The El Peon gold-silver deposit comprises four epithermal, low-sulfidation veins, which contain 2.9 million oz Au and 48 million oz Ag. Provisional data on alteration mineralogy, fluid inclusions, and vein textures constrain the chemistry and hydrology of the hydrothermal system responsible for ore mineralisation at El Peon. Alteration minerals include quartz, adularia, albite, illite, smectite, calcite, chlorite, and pyrite. Alteration minerals suggest the existence of near neutral pH, low salinity hydrothermal fluids, which circulated over an area greater than 18 km2. Complex mineral zonation patterns result from variations in host rock permeability and composition, overprinting alteration possibly produced by thermal waters from surficial and peripheral parts of the hydrothermal system, and supergene weathering in the hyperarid environment. Upflow zones were focused along N-S and NE trending structures, producing crustiform quartz-adularia+carbonate veins. Fluid inclusion data indicate vein deposition from low salinity (<2 wt per cent nacl equiv.), episodically boiling hydrothermal fluids, at temperatures mostly 240-250c. veins range from><50 cm to 4 m true width, exhibit a wide variety of textures, and contain variable gold, electrum, sulfides, and silver sulfosalts with variable supergene products, including iron and manganese oxides, gypsum, and rare alunite.>