Mineralogy and geochemistry of silicate, sulfide, and oxide iron formations in Norway: evidence for fluctuating redox states of early Paleozoic marine basins

摘要

Laterally extensive silicate and sulfide iron formation associated with jasper (hematitic chert) beds and volcanogenic massive sulfide (VMS) deposits in Norway provide evidence of early mineral assemblages and redox conditions within coeval early Paleozoic seawater. Calculated detrital-free compositions record mixed hydrothermal (e.g., Fe, Cu) and seawater +/- biogenic (e.g., Si, Ni, S, REE, P) components. Rare earth element (REE) patterns are characterized by small to large negative Ce anomalies and insignificant to locally large positive Eu anomalies, reflecting seawater REE carried to the seafloor by Fe-P-rich particles later modified by diagenetic processes. Protoliths of silicate iron formation precipitated in anoxic and intermittently euxinic deep waters by the diagenetic modification of amorphous Si-Fe oxyhydroxides and/or Si-Fe-OOH gels, based on possible modern analogues in the Red Sea. Diagenetic minerals include nontronite, greenalite, stilpnomelane, magnetite, manganosiderite, apatite, and iron sulfides. In sulfide iron formation, a local predominance of pyrrhotite over pyrite records highly reducing conditions caused by organic material. The geochemical data provide evidence for Mn-Fe-P shuttle and redox processes in a stratified basin with oxic or suboxic shallow waters and silica concentrations much higher than those of modern seawater. Hydrothermal plume-derived Fe present within the anoxic layer and near the chemocline formed mixed-valence oxyhydroxides and silicates and, intermittently, sulfides by reaction with aqueous Si and H2S, respectively, the latter derived from bacterial reduction of seawater sulfate at the chemocline. Major sustained fluxes of hydrothermally derived reductants (Fe2+, Mn2+, H2S, H-2) produced from large seafloor systems such as LOkken may have changed the redox state of seawater in local, and possibly regional, basins from weakly or moderately oxic to intermittently anoxic or euxinic conditions.