Metals make the modern world go around - in this year alone 18 million tonnes of copper, 10 million tonnes of zinc and 8 million tonnes of lead will be needed by the world's economies to make everything from computers to

Wilkinson Discovers Super-rich Ore Brines

Metals make the modern world go around - in this year alone 18 million tonnes of copper, 10 million tonnes of zinc and 8 million tonnes of lead will be needed by the world's economies to make everything from computers to toasters. These vast quantities of metal must all be mined out of the ground from ore deposits. Understanding how these ore deposits form within the Earth is, therefore, crucially important in finding new sources of metal.

In a paper published in the journal Science, Dr Jamie Wilkinson, of ESE, reports the discovery of super-enriched fluids within zinc-lead ore deposits that dramatically change our understanding of how ore bodies form.

The difficulty in making an ore body is in concentrating the metals. The Earth's crust contains on average only 70 parts per million (ppm) zinc, and a miniscule 12.5 ppm lead. These tiny amounts of metal are much too small to mine and extract. Only in ore bodies where metal is concentrated to abundances of several percent can we obtain metals in any real quantity. The question is, just how do zinc and lead become enriched by factors of a thousand times or more?

The enrichment of metal in many ore deposits is known to occur due to metal-bearing fluids or brines which scavenge metal atoms scattered throughout a large volume of crustal rocks and then precipitate them in ore minerals, such as sulphides, in a small volume of crust. Together with the ore minerals are "waste" minerals with low metal contents, such as quartz, termed "gangue" by miners.

Until now there were two sorts of models that explain why high concentrations of metals occur in some places in the Earth's crust and not in others. The first is that the metal-rich fluids were not particularly unusual but that precipitation was very efficient; an example of a process that can be effective in depositing some metals, like gold, is boiling. This would mean that studying the special features of the deposit are critical to understanding how the ore formed. The second model suggests that significant ore bodies are only formed from "special" fluids that are highly enriched in metals in the first place. This would mean the source of the fluids and metals is most important.

Wilkinson and his team studied small pockets of fluid, known as fluid inclusions, that are trapped within both ore and gangue minerals as they grow, in order to directly measure the metal content of the hydrothermal brines that formed the deposits. They studied samples from two ancient (more than 250 million year old) zinc-lead ore deposits, one in Arkansas, USA, the other in Ireland. To analyse the tiny amounts of fluid they used a laser beam thinner than the width of a hair to drill into the minerals and analyse the fluid released with an inductively coupled plasma mass spectrometer (ICPMS). Their analyses revealed that fluid inclusions within the sphalerite (zinc sulphide) ore minerals contained 10 to 100 times as much metal as fluid inclusions within the quartz.

The results of Wilkinson's study suggest that the composition of the hydrothermal fluids is the most important factor in the formation of world class ore deposits. The influx of fluids highly enriched in metal into the deposits must have occurred to cause the precipitation of the ore minerals whilst most of the rest of the time the fluids flowing through the deposit were barren and only precipitated gangue minerals.
"It is the source of the metal-rich fluids that is key to formation of the best ore deposits", says Wilkinson, "and that is different from deposit to deposit."

For the Arkansas deposits Wilkinson's study indicates that the barren and metal-rich fluids had very different sources. The metal-rich, ore mineral producing, fluids appear to have originated at the Earth's surface by strong evaporation of seawater in the hot, arid environment of the US mid-continent in the Permian around 280 million years ago. This metal-rich brine was then buried and trapped underground, until it was finally expelled and flowed into the ore deposit to precipitate sphalerite and other ore minerals. Both the climate under which the brine formed, and the details of how it flowed through the Earth's crust are, therefore, crucial in how the deposit formed.

"The discovery of these highly metal-enriched fluids could change the way we hunt for new ore deposits", says Wilkinson, "because now we know we must follow the brines to find the metal. We need to find new metal resources to sustain social and economic development around the world but we need to do this efficiently, minimising effects on the environment. Hopefully this work will help".

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