The ore deposits of northwestern and southern Illinois were formed so
many millions of years ago that it is possible to propose only theories
of their origin. Most geologists think that the minerals, dissolved in
warm or hot water, came from deep in the earth. Perhaps the
mineral-bearing water came from, or was associated with, rocks that were
or had been molten (igneous rocks), but it may have had some other
source. Why the ores occur where we now find them is not fully known.
The cooling of the solutions and the lessening of pressure as the
solutions rose toward the surface may have had a part in ore deposition.
Faults and the nature of the rocks encountered by the depositing
solutions also appear to have had an influence.