Archean seafloor hydrothermal vents or Quaternary subaerial springs?
A look at Ironstone pods in South Africa

Aric Olsen

Abstract:

In 1994, ironstone pods in the Archean Barberton greenstone belt, South Africa were interpreted as being Early Archean (>3.2 Ga) seafloor hydrothermal vents.  The ironstone pods were determined to be stratigraphically lower than nearby shales, silicified sandstones, and were shown to be along the same stratigraphic horizon as a banded iron formation just south of the pods.  The mineralogy of the ironstone pods is mostly hematite and goethite, with quartz veins and amorphous Fe-Al silicate.  Fluid inclusion analysis of quartz in the pods has been used to estimate the height of the water column above the vents when they were formed, and also ocean temperatures during the Archean.  Others have used this study to estimate Archean surface temperature, ocean depth and volume, seafloor sedimentation, and surface environments.  Recently (October 2003) these pods were reinterpreted as Quaternary subaerial springs.  The basis for this conclusion comes from the results of  previous studies and recent studies of the ironstone pods in South Africa.  Goethite fills many cavities, and commonly forms dripstone features that indicate formation in their current orientation. In previous studies, the pods were said to have delicate internal textures which suggests a younger age.  Also, goethite becomes unstable at 80-100°C and the surrounding Archean rocks have undergone multiple alterations at temperatures of at least 200-250°C.

Above: Location and geology of the Barberton greenstone belt, South Africa.  Ironstone pods are in areas where the Fig Tree Group is exposed and the arrow pointing to Fig. 1b is the study site that this presentation will cover.  (map from de Ronde et al. 1994)

References:

de Ronde, C.E.J., de Wit, M.J., Spooner, E.T.C. 1994, Early Archean (>3.2 Ga) Fe-oxide-rich, hydrothermal discharge vents in the Barberton greenstone belt, South Africa,  Geological Society of America Bulletin, v. 106, p.86-104.

de Ronde, C.E.J., Ebbesen, Thomas W., 1996, 3.2 b.y. of organic compound formation near sea-floor hot springs, Geology, v.24, no.9, p.791-794.

de Ronde, C.E.J., Channer, D., Faure, K., Bray, C.J., Spooner, E.T.C., 1997, Fluid chemistry of Archean seafloor vents: Implications for the composition of circa 3.2 Ga seawater, v. 71, no.19, p.4025-4042.

Lowe, Donald R., Byerly, Gary R., 2003, Ironstone pods in the Archean Barberton greenstone belt, South Africa: Earth’s oldest seafloor hydrothermal vents reinterpreted as Quaternary subaerial springs, Geology, v. 31, no.10, p.909-912.

Whitfield, John, 2003, Seafloor vents spawn spat, Nature, 3-12-04, www.nature.com/nsu/nsu_pf/031027/031027-6.html