A Distal, High-Grade Irish-Type Orebody: Petrographic, Sulfur Isotope, and Sulfide of the Island Pod Zn-Pb Lisheen, Ireland

Y Irish-type Zn-Pb deposits are important global sources of zinc, but despite a fundamental understanding of ore genesis within the Irish orefield, a detailed understanding of fluid migration and chemical evolution pathways related to sulfide and carbonate precipitation is lacking. We present the first petrographic, paragenetically constrained sulfur isotope and mineral chemistry study of mineralization at the Island Pod orebody, Lisheen deposit. The Island Pod orebody comprises high-grade mineralization that is less deformed than elsewhere in the Irish orefield. Consequently, studies of the Island Pod orebody and its mineralization provide information on the evolving nature of hydrothermal fluids involved in ore deposition. The Island Pod orebody consists almost exclusively of pyrite, sphalerite, and galena, with several stages of calcite and dolomite precipitation. Pre-ore, diagenetic pyrite is commonly overgrown by early main ore-stage pyrite, with both phases frequently replaced by main ore-stage sphalerite. In many cases, early main ore-stage pyrite is texturally zoned and exhibits chemical zoning patterns, reflecting that episodic influxes of hydrothermal fluids contained variable concentrations of As, Co, Ni, and Tl. The main ore stage was dominated by the forma-tion of sphalerite and galena from mineralizing fluids that were depleted in these trace elements (e.g., As, Co, Tl) compared to the early main ore stage. Sulfur isotope analysis reveals four distinctive but slightly overlapping isotopic groupings, corresponding to different mineral and paragenetic stages: (1) delta S-34 values range from -47.7 to -30.7 parts per thousand, associated with diagenetic pyrite; (2) delta S-34 values range from -34.3 to -14.7 parts per thousand, related to early main ore-stage pyrite; (3) delta S-34 values range from -15.5 to +1.7 parts per thousand, corresponding to main ore-stage sphalerite; and (4) delta S-34 values range from -11.1 to +17.4 parts per thousand, associated with galena. Large variations in S isotope composition are common at intragrain and at other small spatial scales. The textures, paragenetic sequence, and ranges in delta S-34 values are consistent with hydrothermal sulfide deposition where the fluids containing bacteriogenic sulfide mixed with metal-bearing fluids. Replacement and remobilization from other Lisheen orebodies may have contributed to some of the higher sulfur isotope ratios observed in the Island Pod orebody. The excellent preservation of sulfide textures in the Island Pod orebody observed during this study demonstrates that it is an ideal location to study hydrothermal fluid evolution, including episodic fluid flow, mixing, precipitation, and compositional variations during the early main ore stage. In other Irish Zn-Pb orebodies, these early-ore textures are often obscured due to more complex dissolution and replacement processes, making interpretation of the early hydrothermal activity challenging. Consequently, the petrographic, mineral chemistry, and sulfur isotope studies of the Island Pod orebody presented here contribute to an enhanced understanding of ore-forming processes in similar deposits, where mineralization is often associated with more complex deformation or repeated pulses of hydrothermal activity.

Automated summary

This study presents the first petrographic, paragenetically constrained sulfur isotope and mineral chemistry study of the Island Pod orebody at the Lisheen deposit. The results reveal distinct characteristics of the early main ore stage and the main ore stage based on sulfur isotope composition and ore textures. The variations in sulfur isotope composition and ore textures are related to the mixing and deposition processes of hydrothermal fluids, which contribute to a better understanding of the ore-forming mechanisms and evolution of Irish-type Zn-Pb deposits.