New early Jurassic U/Pb age in rhyolitic dykes from the Northeastern sector of Gastre (Chubut, Argentina) and its possible relation to the mineralization of the Los Manantiales district

LAGORIO, S. L.; ALICIA BUSTEROS; SILVA NIETO, DIEGO; GIACOSA, RAÚL; CLAUDIA BEATRIZ ZAFFARANA

Salta

Congreso; 15th Symposium of the International Asociation on the Genesis of Ore Deposits (IAGOD); 2018

Servicio Geologico Minero Argentino

In this presentation, both stratigraphic and geochronological results are revealed, which were obtained from cartography and regional geology studies carried out in the Gastre region. The area of study is located in the southwestern sector of the Northpatagonian Massif, in the vicinity of the Los Manantiales mining district, 12 km to the southwest of the Ángela mine, a polymetallic deposit that was active between 1979 and 1992, being, during that period, the main metal exploitation mine of Chubut.Lava, pyroclastic rocks and andesitic brecchia, as well as volcaniclastic deposits assigned to the Lonco Trapial Formation, crop out in this area. Ar/Ar data from lavas of this region indicated ages of 185.4 ± 2 Ma, 185 ± 1.6 / 184 ± 5 Ma (Zaffarana and Somoza 2012), corresponding to the lower Jurassic.These rocks are intruded by numerous WNW, NW, NE, ENE-oriented dykes. The studied dykes are of rhyolitic composition and NE orientation, as well as those from Los Manantiales mining district. They have porphyritic texture, with phenocrysts (2%) of plagioclase and alkali feldspar, with slight sericitic and clay alteration respectively, and minor biotite with zircon inclusions; the groundmass is mainly composed by quartz and feldspar, mostly spherulitic as a devitrification product, with scarce carbonate alteration. These rocks are of subalkaline nature and classify as rhyolites in the TAS diagram as well as using immobile trace element ratios. They are high potassium and peraluminous rhyolites.The results of an U/Pb zircon dating carried out in one of these dykes provide evidence of an age of 177.26 ± 0.96 Ma, corresponding to the lower Jurassic, enabling to relate this event with the Lonco Trapial magmatism, especially with its posthumous activity.In Los Manantiales district there is a clear spatial association between mineralization and rhyolitic dykes, which might have provided heat and fluids to the hydrothermal system (Varela 1994, Márquez 1999). The main mineralization could have been linked to the final stage of the process, characterized by potassium alteration and conditioned by the uprising circulation of fluids through the fractures and faults that are present in the area (Lafont et al. 2003). As regards the setting age of these intrusives, Varela (1994) claims that it might be late Cretaceous-early Tertiary, while Márquez (1999) considers that it could be Jurassic in relation to the Lonco Trapial magmatism or Miocene like the volcanism of the Piré Mahuida. Similarly, Dejonghe et al. (2002) leave the possibility open even though they are more inclined to a Jurassic genesis.Although no alteration zones associated with these dykes could be recognized neither in the study area nor at a superficial level, as can be observed in the epithermal polymetallic deposits of Los Manantiales district, several elements suggest considering the possibility that these dykes might be part of the same intrusive cycle. Apart from the marked lithological similarities, what should be noted is the fact that the studied dykes are part of a NNE- oriented belt of dykes of around 20 km long, being the dykes of the Los Manantiales district located in the northeastern sector of this belt. Therefore, the early Jurassic age obtained from the analyzed dykes allows the assumption that the genesis of the mineralization of Los Manantiales district might be closely related to the final stages of the Lonco Trapial magmatism. ReferencesDejonghe, L., Darras, B., Hughes, G., Muchez, Scoates, J., Weis, D., 2002.Isotopic and fluid-inclusion constraints on the formation of polymetallic vein deposits in the central Argentinian Patagonia. Mineralium Deposita 37: 158?172.Lafont, D., Strazzere, L., Gregori, D., 2003. Diseños y temperaturas de alteración hidrotermal en Mina Ángela, Comarca Nordpatagónica, Argentina. Revista de la Asociación Geológica Argentina 58(3): 391-402.Márquez, M.J., 1999. Los sistemas hidrotermales del distrito Los Manantiales, Chubut. In: Recursos Minerales de la República Argentina (Ed. E.O. Zappettini), Instituto de Geología y Recursos Minerales SEGEMAR, Anales 35: 1167-1175. Buenos Aires.Varela, M.E., 1994. Silicate-melt and fluid inclusions in rhyolitic dykes, Los Manantiales mining district, Argentina. European Journal of Mineralogy 6: 837-854. Zaffarana, C. B., Somoza, R., 2012. Paleomagnetism and 40Ar/39Ar dating from Lower Jurassic rocks in Gastre, central Patagonia: further data to explore tectonomagmatic events associated with Gondwana breakup. Journal of the Geological Society of London 169: 371-379.