Lead and zinc are the only metallic minerals mined in Illinois. Deposits
are found in Jo Daviess County in northwestern Illinois and in the
fluorspar area of Hardin and Pope Counties in extreme southern Illinois.
In northwestern Illinois the ore occurs in rocks of Ordovician age, but
in southeastern Illinois it is associated with fluorspar in rocks of
Mississippian age.
Between 1820 and 1865, the mining area of which northwestern Illinois is
a part was the nation’s principal producer of lead ore. In recent years
production of zinc has increased and that of lead has decreased.