THE HYDROTHERMAL SYSTEM OF VOLCAN PURACE, COLOMBIA

This paper presents chemical and isotopic data for thermal waters, gas es and S deposits from Volcan Purace (summit elevation approximately 4 600 m) in SW Colombia. Hot gas discharges from fumaroles in and around the summit crater, and thermal waters discharge from three areas on i ts flanks. The waters from all areas have deltaD values of -75 +/- 1, indicating a single recharge area at high elevation on the volcano. Ai r-corrected values of He-3/He-4 in thermal waters range from 3.8 to 6. 7 R(A), and approach those for crater fumarole gas (6.1-7.1 R(A)), ind icating widespread addition of magmatic volatiles. An economic S depos it (El Vinagre) is being mined in the Rio Vinagre fault zone at 3600 m elevation. Sulfur isotopic data are consistent with a magmatic origin for S species in thermal waters and gases, and for the S ore deposit. Isotopic equilibration between S species may have occurred at 220 +/- 40-degrees-C, which overlaps possible equilibration temperatures (170 +/- 40-degrees-C) determined by a variety of other geothermometers fo r neutral thermal waters. Apparent CH4-CO2 equilibration temperatures for gases from thermal springs (400 +/- 50-degrees-C) and crater fumar oles (520 +/- 60-degrees-C) reflect higher temperatures deeper in the system. Hot magmatic gas ascending through the Rio Vinagre fault zone is though to have precipitated S and generated thermal waters by inter action with descending meteoric waters.