Science Spin January 2010

Bright outlook for Irish zinc and lead

By Tom Kennedy

Discovering more mineral wealth could help pull Ireland out of the economic depths. Ireland already has the largest zinc lead mine in Europe at Navan, and if prospectors hit lucky, deposits of similar size may yet turn up in the south west. At Kilbricken, Co Clare, high concentrations of lead, zinc and silver have been found, and the old mine workings there are more than likely to get a new lease of life.

Deposits

According to UCD geologist, Julian Menuge, no one knows how much more zinc ore can be discovered in Ireland, but the conditions that led to big mineral deposits at Tynagh, Silvermines and Navan appear to have been repeated right across the country. If we look at the top 55 mineral deposits in Ireland, he said, most are in the Lower Carboniferous limestones in a broad line that extendacross the central plain.

These rocks were laid down as a shallow ocean advanced over the former Devonian age desert, which in turn had covered the massive cracks and fissures familiar to geologists as the Iapetus Suture. This is one of the most ancient features of this island, formed millions of years ago when what is now the southern part of the country drifted up to join the north. That suture extends across Ireland, and the deep cracks along its length allowed extremely hot, mineral
rich, fluids to filter up from the depths towards the upper rocks.

As Julian explained to a recent meeting of the Irish Geological Association, a peculiar chain of events, involving interaction between upper and lower levels, appears to have been responsible for producing these unusually high concentrations of zinc and lead. Although the mineral deposits are at two distinct levels within the carboniferous limestone both groups are thought to have been formed in much the same way. Like Galmoy, Tynagh and Lisheen, the recent discoveries in the south west are in the upper level, while Navan, at about 350 million years old, is in the lower group.

For some time, geologists have speculated on how these deposits came to be there, and while it is taken as a given that the minerals were deposited from hot fluids, opinions have been divided on how and why this happened. One theory is that water filtering down from above heights picked up mineral traces which were then precipitated as hot fluids rose up from the depths to react chemically with the limestone.

MODEL

It is thought that this model could account for some of the mineral deposits in the United States of America, but something more complex seems to have happened in Ireland. A few years ago, the geologist, Mike Russell, now based at the Jet Propulsion Laboratory in California came up with a two fluid model that is now widely accepted as explaining why Ireland has so much zinc and lead.

In this model, hot mineralised fluids are believed to have come up from a considerable depth, and on reaching the basement of the limestone, these fluids came into contact with extremely saline water. At that time conditions on the Earth's surface would have produced oceanic pools of high salinity, and where the two fluids met became a zone of interaction between salt, water and sulphur.

As of now, certain forms of bacteria can thrive under these kind of extreme conditions, needing no oxygen, but harvesting energy by reducing sulphates to sulphides. For them, the mineral rich fluids coming up from below were a feast, and as they fed, the sulphates were converted into insoluble deposits that we find now as sulphide minerals such as galena and sphalerite.

As Julian explained, one part of the evidence to show what was going on is in the form of tiny bubble-like inclusions. When examined under a microscope, these inclusions are seen to contain a fluid, and above this fluid are vapour bubbles. These inclusions are the remaining traces of mineral rich fluids. Originally, these fluids would have been homoginous, so the vapour bubbles only formed later as the temperature dropped. By heating the inclusions on a microscope stage, the temperature at which the vapour and fluid homogenise, reveals that the original fluid would have been between 200 to 300°C.

The fluid was salty, and as salt lowers the freezing temperature, cooling the inclusions until they freeze gives a good idea of just how salty the original was. Evidence of this kind supports the view that these super heated fluids must have come up against much cooler, and much more saline water filtering down from an evaporating sea. Apart from salt and water, sulphur comes into the equation, and this could have come from above and from below. However, when the isotopes of sulphur in mineral deposits are examined, there is a tell-tale peak of isotope 34.

BACTERIA

This particular isotope, 34S, is produced by sulphur reducing bacteria. It seems that the bacteria, like those around present day black smokers in the mid Atlantic, thrived next to the source of hot mineralised fluids, for when the ores at Navan were analysed at, it was found that the levels of 34S were highest next to fault zones. It is quite likely that the same sort of interactions were widespread, and while the deposits at Navan represent more than half, the known total in Ireland comes to about 17 million tons of zinc. "If we consider how much may have been produced by the same process," said Julian, "it could be as much as three times this amount, and if reserves that we do not yet know about are included, this might be as much again."

If this seems amazing, even more so is the fact that zinc and lead could be much more abundant than we usually think. We should not assume that minerals can only occur in concentrated pockets of ore, and as Julian explained, lead and zinc could actually have come from "any old rock'. In the upper crust, he said, there are 67 parts per million zinc, and 17 parts per million lead. "This might not seem much," he commented, "but it would be enough to have produced all these deposits. If you were to take 6.3 metres of upper crust and extract the lead and zinc for deposition in the Carboniferous, you would get 50 million tone of zinc and 12 million tons of lead." Without lower life forms and all those deep and ancient cracks along the Iapetus Suture we might never have been able to recover all these minerals.