CONICET | Buscador de Institutos y Recursos Humanos

Valle Hermoso district (35ºS) is located in the Central Andes and it belongs to a ~200 km-long and 20 km-wide belt (34°S-36°S) which includes volcanic-related Fe, Fe-Cu and Cu-(Ag) mineral deposits and hydrothermal alteration areas.Since 2005, the Instituto de Geología y Recursos Minerales (IGRM) from SEGEMAR has been conducting geological and metalogenetic studies in the 34º-35º20`Andean segment in order to identify and characterize the mineral resources. The main goal is to discriminate among the different magmatic events, linking the hydrothermal alteration and mineralization pulses during Mesozoic and Neogene times (Crosta and Palacio, 2015; Sruoga, et al., 2008; 2016).The study area is located along the río del Cobre, between the río Tordillo and the arroyo Santa Helena. The local stratigraphic record includes sedimentary and volcaniclastic/ volcanic rocks of Middle Jurassic to Middle Cretaceous ages and Neogene volcanic/subvolcanic sequences.Along the eastern margin of the río del Cobre, the so-called en manto Cu deposits (Adriana, Victoria, Sybil, Anita y El Guanaco) are related to the Río Damas Formation, of Kimmeridgian age and widespread distribution in Chile. The chalcocite, chalcopyrite, bornite and pyrite mineralization is hosted in veins, small pockets and vesicle infills in highly-porphyritic andesitic sills and chloritized volcanic breccias (Crosta and Palacio, 2015). Tentatively, these Cu mineral deposits are spatially and genetically related to the Upper Jurassic calcalkaline volcanism.Along the western margin of the río del Cobre, a large-volume subvolcanic body, dioritic in composition and largely affected by calco-sodic hydrothermal alteration, has been recognized for the first time. In the south, it intrudes gypsum beds from the Yeso Auquilco Formation, whereas in the north, sedimentary rocks from the Lotena Formation are intruded. Petrographically, the diorites vary in texture from coarsely granular to slightly porphyritic and they contain Plagioclase and Clinopyroxene, with titanite and apatite as accesory phases. U-Pb dating on zircons from a fresh diorite has been performed and an age of 13.17±0.25 has been obtained.The actinolytic type hydrothermal alteration is well developed on the east slope hill, on the top there is quartz-pyrite±calcite alteration with abundant Fe oxides. Both types of hydrothermal alteration were identified in the ASTER satellite images and is markedly visible in the field.It has been characterized through mapping, petrography, X-ray diffraction and fluorescence and SWIR spectroscopy in order to determine the mineral assemblages.The actinolite ± apatite ± pyrite alteration occurs in net contacts massive veins. These veins are north east orientated, the thickness varies between 0.2 and 0.6 m. It also presents pervasive distribution where the dioritic mafic minerals were replaced by actinolite. This alteration was overprinted by chlorite+epidote+calcite. In some actinolitic veins it was observed that the calcite had formed parallel veins systems. The potassic juxtaposed alteration is suggested by the presence of biotite.This new alteration area is very interesting and is interpreted as a possible IOCG system with an extended sodic calcic (actinolite+ apatite + pyrite) alteration. Over that alteration there is a phyllic alteration (quartz+ pyrite) and a possible potassic alteration.A similar model was previously proposed for Las Choicas deposit (Franchini et al., 2007), located 22 km north of the study area. Based on the Neogene volcanic stratigraphy reported elsewhere (Sruoga et al., 2008, 2016), the radiometric data presented here points out to this magmatic event as unique, in this Andean segment.Near-future studies will allow to better understand the magma sequence and to refine the metalogenetic model, as well as the relationship between Las Choicas and Valle Hermoso districts

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