In reply to Russell Lovett:
This is probably Bog oil or Leptothrix discophora.
https://en.wikipedia.org/wiki/Leptothrix_discophora
It looks like oil flowing from deposits of peat. It is microbial life that 'breathes' iron. The microbes embed themselves in an iridescent film which they manufacture themselves, this is the film that looks like an oil slick, they then proceed to use the iron to maintain their life functions. The colours seen in the 'slick' are a trick of the light being refracted from the surface. You should see it 'shatter' if you run your finger through it whereas an oil slick will flow back together.
Reference: Mike Raine: Nature of Snowdonia.
Post edited at 09:35