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The Vegas Peladas iron prospect (35° 20´ SL - 69° 56´ OL) is one of several Fe, Fe-Cu, Cu (Ag) deposits hosted in the Mesozoic carbonate-rich, sedimentary rocks that are associated with Tertiary intrusions of the Malargüe Fold and Thrust Belt (34-36° SL; Cordillera Principal, SW of Mendoza). The Vegas Peladas iron prospect is hosted in the marine sedimentary rocks of the Puchenque (450 m) and Calabozo (50-100 m) Formations that crop out along both margins of the Vegas Peladas Creek. The Puchenque F. consists of intercalations of calcareous claystone, silttone, shale and sandstone with calcareous cement and the Calabozo F. is a homogeneous mudstone-wackestone. On the NE side of the creek, the gypsum of the Auquilco Formation unconformably overlies these units. The sedimentary units have been affected by Andean tectonism and have been intruded by the following igneous sequence: 1) diorite pluton; 2) granodiorite pluton; 3) granite pluton; 4) andesite dikes and sills. The presence of abundant diorite xenoliths within the granodiorite pluton and the textural evidence of plastic flow suggest that mingling with local chemical exchange occurred between the diorite and granodiorite magmas. Rb-Sr isochrons yielded an age of 15.19 ± 0.24 Ma for the granodiorite pluton with an initial 87Sr/86Sr ratio of 0.704351 ± 0.000044. The 47Sm/144Nd and 143Nd/144Nd ratios are consistent with a mantle source for these calc-alkaline, metaluminous rocks with little crustal contamination. Hydrothermal alteration and mineralization occur in the contact of the four intrusive types. Nevertheless the most important metalogenetic event is an Fe skarn with a wide contact metamorphic aureole (hornfels-marble) that has replaced the Puchenque and Calabozo Formations at the contact with the diorite pluton. The skarn consists of a prograde zoned exoskarn, with an inner pyroxene + magnetite + quartz or garnet (And31-89 Py 0.34-2 Grs68-8.8) ± quartz zone and an intermediate garnet (And 38-51 Py 1-2 Gr 61-47) ± pyroxene massive zone with garnet (And 96.2-100 Py 0.0-0.08 Grs 0.0-3.7) ± pyroxene (Di24-70 Jo4.1-0.7Hd72-29.3) veins. The diorite margin has incipient and selective actinolite ± chlorite ± calcite ± orthoclase ± epidote alteration and a thin massive orthoclase + quartz envelope. Based on the fluid inclusion (FI) data, these zones were formed at depths of 3.3 km under lithostatic pressure of 877.5 bars, by brines (>50% NaCl eq. with NaCl ± KCl ± FeCln ± hematite) with high temperatures (670-400°C); these brines coexisted with vapour of low density. Under these pressures and temperatures, it is likely brines and vapours were formed by immiscibility of a low to moderate salinity magmatic fluids (6-8 % en peso NaCl eq.) that exsolved from the still crystallizing diorite pluton. The isotopic data (δ18O 7.23-8.53 ) of the water in equilibrium with the inner garnet zone of the exoskarn indicates the presence of fluids of magmatic origin. The mineralogical zonation of the exoskarn reflects differences in the solubility of chemical elements (Si > Mg, Fe > Al) in the hydrothermal fluids. During this early stage, the continuous interaction with the wall rock, reduced isobarically the temperature of the brines (up to ~250°C). Superimposed on the prograde exoskarn zones the following retrograde assemblages were recognized: 1) epidote + magnetite ± quartz that replace the inner zone; 2) epidote ± quartz ± albite that replace the massive intermediate zone of garnet ± pyroxene and 3) amphibole ± quartz ± feldspar ± epidote that replace the veins of the intermediate zone and cut hornfels in distal zones. In the diorite pluton, an epidote ± quartz ± amphibole ± pyrite endoskarn, a few centimetres wide, replaces early alteration together with widespread veins and veinlets of amphibole ± quartz ± magnetite ± epidote ± feldspar. The iron mineralization -magnetite, specular hematite and mushketovite- is mainly associated with retrograde mineral assemblages. In the endoskarn, the magnetite is in contact and equilibrium with epidote, quartz and amphibole. In the inner zone of the exoskarn, it is in equilibrium with clinopyroxene ± quartz. But the major iron outcrops (up to 32 x 4.5 m), consist of magnetite in equilibrium with epidote ± quartz replacing the inner zone of the exoskarn. This magnetite contains between 83 to 88 % of FeO total. Specular hematite is in equilibrium with epidote + quartz, forming massive bodies (up to 4 x 1 m) that replace the intermediate zones of the exoskarn. Distal Fe mineralization fills veins several meters long and 5 to 40 cm thick with a core of mushketovite or specularite and magnesio-hornblende + actinolite ± epidote ± quartz selvages. The mushketovite (that replace the specularite) has more iron (93.5 y 95% FeO total) than earlier magnetite. FI in quartz indicate that iron oxides and retrograde minerals precipitated under hydrostatic pressures of 325 to 125 bars from saline (41.6-23 % NaCl eq.) fluids at temperatures between 420°C and 320°C. This change in the pressure