Abstract

We report sulfur and lead isotope analyses of mine concentrates from Navan, the largest zinc deposit in Ireland. These samples, each representing up to a million tonnes of ore, show very limited isotopic variation, with mean δ34S = –13.6 ± 2 per mil (n = 20), and galena concentrate mean 206Pb/204Pb = 18.19 ± 0.03 (n = 7). Calculations suggest >90 percent of the Navan sulfides were derived through bacteriogenic reduction of Mississippian seawater sulfate, whereas metals were acquired from a local, orogenic crustal source beneath the orebody. Enhanced bacterial activity was fundamental to ore deposition at Navan: no bacteria, no giant ore deposit.

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