Iron oxyhydroxide and sulphide mineralization in hydrocarbon seep-related carbonate submarine chimneys, Gulf of Cadiz (SW Iberian Peninsula)

This paper aims to investigate the iron mineralization hosted in the submarine hydrocarbon seep-related carbonate chimneys, from the Gulf of Cadiz continental slope (SW Iberia). Chimneys are made of a general fine groundmass of major Fe-rich dolomite, ankerite and calcite, small grains of quartz and phyllosilicates, and some foraminifer tests and ostracod shells. Primary porosity is frequently generated inside foraminifer chambers and ostracod shells, and is filled with closely packed microcrystals of iron oxyhydroxide and sulphide minerals forming framboids. Some extremely peculiar multiframboidal textures are also detected, corresponding to microcrystal groups, spherical framboids and euhedral crystals without framboidal texture. The mineralogy observed is mainly goethite (pyrite pseudomorphs); traces of tiny grains of pyrite were also observed. Cubic, octahedral and pyritohedral are the habits observed in both minerals. Chemical analyses of framboids and euhedral crystals display high (often erratic) amounts of As, Co, Ni and Mo in oxyhydroxides and Mo, Pb, V and Co in sulphides versus almost always negligible concentrations of Cu and Zn. A textural, geochemical and mineralogical evolution is proposed to explain the coexistence of different morphologies in the multiframboidal texture: (a) growth and aggregation of microcrystals as typical framboidal-type mineralogical associations; (b) development of euhedral habits; (c) coalescence and homogenization of the microcrystal into large size (euhedral to anhedral) crystals, and (d) formation of euhedral crystals or polycrystalline masses, with complete loss of framboidal texture. Along this process, an increase of the concentration of Fe, S, Mn and Ti, linked to a decrease in Mg, Si, Al, As, P, Ca and V was observed. The study of the iron mineralization in the Gulf of Cadiz is a first, and can give clues to understand the complex geobiological interactions in this and other similar extreme hydrocarbon-bearing submarine ecosystems.