Travertine deposits associated with hydrothermal systems constitute relevant sedimentary archives since they record fluid chemistry, depositional processes, tectonic activity, and paleoenvironmental conditions. The present study integrates sedimentological, petrographic, mineralogical, geochemical, and isotopic information from the Northern Puna (Jujuy Province, NW Argentina) active and fossil travertine systems, focusing on those related to the geothermal fields of Rosario de Coyaguayma (RCT), Mina Pirquitas (MPT), and Cono Panizo (CPT). These deposits, developed in a volcanically and tectonically active region, exhibit marked facies variability that is controlled by hydrodynamics, fluid-rock interaction, and biological activity. Facies analysis enabled the recognition of depositional environments of upwelling, proximal fans and terraces, and distal fluvial tufas, each one characterized by distinctive morphologies and microfacies associations. Surge and proximal deposits are dominated by botryoidal, fibrous, radial, oolitic, and sparitic calcite fabrics, locally associated with microbial textures, while distal environments include conglomeratic tufas that reflect interaction with fluvial processes as evidenced by a decreasing supersaturation in CaCO3. Mineralogical analyses reveal a clear predominance of calcite (70–90%), with significant amounts of magnesian calcite (10–63%) in specific samples from RCT and CPT, while aragonite is restricted to currently active, high-temperature springs. Sr concentrations range between 271 and 7275 mg/kg, with markedly higher values in active deposits (>2000 mg/kg at CPT) compared to fossil ones, suggesting progressive recrystallization from aragonite to calcite and temporal weathering. The geochemical trends observed in Sr and Mg suggest the occurrence of progressive recrystallization processes, in addition to temporal variations in fluid temperature and chemistry. The carbon and oxygen isotopic composition (−4.8 to 6; −16 to −1, respectively) indicate a predominantly thermogenic origin, with isotopic fractionation influenced by CO2 degassing, water cooling, and post-depositional alteration. Paleotemperature estimates derived from oxygen isotopes indicate higher hydrothermal temperatures during early depositional stages, followed by progressive cooling and migration of the discharging zones (temperatures of up to 93, 78 and 36 °C were calculated for RCT, CPT and MPT). The integration of facies, geochemical, and isotopic data demonstrates the high sensitivity of travertine systems to sedimentary and hydrothermal influences, highlighting their importance as archives of long-term hydrothermal and environmental evolution in high-altitude volcanic settings. Moreover, travertine deposits have a strong potential to preserve information on the composition and evolution of hydrothermal fluids, which play a key role in mineralizing processes in resource-rich regions such as the Argentine Puna.

The Campanorco Subvolcanic Complex (25 degrees 38 ' S-65 degrees 33 ' W) is a poorly known unit that comprises phenoandesitic sills and dikes located approximately 600 km east of the Nazca Plate subduction trench, an unusual setting for typical Andean back-arc magmatism. This contribution offers new petrographic, geochemical and geochronological data. U-Pb dating on zircon yielded crystallization ages of 6.8 Ma, confirming its belonging to Upper-Miocene back-arc magmatism. This igneous system consists of amphibole-bearing rocks of the high-K calcalkaline and shoshonitic series, related to an enriched mantle source, likely metasomatized by subductionderived fluids. Magmatic differentiation primarily occurred through fractional crystallization of water-rich parental magmas, with low crustal contamination, typical of steady-state andesites. Magmatic enclaves are widespread and are good physical evidence for the influence of multiple episodes of magma mixing on magma evolution. The formation of the Campanorco Subvolcanic Complex is attributed to slab shallowing during Middle to Late Miocene times, likely driven by the subduction of the Taltal and Copiapo aseismic ridges, which facilitated mantle hydration and magma generation far from the volcanic arc. This unit serves as a key indicator of the maximum extent of slab shallowing during this period in the segment between 24 degrees and 26 degrees S. The study of this system provides critical insights into the petrogenesis, tectonic setting, and mineralization potential of the easternmost Neogene back-arc magmatism in the Central Andes.

