Featured Exhibit: Arizona's Mineral History
Arizona’s Mineral History
The unique geology of Arizona not only gives the state some of the most gorgeous vistas in the nation, it also gave the state its copper riches.
Copper Formation: A Scientific Mystery
In the language of geologists, the “Copper Province” of the American Southwest, centered on Arizona, has few counterparts in the world, and none in North America. These deposits are called Porphyry Copper Deposits because they appear in a type of rock called “porphyries.” More than a century of mining and discovery in this region, and a century of geological study, have yet to provide a conclusive answer about the reasons for this unusual concentration of the red metal. However, geologists continue to grow and refine our understanding of the geology and origin or Arizona’s copper deposits.
UA Expert on the Case
Dr. Spencer Titley, Professor Emeritus in the UA Department of Geosciences, has spent his career trying to answer the geologic mystery of Arizona’s copper deposits. He explains what we know like this: most of the deposits formed in a narrow interval of geologic time, between 60 and 65 million years ago. They formed along a continental margin during a period when, in terms of plate tectonics, oceanic and continental plates were converging rapidly, and they are closely related to ancient volcanoes.
Previously, geologists believed that the copper came from the mantle (a hot layer of rock below the Earth’s surface crust), and mid-century ideas involved rock from the ocean crust. However, evidence from Dr. Titley’s research using isotope analysis now reveals that there is a significant contribution to the deposits from local rocks in the Earth’s crust, and that evidence points to a different, yet still uncertain, origin story.
The final story remains to be discovered. But we know that it took millions of years of geologic transformation to create Arizona’s great copper wealth, a resource that humans have mined for about 140 years.
Many of the same geologic forces that created copper ore deposits created the wondrous mineral specimens that we see in this exhibit. Water, heat, pressure, and minerals combined over millions of years to form incredible colors and crystals when chemical elements met the right conditions. Most of the minerals formed in regions where water and air from the surface could seep in.
In much of North and South America, this surface zone was scraped bare by glaciers during the last ice age, a mere 15,000 years ago, so whatever mineral specimens formed were destroyed, but the glaciers did not reach Arizona. Arizona’s underground copper mines were also crucial to the discovery of many amazing Arizona mineral specimens—if miners had not gone deep underground to dig copper ore, many of these incredible specimens would never have been discovered.