The Gruta de las Maravillas (Aracena, South-West Iberia): Setting and origin of a cave in marbles from dissolution of pyrite

Abstract

The Gruta de las Maravillas cave is located at the WNW side of the Cerro del Castillo hill in Aracena (Huelva, SW Spain). The cavity is hosted within marbles included in a strip of high-grade metamorphic rocks belonging to the so-called Aracena Massif in the southernmost Ossa-Morena Zone. The hill is made up of granodiorites, marbles, quartzites, and gneisses, with the foliation trending N110°E and dipping roughly 60–80° towards NE. The marbles appear highly deformed in ductile conditions, with isoclinal folds of different sizes, boudins, porphyroblasts with sigmoidal morphology, and left-lateral S-C shear fabrics. Close to the granodiorite contact, the marbles include a thin band of disseminated and massive pyrite, partially transformed to Fe-oxides. Analysis of the brittle deformation and the associated paleostresses indicates a NE–SW oriented maximum compression, probably related to the latest Variscan collisional tectonics (300 Ma; Late Carboniferous). The Gruta de las Maravillas is divided into three main levels (located at ~ 650, ~ 665 and ~ 685 m a.s.l.), the dissolution having progressed from top to bottom in different stages of stability of the water table. The initial dissolution phases were probably favoured by the presence of pyrite in the host rock, which, in turn, would have caused acidification of the circulating water. Favouring this hypothesis, a thin layer of Fe-oxides, locally including gypsum, covers some parts of the cave walls. The morphology and structure of the cavity result from interaction between the general NNE dipping foliation with sub-perpendicular joints, the pyrite-bearing band in the host marbles, and the descending water table.