Hydrothermal alteration and ore deposition at Chinchillas (northern Puna, Argentina) occurred in pyroclastic deposits that fill an explosive diatreme, as well as in monomictic breccias composed of metasedimentary lithoclasts from the basement that bound the diatreme. Highly porous and permeable rocks acted as reservoir for hydrothermal fluids responsible of alteration and mineralization. Sequential mineral associations can be grouped in three pre- and two syn-to post-mineralization stages, which are frequently superimposed in space. The first stage (adularia-rich) is widespread and develops in a context of alkaline solutions, followed by upward circulation of relatively acidic and reducing fluids (second stage) with B availability (tourmaline +/- pyrite +/- quartz), limited to deep sectors. A third alteration stage is typical of the argillic alteration association and is given by kaolinite replaced by later illite + illite-smectite mixed-layers + quartz +/- anatase +/- pyrite +/- alumino-phosfatesulfates, which indicates the progressive neutralization of fluids during continuous interaction with rocks. The crystal structure characteristics and zonation of clay minerals with depth suggest temperatures between 200 degrees and 170 degrees C and a vertical thermal gradient during illitization. These stages seem to have had minor effects on the reservoir quality of the diatreme, as the Ag-Pb-Zn ore + siderite (mineralization stage) was deposited in two relatively large areas of the diatreme, and late siderite (post-mineralization stage) is ubiquitous in both mineralized and barren areas.

The Zapla Range (Jujuy province) represents the southern edge of the central Andean Subandean Ranges in Argentina. In the northernmost area of the Zapla Range, basaltic rocks are interbedded in the Mealla Formation. These volcanic units form part of the post-rift magmatism of the Salta rift basin, which mainly focused on the Lomas de Olmedo sub-basin to the east. In this paper, six of these basaltic occurrences are characterized on the basis of new geological, petrographic and geochemical data, in order to explore their emplacement, petrogenesis and possible tectonic setting during rift evolution. Peperites in the upper contact of sills evidence the interaction between basalts and unconsolidated or poorly consolidated wet sediments. This implies that there was concomitance between sedimentation and subsurface magma emplacement, suggesting a Paleocene age for the magmatic event according to zircon U-Pb data available for Mealla Formation rocks. Other basaltic units show hipocrystalline textures and lack peperites at the top, which points to their extrusive nature. Additionally, some of these lava flows are spatially associated with pyroclastic deposits that reveal the occurrence of explosive hydrovolcanic eruptions. The study rocks are classified as alkaline basalts and basanites with high Mg# and Niand Cr-rich compositions, close to primitive magmas. They show affinity with continental intraplate settings. The analyses of Ba/Nb, Nb/La, (La/Yb)N, N , (La/Sm)N, N , (Gd/Yb)N N and (Dy/Yb)N N ratios, as well as normalized trace and rare earth element patterns, suggests a genesis by low degrees of partial melting of a residual garnet-bearing mantle source. Therefore, basalts from this part of the Subandean Ranges evidence a magmatic episode probably linked to an extensional reactivation stage of the Salta Group rift system in the Lomas de Olmedo depocenter during the Paleocene (similar to 60 Ma).

The Huachichocana Subvolcanic Complex (HSC) is one of the easternmost Miocene magmatic complexes from the Central Andes back-arc region. This complex comprises three major sheet-like igneous bodies and contiguous minor intrusions of andesitic to dacitic rocks distinguished by their adakite-like high Sr and Sr/Y values. Based on new detailed data of this system, we explore the genesis of intermediate magmas in the far back-arc region of the Central Andes and complement work on similar rocks from the Eastern Cordillera. We add petrography and mineral chemistry studies and we report new whole rock geochemical and Sr–Nd isotopic data, and a U–Pb zircon age for the HSC. The results suggest that these intrusive rocks were derived from melting of a hydrated mantle source and that the adakite-like composition was acquired through amphibole-dominated fractionation at deep to moderate crustal pressures and by the suppression of early plagioclase crystallization in precursor magmas. The presence of mafic microgranular enclaves is frequent and, along with variations in the composition of phenocrysts and common disequilibrium textures, suggests a significant role of mixing between mafic and intermediate magmas during their ascent and storage in the crust. U–Pb radiometric dating yielded a concordia age of 8.343 ± 0.042 Ma for the andesites of the HSC, coinciding with the period of maximum shortening of the eastern border of the Puna plateau. Nevertheless, this igneous complex is located at the southern margin of the Lípez fault system, a zone of lithospheric weakness that should favor magma emplacement so far east of the trench.

