Abstract Shallow-marine, Lower Carboniferous carbonate sequences of the SE Irish Midlands, close to the Leinster Massif, are intensely dolomitized. Fine-crystalline (<50 μm), planar-s (subhedral) dolomite is associated with evidence for evaporites, typical of arid peritidal sequences. However, stable isotope data suggest a diagenetic overprint. Volumetrically more important medium-crystalline (50–200 μm), planar-s and minor planar-e (euhedral) dolomites were precipitated from slightly modified Lower Carboniferous seawater. These dolomites replace open-marine intraclastic and bioclastic packstones and grainstones. Length-slow, fibrous quartz partially replaces crinoids and fills dissolution cavities beneath peritidal strata. Associated dolomites are gradually enriched in 18 O downward through the underlying strata, suggesting vertical brine migration. The widespread occurrence of skeletal material replaced by chalcedony in open-marine wackestones and grainstones further to the west, within the Rathdowney Trend, suggests evaporite cementation in the Zn-Pb mineralized area. Base-metal mineralization in the fractured Waulsortian ‘reservoir’ is associated with chloride-enriched brines (beyond that expected from seawater evaporation alone). The presence of evaporites in the Leinster Massif area suggests a possible source of the excess chloride. The dolomitizing brine may have contributed to the overall chemistry of the Zn-Pb mineralizing fluid and also to the distribution of porosity within the carbonate platform. An Arundian or younger age is suggested for the mineralization, based on the timing of evaporite cement emplacement, and this is compatible with numerical fluid-flow models of brine movement through the carbonate platform. Dolomitization of Lower Carboniferous carbonate rocks of the Irish Midlands is comparable to diagenetic histories of several important dolomite petroleum reservoirs. This study provides an example that may be applied to petroleum exploration in similar geological settings.

Abstract The Irish Midlands host one of the world’s major zinc orefields. The Irish zinc deposits occur in a transgressive sequence of Lower Carboniferous marine carbonate rocks lying above a wedge of Upper Devonian continental red beds. The deposits have enough shared characteristics, as well as differences from other carbonate-hosted zinc-lead deposits worldwide, to have been given the sobriquet “Irish-type.” The Irish deposits share the following features: (1) They occur preferentially in the stratigraphically lowest, non-argillaceous carbonate unit. (2) They occur along, or immediately adjacent to, normal faults which formed conduits for ascending hydrothermal fluids. (3) Sphalerite and galena are the principal sulfides. Iron sulfides occur in variable amounts; some deposits are dominated by iron sulfides while others contain very minor amounts. Barite is present in all the deposits, ranging from a dominant phase to a minor constituent. Many deposits contain minor tennantite, chalcopyrite, and/or Pb-Cu-Ag-As sulfosalt minerals. (4) They are stratabound and many display large-scale stratiform morphologies. (5) They display complex sulfide textures ranging from replacement of host rock by fine-grained, anhedral and colloform sulfides to infill of solution cavities by fine-grained, colloform and medium- to coarse-grained crystalline sulfides. Layered sulfide textures, other than colloform banding, are restricted to geopetal cavity fillings. (6) They formed from the mixing of metal-bearing, moderately saline, slightly acidic, relatively sulfur-poor fluids with relatively sulfur-rich fluids that appear to have been derived from Carboniferous seawater. The Irish orefield is regionally zoned. Copper and silver are most common in deposits located within the southern portion of the country. Pre-mineralization dolomitization is also largely restricted to southern deposits. The age of mineralization is known with certainty only for the Navan deposit which formed several million years after deposition of its host sediments; geologic relationships suggest that the other Irish deposits formed at approximately the same time as the Navan deposit. This period is marked in the Irish Midlands by the establishment of a complex facies mosaic consisting of fault-controlled carbonate basins and high-standing platforms indicating an extensional tectonic environment. Extension was relatively modest and was related to continental collision (the Hercynian Orogeny) occurring to the south of Ireland. The apparent contemporaneity of mineralization and tectonism, together with the regional zoning of metals and dolomitization, suggests that the Hercynian Orogeny was a fundamental driving force for mineralization in the Irish orefield. Topography-driven flow related to the uplift of Hercynian highlands to the south of Ireland could have produced a hydraulic head that drove formation waters northward through the confined Upper Devonian red bed aquifer. Along the flow path these formation waters increased in temperature due to burial and they leached metals. Fluids were focused into the area of present-day Ireland by a high-standing basement block to the east and by the northward thinning of the red bed aquifer. The Irish zinc deposits formed where normal faults tapped the confined red bed aquifer and focused flow of hydrothermal solutions upwards into the Lower Carboniferous carbonate sequence. This focusing allowed the development of discrete thermal anomalies capable of initiating thermal convection cells which mixed formation water from within the Carboniferous sequence with seawater from the overlying ocean. Fluid inclusion studies indicate that the hydrothermal fluid had temperatures of between 150 and 240°C and salinities of between 10 and 23 weight percent NaCl equivalent when it reached the sites of sulfide precipitation. Limited fluid inclusion data suggests that the water pulled into the system from above was significantly cooler (<120°C) and less saline (<10 weight percent NaCl equivalent). Sulfide precipitation occurred as the metal-rich, sulfur-poor, mildly acidic hydrothermal fluids reacted with carbonate sediments causing an increase in fluid pH. Sulfur isotope studies indicate that sulfide precipitation was increased due to the mixing of hydrothermal fluids with the cooler, sulfate-rich water. The host rocks and mineral textures of the Irish deposits are similar to many Mississippi Valley-type deposits. They differ, however, in having a metal suite which includes more copper, silver, and iron than most MVTs and in containing extensive zones of truly massive, often highly iron sulfide-rich, sulfide. These differences are probably the result of higher hydrothermal fluid temperatures which allowed higher metal contents in the fluids and increased reactivity.

