The Andes, copper mines and volcanoes of Chile!

Chile is one of the best countries in the world for geology and metal deposits. Its elongated western margin is bounded by an earthquake-inducing, mountain-building, ore deposit-forming subduction zone. I was part of a field course run by our university that explored the northern-region of the country where the Atacama desert and some of the largest copper deposits in the world are located. In this post I will highlight some of the places I went and talk about the general geology of Chile, its ancient and active volcanic arc, and some of its largest copper deposits.

Mountains, Volcanoes and Copper:

Chile is home to the Andes, the longest mountain chain in the world that runs parallel to the western coastline of South America. The mountain chain started to form around 570 million years ago when the Pacific oceanic crust began subducting eastward, underneath South America (Jordan et al., 1983; Stern, 2004). The compressive forces of the subducting oceanic plate created volcanoes, folding and thrusting of rocks and eventually elevated the Andes mountains to the high elevation they currently reside at. This folding can be seen beautifully at the Valley of the Moon and Salar de Atacama, where are folded evaporate beds and a salt lake currently at > 2500 m above sea level (asl). The Salar de Atacama is also one of the largest modern halite (salt) systems on earth.

The active volcanism in Chile is due to the subducting oceanic plate along the western margin of the continent. This volcanic arc (with associated plutonism and volcanism) has migrated from west to east since its initiation, and thus currently the active volcanoes are at the east margin of Chile/border of Argentina.

We drove near some of these volcanoes, such as Volcan San Pedro (6145 m asl), Volcan San Pablo (6092 m asl), and Cerro Palomna (6023 m asl) in northeast Chile, and Volcan San Jose (5865 m asl) in east-central Chile. However, the Andes story isn’t as simple as pure subduction and volcanoes… they have experience transient periods of accelerated convergence rates of subduction, different angles of convergence, and areas of flat-slab subduction (likely due to the subduction of off-shore sea mounts and other buoyant features; Isacks, 1988). The areas of flat-slab subduction and absence of volcanism are where the majority of the copper and gold deposits in Chile are located.

Northern Chile has some of the largest copper (± gold ± iron) deposits in the world. These copper deposits formed from a combination of factors, but are generally associated with intrusive-extrusive magmatism and tectonic activity on the western boundary of the South American plate (Davidson, 1987; Camus, 1990; Sillitoe and Perelló, 2005).

The eastward migration of the volcanic arc (as mentioned above) has resulted in an west to east zonation of several “metallogentic” belts that developed on the margin (Sillitoe and Perelló, 2005). Belts are interpreted as reflection of changes in the tectonic setting and igneous activity during the evolution of the Andean oreogen (Skewes and Stern, 1994; Mpodozis and Perelló, 2003).

These copper deposits are known as “porphyry copper” deposits (due to their association with a porphyritic intrusions). Most of these formed during the Eocene to Miocene in Chile (~ 43 to 5 Ma; Mpodozis and Cornejo, 2012). Examples of some of the most giant porphyry copper deposits are Chuquicamata and Rio Blanco-Los Bronces. I got to visit some of these copper deposits, which are now active mines… and the sizes of these operations are absolutely incredible.

We also went to a massive magnetite (i.e. iron) mine, Los Colorados. The amount of magnetite in the rocks is so much that the dust is magentic enough to build up on a pen-magnet and show the polarity of the earth’s magnetic fields!

Final thoughts:

I was lucky to be able to participate in a course that took us into the heart of an ancient copper-rich and modern volcano-rich region of earth. The scope of the volcanoes and the amount of copper, gold and other precious metals contained within this skinny country of Chile is awe-inspiring. I only briefly touched on the metal-endowment in this blog post.

As a researcher in copper and gold porphyry deposits I could think of no other place that exemplifies the ideal end-members of our understanding of what these large copper (and sulfur) anomalies are. Chile is a beautiful country to travel and explore, and to be able to also understand the amazing metal endowment makes it all the more fascinating. Well, that’s my opinion… but maybe I’m a bit biased!

– Stephanie

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