Raúl Roberto Fernández

Jurassic epithermal Au–Ag deposits of Patagonia, Argentina

Abstract Important precious metal deposits have been discovered during the last 10 years in the Deseado Massif region of Patagonia, Argentina. This region is a plateau consisting of Middle to Upper Jurassic volcanic rocks that host fracture-controlled epithermal Au–Ag mineralization. These mineral deposits represent low sulfidation type hydrothermal systems and formed following the main period of volcanism, probably during the Late Jurassic period. The presence of silica sinters and veins interpreted to be feeder structures indicates that the tops of some deposits have been preserved, with erosion of younger sedimentary and volcanic rocks now exposing these systems. These deposits represent diverse erosional levels ranging from sinter formed at the paleosurface, to intermediate Au–Ag-rich quartz veins, to base metal-bearing Au–Ag veins that represent deeper levels of the epithermal systems. The best known deposits are in the Cerro Vanguardia district where vein systems occur over a 350 km 2 area and veins can be traced for more than 14 km. Geological mapping of the district has resulted in the definition of over 140 km of veins. Current reserves are over 3.2 million ounces Au equivalent with grades of 10 g/t Au and 120 g/t Ag. The geologic characteristics of the region and the presence of major Au–Ag deposits provide targets for continuing exploration in the Deseado Massif.

Jurassic magmatism and Au-Ag mineralization in the Deseado Massif (Patagonia Argentina): Lead and sulfur isotopic studies

Lead isotope analyses of volcanic rocks and hydrothermal minerals and sulfide isotope analyses of sulfurs from Au-Ag La Josefina prospect (Deseado Massif geological province, Argentina) are reported. Lead isotope ratios for sphalerite and pyrite have mean values of 18.48, 15.69 and 38.61 for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, respectively, and are similar to Pb isotope ratios for the Jurassic volcanic rocks of Bajo Pobre and Chon Aike Formations. δ34S‰ suggesting that the sulfur at La Josefina was primarily of magmatic origin. These results are interpreted to indicate that the Jurassic volcanic rocks are closely related with low sulfidation Au-Ag occurrences.