Abstract

The Lisheen Zn-Pb deposit is one of the major Lower Carboniferous carbonate-hosted base metal deposits of the Irish Midlands. The timing of ore mineralization and, therefore, its genesis remains much debated because of a lack of absolute age determinations. Paleomagnetic analysis of 432 specimens from 12 sites in ore mineralization and 10 sites in host rocks at Lisheen isolated a well-defined stable characteristic remanent magnetization (ChRM) direction using AF step demagnetization, except in one site of massive sulfide mineralization and two sites of postore pink dolomite. Thermal step demagnetization and saturation remanence tests show that the ChRM is carried dominantly by single domain magnetite. A paleomagnetic fold test is negative, indicating that the ChRM postdates burial lithification and deformation. The ChRM directions from eight host-rock and 12 Zn-Pb mineralized sites are indistinguishable at 95 percent confidence. The combined ChRM directions give a mean paleopole at 41.6°S, 18.8°W (semiaxes of the oval of 95% confidence along and perpendicular to the site-pole great circle, dp = 1.7, dm = 3.3) that, when compared to the robust apparent polar wander path for Laurentia rotated into European coordinates, gives an age of 277 ± 7 (2σ) Ma for the ChRM. This 277 ± 7 Ma age is close to the paleomagnetic ages of 290 ± 9 (2σ) Ma for Zn-Pb mineralization at Galmoy and 269 ± 4 (2σ) Ma for the Ba mineralization at Silvermines, which are ~10 km northeast and ~30 km west of Lisheen, respectively. This Early Permian magnetization postdates peak Variscan orogenic heating that is observed in all Carboniferous strata in Ireland. Our preferred interpretation is that the paleomagnetic data, in conjunction with other thermal data, support an entirely epigenetic origin in contrast to the pre-Variscan syn-genetic or syndiagenetic models favored by previous authors. We propose a topographically driven fluid flow, either from the south of Ireland or from closer to the deposit, which was modified by localized convection during the Variscan orogeny.

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