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

摘要

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