Cordilleran arcs are built by long periods of steady-state magmatism punctuated by transient high-flux magmatic episodes or flare-ups. Such flare-ups, manifested as periods of prodigious silicic volcanism and magmatism, result from geodynamic perturbations that cause elevated rates of magma addition to the crust. Questions remain, however, about how magmatic addition rates quantitatively compare between steady-state and flare-up modes of arc magmatism, and how long after the major geodynamic perturbation the flare-up begins. Here, we compute new estimates of erupted volumes over the last 35 Myr for the 22.5–29°S segment of the Central Andes based on a new volcanic geospatial database. These yield magmatic addition rates at least an order of magnitude higher during flare-up compared to steady-state conditions. A lag time of ~8–12 Myr between ocean ridge subduction (the major geodynamic perturbation in the Central Andean arc) and the onset of flare-up conditions is estimated.

Salars with lithium-rich brines are a characteristic feature of the Central Andes, but knowledge about the main sources of lithium and the mobilization processes of lithium in the salar deposits is still incomplete. This work focuses especially on the Salar de Diablillos (southern Puna) as part of a larger area that includes the neighboring Salar Centenario and Salar de Ratones. Building on the ability of Li as a tracer of silicate weathering, we investigate the Li content and isotope composition of samples from the depocenter and catchment of the Diablillos basin (3-D) and conduct a surface reconnaissance in the Centenario and Ratones depocenters to identify the key metallogenic processes. Radiogenic Sr and Nd isotope compositions are also provided to discriminate the main local Li sources. The isotope data in all three depocenters show that most of the Li in the brines and evaporite deposits are derived from Cenozoic volcanic rocks, despite the dominance of the Paleozoic basement in the catchment. In the Centenario and Ratones depocenters, near-surface chemical weathering appears to be the dominant Li mobilization process. In contrast, hydrothermal mobilization of Li also plays a role in the Salar de Diablillos, possibly related to the presence of a fractured basement with pressure zones and artesian conditions in the aquifer at depth. These fluids also show a larger element contribution from the basement.

Evaluation of volcanic hazards typically focusses on single eruptive centres or spatially restricted areas, such as volcanic fields. Expanding hazard assessments across wide regions (e.g., large sections of a continental margin) has rarely been attempted, due to the complexity of integrating temporal and spatial variability in tectonic and magmatic processes. In this study, we investigate new approaches to quantify the hazards of such long-term active and complex settings, using the example of the 22.5–28°S segment of the Central Volcanic Zone of the Andes. This research is based on the estimation of: 1) spatial probability of future volcanic activity (based on kernel density estimation using a new volcanic geospatial database), 2) temporal probability of future volcanic events, and 3) areas susceptible to volcanic flow and fall processes (based on computer modeling). Integrating these results, we produce a set of volcanic hazard maps. We then calculate the relative probabilities of population centres in the area being affected by any volcanic phenomenon. Touristic towns such as La Poma (Argentina), Toconao (Chile), Antofagasta de la Sierra (Argentina), Socaire (Chile), and Talabre (Chile) are exposed to the highest relative volcanic hazard. In addition, through this work we delineate five regions of high spatial probability (i.e., volcanic clusters), three of which correlate well with geophysical evidence of mid-crustal partial melt bodies. Many of the eruptive centres within these volcanic clusters have poorly known eruption histories and are recommended to be targeted for future work. We hope this contribution will be a useful approach to encourage probabilistic volcanic hazard assessments for other arc segments.

Unravelling the 3D architecture of ore shoots and its evolution through time, thereby moving towards a 4D understanding of mineralization processes, requires an interdisciplinary approach based on the capability of carrying out extensive trenching and drilling as well as effectively integrating structural and geochemical analyses. Such conditions are offered in La Paloma district of the Deseado Massif, Argentina. Here, eight epithermal Au-Ag veins are hosted in Middle Jurassic volcanic rocks (Bajo Pobre Formation). The Au-Ag (Zn, Pb, Mo and Cu) Sulfuro Vein, representing the main ore body in the district, is a 750 m long, N to NW striking structure extending 230 m at depth. The geometry and distribution of ore shoots within the Sulfuro Vein are controlled by: (i) lithological and structural features, (ii) metal concentration, (iii) temperature of the fluids at the time of ore deposition, and (iv) remobilization processes. The highest values of Au, Ag, Cu, Mo, Pb, Zn and Sb are concentrated at depths between 50 and 100 m.a.s.l., while high Mo values occur also at greater depths in the southern segment of the vein. Molybdenum distribution in shallower sectors of the vein is controlled by its remobilization by later infill stages. The Au, Ag and Cu ore shoots are widely distributed in the southern and central segments of the vein, as are the areas of greater vein thickness. These ore shoots exhibit a sub-horizontal geometry consistent with dominant extensional faulting during mineralization. In the northern segment of the vein, the Au, Ag and Cu ore shoots are discontinuous and small, and show gentle to moderate plunges probably associated with variable fault kinematics and depletion of the fluid in these metals. Ore-shoots of Mo, Pb and Zn display high values along the longitudinal section and sub-horizontal geometry in the central and northern sectors of the vein, with high-grade Mo ore shoots decreasing to the north. The fact that Pb and Zn high grades extend up to the tip of the northern vein segment suggests that these metals continued to precipitate at lower temperatures, favoured by the permeability of the volcaniclastic units. All of the ore shoots exhibit steeper plunges towards the southern termination of the vein. Here, upward fluid flow may have been enhanced by the dilation associated with oblique-slip along the N-S striking segment of the steeply dipping Sulfuro Vein. The geochemical distribution of metals shows a slight vertical zonation and a distinct lateral zonation, which suggest hydrothermal fluids flowed northward from deeper zones in the southern sector of the vein.

