Le gisement Agsingle bondHg de Zgounder (Jebel Siroua, Anti-Atlas, Maroc) : un épithermal néoprotérozoïque de type Imiter <br />The Agsingle bondHg Zgounder ore deposit (Jebel Siroua, Anti-Atlas, Morocco): a Neoproterozoic epithermal mineralization of the Imiter type

The Zgounder ore deposit (Anti-Atlas, Morocco), is hosted in a PII–PIII Proterozoic volcanosedimentary series. Disseminated mineralization is dominated by mercuriferous native silver (2 to 30 wt.% Hg), with few silver sulfosalts (acanthite, pearceite), arsenopyrite and base-metal sulfides. Arsenic grade of arsenopyrite and homogenisation temperatures of fluid inclusions indicate initial conditions of high temperature (above 400 °C). Lead isotope compositions comfort a Late-Proterozoic age and a crustal origin for metals. Similarities are obvious with the neighbouring silver ore deposit of Imiter and lead to consider Zgounder as another example of Neoproterozoic epithermal deposit in the Anti-Atlas of Morocco, a region that appears more and more as a silver metallogenic province. Le gisement argentifère de Zgounder (Anti-Atlas, Maroc) est encaissé dans une série volcano-sédimentaire protérozoïque, rapportée aux PII et PIII. La minéralisation disséminée est à argent natif mercurifère (2 à 30 % Hg) dominant, sulfosels d'argent (acanthite, pearcéite), arsénopyrite et sulfures de métaux de base. La teneur en arsenic de l'arsénopyrite, ainsi que les températures d'homogénéisation des fluides précoces témoignent de conditions initiales de haute température (plus de 400 °C). Les compositions isotopiques du plomb confirment un âge fini-Protérozoïque et tracent une origine crustale des métaux. Les similitudes avec le gisement argentifère proche d'Imiter sont fortes et amènent à considérer Zgounder comme un autre exemple d'épithermal néoprotérozoïque dans l'Anti-Atlas marocain, qui apparaît ainsi de plus en plus comme une province métallogénique argentifère

Influence des préoccupations de carrière sur la fluctuation des attitudes au travail

La présente étude examine les fluctuations observées au niveau des attitudes au travail selon les étapes de carrière telles que définies initialement par Super (1957), et reprises ultérieurement par plusieurs chercheurs. Comme les modèles basés sur l’âge et l’ancienneté sont hautement critiqués en raison de l’inconsistance des résultats qu’ils génèrent, ils ont été incorporés à la présente étude à des fins de comparaison. Lorsque les étapes de carrière sont établies en fonction des préoccupations de carrière actuelles des individus, les résultats démontrent que les répondants aux périodes d’exploration et de désengagement rapportent, de façon générale, des attitudes au travail plus négatives que ceux qui se situent aux périodes d’établissement et de maintien. Les modèles basés sur l’âge et l’ancienneté dans l’organisation rapportent quant à eux peu de variation dans les attitudes au travail, confirmant ainsi les limites de l’utilisation de ces indicateurs comme variable d’approximation de l’étape de carrière

Hydrothermalismes anté-Hercynien en Sud-Ibérie : apport de la géochimie isotopique du plomb.

International audienceLead isotope study has been performed on massive sulphide deposits of Ossa-Morena and Aracena Belt (South Iberia). Results suggest the existence of at least two ancient hydrothermal events. The first one, Upper Brioverian in age (not, vert, similar600-570 Ma), gave birth to Maria-Luisa and Puebla de la Reina massive sulphide deposits; it thus confirms the existence of a Cadomian orogen in South Iberia. Isotopic compositions indicate a local contribution of mantle-derived material (Maria Luisa mine), confirming the presence of ancient oceanic crust in Aracena Belt. This mineralising event seems to extend till the Armorican Massif. The second episode, Eo-Hercynian in age (not, vert, similar400-350 Ma) has allowed genesis of massive sulphide deposits of la Nava Paredon and Aguas Blancas, and could be coeval with the emplacement of South-Iberian massive sulphide ore deposits in the neighbouring South-Portuguese Zone. A more continental crustal source for later ore deposits could explain the much more important metal accumulation in this zone

The meillers Autunian hydrothermal chalcedony : first evidence of a similar to 295 Ma auriferous epithermal sinter in the Frech Massif Central.

