Deposits of galena (lead) and zinc occur in fractures and bedding planes of the dolomite bedrock. The fractures formed when continental plates were compressed 350–250 million years ago. The lead and zinc formed around 250–150 million years ago when hot, mineral-rich water rose from deep within the Earth to fill the cracks.
Photos of galena and sphalerite
Lead Mining
“Travelers who have been at the upper part of the Missisipi, affirm
they have found mines there of good lead.”
—
Henri Joutel 1680s
Native American lead mines were indicated on early French maps of this
area. Nicholas Perrot visited the mines in 1690 and Pierre Charles Le Sueur
collected samples of ore in 1700. French-backed attempts to develop mines
in the area failed. In 1822, Colonel Richard M. Johnson obtained the first
lease to mine lead around Galena. At their peak in 1845, the mines around
Galena produced about 83 percent of the total United States output of lead.