Paleomagnetism of the Galmoy Zinc-Lead Deposit, Ireland: Evidence of An Epigenetic Origin

The Galmoy Zn-Pb deposit is one of the numerous Lower Carboniferous carbonate-hosted Zn-Pb ore deposits in the Irish Midlands. Galmoy and other major Zn-Pb deposits in the Irish Midlands (Lisheen, Silvermines, Tynagh) are hosted by Courceyan to Chadian (~359 to ~343 Ma) Waulsortian Limestone, implying a major role for stratigraphy in the localization of these deposits. The genesis of Galmoy and other Zn-Pb deposits in the Irish Midlands remains controversial because of the absence of precise geochronological constraints. Another major concern is the effect of the regional Variscan heating episode, observed by the elevated conodont colour alteration indices (CAI) values in virtually all Carboniferous strata of Ireland, on the carbonate-hosted Zn-Pb ore deposits. This paleomagnetic study was undertaken to date the Zn-Pb mineralization at Galmoy, and thereby to constrain models for its genesis. Paleomagnetic analysis of 273 specimens from 14 sites in ore mineralization and 7 sites in host carbonate isolated a well-defined characteristic remanent magnetization (ChRM) direction on alternating field and thermal step-demagnetization. Saturation remanence tests, thermal decay data and paleomagnetic field stability tests indicate a postbrecciation and postfolding ChRM that is carried dominantly by single domain magnetite. The ChRM directions from 7 host rock and 14 mineralized sites are indistinguishable at 95% confidence, give a mean paleopole at 41.5°S, 8.4°W (dp = 1.5°, dm = 3.0°) with an age of 290±9 Ma on the apparent polar wander path for Laurentia in European coordinates. This Early Permian paleomagnetic age for host rocks and Zn-Pb mineralization at Galmoy records Variscan orogenesis and suggests an epigenetic model in which the mineralization event occurred during cooling from the regional Variscan heating episode.