Investigation of the Structurally-Controlled Navidad Mineralization (Argentina): an Integrated Approach

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2014-2015 INVESTIGATION OF THE STRUCTURALLY-CONTROLLED NAVIDAD MINERALIZATION (ARGENTINA): AN INTEGRATED APPROACH Ph.D. candidate: ELISA SAVIGNANO, I course Tutor: Prof. MASSIMILIANO ZATTIN Co-tutors: Prof. STEFANO MAZZOLI (University of Naples); Prof. MARTA FRANCHINI (CONICET, Argentina) Cycle: XXX Abstract Unravelling the timing of deformation and development of geological structures is essential to understand and quantify in terms of rates the geological processes characterizing a given area. This becomes particularly important when the exploitation of mineral resources of economic interests is involved. A satisfactory estimation of cooling paths for a region can be made by producing a thermo-kinematic model integrating structural and thermochronologic methods. This procedure is applied in the Andean and sub-Andean region of North Patagonia (Chubut region) along a transect at 42° S. In this area, the presence of an economically important Ag-deposit (Navidad) may also provide an interesting chance to apply these methodologies to the study of ore-formation processes. During this first year, the research activity has been focused on data collection and preliminary analysis. Fieldwork in the Chubut region involved both sampling for low-temperature thermochronology and gathering of structural data. Subsequent lab work included the petrographic analysis of host rocks and ores, carried out at the University of Comahue (Neuquén and General Roca, Argentina), and apatite concentration procedures performed at Padua University. U-Th/He dating is currently carried out at the University of Paris Sud (Orsay, France). Introduction Coupling balanced and restored cross-section construction with thermochronological analysis allows one to produce detailed studies on deformed areas, providing the possibility to define the various stages of deformation and to quantify both their extent and timing (e.g. Mora et. al., 2014; Castelluccio et. al., 2015). Such an integrated approach will be applied in this research project to investigate a still poorly understood area of northern Patagonia (Argentina), where contrasting geological models are debated. The work starts from the analysis of the present-day tectonic setting and reconstruction of the geological history of the area – by means of a reappraisal of available stratigraphic and geological information integrated with own structural survey and the construction of balanced cross-sections that will be restored – concurrently with low-T thermochronological analysis that can confirm the structural model and constrain the thermal evolution with high detail in the shallower levels of the upper crust. Despite the Andes represent one of the most spectacular orogens on the Earth and much attention has been paid to the interacting processes which led to their formation (i.e. Ramos, 1989; Ramos & Folguera, 2009; Folguera & Ramos, 2011; Montgomery et al., 2001), gaps in knowledge still remain in some sectors. This is particularly important to gain a better understanding of the linked thrust belt-adjacent foreland system. The features exposed in the study area of northern Patagonia provide clear evidence of the complexity and relevance of foreland structures, well beyond the traditional notions of far-field foreland stress or bending-related brittle deformation of foreland sectors. For the comprehension of the coupled thrust belt-foreland system, the relationships between surface deformation and deep geodynamic processes need to be investigated. This may provide further insights into the widespread occurrence and important role of inherited pre-thrusting structures in fold and thrust belts. Generally, in foreland domains, the continental lithosphere is likely to be cooler and stronger with respect to the inner zones of the thrust belt, where the continental crust may be warmer and correspondingly weaker (Butler et al., 2006). Within this scenario, preexisting narrow discontinuities within the foreland crust represent preferential zones of weakness, and they may serve to accommodate and localize contractional deformation (Holdsworth et al., 2001). Therefore, positive inversion tectonics including the compressional reactivation of deep-rooted preexisting faults, especially inherited rift-related normal faults, most likely occur in foreland domains ahead of mountain belts (Ziegler, 1987; Coward, 1994; Ziegler et al., 1995; Lacombe and Mouthereau, 2002; Butler et al., 2004; Butler and Mazzoli, 2006). Moreover, given the relevance of the Andean foreland for its economic resources, precise knowledge of geological structures controlling