HIGH-TEMPERATURE HYDROTHERMAL ACTIVITY BENEATH THE CERRO ESCORIAL VOLCANO, NW ARGENTINA, AS SAMPLED BY AN EXPLOSIVE VOLCANIC ERUPTION

Cerro Escorial forms part of a 15 km-long NW-trending chain of Late Miocene to Quaternary volcanoes known as Corrida de Cori, which forms the border between Chile and Argentina near 25 degrees S. The chain is extensively fumarolically altered to gypsum and native sulfur, and sulfur was mined at Mina Julia until about 20 years ago. Recently active fumarole fields, mud vents, and fresh Strombolian-style volcanic deposits occur on the upper surfaces of the range, attesting to continued activity of the system.

A small (<=0.6 km3) dacitic ignimbrite was erupted from the Cerro Escorial vent at 0.46 +/- 0.01 Ma, followed by extrusion of a viscous andesitic flow at 0.34 +/- 0.03 Ma. Along with pumice blocks, the poorly-welded ignimbrite contains a variety of lithic clasts, including basement granite, supracrustal sedimentary rocks, and fragments of the volcanic edifice. In addition, pebble-sized clasts of quartz were found in the deposit. Some of the clasts consist of poly-crystalline aggregates, but most are single crystals with a form suggestive of hydrothermal vein growth. Primary and pseudosecondary fluid inclusions contained within these clasts are of three types: (1) dark, vapor-rich inclusions; (2) highly concentrated aqueous brines with a high proportion of daughter minerals; (3) internally decrepitated inclusions. Preliminary microthermometric investigations of the brine inclusions show only partial dissolution of daughter minerals by 600 degrees C, suggesting entrapment under two-phase (liquid plus vapor) magmatic-hydrothermal conditions. The internally decrepitated inclusions provide evidence for rapid depressurization, likely during eruption from depth. The quartz clasts and fluid inclusions thus appear to represent “quenched” samples of a deep-seated magmatic-hydrothermal system, perhaps analogous to fossil hydrothermal systems exposed in porphyry-type ore deposits. Unlike fossil systems, however, these samples are pristine and have not been overprinted by later, lower-temperature hydrothermal events. Further detailed study should elucidate early high-temperature magmatic-hydrothermal processes in such systems.