I'm happy with it being a pond, although a little small.
Also it's not shown as a pond on any of the OS maps (although there is one two fields to the southeast) and no feature is shown there at all.
Could indicate an old dew pond (i.e. pre modern OS maps) farming or mining would match. Or a later attempt at a dew pond
However, seems a strange place to put a dew pond - on the rake/vein which a few metres away has been opencut.
There is a feature on google aerial maps but it's not quite round; and on bing maps the feature looks like an open part of the rake.
I went in a mine once.... it was dark and scary..... full of weirdos
When do I get my soapbox, I need to rant on about some b***cks