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Abstract. The Portovelo epithermal vein-system in southwestern Ecuador has produced more than 120 tons of gold and about 250 tons of silver. The veins result from hydrothermal processes close to a Miocene volcano which produced an andesitic to dacitic sequence followed by collapse and post-collapse rhyolitic activity which generated most of the alteration and mineralization. Three main structural segments are defined by NW-trending strike-slip faults, which show later stages of vertical movement. These faults are responsible for development of an extensive N-S dilatational jog within andesitic rocks, which acted as the main host to ore-deposition. A largescale propylitic aureole surrounds a quartz-chloritesericite-adularia core, centered on the rhyolites, within a system of collapse-related ring-structures. A quartzchlorite-sericite-adularia-calcite assemblage is the most common wall-rock alteration close to the veins. The size (4 x 15 km) and vertical range (1400m) of the vein-system is exceptional. Alteration, textures and mineral assemblage, including a quartz-calcite gangue, sulfides, abundant sulfosalts and free gold (electrum), are quite typical of an adularia-sericite epithermal deposit. Spatially, the mineralization is arranged in three zones. In addition, three successive stages can be distinguished. The bulk of economic mineralization was deposited during the second stage, in association with a clear quartz and calcite gangue.Tm-ice and Tm-ctath data of fluid inclusions in the clear quartz indicate a high salinity (--~10.5eq.wt% NaC1). The homogenization temperatures of fluid inclusions in the gangue minerals and in the altered host-rocks vary between 180 ° and 310°C. Quartz 6~s O-values from hydrothermally altered wall-rocks reflect the original isotopic values of the latter. These values show a narrower range in vein quartz (3 018 between + 7.7~o and + 11.5~/oo SMOW). In addition, the 60 18 values of the vein quartz increase systematically with decreasing homogenization temperature. This suggests that quartz was in equilibriumCorrespondence to: F. Van Thournout with a large reservoir of water of constant 1 SO/16 0 composition at decreasing equilibrium temperatures. The estimated isotopic composition of the fluids from which milky quartz and calcite of the main mineralization stage precipitated, lies around -1% SMOW. This value indicates a meteoric rather than a magmatic origin of the ambient fluid. Clear quartz of the second stage seems to have formed from a fluid with a &180 of +3%0 SMOW. This higher value can be due to a more intense water-rock interaction or to mixing of meteoric with magmatic water.

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