Reduction spheroids from northern Switzerland: Mineralogy, geochemistry and genetic models

Abstract Reduction spheroids are small, isolated redox systems occurring in hematite-stained rocks of variable age, origin and provenance. Reduction spheroids from Permo-Triassic continental red-beds and from crystalline rocks of northern Switzerland have been studied mineralogically and geochemically. 48 authigenic minerals were identified in the dark, mineralized cores of the spheroids. Most common are roscoelite, uraninite and Ni-arsenides. Minerals containing Ag, Au, Pd and Pt have been found as well. The bleached spheroids are depleted in ferric iron due to hematite dissolution. Mass-balance calculations show that mobilization of only a minor fraction of the trace elements present in the bulk rock is needed to explain the formation of mineralized spheroids. The relative mobilities were: As > U > Ni > V. The elements precipitated in reduction spheroid cores as mineral phases were probably released from Fe-hydroxides during their alteration to hematite. The reduction spheroid-containing red-beds are unusually rich in As and B compared with underlying organic-rich sediments. Reduction spheroid formation probably took place during Mesozoic time at depths of burial of 400–1100 m. Pore waters were highly saline. Organic matter of unknown origin reacted with dissolved high-valence ions and hematite at discrete sites in the rocks and caused local precipitation of rare elements as mineral phases and dissolution of hematite. The formation of reduction spheroids is not due to detrital organic matter as is demonstrated by their presence in crystalline rocks and by the absence of organic relicts of the needed size. Ore mineral precipitation probably was catalyzed by bacteria because sulfides were formed by in situ, lowtemperature (