Discrimination settings for lead minie balls

I have a Vaquero. I just picked up a self-published book from a local historian that shows there were about 5 civil war camps and forts within a mile of my house (all around). In fact, Fort Weed and Fort Lyon were two blocks away. There was almost no combat, but the magazine at Fort Lyon blew up in 1863 killing several soldiers, and prompting Abraham Lincoln to come inspect the damage.

In any case, there is every possibility of finding relics in my yard. I don't want to run in all-metal mode, but how close is lead to iron on the descriminate dial?

Thanks,

-- Tom