We respond to de Ronde et al. as per their numbered comments. 1. de Ronde et al. suggest that there are two populations of ironstone pods in the Barberton greenstone belt: those on Farm Mendon that are Archean in age and composed primarily of hematite and all other ironstone pods, which are young and composed of goethite. We disagree and regard all of the ironstone pods as related to young spring activity, but will restrict our discussion here to the Mendon ironstone pods. We did not mention pods occurring in “small topographic saddles.” 2. Our observations on the stratigraphic setting of the Mendon iron-stone pods are at odds with those of de Ronde et al.: the Mendon ironstone pods are clearly not “part of the local Archean stratigraphy.” The dimensions provided by de Ronde et al. are those of the present outcrops and provide no information on whether the pods mantle the present surface or are part of Archean stratigraphy.

There are blocks of silicified sediment (chert) and silicified volcanic rock in the Mendon ironstone pods in the old mine and on the surface. In all cases that we have seen, bedding in these blocks is parallel or subparallel to that in surrounding Archean units. These “blocks” have not fallen from cliffs, but are masses of local Archean bedrock remaining after ironstone replacement or protruding through the surface-mantling ironstone veneer.

The Mendon ironstone pods are neither in contact nor interfinger with banded iron formation, as shown by de Ronde's map (de Ronde et al., 1994, their Fig. 2). Although this map shows them surrounded mainly by shale, our mapping (Lowe and Byerly's [2003] Fig. 1) shows them to be developed on a substrate of vertically dipping Archean ultramafic rock and carbonaceous and banded cherts. While strain has been preferentially partitioned into serpentinized ultramafic rocks, cherts and silicified volcanic rocks are strongly fractured, veined, and disrupted. No Archean rocks have escaped strain as suggested by de Ronde et al. for the Mendon ironstone pods. The hematitic portion of the largest Mendon ironstone pods contains open, unfilled cavities up to 70 cm across lined with fresh botryoidal hematite, features unlikely to survive 3.2 billion years of deformation and metamorphism.

The so-called ironstone pods porphyry is irrelevant to establishing the age of the Mendon ironstone pods. de Ronde's map (de Ronde et al., 1994, their Fig. 2) shows that it nowhere cuts or is in contact with the ironstone pods. Their line of reasoning is apparently that the ironstone pods porphyry is related to another small felsic intrusion, the mudpools porphyry, some 10 km away. This second intrusion cuts Fig Tree banded iron formation containing so-called mudpool structures that supposedly reflect hydrothermal activity that might be related to the hydrothermal vents that formed the ironstone pods.

No coarse Archean clastic units are in contact with or directly overlie Mendon ironstone pods. The second largest Mendon ironstone pod, exposed in low cuts along Powerline Road, overlies and is in turn overlain by modern regolith in a number of places. Chunks of ironstone from the pods have been incorporated into the regolith, which de Ronde et al. may have mistaken for Archean clastic material. While clasts of jasper and banded iron formation are common in coarse Fig Tree clastic units, no fragments of ironstone pods have been identified in any Archean sediments.

3. The pods are not composed largely of specular hematite. The eastern half of the largest Mendon ironstone pod is composed largely of hematite, but the western half, including an old mine face some 5 m high (de Ronde et al., 1994, their Fig. 3c), is made up entirely of well-layered goethite containing large open cavities, many with dripstone fill. The other large Mendon ironstone pods (de Ronde et al., 1994, their Figure 2; Lowe and Byerly's [2003] Fig. 1) are composed of 10%–30% hematite and >70% goethite. The vertical open fluid-flow conduit illustrated by Lowe and Byerly (their Fig. 4) from the Mendon ironstone pod along Powerline Road is composed entirely of goethite.

Goethite described by Reyes et al. (2003) occurs from surface temperatures to 290 °C. These authors note that goethite commonly forms at <150 °C and their data clearly indicate that this is a nonequlibrium mineral assemblage. Even if goethite is stable up to ~175 °C at 1 kbar, as stressed by de Ronde et al., rocks of the Barberton greenstone belt, including those around the Mendon ironstone pods, have been altered at sustained temperatures >300 °C (Tice et al., 2004) and goethite is still essentially unknown in the geologic record before the Ordovician.

4. Virtually all quartz veins occur around the margins of the Mendon ironstone pods where iron oxides are replacing chert and other country rocks. Within the main parts of Mendon ironstone pods, quartz occurs mainly as irregular, disrupted masses being replaced by iron oxides, as confirmed by de Ronde et al. Many if not most hydrothermal fluids are brines and represent “modified seawater,” but most do not provide useful data on the specific composition of the ocean or atmosphere because of extensive subsurface modification and exchange. The same is true of the fluids in the quartz masses in the Mendon ironstone pods.

5. The composition of thermal waters in other parts of South Africa is irrelevant to springs discussed by us (Lowe and Byerly, 2003). Bad plaas springs issue from an extensive plutonic terrane and represent waters that have flowed through platform sedimentary sequences, tonalites, and granites. The waters that we suggested to have formed the ironstone pods flowed through thick, vertical sections of sideritic cherts and mafic and ultramafic volcanic rocks. The goethite terrace on Farm Avontuur (Lowe and Byerly, 2003) was clearly formed by surface outflow rather than as ferricrete, as de Ronde et al. suggest, and unambiguously indicates that iron-depositing springs have existed in the Barberton greenstone belt region in the very recent past.

While it is difficult for the reader to evaluate the conflicting interpretations of local Barberton greenstone belt geology, at issue is whether large bodies of finely structured hematite and thermally unstable goethite containing large open cavities and complex organic compounds have survived intact and undeformed since the Archean through multiple episodes of intense deformation and metamorphism at temperatures >300 °C. Alternatively, the Mendon ironstone pods are part of a recognized set of goethite and hematite ironstone pods in the Barberton greenstone belt that show essentially the same characteristics and were deposited by relatively recent springs. Our observations support the latter interpretation and imply that these deposits are irrelevant to understanding Archean Earth.

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