Victoria’s Uranium Secret
BY BILL BIRCH
A unique deposit of beautiful crystals in Victoria reveals how nature has locked up uranium,
providing new clues for the long-term storage of nuclear waste.
s exports of Australian uranium
boom and there is talk of a sixth
deposit being opened up,
Victoria never rates a mention. Yet the
State contains a uranium deposit that is
unique in the world of minerals. As its
secrets are exposed, it is telling us how
nature itself controls the behaviour of
uranium over vast periods of time.
Beneath the sands of the Murray Basin
in north-western Victoria lies a large
mass of granite that crystallised about
370 million years ago during the
Devonian period. Apart from the usual
quartz, feldspars and micas that form in
granites, the Lake Boga Granite, as it is
now called, contains an unusual mix of
other minerals. Tiny grains of uraninite
A
(uranium oxide) were enclosed in mica,
bubbles of copper sulfides formed
between the quartz and feldspar crystals,
and beautiful crystals of apatite (calcium
phosphate fluoride) grew in spaces at the
upper edges of the cooling granite mass.
These ingredients coexisted without
change through the millions of years of
erosion that removed the rocks above the
granite and eventually exposed its upper
surface. Once that occurred, the effects
of atmospheric oxygen in surface waters
kicked in, attacking the minerals in the
granite through the process we call rock
weathering. The timing of these events
is quite vague because we don’t know
when the granite was first exposed to
weathering.
Granite outcrops may have dotted
much of the undulating landscape in this
part of the Murray Basin during the
Tertiary period (2–65 million years ago).
Sediments washed from the land by rivers
slowly filled the basin, and there were
several incursions by shallow seas.
The higher points on outcrops of the
Lake Boga Granite may have been low
islands or reefs during the last invasion
of the sea into the Murray Basin 3.5–6
million years ago. When the seas
retreated, a veneer of sand covered the
granite, effectively locking up its mineralogical secrets once again.
It was only by chance that a very small
outcrop of the granite was found in the
early 1900s in the wheat fields about
10 km south of the Lake Boga township,
and a small family quarrying operation
began supplying crushed granite for the
local roads.
Left: Brilliant green crystals of torbernite
scattered on granite. The field of view is
13 mm across. Below: saléeite crystals
6 mm across.
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| September 2008
The Lake Boga granite quarry is a treasure trove of crystallised minerals, including some that are new to science.
In the late 1950s a government geologist carrying out a who knows what minerals might crystallise?
And that’s exactly what has happened in the Lake Boga
random scintillometer survey found higher-than-normal
radiation levels in the quarry, along with bright-green crystals Granite. More than 20 different minerals have crystallised from
of the copper uranium phosphate mineral torbernite encrusting these solutions, forming beautiful colourful arrays that are best
the granite. There was a brief flurry of excitement among the observed under the microscope. Most are phosphates, including
local population at the prospect of a uranium mine on their some common ones found in many places around the world.
However, four of these minerals are completely new to
doorstep. This soon flickered out as it was realised that the
amount of uranium in the granite was too small to be extracted science, having never been found anywhere on Earth before.
economically. However, the crushed granite itself was a major Amazingly, two of these new minerals are uranium phosphates
that have been named ulrichite and lakebogaite.
resource, so the quarry grew and grew.
Collectors found ulrichite in the late 1980s, and were
Gemstone and mineral fossickers began visiting the quarry
in the 1960s as this hobby began to take off. News had filtered attracted by its striking bright-green, needle-like crystals. Brightout to fossicking clubs of the beautiful crystals of
smoky brown quartz and white feldspar being found
in the Lake Boga granite quarry. Since then the quarry
has been a treasure trove of minerals, offering up its
bounty to not only the amateur collectors but also
professional mineralogists in Australia and overseas.
The reason for this arises from an unusual combination of uraninite, copper sulfide and apatite. When
these are attacked by oxidising groundwater during
weathering, their alteration releases uranium, phosphorus and copper into solution. Surrounding
feldspars and micas are also broken down, adding
iron, magnesium, calcium and sodium.
The watery solutions that percolate through all
the cracks and spaces in the weathering granite are a
rich concentrated mix of these dissolved ingredients. Clusters of green ulrichite needles on red hematite (iron oxide). The field of
All that’s needed is for the solutions to dry out and view is 10 mm across.
September 2008 |
| 23
Metanatroautunite crystals 8 mm across.
A crystal of lakebogaite 0.3 mm high.
yellow crystals of lakebogaite were found
a few years later, but it has taken until
recently for the mineral to be fully characterised and named.
The excitement of discovering entirely
new minerals, especially those containing
uranium, in such an unexpected place
was soon followed by questions of why
and when? To the first question we can
only attribute their existence to a unique
set of environmental conditions, in particular the combination of ingredients.
There is evidence that the Mallee
region has been semi-arid for perhaps a
million years. Periodic wet and dry conditions, either in seasonal or more longterm cycles, may have played a part in
precipitating the minerals. Perhaps the
phosphates were more likely to crystallise
during warm and wet periods than in cool
and dry times.
The Lake Boga uranium minerals
provide a tantalising opportunity to test
this idea. Using modern dating technology it is theoretically possible to determine when they crystallised.
The first experiments were conducted
by Dr Stuart Mills as part of his BSc
(Hons) research at Museum Victoria in
2003. Crystals of the five uranium phos-
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| September 2008
phates found at Lake Boga were carefully
selected for a technique known as
uranium–thorium disequilibrium dating.
Each sample crystal was dissolved in acid,
and the ratios of various isotopes of
uranium and thorium were measured
using a mass spectrometer. Knowing the
half-lives of the various stages in the
U–Th decay series enables an age to be
determined, with an upper limit of about
500,000 years. The highly sensitive
analyses were carried out by Dr Roland
Maas in the geochronology laboratory at
the University of Melbourne, with
funding provided by the Ian Potter Foundation.
The results of about 120 analyses did
not reveal any correlation between climate
cycles and the ages of the uranium
minerals. However, the figures obtained
suggest that most of the crystals show
growth features that formed between
about 100,000 and 500,000 years ago.
So what are the implications of these
figures? The crystals themselves look pristine, as if they had been grown in a laboratory yesterday, yet the data suggest they
are hundreds of thousands of years old.
What nature has done in the Lake
Boga Granite is immobilise uranium by
crystallising it as phosphate minerals
immediately after it was released into
solution from the original uraninite, and
almost in the same place. The key “immobiliser” in the solutions was the high phosphorus content, which came from the
apatite crystals as they were also attacked
and dissolved. Uranium phosphates are
particularly insoluble, so once the
uranium is locked up in this form it may
stay that way for hundreds of thousands
of years.
This ability of phosphate minerals, in
particular torbernite, to sequester
uranium has been recognised by other
mineralogists. However, the Lake Boga
occurrence is the first example where
some idea of the time scale has been determined.
There are obvious implications for the
long-term, near-surface storage of nuclear
waste consisting of uranium oxide. A form
of phosphate-rich envelope may provide
additional security for such a facility.
Nuclear energ y technolog y is
advancing and Australia will inevitably
wrestle with a decision on whether to
build nuclear power stations and store
the resulting waste. The natural uranium
storage system in the Lake Boga Granite
may provide us with valuable evidence
on how to do that more safely.
In any event, the Lake Boga granite
quarry will remain a “world heritage site”
for its new minerals.
Dr Bill Birch is Senior Curator (Geosciences) at
Museum Victoria.