Lowe and Byerly's (2003) description of iron-oxide-depositing spring terraces in the Barberton greenstone belt and how they relate to the iron-stone pods we studied is misleading and inaccurate. Here we demonstrate that the Archean iron deposits we studied have no relationship to the iron deposits studied by Lowe and Byerly. Thus, the ironstone pods we studied are relevant to understanding seawater compositions in the Archean.
1. All our studies on postulated Archean seafloor hydrothermal vents pertain only to the ironstone pods on Mendon farm. This locality excepted, those mentioned by Lowe and Byerly are from small outcrops up to ~20 km away associated with Quaternary landslide deposits and/or they occupy small topographic saddles.
2. Field relations show the Mendon farm ironstone pods are clearly part of the local Archean stratigraphy and are considerably larger than at other localities described by Lowe and Byerly. These ironstone pods have strike lengths up to 120 m, original thicknesses of 25 m, and downdip extensions ≥5 m (de Ronde et al., 1994) and thus are certainly not “slope parallel” in nature. They host large (~1 m), rounded boulders of silicified volcanic rocks (see Figure 3b of de Ronde et al., 1994), similar to those outcropping immediately below the ironstone pods, which could not have been transported to the surface by the springs postulated by Lowe and Byerly, nor is there evidence for eruption breccias. Rather, they are constructional hydrothermal mounds formed on the original Archean seafloor that have incorporated talus from nearby fault scarps. The ironstone pods are massive and are not likely to record deformational features as they occur in an area where the strain is heterogeneous and typically partitioned into zones of bounding, ultramafic-hosted (serpentinite-rich) shear zones. Our 1:5000 scale map over ~9 km2 shows the ironstone pods unit grades into Archean oxide-facies banded iron formation along-strike and is cut by a 3230 Ma porphyry (de Ronde et al., 1994). Clasts of banded iron formation and ironstone pods are found in coarse clastic units stratigraphically overlying the ironstone pods unit. The assertion by Lowe and Byerly (2003, p. 911) that they observed “no stock-work vein complex systematically related to the ironstone pods” is clearly inconsistent with the obvious quartz-hematite-goethite stockwork veins mapped by us (cf. Figures 4a, 4b of de Ronde et al., 1994).
3. The ironstone pods are dominated by specular hematite. Primary goethite is also present within ironstone pods, although it is of lesser importance and has been remobilized in places where it “appears to be late-stage” (de Ronde et al., 1994). Goethite is not a “thermally unstable…mineral” (Lowe and Byerly) but is stable to ~130 °C at 1 bar and ~175 °C at 1 kbar (Garrels and Christ, 1967). Fluid inclusion studies on Philippines geothermal systems indicate formation temperatures for goethite from ambient to 250 °C (Reyes et al., 2003), in contrast to Lowe and Byerly's assertion that goethite would not be recorded in Archean rocks exposed to overprinting temperatures of ~200 °C. Hematite, seen dominating subsurface exposures of iron-stone pods (in adits) but not seen at any other Barberton locality described by Lowe and Byerly, occurs at temperatures >240 °C in the Philippines (Reyes et al., 2003) and other geothermal systems. Together, these results are incompatible with ironstone pod formation by surface discharge of thermal waters.
4. Lowe and Byerly's suggestion that quartz veins in the ironstone pods are Archean yet the pods themselves are Quaternary is untenable. The quartz stockwork veins have a close paragenetic relationship with hematite and goethite, as is clearly shown in Figure 5 of de Ronde et al. (1994). They are intact veins that do not occur as introduced angular fragments. Vug-filled quartz is commonly intergrown with hematite while needles of hematite are prevalent within individual quartz crystals (de Ronde et al., 1994). It is difficult to reconcile the survival of delicate networks of quartz microveinlets (cf. 3d of de Ronde et al., 1994) if ironstone pods were formed by “iron oxide replacement of microquartz in the silicified Archean rocks that were its original hosts” (Lowe and Byerly, 2003, p. 910). The “coarse [Archean] quartz” of ironstone pods is highly unlikely to be spared this replacement. Lowe and Byerly state that all their study areas are “related to young iron-oxide-depositing springs,” implying recent constructional features, but this is contradictory to a replacement origin. Detailed fluid inclusion and stable isotope studies on ironstone pods quartz veins show they have precipitated from modified seawater solutions ≤220 °C, with hydrothermal end-member compositions similar to modern vent fluids (de Ronde et al., 1994).
None of the active warm springs in South Africa (n = 90; T = 25–64 °C; Fe concentrations <3.2 mg/L) appear to form deposits of the size or extent of the ironstone pods (Hoffmann, 1979). Such springs are situated along regional escarpments where meteoric water recharge is obtained from the highveld (plateaus), with gravity-driven discharge focused at the base of the escarpments (e.g., Badplaas hot springs ~45 km W of the Mendon ironstone pods). Formation of the largest ironstone pods (120 × 25 × 5 m, 85 wt% Fe2O3, 15% SiO2; de Ronde et al., 1994) from a Badplaas source (Fe 0.05 mg/L, flow rate 9.5 L/s; Hoffmann, 1979) would require 1165 km3 of focused discharge over ~4 m.y. to form this single pod. These minimum values require 100% Fe extraction efficiency from constantly flowing spring waters, which together with the ironstone pods location along a topographic high (1530 m above sea level), makes the Lowe and Byerly model of subaerial iron-depositing hot springs implausible.
We believe the small, goethite-rich ‘springs’ described by Lowe and Byerly, but not the Archean age ironstone pods, form via normal precipitation of Fe3+ from laterally moving groundwater (Thomas, 1994) and are typical of modern ferricrete deposition in tropical climates (Barberton is at ~26°S). Alternating wet and dry seasons, as seen in Barberton, and a fluctuating water table promote the formation of ferricrete, with various manifestations, such as the dripstones and terraces described by Lowe and Byerly, commonly formed (Thomas, 1994). Abundant Fe2+ in the Barberton greenstone belt provides an iron source for remobilization and deposition as ferricrete. Ferricrete is a standard response to surficial weathering and not a result of hydrothermal activity.