New track to large copper deposits

Aluminum in the rock could betray deeper, rare deposits

The Chino Copper Mine in New Mexico is located above one of the rare porphyry copper deposits. © Marshman / CC-by-sa 3.0
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Aluminum as a signal: Researchers have discovered a new method for finding large, low-lying copper deposits. Because these very rare but sought-after deposits betray themselves by increased aluminum content in the surrounding igneous rock. Because this elemental signal is also detectable near the surface over this occurrence, this could facilitate the detection of such occurrences in the future, according to the researchers in the journal "Nature Communications".

Metals are indispensable for our modern world. Because iron, copper or the rare earth metals are important raw materials for our technologies. But some metal raw materials are now already scarce, because the deposits are either sparse or extremely localized. One of the reasons why metal deposits in the deep sea arouse new desires.

Large deposits: rare but sought after

Although copper is not one of the acutely scarce metals, there are only a few of the world's largest porphyritic copper deposits, which are therefore worth mining. These deposits are located close to the surface and often contain hundreds of millions of tons of metal-bearing rock, which makes extraction in open-pit mining lucrative.

"Porphyry copper deposits today provide 75 percent of the world's copper, 50 percent of molybdenum and 20 percent of gold, " report Ben Williamson of the University of Exeter and his colleagues. But these large deposits are very rare and the near-surface

All occurrences are already considered as discovered. Although geologists suspect more deposits at greater depth, these are extremely difficult to find from the surface. display

Thin section of magmatic rock from a porphyry copper deposit. University of Exeter

Aluminum as an indicator

Williamson and his colleagues have now discovered a way to reveal deeper porphyry copper occurrences. In their study, they systematically compared the chemical composition of igneous rocks from locations with and without copper deposits. In addition, they conducted case studies on the copper mines of La Paloma and Los Sulphatos in Chile.

The researchers found that igneous rocks containing copper ores also have an increased aluminum content in crystals of the mineral plagioclase. The interesting thing is that this excess of aluminum can apparently be detected near the surface even when the copper ores are deeper.

Concentric displacement

The reason for this: In the volcanic phases in which the copper deposits originated, mineral water or water-rich magma briefly infiltrated into these zones of the crust, as the researchers explain. Because aluminum was bound to the rock in a slightly different way than copper, ring-shaped zones of surplus aluminum formed around the copper storehouses.

"This chemical signature can be used as an indicator in the exploration of porphyry copper deposits, " say Williamson and his colleagues. Their findings on the formation of this typical element signature also extends the knowledge of the formation of the large copper deposits. "Our findings provide important insights into why some magmas produce copper deposits rather than others, " the researchers said. (Nature Geoscience, 2016; doi: 10.1038 / ngeo2651)

(University of Exeter, 03.02.2016 - NPO)