Pilar Moreira

Linkages between the southern Patagonia Pre-Permian basements: new insights from detrital zircons U-Pb SHRIMP ages from the Cerro Negro District

The Patagonian basement rocks are dominated by Precambrian to Early Paleozoic metamorphic rocks intruded by Paleozoic granitoids. Recently discovered basement rocks in the Cerro Negro District are characterized mainly by quartz-muscovite-chlorite schists; the metamorphic grade reaches greenschist facies (biotite-garnet grade) with a regional S1 schistosity subparallel to the original sedimentary structure S0 and a secondary non-penetrative S2 foliation. New detrital zircon U-Pb geochronology shows that maximum depositional ages for detrital zircons are Devonian, ages of 379 ± 4Ma. These results suggest that the Cerro Negro basement rocks are the youngest basement in the Deseado Massif, overlapping some detrital zircon ages in the eastern Andean Metamorphic Complex in the Andean region. Most of detrital zircons are igneous in origin with a major peak around ~396Ma, provably sourced from the Devonian granitoids of the Rio Deseado Complex (El Laurel and Bahia Laura granites) and equivalent northern Patagonia granitoids (e.g. Colan Conhue and Lago Lolog granites). Secondary peaks correspond to Ordovician to Silurian ages, being the Rio Deseado Complex and La Modesta Formation (and their igneous contributors) the possible sources of the zircons. The minor oldest peaks yield Cambrian-Neoproterozoic; Mesoproterozoic and Paleoproterozoic-Archean ages, evidencing a common source from the interior of Gondwana. The results provide new insights about the relationships between the pre-Permian metasedimentary rocks of the extra-Andean and Andean region during Mid-Paleozoic ages.

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.