INVESTIGATION OF THE STRUCTURALLY-CONTROLLED ...

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Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2014-2015 1 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

Transcript of INVESTIGATION OF THE STRUCTURALLY-CONTROLLED ...

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2014-2015

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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

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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.

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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 which reflect two different mineralization style: (i) veins, veinlets, stockworks and hydrothermal breccias (Fig. 3A) and (ii) disseminated sulfides filling the porosity of favourable lithologies (Fig. 3B).

Fig. 3 – Drill cores photos of typical mineralization style (A) hydrothermal breccia with clast with early reddish sphalerite (Sph) and late chalcopyrite (Chal) deposited on breccia clast in a matrix of barite and calcite; (B) volcaniclastic sediment with disseminated sulfides.

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The orientation of the deposits along two NW-SE trends – corresponding to the two major tectonic lineament in the district area – has suggested to some authors a relation with the crustal-scale strike-slip fault known in literature as Gastre Fault System (i.e. Rapela & Pankhurst, 1992; Williams, 2010). Methodology

During this first year, the work was mainly focused on the acquisition of the thermochronological and structural dataset. In particular, the first months were spent in bibliographic research and acquisition of geological cartography of the northern Chubut Province in order to plan an efficient sampling strategy and define the location of the more interesting structures to survey during the field activity. At this stage immediately emerged a clear ambiguity in the tectonic models proposed for the evolution of the foreland zone along the chosen transect, especially in the area interested by the mineralization, hosted within the more vast Gastre basin. Previous works propose contrasting scenarios for the tectonic evolution of this foreland basin, involving either: (i) dip-slip activation of rift-inherited extensional structures (Williams, 2010; Pratt, unpublished); or (ii) their re-activation in a crustal shortening regime as a result of a Middle Miocene contractional episode (Bilmes et al., 2013). In some interpretations, the latter is associated with strike-slip tectonics linked to a regional structure known as “Gastre Fault System” (Rapela & Pankhurst, 1992). Fieldwork was carried out following the route from Puerto Madryn (Atlantic coast) to the Esquel Range (Precordillera System), collecting 22 samples throughout the various tectonic domains (Fig. 4) and studying the main structural features in the region.

Fig. 4 – Slice of geological map of Chubut Province indicating the studied area along the 42°S parallel and sampling sites (G01 to G22) for thermochronology. Furthermore, a detailed structural analysis was carried out in the Navidad mineral district for an exhaustive comprehension of all the tectonic structures involved in the geologic configuration of the mineralized area. Subsequently I joined the CONICET research group at the Neuquén and Bahia Blanca Universities, whose research interest is focused on metallogenesis of epithermal mineralizations. All the samples collected were processed for apatite fission-track and apatite (U-Th)/He analyses at the Department of Geosciences of Padua after shipping. Each sample was crushed and then sieved using a

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250 µm sieve. The obtained material was cleaned from the finest component using the washing table and then separated magnetically using Frantz with 1.1 A current. The heavy liquid separation (Na polytungstate) was used to separate apatites from the rest of the diamagnetic material. The apatite-rich mineral fractions were mounted in epoxy resin and then polished for AFT analysis. The grain mounts were etched by HNO3 5M, coupled with external detectors and then sent to the nuclear reactor. Moreover, three apatites for each sample have been handpicked, measured, loaded into a 0.8 Nb tubes for (U-Th)/ He analysis which will be performed at the He dating laboratory of the Paris Sud University under the supervision of Prof. Cécile Gautheron (during October 2015). Preliminary results and future work

