Geoparque Mundial UNESCO
Geoparque Villuercas > Nº35 – Magnesites of Navalvillar
The best outcrops of dolomitic limestones with magnesites are located on the national road from Navalmoral de la Mata to Guadalupe (EX-118), about 2 km north of Navalvillar de Ibor.
It is not easy to get there by car, although there is a small entrance on the right-hand side of the road (if you are heading south). Here you can leave your car, cross the road – very carefully! and make your observations.
Several types of deposits can be observed at this point:
1) Detrital deposits and their sedimentary structures.
2) The dolomites, their initial features (lamination), their crystalline appearance and the brown and grainy appearance they have due to chemical weathering.
3) Magnesites, which are the main objective of this section. In the outcrop on the road, we can see how a mass of large (mm) brown crystals is entering the fractures and stratification planes of the dolomites (grey in colour) and replacing them. The large crystals are magnesite and are replacing the smaller crystals of the grey dolomite. At this point, what can be called a replacement front, in this case a magnesitisation front, is clearly visible.
It should also be noted that it is in these dolomitic rocks with magnesites that the Castañar cave will form. The fact that these rocks are very rich in magnesium is what determines the great mineralogical and morphological variety of the speleothems in this cave.
The magnesites are located in the Ibor anticline, within the Ibor Group (Villarta Formation) of Late Ediacaran age. The dome of the Variscan anticline or anticlinorium disappeared by erosion, leaving today on the surface the oldest materials of the basement, deposited during the Ediacaran period (635-539 Ma) and folded during the Cadomian orogeny.
The Ibor Group includes a great diversity of detrital sedimentary rocks (shales, arkosic sandstones, greywackes and conglomerates) and chemical precipitation rocks (discontinuous carbonate levels that can reach more than 100 metres in thickness). All these sediments were deposited in a mixed platform environment (siliciclastic and carbonate).
The most remarkable feature is the presence in the carbonate levels of Cloudina, one of the first metazoans to secrete a calcareous exoskeleton. Together with Sinotubulites, Namacalathus and other skeletal fossils from the late Ediacaran, they are the precursors of the generalised biomineralisation that occurred in animals at the beginning of the Cambrian, favouring new feeding and defence strategies that allowed the colonisation of new environments. Undoubtedly, the origin of skeletons led to a strong expansion of the marine fauna that has been recorded in the fossil record as a major evolutionary event known as the “Cambrian explosion“.
The carbonate strata of the Ibor Group include limestones that in many areas show intense diagenetic dolomitisation processes. These processes are very important in the Navalvillar de Ibor and Castañar de Ibor area.
In Navalvillar de Ibor these carbonates are mainly dolomites, a calcareous rock composed of calcium-magnesium carbonates, CaMg(CO3)2. In this area we find that the calcium in the dolomites has been practically in totality replaced by magnesium, giving rise to magnesites. Magnesites are formed by the mineral called magnesite (MgCO3), i.e., exclusively magnesium carbonate.