The Meillers "quartzite" deposit represents a major hydrothermal siliceous sinter, some 50 m thick and covering an area of some 30 ha within Autunian sandstone of the northern Massif Central. This siliceous sinter comprises three facies of microcrystalline quartz: (i) a dark-colored facies (black chalcedony), locally rich in pyrite, at the base, (ii) a gray-brown fairly massive facies (gray-brown chalcedony) in the middle; and (iii) a white, finely banded, facies (white chalcedony) at the top. Orientation measurements of the banding have revealed the paleo-flow channels of the silica-saturated fluids. The geochemistry of the deposits shows modest metal values, in particular for gold (average of 0.58 g/t Au, giving a gold metal content of some 20 t). The hydrothermal event is stratigraphically well correlated with the basal Autunian (around 295 Ma); an age that has been confirmed through radiochronological determination (300±21 Ma; SHRIMP on hydrothermal zircon). The lead isotopic composition is not very radiogenic (206Pb/204Pb=18.20) and is similar to that of the Late Hercynian gold lodes in the Massif Central. The mineralized sinter appears to have derived from a geyser-type hot springs. This hydrothermal activity coincides with the auriferous metallogenic peak that occurred in the Hercynides around 300 Ma. This is the first known epithermal-type surface manifestation described for the Hercynian gold event

Delta97/95Mo in molybdenites from the Azegour skarn (Morocco)

International audienceMolybdenum (Mo) isotopes are frequently used to investigate ocean and lake (paleo-)redox conditions. In the frame of mineral ressources, only few studies have been performed regarding Mo-Re-Os isotopes. The aim of this study is to understand the source, processes and mobility of metals concentrations using Mo isotopes on molybdenites in different ore deposits. The present study focuses on the Azegour skarn (Morocco). Located in the High-Atlas, the Azegour site is one of the rare Mo-W-Cu exploited skarns (three historic mines). It is formed by a granitic intrusion (271±3Ma) in cambrian volcano-sedimentary serie composed by schists, volcanic complex (andesites, pyroclastites) and carbonate formations (calcareous and dolomites). The skarn takes place in the carbonate formations where pyroxenites and grenatites occured. The grenatites being the Mo-bearing minerals in the form of molybdenites. Molybdenites sampling has been performed in the main mine (Azegour) and in the Tizgui mine (1km north of the Azegour mine). The Mo isotopic composition has been determined on molybdenites using a MC-ICP-MS Neptune after aquaregia dissolution and adjustment to [Mo] = 1µg.g-1. The δ97/95Mo ratios have been normalized to NBS3134 and a reproducibility of 0.07‰ (2σ) is reached. Presently, we have analysed 12 molybdenites from Azegour and 2 from Tizgui and 14 others are in progress. Regarding the first 14 samples, the δ97/95MoNBS ratios vary between -0.40 and 0.32‰ for Azegour and between 0.08 and 0.30‰ for Tizgui. It is worth noting that variations can occur either at the whole site (difference of about 0.72‰) but also at the cm scale in the same sample (here the largest observed difference is up to 0.40‰). Regarding the Azegour skarn, there is no direct relationship for explaining the Mo fractionation in molydbenites between the facies or the two sites of sampling. Different processes will be discussed to explain the observed variability (redox conditions prevaling during the molydbenites deposits, late metamophism phase...). Further investigations using Pb and S isotopic compositions will help deciphering the oxidation state and the origin of molybdenites regarding the possible different fluids

Anisotropie de susceptibilité magnétique (ASM) et chimie des traces : une nouvelle méthodologie pour démêler processus hydrothermaux et supergènes. Application au gisement à Ba-Fe-F de Chaillac (Indre, France)

