The isotopic compositions of leads in ores and rocks of Butte, Montana, suggest that the ore metals were neither late stage concentrates from the crystallizing magma that produced the Boulder batholith, nor concentrates of trace metals from nearby sedimentary rocks. The ore metals probably were concentrated and isolated in several non-shallow locations about 400 m.y. ago and were then transported into the batholith by the late stage fluids of the magma, after the batholith was emplaced 60 m.y. ago. Mixing of host rock Pb and ore mineral leads occurred in and near the ore zone.