CONICET | Buscador de Institutos y Recursos Humanos

The Fuegian Andes is that portion of the Andean Cordillera south of 50°S. It forms an arcuate orogenic belt, which extends parallel to the continental margin for some 1,000 km. Rocks exposed in this área represent deep-marine volcano-tectonic rift basin environments formed during Gondwana supercontinent fragmentation.The aim of this contribution is to characterize sedimentar/ units in which some massive sulphides are hosted and define their post-depositional evolution in a context of changing tectonic regimes during Jurassic and Cretaceous times. For this study we have singled out the stratigraphicprofíles described by Olivero and Martinioni (1996), which include the main sedimentary facies of the Lemaire and Yahgán formations throughout Isla Grande de Tierra del Fuego. Representative samples of each facies were selected to perform mineralogical analyses of the whole sample and their silt and clay fractions by means of X-ray diffraction (XRD).Whole rock mineralogy of the Lemaire Formation is marked by quartz, feldspars, muscovite and rare calcite, while the Yahgán Formation is composed of quartz, feldspars, chlorite and rare calcite. There is, therefore, a mineralogical change between these formations, chlorite replacing muscovite as the dominant phyllosilicate. This observation confírms the geological differences observed by Olivero et al. (2001, and authors therein).The crystallinity Índex valúes of the phyllosilicates range from 0.16 to 0.22, suggesting that studied samples reached epizone (greenschist facies), in agreement with the observations made by Olivero et al. (2001).This study shows that these sedimentary units host mineral deposits that suffered low-grade metamorphism. Likewise, the observed mineralogical change may be used as a control in cartography in those áreas still under debate.