Hey Turk,
Thats right, Zinc is not a refinable metal the same as Copper or Tin. It's used for making Alloys.
I'll have to double check but, we have one list for everything you can mine, that list is also used for creating the weapons and for the customers to figure out what they want. So we might have not added an exception for Zinc. I'll look into it and see if we can find a fix if it hasnt been fixed already.