International audienceAnisotropy of magnetic susceptibility (AMS) determinations and trace-element analyses have been firstly carried out, at Chaillac, on two ferruginous sandstone sections of hydrothermal and lateritic origins, respectively. The goethite has been identified as the main AMS carrier. Distinct AMS patterns and characteristic Ti-As anomalies have been observed, which allows to quantify the importance of hydrothermal and lateritic processes when both are superimposed. These preliminary results show that the 'Fe-Ba complex' of hydrothermal origin at Chaillac has been lateritized for a few meters from the present erosion surface

Numerical investigation of transient hydrothermal processes around intrusions: heat-transfer and fluid-circulation controlled mineralization patterns.

International audienceNew insights on the circulation of fluids around magmatic intrusions have been obtained through coupled hydrothermal numerical modelling that takes into account i) a continuous variation of permeability with depth, ii) the period of intrusion emplacement, iii) the physical likelihood of ore deposition using a restricted rock alteration index, and iv) the so-far unexplored pluton floor, and then comparing the results against well-constrained natural cases showing different emplacement depths, high permeability zones (cracked thermal aureoles), faults and plutonic apexes. We show that emplacement depth is a key physical parameter controlling the extent and geometries of advective heat dissipation zones, and that shallow apexes strongly modify the fluid-flow pattern by acting as a focus for convective fluids and mineralization zones. We also show that the cooling phase is not the main convective phase for large plutons commonly associated with long-lived magma emplacement; major advective heat dissipation and mineral deposition zones may also develop before and during the hottest phase of the emplacement, i.e. before magma crystallization. The comparison with natural cases shows that we successfully reproduce, in space and time, the physical conditions required for mineral deposition. In particular, extensional detachment is able to restrain and modify classical fluid-flow patterns induced by coeval intrusion. Finally, even though lacking chemical arguments, we conclude that convection induced by granite emplacement plays a major role in the genesis of granite-related Au deposits. Moreover, the formation of this type of deposit is favoured and controlled by the presence of a fractured thermal aureole around the intrusion

Investigating Alternative Acidic Proteases for H/D Exchange Coupled to Mass Spectrometry: Plasmepsin 2 but not Plasmepsin 4 Is Active Under Quenching Conditions

Structural studies of proteins by hydrogen/deuterium exchange coupled to mass spectrometry (DXMS) require the use of proteases working at acidic pH and low temperatures. The spatial resolution of this technique can be improved by combining several acidic proteases, each generating a set of different peptides. Three commercial aspartic proteases are used, namely, pepsin, and proteases XIII and XVIII. However, given their low purity, high enzyme/protein ratios have to be used with proteases XIII and XVIII. In the present work, we investigate the activity of two alternative acidic proteases from Plasmodium falciparum under different pH and temperature conditions. Peptide mapping of four different proteins after digestion with pepsin, plasmepsin 2 (PSM2), and plasmepsin 4 (PSM4) were compared. PSM4 is inactive at pH 2.2 and 0°C, making it unusable for DXMS studies. However, PSM2 showed low but reproducible activity under DXMS conditions. It displayed no substrate specificity and, like pepsin, no strict sequence specificity. Altogether, these results show that PSM2 but not PSM4 is a potential new tool for DXMS studies

Palaeozoic oolitic ironstone of the French Armorican Massif: a chemical and structural trap for orogenic base metal-As-Sb-Au mineralization during Hercynian strike-slip deformation.

