Ranger Uranium Mine

4 Jun

Ranger Uranium Mine was discovered in 1969 and is surrounded by the world heritage listed Kakadu National Park. The mining operation is run by ERA (Energy Resources Australia) who commenced in 1980 and it is one of three mines worldwide to produce more than 100,000 tonnes of uranium oxide. The open cut ceased operation in 2012 but ERA has been given approval to continue its underground Ranger 3 Deeps project.
Kakadu contains many uranium deposits within old (1800 million year old) sandstone and conglomerates of the McArthur River Basin. It is estimated there are more uranium energy reserves in Kakadu than all of the oil reserves in Saudi Arabia and the value of all Australian uranium reserves combined is in the order of $300 billion. These types of unconformity-related uranium deposits form when oxidised groundwaters containing high concentrations of dissolved uranium naturally flow through the porous and permeable sandstone units. When these oxidised waters encounter reduced (low oxygen) deeper groundwaters the uranium precipitates out of solution as pitchblende (UO2) and coffinite. This redox reaction forms the distinct Leisegang rings (dark and pale crescent-shaped stripes) within the sandstones. Interestingly the timing of this mineralising event is not well understood.
Uranium, being soluble in acids, is leached from the ore using sulfuric acid and then recovered from solution by adding kerosene and then burnt to leave a uranium oxide residue. This uranium ore concentrate – yellow cake (U3O8) is exported under strict International Atomic Energy Agency safeguards for electrical power generation or nuclear research only. Australia does not engage in any further enrichment of yellow-cake ore to produce uranium ready for nuclear reactors – a difficult process that greatly increases the value of the product. Uranium is the only naturally occurring fissile element and it has two main isotopes; uranium with an atomic mass of 238 (99.2% of all uranium) and one with a mass of 235 (0.72% of all U). Only the U-235 is fissile when bombarded with neutrons so this lighter isotope of U must be enriched to about 4% to be used in a slow thermal reactor. It has to be enriched to at least 90% U-235 to be used in nuclear weapons. Enrichment of U-235 is extremely difficult because the two isotopes behave the same chemically so enrichment relies on the minute differences in atomic weight and a repeated process of gaseous diffusion or centrifuges of gaseous uranium hexafluoride (UF6).
There are many environmental and social issues associated with uranium mining in a tropical environment like Kakadu in which the worlds oldest living culture (Mirarr Aboriginal people) still hold onto traditions and laws that have been in place for at least 25,000 years but possibly up to 40-50,000 years. Some of the deposits like Jabiluka and Coronation Hill which would be worth many billions of dollars have not been approved by the traditional elders. The recent world heritage listing has locked away these deposits to future mining activities.
Nuclear energy is a topic that divides many people and raises seemingly contradictory philosophical view points; best illustrated by the 3 mines policy of the Hawke era, in which labor were generally anti-uranium mining but not totally so they compromised by allowing 3 uranium mines to operate in Australia. It seemed a little bit of a bad thing was a good policy. Howard removed the 3 mines policy when he came to power and Gillard is now considering opening our uranium market to include India – a country that has not signed the international nuclear non-proliferation treaty. Nuclear energy is also a consideration as an alternative to fossil fuels like coal which are contributing to climate change but obviously events like Chernobyl and Fukushima are etched into the minds of people worldwide. Regardless of this 68 new civil nuclear reactors are under construction in 15 countries; 28 of these are in China so uranium will continue to be a valuable, if somewhat controversial, resource into the foreseeable future.

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Dalgarango: A cute meteorite crater

30 Mar

Dalgarango Meteor Crater is Australia’s smallest and possibly youngest impact crater located 75 km NW of Mt Magnet. It measures only 21 m in diameter and the eastern side is a few metres higher than the western suggesting that a meteorite of about 100 tonnes approached at an oblique angle from the west at a velocity of around 40,000 km/h. An aboriginal stockman, Billy Seward, discovered it in 1920 during a muster and he reported it to the station master Gerard Wellard. Mr Wellard realized the potential scientific importance of the site so collected samples of the meteorite and sent them to the WA Museum. Unfortunately the samples were misplaced and a year later when Mr. Wellard visited the museum they had apparently been lost and things were forgotton.

