The Bou Azzer district in Morocco has a long mining history since the beginning of the XXst century during which it has become the only world producer of Co from primary, hydrothermal Co arsenide ores. Orebodies are structurally controlled, and mainly distributed along fault contacts between Cryogenian ophiolite-related serpentinite bodies and intrusive quartz diorite or, locally, ophiolitic gabbros or Ediacaran volcanic rocks. Ore formation took place through a multi-stage mineralizing process that included an early stage composed by gold, quartz, chlorite, muscovite and calcite, followed by the main arsenide and sulfarsenide stage (subdivided into three substages, IIa: Ni-rich, Co ores, IIb: Co-Fe ores and IIc: Fe-Co ores), and ending with an epithermal stage characterized by the precipitation of sulfides along with quartz and calcite. Field relations and most previous geochronologic dating pointed to a post Pan-African age of ore formation, mainly coincident with the Hercynian orogeny. The isotopic study presented in this paper includes S, Pb, Rb/Sr and Sm/Nd data of a set of ore mineral samples from three deposits (Aghbar, Tamdrost and Ait Ahmane), as well as of regional samples representative of the different lithologies occurring in the Bou Azzer area. The isotope data set was completed with S isotope analyses of arsenide and sulfarsenide minerals from five ore deposits (Filon 7/5, Aghbar, Tamdrost, Ightem and AitAhmane) and of some whole-rock regional samples. Results show that ores formed during multi-episodic hydrothermal events connected with hercynian reactivation of Devonian-Carboniferous faults, supporting previous geochronologic dating. The obtained Pb, Sr, Nd and S isotopic signatures of ore minerals and regional rocks further show that ophiolite-related lithologies became isotopically modified by interaction with crustal material and afterwards acted as the main source of ore-forming elements. Nevertheless, isotopic data do not fully concur with such a simple scenario but are quite consistent with a rather complex interpretation based on multi-source origin of some elements and isotopes scavenged from a number of isotopically different lithologies both from the inferred basement and the volcanic and sedimentary cover.

Numerous small, noneconomic copper deposits are associated with syn-tectonic sandstones of Tertiary age in the south Pyrenean foreland basin. Copper, primarily as sulphides or oxides, replaces woody organic debris and earlier formed pyrite. Sulphide sulphur and organic carbon concentrations are generally low (<2 wt. %) and δ34S values are compatible with bacteriogenic derivation from Pyrenean evaporites, which suggests that Cu-rich fluids were introduced into reduced sediments during diagenesis. Mineralizing processes took place in two different periods: the first one related to the filling of the Jaca basin during erosion of Pyrenean basement and the second one related to the emplacement of a thrust ramp which provoked the erosion of the previously formed cupriferous sandstones.

Chromitites hosted in the serpentinized harzburgite bodies from Los Congos and Los Guanacos (Eastern Pampean Ranges, north Argentina) record a complex metamorphic evolution. The hydration of chromitites during the retrograde metamorphism, their subsequent dehydration during the prograde metamorphism and the later-stage cooling, have resulted in a threefold alteration of chromite: i) Type I is characterized by homogeneous Fe3+- and Cr-rich chromite; ii) Type II chromite contains exsolved textures that consist in blebs and fine lamellae of a magnetite-rich phase hosted in a spinel-rich phase; iii) Type III chromite is formed by variable proportions of magnetite-rich and spinel-rich phases with symplectitic texture. Type I chromite shows lower Ga and higher Co, Zn and Mn than magmatic chromites from chromitites in suprasubduction zone ophiolites as a consequence of the redistribution of these elements between Fe3+-rich non-porous chromite and silicates during the prograde metamorphism. Whereas, the spinel-rich phase in Type III chromite is enriched in Co, Zn, Sc, and Ga, but depleted in Mn, Ni, V and Ti with respect to the magnetite-rich phase, due to the metamorphic cooling from high-temperature conditions. The pseudosection calculated in the fluid-saturated FCrMACaSH system, and contoured for Cr# and Mg#, allows us to constrain the temperature of formation of Fe3+-rich non-porous chromite by the diffusion of magnetite in Fe2+-rich porous chromite at <500 ºC and 20 kbar. The subsequent dehydration of Fe3+-rich non-porous chromite by reaction with antigorite and chlorite formed Type I chromite and Mg-rich olivine and pyroxene at >800 ºC and 10 kbar. The ultimate hydration of silicates in Type I chromite and the exsolution of Type II and Type III chromites would have started at ~600 ºC. These temperatures are in the range of those estimated for ocean floor serpentinization (<300 ºC and <4 kbar), the regional prograde metamorphism in the granulite facies (800 ºC and <10 kbar), and subsequent retrogression to the amphibolite facies (600 ºC and 4-6.2 kbar) in the host ultramafic rocks at Los Congos and Los Guanacos. A continuous and slow cooling from granulite to amphibolite facies produced the exsolution of spinel-rich and magnetite-rich phases, developing symplectitic textures in Type III chromite. However, the discontinuous and relatively fast cooling produced the exsolution of magnetite-rich phase blebs and lamellae within Type II chromite. The P-T conditions calculated in FCrMACaSH system and the complex textural and geochemical fingerprints showed by Type I, Type II and Type III chromites leads us to suggest that continent-continent collisional orogeny better records the fingerprints of prograde metamorphism in ophiolitic chromitites.

