Abstract In the first decade of the 21 st century, surface exploration drilling around the Boliden Tara mine at Navan, Ireland, aimed at ~1-km-deep targets, was becoming ineffective. During 2010, the extensive geologic knowledge of the existing Navan orebody was leveraged in an Experts Meeting to promote near-mine discovery. Two ideas, of many, were of relevance to this paper: (1) undiscovered mineralized fault-related zones were predicted south of the orebody, and (2) seismic surveys could locate subsurface faults. By late 2012, seven 2D seismic lines (totaling 101 km) had been acquired, processed, and initially interpreted. Pre-stack time migration images were used for interpretation, augmented by diamond drill core data where available. The seismic imaging proved a “game changer” in terms of subsurface visualization and a priority target was identified 2 km south of the mine on the footwall crest of a large south-dipping basin-margin fault. The first hole intersected 34 m of mineralized rock with 14% Zn + Pb, but at greater depth than anticipated. Follow-up drilling was initially successful but proved to be challenging. The first hole intersected a deep structurally complex section of the newly discovered zone that required more drilling to establish its location and attitude. Further drilling, utilizing extensive navigational deflection technology, outlined a mineralized zone similar in nature to the Navan 5 Lens at depths of 1 to 2 km. Inferred resources through 2016 were estimated at 10.2 Mt grading 8.5% Zn and 1.8% Pb. Underground exploration development of this zone commenced in April 2017, and will allow accurate delineation of this significant discovery.

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 Vitrinite reflectance (VR) data (R m %) have been compiled from 77 Irish offshore wells and 17 onshore boreholes. This database has facilitated the analysis of vitrinite reflectance v. depth relationships by both basin and stratigraphic interval. In general, VR gradients from the Carboniferous sections are defined by less scattered trends than those from Mesozoic and Cenozoic sections, reflecting the less complex vitrinite populations within Carboniferous coals and shales. A composite approach (display of profiles from a number of wells together) to the interpretation of vitrinite reflectance profiles has been utilized to characterize the thermal history and the prevalent heat transfer mechanisms within the various basins. Calculated peak palaeotemperatures from the wells are used to compute palaeogeothermal gradients and to estimate the magnitude of net exhumation at selected locations. Average palaeogeothermal gradients in the onshore Carboniferous basins range from less than 3 °C km −1 at well IIP-2 in the Clare Basin to 119 °C km −1 at well N998 in the Navan area of the Dublin Basin. Lateral variations in palaeogeothermal gradients recorded in the Carboniferous sections are consistent with a gravity-driven hydrothermal System discharging heated fluids, along fault systems, in a foreland platform area. In general, palaeogeothermal gradients are substantially higher in the Carboniferous sections (mean 60 °C km −1 ) than in the Mesozoic or Cenozoic sections (mean 32 °C km” 1 ). Maturation levels in many of the Carboniferous sections are considered to be the consequence of burial, elevated heat flows and a regional advective System during late Carboniferous to early Permian times rather than Mesozoic or Cenozoic processes. Empirically derived methods of calculating peak palaeotemperature from VR are compared with kinetic models and, although differing in detall, within the resolution of this dataset are shown to produce similar trends. There is a considerable body of evidence to suggest that the extensional evolution of Ireland’s Late Palaeozoic to Cenozoic sedimentary basins has been punctuated by a multiphase inversion history. Regional stratigraphic evidence, combined with VR and apatite fission-track data, suggests at least two periods of pervasive exhumation occurred; one during Late Carboniferous-Late Permian time and another during Tertiary time. Both of these phases are characterized by a component of compressional inversion and the widespread occurrence of extrusive and intrusive igneous rocks. However, the contrasting thermal signature of these regional uplift events suggests that both the basin setting and the mechanism of regional exhumation exerted a fundamental control on processes that determined heat flow distribution within a basin. In terms of the hydrocarbon exploration of Ireland’s sedimentary basins the model presented here has important implications for the timing of maturation of Carboniferous source rocks in these basins. Where Carboniferous source rocks are present, they will make a significant contribution to the hydrocarbon budget only in those basins that experienced relatively low heat flow during Late Carboniferous–Early Permian time and where sufficient Mesozoic burial has occurred to subsequently expose the kerogen to higher temperatures. This observation is consistent with the presence of gas accumulations, which are postulated to have been derived from Carboniferous source rocks, in both the Slyne Basin and the Northwest Carboniferous Basin.