Copper mining

History of the Copper Mountain (Morenci) Mining District, Greenlee County, Arizona


The Arizona Geological Survey has just published a well-written history of the Morenci, Arizona, mining district. The report was written by geologist David F. Briggs and was published as AGS Contributed report Cr-16-C. The 79-page report is available for free download:

The Copper Mountain (Morenci) mining district is located approximately 115 miles northeast of Tucson, Arizona.

Mining began in 1873. This district has produced more than 36 billion pounds of copper from 1873 to 2015. Since 1985 is has been America’s largest domestic copper producer.

The discovery of copper at Morenci during the turbulent years of the American Civil War brought new opportunities for many, but foreshadowed the end of a way of life for Native Americans, who had lived in the region for millennia. A diverse cast of characters has played a role in Morenci’s history, including veterans who ventured west after the war, as well as immigrants eager to make a new life in America.

Briggs provides an interesting narrative of the development of the district as different companies gradually consolidated the mines. Briggs breaks the history into five phases of development as the owner(s) dealt with different types of ore, changing technology, new discoveries, and the sometimes volatile copper market.

Phelps Dodge Corporation operated the district beginning in 1917. Freeport-McMoRan Copper and Gold, Inc. (renamed Freeport-McMoRan, Inc. in July 2014) acquired an 85% interest in the Morenci project through its merger with the Phelps Dodge Corporation in March 2007, and has been operating the mine since then.

The report contains many maps and both current and historical photographs. This report is an interesting read and its story is one that was similar to that of many mines in the West.



 More reports from AZGS:

AZGS field guides to Arizona Geology

A guide to the geology of the Sedona & Oak Creek Canyon area of Arizona

A Guide to the Geology of the Santa Catalina Mountains

A Guide to the Geology of Organ Pipe Cactus National Monument and the Pinacate Biosphere Reserve

A Guide to the Geology of the Flagstaff Area

A Guide to Geology of Petrified Forest National Park

A Guide to Oak Creek-Mormon Lake Graben

AZGS Guides to Northern Arizona Geology

History of the Copper Mountain (Morenci) Mining District, Greenlee County, Arizona

Rosemont’s dry-stacked tailings will be greener than those near Green Valley

Rosemont Copper is planning to use newer technology at its proposed mine in the Santa Rita Mountains south of Tucson to help mitigate the environmental impact of the mine.  One of those newer techniques is to use dry-stacked tailings as opposed to the conventional wet tailing stacks that are used at the mines near Green Valley.

Before getting into a comparison of dry versus wet stacking, I will explain what mine tailings are.  In the copper deposits south of Tucson, most of the copper is contained within sulfide minerals that usually comprise less than 5% of the rock mass.  Copper ore containing chalcopyrite, bornite, or chalcocite is mined and crushed in giant ball or roller mills.  The crushed ore is sent to the concentrator plant for flotation extraction.  The flotation mill consists of a series of tanks where the crushed ore is stirred into a water solution to make a slurry, and air bubbles are injected into the bottom of the tank. The chemistry is adjusted to gain the desired specific gravity of the solution and set the surface tension of the air bubbles.  Sulfide grains stick to the bubbles, which float to the top and are skimmed off.  Rock materials sink to the bottom and must be disposed of as tailings.


In the conventional wet tailings method, after extraction of sulfide minerals, powdered rock slurry is piped to a disposal area.  Typically, this material contains up to 60% water.

The photo shows the mines near Green Valley and their associated tailings piles. In the photo the Mission mine is at the top, the idle Twin Buttes mine in the middle, and the Sierrita mine is on the bottom.  These mines use the conventional wet tailing stacks.  You can see the blue water on the piles.

The principal advantage of wet stacking is that it is less expensive than dry stacking.  Nevertheless, there is a price to pay.  First, lots of water is tied up in the tailing ponds and in this arid climate, much is lost through evaporation.  The rock-powder slurry must be contained within dams which sometimes fail. (Worldwide, there are 2 to 5 tailings dam failure incidents per year.)  Because wet tails contain so much water, there may be seepage problems that must be addressed.  Finally, if the wet tailings are allowed to dry, there is a fugitive dust problem.  Usually reclamation must wait until the tailing pile is completed and no longer used.

In the dry stacking method, filters and vacuum presses remove more of the water, which can be recycled for process water, reused in the flotation step for instance.  Typically, dry tails contain less than 25% water.  Dry tails are transported to the tailing piles by conveyor belt or truck, then dumped and compacted.  Dry tails don’t need a containment structure and are more stable.  Dry stacked tailings are essentially immune to geotechnical “failure” and can be designed to withstand static and seismic forces.  Because there is much less water in the tails, there is less chance of seepage.  Dry stacks usually have a smaller footprint than conventional wet tailings.  Fugitive dust can occur from dry stacks also but is minimized because reclamation can commence very early in the project life.  Dust is also minimized by compaction, and use of binders or wetting agents, or agglomeration. Progressive reclamation often includes covers and re-vegetation of the tailings slopes and surface as part of the annual operating cycle.

For more details, see a report prepared for Rosemont by AMEC Earth & Environmental, Inc here.

The Mission and Sierrita mines used the best technology available at the time those mines were designed, but now, Rosemont has a chance to do it better.

For more mining stories see:

Pima County versus Rosemont

Rare Earths Resources in the US

Sierrita Mine is only U.S. source of Rhenium

Uranium mining and its potential impact on Colorado River water

Mr. Grijalva, why imposing royalties on hard rock mining is a bad idea

Oracle Ridge Mine on Mount Lemmon

Florence Copper another mining controversy

Arizona may become a major producer of potash

Florence Copper another mining controversy

It has long been known that an oxide copper deposit lies buried just across the Gila River from the Town of Florence, Arizona. Now, Curis Resources, a Canadian company, proposes to extract the copper, not by digging a mine, but by in-situ leaching. This process involves pumping a weak acid solution (about the same acidity as vinegar) into the ore zone to dissolve the copper minerals and recovering the solution with extraction wells. The copper-laden solution will then go through a solvent extraction/electrowinning (SX/EW) process to produce metallic copper.

The map below shows the location of the project and the surrounding countryside. Notice that the immediate surrounding land is either desert or agricultural.


The mining project is being opposed, not by environmentalists, but by real estate developers. The developers want to do this:


The Real Estate developers are concerned that the mining operation will detract from the desirability of their plans. They also allege that water quality will be impacted. The Real Estate developers have formed a group called “Protect Our Water Our Future.” See that link for their side of the story.

Curis Resources claims that because the copper deposit occurs below a clay layer, the aquifer will be protected and the acidic copper-laden solutions will be contained. That is according to an extensive hydrological study. Also they will have a network of monitoring wells.

Curis Resources says that “Over a 17-year life-of-project, the Florence Copper Project has the potential to directly and indirectly support 2,090 jobs in the United States, including: 250 jobs in the Town of Florence; 370 jobs in Pinal County; and 1,090 jobs in the State of Arizona. Personal income earned by Florence area residents as a result of the Florence Copper Project is expected to total more than $106 million over the life of the project.” Curis goes on to say, “direct and indirect economic contributions will total more than $290 million. This includes $136 million in local government revenues, $47.6 million in local business revenues and more than $106 million in personal income.”

To see the Curis Resources side of the story visit their main website here.

Among the Curis data is a community presentation which includes how the area will look during operations and after operations, the economic impact, and the water concerns, see here.