Almost all of the copper mined in Arizona comes from porphyry-copper deposits, which are usually associated with intrusive igneous rocks similar to granite. The term porphyry describes the typical texture of these rocks, in which individual mineral grains about a tenth to a half inch in size are surrounded by smaller grains that are barely visible to the naked eye. These deposits, which commonly contain molybdenum and silver as co-products, were formed from saline, metal-bearing fluid that was expelled from cooling magma. Interaction of this fluid and heated groundwater with rocks adjacent to the intrusion caused many cubic miles of rock to be altered chemically. At some localities ore was formed when the hot, metal-bearing fluids reacted with limestone.
All of this intrusive activity took place deep below the land surface. The rich metal deposits were subsequently exposed at the surface millions of years later by weathering and erosion processes. Weathering processes further concentrated the copper. This "secondary enrichment" occurred when pyrite (iron sulfide) oxidized, dissolved in rainwater, and formed an acidic iron sulfate solution that dissolved the main copper-ore mineral, chalcopyrite (copper-iron sulfide). The dissolved copper was redeposited, principally as the mineral chalcocite (copper sulfide). Bacteria may play an important role in both the dissolution and redeposition of the copper.
A second major type of copper deposit in Arizona formed in Precambrian-age volcanic rocks such as those in the United Verde Mine at Jerome. Hot, metal-bearing liquid emerged from springs associated with ancient submarine volcanoes. When this liquid mixed with cold sea water, metallic minerals came out of the solution as tiny particles that fell to the sea floor. If a metal-rich hot spring was active for a long time, a thick layer of metal-rich mud accumulated around the spring. These deposits contained copper, lead, zinc, silver, and gold. Modern "black smokers," hot springs along sea-floor spreading centers, are small-scale analogs of these ancient deposits.
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Photos above by Lee Allison, Director of AZGS and Arizona State Geologist.