A detailed petrographic and isotopic (S, Pb) study of high-grade Zn-Pb mineralisation in the Island Pod orebody, Lisheen deposit, Ireland.

Despite being distal to the main orebody, the 400 000 t 20% Zn, 1.6% Pb Island Pod is still very high-grade and hosts the best-quality ore at Lisheen while showing rapid lateral variation on a small scale, less than10 m, where adjacent boreholes can have zinc concentration differences of c.40%. It also displays a weaker structural control than elsewhere in the southern Irish orefield. The basic sulphide mineralogy for the Island Pod is pyrite, sphalerite and galena, with multiple generations of each observed along with several carbonate phases. Dendritic pyrite/galena, colloform sphalerite and sphalerite/dolomite intergrowth textures suggest an early, rapid mineralisation event, from a supersaturated fluid in a nonequilibrium depositional environment, marking the onset of fluid mixing. As the mineralising system came closer to equilibrium, sulphide textures changed, reflecting a slower precipitation environment. 34-S isotope data suggest a dominantly bacteriogenic source for S in the Island Pod, with minor positive 34 delta-S values recorded. This is consistent with the distal position of the orebody, away from the feeder ramp-relay fault system at Lisheen which is thought to have introduced metal-bearing hydrothermal fluids with 34 delta-S greater than 0. In addition, the orebody and its halo have very similar 34 delta-S values, signifying that a lack of hydrothermal sulphur is not what led to the halo being sub-economic; this may suggest that other factors were responsible for ore grade, such as the availability of bacteriogenic sulphur or stratigraphic controls such as the nature of host-rock permeability.
Despite being distal to the main orebody, the 400 000 t 20% Zn, 1.6% Pb Island Pod is still very high-grade and hosts the best-quality ore at Lisheen while showing rapid lateral variation on a small scale, less than10 m, where adjacent boreholes can have zinc concentration differences of c.40%. It also displays a weaker structural control than elsewhere in the southern Irish orefield. The basic sulphide mineralogy for the Island Pod is pyrite, sphalerite and galena, with multiple generations of each observed along with several carbonate phases. Dendritic pyrite/galena, colloform sphalerite and sphalerite/dolomite intergrowth textures suggest an early, rapid mineralisation event, from a supersaturated fluid in a nonequilibrium depositional environment, marking the onset of fluid mixing. As the mineralising system came closer to equilibrium, sulphide textures changed, reflecting a slower precipitation environment. 34-S isotope data suggest a dominantly bacteriogenic source for S in the Island Pod, with minor positive 34 delta-S values recorded. This is consistent with the distal position of the orebody, away from the feeder ramp-relay fault system at Lisheen which is thought to have introduced metal-bearing hydrothermal fluids with 34 delta-S greater than 0. In addition, the orebody and its halo have very similar 34 delta-S values, signifying that a lack of hydrothermal sulphur is not what led to the halo being sub-economic; this may suggest that other factors were responsible for ore grade, such as the availability of bacteriogenic sulphur or stratigraphic controls such as the nature of host-rock permeability.