4th Swiss Geoscience Meeting, Bern 2006 The low-sulphidation Au-Ag deposit of Rio Blanco (Ecuador): geology, mineralogy, geochronology and

The Rio Blanco Au-Ag prospect (Western Cordillera of southwestern Ecuador), is one of the most important precious metal deposits in Ecuador hosting total ore reserves of 2 Mt at average grades of 8.9 g/t Au and 68 g/t Ag respectively and bonanza grade offshoots of up to 800 g/t Au (Appleyard, 2003). The Au-Ag mineralization at Rio Blanco is hosted by the Tertiary Rio Blanco Formation consisting of volcaniclastics rocks, andesitic lavas, felsic ignimbrites and dacitic tuffs, intruded by diorite intrusions and porphyry andesitic dykes (Appleyard, 2003). In the present study we have dated two intrusions cutting the Rio Blanco Formation by the U-Pb method on zircons, which have yielded ages of 35.7 ± 0.2 Ma and 15.7 ± 0.2 Ma respectively, suggesting a protracted period of magmatism in this area. The volcanic rocks of the Rio Blanco Formation and both intrusions have andesitic to rhyodacite/dacitic compositions with calc- alkaline affinity and, in tectonomagtic discrimination diagrams, fall in the field of volcanic arc rocks. Lead isotope compositions of the volcanic rocks of the Rio Blanco Formation and of the intrusions have elevated 207Pb/204Pb values (15.62-15.70) indicating mixin