The mining installations can be reached from the roundabout at kilometre mark 45.5 on the EX-102 road; some 300 m from here towards the centre of Logrosán we come to the gallery and the main installations: the warehouses, laboratory, museum, and cafeteria. It has an entrance for vehicles and a large car park.
The magmas that gave rise to the Batholite of Logrosán (see the leaflet on the Cerro de San Cristóbal) caused large fractures as they slowly rose among the pre-existing rocks (shales and greywackes) of the Neoproterozoic era (over 600 million years ago) within the Logrosán anticline that no longer exists owing to erosion.
From these magmas, mineralised fluids from phosphates (gases and liquids) were injected into these large fractures or faults which they filled as they crystallised. A fault full of crystallised fluids constitutes a lode when these fluids finally cool and solidify. In this case the phosphorite lode of the Costanaza mine is one of the most easily recognisable filonian deposits in Spain, as it is approximately 5 kilometres in length and varies in thickness from 8 m to 0’10 m.
In its structure the mineralisations of apatite (fluorapatite or phosphorite) alternate with those of quartz, and on occasion carbonates also appear (calcite, siderite, and ankerite). This mineralogy is unique owing to the presence of the fluorapatite that has been discovered in this mine, which makes Logrosán the type locality for this mineral.
The phosphorites of Logrosán were made known by the Irish mining engineer William Bowles, who worked in Spain in the 1750s. The Costanaza mine was exploited sporadically from the late 19th century until its closure in 1944, when it was 210 m deep and had 14 levels; some 200,000 tons of ore had been extracted for the manufacture of superphosphate fertilisers that were exported to much of Europe. The number of parallel galleries in the mine is enormous, but only the upper two have been conditioned for visiting. Inside we can contemplate the mineralised lode of phosphorite, gap areas and fault mirrors, geodes, springs, stalactites, folds, mining support arches, and a workshaft of masonry. Outside we can view installations that still retain their original mining structure in perfect condition, such as the factory of fine aggregates, the factory of superphosphates, the pyrite kiln, and the laboratory, which houses the Interpretation Centre of the Costanaza Mine.
The mine is equally rich in intangible values and elements of our mining heritage, such as the mining support arches of masonry, the pipe vault, the exploitation chamber, the María shaft, and the Calle shaft. During the visit explanations are given of the details of the mining and the extraction method, which was known as “top hole” because the miners gradually extracted the lode from upper layers they could not see. Mention is also made of what the miners were like and their working conditions, the humidity, the very primitive equipment, and the lamps, which were initially traditional Badajoz oil lamps and subsequently carbide lamps. Some of these items decorate the walls of the mine.