The Lindero gold deposit is located in the Southern Puna plateau, northwest Argentina. The deposit is centered in a cluster of six subvolcanic intrusions emplaced at the margin of the Arizaro Basin. Three alteration types were recognized: (i) Ca-Na silicate (clinopyroxene + magnetite +K-feldspar + quartz + calcite plagioclase), (ii) K silicate (K-feldspar + quartz + magnetite biotite +/- anhydrite) and (iii) chlorite-calcite alteration. The highest ore grades are linked to the K-silicate alteration. The proven plus probable reserves of Lindero are 84,226 t with average grades of 0.63 g/t Au and 0.11% Cu. A previous study assigned Lindero to the iron oxide copper-gold (IOCG) deposit type but many features of Lindero suggest that it is a porphyry gold deposit, including (i) the temporal and spatial link between alteration and the intrusive bodies, (ii) the alteration distribution pattern, particularly the small volume of rock affected by Ca-Na silicate alteration, (iii) the Au-rich and Cu-poor mineralization style. The magmatic complex at Lindero comprises an early-mineral unit (FPD), four inter-mineral units (CPD1, PBFD, CPD2 and DDP) and one post-mineral unit (PMI). In-situ U/Pb SIMS dating of the oldest (FPD), an intermediate (PBFD) and the youngest (DDP) intrusive units, confirms a middle Miocene age. The weighted mean ages of the oldest and youngest units are indistinguishable, with 15.36 +/- 0.13 Ma (n = 21) and 15.47 +/- 0.11 Ma (n = 16), respectively. Individual ages from each unit range by similar to 1 m.y. and the overall spread of zircon ages is 15.92 +/- 0.23 to 14.44 +/- 0.33 Ma. We suggest that emplacement of the subvolcanic stocks took place within this span time, likely at the lower end of this range (15.0-14.4 Ma). Two 40Ar/39Ar ages of hydrothermal biotites from the K-silicate alteration (14.99 +/- 0.16 Ma and 14.93 +/- 0.12 Ma) indicate that hydrothermal alteration began practically simultaneously with the emplacement of the porphyry units. All of the intrusive units are similar compositionally. They show a fineto medium-grained porphyric texture (1-4 mm) comprising plagioclase, amphibole, clinopyroxene and scarce quartz phenocrysts (40-55 vol % of phenocrysts) in a K-feldspar +/- quartz microcrystalline (0.02-0.07 mm) groundmass, except in the post-mineral unit which has a cryptocrystalline groundmass. Whole-rock analyses reveal a narrow range of dioritic composition (58.6-61.9 wt % SiO2) and high-K calc-alkaline character for all units. Trace element features (low Ba/Nb ratios, high Nb) of the Lindero magmas indicate a back-arc affinity, similar to those from the Southern Puna and distinct from the Central Volcanic Zone (CVZ) frontal arc. The Sr and Nd isotope ratios (Sr-87/Sr-86 = 0.706042 to 0.706607; Nd-143/Nd-144 = 0.512501 to 0.512582) from Lindero intrusives are also similar to Southern Puna back arc volcanic rocks. The Pb isotope ratios of Lindero (Pb-206/Pb-204 = 18.79 to 18.83; Pb-207/Pb-204 =15.60 to 15.63; Pb-208/Pb-204 = 38.66 to 38.74) overlap with both back-arc and arc magmas in the CVZ. The narrow age range, spatial association and uniform chemical and isotopic composition of Lindero porphyry units suggest that were derived from a common magma source, which underwent fractionation and/or crustal assimilation before emplacement as suggest by the low concentrations of Mg, Cr, Ni and Sr. The Lindero porphyry units show chemical and isotopic similarities with those from porphyry gold deposits in the Maricunga belt, Chile, and with the porphyry copper deposits of Argentina located in a back-arc setting; however, they differ from porphyry copper deposits in the frontal-arc setting of Chile, notably by the lack of an adakite-like signature (high Sr/Y ratio).