This communication provides the first-ever scientific study of the mineralogy and chemistry of magnetite of the Mg-skarn iron deposit of San Manuel, Serrania de Ronda in SW Spain. This iron deposit is hosted in marbles overlying migmatites found forming a tectonic slice along the southern contact of the Ronda ultramafic massif. Prograde and retrograde skam stages have resulted in zoning of magnetite, characterized by variations in the chemistry and typology of mineral inclusions. Using laser-ablation ICPMS we have measured the contents of trace elements in magnetite, fingerprinting the origin of the mineralizing fluid as related to the migmatites that originated during host emplacement of the upper mantle peridotites in the continental crust.

Thrust systems, related to thin-skinned tectonics are the main structural control for the development of Co-Ni deposits (Co-Ni arsenides, diarsenides and sulpharsenides) in the Alpine Central Pyrenees. Co-Ni ores occur in the hanging wall of these thrusts, hosted either by carbonate (San Juan de Plan and Crescencia mines) or siliciclastic rocks (Solite Mine), whereas the footwall is formed by black shales. Previous studies have applied graphite geothermometry and crystal-chemical criteria (the presence of alloclasite and the extent of the cobaltlte gersdorffite solid-solution) to determine that the complex assemblages of the studied deposits were formed at temperatures between 300 degrees C and 600 degrees C. The possible sources of the mineralizing fluids and metals have been investigated with a multi-isotope approach. Stable isotope data (C, O) support a primary magmatic source for the ore-forming fluids although interaction with both the Variscan black shales and carbonate host rocks is also evidenced. Nd and Sr isotope compositions suggest that these fluids reacted with basement siliciclastic and igneous lithologies (and to a lesser extent with carbonate rocks). This reaction scavenged sulfur from the Variscan basement as a whole (positive delta S-34 values) and specifically from the underlying local black shales (negative delta S-34 values) and local magmatic rocks (delta S-34 values close to 0 parts per thousand). Metals were extracted also from the Variscan black shales underneath each deposit, as suggested by the close correspondence between the metal ratios in the Co-Ni deposits and in the underlying black shales. The ore-forming activity responsible for the studied Co-Ni deposits spans an age range (Upper Cretaceous, 86 +/- 27 Ma) similar to that of other high temperature events, developed in a context of thinned crust in the Pyrenean realm (i.e. High T/low P metamorphism, mantle exhumation and alkaline magmatism). These processes promoted high temperature hydrothermalism and formation of ore deposits, prior to the tectonic inversion and thrusting during the compressive stage.

