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The Lough Allen Basin contains approximately 2.9 km of Dinantian (Courccyan) to Silesian (Arnsbergian) limestone, mudstone and sandstone, mostly of marine origin. The basin was initiated during the Courceyan Stage as a result of movement of the basin margin fault complex along the southeast side of the Ox Mountains inlier. The main rifting phase, during the late Courceyan, resulted in the deposition of a southeasterly thinning, initially non-marine, elastic wedge. The intrabasinal Dowra-Macncan High and Slisgarrow Trough developed during the Chadian Stage. Movement on the Curlew fault, which bounds the basin to the south, may have begun in the Courccyan but culminated in the Arundian. There is little direct evidence of fault-controlled sedimentation in the Holkerian and much of the Asbian, although the distribution of carbonate mudmounds of Asbian age in part coincides with earlier structural elements. During the late Asbian, there appears to have been renewed tectonic activity, which led to substantial regional differences of thickness of the lower formations of the Leitrim Group. Thcrc is little evidence of tectonic control on sedimentation from the Brigantian to the Arnsbcrgian.Vitrinite reflectance determinations show that the Carboniferous rocks in the basin arc supra-mature for the generation of oil but are mature with respect to the generation of gas. The maturation values for the youngest preserved rocks suggest that they have been buried under 3-5 km of Upper Palaeozoic cover.

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In the first decade of the 21st 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.

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