Is it not the case that there are so many sites in Cornwall which were worked at different times either for tin or copper, both in a major way, that a mineral type of tin/copper would be the obvious answer to this problem? I can't think of another example of this. There are very few sites where arsenic was worked on equal terms with another product, and the question of lead/silver is answered by the question "what was the mine opened to extract?" Was it lead, and they then discovered good silver content, or was it silver primarily such as the Devon mines? Most lead/silver sites were probably lead mines first and foremost, with one or two exceptions. Tin/copper sticks out as the only exception, like a sore thumb.