The Huayra Huasi Volcanic Complex of Miocene age (11.88 ± 0.15 Ma U–Pb zircon) was emplaced in the Northern Puna plateau of Argentina, spatially associated with ignimbrites erupted from Altiplano–Puna Volcanic Complex calderas. The complex comprises biotite-bearing dacites and low-SiO2 rhyolites in the northern area and high-SiO2 rhyolites in the south, all with peraluminous compositions (A/CNK > 1.0–1.22). The units have broadly similar initial Sr and Nd isotopic ratios (87Sr/86Sr ∼ 0.71013–0.71225 and εNd ∼ −5.4 to −7.0) and are composed of plagioclase, quartz, sanidine and biotite as the main phenocryst phases. All units host macroscopic microgranular enclaves and xenoliths of sillimanite–biotite schists, sillimanite- and sillimanite–garnet gneisses, as well as fibrous alumina-rich microxenoliths, the latter being especially abundant in the southern rhyolites. Petrographic, mineral and whole-rock geochemistry, geothermometric and isotopic data indicate that all units of the complex originated by contamination of andesite magmas through assimilation of upper crustal lithologies in early stages of magma evolution. The fibrous alumina-rich microxenoliths are composed almost entirely of refractory minerals (sill + Kfsp ± Pl ± Bt) and interpreted as peritectic or restitic products of partial melting of assimilated metasedimentary rocks similar to the unmodified metamorphic xenoliths in the complex. Geochemical modeling indicates that, after early-stage contamination, each magmatic unit evolved separately. Whereas the northern dacites and low-SiO2 rhyolites underwent assimilation and fractional crystallization throughout their history, the southern rhyolites mainly evolved via fractional crystallization of felsic phases alone. This study shows that the peraluminous nature of felsic magmas do not necessarily originate by partial melting of crustal material but can be acquired by metaluminous magmas during later evolution. The processes shown here of assimilation and fractional crystallization and pure fractional crystallization has relevance for other igneous bodies of similar compositions in the Puna backarc and worldwide.

In typical Andean arc magmas, amphibole appears as a phenocryst phase only after considerable differentiation. However, some near-primitive volcanic rocks (high-Mg andesites and basalts) from monogenetic centers in the Puna plateau of Argentina also contain amphibole phenocrysts, implying special conditions of hydrous magma generation in this back-arc setting. This study documents typical examples from Southern and Northern Puna regions and uses the major and trace-element compositions of amphibole to constrain a petrogenetic model for the hydrous magmas. There are significant differences in the nature of amphiboles and their host lavas depending on location of the volcanic centers in the Southern and the Northern Puna regions. In the Southern Puna, basaltic andesitic lavas have Sr/Y values>40 and amphiboles showskeletal forms and occur in an assemblagewith olivine and pyroxene. The amphibole compositions are relatively Al- and Ti-poor compared to the Northern Puna. Thermobarometry indicates amphibole crystallization temperatures of 960-1000 degrees C at moderate pressure (< 5 kbar). In contrast, the mafic lavas from centers in the Northern Puna show Sr/Y ratios lower than 20 and amphiboles in these rocks coexist with a plagioclase-orthopyroxene assemblage. The Northern Puna amphiboles have higher Ti and Al contents than those in the southern region and the thermobarometry estimates imply generally higher crystallization temperatures (>1000 degrees C) and pressures (6-8 kbar). Furthermore, the chemical composition of amphibole phenocrysts in the Northern Puna Campo Negro center suggests an alkaline affinity of the parentalmagmas which, together with radiogenic isotope data from earlier studies, indicates a significant contribution of the enriched lithosphere in the magma source. The new data collectively suggest high pressure evolution of hydrousmagmas in the Southern Puna, whereas the Northern Puna magmas underwent more differentiation at higher levels in the crust. This contrast in the evolution history ofmagmas belowboth regions can be connectedwith their position relative to partialmelting zones in the mid-upper crust, which are larger and longer-lived in the north than in the south, thus favoring a slower ascent of magmas in that region. (C) 2020 Elsevier B.V. All rights reserved.

