GEOCHEMICAL CHARACTERIZATION OF HOT SPRINGS OF THE CORDILLERA BLANCA, PERU

The Cordillera Blanca is a ~200 km long NNW trending mountain range hosting peaks between 5 and 6.7 km in elevation. This mountain range is located in a currently amagmatic reach of the Andes above a flat-slab segment of the subducting Nazca Plate. The last period of magmatic activity was the Miocene emplacement of the Cordillera Blanca batholith that creates the core of the mountain range. Bounding the western edge of the range is a ~200 km long, NNW trending and WSW dipping normal fault (Cordillera Blanca detachment). This structure records a progression of top to the west ductile shear to brittle faulting.

Hot springs issue along the trace of this fault and in the adjacent valley paralleling the fault to the east (Callejon de Huaylas). Although rock exposure is limited, valley hot springs appear to issue along steeply dipping normal faults that penetrate the hanging wall of the main detachment. Rock types along hot spring groundwater flow paths include mylonitized granodiorite in the footwall of the detachment and Mesozoic – Miocene sedimentary and volcanic units in the hanging wall. Field measurements made during July 2013 indicate spring temperatures from 17.6 – 78.3 °C, pH from 5.03 – 6.97, and specific conductivity between 723 and 23,200 μS (TDS ~362 – 11,600 ppm). The two hottest springs measured, Baños Huancarhuas (73 °C) and Baños La Pampa (78 °C), are located in the northern part of the range, issuing directly from the trace of the detachment fault. Most of the springs vigorously degas CO₂ at their source and are associated with accumulations of travertine. Major and trace element chemistry, stable isotopes of oxygen, hydrogen and carbon, and the ³He/⁴He ratio of the springs are reported. These data are used to address rock-water interaction controls on the variability in observed salinity and to identify the source of volatiles in the springs. Additionally, as the Peruvian government identified this region as a potential geothermal resource, with preliminary assessments made over a decade ago, we calculate the present-day potential temperature and depth of geothermal resources based on several chemical geoindicators.