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Abstract

The Irish ore field hosts the greatest concentration of Zn per sq km on Earth, and are found mainly as stratabound deposits in Lower Carboniferous carbonates. Nonetheless vein mineralization is common, and can penetrate through Old Red Sandstone sandstones and conglomerates to Lower Palaeozoic basement.
These deposits have relatively simple mineralogy, dominated by sphalerite, galena hosted in occurring with common pyrite, dolomite, calcite and quartz: minor sulfosalts, hosted in galena, add Ag to most economic mines’ economy. In one deposit, Silvermines, an abundance of highly pure barite also occurs.
Most ores are found in the Waulsortian Mudbank sequence, with breccia styles dominant – some breccias are hydrothermal, some are depositional. In contrast, the giant Navan deposit (dominating Europe’s Zn production for nearly 40 years) is hosted by the Pale Beds sequence, stratigraphically below the Waulsortian in the northern part of the Irish Carboniferous basin. In all ores, an association with faulting, often reactivated from more ancient Caledonian fault systems, is clear. Often ore textures are complex, reflecting a dynamic ore depositional environment.
These styles and associations style, together with the sequential occurrence of dolomites, and their stratabound format has led to much controversy on their origin. The arguments centre on the following question: are these deposits they a species of MVT, formed by topographic fluid flow, formed during compressional tectonics associated with the Hercynian, or are they species of SEDEX – the so-called Irish- type – deposits, formed by convection during Lower Carboniferous extensional tectonics.
My talk addresses this question, testing each hypothesis in the face of evidence from geological observation and geochemical analyses, including Pb and S isotope data. No single tool or observation is decisive – as is often the case in ore genetic studies – but key observations and data can rigorously test each hypothesis, and thus enhance exploration and exploitation modeling for current and future prospecting.