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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 ofaqueo_-s¡line composition (without nitro-carbonic phases) have been identified. The eailiest fluids related to the initial mineralizin! stages (gold, adularia, tetrahedrite, galena and early quartz genelations), with a salinity between 9 to l5 wtVo NaCl equivalent and temperatures near 250"C. On the othel hand, late primary fluids related to the latest quartz generations and (probably) hematite precipitation presented lower salinity (<6wt7oNaCl.çquivalenl).qr:d temperatures near 200"C. During the vein formation, an oscillation of pH (between 4.5 and >5.5) and of O, activity (from l0'rr/''* to l0 rtrr¿) occurred. Gold must have precipitated as a consequence ofboiling proðess at pr€ssures near 9 to 36 bars. Adding to the loglJHr/ft7rOl-2.9 value the type of mineralization (gold-electrum{etrahedrite), the small sulfide abundance, presence of adularia, range of salinity ancl temperatures and evidences of fluid boiling indicate the La Josefina district as a low sulfidation type deposit.