Bacteria were responsible for the magnitude of the world-class hydrothermal base metal sulfide orebody at Navan Ireland

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 delta S-34 = -13.6 +/- 2 per mil (n = 20), and galena concentrate mean Pb-206/Pb-264 = 18.19 +/- 0.03 (n = 7). Calculations suggest greater than or equal to 90 percent of the Navan sulfides were derived through bacteriogenic reduction of Mississipian seawater sulfate, whereas metals were acquired from a local, orogenic crustal source beneath the ore-body. Enhanced bacterial activity was fundamental to ore deposition at Navan: no bacteria, no giant ore deposit.