Curiosity has found manganese oxides

[Green Baron]

On earth, atmospheric oxygen is involved to form them. So, applying the principle of uniformitarianism or it's modern descendants we can speculate about oxygen-rich palaeo-oceans on mars. The paper suggests that, if mars had oceans and the planet's magnetic field decayed, radiation could have split water into hydrogen and oxygen, the hydrogen dissipated into space, the oxygen was bound in oxides.

http://onlinelibrary.wiley.com/doi/10.1002/2016GL069109/abstract

Microbes are not needed for the suggested process.

 

Edited June 28, 2016 by Green Baron
's' mising ...

[kerbiloid]

According to my favourite book on this theme, the free oxygen wasn't required, too.
When the volcanic gases (mostly H2O + CO2) got cold and formed an Archaean ocean, this ocean contained high concentrations of H2CO3 (up to pH = 3..5). This acidic solution dissolved rocks, including Mn minerals.
In Proterozoic era the terrestrial climate got very cold, the solved chemicals precipitated out, and that caused the agglomerations of Au, U, Cu, polymetallic, Co, FeSO4 and FeCO3, MnO ores.
An example given by the book is Witwatersrand ore complex in South Africa,