Title: Origin and discovery of Irish-type orebodies: new isotopic methods
Researchers: Dr Steve Hollis, Dr Julian Menuge
Irish-type Zn-Pb-(Cu) ore deposits are the mainstay of the Irish mining industry. There is a consensus that they are mainly epigenetic ores formed by carbonate replacement of Lower Carboniferous limestones, though minor syngenetic seabed ore also occurs. Conditions required for their formation include dense networks of normal faults that allowed ascending, warm, metal-bearing fluids equilibrated with Lower Palaeozoic basement to mix with sinking, cooler, hypersaline brines that carried bacteriogenically reduced dissolved sulphide of ultimate seawater origin. Recent work has shown that zinc isotopes may be fractionated during sphalerite crystallization and that this effect may be used to detect geochemical haloes around Irish-type orebodies where low-grade mineralisation formed from exhaust fluids that remained after orebody crystallisation. Sphalerite and pyrite minor element chemistry correlates with their Zn and S isotope compositions, suggesting inexpensive means to vector towards Irish-type orebodies.