Manganese deposits hosted by the Upper Jurassic limestones (Camarias mine) and the Lower Cretaceous sandstones (Crivillen mine) of the Iberian Range occurs in karstic and pseudo-karstic landforms, sometimes related to E-W and ESE-WNW striking faults. Two distinct morphologies have been observed at Camalias mine: veins and pocket or nest-like bodies, of metric to decimetric scale. This mineralization is marked by an assemblage composed by pyrolusite, romanechite, quartz, and calcite and rare Fe-rich chlorite and barite. In the case of the Crivillen deposit, mineralization consists of cavity fillings of metric to decimetric scale. The internal structure of the orebodies is typically laminated. Ore mineral assemblage is made up of pyrolusite, romanechite and strontiomelane. The presence of hydrothermal minerals and also fluid inclusions in quartz crystals, point to hydrothermal process being responsible for both cavities and ore formation. This ore-forming fluid was reduced and acid since manganese is a soluble and mobile element with a bivalent state (Mn2+) in such conditions. The (pseudo)karst system could provide enhanced fluid percolation and fluid/rock interaction promoted oxidation and increasing pH by cement dissolution that was responsible for oxide and oxyhydroxide deposition. Also, the increasing pH lead to the decoupling of Mn from Fe.

Filon 7/5 is one of the most relevant orebodies in the Bou Azzer mining district. It is composed of a complex sequence of Co-Ni-Fe diarsenides, triarsenides and sulphoarsenides with extensive development of solid solutions between the end-members in each group. The textural relationships and the composition of the phases indicate a general evolution at high temperatures (about 650 degrees C), from Ni-ores to Co-ores through changing fugacities of As and S in the mineralizing fluid. This evolution leads to a very common development of replacement textures. The wide compositional variation of each of the analysed minerals is proposed to be the result of progressive reequilibration with fluids of changing composition, replacing early-formed phases by later ones. This process is supported, for example, by the remnants of high temperature phases (krutovite) inside As-gersdorffite Or by the compositional inheritance of krutovite/rammelsbergite showed by the cobaltite-gersdorffite crystals that replace them.

The Arroyo Rojo deposit, located in Tierra del Fuego, is the most important polymetallic, volcanic-hosted massive sulphide in the rhyolitic belt of the Fuegian Andes. The best intercepts in drill holes indicate a true thickness of 18.6 m and concentrations of 22% Cu, 3.9% Pb, 14.5% Zn, 140 g/t Ag, 1.1 g/t Au). This deposit, located near the town of Ushuaia, is hosted in a Middle Jurassic volcanic and volcanoclastic sequence. Massive and semimassive bodies display stacked lenticular morphologies with disseminated mineralization in both the footwall and hanging wall. The associated hydrothermal alteration system is partially conformable with the layering of the volcanic rocks. The ores and host rocks display a penetrative tectonic foliation and were metamorphosed to greenschist facies.Previous studies have not resulted in a consensus regarding the nature and the source of ore-forming fluids and the style of deposition of the sulphides at Arroyo Rojo. In this study, both stable and radiogenic isotopes were used develop a better understanding of these aspects of the deposit.Hydrogen and oxygen isotopes indicate that an evolved seawater mixed with significant contributions from other fluid reservoirs such as magmatic and/or metamorphic waters was the most likely source of the ore forming fluids. These fluids underwent significant interaction with the underlying volcanic and sedimentary rocks, which promoted partial (Sr isotopes) or full (Pb isotopes) homogenization of radiogenic isotopes. delta S-34(CDT) values suggest that the sulphur was derived from several sources: biogenic reduction of seawater sulphate (BSR) in a restricted to closed basin was mixed with a heavier component derived from inorganic reduction of seawater sulphate (TRS) and possibly from sulphur leached from igneous footwall rocks and/or direct contribution from magmatic fluids.Lateral infiltration of hydrothermal fluids resulted in the formation of a halo of semimassive to disseminated ore due to the replacement of porous, reactive glassy and breccia tuffs.As a result of the hydrothermal circulation, two styles of mineralization are observed in the Arroyo Rojo deposit: a stringer zone and a halo of semimassive to disseminated ore corresponding to sub-seafloor replacement, and syn-sedimentary mineralization consisting of massive sulphides.This model is consistent with the geodynamic context of the study area: a narrow, deep-marine volcano-tectonic rift parallel to the Andean side of South America and related to the initial break-up of Gondwana (ca. 145 Ma). (C) 2016 Elsevier B.V. All rights reserved.

