Geological Explanation of Their Origin
Geologists agree that the Klerksdorp spheres originated as concretions, which formed in volcanic sediments, ash, or both, after they accumulated 3.0 billion years ago. Heinrich argues that the wollastonite nodules formed by the metamorphism of carbonate concretions in the presence of silica-rich fluids generated during the metamorphism of the volcanic deposits containing them into pyrophyllite. It was also argued that the hematite nodules represent pyrite concretions oxidized by weathering of near surface pyrophyllite deposits. Below the near-surface zone of weathering, which has developed in the pyrophyllite, pyrite concretions are unaffected by weathering and, thus, have not been altered to hematite. The radial internal structure of these objects is a pseudomorph after the original crystalline structure of the original carbonate or pyrite concretion.
Both Cairncross and Heinrich argue that the grooves exhibited by these concretions are natural in origin. As proposed by Cairncross, the grooves represent fine-grained laminations within which the concretions grew. The growth of the concretions within the plane of the finer-grained laminations was inhibited because of the lesser permeability and porosity of finer-grained sediments relative to the surrounding sediments. Faint internal lamina, which corresponds to exterior groove, can be seen in cut specimens. A similar process in coarser-grained sediments created the latitudinal ridges and grooves exhibited by innumerable iron oxide concretions found within the Navajo Sandstone of southern Utah called "Moqui Marbles". Latitudinal grooves are also found on carbonate concretions found in Schoharie County, New York. The latitudinal ridges and grooves of the Moqui marbles are more pronounced and irregular than seen in the Klerksdorp (Ottosdal) concretions because they formed in sand that was more permeable than the fine-grained volcanic material in which the Klerksdorp (Ottosdal) concretions grew.
Very similar concretions have been found within strata, as old as 2.7 to 2.8 billion years, comprising part of the Hamersley Group of Australia. The Australian concretions and the Klerksdorp spheres are among the oldest known examples of concretions created by microbial activity during the diagenesis of sediments
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