Pyritic mineralization halo above the Tara Deep Zn-Pb deposit, Navan, Ireland: Evidence for sub-seafloor exhalative hydrothermal processes?

The Tara Deep Zn-Pb deposit, at Navan, Ireland, includes sub-economic pyrite-rich mineralization extending laterally for about 2 x 2 km within the overlying Lower Visean calc-turbidites, known as the 'New-Thin Bedded Unit'. Here, we investigate the genesis of this pyritic mineralization and its links to the limestone-hosted Zn-Pb deposit lying 100 m below it. Four mineral assemblages have been identified: 1) in black shales, laminated pyrite comprises thin framboidrich layers with minor interstitial sphalerite, both showing variable but low delta S-34 values ranging from -37.4 to 3.3 parts per thousand; 2) in calcarenite and calcsiltite layers, pyritized fossils are widely distributed and occur chiefly as biodebris replaced by low delta S-34 pyrite (mean values of -13.2%); 3) a replacive assemblage occurs as late remobilizations exhibiting both crosscutting and bedding-parallel styles, overprinting the early laminated pyrite. It comprises mostly of marcasite, with minor pyrite, sphalerite, chalcopyrite, galena, stibnite and Co-pentlandite, with high delta S-34 values up to 24.5 parts per thousand; and 4) in black shales, bedded sulfide-rich cherts comprising microcrystalline quartz lenses exhibit delta O-18 mean values of 25.3 parts per thousand Cherts are rimmed by dolomite, associated with marcasite, pyrite, sphalerite, chalcopyrite, galena, siegenite and stibnite also with high delta S-34 values up to 44.2 parts per thousand. In general terms, pyrite shows a relatively high Co/Ni ratio > 1 and sphalerite Zn/Cd ratios vary from 268 to 364. Textural analysis indicates overlapping of early-diagenetic and multi-phase hydrothermal sulfide mineralization. Development of laminated pyrite and pyritized calcarenites suggests that this mineralization was generated during early diagenesis, close to the seawater-sediment interface in oxygen-poor conditions under the influence of low-temperature hydrothermal fluid circulation. Later hydrothermal cherts and replacive sulfides suggest discharge of relatively warm hydrothermal fluids during early to mid-diagenesis, presumed to be linked to movements of nearby normal faults. Similarities in mineralogy and S isotope compositions suggest genetic links between the sub-economic pyritic mineralization and the underlying Tara Deep deposit, and consequently, that the former represents a geochemical halo with direct applicability in exploration for Zn-Pb deposits.

Automated summary

The study identified four mineral assemblages, indicating that the Zn-Pb deposit at Tara Deep in Ireland was formed during early diagenesis, under oxygen-poor conditions influenced by low-temperature hydrothermal circulation, and was affected by movements of nearby normal faults.