The Early Proterozoic
1,850,000,000 years ago
(1.85 billion years ago)
Archean Terrane Collision
Shortly after the volcanic island arc collided with the northern Archean terrane a second collision occured with another Archean terrane. This second collision caused more mountain building, thrusting, folding, tilting, faulting and deformation of the rock by high temperatures and pressures. Also, granite plutons intruded into the older volcanic rocks and Archean terranes. These three terranes - the northern Archean terrane, the volcanic island arc, and the southern Archean terrane - formed the basement on which all other rocks in Wisconsin have been deposited or intruded.
Secondary mineral deposits of zinc (sphalerite), copper (chalcopyrite), gold, silver, and other minerals formed in association with the rhyolite centers of the volcanoes of the old island arc and intruded granite. Dissolved in hot fluids, these minerals were deposited by geothermal hot springs and circulating pore water. The hot fluids deposited these minerals preferential in the highly altered volcanic ash and breccias of the volcanic island arcs because of the high solubility and permeability of this rock. The mine at Ladysmith, Wisconsin is an example of this type of ore deposit.