Andes: Mountains and Minerals Geologic Notes

The Andes, the second greatest mountain chain in the world after the Himalaya, jut rapidly up from the coast.    It is a young range still in the process of being uplifted as the Nazca plate (under the Pacific)slides under the South America plate.     They contain an high concentration of peaks higher 6.000 metres, famous mountains like Chimborazo (6.272 m.) in Ecuador, Huascaran (6.768 m.) in Perù, Tocorpuri (6.873 m.) on the border between Bolivia and Chile, Acongaua (6.960 m.) on the border between Argentina and Chile.  The Andes don't stop at the coast; 100 km offshore there is an Ocean trench which is as deep as the Andes are high. Convergence between the continental South American plate and the oceanic Nazca plate gives rise to the Andes and results in the subduction or consumption of the Nazca plate.   As the Nazca plate descends beneath South America into Earth's mantle, the sediments, minerals and rocks carried downward respond to the increasing pressures and temperatures by melting.  In turn, the melt rises toward the surface and erupts in spectacular volcanoes. The South American plate, evolved starting from the collision of microcontinents:  in the South American plate, three big tectonic entities can be recognized: the South American Platform, the  Patagonic  Platform and the Mountain range of The Andes and the mountain  System of the Caribbean.

The South American Platform has a basement from the pre-Paleozoic age and is constituted by Brazil and part of Colombia. This basement is constituted by tectono-metamorphic rocks, and pre-Paleozoic and Paleozoic granitoid rocks. The sequence continues with more recent units constituted by metamorphic rocks of green schist to amphibolite facies  covered by sedimentary and volcanic rocks with some granitoids. 

One of the most profound consequences of the subduction of the Nazca plate is that it introduces hidrated minerals and water that is trapped in the oceanic crust increasing to temperatures and pressures as the slab descends.    At same point the hydrated minerals became unstable and the water is driven from them in a èprocess called dehydration.   The released water rises into the hot mantle overlying the Nazca slab.   The water dramatically lowers the melting point of peridotite, the primary mantle rock, allowing parts of the mantle to melt even though the mantle does not increase in temperature, and the subducted slabs remains cooler then its surroundings.   The water acts as a simple flux to lower the melting point of same minerals.
Melting in the mantle occurs at different pressures and temperatures for different minerals, creating magmas that are concentrated in certain elements and minerals.    As the magma approaches the surface it occasionally erupts and forms vulcanoes.   Rising magmas are the essential component for building the mineral deposits of the Andes.  Magmas do not form in closed systems; they are ever changing as material crystallizes or fluids escape.  Once magmas erupt or cool beneath the surface they are subjected to wethering and interaction with descending meteoric waters.    It is fair to say that many of today's active volcanoes in the Andes will eventually became ore deposits as the forces of nature and geological time concentrate valuable materials.   Why some magmas are rich in silver and others are rich in copper remains a subject of geological research.

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