Irish, but not Irish-type

Abstract

Ireland is well known as a host to many carbonate-hosted zinc-lead deposits dating back to the discovery of the Tynagh deposit in 1961 which ushered in a prospecting rush which resulted in the discovery of several deposits and prospects, e.g., Ballinalack, Boston Hill, Carrickittle, Courtbrown, Harberton Bridge, Keel, Moyvoughly, Navan, Oldcastle and Silvermines. Research into understanding these deposits, in order to further advance successful exploration, initially adopted a Mississippi Valley Type model to direct exploration. How-ever, it soon became apparent that many of the deposits in Ireland did not fit this model and a new paradigm was needed. The term Irish-type was coined to more fully describe those deposits. However, there were some deposits that did not fit this new model notably Abbeytown, Allenwood, Boston Hill and Harberton Bridge (the latter three deposits collectively known as the Kildare District). Unlike most Irish-type deposits they strongly cross-cut the stratigraphy and have a vertical extent that is far greater than the lateral extent. Mineralogically they are significantly different with marcasite being the dominant iron sulphide. Plots of fluid inclusion data fall within fields separate to all the deposits in the Irish Midlands and are similar to the ranges of major MVT deposits from around the world. Similarly sulphur isotopic results show no evidence of the bacterial sulphate reduction that typify the Irish-type deposits and generally all fall with the range of “hydrothermal sulphur”. Fluid inclusion data is indicative of high salinity fluids enriched in chloride relative to bromide and suggests a component of salinity derived via dissolution of halite or from evaporation of seawater. Therefore, the region in which these fluids formed should contain evidence of halite. In this paper we describe the geological features of these deposits and critically assess the evidence which permits us to assign these deposits to the Mississippi Valley class of deposits – notwithstanding that clearly, they are geographically Irish, but they do not possess the typical diagnostic characteristics of Irish-type Zn-Pb deposits.