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Δευτέρα, 10 Σεπτεμβρίου 2012

Arizona Green Valley copper Mines


This astronaut photograph illustrates three open-pit mines located west-northwest of the town of Green Valley, Arizona. While the mines appear to be close to each other, each exploits a separate porphyry copper deposit.

Green Valley is twenty miles (32 km) south of Tucson and 40 miles (64 km) north of Nogales, Sonora, Mexico. Surrounded by copper mines, and near the hiking and birding areas of the Santa Rita Mountains, Green Valley is now an unincorporated retirement community composed of 59 Homeowner Associations.
Copper mines and deposits stretch 20 miles from Green Valley to near Tucson, Arizona. There are just two known original deposits, but one of them got spread out like a fanned deck of cards. From south to north, these mines are Sierrita/Esperanza, Twin Buttes, Mission/Pima, San Xavier, and Saginaw Hill. The deposits were emplaced between 64- and 55 million years ago

The largest of the mines are the Sierrita Mine owned by Phelps Dodge and the Mission Mine of ASARCO.

The mine pits are recognizable by the concentric lines of benches cut into the pit sides. The benches allow equipment and personnel access to the fresh ore (gray) exposed at the bottom of the excavation. Water may also pool at the bottom of inactive pits, such as in the Twin Buttes Mine at image upper right (black areas). The open pit areas are surrounded by an array of sculpted tailings ponds and mine dump areas; these receive mine waste rock for storage and later leaching for further recovery of metals. The green color of the water in the tailings pond at image right is likely due to the presence of leached metals.

The Asarco-Mission complex (first image in the  left) is an active producer of copper and molybdenum (important in making steel and other metal alloys), processing approximately 53,700 tons of ore per day (as of 2008).

 The Twin Buttes Mine (first image top right), also a producer of copper and molybdenum, was closed in 1994 but was recently purchased by a new owner (Freeport-McMoRan Copper & Gold) and may reopen.

The Sierrita Mine at image lower right (also owned by Freeport-McMoRan Copper & Gold) produces copper, molybdenum, and the rare metal rhenium. Rhenium is used to make high-temperature alloys for jet engine parts and in high-octane, lead-free gasoline. This mine can process over 115,000 tons of ore per day (as of 2007).

The current understanding of the geology proposes that the Mission-Pima mine, 7 miles north of Twin Buttes, the San Xavier orebody, and even Saginaw Hill mineralization, some 20 miles north of Twin Buttes, were all originally part of the Twin Buttes deposit. A large, low-angle fault, the San Xavier fault, transported slices of Twin Buttes northward about 28 million years ago. Drilling has confirmed the existence of the fault beneath the Mission-Pima mine.

[by Tzeferis Peter]


History of mining in Green Valley area