MINERALOGY OF FE- AND MN-RICH QUATERNARY SPRING DEPOSITS: BARBERTON GREENSTONE BELT, SOUTH AFRICA
Recent field observations from the 3.5-3.2 Ga Barberton Greenstone Belt, South Africa, do not support previous conclusions that ironstone pods represent Archean seafloor hydrothermal vents. 1) The ironstone pods retain primary textures and structures and a high primary porosity in a terrain of Archean rocks that have been highly sheared and metamorphosed to lower greenschist facies. 2) They are well stratified, composed of layers dipping gently away from discharge sites and down the modern slopes. 3) They contain abundant stalactites, stalagmites, and columnar dripstone, all showing a modern sense of vertical and suggesting deposition in air, not water.
A dozen ironstone pods have been studied at three sites. Goethite is the dominant mineral in all but one, which is composed largely of hematite. Some samples contain significant gibbsite, kaolinite, todorokite (K2Mn3O12·3H2O), and lithiophorite ((Al,Li)MnO2(OH)2). All occur as finely crystalline dripstone and cavity fill. Rare “teardrops” of opal and Ce-phosphate are present. All analyzed minerals were identified based on optical and XRD characteristics. Goethite displays some variation in composition, including SiO2 to 3.70 wt% and P2O5 to 1.76 wt%. Todorokite, close to the potassic endmember, displays minor variation in composition, including Al2O3 to 1.31 wt%. Gibbsite, lithiophorite, and kaolinite display nearly ideal compositions. No evidence for deposition of quartz, carbonate, sulfate or sulfides has been found.
Mineralogy of these deposits is also inconsistent with the original interpretation of the ironstone pods as Archean submarine vent deposits. Goethite is unstable above temperatures of about 100°C. All three Fe and Mn oxyhydroxides require highly oxidizing conditions, the likely complexing by humic acids to obtain solutions, and mediation by bacteria or fungi to promote precipitation. Interaction between nearby Archean banded iron formation and modern groundwater is likely required. Both goethite and hematite of these deposits have been mined by paleolithic peoples. While these deposits have no bearing on early Earth processes, they are interesting examples of unusual reaction pathways previously reported from exotic environments such as caves, and may be important examples of biomarkers that could be used in planetary exploration.