The Ranger Uranium Mine
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| The Ranger Uranium Mine |
Location: Kakadu National Park, Northern Territory, Australia.
Products: Uranium.
Owner: Energy Resources of Australia
Limited.
Deposit Type: Unconformity-related uranium deposits.
Overview: In 1969 the Ranger
orebody was discovered by a Joint Venture of Peko Wallsend Operations Ltd (Peko)
and The Electrolytic Zinc Company of Australia Limited (EZ). In 1974 an
agreement set up a joint venture consisting of Peko, EZ and the Australian
Atomic Energy Commission (AAEC).
In 1978, following a
wide ranging public inquiry (the Ranger Uranium Environmental Inquiry) and
publication of its two reports (the Fox reports), agreement to mine was reached
between the Commonwealth Government and the Northern Land Council, acting on
behalf of the traditional Aboriginal land owners. The terms of the joint
venture were then finalised and Ranger Uranium Mines Pty Ltd was appointed as
manager of the project.
In August 1979 the
Commonwealth Government announced its intention to sell its interest in the
Ranger project. As a result of this, Energy Resources of Australia Ltd (ERA)
was set up with 25% equity holding by overseas customers. In establishing the
company in 1980 the AAEC interest was bought out for $125 million (plus project
costs) and Peko and EZ became the major shareholders. Several customers held 25%
of the equity in non-tradable shares. Ranger Uranium Mines Pty Ltd became a
subsidiary of ERA. During 1987-8 EZ's interest in ERA was taken over by North
Broken Hill Holdings Ltd and that company merged with Peko. Consequently ERA
became a 68% subsidiary of North Limited, and this holding was taken over by
Rio Tinto Ltd in 2000. In 1998 Cameco took over Uranerz, eventually giving it
6.69% of ERA, and Cogema took over other customer shares, giving it (now Areva)
7.76%.
Late in 2005 there was
a rearrangement of ERA shares which meant that Cameco, Cogema and a holding
company (JAURD) representing Japanese utilities lost their special unlisted
status and their shares became tradable. The three companies then sold their
shares, raising the level of public shareholding to 31.61%.
Geological Features:
Features associated with some of the
unconformity-related uranium deposits in the Alligator Rivers, Rum Jungle and
South Alligator Valley uranium fields are as follows (modified after Ewers
& others, 1984; Mernagh, Wyborn & Jagodzinski, 1998): The host rocks
occur in intracontinental or continental margin basins; the deposits are near
to a late Palaeoproterozoic oxidised thick cover sequence (>1 km) of quartz-rich
sandstone;
The basement is chemically reduced,
containing carbonaceous/ferrous iron-rich units or feldspar-bearing rocks;
The deposits are associated with a
Palaeoproterozoic/late Palaeoproterozoic unconformity and with dilatant brecciated
fault structures, which cut both the cover and basement sequences and separate
reduced lithologies from the oxidised cover sequence;
Most of the large deposits in the
Alligator Rivers and the Rum Jungle fields are in stratabound ore zones and
have a regional association with carbonate rock/pelitic rock contact, but an
antipathetic relationship with carbonate in the ore zones;
The major Australian deposits lie
close to an unconformity although the Jabiluka deposit is still open some 550 m
below the unconformity;
The known major uranium deposits are
present where the oxidised cover sequence is in direct contact with the
reducing environments in the underlying pre-1870 Ma Archaean–Palaeoproterozoic
basement and not separated by an intervening sequence, as by the El Sherana and
Edith River Groups in the South Alligator Valley uranium field.
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| Local stratigraphy of The Ranger Mine |
Alteration
Alteration features associated with
the deposits are:
Alteration extends over 1 km from the
deposits,
Alteration is characterised by
sericite–chlorite ± kaolinite ± hematite,
Mg metasomatism and the formation of
late-stage Mg rich chlorite are common,
Strong
desilicification occurs at the unconformity.
