Search

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, H2) produced from large seafloor systems such as Løkken 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. [ABSTRACT FROM AUTHOR]

Precambrian iron formations throughout the world, notably in Australia, Brazil, and South Africa, show evidence of hypogene (≥ 110 °C, mostly > 250 °C) oxidation, alteration, and silica dissolution as a result of tectonic or magmatic activity. Although hydrothermal oxidation has been proposed for the prototype Lake Superior-type iron formation, the Biwabik Iron Formation in Minnesota (USA), it has not been documented there. By examining oxidized and unoxidized Biwabik Iron Formation in three mines, including material from high-angle faults that are associated with oxidation, we document an early hypogene oxidation event (~ 175 °C) involving medium-salinity aqueous fluids (8.4 ± 4.9 wt% NaCl equiv) that infiltrated iron formation along high-angle faults. At the Hibbing Taconite Mine, hydrothermal fluids oxidized iron carbonates and silicates near faults, producing goethite ± quartz. In contrast with much of the oxidized iron ores on the Mesabi Range, silica was not removed but rather recrystallized during this event, perhaps lying in a rock-dominated system at low cumulative fluid flux. During the hydrothermal oxidation event in the Hibbing Taconite deposit, quartz-filled microfractures and irregular inclusions commonly formed in coarse variably oxidized magnetite, currently the ore mineral: these inclusions degrade the ore by introducing excess silica in magnetic concentrate. Hydrothermal oxidation at Hibbing Taconite Mine is overprinted by later, relatively minor supergene oxidation both along faults and near the surface, which locally dissolved quartz. At the Fayal Reserve Mine, widespread silicate and carbonate gangue dissolution and iron oxidation was followed by precipitation of pyrite, Mn-siderite, apatite, and other minerals in void spaces, which prevented post-oxidation compaction and significant volume loss in the sampled rocks. Although definitive temperature data for this assemblage are needed, the weight of evidence indicates that this mineralization is hypogene. The association of oxidation with epithermal conditions constrains the oxidation and subsequent mineralization to have taken place during the Precambrian, the only time when these rocks would have experienced the necessary temperatures. The mineralization at Fayal Reserve shows little supergene overprint: pyrite is largely unoxidized. Hydrothermal oxidation in both mines was likely produced by basinal fluids that were expelled during the 1.83-1.87 Ga Penokean Orogeny, and mixing with meteoric fluids along faults, although a 1.1 Ga rift-related fluid flow event is also possible. Later supergene overprinting of the iron formation was minor. [ABSTRACT FROM AUTHOR]