The mineralogy and the geochemistry of the oolitic iron ore deposits of the region located between Ponferrada and Astorga, in the province of Leon (northwest Spain), were studied. This mineralization constitutes a lithostratigraphic metallotect associated with the formation called the Luarca Slate (middle to upper Ordovician). Magnetite, siderite, and chlorite are the principal constituents of the ore. Ilmenite, apatite, rutile, graphite, pyrite, and arsenopyrite are minor constituents.The deposits have been affected by contact metamorphism. As a consequence important mineralogical and geochemical variations occur which permit differentiation of three zones whose limits are marked by the boundaries of the contact aureole. The characteristics of these zones are the following:Zone P, a zone affected by the post-Hercynian granitic intrusion of San Miguel de Duenas. The mineralization is rich in magnetite and the phosphorus content is above the desirable limits.Zone P-A, not affected by contact metamorphism. The mineralization consists fundamentally of siderite associated with iron silicates. There is minor magnetite and less phosphorous than in zone P.Zone A, also affected by a granitic intrusion. The mineralization is intermediate between the P and P-A zones.Northwest Spain contains the largest resource of iron mineralization in the country, although, at present, the deposits contribute only a small percentage to the national production of iron. The zone of mineralization is between Ponferrada and Astorga. The Vivaldi, Wagner, and S. Bernardo mineral districts are the important centers of mining.