Abstract The sediments of the Moyvoughly Beds within the Moyvoughly-Moate area of the Irish Midlands comprise ooid grainstones, skeletal ooid grainstones, and marine sandstones, punctuated by rarer calcareous mudstones. These sediments were deposited in agitated environments on a gently sloping south- or southeast-facing ramp. Core investigations and construction of isopach maps have shown changes of thicknesses within individual sedimentary units across known faults. These are interpreted as the result of synsedimentary faulting during sediment deposition. Changes in sediment lithologies can be related both to relative changes in sea level plus faulting activity, with the sandstones being sourced from erosion of local, active, fault-bounded highs. Relative sea-level highs are characterized by calcareous mudstones, with periods of lower sea level being characterized by grainstones or sandstones dependent on local siliciclastic sourcing. No major sequence breaks have been recognized with the Moyvoughly Beds. However, a regional change in sedimentation occurs at the base of the Moyvoughly Beds, where corals colonized the lithified surface of the underlying Micrite Unit. These lower sediments were deposited in a shallow lagoon, with periodic marsh progradation and subsequent exposure with paleosol formation. The Micrite Unit can be traced across the Irish Midlands and, although no regional detailed studies have yet been carried out, appears to be of similar lithologies throughout. In contrast, units within the Moyvoughly Beds, although correlatable within the area studied, cannot be traced to other areas, and their characteristics are more controlled by local tectonic movements.

Abstract Introduction The Lower Carboniferous carbonate rocks of the Irish Midlands host one of the world’s major orefields. The Midlands area stretches from the Mallow area in the south to the Navan-Oldcastle area in the north and is bound to the east by the Leinster Massif (Fig. 1). Although limited medieval mining for silver took place at Silvermines, it was not until the 1960s that Ireland became a significant world producer of zinc. The deposits and prospects discovered in the 1960s and 1970s were mostly subcropping; the discovery of concealed orebodies at Galmoy (1986) and Lisheen (1990) marked a new phase in Irish base metal exploration. These recent discoveries will ensure that Ireland remains a major zinc producer.