Story: Radioactive minerals

Page 1. Prospecting for uranium

All images & media in this story

Radioactive minerals

A small number of naturally occurring minerals give off invisible radiation as they gradually decay into a more stable form. The term radioactivity was coined by the French physicists Marie and Pierre Curie, who investigated this property in the late 19th century. Radioactivity in rocks and minerals is due to the presence of tiny amounts of radioactive elements, particularly uranium and thorium.

Uses

Radioactive elements have a range of applications in medicine and agriculture. In the first part of the 20th century scientists theorised that they could be used to generate heat for power stations, and to build a powerful type of bomb. Trace amounts of uranium compounds are found in almost all rock, soil and water, and one uranium isotope (U-235) is highly radioactive. For use in nuclear reactors and weapons, natural uranium must first be enriched with U-235. This form of uranium is capable of sustaining a chain reaction, releasing large amounts of heat.

During the Second World War, uranium was used to make the bomb that was dropped on the Japanese city of Hiroshima on 6 August 1945, and to make plutonium for the bomb dropped on the city of Nagasaki three days later. Development of nuclear power followed and uranium became a sought-after global commodity that would take the world – including New Zealand – into the atomic age.

The wartime search

New Zealand’s first uranium survey was inspired by the British and American demand for uranium for their nuclear weapons programmes in the Second World War.

Ernest Marsden, head of the Department of Scientific and Industrial Research (DSIR), initiated New Zealand’s first uranium survey during the war. In 1943, British and American scientists began work on the Manhattan Project to isolate U-235 and make an atomic bomb. While the UK had initiated a Commonwealth search for uranium in 1942, they did not consider New Zealand promising enough to include in the survey. Marsden, however, planned his own search for radioactive minerals in the South Island.

In the winter of 1944 a team of DSIR physicists assembled at the Dominion Physical Laboratory in Wellington to prepare equipment for the survey. The following summer, two teams of geologists and physicists began secretly exploring the South Island. Each was armed with a large, unwieldy Geiger counter – used to detect radioactivity.

An earlier study of New Zealand soils and rocks had suggested that granitic rocks and the beach sands derived from them would be the most likely source of uranium and other radioactive minerals. Although radioactivity levels were low, the DSIR teams found the most significant levels in the black sand concentrates of the West Coast gold dredges, and spent two months working on the Gillespies Beach dredge sands near Franz Josef.

Field day

Physicist Jim McCahon became an unwitting target while searching for uranium on the West Coast:

‘It was very hot so we worked most of the time nearly naked, and any time a cloud came over the sun we were free feed for the sandflies. When we had been there 10 days Ernie Marsden came to see how we were getting on. We told him of the sandfly troubles. He went away to his car and came back with a bottle labelled dimethyl phthalate – the stuff that is nowadays called ‘dimp’. He had bludged this from the airforce up in the islands and he gave it to us for protection. It was an absolute godsend.’ 1

First voyage of the New Golden Hind

In March 1945 the DSIR chartered the government ship New Golden Hind, and the secret uranium survey was extended to the granitic rocks of Fiordland. The scientists on board investigated the eight sounds from Milford Sound to Nancy Sound, but failed to find any promising sources of radioactive minerals.

In August 1945 the United States dropped atomic bombs on Hiroshima and Nagasaki. In recognition of the military and economic importance of uranium, New Zealand’s Atomic Energy Act was passed on 7 December 1945 to give the state full ownership and control over uranium and other radioactive elements.

Second voyage of the New Golden Hind

In January 1946 a second New Golden Hind expedition – this one was not secret – continued the survey in the southern sounds from Preservation Inlet up to Nancy Sound.

Once again, they found no significant radioactivity. Over the course of the two-year survey, the scientists had tested the radioactivity of rocks and beaches along nearly 1,600 kilometres of coast, the sands and gravels from more than 100 streams and rivers, and the concentrates from more than 20 sluicing and gold-dredging claims in Nelson, Westland, Otago and Southland. While uranium-bearing minerals – including some previously unknown to science – were found concentrated in tailings left by West Coast gold dredges, the quantity and concentration was considered insufficient for a mining venture.

Footnotes:
  1. Jim McCahon, personal recollection, recorded 14–16 August 1998. › Back
How to cite this page:

Rebecca Priestley, 'Radioactive minerals - Prospecting for uranium', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/radioactive-minerals/page-1 (accessed 22 November 2018)

Story by Rebecca Priestley, published 12 Jun 2006