NEUTRON COMPUTED TOMOGRAPHY AND DESCRIPTION OF MOLAR TOOTH STRUCTURES IN THE NEOARCHEAN BOOMPLAAS AND MONTEVILLE FORMATIONS, AND THE PALEOPROTEROZOIC HOTAZEL IRON-FORMATION, SOUTH AFRICA

Molar tooth structures (MTS) are crumpled veins of CaCO3 microspar that formed in fine grained host rocks during early diagenesis, mostly in Meso- and Neoproterozoic time. However, MTS are present in South Africa in two Neoarchean shales, and MTS-like veins occur in Paleoproterozoic iron-formation. MTS are present in Agouron core GKP-1 through the ~2.6 Ga Boomplaas Formation, and in outcrop exposure of the slightly younger Monteville Formation. In these examples, MTS formed in shale host rocks that are intercalated with wave and current cross-stratified grainstone lenses. MTS are filled with microspar and varying proportions of pyrite. MTS-like veins also occur in core through the ~ 2.2 Ga Hotazel Formation. Host rocks are carbonate-rich, granular facies BIF that were deposited above wave base. MTS-like veins are filled with calcite microspar, coarser calcite rhombs, and pyrite. Ptygmatic folding caused tension gashes near fold hinges in the vein, and the surrounding iron formation is differentially compacted around the vein. Veins are interpreted as MTS because they are sharp walled, filled with microspar and other authigenic minerals, and formed during early diagenesis.

These examples are consistent with our proposed model for MTS formation: (1) deposition of muds and sand lenses; (2) impermeable seal formation over the sediment pile and diagenetic evolution of pore fluids; (3) formation of interconnected network of MTS cracks, open to pores in sand lenses; (4) removal of sediment seal by scour followed by rapid wave-induced mixing of seawater with pore fluids, causing nucleation, Ostwald Ripening, and growth of microspar cores; (5) compaction of the sediment pile, plastically deforming MTS (6) polygonal overgrowth of microspar cores; and (7) brittle deformation during additional compaction. A key requirement of this model is that the MTS cracks are sufficiently interconnected to allow rapid mixing of seawater and pore waters. This interconnectivity has been demonstrated for several samples of Monteville MTS that have been imaged using neutron computed tomography to create 3D images of MTS and host rock. The calcite MTS are distinct from host rock in the images because they attenuate fewer neutrons than shale. These images show that MTS are interconnected in 3D and form an anastomizing network as required by the model.