Precious metals and cenozoic volcanism in the Chilean Andes

The Cenozoic evolution of the Andean Cordillera is dominated by subaerial volcanism of calc-alkalic composition generated in a magmatic are system related to subduction of the Pacific oceanic plate along the western border of the South American continent. Granodioritic intrusives and dacitic subvolcanic bodies are comagmatic with the extrusive suite. Several compressive and extensional stages related to the evolution of the arc are identified, as well as a general eastward migration of magmatism. Normal faulting, plus block tectonism gave rise to modern physiograpic features, younger volcanic cone alignments and control of geothermal fields.

Four precious-metal occurrences - Choquelimpie (Ag-Au), La Coipa (Ag-Au), El Indio (Au-Ag-Cu), and Minas del Prado (Au-Ag) - hosted by volcanic rocks are considered as examples of the relationship between metallic mineralization and late Cenozoic volcanism.

On the basis of geological environment, hydrothermal alteration patterns and oregangue mineral assemblages, these deposits may be classified as subvolcanic, high-level, epithermal type. In the areas of precious-metal mineralization, andesitic to dacitic extrusive rocks are widespread, as well as dacitic-dioritic and granodioritic intrusions. A relationship with structures, locally of possible caldera type, is evident. Hydrothermal alteration assemblages are: advanced argillic, quartz-sericite and silicification (including chalcedonic type), with external propylitic halos. The most common ore minerals are native gold and silver, base-metal sulfides, and sulfosalts, all in a quartz-pyrite gangue. Orebodies are described mainly as vein type, but disseminations and mineralized hydrothermal breccias are also known.

The presence of these occurrences in north and central Chile, and the existence of similar mineralization in comparable geological environments in the Andes of Peru, Bolivia and Argentina, suggest the existence of a volcanogenic precious-metal province genetically related to plate convergence.