The Co–Ni arsenides from the Bou-Azzer mining district contain disseminated chromian spinels with the highest Zn, Mn and Co contents ever reported up to date in any geological environment. The rationale behind this study was checking the role of Zn, Mn and Co contents in chromian spinel as possible indicators of mineralized environments. To tackle this issue the chemical compositional variations of chromian spinel disseminated in barren serpentinite, in Co arsenide ores and in Cu sulphide ores from three different deposits (Aghbar, Tamdrost and Aït-Ahmane mines) were studied focusing on the alteration patterns of chromian spinel grains, their fracturing degree and relationship with the precipitation of ore minerals. Results show that chromian spinel crystals are zoned and strongly fractured. They record, at least, two fracturation events: an early one developed before or coeval with the alteration process that gave rise to the zoning, and a second one that disrupted the zoning pattern splitting the altered grains in fragments which became included and partly dissolved in arsenide minerals. The early fracturing and alteration of chromite occurred during the Pan-African orogenesis and became fractured again during the Variscan tectono-metamorphic evolution of the Bou-Azzer ophiolite, just before the formation of arsenide ores. Maximum ZnO contents (up to 19.7wt.%) occur in cores of chromian spinels associated with Co minerals from Aghbar, MnO reaches its maximum (21.4wt.%) in rims of crystals included in chalcopyrite and CoO (up to 2.3wt.%) concentrates in cores of grains hosted by skutterudite (CoAs3), all them from Aghbar mine. Chromian spinels from Tamdrost and Aït-Ahmane ores have much lower contents in these elements. Zn and Mn concentration in chromian spinel are neither related with the ore type nor with the mineralization degree of the host suggesting that these elements became enriched in chromian spinel during its early, ocean-floor alteration in a metal-rich environment characterized by the nearby presence of hydrothermal vent fields and forming volcano-sedimentary massive sulphide deposits (e.g. the Bleida deposit). In contrast, Co cannot be upgraded up to the levels measured in these chromian spinel grains in this ocean floor environment but its high contents seem to be related with the formation of the arsenide ores.

Vein fluorite deposits (Tebarray, Lanuza, Bielsa-Parzan, Bizielle and Yenefrito) as well as one MVT-style fluorite mineralization (Portalet) in the Central Pyrenees are the focus in this contribution. These deposits are made up of fluorite, barite, base metal sulphides, calcite, and quartz and are hosted in sedimentary rocks and granites of Palaeozoic age. Generally, these mineral occurrences, typically associated with Late Palaeozoic steeply dipping faults are similar with respect to geologic setting, mineralogy and geochemical trends to other fluorite and base metal veins located in the Central Pyrenees. Veins occurring along such faults most likely represent channelways used by mineralizing solutions that were expelled from the basement. Previous work argued for genetic processes involving circulation of mineralising fluids during the Triassic-Lower Cretaceous period, which is often considered to represent a period of heat, fluid, and mass transfers related to rifting events in the western European basins, which is related to the opening of the Atlantic.A major goal of this study was to decipher the timing of fluid flow and ore formation on the basis of Nd-Sm dating of fluorite sampled from a number of deposits sharing a similar geological framework. No precise age(s) could be obtained due to a scatter in data, but results from the Portalet MVT-style deposit point to a mid-Triassic age (around 220 Ma) for this mineralization. The model that best explains the diagenetic stratabound mineralization at Portalet is gravity-driven fluid flow involving basinal brines during a rifting stage. Indeed, the formation of horst and graben structures during Early Alpine extensional tectonics favoured the infiltration of meteoric water into uplifted blocks, followed by fluid migration through the deeper parts of the basins whereby heat and dissolved components were acquired. This model also explains diagenetic changes recorded in the host limestone at Portalet. Also, overall Pb, Sr and Nd isotopic ratios measured in galena and fluorite suggest that differences in host rock and in the lithology of the basement seem to have exerted control on the chemistry of mineralizing fluids providing each deposit with distinctive characteristics. (C) 2015 Elsevier B.V. All rights reserved.