The lithium-rich brines of the Salar de Olaroz in the Central Andes of NW Argentina are considered to be of great economic and strategic interest. This study focused on the fluid source(s) and geochemical processes governing the chemical and isotopic characteristics of the surficial waters of Olaroz (residual brines, ephemeral lakes, rivers and tributary streams), aiming to define the mechanisms leading to such a huge Li reservoir. The chemistry of the Rosario River, which is one of the main sources of recharge of the Salar de Olaroz, is mostly controlled by fluid inputs from hydrothermal systems located north of the salar (in the volcanic areas of Rosario de Coyaguayma, Pairique, and Cono Panizo). The hydrothermal fluids are characterized by relatively high Li concentrations, as they interact with Li-rich rocks pertaining to Miocene – Pliocene volcanic formations, Ordovician sedimentary deposits, and, possibly, pre-Ordovician crystalline basement. In the salar, the hyperarid climate regulates the relative proportion between supplied waters and evaporation, inducing deposition/dissolution of salts, which controls the concentrations of main ions in brines and ephemeral lakes. Hence, the peculiar combination of a Li-rich primary source, the hydrothermal scavenging by geothermal fluids that feed the Rosario River, and secondary concentration processes affecting the surficial water within the salar leads to the formation of the huge Li reservoir characterizing this area.

Fluid primary source(s) and chemical-physical processes controlling water and gas chemistry of thermal springs from Eastern Cordillera, sub-Andean Ranges and Santa Bárbara (Jujuy Province, northern Argentina) were investigated to provide information for a preliminary evaluation of the geothermal potential in these areas. Thermal manifestations in Eastern Cordillera (Reyes) and part of those in the western sector of sub-Andean Range (Aguas Calientes) are fed by shallow aquifers, interacting with Quaternary- Neogene rocks and the upper portion of Pliocene-Miocene formations (Orán Group), whereas the meteoric water recharge area is located at >2500 m a.s.l., corresponding to Chañi hill. Differently, El Jordán thermal spring in the sub-Andean Range is fed by a hydrothermal aquifer hosted within highly porous and fractured formations of the Salta Group (Yacoraite Formation) and recharged by meteoric water from Sierra de Calilegua (∼1500 m a.s.l.). The latter is the recharge area of the La Quinta geothermal waters as well, but these have been fed at higher altitudes (>2500 m a.s.l.) in the range. The hydrothermal reservoir feeding the other thermal springs from the Santa Barbara system (Caimancito, El Palmar, and Siete Aguas) is recharged by meteoric water from Zapla Ranges and Santa Barbara Hill at <2500 m a.s.l. The high-TDS (>16,000 mg/L) Na+-Cl- La Quinta thermal springs are produced by interaction with the evaporite deposits of Salta Group, including halite, whereas the chemistry of El Palmar, El Jordán and Caimancito thermal springs, showing a Na+-SO42-(Cl−) composition, depends on mixing with shallower SO42--rich waters interacting with gypsum deposits of Anta Formation. Dissolved and bubbling gases from all the investigated provinces are related to CO2- and CH4-rich crustal fluids produced by both thermogenic processes occurring within the hydrothermal systems and microbial activity at relatively low depth, with low to negligible mantle contribution, as indicated by the 3He/4He values ≤ 0.21 Ra. The fluid reservoir feeding the Quinta thermal springs shows the highest estimated temperatures (>200 °C), which, considering the depth of Salta Group in the Santa Barbara system (~2000 m), support the idea, suggested by previous authors, of an anomalous geothermal gradient for this area, a promising pre-requisite for future exploitation of the geothermal resource.

We investigate the Li isotope composition and the Li concentrations of metamorphic and sedimentary rocks of the Palaeozoic (Pz) basement in the Central Andes and follow the trace of the Li in the Cenozoic volcanic rocks at the active continental margin. The average Li isotope composition of Pz-basement closely resembles global averages of upper crustal rocks with overlapping, but higher average Li content in the Pz-basement. Lithium isotope composition and content in the Cenozoic volcanic rocks of the Central Volcanic Zone (CVZ) range from mantle-like signatures to Pz-basement compositions with high δ 7 Li values and high Li contents. Evolutionary trends of the Li isotope composition in the CVZ volcanic rocks can be explained by assimilation of the Pz-basement. At a margin-wide scale, the abundance of Li in the CVZ volcanic rocks is higher than that of the Cenozoic volcanic rocks of the active Andean arc north and south of the CVZ. The CVZ volcanic and Pz-basement rocks are considered to be the primary source of Li in world-class Li-deposits in evaporates of the Altiplano-Puna high plateau and its western slope between ca 27° and 20° S. These deposits define the so-called “Lithium-Triangle”, between southern Bolivia, NW Argentina and NE Chile. The pivotal processes of extraction of Li from its primary rock sources and of Li migration from the source rocks to the deposits still await detailed investigation.

