Procesos de dolomitización-dedolomitización de la formación Láncara (Cámbrico inferior-medio) en la región de pliegues y mantos de la zona Cantábrica, NW de España

Abstract

The Láncara Formation is a carbonate sequence within the Cantabrian zone with two members, the lower of primary dolomite nature, and the upper mainly composed of limestone beds. This formation has two basic features worth mentioning: 1) Represents the sliding and fracturing surface of most nappes and thrust-sheets of the Cantabrian zone; 2) Hosts several ore deposits, mainly Pb, Zn, Ba and Hg. A representative section of this Formation has been defined at Caranga de Arriba, Asturias (Spain). The samples obtained from this section have been characterized using several analytic techniques (transmission microscopy, X-ray diffraction, electron microprobe and chemical analysis). A certain dedolomitization afecting the singenetic dolomites has been observed in the lower member, whereas the upper member shows a complete dolomitization. On the other hand, in the surroundings of the site described, and hosted by the refered Formation, the stratabound Pb-Zn ore deposito named Grupo Cantabria, is found. These facts seem to indicate that the Lancara Formation has undergone a great circulation of connate fluids, from the bottom to the top of the Formation, resulting in an intense cation leaching (such as magnesium, very abundant in the lower member) towards the upper member. This is probably due to the fact that the Lancara Formation represents the sliding surface of most nappes of the Cantabrian zone.