In the Iberian Chains of northeastern Spain, the Hirnantian Orea Shale comprises three erosive glaciogenic unconformities punctuating two transgressive glaciomarine sequences. These sequences represent ice retreat episodes of grounded ice on a North Gondwanan region of 50-60 degrees S of estimated palaeolatitude. In contrast, the same formation recorded in the Hesperian Chains repeated episodes of extensional tectonic activity: diamictites were associated with slope-related debris flows and slumps commonly interrupted by truncating discontinuities.An analysis of delta C-13(org) in the Orea Shale has revealed that the lower Orea sequence displays isotopically light baseline values (similar to -27%0) punctuated by minor (2-3 parts per thousand) shifts. In distal parts, the upper sequence is characterised by a rapid rise in delta C-13(org) values, which mark a positive excursion, in the range of 2.5 parts per thousand to 7 parts per thousand over 40 cm of thickness. The stratigraphic gap involved in the intra-Orea erosive unconformity appears to be greater both in proximal exposures and slope-related (Hesperian) areas, where the chemostratigraphic shift was not recorded. Upsection in the three studied sections, the delta C-13 values display upsection a sharp wreturn to baseline values of -26 parts per thousand to -28 parts per thousand.The Hirnantian strata of the Iberian and Hesperian Chains have recorded three major correlatable events: (i) a the karstic surface that caps upper Katian limestones and reflects the maximum glacial extension; (ii) a single delta C-13(org) positive peak of-6 parts per thousand that is recognisable in distal ramp settings, marking the beginning of the second ice retreat episode and probably representing the late Hirnantian HICE shift; and (iii) the erosive unconformity marked by the progradation of the Los Puertos shoreline complexes. (C) 2015 Elsevier GmbH. All rights reserved.

The complexity of high-grade ore-shoot formation is addressed through a detailed study of the Martha mine epithermal deposit (Deseado Massif, Patagonia, Argentina). The mine was active from 2002 to 2012, producing over 21millionoz of silver from an intermediate sulfidation vein system hosted in a pyroclastic sequence erupted during Upper Jurassic times (40Ar–39Ar age: 157.6±1Ma). The Martha deposit is composed of more than 15 different veins, 8 of which show high grade ore-shoots (locally up to 45,000g/t Ag). Veins are hosted in a sinistral transtensive horst structure delimited by two NW–SE trending first-order veins (up to 600m long and 5m thick). Inside this horst, a series of E–W trending veins (up to 450m long and 2.5m thick) developed as second-order structures in response to growth and linkage of the two overlapping first-order vein structures. Ore is characterized by a complex paragenetic sequence of Ag–As–Sb sulfosalts and Cu–Pb–Zn sulfides, with an adularia-quartz-illite gangue. Hydrothermal alteration is zoned outward and rarely extends for more than 15m away from the vein walls; five alteration zones were defined: an adularia zone close to the veins, an intermediate halo characterized by illite and illite–smectite zones, and a distal smectite zone, additionally a late and supergene kaolinite zone can be found near the surface in close association with the mineralized structures. High grade “bonanza” ore-shoots are the result of the interplay of a series of structural, hydrothermal (mostly physicochemical) and supergene processes. The mineralization evolved through three main phases: a main hydrothermal tectonically controlled phase, a late tectonic–hydrothermal phase and a post-mineral supergene phase. The main hydrothermal phase was the result of mineral deposition from low salinity (0.5 to 3.5wt.% NaCl eq.) near-neutral to weakly alkaline chloride aqueous hydrothermal solutions with estimated temperatures ranging from 215.5°C to 316.5°C, and isotopic composition suggesting high proportions of meteoric waters (δ18OH2O from −8.1‰ to −2.7‰). Relatively constant fluid inclusion liquid-to-vapor ratios and a trend of decreasing Th and δ18OH2O with increasing time suggest that cooling and dilution were the main mechanisms controlling ore deposition. These physicochemical conditions, combined with an efficient network of structural conduits (vein intersections, jogs and step-over zones) were the first step in the construction of the high grade ore shoots that characterize the vein system. 40Ar–39Ar dating in adularia crystals constrains the hydrothermal activity to the Upper Jurassic (156.5±0.9Ma). Afterwards, a late tectonic–hydrothermal phase occurred through the development of irregularly-shaped bodies of massive fault breccias and foliated cataclasites related to tectonic reactivations of the vein system in synchronism with the waning stages of the hydrothermal system. The interplay between deformation and fluid circulation resulted in mechanical and chemical remobilization of the previously deposited Ag-rich sulfosalts by lower temperature (138°C to 168°C) and higher salinity fluids (7.7 and 9.9wt.%. NaCl eq.), possibly reflecting a late magmatic input into the waning system. This late tectonic–hydrothermal phase strongly contributes to increase the grades in some veins, and therefore is considered an essential process in order to build the highest-grade portions of the ore-shoots found at the mine. Finally, a post-mineral supergene phase defined a 20m thick oxidized zone close to the surface, followed by a 20–30m thick secondary enrichment zone below it, developed in response to the circulation of descending cold meteoric waters. Despite its small size and vertical extension, the secondary enrichment level at Martha mine constitutes an important economic factor for the mining operations as it helped to increase the silver grades in the shallower portion of the vein system. The Martha mine vein system constitutes an excellent example of how the interplay of hydrothermal processes controlling mineral deposition, structural factors building up an efficient plumbing system, tectonic ore-remobilization, and late supergene secondary enrichment processes, are combined in order to build “bonanza”-grade ore shoots in the epithermal environment.