regimen was induced by exsolution of volatiles from the magma, sealing of conducts by silicate mineral precipitation, and consequent fracturing of the exoskarn and boiling of the fluids. Under hydrostatic pressures, the increase of permeability allowed the infiltration of external fluids (formations water ± meteoric water?) to the hydrothermal system, their mixing with the magmatic fluids and cooling, promoting early silicate mineral instability and their replacement by hydrous minerals, quartz and the massive precipitation of most iron oxides, that also filled fractures, veins and cavities. The δ18O (~5 -13) values obtained for a fluid in equilibrium with magnetite, suggest external influence, like formation waters with high δ18O values of the sedimentary sequence. The δ18O (~ 4.67-6.69 ) values of the fluid in equilibrium with ferropargasite, its texture and chemical composition, indicate that this mineral is early among the retrograde minerals assemblage. The trend toward lower δ18O values in the fluid in equilibrium with retrograde epidote ((-0.55)-3.67), quartz ((-2.54)-4.31) and actinolite ((-0.55)-3.67 ) than the previous assemblages might be due to a temperature decrease after boiling and/or an influx of meteoric water into the system. The isotopic data of water in equilibrium with epidote of the endoskarn, indicates that epidote ± quartz ± amphibole ± pyrite and the veins of amphibole ± magnetite ± epidote ± quartz ± feldspar were formed under similar physical-chemical conditions as the retrograde mineral assemblages of the exoskarn. In this retrograde stage, the external zones collapsed downward and inside the hydrothermal system, in a reverse sequence that followed the prograde zones during their formation, and fluids that generated the retrograde paragenesis in the exoskarn, gradually invaded the heart of hydrothermal system. Later feldspar ± epidote ± quartz ± calcite ± titanite ± chlorite ± pyrite veins cut all the prograde and retrograde assemblages and hornfels and marble. FI in calcite indicate fluids with lower temperatures (165-315°C) and salinities (8.4 y 13.5 % of NaCl eq.) than previous stages. The δ18O values for the water in equilibrium with this epidote (-4.66) - (-0.19) and calcite ((-3.9) (-2.68) ), suggest mixing and dilution of pervious fluids with meteoric water (with a dominance of the later) during cooling and collapse of the hydrothermal system. Another metasomatic event formed an Fe skarn in association with the granite pluton and associated rhyolitic dikes. This skarn consist of: 1) a hornfels aureole a few meters wide; 2) an inner garnet (And30-81 Py0.6-0 Gr69-19) ± pyroxene (Di82-93 Jo3.7-1,7 Hd14-5,1) exoskarn zone with abundant scapolite (Me28-36); 3) an intermediate scapolite ± clinopyroxene (Di 18.5-4.1Jo 8.8-2.2Hd 74-93.7) zone that cut the hornfels; 4) a distal garnet (And10.5-83,1 Py 0..72-1.4 Grs 89-15,49) ± pyroxene (Di42,6-96 Jo2,6-1,27 Hd54,8-2,73) zone; 5) veins of scapolite (Me25-36) ± ferroactinolite ± pyrite cut the previous alterations; 6) mushketovite and mushketovite ± calcite veins cut the inner zones of the exoskarn, and 7) epidote + calcite + titanite ± quartz ± chlorite and pyrite also replace the distal zone. The intrusion of the granite pluton increased the temperature of the adjacent wall rock (>550°C) and generated saline fluids and vapour by immiscibility that caused dissolution of Fe from previous skarn. These brines carried Fe in solution and, when cooled, precipitated it as iron oxides. This second metasomatic event is minor in magnitude and has little metallogenic importance. The Vegas Peladas and Hierro Indio Fe skarns, located in southern Mendoza, present similar alteration and mineralization styles to world-class iron calcic skarns. The presence of several intrusive events in Vegas Peladas, located deeper than the Hierro Indio plutons, can explain its wider alteration envelope with several minerals assemblages and hydrothermal processes. Previous works assigned a Palaeogene age to the Fe skarns of southern Mendoza. New ages and the present study demonstrate that the iron skarns are associated with the least differentiated plutons, dikes and sills of the voluminous and ubiquitous magmatism of the Upper Miocene (Andesita Huincán). These Neogene igneous rocks derived from primitive magmas originated in the mantle, with little to no crustal contamination; their emplacement was structurally controlled. Thus, these plutons at the intersection of the main lineaments, thrusts, and fold cores in poorly explored areas, can host iron mineralization in associated skarn. The abundance of primary amphibole and magnetite in these igneous rocks and in the skarn gives them a strong magnetic response which is important from a prospective point of view. The earlier assemblages distribution in the igneous rocks associated with the iron skarn and their mineralogical zonation toward the sedimentary protolith contacts can be used as an exploration guide, in regions with poor exposure or without outcrops.

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