Hot springs in the Peruvian Andes

March 19 “Hot springs in the Peruvian Andes – deep fluids in young mountains & possible analog for Wyoming’s mountains and basins”, Presented by Dennis Newell, Utah State University.

 The mountains and intervening basins of Wyoming were formed during the “Laramide” event, some ~65-45 million years ago, and are commonly believed to be a result of flat-slab subduction of the Farallon plate. The best modern analog for this type of plate tectonics is along the Peru segment of the Andean (South American) subduction zone. Study of this modern Andean flat-slab subduction zone may well inform our understanding of the Laramide features in Wyoming.

Hot springs in continental arc settings provide a direct window into the source and chemistry of deeply-ciculated “geofluids” in subductions zones. These groundwaters and volatiles play a primary role in modifying the chemistry and strength of the lithosphere, the generation of magmatism, and the distribution of geothermal resources and ore deposits. Much of what we know about these fluids is from research on active volcanic arcs, but little is known from flat-slab subduction segments lacking active magmatism, where the subducting oceanic plate is in contact with the overriding continental plate.

The Peruvian segment of the Andean subduction zone is an ideal location to document contemporaneous mantle-to-crust migration of fluids in a flat-slab subduction setting compared to an adjacent active volcanic arc. Hot spring water, gas, and isotope geochemistry are presented from a traverse from the flat-slab subduction segment in north-central Peru, across the transition from flat-slab subduction to the magmatically active part of the arc in southern Peru. Regions investigated include the Altiplano region of southern Peru, the Cordillera Blanca, and Cordillera Huayhuash mountain ranges of central Peru. Springs investigated range from 17 to 89 °C, have slightly acidic to neutral pH, have a wide range of salinity from fresh-water to brackish, carry high-levels of dissolved CO2, and contain dissolved arsenic up to 23 ppm (2300 times the drinking water standard). Helium and carbon isotope tracers detect the presence of mantle-derived volatiles in deeply-circulated geofluids.

An understanding of the flat-slab subduction in the Andes and its associated fluid regime and associated mineralization may help to inform our understanding of the distribution of mineral deposits in Wyoming, both known and as yet unknown, and in other areas of past and current flat-slab subduction.