RE-OS DATING OF SULFIDE MINERALIZATION (PYRITE) FROM THE LISHEEN PB-ZN DEPOSIT, IRELAND

The Lisheen deposit is a significant resource within the Irish Midlands Pb-Zn province, and is a typical example of carbonate-hosted base metal mineralization in the region. Like the majority of the Irish Pb-Zn deposits, Lisheen is hosted by Lower Carboniferous strata, but the timing of sulfide mineralization is only poorly constrained by geochronologic methods due to the absence of minerals useful for dating by commonly-applied methods such as K-Ar, U-Pb and Rb-Sr. As such, the genetic models for this important type of sedimentary-hosted deposit remain controversial, with timing constraints provided primarily by geologic relationships and, more recently, paleomagnetic data. The widely proposed syngenetic and syndiagenetic models require that mineralization coincided with or occurred fairly soon after the ca. 346 Ma stratigraphic age of the host sequence at Lisheen and elsewhere, but none of the available age constraints for sulfide mineralization match this age, a problem complicated by younger Variscan events. Recent paleomagnetic data from Lisheen have been interpreted to indicate post-Variscan epigenetic mineralization at ca. 277 Ma, much younger than previously proposed and in apparent contradiction with Variscan thrust deformation of the ores. In order to address this problem we have applied Re-Os dating of main-stage pyrite ore at Lisheen because pyrite has been found to retain its primary Re-Os age information despite younger thermal disturbances. From the Bog Zone at Lisheen we have located main ore stage pyrite with Re contents suitable for Re-Os geochronology. This pyrite shows very high Re/Os ratios, from 3000 - 6000, with low common Os contents - the majority of Os present is radiogenic 187Os (>94%). Nine Re-Os analyses of pyrite yield an isochron age of ca. 340 Ma, within uncertainty of the stratigraphic age. These data provide strong support for syngenetic/syndiagenetic models for sulfide mineralization at Lisheen, and indicate that the ca. 277 Ma paleomagnetic age at this deposit does not appear to record the timing of primary sulfide mineralization