Contrasting sources for lead in Cu-polymetallic and Zn–Pb mineralisation in Ireland: constraints from lead isotopes

Abstract

Devonian red bed-hosted deposits in SW Ireland occur either as low-grade stratiform disseminations and minor veinlets with bornite and djurleite, or as major quartz veins with copper-polymetallic deposits. Pb isotopic ratios vary according to the style of mineralisation. For disseminated sulphides, there is a considerable scatter of lead isotope compositions. These appear to define a mixing line between a basement source and a more radiogenic component, which may have been granite-derived detrital minerals in the red beds. For minor veinlets there is a more restricted distribution of lead isotopic data, which could be interpreted in terms of fluid flow homogenising the wide spread of Pb isotope ratios from the disseminated sulphides. This is consistent with published sulphur isotope data. Major quartz veins show a clustered array of Pb isotopic ratios and it is suggested that the bulk of the metals were derived after Variscan compression, from a basement source with a minor contribution from red beds. Lower Carboniferous limestone-hosted deposits form three groups: Cu–Ag ± Hg–As–Sb–U epigenetic vein-hosted deposits; Cu–As ± Zn, Pb, Ag, Co, Ni stratabound deposits; and Cu–As ± Pb, Ag, Mo, Co, Ni epigenetic vein deposits. Lead isotope compositions for the stratabound deposits have a very restricted range for each locality and plot close to the same mixing line as the Carboniferous Zn–Pb deposits of Central Ireland. It is suggested that the lead in both areas has been derived from a similar basement source. The model age of ca. 350±10 Ma for stratabound mineralisation is close to that of their host rocks. Lead isotope data for the epigenetic Cu–As vein deposits suggest a similar source, but a younger age of 280±10 Ma for mineralisation. This 280±10-Ma age has important implications for the timing of the Variscan orogeny in SW Ireland. Because chalcopyrite–tennantite occurs along pressure solution cleavage planes that have been deformed by thrusting, the emplacement of the mineralised tectonic Ross Island sheet, west of Killarney, must post-date 280 Ma. The difference in the ore assemblage between the limestone-hosted Cu-polymetallic deposits of Ireland, and the penecontemporaneous Zn–Pb deposits of Central Ireland may be because of differences in the underlying lithology. In the SW, up to 6 km of red beds underlie the limestone-hosted stratabound Cu-polymetallic deposits whereas, in Central Ireland, the Zn–Pb ore-bearing carbonate rocks are underlain by lower Palaeozoic greywackes with a minimal thickness of late Devonian red beds. However, the isotopic evidence only indicates a major red bed source of metals at Ardtully. Elsewhere, unless the metals in the red beds originated from a basement rather than a granite source, a significant red bed contribution to the metals seems unlikely. An alternative possibility is that copper ores were derived from an extension of the Ordovician island arc of SE Ireland.