Abstract
The La Josefina Au-Ag epithermal deposit is located in the Jurassic Macizo del Deseado volcano-sedimentary complex. In the present work mineralized vein formation fluids and dominant mineral stability conditions during their percolation have been studied. Primary fluids of aqueo-saline composition (without nitro-carbonic phases) have been identified. The earliest fluids related to the initial mineralizing stages (gold, adularia, tetrahedrite, galena and early quartz generations), with a salinity between 9 to 15 wt% NaCl equivalent and temperatures near 250°C. On the other hand, late primary fluids related to the latest quartz generations and (probably) hematite precipitation presented lower salinity ( 5.5) and of O, activity (from io -33/-34 to 10 -31/-32 ) occurred. Gold must have precipitated as a consequence of boiling process at pressures near 9 to 36 bars. Adding to the log[/H 2 //HJO]~2.9 value the type of mineralization (gold-electrum-tetrahedrite), the small sulfide abundance, presence of adularia, range of salinity and temperatures and evidences of fluid boiling indicate the La Josefina district as a low sulfidation type deposit.
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