Manganese Mineralization – Pilbara Region, WA

Manganese (Mn) oxidation requires free dissolved oxygen (O2) or other reactive oxygen species (ROS) in the water column, and evidence for widespread Mn oxide deposition in ancient sedimentary rocks has long been used as a proxy for atmospheric-ocean oxygenation or Mn-oxidizing photosynthesis. The oxygenation of Earth’s atmosphere and oceans across the Archean-Proterozoic boundary (~2.5-2.3 billion years ago [Ga]), caused widespread deposition of massive marine Mn deposits into the Paleoproterozoic, to approximately 1.8 Ga. The following period, through the end of the Paleoproterozoic and the Mesoproterozoic (1.8-1.0 Ga) is often regarded as a time of redox stasis, with persistently low atmospheric oxygen, and an apparent hiatus in sedimentary Mn deposition in the oceans before deposition of Mn in the Neoproterozoic linked to global glaciations. However, a model of a more dynamic and heterogeneous redox structure in the Mesoproterozoic is emerging, with evidence for at least localized or transient oxidized environments. Recently discovered massive deposits of stratiform Mn in the Mesoproterozoic rocks of the Collier Basin in the southern Pilbara region offer an excellent opportunity to understand the controls on Mn deposition during this time in earth history.

We aim to understand the drivers behind sedimentary Mn deposition during the Mesoproterozoic by studying the Mn minerals and paleoredox conditions of the oceans at the time using a combination of advanced petrographic techniques and metal isotopes.

Here, we report geochemical and mineralogical analyses from recently discovered 1.1 Ga manganiferous marine-shelf siltstones from the Bangemall Supergroup, Western Australia. Layers bearing Mn carbonate microspheres, comparable with major global Mn deposits, reveal that intense periods of sedimentary Mn deposition occurred in the late Mesoproterozoic. Iron geochemical data suggest anoxic-ferruginous seafloor conditions at the onset of Mn deposition, followed by oxic conditions in the water column as Mn deposition persisted and eventually ceased. These data imply there was spatially-widespread surface oxygenation~1.1 Ga with sufficiently oxic conditions in shelf environments to oxidize marine Mn(II). Comparable large stratiform Mn carbonate deposits also occur in ~1.4 Ga marine siltstones hosted in an older unit of the same superbasin, indicating it was a long term depocenter for sedimentary manganese. Furthermore, we demonstrate these periods of manganogenesis are greater or at least commensurate in scale (tonnage) to those that followed the major oxygenation transitions. Such a period of sedimentary manganogenesis may be inconsistent with a model of persistently low O2 throughout the Mesoproterozoic and provides robust evidence for transient oxidative surface environments in the mid to late Mesoproterozoic.

Publications/reports:

Spinks, S., Thorne, R., Sperling, E., White, A., Armstrong, J., Brant, F., leGras, M., Birchall, R., Munday, T., 2018. Sedimentary Manganese as Precursors to the Supergene Manganese Deposits of the Collier Group; Capricorn Orogen, Western Australia. Perth, Australia: CSIRO. https://doi.org/10.25919/5ba53fb6906b0

Spinks, S., Sperling, E., Thorne, R., White, A., Armstrong, J., 2018. Late Mesoproterozoic Oceanic Oxygenation caused Widespread Sedimentary Manganogenesis. In: Goldschmidt 2018 Conference; 12-17 August 2018; Boston, USA. Conference Organisers. 1p. http://hdl.handle.net/102.100.100/86618?index=1