The Bizielle vein has some unique features among the Pyrenean alpine veins that allow us to address the question of the nature of iron oxides transformations under low temperature hydrothermal conditions, which is well known to prevailed over wide areas of western Europe between early Triassic to early Cretaceous times. Isotopic studies indicate a deep-seated origin of the ore-forming fluids and suggest that the metals were leached from the Variscan basement (mainly from granites). Isotopic geothermometry and regional evidences point to a 250°C and reducing fluid, being SH2 the predominant S specie. Under such conditions, the proposed in situ deposition of hematite is a consequence of Fe carbonates dissolution and oxidation involving dissolution/precipitation processes in the sense of Putnis. Non-redox model is a quite plausible origin for subsequent hematite to magnetite conversion.

•Discussion about Bajada del Diablo astrobleme-strewn field (BdD).•BdD includes two different but contiguous geologic environments.•Geophysical research over both formations is characteristic of impact structures.

The Arroyo Rojo deposit is the most important polymetallic, volcanic-hosted massive sulphide close to the town of Ushuaia in the rhyolitic belt of the Andes of the Tierra del Fuego. This deposit is hosted by a volcanic and volcanoclastic sequence, Middle Jurassic in age. This Zn-Pb-Cu deposit display a lenticular morphology developed as a stacked lenses-style, with disseminated mineralization at both the footwall and hanging-wall. The ores and host-rocks have one penetrative tectonic foliation and have been metamorphosed to greenschist facies. Associated hydrothermal alteration follows a stratabound model. Stable isotopes geochemistry points towards the mineralizing fluids being derived from the interaction and re-equilibration with the siliciclastic portion of the host rocks, with sulphur being derived from a mixture of biogenic reduction of seawater sulphate in some restricted to closed ambient with probably magmatic influence, and heavier delta S-34(CDT) values derived by leaching of H2S from the detrital rocks. Lead and strontium isotopes indicate that the metals were leached from the sedimentary rocks. This process allowed re-equilibration and homogenization of hydrothermal fluids.

