Pb-Zn mineralisation within the Limerick Basin (SW Ireland): a role for volcanism?

Abstract

Lead-zinc exploration drilling within the Limerick Basin (SW Ireland) has revealed the deep internal architecture and extra-crater deposits of five alkali-basaltic maar-diatremes. Base metal mineralisation, appearing to be spatially and temporarily associated with these diatremes, overprints adjacent hydrothermal Black Matrix Breccia (BMB) horizons. Diatremes and extra-crater deposits were emplaced during the Carboniferous Period as part of a regional tectonomagmatic trend across NW Europe. Trace element data indicates a genetic relationship between the diatremes and sequences of the extra-crater Knockroe Formation. Field relationships and textural evidence from diatreme and Knockroe deposits, suggest eruptions occurred in a shallow submarine environment (<120 m). Eruptions were dominated by phreatomagmatic activity, however emergence above sea level and subsequent drying out led to a decline in seawater ingress, which corresponded to a late magmatic phase. This study utilises a rare opportunity to investigate both the deep architecture and extra-crater surface deposits, providing detailed volcanic lithofacies descriptions of the submarine maar-diatreme systems at Limerick.

The pyroclastic sequences are highly altered, therefore a principal objective was to analytically distinguish between primary and secondary alteration mineral phases and elemental trends. Rare earth element (REE) patterns suggest the magma was sourced by partial melting of an enriched and metasomatised mantle, which erupted in a within-plate continental rift environment. The lower diatreme is overprinted by a greenschist metamorphic assemblage of minerals, in addition to pervasive dolomitisation of the volcanic deposits. Dolomite is a key mineral in BMBs and the presence of diatreme clasts within polymict BMB horizons, combined with small concentrations of ore-forming minerals (sphalerite, galena and pyrite) in the lower diatremes, suggest that hydrothermal fluids utilised the diatremes as conduits.

The Irish Orefield is a base metal source of global importance, but evidence has not previously been documented to link the large-scale Lower Carboniferous volcanic activity to the Pb-Zn mineralisation. Sulphur isotope data indicates that diatreme formation in Limerick had a significant impact on mineralisation within the basin. Increased permeability and porosity of the diatremes, compared to the traditional Irish-type fault fluid pathways, allowed enhanced formation of BMB mineralisation hosts adjacent to the diatremes and an increased flow of metal-rich hydrothermal fluid from the basement. The presence of large volumes of volcanic material also provided additional magmatic sulphur as well as ore-forming constituents.

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ORCID for Thomas Gernon: orcid.org/0000-0002-7717-2092

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