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Source of Uranium mineralization
Archaean and
Palaeoproterozoic granites of the Alligator Rivers and South Alligator Valley
uranium fields have uranium contents which are well above the crustal average
of 2.8 ppm U (Wyborn, 1990a). Granites and granitic gneisses of the Nanambu
complex contain 3–50 ppm U; tonalites, granitic gneisses and granitic migmatites
of the Nimbuwah complex have 1–10 ppm U. The Nabarlek Granite that has been
intersected in drill holes below the Nabarlek deposit has 3–30 ppm U, and the
Tin Camp and Jim Jim Granites also have high uranium contents. The Malone Creek
Granite (South Alligator Valley) has 11–28 ppm U. Wyborn (1990b) suggested that
the underlying crust in the region of these uranium fields is enriched in
uranium. Maas (1989) concluded from Nd–Sr isotopic studies that for Jabiluka,
Nabarlek and Koongarra, the uranium was derived from two sources: the
Palaeoproterozoic metasediments and a post-unconformity source, probably highly
altered volcanics within the Kombolgie Subgroup. Maas (1989) also proposed that
these orebodies formed when hot oxidising meteoric waters, which contained
uranium derived from volcano-sedimentary units within the Kombolgie, reacted
with reducing metasediments of the Palaeoproterozoic basement.
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| Uranium mineralization |
Processing: Following crushing,
the ore is ground and processed through a sulfuric acid leach to recover the
uranium. The pregnant liquor is then separated from the barren tailings and in
the solvent extraction plant the uranium is removed using kerosene with an
amine as a solvent. The solvent is then stripped, using an ammonium sulphate
solution and injected gaseous ammonia. Yellow ammonium diuranate is then
precipitated from the loaded strip solution by raising the pH (increasing the
alkalinity), and removed by centrifuge. In a furnace the diuranate is converted
to uranium oxide product (U3O8).
Reserves & Resources: The Ranger 1 orebody,
which was mined out in December 1995, started off with 17 million tonnes of ore
some of which is still stockpiled. The Ranger 3 nearby is slightly larger, and
open pit mining of it took place over 1997 to 2012.
In 1991 ERA bought
from Pancontinental Mining Ltd the richer Jabiluka orebody (briefly known as
North Ranger), 20 km to the north of the processing plant and with a lease
adjoining the Ranger lease. ERA was proposing initially to produce 1000 t/yr
from Jabiluka concurrently with Ranger 3. The preferred option involved
trucking the Jabiluka ore to the existing Ranger mill, rather than setting up a
new plant, tailings and waste water system to treat it on site as envisaged in
an original EIS approved in 1979. However, all these plans are now superseded –
see Australia's Uranium Deposits and Prospective Mines paper.
In the Ranger 3 Pit
and Deeps the upper mine sequence consists of quartz-chlorite schists and the
lower mine sequence is similar but with variable carbonate (dolomite, magnesite
and calcite). The primary ore minerals have a fairly uniform uranium mineralogy
with around 60% coffinite, 35% uraninite and 5% brannerite. In weathered and
lateritic ores the dominant uranium mineralogy is the secondary mineral
saleeite with lesser sklodowskite.
In the second half of
2008 a $44 million processing plant was commissioned to treat 1.6 million
tonnes of stockpiled lateritic ore with too high a clay content to be used
without this pre-treatment. Following initial treatment the treated ore is fed
into the main plant, contributing 400 t/yr U3O8 production for seven years. A
new $19 million radiometric ore sorter was commissioned at the same time, to
upgrade low-grade ore and bring it to sufficient head grade to go through the
mill. It will add about 1100 tonnes U3O8 to production over the life of the
mine, and be essential for beneficiating carbonate ore from the lower mines
sequence of the Ranger 3 Deeps.
A feasibility study
into a major heap leach operation for 10 Mt/yr of low-grade ore showed the
prospect of recovering up to 20,000 t U3O8 in total. Column leach trials were
encouraging, yielding extractions of greater than 70% at low rates of acid
consumption. The facility would consist of fully lined heaps of material about
5m high and covering about 60-70 ha. These will be built and removed on a
regular cycle and the residues stored appropriately after leaching is
completed. The acid leach solutions would be treated in a process similar to
that used in the existing Ranger plant and recycled after the uranium is
removed from the pregnant liquor. ERA applied for government (including
environmental) approval for the project, which was expected to begin operation
in 2014, but in August 2011 ERA announced that the plan was shelved due to high
capital costs and uncertain stakeholder support. As a result, ore reserves of
7,100 tonnes of uranium oxide were reclassified as resources.
In 2006 the projected operating life of the
Ranger plant was extended to 2020 due to an improvement in the market price
enabling treatment of lower grade ores, and in 2007 a decision to extend the
operating Ranger 3 open pit at a cost of $57 million meant that mining there
continued to 2012. However, reassessment of the low-grade stockpile in 2011
resulted in downgrading reserves by 6100 t U3O8. The #3 pit is now being
backfilled, and to mid-2014, 31 million tonnes of waste material had been moved
there. It will then be used as a tailings dam.
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