Interpreting hydrothermal clumped isotope temperatures in the Irish Zn-Pb ore field

With the introduction of climate action plans by many countries globally, the development of green technologies like electric vehicles and renewable infrastructure is expected to increase. These technologies are resource intensive, meaning we will require increased production of metals to meet the growing demands of society. However, discovery and exploration rates are not increasing at the same rate as demand. Improving understanding of ore system formation and evolution is a crucial step in aiding future exploration, to help supply these critical resources.

In hydrothermal systems, carbonate minerals (e.g., calcite and dolomite) are often associated with all stages of ore formation, with fluid inclusion thermometry and carbon-oxygen (C-O) isotope ratios traditionally used to study fluid temperature and composition. However, there are several challenges still remaining with these techniques, with fluid inclusions often too small, ruptured or deformed for adequate study. In carbonate minerals, the rare, heavy isotopes ¹³C and ¹⁸O bond or clump more frequently at lower temperatures, with the magnitude of clumping inversely temperature-dependent. Measurement of clumped C-O isotope ratios, using gas source isotope spectrometry, simultaneously yields carbonate δ¹³C and δ¹⁸O values and generates mineral precipitation temperatures, allowing fluid δ¹⁸O to be directly calculated. While traditionally applied to low temperate environments, recent applications have included hydrothermal ore systems to study fluid temperature and mixing. When combined with other techniques, such as strontium isotopes, new understanding of the sources, movement and compositional evolution of fluids can be deciphered.

Recent clumped C-O and strontium isotope analyses of ore-related carbonates from the Lisheen and Galmoy deposits, southern Irish Zn-Pb ore field, have facilitated the study of fluid sources, temperatures, mixing, and modification. Lisheen and Galmoy are  hosted in a belt of regionally dolomitized Lower Carboniferous (Mississippian) marine limestones, cut by a series of NE-SW-trending ramp-relay normal faults. Study of these deposits reveals that early dolomitizing and later hydrothermal fluids are part of a complex multistage continuum, with phases of fluid mixing, compositional buffering due to dissolution, and isotope resetting. Consequently, studies of carbonates in other deposits may yield new insights into ore formation, ultimately helping exploration for crucial resources.