside the region bounded by
these minerals are usually due to the presence of U and
Th, explainable in terms of the substitution of these
elements in accessory minerals and other rock-forming
species.

Mineral K (%) U (ppm) Th (ppm)

Amphibole 0 - 0.3 0 - 0.5

Ilmenite 0 - 50

Olivine 0 - 1.5 0 - 4

4.6.1 Disequilibrium in the geological
environment

Plagioclase 0 - 0.5 0 - 5 0 - 3

- -

Pyroxene 0 40 0 25

A significant feature of both the 238 U and 232 Th decay
series is that neither of these isotopes decays by -emission

(see Section 4.2.5 ) and so it is the emission products of
daughter elements that are detected by radiometric
surveys and used to indirectly infer concentrations of
the parent. However, and importantly, an assumption is
made that daughter isotopes are neither added to nor
removed from the system, i.e. the decay series is closed
and in equilibrium. As explained in Section 4.2.2 ,thetime
for the whole decay series to reach equilibrium is
governed by the longest half-life in the series. Members
of 232 Th decay series have half-lives ranging from nano-
seconds to a few years, with the exception of 232 Th itself,
so equilibrium can be re-established instantaneously in
terms of the geological time scale. The

γ Quartz 0 - 5 0 - 6

Rutile 0 - 194

Sphene 0 - 700 0 - 1000

Zircon 0 - 6000 0 - 4000

The behaviour of U and Th in the geological environ-
ment is described in detail by Gascoyne ( 1992 ). Under
chemically reducing conditions, both U and Th exist in a
tetravalent state but, importantly, under oxidising condi-
tions U occurs in a hexavalent state. Tetravalent U and
Th have similar ionic radii, equal coordination number
with respect to oxygen (8) and complete outermost elec-
tron shells. Consequently, they tend to remain together
in geological processes occurring in a reducing environ-
ment. Both tetravalent ions are relatively insoluble,
but the hexavalent uranyl ion (UO 2 2+ )issolublein
water. Uranium can form complexes with a wide variety
of ions in aqueous environments. Organic compounds
may enhance Th solubility in neutral conditions, but
normally Th has very low solubility in natural waters
and is largely transported in particulate matter. The
mobility of U in its hexavalent state is also affected
by adsorption on hydrous iron oxides, clay minerals,
zeolites and colloids. A commonly cited mechanism
for uranium concentration involves precipitation from
oxidised groundwater when reducing environments are
encountered.
Concentrations of K, U and Th for various lithotypes
and mineral species are plotted in Figs. 4.12 , 4.13 and
4.14 . An important characteristic of the three radioele-
ments is that their concentrations tend to be correlated
within most lithotypes: see, for example, Galbraith and
Saunders ( 1983 ) and references therein. The data from the
different rocks types are mostly explainable in terms of
the radioelement content of the common rock-forming
minerals, with micas and feldspars being the dominant

232 Th series is

virtually always in equilibrium.
The half-lives of members of the 238 U decay series range
from fractions of a second to thousands of years, and a
variety of physical and chemical mechanisms can remove
isotopes from the series, or introduce isotopes created
elsewhere ( Fig. 4.15 ) . Consequently, disequilibrium is
common for the 238 U decay series. Uranium and radium
(Ra) are both soluble so they can be removed by ground-
water. Radium can be mobilised by most groundwater,
with its mobility restricted by co-precipitation with barium
sulphates, iron-manganese oxides or sulphates, or through
adsorption by organic matter. The presence of radon (Rn),
a gas albeit an inert one, also encourages disequilibrium
since there are ample opportunities for movement of gas in
the geological environment. In terms of the ' theoretically

decay series, the loss of 222 Rn will require about 27
days for equilibrium to be re-established. However, if
234 U is leeched relative to Ra, equilibrium would take as
long as 1.74 million years to re-establish. The long half-life
of 230 Th (next in the series) ensures that the effects are not
felt further down the decay series for a considerable time. If
230 Th were lost it would require about 530,000 years for
equilibrium to re-establish. Clearly then, the age of a
U occurrence is a critical

closed '

factor in determining the