the origin and distribution of ore deposits is fundamental for the 1 Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2014-2015 exploitation of mining districts and future exploration. The study area here considered encompasses the world-class Ag-deposit of Navidad (Gastre, Chubut Province), which is now in advanced exploration. Nowadays a good estimate of the held resources (measured and indicated) and location of the more fertile sectors in the district is well defined in contrast with a robust knowledge about its metallogenesis. Here, the thermochronometric techniques will be a powerful tool to elucidate the processes involved in the ore formation – i.e. timing, duration of mineralization process and temperatures – as suggested by McInnes et al. (2005) together with the structural analysis which in turn will unravel the favourable feeder structures acting during the emplacement of the mineralization. Geological background of the study area The Southern Andes are a mostly linear orogenic belt formed at the convergent plate margin between the Nazca and the South American Plates. The building processes that led to the formation of this orogen have been interpreted as progressive and non-steady shortening phenomena, acting in several pulses since the Late Cretaceous until Late Miocene (Folguera and Ramos, 2011). Although the orogeny is continuous along strike, a tectonic segmentation is easily recognizable looking at the different extent of deformation of the foreland and position of the magmatic arc moving to the East. This variability along strike has been related to the alternation of flat and steep segments of the slab (e.g. Jordan et al., 1983). Variation of the subduction angle is possibly induced by variability of crustal buoyancy, independent of the age of the subducted slab and influenced by the local morphology and tectonics of the subducting plate (Gutscher et al., 2000). Major changes in the tectonic style can be recognized and characterized at present by direct observation and deep geophysical survey. However, their identification in past times requires detailed observations on a number of geologic processes such as the evolution of sedimentary basins, cycles of magmatism, structures marked by the shift from extensional to compressional regimes (and vice versa), exhumation and mountain building. A wealth of studies on these subjects is today available for the Central Andes (e.g. Allmendinger et al., 1997). In contrast, comparable studies for the North Patagonian Andes are rather scarce and large uncertainties concerning the magnitude of orogenic shortening, deformational processes, and foreland basin formation still remain. More in detail, the investigated area comprises the transect along the latitude 42°S (Fig. 1), in which the following morphostructural regions can be distinguished from west to east: the Main North Patagonian Fig. 1 – Location of the study area marked by the red box in the map from Bilmes (2013) and Google Earth satellite image. 2 Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2014-2015 Andes, the Precordilleran system, the wedge-top and foreland area. The Main Andes are characterized by Jurassic to Cretaceous granitoids intruding Mesozoic volcaniclastic successions covered by localized Neogene sequences. The Precordilleran system is formed by Palaeozoic sequences, intruded by Early Jurassic granitoids, unconformably covered by Middle Jurassic to Cretaceous and Tertiary successions. Moving to the East, Miocene synorogenic strata belonging to the wedge-top basins outcrop, while the less deformed foreland area host Cenozoic volcaniclastic successions and locally blocks of uplifted basement (Fig. 2; Orts et al., 2012 and reference therein). Fig. 2 – Balanced cross-sections of interpreted geological structures by Orts et al. (2012) in a close and parallel area respect to the studied transect. Noteworthy, the chosen transect crosses in its eastern part the middle Jurassic to early Cretaceous non- marine depocenter of the Cañadon Asfalto formed in an active continental rift environment and where the Navidad deposit is hosted (Figari, 2005). The Cañadon Asfalto Fm. is a dominantly sedimentary succession, with subordinate intermediate volcanic rocks, which age is not well constrained by stratigraphic relationships or fossils. The host rock for the main silver-lead mineralization consists of a latite flow sequence. The genesis of this giant epithermal Ag- deposit is supposed to be related to a Late Jurassic geothermal system that fed hot springs into a lake via a network of epithermal veins and permeable autobreccias (Williams, 2010). Mineralization shows both structural and lithostratigraphic controls on its deposition

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