In the Saint-Aubin-des-Châteaux quarry (Armorican Hercynian belt, western France), an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the Lower Ordovician Grès armoricain Formation. The hydrothermal overprint produced pervasive and massive sulphidation with stratoid pyritized lenticular bodies within the oolitic ironstone layer. These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. Following the massive sulphidation stage (Fe-As, stage 1), subsequent fracturing allowed the deposition of base metals (stage 2) and Pb-Sb-Au (stage 3) parageneses in veins. The dominant brittle structures are vertical extension veins, conjugate shear veins and strike-slip faults of various orders. All these structures are filled with the same paragenetic sequence. Deformation analysis allows the identification of structures that developed incrementally via right lateral simple shear compatible with bulk strain affecting the Central Armorican Domain. Each increment corresponds to a fracture set filled with specific parageneses. Successive hydrothermal pulses reflect clockwise rotation of the horizontal shortening direction. Geothermometry on chlorite and arsenopyrite shows an input of hot hydrothermal fluids (maximum of 390-350°C) during the main sulphide stage 1. The subsequent stages present a marked temperature drop (300-275°C). Lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds to the underlying Neo-proterozoic basement. Lead isotope data, relative ages of deformation and comparison with neighbouring deposits suggest large-scale fluid pulses occurred during the whole Hercynian orogeny rather than pulses restricted to the late Hercynian period. The vicinity of the Hercynian internal domain appears as a key-control for deformation and fluid flow in the oolitic ironstones which acted as a chemical and structural trap for the hydrothermal fluids. The epigenetic mineralization of Saint-Aubin-des-Châteaux appears to be very similar to epigenetic sulphidation described in BIF-hosted gold deposits

Géochronologie revisitée du dôme du Léon (Massif armoricain, France)

National audienceDe nouvelles datations réalisées sur les principaux ensembles granitiques et métamorphiques du Léon permettent de préciser l'histoire de ce domaine de la chaîne hercynienne. Dans les domaines métamorphiques, les trois âges obtenus sur des zircons des orthogneiss granodioritiques de Brest s'échelonnent de 529 à 504 Ma, confirmant le caractère polyphasé de ce complexe orthodérivé cambrien. Dans l'unité autochtone, l'âge dévonien inférieur (385 - 391 Ma) des orthogneiss de Plounévez-Lochrist est confirmé par une nouvelle datation U-Pb sur zircon réalisée dans l'ensemble oriental. Dans les unités allochtones du Nord, les monazites du complexe migmatitique de Plouguerneau livrent un âge de 331 ± 7 Ma, interprété comme celui de la migmatitisation et conforme à celui proposé récemment par Faure et al. (335 - 327 Ma). L'âge des événements magmatiques hercyniens a également été précisé. Le granite porphyroïde de Kersaint se confirme comme le plus ancien du Léon avec un âge sur monazite de 331 ± 4 Ma. Cependant, les nouvelles datations réalisées montrent que les granites léonards sont essentiellement stéphaniens. Le granite porphyroïde rose de Guissény, satellite du granite de l'Aber-Ildut, est ainsi daté par U-Pb sur zircons à 301,4 ± 4,3 Ma. La monzodiorite de Plounévez-Lochrist, satellite mafique du grand massif de Brignogan-Plouescat, donne un âge de 296 ± 3 Ma, cohérent avec celui de 292 ± 15 Ma obtenu par Rb/Sr sur le monzogranite de Brignogan-Plouescat. Le leucogranite à tourmaline de Sainte-Catherine, considéré comme tardif par son allure en dykes sécants, est en fait également stéphanien ; le faciès le plus répandu est daté à 302,7 ± 0,3 Ma, le faciès sécant plus tardif riche en tourmaline à 301,5 ± 0,3 Ma (Ar-Ar sur muscovites). L'âge obtenu sur la mylonite formée aux dépens du leucogranite de Kernilis (292,5 ± 2,7 Ma par Ar-Ar sur muscovite) correspond vraisemblablement au jeu de l'accident mylonitique de Porspoder. Les granitisations du Léon débutent donc immédiatement après le pic métamorphique hercynien, comme dans le Massif central. La principale période de granitisation du Léon s'effectue cependant au Stéphanien, c'est-à-dire de la fin de l'extension syn-collisionnelle (300-310 Ma) à la fin de l'extension post-collisionnelle (post-300 Ma)