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!
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
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.
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