hi,the gold mine you refer to in the forest of dean was the bailey level,the chap who owned the mine at the time said that he had struck gold,and like you said came up with the shares idea,people bought shares in this "gold" mine and promptly did a runner.
I think this mine was originally an old iron mine as it is very close to a much bigger iron mine called wigpool iron mine,which as a member of the forest of dean caving club I spend a lot of time in as its my favourite mine to explore.