The best outcrops of dolomitic limestones or magnesites are located on the road from Navalmoral de la Mata to Guadalupe, some 2 km to the north of Navalvillar de Ibor. It is not easy to stop a car there, although there is a small entrance to the right of the road (heading south) where is is possible to park; cross the road with great care in order to make your observations.
The magnesites are to be found in the central area of the Ibor-Guadalupe anticline, the cupola of which as can be seen in its description (Geosite 23) disappeared owing to erosion to leave on the surface only the oldest materials deposited during the Ediacaran period (630 a 542 m.y.). They represent a set of detrital deposits (formed by fragments or debris) of shales, sandstones, and greywackes known to geologists as the Schist-Greywacke Complex. This area is classed as “Grupo Ibor” and also includes some carbonated strata some 5 m thick that are brown in colour on their outcrops.
These carbonates are above all dolomites, calcareous rock made up of calcium-magnesium carbonates Ca.Mg(CO3)2. The visitor can observe well laminated levels that include gaps with flat stones in the upper part of the stratum. The sedimentary structures of sand, mud, and clay that accompany it have allowed the interpretation of these deposits as shallow marine sediments laid down on a mixed platform of carbonates with more siliceous materials (sand, mud, and clay). The most notable feature is the presence of Cloudina fossils: Cloudina carinata is the earliest animal to produce a mineralised exoskeleton and is the precursor of the generalised biomineralisation that occurred in animals in the early Palaeozoic so that they could colonise new habitats in which to feed and reproduce. There is no doubt that this gave rise to a great expansion of marine fauna that is reflected in the fossil record as the great evolutionary event known as “The Cambrian Explosion”.
In this area the calcium of the dolomites has been replaced by magnesium to produce magnesites. Magnesites are formed by the mineral known as magnesite (CO3Mg), i.e. exclusively magnesium carbonate.
Many of the deposits that have been described can be observed here, such as:
1) Detrital deposits and their sedimentary structures (Figure 1).
2) The dolomites, their initial features (lamination, flat stones,…), their crystalline, brown, and grainy appearance owing to chemical meteorisation (Figure 1).
3) Magnesites, which are the main point of interest. On the outcrop on the road it can be seen how a mass of large brown crystals (mm) enter the fractures and the stratification planes of the dolomites (grey in colours) and gradually replace them. The large crystals are of magnesite and are replacing the smaller crystals of grey dolomite. At this point what is known as a front of replacement can clearly be seen, in this case a magnesitisation front (Figure 2).
It should also be noted that it is in these rocks that the Cueva de Castañar is formed; the fact that these rocks are very rich in magnesium is what conditions the great mineralogical and morphological variety of the speleothems of this cave.