La Donna Fredericks, PhD
L. Fredericks1,2, J. Walsh1,2, K. Torremans1,2, V. Monchal3, H. Othen1,2, K. Dorst2,3,J. Güven4, D. Chew2,3, V. Roche1,2
1School of Earth Sciences, University College Dublin, Belfield Dublin 4, Ireland
2Research Ireland Centre in Applied Geoscience (iCRAG), Belfield Dublin 4, Ireland
3Department of Geology, Trinity College Dublin, Dublin, Ireland
4Shanoon Resources Ltd.,Galmoy, Ireland
Inversion-related deformations are associated with N-S compression during the Variscan and Alpine. Orogenies on the pre-existing mineral endowed NE-SW Rathdowney fault Trend. The Rathdowney fault Trend in south-central Ireland, hosts world-class Zn-Pb-Ag deposits at Lisheen and Galmoy mines and Rapla prospect. Our study combines the seismic, aeromagnetics, borehole and outcrop data, supplemented by petrographic, cathodoluminescence, micro XRF-EDS mapping and U-Pb LA-ICP-MS geochronology of associated fault rocks and vein infills to determine the geometry, nature, timing and effects of inversion structures on the Irish Ore Field.
Mineralization occurs as massive semi-massive and disseminated galena, sphalerite and pyrite or marcasite, and to a lesser extent chalcopyrite, niccolite and tennantite directly linked to the syn-sedimentary Mississippian N-S extension phase. The stratabound mineralization is hosted in dissolution-precipitation breccias of clean dolomitized limestones predominantly in the hanging wall of Mississippian ENE-WSW laterally discontinuous segmented normal fault arrays. The fault zones show with 5 – 250 m displacement and associated extensional calcite veins were dated to 342.0 ± 9.6/14.5 Ma.
The mineralization was later folded, duplicated, offset and truncated by large NE-SW oriented oblique-slip dextral Variscan related and later Alpine related NNW-SSE dextral strike-slip faults. Variscan inversion takes the form of folds, reverse faults, hangingwall buttressing and reverse reactivation of mineralized normal faults. This deformation was accompanied by a complex set of NE - SW trending, dextral reverse faults that laterally offset the mineralised normal faults by > 100m, resulting in displacements between 150 – 400 m on individual reverse faults. Where reverse faults localise on pre-existing normal fault segments, the faults and relay ramp zones are offset by dextral oblique-slip. Associated grey-white hybrid extensional-shear calcite veins were dated to 302.4 ± 6.3/11.5 Ma. Additionally, N-S inversion during the Alpine Orogeny is indicated by fold tightening, NNW-SSE dextral and NE-SW sinistral strike-slip faults, and horizontal to near-vertical dated to 67 ± 9.6/9.8 – 28.9 ± 7 Ma. The NNW-SSE strike slip faults also offsets the mineralized ore bodies. In conclusion, the deposits were deformed and affected by the later inversion related orogenies and these effects should be considered during exploration.