Arizona's Copper History

Pure copper, element #29 on the periodic table.
PC: Jonathan Zander 2009, used under the Creative
Commons license CC BY-SA 3.0.

One of the most important exports of Arizona is the element copper. As of 2007, Arizona was the leading copper-producing state in the U.S. and accounted for 60% of the total for the entire country. According to the University of Arizona's Geological Survey, prospecting and mining in Arizona begin as far back as 1583, though their targets were gold and silver rather than copper. In the 1850s, shortly after the land that became Arizona was acquired by the U.S. in the signing of the Treaty of Guadalupe Hidalgo, hardrock mining in the area helped build up the local economies, towns, and cities, many of which still exist today. By the time that Arizona became a state in 1912, "there were 445 active mines, 72 concentrating facilities, and 11 smelters with a gross value of nearly $67 million," an amount equivalent to $1.76 billion in 2018 dollars. Not too shabby for the 29th element!

Copper mining is part of Arizona's recent history, and its roots in the community stretch deep. In November 2018, the magma smelter of Resolution Copper in Superior, AZ was demolished as part of a larger effort to reduce the environmental impact of mining and to remove toxins from the area. The smelter is named after the old Magma Copper Co., which began mining the area around Superior in 1911. The smelter was opened in 1924 and immediately got to melting down ore to extract the valuable copper. In 1971 Magma Copper started to use another smelter, and this one was left in honor of its standing as a literal pillar of the community. Eventually it became too dangerous and unstable to keep it, and the choice was made to demolish it safely. It crashed down on November 10, 2018.

PC: Mark Henle/The Republic, c/o the Arizona Republic.

On a larger scale, what is it about Arizona that makes it so wonderful for copper mining? To answer this I first need to explain where the copper comes from. Copper needs to be extracted from copper-rich ores, or the natural materials that contains valuable metals or minerals. Copper-bearing rocks usually have a blue or green hue. When the copper-rich rocks interact with weathering agents such as water and Earth's atmosphere, the copper reacts with the other chemicals and can form other minerals. For example, if there is a lot of carbonate (a salt component of carbon and oxygen) in the water that touches the rocks, then other famous Arizona minerals such as malachite and azurite can form. Chrysocolla is another ore of copper. Though it is generally thought of as a phosphate-rich mineral, turquoise also contains copper and is found in Arizona and New Mexico.

The rocks that are richest in copper in Arizona are igneous rocks, meaning they formed either when a volcano erupted and deposited lava or ash on Earth's surface, or deep under the ground in a magma chamber. The rocks around Superior are a type of volcanic rock called tuff (specifically the Apache Leap Tuff). Tuff is formed from solidified ash that erupted from a vent during a volcanic eruption. The Apache Leap tuff is estimated to have formed 20 million years ago, plenty of time to interact with groundwater and form copper-rich minerals. Other volcanic rocks in the Superior area are rhyolite, a silica-rich igneous rock, and porphyry, a general term for rocks that have large crystals in them.

The copper rich minerals azurite (blue) and malachite
(green). PC: BYU Geology Dept.

Eastern and southeastern Arizona between 55 and 5 million years ago was an incredibly volcanically active place. A major geologic event called orogeny, or mountain building, was developing as the ancient Farallon tectonic plate was slowly moving underneath the larger North American plate. This subduction led to major eruptions that spread enormous amounts of ash all over what later became Arizona. The ash eventually solidified and became tuff, a rock we're already familiar with. These tuff rocks formed large mountain ranges like the Superstitions, Galiuros, Chiricahuas and Tumacacoris. This tuff reacted with groundwater and formed the copper-bearing minerals and ores that are mined today.

All of the volcanic deposits in Arizona make it a rich environment for copper mining. The unique geology here is the reason for why this state is the most copper-rich in the country, which has important economic implications for the state's continued success.
From a science perspective, studying and understanding the environments where certain minerals form is an important step in learning more about Earth's history.

Resources: 
http://azgs.arizona.edu/minerals/mining-arizona


https://www.azcentral.com/story/news/local/pinal/2018/11/10/cheers-tears-historic-copper-smelter-superior-demolished/1808363002/


USGS Geologic map of the Superior, AZ quadrangle: https://ngmdb.usgs.gov/Prodesc/proddesc_2120.htm

https://en.wikipedia.org/wiki/Copper_mining_in_Arizona