An Ireland mining primer

While Ireland has a long tradition of mining dating back to the Bronze Age, today the Emerald Isle is internationally renowned as a major zinc-lead mining province.

Geologically, the widespread carbonate succession in Ireland plays host to a significant ore field: the Central Ireland basin mineral province.

With such bounty, Ireland is the largest producer of zinc concentrates in Europe and ranks as the eighth-largest producer of zinc concentrates in the world, having three underground zinc-lead mines: Navan, Galmoy and Lisheen. Annual total production from these three mines is in the order of 700,000 tonnes of zinc concentrate and 110,000 tonnes of lead concentrate.

Known carbonate-hosted deposits contain some 11 million tonnes of zinc metal, or about 1.5% of the world’s zinc found to date.

In fact, Ireland is ranked first in the world in terms of zinc discovered per square kilo-metre, and ranked second with respect to lead.

Since 1960, 14 significant zinc-lead resources of greater than 1 million tonnes have been discovered, including the world-class orebody at Navan, which tips the scales at more than 80 million tonnes.

Moreover, the high-grade, shallow occurrence and clean metallurgy of the orebodies result in a relatively low cost of mining for the Irish-type zinc-lead deposits.

There is stratigraphic control of the major deposits in Ireland. Shallow-water carbonates of the 50-to 400-metre-thick Navan group that hosts the Navan deposit and several sub-economic deposits in the northern part of the province. The Waulsortian limestone– a mudbank complex up to 1.5-km thick which thins into isolated units set in an argillaceous bioclastic envelope to the north — hosts the Tynagh, Silvermines, Galmoy, Lisheen and Harberton Bridge. Significant mineralization has also been found in the underlying sub-Waulsortian strata (Silvermines, Lisheen).

The overlying shelf limestone facies of the supra-Waulsortian strata, in which crosscutting deposits at Harberton Bridge are developed.

In terms of structural control, a pattern of widespread normal faulting is also favourable, in that the deposits found to date are all associated with east-northeast-trending faults that often appear to have acted as conduits for hydrothermal fluids, and control by known basement lineaments has also been suggested.

Other regions also have potentially favourable geological settings, although with limited discovery success to date.

The two most significant styles of mineralization are: stratabound (and stratiform) deposits and cross-cutting Mississippi Valley-type (MVT) deposits.

Stratabound deposits are hosted in the Navan group and Waulsortian limestone and are preferentially located in the stratigraphically lowest non-argillaceous carbonate strata.

At Navan, the mineralization occurs as stacked stratabound lenses, while the Waulsortian-hosted deposits have a variety of form, with regular (Lisheen, Garrycam) or irregular tabular lenses (Galmoy, Upper Silvermines), cross-cutting epigenetic zones (Lower Silvermines) or discrete pods (Tynagh, Ballinalack). Enhanced zones of mineralization are associated with northeast-or east-northeast-trending faults or mineralized fractures.

Cross-cutting MVT deposits typically found in the Waulsortian limestone and supra-Waulsortian lithologies. The form of the MVT deposits varies from tabular, dome or pipe style, but is typically as tabular bodies at the base of the Waulsortian limestone, and in some cases extend upwards for up to 500 metres into overlying strata.

Several key target criteria are recognized: the lowest non-argillaceous carbonates of the Navan group in the northern part of the basin; the Waulsortian in the central and southern parts; major northeast-or east-northeast-trending fault structures that displace the Navan group or Waulsortian limestone; and locations near or against inliers are targets, as are the points of maximum throw on faults.

All modern discoveries have been at shallow levels of less than 200 metres, suggesting that deeper deposits could yet be discovered.

With respect to exploration and development licenses, the Irish government’s broad policy is to support the development of Ireland’s mineral resources provided it’s done in an environmentally and socially responsible way.

The government’s Exploration and Mining Division regulates the minerals industry on behalf of the Minister for Communications, Energy and Natural Resources, by means of a system of a prospecting licences, and mining leases and licences.

A prospecting licence gives the holder the right to explore for certain specific minerals, typically covering some 35 sq. km and normally issued for six years, with the option of renewal if the holder has met agreed requirements such as minimum expenditures.

Prospecting work reports are required every two years, and are held confidential for six years or until surrender of the licence (if earlier).

Minerals can be in state ownership or privately owned, but regardless of ownership, minerals development requires a State Mining Facility from the minister.

When it comes to environmental issues, the Irish system has evolved over many years following a typical progression from concern mainly with economic development to a greater consideration of the need for environmental protection.

All development of minerals governed by the Minerals Development Acts is subject to an environmental impact assessment through the planning process.

— The author is a member of the Irish Mining Co-operative (IMC), which is offering interested companies an opportunity to invest in exploration in the Irish base metal province. He can be reached at glenn. millar@exmap. ie. Other members of the IMC include Hugh Gibney, Gareth Jones and Tom Davitt.