There is not a lot of context here - it's not a complete sentence so it's hard to say. However, given what you have provided, this would be a revision: "A mixture of metals". It would simply mean that whatever the object being referred to is, is a mixture of metals. Like a mixture of steel and titanium or a mixture of brass and gold. Again, this is not a complete sentence and it lacks context so there is a limited amount of help I can provide.