APATITE (U-Th)/HE THERMOCHRONOLOGY IN THE CENTRAL ANDES (31°30?S), MAIN CORDILLERA SAN JUAN, ARGENTINA: IMPLICATIONS FOR PORPHYRY TYPE Cu (Au) MINERALIZATION

LAURA MAYDAGÁN; MASSIMILIANO ZATTIN; MARTA FRANCHINI

Salta

Simposio; 15TH QUADRENNIAL IAGOD INTERNATIONAL ASSOCIATION ON THE GENESIS OF ORE DEPOSITS SYMPOSIUM; 2018

SEGEMAR, AGA

The study region (31°30 S, 70° 15 W) is located in the southwestern sector of San Juan Province (Argentina), inthe southern portion of the Pampean flat-slab segment and northeast end of the la Ramada fold and thrust belt of theAndes Main Cordillera. This region is a continuation of the Miocene and Pliocene porphyry copper belt of Chile that hoststhree of the largest copper deposits in the world (El Teniente, Río Blanco - Los Bronces and Los Pelambres). NumerousCu (Au) prospects with high mining potential have been discovered in this area. Some of them show the overlapbetween porphyry-type and high sulfidation epithermal deposits, indicating a high degree of uplift during their formation.The Altar deposit is a large porphyry Cu (Au-Mo) with associated high sulfidation epithermal veins (measuredresources of 995 Mt @ 0.35% Cu and 0.083 g/t Au, May. The basement of the area is characterizedby a late Carboniferous tonalitic batholith (~ 297 Ma, U-Pb, Maydagán, 2012) that crops out to the east of the Altar district. Cretaceous volcanic rocks outcrop to the west, in the Chile-Argentina border, between the Pantanosa Faultto the east and Tres Quebradas Fault to the west. Early Miocene volcanic rocks that crop out in the study area were intruded by porphyritic stocks of andesitic-dacitic composition in the middle to late Miocene that contains most of the Cu-Au mineralization.Igneous rocks of Altar region were grouped in the lower Miocene early volcanic complex (21.6 ± 1.2 Ma to20.8 ± 0.3 Ma, U-Pb zircon), that consist of an intercalation of basaltic andesite andporphyritic andesite-dacite lavas, levels of andesitic- dacitic lapilli tuff, and pyroclastic breccia that grade upwards toan upper unit of compacted and thick rhyolitic tuff, and the upper subvolcanic suite of middle-late Miocene age(11.75 ± 0.24 Ma, 8.9 ± 0.4 Ma, U-Pb zircon) that consists of a series of porphyritic intrusions,dykes, and magmatic-hydrothermal breccias.LA-ICPMS U-Pb zircon ages from the Altar porphyries indicate four discrete events of intrusions over an extendedmagmatic life time of ca. 3 m.y. It comprises a pre- mineralization porphyry (11.75 ±0.24 Ma), three mineralized porphyries (11.62 ± 0.21 and 11.68 ± 0.27 Ma, 11.13 ± 0.26 Ma, 10.35 ± 0.32 Ma) related to hydrothermal breccias,two post-mineralization intrusions, and a post-mineralization breccia (8.9 ± 0.4 Ma).In this study we present new apatite (U-Th)/He data (AHe) in order to quantify the extent of tectonic-relatedexhumation in the Andes Main Cordillera in Argentina and compare the results with the exhumation patterns fromthe Coastal, Frontal and Main Cordilleras at similar latitudes in Chile. The aim of our research is to identify possiblepulses of tectonic exhumation in the region given their importance to emplacement of subvolcanic intrusions relatedto Cu (Au) mineralization.AHe ages present a broad distribution ranging from 9.96 to 56.83 Ma, but the majority of them show middleMiocene ages. Two samples from the tonalitic batholith of Late Carboniferous age (~ 297 Ma dating U-Pb in zircons,Maydagán, 2012) showed AHe ages of 14.3 and 11.87 Ma. AHe ages from the subvolcanic stocks from the middle tolate Miocene showed ages between 13.45 and 11.17 Ma.The U-Th/He ages ranging between 14.3 and 11.17 Ma obtained for the late Carboniferous tonalite and forthe subvolcanic stocks indicate an exhumation pulse of the region during the middle Miocene. This lapse of time partially overlaps with the period of emplacement of the subvolcanic porphyries in the Altar deposit, which have UPbcrystallization ages between 12.0-10.3 Ma (Maydagán et al., 2011, 2014). Therefore, thermochronological dataindicate that the intrusion of subvolcanic bodies in the study region coincide with the final stage of the regionalexhumation event.In the study area, Triassic and Jurassic sedimentary rocks are tectonically overlain by the late Carboniferoustonalite along the E-vergent Mondaquita reverse fault (Perelló et al., 2012). U-Th/He ages obtained in the tonalitecould be therefore related to vertical displacements associated to the Mondaquita Fault. To the west, the PachónFault brings the Miocene sequence (early volcanic rocks and middle-late Miocene subvolcanic stocks) on the lateCarboniferous tonalite. As the AHe ages obtained from both sides of the Pachon Fault are similar we can hypothesizethat the entire area (irrespective of the Pachon Fault) was exhuming between 14 and 11 Ma.The overlap of U-Pb and AHe ages recognized in the samples of the middle Miocene porphyritic intrusions analyzed (Altar North and Altar East) suggests that these AHe ages are related not to exhumation, but rather toigneous cooling and, thus, that the intrusion depth of these porphyritic stocks was shallow, probably in the upper ~2-3 km.In the Frontal Cordillera of Chile, to the west, Rodríguez Montecinos (2013) obtained fission-track agessignificantly older than AHe ages in Late Cretaceous, Paleocene, and Eocene intrusions, therefore demonstratingthat these magmatic bodies were intruded at deeper levels. However, the AHe ages obtained by these authors areMiocene (18.1-6.9 Ma) reflecting a similar time of exhumation in the Frontal Cordillera of Chile and the study area.The period of exhumation reflected by AHe ages also coincide with the collision and passage of the JuanFernández Ridge below the flat-slab segment at these latitudes. The new data confirm a temporal connectionbetween ridge arrival, exhumation, and porphyry copper formation in the study region.