Lead isotope systematics in Irish base metal deposits and basement rocks indicate that lead and other metals were derived from local basement rocks with little or no addition from basinal sediments hosting the deposits. Thus, a fluid circulation model involving simple basin brine expulsion and an impermeable boundary at the basin-basement interface cannot be invoked to explain the origin of the mineralization. Production of ore deposits as a result of transport out of the basement would involve a relatively trivial integrated heat addition as compared to the total heat transported within the basin on the time scale of deposit formation. ¿American Geophysical Union 1991