Salars of the Andean Plateau in the Central Andes are the largest lithium deposits on Earth. The most notorious are the Chilean Salar de Atacama, and Uyuni in the Bolivian Altiplano. Despite the relevance of the region concerning the lithium resources, there is still scarce scientific literature on the hydrochemistry of lithium deposits in the Argentine portion of the Andean Plateau. In this article we present new hydrochemical data from the first regional-scale reconnaissance exploration of the four major salars in the northernmost Argentine Andes. Data revealed that brines in the studied salars are characterized by mean Li concentrations ranging between 82 and 1014 mg L-1, and mean Li:Mg ratios from 0.92 to 0.54. The size of the study salars becomes a potential limitation for the whole Li resources in comparison with the giant Atacama and Uyuni. Nonetheless, when considering the Li grade and the Li:Mg ratio of brines, the Northern Puna salars turn out to be very remarkable lithium prospects. Data emerged from this survey represent a valuable tool for: 1) private investment projects by defining Li mining targets, 2) for the administration of natural resources and the definition of the State's politics, and 3) for scientific purposes, especially in investigations meant to better understand the processes involved in the formation of Li brine deposits, salars, and endorheic basins.

Fil: Lopez Steinmetz, Romina Lucrecia. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Salta. Instituto de Ecorregiones Andinas; Argentina. Universidad Nacional de Jujuy. Instituto de Geologia Minera; Argentina

This study focused on the geochemical and isotopic features of thermal fluids discharged from five zones located in the high altitude Puna plateau (Jujuy Province between S 22°20′–23°20′ and W 66°–67°), i.e. Granada, Vilama, Pairique, Coranzulí and Olaroz. Partially mature waters with a Na+-Cl− composition were recognized in all the investigated zones, suggesting that a deep hydrothermal reservoir hosted within the Paleozoic crystalline basement represents the main hydrothermal fluid source. The hydrothermal reservoirs are mainly recharged by meteoric water, although based on the δ18O-H2O and δD-H2O values, some contribution of andesitic water cannot be completely ruled out. Regional S-oriented faulting systems, which generated a horst and graben tectonics, and NE-, NW- and WE-oriented transverse structures, likely act as preferentially uprising pathways for the deep-originated fluids, as also supported by the Rc/Ra values (up to 1.39) indicating the occurrence of significant amounts of mantle He (up to 16%). Carbon dioxide, the most abundant compound in the gas phase associated with the thermal waters, mostly originated from a crustal source, although the occurrence of CO2 from a mantle source, contaminated by organic-rich material due to the subduction process, is also possible. Relatively small and cold Na+-HCO3−-type aquifers were produced by the interaction between meteoric water and Cretaceous, Palaeogene to Miocene sediments. Dissolution of evaporitic surficial deposits strongly affected the chemistry of the thermal springs in the peripheral zones of the study area. Geothermometry in the Na-K-Ca-Mg system suggested equilibrium temperatures up to 200°C for the deep aquifer, whereas lower temperatures (from 105 to 155°C) were inferred by applying the H2 geothermometer, likely due to re-equilibrium processes during the thermal fluid uprising within relatively shallow Na-HCO3 aquifers. The great depth of the geothermal resource (possibly>5000m b.g.l.) is likely preventing further studies aimed to evaluate possible exploitation, although the occurrence of Li- and Ba-rich deposits associated may attract financial investments, giving a pulse for the development of this remote region.

