Mantle heat drives hydrothermal fluids responsible for carbonate-hosted base metal deposits: evidence from 3He/4He of ore fluids in the Irish Pb-Zn ore district

There is little consensus on whether carbonate-hosted base metal deposits, such as the world-class Irish Zn + Pb ore field, formed in collisional or extensional tectonic settings. Helium isotopes have been analysed in ore fluids trapped in sulphides samples from the major base metal deposits of the Irish Zn-Pb ore field in order to quantify the involvement of mantle-derived volatiles that require melting to be realised, as well as test prevailing models for the genesis of the ore fields. He-3/He-4 ratios range up to 0.2 R (a), indicating that a small but clear mantle helium contribution is present in the mineralising fluids trapped in galena and marcasite. Sulphides from ore deposits with the highest fluid inclusion temperatures (similar to 200 A degrees C) also have the highest He-3/He-4 (> 0.15 R (a)). Similar He-3/He-4 are recorded in fluids from modern continental regions that are undergoing active extension. By analogy, we consider that the hydrothermal fluids responsible for the carbonate-hosted Irish base metal mineralization circulated in thinned continental crust undergoing extension and demonstrate that enhanced mantle heat flow is ultimately responsible for driving fluid convection.