El objetivo de esta tesis es avanzar en el conocimiento de la geologia y modelos de alteracion hidrotermal de los yacimientos de sulfuros masivos de los Andes Fueguinos. Para ello se ha seleccionado el deposito de Arroyo Rojo, el mas representativo del cinturon Rio Encajado-Rio Hambre (Tierra del Fuego, Argentina). Este deposito encaja en la parte volcanica y volcanoclastica de la Formacion Lemaire, de edad jurasica media, compuesta de base a techo por: riolitas, dacitas, ignimbritas, tobas y lavas, que yace sobre el basamento pre-jurasico y bajo las andesitas hialoclasticas de la Formacion Yahgan. Este deposito de tipo Zn-Pb-Cu presenta morfologia lenticular con facies masivas, semimasivas y bandeadas, intercaladas con zonas de stringer, brechas y de mineralizacion diseminada, dando lugar a un deposito de lentes apiladas (stacked lenses-style o stacked), con mineralizacion diseminada a muro y techo. La mineralogia principal del mismo es pirita y esfalerita, con cantidades menores de galena y calcopirita, y trazas de pirrotita, arsenopirita, tetraedrita y bournonita. El grado de metamorfismo alcanzado en el deposito, facies prenhita-pumpellita, dio lugar al desarrollo de una foliacion tectonica muy penetrativa, generando una intensa cataclasis en la pirita, y la removilizacion de los sulfuros mas ductiles, que precipitaron como cemento entre granos y relleno de fracturas. La deformacion asociada dio lugar en estos ultimos al desarrollo de texturas de deformacion, recristalizacion y annealing, y en las rocas de caja a la formacion metariolitas, milonitas y ultramilonitas. La alteracion regional de fondo marino, caracterizada por la presencia de titanita, clinozoisita y clorita ferrosa, junto a cuarzo y albita, esta parcialmente obliterada por la alteracion hidrotermal del deposito. El modelo de alteracion hidrotermal es de tipo estratoligado o concordante, desarrollando de muro a techo: (1) Cuarzo-Clorita ± Sericita, (2) Cuarzo-Clorita, (2) Clorita ± Cuarzo ± Sericita-Calcita, (3) Cuarzo-Clorita ± Calcita y (4) Sericita + Cuarzo ± Clorita ± Calcita. En el que las composiciones de clorita magnesica y mica blanca fengitica predominan en las proximidades de las lentes mineralizadas. La accion combinada de estas dos alteraciones, reflejada en el balance de masas, muestra una elevada movilidad de los oxidos mayores principalmente en la alteracion cloritica con ganancias importantes en MgO, Fe2O3, Cu y S, mientras que en la sericitica tiene lugar una ganancia importante de MgO, y perdida de Na2O y As, siendo en ambos casos la mayor transferencia de masa en MgO. A pesar de ello, se ha preservado la estratificacion primaria en los contactos entre facies, y algunos vestigios del origen magmatico, como los fenocristales y fenoclastos de la secuencia volcanica encajante. La mayoria de las rocas volcanicas y volcanoclasticas del deposito presentan una composicion quimica de riolitica a riodacitica y una afinidad geoquimica de toleitica a intermedia, mostrando en general un caracter subalcalino. El ambiente geotectonico de formacion de las mismas corresponde a un arco volcanico y el patron de distribucion y geoquimica de las REE y elementos traza indican un ambiente de estadio inicial de cuenca de tras-arco, sugiriendo un origen mixto para este magmatismo por fusion parcial de la corteza continental asociada al rifting, e influencia del arco magmatico. La geoquimica isotopica de Pb, Sr y Nd indica un origen de las rocas igneas y sedimentarias de la Formacion Lemaire orogenico relacionado con la apertura de la cuenca marginal de tras-arco (Rocas Verdes). Las composiciones isotopicas Sr-Nd de la alteracion hidrotermal y rocas asociadas al deposito de Arroyo Rojo indican un origen mixto para el mismo, entre mantelico y cortical, asociado a esta cuenca de tras-arco. Los datos de geoquimica isotopica de S, H/D, Pb y Sr e inclusiones fluidas sugieren un modelo hidrotermal para este deposito en el que los fluidos mineralizadores se almacenaron en la serie sedimentaria infrayacente, correspondiente a la Formacion Lemaire, que actuo como un acuifero permeable, produciendose el lavado continuo de metales y S. La intrusion de un pluton subvolcanico ejercio de fuente de calor desarrollando un sistema hidrotermal de conveccion (magmatic floor heating) de descarga difusa. El flujo de estos fluidos salinos y densos se canalizo a traves de las rocas mas permeables (tobas y brechas) hasta alcanzar la superficie marina por la que se desplazaron a favor de la pendiente, acumulandose en el fondo cuencas de tipo brine pool. Las condiciones climaticas y escarpada orografia de los Andes Fueguinos condicionan la prospeccion en este distrito (Fin del Mundo). En este sentido el PIMA se ha revelado como una herramienta muy util en la exploracion de otros VMS del cinturon mineralizado, mostrando una muy buena correlacion de la composicion quimica de la clorita y el contenido fengitico y paragonitico de la mica blanca con las longitudes de onda de absorcion de Fe-OH y Al-OH, respectivamente, y con la proximidad a las lentes mineralizadas. Los resultados de este estudio son de gran ayuda para la identificacion de asociaciones de facies volcanoclasticas felsicas proximales, nuevos horizontes mineralizados y zonas de alteracion hidrotermal preferentes en otras areas del cinturon mineralizado Rio Encajonado-Rio Hambre.