White Mountains Field Trip

Our field trip in spring of 1996 was to the White Mountains in Eastern Arizona. We examined glacial and volcanic geological features, and also the general geology of the region.

Click here for our White Mountains field trip home page.

Click here for the exciting Copper Sweepstakes page.

On our way to the White Mountains on 26 Apr, we drove past the large open-pit copper mine at Morenci, which was the subject of my field trip report:

Copper - Mining Porphyry Deposits or Red Giants to Arizona to Your Pocket

Copper is a nice, shiny, copper-colored metal. It is nice because it is very electrically and thermally conductive, ductile and malleable, and resistant to corrosion. Because of these properties, it is one of the more useful elements, as opposed to, say, yttrium, or rhenium, or those boring non-descript rare earth elements. Copper is ideal for use in electrical devices, and indeed, at present over 75% of all copper is used in electronics and electrical applications.

All the copper in the world was created in the advanced nuclear burning stages of red giant stars billions of years ago. How some of it came to be part of our planet is beyond the scope of this work. How much of it ended up in Arizona would be slightly more relevant. How it comes to be dug out of the ground, separated from the surrounding rock, purified, and fashioned into nice things such as pennies and electrical wiring is bang on, so it is this last story on which we shall focus. As one chronicler of the history of copper mining said, "Whatever Power is responsible for the Universe made ore deposits; but mines are made by the genius of men."(1)

Arizona's extensive deposits of copper-bearing rock have made it the largest copper producing state in the U.S., with more production than all the other states combined. The oldest and most productive mine in the state is located at Morenci, near the New Mexico border. Copper was first mined at Morenci in 1872 (see chronology below), from the very richest 'bonanza' sources, veins or sheets near the surface. As these high-yield deposits were quickly used up, more intensive techniques had to be employed to mine the copper. The bulk of the deposits are low-grade porphyry deposits, which contain much smaller amounts (less than 2% by mass) of copper, in crystals which are distributed fairly evenly throughout the deposit, and in thin coats along fracture planes.

As early as 1893 a struggling Arizona Copper Co. had to consider mining the low-grade porphyry copper at Morenci. The term porphyry refers to igneous rock containing conspicuous crystals (phenocrysts) in a fine-grained groundmass. It is formed when a molten magma intrudes or invades existing rock, and as it solidifies, develops the porphyritic structure of relatively prominent (0.1 mm sized) crystals in a much finer-grained groundmass. This genesis is followed by a period of secondary (or supergene) enrichment, as acidic solutions leach copper and repreciptate it lower down. The result is a porphyry deposit enriched with chalcocite under a leached capping layer.

In the case of Morenci, a predominately quartz monzonite magma intruded on the regional limestone and shale to create a porphyry deposit about 1400 x 900 m and averaging 250 m thick, with chalcocite (Cu2S) grains, and pyrite (FeS2), chalcopyrite (CuFeS2), and sphalerite (FeS) veins, beneath a capping layer about 80 m thick.(2,3) Although it is a geological term, the word Porphyry is also commonly used to refer to the major copper mines, even if the ore is not porphyry, and Morenci is considered one of the original 'Porphyries'.

Morenci

The extent of a deposit which can be profitably mined is determined by boring for numerous samples, and using the assay results to delineate the region to be excavated. Obviously though, the definition of what is profitable varies from mine to mine, and changes with increasing technology and with the supply and demand for copper. As the percent yield of the remaining ores decreases, it becomes necessary to make use of lower grade ore through improvements in technology and efficiency.(2) Even the richest porphyry ores remaining today contain less than 1% copper. For this reason, economies of scale usually dictate the necessity for open pit mining, the removal of enormous quantities of overlying 'waste' rock to reach the porphyry ore and remove it in bulk quantities. As a result, Morenci and other open pit copper mines are among the largest man-made things on Earth.

Open pit mining commenced at Morenci in 1937, and large scale exploitation of the orebody began in 1942 with the construction of a new concentration facility. To profitably extract the copper from the deposit, it is necessary to remove great amounts (up to 2.5 parts) of waste rock to get to good ore. Great diesel trucks, arguably one of the most significant developments in open pit mining, are used to haul the ore out of the huge chasm, up to 190 tons per load. The mine grows deeper and wider, a gaping man-made crater marked by tiers of horizontal benches 50 feet (15.2 m) high. As one observer remarked, "Perhaps the principle impression is one of hugeness; of wonder in the capacity of mere man to make this tremendous scar on the bosom of the eternal Earth . . ."(1)

Morenci

The concentrate is then sent to a smelter, where it is roasted to drive off some of the sulfur and volatile impurities, then melted in large reverberatory furnaces. The resulting mixture of iron sulfide (FeS) and copper sulfide (Cu2S), called matte, is transferred to a Bessemer converter where the iron is slagged and the sulfur burned off, leaving 99.5% pure 'blister' copper. This is remelted and cast into large anodes, which are sent to an electrolytic refinery, where they are dissolved in solution and reprecipitated at a cathode, yielding 99.9% pure copper.

For ores rich in copper oxides (e.g. as a result of weathering), an alternate technique is leaching, percolating through sulfuric acid to dissolve the copper, after which the copper sulfate solution is directly electroplated onto cathodes. This is known as the solvent extraction / electrowinning (SX / EW) process. A new facility for this process was constructed at Morenci in 1987 to continue supplying high quality copper for decades to come.(4)

Morenci

The world's supply of new copper comes from a surprisingly few copper mines, most of them open-pit porphyry mines. Undoubtedly there exist deposits still hidden, but the overall supply is obviously limited. Fortunately, copper is one of the most easily recyclable metals, and 'used' copper can now supply over half of our copper needs. Still, the mining of copper is an impressive feat, which has undergone great technological advancements. Every effort should be made to pull all of this metal out of the earth. After all, "If it had not been for the application of brains and capital to the development of this copper, these properties would be so much worthless rock in mountains of scenic value only."(3)

A brief chronology of the Morenci mines:

1872 - Federal Mining Law; First claims at Morenci; town of Clifton founded
1874 - Detroit Copper Mining Company organized
1875 - First smelter (led to first railroad in Arizona)
1881 - Phelps, Dodge & Co. invests in new smelter for Detroit Copper
1884 - Arizona Copper Company organized
1886 - Bonanza ores exhausted; First concentrator in Arizona built
1892 - James Colquhoun appointed General Manager of a struggling Arizona Copper Co.; he institutes leaching of porphyry copper with sulfuric acid, leads to company's first profits
1897 - Phelps, Dodge & Co. buys Detroit Copper Mining Co.
1922 - Phelps, Dodge & Co. buys Arizona Copper Co. (with its Metcalf mine)
1932 - All mining temporarily halted due to the Great Depression
1937 - Phelps Dodge begins open-pit mining at Morenci
1942 - New concentrator and smelter built between Morenci and Clifton; large-scale mining begins in earnest
1948 - Molybdenum separator added
1974 - New Metcalf concentrator built
1985 - Phelps Dodge Inc. sells 15% of Morenci to Sumitomo Metal Mining, of Japan
1987 - New SX / EW plant constructed
1989 - Movable in-pit crushers and conveyors transport ore more efficiently

References:
1 A.B. Parsons, The Porphyry Coppers, 1933
2 A.B. Parsons, The Porphyry Coppers in 1956, 1956
3 F.J. Tuck, Stories of Arizona Copper Mines, 1957
4 R.R. Beard, The Primary Copper Industry of Arizona, 1985