However, in 1938 one of the rock specimens was found and analysed. A report was written authenticating the crater but the discovery was attributed to Mr Willard, not Mr Wellard (even Microsoft Word wants me to correct Wellard to Willard!). This simple spelling error was to create a problem that wasn’t solved until an extraordinary coincidence 23 years later.

After publication of a scientific paper in 1960, a US scientist, Prof. Pearl, flew to WA to inspect Dalgarango crater and to talk to Mr Willard but because of the misspelling of his name didn’t find him. Dissappointed, Prof. Pearl boarded a cruise ship to return home. Three days into the cruise his wife was talking to another lady and the topic of meteorites was raised. Incredibly, this woman turned out to be Mrs. Wellard and the connection was made!

The meteorite responsible for the Dalgarango crator is a rare stony-iron type and it is thought that it impacted only about 3000 years ago. Although it is only small, the crater displays the typical characteristics of meteorite impacts with strata tilting away from the crater as a result of the ‘rebound’ effect after the meteorite buries itself, it then explodes. Fragments of the unweathered granite buried beneath the laterite cap are strewn about the crator.

Dalgaranga Crator

Mudcracks in a gold mine

30 Mar

Mudcracks in a gold mine
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Gallery: Nullarbor

22 Mar

A gallery: Nullarbor

Land of the giants! These cliffs stretch for almost a thousand km’s along the Great Australian Bite and are composed of 3 sedimentary layers. The lowest and oldest white unit is the Wilsons Bluff Limestone, which was deposited in a shallow warm sea 44-38 million years ago (Eocene) as Australia started to rift away from Antarctica and the rift valley was flooded by the ocean. The middle layer is the Nullarbor Limestone which was deposited 15-5 million years ago (Miocene) and the uppermost unit is the Bridgewater Formation which consists of wind blown sands (aeolianites) deposited in the past 2 million years (Pleistocene). Together these 3 units form part of the Eucla Basin and lie at almost the exact same elevation right across the Nullarbor indicating there has been no significant tectonic faulting or folding in the past 50 million years. This raises a conundrum though as these limestone units now sit well above present and past sea levels responsible for their formation so how have they been uniformly uplifted without any faulting or tectonic dislocation? Some suggest it is a result of tilting of the Australian continent with the northern margin sinking and the southern margin rising – but why? It could be due to the subduction zone beneath Indonesia pulling the northern margin of Australia downwards or more likely it is the fact that the Australian continent is moving northward (at 6 cm per year) over a geoid high in the south and a geoid low in the north. The Earth is not a perfect sphere but in fact an irregular geoid with subtle bumps and troughs caused by variations in density of the underlying mantle and as a result variations in the gravitation field which in turn affect the height of the oceans. The oldest limestones were deposited when southern Australia was positioned on a geoid high nearer the South Pole and sea levels were relatively high. However since rifting away from Antarctica 50 million years ago Australia has drifted across a geoid low and the sea level has dropped relative to where it was initially. In northern Australia, the continent is entering another geoid high and so much of the continental shelf is submerged as the relative level of the sea rises again, depositing a new layer of limestone across the flooded Gulf of Carpenteria.
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Gallery: Earthdownunder

13 Mar

A gallery: Earthdownunder

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Gallery: Crevasse

11 Mar

A gallery: Earthdownunder

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Rhizomorphs. Fossilised roots

10 Mar

Rhizomorph1 Rhizomorph2

Rhizomorphs. Fossilised roots preserved in a calcrete layer developed below the soil horizon. These are formed in arid climates as plant roots draw in water but exclude dissolved salts such as calcium. The calcium ions accumulate in high concentrations around the living root and combine with carbon dioxide (dissolved in rain water as a week carbonic acid HCO3-), to precipitate calcite (CaCO3). Eventually the root is suffocated by the calcite and the rhizomorph remains fossilised in the calcrete layer just below the surface. These calcrete layers are common in the arid coastal region of Port Lincoln, South Australia. These calcretes probably formed more extensively during dry, cold interglacial periods but they are notoriously difficult to date and as a result the rates of formation are not well understood. Most of arid Australia is covered by calcrete just below the surface and it represents a huge reservoir of solid carbon dioxide locked up as calcite.

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