Genetic Links between Irish-type Zn-Pb Deposits and Related Geochemical Halos

A broad geochemical dispersion halo has been identified with a direct link to the underlying Tara Deep deposit at Navan, Ireland. In situ laser S isotope analyses have been performed on petrographically well characterized samples from the halo. Four mineral assemblages have been identified. 1) In black shales, laminated pyrite comprising thin layers of framboidal low-delta S-34 pyrite with minor interstitial sphalerite. 2) Pyritized calcarenites are widely distributed and occur chiefly as biodebris replaced by low-delta S-34 pyrite. 3) A replacive assemblage occurs as late remobilizations exhibiting both crosscutting and bedding-parallel styles, overprinting the early laminated pyrite. It comprises mostly marcasite, with minor pyrite, sphalerite, chalcopyrite, galena, stibnite, arsenopyrite and pentlandite, with high delta S-34 values. 4) Hydrothermal cherts comprise thick microcrystalline quartz bands rimmed by dolomite, associated with marcasite, pyrite, sphalerite, chalcopyrite, galena, Ni-sulfosalts and stibnite with high delta S-34 values. These results indicate overlapping diagenetic and multi-phase hydrothermal sulfide mineralization. First, laminated pyrite and pyritized calcarenites suggests a bacterial origin within sediments during early diagenesis. Later, hydrothermal chert and replacive sulfides suggest hydrothermal exhalation during early-mid diagenesis. Similarities in mineralogy and S isotope compositions suggest genetic links between the halo and the underlying Tara Deep deposit