The transect chosen for this study comprises various structural domains from the Andean Cordillera to the undeformed foreland. As noted by several authors, the deformation linked to subduction affects areas relatively far away from the orogenic front, producing exhumation of fault blocks and the development of intramontane basins in the Andean foreland, several hundred kilometres away from the trench (Bilmes et al. 2013). This far field deformation, which produces the so called “broken foreland” and a widening of the orogenic wedge, seems to be related to flat-slab configuration of the subducted plate, according to the model proposed by Humphreys (2009). However, the kinematic significance of these Neogene structures is strongly debated, as they have been interpreted as the result of: (i) extensional tectonics (Volkheimer, 1965), (ii) strike-slip tectonics (Coira et al., 1975; Dalla Salda and Franzese, 1987; Rapela et al., 1991), and (iii) thrust tectonics (Dessanti, 1956; Figari et al., 1996; Giacosa and Heredia, 2004). Furthermore, Andean tectonics is strongly influenced by pre-Neogene crustal heterogeneities produced since the Late Paleozoic. These inherited structures are the result of a complex tectonic history related to the continental amalgamation of magmatic belts from Carboniferous to Triassic times (Pankhurst et al., 2006; von Gosen, 2009; Zaffarana et al., 2010), rifting and basin formation during the Jurassic–Early Cretaceous (Figari et al., 1996), Cretaceous shortening (Allard et al., 2011; Folguera and Ramos, 2011; Orts et al., 2012; Proserpio, 1978) and Paleogene extension and volcanism (Rapela et al., 1984). Anyway, in the area of the detailed structural survey (Gastre), field evidence points out dominant extensional deformation and a subordinate shortening (possibly related to a contractional pulse in the Late Cretaceous). Strike-slip structures, instead, appear to have acted as transfer faults related to the main extensional structures.

Fig. 5 – Field photos showing: (A) mineralized barite-calcite veins at Navidad (Calcite Hill) and (B) extensional fractures in Paso del Sapo Fm (upper Cretaceous).

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More in detail, in the mining district, dominantly NW-trending, Mesozoic normal faults controlled the emplacement of the intermediate sulfidation epithermal deposits, allowing the rise of mineralized brines of a probably Jurassic hydrothermal system (Fig. 5A). Open folds and locally developed cleavage in limestones of the Cañadon Asfalto Fm. record a minor NE-SW shortening event resulting in a mild basin inversion. Field evidences in the district, thus, do not support the idea of a strike-slip regime controlling the major structures and their linking with the so-called “Gastre Fault System”. The Campanian succession, well exposed in the western portion of the study area (Paso del Sapo area), is exclusively affected by normal faults and does not show any evidence of folding/reverse faulting (Fig. 5B), thus suggesting that the shortening pulse occurred in the early part of the Late Cretaceous (i.e. pre-Campanian inversion) and was both preceded and followed by dominantly extensional deformation. Results from thermochronological analysis of samples will be provided in the next months. The exhumation history of each sampling site will be characterized by double dating and thermal modelling. These data will thus allow one to identify major cooling events that, in turn, will be related to the development of the thrust and fold belt and to the deformation in the adjacent “broken” foreland sector. Regarding the sampling within the Ag-deposit, we will also explore the possibility to use U-Th/He technique to date Fe-oxide mineralizations, therefore providing an absolute age for ore formation. A second field survey is planned for the next year in order to integrate the sampling and the structural data and to reach the Main Cordillera area for a complete analysis of deformation close to the thrust front. We are also evaluating the move to the North in order to examine the broken foreland of the Puna region (Central Andes) due to the affinity shown with the already selected area for this research. References

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Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2014-2015

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SUMMARY OF ACTIVITY IN THIS YEAR Courses: M. BORG: “Academic English: Consolidating Communicative Skills for the Earth Sciences”, Dipartimento di Geoscienze, Università degli Studi di Padova. L. SALMASO, P. STARK: “PhD Course: Statistics for Engineers 2015” – Dipartimento di Ingegneria Industriale, Università degli Studi di Padova. Publications: SAVIGNANO, E, BRIDGES, J, REDDY, S. M., MAZZOLI, S. – Quartz fabric variations across a natural greenschist facies shear zone, Upper Val Gressoney, Western Alps, Italian Journal of Geosciences, submitted Teaching activities: Teaching assistant: 25 hours, “Geologia”, Laurea di primo livello in Scienze Naturali (A.A. 2014/2015). Field activity: February 2015: Fieldwork for structural analysis and samples collection in the Chubut region, North Patagonia, Argentina. Lab activity abroad: February- March 2015: Research period in Argentina at Universdad del Comahue (Neuquen and General Roca) and at Universidad del Sur (Bahia Blanca) to study mineralized well log samples from Navidad deposit under the supervision of Prof. Marta Franchini and CONICET research group. . October 2015: LA-ICP-MS analysis for U-Th/He dating on collected samples at Universitè Paris Sud – Départment de science de la Terre – under the supervision of Prof. Cecile Gautheron and Dr. Rosella Pinna.