The coin randygeki posted is an extremely commonly encountered brass replica of a silver coin from the city of Gela, Sicily. You can find a dozen threads with people posting them here on this forum (just do a forum search for "Gela" and you should find them all). They were apparently originally stuck to pieces of cardboard and mass-mailed out by Readers Digest or some such bookseller, promoting a book on ancient history. The huge hole in the man-headed-bull's neck was placed there by the people that made the replicas, to allow a big blob of glue to be used to attach the coin to the card; genuine silver tertradrachms of Gela look more like the one jwharper posted, with no crater in the neck.
Some of these replicas have the "COPY" stamp very lightly applied; on others, someone's actually attempted to scrape away the word "COPY" in order to try to pass it off as genuine. Here's one
with the word "COPY" either erased or barely visible.
Frankly, unless we see actual pictures of your actual coin and it looks radically different to the brass one posted above, then we'd have to assume yours is one of these brass replicas, too.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis