A ZINC ISOTOPE STUDY OF THE IRISH ZN-PB OREFIELD

This study utilizes the extensively characterized Irish carbonate hosted Zn-Pb orefield as a natural laboratory in which to study the behaviour of Zn isotopes in hydrothermal ore-forming systems. Deep circulation of modified seawater within the continental crust underlying the Irish orefield was an important process in the generation of hydrothermal fluids, which are considered to have transported the bulk of the ore metals and minor, isotopically heavy sulphur to the site of ore deposition. Ore formation occurred in the shallow subsurface due to mixing of these hydrothermal fluids with surface derived evaporitic brines containing bacteriogenically reduced isotopically light sulphur. Analyses of 35 sphalerite samples selected from the Galmoy deposit, smaller sub economic prospects and underlying feeder veins in the Irish orefield indicate that the mechanisms controlling Zn isotope variation are operating primarily on a deposit scale, since the full range of measured isotopic compositions is observed in the CW orebody at Galmoy. A limited correlation exists between sulphur isotope composition and 66/64Zn, presenting the possibility that Zn is in fact derived from two isotopically distinct sources: the deeper crustal Zn component transported by the principal hydrothermal fluid, supplemented by a second Zn component derived from the near surface and transported by evaporitic brines. Alternatively, coherent variations in sphalerite 66/64Zn observed from underlying feeder veins, through main ore-stage mineralization to late, post ore-stage sphalerites could indicate isotopic fractionation during precipitation of sphalerite from Zn bearing hydrothermal fluids alone. In order to test these competing hypotheses, the isotopic composition of leachable trace Zn was measured for a variety of lithologies with which the mineralising fluids are known to have interacted (Lower Palaeozoic basement rocks, Old Red Sandstone and Waulsortian carbonates). In addition, sphalerite was analysed both proximal and distal to hydrothermal feeder structures within a single orebody, in order to assess the role of Zn isotopic fractionation during ore precipitation from a single Zn-bearing fluid.