in mineral sands: measurement and uses
I.R. Duddy and P.R. Kelly
Australian Institute of Geoscientists Bulletin No. 26 1999
The uranium content of minerals
such as zircon and rutile is a key parameter in the economics of mineral
sand production as high uranium contents may significantly down grade
the value of the mineral sand product. In this short paper the application
of fission track methods to the highly accurate determination of uranium
in individual mineral grains (eg Lovering & Kleeman 1970) is described
and discussed. In the early exploration phase, the technique allows
the rapid identification of the minerals hosting uranium and the determination
of the absolute U-content in individual grains. In the development and
production phase, use of the single grain technique permits the development
of strategies for adjusting mineral processing procedures to separate
desirable low-uranium mineral populations. The paper will also briefly
address more general uses of the fission track method in provenance
studies for heavy minerals, diamond exploration and geochronology.
Basics of the fission
track method for Uranium determination
Uranium is a common trace
constituent of many heavy minerals. Apart from uranium minerals sensu
stricto, monazite and zircon are typically the main uranium hosts in
mineral sand deposits, containing around 100 to 1000 ppm. Rutile and
apatite typically contain between 10 and 100 ppm U, but higher amounts
Uranium decays spontaneously
by two main processes: Alpha decay and fission decay. In this application
we are only concerned with fission decay. Natural spontaneous fission
occurs only for the most abundant uranium isotope, 238U
(~99% of natural uranium). The second most abundant Uranium isotope,
(~0.7% of natural uranium), does not undergo natural fission but it
may be artificially induced to fission by bombardment with thermal neutrons.
The ability of 235U
to undergo induced fission provides the basis for the quantitative determination
of uranium content in mineral grains discussed here.
The process of induced fission
is illustrated in Figure 1. Collision of a thermal neutron with 235U
causes the atom to split into two sub-equal fragments that have very
high positive charges. These fragments move apart damaging the crystal
lattice of the host mineral by an ionisation process, essentially the
removal of electrons from the lattice atoms. With the passage of the
fission fragments through the lattice, the now positively charged lattice
atoms repel each other, forming a disrupted zone in the crystal. This
disrupted, glass-like, zone is known as a fission track. In zircon,
these tracks are approximately 50 angstroms wide and 12 µm long whereas
in apatite they are around 16 µm long and in mica around 20 µm long.
The key aspect of the exploitation
of this process is that the number of induced fission tracks is directly
proportional to the uranium content of the mineral host.
Figure 1: Formation of
induced fission tracks in a crystal lattice.
A. Bombardment of 235U with
thermal neutrons causes fission.
B. The highly positive charged 235U
fission fragments fly apart, ionising the crystal lattice, essentially
by removing electrons.
C. The remaining positively charged lattice
atoms repel each other causing a damaged zone in the crystal - the
fission track. In zircon, the damage zone is ~12 µm long and 50
in individual mineral grains by the external detector method
Uranium content (U) can be
measured by irradiating the mineral sample with a calibrated dose of
thermal neutrons and measuring the number of tracks produced by induced
fission of 235U per unit area of the apatite grain surface (the "induced
track density", rhoi).
U unknown = rhoi
unknown x U std / rhoi std
rhoi is measured in a muscovite
"external detector" attached to the surface of the unknown
Induced fission tracks leave
the grain and form tracks in the mica external detector, which is subsequently
etched (in hydrofluoric acid) so that rhoi can be measured using an
optical microscope. The procedure is illustrated in Figure 2.
In practice, the mineral
grains for U-determination are mounted as a strewn mount on a glass
slide in epoxy resin and polished as for a rock thin section. A mica
external detector is placed over the grain mount and held in place with
a heat shrink plastic. Groups of grain mounts are stacked together with
U-bearing calibration standard glasses with attached mica detectors,
at each end. The complete package is then subjected to irradiation with
thermal neutrons after which the detectors from the samples and standards
are removed and etched and the number of tracks counted.
Mica external detector held against mineral grain mount during
irradiation with thermal neutrons (view in section). Fission
tracks are induced throughout the body of the crystal as shown
but only those within range of the surface (~12 µm) leave the
crystal and a recorded on the mica external detector.
Mica external detector removed from grain mount and etched to
reveal induced fission tracks from the zircon grain (view in
Microscope plan view of mica external detector showing etched
tracks from the fission of uranium in the zircon grain that
was in contact during irradiation. The number of tracks in a
given area is directly proportional to the uranium content.
