Geology of the Stonepark Zn-Pb prospects, County Limerick, Ireland

Abstract

The Zn-Pb prospects in the Stonepark area of southwest Ireland are Irish-type carbonate replacement deposits. The k prospects are hosted in a Lower Carboniferous transgressive marine carbonate sequence along the eastern margin of the east-west-trending Limerick Syncline, which contains a number of volcanic centers. Unlike other Irish-type deposits, those at Stonepark display a close spatial and temporal association with volcanism. Zinc-lead massive sulfides occur in strata-bound lenses and replaced hydrothermally dolomitized and brecciated intervals within Courceyan-age, non-argillaceous micritic mudmounds of the Waulsortian Limestone Formation. The Waulsortian Limestone in the Stonepark area is crosscut by Chadian-age dikes, sills, and maar-diatremes of alkali basalt to trachy andesite composition that were emplaced pre-, syn-, and post-mineralization. Unlike other well-described Irish deposits, the prospects at Stonepark contain an early breccia type informally designated as 'igneous dissolution breccia.' This breccia is composed of undolomitized angular clasts of Waulsortian Limestone hosted in a dark-black argillaceous and bituminous matrix cemented by calcite and locally by ferroan dolomite. The matrix of the igneous dissolution breccia contains abundant quartz grains and minor amounts of tourmaline, zircon, and garnet grains. The composition and texture of the igneous dissolution breccias suggest they formed from dissolution of carbonate rocks by fluids derived from alkali basalt magmatism. The igneous dissolution breccias grade into 'siliceous lithic breccias' near the base of the Waulsortian Limestone. Siliceous lithic breccia contains rounded lithic clasts of shale and sandstone in a dark-green matrix composed largely of quartz grains cemented by either calcite or ferroan dolomite. The quartz and heavy mineral grains in the igneous dissolution breccia as well as the lithic clasts and quartz grains in the siliceous lithic breccia appear to have been derived from underlying carbonate units as well as the Old Red Sandstone and basement rocks; movement of this material upward was associated with alkali basalt diatreme activity. Zn-Pb massive sulfides in the Stonepark prospects are hosted in a dark colored dolomite breccia referred to as 'black matrix breccia.' This breccia was formed by in situ carbonate replacement of the Waulsortian Limestone by ferroan dolomite; dissolution associated with breccia formation resulted in minor solution collapse. The black matrix breccia in the Stonepark area is similar to the black matrix breccia found at other Waulsortian-hosted Irish-type deposits. Black matrix breccia in the Stonepark prospects is less laterally and vertically extensive than the earlier formed igneous dissolution breccia. Intrusive rocks, igneous-associated breccias, black matrix breccia, and mineralized zones display a strong structural control and strike parallel to east-northeast-trending, west dipping normal faults. Intrusions appear to have acted as an aquitards that channeled and trapped ascending hydrothermal fluids while black matrix breccia formed aquifers for hydrothermal fluids. Sulfides from the Stonepark area have a wide spread of [delta]34S isotope values that range from -42 0/00 to +13 0/00. Ore stage Zn-Pb massive sulfides display a more restricted range of -21 0/00 to -1 0/00 (average -7 0/00). These values are similar to those reported at other Irish-type deposits and suggest ore stage sulfides at Stonepark precipitated from biogenically reduced seawater sulfate. Magmatism in the Stonepark area was not a significant source of reduced sulfur.