The Speleogenesis of Vallgornera Cave (Mallorca, Spain): a Mineralogical and Morphological Study

Cova des Pas de Vallgornera (CPV) is morphologically and scientifically, the most prominent karst feature of Mallorca Island (Spain). It consists of over 74 km of passages developed within two carbonate lithologies (reef front and back reef facies) of Upper Miocene age. Two distinct cave patterns are recognized, both tightly controlled by the type of facies; spongework mazes and collapse chambers are characteristic for the reef front unit, whereas linear, fracture-guided galleries develop in the back reef carbonates. CPV is abundantly decorated with a variety of speleothems. The overarching goal of this study is to provide evidence towards the cave\u27s major speleogenetic pathways using diagnostic mineral assemblages and a set of micro- and macro-scale morphological features. In particular, this research investigates whether hypogene processes were/are the main driving force in CPV\u27s speleogenesis and how mineral assemblages and cave morphologies help untangle the complex evolution of the cave. The mineralogy of speleothems (crusts, nodules, crystals, earthy masses) deposited in the vicinity of hypogene features in the reef front is characterized by the presence of aragonite, ankerite, huntite, clay minerals, and quartz. In the back reef limestone, however, the dominant mineral is dolomite, along with aragonite, celestine, huntite, clay minerals, and quartz. Calcite is by far the most ubiquitous mineral throughout the cave. A total of twenty-six minerals were identified throughout the cave; the highest diversity (20 species) is around or inside typical hypogene features occurring within the back reef limestone, whereas in the seaward part only 13 minerals occur. Detailed macroscopic and scanning electron micro-chemical analysis and imaging have permitted the investigation of textural relationships between the minerals associated with vents, rims, and corrosion cupolas above them. These studies along with morphological and stable isotope analyses (δ13C, δ18O, δ34S), confirm that not all minerals are connected with a hypogene stage in the cave evolution, and furthermore, none of them appears to be sulfuric acid by-products. Instead, the mineral assemblages documented in speleothems from CPV clearly support at least three speleogenetic pathways, namely seacoast mixing, ascending of warm groundwaters (basal recharge), and vadose (meteoric recharge). In summary, we have shown that cave minerals (in association with particular cave morphologies) in CPV hold the keys to understanding mineral-forming processes, conditions, and events, allowing establishing their relationship with various speleogenetic pathways