J.S. Mason - Central Wales Orefield: Framboidal Pyrite - "Mineralised Bacteria"?

What is framboidal pyrite? That is a question that has stretched great minds for many decades. Paul Ramdohr used to describe framboids as "mineralised bacteria", which I am inclined to sympathise with. Look at these knobbly clumps, clustered together - they look like a colony of something!

Framboidal pyrite is common in Central Wales, particularly in pelagic sediments such as the black shales of the Cwmere Formation. Framboids run up to about 1mm in size - mostly much smaller, and share their host rock with pyritised graptolites and, less commonly, orthocones. In other words, they have formed in an environment where all organic matter has been pyritised, in some cases only shortly after burial, so that they are still preserved three-dimensionally.

Pyrite framboids were at one time thought, in the case of Central Wales, to be an effect of the vein mineralisation but having looked at black shale sequences both proximal and distal to veins, I cannot share that view. They are everywhere! But this is epigenetic vein country, not black shale-hosted sedex country, where the picture may be rather different.