Uranium in mineral
sands - know where the uranium is early in a project
With the ability to determine
the U-content of individual grains, comes the ability to closely monitor
the fate of Uranium in mineral processing. For example, there may be
a tendency for grains of a particular grain size to have unacceptably
high U contents. Figure 3a shows a photomicrograph of a mineral separate
largely comprised of zircon, but with a few contaminant grains. Figure
3b shows the mica external detector print with induced fission tracks
from all of the grains present. Variation in U-content of individual
grains is easily recognised by the variation in track density as is
U-zoning (particularly in grain Z1). Two transparent contaminant grains
(A and B) contain no uranium while an opaque grain (O) contains a small
but measurable amount.
The fission track method
allows this U-variation to be readily identified and quantified. In
the mineral sands exploration phase, early knowledge of the U-content
of potentially economic deposits allows appropriate action to be defined
before expenditure on plant and equipment. In the production phase,
U-content of the various products can be monitored on a single grain
basis thus making it possible to adjust processes to maximise quality
of the refined product.
Photomicrograph of mineral grain mount. Zircons (varying from
euhedral to rounded shapes) are labelled Z1 to Z7. Other transparent
minerals are labelled A and B and an opaque mineral, O.
Figure 3B: Photomicrograph
of Uranium fission-track print of mineral grain mount on mica
external detector. Note that all of the zircons are revealed
by a concentration of fission tracks that mirror the shape
of the host grain. Note also that variation in density of
fission tracks reflects variation in uranium content. Variation
in track density within some grains reflects uranium zoning.
The other transparent minerals
labelled A and B in Figure 3A have no fission track print
and therefore contain no uranium, while the opaque mineral
(O) has a very low density print reflecting a very low uranium
content. Quantitative analysis of the area density of tracks
in each grain print allows the absolute uranium content in
each grain to be determined.
Uranium in zircons
as a provenance indicator
The uranium content of detrital
zircons can be used as a provenance indicator. In general terms zircons
from crystalline mantle and crustal rocks can be easily distinguished
by their U-content as illustrated in Figure 4. Mantle derived zircons,
including those from diamondiferous kimberlitic sources, typically contain
less than 60 ppm U (median ~10 ppm), while those from crustal sources
contain much more (median ~300 ppm). This observation is particularly
useful in alluvial and soil surveys for diamonds where the more traditional
diamond indicator minerals have been removed by weathering processes.
It also provides a possible exploration strategy for possible high grade,
low-U zircon deposits.
Uranium content in zircon grains from crustal and mantle sources.
Zircons derived from mantle (including kimberlitic) rocks have
significantly lower uranium than those from crustal igneous
rocks. This observation forms the basis for the use of zircons
as a mantle source indicator in diamond exploration. (Uranium
in individual grains determined by the fission track external
detector method - Geotrack unpublished data)
Geochronology - zircon
fission track analysis - ZFTA
Detailed discussion of ZFTA
is beyond the scope of this short paper. However, it should be noted
that it is possible to use similar fission track methods to those described
above to determine the age of individual zircon grains (eg Fleischer
et al. 1975) from an alluvial sample. The resulting fission track age
is numerically very similar to a K-Ar age on the original crystalline
rock. Thus it is possible to relate ZFTA ages back to particular provenance
areas. As a word of caution, some zircons contain so much uranium (or
are so old) that the fission track radiation damage is so severe that
they cannot be etched without dissolving the entire grain. This is a
part of the process of metamictization. In such cases, a particular
provenance age may not be revealed even though it makes up a high proportion
of the zircons present.
The fission track technique
allows both the rapid identification of the minerals hosting uranium
and the determination of the absolute U content in single mineral grains.
Such knowledge permits the development of strategies for adjusting mineral
processing procedures to separate desirable low-uranium mineral populations.
The technique can also provide valuable provenance information for sand-sized
detrital products and in the case of zircon, direct age dating of individual
FLEISCHER R. L., PRICE P.
B. & WALKER R. M. 1975. Nuclear Tracks in Solids. Principles and
Applications. University of California Press.
LOVERING J. F. and KLEEMAN
J. D. 1970. Fission track Uranium distribution studies on Apollo 11
Lunar samples. Proceedings Apollo 11 Lunar Conference, 1, 151-158. Cambridge,
of Geoscientists Bulletin No. 26 1999