Geology, mineralogy and paragenesis of cobalt-nickel deposits from central Pyrenees, Spain

Two distinct episodes of Co-Ni mineralization have been distinguished in Central Pyrenees of Spain: (1) A Hercynian one marked by a complex assemblage of diarsenides, triarsenides and sulpharsenides and (2) an alpine one marked by the presence of siegenite. Hercynian ores fill Hercynian E-W faults, which constitute the contact between black shales and metasomatized limestones. One main feature of these deposits is the occurrence of an intermediate member of gersdorffite-rammelsbergite solid solution. Ore genesis seems to be related to granite-derived hydrothermal fluids. On the other hand, the occurrence of siegenite in alpine Pb-Zn-F-(Ba) Parzan vein could have been linked to higher temperatures of formation and a magmatic contribution to the ore-forming fluids in a model invoking fluid mixing between deep-seated metal-bearing basement brines and cooler sulphate-rich meteoric waters to explain mineral deposition.