Localization of lead-zinc deposits in the mid-Wales ore field, United Kingdom, occurred where cross-cutting mineral veins were deflected by pre-existing sets of ac joints. Such deposits are frequently attributed to the infilling of open spaces formed by the intersection of premineralization fractures. This concept is not supported by the presence of analogous open spaces in unmineralized fracture systems. It is suggested that some ore shoots in the mid-Wales lodes were formed by the opening of ac fractures under the hydraulic pressure of the ore fluids during the mineralization process. Any planar anisotropy in a rock can affect the development of a hydraulically formed mineral vein, and the suggested mechanism probably operated during the formation of deposits in other ore fields.