THE TAG active mound(1-13), on the Mid-Atlantic Ridge near 26 degrees N, is one of the largest known, actively forming volcanogenic massive sulphide deposits. Construction of such a deposit requires that the upflow of hydrothermal fluids be localized at this site over tens of thousands of years, but the cause of this localization has been controversial. Two popular hypotheses propose that it results from young volcanism immediately adjacent to the mound(6), or from the intersection of a ’transfer fault’, orthogonally crossing the rift-valley floor, with ridge-parallel faults(8). Here we present high-resolution sonar and photographic data which provide no evidence for young eruptions nearby, or for transverse, through-going faults intersecting the TAG mound. Instead, our data suggest that the localization of hydrothermal activity at the TAG mound is strongly controlled by permeable conduits at the intersection of an actively developing ridge-parallel fissure/fault complex with a newly recognized but older, oblique fault system, These observations may aid the interpretation of similar structures in ancient sea-floor sulphide deposits, where the tectonic disruption caused by emplacement on land has obscured the original geometry.