Neogene back-arc mafic volcanism in the northern Puna Plateau produced a suite of mainly high-Mg, calc-alkaline, basaltic andesites to andesites that form small scoria cones and lava fields. We present the first comprehensive geochemical study of the mafic suite in the northern Puna that complements work on similar rocks from the southern Puna Plateau. The emphasis is on magma genesis and evolution in both areas, and on a combined interpretation of the two regional datasets in the geodynamic context of back-arc magmatism in the central Andes. The results from the northern Puna suite (bulk-rock and mineral compositions, thermobarometry and radiogenic isotope ratios) are consistent with a predominantly asthenospheric source for the mafic magmas, with variable but locally significant contamination by crustal material. Quantifying the crustal and mantle input fails in most cases because the data display contradictory features, such as high compatible element contents (Mg, Ni, Cr) paired with moderate contents of silica and incompatible lithophile elements that defy classical models of magma mixing, fractionation and assimilation. We suggest that magma evolution involved selective assimilation during turbulent flow, probably at more than one level in the crust. Comparison with the southern Puna mafic suite reveals many features in common (high magma temperatures, textural evidence of rapid magma ascent and cooling, assimilation of crust at different depths). However, the volume of erupted magma is greater in the south than in the north and the volcanism in the south is slightly younger. There is much compositional overlap between the two regions, but the southern Puna suite extends to more primitive compositions. These differences suggest a stronger crustal influence in the northern Puna andesites, which we suggest is due to the presence of an extended upper-crustal melt zone associated with the Altiplano-Puna ignimbrite province. Radiogenic Sr and Nd isotope data from both suites define two diverging trends of variation with MgO that can be explained with a crustal component common to both trends, similar to the silicic ignimbrites, and two contrasting mantle components. The more common and regionally more widespread of the two mantle components (also seen in the frontal arc magmas) has Sr-87/Sr-86 and Nd-143/Nd-144 values of 0 center dot 705 and 0 center dot 5126, respectively, which we attribute to an asthenospheric source enriched by subduction erosion. The less common of the two has Sr and Nd initial ratios (0 center dot 708 and 0 center dot 51235) that we attribute to melting or assimilation of enriched lithosphere. This component has been found only in the northern Puna and it may have an origin in delaminated lithosphere.

We present the first study of the volcanic rocks of the Canadon Asfalto Formation that host the Navidad world class Ag + Pb epithermal district located in the North Patagonian Massif, Patagonia, Argentina. These volcanic and sedimentary rocks were deposited in a lacustrine environment during an extensional tectonic regime associated with the breakup of Gondwana and represent the mafic to intermediate counterparts of the mainly silicic Jurassic Chon Aike Volcanic Province. Lava flows surrounded by autobrecciated carapace were extruded in sub aerial conditions, whereas hyaloclastite and peperite facies suggest contemporaneous subaqueous volcanism and sedimentation. LA-ICPMS U-Pb ages of zircon crystals from the volcanic units yielded Middle Jurassic ages of 173.9 +/- 1.9 Ma and 170.8 +/- 3 Ma. In the Navidad district, volcanic rocks of the Cafiackin Asfalto Formation show arc-like signatures including high-K basaltic-andesite to high-K dacite compositions, Rb, Ba and Th enrichment relative to the less mobile HFS elements (Nb, Ta), enrichment in light rare earth elements (LREE), Y-Ti depletion, and high Zr contents. These characteristics could be explained by assimilation of crustal rocks in the Jurassic magmas, which is also supported by the presence of zircon xenocrysts with Permian and Middle Upper Triassic ages (2813 Ma, 246.5, 218.1, and 201.3 Ma) and quartz xenocrysts recognized in these volcanic units. Furthermore, Sr and Nd isotope compositions suggest a contribution of crustal components in these Middle Jurassic magmas. High-K basaltic andesite has initial Sr-87/Sr-86 ratios of 0.70416-0.70658 and xi Nd(t) values of -5.3 and -4. High-K dacite and andesite have initial Sr-87/Sr-86 compositions of 0.70584-0.70601 and Nd(t) values of -4,1 and -3,2. The range of Pb isotope values ((206)pb/(204)pb = 18.28-18.37, Pb-207/Pb-204 = 15.61-15.62, and (208)pb/(204)pb = 38.26-38.43) of Navidad volcanic rocks and ore minerals suggest mixing Pb sources with contributions of mantle and crust. Pb-206/Pb-284 isotopic ratios of Jurassic volcanic rocks of the Chon Aike Volcanic Province and sulfides of associated epithermal deposits increase with time from the volcanic event V1 (188-178 Ma) to volcanic events V2 (172-162 Ma) and V3 (157-153 Ma), reflecting variations in the radiogenic Pb source as volcanism was migrating towards the Proto Pacific margin of Gondwana. (C) 2017 Elsevier B.V. All rights reserved.