Abstract

Ironstone pods (Fe2O3 = 72.5-97.2 wt%), interpreted to have formed by early Archean, sea-floor-related hydrothermal activity, are described from the ca. 3.5-3.1 Ga Barberton greenstone belt, South Africa. Most of the pods are elliptical in shape and have their longest dimensions subparallel to the local stratigraphy. They rest on silicified ultramafic rocks and are overlain by ferruginous shales, silicified sand-stones, other coarse clastics, and barite horizons. The ironstone pods grade along strike into laminated iron-oxide facies banded iron-formation that is inferred to represent periodic discharge of iron-oxide- and silica-rich flocculates from the pods. The massive texture of the iron-stone pods and their lack of internal sedimentary features suggest that they formed directly on the sea floor; relict hydrothermal Discharge chimney structures have been recognized. Honeycomb-like cavities and possible fluid-flow channel textures attest to primary porosity. The iron-stone pods are dominated by massive, locally coarse-grained, specular hematite and goethite, with lesser amounts of quartz and an X-ray amorphous Fe-AI-bearing silicate.

The pods are distinct from typical massive sulfide gossans in total trace-element concentrations, including precious and base metals, indicating a different origin. In contrast, the ironstone pods have similarities in major oxide and trace-metal concentrations to iron-oxide deposits presently forming, in part, from low-temperature hydrothermal fluids on the sea floor (for example, Red Seamount iron-oxide deposit).

Fluid-inclusion studies on quartz show dominant type I primary fluid inclusions with salinities of 4.7 to 15.8 wt% NaCl equiv. and homogenization temperatures (Th) of ∼90 to 150 °C; no evidence for boiling is seen. Type I fluid inclusions are most likely dominated by NaCl-CaCl2-H2O, although their measured eutectic temperatures are consistent with the presence of FeCl2; these inclusions represent the end-member hydrothermal fluid. More saline (24-29.6 wt% CaCl2 equiv.), lower temperature (Th = 33-109 °C), type II and Ila fluid inclusions represent pulses of a hydrothermal fluid of NaCl-CaCl2-FeCl2 (or MgCl2)-H2O, or possibly FeCl3-bearing composition, mixing with ambient sea water in the case of type II inclusions. The various fluid types are interpreted to be indicative of intermediate compositions of an evaporitic brine. The lowest salinity measurement for a type I inclusion of 3.1 wt% NaCl equiv. provides a constraint on the salinity of unenriched (by evaporation) sea water.

The Th data give a minimum trapping pressure of ∼7 bars, which equates to a minimum sea-water depth of ∼60 m. The lack of precise pressure estimates has not enabled calculation of maximum water depths above the pods.

Quartz δ18O analyses average 17.3 ± 0.6‰ (1σ n = 5). Calculated δ18OH2O, for two samples are -1.4‰ and -1.2‰, respectively. These estimates are within error of the postulated Barberton early Archean sea water value of ∼0‰ Quartz/hematite-goethite mineral pairs yield temperatures inconsistent with fluid-inclusion Th data, suggesting isotopic disequilibrium.

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